GEORGIA
STORMWATER MANAGEMENT MANUAL
VOLUME 2: TECHNICAL HANDBOOK
2016 EDITION
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Volume 2: Technical Handbook
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1
1.1 Objective of the Manual
1.2 Organization of the Manual
1.3 Target Audience
1.4 How to Use This Volume
1.5 Regulatory Status of the Manual
1.6 How to Find the Manual on the Internet
1.7 Contact Information
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2. Stormwater Management Planning, Design, & Implementation
2.1 The Need for Stormwater Management
4
2.1.1 Impacts of Development and Stormwater Runoff
2.1.1.1 Development Changes Land and Runoff
2.1.2 Addressing Stormwater Impacts
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2.2.2 Recommended Standards for Development
2.2.2.1 Applicability
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2.2.2.2 Recommended Stormwater Management Standards
2.2.3 Numerical Sizing Criteria Overview
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2.2 Stormwater Management Standards & Numerical Sizing Criteria
2.2.1 Overview
4
2.2.4 Description of Unified Stormwater Sizing Criteria
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2.2.4.1 Water Quality Volume (WQv)14
Volume 2: Technical Handbook - Table of Contents
1. Introduction
2.2.4.2 Channel Protection (CPv)16
2.2.4.3 Overbank Flood Protection (Qp25)17
2.2.4.4 Extreme Flood Protection (Qf)18
2.2.5 Meeting the Unified Stormwater Sizing Criteria Requirements
2.2.5.1 Introduction
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2.2.5.2 Site Design as the First Step in Addressing Unified Stormwater Sizing Criteria Requirements
19
2.2.5.3 Recommended Best Management Practices
20
2.2.5.4 Using Best Management Practices to Meet Unified Stormwater Sizing Criteria Requirements
20
2.2.5.5 Typical Steps in Addressing the Unified Stormwater Sizing Criteria
2.3 Stormwater Better Site Design & Techniques
2.3.1 Overview
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2.3.1.1 Introduction
21
2.3.1.2 List of Stormwater Better Site Design Practices and Techniques
22
2.3.1.3 Using Stormwater Better Site Design Practices
22
2.3.2 Better Site Design Practices
23
2.3.2.1 Conservation of Natural Features and Resources
23
2.3.2.2 Lower Impact Site Design Techniques
30
2.3.3 Site Design Stormwater Credits
2.3.3.1 Introduction
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2.3.3.2 Stormwater Credits and the Site Planning Process
38
2.3.3.3 Site Design Credit - Natural Area Conservation
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VOL 2
TOC
2.4.1 Stormwater Management and Site Planning
2.4.1.1 Introduction
2.4.1.2 Principles of Stormwater Management Site Planning
2.4.2 Preparation of Stormwater Management Site Plans
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2.4.2.1 Introduction
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2.4.2.2 Pre-consultation Meeting and Joint Site Visit
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2.4.2.3 Review of Local Requirements & Permitting Guidance
43
2.4.2.4 Perform Site Analysis and Inventory
45
2.4.2.5 Prepare Stormwater Concept Plan
45
2.4.2.6 Prepare Preliminary Stormwater Site Plan
46
2.4.2.7 Complete Final Stormwater Site Plan
48
2.4.2.8 Obtain Non-Local Permits
48
2.4.3 Stormwater Planning in the Development Process
2.4.3.1 General Site Development Process
2.4.3.2 Stormwater Site Planning and Design
3. Stormwater Hydrology
3.1 Methods for Estimating Stormwater Runoff
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3.1.1 Introduction to Hydrologic Methods
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3.1.2 Symbols and Definitions
59
3.1.3 Rainfall Estimation
60
3.1.4 Rational Method
61
3.1.4.1 Introduction
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3.1.4.2 Application
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3.1.4.3 Equations
62
3.1.4.4 Time of Concentration
63
3.1.4.5 Rainfall Intensity (I)
65
3.1.4.6 Runoff Coefficient (C)
66
3.1.4.7 Example Problem
3.1.5 NRCS TR-55 Hydrologic Method
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3.1.5.1 Introduction
68
3.1.5.2 Application
68
3.1.5.3 Equations and Concepts
68
3.1.5.4 Runoff Factor
70
3.1.5.5 Urban Modifications of the NRCS TR-55 Method
71
3.1.5.6 Travel Time Estimation
73
3.1.5.7 Simplified NRCS TR-55 Peak Runoff Rate Estimation
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3.1.5.8 Example Problem 1
80
3.1.5.9 Hydrograph Generation
80
3.1.5.10 Example Problem 2
3.1.6 U.S. Geological Survey Peak Flow and Hydrograph Method
Volume 2: Technical Handbook - Table of Contents
2.4 Stormwater Site Planning & Design
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3.1.6.1 Introduction
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3.1.6.2 Application
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3.1.6.4 Peak Discharge Limitations for Urban and Rural Basins
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3.1.6.5 Hydrographs
85
3.1.6.7 Example Problem
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3.1.7 Water Quality Volume and Peak Flow
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3.1.7.1 Water Quality Volume Calculation
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3.1.7.2 Water Quality Volume Peak Flow Calculation
88
3.1.7.3 Example Problem
89
3.1.7.4 Runoff Reduction Volume Calculation
89
3.1.7.5 Adjusted Curve Number Procedure for Peak Flow Reduction
89
3.1.8 Water Balance Calculations
91
3.1.8.1 Introduction
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3.1.8.2 Basic Equations
91
3.1.8.3 Example Problem
94
3.1.9 Downstream Hydrologic Assessment
3.1.9.1 Reasons for Downstream Problems
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3.1.9.2 The Ten-Percent Rule
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3.1.9.3 Example Problem
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3.2 Methods for Estimating Stormwater Volume Reduction
3.2.1 Introduction
99
3.3 Storage Design
3.3.1 General Storage Concepts
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3.3.1.1 Introduction
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3.3.1.2 Storage Classification
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3.3.1.3 Stage-Storage Relationship
102
3.3.1.4 Stage-Discharge Relationship
103
3.3.2 Symbols and Definitions
103
3.3.3 General Storage Design Procedures
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3.3.3.1 Introduction
104
3.3.3.2 Data Needs
104
3.3.3.3 Design Procedure
3.3.4 Preliminary Detention Calculations
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3.3.4.1 Introduction
106
3.3.4.2 Storage Volume
106
3.3.4.3 Alternative Method
106
3.3.4.4 Peak Flow Reduction
3.3.5 Channel Protection Volume Estimation
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3.3.5.1 Introduction
108
3.3.5.2 Basic Approach
108
3.3.5.3 Example Problem
109
3.3.6 The Modified Rational Method
3.3.6.1 Introduction
Volume 2: Technical Handbook - Table of Contents
3.1.6.3 Peak Discharge Equations
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3.3.6.2 Design Equations
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3.3.6.3 Example Problem
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TOC
113
3.4.1 Symbols and Definitions
113
3.4.2 Primary Outlets
113
3.4.2.1 Introduction
113
3.4.2.2 Outlet Structure Types
114
3.4.2.3 Orifices
115
3.4.2.4 Perforated Risers
117
3.4.2.5 Pipes and Culverts
117
3.4.2.6 Sharp-Crested Weirs
118
3.4.2.7 Broad-Crested Weirs
119
3.4.2.8 V-Notch Weirs
120
3.4.2.9 Proportional Weirs
120
3.4.2.10 Combination Outlets
120
3.4.3 Extended Detention (Water Quality and Channel Protection) Outlet Design
121
3.4.3.1 Introduction
121
3.4.3.2 Method 1: Maximum Hydraulic Head with Routing
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3.4.3.3 Method 2: Average Hydraulic Head and Average Discharge
122
3.4.4 Multi-Stage Outlet Design
123
3.4.4.1 Introduction
123
3.4.4.2 Multi-Stage Outlet Design Procedure
123
3.4.5 Extended Detention Outlet Protection
125
3.4.6 Trash Racks and Safety Grates
127
3.4.6.1 Introduction
127
3.4.6.2 Trash Rack Design
128
3.4.7 Secondary Outlets
129
3.4.7.1 Introduction
129
3.4.7.2 Emergency Spillway Design
130
4. Stormwater Best Management Practices
4.1 Stormwater Best Management Practices Overview
4.1.1 Best Management Practices
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4.1.1.1 Introduction
132
4.1.1.2 Types of Best Management Practices
132
4.1.1.3 Using Other or New Best Management Practices
135
4.1.2 Best Management Practice Pollutant Removal Capabilities
136
4.1.3 Best Management Practice Selection
136
4.1.3.1 BMP Screening Process
136
4.1.3.2 Design Process
136
4.1.3.3 Example Application
4.1.4 On-Line Versus Off-Line Best Management Practices
Volume 2: Technical Handbook - Table of Contents
3.4 Outlet Structures
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4.1.4.1 Introduction
144
4.1.4.2 Flow Regulators
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4.1.5.1 Introduction
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4.1.5.2 Advantages and Disadvantages of Regional Best Management Practices
147
4.1.5.3 Important Considerations for The Use of Regional Stormwater Management Facilities
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4.1.6 Using Best Management Practices in Series
149
4.1.6.1 Stormwater Treatment Trains
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4.1.6.2 Use of Multiple Best Management Practices in Series
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4.1.6.3 Calculation of Pollutant Removal for Water Quality Best Management Practices in Series
4.2 Bioretention Areas
4.2.1 General Description
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4.2.2 Stormwater Management Suitability
155
4.2.3 Pollutant Removal Capabilities
156
4.2.4 Application and Site Feasibility Criteria
156
4.2.5 Planning and Design Criteria
157
4.2.5.1 Location and Layout
157
4.2.5.2 General Design
158
4.2.5.3 Physical Specifications/Geometry
158
4.2.5.4 Pretreatment/Inlets
160
4.2.5.5 Outlet Structures
160
4.2.5.6 Safety Features
161
4.2.5.7 Landscaping
161
4.2.5.8 Construction Considerations
161
4.2.5.9 Construction and Maintenance Costs
4.2.6 Design Procedures
4.2.7 Inspection and Maintenance Requirements
4.3 Bioslope
4.3.1 General Description
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4.3.2 Stormwater Management Suitability
167
4.3.3 Pollutant Removal Capabilities
168
4.3.4 Application and Feasibility Criteria
168
4.3.5 Planning and Design Criteria
169
4.3.5.1 Location and Layout
169
4.3.5.2 General Design
169
4.3.5.3 Physical Specifications/Geometry
170
4.3.5.4 Pretreatment/Inlets
171
4.3.5.5 Outlet Structures
171
4.3.5.6 Emergency Spillway
171
4.3.5.7 Maintenance Access
171
4.3.5.8 Landscaping
171
4.3.5.9 Additional Site-Specific Design Criteria and Issues
171
4.3.5.10 Construction Considerations
Volume 2: Technical Handbook - Table of Contents
4.1.5 Regional vs. On-site Stormwater Management
171
4.3.6 Design Procedures
172
4.3.7 Inspection and Maintenance Requirements
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4.4.1 General Description
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4.4.2 Stormwater Management Suitability
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4.4.3 Pollutant Removal Capabilities
178
4.4.4 Application and Site Feasibility Criteria
178
4.4.5 Planning and Design Criteria
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4.4.5.1 Location and Layout
179
4.4.5.2 General Design
180
4.4.5.3 Landscaping
180
4.4.5.4 Construction Considerations
180
4.4.5.5 Construction and Maintenance Costs
180
4.4.6 Design Procedures
181
4.4.7 Inspection and Maintenance Requirements
182
4.5 Dry Detention Basins
183
4.5.1 General Description
184
4.5.2 Stormwater Management Suitability
185
4.5.3 Pollutant Removal Capabilities
185
4.5.4 Application and Site Feasibility Criteria
185
4.5.5 Planning and Design Criteria
186
4.5.5.1 Location and Layout
186
4.5.5.2 General Design
187
4.5.5.3 Physical Specifications/Geometry
187
4.5.5.4 Pretreatment/Inlets
187
4.5.5.5 Outlet Structures
188
4.5.5.6 Safety Features
188
4.5.5.7 Landscaping
188
4.5.5.8 Construction Considerations
188
4.5.5.9 Construction and Maintenance Costs
189
4.5.6 Design Procedures
189
4.5.7 Inspection and Maintenance Requirements
190
4.6 Dry Extended Detention Basins
Volume 2: Technical Handbook - Table of Contents
4.4 Downspout Disconnects
191
4.6.1 General Description
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4.6.2 Stormwater Management Suitability
193
4.6.3 Pollutant Removal Capabilities
193
4.6.4 Application and Site Feasibility Criteria
193
4.6.5 Planning and Design Criteria
194
4.6.5.1 Location and Layout
194
4.6.5.2 General Design
195
4.6.5.3 Physical Specifications/Geometry
195
4.6.5.4 Pretreatment/Inlets
195
4.6.5.5 Outlet Structures
196
4.6.5.6 Safety Features
196
4.6.5.7 Landscaping
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4.6.5.9 Construction and Maintenance Costs
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4.6.6 Design Procedures
197
4.6.7 Inspection and Maintenance Requirements
198
4.7 Dry Wells
199
4.7.1 General Description
200
4.7.2 Stormwater Management Suitability
201
4.7.3 Pollutant Removal Capabilities
201
4.7.4 Application and Site Feasibility Criteria
202
4.7.5 Planning and Design Criteria
203
4.7.5.1 Location and Layout
203
4.7.5.2 General Design
204
4.7.5.3 Pretreatment/Inlets
205
4.7.5.4 Outlet Structures
205
4.7.5.5 Construction and Maintenance Costs
205
4.7.7.6 Safety Features
205
4.7.5.7 Landscaping
205
4.7.5.8 Construction Considerations
4.7.6 Design Procedures
4.7.7 Inspection and Maintenance Requirements
4.8 Dry Enhanced Swales/Wet Enhanced Swales
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4.8.1 General Description
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4.8.2 Stormwater Management Suitability
211
4.8.3 Pollutant Removal Capabilities
211
4.8.4 Application and Feasibility Criteria
212
4.8.5 Planning and Design Criteria
212
4.8.5.1 Location and Layout
212
4.8.5.2 General Design
213
4.8.5.3 Physical Specifications/Geometry
213
4.8.5.4 Pretreatment/Inlets
215
4.8.5.5 Outlet Structures
215
4.8.5.6 Emergency Spillway
215
4.8.5.7 Maintenance Access
215
4.8.5.8 Safety Features
215
4.8.5.9 Landscaping
215
4.8.5.10 Additional Site-Specific Design Criteria and Issues
216
4.8.5.11 Construction Considerations
216
4.8.6 Design Procedures
217
4.8.7 Inspection and Maintenance Requirements
220
4.9 Grass Channel
Volume 2: Technical Handbook - Table of Contents
4.6.5.8 Construction Considerations
221
4.9.1 General Description
222
4.9.2 Stormwater Management Suitability
222
4.9.3 Pollutant Removal Capabilities
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4.9.5 Planning and Design Criteria
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4.9.5.1 Location and Layout
224
4.9.6 Design Procedures
225
4.9.7 Inspection and Maintenance Requirements
226
4.10 Gravity (Oil-Grit) Separator
227
4.10.1 General Description
228
4.10.2 Stormwater Management Suitability
228
4.10.3 Pollutant Removal Capabilities
229
4.10.4 Application and Site Feasibility Criteria
229
4.10.5 Planning and Design Criteria
230
4.10.5.1 Location and Layout
231
4.10.5.2 General Design
231
4.10.5.3 Physical Specifications/Geometry
231
4.10.5.4 Pretreatment/Inlets
231
4.10.5.5 Outlet Structures
231
4.10.5.6 Safety Features
232
4.10.5.7 Construction Considerations
232
4.10.5.8 Construction and Maintenance Costs
232
4.10.6 Design Procedures
4.10.7 Inspection and Maintenance Requirements
4.11 Green Roof
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4.11.1 General Description
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4.11.2 Stormwater Management Suitability
237
4.11.3 Pollutant Removal Capabilities
237
4.11.4 Application and Site Feasibility Criteria
238
4.11.5 Planning and Design Criteria
239
4.11.5.1 Location and Layout
239
4.11.5.2 General Design
239
4.11.5.3 Physical Specifications/Geometry
240
4.11.5.4 Pretreatment/Inlets
240
4.11.5.5 Outlet Structures
240
4.11.5.6 Safety Features
240
4.11.5.7 Landscaping
240
4.11.5.8 Construction Considerations
241
4.11.5.9 Construction and Maintenance Costs
4.11.6 Design Procedures
4.11.7 Maintenance Requirements
4.12 Infiltration Practices
Volume 2: Technical Handbook - Table of Contents
4.9.4 Application and Site Feasibility Criteria
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4.12.1 General Discussion
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4.12.2 Stormwater Management Suitability
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4.12.3 Pollutant Removal Capabilities
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4.12.4 Application and Site Feasibility Criteria
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4.12.5.1 Location and Layout
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4.12.5.2 General Design
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4.12.5.3 Physical Specifications/Geometry
250
4.12.5.4 Pretreatment/Inlets
251
4.12.5.5 Outlet Structures
251
4.12.5.6 Emergency Spillway
251
4.12.5.7 Maintenance Access
251
4.12.5.8 Safety Features
251
4.12.5.9 Landscaping
251
4.12.5.10 Additional Site-Specific Design Criteria and Issues
252
4.12.6 Design Procedures
253
4.12.7 Inspection and Maintenance Requirements
256
4.13 Multi-Purpose Detention Areas
257
4.13.1 General Description
258
4.13.2 Design Criteria and Specifications
258
4.13.3 Inspection and Maintenance Requirements
4.14 Organic Filter
259
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4.14.1 General Description
261
4.14.2 Pollutant Removal Capabilities
261
4.14.3 Design Criteria and Specifications
261
4.14.4 Inspection and Maintenance equirements
262
4.15 Permeable Paver Systems
263
4.15.1 General Description
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4.12.2 Stormwater Management Suitability
264
4.15.3 Pollutant Removal Capabilities
265
4.15.4 Design Criteria and Specifications
265
4.15.5 Design Procedures
267
4.15.6 Inspection and Maintenance Requirements
270
4.16 Pervious Concrete
271
4.16.1 General Description
272
4.16.2 Stormwater Management Suitability
273
4.16.3 Pollutant Removal Capabilities
274
4.16.4 Application and Site Feasibility Criteria
274
4.16.5 Planning and Design Criteria
275
4.16.6 Design Procedures
278
4.16.7 Inspection and Maintenance Requirements
4.17 Porous Asphalt
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4.12.5 Planning and Design Criteria
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4.17.1 General Description
282
4.17.2 Stormwater Management Suitability
283
4.17.3 Pollutant Removal Capabilities
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4.17.5 Planning and Design Criteria
284
4.17.6 Design Procedures
286
4.17.7 Inspection and Maintenance Requirements
289
4.18 Proprietary Systems
290
4.18.1 General Description
291
4.18.2 Guidelines for Using Proprietary Systems
291
4.18.3 Inspection and Maintenance Requirements
292
4.19 Rainwater Harvesting
293
4.19.1 General Description
294
4.19.2 Stormwater Management Suitability
295
4.19.3 Pollutant Removal Capabilities
295
4.19.4 Application and Site Feasibility Criteria
295
4.19.5 Planning and Design Criteria
296
4.19.5.1 Location and Layout
296
4.19.5.2 General Design
296
4.19.5.3 Physical Specifications/Geometry
296
4.19.5.4 Pretreatment/Inlets
297
4.19.5.5 Outlet Structures
297
4.19.5.6 Safety Features
297
4.19.5.7 Landscaping
298
4.19.5.8 Construction Considerations
298
4.19.6 Design Procedures
299
4.19.7 Maintenance Requirements
300
4.20 Regenerative Stormwater Conveyance
301
4.20.1 General Description
302
4.20.2 Stormwater Management Suitability
302
4.20.3 Pollutant Removal Capabilities
303
4.20.4 Application and Site Feasibility Criteria
303
4.20.5 Planning and Design Criteria
304
4.20.5.1 Location and Layout
304
4.20.5.2 General Design
304
4.20.5.3 Physical Specifications/Geometry
304
4.20.5.4 Pretreatment/Inlets
306
4.20.5.5 Outlet Structures
306
4.20.5.6 Safety Features
306
4.20.5.7 Landscaping
306
4.20.5.8 Construction Considerations
306
4.20.6 Design Procedures
307
4.20.7 Inspection and Maintenance Requirements
308
4.21 Sand Filters
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4.17.4 Application and Site Feasibility Criteria
309
4.21.1 General Description
310
4.21.2 Stormwater Management Suitability
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4.21.4 Application and Site Feasibility Criteria
312
4.21.5 Planning and Design Criteria
313
4.21.5.1 Location and Layout
313
4.21.5.2 General Design
314
4.21.5.3 Physical Specifications/Geometry
314
4.21.5 Pretreatment/Inlets
317
4.21.5.5 Outlet Structures
317
4.21.5.6 Emergency Spillway
317
4.21.5.7 Maintenance Access
319
4.21.5.8 Safety Features
319
4.21.5.9 Landscaping
319
4.21.5.10 Additional Site-Specific Design Criteria and Issues
4.21.6 Design Procedures
4.21.7 Inspection and Maintenance Requirements
4.22 Site Reforestation/Revegetation
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4.22.1 General Description
324
4.22.2 Stormwater Management Suitability
324
4.22.3 Applications and Site Feasibility Criteria
325
4.22.4 Planning and Design Criteria
326
4.22.5 Construction Considerations
327
4.22.6 Inspection and Maintenance Requirements
4.23 Soil Restoration
4.23.1 General Description
327
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4.23.2 Stormwater Management Suitability
329
4.23.3 Applications and Site Feasibility Criteria
330
4.23.4 Planning and Design Criteria
331
4.23.5 Construction Considerations
332
4.23.6 Inspection and Maintenance Requirements
4.24 Stormwater Planters/Tree Boxes
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4.21.3 Pollutant Removal Capabilities
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4.24.1 General Description
334
4.24.2 Stormwater Management Suitability
335
4.24.3 Pollutant Removal Capabilities
335
4.24.4 Application and Site Feasibility Criteria
335
4.24.5 Planning and Design Criteria
336
4.24.5.1 Location and Layout
337
4.24.5.2 General Design
337
4.24.5.3 Pretreatment/Inlets
338
4.24.5.4 Outlet Structures
338
4.24.7.5 Safety Features
338
4.24.5.6 Landscaping
338
4.24.5.7 Construction Considerations
339
4.24.5.8 Construction and Maintenance Costs
339
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4.24.7 Maintenance Requirements
4.25 Stormwater Ponds
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4.25.1 General Description
344
4.25.2 Stormwater Management Suitability
348
4.25.3 Pollutant Removal Capabilities
348
4.25.4 Application and Site Feasibility Criteria
348
4.25.5 Planning and Design Criteria
350
4.25.5.1 Location and Layout
350
4.25.5.2 General Design
350
4.25.5.3 Physical Specifications/Geometry
351
4.25.5.4 Pretreatment/Inlets
352
4.25.5.5 Outlet Structures
352
4.25.5.6 Emergency Spillways
353
4.25.5.7 Maintenance Access
353
4.25.5.8 Safety Features
354
4.25.5.9 Landscaping
354
4.25.5.10 Construction Considerations
354
4.25.5.11 Construction and Maintenance Costs
4.25.6 Design Procedures
4.25.7 Inspection and Maintenance Requirements
4.26 Stormwater Wetlands
4.26.1 General Description
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4.26.2 Stormwater Management Suitability
361
4.26.3 Pollutant Removal Capabilities
362
4.26.4 Application and Site Feasibility Criteria
362
4.26.5 Planning and Design Criteria
364
4.26.5.1 Location and Layout
364
4.26.5.2 General Design
364
4.26.5.3 Physical Specifications/Geometry
365
4.26.5.4 Pretreatment/Inlets
366
4.26.5.5 Outlet Structures
366
4.26.5.6 Emergency Spillway
367
4.26.5.7 Maintenance Access
368
4.26.5.8 Safety Features
368
4.26.5.9 Landscaping
368
4.26.5.10 Additional Site Specific Design Criteria and Issues
369
4.26.6 Design Procedures
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4.24.6 Design Procedures
370
4.26.7 Inspection and Maintenance Requirements
371
4.26.8 Example Schematics
372
4.27 Submerged Gravel Wetlands
374
4.27.1 General Description
375
4.27.2 Stormwater Management Suitability
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376
4.27.4 Application and Site Feasibility Criteria
376
4.27.5 Planning and Design Criteria
378
4.27.5.1 Location and Layout
378
4.27.5.2 General Design
378
4.27.5.3 Physical Specifications/Geometry
378
4.27.5.4 Pretreatment/Inlets
378
4.27.5.5 Outlet Structures
378
4.27.5.6 Safety Features
379
4.27.5.7 Landscaping
379
4.27.5.8 Construction Considerations
379
4.27.5.9 Construction and Maintenance Costs
4.27.6 Design Procedures
4.27.7 Inspection and Maintenance Requirements
4.28 Underground Detention
379
380
381
382
4.28.1 General Description
383
4.28.2 Stormwater Management Suitability
384
4.28.3 Pollutant Removal Capabilities
384
4.28.4 Application and Site Feasibility Criteria
384
4.28.5 Planning and Design Criteria
385
4.28.5.1 Location and Layout
385
4.28.5.2 General Design
385
4.28.5.3 Physical Specifications/Geometry
386
4.28.5.4 Pretreatment/Inlets
386
4.28.5.5 Outlet Structures
386
4.28.5.6 Safety Features
386
4.28.5.7 Construction Considerations
386
4.28.5.8 Construction and Maintenance Costs
386
4.28.6 Design Procedures
4.28.7 Inspection and Maintenance Requirements
4.29 Vegetated Filter Strip
4.29.1 General Description
Volume 2: Technical Handbook - Table of Contents
4.27.3 Pollutant Removal Capabilities
387
387
388
389
4.29.2 Stormwater Management Suitability
389
4.29.3 Pollutant Removal Capabilities
390
4.29.4 Application and Site Feasibility Criteria
390
4.29.5 Planning and Design Criteria
391
4.29.5.1 Location and Layout
391
4.29.5.2 General Design
391
4.29.5.3 Physical Specifications/Geometry
392
4.29.6 Design Procedures
394
4.29.7 Inspection and Maintenance Requirements
396
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398
5.1 Stormwater Drainage Design Overview
398
5.1.1 Stormwater Drainage System Design
398
5.1.1.1 Introduction
398
5.1.1.2 Drainage System Components
398
5.1.1.3 Checklist for Drainage Planning and Design
398
5.1.2 Key Issues in Stormwater Drainage Design
399
5.1.2.1 Introduction
399
5.1.2.2 General Drainage Design Considerations
399
5.1.2.3 Street and Roadway Gutters
399
5.1.2.4 Inlets and Drains
400
5.1.2.5 Storm Drain Pipe Systems (Storm Sewers)
400
5.1.2.6 Culverts
401
5.1.2.7 Open Channels
401
5.1.2.8 Energy Dissipators
401
5.1.3 Design Storm Recommendations
402
5.2 Minor Drainage System Design
402
5.2.1 Overview
402
5.2.1.1 Introduction
402
5.2.1.2 General Criteria
403
5.2.2 Symbols and Definitions
5.2.3 Street and Roadway Gutters
403
404
5.2.3.1 Formula
404
5.2.3.2 Nomograph
404
5.2.3.3 Manning’s n Table
404
5.2.3.4 Uniform Cross Slope
404
5.2.3.5 Composite Gutter Sections
405
5.2.4 Stormwater Inlets
408
5.2.5 Grate Inlet Design
408
5.2.5.1 Grate Inlets on Grade
5.2.5.2 Grate Inlets in Sag
5.2.6 Curb Inlet Design
408
412
414
5.2.6.1 Curb Inlets on Grade
414
5.2.6.2 Curb Inlets in Sump
416
5.2.7 Combination Inlets
419
5.2.7.1 Combination Inlets On Grade
419
5.2.7.2 Combination Inlets In Sump
419
5.2.8 Storm Drain Pipe Systems
Volume 2: Technical Handbook - Table of Contents
5. Stormwater Drainage System Design
419
5.2.8.1 Introduction
419
5.2.8.2 General Design Procedure
419
5.2.8.3 Design Criteria
419
5.2.8.4 Capacity Calculations
421
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421
5.2.8.6 Hydraulic Grade Lines
421
5.2.8.7 Minimum Grade
427
5.2.8.8 Storm Drain Storage
5.3 Culvert Design
427
428
5.3.1 Overview
428
5.3.2 Symbols and Definitions
428
5.3.3 Design Criteria
428
5.3.3.1 Frequency Flood
428
5.3.3.2 Velocity Limitations
428
5.3.3.3 Buoyancy Protection
429
5.3.3.4 Length and Slope
429
5.3.3.5 Debris Control
429
5.3.3.6 Headwater Limitations
429
5.3.3.7 Tailwater Considerations
429
5.3.3.8 Storage
430
5.3.3.9 Culvert Inlets
430
5.3.3.10 Inlets with Headwalls
430
5.3.3.11 Wingwalls and Aprons
430
5.3.3.12 Improved Inlets
430
5.3.3.13 Material Selection
430
5.3.3.14 Culvert Skews
430
5.3.3.15 Culvert Sizes
430
5.3.3.16 Weep Holes
430
5.3.3.17 Outlet Protection
430
5.3.3.18 Erosion and Sediment Control
430
5.3.3.19 Environmental Considerations
430
5.3.4 Design Procedures
432
5.3.4.1 Types of Flow Control
432
5.3.4.2 Procedures
432
5.3.4.3 Nomographs
432
5.3.4.4 Design Procedure
434
5.3.4.5 Performance Curves - Roadway Overtopping
434
5.3.4.6 Storage Routing
436
5.3.5 Culvert Design Example
436
5.3.5. Introduction
436
5.3.5.2 Example
436
5.3.5.3 Example Data
436
5.3.5.4 Computations
436
5.3.6 Design Procedures for Beveled-Edged Inlets
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5.2.8.5 Nomographs and Table
439
5.3.6.1 Introduction
439
5.3.6.2 Design Figures
439
5.3.6.3 Design Procedure
439
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439
5.3.6.5 Multibarrel Installations
439
5.3.6.6 Skewed Inlets
440
5.3.7 Flood Routing and Culvert Design
5.3.7.1 Introduction
5.3.7.2 Design Procedure
5.4 Open Channel Design
5.4.1 Overview
5.4.1.1 Introduction
5.4.1.2 Open Channel Types
440
440
440
441
441
441
441
5.4.2 Symbols and Definitions
442
5.4.3 Design Criteria
443
5.4.3.1 General Criteria
5.4.3.2 Velocity Limitations
443
443
5.4.4 Manning’s Values
444
5.4.5 Uniform Flow Calculations
444
5.4.5.1 Design Charts
444
5.4.5.2 Manning’s Equation
444
5.4.5.3 Geometric Relationships
444
5.4.5.4 Direct Solutions
444
5.4.5.5 Trial and Error Solutions
450
5.4.6 Critical Flow Calculations
5.4.6.1 Background
5.4.6.2 Semi-Empirical Equations
452
452
452
5.4.7 Vegetative Design
454
5.4.7.1 Introduction
454
5.4.7.2 Design Stability
454
5.4.7.3 Design Capacity
5.4.8 Riprap Design
455
456
5.4.8.1 Assumptions
456
5.4.8.2 Procedure
456
5.4.9 Uniform Flow - Example Problems
459
5.4.10 Gradually Varied Flow
461
5.4.11 Rectangular, Triangular, and Trapezoidal Open Channel Design Figures
461
5.4.11.1 Introduction
461
5.4.11.2 Description of Figures
461
5.4.11.3 Instructions for Rectangular and Trapezoidal Figures
462
5.4.11.4 Grassed Channel Figures
467
5.4.11.5 Description of Figures
467
5.4.11.6 Instructions for Grassed Channel Figures
5.5 Energy Dissipation Design
5.5.1 Overview
5.5.1.1 Introduction
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5.3.6.4 Design Figure Limits
467
471
471
471
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5.5.1.3 Recommended Energy Dissipators
471
471
5.5.2 Symbols and Definitions
472
5.5.3 Design Guidelines
472
5.5.4 Riprap Aprons
473
5.5.4.1 Description
473
5.5.4.2 Design Procedure
473
5.5.4.3 Design Considerations
476
5.5.4.4 Example Designs
476
5.5.5 Riprap Basins
477
5.5.5.1 Description
477
5.5.5.2 Basin Features
477
5.5.5.3 Design Procedure
477
5.5.5.4 Design Considerations
482
5.5.5.5 Example Designs
482
5.5.6 Baffled Outlets
5.5.6.1 Description
485
485
5.5.6.2 Design Procedure
485
5.5.6.3 Example Design
486
Appendix A: Rainfall Tables for Georgia
489
Appendix B: Best Management Practices Design Examples
490
Appendix B-1: Stormwater Pond Design Example
491
Appendix B-2: Bioretention Area Design Example
505
Appendix B-3: Sand Filter Design Example
510
Appendix B-4: Infiltration Trench Design Example
519
Appendix B-5: Enhanced Swale Design Example
524
Appendix C: Nomographs & Design Aids
Volume 2: Technical Handbook - Table of Contents
5.5.1.2 General Criteria
532
C-1 Culvert Design Charts and Nomographs
533
C-2 Open Channel Design Figures
593
C-3 Triangular Channel Nomograph
621
C-4 Grassed Channel Design Figures
622
Appendix D: Planting & Soil Guidance
627
Appendix E: Best Management Practice Operations & Maintenance
654
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1. Introduction
ter quantity and quality through recommended
Local jurisdictions may adopt and apply the
The objective of the Georgia Stormwater Man-
stormwater management standards and local
recommended standards for new development
agement Manual is to provide guidance on the
stormwater programs. It also provides an over-
and redevelopment in this Manual directly as part
best post-construction stormwater management
view of integrated stormwater management and
of their local development code. Further, local
practices available to Georgia communities to
technologies and tools for implementing storm-
jurisdictions may use Volume 2 to review storm-
minimize the negative impacts of increasing
water management programs.
water site plans and provide technical advice, and
may adopt any part of the guidance and design
stormwater runoff and its associated pollutants.
The goal is to provide an effective tool for local
Volume 2 of the Manual, the Technical Hand-
criteria for best management practices and drain-
governments and the development community
book, provides guidance on the techniques and
age design contained in this Manual as their local
to reduce both stormwater quality and quanti-
measures that can be implemented to meet a
engineering design requirements. Check with the
ty impacts and protect downstream areas and
set of recommended stormwater management
local review authority for more information.
receiving waters.
standards for new development and redevelopment. Volume 2 is designed to provide the site
Those parties involved with site development
This Manual does not cover construction site sed-
designer or engineer, as well as the local plan
will utilize Volume 2 for technical guidance and
iment and erosion control practices. Guidance on
reviewer or inspector, with all of the information
information on the preparation of stormwater site
these practices can be found in the latest edition
on best management practices (BMPs) required to
plans, the use of better site design techniques,
of the Manual for Erosion and Sediment Control
effectively address and control both water quality
hydrologic techniques, selection and design of
in Georgia.
and quantity on a development site. This includes
appropriate best management practices, and
guidance on site planning, better site design
drainage (hydraulic) design.
1.2 Organization of the Manual
The Georgia Stormwater Management Manual is
organized as a three volume set, with each volume published as a separate document. You are
currently reading Volume 2 of the Manual.
Volume 1 of the Manual, the Local Government
Guide to Stormwater Management, is designed
to provide guidance for local jurisdictions on the
basic principles of effective urban stormwater
Introduction
1.1 Objective of the Manual
practices, hydrologic techniques, criteria for the
selection and design of stormwater BMPs, and
1.4 How to Use This Volume
drainage system design, as well as construction
The following provides a guide to the various
and maintenance information.
chapters of Volume 2 of the Manual.
Volume 3, the Pollution Prevention Guidebook, is
a compendium of pollution prevention practices
for stormwater quality for use by local jurisdictions, businesses and industry, and local citizens.
management. Volume 1 covers the environ-
1.3 Target Audience
mental, economic and social problems resulting
The users of Volume 2 will be site planners, engi-
from urban stormwater runoff and the need for
neers, contractors, plan reviewers, and inspectors
local communities to address urban stormwa-
from local government and the development
• Chapter 1 (Introduction). This chapter
discusses the overall purpose and organization
of the Manual, the intended use and other
background information.
• Chapter 2 (Stormwater Management Planning,
Design, & Implementation). This chapter
provides the framework for addressing
stormwater runoff on new development and
redevelopment sites. This chapter includes the
following sections:
community.
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»» Section 2.2 – Stormwater Management
Standards & Numerical Sizing Criteria.
This section contains the stormwater
management recommended standards for
new development and redevelopment sites,
explains the four sizing criteria for water
quality, channel protection, overbank flood
protection, and extreme flood protection,
and describes the approaches for meeting
the criteria through the use of better site
design practices and best management
practices (BMPs).
»» Section 2.3 – Stormwater Better Site Design
& Techniques. This section covers the toolkit
of better site design practices and techniques
that can be used to reduce the amount of
stormwater runoff and pollutants generated
from a site.
»» Section 2.4 – Stormwater Site Planning &
Design. This section outlines the typical
contents and procedures for preparing a
stormwater site plan.
• Chapter 3 (Stormwater Hydrology). This
chapter presents engineering topics and
methods used in stormwater drainage,
conveyance and facility design.
»» Section 3.1 – Methods for Estimating
Stormwater Runoff. This section provides an
overview of the different hydrologic methods
and their application.
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»» Section 3.2 – Methods for Estimating
Stormwater Volume Reduction – Design
Worksheet. This section provides an overview
of the spreadsheet developed to calculate
runoff reduction and TSS removal.
»» Section 3.3 – Storage Design. This section
covers the criteria and general procedures
for the design and evaluation of stormwater
storage (detention and retention) facilities.
»» Section 3.4 – Outlet Structures. This section
outlines various stormwater facility outlet
types and provides criteria and procedures
for water quality outlet design.
• Chapter 4 (Stormwater Best Management
Practices). This chapter contains the
information and guidance for the selection and
design of best management practices (BMPs)
for managing stormwater quantity and quality.
It is divided into the following sections:
»» Section 5.1 – Stormwater Drainage Design
Overview.
»» Section 5.2 – Minor Drainage System Design.
This section provides guidelines and design
criteria for gutter and inlet hydraulics, and
provides an overview of storm drain pipe
system design.
»» Section 5.3 – Culvert Design. This section
covers criteria and procedures for the design
and evaluation of culverts.
»» Section 5.4 – Open Channel Design. This
section describes the criteria and calculations
for the design of open stormwater drainage
channels.
»» Section 5.5 – Energy Dissipation Design.
This section includes information and design
criteria for a number of energy dissipators,
including riprap aprons, riprap basins and
baffled outlets.
»» Section 4.1 – Stormwater BMP Overview.
This section provides an overview of the best
management practices that can be used to
reduce and/or treat stormwater runoff and/or
mitigate the effects of increased runoff peak
rates, volumes, and velocities.
• Appendix A – Rainfall Tables for Georgia. This
appendix contains a weblink to the National
Oceanic and Atmospheric Administration’s
website where users can access the most
recent rainfall data available.
»» Section 4.2 through 4.29 – Individual BMP
Sections. These sections contain detailed
information and design criteria for each best
management practice and their applicability
for sites with a demonstrated ability to meet
stormwater management goals.
• Appendix B – Best Management Practice
Design Examples. This appendix includes
design examples for different best
management practices: stormwater pond,
bioretention area, surface sand filter, infiltration
trench, vegetated filter strip, and enhanced
(dry) swale.
• Chapter 5 (Stormwater Drainage System
Design). This chapter provides technical
guidance on the various elements of
stormwater drainage design. This chapter
includes the following sections:
Introduction
»» Section 2.1 – The Need for Stormwater
Management. This section provides an
overview of the impacts of stormwater
runoff.
• Appendix C – Nomographs & Design Aids.
This appendix includes various nomographs
and other design tools primarily referenced
throughout Chapter 5.
VOL 2
2
(1) Codes and ordinances established by local
• Appendix E – Best Management Practice
Operations & Maintenance. The appendix
provides general information and checklists
for the inspection and maintenance of postconstruction stormwater best management
practices. A site specific checklist regarding the
inspection and maintenance needs for each
BMP is recommended.
It is recommended that all Georgia communities
governments
1.7 Contact Information
If you have any technical questions or comments
on the Manual, please send an email to:
(2) Rules, regulations, and permits established by
local, state and federal agencies
info@georgiastormwater.com
Introduction
• Appendix D – Planting & Soil Guidance.
This appendix provides landscaping
criteria and plant selection guidance for
stormwater management facilities. A site
specific landscaping plan for each BMP is
recommended.
use the information presented in this manual, or
an equivalent post-construction stormwater management manual, to regulate new development
and redevelopment activities. For those communities that are covered under the National Pollutant Discharge Elimination System (NPDES) Municipal Stormwater Program, adoption of portions of
this manual (or an equivalent) are required.
1.5 Regulatory Status of the
Manual
Communities are encouraged to review and mod-
The Georgia Stormwater Management Manual is
ify the contents of this manual, as necessary, to
designed to provide Georgia communities with
meet local watershed and stormwater manage-
comprehensive guidance on a Low Impact Devel-
ment goals and objectives while still maintaining
opment (LID)-based approach to natural resource
the essence of a Low Impact Development (LID)-
protection, stormwater management and site de-
based approach for stormwater management.
sign that they can use to better protect the state’s
pacts of land development and nonpoint source
1.6 How to Find the Manual on
the Internet
pollution. Although communities may choose to
All three volumes of the Georgia Stormwater
use the information presented in this manual to
Management Manual are available in Adobe Ac-
regulate new development and redevelopment
robat PDF document format for download at the
activities, the document itself has no independent
following Internet address:
valuable natural resources from the negative im-
regulatory authority. The approach to natural resource protection, stormwater management and
http://www.georgiastormwater.com
site design detailed in the Georgia Stormwater
Management Manual can only become required
through:
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3
2. Stormwater Management Planning, Design, & Implementation
effect is further exacerbated by drainage systems
Finally, development and urbanization affect
such as gutters, storm sewers and lined channels
not only the quantity of stormwater runoff, but
that are designed to quickly carry runoff to rivers
also its quality. Development increases both the
2.1.1 Impacts of Development and
Stormwater Runoff
and streams.
concentration and types of pollutants carried by
runoff. As it runs over rooftops and lawns, parking
lots and industrial sites, stormwater picks up and
Land development changes not only the physical,
transports a variety of contaminants and pollut-
but also the chemical and biological conditions
ants to downstream waterbodies. The loss of the
of Georgia’s waterways and water resources. This
original topsoil and vegetation removes a valuable
chapter describes the changes that occur due to
filtering mechanism for stormwater runoff.
development and the resulting stormwater runoff
impacts.
The cumulative impact of development and
urban activities, and the resultant changes to both
2.1.1.1 DEVELOPMENT CHANGES LAND AND
RUNOFF
When land is developed, the hydrology, or the
natural cycle of water is disrupted and altered.
stormwater quantity and quality in the entire land
area that drains to a stream, river, lake or estuary
Figure 2.1.1-1 Clearing and Grading
Alter the Hydrology of the Land
determines the conditions of the waterbody. This
land area that drains to the waterbody is known
Clearing removes the vegetation that intercepts,
Development and impervious surfaces also re-
as its watershed. Urban development within a
slows and returns rainfall to the air through
duce the amount of water that infiltrates into the
watershed has a number of direct impacts on
evaporation and transpiration. Grading flattens
soil and groundwater, thus reducing the amount
downstream waters and waterways. These im-
hilly terrain and fills in natural depressions that
of water that can recharge aquifers and feed
pacts include:
slow and provide temporary storage for rainfall.
streamflow during periods of dry weather.
The topsoil and sponge-like layers of humus are
Stormwater Management Planning, Design & Implementation
2.1 The Need for Stormwater
Management
• Changes to stream flow
scraped and removed and the remaining subsoil
• Changes to stream geometry
is compacted. Rainfall that once seeped into the
ground now runs off the surface. The addition
• Degradation of aquatic habitat
of buildings, roadways, parking lots and other
• Water quality impacts
surfaces that are impervious to rainfall further
reduces infiltration and increases runoff.
The remainder of this section discusses these imDepending on the magnitude of changes to the
pacts and why effective stormwater management
land surface, the total runoff volume can increase
is needed to address and mitigate them.
dramatically. These changes not only increase
the total volume of runoff, but also accelerate the
rate at which runoff flows across the land. This
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Figure
2.1.1-2 Impervious Cover Increases Runoff Peak
Flows and Volumes While Reducing Recharge
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4
The remainder of Chapter 2 outlines a tech-
The focus of this Manual is how to effectively
nical approach for incorporating all of these
deal with the impacts of urban stormwater runoff
stormwater management approaches into the
through effective and comprehensive storm-
development process. The next section dis-
water management. Stormwater management
cusses stormwater management standards and
involves the reduction, prevention, and mitigation
numerical sizing criteria for new development and
of stormwater runoff quantity and quality impacts
redevelopment in Georgia that aim to meet the
It should be noted that the standards presented
as described in this chapter through a variety of
objectives above.
here are recommended for all communities in
2.
Managing and treating stormwater runoff
though the use of best management
practices (BMPs);
3.
Implementing pollution prevention practices
to limit potential stormwater contaminants.
Georgia. They may be adopted by local jurisdic-
methods and mechanisms.
tions as stormwater management development
Volume 2 of this Manual deals with ways that
developers in Georgia can effectively implement
stormwater management to address the impacts
2.2 Stormwater Management
Standards & Numerical Sizing
Criteria
requirements and/or may be modified to meet
local or watershed-specific stormwater management goals and objectives. Please consult your
local review authority for more information.
of new development and redevelopment, and reduce, prevent, and mitigate problems associated
2.2.1 Overview
with stormwater runoff. This is accomplished by:
This section presents a comprehensive set of
The recommended standards for development
recommended performance standards for storm-
are designed to assist local governments in com-
water management for development activities in
plying with regulatory and programmatic require-
the state of Georgia. The overall aim is to provide
ments for various state and Federal programs
an integrated approach to address both the water
including the National Pollutant Discharge Elimi-
quality and quantity problems associated with
nation System (NPDES) Municipal Separate Storm
stormwater runoff due to development.
Sewer System (MS4) permit program and the
❏❏Developing land in a way that minimizes its
impact on a watershed, and reduces both the
amount of runoff and pollutants generated
through infiltration, evapotranspiration, and
reuse.
❏❏Using the most current and effective erosion
and sedimentation control practices during the
construction phase of development
❏❏Reducing and controlling stormwater runoff
peaks, volumes, and velocities to prevent both
downstream flooding and streambank channel
erosion
❏❏Treating post-construction stormwater runoff
before it is discharged to a waterway
❏❏Implementing pollution prevention practices
to prevent stormwater from becoming
contaminated in the first place
❏❏Using various techniques to maintain
groundwater recharge
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Stormwater Management Planning, Design & Implementation
2.1.2 Addressing Stormwater Impacts
National Flood Insurance Program under FEMA.
The goal of a set of recommended stormwater
management standards for areas of new development and significant redevelopment is to reduce
the impact of post-construction stormwater runoff on the watershed. This can be achieved by:
1.
Maximizing the use of site design and
nonstructural methods such as canopy
interception, infiltration, evapotranspiration
and reuse to reduce the generation of runoff
and pollutants;
VOL 2
5
in urban stormwater runoff. Examples of
hotspots include gas stations, vehicle service
and maintenance areas, industrial facilities
such as salvage yards (both permitted under
the Industrial General Permit and others),
material storage sites, garbage transfer
facilities, and commercial parking lots with
high-intensity use.
stormwater runoff coming from the site will cause
significant impacts to the receiving waters due to
the type or location of the development site or
2.2.2.1 APPLICABILITY
other circumstances.
It is recommended that the stormwater management standards listed below be required for
any new development or redevelopment site that
meets one or more of the following criteria:
1.
2.
3.
New development that includes the creation
or addition of 5,000 square feet or greater of
new impervious surface area, or that involves
land disturbing activity of 1 acre of land or
greater.
Redevelopment that includes the creation or
addition of 5,000 square feet or greater of
new impervious surface area, or that involves
land disturbing activity of 1 acre or more.
Any commercial or industrial new
development or redevelopment, regardless
of size, with a Standard Industrial
Classification (SIC) code that falls under
the NPDES Industrial Stormwater Permit
program, or is a hotspot land use as defined
below.
Since runoff from smaller developments can
cause water quality and quantity impacts as well,
an individual community may choose to adopt
more stringent area criteria, especially if it determines that a significant amount of development
Definitions
»» New development is defined as land
disturbing activities, structural development
(construction, installation or expansion of a
building or other structure), and/or creation
of impervious surfaces on a previously
undeveloped site.
The goals and policies of individual communities
should determine the specific definition of pre-development. It is recommended that pre-development be defined as “natural, undisturbed conditions.”
This can be simplified to a set type of vegetative
»» Redevelopment is defined as structural
development (construction, installation, or
expansion of a building or other structure),
creation or addition of impervious surfaces,
replacement of impervious surfaces not
as part of routine maintenance, and land
disturbing activities associated with structural
or impervious development on a previously
developed site. Redevelopment does not
include such activities as exterior remodeling.
»» Previously developed site is defined as a site
that has been altered by paving, construction,
and/or land use that would typically have
required regulatory permitting to have been
initiated (alterations may exist now or in the
past).
condition, such as “woods in good condition,”
if appropriate. However, where redevelopment
incentives are desired, or where flooding concerns
do not currently exist, pre-development may be defined as the condition of the site immediately prior to
the implementation of the proposed project.
Exemptions
In order to avoid excessive regulation on individual residential lots, maintenance and repair
efforts, and environmental projects, the following
development activities are recommended to be
exempted from the stormwater management
standards:
1.
Individual single family residential lots (single
family lots that are part of a subdivision or
phased development project should not be
exempt from the standards);
2.
Additions or modifications to existing singlefamily structures;
in the community falls below these thresholds.
In addition, a community may choose to apply
stormwater management standards on a case-bycase basis to smaller developments if it determines that the quantity, quality, and or rate of
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»» A hotspot is defined as a land use or activity
on a site that produces higher concentrations
of trace metals, hydrocarbons, or other
priority pollutants than are normally found
Stormwater Management Planning, Design & Implementation
2.2.2 Recommended Standards for Development
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6
Duplex residential units that do not meet the
criteria listed above.
4.
Land disturbing activity conducted by local,
state, authority, or federal agencies, solely to
respond to an emergency need to protect
life, limb, or property or conduct emergency
repairs;
5.
Land disturbing activity that consists solely of
cutting a trench for utility work and related
pavement replacement; and
6.
Land disturbing activity conducted by local,
state, authority, or federal agencies, whose
sole purpose is to implement stormwater
management or environmental restoration.
1.
Off-Site Mitigation – Runoff reduction
practices at a redevelopment or retrofit site
are implemented at another location within
the same watershed. The off-site project
would likely be initiated by the site developer,
and the MS4 plays a coordinating and/or
project approval role.
2.
Fee in Lieu – The developer pays the MS4 (or
its assigned entity) an appropriate fee. Fees
from multiple sites are aggregated by the
MS4 to construct public stormwater projects.
This requires an active role for the MS4.
These alternatives are discussed in further detail in
Section 5.7 of Vol. 1.
Additional Requirements
2.2.2.2 RECOMMENDED STORMWATER MANAGEMENT STANDARDS
New development or redevelopment in critical or
It is recommended that the following stormwater
sensitive areas, or as identified through a water-
management performance standards be adopt-
shed study or plan, may be subject to additional
ed for new development or redevelopment sites
performance and/or regulatory criteria. Further-
falling under the applicability criteria above. It
more, these sites may need to utilize or restrict
is further recommended that these twelve (12)
certain structural practices in order to protect a
standards be adopted in whole to create a com-
special resource or address certain water quality
prehensive stormwater management approach.
or drainage problems identified for a drainage
However, an individual community may choose
area.
to adopt some of the standards rather than the
entire set, or modify individual standards, depending upon its regulatory requirements and specific
Off-Site Compliance Alternatives
Where site conditions do not permit the achievement of the minimum runoff reduction goals for
development or redevelopment onsite, alternatives exist for compliance as follows:
local approach to stormwater management. Specific required criteria for communities covered by
a Municipal Separate Storm Sewer System (MS4)
permit are delineated in the permit.
❏❏Standard #1 – Natural Resource Inventory
Prior to the start of any land disturbing activities
(including any clearing or grading activities),
acceptable site reconnaissance and surveying
techniques shall be used to complete a
thorough assessment of the natural resources,
both terrestrial and aquatic, found on a
development site.
The site’s critical natural features and drainage
patterns shall be identified early in the site
planning process. The natural resources
inventory shall be used to identify and map the
natural resources on site, as they exist prior to
the start of any land disturbing activities. The
identification, and subsequent preservation
and/or restoration of these natural resources,
through the use of better site design practices,
helps reduce the negative impacts of the land
development process “by design”.
Resources to be identified and mapped during
the natural resources inventory, include, at a
minimum (as applicable):
Stormwater Management Planning, Design & Implementation
3.
»» Topography and Steep Slopes (i.e., Areas with
Slopes Greater Than 15%)
»» Natural Drainage Divides and Patterns
»» Natural Drainage Features (e.g., swales,
basins, depressional areas)
»» Wetlands
»» Water Bodies
»» Floodplains
»» Aquatic Buffers
»» Shellfish Harvesting Areas
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VOL 2
7
»» Erodible Soils
»» Groundwater Recharge Areas
generation of additional stormwater runoff and
pollutants to the maximum extent practicable.
More information on Better Site Design is
provided in Section 2.3.
»» Wellhead Protection Areas
»» Trees and Other Existing Vegetation
»» High Quality Habitat Areas
»» Protected River Corridors
»» Protected Mountains
»» Karst Areas
The use of certain better site design practices
that provide water quality benefits allows for
a reduction (known as a “credit”) of the water
quality volume. The applicable design practices
and stormwater site design credits are covered
in Section 2.3.2.
All relevant resources shall be shown on the
Stormwater Management Site Plan (Standard #12).
❏❏Standard #2 –Better Site Design Practices for
Stormwater Management
All site designs shall implement a combination
of approaches collectively known as
stormwater better site design practices to the
maximum extent practicable. Through the use
of these practices and techniques, the impacts
of urbanization on the natural hydrology of
the site and water quality can be significantly
reduced. The goal is to reduce the amount
of stormwater runoff and pollutants that are
generated, provide for natural on-site control
and treatment of runoff, and optimize the
location of stormwater management facilities.
Better site design concepts can be viewed
as both water quantity and water quality
management tools and can reduce the size and
cost of required BMPs.
Site designs shall preserve the natural drainage
and treatment systems and reduce the
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❏❏Standard #3 – Runoff Reduction
Runoff reduction practices shall be sized and
designed to retain the first 1.0 inch of rainfall
on the site to the maximum extent practicable.
Runoff reduction practices are stormwater
BMPs used to disconnect impervious
and disturbed pervious surfaces from the
storm drain system, thereby reducing postconstruction stormwater runoff rates, volumes,
and pollutant loads. Since runoff reduction
practices actually eliminate stormwater runoff
(and the pollutants associated with it), rather
than simply treating or detaining runoff,
they can contribute to several of the other
performance standards, while providing many
additional benefits. If the entire 1.0 inch of
rainfall can be retained onsite using runoff
reduction methods, the community may
choose to waive the water quality treatment
volume in Standard #4. If the entire 1.0-inch
runoff reduction standard cannot be achieved,
the remaining runoff from the 1.2-inch rainfall
event must be treated by BMPs to remove at
least 80% of the calculated average annual
post-development TSS loading from the site
per Standard #4 Water Quality.
Runoff reduction percentages are assigned
to applicable BMPs that reduce the amount
of stormwater required for treatment, and
subsequently reduce the other stormwater
management volumes, incentivizing their
use. Runoff reduction practices inherently
reduce TSS and other pollutants to provide
water quality treatment (i.e. 100% pollutant
removal for stormwater retention, infiltration,
evaporation, transpiration, or rainwater
harvesting and reuse). This standard
is quantified and expressed in terms of
engineering design criteria through the
specification of the runoff reduction volume
(RRV), which is equal to the runoff generated
on a site from 1.0 inches of rainfall. Individual
runoff reductions specific to each practice are
described in detail in Chapter 4.
While runoff reduction practices provide
important water quality benefits, as described
in Volume 1, Chapter 2, certain conditions,
such as karst topography, soils with very low
infiltration rates, high groundwater, or shallow
bedrock, may lead a community to choose
to waive or reduce the runoff reduction
requirement. Alternatively, these conditions can
be addressed on a site-specific basis. If the RRV
of 1.0 inches of rainfall cannot be achieved,
adequate documentation should be provided
to the local development review authority
to show that no additional runoff reduction
practices can be used on the development site.
Stormwater Management Planning, Design & Implementation
»» Soils
VOL 2
8
Communities that choose to adopt runoff
reduction may choose to waive the water
quality treatment volume from this standard if
100% of the 1.0 inch runoff reduction volume
is achieved. If the entire 1.0-inch runoff
reduction standard cannot be achieved, the
remaining runoff from the 1.2-inch rainfall
event must be treated by BMPs to remove at
least 80% of the calculated average annual
post-development TSS loading from the site.
This standard is quantified and expressed in terms
of engineering design criteria through specification
of the water quality volume (WQv), which is equal
to the runoff generated on a site from 1.2 inches of
rainfall. The WQV must be treated to the 80% TSS
removal performance goal.
This standard assumes that BMPs will be
designed, constructed and maintained according
to the criteria in this Manual. Stormwater
discharges from land uses or activities with
higher or special potential pollutant loadings may
require the use of specific structural practices
and pollution prevention practices. A detailed
overview of BMPs is provided in Chapter 4.
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❏❏Standard #5 – Stream Channel Protection
Stream channel protection shall be provided by
using all of the following three approaches:
1. 24-hour extended detention storage of the
1-year, 24-hour return frequency storm event
2. Erosion prevention measures, such as energy
dissipation and velocity control
3. Preservation of the applicable stream buffer.
Stream channel protection requirements are
further described in Section 2.2.4.2.
The first method of providing stream bank
protection is the extended detention of the
1-year, 24-hour storm for a period of 24 hours
using BMPs. It is known that the increase in
runoff due to development can dramatically
increase stream channel erosion. This standard
is intended to reduce the frequency, magnitude
and duration of post-development bankfull
flow conditions. The volume to be detained
is also known as the channel protection
volume (CP v). The use of nonstructural site
design practices and runoff reduction BMPs
that reduce the total amount of runoff may
also reduce CP v by a proportional amount.
Refer to Table 4.1.3-1 (BMP Selection Guide)
for applicable BMPs. Where runoff reduction
practices are used, an adjusted curve number
(CN) is computed that is lower than the original
CN based on an actual stormwater volume
removed from the total runoff, see Section
3.1.7.5 for additional information.
This requirement may be waived by a local
jurisdiction for sites that discharge directly or
through piped stormwater drainage systems
into larger streams, rivers, wetlands, lakes,
estuaries, tidal waters, or other situations where
the reduction in the smaller flows will not have
an impact on stream bank or channel integrity.
The second stream bank protection method
is to implement velocity control, energy
dissipation, stream bank stabilization, and
erosion prevention practices and structures
as necessary in the stormwater management
system to prevent downstream erosion and
stream bank damage. Energy dissipation and
velocity control methods are discussed in
Section 5.5.
The third method of providing for stream
channel protection is through the
establishment of riparian stream buffers on
the development site. Stream buffers not only
provide channel protection but also water
quality benefits and protection of streamside
properties from flooding. It is recommended
that 100-foot buffers be established where
feasible. Additional stream buffer guidelines are
presented in Section 2.3.
Stormwater Management Planning, Design & Implementation
❏❏Standard #4 – Water Quality
Stormwater management systems shall be
designed to retain or treat the runoff from 85%
of the storms that occur in an average year, and
reduce average annual post-development total
suspended solids loadings by 80%. Averaged
from rainfall events across the state of Georgia,
this equates to treating storm events of 1.2
inches or less, as well as the first 1.2 inches of
runoff for all larger storm events.
❏❏Standard #6 – Overbank Flood Protection
Overbank flood protection shall be provided
by controlling the post-development peak
discharge rate to the pre-development rate
(natural or existing condition, as applicable) for
the 25-year, 24-hour return frequency storm
event. If control of the 1-year, 24-hour storm
(Standard #5) is exempted, then overbank flood
protection shall be provided by controlling
the post-development peak discharge rate to
VOL 2
9
The use of nonstructural site design practices
and runoff reduction BMPs that reduce the
total amount of runoff will also reduce Qp25 by
a proportional amount. Refer to Table 4.1.3-1
(BMP Selection Guide) for applicable BMPs.
Where runoff reduction practices are used,
an adjusted curve number (CN) is computed
that is lower than the original CN based on an
actual stormwater volume removed from the
total runoff, see Section 3.1.7.5 for additional
information.
Smaller storm events (e.g., 2-year and 10year) are effectively controlled through a
combination of extended detention for the
1-year, 24-hour event (channel protection) and
control of the 25-year peak rate for overbank
flood protection. These design standards,
therefore, are intended to be used in unison.
This standard may be adjusted by a local
jurisdiction for areas where all downstream
conveyances and receiving waters have the
natural capacity to handle the full build-out
25-year storm through a combination of
channel capacity and overbank flood storage
without increasing flood stages above predevelopment flood levels.
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❏❏Standard #7 – Extreme Flood Protection
Extreme flood protection shall be provided
by controlling and/or safely conveying the
100-year, 24-hour storm event (denoted Qf).
This is accomplished either by (1) controlling
Qf through BMPs to maintain the existing
100-year floodplain, or (2) by sizing the onsite conveyance system to safely pass Qf and
allowing it to discharge into a receiving water
whose protected floodplain is sufficiently sized
to account for extreme flow increases without
causing damage. In this case, the extreme
flood protection criterion may be waived by a
local jurisdiction in lieu of provision of safe and
effective conveyance to receiving waters that
have the capacity to handle flow increases to
maintain 100-year level.
The use of nonstructural site design practices
and runoff reduction BMPs that reduce the
total amount of runoff will also reduce Qf by
a proportional amount. Refer to Table 4.1.3-1
(BMP Selection Guide) for applicable BMPs.
Where runoff reduction practices are used,
an adjusted curve number (CN) is computed
that is lower than the original CN based on an
actual stormwater volume removed from the
total runoff, see Section 3.1.7.5 for additional
information.
Existing and future floodplain areas shall be
preserved to the extent possible. Extreme flood
protection requirements are further described
in Section 2.2.3.
❏❏Standard #8 – Downstream Analysis
Due to peak flow timing and runoff volume
effects, some structural practices fail to
reduce discharge peaks to pre-development
levels downstream from the development
site. A downstream peak flow analysis shall
be provided to the point in the watershed
downstream of the site or the stormwater
management system where the area of the
site comprises 10% of the total drainage area.
This is to help ensure that there are minimal
downstream impacts from the developed site.
The downstream analysis may result in the
need to resize BMPs, or may allow the waiving
of some unnecessary peak flow controls
altogether. The use of a downstream analysis
and the “ten-percent” rule are discussed in
Section 3.1.9.
❏❏Standard #9 – Construction Erosion and
Sedimentation Control
Erosion and sedimentation control practices
shall be utilized during the construction phase
of development or during any land disturbing
activities.
Stormwater Management Planning, Design & Implementation
the pre-development rate (natural or existing
condition, as applicable) for the 2-year through
the 25-year return frequency storm events.
Overbank flood protection requirements are
further described in Section 2.2.4.3.
All new development and redevelopment sites
must meet the regulatory requirements for
land disturbance activities under the Georgia
Erosion and Sedimentation Control Act and/
or the NPDES General Permit for Construction
Activities. This involves the preparation and
implementation of an approved erosion
and sedimentation control plan, including
appropriate best management practices, during
the construction phase of development.
VOL 2
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operation and maintenance plan must provide:
Better site design practices and techniques
that can reduce the total amount of area that
needs to be cleared and graded should be
implemented wherever possible. It is essential
that erosion and sedimentation control be
considered and implemented in stormwater
concept plans and throughout the construction
phase to prevent damage to natural stormwater
drainage systems and previously constructed
best management practices and conveyance
facilities.
2. The routine and non-routine maintenance
tasks to be undertaken
❏❏Standard #10 – Stormwater Management
System Operation and Maintenance
The stormwater management system,
including all best management practices and
conveyances, shall have an operation and
maintenance plan to ensure that it continues
to function as designed. See Appendix E for
more information on stormwater operation and
maintenance.
All new development and redevelopment sites
are to prepare a comprehensive operation and
maintenance plan for the on-site stormwater
management system. This is to include all
of the stormwater management system
components, including drainage facilities,
BMPs, and conveyance systems. To ensure that
stormwater management systems function
as they were designed and constructed, the
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1. A clear assignment of stormwater inspection
and maintenance responsibilities
3. A schedule for inspection and maintenance
4. Any necessary legally binding maintenance
agreements
❏❏Standard #11 – Pollution Prevention
To the maximum extent practicable, the
development or redevelopment project shall
implement pollutant prevention practices and
have a stormwater pollution prevention plan.
All new development and redevelopment sites
are to consider pollution prevention in the
design and operation of the site, and prepare
a formal stormwater pollution prevention plan
if circumstances warrant it. Specific land use
types and hotspots may need to implement
more rigorous pollution prevention practices.
The preparation of pollution prevention
plans and the full set of pollution prevention
practices are covered in Volume 3 of this
Manual.
government review that addresses Standard #1
through Standard #11.
All new development and redevelopment
sites will require the preparation of a
stormwater management site plan for
development activities. A stormwater site plan
is a comprehensive report that contains the
technical information and analysis to allow a
local review authority to determine whether a
proposed new development or redevelopment
project meets local stormwater regulatory
requirements. See, Volume 1, Section 4.3 and
other local stormwater regulatory requirements
for specific guidance.
Stormwater Management Planning, Design & Implementation
Further guidance on practices for construction
site erosion and sedimentation control can be
found in the latest version of the Manual for
Erosion and Sediment Control in Georgia.
❏❏Standard #12 – Stormwater Management Site
Plan
The development project shall prepare a
stormwater management site plan for local
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11
Table 2.2.3-1 Summary of the Statewide Stormwater Sizing Criteria for Stormwater Control and
This section presents an integrated approach for
Mitigation
meeting the volume-based stormwater runoff
Sizing Criteria
Description
quality and quantity management standards for
Retain or reduce the runoff for the first 1.0 inch of rainfall, or to the
development (see Section 2.2.2) by addressing the
maximum extent practicable. Since runoff reduction practices elim-
key adverse impacts of stormwater runoff from
a development site. The purpose is to provide a
framework for designing a stormwater management system to:
Runoff
inate stormwater runoff, and the pollutants associated with it, rather
Reduction, RRv
than treating or detaining, they can contribute to other stormwater
(Standard #3)
management standards. If the entire 1.0 inch runoff reduction cannot
be achieved, the remaining runoff from the 1.2 inch rainfall must be
Water Quality
• Improve water quality through runoff reduction
and/or water quality treatment (Recommended
Standard #3 and #4);
• Prevent downstream stream bank and channel
erosion (Recommended Standard #5);
• Reduce downstream overbank flooding
(Recommended Standard #6); and
treated, as described in Standard #4.
Retain or treat the runoff from 85% of the storms that occur in an
Treatment, WQv
(Standard #4)
average year. For Georgia, this equates to providing water quality
treatment for the runoff resulting from a rainfall depth of 1.2 inches.
The water quality treatment goal is to reduce average annual post-development total suspended solids loadings by 80%.
Provide extended detention of the 1-year, 24 hour storm event re-
Channel Protection
leased over a period of 24 hours to reduce bankfull flows and protect
downstream channels from erosive velocities and unstable conditions.
• Safely pass or reduce the runoff from extreme
storm events (Recommended Standard #7).
Provide peak discharge control of the 25-year, 24 hour storm event
Overbank Flood Protection
velopment rate to reduce overbank flooding.
For these objectives, an integrated set of engi-
Evaluate the effects of the 100-year, 24 hour storm on the stormwater
neering criteria, known as the Unified Stormwater Sizing Criteria, have been developed which
are used to size and design best management
practices. Table 2.2.3-1 briefly summarizes the
such that the post-development peak rate does not exceed the prede-
Stormwater Management Planning, Design & Implementation
2.2.3 Numerical Sizing Criteria Overview
management system, adjacent property, and downstream facilities and
Extreme Flood Protection
property. Manage the impacts of the extreme storm event through
detention controls and/or floodplain management.
criteria. Recommended standard #3 and #4 (water quality) can be achieved by using one of the
two standards listed below. The local community
will select and require either Runoff Reduction
(Standard #3) and/or Water Quality Treatment
(Standard #4). See Section 4.2.3 of Volume 1 for
more information.
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VOL 2
12
are intended to be used in conjunction with the
others to address the overall stormwater impacts
from a development site. When used as a set, the
unified criteria control the entire range of hydrologic events, from the smallest runoff producing
rainfalls to the 100-year, 24 hour storm. Figure
2.2.3-1 graphically illustrates the relative volume
requirements of each of the unified stormwater
sizing criteria as well as demonstrates that the
criteria are “nested” within one another, i.e., the
extreme flood protection volume requirement
also contains the overbank flood protection
volume, the channel protection volume and the
water quality treatment volume. Figure 2.2.3-2
shows how these volumes would be nested in a
typical stormwater wet pond designed to handle
all four criteria.
Also shown is the sediment storage volume.
The pond must be dredged when the sediment
Figure 2.2.3-1 Representation of the Unified Stormwater Sizing Criteria
storage volume is full to maintain its sediment
Stormwater Management Planning, Design & Implementation
Each of the unified stormwater sizing criteria
removal effectiveness, and its channel, overbank
flood and extreme flood protection capabilities.
Extreme Flood Protection (100-year) Level
The following pages describe the four sizing
Overbank Flood Protection (25-year) Level
criteria in detail and present guidance on how to
Channel Protection Level
properly compute and apply the required storage
volumes.
Permanent Pool
(Water Quality Volume)
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Figure 2.2.3-2 Unified Sizing Criteria Water Surface Elevations in a Stormwater (Wet) Pond
VOL 2
13
Rv = 0.05 + 0.009(I) where I is percent
Discussion
impervious cover
Hydrologic studies show that small-sized, fre-
A = site area in square feet
quently occurring storms account for the majority
2.2.4.1 WATER QUALITY VOLUME (WQV)
of rainfall events that generate stormwater runoff.
As noted in Recommended Standard #3 and #4,
Consequently, the runoff from these storms also
the water quality goal can be accomplished either
The Water Quality sizing criterion, denoted WQv,
accounts for a major portion of the annual pollut-
through runoff reduction or water quality treat-
specifies the retention and/or treatment required
ant loadings. Therefore, by eliminating, retaining,
ment or some combination of the two. For more
to remove a significant percentage of the total
or treating these frequently occurring smaller
information on either water quality approach, see
pollution load inherent in stormwater runoff by
rainfall events and a portion of the stormwater
Section 2.2.2.2.
intercepting and retaining or treating the 85th
runoff from larger events, it is possible to effec-
percentile storm event, which is equal to 1.2
tively mitigate the water quality impacts from a
The Runoff Reduction approach to the Water
inches (i.e., all the runoff from 85% of the storms
developed area.
Quality sizing criterion, denoted RRv, specifies the
that occur on average during the course of a year
reduction or elimination of the total pollution load
and a portion of the runoff from all storms greater
A water quality volume (WQv) is specified to size
inherent in stormwater runoff by intercepting and
than 1.2 inches). The Water Quality Volume is
best management practices (BMPs) to reduce/
reducing or eliminating the first 1.0 inch of rainfall,
a runoff volume that is directly related to the
eliminate or treat these small storms up to a maxi-
or to the maximum extent practicable. The Runoff
amount of impervious cover at a site.
mum runoff depth and the “first flush” of all larger
Reduction Volume is a runoff volume that is
storm events. For Georgia, this maximum depth
directly related to the amount of impervious cover
In numerical terms, it is equivalent to a rainfall
was determined to be the runoff generated from
at a site.
depth of 1.2 inches multiplied by the volumetric
the 85th percentile storm event (i.e., the storm
runoff coefficient (Rv) and the site area, and is
event that is greater than 85% of the storms that
calculated using the formula below:
occur within an average year). The 85th percentile
In numerical terms, it is equivalent to a rainfall depth of 1.0 inches (or other target rainfall
volume was considered the point of optimization
amount specified by a local community) multi-
WQv = 1.2RvA
12
plied by the volumetric runoff coefficient (Rv) and
the site area, and is calculated using the formula
below:
between pollutant removal ability and cost-effectiveness. Capturing and treating a larger percentage of the annual stormwater runoff would
Where:
RRv = P x Rv x A
12
Stormwater Management Planning, Design & Implementation
2.2.4 Description of Unified Stormwater
Sizing Criteria
provide only a small increase in additional pollut-
WQv = water quality volume (in cubic ft)
ant removal, but would considerably increase the
Rv = 0.05 + 0.009(I) where I is percent
required size (and cost) of the best management
impervious cover
practices.
A = site area in square feet
Where:
RRv = runoff reduction volume (cubic ft)
P = target runoff reduction rainfall (often
1.0 inches)
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VOL 2
14
Water Quality Volume (inches)
for the 85th percentile storm was selected as an
average for the entire state. Thus, the statewide
water quality treatment volume criterion is equal
to the runoff from the first 1.2 inches of rainfall.
A stormwater management system designed for
the WQv treatment will retain or treat the runoff
from all storm events of 1.2 inches or less, as well
as the first 1.2 inches of runoff for all larger storm
events. For the runoff reduction approach, a best
management practice eliminating or reducing
the stormwater runoff from all storm events of
1.0 inches or less, as well as the first 1.0 inches of runoff for all larger storm events will also
1.2
1
0.8
0.6
0.4
0.2
0
0
meet the Water Quality criterion. The correlation
10
20
30
water quality volume and the 1.0 inch volume
Given that an 80% TSS removal rate for the 1.2
inch rainfall event is the standard for addressing water quality, 100% TSS removal through
volume reduction of the 1.0 inch rainfall event
will address the same requirement. In another
method of describing total TSS removal, 80% of
1.2 inches (0.96) approximately equates to 100%
of 1.0 inches.
Figure 2.2.4-1 shows a plot of the Water Quality
70
80
90
100
1.
The use of TSS as an “indicator” pollutant is
well established.
2.
Sediment and turbidity, as well as other
pollutants of concern that adhere to
suspended solids, are a major source of
water quality impairment due to urban
development in Georgia watersheds.
3.
A large fraction of many other pollutants of
concern are either removed along with TSS,
or at rates proportional to the TSS removal.
4.
The 80% TSS removal level is reasonably
attainable using well-designed best
management practices (for typical ranges
of TSS concentration found in stormwater
runoff).
Volume versus impervious area percentage.
TSS Reduction Goal
This Manual follows the philosophy of removing
pollutants to the “maximum extent practicable”
mance goal. The approach taken in this Manual
from a regression analysis performed on rain-
is to require treatment of the WQv from a site to
fall runoff volume data from a number of cities
reduce post-development total suspended solids
nationwide and is a shortcut method consid-
(TSS) loadings by 80%, as measured on an average
ered adequate for runoff volume calculation for
annual basis. TSS was chosen as the representa-
the type of small storms normally considered in
tive stormwater pollutant for measuring treatment
stormwater quality calculations.
effectiveness for several reasons:
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60
Figure 2.2.4-1 Water Quality Volume versus Percent Impervious Area
through the use of a percentage removal perforThe volumetric runoff coefficient (Rv) was derived
50
Percent Impervious
between the 1.2 inch treatment requirement for
based reduction is the following:
40
Stormwater Management Planning, Design & Implementation
Based on a rainfall analysis, a value of 1.2 inches
VOL 2
15
pollutants. However, the removal performance for
pollutants that are soluble or that cannot be removed by settling will vary depending on the best
management practice. For pollutants of specific
concern, individual analyses of specific pollutant
sources and the appropriate removal mechanisms
should be performed.
OR
-- Calculate the WQv for the entire site. To
satisfy the WQv for the project, the WQv
should be met for all drainage areas within
the site. Should a portion of the site not
be managed by a BMP or a drainage area
not providing the WQv, the other drainage
areas will be required to provide greater
than the WQv to ensure the entire site WQv
goal is met.
Determining the Runoff Reduction (RRV) and
Water Quality Volumes (WQv)
»» Measuring Impervious Area: The area of
impervious cover can be taken directly off a
set of plans or appropriate mapping. Where
this is impractical, NRCS TR-55 land use/
impervious cover relationships can be used
to estimate impervious cover. I is expressed
as a percent value not a fraction (e.g., I = 30
for 30% impervious cover).
»» Multiple Drainage Areas: When a
development project contains or is divided
into multiple drainage areas, RRV and
WQv should be calculated and addressed
separately for each drainage area.
-- Calculate the target RRv for the entire site.
To receive the runoff reduction credit
for the project, the target RRv should
be met for all drainage areas within the
site. Should a portion of the site not be
managed by a BMP or a drainage area not
providing the target RRv, the other drainage
areas will be required to have RRv values
greater than the target to ensure the entire
site RRv requirement is met.
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»» Off-site Drainage Areas: Off-site drainage
may be excluded from the calculation of the
RRV and WQv if the drainage is routed around
the site.
»» Credits for Site Design Practices: The use
of certain better site design practices may
allow the WQv to be reduced through the
subtraction of a site design “credit.” These
site design credits are described in Section
2.3.
»» Determining the Peak Discharge for the
Water Quality Storm: When designing
off-line structural control facilities, the
peak discharge of the water quality storm
(Qwq) can be determined using the method
provided in Section 3.1.
»» Extended Detention of the Water Quality
Volume: The water quality treatment
requirement can be met by providing a
24-hour drawdown of a portion of WQv in
a wet stormwater pond or wetland system
(as described in Section 4.25). Referred to as
water quality ED (extended detention), it is
different than providing extended detention
of the 1-year, 24 hour storm for the channel
protection volume (CP v). The ED portion of
the WQv may be included when routing the
CP v.
»» RRV and WQv can be expressed in cubic feet
by multiplying by 43,560.
»» RRV and WQv can also be expressed in
watershed-inches by removing the area (A)
and the “12” in the denominator.
2.2.4.2 CHANNEL PROTECTION (CPV)
The Channel Protection sizing criterion specifies
that 24 hours of extended detention be provided for runoff generated by the 1-year, 24-hour
rainfall event to protect downstream channels.
The required volume needed for 1-year extended
detention, denoted CPv, is roughly equivalent to
the required volume needed for peak discharge
control of the 5- to 10-year storm.
• CP v control is not required for postdevelopment discharges less than 2.0 cfs at
each individual discharge location
Stormwater Management Planning, Design & Implementation
TSS is a good indicator for many stormwater
• The use of nonstructural site design practices
and runoff reduction practices that reduce
the total amount of runoff will also reduce the
channel protection volume by a proportional
amount.
• The channel protection criteria may be waived
by a local jurisdiction for sites that discharge
directly into larger streams, rivers, wetlands,
lakes, estuaries, or tidal waters where the
reduction in the smaller flows will not have an
impact on stream bank or channel integrity.
VOL 2
16
The increase in the frequency and duration of
bankfull flow conditions in stream channels due
to urban development is the primary cause of
stream bank erosion and the widening and downcutting of stream channels. Therefore, channel
erosion downstream of a development site can
be significantly reduced by storing and releasing
stormwater runoff from the channel-forming
runoff events (which corresponds approximately
to the 1-year, 24 hour storm event) in a gradual
manner to ensure that critical erosive velocities
and flow volumes are not exceeded.
Determining the Channel Protection Volume
(CP v)
»» CP v Calculation Methods: Several methods
can be used to calculate the CP v storage
volume required for a site. Subsection 3.1.5 and
Appendix B illustrate the recommended average
outflow method for volume calculation.
»» Hydrograph Generation: The NRCS TR-55
hydrograph methods provided in Section
3.1 can be used to compute the runoff
hydrograph for the 1-year, 24-hour storm.
»» Rainfall Depths: The rainfall depth of the
1-year, 24-hour storm will vary depending on
location and can be determined from rainfall
data found in the National Oceanic and
Atmospheric Administration (NOAA) Atlas 14
publication, or online using the Precipitation
Frequency Data Server database for any
location across Georgia (http://hdsc.nws.
noaa.gov/hdsc/pfds/).
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»» Multiple Drainage Areas: When a
development project contains or is divided
into multiple drainage areas, CP v may be
distributed proportionally to each drainage
area.
»» Off-site Drainage Areas: Off-site drainage
areas should be modeled as “present
condition” for the 1-year storm event. If there
are adequate upstream channel protection
controls, then the off-site area can be
modeled as “forested” or “natural” condition.
A best management practice located “on-line”
will need to safely bypass any off-site flows.
(channel protection CPv control) and the control of
Qp25 for overbank channel protection.
• Larger storms (> 25-year) are partially
attenuated through the control of Qp25.
• The use of nonstructural site design practices
and runoff reduction practices that reduce the
total amount of runoff will also reduce Qp25 by
a proportional amount.
Control of Qp25 is not intended to serve as a
stand-alone design standard, but is intended to
be used in conjunction with the channel pro-
»» Routing/Storage Requirements: The
required storage volume for the CP v may
be provided above the WQv storage in
stormwater ponds and wetlands with
appropriate hydraulic control structures for
each storage requirement.
tection AND extreme flood protection criteria. If
»» Control Orifices: Orifice diameters for
CP v control of less than 3 inches are not
recommended without adequate clogging
of the 2-year (Qp2) through the 25-year (Qp25) re-
protection (see Section 3.3).
detention is designed for only the 25-year storm,
smaller runoff events will simply pass through
the outlet structure with little attenuation. If the
channel protection criterion is not used, then for
overbank flood protection, peak flow attenuation
turn frequency storm events must be provided.
Stormwater Management Planning, Design & Implementation
Discussion
Discussion
The purpose of overbank flood protection is to
2.2.4.3 OVERBANK FLOOD PROTECTION (QP25)
prevent an increase in the frequency and magni-
The Overbank Flood Protection criterion speci-
tude of damaging out-of-bank flooding (i.e., flow
fies that the post-development 25-year, 24-hour
storm peak discharge rate, denoted Qp25, not
exceed the pre-development (natural or existing
events that exceed the capacity of the channel
condition, as applicable) discharge rate. This is
dle-frequency storm events.
and enter the floodplain). It is intended to protect
downstream properties from flooding at mid-
achieved through detention of runoff from the
25-year event.
This criterion may be adjusted by a local jurisdic-
• Smaller storm events (e.g., 2-year and 10-year) are
effectively controlled through the combination
of the extended detention for the 1-year event
tion for areas where all downstream conveyances
are designed to handle runoff from the full buildout
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17
2.2.4.4 EXTREME FLOOD PROTECTION (Qf)
ing of runoff through the drainage system and
that no downstream flooding will occur as a result
The Extreme Flood Protection criterion specifies
stormwater management facilities to determine
of a proposed development (see Section 3.1.9). In
that all stormwater management facilities be de-
the effects on the facilities, adjacent property, and
this case, the overbank flood protection criterion
signed to safely handle the runoff from the 100year, 24-hour return frequency storm event,
downstream. Emergency spillways of best man-
may be waived by a local jurisdiction in lieu of
provision of safe and effective conveyance to a
denoted Qf. This is accomplished either by:
ately to safely pass the resulting flows.
major river system, lake, wetland, estuary, or tidal
1.
waters that have capacity to handle flow increases
at the 25-year level.
Determining the Overbank Flood Protection
Volume (Qp25)
»» Peak-Discharge and Hydrograph Generation:
The NRCS TR-55 or USGS hydrograph
methods provided in Section 3.1 can be used
to compute the peak discharge rate and
runoff for the 25-year, 24-hour storm.
»» Rainfall Depths: The rainfall depth of the 25-year,
24-hour storm will vary depending on location
and can be determined from rainfall data found
in the National Oceanic and Atmospheric
Administration (NOAA) Atlas 14 publication, or
2.
Controlling Qf through on-site or regional
best management practices to maintain the
existing 100-year floodplain. This is done
where residences or other structures have
already been constructed within the 100year floodplain fringe area; or
By sizing the on-site conveyance system to
safely pass Qf and allowing it to discharge
into a receiving water whose protected full
buildout floodplain is sufficiently sized to
account for extreme flow increases without
causing damage.
Local flood protection (levees, floodwalls, floodproofing, etc.) and/or channel enlargements may
be substituted as appropriate, as long as adequate
online using the Precipitation Frequency Data
conveyance and structural safety is ensured
Server database for any location across Georgia
(http://hdsc.nws.noaa.gov/hdsc/pfds/).
through the measure used, and stream environ-
»» Off-site Drainage Areas: Off-site drainage areas
should be modeled as “present condition” for
the 25-year storm event and do not need to be
included in Qp25 estimates, but can be routed
through a best management practice.
»» Downstream Analysis: Downstream areas
should be checked to ensure there is no
peak flow increase above pre-development
conditions to the point where the site area is
10% of the total drainage to that point.
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mental integrity is adequately maintained.
Discussion
The intent of the extreme flood protection is to
prevent flood damage from infrequent but large
storm events, maintain the boundaries of the
mapped 100-year floodplain, and protect the
physical integrity of the best management practices as well as downstream stormwater and flood
control facilities.
agement practices should be designed appropri-
Determining the Extreme Flood Protection
Criteria (Qp25)
»» Peak-Discharge and Hydrograph
Generation: The NRCS TR-55 or USGS
hydrograph methods provided in Section 3.1
can be used to compute the peak discharge
rate and runoff for the 100-year, 24-hour
storm.
»» Rainfall Depths: The rainfall depth of the
100-year, 24-hour storm will vary depending
on location and can be determined from
rainfall data found in the National Oceanic
and Atmospheric Administration (NOAA)
Atlas 14 publication, or online using the
Precipitation Frequency Data Server database
for any location across Georgia (http://hdsc.
nws.noaa.gov/hdsc/pfds/).
Stormwater Management Planning, Design & Implementation
25-year storm, or where it can be demonstrated
»» Off-site Drainage Areas: Off-site drainage
areas should be modeled as “full buildout
condition” for the 100-year storm event to
ensure safe passage of future flows.
»» Downstream Analysis: If Qf is being detained,
downstream areas should be checked
to ensure there is no peak flow increase
above pre-development conditions to the
point where the site area is 10% of the total
drainage to that point.
It is recommended that Qf be used in the routVOL 2
18
as well as provide for at least some nonstructural
For each of the above categories, there are a
on-site treatment and control of runoff. Better site
number of practices and techniques that aim to
design concepts can be viewed as both water quan-
reduce the impact of urban development and
2.2.5.1 INTRODUCTION
tity and water quality management tools and can
stormwater runoff from the site. These better site
There are two primary approaches for managing storm-
reduce the size and cost of required best manage-
design practices are described in detail in Section
water runoff and addressing the unified stormwater
ment practices—sometimes eliminating the need for
2.3.
sizing criteria requirements on a development site:
them entirely. The site design approach can result
1.
2.
The use of better site design practices to reduce
the amount of stormwater runoff and pollutants
generated and/or provide for natural runoff
reduction, treatment, and control of runoff; and
The use of best management practices to
provide runoff reduction, treatment, and
control of stormwater runoff
This subsection introduces both of these approaches. Stormwater better site design practices
are discussed in-depth in Section 2.3, while best
management practices are covered in Chapter 4.
in a more natural and cost-effective stormwater
For several of the better site design practices,
management system that better mimics the natural
there is a direct economic benefit to their imple-
hydrologic conditions of the site, has a lower main-
mentation for both stormwater quality and quan-
tenance burden and provides for more sustainability.
tity through the application of site design “credits.”
In terms of the unified stormwater sizing criteria,
»» Better site design includes:
Table 2.2.5-1 shows how the use of nonstructural
-- Conserving natural features and resources
site design practices can provide a reduction in
-- Using lower impact site design techniques
the amount of stormwater runoff that is required
to be treated and/or controlled through the appli-
-- Reducing impervious cover
cation of site design credits.
-- Utilizing natural features for stormwater
management
Table 2.2.5-1 Reductions or “Credits” to the Unified Stormwater Sizing Criteria through the Use of Better Site Design Practices
2.2.5.2 SITE DESIGN AS THE FIRST STEP IN
ADDRESSING UNIFIED STORMWATER SIZING
CRITERIA REQUIREMENTS
Using the site design process to reduce stormwa-
Sizing Criteria
Potential Benefits of the Use of Better Site Design Practices
Water Quality
•
(RRV & WQV)
Stormwater Management Planning, Design & Implementation
2.2.5 Meeting the Unified Stormwater
Sizing Criteria Requirements
Better site design practices that reduce the total amount of
runoff will also reduce RRV & WQv by a proportional amount,
through the overall volume and TSS removed.
ter runoff and pollutants should always be the first
•
consideration of the site designer and engineer
Certain site design practices will allow for a further reduction
to the RRV and WQV. The site design credits are discussed in
in the planning of the stormwater management
Section 2.3.
system for a development.
Through the use of a combination of approaches
collectively known as stormwater better site design
practices and techniques, it is possible to reduce the
amount of runoff and pollutants that are generated,
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TOC
Channel Protection, Overbank
•
tional amount.
Flood Protection
(CPv, Qp25, Qf)
The use of better site design practices that reduce the total
amount of runoff will also reduce CPv, Qp25, and Qf by a propor-
Flood Protection, and Extreme
•
Floodplain preservation may allow waiving of overbank flood
and/or extreme flood protection requirements.
VOL 2
19
In Georgia, the state requirement for Water
the unified stormwater sizing criteria. Guidance
Quality is to remove 80% TSS from the 1.2” rainfall
for choosing the appropriate best management
Best management practices (sometimes referred
event. To comply with this requirement, there are
practice(s) for a site is provided in Section 4.1.
to as structural practices or BMPs) are construct-
several ways to incorporate better site design,
ed stormwater management facilities designed to
runoff reduction practices, and best management
retain or treat stormwater runoff and/or mitigate
practices in a design.
the effects of increased stormwater runoff peak
Each development site is unique in how storm-
rate, volume, and velocity due to urbanization.
This Manual recommends a number of best management practices for meeting unified stormwater
sizing criteria. The recommended practices are
divided into three categories: runoff reduction (RRv), stormwater treatment (water quality
volume, WQv), and detention controls. Some of
the best management practices provide multiple
benefits and may even fit into more than one of
the previously mentioned categories.
Some stormwater practices are best suited to ad-
2.2.5.5 TYPICAL STEPS IN ADDRESSING THE
UNIFIED STORMWATER SIZING CRITERIA
2.2.5.4 USING BEST MANAGEMENT PRACTICES
TO MEET UNIFIED STORMWATER SIZING CRITERIA REQUIREMENTS
water management objectives are met. The type
of development, physical site conditions, location
in the watershed, and other factors determine
Best management practices should be considered
how the recommended stormwater management
after all reasonable attempts have been made
standards and unified stormwater sizing criteria
to minimize stormwater runoff and maximize its
are addressed.
control and treatment through the better site design methods. Once the need for structural prac-
Figure 2.2.5-2 provides a flowchart for the typical
tices has been established, one or more appro-
steps in stormwater management system design
priate best management practices will need to be
using the unified stormwater sizing criteria. This is
selected to handle the stormwater runoff storage
a subset of the stormwater site planning process
and treatment requirements calculated using
detailed in Section 2.4.
dress runoff quality, while others are best suited to
address runoff quantity. However, better site design and runoff reduction practices address both
simultaneously. As discussed in Volume 1, Chapter
3, by reducing the impervious cover associated
Concept Plan
Stormwater
Unified Sizing
Natural
Developed Using
“Credits” for Natural
Criteria Used to
Resources
Better Site Design
Area Conservation
Determine Storm-
Practices
Applied to Reduce
water Control
Volumes
Treatment
Inventory
Stormwater Management Planning, Design & Implementation
2.2.5.3 RECOMMENDED BEST MANAGEMENT
PRACTICES
Volumes
with a development, better site design techniques
reduce the amount of runoff being generated by
a development in the first place. Runoff reduction
practices, on the other hand eliminate some of
the runoff after it is generated. Instead of treating
the runoff like a typical water quality practice, or
detaining it like a typical water quality practice,
they remove it – removing the pollutants along
Final Site
Plan
Downstream
Assessment
Performed
Structural Controls
are Sized,
Runoff Reduction
Designed, and
Practices are Utilized
Sited
with it.
Figure 2.2.5-2 Typical Stormwater Management System Design Process
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VOL 2
20
Better site design for stormwater management
One of the site design practices described in this
includes a number of site design techniques such
section provide a calculable reduction or site
as preserving natural features and resources,
design “credit” which can be applied to the unified
2.3.1 Overview
effectively laying out the site elements to re-
stormwater sizing criteria requirements. Section
duce impact, reducing the amount of impervious
2.3.2 discusses these practices and provide exam-
2.3.1.1 INTRODUCTION
surfaces, and utilizing natural features on the
ples of their application.
The first step in addressing stormwater manage-
site for stormwater management. The aim is to
ment begins with the site planning and design
reduce the environmental impact “footprint” of
The use of stormwater better site design can also
process. Development projects can be designed
the site while retaining and enhancing the owner/
have a number of other ancillary benefits includ-
to reduce their impact on watersheds when
developer’s purpose and vision for the site. Many
ing:
careful efforts are made to conserve natural
of the better site design concepts can reduce the
areas, reduce impervious cover and better inte-
cost of infrastructure while maintaining or even
grate stormwater treatment. By implementing a
increasing the value of the property.
• Increased property values
combination of these nonstructural approaches
collectively known as stormwater better site de-
Reduction of adverse stormwater runoff impacts
sign practices, it is possible to reduce the amount
through the use of better site design should be
of runoff and pollutants that are generated from
the first consideration of the design engineer.
a site and provide for some nonstructural on-site
Operationally, economically, and aesthetically, the
treatment and control of runoff. The goals of
use of better site design practices offers signifi-
better site design include:
cant benefits over treating and controlling runoff
downstream. Therefore, feasible opportunities
• Managing stormwater (quantity and quality)
as close to the point of origin as possible and
minimizing collection and conveyance
• Preventing stormwater impacts rather than
mitigating them
• Utilizing simple, nonstructural methods for
stormwater management that are lower cost
and lower maintenance than structural controls
(best management practices)
• Reduced construction costs
for using these methods should be explored and
exhausted before considering best management
practices.
• More open space for recreation
• More pedestrian friendly neighborhoods
• Protection of sensitive forests, wetlands and
habitats
• More aesthetically pleasing and naturally
attractive landscape
Stormwater Management Planning, Design & Implementation
2.3 Stormwater Better Site Design & Techniques
• Easier compliance with wetland and other
resource protection regulations
The reduction in runoff and pollutants using
better site design can reduce the required runoff
peak and volumes that need to be conveyed and
controlled on a site and, therefore, the size and
cost of necessary drainage infrastructure and best
management practices. In some cases, the use of
better site design practices may eliminate the need
• Creating a multifunctional landscape
for structural controls entirely. Hence, better site
• Using hydrology as a framework for site design
design concepts can be viewed as both a water
quantity and water quality management tool.
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VOL 2
21
The stormwater better site design practices and
techniques covered in this Manual are grouped
into four categories and are listed below:
❏❏Conservation of Natural Features and
Resources
»» Preserve Undisturbed Natural Areas
»» Preserve Riparian Buffers
»» Avoid Developing in Floodplains
»» Avoid Developing on Steep Slopes
»» Minimize Siting on Porous or Erodible Soils
❏❏Utilization of Natural Features for Stormwater
Management
»» Use Buffers and Undisturbed Areas
»» Use Natural Drainageways Instead of Storm
Sewers
»» Use Vegetated Swale Instead of Curb and
Gutter
»» Use soil restoration practices to improve
native soils
»» Drain Rooftop Runoff to Pervious Areas
»» Locate Development in Less Sensitive Areas
ed in the Stormwater Better Site Design Practice
maries provide the key benefits of each practice,
examples and details on how to apply them in site
»» Consider Creative Development Design
2.3.1.3 USING STORMWATER BETTER SITE DESIGN PRACTICES
Site design should be done in unison with the
❏❏Reduction of Impervious Cover
»» Reduce Roadway Lengths and Widths
»» Reduce Building Footprints
»» Reduce the Parking Footprint
design and layout of stormwater infrastructure in
attaining stormwater management goals. Fig-
»» Create Parking Lot Stormwater “Islands”
design process that utilizes the four better site
design categories.
The first step in stormwater better site design
involves identifying significant natural features
and resources on a site such as undisturbed forest
areas, stream buffers and steep slopes that should
be preserved to retain some of the original hydrologic function of the site.
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Utilize Natural Features
and Conservation Areas
to Manage Stormwater
Quantity and Quality
Figure 2.3.1-1 Stormwater Better Site Design Process
ure 2.3.1-1 illustrates the stormwater better site
»» Reduce Setbacks and Frontages
»» Use Fewer or Alternative Cul-de-Sacs
Use Various Techniques
to Reduce Impervious
Cover in the Site Design
design.
»» Reduce Limits of Clearing and Grading
»» Utilize Open Space Development
Design Site Layout to
Preserve Conservation
Areas and Minimize
Stormwater Impacts
More detail on each site design practice is providSummary Sheets in Subsection 2.3.2. These sum-
❏❏Lower Impact Site Design Techniques
»» Fit Design to the Terrain
Identify Natural Features
and Resources –
Delineate Site
Conservation Areas
Stormwater Management Planning, Design & Implementation
2.3.1.2 LIST OF STORMWATER BETTER SITE
DESIGN PRACTICES AND TECHNIQUES
Next, the site layout is designed such that these
conservation areas are preserved and the impact
of the development is minimized. A number of
techniques can then be used to reduce the overall
imperviousness of the development site.
Finally, natural features and conservation areas
can be utilized to serve stormwater quantity and
quality management purposes.
VOL 2
22
Stormwater Management Planning, Design & Implementation
2.3.2 Better Site Design Practices
2.3.2.1 CONSERVATION OF NATURAL FEATURES AND RESOURCES
Conservation of natural features is integral to better site design. The first step
in the better site design process is to identify and preserve the natural features
and resources that can be used in the protection of water resources by reducing stormwater runoff, providing runoff storage, reducing flooding, preventing soil erosion, promoting infiltration, and removing stormwater pollutants.
Some of the natural features that should be taken into account include:
• Areas of undisturbed vegetation
• Floodplains and riparian areas
• Ridgetops and steep slopes
• Natural drainage pathways
• Intermittent and perennial streams
• Wetlands / tidal marshes
• Aquifers and recharge areas
• Soils
• Shallow bedrock or high water table
• Other natural features or critical areas
Some of the ways used to conserve natural features and resources described
over the next several pages include the following methods:
• Preserve Undisturbed Natural Areas
• Preserve Riparian Buffers
Figure 2.3.2-1 Example of Natural Feature Delineation
(Source: MPCA, 1989)
Delineation of natural features is typically done through a comprehensive site
analysis and inventory before any site layout design is performed (see Section
2.4). From this site analysis, a concept plan for a site can be prepared that
provides for the conservation and protection of natural features. Figure 2.3.21 shows an example of the delineation of natural features on a base map of a
development parcel.
• Avoid Floodplains
• Avoid Steep Slopes
• Minimize Siting on Porous or Erodible Soils
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VOL 2
23
Discussion
Conservation areas should be incorporated into
Preserve Undisturbed Natural Areas
Preserving natural conservation areas such as
site plans and clearly marked on all construction
undisturbed forested and vegetated areas, natural
and grading plans to ensure that equipment is
drainageways, stream corridors and wetlands on
kept out of these areas and that native vegetation
a development site helps to preserve the original
is kept in an undisturbed state. The boundaries
hydrology of the site and aids in reducing the
of each conservation area should be mapped by
generation of stormwater runoff and pollutants.
carefully determining the limit which should not
Undisturbed vegetated areas also promote soil
be crossed by construction activity.
»» Description: Important natural features
and areas such as undisturbed forested
and vegetated areas, natural drainageways,
stream corridors, wetlands and other
important site features should be delineated
and placed into conservation areas.
stabilization and provide for filtering, infiltration
and evapotranspiration of runoff.
KEY BENEFITS
Once established, natural conservation areas must
be protected during construction and managed
Natural conservation areas are typically identified
after occupancy by a responsible party able to
helps to preserve a portion of the site’s
through a site analysis using maps and aerial/sat-
maintain the areas in a natural state in perpetuity.
natural pre-development hydrology
ellite photography, or by conducting a site visit.
Typically, conservation areas are protected by
These areas should be delineated before any site
legally enforceable deed restrictions, conservation
design, clearing or construction begins. When
easements, and maintenance agreements.
• Conserving undisturbed natural areas
• Can be used as nonstructural stormwater
filtering and infiltration zones
• Helps to preserve the site’s natural
done before the concept plan phase, the planned
character and aesthetic features
conservation areas can be used to guide the layout of the site. Figure 2.3.2-2 shows a site map
• May increase the value of the developed
property
• A stormwater site design credit can
be taken if allowed by the local review
authority (see subsection 2.3.2)
with undisturbed natural areas delineated.
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #1:
• Delineate natural areas before performing
site layout and design
• Ensure that conservation areas and
Natural Drainageway
native vegetation are protected in
Wetland
an undisturbed state throughout
Undisturbed Forest
Conservation Area
construction and occupancy
Figure 2.3.2-2 Delineation of Natural Conservation Areas
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VOL 2
24
Discussion
Ideally, riparian buffers should be sized to include
Preserve Riparian Buffers
A riparian buffer is a special type of natural con-
the 100-year floodplain as well as steep banks and
servation area along a stream, wetland or shore-
freshwater wetlands. The buffer width needed
line where development is restricted or prohibit-
to perform properly will depend on the size of
ed. The primary function of buffers is to protect
the stream and the surrounding conditions, but a
and physically separate a stream, lake or wetland
minimum 25-foot undisturbed vegetative buffer
from future disturbance or encroachment. If
is needed for even the smallest perennial streams
properly designed, a buffer can provide stormwa-
and a 50-foot or larger undisturbed buffer is ideal.
ter management functions, can act as a right-of-
Even with a 25-foot undisturbed buffer, additional
way during floods, and can sustain the integrity of
zones can be added to extend the total buffer to
stream ecosystems and habitats. An example of a
at least 75 feet from the edge of the stream. The
riparian stream buffer is shown in Figure 2.3.2-3.
three distinct zones within the 75-foot width are
»» Description: Naturally vegetated buffers
should be delineated and preserved along
perennial streams, rivers, lakes, and wetlands.
KEY BENEFITS
• Riparian buffers can be used as
nonstructural stormwater filtering and
infiltration zones
• Keeps structures out of the floodplain and
shown in Figure 2.3.2-4. The function, vegetative
provides a right-of-way for large flood
target and allowable uses vary by zone as de-
events
scribed in Table 2.3.2-1.
• Helps to preserve riparian ecosystems and
habitats
These recommendations are minimum standards
to apply to most streams. Some streams and
watershed may require additional measures to
achieve protection. In some areas, specific state
laws or local ordinances already require stricter buffers than are described here. The buffer
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #2:
widths discussed are not intended to modify or
supersede deeper or more restrictive buffer re-
• Delineate and preserve naturally
vegetated riparian buffers
• Ensure that buffers and native vegetation
Figure 2.3.2-3 Riparian Stream Buffer
are protected throughout construction
Forested riparian buffers should be maintained
and occupancy
and reforestation should be encouraged where no
quirements that are already in place.
wooded buffer exists. Proper restoration should
include all layers of the forest plant community,
including understory, shrubs and groundcover,
not just trees. A riparian buffer can be of fixed or
variable width, but should be continuous and not
interrupted by impervious areas that would allow
stormwater to concentrate and flow into the
stream without first flowing through the buffer.
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VOL 2
25
should consist of a minimum of 25 feet of undisturbed mature forest. In addition to runoff
protection, this zone provides bank stabilization
as well as shading and protection for the stream.
This zone should also include wetlands and any
critical habitats, and its width should be adjusted
accordingly. The middle zone provides a transition between upland development and the inner
zone and should consist of managed woodland
that allows for infiltration and filtration of runoff.
STREAM
An outer zone allows more clearing and acts as
a further setback for impervious surfaces. It also
functions to prevent encroachment and filter runoff. It is here that flow into the buffer should be
STREAMSIDE
ZONE
MIDDLE ZONE
OUTER ZONE
Figure 2.3.2-4 Three-Zone Stream Buffer System
transformed from concentrated flow into sheet
flow to maximize ground contact with the runoff.
Development within the riparian buffer should be
limited only to those structures and facilities that
are absolutely necessary. Such limited development should be specifically identified in any
Table 2.3.2-1 Riparian Buffer Management Zones
codes or ordinances enabling the buffers. When
construction activities do occur within the riparian
corridor, specific mitigation measures should be
required, such as deeper buffers or riparian buffer
Width
Streamside Zone
Middle Zone
Outer Zone
Minimum 25 feet plus
Variable depending on
25-foot minimum setback
wetlands and critical
stream order, slope, and
from structures
habitat
100-year floodplain (min.
25 ft)
improvements.
Generally, the riparian buffer should remain in its
natural state. However, some maintenance is pe-
Stormwater Management Planning, Design & Implementation
As stated above, the streamside or inner zone
Vegetative Target
Undisturbed mature
Managed forest, some
Forest encouraged, but
forest. Reforest if nec-
clearing allowed.
usually turf grass.
Restricted
Unrestricted
essary.
riodically necessary, such as planting to minimize
Very Restricted
concentrated flow, the removal of exotic plant
species when these species are detrimental to the
vegetated buffer and the removal of diseased or
damaged trees.
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Allowable Uses
e.g., flood control, utility
e.g., some recreational
e.g., residential uses in-
easements, footpaths
uses, some stormwater
cluding lawn, garden, most
controls, bike paths
stormwater controls
VOL 2
26
Discussion
As such, floodplain areas should be avoided on
Avoid Floodplains
Floodplains are the low-lying flat lands that bor-
a development site. Ideally, the entire 100-year
der streams and rivers. When a stream reaches
full-buildout floodplain should be avoided for
its capacity and overflows its channel after storm
clearing or building activities, and should be
events, the floodplain provides for storage and
preserved in a natural undisturbed state where
conveyance of these excess flows. In their natural
possible. Floodplain protection is complementary
state they reduce flood velocities and peak flow
to riparian buffer preservation. Both of these bet-
rates by the passage of flows through dense veg-
ter site design practices preserve stream corridors
etation. Floodplains also play an important role in
in a natural state and allow for the protection of
reducing sedimentation and filtering runoff, and
vegetation and habitat. Depending on the site
provide habitat for both aquatic and terrestrial
topography, 100-year floodplain boundaries may
• Preserving floodplains provides a natural
life. Development in floodplain areas can reduce
lie inside or outside of a preserved riparian buffer
right-of-way and temporary storage for
the ability of the floodplain to convey stormwater,
corridor, as shown in Figure 2.3.2-5.
large flood events
potentially causing safety problems or significant
»» Description: Floodplain areas should be
avoided for homes and other structures to
minimize risk to human life and property
damage, and to allow the natural stream
corridor to accommodate flood flows.
KEY BENEFITS
• Keeps people and structures out of harm’s
way
• Helps to preserve riparian ecosystems and
habitats
• Can be combined with riparian buffer
protection to create linear greenways
damage to the site in question, as well as to both
Maps of the 100-year floodplain can typically be
upstream and downstream properties. Most
obtained through the local review authority, through
communities regulate the use of floodplain areas
FEMA’s website, or through the Georgia Flood M.A.P.
to minimize the risk to human life as well as to
website. Developers and builders should also ensure
avoid flood damage to structures and property.
that their site design comply will any other rele-
vant local floodplain and FEMA requirements.
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #3:
• Obtain maps of the 100-year floodplain
from the local review authority
• Ensure that all development activities
do not encroach on the designated
floodplain areas
Figure 2.3.2-5 Floodplain Boundaries in Relation to a Riparian Buffer
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27
Discussion
Excessive grading should be avoided on all slopes,
Avoid Steep Slopes
Developing on steep slope areas has the potential
as should the flattening of hills and ridges. Steep
to cause excessive soil erosion and stormwater
slopes should be kept in an undisturbed natural
runoff during and after construction. Past studies
condition to help stabilize hillsides and soils
»» Description: Steep slopes should be avoided
due to the potential for soil erosion and
increased sediment loading. Excessive
grading and flattening of hills and ridges
should be minimized.
KEY BENEFITS
• Preserving steep slopes helps to
prevent soil erosion and degradation of
stormwater runoff
by the NRCS and others have shown that soil
erosion is significantly increased on slopes of 15%
On slopes greater than 25%, no development,
or greater. In addition, the nature of steep slopes
regrading, or stripping of vegetation should be
means that greater areas of soil and land area are
considered unless the disturbance is for roadway
disturbed to locate facilities on them compared to
crossings or utility construction and it can be
flatter slopes as demonstrated in Figure 2.3.2-6.
demonstrated that the roadway or utility improve-
ments are absolutely necessary in the sloped area.
Large Impact Area
• Steep slopes can be kept in an
undisturbed natural condition to help
stabilize hillsides and soils
• Building on flatter areas will reduce the
need for cut-and-fill and grading
• Avoid development on steep slope areas,
especially those with a grade of 15% or
greater
• Minimize grading and flattening of hills
and ridges
Small Impact Area
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #4:
Figure 2.3.2-6 Flattening Steep Slopes
for Building Sites Uses More Land Area
Than Building on Flatter Slopes
(Source: MPCA, 1989)
Therefore, development on slopes with a grade
of 15% or greater should be avoided if possible
to limit soil loss, erosion, excessive stormwater
runoff, and the degradation of surface water.
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28
Discussion
Soils on a development site should be mapped
Minimize Siting on Porous or Erodible
Soils
Infiltration of stormwater into the soil reduces both
in order to preserve areas with porous soils, and
the volume and peak discharge of runoff from a giv-
to identify those areas with unstable or erodible
en rainfall event, and also provides for water quality
soils as shown in Figure 2.3.2-7. Soil surveys can
treatment and groundwater recharge. Soils with
provide a considerable amount of information
maximum permeabilities (hydrologic soil group A
relating to all relevant aspects of soils. Appendix D
and B soils such as sands and sandy loams) allow for
of this Manual provides permeability, shrink-swell
the most infiltration of runoff into the subsoil. Thus,
potential and hydrologic soils group informa-
areas of a site with these soils should be conserved
tion for all Georgia soil series. General soil types
as much as possible and these areas should ideally
should be delineated on concept site plans to
be incorporated into undisturbed natural or open
guide site layout and the placement of buildings
space areas. Conversely, buildings and other imper-
and impervious surfaces.
»» Description: Porous soils such as sand
and gravels provide an opportunity for
groundwater recharge of stormwater runoff
and should be preserved as a potential
stormwater management option. Unstable
or easily erodible soils should be avoided due
to their greater erosion potential.
KEY BENEFITS
vious surfaces should be located on those portions
of the site with the least permeable soils.
• Areas with highly permeable soils can
be used as nonstructural stormwater
Similarly, areas on a site with highly erodible or
infiltration zones.
unstable soils should be avoided for land disturb-
• Avoiding high erodible or unstable soils
ing activities and buildings to prevent erosion
can prevent erosion and sedimentation
and sedimentation problems as well as potential
problems and water quality degradation
future structural problems. These areas should be
left in an undisturbed and vegetated condition.
Area with
Area erodible
with erodible
soils soils.
C
B
• Use soil surveys to determine site soil
types
A
C
D
“A”and
and“B”
“B”
“A”
soils
soils
are
more
are more porous–
porous –undisturbed
preserve
preserve
if
possible.
B
C
• Leave areas of porous or highly erodible
undisturbed
if possible
soils as undisturbed conservation areas
A
“C” and “D” soils
“C” and “D”
soils should be
impervious
usedsurfaces
for impervious
and buildings.
surfaces and buildings
should be used for
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Stormwater Management Planning, Design & Implementation
Better Site Design Practice #5:
A
B
A
Figure 2.3.2-7 Soil Mapping Information Can Be Used to Guide Development
VOL 2
29
Stormwater Management Planning, Design & Implementation
2.3.2.2 LOWER IMPACT SITE DESIGN TECHNIQUES
After a site analysis has been performed and
conservation areas have been delineated, there
are numerous opportunities in the site design and
layout phase to reduce both water quantity and
quality impacts of stormwater runoff. These primarily deal with the location and configuration of
impervious surfaces or structures on the site and
include the following practices and techniques
covered over the next several pages:
»» Fit the Design to the Terrain
»» Locate Development in Less Sensitive Areas
»» Reduce Limits of Clearing and Grading
»» Utilize Open Space Development
»» Consider Creative Development Design
The goal of lower impact site design techniques is
to lay out the elements of the development project
in such a way that the site design (i.e. placement
of buildings, parking, streets and driveways, lawns,
undisturbed vegetation, buffers, etc.) is optimized
for effective stormwater management. That is, the
Figure 2.3.2-8 Development Design Utilizing Several Lower Impact Site Design Techniques
site design takes advantage of the site’s natural features, including those placed in conservation areas,
as well as any site constraints and opportunities
(topography, soils, natural vegetation, floodplains,
shallow bedrock, high water table, etc.) to prevent
both on-site and downstream stormwater impacts.
Figure 2.3.2-8 shows a development that has utilized several lower impact site design techniques
in its overall layout and design.
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30
Discussion
In flatter areas, a traditional grid pattern of streets
Fit Design to the Terrain
All site layouts should be designed to conform with
or “fluid” grids which bend and may be interrupted
or “fit” the natural landforms and topography of a
by natural drainageways may be more appropriate
site. This helps to preserve the natural hydrology
(see Figure 2.3.2-11). In either case, buildings and
and drainageways on the site, as well as reduces
impervious surfaces should be kept off of steep
the need for grading and disturbance of vegetation
slopes, away from natural drainageways, and out of
and soils. Figure 2.3.2-9 illustrates the placement
floodplains and other lower lying areas. In addi-
of roads and homes in a residential development.
tion, the major axis of buildings should be oriented
»» Description: The layout of roadways
and buildings on a site should generally
conform to the landforms on a site. Natural
drainageways and stream buffer areas
should be preserved by designing road
layouts around them. Buildings should
be sited to utilize the natural grading and
drainage system and avoid the unnecessary
disturbance of vegetation and soils.
parallel to existing contours.
Roadway patterns on a site should be chosen to
provide access schemes which match the terrain.
In rolling or hilly terrain, streets should be designed
to follow natural contours to reduce clearing
KEY BENEFITS
• Helps to preserve the natural hydrology
and drainageways of a site
• Reduces the need for grading and land
and grading. Street hierarchies with local streets
branching from collectors in short loops and
cul-de-sacs along ridgelines help to prevent the
crossing of streams and drainageways as shown in
Figure 2.3.2-10.
disturbance
• Provides a framework for site design and
layout
Vegetated
drainage swales
• Develop roadway patterns to fit the site
terrain. Locate buildings and impervious
Natural drainageways
preserved
surfaces away from steep slopes,
drainageways and floodplains
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Houses located on
“brow” of ridge
Roads on ridge lines
or upland areas
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #6:
Undisturbed
vegetation
on slopes
Figure 2.3.2-9 Preserving the Natural Topography of the Site
(Adapted from Sykes, 1989)
VOL 2
31
Stormwater Management Planning, Design & Implementation
Figure 2.3.2-10 Subdivision Design for Hilly or Steep Terrain Utilizes Branching Streets
From Collectors that Preserves Natural Drainageways and Stream Corridors
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2.3.2-11 A Subdivision Design for Flat Terrain
Uses a Fluid Grid Layout that is Interrupted by the Stream Corridor
VOL 2
32
project. This is accomplished by steering develop-
Locate Development in Less Sensitive
Areas
ment to areas of the site that are less sensitive to
»» Description: To minimize the hydrologic
impacts on the existing site land cover, the
area of development should be located in
areas of the site that are less sensitive to
disturbance or have a lower value in terms of
hydrologic function.
KEY BENEFITS
• Helps to preserve the natural hydrology
and drainageways of a site
• Makes most efficient use of natural site
features for preventing and mitigating
land disturbance or have a lower value in terms of
hydrologic function using the following methods:
• Locate buildings and impervious surfaces away
from stream corridors, wetlands and natural
drainageways. Use buffers to preserve and
protect riparian areas and corridors.
• Areas of the site with porous soils should left
in an undisturbed condition and/or used as
stormwater runoff infiltration zones. Buildings
and impervious surfaces should be located in
areas with less permeable soils.
• Avoid land disturbing activities or construction
on areas with steep slopes or unstable soils.
• Minimize the clearing of areas with dense tree
canopy or thick vegetation, and ideally preserve
them as natural conservation areas
• Ensure that natural drainageways and flow
paths are preserved, where possible. Avoid the
filling or grading of natural depressions and
ponding areas.
Figure 2.3.2-12 shows a development site where
the natural features have been mapped in order
to delineate the hydrologically sensitive areas.
Through careful site planning, sensitive areas can
be set aside as natural open space areas (see Better
Site Design Practice #9). In many cases, such
areas can be used as buffer spaces between land
uses on the site or between adjacent sites.
contours.
stormwater impacts
• Provides a framework for site design and
layout
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #7:
»»
• Lay out the site design to minimize the
»»
hydrologic impact of structures and
»» impervious surfaces
»»
Discussion
In much the same way that a development should
be designed to conform to terrain of the site,
a site layout should also be designed so that the
areas of development are placed in the locations of
Figure 2.3.2-12 Guiding Development Away
from Sensitive Areas of a Site
the site that minimize the hydrologic impact of the
(Source: Prince George’s County, MD, 1999)
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33
Discussion
Reduce Limits of Clearing and Grading
Minimal disturbance methods should be used to
»» Description: Clearing and grading of the site
should be limited to the minimum amount
needed for the development and road
access. Site footprinting should be used to
disturb the smallest possible land area on a
site.
KEY BENEFITS
• Preserves more undisturbed natural areas
on a development site
• Techniques can be used to help protect
natural conservation areas and other site
features
limit the amount of clearing and grading that takes
place on a development site, preserving more of
the undisturbed vegetation and natural hydrology
of a site. These methods include:
• Establishing a limit of disturbance (LOD)
based on maximum disturbance zone radii/
lengths. These maximum distances should
reflect reasonable construction techniques and
equipment needs together with the physical
situation of the development site such as
slopes or soils. LOD distances may vary by
type of development, size of lot or site, and by
the specific development feature involved.
Figure 2.3.2-13
Establishing Limits of Clearing
(Source: DDNREC, 1997)
• Using site “footprinting” which maps all of the
limits of disturbance to identify the smallest
possible land area on a site which requires
clearing or land disturbance. Examples of site
footprinting is illustrated in Figures 2.3.2-13
and 2.3.2-14.
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #8:
• Fitting the site design to the terrain.
• Using special procedures and equipment which
reduce land disturbance.
• Establish limits of disturbance for all
Figure 2.3.2-14 Example of Site Footprinting
development activities
• Use site footprinting to minimize clearing
and land disturbance
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VOL 2
34
Discussion
Open space developments have many benefits
Utilize Open Space Development
Open space development, also known as con-
compared with conventional commercial de-
servation development or clustering, is a better
velopments or residential subdivisions: they can
site design technique that concentrates structures
reduce impervious cover, stormwater pollution,
and impervious surfaces in a compact area in one
construction costs, and the need for grading and
portion of the development site in exchange for
landscaping, while providing for the conservation
providing open space and natural areas elsewhere
of natural areas. Figures 2.3.2-15 and 2.3.2-16
on the site. Typically smaller lots and/or nontradi-
show examples of open space developments.
»» Description: Open space site designs
incorporate smaller lot sizes to reduce overall
impervious cover while providing more
undisturbed open space and protection of
water resources.
tional lot designs are used to cluster development
KEY BENEFITS
• Preserves conservation areas on a
development site
• Can be used to preserve natural hydrology
and create more conservation areas on the site.
and drainageways
• Can be used to help protect natural
conservation areas and other site features
• Reduces the need for grading and land
disturbance
• Reduces infrastructure needs and overall
development costs
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #9:
• Use a site design which concentrates
development and preserves open space
and natural areas of the site
Figure 2.3.2-15 Open Space Subdivision Site Design Example
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VOL 2
35
to 50 percent of the development site in conserva-
Several studies estimate that residential properties
designs provide a host of other environmental
tion areas that would not otherwise be protected.
in open space developments garner premiums
benefits lacking in most conventional designs.
Open space developments can also be signifi-
that are higher than conventional subdivisions and
These developments reduce potential pressure
cantly less expensive to build than conventional
moreover, sell or lease at an increased rate.
to encroach on conservation and buffer areas
projects. Most of the cost savings are due to
because enough open space is usually reserved to
reduced infrastructure cost for roads and storm-
Once established, common open space and
accommodate these protection areas. As less land
water management controls and conveyances.
natural conservation areas must be managed by
is cleared during the construction process, alter-
While open space developments are frequent-
a responsible party able to maintain the areas in a
ation of the natural hydrology and the potential for
ly less expensive to build, developers find that
natural state in perpetuity. Typically, the conser-
soil erosion are also greatly diminished. Perhaps
these properties often command higher prices
vation areas are protected by legally enforceable
most importantly, open space design reserves 25
than those in more conventional developments.
deed restrictions, conservation easements, and
maintenance agreements.
Stormwater Management Planning, Design & Implementation
Along with reduced imperviousness, open space
Figure 2.3.2-16 Aerial View of an Open Space Subdivision
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36
Discussion
A developer or site designer should consult their
Consider Creative Development Design
A Planned Unit Development (PUD) is a type of
local review authority to determine whether
planning approval available in some communi-
the community supports PUD approvals. If so,
ties which provides greater design flexibility by
the type and nature of deviations allowed from
allowing deviations from the typical development
individual development requirements should be
standards required by the local zoning code with
obtained from the review authority in addition to
additional variances or zoning hearings.
any other criteria that must be met to obtain a
The intent is to encourage better designed proj-
PUD approval.
»» Description: Planned Unit Developments
(PUDs) allow a developer or site designer the
flexibility to design a residential, commercial,
industrial, or mixed-use development in
a fashion that best promotes effective
stormwater management and the protection
of environmentally sensitive areas.
ects through the relaxation of some development
requirements, in exchange for providing greater
benefits to the community. PUDs can be used to
KEY BENEFITS
implement many of the other stormwater better site design practices covered in this Manual
and to create site designs that maximize natural
• Allows flexibility to developers to
implement creative site designs which
nonstructural approaches to stormwater management.
include stormwater better site design
practices
• May be useful for implementing an open
space development
Examples of the types of zoning deviations which
are often allowed through a PUD process include:
• Allowing uses not listed as permitted,
conditional or accessory by the zoning district
in which the property is located
Stormwater Management Planning, Design & Implementation
Better Site Design Practice #10:
• Modifying lot size and width requirements
• Reducing building setbacks and frontages from
property lines
• Check with your local review authority
to determine if the community supports
PUDs
• Altering parking requirements
• Increasing building height limits
• Determine the type and nature of
deviations allowed and other criteria for
receiving PUD approval
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VOL 2
37
and techniques that reduce the overall impervious
2.3.3.2 STORMWATER CREDITS AND THE SITE
PLANNING PROCESS
2.3.3.1 INTRODUCTION
area on a site already implicitly reduce the total
During the site planning process described in
Non-structural stormwater control practices are
amount of stormwater runoff generated by a site
Section 2.4 there are several steps involved in site
increasingly recognized as a critical feature in
(and thus reduce WQv) and are not further credit-
layout and design, each more clearly defining the
every site design. As such, a set of stormwater
ed under this system.
It should be noted that better site design practices
location and function of the various components
of the stormwater management system. The
“credits” has been developed to provide developers and site designers an incentive to implement
For each potential credit, there is a minimum set
integration of site design credits can be integrated
better site design practices that can reduce the
of criteria and requirements which identify the
with this process as shown in Table 2.3.3-2.
volume of stormwater runoff and minimize the
conditions or circumstances under which the
pollutant loads from a site. The credit system di-
credit may be applied. The intent of the sug-
rectly translates into cost savings to the developer
gested numeric conditions (e.g., flow length,
by reducing the size of best management practic-
contributing area, etc.) is to avoid situations that
es and conveyance facilities.
could lead to a credit being granted without the
corresponding reduction in pollution attributable
The basic premise of the credit system is to rec-
to an effective site design modification.
ognize the water quality benefits of certain site
design practices by allowing for a reduction in the
Site designers are encouraged to utilize credits on
water quality treatment volume (WQv). If a de-
a site. Greater reductions in stormwater storage
veloper incorporates one or more of the credited
volumes can be achieved when many credits
practices in the design of the site, the requirement
are combined (e.g., disconnecting rooftops and
for capture and treatment of the water quality
protecting natural conservation areas). However,
volume will be reduced.
credits cannot be claimed twice for an identical
Stormwater Management Planning, Design & Implementation
2.3.3 Site Design Stormwater Credits
area of the site (e.g. claiming credit for natural
The Natural Area Conservation better site de-
conservation areas and disconnecting rooftops
sign practice provides a stormwater credit, given
over the same site area).
certain site-specific conditions are met. For
example, natural area conservation credits must
Due to local safety codes, soil conditions, and
be protected by a conservation easement. Natural
topography, some of these site design credits may
conservation areas are defined as undisturbed
be restricted. Designers are encouraged to consult
natural areas that are conserved on a site, thereby
with the appropriate approval authority to ensure
retaining their pre-development hydrologic and
if and when a credit is applicable and to determine
water quality characteristics.
restrictions on non-structural strategies.
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VOL 2
38
Site Development Phase
Feasibility Study
Site Design Credit Activity
•
Determine stormwater management requirements
•
Perform site reconnaissance to identify potential areas for and types of
credits
Site Analysis
Concept Plan
•
Identify and delineate natural feature conservation areas (natural areas
and stream buffers)
•
Preserve natural areas and stream buffers during site layout
•
Reduce impervious surface area through various techniques
•
Identify locations for use of vegetated channels and groundwater recharge
•
Look for areas to disconnect impervious surfaces
•
Document the use of site design credits.
•
Perform layout and design of credit areas – integrating them into treatment trains
Preliminary and Final Plan
•
Ensure unified stormwater sizing criteria are satisfied
•
Ensure appropriate documentation of site design credits according to
local requirements.
Construction
Final Inspection
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•
Ensure protection of key areas
•
Ensure correct final construction of areas needed for credits
•
Develop maintenance requirements and documents
•
Ensure long term protection and maintenance
•
Ensure credit areas are identified on final plan and plat if applicable
Stormwater Management Planning, Design & Implementation
Table 2.3.3-2 Integration of Site Design Credits with Site Development Process
VOL 2
39
A stormwater credit can be taken when undisturbed natural areas are conserved on a site,
thereby retaining their pre-development hydrologic and water quality characteristics. Under this
credit, a designer would be able to subtract conservation areas from total site area when com-
[Note: managed turf (e.g., playgrounds,
regularly maintained open areas) is not an
acceptable form of vegetation management]
• Shall have a minimum contiguous area
requirement of 10,000 square feet
• Rv is kept constant when calculating WQv
puting water quality volume requirements. An
added benefit will be that the post-development
peak discharges will be smaller, and hence water
quantity control volumes (CPv, Qp25, and Qf) will
be reduced due to lower post-development curve
numbers or rational formula “C” values.
Rule: Subtract conservation areas from total
site area when computing water quality volume
EXAMPLE
requirements.
Residential Subdivision
Area = 38 acres
Criteria:
Natural Conservation Area = 7 acres
• Conservation area cannot be disturbed during
project construction
Impervious Area = 13.8 acres
• Conservation area must remain undisturbed (if
disturbed in the future, the credit is no longer
valid and additional water quality measures will
be required)
• Shall be protected by limits of disturbance
clearly shown on all construction drawings
• Shall be located within an acceptable
conservation easement instrument that ensures
perpetual protection of the proposed area.
The easement must clearly specify how the
natural area vegetation shall be managed and
boundaries will be marked
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Stormwater Management Planning, Design & Implementation
2.3.3.3 SITE DESIGN CREDIT - NATURAL AREA
CONSERVATION
Rv = 0.05 + 0.009 (I) = 0.05 + 0.009 (36.3%) = 0.37
Credit:
7.0 acres in natural conservation area
New drainage area = 38 – 7 = 31 acres
Before credit:
WQv = (1.2)(0.37)(38)/12 = 1.40 ac-ft
With credit:
WQv = (1.2)(0.37)(31)/12 = 1.15 ac-ft
(18% reduction in water quality volume)
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• What are the opportunities for utilizing better
site design practices to minimize the need for
best management practices?
2.4.1 Stormwater Management and Site
Planning
• What are the development site constraints that
preclude the use of certain best management
practices?
2.4.1.1 INTRODUCTION
In order to most effectively address stormwater
management objectives, consideration of storm-
2.
Almost all sites contain natural features
which can be used to help manage and
mitigate runoff from development. Features
on a development site might include natural
drainage patterns, depressions, permeable
soils, wetlands, floodplains, and undisturbed
vegetated areas that can be used to reduce
runoff, provide infiltration and stormwater
filtering of pollutants and sediment, recycle
nutrients, and maximize on-site storage
of stormwater. Site design should seek to
improve the effectiveness of natural systems
rather than to ignore or replace them.
Further, natural systems typically require
low or no maintenance, and will continue to
function many years into the future.
• What best management practices are most
suitable and cost-effective for the site?
water runoff needs to be fully integrated into the
site planning and design process. This involves a
more comprehensive approach to site planning
and a thorough understanding of the physical
characteristics and resources of the site. The
purpose of this section is to provide a framework for including effective and environmentally
sensitive stormwater management into the site
development process and to encourage a greater
2.4.1.2 PRINCIPLES OF STORMWATER MANAGEMENT SITE PLANNING
The following principles should be kept in mind
in preparing a stormwater management plan for a
development site:
1.
uniformity in stormwater management site plan
preparation.
When designing the stormwater management
system for a site, a number of questions need
to be answered by the site planners and design
engineers, including:
• How can the stormwater management system
be designed to most effectively meet the
stormwater management recommended
standards (and any additional needs or
objectives)?
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.
The site design should utilize an integrated
approach to deal with stormwater
quantity, quality and streambank (channel)
protection requirements.
The stormwater management infrastructure
for a site should be designed to integrate
drainage and water quantity control,
water quality protection, and downstream
channel protection. Site design should
be done in unison with the design and
layout of stormwater infrastructure to
attain stormwater management goals.
Together, the combination of better site
design practices and effective infrastructure
layout and design can mitigate the worst
stormwater impacts of most urban
developments while preserving stream
integrity and aesthetic attractiveness.
Stormwater management practices should
strive to utilize the natural drainage system
and require as little maintenance as possible.
3.
Best management practices should be
implemented only after all site design and
nonstructural options have been exhausted.
.
Operationally, economically, and
aesthetically, stormwater better site design
and the use of natural techniques offer
significant benefits over best management
practices. Therefore, all opportunities for
utilizing these methods should be explored
before implementing best management
practices such as wet ponds and sand filters.
Stormwater Management Planning, Design & Implementation
2.4 Stormwater Site Planning &
Design
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41
.
Structural stormwater solutions should
attempt to be multi-purpose and be
aesthetically integrated into a site’s design.
2.4.2 Preparation of Stormwater Management Site Plans
2.4.2.2 PRE-CONSULTATION MEETING AND
JOINT SITE VISIT
A structural stormwater facility need not
be an afterthought or ugly nuisance on a
development site. A parking lot, soccer
field or city plaza can serve as a temporary
storage facility for stormwater. In addition,
water features such as ponds and lakes,
when correctly designed and integrated into
a site, can increase the aesthetic value of a
development.
2.4.2.1 INTRODUCTION
at the beginning of the development project is
A stormwater management site plan is a com-
a pre-consultation meeting between the local
prehensive report that contains the technical
review authority and the developer and their team
information and analysis to allow a local review
to outline the stormwater management require-
authority to determine whether a proposed new
ments and other regulations, and to assist the
development or redevelopment project meets the
developer in assessing constraints, opportunities,
local stormwater regulatory requirements and/
and potential for stormwater design concepts
or the recommended stormwater management
utilizing low impact development and runoff
standards contained in this Manual. See Figure
reduction techniques.
The most important action that can take place
2.2.5-2 for information on the stormwater design
5.
“One size does not fit all” in terms of
This recommended step helps to establish a con-
process.
structive partnership for the entire development
stormwater management solutions.
Although the basic problems of stormwater
runoff and the need for its management
remain the same, each site, project, and
watershed presents different challenges
and opportunities. For instance, an infill
development in a highly urbanized town
center or downtown area will require a much
different set of stormwater management
solutions than a low-density residential
subdivision in a largely undeveloped
watershed. Therefore, local stormwater
management needs to take into account
differences between development sites,
different types of development and land use,
various watershed conditions and priorities,
the nature of downstream lands and waters,
and community desires and preferences.
This section describes the typical contents and
process. A joint site visit, if possible, can yield a
general procedure for preparing a stormwater
conceptual outline of the stormwater manage-
management site plan. The level of detail in-
ment plan and strategies. By walking the site, the
volved in the plan will depend on the project size
two parties can identify and anticipate problems,
and the individual site and development charac-
define general expectations and establish general
teristics.
boundaries of natural feature protection and conservation areas. A major incentive for pre-con-
The preparation of a stormwater site plan ideally
sultation is that permitting and plan approval
follows these steps: ‘
requirements will become clear at an early stage,
Stormwater Management Planning, Design & Implementation
4.
increasing the likelihood that the approval process
(Step 1) Pre-consultation Meeting and
will proceed faster and more smoothly.
Joint Site Visit
(Step 2) Review of Local Requirements
(Step 3) Perform Site Analysis
(Step 4) Prepare Stormwater Concept
Plan
(Step 5) Prepare Preliminary Stormwater
Site Plan
(Step 6) Complete Final Stormwater Site
Plan
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42
oper or the developer’s design team to submit the
following information for familiarity and review:
• Wetland provisions
• Watershed-based criteria
• Existing conditions plan (boundary information,
topography, site features, etc.)
• Erosion and sedimentation control
requirements
• Proposed site plan (proposed site features)
• Maintenance requirements
• Soil characteristics, infiltration rates, etc. (if
available)
• Need for physical site evaluations (infiltration
tests, geotechnical evaluations, etc.)
• Natural resources inventory
• Conceptual stormwater management plans
Much of this guidance can be obtained at the
pre-consultation meeting with the local review
authority and should be detailed in various local
ordinances (e.g., subdivision codes, stormwater
2.4.2.3 REVIEW OF LOCAL REQUIREMENTS &
PERMITTING GUIDANCE
and drainage codes, etc.)
The site developer should be made familiar with
Current land use plans, comprehensive plans,
the local stormwater management and develop-
zoning ordinances, road and utility plans, water-
ment requirements and design criteria that apply
shed or overlay districts, and public facility plans
to the site. These requirements may include:
should all be consulted to determine the need for
• The recommended standards for stormwater
management included in this Manual (see
Section 2.2)
compliance with other local and state regulatory
• Design storm frequencies
• Conveyance design criteria
• Floodplain criteria
• Buffer/setback criteria
The checklist on the following page (Table 2.4.21) provides a condensed summary of current
restrictions as they relate to common site features
that may be regulated under local, state or federal
law. These restrictions fall into one of three general categories:
• Locating a best management practice within an
area that is expressly prohibited by law.
• Locating a best management practice within
an area that is strongly discouraged, and is only
allowed on a case by case basis. Local, state
and/or federal permits shall be obtained, and
the applicant will need to supply additional
documentation to justify locating the
stormwater practice within the regulated area.
• Best management practices must be setback a
fixed distance from the site feature.
This checklist is only intended as a general guide
requirements.
to location and permitting requirements as they
Opportunities for special types of development
Consultation with the appropriate regulatory
(e.g., clustering) or special land use opportunities
Stormwater Management Planning, Design & Implementation
Prior to the joint site visit, it is helpful for the devel-
relate to siting of best management practices.
agency is the best strategy.
(e.g., conservation easements or tax incentives)
should be investigated. There may also be an
ability to partner with a local community for the
development of greenways, or other riparian corridor or open space developments.
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43
Site Feature
Location and Permitting Guidance
Jurisdictional Wetland
(Waters of the U.S)
U.S. Army Corps of Engineers
Section 404 Permit
•
•
•
•
•
•
Jurisdictional wetlands should be delineated prior to siting structural practice.
Use of natural wetlands for stormwater quality treatment is contrary to the goals of the Clean Water Act and should be avoided.
Stormwater should be treated prior to discharge into a natural wetland.
Structural practices may also be restricted in local buffer zones, although they may be utilized as a non-structural filter strip (i.e., accept sheet flow).
Should justify that no practical upland treatment alternatives exist.
Where practical, excess stormwater flows should be conveyed away from jurisdictional wetlands.
Stream Channel
(Waters of the U.S)
U.S. Army Corps of Engineers
Section 404 Permit
•
•
•
•
•
•
•
All Waters of the U.S. (streams, ponds, lakes, etc.) should be delineated prior to design.
Use of any Waters of the U.S. for stormwater quality treatment is contrary to the goals of the Clean Water Act and should be avoided.
Stormwater should be treated prior to discharge into Waters of the U.S.
In-stream ponds for stormwater quality treatment are highly discouraged.
Must justify that no practical upland treatment alternatives exist.
Temporary runoff storage preferred over permanent pools.
Implement measures that reduce downstream warming.
Georgia Planning Act
Groundwater Recharge Areas
•
•
•
Prevention of groundwater contamination
Covers about 23% of State. Detailed mapping available at Regional Commissions of the Department of Community Affairs.
Permanent stormwater infiltration devices are prohibited in areas having high pollution susceptibility.
Georgia Planning Act
Water Supply Watersheds
•
•
•
Specific stream and reservoir buffer requirements.
May be imperviousness limitations
May be specific best management practice requirements.
100 Year Floodplain
Local Stormwater Review Authority
•
Grading and fill for structural practice construction is generally discouraged within the ultimate 100 year floodplain, as delineated by FEMA flood insurance rate
maps, FEMA flood boundary and floodway maps, or more stringent local floodplain maps.
Floodplain fill cannot raise the floodplain water surface elevation by more than a tenth of a foot.
•
Stream Buffer
•
•
•
Consult local authority for stormwater policy.
Structural practices are discouraged in the streamside zone (within 25 feet or more of streambank, depending on the specific regulations).
There are specific additional requirements related to River Corridor Protection, the Metropolitan River Protection Act, the Metropolitan North Georgia Water Planning
District, and the Georgia Scenic Rivers Act (which include wider and more stringent buffers).
Utilities
Local Review Authority
•
•
•
Call appropriate agency to locate existing utilities prior to design.
Note the location of proposed utilities to serve development.
Structural practices are discouraged within utility easements or rights of way for public or private utilities.
Roads
Local DOT, DPW, or
State DOT
•
•
•
Consult local DOT or DPW for any setback requirement from local roads.
Consult DOT for setbacks from State maintained roads.
Approval must also be obtained for any stormwater discharges to a local or state-owned conveyance channel.
Structures
Local Review Authority
•
Consult local review authority for structural practice setbacks from structures.
Septic Drain fields
Local Health Authority
•
•
Consult local health authority.
Recommended setback is a minimum of 50 feet from drain field edge.
Water Wells
Local Health Authority
•
•
100-foot setback for stormwater infiltration.
50-foot setback for all other structural practices.
Check with appropriate review
authority whether stream buffers
are required
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Stormwater Management Planning, Design & Implementation
Table 2.4.2-1 Location and Permitting Checklist
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44
requires a permit under the Erosion and Sedimentation Act, most of the resource protection features
2.4.2.5 PREPARE STORMWATER CONCEPT
PLAN
Using approved field and mapping techniques, the
will likely have been mapped as part of the land
Based upon the review of existing conditions and
site engineer should collect and review information
disturbance activity plan. Other recommended
site analysis, the design engineer should develop a
on the existing site conditions and map the follow-
site information to map or obtain includes utilities
concept site layout plan for the project.
ing site features:
»» Topography
information, seasonal groundwater levels, and
geologic mapping.
During the concept plan stage the site designer will
perform most of the layout of the site including
»» Drainage patterns and basins
Individual map or geographic information system
the preliminary stormwater management system
(GIS) layers can be designed to facilitate an analysis
design and layout. The stormwater concept plan
of the site through what is known as map overlay,
allows the design engineer to propose a potential
»» Ground cover and vegetation
or a composite analysis. Each layer (or group of
site layout and gives the developer and local review
»» Existing development
related information layers) is placed on the map in
authority a “first look” at the stormwater manage-
»» Existing stormwater facilities
such a way as to facilitate comparison and con-
ment system for the proposed development. The
trast with other layers. A composite layer is often
stormwater concept plan should be submitted to
developed to show all the layers at the same time
the local plan reviewer before detailed preliminary
(see Figure 2.4.2-1). This composite layer can be a
site plans are developed.
»» Intermittent and perennial streams
»» Soils
»» Adjacent areas
In addition, the site engineer should identify and
useful tool for defining the best buildable areas and
map all previously unmapped natural features such
delineating and preserving natural feature conser-
The following steps should be followed in develop-
as:
»» Wetlands
vation areas.
ing the stormwater concept plan:
(Step 1) Use better site design approaches (see
Section 2.3) as applicable to develop the
site layout, including:
»» Critical habitat areas
»» Boundaries of wooded areas
»» Floodplain boundaries
»» Preserving the natural feature
conservation areas defined in the site
analysis
»» Steep slopes
»» Required buffers
»» Fitting the development to the terrain
and minimizing land disturbance
»» Proposed stream crossing locations
»» Other required protection areas (e.g., well
setbacks)
»» Reducing impervious surface area
through various techniques
»» Preserving and utilizing the natural
drainage system wherever possible
Some of this information may be available from
previously performed studies or from the previous
feasibility study. For example, if a development site
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Stormwater Management Planning, Design & Implementation
2.4.2.4 PERFORM SITE ANALYSIS AND INVENTORY
Figure 2.4.2-1 Composite Analysis
(Source: Marsh, 1983)
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(Step 3) Determine the site design stormwater
credits to be accounted for in the design
of best management practices handling
the water quality volume (Section 2.3)
(Step 4) Perform screening and preliminary
selection of appropriate best
management practices and identification
of potential siting locations (Section 4.1).
It is extremely important at this stage that stormwater design is integrated into the overall site design concept in order to best reduce the impacts
of the development as well as provide for the
most cost-effective and environmentally sensitive approach. Using hydrology calculations, the
goal of mimicking pre-development (natural or
existing condition, as applicable) conditions can
serve a useful purpose in planning the stormwater
3.
Existing conditions and proposed site layout
mapping and plans (recommended scale of
1” = 50’), which illustrate at a minimum:
-- Existing and proposed topography
(minimum of 2-foot contours
recommended)
-- Mapping of predominant soils from USDA
soil surveys
-- Boundaries of existing predominant
vegetation and proposed limits of clearing
and grading
-- Location and boundaries of other natural
feature protection and conservation areas
such as wetlands, lakes, ponds, floodplains,
stream buffers and other setbacks (e.g.,
drinking water well setbacks, septic
setbacks, etc.)
-- Location of existing and proposed
roads, buildings, parking areas and other
impervious surfaces
-- Existing and proposed utilities (e.g., water,
sewer, gas, electric) and easements
-- Preliminary estimates of unified stormwater
sizing criteria requirements
For local review purposes, the stormwater con-
-- Identification and calculation of
stormwater site design credits
1.
2.
Common address and legal description of
site
Vicinity map
-- Preliminary location and dimensions of
proposed channel modifications, such as
bridge or culvert crossings
-- Perennial and intermittent streams
management system.
cept plan should include the following elements:
-- Location of floodplain/floodway limits
and relationship of site to upstream and
downstream properties and drainages
-- Preliminary selection and location, size,
and limits of disturbance of proposed best
management practices
-- Location of existing and proposed
conveyance systems such as grass
channels, swales, and storm drains
4.
Identification of preliminary waiver
requests
2.4.2.6 PREPARE PRELIMINARY STORMWATER
SITE PLAN
The preliminary plan ensures that requirements
and criteria are being complied with and that
opportunities are being taken to minimize adverse
impacts from the development.
The preliminary stormwater management site
plan should consist of maps, narrative, and
supporting design calculations (hydrologic and
hydraulic) for the proposed stormwater manage-
Stormwater Management Planning, Design & Implementation
(Step 2) Calculate preliminary estimates of
the unified stormwater sizing criteria
requirements for water quality, channel
protection, overbank flooding protection
and extreme flood protection based on
the concept plan site layout (Section 2.2)
ment system, and should include the following
sections:
1.
Natural Resources Inventory
-- Natural Drainage Divides
-- Natural Drainage Features (e.g., swales,
basins, depressional areas)
-- Wetlands
-- Water Bodies
-- Floodplains
-- Aquatic Buffers
-- Flow paths
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3.
-- Soils
-- Erodible Soils
-- Steep Slopes (i.e., Areas with Slopes
Greater Than 15%)
-- Groundwater Recharge Areas
-- Wellhead Protection Areas
-- Trees and Other Existing Vegetation
-- High Quality Habitat Areas
2.
Natural Conditions Hydrologic Analysis
(where applicable)
-- In communities where pre-development
is defined as natural conditions rather
than existing conditions, or where natural
conditions are a more appropriate
hydrologic standard, such as discharges
to impaired streams or floodprone areas,
a natural conditions hydrologic analysis
will be necessary. The natural conditions
hydrologic analysis should include all
of the elements described for Existing
Conditions Hydrologic Analysis above.
Existing Conditions Hydrologic Analysis
-- A topographic map of existing site
conditions (minimum 2-foot contour
interval recommended) with the basin
boundaries indicated
-- In some cases, the existing topography
may not be representative of natural
conditions, and the hydrologic analysis
should be modified for leveling or grading
that has occurred.
-- Acreage, soil types, and land cover of areas
for each sub-basin affected by the project
-- A set type of vegetative condition such
as “woods in good condition” may be
used in the natural conditions hydrologic
analysis. The Georgia Stormwater Quality
Site Development Review Tool is used
to calculate these conditions and can be
found at the following website: www.
gastormwater.com
-- All perennial and intermittent streams and
other surface water features
-- All existing stormwater conveyances and
structural control facilities
-- Direction of flow and exits from the site
-- Analysis of runoff provided by off-site areas
upstream of the project site
-- Infiltration rates of existing soils
-- Methodologies, assumptions, site
parameters, and supporting design
calculations used in analyzing the existing
conditions and site hydrology
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4.
Post-Development Hydrologic Analysis
-- A topographic map of developed site
conditions (minimum 2-foot contour
interval recommended) with the postdevelopment basin boundaries indicated
-- Total area of post-development impervious
surfaces and other land cover areas for
each sub-basin affected by the project
-- Unified stormwater sizing criteria runoff
calculations for water quality, channel
protection, overbank flooding protection,
and extreme flood protection for each
sub-basin
-- Location and boundaries of proposed
natural feature protection areas
-- Documentation and calculations for any
applicable site design credits that are being
utilized
-- Methodologies, assumptions, site
parameters and supporting design
calculations used in analyzing the existing
conditions site hydrology
5.
Stormwater Management System
-- Drawing or sketch of the stormwater
management system including the location
of non-structural site design features and
the placement of existing and proposed
structural stormwater controls. This
drawing should show design water surface
elevations, storage volumes available from
zero to maximum head, location of inlets
and outlets, location of bypass and discharge
systems, and all orifice/restrictor sizes.
Stormwater Management Planning, Design & Implementation
-- Shellfish Harvesting Areas
-- Narrative describing that appropriate and
effective structural stormwater controls
have been selected
-- Cross-section and profile drawings and
design details for each of the structural
stormwater controls in the system. This
should include supporting calculations to
show that the facility is designed according
to the applicable design criteria.
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ed buildout/future condition” when sizing and
-- Documentation and supporting
calculations to show that the stormwater
management system adequately meets the
unified stormwater sizing criteria
2.4.2.7 COMPLETE FINAL STORMWATER SITE
PLAN
-- Drawings, design calculations, and
elevations for all existing and proposed
stormwater conveyance elements
including stormwater drains, pipes,
culverts, catch basins, channels, swales,
and areas of overland flow
6.
In calculating runoff volumes and discharge
rates, consideration may need to be given to
any planned future upstream land use changes.
Depending on the site characteristics and given design criteria, upstream lands may need to
Operations and Maintenance Plan
-- Description of maintenance tasks,
responsible parties for maintenance,
funding, access and safety issues
4.
Evidence of Acquisition of Applicable Local
and Non-local Permits
5.
Waiver Requests
designing on-site conveyances and stormwater
practices.
The final stormwater management site plan adds
further detail to the preliminary plan and reflects
changes that are requested or required by the
local review authority. The final stormwater site
plan should include all of the revised elements of
The completed final stormwater site plan should
the preliminary plan as well as the following items:
be submitted to the local review authority for final
approval prior to any construction activities on
1.
Downstream Analysis
-- Supporting calculations for a downstream
peak flow analysis using the ten-percent
rule necessary to show safe passage
of post-development design flows
downstream
-- In some instances, the results from the
downstream analysis may indicate a
detention practice has an adverse impact
on the watershed as a whole. In such
cases, the local community may determine
the detention practice is not warranted.
3.
Erosion and Sedimentation Control Plan
-- Must contain all the elements specified in
the Georgia Erosion and Sediment Control
Act and local ordinances and regulations
-- Sequence/phasing of construction and
temporary stabilization measures
2.
the development site.
2.4.2.8 OBTAIN NON-LOCAL PERMITS
The developer should obtain any applicable
non-local environmental permit such as 404
wetland permits, 401 water quality certification,
-- Temporary structures that will be
converted into permanent best
management practices
or construction NPDES permits prior to or in con-
Landscaping Plan
-- Arrangement of planted areas, natural
areas and other landscaped features on the
site plan
conditions that require the original concept plan
junction with final plan submittal. In some cases,
Stormwater Management Planning, Design & Implementation
-- Hydrologic and hydraulic analysis of the
stormwater management system for all
applicable design storms (should include
stage-storage or outlet rating curves, and
inflow and outflow hydrographs)
a non-local permitting authority may impose
to be changed. Developers and engineers should
be aware that permit acquisition can be a long,
time-consuming process.
-- Information necessary to construct the
landscaping elements shown on the plan
drawings
-- Descriptions and standards for the
methods, materials and vegetation that are
to be used in the construction
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Feasibility Study
ment process from the perspective of the land
Stormwater site planning and design is a subset
of overall site development and must fit into the
2.4.3.1 GENERAL SITE DEVELOPMENT PROCESS
Figure 2.4.3-1 depicts a typical site develop-
2.4.3.2 STORMWATER SITE PLANNING AND
DESIGN
overall process if it is to be successful. Table
Site Analysis
2.4.3-1 on the next several pages shows how
planning for the stormwater management system
developer. After an initial site visit the developer
fits into the site development process from the
assesses the feasibility of the project. If the proj-
perspective of the developer and site planner/
ect is deemed workable, a survey is completed.
The design team prepares a concept plan (often
Concept Plan
engineer. For each step in the development
process, the stormwater-related objectives are
called a sketch plan) for consultation with the
described, along with the key actions and major
local review authority. A preliminary plan is then
activities that are typically performed to meet
prepared and submitted for necessary reviews and
Preliminary Plan
approvals. Federal, state and local permits are
applied for at various stages in the process.
After review by the local authority and possible
Final Plan
public hearings, necessary revisions are made
and a final construction plan is prepared. There
may be several iterations between plan submittal and plan approval. Bonds are set and placed,
Construction
those objectives.
Stormwater Management Planning, Design & Implementation
2.4.3 Stormwater Planning in the Development Process
contractors are hired, and construction of the
project takes place. During and after construction
numerous types of inspections take place. At the
end of construction, there is a final inspection
Final Inspections
and a use and occupancy permit is issued for the
structure itself.
Figure 2.4.3-1
Typical Site Development Flowchart
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Table 2.4.3-1 Stormwater Planning in the Site Development Process
Description:
A feasibility study is performed to determine the factors that may influence the decision to proceed with the site
development, including the basic site characteristics, local and other governmental requirements, area information,
surrounding developments, etc.
Stormwater-Related Objectives:
•
•
Understand major site constraints and opportunities
Understand local and other requirements
Key Actions:
•
•
•
Initiate discussions with local review authority
Pre-consultation between developer and plan reviewer
Determine local stormwater management requirements
Major Activities:
•
•
•
•
•
•
•
Base map development
Review of project requirements
Review of local development and stormwater management requirements
Review of local stormwater master plans or comprehensive plans
Joint site visit with local review authority
Collection of secondary source information
Determination of other factors or constraints impacting feasibility
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Stormwater Management Planning, Design & Implementation
Feasibility Study
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Table 2.4.3-1 Stormwater Planning in the Site Development Process (continued)
Description:
A site analysis is used to gain an understanding of the constraints and opportunities associated with the site
through identification, mapping and assessment of natural features and resources. Potential conservation and
resource protection areas are identified at this stage.
Stormwater-Related Objectives:
•
•
Identify key site physical, environmental, and other significant resources
Develop preliminary vision for stormwater management system
Key Actions:
•
Site evaluation and delineation of natural feature protection areas
Major Activities:
•
•
•
•
•
•
•
•
Mapping of natural resources: soils, vegetation, streams, topography, slope, wetlands, floodplains, aquifers
Identification of other key cultural, historic, archaeological, or scenic features, orientation and exposure
Identification of adjacent land uses
Identification of natural feature protection and conservation areas
Mapping of easements and utilities
Integration of all layers – map overlay
Other constraints and opportunities
Identification of adjacent transportation and utility access
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Stormwater Management Planning, Design & Implementation
Site Analysis
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51
Table 2.4.3-1 Stormwater Planning in the Site Development Process (continued)
Description:
A concept plan is used to provide both the developer and reviewer a preliminary look at the development and
stormwater management concept. Based on the site analysis, a concept plan should take into account the constraints and resources available on the site. Several alternative “what if” concept plans can be created.
Stormwater-Related Objectives:
•
•
Develop concept for stormwater management system
Gain approval from developer and local review authority of concept plan
Key Actions:
•
•
•
•
Develop site layout concept using better site design techniques where possible
Perform initial runoff characterization based on site layout concept
Determine necessary site design and/or best management practices needed to meet
stormwater management requirements
Major Activities:
•
•
•
•
•
•
Prepare sketches of functional land uses including conservation areas
“What if” analysis of different design concepts
Unified stormwater sizing criteria preliminary calculations
Utilization of better site design concepts and crediting mechanisms in layout concept
Preliminary selection and siting of best management practices
Location of drainage/conveyance facilities
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Stormwater Management Planning, Design & Implementation
Concept Plan
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52
Table 2.4.3-1 Stormwater Planning in the Site Development Process (continued)
Description:
A preliminary site plan is created for local review, which includes roadways, building and parking locations,
conservation areas, utilities, and stormwater management facilities. Following local approval, a final set of
construction plans are developed.
Stormwater-Related Objectives:
•
•
Prepare preliminary and final stormwater management site plan
Secure local and non-local permits
Key Actions:
•
•
•
Perform runoff characterization based on preliminary/final site plan
Design best management practices and conveyance systems
Perform downstream analysis
Major Activities:
•
•
•
•
•
•
•
Preliminary/final site layout plan
Unified stormwater sizing criteria calculations
Calculation of site design credit
Selection, siting and design of best management practices
Development of erosion and sedimentation control plan and landscaping plan
Applications for needed permits and waivers
Design of drainage and conveyance facilities
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Stormwater Management Planning, Design & Implementation
Preliminary and Final Plan
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53
Table 2.4.3-1 Stormwater Planning in the Site Development Process (continued)
Summary:
During the construction stage, the site must be inspected regularly to ensure that all elements are being built according to plan, and that all resource or conservation areas are suitably protected during construction.
Stormwater Objectives:
•
Ensure that stormwater management facilities and site design practices are built as designed
Key Actions:
•
•
Pre-construction meeting
Inspection during construction
Major Activities:
•
•
•
•
•
•
Execution of bonds
Inspection during key phases or key installations
Protection of best management practices
Protection of conservation areas
Erosion and sedimentation control
Proper sequencing
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Stormwater Management Planning, Design & Implementation
Construction
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54
Table 2.4.3-1 Stormwater Planning in the Site Development Process (continued)
Summary:
After construction, the site must be inspected to ensure that all elements are completed according to plan.
Long-term maintenance agreements should be executed.
Stormwater Objectives:
•
•
•
Ensure that stormwater management facilities and site design practices are built and operating as designed
Ensure long-term maintenance of best management practices and conveyances
Ensure long-term protection of conservation and resource protection areas
Key Actions:
•
•
Final inspection and submission of record drawings
Maintenance inspections
Major Activities:
•
•
•
•
Final stabilization
As-built survey
Final inspection and use permit
Execution of maintenance agreements
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Stormwater Management Planning, Design & Implementation
Final Inspection
VOL 2
55
Stream Protection. Prepared by Center for Water-
Center for Watershed Protection. 1998. Better Site
shed Protection (CWP). Metropolitan Washington
Design: A Handbook for Changing Development
Council of Governments. Washington D.C.
Rules in Your Community. Center for Watershed
Protection (CWP). Ellicott City, MD.
Schueler, Thomas R. and Heather K. Holland.
2000. The Practice of Watershed Protection.
Center for Watershed Protection. 2000. Maryland
Center for Watershed Protection (CWP), Ellicott
Stormwater Design Manual, Volumes I and II.
City, MD.
Center for Watershed Protection (CWP). Ellicott
City, MD.
Sykes, R. D. 1989. Chapter 31 – Site Planning. University of Minnesota. Minneapolis, Minnesota.
Delaware Department of Natural Resources and
Environmental Control, 1997. Conservation Design for Stormwater Management. Prepared by
the Brandywine Conservancy.
Department of Environmental Resources. 1999.
Low-Impact Development Design Strategies, An
Integrated Design Approach. Prince George’s
County, Maryland.
Marsh, W. 1983. Landscape Planning: Environ-
Stormwater Management Planning, Design & Implementation
References
mental Applications. John Wiley & Sons, New
York.
Minnesota Pollution Control Agency. 1989. Protecting Water Quality in Urban Areas. Saint Paul,
Minnesota.
Schueler, Thomas R. 1987. Controlling Urban
Runoff: A Practical Manual for Planning and
Designing Urban BMPs. Metropolitan Washington
Council of Governments. Washington D.C.
Schueler, Thomas R. 1995. Site Planning for Urban
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56
3. Stormwater Hydrology
3.1.1 Introduction to Hydrologic Methods
Hydrology deals with estimating peak flows,
volumes, and time distributions of stormwater
runoff. The analysis of these parameters is funda-
site analysis for the design methods and proce-
These methods were selected based upon a
dures included in the Manual:
❏❏Rational Method
❏❏NRCS TR-55 Unit Hydrograph Method
❏❏U.S. Geological Survey (USGS) Regression
Equations
❏❏Water Quality Treatment Volume Calculation
(which includes Runoff Reduction Calculations)
❏❏Water Balance Calculations
verification of their accuracy in duplicating local
mental to the design of stormwater management
hydrologic estimates for a range of design storms
throughout the state and the availability of equations, nomographs, and computer programs to
support the methods.
Table 3.1.1-1 lists the hydrologic methods and the
Stormwater Hydrology
3.1 Methods for Estimating
Stormwater Runoff
circumstances for their use in various analysis and
design applications. Table 3.1.1-2 provides some
facilities, such as storm drainage systems and best
limitations on the use of several methods.
management practices. In the hydrologic analysis
of a development site, there are a number of
factors that affect the nature of stormwater runoff
from the site. Some of the factors that need to be
considered include:
• Rainfall amount and storm distribution
Table 3.1.1-1 Applications of the Recommended Hydrologic Methods
• Drainage area size, shape and orientation
Method
Manual
Section
Rational
Method
NRCS TR-55
Method
USGS
Equations
• Ground cover and soil type
• Slopes of terrain and stream channel(s)
• Antecedent moisture condition
• Storage potential (floodplains, ponds, wetlands,
reservoirs, channels, etc.)
• Watershed development potential
• Characteristics of the local drainage system
There are a number of empirical hydrologic
methods that can be used to estimate runoff
characteristics for a site or drainage subbasin;
however, the following methods presented in this
Water Quality Volume (WQv)
2.2
Channel Protection Volume (CPv)
2.2
p
Overbank Flood Protection (Qp25)
2.2
p
p
Extreme Flood Protection (Qf)
2.2
p
p
Storage Facilities
3.3
p
p
Outlet Structures
3.4
p
p
Gutter Flow and Inlets
5.2
p
Storm Drain Pipes
5.2
p
p
p
Culverts
5.3
p
p
p
Small Ditches
5.4
p
p
p
Open Channels
5,4
p
p
Energy Dissipation
5.5
p
p
Water
Quality
Volume
p
section have been selected to support hydrologic
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57
• The Rational Method is recommended for
small highly impervious drainage areas such as
parking lots and roadways draining into inlets
and gutters.
Table 3.1.1-2 Constraints on Using Recommended Hydrologic Methods
Method
Rational
Comments
Size Limitations1
0-200 acres
Method can be used for estimating peak flows and the design
of small site or subdivision storm sewer systems. Shall not be
used for storage design.
• The USGS regression equations are
recommended for drainage areas with
characteristics within the ranges given for the
equations. The USGS equations should be
used with caution when there are significant
storage areas within the drainage basin or
where other drainage characteristics indicate
that general regression equations might not be
appropriate.
NRCS TR-55
2
0-2,000 acres*
Method can be used for estimating peak flows and hydrographs for all design applications.
USGS
USGS
25 acres to 25
Method can be used for estimating peak flows for all design
mi2
applications.
128 acres to 25
Method can be used for estimating hydrographs for all design
mi
applications.
Limits set for
Method used for calculating the Water Quality Volume (WQv)
2
Water Quality
Stormwater Hydrology
In general:
each BMP
Size limitation refers to the drainage basin for the stormwater management facility (e.g., culvert, inlet).
1
If other hydrologic methods are to be considered
2There are many readily available programs (such as HEC-1) that utilize this methodology
*2,000-acre upper size limit applies to single basin simplified peak flow only.
and used by a local review authority or design
engineer, the method should first be calibrated
to local conditions and tested for accuracy and
reliability. If local streamgage data are available,
these data can be used to develop peak discharges and hydrographs. The user is referred
to standard hydrology textbooks for statistical
procedures that can be used to estimate design
flood events from stream gage data.
Note: It must be realized that any hydrologic
analysis is only an approximation. The relationship between the amount of precipitation on a
drainage basin and the amount of runoff from the
basin is complex and too little data are available
on the factors influencing the rainfall-runoff relationship to expect exact solutions.
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To provide consistency within this section as well as throughout this Manual, the symbols listed in Table 3.1.2-1 will be used. These symbols were selected because
of their wide use in technical publications. In some cases, the same symbol is used in existing publications for more than one definition. Where this occurs in this
section, the symbol will be defined where it occurs in the text or equations.
Table 3.1.2-1 Symbols and Definitions
Symbol
Definition
Units
Symbol
Definition
Units
A
Drainage area
acres
Qf
Extreme Flood Protection Volume
ft3
Bf
Baseflow
acres-feet
Qi
Peak inflow discharge
cfs
C
Runoff coefficient
-
Qo
Peak outflow discharge
cfs
Cf
Frequency factor
-
Qp
Peak rate of discharge
cfs
CN
NRCS TR-55 runoff curve number
-
Qp25
Overbank Flood Protection Volume
ft3
CPv
Channel Protection Volume
ft3
Qwq
Water Quality peak rate of discharge
cfs
d
Time interval
hours
q
Storm runoff during a time interval
in
E
Evaporation
ft
qu
Unit peak discharge
cfs (or cfs/mi2/
inch)
Et
Evapotranspiration
ft
R
Hydraulic radius
ft
ft3
Fp
Pond and swamp adjustment factor
-
Ro
Runoff
Gh
Hydraulic gradient
-
Rv
Runoff Coefficient
I or i
Runoff intensity
in/hr
RRv
Runoff Reduction Volume
ft3
I
Percent of impervious cover
%
S
Ground slope
ft/ft or %
I
Infiltration
ft
S
Potential maximum retention
in
Ia
Initial abstraction from total rainfall
in
S
Slope of hydraulic grade line
ft/ft
kh
Infiltration rate
ft/day
T
Channel top width
ft
L
Flow length
ft
TL
Lag time
hours
n
Manning roughness coefficient
-
Tp
Time to peak
hr
NRCS
Natural Resources Conservation Service
-
t
Time
min
Of
Overflow
ft3
tc
Time of concentration
min
P
Accumulated rainfall
in
TIA
Total impervious area
%
P2
2-year, 24-hour rainfall
in
V
Velocity
ft/s
Pw
Wetted perimeter
ft
V
Pond volume
ft3
PF
Peaking factor
-
Vr
Runoff volume
ft3
Q
Rate of runoff
cfs (or inches)
Vs
Storage volume
ft3
Qd
Developed runoff for the design storm
in
WQv
Water Quality Volume
ft3
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Stormwater Hydrology
3.1.2 Symbols and Definitions
VOL 2
59
Stormwater Hydrology
3.1.3 Rainfall Estimation
The first step in any hydrologic analysis is an estimation of the rainfall that will fall on the site for a
given time period. The amount of rainfall can be
quantified with the following characteristics:
• Duration (hours) – Length of time over which
rainfall (storm event) occurs
• Depth (inches) – Total amount of rainfall
occurring during the storm duration
• Intensity (inches per hour) – Depth divided by
the duration
The Frequency of a rainfall event is the recurrence
interval of storms having the same duration and
volume (depth). This can be expressed either in
terms of exceedence probability or return period.
• Exceedence Probability – Probability that a
storm event having the specified duration and
volume will be exceeded in one given time
period, typically 1 year
• Return Period – Average length of time
between events that have the same duration
and volume
Thus, if a storm event with a specified duration
and volume has a 1% chance of occurring in any
given year, then it has an exceedence probability
of 0.01 and a return period of 100 years.
Figure 3.1.3-1 Example DDF Curve (Athens, Georgia)
(Source: NOAA Atlas 14, Volume 9, Version 2, 2013)
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The Rational Formula follows the assumption that:
can be obtained through the National Oceanic
• The predicted peak discharge has the same
probability of occurrence (return period) as the
used rainfall intensity (I)
and Atmospheric Administration (NOAA) Atlas
14 publication, or online using the Precipitation
Frequency Data Server database (http://hdsc.nws.
• The runoff coefficient (C) is constant during the
storm event
noaa.gov/hdsc/pfds). NOAA precipitation data
should be used for all hydro¬logic analysis at the
given locations. Additional information regarding
how the values in this database were derived can
When using the Rational Method some precau-
be accessed using the link above.
tions should be considered:
❏❏In determining the C value (runoff coefficient
based on land use) for the drainage area,
hydro-logic analysis should take into account
any future changes in land use that might occur
during the service life of the proposed facility.
❏❏Since the Rational Method uses a composite
C and a single tc value for the entire drainage
area, if the distribution of land uses within
the drainage basin will affect the results of
hydrologic analysis (e.g., if the impervious areas
are segregated from the pervious areas), then
basin should be divided into sub-drainage
basins.
❏❏The charts, graphs, and tables included in this
section are given to assist the design engineer
in applying the Rational Method. The design
engineer should use sound engineering
judgment in applying these design aids and
should make appropriate adjustments when
specific site characteristics dictate that these
adjustments are appropriate.
The tabular precipitation data provided within the
database are applicable for storm durations from
5 minutes to 60 days. In addition to the tabular
data, the NOAA precipitation database also has a
graphical display that shows the intensity duration
frequency curve of any given precipitation data.
Figure 3.1.3-1 shows an example Intensity-Duration-Frequency (IDF) Curve for Athens, Georgia,
for up to 10 storms (1-year – 1,000-year). These
curves are plots of the tabular values. No values
are given for times less than 5 minutes.
Figure 3.1.3-2 (included as the 10-year 24-hour
values from TP40) shows that the rainfall values
vary south to north with generally constant values
in a “V” pattern from east to west in central and
south Georgia.
Figure 3.1.3-2 Rainfall Isohyetal Lines (10-year, 24-hour
values)
3.1.4 Rational Method
3.1.4.1 INTRODUCTION
An important formula for determining the peak
runoff rate is the Rational Formula. It is characterized by:
• Consideration of the entire drainage area as a
single unit
• Estimation of flow at the most downstream
point only
Stormwater Hydrology
Rainfall intensities for any location across Georgia
• The assumption that rainfall is uniformly
distributed over the drainage area and is
constant over time
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concentration, tc (the time required for water to
The coefficients given in Table 3.1.4-2 are appli-
The Rational Method can be used to estimate
flow from the most remote point of the basin to
cable for storms of 5 year to 10 year frequencies.
stormwater runoff peak flows for the design of
the location being ana¬lyzed).
Less frequent, higher intensity storms may require
gutter flows, drainage inlets, storm drain pipe, culverts, and small ditches. It is most applicable to
modification of the coefficient because infiltration
The Rational Formula is expressed as follows:
small, highly impervious areas. The recommend-
and other losses have a proportion¬ally smaller
effect on runoff (Wright ¬McLaughlin Engineers,
Q = CIA
ed maximum drainage area that should be used
(3.1.1)
with the Rational Method is 200 acres.
1969). The adjustment of the Rational Method for
use with major storms can be made by multi-
Where:
Stormwater Hydrology
3.1.4.2 APPLICATION
plying the right side of the Rational Formula by a
The Rational Method should not be used for
Q = maximum rate of runoff (cfs)
frequency factor Cf. The Rational Formula now
storage design or any other application where
C = runoff coefficient representing a
becomes:
a more detailed routing procedure is required.
ratio of runoff to rainfall
However, due to the popularity of the Modified
I = average rainfall intensity for a duration
Q = CfCIA
(3.1.2)
Rational method among Georgia practitioners for
equal to the tc (in/hr)
design of small detention facilities, a method has
A = drainage area contributing to the
The Cf values that can be used are listed in Table
been included in Section 3.3. The normal use of
design location (acres)
3.1.4-1. The product of Cf times C shall not exceed 1.0.
the Modified Rational method significantly under
predicts detention volumes, but the improved
method in Section 3.3 corrects this deficiency in
the method and can be used for detention design
Table 3.1.4-1 Frequency Factors for Rational Formula
for drainage areas up to 5 acres.
Recurrence Interval (years)
Cf
10 or less
1.0
The Rational Method should also not be used for
25
1.1
calculating peak flows downstream of bridges,
50
1.2
culverts or storm sewers that may act as restric-
100
1.25
tions and impact the peak rate of discharge.
3.1.4.3 EQUATIONS
The Rational Formula estimates the peak rate of
runoff at any location in a watershed as a function
of the drainage area, runoff coefficient, and mean
rainfall intensity for a duration equal to the time of
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Use of the Rational Formula requires the time of
concentration (tc) for each design point within the
drainage basin. The duration of rainfall is then set
equal to the time of concentration and is used to
estimate the design average rainfall intensity (I).
The time of concentration consists of an overland flow time to the point where the runoff is
Stormwater Hydrology
3.1.4.4 TIME OF CONCENTRATION
concentrated or enters a defined drainage feature
(e.g., open channel) plus the time of flow in a
closed conduit or open channel to the design
point.
Figure 3.1.4-1 can be used to estimate overland
flow time. For each drainage area, the distance
is determined from the inlet to the most remote
point in the tributary area. From a topographic map, the average slope is determined for the
same distance. The runoff coefficient (C) is determined by the procedure described in a subsequent section of this chapter.
To obtain the total time of concentration, the pipe
or open channel flow time must be calculated
and added to the inlet time. After first determining the average flow velocity in the pipe or
channel, the travel time is obtained by dividing
velocity into the pipe or channel length. Velocity
can be estimated by using the nomograph shown
in Figure 3.1.4-2. Note: time of concentration
cannot be less than 5 minutes.
Figure 3.1.4-1 Rational Formula Overland Time of Flow Nomograph
(Source: Airport Drainage, Federal Aviation Administration, 1965)
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must be calculated and added to the inlet time. After first determining the
average flow velocity in the pipe or channel, the travel time is obtained by
dividing velocity into the pipe or channel length. Velocity can be estimated by
using the nomograph shown in Figure 3.1.4-2. Note: time of concentration
cannot be less than 5 minutes.
Another method that can be used to determine the overland flow portion of
Stormwater Hydrology
To obtain the total time of concentration, the pipe or open channel flow time
the time of concentration is the “Kinematic Wave Nomograph” (Figure 3.1.43). The kinematic wave method incorporates several variables including rainfall intensity and Manning’s “n”. In using the nomograph, the design engineer
has two unknowns starting the computations: the time of concentration and
the rainfall intensity. A value for the rainfall intensity “I” must be assumed. The
travel time is determined iteratively.
If one has determined the length, slope and roughness coefficient, and selected a rainfall intensity table, the steps to use Figure 3.1.4-3 are as follows:
(Step 1) Assume a rainfall intensity.
(Step 2) Use Figure 3.1.4-3 (or the equation given in the figure) to
obtain the first estimate of time of concentration.
(Step 3) Using the time of concentration obtained from Step 2, use
the appropriate rainfall intensity from NOAA Atlas 14 and find
the rainfall intensity corresponding to the computed time of
concentration. If this rainfall intensity corresponds with the
assumed intensity, the problem is solved. If not, proceed to
Step 4.
(Step 4) Assume a new rainfall intensity that is between that assumed
in Step 1 and that determined in Step 3.
(Step 5) Repeat Steps 1 through 3 until there is good agreement between the assumed rainfall intensity and that obtained from
the rainfall intensity tables.
Figure 3.1.4-2 Manning’s Equation Nomograph
(Source: USDOT, FHWA, HDS-3 (1961))
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overall design problem. Often one encounters swale flow, confined channel
flow, and closed conduit flow-times that must be added as part of the overall time of concentration. When this situation is encountered, it is best to
compute the confined flow-times as the first step in the overall determination
of the time of concentration. This will give the designer a rough estimate of
the time involved for the overland flow, which will give a better first start on
the rainfall intensity assumption. For example, if the flow time in a channel is
Stormwater Hydrology
Generally, the time of concentration for overland flow is only a part of the
15 minutes and the overland flow time from the ridge line to the channel is 10
minutes, then the total time of concentration is 25 minutes.
Other methods and charts may be used to calculate overland flow time if
approved by the local review authority.
Two common errors should be avoided when calculating time of concentration. First, in some cases runoff from a portion of the drainage area which is
highly impervious may result in a greater peak discharge than would occur if
the entire area were considered. Second, when designing a drainage system,
the overland flow path is not necessarily the same before and after development and grading operations have been completed. Selecting overland
flow paths in excess of 50 feet for impervious areas should be done only after
careful consideration.
3.1.4.5 RAINFALL INTENSITY (I)
The rainfall intensity (I) is the average rainfall rate in in/hr for a duration equal
to the time of concentration for a selected return period. Once a particular
return period has been selected for design and a time of concentration calculated for the drainage area, the rainfall intensity can be determined from data
given from NOAA Atlas 14.
Figure 3.1.4-3 Kinematic Wave Nomograph
(Source: Manual For Erosion And Sediment Control In Georgia)
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It may be that using only the impervious area from
The runoff coefficient (C) is the variable of the Ra-
a highly impervious site (and the corresponding
tional Method least susceptible to precise deter-
high C factor and shorter time of concentration)
mination and requires judgment and understand-
will yield a higher peak runoff value than by using
ing on the part of the design engineer.
the whole site. This should be checked particu-
Stormwater Hydrology
3.1.4.6 RUNOFF COEFFICIENT (C)
larly in areas where the overland portion is grassy
While engineering judgment will always be re-
(yielding a long tc) to avoid underestimating peak
quired in the selection of runoff coefficients, typ-
runoff.
ical coefficients represent the integrated effects
of many drainage basin parameters. Table 3.1.4-2
gives the recommended runoff coefficients for
the Rational Method.
It is often desirable to develop a composite runoff
coefficient based on the percentage of different
types of surfaces in the drainage areas. Com-
Table 3.1.4-2 Recommended Runoff Coefficient Values
posites can be made with the values from Table
Description of Area
be made with coefficients for different surface
types such as rooftops, asphalt, and concrete
streets and sidewalks. The composite procedure
can be applied to an entire drainage area or to
typical “sample” blocks as a guide to the selection of reasonable values of the coefficient for an
entire area.
It should be remembered that the Rational Method assumes that all land uses within a drainage
Description of Area
Runoff
Coefficients (C)
Coefficients (C)
3.1.4-2 by using percentages of different land
uses. In addition, more detailed composites can
Runoff
Industrial
Lawns
Light areas
0.70
Heavy areas
0.80
Sandy soil, flat, 2%
0.10
Sandy soil, average, 2-7%
0.15
Sandy soil, steep, >7%
0.20
Parks, cemeteries
0.25
Clay soil, flat, 2%
0.17
Playgrounds
0.35
Clay soil, average, 2-7%
0.22
Railroad yard areas
0.40
Clay soil, steep, >7%
0.35
Streets
Unimproved areas (forest)
0.15
Asphalt and concrete
0.95
Brick
0.85
Downtown areas
0.95
Drives, walks, and roofs
0.95
Neighborhood areas
0.70
Gravel areas
0.50
Business
Graded or no plant cover
Residential
area are uniformly distributed throughout the
Single-family areas
0.50
Sandy soil, flat, 0-5%
0.30
area. If it is important to locate a specific land use
Multi-units, detached
0.60
Sandy soil, flat, 5-10%
0.40
within the drainage area then another hydrologic
Multi-units, attached
0.70
Clay soil, flat, 0-5%
0.50
method should be used where hydrographs can
Suburban
0.40
Clay soil, average, 5-10%
0.60
be generated and routed through the drainage
Apartment dwelling areas
0.70
system.
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Overland Flow
Following is an example problem that illustrates
A runoff coefficient (C) for the overland flow area
the application of the Rational Method to estimate
is determined from Table 3.1.4-2 to be 0.10.
peak discharges.
Time of Concentration
Estimates of the maximum rate of runoff are
From Figure 3.1.4-1 with an overland flow length
needed at the inlet to a proposed culvert for a
of 50 ft, slope of 2% and a C of 0.10, the over-
25-year return period.
land flow time is 10 min. Channel flow velocity is
Stormwater Hydrology
3.1.4.7 EXAMPLE PROBLEM
determined from Figure 3.1.4-2 to be 3.1 ft/s (n =
Site Data
0.090, R = 1.62 (from channel dimensions) and S
From a topographic map of the City of Roswell
= .018). Therefore,
and a field survey, the area of the drainage
basin upstream from the point in question is
found to be 23 acres. In addition the following
Flow Time =
2,250 feet
= 12.1 minutes
(3.1 ft/s)/(60 s/min)
data were measured:
• Average overland slope = 2.0%
and tc = 10 + 12.1 = 22.1 min (use 22 min)
• Length of overland flow = 50 ft
• Length of main basin channel = 2,250 ft
• Slope of channel .018 ft/ft = 1.8%
• Roughness coefficient (n) of channel was
estimated to be 0.090
• Roughness coefficient (n) of channel was
estimated to be 0.090
• From existing land use maps, land use for the
drainage basin was estimated to be:
»» Residential (single family) - 80%
»» Graded - sandy soil, 3% slope - 20%
From existing land use maps, the land use for the
overland flow area at the head of the basin was
estimated to be: Lawn - sandy soil, 2% slope
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VOL 2
67
3.1.5.3 EQUATIONS AND CONCEPTS
that decreases during the course of a storm. De-
The hydrograph of outflow from a drainage basin
From NOAA Atlas 14, and using a duration equal
tails of the methodology can be found in the
is the sum of the elemental hydrographs from all
to 22 minutes,
NRCS National Engineering Handbook, Part 630,
the sub-areas of the basin, modified by the effects
I25 (25 yr return period) = 4.88 in/hr
Hydrology.
of transit time through the basin and storage in
Runoff Coefficient
A typical application of the NRCS TR-55 method
acteristics of the basin including shape, size and
A weighted runoff coefficient (C) for the total
includes the following basic steps:
slope are constant, the unit hydrograph approach
the stream channels. Since the physical char-
drainage area is determined below by utilizing the
values from Table 3.1.4-2.
Land Use
Residential
Percent of
Runoff
Weighted Runoff
Total Land Area
Coefficient
Coefficient*
0.80
0.50
0.40
(single family)
(Step 1) Determination of curve numbers
that represent different land
the shape of hydrographs from storms of similar
uses within the drainage area.
rainfall characteristics. Thus, the unit hydrograph
(Step 2) Calculation of time of concentration to the study point.
(Step 3) Using the Type II or Type III rain-
Graded area
0.20
0.30
0.06
Total Weighted Runoff Coefficient - 0.46
*Column 3 equals Column 1 multiplied by Column 2
fall distribution, total and excess
rainfall amounts are determined.
Note: See Figure 3.1.5-1 for the
Peak Runoff
geographic boundaries for the
The estimate of peak runoff for a 25-yr design
different NRCS TR-55 rainfall
storm for the given basin is:
distributions.
(Step 4) Using the unit hydrograph
Q25 = CfCIA = (1.10)(.46)(4.88)(23) = 57 cfs
approach, the hydro¬graph of
direct runoff from the drainage
basin can be developed.
3.1.5 NRCS TR-55 Hydrologic Method
3.1.5.1 INTRODUCTION
The Soil Conservation Service (NRCS TR-55) hydrologic method requires basic data similar to the
Rational Method: drainage area, a runoff factor,
time of concentration, and rainfall. The NRCS
TR-55 approach, however, is more sophisticated
in that it also considers the time distribution of the
rain¬fall, the initial rainfall losses to interception
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assumes that there is considerable similarity in
Stormwater Hydrology
and depression storage, and an infiltration rate
Rainfall Intensity
is a typical hydrograph for the basin with a runoff
volume under the hydrograph equal to one (1.0)
inch from a storm of specified duration. For a
storm of the same duration but with a different
amount of runoff, the hydrograph of direct runoff
can be expected to have the same time base as
the unit hydrograph and ordinates of flow proportional to the runoff volume. Therefore, a storm
that produces 2 inches of runoff would have a
hydrograph with a flow equal to twice the flow of
the unit hydrograph. With 0.5 inches of runoff,
the flow of the hydrograph would be one-half of
the flow of the unit hydrograph.
3.1.5.2 APPLICATION
The NRCS TR-55 method can be used for both
the estimation of stormwater runoff peak rates
and the generation of hydrographs for the routing
of stormwater flows. The simplified method of
Subsection 3.1.5.7 can be used for drainage areas
up to 2,000 acres. Thus, the NRCS TR-55 method
can be used for most design applications, including storage facilities and outlet structures, storm
drain systems, culverts, small drainage ditches and
open channels, and energy dissipators.
VOL 2
68
Where:
This is an average value that could be adjusted
and basin concepts used in the NRCS TR-55
Q = accumulated direct runoff (in)
for flatter areas with more depressions if there are
method.
P = accumulated rainfall (potential maxi-
calibration data to substantiate the adjustment.
mum runoff) (in)
Drainage Area - The drainage area of a watershed
Ia = initial abstraction including surface
Substituting 0.2S for Ia in equation 3.1.3, the
is determined from topographic maps and field
storage, interception, evaporation, and
equation becomes:
surveys. For large drainage areas it might be nec-
infiltration prior to runoff (in)
essary to divide the area into sub-drainage areas
S = potential maximum soil retention (in)
Q = (P-0.2S)2
(P + 0.8S)
to account for major land use changes, obtain
analysis results at different points within the drain-
An empirical relationship used in the NRCS TR-55
age area, combine hydrographs from different
method for estimating Ia is:
Where:
S = 1000/CN - 10
sub-basins as applicable, and/or route flows to
Ia = 0.2S
points of interest.
(3.1.5)
Stormwater Hydrology
The following discussion outlines the equations
(3.1.4)
CN = NRCS TR-55 curve number.
Rainfall - The NRCS TR-55 method applicable to
the State of Georgia is based on a storm event
that has a Type II or Type III time distribution.
These distributions are used to distribute the
24-hour volume of rainfall for the different storm
frequencies (Figure 3.1.5-1).
Rainfall-Runoff Equation - A relationship between
accumulated rainfall and accumulated runoff was
derived by NRCS TR-55 from ex¬perimental plots
for numerous soils and vegetative cover conditions. The following NRCS TR-55 runoff equation
is used to estimate direct runoff from 24 hour or 1
day storm rainfall. The equation is:
(P − I ) 2
(P − Ia ) + S
a
Q=
(3.1.3)
Figure 3.1.5-1 Approximate Geographic Boundaries
for NRCS TR-55 Rainfall Distributions
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VOL 2
69
3.1.5.4 RUNOFF FACTOR
Based on infiltration rates, the NRCS TR-55 has
equation. For example, 4.1 inches of direct runoff
The principal physical watershed characteristics
divided soils into four hydrologic soil groups.
would result if 5.8 inches of rainfall occurs on a
affecting the relationship between rainfall and
watershed with a curve number of 85.
runoff are land use, land treatment, soil types,
Group A: Soils having a low runoff potential due
and land slope. The NRCS TR-55 method uses
to high infiltration rates. These soils consist pri-
Equation 3.1.5 can be rearranged so that the
a combination of soil conditions and land uses
marily of deep, well-drained sands and gravels.
curve number can be estimated if rainfall and
(ground cover) to assign a runoff factor to an area.
runoff volume are known. The equation then
These runoff factors, called runoff curve numbers
Group B: Soils having a moderately low runoff
becomes (Pitt, 1994):
(CN), indicate the runoff potential of an area. The
potential due to moderate infiltration rates. These
higher the CN, the higher the runoff potential.
soils consist primarily of moderately deep to deep,
Soil properties influence the relationship between
moderately well to well drained soils with moder-
runoff and rainfall since soils have differing rates
ately fine to moderate¬y coarse textures.
CN = 1000/[10 + 5P + 10Q –
10(Q2 + 1.25QP)1/2]
(3.1.6)
Stormwater Hydrology
Figure 3.1.5-2 shows a graphical solution of this
of infiltration.
Group C: Soils having a moderately high runoff
potential due to slow infiltration rates. These soils
consist primarily of soils in which a layer exists
near the surface that impedes the down¬ward
movement of water or soils with moderately fine
to fine texture.
Group D: Soils having a high runoff potential due
to very slow infiltration rates. These soils consist
primarily of clays with high swelling potential,
soils with permanently high water tables, soils
with a claypan or clay layer at or near the surface,
and shallow soils over nearly impervious parent
material.
Figure 3.1.5-2 NRCS TR-55 Solution of the Runoff Equation
(Source: NRCS TR-55, NEH630, 2004)
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VOL 2
70
Composite curve numbers for a drainage area can be calculated by using the
tion can be found in the publication Urban Hydrology for Small Watersheds,
weighted method as presented below.
2nd Edition, Technical Release Number 55, 1986. Soil Survey maps can be
Composite Curve Number Calculation Example
obtained online at the United States Department of Agriculture (USDA) Natural
Resource Conservation Commission’s (NRCS) web soil survey online tool to
classify the soil type. (http://websoilsurvey.sc.egov.usda.gov/App/HomePage.
htm)
Land Use
Percent of Total
Curve Number
Land Area
Residential
Weighted Curve Number (%area x CN)
80%
85
68
20%
71
14
1/8 acre
Soil group B
Stormwater Hydrology
A list of soils throughout the State of Georgia and their hydrologic classifica-
Consideration should be given to the effects of urbanization on the natural
hydrologic soil group. If heavy equipment can be expected to compact the
soil during construction or if grading will mix the surface and subsurface soils,
appropriate changes should be made in the soil group selected. Also, runoff
Meadow
Good conditions
Soil group C
Total Weighted Curve Number = 68 + 14 = 82
curve numbers vary with the antecedent soil moisture conditions. Average antecedent soil moisture conditions (AMC II) are recommended for most hydro-
The different land uses within the basin should reflect a uniform hydrologic
logic analysis, except in the design of state-regulated Category I dams where
group represented by a single curve number. Any number of land uses can be
AMC III may be required. Areas with high water table conditions may want to
included, but if their spatial distribution is important to the hydrologic analysis,
consider using AMC III antecedent soil moisture conditions. This should be
then sub-basins should be developed and separate hydrographs developed
considered a calibration parameter for modeling against real calibration data.
and routed to the study point.
Table 3.1.5-1 gives recommended curve number values for a range of different
land uses.
3.1.5.5 URBAN MODIFICATIONS OF THE NRCS TR-55 METHOD
When a drainage area has more than one land use, a composite curve num-
Several factors, such as the percentage of impervious area and the means of
ber can be calculated and used in the analysis. It should be noted that when
conveying runoff from impervious areas to the drainage system, should be
composite curve numbers are used, the analysis does not take into account
considered in computing CN for developed areas. For example, do the im-
the location of the specific land uses but sees the drainage area as a uniform
pervious areas connect directly to the drainage system, or do they outlet onto
land use represented by the composite curve number.
lawns or other pervious areas where infiltration can occur?
The curve number values given in Table 3.1.5-1 are based on directly connected impervious area. An impervious area is considered directly connected
if runoff from it flows directly into the drainage system. It is also considered
directly connected if runoff from it occurs as concentrated shallow flow
that runs over pervious areas and then into a drainage system. It is possible
that curve number values from urban areas could be reduced by not directly
connecting impervious surfaces to the drainage system, but allowing runoff to
flow as sheet flow over significant pervious areas.
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VOL 2
71
Table 3.1.5-1 Runoff Curve Numbers1
for different types of impervious areas.
Cover description
Connected Impervious Areas
Curve numbers for
hydrological soil groups
Cover type and hydrologic condition
A
B
C
D
72
62
81
71
88
78
91
81
poor condition
good condition
68
39
79
61
86
74
89
80
good
30
58
71
78
45
25
66
55
77
70
83
77
68
79
86
89
49
69
79
84
39
61
74
80
The CNs provided in Table 3.1.5-1 for various land cover types were developed
for typical land use relationships based on specific assumed percentages of
Average
percent
impervious
area2
Cultivated land: without conservation treatment
with conservation treatment
impervious area. These CN values were developed on the assumptions that:
Pasture or range land:
(a) Pervious urban areas are equivalent to pasture in good hydrologic
Meadow:
condition, and
Wood or forest land: thin stand, poor cover
good cover
Open space (laws, Poor condition (grass cover
parks, golf courses, <50%)
cemeteries, etc)3: Fair condition (grass cover 50%
to 75%)
Good condition (grass cover
>75%)
(b) Impervious areas have a CN of 98 and are directly connected to the
drainage system.
If all of the impervious area is directly connected to the drainage system, but
the impervious area percentages or the pervious land use assumptions in Ta-
Impervious areas:
Paved parking lots, roofs,
driveways, etc (excluding rightof-way)
98
98
98
98
Streets and roads:
Paved; curbs and storm drains
(excluding right-of-way)
Paved; open ditches (including
right-of-way)
Gravel (including right-of-way)
Dirt (including right-of-way)
98
98
98
98
83
89
92
93
76
72
85
82
89
87
91
89
ble 3.1.5-1 are not applicable, use Figure 3.1.5-3 to compute a composite CN.
For example, Table 3.1.5-1 gives a CN of 70 for a 1/2-acre lot in hydrologic soil
group B, with an assumed impervious area of 25%. However, if the lot has 20%
impervious area and a pervious area CN of 61, the composite CN obtained
from Figure 3.1.5-3 is 68. The CN difference between 70 and 68 reflects the
difference in percent impervious area.
Urban Districts:
Unconnected Impervious Areas
Residential districts by
average lot size:
Runoff from these areas is spread over a pervious area as sheet flow. To determine CN when all or part of the impervious area is not directly connected to
the drainage system, (1) use Figure 3.1.5-4 if total impervious area is less than
30% or (2) use Figure 3.1.5-3 if the total impervious area is equal to or greater
Developing urban areas and newly graded
areas (pervious areas
only, no vegetation)
than 30%, because the absorptive capacity of the remaining pervious areas will
not significantly affect runoff.
Commercial and business
Industrial
85%
72%
89
81
92
88
94
91
95
93
1/8 acre or less (townhouses)
1/4 acre
1/3 acre
1/2 acre
1 acre
2 acres
65%
38%
30%
25%
20%
12%
77
61
57
54
51
46
85
75
72
70
68
65
90
83
81
80
79
77
92
87
86
85
84
82
77
86
91
94
Stormwater Hydrology
The following discussion will give some guidance for adjusting curve numbers
Average runoff condition, and Ia = 0.2S
The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as
follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and
pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not
connected, the NRCS TR-55 method has an adjustment to reduce the effect.
3
CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of
open space cover type.
1
2
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VOL 2
72
Stormwater Hydrology
When impervious area is less than 30%, obtain the composite CN by entering
the right half of Figure 3.1.5-4 with the percentage of total impervious area
and the ratio of total unconnected impervious area to total impervious area.
Then move left to the appropriate pervious CN and read down to find the
composite CN. For example, for a 1/2-acre lot with 20% total impervious area
Figure 3.1.5-4 is 66. If all of the impervious area is connected, the resulting
CN (from Figure 3.1.5-3) would be 68.
3.1.5.6 TRAVEL TIME ESTIMATION
Composite CN
(75% of which is unconnected) and pervious CN of 61, the composite CN from
Travel time (Tt) is the time it takes water to travel from one location to another
within a watershed, through the various components of the drainage system. Time of concentration (tc) is computed by summing all the travel times
for consecutive components of the drainage conveyance system from the
hydraulically most distant point of the watershed to the point of interest within
the watershed. Following is a discussion of related procedures and equations
(USDA, 1986).
Connected impervious area (percent)
Figure 3.1.5-3 Composite CN with Connected Impervious Areas
(Source: NRCS TR-55, NEH630, 2004)
Travel Time
Water moves through a watershed as sheet flow, shallow concentrated flow,
open channel flow, or some combination of these. The type that occurs is a
function of the conveyance system and is best determined by field inspection..
Travel time is the ratio of flow length to flow velocity:
Tt
=
L
3600V
(3.1.7)
Where:
Tt = travel time (hr)
Composite CN
L = flow length (ft)
V = average velocity (ft/s)
3600 = conversion factor from seconds to hours
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Total impervious
area (percent)
Figure 3.1.5-4 Composite CN with Unconnected Impervious Areas
(Total Impervious Area Less Than 30%)
(Source: NRCS TR-55, NEH630, 2004)
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73
Sheet flow can be calculated using the following
formula:
Tt =
0.42(nL)0.8
60(P2)0.5(S)0.4
(3.1.8)
Where:
Tt = travel time (hr)
Table 3.1.5-2 Roughness Coefficients (Manning’s n) for Sheet Flow1
Surface description
n
Smooth surfaces (concrete, asphalt, gravel, or bare soil):
0.011
Fallow (no residue):
0.05
Cultivated soils:
Residue cover <20%
Residue cover >20%
0.06
0.17
Grass:
Short grass prairie
Dense grasses2
Bermuda grass
0.15
0.24
0.41
n = Manning roughness coefficient (see
Table 3.1.5-2)
Range (natural):
L = flow length (ft)
Woods :
3
P2 = 2-year, 24-hour rainfall
S = land slope (ft/ft)
Stormwater Hydrology
Sheet Flow
0.13
Light underbrush
Dense underbrush
0.40
0.80
The n values are a composite of information by Engman (1986).
Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native grass mixtures.
3
When selecting n, consider cover to a height of about 0.1 ft. This is the only part of the plant cover that will obstruct sheet flow.
1
2
Shallow Concentrated Flow
Source: NRCS TR-55, TR-55, Second Edition, June 1986.
After a maximum of 50 to 100 feet, sheet flow
usually becomes shallow concentrated flow. The
average velocity for this flow can be determined
from Figure 3.1.5-5, in which average velocity
Open Channels
Where:
is a function of water¬course slope and type of
V = average velocity (ft/s)
Velocity in channels should be calculated from
channel.
S = slope of hydraulic grade line (water-
the Manning equation. Open channels are
course slope, ft/ft)
assumed to begin where surveyed cross section
Average velocities for estimating travel time for
information has been obtained, where channels
shallow concentrated flow can be comput-
After determining average velocity using Figure
are visible on aerial photographs, where channels
ed from using Figure 3.1.5-5, or the following
3.1.5-5 or Equations 3.1.9 or 3.1.10, use Equation
have been identified by the local municipality, or
equations. These equations can also be used for
3.1.7 to estimate travel time for the shallow con-
where blue lines (indicating streams) appear on
slopes less than 0.005 ft/ft.
centrated flow segment.
United States Geological Survey (USGS) quadrangle sheets. Manning’s equation or water surface
Unpaved: V = 16.13(S)0.5
(3.1.9)
profile information can be used to estimate average flow velocity.
Paved: V = 20.33(S)0.5
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(3.1.10)
VOL 2
74
Stormwater Hydrology
Average flow velocity for travel time calculations
is usually determined for bank-full elevation assuming low vegetation winter conditions.
Manning’s equation is:
V = 1.49(R)2/3(S)1/2
n
(3.1.11)
Where:
V = average velocity (ft/s)
R = hydraulic radius (ft) and is equal to A/Pw
A = cross sectional flow area (ft2)
Pw = wetted perimeter (ft)
S = slope of the hydraulic grade line (ft/ft)
n = Manning’s roughness coefficient for
open channel flow
After average velocity is computed using Equation
3.1.11, Tt for the channel segment can be estimated using Equation 3.1.7.
Limitations
❏❏Equations in this section should not be used for
sheet flow longer than 50 feet for impervious
land uses.
❏❏In watersheds with storm sewers, carefully
identify the appropriate hydraulic flow path to
estimate tc.
❏❏A culvert or bridge can act as detention
structure if there is significant storage behind
it. Detailed storage routing procedures should
be used to determine the outflow through the
culvert or bridge.
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Figure 3.1.5-5 Average Velocities - Shallow Concentrated Flow
(Source: NRCS TR-55, NEH630, 2004)
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75
Computations for the peak discharge method proceed as follows:
The following NRCS TR-55 procedures were taken from the NRCS TR-55
Technical Release 55 (USDA, 1986) which presents simplified procedures to
(Step 1) The 24-hour rainfall depth is determined from the precipi-
calculate storm runoff volume and peak rate of discharges. These procedures
tation data in the NOAA Atlas 14 publication, or online using
are applicable to small drainage areas (typically less than 2,000 acres) with
the Precipitation Frequency Data Server database (http://
homogeneous land uses that can be described by a single CN value. The peak
hdsc.nws.noaa.gov/hdsc/pfds/).
(Step 2) The runoff curve number, CN, is estimated from Table 3.1.5-
discharge equation is:
1 and direct runoff, Qp, is calculated using Equation 3.1.12.
Qp = quAQFp
(3.1.12)
Stormwater Hydrology
3.1.5.7 SIMPLIFIED NRCS TR-55 PEAK RUNOFF RATE ESTIMATION
(Step 3) The CN value is used to determine the initial abstraction, Ia,
from Table 3.1.5-3, and the ratio Ia/P is then computed (P =
Where:
accumulated 24-hour rainfall).
Qp = peak discharge (cfs)
qu = unit peak discharge (cfs/mi2/in)
(Step 4) (The watershed time of concentration is computed using the
procedures in Subsection 3.1.4.4 and is used with the ratio
A = drainage area (mi )
Ia/P to obtain the unit peak discharge, qup, from Figure 3.1.5-
Q = runoff (in)
6 for the Type II rainfall distribution and Figure 3.1.5-7 for
2
Fp = pond and swamp adjustment factor
the Type III rainfall distribution. If the ratio Ia/P lies outside
the range shown in the figures, either the limiting values
The input requirements for this method are as follows:
or another peak discharge method should be used. Note:
• tc – hours
Figures 3.1.5-6 and 3.1.5-7 are based on a peaking factor
• Drainage area – mi2
are not applicable and the simplified NRCS TR-55 method
• Type II or type III rainfall distribution
• 24-hour design rainfall
• CN value
• Pond and Swamp adjustment factor (If pond and swamp areas are spread
throughout the watershed and are not considered in the tc computation, an
adjustment is needed.)
of 484. If a peaking factor of 300 is needed, these figures
should not be used.
(Step 5) The pond and swamp adjustment factor, Fp, is estimated
from below:
Pond and Swamp Areas (%)*
Fp
0
1.00
0.2
0.97
1.0
0.87
3.0
0.75
5.0
0.72
*Percent of entire drainage basin
(Step 6) The peak runoff rate is computed using Equation 3.1.12.
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VOL 2
76
Curve Number
Ia (in)
Curve Number
Ia (in)
40
3.000
70
0.857
41
2.878
71
0.817
42
2.762
72
0.778
43
2.651
73
0.740
44
2.545
74
0.703
45
2.444
75
0.667
46
2.348
76
0.632
47
2.255
77
0.597
48
2.167
78
0.564
49
2.082
79
0.532
50
2.000
80
0.500
51
1.922
81
0.469
52
1.846
82
0.439
53
1.774
83
0.410
54
1.704
84
0.381
55
1.636
85
0.353
56
1.571
86
0.326
57
1.509
87
0.299
58
1.448
88
0.273
59
1.390
89
0.247
60
1.333
90
0.222
61
1.279
91
0.198
62
1.226
92
0.174
63
1.175
93
0.151
64
1.125
94
0.128
65
1.077
95
0.105
66
1.030
96
0.083
67
0.985
97
0.062
68
0.941
98
0.041
69
0.899
Stormwater Hydrology
Table 3.1.5-3 Ia Values for Runoff Curve Numbers
Source: NRCS TR-55, TR-55, Second Edition, June 1986
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VOL 2
77
Stormwater Hydrology
Figure 3.1.5-6 NRCS TR-55 Type II Unit Peak Discharge Graph
(Source: NRCS TR-55, TR-55, Second Edition, June 1986)
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VOL 2
78
Stormwater Hydrology
Figure 3.1.5-7 RCS TR-55 Type III Unit Peak Discharge Graph
(Source: NRCS TR-55, TR-55, Second Edition, June 1986)
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VOL 2
79
2.
Compute the 100-year peak discharge for a
The hydrologic flow path for this watershed =
50-acre wooded watershed located in Peachtree
1,890 ft
City, which will be developed as follows:
• Forest land - good cover (hydrologic soil group
B) = 10 ac
• Forest land - good cover (hydrologic soil group
C) = 10 ac
• 1/3 acre residential (hydrologic soil group B) =
20 ac
Calculate time of concentration
Segment
Type of Flow
Length (ft)
Slope (%)
1
Overland n=0.24
40
2.0
2
Shallow channel
750
1.7
3
Main channel*
1100
0.5
4.
Unit discharge qu (100-year) from Figure
3.1.5-6 = 650 csm/in, qu (1-year) = 580 csm/
in
5.
Calculate peak discharge with Fp = 1 using
equation 3.1.12
Q100 = 650 (50/640)(4.89)(1) = 248 cfs
*For the main channel, n = .06 (estimated), width = 10 feet,
depth = 2 feet, rectangular channel
Segment 1 - Travel time from Equation 3.1.8 with
3.1.5.9 HYDROGRAPH GENERATION
P2 = 3.84 inches
In addition to estimating the peak discharge, the
NRCS TR-55 method can be used to estimate
(From NOAA Atlas 14)
• Industrial development (hydrological soil group
C) = 10 ac
Stormwater Hydrology
3.1.5.8 EXAMPLE PROBLEM 1
Tt = [0.42(0.24 X 40)0.8] / [(3.84)
0.5
(.020) ] =
0.4
6.26 minutes
the entire hydrograph from a drainage area. The
NRCS TR-55 has developed a Tabular Hydrograph procedure that can be used to generate
Other data include the following: Total impervi-
Segment 2 - Travel time from Figure 3.1.5-5 or
the hydrograph for small drainage areas (less than
ous area = 18 acres, % of pond / swamp area = 0
Equation 3.1.9
2,000 acres). The Tabular Hydrograph procedure
Computations
1.
V = 2.1 ft/sec (from equation 3.1.9)
uses unit discharge hydrographs that have been
Tt = 750 / 60(2.1) = 5.95 minutes
generated for a series of time of concentrations.
Calculate rainfall excess:
• The 100-year, 24-hour rainfall is 8.22 inches
(From NOAA Atlas 14).
Segment 3 - Using equation 3.1.11
• The 1-year, 24-hour rainfall is 3.37 inches (From
NOAA Atlas 14).
• Composite weighted runoff coefficient is:
Dev #
Area
% Total
CN
Composite CN
1
10 ac
20%
55
11.0
2
10 ac
20%
70
14.0
3
20 ac
40%
72
28.8
4
10 ac
20%
91
18.2
Total
50 ac
100%
72
In addition, NRCS TR-55 has developed hydrograph procedures to be used to generate com-
V = (1.49/.06) (1.43)0.67 (.005)0.5 = 2.23 ft/
posite flood hydrographs. For the development
sec
of a hydrograph from a homogeneous developed
Tt = 1100 / 60 (2.23) = 8.22 minutes
drainage area and drainage areas that are not
homoge¬neous, where hydro¬graphs need to
tc = 6.26 + 5.95 + 8.22 = 20.43 minutes (.34
hours)
be generated from sub-areas and then routed
and combined at a point downstream, the design
engineer is referred to the procedures outlined
3.
Calculate Ia/P for Cn = 72 (Table 3.1.5-1),
Ia = .778 (Table 3.1.5-3)
Ia/P = (.778 / 8.23) = .095 (Note: Use
Ia/P = .10 to facilitate use of Figure 3.1.5-6.
Straight line interpolation could also be used.)
by the NRCS TR-55 in the 1986 version of TR-55
available from the National Technical Information
Service in Springfield, Virginia 22161. The catalog
number for TR-55, “Urban Hydrology for Small
Watersheds,” is PB87-101580.
*from Equation 3.1.5, Q (100-year) = 4.89 inches
Qd (1-year developed) = 1.0 inches
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VOL 2
80
TR-55 method for generating hydrographs includes a constant to account for the general land
slope in the drainage area. This constant, called
a peaking factor, can be adjusted when using the
❏❏Mild Slopes (less than 2% slope)
❏❏Significant surface storage throughout the
watershed in the form of standing water
during storm events or inefficient drainage
system
Subsections 3.1.5.4,3.1.5.5, and
3.1.5.6.
(Step 3) Development of a unit hydrograph from either the standard
(peaking factor of 484) or coast-
method. A default value of 484 for the peaking
al area (peaking factor of 300)
factor represents rolling hills – a medium level
The NRCS TR-55 method can be similarly adjust-
of relief. NRCS TR-55 indicates that for moun-
ed for any watershed that has flow and storage
tainous terrain the peaking factor can go as high
characteristics similar to a typical Region 3 stream
dimensionless unit hydrographs.
See discussion below.
(Step 4) Step-wise computation of the
as 600, and as low as 300 for flat (coastal) areas.
initial and infiltration rainfall
Referring to Figure 3.1.6-1, which shows the dif-
The development of a runoff hydrograph from
losses and, thus, the excess
ferent hydrologic regions developed by the USGS
a watershed is a laborious process not normally
rainfall hyetograph using a de-
for the state of Georgia, Region 3 represents the
done by hand. For that reason only an overview
rivative form of the NRCS TR-55
primary region of the state where modification of
of the process is given here to assist the designer
rainfall-runoff equation (Equa-
the peaking factor from 484 to 300 is most often
in reviewing and understanding the input and out-
tion 3.1.5)
warranted if the individual watershed possesses
put from a typical computer program. There are
flat terrain.
choices of computational interval, storm length (if
of excess rainfall to the unit
the 24-hour storm is not going to be used), and
hydrograph to develop a series
As a result of hydrologic/hydraulic studies com-
other “administrative” parameters that are peculiar
of runoff hydrographs, one for
pleted in the development of this Manual, the
to each computer program.
each increment of rainfall (this is
(Step 5) Application of each increment
following are recommendations related to the use
of different peaking factors:
• The NRCS TR-55 method can be used without
modification (peaking factor left at 484) in
Regions 1, 2 and 4 generally when performing
modeling analysis.
• The NRCS TR-55 method can be modified in
that a peaking factor of 484 to 300 can be
used for modeling generally in Region 3 when
watersheds are flat and have significant storage
in the overbanks. These watersheds would be
characterized by:
Stormwater Hydrology
The unit hydrograph equations used in the NRCS
called “convolution.”
The development of a runoff hydrograph for a
(Step 6) Summation of the flows from
watershed or one of many sub-basins within
each of the small incremental
a more complex model involves the following
hydrographs (keeping proper
steps:
track of time steps) to form a
runoff hydrograph for that wa(Step 1) Development or selection of a
tershed or sub-basin.
design storm hyetograph. Often
the NRCS TR-55 24-hour storm
To assist the designer in using the NRCS TR-55
described in Subsection 3.1.5.3
unit hydrograph approach with a peaking factor of
is used. This storm is recom-
300, Figure 3.1.5-8 and Table 3.1.5-4 have been
mended for use in Georgia.
developed. The unit hydrograph is used in the
(Step 2) Development of curve numbers
and lag times for the watershed
same way as the unit hydrograph with a peaking
factor of 484.
using the methods described in
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VOL 2
81
Table 3.1.5-4 Dimensionless Unit Hydrographs
the dimensionless unit hydrographs in the table
below is to multiply each time ratio value by the
time-to-peak (Tp) and each value of q/qu by qu
calculated as:
qu = (PF A ) / (Tp)
(3.1.13)
Where:
484
t/Tt
q/qu
300
Q/Qp
q/qu
484
Q/Qp
t/Tt
q/qu
300
Q/Qp
q/qu
Q/Qp
0.0
0.0
0.0
0.0
0.0
3.2
0.020
0.995
0.211
0.919
0.1
0.005
0.000
0.122
0.006
3.3
0.015
0.996
0.190
0.928
0.2
0.046
0.004
0.296
0.019
3.4
0.012
0.997
0.171
0.936
0.3
0.148
0.015
0.469
0.041
3.5
0.009
0.998
0.153
0.943
0.4
0.301
0.038
0.622
0.070
3.6
0.007
0.998
0.138
0.949
0.5
0.481
0.075
0.748
0.105
3.7
0.005
0.999
0.124
0.955
0.6
0.657
0.125
0.847
0.144
3.8
0.004
0.999
0.111
0.960
charge (cfs)
0.7
0.807
0.186
0.918
0.186
3.9
0.003
0.999
0.099
0.965
PF = peaking factor (either 484 or 300)
0.8
0.916
0.255
0.966
0.231
4.0
0.002
1.000
0.089
0.969
A = area (mi )
0.9
0.980
0.330
0.992
0.277
4.1
0.079
0.972
d = rainfall time increment (hr)
1.0
1.000
0.406
1.000
0.324
4.2
0.071
0.976
Tp = time to peak = d/2 + 0.6 Tc (hr)
1.1
0.982
0.481
0.993
0.370
4.3
0.063
0.979
1.2
0.935
0.552
0.974
0.415
4.4
0.056
0.981
For ease of spreadsheet calculations, the dimen-
1.3
0.867
0.618
0.945
0.459
4.5
0.050
0.984
sionless unit hydrographs for 484 and 300 can be
1.4
0.786
0.677
0.909
0.501
4.6
0.044
0.986
qu= unit hydrograph peak rate of dis-
2
approximated by the equation:
X
⎡
⎜ 1− t ⎟ ⎤
⎛
⎞
q ⎢ t ⎜⎝ T p ⎟⎠ ⎥
=
e
⎥
qu ⎢ T p
⎢⎣
⎥⎦
(3.1.14)
1.5
0.699
0.730
0.868
0.541
4.7
0.039
0.987
1.6
0.611
0.777
0.823
0.579
4.8
0.035
0.989
1.7
0.526
0.817
0.775
0.615
4.9
0.031
0.990
1.8
0.447
0.851
0.727
0.649
5.0
0.028
0.992
1.9
0.376
0.879
0.678
0.680
5.1
0.024
0.993
2.0
0.312
0.903
0.631
0.710
5.2
0.022
0.994
0.995
2.1
0.257
0.923
0.584
0.737
5.3
0.019
2.2
0.210
0.939
0.539
0.762
5.4
0.017
0.996
Where X is 3.79 for the PF=484 unit hydrograph
2.3
0.170
0.951
0.496
0.785
5.5
0.015
0.996
and 1.50 for the PF=300 unit hydrograph.
2.4
0.137
0.962
0.455
0.806
5.6
0.013
0.997
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2.5
0.109
0.970
0.416
0.825
5.7
0.012
0.997
2.6
0.087
0.977
0.380
0.843
5.8
0.010
0.998
2.7
0.069
0.982
0.346
0.859
5.9
0.009
0.998
2.8
0.054
0.986
0.314
0.873
6.0
0.008
0.999
2.9
0.042
0.989
0.285
0.886
6.1
0.007
0.999
3.0
0.033
0.992
0.258
0.898
6.2
0.006
0.999
3.1
0.025
0.994
0.233
0.909
6.3
0.006
1.000
Stormwater Hydrology
The procedure to develop a unit hydrograph from
VOL 2
82
Time
300
time (min)
q/qpu
q
q/qpu
q
0
0
0
0.00
0
0.00
0.22
3.0
0.06
10.26
0.33
34.18
0.44
6.0
0.37
61.74
0.68
69.60
0.66
9.0
0.75
124.79
0.89
91.99
0.88
12.0
0.97
161.37
0.99
101.85
1
13.64
1
166
1
103
1.10
15.0
0.98
163.39
0.99
102.35
96.74
1.32
18.0
0.85
141.70
0.94
1.54
21.0
0.66
110.45
0.85
87.64
1.76
24.0
0.48
79.61
0.75
76.98
66.03
1.98
27.0
0.33
54.06
0.64
2.20
30.0
0.21
35.02
0.54
55.59
2.42
33.0
0.13
21.84
0.45
46.10
2.64
36.0
0.08
13.19
0.37
37.76
2.86
39.0
0.05
7.77
0.30
30.60
3.08
42.0
0.03
4.47
0.24
24.58
3.30
45.0
0.02
2.52
0.19
19.60
3.52
48.0
0.01
1.40
0.15
15.52
3.74
51.0
0.00
0.76
0.12
12.21
3.96
54.0
0.00
0.41
0.09
9.57
4.18
57.0
0.00
0.22
0.07
7.46
4.40
60.0
0.00
0.12
0.06
5.79
4.48
4.62
63.0
0.00
0.06
0.04
4.84
66.0
0.00
0.03
0.03
3.45
5.06
69.0
0.00
0.02
0.03
2.65
5.28
72.0
0.00
0.01
0.02
2.03
5.50
75.0
0.00
0.00
0.02
1.55
3.1.5.10 EXAMPLE PROBLEM 2
5.72
78.0
0.01
1.18
Compute the unit hydrograph for the 50-acre
5.94
81.0
0.01
0.90
0.68
wooded watershed in Subsection 3.1.5.8.
Computations
1.
Calculate Tp and time increment
The time of concentration (Tc) is calculated to be
20.24 minutes for this watershed. If we assume
a computer calculation time increment (d) of 3
minutes then:
Tp = d/2 + 0.6Tc = 3/2 + 0.6 * 20.24 = 13.64 minutes (0.227 hrs)
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2.
Calculate qpu
qpu = PF A/Tp = (484 * 50/640)/(0.227) = 166 cfs
For a PF of 300 qpu would be:
qpu = PF A/Tp = (300 * 50/640)/(0.227) = 103 cfs
3.
Calculate unit hydrograph for both 484 and
300.
Based on spreadsheet calculations using Equations 3.1.13 and 3.1.14, the table to the right has
been derived.
6.16
84.0
0.01
6.38
87.0
0.01
0.52
6.60
90.0
0.00
0.39
6.82
93.0
0.00
0.30
7.04
96.0
0.00
0.22
7.26
99.0
0.00
0.17
7.48
102.0
0.00
0.13
0.09
7.70
105.0
0.00
7.92
108.0
0.00
0.07
8.14
111.0
0.00
0.05
0.04
8.36
114.0
0.00
8.58
117.0
0.00
0.03
8.80
120.0
0.00
0.02
0.02
9.01
123.0
0.00
9.23
126.0
0.00
0.01
9.45
129.0
0.00
0.01
9.67
132.0
0.00
0.01
9.89
135.0
0.00
0.01
10.11
138.0
0.00
0.00
Stormwater Hydrology
Figure 3.1.5-8 Dimensionless Unit Hydrographs for
Peaking Factors of 484 and 300
484
t/Tp
VOL 2
83
EXPLANATION
Hydrologic Region
Region 1
Region 2
Region 3
Region 4
Region 5
Undefined
3.1.6.1 INTRODUCTION
For the past 30 years the USGS has been collecting rain and streamflow data at various sites
throughout the state of Georgia. The data from
these efforts have been used to calibrate a USGS
Stormwater Hydrology
3.1.6 U.S. Geological Survey Peak Flow
and Hydrograph Method
rainfall-runoff model. The U.S. Geological Survey
Model was then used to develop peak dis-charge
regression equations for the 2-, 5-, 10-, 25-,
50- and 100-year floods. In addition, the USGS
used the statewide database to develop a dimensionless hydrograph that can be used to simulate
flood hydrographs from rural and urban streams
in Georgia. This USGS information is specific to
geographical regions of Georgia. Figure 3.1.6-1
shows the locations of these different regions.
3.1.6.2 APPLICATION
The USGS regression method is used for both the
estimation of stormwater runoff peak rates and
the generation of hydrographs for the routing of
stormwater flows for larger drainage areas:
• 25 acres and larger for peak flow estimation
• 128 acres and larger for hydrograph generation
The USGS method can be used for most design
applications, including the design of storage facilities and outlet structures, storm drain systems,
Base modified from U.S. Geological Survey
1:100,000 and 1:250,000-scale digital data
0
0
25
25
50 MILES
50 KILOMETERS
culverts, small drainage ditches and open channels, and energy dissipators.
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Figure 3.1.6-1 USGS Hydrologic Regions in Georgia
(Source: USGS, 2011)
VOL 2
84
For a complete description of the USGS regression equations presented below, consult the
latest USGS publications regarding both rural and
urban flood frequencies. Based on the current
USGS publications, a watershed is determined to
be urban if 10% or more of the watershed basin is
impervious. USGS regression equations have been
removed from this Manual due to their periodic
update. Check the USGS publications website
for the most recent publications and regression
3.1.6.5 HYDROGRAPHS
• Drainage area (DRNAREA, DA, or A)
The USGS has developed a dimensionless hydro-
• Percent impervious cover; and
graph for Georgia streams having drainage areas
of less than 500 mi2. This dimensionless hydro-
• Percent developed land
graph can be used to simulate flood hydrographs
The most recent version of the USGS publications
for rural and urban streams throughout the State
should also be used to verify the limitations of
of Georgia. For a complete description of the
these variables within the peak discharge equa-
USGS dimension¬less hydrograph, consult the
tions. These equations should not be used on
USGS publication Simulation of Flood Hydro-
any variables which have physical characteris¬tics
graphs for Georgia Streams, Water-Resources
outside of their appropriate range.
Investigation Report 86-4004. Table 3.1.6-1 lists
equations. At the time of this Manual update, the
the time and discharge ratios for the dimension-
following publications were available:
less hydrograph.
• Urban Method: Methods for Estimating the
Magnitude and Frequency of Floods for Urban
and Small, Rural Streams in Georgia, South
Carolina, and North Carolina, 2011 (http://pubs.
usgs.gov/sir/2014/5030/)
• Rural Method: Magnitude and Frequency of
Rural Floods in the Southeastern United States,
2006: Volume 1, Georgia (http://pubs.usgs.gov/
Table 3.1.6-1 Dimensionless USGS Hydrograph
Time Ratio (t/TL)
Discharge Ratio (Q/Qp)
Time Ratio (t/TL)
Discharge Ratio (Q/Qp)
0.25
0.12
1.35
0.62
0.30
0.16
1.40
0.56
0.35
0.21
1.45
0.51
0.40
0.26
1.50
0.47
0.45
0.33
1.55
0.43
0.50
0.40
1.60
0.39
0.36
0.55
0.49
1.65
0.60
0.58
1.70
0.33
In addition to the publications, USGS has also de-
0.65
0.67
1.75
0.30
veloped a spreadsheet tool to assist the designer
0.70
0.76
1.80
0.28
in computing flood-frequency characteristics, for
0.75
0.84
1.85
0.26
both the urban and rural methods. The spread-
0.80
0.90
1.90
0.24
sir/2009/5043/)
sheets are downloadable, using the links provided
0.85
0.95
1.95
0.22
above, as Microsoft Excel documents
0.90
0.98
2.00
0.20
0.95
1.00
2.05
0.19
1.00
0.99
2.10
0.17
3.1.6.4 PEAK DISCHARGE LIMITATIONS FOR
URBAN AND RURAL BASINS
1.05
0.96
2.15
0.16
1.10
0.92
2.20
0.15
Each USGS regression equation uses variables that
1.15
0.86
2.25
0.14
represent the following:
1.20
0.80
2.30
0.13
1.25
0.74
2.35
0.12
1.30
0.68
2.40
0.11
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Stormwater Hydrology
3.1.6.3 PEAK DISCHARGE EQUATIONS
Source: USGS, 1986
VOL 2
85
sionless hydrograph are:
North of the Fall Line (rural):
TL = 4.64A0.49S-0.21
(3.1.15)
• Drainage Area = 175 acres = 0.273 mi2
• Main Channel Slope = 117 ft/mi
South of the Fall Line (rural):
TL = 13.6A
the following drainage area located in Region
1 in the Atlanta metro area. For the developed
conditions, develop the flood hydrograph for this
drainage area.
S
(3.1.16)
0.43 -0.31
• Percent Impervious Area = 32%
Lag Time Calculations
TL = 7.86A0.35TIA-0.22S-0.31 = 7.86 (0.273)0.35
(32)-0.22 (117)-0.31 = 0.53 hours
Hydrograph Calculations
Using the dimensionless USGS hydrograph given
in Table 3.1.6-1, the following calculations are
done to determine the coordinates of the flood
Stormwater Hydrology
The lag time equations for calculating the dimen-
hydrograph.
Peak Discharge Calculations
Regions 1, 2 and 3 (urban):
TL = 7.86A0.35TIA-0.22S-0.31
(3.1.17)
100-year Rural Peak Discharge:
Time (t) = t/TL x 0.53
Q100 = 776(DA)0.594 (Taken from the most recent
on previous page
t/TL from Table 3.1.6-1
publication)
Region 4 (urban):
TL = 6.10A0.35TIA-0.22S-0.31
(3.1.18)
Q100 = 776(0.273)0.594 = 359 cfs
Discharge (Q) = Q/Qp x 561
Q/Qp from Table
3.1.6-1 on previous page
Where:
TL = lag time (hours)
100-year Developed (Urban) Peak Flow:
Coordinates for the flood hydrograph are given in
Q100 = 753(DRNAREA)0.803810(0.0024*IMPNLCD06) (Taken
Table 3.1.6-2 on the next page.
A = drainage area (mi )
from the most recent publication)
S = main channel slope (ft/mi)
Q100 = 753(0.273)0.803810(0.0024*32)= 317 cfs
2
TIA = total impervious area (percent)
Using these lag time equations and the dimensionless hydrograph, a runoff hydrograph can be
determined after the peak discharge is calculated.
3.1.6.6 HYDROGRAPH LIMITATIONS
List in the table shown at the right are the limitations of the variables within the lag time equations. The lag time equation should not be used
for drainage areas that have physical characteristics outside the limits listed on this page.
Physical Characteristics
North of the Fall Line A - Drainage Area
(rural) S - Main Channel Slope
South of the Fall Line A - Drainage Area
(rural) S - Main Channel Slope
Regions 1, 2, & 3 A - Drainage Area
(urban) S - Main Channel Slope
TIA - Total Impervious Area
3.1.6.7 EXAMPLE PROBLEM
For the 100-year flood, calculate the peak
discharge for rural and developed conditions for
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Region 4 (urban) A - Drainage Area
Minimum
Maximum
0.3
500
mi2
Units
5.0
200
feet per mile
0.2
500
mi2
1.3
60
feet per mile
0.04
19.1
mi2
9.4
772
feet per mile
1.0
61.6
percent
0.12
2.9
mi2
S - Main Channel Slope
19.4
110
feet per mile
TIA - Total Impervious Area
6.1
42.4
percent
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86
Time Ratio
Time (t)
Discharge Ratio
Discharge
Time Ratio
Time (t)
Discharge Ratio
Discharge
(t/TL)
(Hours)
(Q/Qp)
(cfs)
(t/TL)
(Hours)
(Q/Qp)
(cfs)
0.25
0.13
0.12
67
1.35
0.72
0.62
348
0.30
0.16
0.16
90
1.40
0.74
0.56
314
0.35
0.19
0.21
118
1.45
0.77
0.51
286
0.40
0.21
0.26
146
1.50
0.80
0.47
264
0.45
0.24
0.33
185
1.55
0.82
0.43
241
0.50
0.27
0.40
224
1.60
0.85
0.39
219
0.55
0.29
0.49
275
1.65
0.87
0.36
202
0.60
0.32
0.58
325
1.70
0.90
0.33
185
0.65
0.34
0.67
376
1.75
0.93
0.30
168
0.70
0.37
0.76
426
1.80
0.95
0.28
157
0.75
0.40
0.84
471
1.85
0.98
0.26
146
0.80
0.42
0.90
505
1.90
1.01
0.24
135
0.85
0.45
0.95
533
1.95
1.03
0.22
123
0.90
0.48
0.98
550
2.00
1.06
0.20
112
0.95
0.50
1.00
561
2.05
1.09
0.19
107
1.00
0.53
0.99
555
2.10
1.11
0.17
95
1.05
0.56
0.96
539
2.15
1.14
0.16
90
1.10
0.58
0.92
516
2.20
1.17
0.15
84
1.15
0.61
0.86
482
2.25
1.19
0.14
79
1.20
0.64
0.80
449
2.30
1.22
0.13
73
1.25
0.66
0.74
415
2.35
1.25
0.12
67
1.30
0.69
0.68
381
2.40
1.27
0.11
62
Stormwater Hydrology
Table 3.1.6-2 Flood Hydrograph
Source: U.S.G.S., 1986
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VOL 2
87
Where:
WQv = water quality volume (acre-feet)
In addition to the discussion of the calculations
Rv = volumetric runoff coefficient
provided below, full examples for five stormwater
A = total drainage area (acres)
CN = 1000/[10 + 5P +10Qwv - 10(Qwv² +
1.25QwvP)½]
Where:
P = rainfall, in inches (use 1.2 inches for
best management practices have been provided
in Appendix B.
3.1.7.1 WATER QUALITY VOLUME CALCULATION
The Water Quality Volume (WQv) is the reten-
WQv can be expressed in inches simply as 1.2(Rv)
the Water Quality Storm in Georgia)
= Qwv
Qwv = Water Quality Volume, in inches
3.1.7.2 WATER QUALITY VOLUME PEAK FLOW
CALCULATION
tion or treatment volume required to remove a
The peak rate of discharge for the water quality
significant percentage of the stormwater pollution
design storm is needed for the sizing of off-line
load, defined in this Manual as an 80% removal
diversion structures, such as for sand filters and
of the average annual post-development total
infiltration trenches. An arbitrary storm would
suspended solids (TSS) load. This is achieved by
need to be chosen using the Rational Method,
intercepting and retaining or treating a portion of
and conventional NRCS TR-55 methods have
the runoff from all storms and all the runoff from
been found to underestimate the volume and rate
85% of the storms that occur on average during
of runoff for rainfall events less than 2 inches. This
the course of a year.
discrepancy in estimating runoff and discharge
rates can lead to situations where a signifi-
The water quality treatment volume is calculated
cant amount of runoff by-passes the treatment
by multiplying the 85th percentile annual rainfall
practice due to an inadequately sized diversion
event by the volumetric runoff coefficient (Rv) and
structure and leads to the design of undersized
the site area. Rv is defined as:
Rv = 0.05 + 0.009(I)
Where:
(3.1.19)
bypass channels.
(1.2Rv)
2.
3.
Once a CN is computed, the time of
concentration (tc) is computed (based on the
methods described in this section).
Using the computed CN, tc and drainage
area (A), in acres; the peak discharge (Qwq) for
the water quality storm event is computed
using a slight modification of the Simplified
NRCS TR-55 Peak Runoff Rate Estimation
technique of Subsection 3.1.5.7. Use
appropriate rainfall distribution type (either
Type II or Type III in Georgia).
»» Read initial abstraction (Ia), compute Ia/P
»» Read the unit peak discharge (qu) for
appropriate tc
»» Using WQv, compute the peak discharge
(Qwq)
The following procedure can be used to estimate
Qwq = qu * A * Qwv
peak discharges for small storm events.
I = percent of impervious cover (%)
It relies on the Water Quality Volume and the
simplified peak flow estimating method above. A
Where:
brief description of the calculation procedure is
Qwq = the water quality peak discharge
tile annual rainfall event is 1.2 inches. Therefore,
presented below:
(cfs)
WQv is calculated using the following formula:
1.
For the state of Georgia, the average 85th percen-
WQv = 1.2 Rv A
12
(3.1.20)
Stormwater Hydrology
3.1.7 Water Quality Volume and Peak
Flow
Using WQv, a corresponding Curve Number
(CN) is computed utilizing the following
equation:
qu = the unit peak discharge (cfs/mi²/
inch)
A = drainage area (mi2)
Qwv = Water Quality Volume, in inches
(1.2Rv)
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VOL 2
88
Using the data and information from the example
3.1.7.4 RUNOFF REDUCTION VOLUME CALCULATION
3.1.7.5 ADJUSTED CURVE NUMBER PROCEDURE FOR PEAK FLOW REDUCTION
problem in Subsection 3.1.5.8 calculate the water
The Runoff Reduction Volume (RRv) is the reten-
The following method utilizes the Natural Re-
quality volume and the water quality peak flow.
tion volume calculated to infiltrate, evapotrans-
source Conservation Service runoff equations
pirate, or otherwise removed from a post-de-
originally provided in Urban Hydrology for Small
veloped condition to more naturally mimic the
Watersheds (USDA 1986) to compute a curve
natural hydrologic conditions. For additional
number adjustment that effectively reduces the
information, see Section 2.2.2.2 (Standard #3).
peak flow of other storm events. A simplified
Calculate water quality volume (WQv)
Compute volumetric runoff coefficient, Rv
RV = 0.05 + (0.009) = 0.05 + (0.009)(18/50 x
approach has been provided that combines these
100%) = 0.37
The runoff reduction volume is calculated by mul-
runoff equations. The NRCS runoff equations are
Compute water quality volume, WQv
tiplying the target runoff reduction rainfall event
discussed in Subsection 3.1.5 and include Equa-
(typically 1.0 inches) by the volumetric runoff co-
tions 3.1.3, 3.1.4, and 3.1.5. The following modi-
efficient (Rv) and the site area. Rv is again defined
fied equation is discussed in the 2010 Center for
as:
Watershed Protection journal article titled, The
WQv = 1.2(RV)(A)/12 = 1.2(.37)(50)/12 = 1.85 acrefeet
Runoff Reduction Method:
Calculate water quality peak flow
Rv = 0.05 + 0.009(I)
Compute runoff volume in inches, Qwv:
Qwv = 1.2Rv = 1.2 * 0.37 = 0.44 inches
Computer curve number:
(3.1.19)
(P − 0.2S ) 2
Q-R =
Where:
I = percent of impervious cover (%)
CN = 1000/[10 + 5P +10Q - 10(Qwv² + 1.25Qwv
(P + 0.8S)
CN = 1000/[10 + 5*1.2 +10*0.252 - 10(0.252² +
For the state of Georgia, the recommended target
Q = runoff depth (in),
1.25*0.252*1.2)½] = 84
runoff reduction rainfall is 1.0 inches. Therefore,
P = rainfall depth (in),
RRv is calculated using the following formula:
Ia = Initial abstraction (in),
S = 1000/CN – 10 = 1000/84 – 10 = 1.90 inches
0.2S = Ia = 0.38 inches
Ia/P = 0.38/1.2 = 0.317
Find qu:
From Figure 3.1.5-6 for Ia/P = 0.317
qu = 535 cfs/mi2/in
(Modified
Eq. 3.1.5)
Where:
P)½]
tc = 0.34 (computed previously)
Stormwater Hydrology
3.1.7.3 EXAMPLE PROBLEM
S = potential maximum retention after
RRv = P Rv A
12
runoff begins (in),
Where:
R = Retention storage provided by runoff
RRv = runoff reduction volume (cubic
CN = Runoff Curve Number, and
reduction practices (in).
feet)
P = target runoff reduction rainfall
Rv = volumetric runoff coefficient
A = total drainage area (square feet)
Compute water quality peak flow:
Qwq = qu * A * Qwv = 535 * 50/640 * 0.44 = 18.4
cfs
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VOL 2
89
tice(s) can be calculated by the following:
R = (VP)(RR%) / Area
• P1YR: 3.4 inches
• S: 1.36 (1000/CN – 10)
• Post-Q1YR:2.18 inches (See Equation 3.1.5)
Where:
Calculate the amount of Runoff Reduction (RRV
provided) by the practice. This information will
be needed in the adjusted curve number calculation (converted to the variable “R”):
RRV (provided) = (RR%) (VP)
= (50%) (13,504 ft3)
VP = Total Volume Provided by the BMP
(See Chapter 4 for BMP sizing information)
RR% = runoff reduction credit provided
by the BMP (See Table 4.1.3-2 for RR%)
By solving the modified equation above for a
new potential maximum retention value, S, the
adjusted curve number can be back-calculated as
a representation of the runoff reduction achieved
in any particular storm event.
Runoff Reduction/Adjusted Curve Number
Example
Using the given data and information provided
below, calculate the runoff reduction volume and
the adjusted curve number for channel protection
assuming a best management practice is used
that provides a runoff reduction removal percentage (RR%) of 50%.
Calculate water runoff reduction volume (RRv)
= 6,752 ft3
Compute volumetric runoff coefficient, Rv
RV = 0.05 + (0.009)(I)
= 0.05 + (0.009)(1.9/3 x 100%)
= 0.62
Compute runoff reduction volume, RRv
RRv = 1.0(RV)(A)/12
= 1.0(.62)(3)/12
= 0.155 acre-feet (6,752 cubic feet)
Calculate the minimum volume of the practice
(VPMIN)
(VPMIN) ≥ RRV / (RR%)
= (6,752 ft3) / (50%)
= 13,504 ft3
Note: The Volume Provided (VP) of this practice
must be a minimum of 13,504 ft3.
Stormwater Hydrology
To calculate “R”, the provided RRV of the prac-
Adjusted Curve Number Procedure for Peak
Flow Reduction of CPV
Given Q = 2.18 in. and P = 3.4 in., Find “R” and “S”
to back calculate an adjusted CN
(Modified
Equation
3.1.5)
Retention storage (expressed in inches) for this
basin is calculated by the following formula:
R = RRV (provided) / Basin Area
= (6,752 ft3) / A
= 6,752 ft3 / 130,680 ft2 (12 in / 1 ft)
= 0.62 inches
Solve for “S” to back calculate CN: S = 2.49
S = 1000/CN-10: therefore, Adjusted CN = 80.1
Given Information:
• Site Area: 3.0 ac (130,680 ft2)
• Impervious Area: 1.9 ac; or I=1.9/3.0 = 63.3%
• Pre-developed CN: 70
• Post-developed CN: 88
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The inflows consist of rainfall, runoff and baseflow
modeling using rainfall records and a watershed
into the pond. The outflows consist of infiltra-
model. Two other methods have been proposed.
3.1.8.1 INTRODUCTION
tion, evaporation, evapotranspiration, and surface
Water balance calculations help determine if a
overflow out of the best management practice.
Equation 3.1.19 gives a ratio of runoff to rain-
drainage area is large enough, or has the right
Equation 3.1.21 can be changed to reflect these
fall volume for a particular storm. If it can be
characteristics, to support a permanent pool
factors.
assumed that the average storm that produces
of water during average or extreme conditions.
be advisable for any best management practice
∆ V = P + Ro + Bf – I – E – Et – Of
(3.1.22)
that maintains a permanent volume of stormwa-
Where:
When in doubt, a water balance calculation may
ter.
runoff has a similar ratio, then the Rv value can
serve as the ratio of rainfall to runoff. Not all
storms produce runoff in an urban setting. Typical initial losses (often called “initial abstractions”)
P = precipitation (ft)
are normally taken between 0.1 and 0.2 inches.
Ro = runoff (ac-ft)
When compared to the rainfall records in Georgia,
The details of a rigorous water balance are
Bf = baseflow (ac-ft)
this is equivalent of about a 10% runoff volume
beyond the scope of this manual. However, a
I = infiltration (ft)
loss. Thus a factor of 0.9 should be applied to
simplified procedure is described herein that will
E = evaporation (ft)
the calculated Rv value to account for storms that
provide an estimate of pool viability and point to
Et = evapotranspiration (ft)
produce no runoff. Equation 3.1.23 reflects this
the need for more rigorous analysis. Water bal-
Of = overflow (ac-ft)
approach. Total runoff volume is then simply the
ance can also be used to help establish planting
zones in a wetland design.
Stormwater Hydrology
3.1.8 Water Balance Calculations
product of runoff depth (Q) times the drainage
Rainfall (P) – Rainfall values can be obtained from
area to the pond.
NOAA Atlas 14 at: http://hdsc.nws.noaa.gov/hdsc/
Water balance is defined as the change in volume
of the permanent pool resulting from the total inflow minus the total outflow (actual or potential):
∆V= ΣI-ΣO
Q = 0.9 PRv
pfds/
3.1.8.2 BASIC EQUATIONS
(3.1.21)
Monthly values are commonly used for calcu-
(3.1.23)
Where:
lations of values over a season. Rainfall is then
P = precipitation (in)
the direct amount that falls on the permanent
Q = runoff volume (in)
pool surface for the period in question. When
Rv = volumetric runoff coefficient [see
multiplied by the permanent pool surface area (in
Equation 3.1.19]
acres) it becomes acre-feet of volume.
Where:
∆
= “change in”
V = permanent pool volume (ac-ft)
Σ = “sum of”
I = Inflows (ac-ft)
O = Outflows (ac-ft)
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Runoff (Ro) – Runoff is equivalent to the rainfall for the period times the “efficiency” of the
watershed, which is equal to the ratio of runoff to
rainfall. In lieu of gage information, Q/P can be
estimated one of several ways. The best method would be to perform long-term simulation
VOL 2
91
the water infiltrates (ac)
eling in an attempt to quantify an average ratio
Kh = saturated hydraulic conductivity or infiltration rate (ft/day)
on a monthly basis. For the Atlanta area he has
Gh = hydraulic gradient = pressure head/distance
developed the following equation:
Gh can be set equal to 1.0 for pond bottoms and 0.5 for pond sides steeper than about 4:1. Infiltration rate
Q = 0.235P/S
0.64
– 0.161
(3.1.24)
can be established through testing, though not always accurately. As a first cut estimate Table 3.1.8-1 can
be used.
Where:
P = precipitation (in)
Table 3.1.8-1 Saturated Hydraulic Conductivity
Q = runoff volume (in)
Material
S = potential maximum retention (in) [see
Equation 3.1.5]
Baseflow (Bf) – Most stormwater ponds and wetlands have little, if any, baseflow, as they are rarely
placed across perennial streams. If so placed,
baseflow must be estimated from observation or
through theoretical estimates. Methods of estimation and baseflow separation can be found in
most hydrology textbooks.
Hydraulic Conductivity
in/hr
ft/day
ASTM Crushed Stone No. 3
50,000
100,000
ASTM Crushed Stone No. 4
40,000
80,000
ASTM Crushed Stone No. 5
25,000
50,000
ASTM Crushed Stone No. 6
15,000
30,000
Sand
8.27
16.54
Loamy sand
2.41
4.82
Sandy loam
1.02
2.04
Loam
0.52
1.04
Silt loam
0.27
0.54
Infiltration (I) – Infiltration is a very complex
Sandy clay loam
0.17
0.34
subject and cannot be covered in detail here. The
Clay loam
0.09
0.18
amount of infiltration depends on soils, water
Silty clay loam
0.06
0.12
table depth, rock layers, surface disturbance, the
Sandy clay
0.05
0.10
presence or absence of a liner in the pond, and
Silty clay
0.04
0.08
other factors. The infiltration rate is governed by
Clay
0.02
0.04
the Darcy equation as:
Source: Ferguson and Debo, “On-Site Stormwater Management,” 1990
I = AkhGh
Stormwater Hydrology
Ferguson (1996) has performed simulation mod-
(3.1.25)
Evaporation (E) – Evaporation is from an open water surface. Evaporation rates are dependent on differences in vapor pressure, which, in turn, depend on temperature, wind, atmospheric pressure, water purity,
Where:
and shape and depth of the pond. It is estimated or measured in a number of ways, which can be found in
I = infiltration (ac-ft/day)
most hydrology textbooks. Pan evaporation methods are also used though there are only two pan evap-
A = cross sectional area through which
oration sites active in Georgia (Lake Allatoona and Griffin). A pan coefficient of 0.7 is commonly used to
convert the higher pan value to the lower lake values.
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Evaporation Monthly Distribution
J
F
M
A
M
J
J
A
S
O
N
D
3.2%
4.4%
7.4%
10.3%
12.3%
12.9%
13.4%
11.8%
9.3%
7.0%
4.7%
3.2%
Table 3.1.8-2 gives pan evaporation rate distribu-
when wetlands are being designed and emergent
tions for a typical 12-month period based on pan
vegetation covers a significant portion of the
evaporation information from five stations in and
pond surface. In these cases conservative esti-
around Georgia. Figure 3.1.8-1 depicts a map of
mates of lake evaporation should be compared
annual free water surface (FWS) evaporation aver-
to crop-based Et estimates and a decision made.
ages for Georgia based on a National Oceanic and
Crop-based Et estimates can be obtained from
Atmospheric Administration (NOAA) assessment
typical hydrology textbooks or from the web sites
done in 1982. FWS evaporation differs from lake
mentioned above.
Stormwater Hydrology
Table 3.1.8-2
evaporation for larger and deeper lakes, but can
be used as an estimate for the type of structural
Overflow (Of) – Overflow is considered as excess
stormwater ponds and wetlands being designed
runoff, and in water balance design is either not
in Georgia. Total annual values can be estimated
considered, since the concern is for average
from this map and distributed according to Figure
values of precipitation, or is considered lost for all
3.1.8-1.
volumes above the maximum pond storage. Obviously, for long-term simulations of rainfall-run-
Evapotranspiration (Et) – Evapotranspiration
off, large storms would play an important part in
consists of the combination of evaporation and
pond design.
transpiration by plants. The estimation of Et for
crops in Georgia is well documented and has
become standard practice. However, for wetlands the estimating methods are not documented, nor are there consistent studies to assist the
designer in estimating the demand wetland plants
would put on water volumes. Literature values
for various places in the United States vary around
the free water surface lake evaporation values.
Estimating Et only becomes important
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Austin Acres, a 26-acre site in Augusta, is being
developed along with an estimated 0.5-acre
surface area pond. There is no baseflow. The
desired pond volume to the overflow point is
2 acre-feet. Will the site be able to support the
pond volume? From the basic site data we find
that the site is 75% impervious with sandy clay
Stormwater Hydrology
3.1.8.3 EXAMPLE PROBLEM
loam soil.
• From Equation 3.1.19, Rv = 0.05 + 0.009 (75)
= 0.73. With the correction factor of 0.9 the
watershed efficiency is 0.65.
• The annual lake evaporation from Figure 3.1.81 is about 42 inches.
• For a sandy clay loam the infiltration rate is I =
0.34 ft/day (Table 3.1.8-1).
• From a grading plan it is known that about 10%
of the total pond area is sloped greater than
1:4.
• Monthly rainfall for Augusta was found from
the Web site provided above.
Figure 3.1.8-1 Average Annual Free Water Surface Evaporation (in inches)
(Source: NOAA, 1982)
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94
1
2
Days/mo
J
F
M
A
M
J
J
A
S
O
N
D
31
28
31
30
31
30
31
31
30
31
30
31
3
Precipitation (in)
4.05
4.27
4.65
3.31
3.77
4.13
4.24
4.5
3.02
2.84
2.48
3.40
4
Evap Dist
3.2%
4.4%
7.4%
10.3%
12.3%
12.9%
13.4%
11.8%
9.3%
7.0%
4.7%
3.2%
5
Ro (ac-ft)
5.70
6.01
6.55
4.66
5.31
5.82
5.97
6.34
4.25
4.00
3.49
4.79
6
P (ac-ft)
0.17
0.18
0.19
0.14
0.16
0.17
0.18
0.19
0.18
0.12
0.10
0.14
7
E (ac-ft)
0.06
0.08
0.13
0.18
0.22
0.29
0.23
0.21
0.16
0.12
0.08
0.05
8
I (ac-ft)
5.01
4.52
5.01
4.85
5.01
4.85
5.01
5.01
4.85
5.01
4.85
5.01
Stormwater Hydrology
Table 3.1.8-3 Summary Calculations for Each Month of the Year
9
10
Balance (ac-ft)
0.81
1.59
1.61
-0.23
0.24
0.92
0.91
1.31
-0.63
-1.01
-1.33
-0.13
11
Running Balance (ac-ft)
0.81
2.00
2.00
1.77
2.00
2.00
2.00
2.00
1.87
0.36
0.00
0.00
Explanation of Table:
1.
Months of year
2.
Days per month
3.
Monthly precipitation from web site is shown in Figure 3.1.8-2.
4.
Distribution of evaporation by month from Table 3.1.8-2.
5.
Watershed efficiency of 0.65 times the rainfall and converted to acre-feet.
6.
Precipitation volume directly into pond equals precipitation depth times
pond surface area divided by 12 to convert to acre-feet
7.
Evaporation equals monthly percent of 42 inches from line 4 converted
to acre-feet
8.
Infiltration equals infiltration rate times 90% of the surface area plus
infiltration rate times 0.5 (banks greater than 1:4) times 10% of the pond
area converted to acre-feet
9.
Lines 5 and 6 minus lines 7 and 8
10. Accumulated total from line 10 keeping in mind that all volume above 2
acre-feet overflows and is lost in the trial design
Figure 3.1.8-2 Augusta Precipitation Information
It can be seen that for this example the pond has potential to go dry in winter
months. This can be remedied in a number of ways including compacting the
pond bottom, placing a liner of clay or geosynthetics, and changing the pond
geometry to decrease surface area.
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Stormwater Hydrology
3.1.9 Downstream Hydrologic Assessment
The purpose of the overbank flood protection and extreme flood protection criteria is to protect downstream properties from flood increases due to
upstream development. These criteria require the designer to control peak
flow at the outlet of a site such that post-development peak discharge equals
pre-development peak discharge. It has been shown that in certain cases this
does not always provide effective water quantity control downstream from
the site and may actually exacerbate flooding problems downstream. The
reasons for this have to do with (1) the timing of the flow peaks, and (2) the
total increase in volume of runoff. Further, due to a site’s location within a
watershed, there may be very little reason for requiring overbank flood control
from a particular site. This section outlines a suggested procedure for determining the impacts of post-development stormwater peak flows and volumes
on downstream flows that a community may require as part of a developer’s
stormwater management site plan. In summary, a downstream analysis may
warrant a development to over-detain to protect downstream properties or
may even warrant a reduction/elimination of detention because of the timing
Figure 3.1.9-1 Detention Timing Example
of peak discharges within the watershed.
3.1.9.1 REASONS FOR DOWNSTREAM PROBLEMS
Flow Timing
If water quantity control (detention) structures are indiscriminately placed in a
watershed and changes to the flow timing are not considered, the structural
control may actually increase the peak discharge downstream. The reason
for this may be seen in Figure 3.1.9-1. The peak flow from the site is reduced
appropriately, but the timing of the flow is such that the combined detained
peak flow (the larger dashed triangle) is actually higher than if no detention
were required. In this case, the shifting of flows to a later time brought about
by the detention pond actually makes the downstream flooding worse than if
the post-development flows were not detained.
Figure 3.1.9-2 Effect of Increased Post-Development Runoff Volume with Detention
on a Downstream Hydrograph
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The ten-percent rule recognizes the fact that a
An important impact of new development is an
structural control providing detention has a “zone
increase in the total runoff volume of flow. Thus,
of influence” downstream where its effectiveness
even if the peak flow is effectively attenuated,
can be felt. Beyond this zone of influence the
the longer duration of higher flows due to the
structural control becomes relatively small and
increased volume may combine with downstream
insignificant compared to the runoff from the
tributaries to increase the downstream peak flows.
total drainage area at that point. Based on studies
and master planning results for a large number of
Figure 3.1.9-2 illustrates this concept. The figure
sites, that zone of influence is considered to be
shows the pre- and post-development hydro-
the point where the drainage area controlled by
graphs from a development site (Tributary 1). The
the detention or storage facility comprises 10% of
post-development runoff hydrograph meets the
the total drainage area. For example, if the struc-
flood protection criteria (i.e., the post-develop-
tural control drains 10 acres, the zone of influence
ment peak flow is equal to the pre-development
ends at the point where the total drainage area is
peak flow at the outlet from the site). However,
100 acres or greater.
the post-development combined
flow at the first downstream tributary (Tributary 2)
Typical steps in the application of the ten-percent
is higher than pre-development combined flow.
rule are:
This is because the increased volume and timing
1.
Determine the target peak flow for the site
for predevelopment conditions.
2.
Using a topographic map determine the
lower limit of the zone of influence (10%
point).
3.
Using a hydrologic model determine the predevelopment peak flows and timing of those
peaks at each tributary junction beginning
at the pond outlet and ending at the next
tributary junction beyond the 10% point.
of runoff from the developed site increases the
combined flow and flooding downstream. In this
case, the detention volume would have to have
been increased to account for the downstream
timing of the combined hydrographs to mitigate
the impact of the increased runoff volume.
3.1.9.2 THE TEN-PERCENT RULE
In this Manual the “ten percent” criterion has
been adopted as the most flexible and effective
4.
Change the land use on the site to postdevelopment and rerun the model.
5.
Design the structural control facility such that
the overbank flood protection (25-year) postdevelopment flow does not increase the
peak flows at the outlet and the determined
tributary junctions.
approach for ensuring that stormwater quantity
detention ponds actually attempt to maintain
pre-development peak flows throughout the
system downstream.
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6.
If it does increase the peak flow, the
structural control facility must be redesigned
or one of the following options considered:
-- Control of the overbank flood volume
(Qp25) may be waived by the local authority
saving the developer the cost of sizing a
detention basin for overbank flood control.
In this case the ten-percent rule saved the
construction of an unnecessary structural
control facility that would have been
detrimental to the watershed flooding
problems. In some communities this
situation may result in a fee being paid to
the local government in lieu of detention.
That fee would go toward alleviating
downstream flooding or making channel
or other conveyance improvements.
Stormwater Hydrology
Increased Volume
-- Work with the local government to reduce
the flow elevation through channel or
flow conveyance structure improvements
downstream.
-- Obtain a flow easement from downstream
property owners to the 10% point.
Even if the overbank flood protection requirement
is eliminated, the water quality treatment (WQv),
channel protection (CPv), and extreme flood protection (Qf) criteria will
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Discussion
Figure 3.1.9-3 illustrates the concept of the ten-percent rule for two sites in a
Site A is a development of 10 acres, all draining to a wet ED stormwater pond.
watershed.
The overbank flooding and extreme flood portions of the design are going
to incorporate the ten-percent rule. Looking downstream at each tributary
in turn, it is determined that the analysis should end at the tributary marked
“80 acres.” The 100-acre (10%) point is in between the 80-acre and 120-acre
tributary junction points.
Stormwater Hydrology
3.1.9.3 EXAMPLE PROBLEM
The assumption is that if there is no peak flow increase at the 80-acre point
then there will be no increase through the next stream reach downstream
through the 10% point (100 acres) to the 120-acre point. The designer
constructs a simple HEC-1 model of the 80-acre areas using single existing
condition sub-watersheds for each tributary. Key detention structures existing
in other tributaries must be modeled. An approximate curve number is used
since the actual peak flow is not key for initial analysis; only the increase or
decrease is important. The accuracy in curve number determination is not as
significant as an accurate estimate of the time of concentration. Since flooding is an issue downstream, the pond is designed (through several iterations)
until the peak flow does not increase at junction points downstream to the
80-acre point.
Figure 3.1.9-3 Example of the Ten-Percent Rule
Site B is located downstream at the point where the total drainage area is 190
acres. The site itself is only 6 acres. The first tributary junction downstream
from the 10% point is the junction of the site outlet with the stream. The total
190 acres is modeled as one basin with care taken to estimate the time of
concentration for input into the TR-20 model of the watershed. The model
shows that a detention facility, in this case, will actually increase the peak flow
in the stream.
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rainfall event. The modified equation discussed
3.2.1 Introduction
article titled, The Runoff Reduction Method.
The runoff reduction approach to addressing
Water Quality requirements is discussed in detail
in Volume 1 and Subsections 2.2.2 and 2.2.3
of Volume 2. Best management practices that
incorporate stormwater runoff reduction are provided in Chapter 4. Within each BMP section of
the manual, design steps have been provided for
each unique application and practice, and detailed
design examples are provided in Appendices B-2
(bioretention area), B-4 (infiltration trench), and
B-5 (enhanced swale). Refer to Table 4.1.3-1 (BMP
Selection Guide) for applicable BMPs that can
provide runoff reduction.
Where runoff reduction practices are used, an
adjusted curve number (CN) is computed that is
in Subsection 3.1.7.5 was originally presented in
the 2010 Center for Watershed Protection journal
As a Georgia Stormwater Management Manual
design aid, a site development review tool has
been created to aid in the design and documen-
Stormwater Hydrology
3.2 Methods for Estimating
Stormwater Volume Reduction
tation of stormwater management requirements.
This tool can be accessed and downloaded at the
following location: www.georgiastormwater.com.
Within the tool, an “Instructions” and “Tool Flowchart” tab have been created to provide specific
input requirements and responsibilities of the end
user. These same instructions and workflow process are particularly beneficial for local communities who will ultimately review the information
as part of their stormwater management review
process.
lower than the original CN based on an actual
stormwater volume removed from the total runoff, see Subsection 3.1.7.5 for additional information. This adjusted CN calculation is performed for
each storm event being analyzed after a portion
of the stormwater runoff has been quantified and
removed from the site. In other words, an adjusted CN can be computed separately for stream
channel protection, overbank flood protection,
and extreme flood protection storm events since
the volume of runoff changes for any given
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Stormwater Hydrology
3.3 Storage Design
3.3.1 General Storage Concepts
3.3.1.1 INTRODUCTION
This section provides general guidance on stormwater runoff storage for meeting stormwater
management control requirements (i.e., water
quality treatment, downstream channel protection, overbank flood protection, and extreme
flood protection).
Storage of stormwater runoff within a stormwater management system is essential to providing
the extended detention of flows for water quality
treatment and downstream channel protection,
as well as for peak flow attenuation of larger
flows for overbank and extreme flood protection.
Runoff storage can be provided within an on-site
system through the use of best management
practices and/or nonstructural features and landscaped areas. Figure 3.3.1-1 illustrates various
storage facilities that can be considered for a
development site.
Figure 3.3.1-1 Examples of Typical Stormwater Storage Facilities
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Stormwater
Conveyance
Stormwater storage(s) can be classified as detention, extended detention or retention. Some
facilities include one or more types of storage.
Flow Diversion
Structure
Stormwater detention is used to reduce the peak
discharge and detain runoff for a specified short
period of time. Detention volumes are designed
to completely drain after the design storm has
passed. Detention is used to meet overbank flood
Storage
Facility
Storage
Facility
Stormwater Hydrology
3.3.1.2 STORAGE CLASSIFICATION
protection criteria, and extreme flood criteria
where required.
Extended detention (ED) is used to drain a runoff
volume over a specified period of time, typically
24 hours, and is used to meet channel protection
criteria. Some best management practices (wet
ED pond, micropool ED pond, and wetlands) also
include extended detention storage of a portion
of the water quality volume.
On-Line Storage
Off-Line Storage
Figure 3.3.1-2 On-Line versus Off-Line Storage
Retention facilities are designed to contain a per-
Storage can also be categorized as on-line or
manent pool of water, such as stormwater ponds
off-line. On-line storage uses a structural control
and wetlands, which is used for water quality
facility that intercepts flows directly within a con-
treatment.
veyance system or stream. Off-line storage is a
separate storage facility to which flow is diverted
Storage facilities are often classified on the basis
from the conveyance system. Figure 3.3.1-2 illus-
of their location and size. On-site storage is
trates on-line versus off-line storage.
constructed on individual development sites.
Regional storage facilities are constructed at the
lower end of a subwatershed and are designed to
manage stormwater runoff from multiple projects and/or properties. A discussion of regional
stormwater controls is found in Section 4.1.
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Where:
A stage-storage curve defines the relationship be-
V = volume of trapezoidal basin (ft3)
tween the depth of water and storage volume in
L = length of basin at base (ft)
a storage facility (see Figure 3.3.1-3). The volume
W= width of basin at base (ft)
of storage can be calculated by using simple geo-
D = depth of basin (ft)
metric formulas expressed as a function of depth.
Z = side slope factor, ratio of horizontal
to vertical
12.0
Storage (ac-ft)
10.0
The circular conic section formula is:
8.0
V = 1.047 D(R12 + R22 + R1R2)
6.0
2.0
100
(3.3.4)
V = 1.047 D(3 R12 +3ZDR1 + Z2D²)
(3.3.5)
4.0
0.0
Stormwater Hydrology
3.3.1.3 STAGE-STORAGE RELATIONSHIP
Figure 3.3.1-4 Double-End Area Method
101
102 103 104
105 106
107
Stage (ft)
The frustum of a pyramid formula is expressed as:
Where:
R1 , R2 = bottom and surface radii of the
conic section (ft)
Figure 3.3.1-3 Stage-Storage Curve
D = depth of basin (ft)
The storage volume for natural basins may be
V = d/3 [A1 + (A1 x A2)0.5 + A2]/3
(3.3.2)
developed using a topographic map and the dou-
Where:
to vertical
ble-end area, frustum of a pyramid, prismoidal or
V = volume of frustum of a pyramid (ft3)
circular conic section formulas.
d = change in elevation between points
The double-end area formula (see Figure 3.3.1-4)
1 and 2 (ft)
is expressed as:
A1 = surface area at elevation 1 (ft2)
Z = side slope factor, ratio of horizontal
A2 = surface area at elevation 2 (ft2)
V1,2 = [(A1 + A2)/2]d
(3.3.1)
Where:
The prismoidal formula for trapezoidal basins is
expressed as:
V1,2 = storage volume (ft ) between eleva3
tions 1 and 2
A1 = surface area at elevation 1 (ft2)
V = LWD + (L + W) ZD2 + 4/3 Z2 D3
(3.3.3)
A2 = surface area at elevation 2 (ft2)
d = change in elevation between points
1 and 2 (ft)
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3.3.2 Symbols and Definitions
AA stage-discharge curve defines the relationship
To provide consistency within this section as well
between the depth of water and the discharge or
as throughout this Manual, the symbols listed in
outflow from a storage facility (see Figure 3.3.1-
Table 3.3.2-1 will be used. These symbols were
5). A typical storage facility has two outlets or
selected because of their wide use in technical
spillways: a principal outlet and a secondary (or
publications. In some cases, the same symbol is
emergency) outlet. The principal outlet is usually
used in existing publications for more than one
designed with a capacity sufficient to convey the
definition. Where this occurs in this section, the
design flows without allowing flow to enter the
symbol will be defined where it occurs in the text
emergency spillway. A pipe culvert, weir, or other
or equations.
Stormwater Hydrology
3.3.1.4 STAGE-DISCHARGE RELATIONSHIP
appropriate outlet can be used for the principal
Table 3.3.2-1 Symbols and Definitions
spillway or outlet.
Symbol
Definition
Units
The emergency spillway is sized to provide a
A
Cross sectional or surface area
ft2
bypass for floodwater during a flood that exceeds
Am
Drainage area
mi2
the design capacity of the principal outlet. This
C
Weir coefficient
-
spillway should be designed taking into account
d
Change in elevation
ft
the potential threat to downstream areas if the
D
Depth of basin or diameter of pipe
ft
storage facility were to fail. The stage-discharge
t
Routing time period
sec
curve should take into account the discharge
g
Acceleration due to gravity
ft/s2
characteristics of both the principal spillway and
H
Head on structure
ft
the emergency spillway. For more details, see
Hc
Height of weir crest above channel bottom
ft
Section 3.4, Outlet Structures.
K
Coefficient
-
l
Inflow rate
cfs
L
Length
ft
Discharge (cfs)
300
250
Q, q
Peak inflow or outflow
cfs, in
200
R
Surface radii
ft
150
S, Vs
Storage volume
ft3
tb
Time base on hydrograph
hrs
TI
Duration of basin inflow
hrs
tP
Time to peak
hrs
Vs, S
Storage Volume
ft3, in, acre-ft
Vr
Volume of runoff
ft3, in, acre-ft
W
Width of basin
ft
Z
Side slope factor
-
100
50
0
100
101
102 103 104
105 106
Stage (ft)
Figure 3.3.1-5 Stage-Discharge Curve
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An important consideration in these designs is
3.3.3.2 DATA NEEDS
the sediment volume that the system is capable
The following data are needed for storage design
of storing before performance and/or capacity
and routing calculations:
3.3.3.1 INTRODUCTION
are reduced. For larger watersheds, or region-
This section discusses the general design proce-
al detention facilities, the sediment volume is
• Inflow hydrograph for all selected design
storms
dures for designing storage to provide standard
estimated from the sediment produced per year,
detention of stormwater runoff for overbank and
times the years between dredging or similar main-
extreme flood protection (Qp25 and Qf).
tenance. Provisions should be made in the layout
to facilitate access for dredging equipment to the
The design procedures for all structural control
storage area and the maximum sediment depth
storage facilities are the same whether or not they
should be defined. Maintenance plans should
include a permanent pool of water. In the latter
discuss dredging and set a time interval for evalu-
case, the permanent pool elevation is taken as the
ation such as once per year.
• Stage-storage curve for proposed storage
facility
Stormwater Hydrology
3.3.3 General Storage Design Procedures
• Stage-discharge curve for all outlet control
structures
• Estimate of sediment deposited per year and
the number of years desired before dredging.
“bottom” of storage and is treated as if it were a
solid basin bottom for routing purposes.
For smaller watersheds, and all other stormwater
detention facilities, sedimentation can be ad-
It should be noted that the location of structural
dressed by a local community requirement for an
stormwater controls is very important as it relates
as-built pond survey and/or certification process.
to the effectiveness of these facilities to con-
During the construction phase of a development
trol downstream impacts. In addition, multiple
or redevelopment, sedimentation occurs and can
storage facilities located in the same drainage
reduce the storage capacity of the post-con-
basin will affect the timing of the runoff through
struction stormwater detention basin drastically.
the conveyance system, which could decrease
Once final stabilization of a site has occurred, the
or increase flood peaks in different down¬stream
accumulated sediment should be removed and
locations. Therefore, a downstream peak flow
the detention basin surveyed to comply with the
analysis should be performed as part of the stor-
originally approved design volume. It is under-
age facility design process (see Subsection 3.1.9).
stood that sedimentation of a fully stabilized site
over time is minimal for smaller watersheds, when
In multi-purpose multi-stage facilities such as
compared to a larger watershed during active
stormwater ponds, the design of storage must be
construction activities.
integrated with the overall design for water quality
treatment objectives. See Chapter 4 for further
guidance and criteria for the design of structural
stormwater controls.
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flooding problems. If problems will be created (e.g., flooding
A general procedure for using the above data in the design of storage facilities
of habitable dwellings, property damage, or public access
is presented below.
and/or utility interruption) then the storage facility must be
designed to control the increased flows from the 100-year
(Step 1) Compute inflow hydrograph for run¬off from the 25- (Qp25),
storm. If not then consider emergency overflow from runoff
and 100-year (Qf) design storms using the hydrologic meth-
due to the 100-year (or larger) design storm and established
ods outlined in Section 3.1. Both existing- and post-devel-
freeboard requirements.
opment hydrographs are required for 25-year design storm.
Stormwater Hydrology
3.3.3.3 DESIGN PROCEDURE
(Step 7) Evaluate the downstream effects of detention outflows
(Step 2) Perform preliminary calculations to evaluate detention
for the 25- and 100-year storms to ensure that the routed
storage requirements for the hydrographs from Step 1 (see
hydrograph does not cause downstream flooding prob-
Subsection 3.3.4).
lems. The exit hydrograph from the storage facility should
be routed though the downstream channel system until a
(Step 3) Determine the physical dimensions necessary to hold the es-
confluence point is reached where the drainage area being
timated volume from Step 2, including free¬board. Include
analyzed represents 10% of the total drainage area (see Sub-
the estimated volume of sediment storage (if appropriate).
section 3.1.9).
The maximum storage requirement calculated from Step
2 should be used. From the selected shape determine the
maximum depth in the pond.
(Step 8) Evaluate the control structure outlet velocity and provide
channel and bank stabilization if the velocity will cause erosion problems downstream.
(Step 4) Select the type of outlet and size the outlet structure. The
estimated peak stage will occur for the estimated volume
Routing of hydrographs through storage facilities is critical to the proper
from Step 2. The outlet structure should be sized to convey
design of these facilities. Although storage design procedures using inflow/
the allow¬able discharge at this stage.
outflow analysis without routing have been developed, their use in designing
detention facilities has not produced acceptable results in many areas of the
(Step 5) Perform routing calculations using inflow hydrographs from
country, including Georgia.
Step 1 to check the preliminary design using a storage routing computer model. If the routed post-development peak
Although hand calculation procedures are available for routing hydrographs
discharges from the 25-year design storm exceed the exist-
through storage facilities, they are very time consuming, especially when
ing-development peak discharges, then revise the available
several different designs and iterations are evaluated. Many standard hydrology
storage volume, outlet device, etc., and return to Step 3.
and hydraulics textbooks give examples of hand-routing techniques. For this
Manual, it assumed that designers will be using one of the many computer
(Step 6) Perform routing calculations using the 100-year hydrograph
to determine if any increases in downstream flows from
programs available for storage routing and thus other procedures and example
applications will not be given here.
this hydrograph will cause damages and/or drainage and
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(Step 2) Calculate a preliminary estimate
of the ratio VS/Vr using the input
data from Step 1 and the follow-
3.3.4.1 INTRODUCTION
ing equation:
Procedures for preliminary detention calculations
are included here to provide a simple method
that can be used to estimate storage needs and
also provide a quick check on the results of using
(3.3.7)
Stormwater Hydrology
3.3.4 Preliminary Detention Calculations
different computer programs. Standard routing
should be used for actual (final) storage facility
calculations and design.
Where:
3.3.4.2 STORAGE VOLUME
VS = volume of storage (in)
For small drainage areas, a preliminary estimate of
Vr = volume of runoff (in)
the storage volume required for peak flow atten-
QO = outflow peak flow (cfs)
uation may be obtained from a simplified design
Qi = inflow peak flow (cfs)
procedure that replaces the actual inflow and
tb = time base of the inflow hydrograph (hr) [De-
outflow hydrographs with the standard triangular
shapes shown in Figure 3.3.4-1.
termined as the time from the beginning of rise to
Figure 3.3.4-1 Triangular-Shaped Hydrographs
(For Preliminary Estimate of Required Storage Volume)
The required storage volume may be estimated
from the area above the outflow hydrograph and
3.3.4.3 ALTERNATIVE METHOD
inside the inflow hydrograph, expressed as:
An alternative preliminary estimate of the storage
volume required for a specified peak flow reduc-
VS = 0.5TI (Qi - QO)
(3.3.6)
Qi = peak inflow rate (cfs)
of the peak]
tp = time to peak of the inflow hydrograph (hr)
(Step 3) Multiply the volume of runoff, Vr,
times the ratio VS/Vr, calculated
tion can be obtained by the following regression
in Step 2 to obtain the estimated
equation procedure (Wycoff and Singh, 1976).
storage volume VS.
Where:
VS = storage volume estimate (ft3)
a point on the recession limb where the flow is 5%
(Step 1) Determine input data, including
the allowable peak outflow rate,
QO = peak outflow rate (cfs)
QO, the peak flow rate of the
Ti = duration of basin inflow (s)
inflow hydrograph, Qi, the time
base of the inflow hydro¬graph,
tb, and the time to peak of the
inflow hydrograph, tp.
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A preliminary es¬timate of the potential peak flow reduction for a selected
storage volume can be obtained by the following procedure.
(Step 1) Determine volume of runoff, Vr, peak flow rate of the inflow
hydrograph, Qi, time base of the inflow hydrograph, tb, time
to peak of the inflow hydrograph, tp, and storage volume VS.
Stormwater Hydrology
3.3.4.4 PEAK FLOW REDUCTION
(Step 2) Calculate a preliminary estimate of the potential peak flow
reduction for the selected storage volume using the following equation (Wycoff and Singh, 1976):
QO/Qi = 1 - 0.712(VS/Vr)1.328(tb/tp)0.546
(3.3.8)
Where:
QO= outflow peak flow (cfs)
Qi = inflow peak flow (cfs)
VS = volume of storage (in)
Vr = volume of runoff (in)
tb = time base of the inflow hydrograph (hr) [Determined as the time
from the beginning of rise to a point on the recession limb where the
flow is 5 percent of the peak]
tp = time to peak of the inflow hydrograph (hr)
(Step 3) Multiply the peak flow rate of the inflow hydrograph, Qi,
times the potential peak flow reduction calculated from
Step 2 to obtain the estimated peak outflow rate, QO, for the
selected storage volume.
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The required storage volume can then be calculated by:
3.3.5.1 INTRODUCTION
VS = (VS/Vr)(Qd)(A)
12
The Simplified NRCS TR-55 Peak Runoff Rate Estimation approach (see Sub-
(3.3.10)
section 3.1.5.7) can be used for estimation of the Channel Protection Volume
(CPv) for storage facility design.
Where:
VS and Vr are defined above
This method should not be used for standard detention design calculations.
Qd = the developed runoff for the design storm (inches)
See either Subsection 3.3.4 or the modified rational method in Subsection
A = total drainage area (acres)
Stormwater Hydrology
3.3.5 Channel Protection Volume Estimation
3.3.6 for preliminary detention calculations without formal routing.
While the TR-55 short-cut method reports to incorporate multiple stage
3.3.5.2 BASIC APPROACH
For CPv estimation, using Figures 3.1.5-6 and 3.1.5-7 in Section 3.1, the unit
peak discharge (qU) can be determined based on Ia/P and time of concentra-
structures, experience has shown that an additional 10-15% storage is required
when multiple levels of extended detention are provided inclusive with the
25-year storm.
tion (tC). Knowing qU and T (extended detention time, typically 24 hours), the
qO/qI ratio (peak outflow discharge/peak inflow discharge) can be estimated
from Figure 3.3.5-1.
Using the following equation from TR-55 for a Type II or Type III rainfall distribution, VS/Vr can be calculated.
Note: Figure 3.3.4-1 can also be used to estimate VS/Vr.
VS/Vr = 0.682 – 1.43(qO/qI) + 1.64 (qO/qI)2 – 0.804(qO/qI)3 (3.3.9)
Where:
VS = required storage volume (acre-feet)
Vr = runoff volume (acre-feet)
qO = peak outflow discharge (cfs)
qI = peak inflow discharge (cfs)
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Other data include the following:
Computations
Compute the 100-year peak discharge for a
• Total impervious area = 18 acres
1.
Calculate rainfall excess:
50-acre wooded watershed located in Peachtree
• % of pond and swamp area = 0
City, which will be developed as follows:
»» The 100-year, 24-hour rainfall is 8.22 inches
(From NOAA Atlas 14).
• Forest land - good cover (hydrologic soil group
B) = 10 ac
»» The 1-year, 24 hour rainfall is 3.37 inches
(From NOAA Atlas 14).
• Forest land - good cover (hydrologic soil group
C) = 10 ac
• 1/3 Acre residential (hydrologic soil group B) =
20 ac
Industrial development (hydrological soil group C)
= 10 ac
»» Composite weighted runoff coefficient is:
Dev #
Area
% Total
CN
Composite CN
1
10 ac
20%
55
11.0
2
10 ac
20%
70
14.0
3
20 ac
40%
72
28.8
4
10 ac
20%
91
18.2
Total
50 ac
100%
Stormwater Hydrology
3.3.5.3 EXAMPLE PROBLEM
72
*from Equation 3.1.6, Q (100-year) = 4.89 inches
Qd (1-year developed) = 1.0 inches
2.
Calculate time of concentration
Segment
Type of Flow
Length (ft)
Slope (%)
1
Overland n=0.24
40
2.0%
2
Shallow channel
750
1.7%
3
Main channel
1100
0.5%
*For the main channel, n = .06 (estimated), width = 10 feet,
depth = 2 feet, rectangular channel
The hydrologic flow path for this watershed =
1,890 ft
Segment 1 - Travel time from equation 2.1.9 with
P2 = 3.84 in
(From NOAA Atlas 14)
Tt = [0.42(0.24 X 40)0.8] / [(3.84)0.5 (.020)0.4] = 6.26
Figure 3.3.5-1 Detention Time vs. Discharge Ratios
minutes
(Source: MDE, 1998)
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VOL 2
109
3.
equation 3.1.9
Calculate Ia/P for Cn = 72 (Table 3.1.5-1), Ia =
.778 (Table 3.1.5-3)
5.
Calculate peak discharge with Fp = 1 using
equation 3.1.12
Q100 = 650 (50/640)(4.89)(1) = 248 cfs
V = 2.1 ft/sec (from equation 3.1.9)
Tt = 750 / 60 (2.1) = 5.95 minutes
Ia/P = (.778 / 8.23 = .095
Note: Use Ia/P = .10 to facilitate use of Figure
6.
Segment 3 - Using equation 3.1.11
3.1.5-6. Straight line interpolation could also be
V = (1.49/.06) (1.43)0.67 (.005)0.5 = 2.23 ft/sec
used.
Compute runoff coefficient, Rv
Tt = 1100 / 60 (2.23) = 8.22 minutes
Calculate water quality volume (WQv)
RV = 0.50 + (IA)(0.009)
= 0.50 + (18)(0.009)
4.
tc = 6.26 + 5.95 + 8.22 = 20.43 minutes (.34
hours)
Unit discharge qu (100-year) from Figure
3.1.5-6 = 650 csm/in, qu (1-year) = 580 csm/
in
Stormwater Hydrology
Segment 2 - Travel time from Figure 3.1.5-5 or
= 0.21
Compute water quality volume, WQv
WQv = 1.2(RV)(A)/12
= 1.2(.21)(50)/12
= 1.05 acre-feet
7.
(Calculate channel protection volume (CP v =
VS )
Knowing qu (1-year) = 580 csm/in from Step 3 and
T (extended detention time of 24 hours), find qO/qI
from Figure 3.3.5-1.
qO/qI = 0.03
For a Type II rainfall distribution:
VS/Vr = 0.682 – 1.43 (qO/qI) + 1.64 (qO/qI)2 – 0.804
(qO/qI)3
= 0.682 – 1.43 (0.03) + 1.64 (0.03) – 0.804
(0.03)
= 0.64
Therefore, stream channel protection volume
with Qd (1-year developed) = 1.0 inches, from
Step 1, is CPv = VS = (VS/Vr)(Qd)(A)/12 = (0.64)(1.0)
Figure 3.3.5-2 Approximate Detention Basin Routing for Rainfall Types I, IA, II, and III
(50)/12 = 2.67 acre-feet
(Source: TR-55, 1986)
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VOL 2
110
a, b = rainfall factors dependent on
Discharge
location and return period taken from
3.3.6.1 INTRODUCTION
Table 3.3.6-1.
For drainage areas of less than 5 acres, a modification of the Rational Method can be used for
The required storage volume, in cubic feet can be
the estimation of storage volumes for detention
obtained from Equation 3.3.13.
calculations.
Vpreliminary = 60 [CAa – (2CabAQa)1/2 +
(Qa/2) (b-tc)]
(3.3.13a)
Qa
The Modified Rational Method uses the peak flow
Stormwater Hydrology
3.3.6 The Modified Rational Method
calculating capability of the Rational Method
paired with assumptions about the inflow and
tc
outflow hydrographs to compute an approximation of storage volumes for simple detention
calculations. There are many variations on the
td
Vmax = Vpreliminary * P180/Ptd
Time
Figure 3.3.6-1 Modified Rational Definitions
(3.3.13b)
Where:
Vpreliminary = preliminary required storage
Where:
approach. Figure 3.3.6-1 illustrates one appli-
Qa = allowable release rate (cfs)
(ft3)
cation. The rising and falling limbs of the inflow
Ca = predevelopment Rational Method
Vmax = required storage (ft3)
hydrograph have a duration equal to the time of
runoff coefficient
tc = time of concentration for the devel-
concentration (tc). An allowable target outflow is
i = rainfall intensity for the correspond-
oped condition (min)
set (Qa) based on pre-development conditions.
ing time of concentration (in/hr)
P180 = 3-hour (180-minute) storm depth
The storm duration is td, and is varied until the
A = area (acres)
(in)
storage volume (shaded gray area) is maximized.
Ptd = storm depth for the critical dura-
It is normally an iterative process done by hand
The critical duration of storm, the time value to
tion (in)
or on a spreadsheet. Downstream analysis is not
determine rainfall intensity, at which the storage
All other variables are as defined above
possible with this method as only approximate
volume is maximized is:
graphical routing takes place.
2CAab
−b
Td =
Qa
3.3.6.2 DESIGN EQUATIONS
The design of detention using the Modified Rational Method is presented as a noniterative approach suitable for spreadsheet calculation (Debo
& Reese, 1995).
The allowable release rate can be determined
from:
Qa = Ca i A
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(3.3.11)
The equations above include the use of an ad-
(3.3.12)
Where:
justment factor to the calculated storage volume
to account for undersizing. The factor (P180/Ptd) is
the ratio of the 3-hour storm depth for the return
frequency divided by the rainfall depth for the
Td = critical storm duration (min)
critical duration calculated in Equation 3.3.12.
Qa = allowable release rate (cfs)
The Modified Rational Method also often undersizes storage facilities in flat and more sandy areas
C = developed condition Rational Meth-
where the target discharge may be set too large,
od runoff coefficient
resulting in an oversized orifice. In these loca-
A = area (acres)
tions a C factor of 0.05 to 0.1 should be used.
VOL 2
111
Table 3.3.6-1 Rainfall Factors “a” and “b” for the Modified Rational Method
A 5-acre site is to be developed in Atlanta. Based
on site and local information, it is determined that
channel protection is not required and that limiting the 25-year and 100-year storm is also not
required. The local government has determined
that the development must detain the 2-year and
(1-year through 100-year return periods)
Return Interval
City
Albany
Atlanta
1
2
5
10
25
50
100
a
126.72
159.17
198.14
230.00
271.84
305.29
b
16.02
19.72
22.52
24.49
26.00
26.97
341.98
28.23
a
97.05
123.19
157.99
184.23
219.21
249.86
278.71
10-year storms. Rainfall values are taken from
b
12.88
15.91
18.44
19.96
21.13
22.28
23.01
NOAA Atlas 14. The following key information is
a
106.01
126.29
162.23
187.80
224.41
253.05
281.69
b
15.41
16.95
19.57
20.87
22.19
22.99
23.68
271.24
Athens
obtained:
Augusta
A = 5 acres
Slope is about 5%
Bainbridge
Pre-development tc = 21 minutes and C
Brunswick
factor = 0.22
Post-development tc = 10 minutes and C
factor = 0.80
Steps
2 - year
10 - year
tc (min)
21
21
i (in/hr)
3.34
4.51
Qa (Equation 3.3.11) (cfs)
3.67
4.96
a (from Table 3.3.6-1)
123.19
184.23
b (from Table 3.3.6-1)
15.91
19.96
Vmax (Equation 3.3.13) (ft3)
16,017
23,199
2.43
3.42
P180 (from NOAA Atlas 14) (in)
Td (Equation 3.3.12) (min)
Ptd (from NOAA Atlas 14) (in)
Vmax (Equation 3.3.13) (ft )
3
49
57
1.62
2.70
24,025
29,385
Columbus
Macon
119.32
142.78
171.04
192.10
221.48
247.98
17.05
19.12
20.34
20.96
21.40
22.10
22.32
a
128.79
171.90
215.02
245.38
291.64
329.59
367.38
b
16.39
21.13
24.33
25.87
27.73
29.12
30.26
a
177.81
191.06
233.75
266.24
314.79
352.59
367.38
b
26.30
24.13
27.51
29.49
31.77
33.16
34.22
306.45
a
113.09
142.00
177.92
205.63
246.52
273.92
b
15.67
17.87
20.34
21.88
23.63
24.11
25.13
a
111.40
139.06
176.78
203.43
242.56
272.93
306.45
b
15.48
17.68
20.55
21.94
23.47
24.38
25.59
Metro
a
93.15
116.20
148.58
171.22
201.95
227.07
254.06
Chattanooga
b
14.25
15.97
18.00
18.91
19.60
20.12
20.84
277.86
Peachtree City
Rome
Roswell
Savannah
Toccoa
Valdosta
Vidalia
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a
b
a
101.63
125.43
160.73
185.58
219.86
250.95
b
13.72
15.94
18.64
19.91
21.02
22.25
22.81
a
88.91
120.41
159.75
188.99
229.97
264.15
292.64
b
12.10
16.05
19.06
20.82
22.51
23.81
24.21
273.06
a
93.33
126.28
159.12
182.23
219.74
246.68
b
12.28
16.92
19.00
19.96
21.54
22.17
22.67
a
135.97
178.06
230.29
266.68
325.90
373.89
418.97
b
19.41
23.22
28.28
30.80
34.41
36.82
38.60
a
114.77
124.54
164.15
192.50
234.48
266.57
299.01
b
19.58
17.40
20.33
21.85
23.67
24.65
25.51
333.57
a
132.93
165.35
203.32
229.47
269.41
301.00
b
16.72
19.94
22.63
23.79
25.20
26.10
26.98
a
120.40
161.23
201.42
230.71
272.84
310.23
343.58
b
15.00
20.17
23.69
25.24
26.80
28.32
29.15
Stormwater Hydrology
3.3.6.3 EXAMPLE PROBLEM
VOL 2
112
3.4.1 Symbols and Definitions
To provide consistency within this section as well
as throughout this Manual, the symbols listed in
In some cases, the same symbol is used in existing
publications for more than one definition. Where
this occurs in this section, the symbol will be defined where it occurs in the text or equations.
Table 3.4.1-1 will be used. These symbols were
3.4.2 Primary Outlets
3.4.2.1 INTRODUCTION
Primary outlets provide the critical function of the
regulation of flow for structural stormwater controls. There are several different types of outlets
selected because of their wide use in technical
that may consist of a single stage outlet structure,
publications.
or several outlet structures combined to provide
multi-stage outlet control.
Stormwater Hydrology
3.4 Outlet Structures
Table 3.4.1-1 Symbols and Definitions
Symbol
Definition
Units
For a single stage system, the stormwater facility
A, a
Cross sectional or surface area
ft2
can be designed as a simple pipe or culvert. For
Am
Drainage area
mi
multistage control structures, the inlet is designed
D
Breadth of weir
ft
considering a range of design flows.
C
Weir coefficient
-
d
Change in elevation
ft
A stage-discharge curve is developed for the full
D
Depth of basin or diameter of pipe
ft
range of flows that the structure would experi-
g
Acceleration due to gravity
ft/s2
ence. The outlets are housed in a riser structure
H
Head on structure
ft
connected to a single outlet conduit. An alterna-
Hc
Height of weir crest above channel bottom
ft
tive approach would be to provide several pipe or
K, k
Coefficient
-
culvert outlets at different levels in the basin that
L
Length
ft
are either discharged separately or are combined
n
Manning’s n
-
to discharge at a single location.
Q, q
Peak inflow or outflow
cfs, in
Vu
Approach velocity
ft/s
This section provides an overview of outlet struc-
WQv
Water quality volume
ac-ft
ture hydraulics and design for stormwater storage
w
Maximum cross sectional bar width facing the flow
in
facilities. The design engineer is referred to an
x
Minimum clear spacing between bars
in
appropriate hydraulics text for additional infor-
Θ
Angle of v-notch
degrees
mation on outlet structures not contained in this
Θg
Angle of the grate with respect to the horizontal
degrees
section.
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2
VOL 2
113
There are a wide variety of outlet structure types, the most common of which
are covered in this section. Descriptions and equations are provided for the
(a) PIPE OR BOX CULVERT
(d) WEIR OVERFLOW SPILLWAY
following outlet types for use in stormwater facility design:
• Orifices
• Perforated risers
(b) RISER STRUCTURE
(single and multi-level outlets)
Side Elevation
• Pipes / Culverts
Stormwater Hydrology
3.4.2.2 OUTLET STRUCTURE TYPES
• Sharp-crested weirs
• Broad-crested weirs
Front Elevation
(e) SLOTTED OUTLET
• V-notch weirs
• Proportional weirs
(c) DROP INLET
• Combination outlets
Figure 3.4.2-1 Typical Primary Outlets
Each of these outlet types has a different design purpose and application:
• Water quality and channel protection flows are normally handled with
smaller, more protected outlet structures such as reverse slope pipes,
hooded orifices, orifices located within screened pipes or risers, perforated
plates or risers, and V-notch weirs.
• Larger flows, such as overbank protection and extreme flood flows, are
typically handled through a riser with different sized openings, through an
overflow at the top of a riser (drop inlet structure), or a flow over a broad
crested weir or spillway through the embankment. Overflow weirs can
also be of different heights and configurations to handle control of multiple
design flows.
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VOL 2
114
Stormwater Hydrology
3.4.2.3 ORIFICES
An orifice is a circular or rectangular opening of a
prescribed shape and size. The flow rate depends
on the height of the water above the opening and
the size and edge treatment of the orifice.
H
D
For a single orifice, as illustrated in Figure 3.4.22(a), the orifice discharge can be determined
using the standard orifice equation below.
Q = CA (2gH)0.5
(a)
(3.4.1)
Headwater
Where:
H
Q = the orifice flow discharge (cfs)
Tailwater
D
C = discharge coefficient
A = cross-sectional area of orifice or
pipe (ft2)
(b)
g = acceleration due to gravity (32.2 ft/
s2)
D = diameter of orifice or pipe (ft)
H1
H = effective head on the orifice, from
orifice center to the water surface
D
H2
H3
If the orifice discharges as a free outfall, then the
effective head is measured from the center of the
orifice to the upstream (headwater) surface ele-
(c)
vation. If the orifice discharge is submerged, then
the effective head is the difference in elevation of
the headwater and tailwater surfaces as shown in
Figure 3.4.2-2(b).
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Figure 3.4.2-2 Orifice Definition
Figure 3.4.2-3 Perforated Riser
VOL 2
115
Table 3.4.2-1 Circular Perforation Sizing
diameter, with sharp edges, a coefficient of 0.6
should be used. For square-edged entrance
conditions the generic orifice equation can be
Flow Area per Row (in2)
Minimum Column
Hole Diameter
Hole Centerline
(in)
Spacing
1 column
2 columns
3 columns
0.1
0.15
(in)
simplified:
1/4
1
0.05
5/16
2
0.08
0.15
0.23
3/8
2
0.11
0.22
0.33
7/16
2
0.15
0.3
0.45
1/2
2
0.2
0.4
0.6
9/16
3
0.25
0.5
0.75
5/8
3
0.31
0.62
0.93
be used.
11/16
3
0.37
0.74
1.11
3/4
3
0.44
0.88
1.32
Flow through multiple orifices, such as the per-
13/16
3
0.52
1.04
1.56
forated plate shown in Figure 3.4.2-2(c), can be
7/8
3
0.6
1.2
1.8
15/16
3
0.69
1.38
2.07
1
4
0.79
1.58
2.37
1 1/16
4
0.89
1.78
2.67
head, the total flow can be determined by mul-
1 1/8
4
0.99
1.98
2.97
tiplying the discharge for a single orifice by the
1 3/16
4
1.11
2.22
3.33
number of openings.
1 1/4
4
1.23
2.46
3.69
1 5/16
4
1.35
2.7
4.05
1 3/8
4
1.48
2.96
4.44
1 7/16
4
1.62
3.24
4.86
1 1/2
4
1.77
3.54
5.31
1 9/16
4
1.92
3.84
5.76
1 5/8
4
2.07
4.14
6.21
1 11/16
4
2.24
4.48
6.72
7.23
Q = 0.6A (2gH)0.5 = 3.78D2H0.5 (3.4.2)
When the material is thicker than the orifice
diameter a coefficient of 0.80 should be used. If
the edges are rounded, a coefficient of 0.92 can
computed by summing the flow through individual orifices. For multiple orifices of the same
size and under the influence of the same effective
Perforated orifice plates for the control of discharge can be of any size and configuration.
However, the Denver Urban Drainage and Flood
Control District has developed standardized
dimensions that have worked well. Table 3.4.2-1
gives appropriate dimensions. The vertical spacing between hole centerlines is always 4 inches.
For rectangular slots the height is normally 2
inches with variable width. Only one column of
rectangular slots is allowed.
1 3/4
4
2.41
4.82
1 13/16
4
2.58
5.16
7.74
1 7/8
4
2.76
5.52
8.28
1 15/16
4
2.95
5.9
8.85
2
4
3.14
6.28
9.42
Stormwater Hydrology
When the material is thinner than the orifice
Number of columns refers to parallel columns of holes
Minimum steel plate thickness
1/4”
5/16”
3/8”
Source: Urban Drainage and Flood Control District, Denver, CO
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VOL 2
116
Ap = cross-sectional area of all the holes (ft2)
wet ED pond showing the design pool elevations and the flow control mech-
Hs = distance from S/2 below the lowest row of holes to S/2 above
anisms.
the top row (ft)
3.4.2.5 PIPES AND CULVERTS
Discharge pipes are often used as outlet structures for stormwater control
facilities. The design of these pipes can be for either single or multi-stage
Stormwater Hydrology
Figure 3.4.2-4 provides a schematic of an orifice plate outlet structure for a
discharges. A reverse-slope underwater pipe is often used for water quality or
channel protection outlets.
Pipes smaller than 12 inches in diameter may be analyzed as a submerged orifice as long as H/D is greater than 1.5. Note: For low flow conditions when the
flow reaches and begins to overflow the pipe, weir flow controls (see Subsection 3.4.2.6). As the stage increases the flow will transition to orifice flow.
Pipes greater than 12 inches in diameter should be analyzed as a discharge
Figure 3.4.2-4 Schematic of Orifice Plate Outlet Structure
pipe with headwater and tailwater effects taken into account. The outlet
3.4.2.4 PERFORATED RISERS
hydraulics for pipe flow can be determined from the outlet control culvert
A special kind of orifice flow is a perforated riser as illustrated in Figure 3.4.2-3.
nomographs and procedures given in Section 5.3, Culvert Design, or by using
In the perforated riser, an orifice plate at the bottom of the riser, or in the out-
Equation 3.4.4 (NRCS, 1984).
let pipe just downstream from the elbow at the bottom of the riser, controls
the flow. It is important that the perforations in the riser convey more flow
The following equation is a general pipe flow equation that is derived through
than the orifice plate so as not to become the control.
the use of the Bernoulli and continuity principles.
Referring to Figure 3.4.2-3, a shortcut formula has been developed to estimate
Q = a[(2gH) / (1 + km + kpL)]0.5
the total flow capacity of the perforated section (McEnroe, 1988):
Where:
Q = discharge (cfs)
Q =C
2 Ap
2g H
3
2
p
s
3H
(3.4.4)
(3.4.3)
a = pipe cross sectional area (ft2)
g = acceleration of gravity (ft/s2)
H = elevation head differential (ft)
km = coefficient of minor losses (use 1.0)
Where:
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Q = discharge (cfs)
kp = pipe friction coefficient = 5087n2/D4/3
Cp = discharge coefficient for perforations (normally 0.61)
L = pipe length (ft)
VOL 2
117
A sharp-crested weir with two end con¬tractions
If the overflow portion of a weir has a sharp,
is illustrated in Figure 3.4.2-5(b). The discharge
thin leading edge such that the water springs
equation for this configuration is (Chow, 1959):
clear as it overflows, the overflow is termed a
sharp-crested weir. If the sides of the weir also
cause the through flow to contract, it is termed an
Q = [(3.27 + 0.04(H/HC)] (L - 0.2H) H1.5
(3.4.6)
end-contracted sharp-crested weir. Sharp-crest-
Where:
ed weirs have stable stage-discharge relations
Q = discharge (cfs)
and are often used as a measurement device. A
H = head above weir crest excluding
sharp-crested weir with no end contractions is
velocity head (ft)
illustrated in Figure 3.4.2-5(a). The discharge
HC = height of weir crest above channel
equation for this configuration is (Chow, 1959):
Stormwater Hydrology
3.4.2.6 SHARP-CRESTED WEIRS
bottom (ft)
L = horizontal weir length (ft)
Q = [(3.27 + 0.4(H/HC)] LH1.5
(3.4.5)
A sharp-crested weir will be affected by sub-
Where:
mergence when the tailwater rises above the
Q = discharge (cfs)
weir crest elevation. The result will be that the
H = head above weir crest excluding
discharge over the weir will be reduced. The dis-
velocity head (ft)
charge equation for a sharp-crested submerged
HC = height of weir crest above channel
weir is (Brater and King, 197
bottom (ft)
L = horizontal weir length (ft)
L
QS = Qf (1 - (H2/H1)1.5)0.385
L
(3.4.7)
Where:
QS = submergence flow (cfs)
Qf = free flow (cfs)
(a) No end contractions
H1 = upstream head above crest (ft)
(b) With end contractions
H2 = downstream head above crest (ft)
H
H1
Hc
Hc
H2
(c) Section view
Figure 3.4.2-5 Sharp-Crested Weir
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VOL 2
118
Stormwater Hydrology
3.4.2.7 BROAD-CRESTED WEIRS
A weir in the form of a relatively long raised channel control crest section is a
broad-crested weir. The flow control section can have different shapes, such
as triangular or circular. True broad-crested weir flow occurs when upstream
head above the crest is between the limits of about 1/20 and 1/2 the crest
length in the direction of flow. For example, a thick wall or a flat stop log can
act like a sharp-crested weir when the approach head is large enough that
Figure 3.4.2-6 Broad-Crested Weir
the flow springs from the upstream corner. If upstream head is small enough
relative to the top profile length, the stop log can act like a broad-crested weir
(USBR, 1997).
Table 3.4.2-2 Broad-Crested Weir Coefficient (C) Values
The equation for the broad-crested weir is (Brater and King, 1976):
Q = CLH1.5
(3.4.8)
Where:
Q = discharge (cfs)
C = broad-crested weir coefficient
L = broad-crested weir length perpendicular to flow (ft)
H = head above weir crest (ft)
Measured
Head (H)*
Weir Crest Breadth (b) in feet
In feet
0.50
0.75
1.00
1.50
2.00
2.50
3.00
4.00
5.00
10.00
15.00
0.2
2.80
2.75
2.69
2.62
2.54
2.48
2.44
2.38
2.34
2.49
2.68
0.4
2.92
2.80
2.72
2.64
2.61
2.60
2.58
2.54
2.50
2.56
2.70
0.6
3.08
2.89
2.75
2.64
2.61
2.60
2.68
2.69
2.70
2.70
2.70
0.8
3.30
3.04
2.85
2.68
2.60
2.60
2.67
2.68
2.68
2.69
2.64
1.0
3.32
3.14
2.98
2.75
2.66
2.64
2.65
2.67
2.68
2.68
2.63
1.2
3.32
3.20
3.08
2.86
2.70
2.65
2.64
2.67
2.66
2.69
2.64
2.64
1.4
3.32
3.26
3.20
2.92
2.77
2.68
2.64
2.65
2.65
2.67
1.6
3.32
3.29
3.28
3.07
2.89
2.75
2.68
2.66
2.65
2.64
2.63
1.8
3.32
3.32
3.31
3.07
2.88
2.74
2.68
2.66
2.65
2.64
2.63
2.0
3.32
3.31
3.30
3.03
2.85
2.76
2.27
2.68
2.65
2.64
2.63
2.5
3.32
3.32
3.31
3.28
3.07
2.89
2.81
2.72
2.67
2.64
2.63
3.0
3.32
3.32
3.32
3.32
3.20
3.05
2.92
2.73
2.66
2.64
2.63
3.5
3.32
3.32
3.32
3.32
3.32
3.19
2.97
2.76
2.68
2.64
2.63
contraction and if the slope of the crest is as great as the loss of head due to
4.0
3.32
3.32
3.32
3.32
3.32
3.32
3.07
2.79
2.70
2.64
2.63
friction, flow will pass through critical depth at the weir crest; this gives the
4.5
3.32
3.32
3.32
3.32
3.32
3.32
3.32
2.88
2.74
2.64
2.63
maximum C value of 3.087. For sharp corners on the broad-crested weir, a
5.0
3.32
3.32
3.32
3.32
3.32
3.32
3.32
3.07
2.79
2.64
2.63
5.5
3.32
3.32
3.32
3.32
3.32
3.32
3.32
3.32
2.88
2.64
2.63
If the upstream edge of a broad-crested weir is so rounded as to prevent
minimum C value of 2.6 should be used. Information on C values as a func-
* Measured at least 2.5H upstream of the weir.
Source: Brater and King (1976)
tion of weir crest breadth and head is given in Table 3.4.2-2.
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3.4.2.9 PROPORTIONAL WEIRS
3.4.2.10 COMBINATION OUTLETS
The discharge through a V-notch weir (Figure
Although more complex to design and construct,
Combinations of orifices, weirs and pipes can be
3.4.2-7) can be calculated from the following
a proportional weir may significantly reduce the
used to provide multi-stage outlet control for
equation (Brater and King, 1976).
required storage volume for a given site. The
different control volumes within a storage facility
proportional weir is distinguished from other
(i.e., water quality volume, channel protection
control devices by having a linear head-discharge
volume, overbank flood protection volume, and/
relationship achieved by allowing the discharge
or extreme flood protection volume).
Q = 2.5 tan (θ/2) H
(3.4.9)
2.5
Where:
area to vary nonlinearly with head. A typical proQ = discharge (cfs)
portional weir is shown in Figure 3.4.2-8. Design
They are generally of two types of combination
θ = angle of V-notch (degrees)
equations for proportional weirs are (Sandvik,
outlets: shared outlet control structures and
H = head on apex of notch (ft)
1985):
separate outlet controls. Shared outlet control is
Stormwater Hydrology
3.4.2.8 V-NOTCH WEIRS
typically a number of individual outlet openings
A
Q/2
Q = 4.97 a0.5 b (H - a/3)
(3.4.10)
(orifices), weirs or drops at different elevations on
a riser pipe or box which all flow to a common
x/b = 1 - (1/3.17) (arctan (y/a)0.5
(3.4.11)
larger conduit or pipe. Figure 3.4.2-9 shows an
example of a riser designed for a wet ED pond.
The orifice plate outlet structure in Figure 3.4.2-4
Where:
is another example of a combination outlet.
Q = discharge (cfs)
A
Dimensions a, b, H, x, and y are shown in
Separate outlet controls are less common and
Figure 3.4.2-8
may consist of several pipe or culvert outlets
at different levels in the storage facility that are
either discharged separately or are combined to
discharge at a single location.
H
The use of a combination outlet requires the
construction of a composite stage-discharge
curve (as shown in Figure 3.4.2-10) suitable for
control of multiple storm flows. The design of
multi-stage combination outlets is discussed later
Section A-A
Figure 3.4.2-7 V-Notch Weir
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in this section.
Figure 3.4.2-8 Proportional Weir Dimensions
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120
In an extended detention facility for water quality
treatment or downstream channel protection,
however, the storage volume is detained and
3.4.3.1 INTRODUCTION
released over a specified amount of time (e.g.,
Extended detention orifice sizing is required in
24-hours). The release period is a brim draw-
design applications that provide extended deten-
down time, beginning at the time of peak storage
tion for downstream channel protection or the ED
of the water quality volume until the entire cal-
portion of the water quality volume. In both cases
culated volume drains out of the basin. This as-
an extended detention orifice or reverse slope
sumes that the brim volume is present in the basin
pipe can be used for the outlet. For a structur-
prior to any discharge. In reality, however, water
al control facility providing both WQv extended
is flowing out of the basin prior to the full or brim
detention and CPv control (wet ED pond, microp-
volume being reached. Therefore, the extended
ool ED pond, and shallow ED wetland), there will
detention outlet can be sized using either of the
be a need to design two outlet orifices – one for
following methods:
the water quality control outlet and one for the
1.
Use the maximum hydraulic head associated
with the storage volume and maximum
flow, and calculate the orifice size needed
to achieve the required drawdown time, and
route the volume through the basin to verify
the actual storage volume used and the
drawdown time.
2.
Approximate the orifice size using the
average hydraulic head associated with the
storage volume and the required drawdown
time.
channel protection
(The following procedures are based on the water
Figure 3.4.2-9 Schematic of Combination Outlet
Structure
103.0
The outlet hydraulics for peak control design
(overbank flood protection and extreme flood
102.5
Elevation (ft)
Virginia Stormwater Management Handbook,
1999)
103.5
protection) is usually straightforward in that an
outlet is selected that will limit the peak flow to
102.0
Secondary Outlet
(Spillway)
101.5
Riser Capacity
101.0
100.5
100.0
quality outlet design procedures included in the
Stormwater Hydrology
3.4.3 Extended Detention (Water Quality
and Channel Protection) Outlet Design
some predetermined maximum. Since volume
These two procedures are outlined in the exam-
and the time required for water to exit the storage
ples below and can be used to size an extended
facility are not usually considered, the outlet de-
detention orifice for water quality and/or channel
Primary Outlet
sign can easily be calculated and routing proce-
protection.
Total Outflow
dures used to determine if quantity design criteria
are met.
0
5
10
15
20
25
30
Discharge (cfs)
Figure 3.4.2-10 Composite Stage-Discharge Curve
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121
should be used to determine whether the orifice size should be reduced to
A wet ED pond sized for the required water quality volume will be used here to
achieve the required 24 hours or if the actual time achieved will provide ade-
illustrate the sizing procedure for an extended-detention orifice.
quate pollutant removal.
Given the following information, calculate the required orifice size for water
3.4.3.3 METHOD 2: AVERAGE HYDRAULIC HEAD AND AVERAGE DISCHARGE
quality design.
Given: Water Quality Volume (WQv) = 0.76 ac ft = 33,106 ft3
Maximum Hydraulic Head (Hmax) = 5.0 ft (from stage vs. storage data)
(Step 1) Determine the maximum discharge resulting from the 24hour drawdown requirement. It is calculated by dividing the
Water Quality Volume (or Channel Protection Volume) by the
required time to find the average discharge, and then multi-
Using the data from the previous example (3.4.3.2) use Method 2 to calculate
the size of the outlet orifice.
Given: Water Quality Volume (WQv) = 0.76 ac ft = 33,106 ft3
Average Hydraulic Head (havg) = 2.5 ft (from stage vs storage data)
(Step 1) Determine the average release rate to release the water
quality volume over a 24-hour time period.
plying by two to obtain the maximum discharge.
Q = 33,106 ft3 / (24 hr)(3,600 s/hr) = 0.38 cfs
Qavg = 33,106 ft3 / (24 hr)(3,600 s/hr) = 0.38 cfs
Qmax = 2 * Qavg = 2 * 0.38 = 0.76 cfs
(Step 2) Determine the required orifice diameter by using the orifice
Equation (3.4.8) and the average head on the orifice:
(Step 2) Determine the required orifice diameter by using the orifice
Equation (3.4.8) and Qmax and Hmax:
Q = CA(2gH)0.5, or A = Q / C(2gH)0.5
A = 0.38 / 0.6[(2)(32.2)(2.5)]0.5 = 0.05 ft3
Q = CA(2gH)0.5, or A = Q / C(2gH)0.5
A = 0.76 / 0.6[(2)(32.2)(5.0)]0.5 = 0.071 ft3
Determine pipe diameter from
A = 3.14d2/4, then d = (4A/3.14)0.5
D = [4(0.071)/3.14]0.5 = 0.30 ft = 3.61 in
Use a 3.6-inch diameter water quality orifice.
Routing the water quality volume of 0.76 ac ft through the 3.6-inch water
quality orifice will allow the designer to verify the drawdown time, as well as
the maximum hydraulic head elevation. The routing effect will result in the
Stormwater Hydrology
3.4.3.2 METHOD 1: MAXIMUM HYDRAULIC HEAD WITH ROUTING
Determine pipe diameter from
A = 3.14r2 = 3.14d2/4, then d = (4A/3.14)0.5
D = [4(0.05)/3.14]0.5 = 0.252 ft = 3.03 in
Use a 3-inch diameter water quality orifice.
Use of Method 1, utilizing the maximum hydraulic head and discharge and
routing, results in a 3.6-inch diameter orifice (though actual routing may result
in a changed orifice size) and Method 2, utilizing average hydraulic head and
average discharge, results in a 3.0-inch diameter orifice.
actual drawdown time being less than the calculated 24 hours. Judgment
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122
(Step 3)
Design Water Quality Outlet.
Design the water quality extended detention (WQv-ED) orifice
3.4.4.1 INTRODUCTION
using either Method 1 or Method 2 outlined in Subsection
A combination outlet such as a multiple orifice plate system or multi-stage ris-
3.4.3. If a permanent pool is incorporated into the design of
er is often used to provide adequate hydraulic outlet controls for the different
the facility, a portion of the storage volume for water quality
design requirements (e.g., water quality, channel protection, overbank flood
will be above the elevation of the permanent pool. The outlet
protection, and/or extreme flood protection) for stormwater ponds, stormwa-
can be protected using either a reverse slope pipe, a hooded
ter wetlands and detention-only facilities. Separate openings or devices at dif-
protection device, or another acceptable method (see Sub-
ferent elevations are used to control the rate of discharge from a facility during
section 3.4.5).
Stormwater Hydrology
3.4.4 Multi-Stage Outlet Design
multiple design storms. As discussed in the previous section, Figures 3.4.2-4
and 3.4.2-9 are examples of multi-stage combination outlet systems.
(Step 4)
Design Channel Protection Outlet.
Design the stream channel protection extended detention
A design engineer may be creative to provide the most economical and
outlet (CPv-ED) using either method from Subsection 3.4.3.
hydraulically efficient outlet design possible in designing a multi-stage outlet.
For this design, the storage needed for channel protection will
Many iterative routings are usually required to arrive at a minimum structure
be “stacked” on top of the water quality volume storage eleva-
size and storage volume that provides proper control. The stage-discharge
tion determined in Step 3. The total stage-discharge rating
table or rating curve is a composite of the different outlets that are used for
curve at this point will include water quality control orifice
different elevations within the multi-stage riser (see Figure 3.4.2-10).
and the outlet used for stream channel protection. The outlet
should be protected in a manner similar to that for the water
quality orifice.
3.4.4.2 MULTI-STAGE OUTLET DESIGN PROCEDURE
Below are the steps for designing a multi-stage outlet. Note that if a structural
control facility will not control one or more of the required storage volumes
(WQv, CPv, Qp25, and Qf), then that step in the procedure is skipped
(Step 1)
Determine Stormwater Control Volumes.
Using the procedures from Sections 3.1 and 3.3, estimate the
required storage volumes for water quality treatment (WQv),
channel protection (CPv), and overbank flood control (Qp25)
and extreme flood control (Qf).
(Step 2)
Develop Stage-Storage Curve.
Using the site geometry and topography, develop the
stage-storage curve for the facility in order to provide suffi-
(Step 5)
Design Overbank Flood Protection Outlet.
The overbank protection volume is added above the water
quality and channel protection storage. Establish the Qp25
maximum water surface elevation using the stage-storage
curve and subtract the CPv elevation to find the 25-year
maximum head. Select an outlet type and calculate the initial
size and geometry based upon maintaining the predevelopment 25-year peak discharge rate. Develop a stage-discharge
curve for the combined set of outlets (WQv, CPv and Qp25).
This procedure is repeated for control (peak flow attenuation)
of the 100-year storm (Qf), if required.
cient storage for the control volumes involved in the design.
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Perform a hydraulic analysis of the multi-stage outlet struc-
the outlet devices and emergency spillway to ensure the
hydraulics of the system will operate as designed.
ture using reservoir routing to ensure that all outlets will
function as designed. Several iterations may be required to
(Step 8) Design Outlet Protection.
calibrate and optimize the hydraulics and outlets that are
Design necessary outlet protection and energy dissipation
used. Also, the structure should operate without excessive
facilities to avoid erosion problems downstream from outlet
surging, noise, vibration, or vortex action at any stage. This
devices and emergency spillway(s). See Section 5.5, Energy
usually requires that the structure have a larger cross-sec-
Dissipation Design, for more information.
Stormwater Hydrology
(Step 6) Check Performance of the Outlet Structure.
tional area than the outlet conduit.
(Step 9) Perform Buoyancy Calculations.
The hydraulic analysis of the design must take into account
Perform buoyancy calculations for the outlet structure and
the hydraulic changes that will occur as depth of storage
footing. Flotation will occur when the weight of the struc-
changes for the different design storms. As shown in Figure
ture is less than or equal to the buoyant force exerted by the
3.4.4-1, as the water passes over the rim of a riser, the riser
water.
acts as a weir. However, when the water surface reaches a
certain height over the rim of a riser, the riser will begin to
(Step 10) Provide Seepage Control.
act as a submerged orifice. The designer must compute the
Seepage control should be provided for the outflow pipe or
elevation at which this transition from riser weir flow control
culvert through an embankment. The two most common
to riser orifice flow control takes place for an outlet where
devices for controlling seepage are (1) filter and drainage
this change in hydraulic conditions will change. Also note
diaphragms and (2) anti-seep collars.
in Figure 3.4.4-1 that as the elevation of the water increases
further, the control can change from barrel inlet flow control
to barrel pipe flow control. Figure 3.4.4-2 shows another
condition where weir flow can change to orifice flow, which
must be taken into account in the hydraulics of the rating
curve as different design conditions results in changing water
surface elevations.
(Step 7) Size the Emergency Spillway.
It is recommended that all stormwater impoundment structures have a vegetated emergency spillway (see Subsection
3.4.6). An emergency spillway provides a degree of safety to
prevent overtopping of an embankment if the primary outlet
or principal spillway should become clogged, or otherwise
inoperative. The 100-year storm should be routed through
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Figure 3.4.4-1 Riser Flow Diagrams
(Source: VDCR, 1999)
VOL 2
124
• Internal orifice protection through the use of an
over-perforated vertical stand pipe with ½-inch
orifices or slots that are protected by wirecloth
and a stone filtering jacket (see Figure 3.4.5-4).
Figure 3.4.4-2 Weir and Orifice Flow
(Source: VDCR, 1999)
Stormwater Hydrology
• The use of a hooded outlet for a stormwater
pond or wetland with a permanent pool (see
Figures 3.4.5-2 and 3.4.5-3).
• Internal orifice protection through the use of
an adjustable gate valves can to achieve an
equivalent orifice diameter.
3.4.5 Extended Detention Outlet Protection
Small, low flow orifices such as those used for
extended detention applications can easily clog,
preventing the structural control from meeting its
design purpose(s) and potentially causing adverse
impacts. Therefore, extended detention orifices
need to be adequately protected from clogging.
There are a number of different anti-clogging
designs, including:
• The use of a reverse slope pipe attached to a
riser for a stormwater pond or wetland with a
permanent pool (see Figure 3.4.5-1). The inlet
is submerged 1 foot below the elevation of the
permanent pool to prevent floatables from
clogging the pipe and to avoid discharging
warmer water at the surface of the pond.
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Stormwater Hydrology
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Figure 3.4.5-1 Reverse Slope Pipe Outlet
Figure 3.4.5-3 Half-Round CMP Orifice Hood
Figure 3.4.5-2 Hooded Outlet
Figure 3.4.5-4 Internal Control for Orifice Protection
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126
An example of trash racks used on a riser outlet
The inclined vertical bar rack is most effective for
structure is shown in Figure 3.4.6-1. Trash rack
lower stage outlets. Debris will ride up the trash
3.4.6.1 INTRODUCTION
design should be based on the effective opening
rack as water levels rise. This design also allows
The susceptibility of larger inlets to clogging by
of the trash rack compared to the orifice, or outlet
for removal of accumulated debris with a rake
debris and trash needs to be considered when
size. The outlet size should be the controlling or
while standing on top of the structure.
estimating their hydraulic capacities. In most
smaller open area, when compared to the effec-
instances trash racks will be needed. Trash racks
tive open area of the trash rack.
and safety grates are a critical element of out-
Stormwater Hydrology
3.4.6 Trash Racks and Safety Grates
let structure design and serve several important
functions:
• Keeping debris away from the entrance to
the outlet works where they will not clog the
critical portions of the structure
• Capturing debris in such a way that relatively
easy removal is possible
• Ensuring that people and large animals are kept
out of confined conveyance and outlet areas
• Providing a safety system that prevents anyone
from being drawn into the outlet and allows
them to climb to safety
When designed properly, trash racks serve these
purposes without interfering significantly with
the hydraulic capacity of the outlet (or inlet in
the case of conveyance structures) (ASCE, 1985;
Allred-Coonrod, 1991). The location and size of
the trash rack depends on a number of factors,
including head losses through the rack, structural
convenience, safety and size of outlet. Well-designed trash racks can also have an aesthetically
pleasing appearance.
Figure 3.4.6-1 Example of Various Trash Racks Used on a Riser Outlet Structure
(Source: VDCR, 1999)
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127
low the ground level to minimize clogging due to
Trash racks must be large enough such that partial
sedimentation. Depressing the outlet bottom to a
plugging will not adversely restrict flows reach-
depth below the ground surface at least equal to
Metcalf & Eddy (1972) give the following equation
ing the control outlet. There are no universal
the diameter of the outlet is recommended.
(based on German experiments) for losses. Grate
guidelines for the design of trash racks to protect
be given to allow for comparison.
openings should be calculated assuming a certain
detention basin outlets, although a commonly
Trash racks at entrances to pipes and conduits
percentage blockage as a worst case to deter-
used “rule-of-thumb” is to have the trash rack
should be sloped at about 3H:1V to 5H:1V to
mine losses and upstream head. Often 40 to 50%
area at least ten times larger than the control
allow trash to slide up the rack with flow pressure
is chosen as a working assumption.
outlet orifice.
and rising water level—the slower the approach
The surface area of all trash racks should be
of-thumb are found in literature. Figure 3.4.6-2
Hg = Kg1 (w/x)4/3 (Vu2/2g) sin θg
(3.4.12)
maximized and the trash racks should be located
gives opening estimates based on outlet diameter
Where:
a suitable distance from the protected outlet to
(UDFCD, 1992). Judgment should be used in that
Hg = head loss through grate (ft)
avoid interference with the hydraulic capacity of
an area with higher debris (e.g., a wooded area)
Kg1 = bar shape factor:
the outlet. The spacing of trash rack bars must
may require more opening space.
flow, the flatter the angle. Rack opening rules-
2.42 - sharp edged rectangular
1.83 - rectangular bars with semicir-
be proportioned to the size of the smallest outlet
protected. However, where a small orifice is pro-
The bar opening space for small pipes should
cular upstream
vided, a separate trash rack for that outlet should
be less than the pipe diameter. For larger diam-
1.79 - circular bars
be used, so that a simpler, sturdier trash rack with
eter pipes, openings should be 6 inches or less.
1.67 - rectangular bars with semi-
more widely spaced members can be used for
Collapsible racks have been used in some places
circular up- and down¬stream
the other outlets. Spacing of the rack bars should
if clogging becomes excessive or a person be-
faces
be wide enough to avoid interference, but close
comes pinned to the rack.
w = maximum cross-sectional bar width
facing the flow (in)
enough to provide the level of clogging protecAlternately, debris for culvert openings can be
x = minimum clear spacing between bars
caught upstream from the opening by using pipes
(in)
To facilitate removal of accumulated debris and
placed in the ground or a chain safety net (USBR,
Vu = approach velocity (ft/s)
sediment from around the outlet structure, the
1978; UDFCD, 1992). Racks can be hinged on top
θg = angle of the grate with respect to
racks should have hinged connections. If the rack
to allow for easy opening and cleaning.
the horizontal (degrees)
tion required.
Stormwater Hydrology
3.4.6.2 TRASH RACK DESIGN
is bolted or set in concrete it will preclude removal of accumulated material and will eventually
The control for the outlet should not shift to the
adversely affect the outlet hydraulics.
grate, nor should the grate cause the headwater to rise above planned levels. Therefore head
Since sediment will tend to accumulate around
losses through the grate should be calculated. A
the lowest stage outlet, the inside of the outlet
number of empirical loss equations exist though
structure for a dry basin should be depressed be-
many have difficult to estimate variables. Two will
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Stormwater Hydrology
The Corps of Engineers (HDC, 1988) has developed curves for trash racks
based on similar and additional tests. These curves are for vertical racks but
presumably they can be adjusted, in a manner similar to the previous equation,
through multiplication by the sine of the angle of the grate with respect to the
horizontal.
Hg = Kg2Vu2
2g
(3.4.13)
Where Kg2 is defined from a series of fit curves as:
• Sharp edged rectangular (length/thickness = 10)
Kg2 = 0.00158 - 0.03217 Ar + 7.1786 Ar2
• Sharp edged rectangular (length/thickness = 5)Kg2
= -0.00731 + 0.69453 Ar + 7.0856 Ar2
• Round edged rectangular (length/thickness = 10.9)
Kg2 = -0.00101 + 0.02520 Ar + 6.0000 Ar2
• Circular cross section
Kg2 = 0.00866 + 0.13589 Ar + 6.0357 Ar2
Figure 3.4.6-2 Minimum Rack Size vs. Outlet Diameter
(Source: UDCFD, 1992)
• Ar is the ratio of the area of the bars to the area of the grate section.
3.4.7 Secondary Outlets
3.4.7.1 INTRODUCTION
The purpose of a secondary outlet (emergency spillway) is to provide a controlled overflow for flows in excess of the maximum design storm for a storage
facility. Figure 3.4.7-1 shows an example of an emergency spillway.
In many cases, on-site stormwater storage facilities do not warrant elaborate
studies to determine spillway capacity. While the risk of damage due to failure
is a real one, it normally does not approach the catastrophic risk involved
in the overtopping or breaching of a major reservoir. By contrast, regional
facilities with homes immediately downstream could pose a significant hazard
if failure were to occur, in which case emergency spillway considerations are a
major design factor.
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Stormwater Hydrology
3.4.7.2 EMERGENCY SPILLWAY DESIGN
Emergency spillway designs are open channels,
usually trapezoidal in cross section, and consist
of an inlet channel, a control section, and an exit
channel (see Figure 3.4.7-1). The emergency
spillway is proportioned to pass flows in excess of
the design flood (typically the 100-year flood or
greater) without allowing excessive velocities and
without overtopping of the embankment. Flow
in the emergency spillway is open channel flow
(see Section 5.4, Open Channel Design, for more
information). Normally, it is assumed that critical
depth occurs at the control section.
NRCS (NRCS TR-55) manuals provide guidance
for the selection of emergency spillway characteristics for different soil conditions and different
types of vegetation. The selection of degree of
retardance for a given spillway depends on the
vegetation. Knowing the retardance factor and
the estimated discharge rate, the emergency
spillway bottom width can be determined. For
erosion protection during the first year, assume
minimum retardance. Both the inlet and exit
channels should have a straight alignment and
grade. Spillway side slopes should be no steeper
the 3:1 horizontal to vertical.
The most common type of emergency spillway
used is a broad-crested overflow weir cut through
original ground next to the embankment. The
transverse cross section of the weir cut is typically trapezoidal in shape for ease of construction.
Such an excavated emergency spillway is illustrated to the right.
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Figure 3.4.7-1 Emergency Spillway
(Source: VDCR, 1999)
VOL 2
130
NOAA. 1997. Five- to 60-Minute Precipitation
U.S. Department of Transportation, Federal
Battiata, Joseph, Kelly Collins, David Hirschman,
Frequency for the Eastern and Central United
Highway Administration. 1984. Hydrology.
and Greg Hoffman. The Runoff Reduction
States, NOAA Technical Memo NWS HYDRO-35.
Hydraulic Engineering Circular No. 19.
Method. Center for Watershed Protection,
National Oceanic and Atmospheric Administration
Mechanicsville, VA.
(NOAA).
Brater, Ernist F., Horace W. King. 1976. Handbook
NOAA. 2013. Atlas 14, Version 9. National Oceanic
of Hydraulics. 6th ed. NY: McGraw Hill Book Co.
and Atmospheric Administration (NOAA).
USGS. 2011. Flood-frequency Relations for Urban
Streams in Georgia. Water Resources Investigation
Report. U.S. Geological Survey.
Stormwater Hydrology
References
USGS. 2003. Techniques for Estimating Magnitude
Chow, C. N. 1959. Open Channel Hydraulics. New
Pitt, Robert. 1994. Small Storm Hydrology.
and Frequency of Floods in Rural Basins of
York: McGraw Hill Book Company.
Unpublished report. Department of Civil
Georgia. Water Resources Investigation Report.
Engineering, University of Alabama. Birmingham,
U.S. Geological Survey.
Debo, Thomas N. and Andrew J. Reese. 1995.
Alabama.
Virginia Department of Conservation and
Municipal Stormwater Management. Lewis
Publishers: CRC Press, Inc., Boca Raton, Florida.
Ferguson, Bruce K. and Thomas Neil Debo. 1990.
Sandvik, A. 1985. Proportional Weirs for
Recreation. 1999. Virginia Stormwater
Stormwater Pond Outlets. Civil Engineering,
Management Handbook.
March 1985, ASCE pp. 54-56.
Wycoff, Ronald L. and Udai P. Singh. 1976.
On-site Stormwater Management.
Hershfield, David M. 1961. Rainfall Frequency
Urban Drainage and Flood Control District. 1999.
Preliminary Hydrologic Design of Small Flood
Criteria Manual. Denver, CO.
Detention Reservoirs. Water Resources Bulletin.
Vol. 12, No. 2, pp 337-47.
Atlas of the United States, Technical Paper No. 40.
Engineering Division, Soil Conservation Service
U.S. Department of Agriculture, Natural Resource
U.S. Department of Agriculture. Washington D.C.
Conservation Service, Engineering Division. 2004.
NRCS National Engineering Handbook 630.
Maryland Department of the Environment. 2000.
Natural Resource Conservation Service (NRCS).
Maryland Stormwater Design Manual, Volumes
I and II. Center for Watershed Protection (CWP),
U.S. Department of Agriculture, Natural Resource
Ellicott City, MD.
Conservation Service, Engineering Division. 1986.
Urban Hydrology for Small Watersheds. Technical
McEnroe, B.M., J.M. Steichen and R.M. Schweiger.
Release 55. Natural Resource Conservation
1988. Hydraulics of Perforated Riser Inlets for
Service (NRCS).
Underground Outlet Terraces. Trans ASAE, Vol. 31,
No. 4, 1988.
U.S. Department of the Interior. 1997. Water
Measurement Manual. 3rd ed. Bureau of
Reclamation. Washington D.C.
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4. Stormwater Best Management Practices
4.1.1 Best Management Practices
4.1.1.1 INTRODUCTION
Stormwater Best Management Practices (BMPs)
• Provide for Extreme Flood Protection by either:
(1) control of the peak discharge increase from
the 100-year, 24-hour storm event, Qf, through
detention; or (2) safely pass Qf through the
structural control and allow it to discharge into
a receiving water whose protected floodplain
is sufficiently sized to account for extreme flow
increases without causing damage.
are engineered facilities designed to reduce and/
or treat stormwater runoff, which mitigate the
effects of increased stormwater runoff peak rate,
volume, and velocity due to urbanization. This
section provides an overview of BMPs that can be
used to address the minimum stormwater management standards outlined in Section 2.2.
In terms of the Unified Stormwater Sizing Crite-
Localities choosing to adopt the GSMM as a
guidance document may have more stringent
local requirements for Flood Protection, such
as detention of the post-development 50-year,
24-hour storm peak discharge rate to the predevelopment rate. These localities may wish to
develop their own Technical Reference Sections
(or Manual).
Stormwater Best Management Practices
4.1 Stormwater Best Management Practices Overview
ria, a BMP, or treatment train that includes two or
more BMPs, should be designed to meet one or
more of the following requirements:
• Reduce total runoff from the drainage area
using the Runoff Reduction Volume, RRv (the
runoff generated by a target rainfall event);
• Treat the Water Quality Volume, WQv (the
runoff generated by first 1.2 inches of rainfall);
4.1.1.2 TYPES OF BEST MANAGEMENT PRACTICES
Table 4.1.1-1 lists many types of BMPs. These
BMPs are recommended for use in a wide variety
of applications. A detailed discussion of each of
the BMPs, as well as design criteria and procedures for each, can be found in Sections 4.2
through 4.29.
• Control the Channel Protection Volume, CP v
(24 hours of extended detention for the oneyear, 24-hour rainfall event), where necessary
or required;
• Control for Overbank Flood Protection, Qp25
(detention of the post-development 25-year,
24-hour storm peak discharge rate to the predevelopment rate), where required; and
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132
BMP
Description
BMP
Description
Bioretention Areas
Bioretention areas are shallow stormwater basins
or landscaped areas that utilize engineered soils
and vegetation to capture and treat stormwater
runoff. Bioretention areas may be designed with an
underdrain that returns runoff to the conveyance
system or designed without an underdrain to exfiltrate runoff into the soil.
Grass Channels
Grass channels are vegetated open channels that provide
“biofiltering” of stormwater runoff as it flows across the
grass surface.
Gravity Oil / Grit
Separators
Gravity oil / grit separators are hydrodynamic controls
that use the movement of stormwater runoff through a
specially-designed structure to remove target pollutants.
They are typically used on smaller, impervious, commercial sites and urban hotspots.
Green Roofs
Green roofs represent an alternative to traditional impervious roof surfaces and typically consist of underlying
water proofing, drainage systems, and an engineered
planting media. Stormwater runoff is captured and temporarily stored in the engineered planting media, where it
is subjected to evaporation and transpiration before being
conveyed back into the storm drain system. There are
two different types of green roof systems. Intensive green
roofs have a thick layer of soil, can support a diverse
plant community, and may include trees. Extensive green
roofs have a much thinner layer of soil that is comprised
primarily of drought tolerant vegetation.
Infiltration Practices
An infiltration practice is a shallow excavation, typically filled with stone or an engineered soil mix, which is
designed to temporarily hold stormwater runoff until it
infiltrates into the surrounding soils. Infiltration practices are able to reduce stormwater quantity, recharge the
groundwater, and reduce pollutant loads.
Multi-Purpose Detention
Basins
Multi-purpose detention basins are on-site areas used for
one or more specific activities, such as parking lots and
rooftops, as well as for the temporary storage of runoff.
Organic Filters
Organic filters are surface media filters that use organic
materials, such as leaf compost or a peat/sand mixture,
as the filter media. Runoff is filtered through the media
prior to discharging through an underdrain system. The
organic media may be able to provide enhanced removal
of some contaminants, such as heavy metals.
Bioslopes
Bioslopes are linear, non-structural BMPs with a
permeable media that allows stormwater runoff
to infiltrate and filter through the practice before
exiting through an underdrain. Generally, a pretreatment device, such as filter strip, grass shoulder,
or pea gravel diaphragm, is placed upstream of the
bioslope to capture sediment and debris.
Downspout Disconnects
A downspout disconnect spreads rooftop runoff
from individual downspouts across lawns, vegetated areas, and other pervious areas, where the
runoff is slowed, filtered, and can infiltrate into the
native soils.
Dry Detention / Dry
Extended Detention Basins
Dry detention basins and dry extended detention
(ED) basins are surface facilities intended to provide
temporary storage of stormwater runoff to reduce
downstream water quantity impacts.
Dry Wells
Dry wells are shallow excavations, typically filled
with stone, that are designed to intercept and temporarily store post-construction stormwater runoff
under the ground surface until it infiltrates into
the underlying and surrounding soils. If properly
designed, they can provide significant reductions
in post-construction stormwater runoff rates, volumes, and pollutant loads on development sites.
Enhanced Dry or Wet
Swales
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Enhanced swales are vegetated open channels that
are designed and constructed to capture and treat
stormwater runoff within dry or wet cells formed by
check dams or other structures.
Stormwater Best Management Practices
Table 4.1.1-1 Best Management Practices
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133
BMP
Description
BMP
Description
Permeable Paver Systems
A permeable paver system is a pavement surface composed
of structural units with void areas that are filled with pervious
materials such as gravel, sand, or grass turf. The system is
installed over a gravel base course that provides structural
support and stores stormwater runoff that infiltrates through
the system into underlying permeable soils.
Sand Filter:
»» Surface Sand Filter
Sand filters are multi-chamber structures designed to
treat stormwater runoff through filtration, using a sand
bed as its primary filter media. Filtered runoff may be
returned to the conveyance system through an underdrain system, or allowed to partially exfiltrate into the soil.
A surface sand filter is a ground-level open air structure
that consists of a pretreatment sediment forebay and a
filter bed chamber. A perimeter sand filter is an enclosed
system typically just below the ground in a vault along
the edge of an impervious area such as a parking lot.
Underground sand filters are sand filter systems located
in an underground vault.
Pervious Concrete
Pervious concrete is the term for a mixture of coarse aggregate, portland cement, and water that allows for rapid
infiltration of water. The concrete overlays a stone aggregate
reservoir that provides temporary storage as runoff infiltrates
into underlying permeable soils and/or out through an underdrain system.
Porous Asphalt
Porous asphalt is asphalt with large void spaces to allow
water to drain through it. Porous asphalt allows water to
infiltrate into the subsoil through the paved surface and a
base, aggregate layer that acts as both a structural layer and
container to temporarily hold water. Porous asphalt is generally used on sidewalks, bicycle paths, or roads with low traffic
volumes.
Proprietary Systems
Proprietary controls are manufactured structural control
systems available from commercial vendors that are designed
to treat stormwater runoff and/or provide water quantity
control. Proprietary systems often can be used on small sites
and in space-limited areas.
Rainwater Harvesting
Rainwater harvesting is a common stormwater management
practice used to catch rainfall and store it for later use. Typically, gutters and downspout systems are used to collect the
water from roof tops and direct it to a storage tank. Rainwater
harvesting systems can be either above or below the ground.
Once captured in the storage tank, the water may be used for
non-potable indoor (requires treatment) and outdoor uses.
Regenerative Stormwater
Conveyance
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A regenerative stormwater conveyance (RSC) is a practice
that provides treatment, infiltration, and conveyance to
stormwater runoff through a combination of pools, riffles
(with either cobble rocks or boulders), native vegetation, an
underlying sand layer, and wood chips. RSCs can also be
used to repair areas with large amounts of erosion.
»» Perimeter Sand Filter
»» Underground Sand
Filter
Site Reforestation /
Revegetation
Site reforestation/revegetation is a process of planting
trees, shrubs and other native vegetation in disturbed
pervious areas to restore the area to pre-development or
better conditions. The process can be used to establish
mature native plant communities, such as forests, in pervious areas that have been disturbed by clearing, grading
and other land disturbing activities. These plant communities intercept rainfall and slow and filter the stormwater
runoff to improve infiltration in the ground. Areas that
have been reforested or revegetated should be maintained in an undisturbed, natural state over time. These
areas must be designated as conservation areas and
protected in perpetuity through a legally enforceable
conservation instrument (e.g., conservation easement,
deed restriction).
Soil Restoration
Soil restoration is the process of tilling and adding
compost and other amendments to soils to restore them
to their pre-development conditions. This improves the
soil’s ability to reduce post-construction stormwater
runoff rates, volumes and pollutant loads. This process
is ideal for areas that have been disturbed by clearing,
grading and other land disturbing activities. This process
is generally used in conjunction with other practices
including, but not limited to, vegetated filter strips, grass
channels, and simple downspout disconnections.
Stormwater Best Management Practices
Table 4.1.1-1 Best Management Practices (continued)
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134
BMP
Description
Stormwater Planters / Tree
Boxes
Stormwater planters are similar to bioretention areas
in their design purpose to detain, filter, and infiltrate
stormwater. In addition, stormwater planters utilize native or non-invasive flowers, shrubs and trees to provide
aesthetic qualities to the site. Planters and tree boxes
receive stormwater from a variety of sources such as,
rooftops, downspouts and runoff from streets.
Stormwater Ponds:
»» Wet Pond
»» Wet Extended
Detention Pond
»» Micropool Extended
Detention Pond
»» Multiple Pond
Systems
Stormwater Wetlands:
»» Shallow Wetland
»» Extended Detention
Shallow Wetland
»» Pond/Wetland
Systems
»» Pocket Wetland
Stormwater ponds are constructed stormwater retention basins that have a permanent pool (or micropool)
of water. Some runoff reduction is achieved within a
stormwater pond or detention system through evaporation and transpiration. Stormwater ponds provide water
quality treatment through sediment precipitation in the
permanent pool.
Innovative technologies should be allowed and encouraged providing there is
sufficient documentation as to their effectiveness and reliability. Communities
can allow the use of controls not included in this Manual at their discretion,
but should not do so without independently derived information concerning
performance, maintenance, application requirements, and limitations.
More specifically, new BMP designs will not be accepted for inclusion in the
Manual until independent pollutant removal performance monitoring data
determine that the practice can meet the TSS and other selected pollutant
concentration removal targets, and that the practice conforms with all necessary criteria for treatment, maintenance, and environmental impact.
Stormwater wetlands are constructed wetland systems
used for stormwater management. Stormwater wetlands
consist of a combination of shallow marsh areas, open
water, and semi-wet areas above the permanent water
surface. As stormwater runoff flows through a wetland,
it is treated, primarily through gravitational settling and
biological uptake.
Submerged Gravel
Wetlands
Submerged gravel wetland systems use wetland plants
in submerged gravel or crushed rock media to remove
stormwater pollutants. An anaerobic zone is created
within the gravel wetland that provides additional treatment for contaminants.
Underground Detention
Underground detention tanks and vaults provide temporary storage of stormwater runoff for space-limited areas where there is not adequate land for a dry detention
basin or multi-purpose detention area.
Vegetated Filter Strip
Vegetated filter strips are uniformly graded and densely
vegetated sections of land that provide “biofiltering” of
stormwater runoff as it flows across the surface.
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4.1.1.3 USING OTHER OR NEW BEST MANAGEMENT PRACTICES
Stormwater Best Management Practices
Table 4.1.1-1 Best Management Practices (continued)
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135
accordance with recommended specifications in
this Manual.
Best Management Practices (BMPs) are intended
to provide water quality treatment for stormwater
Where the pollutant removal capabilities of an
runoff. Though each of these practices provides
individual BMP are not deemed sufficient for a
pollutant removal capabilities, the relative capabil-
given site application, additional practices may
ities vary between BMPs and for different pollut-
be used in series in a “treatment train” approach.
ant types.
More details on using BMPs in series are provided
in Subsection 4.1.6.1.
• Runoff Reduction Capability
• Water Quality Performance
• Site Applicability
• Cost Considerations - both capital and
maintenance
In addition, for a given site, the following factors
should be considered and any specific design
Pollutant removal capabilities for a given BMP
are based on a number of factors including the
For additional information and data on the range
criteria or restrictions need to be evaluated:
physical, chemical and/or biological processes
of pollutant removal capabilities for various
• Physiographic Factors
that take place in the practice and the design and
BMPs, refer to the National Pollutant Removal
size of the facility. In addition, pollutant removal
Performance Database (3rd Edition) available at
efficiencies for the same BMP type and facility
www.cwp.org and the National Stormwater Best
design can vary widely depending on the tributary
Management Practices (BMP) Database at www.
land use and area, incoming pollutant concentra-
bmpdatabase.org.
tions, rainfall pattern, time of year, maintenance
• Soils
• Special Watershed or Stream Considerations
Finally, environmental regulations that may
4.1.3 Best Management Practice Selection
influence the location of a BMP on-site, or may
pollutant removal performance of the various
4.1.3.1 BMP SCREENING PROCESS
permitting information.
structural control options, Table 4.1.3-1 provides
Outlined below is a screening process for BMPs.
design removal efficiencies for each of the above
This process is intended to assist the site design-
listed BMPs. It should be noted that these values
ers and engineers in selecting the most appro-
are conservative average pollutant reduction
priate BMPs for a development site, and provides
percentages for design purposes derived from
guidance on factors to consider in their location.
frequency, and numerous other factors.
To assist designers in evaluating the relative
sampling data, modeling, and professional judgment. A BMP design may be capable of exceeding
In general the following five criteria should be
these performances; however, the values in the
evaluated in order to select the appropriate struc-
table are minimum reasonable values that can
tural control(s) or group of controls for a devel-
be assumed to be achieved when the practice is
opment:
sized, designed, constructed, and maintained in
• Stormwater Management and Treatment
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Stormwater Best Management Practices
4.1.2 Best Management Practice Pollutant Removal Capabilities
require a permit, need to be considered. See
Section 2.4 for additional information regarding
4.1.3.2 DESIGN PROCESS
Using Table 4.1.3-1, the site designer can evaluate and screen the overall applicability of BMPs,
as well as the constraints of the site in question.
Following are details of the various screening
categories and individual characteristics used to
evaluate structural controls.
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136
The first group of columns in Table 4.1.3-1 examines the capability of each BMP option to provide
runoff reduction. The second set of columns
references water quality treatment, downstream
channel protection, overbank flood protection, extreme flood protection, and provides an
overview of the pollutant removal performance
of each BMP option when designed, constructed, and maintained according to the criteria and
specifications in this Manual. The presence of a
check mark indicates that the structural control
can be used to meet a unified stormwater sizing
criterion. An ’X’ entry in the table means that the
practice cannot or is not typically used to meet
a unified stormwater sizing criterion. This does
not necessarily mean that it should be eliminated
from consideration, but rather is a reminder that
more than one BMP may be needed at a site (e.g.,
a bioretention area used in conjunction with dry
detention storage). A ‘=’ entry indicates that a BMP
may be able to meet the Unified Sizing Criterion
depending upon size, configuration, and design
constraints.
• Runoff Reduction Volume (RRv) – Indicates
whether a BMP reduces and/or removes a
portion of the Runoff Reduction Volume (RRv)
through storage, infiltration, exfiltration, and
root uptake. Runoff reduction percentages are
listed in Table 4.1.2.2 BMP Runoff Reduction
Credits.
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• Water Quality Volume (WQv) – Indicates
whether a BMP provides treatment of the water
quality volume (WQv).
• Channel Protection (CP v) – Indicates whether
the BMP can be used to provide extended
detention of the channel protection volume
(CP v).
• Overbank Flood Protection (Qp25) – Indicates
whether a BMP can be used to meet the
overbank flood protection criteria.
• Extreme Flood Protection (Qf) – Indicates
whether a BMP can be used to meet the
extreme flood protection criteria.
present in stormwater runoff from designated
hotspots, or where subsurface soil and/or
groundwater contamination may be present.
Examples of hotspots include: gas stations,
convenience stores, marinas, public works
storage areas, vehicle service and maintenance
areas, commercial nurseries, and auto recycling
facilities.
Please see the specific design criteria of each BMP
for more information on the capability of each
practice to treat runoff from designated hotspots.
Stormwater Best Management Practices
Stormwater Management and Treatment
• Total Phosphorous Removal – Indicates the
capability of a BMP to remove phosphorus in
runoff, which may be of particular concern with
certain downstream receiving waters.
• Total Nitrogen Removal – IIndicates the
capability of a BMP to remove nitrogen in
runoff, which may also be of particular concern
with certain downstream receiving waters.
• Fecal Coliform Removal – IIndicates the
capability of a BMP to remove fecal coliform
and associated bacteria in runoff. This
capability may be of particular focus in areas
with public beaches, shellfish beds, and/or
water regulatory quality criteria under the Total
Maximum Daily Load (TMDL) program.
• Metals Removal – Indicates the capability of
a BMP to remove trace metals, which may be
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137
The third group of columns in Table 4.1.3-1
provides an overview of specific site conditions or
criteria that must be met for a particular BMP to
be suitable. In some cases, these values are recommended values or limits that can be exceeded
or reduced with proper design or depending on
specific circumstances. Please see the specific
criteria section of each practice for more details.
• LID/GI – Indicates that the BMP is a LID/
GI structure and can be used in those
applications.
• Drainage Area – Indicates the approximate
minimum or maximum drainage area that is
considered suitable for the BMP. If the drainage
area present at a site is slightly greater than
the maximum allowable drainage area for
a practice, some leeway can be permitted
if more than one practice is installed. The
minimum drainage areas indicated for ponds
and wetlands should not be considered
inflexible limits, and may be increased or
decreased depending on water availability
(baseflow or groundwater), the mechanisms
employed to prevent outlet clogging, and/or
design variations used to maintain a permanent
pool (e.g., liners).
• Space Required (% of Impervious Area) –
Comparative index expresses how much space
a BMP typically consumes at a site in terms of
the approximate area required as a percentage
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of impervious area draining to the control.
BMP design constraints affecting the site area
consumed by the practice are also included
in this column. For example, the flow path
for a downspout disconnect must be 15 feet;
the total area of the downspout disconnect
can vary as long as the BMP meets this design
constraint.
• Maximum Site Slope – Evaluates the effect
of slope on the best management practice.
Specifically, the slope restrictions refer to how
flat the area where the facility is installed must
be and/or how steep the contributing drainage
area or flow length can be.
• Maintenance – Assesses the relative
maintenance effort needed for a BMP in terms
of three criteria: frequency of scheduled
maintenance, chronic maintenance problems
(such as clogging), and reported failure rates.
It should be noted that best management
practices require routine inspection and
maintenance.
Table 4.1.3-2 provides the runoff reduction values
for each of the BMP Practices.
Stormwater Best Management Practices
Site Applicability
• Minimum Head (Elevation Difference) –
Provides an estimate of the minimum elevation
difference needed at a site (from inflow to
outflow) to allow for gravity operation within
the BMP.
• Water Table – Indicates the minimum depth
to the seasonally high water table from the
bottom, or floor, of a BMP.
Cost Considerations
The last group of columns in Table 4.1.3-1 provides cost considerations and an estimate of the
maintenance burden for each BMP option.
• Construction Cost – Structural controls are
ranked according to their relative construction
cost per impervious acre treated, as estimated
from cost surveys.
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138
Table 4.1.3-1 BMP Selection Guide
BMP
Stormwater Management & Treatment
RR ***
WQv / TSS
CPv
Bioretention Basins 3, 5, 6
Yes
85%
Bioslopes 7
Yes
85%
Downspout Disconnects 2
Yes
No
Dry Detention Basins 6
Dry Extended Detention Basins
Qp25 / Qf
Total
Phosphorus
Total
Nitrogen
=
=
80%
=
X
60%
80%
X
X
60%
X
p
Site Applicability
Fecal
Coliform
Metals
LID/GI
Drainage Area (ac)
Space Req’d (% of Imperv. Drainage Area)
Max Site
Slope
60%
90%
25%
60%
95%
Yes
5 max
3-6%
20%
75%
Yes
N/A
N/A
5%
25%
25%
N/A**
40%
Yes
2,500 ft2
Min. length of flow
path 15'
6%
10%
30%
N/A**
50%
No
75 max
N/A
Cost Considerations
Minimum Head
(Elevation Difference)
Depth to Water
Table
Construction
Cost
Maintenance
Burden
3 ft
2 ft
Med-High
Med
N/A
2 ft
Med
Med
N/A
No restrictions
Low
Low
15%
3 ft
2 ft
Low
Low
No
60%
p
p
10%
30%
N/A**
50%
No
No restrictions
1-3%
15%
4-8 ft
2 ft
Low
Low
Dry Wells 2
Yes
100%
=
X
100%
100%
100%
100%
Yes
2,500 ft2
5-10%
6%
2 ft
2 ft
Med
Med
Enhanced Dry Swales 1
Yes
80%
=
X
50%
50%
X
40%
Yes
5 max
10-20%
4%
3-5 ft
2 ft
Med
Low
Enhanced Wet Swales 1
No
80%
=
X
25%
40%
X
20%
Yes
5 max
10-20%
4%
1 ft
Below
Med
Low
2
Grass Channels 1
Minimal
50%
=
X
25%
20%
X
30%
Yes
5 max
10%
4%
<1 ft
2 ft
Low
Low
Gravity (oil-grit) Separators 2
No
40%
X
X
5%
5%
N/A
N/A
No
5
N/A
6%
4 ft
2 ft
High
High
Green Roofs 2
Yes
80%
X
X
50%
50%
N/A**
N/A**
Yes
N/A
No restrictions
25%
6-12 in
N/A
High
Low
Infiltration Trenches 10
Yes
100%
=
=
100%
100%
100%
100%
Yes
5 max
2-3%
6%
1 ft
2 ft
High
High
Multi-Purpose Detention Basins 2
No
Varies
X
=
N/A**
N/A**
N/A**
N/A**
No
No restrictions
1-3%
15%
4-8 ft
2 ft
Low
Low
Organic Filters 2
No
80%
=
X
60%
40%
50%
75%
Yes
10
3-5%
6%
5-8 ft
2 ft
High
High
Permeable Paver Systems 2
Yes
80%
=
=
50%
50%
N/A**
60%
Yes
N/A
No restrictions
6%
2-4 ft
2 ft
High
High
Pervious Concrete
Yes
80%
=
=
50%
65%
N/A**
60%
Yes
N/A
No restrictions
6%
2-4 ft
2 ft
High
High
Yes
80%
=
=
50%
50%
X
60%
Yes
N/A
0%
N/A
N/A
2 ft
Med
Med
Varies
Varies
Varies
Varies
Varies
Varies
Varies
Varies
No
Varies
Varies
Varies
Varies
Varies
Varies
Varies
Based on
Demand
Varies
=
X
Varies
Varies
Varies
Varies
Yes
No restrictions
Varies
No
restritions
N/A
N/A
Med
High
Regenerative Stormwater Conveyance 8
No
80%
X
X
70%
70%
N/A**
N/A**
Yes
50 max
Varies
10%
Varies
Above
High
Med
Sand Filters 1
No
80%
=
X
50%
25%
40%
50%
Yes
2-10 max
2-3%
6%
2-5 ft
2 ft
High
High
Site Reforestation/Revegetation 2
No**
N/A**
N/A**
N/A**
N/A**
N/A**
N/A**
N/A**
Yes
N/A
10,000 ft2 Min.
25%
N/A
No restrictions
Med
Low
Soil Restoration
2
Porous Asphalt (excludes OGFC) 2
Proprietary Systems 2
Rainwater Harvesting 2
No**
N/A**
N/A**
N/A**
N/A**
N/A**
N/A**
N/A**
Yes
N/A
No restrictions
10%
N/A
1.5 ft
Med
Low
Stormwater Planters / Tree Boxes 2
Yes
80%
X
X
60%
60%
80%
N/A
Yes
2,500 ft2
5%
6%
2 ft
2 ft
High
Med
Stormwater Ponds 2
No
80%
p
p
50%
30%
70%
50%
No
10-25 min
2-3%
15%
6-8 ft
2 ft (if aquifer)
Low
Low
Stormwater Wetlands – Level 1 1
No
80%
p
p
40%
30%
70%
50%
Yes
25 min
3-5%
8%
2-3 ft
2 ft (if aquifer)
Med
Med
Stormwater Wetlands – Level 2 4
No
85%
X
X
75%
55%
85%
60%
Yes
5 min
3-5%
Flat
2-3 ft
2 ft (if aquifer)
Med-High
Med
Submerged Gravel Wetlands 2
No
80%
X
X
50%
20%
70%
50%
No
5
3-5%
4%
2-5 ft
No restrictions
High
High
2
Underground Detention 2
No
0%
p
p
0%
0%
0%
0%
No
25 max
N/A
15%
4-8 ft
2 ft
High
Med
Vegetated Filter Strips 1
Yes
60%
=
X
20%
20%
X
40%
Yes
5 max
20%
6%
<1 ft
1-2 ft
Low
Low
p - BMP can meet the stormwater management or treatment requirement
= - BMP may meet the stormwater management or treatment requirement depending on size,
configuration, and site constraints
X - BMP may contribute but is not likely to fully meet the stormwater management or treatment requirement
Stormwater Best Management Practices
Runoff
Reduction
* - Minimum drainage area of ten acres is required to maintain the permanent pool (unless groundwater is present).
**- Helps restore pre-development hydrology, which implicitly reduces post-construction stormwater runoff rates, volumes and pollutant loads
*** - Runoff reduction percentages are listed in Table 4.1.3-2 (BMP Runoff Reduction Credits)
Pollutant Removal References:
1: Original Georgia Stormwater Management
Manual, 2001
2: Coastal Stormwater Supplement to the Georgia
Stormwater Management Manual, 2009
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3: Bioretention - Watershed Benefits. Low
Impact Development Urban Design Tools. 04
April 2014.
4: The Next Generation of Stormwater Wetlands. EPA Wetlands and Watersheds Article
Series (2008) Center for Watershed Protection
5: Bioretention Performance, Design, Construction, and Maintenance. North Carolina Cooperative
Extension Service. Hunt, William. 2006
6: North Carolina Department of Environment and
Natural Resources Stormwater Best Management
Practices Manual. 2007
7: Washington State Department of Transportation (WSDOT) Highway Runoff
Manual, 2011.
8: West Virginia Stormwater Management Design Guidance Manual, 2012
9: Georgia Department of Transportation
(GDOT) Drainage Manual, 2014
10: Pollutant removal rates based on 100%
infiltration with no underdrain
VOL 2
139
Runoff Reduction
BMP
RR (%) *
100%
Bioretention Area (w/o underdrain) 1
Runoff Reduction
BMP
RR (%) *
Permeable Paver System (w/underdrain) 1
50%
75%
Pervious Concrete (w/o underdrain)
1
50%
Pervious Concrete (w/upturned underdrain)
Bioslopes (A & B hydrologic soils) 1
50%
Pervious Concrete (w/underdrain) 1
Bioslopes (C & D hydrologic soils) 1
25%
Porous Asphalt (w/o underdrain) 1
2
50%
Porous Asphalt (w/upturned underdrain)
Downspout Disconnects (C & D hydrologic soils) 1
25%
Porous Asphalt (w/underdrain) 1
50%
Dry Detention Basins
0%
Porous Asphalt (OGFC, PEM) 3
0%
Dry Extended Detention Basins
0%
Proprietary Systems
Bioretention Area (w/upturned underdrain)
Bioretention Area (w/underdrain)
1
Downspout Disconnects (A & B hydrologic soils)
Dry Wells 1
Enhanced Dry Swales (w/underdrain) 1
Enhanced Dry Swales (w/o underdrain)
1
Grass Channels (C & D hydrologic soils)
Gravity (oil-grit) Separators
50%
100%
75%
1
Varies
3
Based on Demand
100%
Rainwater Harvesting 3
50%
Regenerative Stormwater Conveyance
0%
100%
Sand Filters
0%
2
Site Reforestation/Revegetation 1
2
25%
Soil Restoration (Can be used to remediate C & D Soils)
1
10%
Stormwater Planters / Tree Boxes
1
Green Roofs 1
Infiltration Trenches
75%
1
0%
Enhanced Wet Swales 2
Grass Channels (A & B hydrologic soils)
100%
1
1
0%
1
0%
50%
1
0%
Stormwater Ponds
60%
Stormwater Wetlands – Level 1 2
0%
0%
100%
Stormwater Wetlands – Level 2
0%
2
2
0%
Multi-Purpose Detention Basins
0%
Submerged Gravel Wetlands
Organic Filters 2
0%
Underground Detention (Not including infiltration) 2
0%
100%
Vegetated Filter Strips (A & B hydrologic soils)
2
50%
75%
Vegetated Filter Strips (C & D hydrologic soils)
1
25%
Permeable Paver System (w/o underdrain)
1
Permeable Paver System (w/ upturned underdrain)
1
Stormwater Best Management Practices
Table 4.1.3-2 BMP Runoff Reduction Credits
2
BMP pollutant removal rates and other BMP specific design criteria are listed in Table 4.1.3-1 (BMP Selection Guide)
* Runoff reduction percentages listed are maximum allowable credit values. These values, similar to other pollutant removal rates, are based on the performance and operating efficiency of the BMP.
Runoff reduction percent removals are based on values from the former Georgia Stormwater Management Manual's Coastal Stormwater Supplement, 2009.
1
Runoff reduction percent removals are based on values from the Center for Watershed Protection, 2008.
2
Runoff reduction percent removals are not available
3
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VOL 2
140
Soils
account when designing a BMP. They include
Key evaluation factors are based on an initial
physiographic factors, soils, special watershed or
investigation of the NRCS hydrologic soils groups
stream considerations, and temperature.
at the site. Note that more detailed geotechnical
tests are usually required for infiltration feasibility
Physiographic Factors
and during design to confirm permeability and
Three key factors to consider are low-relief,
other factors.
high-relief, and karst terrain. In the state of
Georgia, low-relief (very flat) areas are primarily
Special Watershed or Stream Considerations
located in the Coastal Plain and along the Atlantic
The design of BMPs is fundamentally influenced
coast. High-relief (steep and hilly) areas are found
by the nature of downstream receiving water
throughout the Piedmont and far northern parts
bodies. Consequently, designers should deter-
of the state. Karst and major carbonaceous rock
mine the Use Classification of the watershed in
areas are generally found in the western portions
which their project is located prior to design (see
of the state. Special geotechnical testing require-
Georgia Department of Natural Resources Envi-
ments may be needed in karst areas. The local
ronmental Protection Division Water Quality Con-
reviewing authority should be consulted to deter-
trol Rules Chapter 391-3-6). In addition, designers
mine if a project is subject to terrain constraints..
should consult with appropriate review authori-
• Low-relief areas need special consideration
because many BMPs require a hydraulic head
to move stormwater runoff through the facility.
ties to determine if their development project is
• High-relief areas may limit the use of practices
that need flat or gently sloping areas to reduce
sediment and/or runoff flow velocities. In
other cases high-relief terrain may impact dam
heights to the point that the use of a practice
becomes infeasible.
• Karst terrain can limit the use of some BMPs as
the infiltration of polluted waters directly into
underground streams found in karst areas may
be prohibited. In addition, ponding areas may
not reliably hold water in karst areas.
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subject to additional BMP criteria as a result of an
adopted local watershed plan or special provision.
In some cases, higher pollutant removal or environmental performance is needed to fully protect
aquatic resources and/or human health and safety
within a particular watershed or receiving water.
Therefore, special design criteria for a particular
practice or the exclusion of one or more practices
may need to be considered within these watersheds or protected areas. Examples of important
watershed factors to consider include:
• Primary and Secondary Trout Streams – Cold
and cool water streams have habitat qualities
capable of supporting trout and other sensitive
aquatic organisms. Therefore, the design
objective for these streams is to maintain
habitat quality by preventing stream warming,
maintaining natural recharge, preventing bank
and channel erosion, and preserving the natural
riparian corridor. Some BMPs can have adverse
downstream impacts on cold-water streams,
and their design may need to be modified or
use restricted. BMPs that result in reduced
thermal and sediment loading, such as shaded
or underground stormwater detention and
green roof technologies should be considered
where stormwater runoff may impact trout
streams.
• High Quality Streams (High quality streams
with a watershed impervious cover less than
approximately 15%) – These streams may also
possess high quality cool water or warm water
aquatic resources and/or endangered species.
The design objectives are to maintain habitat
quality through the same techniques used for
cold-water streams, with the exception that
stream warming is not as severe of a design
constraint. These streams may also be specially
designated by local authorities.
Stormwater Best Management Practices
Additional considerations should be taken into
• Wellhead Protection –Areas that recharge
existing public (or known private) water supply
wells present a unique management challenge.
The key design constraint is to prevent possible
groundwater contamination by preventing
infiltration of hotspot runoff. At the same
time, recharge of unpolluted stormwater is
encouraged to maintain flow in streams and
wells during dry weather.
VOL 2
141
contaminate drinking water.
• Swimming/Shellfish – Watersheds that drain to
public swimming waters or shellfish harvesting
areas require a higher level of stormwater
treatment to prevent closings caused by
bacterial contamination from stormwater
runoff. In these watersheds, BMPs should
be explicitly designed to maximize bacteria
removal.
• Temperature – In many cases, the water
running off the site may flow over an
impervious surface that has absorbed the
sun’s heat, and can potentially cause thermal
pollution to streams. Water temperatures
are also increased due to shallow ponds and
impoundments along a watercourse and fewer
trees along streams to shade the water. This
may have an adverse effect on aquatic life,
particularly in trout streams as mentioned
above. Table 4.1.3-3 shows a list of the BMPs
and if they can potentially reduce thermal
pollution. BMPs that encourage infiltration
appear to be the most effective in mitigating
Additionally, Better Site Design principles (see
Section 2.3 for more information), including conservation of natural buffers and stream canopy
can maintain shade and reduce thermal pollution.
Table 4.1.3-3 – List of BMPs with Temperature Reduction Possibility
BMP
*Temperature
BMP
Reduced?
*Temperature
Reduced?
Bioretention Areas
Yes
Pervious Concrete
No
Bioslopes
Yes
Porous Asphalt
No
Downspout Disconnects
No
Proprietary Systems
No
Dry Detention Basins
No
Rainwater Harvesting
No
Dry Enhanced Swales/Wet En-
No
Regenerative Stormwater
Yes
hanced Swales
Stormwater Best Management Practices
• Reservoir or Drinking Water Protection –
Watersheds that deliver surface runoff to a
public water supply reservoir or impoundment
are a special concern. Depending on the
treatment available at the water intake, it may
be necessary to achieve a greater removal rates
for pollutants of concern, such as bacteria
pathogens, nutrients, sediment, and/or metals.
One particular management concern for
reservoirs is ensuring that stormwater hotspots
are adequately treated so that they do not
Conveyance
Dry Extended Detention Basins
No
Sand Filter
Yes
Dry Wells
Yes
Site Reforestation/Reveg-
Yes
etation
Grass Channel
Yes
Soil Restoration
Yes
Gravity (Oil-Grit) Separators
No
Stormwater Planters/Tree
Yes
Boxes
Green Roofs
Yes
Stormwater Ponds
No
Infiltration Practices
Yes
Stormwater Wetlands
No
Multi-Purpose Detention Basin
No
Submerged Gravel Wet-
Yes
lands
Organic Filter
Yes
Underground Detention
Yes
Permeable Paver System
No
Vegetated Filter Strip
Yes
*The effects of BMPs on temperature can be dependent on many factors, including the sizing of the BMP and the presence of an
underdrain.
thermal impacts.
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VOL 2
142
While there is a downstream reservoir to consider,
erty. However, the compacted subsurface soils
A 20-acre institutional area (e.g., church and
there are no special watershed factors or physio-
may reduce the effectiveness of installing porous
associated buildings) is being constructed in a
graphic factors that preclude the use of any of the
surfaces and can require the use of an extensive
dense urban area within metropolitan Atlanta. The
practices from the BMP list. However, due to the
underdrain system that may become expensive.
existing impervious coverage of the site is 40%.
size of the drainage area, most stormwater ponds
Soil restoration and site revegetation may be used
The site drains to an urban stream that is highly
and wetlands are removed from consideration. In
to mitigate the effects of the urbanized soils. In-
impacted from hydrologic alterations (accelerated
addition, the site’s impermeable soils remove an
stallation of a green roof can increase the build-
channel erosion). The stream channel is deeply
infiltration trench from being considered. Veg-
ing’s construction costs and should be considered
incised; consequently, flooding is not a prob-
etated filter strips and downspout disconnects
early in the design process.
lem. The channel drains to an urban river that is
could be considered for pretreatment; however,
a tributary to a phosphorus limited drinking water
the steep slope may reduce their effectiveness
To provide overbank flood control, as well as
reservoir. Low permeability soils limit infiltration
and preclude their use.
channel protection storage, a micropool ED pond,
practices.
• Objective: Avoid additional disruptions to
receiving channels and reduce pollutant loads
for sediment and phosphorus to receiving
waters.
• Target Removals: Provide stormwater
management to mitigate for accelerated
channel incision and reduce loadings of key
pollutants by:
»» Sediment: 80%
»» Phosphorus: 40%
• Activity/Runoff Characteristics: The proposed
site will have large areas of impervious surface
in the form of parking and structures. However,
there will be a large contiguous portion of turf
grass proposed for the front of the parcel that will
have a relatively steep slope (approximately 10%)
and will drain to the storm drain system associated
with the entrance drive. Stormwater runoff from
the site is expected to exhibit fairly high sediment
levels and seasonally high phosphorus levels (due
to turf grass management).
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dry extended detention basin, or multi-purpose
To provide additional pollutant removal capa-
detention basin will likely be needed, unless some
bilities in an attempt to better meet the target
downstream regional storage is available to con-
removals, bioretention, surface sand filters, and/
trol the overbank flood. The site drainage system
or perimeter sand filters can be used to treat the
can be designed so that the bioretention and/
parking lot and driveway runoff. Bioretention
or sand filters drain to the micropool ED pond or
provides some removal of phosphorus while
detention basin for redundant treatment. Vegetat-
improving the aesthetics of the site. Surface sand
ed dry swales could also be used to convey runoff
filters provide higher phosphorus removal at a
to the pond, which would provide pretreatment,
comparable unit cost to bioretention, but are not
runoff reduction, TSS removal, and some water
as aesthetically pleasing. The perimeter sand filter
quality volume reduction. Pocket wetlands, wet
is a flexible, easy to access practice (but at higher
swales, and submerged gravel wetlands were
cost) that provides good phosphorus removal and
eliminated from consideration due to the poten-
high oil and grease trapping ability. A bioslope
tial for nuisance conditions. Underground sand
may be an ideal BMP for use in higher slope sit-
filters could also be used at the site; however,
uations and should be designed to complement
cost and aesthetic considerations were significant
the site‘s aesthetics and accommodate pedestrian
enough to eliminate them from consideration.
Stormwater Best Management Practices
4.1.3.3 EXAMPLE APPLICATION
infrastructure.
Designers may consider the feasibility of using
green roofs, pervious concrete, porous asphalt,
and/or permeable pavers wherever possible to
reduce the overall imperviousness of the propVOL 2
143
Stormwater Best Management Practices
4.1.4 On-Line Versus Off-Line Best Management Practices
4.1.4.1 INTRODUCTION
Stormwater best management practices (BMPs)
are designed to be either “on-line” or “off-line.”
On-line BMPs are designed to receive, but not
necessarily control or treat, entire runoff volumes
up to the Qp25 or Qf event. On-line BMPs must be
able to handle the entire range of storm flows.
Off-line facilities are designed to receive only a
specified flow rate through the use of a flow regulator (diversion structure, flow splitter, etc.). Flow
regulators are typically used to divert the water
quality volume (WQv) or runoff reduction volume
(RRV) to an off-line BMP sized and designed to
treat and control the WQv. After the design runoff
flow has been treated and/or controlled it is
returned to the conveyance system. Figure 4.1.4.1
shows an example of an off-line sand filter and an
on-line enhanced dry swale.
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Figure 4.1.4-1 Example of On-Line versus Off-Line BMPs
(Source: CWP, 1996)
VOL 2
144
Flow regulation to off-line best management
EXCESS RUNOFF TO
STORM DRAIN SYSTEM
practices can be achieved by either:
• Diverting the water quality volume or other
specific maximum flow rate to an off-line BMP,
or
TO TREATMENT CONTROL
MEASURE
• Bypassing flows in excess of the design flow
rate.
The peak water quality flow rate (Qwq) can be calculated using the procedure found in Subsection
3.1.7.2.
Flow regulators can be flow splitter devices, diversion structures, or overflow structures. Several
CONTRIBUTING
AREA RUNOFF
examples are shown on the right and on the
Stormwater Best Management Practices
4.1.4.2 FLOW REGULATORS
WEIR HEIGHT EQUAL TO MAXIMUM
DEPTH OF WQV BEING RETAINED IN
CONTROL MEASURE
following pages.
PIPE INTERCEPTOR ISOLATION/DIVERSION STRUCTURE
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Figure 4.1.4-2 Pipe Interceptor Diversion Structure
(Source: City of Sacramento, 2000
VOL 2
145
TOP OF SLOTS “HEIGHT OF WATER
QUALITY VOLUME IN BASIN” OF
DIVERSION WEIR (NOT MANDATORY).
Stormwater Best Management Practices
TOP OF ISOLATION BAFFLE MUST BE GREATER THAN
MAXIMUM WATER SURFACE ELEVATION OVER DIVERSION
WEIR FOR DESIGN STORM ESTABLISHED BY LOCAL AGENCY
SURFACE CHANNEL DIVERSION STRUCTURE
Figure 4.1.4-3 Surface Channel Diversion Structure
(Source: City of Sacramento, 2000)
USE OF ORIFICE FOR BYPASS
IN LIEU OF WEIR SHOWN IN
FIGURE 4-3.
CLEARWELL
ORIFICE INLET ELEVATION EQUAL TO MAXIMUM
WATER ELEVATION WHEN ENTIRE WQV BEING
RETAINED IN BASIN OR CONTROL MEASURE.
TREATMENT CONTROL
MEASURE
TO CLEARWELL OR STORM
DRAIN SYSTEM
Figure 4.1.4-4 Outlet Flow Regulator
(Source: City of Sacramento, 2000)
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VOL 2
146
4.1.5.2 ADVANTAGES AND DISADVANTAGES OF
REGIONAL BEST MANAGEMENT PRACTICES
Conversely, an upstream developer may have
Regional stormwater facilities can be more
regional facility is not in place before construc-
4.1.5.1 INTRODUCTION
cost-effective because it is easier and less ex-
tion. Maintenance responsibilities generally shift
Using individual, on-site best management prac-
pensive to build, operate, and maintain one large
from the homeowner or developer to the local
tices or a treatment train of one or more BMPs
facility than several small ones. Regional storm-
government when a regional approach is select-
for each development is the typical approach for
water facilities can also be better maintained than
ed. The local government would need to establish
controlling stormwater quantity and quality. The
individual site BMPs because they are large, highly
a stormwater utility or some other program to
developer finances the design and construction
visible, and typically the responsibility of local
fund and implement stormwater management
of these controls and, initially, is responsible for all
government entities.
facilities. Finally, a large in-stream facility can pose
operation and maintenance.
to establish temporary control structures if the
a greater disruption to the natural flow network
There are also several disadvantages to regional
and is more likely to affect wetlands within the
A potential alternative approach is for a commu-
stormwater BMPs. In many cases, a community
watershed.
nity to install a few strategically located regional
must provide capital construction funds for a
stormwater BMPs in a subwatershed rather than
regional facility, including the costs of land ac-
requiring on-site controls (see Figure 4.1.5-1).
quisition. However, if a downstream developer is
For this Manual, regional BMPs are defined as
the first to build, that person could be required to
facilities designed to manage stormwater runoff
construct the facility and later be compensated by
from multiple projects and/or properties through
upstream developers for the capital construction
a local jurisdiction-sponsored program, where the
costs and annual maintenance expenditures.
Stormwater Best Management Practices
4.1.5 Regional vs. On-site Stormwater
Management
individual properties may assist in the financing of
the facility, and the requirement for on-site practices is either eliminated or reduced
Structural Controls on Each
Development Site
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Regional Structural
Stormwater Control
Figure 4.1.5-1 On-site versus Regional Stormwater Management
VOL 2
147
“cons” of regional stormwater practices.
• Other Benefits – Well-sited regional
stormwater facilities can serve as a recreational
and aesthetic amenity for a community.
Advantages of Regional Stormwater Management Facilities
Disadvantages of Regional Stormwater Controls
• Reduced Construction Costs – Design and
construction of a single regional stormwater
management facility can be more costeffective than numerous individual on-site
BMPs.
• Location and Siting – Regional stormwater
facilities may be difficult to site, particularly
for large facilities or in areas with existing
development.
• Reduced Operation and Maintenance Costs –
Rather than multiple owners and associations
being responsible for the maintenance
of several stormwater facilities on their
developments, it is simpler and more cost
effective to establish scheduled maintenance
of a single regional facility.
• Higher Assurance of Maintenance – Regional
stormwater facilities are far more likely to be
adequately maintained as they are large and
have a higher visibility, and are typically the
responsibility of local government.
• Capital Costs – The community must typically
provide capital construction funds for a
regional facility, including the costs of land
acquisition.
• Maintenance – The local government is
typically responsible for the operation and
maintenance of a regional stormwater facility.
For in-stream regional facilities:
• Water Quality and Channel Protection –
Without on-site water quality and channel
protection, regional controls do not protect
smaller streams upstream from the facility
from degradation, streambank erosion, and
thermal impacts. Further, diverting stormwater
runoff to a stormdrain system for discharge to
a regional facility eliminates the opportunity to
provide local groundwater recharge and runoff
reduction.
• Ponding Impacts – Upstream inundation from
a regional facility impoundment can eliminate
floodplains, wetlands, and other habitat.
Stormwater Best Management Practices
Below are summarized some of the “pros” and
• Additional Planning – The implementation of
in-stream regional facilities requires substantial
planning, permitting and increased regulatory
• Need for Planning – The implementation of
regional stormwater management facilities
requires substantial planning, financing, and
permitting. Land acquisition must be in place
compliance.
ahead of future projected growth.
• Maximum Utilization of Developable Land
– Developers would be able to potentially
maximize the utilization of the proposed
development for the purpose intended by
minimizing the land normally set aside for the
construction of stormwater BMPs.
• Retrofit Potential – Regional facilities can
be used by a community to mitigate existing
developed areas that have insufficient or no
best management practices for water quality
and/or quantity, as well as provide for future
development.
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VOL 2
148
If a community decides to implement a regional
stormwater facility, then it must ensure that the
conveyances between the individual upstream
developments and the regional facility can handle the design peak flows and volumes without
causing adverse impact or property damage.
Full-buildout conditions in the regional facility
drainage area should be used in the analysis.
In addition, unless the system consists of completely non-erodable conveyances (storm drains,
pipes, concrete channels, etc.), on-site best management practices for water quality and downstream channel protection should be required
for all developments within the regional facility’s
drainage area. Federal water quality provisions do
not allow the degradation of water bodies from
untreated stormwater discharges, and it is U.S.
EPA policy to not allow regional stormwater facilities that would degrade stream quality between
the upstream development and the regional facility. Further, without adequate channel protection,
aquatic habitats and water quality in the channel
network upstream of a regional facility may be
It is important to note that siting and designing
Pretreatment – The next step in the treatment
regional facilities should ideally be done within
train consists of pretreatment measures. These
the context of stormwater master planning or
measures typically do not provide sufficient
watershed planning to be effective.
pollutant removal to meet the runoff reduction or
80% TSS reduction goal, but do provide calculable
4.1.6 Using Best Management Practices
in Series
water quality benefits that may be applied towards
meeting the WQv treatment requirement. These
measures include:
4.1.6.1 STORMWATER TREATMENT TRAINS
TThe minimum stormwater management standards are an integrated planning and design approach, the components of which work together
to limit adverse impacts of urban development on
downstream waters and riparian areas. This approach is sometimes called a stormwater “treatment train”. When considered comprehensively, a
treatment train consists of all the design concepts, better site design practices, and BMPs that
work to attain water quality and quantity goals.
• The use of stormwater better site design
practices and site design credits to reduce the
water quality volume (WQv)
• Using best management practices such as
downspout disconnects, vegetated filter strips,
and/or soil restoration and site revegetation to
provide pretreatment and runoff reduction
Stormwater Best Management Practices
4.1.5.3 IMPORTANT CONSIDERATIONS FOR
THE USE OF REGIONAL STORMWATER MANAGEMENT FACILITIES
• Pretreatment facilities, such as sediment
forebays, on best management practices that
provide storage and/or filtration of runoff
This is illustrated in Figure 4.1.6-1.
Runoff and Load Generation – The initial part of
the “train” is located at the source of runoff and
pollutant load generation, and consists of better
site design and pollution prevention practices that
reduce runoff and stormwater pollutants.
degraded by streambank erosion if they are not
protected from bankfull flows and high velocities.
Based on these concerns, both the EPA and the
Runoff & Load
Generation
U.S. Army Corps of Engineers have expressed opposition to in-stream regional stormwater control
facilities. In-stream facilities should be avoided if
possible and will likely be permitted on a case-bycase basis only.
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Pretreatment
Primary Treatment
and/or
Quantity Control
Figure 4.1.6-1 Generalized Stormwater Treatment Train
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Stormwater Best Management Practices
Primary Treatment and/or Quantity Control –
The last step is primary water quality treatment
and/or quantity (channel protection, overbank
flood protection, and/or extreme flood protection) control. This is achieved through the use of
one or more of the best management practices
shown in Table 4.1.1-1 as a stand-alone stormwater management practice or in series.
4.1.6.2 USE OF MULTIPLE BEST MANAGEMENT
PRACTICES IN SERIES
Many combinations of best management practices in series may exist for a site. The combinations
of best management practices are limited only by
the need to employ measures of proven effectiveness and meet local regulatory and physical
site requirements. Figures 4.1.6-3 through 4.1.6-5
illustrate the application of the treatment train
concept for: a moderate density residential neighborhood, a small commercial site, and a large
shopping mall site.
Figure 4.1.6-3 Example Treatment Train – Residential Subdivision
(Adapted from: NIPC, 2000)
In Figure 4.1.6-3 rooftop runoff drains over
grassed yards to backyard grass channels. Runoff
from front yards and driveways reaches roadside
grass channels. Finally, all stormwater flows drain
to a micropool ED stormwater pond.
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Stormwater Best Management Practices
A gas station and convenience store is depicted
in Figure 4.1.6-4. In this case, the decision was
made to intercept hydrocarbons and oils using a
commercial gravity (oil-grit) separator located on
the site prior to draining to a perimeter sand filter
for removal of finer particles and TSS. No BMP
for channel protection is required as the system
drains to the municipal storm drain pipe system.
Overbank and extreme flood protection is provided by a regional stormwater control downstream.
Figure 4.1.6-5 shows an example treatment train
for a commercial shopping center. In this case,
runoff from rooftops and parking lots drains to a
depressed parking lot, perimeter grass channels,
and bioretention areas. Slotted curbs are used at
the entrances to these swales to better distribute
the flow and settle out the very coarse particles
at the parking lot edge for sweepers to remove.
Runoff is then conveyed to a wet ED pond for
additional pollutant removal and channel protection. Overbank and extreme flood protection is
provided through parking lot detention.
Existing
Storm Sewer
Oil-Grit
Separator
Perimeter
Sand Filter
Gas Pumps
Figure 4.1.6-5 Example Treatment Train – Commercial Development
(Source: NIPC, 2000)
Convenience
Store
Gas Pumps
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Figure 4.1.6-4
Example Treatment
Train - Commercial
Development
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PRACTICES IN SERIES
To estimate the pollutant removal rate of water
quality based best management practices in series, the following steps are used to determine the
pollutant removal:
• For each drainage area, list the BMPs in order,
upstream to downstream, along with their
expected average pollutant removal rates from
Table 4.1.3-1 for the pollutants of concern.
• Apply the following equation for calculation
of approximate total accumulated pollution
removal for BMPs in series:
Stormwater Best Management Practices
4.1.6.3 CALCULATION OF POLLUTANT REMOVAL FOR WATER QUALITY BEST MANAGEMENT
Final Pollutant Removal = BMP1 removal rate +
(remaining pollutant load * BMP2 removal rate) +
… for other Controls in series
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Maximum contributing drainage area of 5 acres
• Treatment area consists of ponding area, organic/mulch layer,
planting media, and vegetation
• Requires landscaping plan
• Standing water has a maximum drain time of 24 hours
• Pretreatment recommended to prevent clogging of underdrains or
native soil
• Ponding depth should be a maximum of 12 inches, preferably 9
inches
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
Description: Shallow stormwater basin or landscaped area that utilizes engineered soils or native, well-draining soil and vegetation to capture
and treat runoff.
LID/GI Consideration: Low land requirement,
adaptable to many situations, and often a small
BMP used to treat runoff close to the source.
ADVANTAGES / BENEFITS
• Applicable to small drainage areas
• Effective pollutant removals
• Appropriate for small areas with high impervious cover, particularly
parking lots
• Natural integration into landscaping for urban landscape
enhancement
• Good retrofit capability
• Can be planned as an aesthetic feature and meet local planting
requirements
DISADVANTAGES / LIMITATIONS
• Requires landscaping
• Not recommended for areas with steep slopes
• Medium to high capital cost
• Medium cost maintenance burden
• Soils may clog over time (may require cleaning or replacing)
MAINTENANCE REQUIREMENTS
• Inspect and repair or replace treatment area components such as
mulch, plants, and scour protection, as needed
• Ensure bioretention area is draining properly so it does not become
a breeding ground for mosquitos
• Remove trash and debris
• Ensure mulch is 3-4 inches thick in the practice
• Requires plant maintenance plan
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.2 Bioretention Areas
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes
Soils: Engineered soil media is composed of
sand, fines, and organic matter
Other Considerations: Use of native plants is
recommended
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 100% of the runoff reduction volume provided
(no underdrain)
• 75% of the runoff reduction volume provided
(upturned underdrain system)
• 50% of the runoff reduction volume provided
(underdrain)
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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Bioretention areas are structural stormwater
controls that capture and infiltrate, or at least
temporarily store the water quality volume (WQv)
using soils and vegetation in shallow basins or
Left: Figure 4.2-1 Landscaped Bioretention Area
Middle Left: Figure 4.2-2 Landscaped Island
landscaped areas.
Bioretention areas are engineered controls that
convey runoff to the “treatment area,” which consists of a ponding area, organic or mulch layer,
planting soil, and vegetation. If the native soils
are adequate, the captured stormwater runoff
will infiltrate into the surrounding soils. If not, the
filtered runoff is typically collected and returned
Bottom: Figure 4.2-3 Bioretention Area near Parking
Lot
Middle Right: Figure 4.2-4 Bioretention Area after
Storm
Stormwater Best Management Practices
4.2.1 General Description
to the conveyance system, through an underdrain system. Bioretention areas slightly differ
from rain gardens in that they are an engineered
structure that has a larger drainage area and may
include an underdrain. For additional information
of designing a Rain Garden in a residential lot, see
the following website: https://www.atlantawatershed.org/greeninfrastructure/atlanta-residential-gi-nov-2012022013/
There are numerous design applications for
bioretention areas including along highway and
roadway drainage swales, within larger landscaped pervious areas, and as landscaped islands
in impervious or high-density environments. A
variety of bioretention areas are shown Figure
4.2-1 through Figure 4.2-4.
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Bioretention areas can be designed for water
quantity and quality, i.e. the removal of stormwater pollutants, depending upon the native soils.
Bioretention areas can provide runoff quantity
control, particularly for smaller runoff volumes
such as those generated by the water quality
within 72 hours. A bioretention area can be
designed with an upturned underdrain to
provide 75% of the runoff reduction volume, if
properly maintained. An upturned underdrain
is recommended in decent subsoils due to the
clogging potential created during construction.
Finally a bioretention area can be designed
with an underdrain to provide 50% of the runoff
reduction volume, if properly maintained.
storm event (1.2 inches). These facilities may
sometimes be used to partially or completely
meet channel protection requirements on smaller
sites. However, bioretention areas will typically need to be used in conjunction with another
control to provide channel protection, as well as
overbank flood protection. Bioretention areas
need to be designed and maintained to safely
bypass higher flows.
• Runoff Reduction
Bioretention areas are one of the most effective
low impact development (LID) practices
that can be used in Georgia to reduce postconstruction stormwater runoff and improve
stormwater runoff quality. Like other LID
practices, they become even more effective
with a higher infiltration rate of native soils. A
bioretention area, with no underdrain, can
be designed to provide 100% of the runoff
reduction volume, if properly maintained.
In order to provide runoff reduction for a
bioretention area that is designed without
an underdrain, a soils test or other reliable
resource must indicate that the ponding area
of the bioretention area will drain within 24
hours and entire bioretention area will drain
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• Water Quality
A bioretention area is an excellent stormwater
treatment practice due to its variety of
pollutant removal mechanisms. Each of
the components of the bioretention area
is designed to perform a specific function.
The grass filter strip (or grass channel) pretreatment component reduces incoming runoff
velocity and filters particulates from the runoff.
The ponding area provides for temporary
storage of stormwater runoff prior to its
evaporation, infiltration, or uptake and provides
additional settling capacity. The organic or
mulch layer provides filtration as well as an
environment conducive to the growth of
microorganisms that degrade hydrocarbons
and organic material. The planting soil in the
bioretention area acts as a filtration system,
and clay in the soil provides adsorption sites
for hydrocarbons, heavy metals, nutrients and
other pollutants. Both woody and herbaceous
plants in the ponding area provide vegetative
uptake of runoff and pollutants and also serve
to stabilize surrounding soils.
• Channel Protection
For smaller sites, a bioretention area may
be designed to capture the entire channel
protection volume (CP v). Given that a
bioretention area facility is typically designed
to completely drain over 48-72 hours, the
requirement of extended detention for the
1-year, 24-hour storm runoff volume will be
met. For larger sites, or where only the WQv
is diverted to the bioretention area, another
control must be used to provide CP v extended
detention.
• Overbank Flood Protection
Another control in conjunction with a
bioretention area will likely be required to reduce
the post-development peak flow of the 25-year
storm (Qp) to pre-development levels (detention).
Stormwater Best Management Practices
4.2.2 Stormwater Management Suitability
• Extreme Flood Protection
Bioretention areas must provide flow diversion
and/or be designed to safely pass extreme
storm flows and protect the ponding area,
mulch layer and vegetation.
Credit for the volume of runoff reduced in the
bioretention area may be taken in the overbank
flood protection and extreme flood protection
calculations. If the practice is designed to provide
Runoff Reduction for Water Quality compliance,
then the practice is given credit for Channel
Protection and Flood Control requirements by
allowing the designer to compute an Adjusted CN
(see Subsection 3.1.7.5 for more information).
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General Feasibility
Bioretention areas are presumed to be able to
4.2.4 Application and Site Feasibility
Criteria
remove 85% of the total suspended solids (TSS)
Bioretention areas are suitable for many types of
- Suitable for High Density/Ultra Urban Areas – YES
load in typical urban post-development runoff
development, from single-family residential to
- Regional Stormwater Control – NO
when sized, designed, constructed, and main-
high-density commercial projects. Because of
tained in accordance with the recommended
its ability to be incorporated in landscaped areas,
specifications. Other pollutants bioretention areas
the use of bioretention areas is extremely flexible
Physical Feasibility - Physical Constraints at
can remove include Phosphorus, Nitrogen, metals
whether they are placed along roadways or in ar-
Project Site
(such as Cadmium, Copper, Lead, and Zinc), and
eas undergoing development or re-development.
Pathogens (such as Fecal Coliform). A bioret-
Bioretention areas are an ideal stormwater control
ention area that is undersized, poorly designed,
for use as roadway median strips and parking
or maintained improperly can have reduced
lot islands and are also good candidates for the
• Drainage Area – 5 acres or less. If the drainage
area is greater than 5 areas, the drainage area
can be divided up into multiple areas with each
drainage area having a bioretention area.
pollutant removal performance. Proper design
treatment of runoff from pervious areas, such as a
of the bioretention area is critical to ensure that
golf course. Bioretention areas can also be used
pollutants can be properly removed from storm-
to retrofit existing development with stormwater
water that drains into the ground or storm sewer
quantity and quality treatment capacity. Curbs are
system. Proper designs improve the quality of
not required for this type of practice.
our environment. One of the benefits of using an
Upturned Underdrain (see Figure 4.2-6) is that this
Because of the many design constraints including
particular type of design will increase the removal
limited ponding depths and inlet velocities, bioret-
of nitrogen in the soil.
ention areas generally have a maximum drainage
- Suitable for Residential Subdivision Usage – YES
• Space Required – Rough rule of thumb of
3-6% of the contributing drainage area
Stormwater Best Management Practices
4.2.3 Pollutant Removal Capabilities
• Site Slope – Slopes should be a maximum of
20%, 5% preferred
• Minimum Depth to Water Table – A separation
distance of 2 feet is recommended between
the bottom of the bioretention area and the
elevation of the seasonally high water table.
area of 5 acres or less. The size of the bioretenFor additional information and data on pollutant
tion area is generally 3-6% of the contributing
removal capabilities for bioretention areas, see the
drainage area but varies significantly depending
National Pollutant Removal Performance Data-
on each component of the bioretention area’s
base (3rd Edition) available at www.cwp.org and
ability to capture and infiltrate stormwater runoff
the National Stormwater Best Management Prac-
and the percent imperviousness of the drainage
tices (BMP) Database at www.bmpdatabase.org.
area.
The following criteria should be evaluated to
ensure the suitability of a bioretention area for
• Soils – Native soils if they have at least
0.5 inch/hr infiltration ability. Otherwise
engineered media is needed including coarse
sand, silt, clay, and other organic matter. The
recommended standard media depth is 36
inches, with a minimum depth of 18 inches and
a maximum depth of 48 inches. A qualified,
licensed professional should test the soils
to determine the best depth of soil for the
practice.
meeting stormwater management objectives on a
site or development.
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• Hot spots – Do not use for hot spot runoff.
• Damage to existing structures and facilities –
Consideration should be given to the impact
of water exfiltrating the bioretention areas on
nearby road bases.
• Proximity – The following is a list of specific
setback requirements for the location of a
bioretention area:
»» 10 feet from building foundations
»» 100 feet from private water supply wells
»» 200 feet from public water supply reservoirs
(measured from edge of water)
»» 1,200 feet from public water supply wells
• Flat Terrain—May be difficult to provide
adequate drainage so multiple smaller
bioretention areas may be needed.
• Infiltration testing of native soils at proposed
elevation of bottom of bioretention area.
• Shallow Water Table—This can prevent the
provision of 2 feet of clearance between
the bottom of the bioretention area and
the top of the water table which may cause
stormwater runoff to pond in the bioretention
area. Possible solutions are to minimize the
depth of the planting media, or consider using
stormwater ponds, wetlands, or wet swales to
intercept and treat stormwater runoff.
imum standards for the design of a bioretention
• Karst topography—This condition usually
warrants the use of an underdrain or
impermeable liner to avoid infiltration into karst
subsoils.
The following criteria are to be considered minarea. Consult with the local review authority to
determine if there are any variations to these criteria or additional standards that must be met.
4.2.5.1 LOCATION AND LAYOUT
Bioretention areas vary based on site constraints
such as proposed and existing infrastructure, soils,
existing vegetation, contributing drainage area,
and utilities. Bioretention area systems are designed for intermittent flow and must be allowed
to drain and reaerate between rainfall events.
They should not be used on sites with a contin-
• Trout Stream – Evaluate for stream warming
when an underdrain system is used.
uous flow from groundwater, sump pumps, or
other sources. Bioretention area locations should
In addition, careful consideration should be given
4.2.5 Planning and Design Criteria
to the potential of perched or raised groundwater
Before designing the bioretention area, the fol-
levels. Provide adequate distance from building
lowing data is necessary:
foundations or use impermeable liner on side of
• Existing and proposed site, topographic and
location maps, and field reviews.
Elevations must be carefully worked out to ensure
• Impervious and pervious areas. Other means
may be used to determine the land use data.
no more than the maximum design depth and
excavated area nearest to structure.
Challenges and Potential Solutions for Coastal
Areas
• Poorly Drained Soils—This condition minimizes
the ability of bioretention areas to reduce
stormwater runoff rates and volumes. One
solution would be to include an underdrain
system. An alternative would be to use a small
stormwater wetlands or wet swales to intercept
and treat stormwater runoff.
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Stormwater Best Management Practices
Other Constraints / Considerations
be integrated into the site planning process and
aesthetic and maintenance considerations should
be taken into account in their siting and design.
that the desired runoff flow enters the facility with
velocity.
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site.
• Design data from nearby storm sewer
structures.
• Water surface elevation of nearby water
systems as well as the depth to seasonally high
groundwater.
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See Figure 4.2-5 and Figure 4.2-6 for an overview
• A bioretention area consists of the following:
of the various components of a bioretention area.
1. A pretreatment area, usually consisting of
a grass filter strip between the contributing
drainage area and the ponding area or a
forebay to ease maintenance of the mulch,
sand, or soil layers.
2. Ponding area containing vegetation with an
engineered planting media.
3. Organic/mulch layer to protect planting
media.
4. Native soils to infiltrate the treated runoff,
(see description of infiltration trenches,
Section 4.12, for infiltration criteria).
5. Where native soils have low infiltration
rates include gravel and perforated pipe
underdrain system to collect runoff that
has filtered through the soil layers and pipe
it to the storm sewer system. An upturned
underdrain system can be used, however, the
system should be 12-18” below the bottom
of the planted area to reduce saturated
conditions in root zone.
6. Overflow, diversion or bypass structure to
safely route larger storms through or around
the bioretention area.
• A bioretention area design may include some
of the following:
»» Optional level spreader to spread and filter
runoff.
»» For curbed pavements use an inlet deflector
to direct flow into the practice.
»» A splash/erosion prevention pad at the inlet
to the practice.
4.2.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Recommended minimum dimensions of a
bioretention area are 3-6% of the total drainage
area, though modeling is recommended to
accurately size the area.
»» Topsoil Content: Soils should contain 20%30% topsoil
»» Organic Matter Content: Soils should
contain 10%-25% organic matter
»» Clay: Soils should contain less than 15%
»» Infiltration Rate: Soils should have an
infiltration rate of at least 0.50 inches per
hour (in/hr), although an infiltration rate of
between 2 and 4 in/hr is preferred
• The maximum recommended ponding depth
of the bioretention areas is 12 inches.
»» Phosphorus Index (P-Index): Soils should
have a P-Index of less than 30
• A grass filter strip or channel can be used for
pretreatment. The length of the grass channel
or width of the grass filter strip depends on
the drainage area, land use, and channel
slope. Design guidance on grass channels
for pretreatment can be found in Section 4.9
(Grass Channel) and filter strips can be found
in Section 4.29 (Vegetated Filter Strip). A pea
gravel diaphragm flow spreader can also be
used.
»» Exchange Capacity (CEC): Soils should have
a CEC that exceeds 10 milliequivalents (meq)
per 100 grams of dry weight
Stormwater Best Management Practices
4.2.5.2 GENERAL DESIGN
»» pH: Soils should have a pH of 6-8
For additional information on the soils for a Bioretention Area, refer to Appendix D.
• The mulch layer should consist of 3 to 4 inches
of triple-shredded hardwood mulch. This
provides additional benefits such as removing
sediment and metals and retaining soil
moisture.
• If the native soils cannot suffice for the planting
media used within the bioretention area
planting beds, then an engineered soil mix
should be provided that meets the following
specifications:
»» Texture: Sandy loam or loamy sand
»» Sand Content: Soils should contain 35%-60%
clean, washed sand
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Stormwater Best Management Practices
Figure 4.2.-5 Schematic of Typical Bioretention Area Without an Underdrain
(Source: AECOM 2015)
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Figure 4.2.-6 Schematic of a Typical Bioretention Area with an Upturned Underdrain
(Source: AECOM, 2015)
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internal water levels, should be equipped with a 4-6-inch perforated PVC pipe
(AASHTO M 252) in an 8-inch gravel layer. The pipe should have 3/8-inch
perforations, spaced at a minimum of 6-inch centers, with a minimum of 4
holes per row. The pipe is spaced at a maximum of 10 feet on center and a
minimum grade of 0.5% must be maintained. Should the size of the pipe need
to be bigger, a qualified licensed professional should model the area to ensure
the size of the pipe is sufficient. It is recommended to separate the stone and
the soil in the practice. This can be done by placing a permeable geotextile
fabric between the gravel layer and the planting media. Alternatively, a stone
choker layer 2-3 inches deep of #89 stone could be used to separate the
practice from the soil. It is recommended that a qualified licensed professional
be consulted to determine how this should be done based on the native soils
and other physical properties around the practice.
Stormwater Best Management Practices
It is recommended that the underdrain collection system, to monitor the
4.2.5.4 PRETREATMENT/INLETS
• Adequate pretreatment is provided when a forebay, grass filter strip, or grass
channel is provided.
• Inlet protection should be designed to reduce the velocity and energy
of stormwater entering the practice and prevent scour of the mulch and
plantings. Inlet protection may include splash blocks, a stone diaphragm, a
level spreader or another similar device.
Figure 4.2.-7 Schematic of a Typical Bioretention Area with Underdrain
(Source: AECOM, 2015)
4.2.5.5 OUTLET STRUCTURES
• Outlet structures should be included in the design of a bioretention
area configuration to ensure that larger storms can be bypassed without
damaging the practice. Exceptions include small bioretention areas with
flow bypass structures. Outlet configurations can include riser boxes and/
or emergency spillway channels.
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• Bioretention areas generally do not require any
special safety features, provided side slopes
are maintained at 3:1 or flatter. Fencing of
bioretention area facilities is not generally
desired.
4.2.5.7 LANDSCAPING
• Landscaping is critical to the performance and
function of bioretention areas; the vegetation
filters and transpires runoff and the root
systems encourage infiltration.
• Vegetation should be selected to match the
look and maintenance effort desired by locals
and those responsible for maintaining the
facility.
• The bioretention area should be vegetated to
resemble a terrestrial forest ecosystem, with a
mature tree canopy, subcanopy of understory
trees, shrub layer, and herbaceous ground
cover. Three species each of trees, shrubs,
and grass/herbaceous species should be
planted to avoid creating a monoculture. When
determining what trees should be planted
in the bioretention area, remember that tree
leaves can clog the bioretention area. Consider
using trees that only drop their leaves once in
the fall.
• Choose plants based on factors such as
whether they are native or not, resistance
to drought and inundation, cost, aesthetics,
maintenance, etc. Planting recommendations
for bioretention area facilities are as follows:
»» Native plant species should be preferred over
non-native species.
»» Vegetation should be selected based on a
specified zone of hydric tolerance.
»» A selection of trees with an understory of
shrubs and herbaceous materials should be
provided.
• Additional information and guidance on the
appropriate woody and herbaceous species
appropriate for bioretention areas in Georgia,
and their planting and establishment, can be
found in Appendix D.
4.2.5.9 CONSTRUCTION AND MAINTENANCE
COSTS
• A budget level construction cost estimate is
between $3.00 and $10.00 per square foot.
• A budget level maintenance cost estimate
is between $1.50 and $3.50 per square foot
annually.
Stormwater Best Management Practices
4.2.5.6 SAFETY FEATURES
4.2.5.8 CONSTRUCTION CONSIDERATIONS
• Construction equipment should be restricted
from the bioretention area to prevent
compaction of the native soils.
• A dense and vigorous vegetative cover should
be established over the contributing pervious
drainage areas before runoff can be accepted
into the facility. Otherwise the sediment from
the stormwater runoff will clog the pores in the
planting media and native soils.
• Woody vegetation should not be specified at
inflow locations.
• Plants should be installed prior to mulch.
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(Step 1) Determine if the development site and conditions are ap-
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
propriate for the use of a bioretention area.
the design elements of the practice can be determined (i.e.
Consider the application and site feasibility criteria in this
planting media, underdrain, inlet/outlet, overflow, etc.)
chapter. In addition, determine if site conditions are suitable
for an bioretention area. Create a rough layout of the bio-
Complete Step 3A, 3B, and 3C for a runoff reduction approach, or
retention area dimensions taking into consideration existing
skip Step 3 and complete Steps 4A and 4B for a water quality (treat-
trees, utility lines, and other obstructions.
ment) approach. Refer to your local community’s guidelines for any
additional information or specific requirements regarding the use of
(Step 2) Determine the goals and primary function of the bioreten-
either method.
tion area.
Consider whether the bioretention area is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Calculate the Runoff Reduction Volume using the following
formula:
Stormwater Best Management Practices
4.2.6 Design Procedures
RRV = (P) (RV) (A) / 12
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
»» Provide a possible solution to a drainage problem
RV = 0.05+0.009(I)
RV = Volumetric runoff coefficient which can be
found by:
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
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Provide pretreatment by using a grass filter strip or pea gravel
up the appropriate runoff reduction percentage (or credit)
diaphragm, as needed, (sheet flow), or a grass channel or
provided by the practice:
forebay (concentrated flow). Where filter strips are used,
100% of the runoff should flow across the filter strip. Pre-
Using the RRV calculated above, determine the minimum
treatment may also be desired to reduce flow velocities or
Volume of the Practice (VP)
assist in sediment removal and maintenance. Pretreatment
can include a forebay, weir, or check dam. Splash blocks
(VPMIN) ≥ RRv (target) / (RR%)
or level spreaders should be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour.
Where:
Forebays should be sized to contain 0.1 inches per imperRR% = Runoff Reduction percentage, or credit,
vious acre of contributing drainage. Refer to Section 4.9
assigned to the specific practice
for design criteria for a grass channel and Section 4.29 for
VPMIN = Minimum storage volume required to
vegetated filter strips.
provide Runoff Reduction Target Volume (ft )
3
RRv (target) = Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
(Step 3C) Determine whether the minimum storage volume was met
When the VP ≥ VPMIN, then the Runoff Reduction require-
(Step 3B) Determine the storage volume of the practice and the
ments are met for this practice. Proceed to Step 5.
Pretreatment Volume
To determine the actual volume provided in the bioretention
When the VP < VPMIN, then the BMP must be sized according
area, use the following equation:
to the WQV treatment method (See Step 4).
VP = (PV + VES (N))
Where:
VP = Volume provided (temporary storage)
PV = Ponding Volume
VES = Volume of Engineered Soils
N = Porosity
To determine the porosity, a qualified licensed professional
should be consulted to determine the proper porosity based
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
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Calculate the Water Quality Volume using the following for-
To determine the minimum surface area of the bioretention
area, use the following formula:
mula:
Af = (WQv) (df) / [ (k) (hf + df) (tf)]
WQV = (1.2) (RV) (A) / 12
Where:
Where:
Af = surface area of ponding area (ft2)
WQV = Water Quality Volume (ft3)
WQv = water quality volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
df = media depth (ft)
RV = Volumetric runoff coefficient which can be
k = coefficient of permeability of planting media
found by:
(ft/day) (use 1 ft/day for silt-loam if engineered soils
is being used)
RV = 0.05+0.009(I)
hf = average height of water above bioretention
area bed (ft)
Where:
tf = design planting media drain time (days) (1 day is
I = new impervious area of the contributing
recommended maximum)
Stormwater Best Management Practices
(Step 4A) Calculate the Target Water Quality Volume
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
storm). See Subsection 3.1.7.5 or Appendix B-2 for a detailed
(Step 4B) If using the practice for Water Quality treatment, deter-
bioretention area design example
mine the footprint of the bioretention area practice and the
pretreatment volume required
(Step 6) Size flow diversion structure, if needed
The peak rate of discharge for the water quality design storm
A flow regulator (or flow splitter diversion structure) should
is needed for sizing of off-line diversion structures (see Sub-
be supplied to divert the WQv (or RRv) to the bioretention
section 3.1.7). If designing off-line, follow steps (a) through
area.
(d) below:
Size low flow orifice, weir, or other device to pass Qwq.
.
(a) Using WQv, compute CN
(b) Compute time of concentration using TR-55 method
(c) Determine appropriate unit peak discharge from time of
(Step 7) Size underdrain system
See Subsection 4.2.5.3 (Physical Specifications/Geometry)
concentration
(d) Compute Qwq from unit peak discharge, drainage area,
and WQv.
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An overflow must be provided to bypass and/or convey
larger flows to the downstream drainage system or stabilized
watercourse. Non-erosive velocities need to be ensured at
the outlet point. The overflow should be sized to safely pass
the peak flows anticipated to reach the practice, up to a 100year storm event.
(Step 9) Prepare Vegetation and Landscaping Plan
A landscaping plan for the bioretention area should be prepared to indicate how it will be established with vegetation.
See Subsection 4.2.5.7 (Landscaping) and Appendix D for
more details.
Stormwater Best Management Practices
(Step 8) Design emergency overflow
See Appendix B-2 for a Bioretention Area Design Example
4.2.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Longitudinal slopes must be less than 5%
• Minimum 2 foot width
• Side slopes 3:1 or flatter; 4:1 recommended
• Length is usually the length of adjacent paved area being treated
• Sized to capture the water quality peak flow rate of discharge
• Pretreatment most commonly provided through a filter strip
ADVANTAGES / BENEFITS
• Requires minimal land
• Reduces runoff volume and velocity
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
Description: Specialized media filtration BMP
typically used in longitudinal applications to treat
DISADVANTAGES / LIMITATIONS
• Limited to sheet flow uses only
• Not suitable for embankment slopes steeper than 3:1
• Does not meet quantity control stormwater requirements
stormwater along an impervious area (road, parking lot, etc.).
LID/GI Consideration: Adaptable to many linear
situations, and often a small BMP used to treat
runoff close to the source.
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.3 Bioslope
Capital Cost
MAINTENANCE REQUIREMENTS
• Avoid damaging or rutting the permeable soil layer when mowing
grass
• Remove sediment and debris from filter strip and adjacent areas
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: No
Drainage Area: Contributing
upstream flow must be less than 150 feet
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
Soils: No restrictions
Other Considerations: Permeable soil layer
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• HSG A & B: 50% of the runoff reduction volume
provided
• HSG C & D: 25% of the runoff reduction volume
provided
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Bioslopes (also referred to as ecology embank-
4.3.2 Stormwater Management Suitability
ments) are water quality best management prac-
Bioslopes are designed primarily for stormwater
tices that use a permeable engineered soil media
quality and have only a limited ability to provide a
to capture and treat stormwater runoff from adja-
small amount of channel protection volume.
cent paved areas (see Figure 4.3-1). Bioslopes are
typically installed along embankments or other
slopes and designed to treat sheet flow stormwater runoff.
Bioslopes are designed with limited longitudinal
slopes to force the flow to be slow and uniform,
thus allowing for particulates to settle and limiting the effects of erosion. Once infiltrated into
the highly permeable engineered soil layer, an
underdrain is typically used to remove the treated
stormwater from the embankment or slope.
Larger flow rates, from less frequent storm events,
• Runoff Reduction
Bioslopes are an effective low impact
development (LID) practice that can be used
in Georgia to reduce post-construction
stormwater runoff and improve stormwater
runoff quality. Like other LID practices, they
become even more effective the higher the
infiltration rate of the native soils. Bioslopes
can be designed to provide 50% of the runoff
reduction volume for type A and B hydrologic
soils or 25% of the runoff reduction volume for
type C and D hydrologic soils.
in the form of sheet flow bypass the engineered
soil media by overtopping and continuing down
the embankment or slope.
Figure 4.3-1 Bioslope Example
• Water Quality
Bioslopes rely primarily on filtration through
an engineered media to provide removal of
stormwater contaminants. The pretreatment
component, commonly a vegetated filter strip,
is most effective at sediment/debris removal,
whereas the engineered media is capable of
removing other pollutants. Subsection 4.3.3
provides median pollutant removal efficiencies
that can be used for planning and design
purposes.
• Channel Protection
Generally, only the WQv is treated by a
bioslope, so another BMP must be used to
provide CP v extended detention. However, for
some smaller sites, a bioslope could provide
some benefit towards detaining a portion of
the full CP v.
Stormwater Best Management Practices
4.3.1 General Description
• Overbank Flood Protection
Bioslopes do not provide stormwater quantity
control and should be designed to safely pass
overbank flood flows. Another BMP must be
used in conjunction with a bioslope to reduce
the post-development peak flow of the 25year storm (Qp25) to pre-development levels
(detention).
• Extreme Flood Protection
Bioslopes do not provide stormwater quantity
control and should be designed to safely pass
overbank flood flows. Another BMP must be
used in conjunction with a bioslope to reduce
the post-development peak flow of the 100year storm (Qf) to pre-development levels
(detention).
(adapted from NCHRP, 2006)
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4.3.4 Application and Feasibility Criteria
The following criteria should be evaluated to
Bioslopes are presumed to be able to remove 85%
Bioslopes can be used in a variety of development
ensure the suitability of a bioslope for meeting
of the total suspended solids load in typical urban
types; however, they are primarily applicable to
stormwater management objectives on a site or
post-development runoff when sized, designed,
linear roadway applications where a rural (no curb
development.
constructed and maintained in accordance with
and gutter) cross section is utilized. The impervi-
the recommended specifications. Undersized or
ous cover in the contributing drainage area is rel-
General Feasibility
poorly designed bioslopes can reduce TSS remov-
atively small, and consists of paved areas adjacent
- Suitable for Residential Subdivision Usage – YES
al performance.
to the embankment or slope where the practice
- Suitable for High Density/Ultra Urban Areas – NO
has been installed.
- Regional Stormwater Control – NO
conservative average pollutant reduction percent-
Since bioslopes require a relatively small amount
Physical Feasibility - Physical Constraints at
ages for design purposes derived from sampling
of land, they are more commonly selected when
Project Site
data, modeling and professional judgment. In
right-of-way availability is limited. Other prac-
a situation where a removal rate is not deemed
tices that would be required at the bottom of the
• Drainage Area – Contributing upstream flow
must be less than 150 feet
sufficient, additional controls may be put in place
embankment or slope, such as enhanced swales
at the given site in a series or “treatment train”
or grass channels, most always require a larger
approach.
amount of land. Bioslopes may not be desirable
The following design pollutant removal rates are
Stormwater Best Management Practices
4.3.3 Pollutant Removal Capabilities
• Space Required – N/A
• Site Slope – Typically no more than 5%
in some more aesthetically landscaped or sodded
-- Total Suspended Solids – 85%
areas, due to the need for keeping the media layer
• Minimum Head – N/A
-- Total Phosphorus –60%
free and clear of plantings and other obstructions.
• Minimum Depth to Water Table – 2 feet
required between the bottom of the media
layer and the elevation of the seasonally high
water table
-- Total Nitrogen –25%
-- Fecal Coliform – 60%
-- Heavy Metals – 75%
Only when the same infiltration rate of the media
could be preserved, can grass be planted directly
on top of the engineered media. This may include
frequently maintained “half-cut” sod or similar
vegetation.
For additional information and data on pollutant
• Soils – Engineered media
removal capabilities for bioslopes, see research
The topography and location of a site will de-
Other Constraints / Considerations
and testing results from the Washington State De-
termine the applicability of the use of bioslopes.
partment of Transportation and published removal
Overall, the topography should allow for the
• Aquifer Protection – Exfiltration should not be
allowed in hotspot areas
rates from a synthesis performed by the National
design of a bioslope with sufficient slope and
Cooperative Highway Research Program.
cross-sectional area to maintain non-erosive
velocities.
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The following equation and variables are used for
Qwq = water quality volume peak flow
The following criteria are to be considered
minimum standards for the design of a bioslope.
Consult with the local review authority to determine if there are any variations to these criteria or
the sizing of a bioslope:
(ft3/s, refer to Subsection 3.1.7.2)
SF = safety factor equal to 1 (unitless,
W = C Qwq SF
KL
typical throughout Georgia)
k = infiltration rate, use long-term
additional standards that must be followed.
infiltration rate of 10 (inches/hour)
L = bioslope length (perpendicular with
Where:
4.3.5.1 LOCATION AND LAYOUT
• A bioslope should be sited such that the
topography allows for the design with
sufficiently mild slope and cross-sectional area
to maintain non-erosive velocities.
W = bioslope width (parallel with flow
flow path) (feet)
path) (feet)
C = conversion factor = 43,200 [(in/hr)/(ft/s)]
• Bioslopes should have a maximum contributing
upstream flow length of 150 feet, or less.
Stormwater Best Management Practices
4.3.5 Planning and Design Criteria
4.3.5.2 GENERAL DESIGN
• Bioslopes are designed to treat the WQv
through a flow rate-based design, and to safely
pass larger storm flows by checking velocities.
Flow enters the bioslope via sheet flow through
a pretreatment filter strip area, or a minimum 2
foot wide grass strip.
• A bioslope consists of a permeable soil mixture
that overlays an underdrain system. Flow enters
the media layer where it is filtered through the
soil bed. Runoff is collected and conveyed by a
perforated pipe and gravel underdrain system to
the outlet. Figure 4.3-2 provides a schematic
for typical components of a bioslope.
Figure 4.3-2 Schematic of Bioslope
(Source: GDOT Drainage Manual, 2014)
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• Embankment slopes should be 3:1 or flatter.
When slopes steeper than 4:1 are used,
additional measures should be taken to ensure
stabilization of vegetation along the slope.
• Longitudinal slopes (parallel with the
embankment) should be no more than 5%.
• The area between the impervious surface, or
paved area, should preferably be no more than
30 feet to avoid reconcentration of stormwater
runoff creating erosion and scour through the
engineered media.
• The bioslope consists of a permeable soil
layer of at least 12 inches in depth, above an
underdrain. The soil media should contain a
mixture of crushed rock, dolomite, gypsum,
and perlite as shown in Table 4.3-1. This
mixture has an initial infiltration rate of 50
inches per hour and an infiltration rate of 28
inches per hour long-term. For sizing, an
infiltration rate of 10 inches per hour is used in
calculations as a factor of safety. A permeable
filter fabric is placed between the media and
underlying soil. Where an underdrain collection
system is utilized, it should be equipped with
at least a 6-inch diameter perforated PVC pipe
longitudinal underdrain in a gravel layer.
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Table 4.3-1 Bioslope Media Mixture
(Source: adapted from WSDOT, 2011)
Engineered Media Amendment
Aggregate:
»» #89 stone
Quantity
3 cubic yards (CY)
(Note: 3 CY is used as a baseline for other
»» No recycled material
mixture components; adjust total quantity
»» Non-limestone material mineral aggregate
based on bioslope dimensions)
Perlite:
»» Horticultural grade, free of any toxic materials
»» 0-30% passing US No. 18 Sieve
1 CY per 3 CY of mineral aggregate
»» 0-10% passing US No. 30 Sieve
Dolomite: CaMg(CO3)2 (calcium magnesium carbonate)
»» Agricultural grade, free of any toxic materials
»» 100% passing US No. 8 Sieve
10 pounds per CY of perlite
Stormwater Best Management Practices
4.3.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
»» 0% passing US No. 16 Sieve
Gypsum: Non-calcined, agricultural gypsum CaSO4•2H2O
(hydrated calcium sulfate)
»» Agricultural grade, free of any toxic materials
1.5 pounds per CY of perlite
»» 100% passing US No. 8 Sieve
»» 0% passing US No. 16 Sieve
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4.3.5.8 LANDSCAPING
4.3.5.10 CONSTRUCTION CONSIDERATIONS
• Where space allows, vegetated filter strips
should be used as the pretreatment device
for bioslopes. See Section 4.29 for more
information on vegetated filter strips.
• Surrounding vegetation is typically a grassed or
vegetated filter strip, Vegetation should be well
established and thriving to prevent erosion and
sedimentation.
• Construction equipment should be restricted
from the bioslope area to prevent compaction
of the native soils.
• Where space does not allow for a full width
vegetated filter strip, a grassed area can still
be used as pretreatment to the best extent
possible.
• A pea gravel diaphragm can also be used for
pretreatment where space constraints do not
allow for enough of a grassed area between
the bioslope and the paved surface.
4.3.5.9 ADDITIONAL SITE-SPECIFIC DESIGN
CRITERIA AND ISSUES
Physiographic Factors - Local terrain design
constraints
• Low Relief – Ideal for embankment sheet flow
conditions
• High Relief – Often infeasible if longitudinal
slopes are greater than 5%
• A dense and vigorous vegetative cover should
be established over the contributing pervious
drainage areas before runoff can be accepted
into the bioslope. Otherwise the sediment
from the stormwater runoff will clog the pores
in the planting media and native soils.
Stormwater Best Management Practices
4.3.5.4 PRETREATMENT/INLETS
4.3.5.5 OUTLET STRUCTURES
• The underdrain system should discharge to a
storm drainage structure or a stable outfall.
• Karst – No exfiltration of hotspot runoff from
bioslopes; use impermeable liner
Soils
4.3.5.6 EMERGENCY SPILLWAY
• Bioslopes must be adequately designed to
safely pass flows that exceed the design storm
flows.
No additional criteria
Special Downstream Watershed Considerations
• Aquifer Protection – No exfiltration of hotspot
runoff from bioslopes; use impermeable liner
4.3.5.7 MAINTENANCE ACCESS
• Adequate access should be provided for all
bioslopes for inspection and maintenance.
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(Step 1) Determine if the development site and conditions are appropriate for the use of an bioslope
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Consider the application and site feasibility criteria in this
chapter. In addition, determine if site conditions are suitable
Complete Step 3A, 3B, and 3C for a runoff reduction ap-
for a bioslope. Create a rough layout of the bioslope dimen-
proach, or skip Step 3 and complete Step 4A and 4B for a
sions taking into consideration existing trees, utility lines, and
water quality (treatment) approach. Refer to your local com-
other obstructions.
munity’s guidelines for any additional information or specific
requirements regarding the use of either method.
(Step 2) Determine the goals and primary function of the bioslope.
Consider whether the bioslope is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3A)Calculate the Stormwater Runoff Reduction Target Volume
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Stormwater Best Management Practices
4.3.6 Design Procedures
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
RV = Volumetric runoff coefficient which can be
»» Provide a possible solution to a drainage problem
RV = 0.05+0.009(I)
found by:
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
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Provide pretreatment by using a grass filter strip or pea gravel
up the appropriate runoff reduction percentage (or credit)
diaphragm, as needed, (sheet flow), or a grass channel or
provided by the practice:
forebay (concentrated flow). Where filter strips are used,
100% of the runoff should flow across the filter strip. Pre-
Using the RRV calculated above, determine the minimum
treatment may also be desired to reduce flow velocities or
Volume of the Practice (VP)
assist in sediment removal and maintenance. Pretreatment
can include a forebay, weir, or check dam. Splash blocks
(VPMIN) ≥ RRv (target) / (RR%)
or level spreaders should be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour.
Where:
Forebays should be sized to contain 0.1 inches per imperRR% = Runoff Reduction percentage, or credit,
vious acre of contributing drainage. Refer to Section 4.9
assigned to the specific practice
for design criteria for a grass channel and Section 4.29 for
VPMIN = Minimum storage volume required to
vegetated filter strips.
provide Runoff Reduction Target Volume (ft )
3
RRv (target) = Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
(Step 3C) Determine whether the minimum storage volume was met
When the VP ≥ VPMIN, then the Runoff Reduction require-
(Step 3B)Determine the storage volume of the practice and the Pre-
ments are met for this practice. Proceed to Step 5.
treatment Volume
To determine the actual volume provided in the bioslope, use
When the VP < VPMIN, then the BMP must be sized according
the following equation:
to the WQV treatment method (See Step 4A).
VP = (PV + VES (N))
Where:
VP = Volume provided (temporary storage)
PV = Ponding Volume
VES = Volume of Engineered Soils
N = Porosity
To determine the porosity, a qualified licensed professional
should be consulted to determine the proper porosity based
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
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Calculate the Water Quality Volume using the following
See Subsection 4.3.5.3 (Physical Specifications / Geometry)
for more details
formula:
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
WQV = (1.2) (RV) (A) / 12
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
storm). See Subsection 3.1.7.5 for more information
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
(Step 6) Size underdrain system
See Subsection 4.2.5.3 (Physical Specifications/Geometry)
RV = Volumetric runoff coefficient which can be
found by:
4.3.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
RV = 0.05+0.009(I)
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for main-
Where:
Stormwater Best Management Practices
(Step 4A) Calculate the Target Water Quality Volume
tenance and maintenance access. For additional information on inspection
I = new impervious area of the contributing
and maintenance requirements, see Appendix E.
drainage area (%)
A = Area draining to this practice (ft2)
Regular inspection and maintenance is critical to the effective operation of
12 = Unit conversion factor (in/ft)
a bioslope as designed. Maintenance responsibility for a bioslope should be
vested with a responsible authority by means of a legally binding and enforce-
(Step 4B) If using the practice for Water Quality treatment, deter-
able maintenance agreement that is executed as a condition of plan approval.
mine the length and width of the bioslope practice and the
Pretreatment Volume required
Determine Bioslope Dimensions:
Refer to Subsection 3.1.7.2 and 3.1.7.5 to calculate the water
quality volume peak flow rate using a reduced CN for any
RRV provided, then find either length or width using the
equation in Subsection 4.3.5. In most applications, site restrictions limit the available length.
»» Longitudinal slope cannot exceed 5% (2 to 4%
recommended)
»» Width should be a minimum of 2 feet
»» Ensure that side slopes are no steeper than 3:1 (4:1
recommended)
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Figure 4.4.1 Downspout Disconnect
Source: (Center for Watershed Protection)
Description: Where site characteristics permit,
KEY CONSIDERATIONS
DESIGN CRITERIA
• Maximum length of flow path in contributing drainage areas is 75 feet
• Minimum length of flow path in pervious areas below downspout
disconnects is 15 feet and equal to or greater than the length of the flow path
in the contributing drainage area
• Maximum impervious rooftop drainage area to one disconnected downspout
is 2,500 square feet
• Maximum slope of pervious area beneath the downspout is 6 percent
• Runoff must be conveyed as sheet flow from the downspout and across open
areas to maintain proper disconnect
• Downspout disconnects should be designed to convey stormwater runoff
away from buildings to prevent damage to building foundations
STORMWATER MANAGEMENT
SUITABILITY
ADVANTAGES / BENEFITS
• Helps restore pre-development hydrology on development sites
• Reduces post-construction stormwater runoff rates, volumes and pollutant
loads
• Relatively low construction cost and long-term maintenance burden
• Encourages groundwater recharge
IMPLEMENTATION CONSIDERATIONS
downspout disconnects can be used to spread
rooftop runoff from individual downspouts
across lawns and other pervious areas, where it is
slowed, filtered and allowed to infiltrate into the
native soils.
DISADVANTAGES / LIMITATIONS
• Provides greater stormwater management benefits on sites with permeable
soils (i.e., hydrologic soil group A and B soils)
• Level spreaders must be needed at the downspout to dissipate flow
• Clay soils or soils that have been compacted by construction equipment
greatly reduce the effectiveness of this practice, and soil amendments may
be needed
LID/GI Considerations: If properly designed,
downspout disconnects can provide measurable
reductions in post-construction stormwater runoff rates, volumes and pollutant loads on development sites.
ROUTINE MAINTENANCE REQUIREMENTS
• Maintenance of areas receiving disconnected runoff is generally the same as
that required for other lawn or landscaped areas.
• Areas receiving runoff should be protected from future compaction (e.g., by
planting trees or shrubs along the perimeter).
• Gutters and downspouts should be kept clear of dirt, debris, vegetation, and
other buildup.
• Downspout disconnects are often used in conjunction with other BMPs.
Ensure that upstream and/or downstream BMPs are maintained in
accordance with this manual.
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
Land Requirement
Stormwater Best Management Practices
4.4 Downspout Disconnects
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Not recommended
Roadway Projects: Not applicable
Soils: Disconnects should be directed
over HSG A, B, or C (e.g., sands, sandy
loams, loams).
Other Considerations: Erosion and
sediment control practices should not
be located in vegetated areas receiving disconnected runoff. Construction
vehicles and equipment should avoid
areas receiving disconnected runoff to
minimize disturbance and compaction.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 50% of the RRV conveyed to the
practice (A & B hydrologic soils)
• 25% of the RRV conveyed to the
practice (C & D hydrologic soils)
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Stormwater Best Management Practices
4.4.1 General Description
As the name implies, a downspout disconnect
is the most basic of all low impact development
practices that can be used to “receive” rooftop
runoff. Where site characteristics permit, they can
be used to spread rooftop runoff from individual downspouts across lawns and other pervious
areas, where it is slowed, filtered, and allowed
to infiltrate into the soil. If properly designed,
downspout disconnects can provide measurable
reductions in post-construction stormwater runoff rates, volumes, and pollutant loads on development sites.
In order to use downspout disconnects to receive
post-construction stormwater runoff, down-
Figure 4.4.2 Downspout Disconnects to Pervious Areas
(Source: Center for Watershed Protection)
spouts must be designed to discharge to a lawn
or other pervious area (Figure 4.4.2). The pervious
consider using other low impact development
area located below the downspout disconnect
practices. A typical schematic for a downspout
should slope away from buildings and other im-
disconnect is shown in Figure 4.4-3.
pervious surfaces to prevent damage to building
foundations and discourage rooftop runoff from
entering the storm drain system.
The primary concern associated with a downspout disconnect (Figure 4.4.3) is the length of
the flow path over the lawn or other pervious
area below the disconnection point. To provide
adequate residence time for stormwater runoff,
the length of the flow path in the pervious area
below a downspout disconnect should be equal
to or greater than the length of the flow path
of the contributing drainage area. If this cannot
be accomplished, due to site characteristics or
constraints, site planning and design teams should
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• Runoff Reduction
Runoff reduction credit can be applied to the stormwater runoff generated
from the rooftop drainage area that drains to a properly designed, installed,
and maintained downspout disconnect. As shown in Table 4.1.3-2, the
runoff reduction volume (RRv) conveyed through a downspout disconnect
located on A/B or amended soils is reduced by 50%. Reduce the RRv
conveyed through a downspout disconnect located on C/D soils by 25%.
• Water Quality
If installed as per the recommended design criteria and properly
maintained, 80% total suspended solids removal will be applied to the water
quality volume (WQv) flowing to the disconnected downspout.
• Channel Protection
No channel protection volume (CP v) storage is provided by a disconnected
downspout. Stormwater runoff generated by the impervious rooftop area
and pervious receiving area should be routed to a downstream regional
BMP that provides storage and treatment of the CP v. Proportionally adjust
the post-development runoff curve number (CN) to account for the runoff
reduction provided by a downspout disconnect for the contributing
drainage area when calculating the channel protection volume for the
regional BMP.
Stormwater Best Management Practices
4.4.2 Stormwater Management Suitability
• Overbank Flood Protection
Proportionally adjust the post-development runoff CN to account for the
runoff reduction provided by a downspout disconnect for the contributing
drainage area when calculating the overbank peak discharge (Qp25) on a
development site.
Figure 4.4-3 Schematic of Downspout Disconnect
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• Extreme Flood Protection
No Extreme Flood Protection volume is provided by a downspout
disconnect. Proportionally adjust the post-development runoff curve
number (CN) to account for the runoff reduction provided by a downspout
disconnect for the contributing drainage area when calculating the extreme
peak discharge (Qf) on a development site.
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designers, and developers should ensure that the
Downspout disconnects are presumed to remove
pervious area beneath the downspout is at least
80% of the total suspended solids (TSS) in typi-
the same length as the flow path of the contribut-
cal urban post-development runoff when sized,
ing runoff. Additionally, disconnected stormwater
designed, constructed, and maintained in ac-
runoff should not be allowed to “reconnect”, or
cordance with the recommended specifications.
flow across impervious areas, before reaching a
Other pollutants that can be removed by down-
downstream regional BMP or being discharged
spout disconnects include Phosphorus, Nitrogen,
off-site.
and metals (such as Cadmium, Copper, Lead, and
Zinc).
General Feasibility
- Suitable for Residential Subdivision Usage – YES
In order to provide the most efficient pollutant
- Suitable for High Density/Ultra Urban Areas – NO
removal, downspout disconnects should be sized
- Regional Stormwater Control – NO
with the appropriate length, width, and slope. The
maximum drainage area should not be exceeded,
Physical Feasibility - Physical Constraints at
and rooftop runoff should enter the disconnect-
Project Site
ed area as sheet flow to ensure proper pollutant
• Drainage Area – The contributing impervious
rooftop drainage area should be 2,500
square feet or less per downspout. If the total
rooftop area is greater than 2,500 square feet,
the rooftop can be divided up into multiple
drainage areas with each drainage area flowing
to a separate, properly sized, downspout
disconnect.
removal.
For additional information and data on pollutant
removal capabilities for downspout disconnects,
see the National Pollutant Removal Performance
Database (Version 3) available at www.cwp.org
and the National Stormwater Best Management
Practices (BMP) Database at www.bmpdatabase.
org.
4.4.4 Application and Site Feasibility
Criteria
Downspout disconnects are ideal for residential,
• Site Slope – Slope of the pervious area
below the downspout should be a maximum
of 6% and a minimum of 0.5% (1% to 5% is
recommended).
• Minimum Depth to Water Table – Separation
from the water table is not required for a
downspout disconnect.
• Soils – Disconnects should be directed over
HSG A, B, or C (e.g., sands, sandy loams,
loams). Clayey soils or soils that have been
compacted by construction equipment greatly
reduce the effectiveness of this practice, such
that soil amendments may be needed.
Stormwater Best Management Practices
4.4.3 Pollutant Removal Capabilities
• Space Required – The length of the disconnect
should be a minimum of 15 feet and equal
to or greater than the contributing flow path
length. The length of the flow path within the
contributing drainage area should be 75 feet
or less.
commercial, industrial, and municipal development projects where buildings are surrounded
by fairly level lawn and grass areas. Planners,
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• Hot spots – May be used for hot spot runoff
• Damage to existing structures and facilities –
Downspout disconnects should be designed to
convey stormwater runoff away from buildings
to prevent damage to building foundations
• Proximity – Downspout disconnects may be
used without restriction near:
»» Private water supply wells
• Flat Terrain – Downspout disconnects require
minimal slope to treat and convey stormwater
runoff; therefore, they may be used in areas of
flatter terrain.
• Shallow Water Table – Downspout disconnects
may be used with a shallow water table.
4.4.5 Planning and Design Criteria
Before designing the downspout disconnect, the
»» Open water
following data is necessary:
»» Public water supply reservoirs
• Existing and proposed site, topographic, and
location maps, as well as field reviews
»» Public water supply wells
• Property Lines – Downspout disconnects
may be used near property lines; however,
ensure that stormwater runoff is not redirected
to cause additional flooding to another
homeowner.
• Trout Stream – Downspout disconnects help
treat stormwater for pollutants and reduce
the volume and velocity of runoff. Therefore,
downspout disconnects are an effective BMP
for use where trout streams or other protected
waters may receive stormwater runoff.
Coastal Areas
• Poorly Drained Soils – Downspout disconnects
may be used with poorly drained soils;
however, less runoff reduction will be provided
in these cases.
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• The proposed site design including, buildings,
parking lots, sidewalks, stairs, handicapped
ramps, and landscaped areas
• Architectural roof plan for rooftop pitches and
downspout locations
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Information about downstream BMPs and
receiving waters
• Design data from nearby storm sewer
structures
4.4.5.1 LOCATION AND LAYOUT
• Downspout disconnects should be located
where there is at least 15 linear feet of
managed turf, grass, lawn, or landscaped area
at the receiving end of the downspout.
• Disconnects should not be upstream of an
impervious area, which will allow rooftop
runoff to “reconnect”. Locate disconnects
upstream of vegetated or forested areas,
regional BMPs, and/or stormwater drainage
systems.
• The slope of the pervious receiving area should
be between 0.5% and 6%, with 1% to 5% slopes
being ideal.
Stormwater Best Management Practices
Other Constraints / Considerations
• Ensure that the length of each disconnected
area is equal to or greater than the contributing
flow path of the rooftop runoff.
• The length of the contributing flow path should
no greater than 75 feet.
• Rooftop runoff should enter the disconnected
pervious area as sheetflow. Level spreaders
should be used at the discharge end of the
downspout to dissipate energy and spread out
the flow.
The following criteria are to be considered
minimum standards for the design of a downspout disconnect. Consult with the local review
authority to determine if there are any variations
to these criteria or additional standards that must
be met.
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4.4.5.4 CONSTRUCTION CONSIDERATIONS
A level (0.5 % to 6% slope) managed turf, grass,
To help ensure that downspout disconnects are
lawn, or landscaped area at the receiving end of
properly installed on a development site, planning
the downspout, between 15 and 75 feet long.
and design teams should consider the following
Downspout disconnects can be constructed on
recommendations:
HSG A, B, C or D soils.
• Simple erosion and sediment control measures,
such as temporary seeding and erosion control
mats, should be used within the pervious areas
located below downspout disconnects.
4.4.5.3 LANDSCAPING
• Vegetation commonly planted in the pervious
areas located below downspout disconnects
includes turf, shrubs, trees, and other
herbaceous vegetation. Although managed
turf is most commonly used, site planning and
design teams are encouraged to use trees,
shrubs and/or other native vegetation to help
establish mature native plant communities in
the pervious areas located below downspout
disconnects.
• Methods used to establish vegetative cover
within the pervious area below a downspout
disconnect should achieve at least 75 percent
vegetative cover within one year of installation.
• To help prevent soil erosion and sediment loss,
landscaping should be provided immediately
after the downspout disconnect has been
completed. Temporary irrigation may be
needed to quickly establish vegetative cover
within the pervious areas below downspout
disconnects.
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• To help prevent soil compaction, heavy
vehicular and foot traffic should be kept out of
the pervious areas located below downspout
disconnects during and after construction.
Stormwater Best Management Practices
4.4.5.2 GENERAL DESIGN
• Construction contracts should contain a
replacement warranty that covers at least three
growing seasons to help ensure adequate
growth and survival of the vegetation planted
within the pervious area located below a
downspout disconnect.
4.4.5.5 CONSTRUCTION AND MAINTENANCE
COSTS
Construction and maintenance of areas receiving
disconnected runoff is generally no different than
that required for other lawn or landscaped areas.
Soil amendments may be required for downspout
disconnects constructed on HSG C or D soils or
on previously compacted areas, such as gravel
driveways or grassed parking areas.
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(Step 1) Determine the goals and primary function of the downspout
Complete Steps 3A and 3B for a runoff reduction approach,
disconnect.
or skip Step 3 and complete Step 4 for a water quality (treat-
Consider whether the downspout disconnect is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
ment) approach. Refer to your local community’s guidelines
»» Provide a possible solution to a drainage problem.
for any additional information or specific requirements
regarding the use of either method.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume.
Calculate the Runoff Reduction Volume using the following
formula:
Check with local officials and other agencies to determine if
RRV = (P) (RV) (A) / 12
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the downspout disconnect has any special site-specific
Where:
RRV = Runoff Reduction Target Volume (ft3)
design conditions or criteria. List any restrictions or other
P = Target runoff reduction rainfall (inches)
requirements that may apply or affect the design.
RV = Volumetric runoff coefficient which can be
found by:
The design of the downspout disconnect should be centered
on the restrictions/requirements, goals, targets, and primary
RV = 0.05+0.009(I)
function(s) of a downspout disconnect. By considering the
primary function, as well as, topographic and soil conditions,
the design elements of the downspout disconnect can be
Where:
I = new impervious area of the contributing
determined (i.e. planting media, outlet, etc.).
(Step 2) Determine if the development site and conditions are appropriate for the use of a downspout disconnect.
Consider the application and site feasibility criteria in this chapter
Stormwater Best Management Practices
4.4.6 Design Procedures
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
to determine if site conditions are suitable for a downspout
disconnect. Ensure that the drainage area and ground surface
slope do not exceed the maximum criteria. Calculate the drainage area flow path and determine if the disconnected pervious
area is of equal or greater length. Finally, review the NRCS data
and perform site and geotechnical investigations, if necessary, to
determine soil type and condition.
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Where:
Runoff Reduction Credit to the Runoff Reduction Volume
WQV = Water Quality Volume (ft3)
calculated in Step 3A.
1.2 = Target rainfall amount to be treated (inches)
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
RV = Volumetric runoff coefficient which can be
up the appropriate runoff reduction percentage (or credit)
found by:
provided by the downspout disconnect – either 50% for HSG
RV = 0.05+0.009(I)
A/B soils, or 25% for HSG C/D soils. If the practice meets
the 2500 square foot rooftop restriction, the minimum and
maximum slope requirement, and the 15 – 75 foot flow path
Where:
requirement then a credit of 50% or 25% (depending upon
I = new impervious area of the contributing
soil type) of the runoff volume to this practice is awarded.
drainage area (%)
This volume reduction can be used to calculate a reduced
A = Area draining to this practice (ft2)
CN for CPv, Qp25, and Qf calculations.
12 = Unit conversion factor (in/ft)
Stormwater Best Management Practices
(Step 3B) If using the practice for Runoff Reduction, apply the BMP
Multiply the Runoff Reduction Credit by the Runoff Reduc(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
tion Volume (RRv)
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
RRv x Runoff Reduction Credit (%) =
RRv (Credited)
storm). See Subsection 3.1.7.5 for more information.
(Step 6) Prepare a site vegetation and planting plan.
A planting plan for the downspout disconnect area should be
Where:
RRV = Runoff Reduction Volume conveyed to the
prepared to indicate how it will be established with vegeta-
practice
tion.
RR% = Runoff Reduction percentage, or credit,
assigned to the specific practice
See Subsection 4.4.5.3 (Landscaping) and Appendix D for
RRv (Credited) = Runoff Reduction Volume
more details.
provided (ft )
3
4.4.7 Inspection and Maintenance Requirements
(Step 4) Calculate the Target Water Quality Volume.
All best management practices require proper maintenance. Without proper
Calculate the Water Quality Volume using the following
maintenance, BMPs will not function as originally designed and may cease to
formula:
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
WQV = (1.2) (RV) (A) / 12
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and maintenance requirements, see Appendix E.
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Description: A surface storage basin or facility designed to provide water quality treatment
and water quantity control through detention of
stormwater runoff. Dry detention basins differ
from dry extended detention (ED) basins (Section
4.6) in that they do not provide 24-hour detention
of the channel protection volume (CPv).
LID/GI Considerations: While dry detention
basins are primarily used for storage and control
of larger storm events, their use as a BMP contributes to a site’s overall perviousness and aesthetics. The open grassed area of a dry detention
basin can be used for multiple purposes, such as
landscaped or recreational areas. Additionally, dry
detention basins dissipate energy in the stormwater runoff it receives and provide opportunities for
some sedimentation of suspended solids.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Embankments should be less than 20 feet in height and should have side
slopes no steeper than 2:1 (horizontal to vertical) although 3:1 is preferred.
• The depth of the basin should not exceed 10 feet.
• Geotechnical slope stability analysis is recommended for embankments
greater than 10 feet in height.
• All embankments must be designed to State of Georgia guidelines for dam
safety.
• Storage volumes greater than 100 acre-feet are subject to the requirements
of the Georgia Safe Dams Act (Georgia Annotated Code 12-5-370) unless the
facility is excavated to this depth.
• The dry detention basin bottom should be graded toward the outlet to
prevent standing water conditions.
• The outfall(s) of dry detention basins should always be stabilized to prevent
scour.
• An emergency spillway should be provided to safely convey large flood
events.
STORMWATER MANAGEMENT
SUITABILITY
ADVANTAGES / BENEFITS
• Moderate removal rate of urban pollutants
• High community acceptance
• Useful for water quality treatment and flood control
• Dry detention basins can serve multiple use purposes on a development site.
Maintenance Burden
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Capital Cost
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Not recommended
Roadway Projects: Yes
DISADVANTAGES / LIMITATIONS
• Potential for thermal impacts/downstream warming
• Dam height restrictions for high relief areas
• Detention basin drainage can be problematic for low relief terrain.
Soils: Dry detention basins can be
used with almost all soils and geology,
with minor design adjustments for regions of karst topography or in rapidly
percolating soils, such as sand.
ROUTINE MAINTENANCE REQUIREMENTS
• Remove debris from inlet and outlet structures
• Maintain side slopes and outlet structure
• Remove invasive vegetation
• Monitor sediment accumulation and remove periodically
Other Considerations: In order to
convey low flows through dry detention basins, designers should provide a
pilot channel. Plantings on the bank of
the detention pond to provide shade
around the pilot channel and the basin
outlet or designing the pilot channel
as a grass channel can reduce thermal
impacts.
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
Stormwater Best Management Practices
4.5 Dry Detention Basins
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 0% of the runoff reduction volume
provided
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Stormwater Best Management Practices
4.5.1 General Description
Dry detention basins are surface facilities intended
to provide for the temporary storage of stormwater runoff to reduce downstream water quantity
impacts. These facilities temporarily detain stormwater runoff, releasing the flow over a period
of time. They are designed to completely drain
following a storm event and are normally dry
between rain events. See Figure 4.5-1 for a typical
dry detention basin schematic.
Dry detention basins provide limited pollutant
removal benefits and are not intended for runoff
reduction. Detention-only facilities should be
used in a treatment train approach with other
BMPs that provide runoff reduction, additional
water quality treatment, and channel protection
(see Subsection 4.1.6).
Figure 4.5-1 Schematic of Dry Detention Basin
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4.5.3 Pollutant Removal Capabilities
• Runoff Reduction
Another BMP should be used in a treatment
train with dry detention basins to provide runoff
reduction as they are not designed to provide
RRV as a stand-alone BMP.
60% of the total suspended solids (TSS) load in
The following criteria should be evaluated to
ensure the suitability of dry detention basins for
meeting stormwater management objectives on
typical urban post development runoff when
a site.
• Water Quality
If installed as per the recommended design
criteria and properly maintained, 60% total
suspended solids removal will be applied to the
water quality volume (WQv) flowing to the dry
detention basin. Another BMP should be used
in a treatment train with dry detention basins
to provide the additional required water quality
treatment.
• Channel Protection
Dry detention basins are not generally used to
store and treat the channel protection volume
(CP v). A dry extended detention basin (Section
4.6) should be used if the CP v must be treated
within the facility.
• Overbank Flood Protection
Dry detention basins are intended to provide
overbank flood protection (peak flow reduction
of the 25-year storm, Qp25).
• Extreme Flood Protection
Dry detention basins can be designed to
control the extreme flood (100-year, Qf) storm
event.
Dry detention basins are presumed to remove
sized, designed, constructed, and maintained in
accordance with the recommended specifications. Although they can be effective at removing
some pollutants through settling, they are less
effective at removing soluble pollutants because
of the absence of a permanent pool. Other
pollutants that dry detention basins can remove
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas – NO
- Regional Stormwater Control – YES
Physical Feasibility – Physical Constraints at
include Phosphorus, Nitrogen, and metals (such
Project Site
as Cadmium, Copper, Lead, and Zinc).
• Drainage Area – In general, dry detention
basins should be used on sites with a minimum
drainage area of 10 acres.
For additional information and data on pollutant
removal capabilities for dry detention basins, see
the National Pollutant Removal Performance Da-
Stormwater Best Management Practices
4.5.2 Stormwater Management Suitability
• Space Required – Approximately of 2-3% of the
contributing drainage area.
tabase (3rd Edition) available at www.cwp.org and
tices (BMP) Database at www.bmpdatabase.org.
• Site Slope – Dry detention basins can be used
on sites with slopes up to about 15%.
4.5.4 Application and Site Feasibility
Criteria
• Vegetated and rip rap embankments should be
less than 20 feet in height and have side slopes
no steeper than 2:1 (horizontal to vertical),
although 3:1 is preferred.
the National Stormwater Best Management Prac-
Dry detention basins have traditionally been one
of the most widely used stormwater BMPs. Dry
detention basins can easily be designed for flood
control, and this is actually the primary purpose of
• Minimum Depth to Water Table – The base of
the detention facility should not intersect the
groundwater table
most detention basins in the ground today. However, if pollutant removal efficiency is an important consideration, then dry detention basins may
not be the most appropriate choice.
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sinkhole formation.
• Trout Stream – In cold water streams, dry
detention basins should be designed to detain
stormwater for a relatively short time (i.e., less
than twelve hours) to minimize the potential
amount of stream warming that occurs in the
practice. In addition, careful consideration
should be given to the potential of perched or
raised groundwater levels.
Other Constraints / Considerations
Coastal Areas
• Hot spots – Dry detention basins can accept
runoff from stormwater hotspots, but need
significant separation from groundwater when
used for this purpose.
• Poorly Drained Soils – Poorly draining soils
do not generally inhibit a dry detention basin’s
ability to temporarily store and treat stormwater
runoff and completely drain between rain
events, as the bottom of the BMP is sloped to
provide for flow.
• Damage to existing structures and facilities
– Dry Detention basins should be designed to
safely store and/or bypass the overbank flood
(Qp25) and extreme flood (Qf) storms to prevent
overflow or failure, which may cause damage
to site structures and facilities.
• Proximity – The following is a list of specific
setback requirements for the location of a dry
detention basin:
»» 10 feet from building foundations
• Flat Terrain – The local slope should be
relatively flat in order to maintain reasonably
flat side slopes
• Shallow Water Table – Except for the case
of hot spot runoff, the only consideration
regarding ground water is that the base of
the detention facility should not intersect the
ground water table.
4.5.5 Planning and Design Criteria
Before designing dry detention basins, the following data is necessary:
• Existing and proposed site, topographic, and
location maps, as well as field reviews
• Impervious and pervious areas. Other means
may be used to determine the land use data.
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site.
• Design data from nearby storm sewer
structures.
• Water surface elevation of nearby water
systems as well as depth to the seasonally high
groundwater level.
The following criteria are to be considered minimum standards for the design of a dry detention
basin. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be
followed.
4.5.5.1 LOCATION AND LAYOUT
»» 10 feet from property lines
Dry detention basins should be located down-
»» 100 feet from private water supply wells
stream of other BMPs providing runoff reduction
»» 100 feet from open water (measured from
edge of water)
»» 200 feet from public water supply reservoirs
(measured from edge of water)
Stormwater Best Management Practices
• Soils – Dry detention basins can be used in
almost all soils and geology, with minor design
adjustments for regions of karst (i.e., limestone)
topography or in rapidly percolating soils,
such as sand. In these areas, detention basins
should be designed with an impermeable liner
to prevent groundwater contamination or
and/or additional treatment of the water quality
volume (WQv). See Subsection 4.1.6 for more
information on the use of multiple BMPs in a
treatment train.
»» 1,200 feet from public water supply wells
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• Dry detention basins are sized to provide
storage and control for multiple rain events,
including:
»» Storage of the water quality volume (WQv);
»» Temporary storage of the volume of runoff
required to provide overbank flood (Qp25)
protection (i.e., reduce the post-development
peak flow of the 25-year storm event to the
pre-development rate);
»» Control of the 100-year storm (Qf), if
required.
• Routing calculations must be used to
demonstrate that the storage volume is
adequate. See Section 3.3 (Storage Design) for
procedures on the design of detention storage.
• Storage volumes greater than 100 acre-feet are
subject to the requirements of the Georgia Safe
Dams Act (Georgia Annotated Code 12-5-370)
unless the facility is excavated to this depth.
4.5.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Vegetated embankments should be less than
20 feet in height and should have side slopes
no steeper than 2:1 (horizontal to vertical)
although 3:1 is preferred. Riprap-protected
embankments should be no steeper than
2:1. Geotechnical slope stability analysis is
recommended for embankments greater
than 10 feet in height and is mandatory for
embankment slopes steeper than those given
above. All embankments must be designed to
State of Georgia guidelines for dam safety.
• The depth of the basin should not exceed 10
feet.
• Areas above the normal high water elevations
of the detention facility should be sloped
toward the basin to allow drainage. Careful
finish grading is required to avoid creation of
upland surface depressions that may retain
runoff. The bottom area of storage facilities
should be graded toward the outlet to prevent
standing water conditions.
• Designing dry detention basins with a high
length to width ratio (i.e., at least 1.5:1)
and incorporating other design features to
maximize the flow path effectively increases
detention time by eliminating the potential of
flow to short circuit the basin. Designing basins
with relatively flat side slopes can also help to
lengthen the effective flow path.
4.5.5.4 PRETREATMENT/INLETS
• Inflow channels are to be stabilized with flared
aprons, or the equivalent.
• Pretreatment for a dry detention basin is usually
provided by a sediment forebay. The sediment
forebay should be sized to 0.1 inches of runoff
per impervious acre of contributing drainage
area for dry detention basins
• Pretreatment may also be provided by using a
grass filter strip, pea gravel diaphragm, or grass
channel. Where filter strips are used, 100% of
the contributing runoff should flow across
the filter strip. Refer to Section 4.9 for design
criteria for a grass channel and Section 4.29 for
vegetated filter strips.
Stormwater Best Management Practices
4.5.5.2 GENERAL DESIGN
• While there is no minimum slope requirement,
enough elevation drop is needed from the
basin inlet to the basin outlet to ensure that
flow can move through the system.
• A low flow or pilot channel across the
facility bottom from the inlet to the outlet
is recommended to convey low flows and
prevent standing water conditions. To prevent
stream warming, designers should place
landscaping to provide shade around the pilot
channel and the basin outlet. Designing the
pilot channel as a grass channel also reduces
thermal impacts. A minimum slope of 1% is
recommended for grass swales and 0.5% for
armored pilot channels.
• Adequate maintenance access must be
provided for all dry detention basins.
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4.5.5.6 SAFETY FEATURES
4.5.5.7 LANDSCAPING
• For a dry detention basin, the outlet structure is
sized for Qp25 control (based upon hydrologic
routing calculations) and can consist of a weir,
orifice, outlet pipe, combination outlet, or other
acceptable control structure. Small outlets that
will be prone to clogging or difficult to maintain
are not acceptable.
• An emergency spillway should be included in
the dry detention basin design to safely pass
the extreme flood flow. The spillway prevents
basin water levels from overtopping the
embankment and causing structural damage.
The emergency spillway must be designed to
State of Georgia guidelines for dam safety and
located so that downstream structures will not
be impacted by spillway discharges.
• Designers should maintain a vegetated buffer
around the dry detention basin and select
plants within the detention zone (i.e., the
portion of the basin up to the elevation where
stormwater is detained) that can withstand
both wet and dry periods. The side slopes of
dry basins should be relatively flat to reduce
safety risks.
• The water quality orifice should have a
minimum diameter of 3 inches and be
adequately protected from clogging by an
external trash rack. The orifice diameter may be
reduced to 1 inch if internal orifice protection is
used (e.g., a perforated vertical stand pipe with
0.5-inch orifices or slots that are protected
by wirecloth and a stone filtering jacket).
Adjustable gate valves can also be used to
achieve this equivalent diameter.
• Seepage control or anti-seep collars should be
provided for all outlet pipes.
• Riprap, plunge pools or pads, or other energy
dissipators are to be placed at the end of the
outlet to prevent scouring and erosion. If the
basin discharges to a channel with dry weather
flow, care should be taken to minimize tree
clearing along the downstream channel, and
to reestablish a forested riparian zone in the
shortest possible distance.
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• A minimum of 1 foot of freeboard must be
provided, measured from the top of the water
surface elevation for the extreme flood, to
the lowest point of the dam embankment,
not counting the emergency spillway. A safety
bench shall be provided for embankments
greater than 10 feet in height and having a side
slope steeper than 3:1. For large basins, the
safety bench shall extend no less than 15 feet
outward from the normal water edge to the toe
of the basin side slope. The slope of the safety
bench shall not exceed 6%.
• Stormwater should be conveyed to and from
dry detention basins safely and to minimize
erosion potential.
• Trees planted on or near the side slopes of
the dry detention basin can intercept and
slow rainfall, reducing its erosive force before
hitting the ground. The trees also transpire soil
moisture within the basin. There is an added
benefit of rainfall storage that is held within the
tree canopy and does not reach the ground.
Stormwater Best Management Practices
4.5.5.5 OUTLET STRUCTURES
• Plantings should be designed not to conflict
with the current drainage of the basin.
• All trees should be kept away from any drainage
structures to allow for maintenance access and
repairs as needed.
4.5.5.8 CONSTRUCTION CONSIDERATIONS
Construction equipment should be restricted
from the dry detention basin to prevent compaction of soils.
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(Step 2) Determine the goals and primary function of the dry deten-
Construction costs associated with dry extended detention basins range con-
tion basin.
siderably and are based on a cost per unit area treated. One study evaluated
Consider whether the dry detention basin is intended to:
»» Meet a water quality (treatment) target. See Step 3
to size the BMP utilizing the water quality treatment
approach.
the cost of all basin systems (Brown and Schueler, 1997). Adjusting for inflation
using a RSMeans Construction Cost Index, the cost of dry ED basins can be
estimated with the equation:
»» Provide a possible solution to a drainage problem
C = 22.7V
0.760
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
Where:
C = Total construction cost including design and permitting cost
V = Total volume required to control the 10-year storm (cubic ft)
Using this equation, typical construction costs are:
$ 76,200 for a 1 acre-foot basin
$ 438,000 for a 10 acre-foot basin
$ 2,530,000 for a 100 acre-foot basin
Dry ED basins are typically less costly than stormwater (wet) basins, for example, that would provide equivalent flood storage, as less excavation is required.
4.5.6 Design Procedures
(Step 1) Determine if the development site and conditions are appropriate for the use of a dry detention basins.
Consider the application and site feasibility criteria in this
chapter. In addition, determine if site conditions are suitable
for a dry detention basin. Create a rough layout of the dry
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Stormwater Best Management Practices
4.5.5.9 CONSTRUCTION AND MAINTENANCE COSTS
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Complete Step 3 for a water quality (treatment) approach.
Refer to your local community’s guidelines for any additional
information or specific requirements regarding the use of
either method.
detention basin dimensions taking into consideration existing
trees, utility lines, and other obstructions.
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(Step 7) Compute WQv orifice release rate and size WQv orifice.
Calculate the Water Quality Volume using the following
»» Size a water quality orifice to release the calculated WQv.
formula:
»» The WQv elevation is then determined from the stage-
WQV = (1.2) (RV) (A) / 12
storage relationship. The invert of the WQv orifice is
located at the water quality detention elevation, and
the orifice is sized to allow for temporary storage of the
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
RV = Volumetric runoff coefficient which can be
water quality storage volume
»» The water quality orifice should be adequately protected
from clogging by an acceptable external trash rack.
found by:
(Step 8) Design embankment(s) and spillway(s).
RV = 0.05+0.009(I)
Size emergency spillway, calculate the 100-year, 24-hour
storm water surface elevation, set the top of the embank-
Stormwater Best Management Practices
(Step 3) Calculate the Target Water Quality Volume
ment elevation, and analyze safe passage of the Qf. Set the
Where:
I = new impervious area of the contributing
invert elevation of the emergency spillway 0.1 foot above the
100-year, 24-hour storm water surface elevation.
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
(Step 9) Investigate potential basin hazard classification
Design and construction of the detention facility may be
required to meet the Georgia Dam Safety standards.
(Step 4) Size flow diversion structure, if needed
A flow regulator (or flow splitter diversion structure) should be
(Step 10)Prepare a site vegetation and landscaping plan.
supplied to divert the WQv to the dry detention basins facility.
A vegetation scheme for the dry detention basin should be
Size low flow orifice, weir, or other device to pass Qwq.
prepared to indicate how the basin bottom, side slopes, and
embankment will be stabilized and established with vegeta-
(Step 5) Determine pretreatment volume.
tion.
A sediment forebay is provided at each inlet, unless the inlet
provides less than 10% of the total design storm inflow to the
4.5.7 Inspection and Maintenance Requirements
basin. The forebay should be sized to contain 0.1 inch per
All best management practices require proper maintenance. Without proper
impervious acre of contributing drainage.
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for main-
(Step 6) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
tenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
storm).
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Embankments should be less than 20 feet in height and should have side
slopes no steeper than 2:1 (horizontal to vertical) although 3:1 is preferred.
• The depth of the basin should not exceed 10 feet.
• Geotechnical slope stability analysis is recommended for embankments
greater than 10 feet in height.
• All embankments must be designed to State of Georgia guidelines for dam
safety.
• Storage volumes greater than 100 acre-feet are subject to the requirements
of the Georgia Safe Dams Act (Georgia Annotated Code 12-5-370) unless the
facility is excavated to this depth.
• The bottom area of storage facilities should be graded toward the outlet to
prevent standing water conditions.
• The outfall of dry ED basins should always be stabilized to prevent scour.
• An emergency spillway should be provided to safely convey large flood
events.
Description: A surface storage basin or facility
designed to provide water quality treatment and
water quantity control through extended detention (ED) of stormwater runoff. Dry ED basins
ADVANTAGES / BENEFITS
• Moderate removal rate of urban pollutants
• High community acceptance
• Useful for water quality treatment, channel protection, and flood control
• Dry ED basins can serve multiple purposes on a development site.
• Settling pools within the dry ED basin mitigate potential thermal impacts.
DISADVANTAGES / LIMITATIONS
• Dam height restrictions for high relief areas
• Drainage from the ED basin can be problematic for low relief terrain.
differ from dry detention basins in that they provide 24-hour detention of the channel protection
volume (CPv).
LID/GI Considerations: Similar to dry detention,
dry ED basins contribute to a site’s overall pervi-
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.6 Dry Extended Detention
Basins
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Not recommended
Roadway Projects: Yes
Soils: Dry ED basins can be used with
nearly all soils and geology, with minor
design adjustments for regions of karst
topography or in rapidly percolating
soils, such as sand.
ROUTINE MAINTENANCE REQUIREMENTS
• Remove debris from inlet and outlet structures
• Maintain side slopes and outlet structure
• Remove invasive vegetation
• Monitor sediment accumulation and remove periodically
Other Considerations: In order to
convey low flows through dry ED basins, designers should provide a pilot
channel. Designers should provide
shade around the pilot channel and
the basin outlet to prevent stream
warming.
POLLUTANT REMOVAL
L=Low M=Moderate H=High
ousness and aesthetics. The open grassed area of
a dry ED basin can also be used for multiple purposes, such as landscaped or recreational areas.
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Total Suspended Solids
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Metals - Cadmium, Copper,
Lead, and Zinc removal
Pathogens – Fecal Coliform
RUNOFF REDUCTION CREDIT
• 0% of the runoff reduction volume
provided
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Stormwater Best Management Practices
4.6.1 General Description
Dry ED basins are surface facilities intended to
provide for the temporary storage and treatment
of stormwater runoff to reduce downstream
water quality and water quantity impacts. These
facilities temporarily detain stormwater runoff,
releasing the flow over a period of time. They
are designed to completely drain within 24 – 72
hours after a storm event and are normally dry
between rain events. A typical schematic for a dry
ED basin is shown in Figure 4.6-1. Dry ED basins
provide downstream channel protection through
extended detention of the channel protection
volume (CPv). Dry ED basins are intended to
provide overbank flood protection (peak flow
reduction of the 25-year, 24-hour storm, Qp25)
and can be designed to control the extreme flood
(100-year, 24-hour, Qf) storm event.
Dry ED basins provide limited pollutant removal
benefits and are not intended for runoff reduction. Dry ED-basins should be used in a treatment
train approach with other BMPs that provide
runoff reduction and/or additional water quality
treatment (see Subsection 4.1.6).
Figure 4.6-1 Schematic of dry ED Basin
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4.6.3 Pollutant Removal Capabilities
The following criteria should be evaluated to
Dry ED basins are presumed to remove 60% of the
gage the suitability of dry ED basins for meeting
• Runoff Reduction
Another BMP should be used in a treatment
train with dry ED basins to provide runoff
reduction as they are not designed to provide
RRV as a stand-alone BMP.
total suspended solids (TSS) load in typical urban
stormwater management objectives on a site or
post development runoff when sized, designed,
development.
• Water Quality
If installed as per the recommended design
criteria and properly maintained, 60% total
suspended solids removal will be applied to
the water quality volume (WQv) flowing to the
dry ED basin. Another BMP should be used in
a treatment train with dry ED basins to provide
the additional required water quality treatment.
• Channel Protection
Dry ED basins can be sized to store the
Channel Protection volume (CP v) and to
completely drain over 24-72 hours, meeting
the requirement of extended detention of the
1-year, 24-hour stormwater runoff volume.
constructed, and maintained in accordance with
the recommended specifications. Although they
can be effective at removing some pollutants
General Feasibility
through settling, they are less effective at remov-
- Suitable for Residential Subdivision Usage – YES
ing soluble pollutants because of the absence of
- Suitable for High Density/Ultra Urban Areas – NO
a permanent pool. Other pollutants that dry ED
- Regional Stormwater Control – YES
basins can remove include Phosphorus, Nitrogen,
and metals (such as Cadmium, Copper, Lead, and
Zinc).
Physical Feasibility – Physical Constraints at
Project Site
For additional information and data on pollutant
removal capabilities for dry ED basins, see the
National Pollutant Removal Performance Database (Version 3) available at www.cwp.org and the
National Stormwater Best Management Practices
(BMP) Database at www.bmpdatabase.org.
Stormwater Best Management Practices
4.6.2 Stormwater Management Suitability
• Drainage Area – In general, dry ED basins
should be used on sites with a minimum
drainage area of 10 acres.
• Space Required – Roughly 2-3% of the
contributing drainage area.
• Site Slope – Dry ED basins can be used on sites
with slopes up to about 15%.
• Overbank Flood Protection
Dry ED basins are intended to provide overbank
flood protection (peak flow reduction of the
25-year, 24-hour storm, Qp25).
4.6.4 Application and Site Feasibility
Criteria
• Extreme Flood Protection
Dry ED basins can be designed to control the
extreme flood (100-year, 24-hour storm, Qf)
rainfall event.
specified period of time. By definition, dry ED
A dry ED basin temporarily stores runoff and
releases that runoff at a controlled rate over a
basins are dry structures during non-precipitation
periods. Dry ED basins are capable of providing
water quality improvement, downstream flood
control, channel erosion control, and mitigation
of post-development runoff to pre-development
• Vegetated and rip rap embankments should
be less than 20 feet in height and should have
side slopes no steeper than 2:1 (horizontal to
vertical), although 3:1 is preferred.
• Minimum Depth to Water Table – Except
for the case of hotspot runoff, the only
consideration regarding groundwater is that the
base of the dry ED basin should not intersect
the groundwater table.
levels. A dry ED facility improves runoff quality primarily through the gravitational settling of
pollutants.
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formation.
Other Constraints / Considerations
• Hot spots – Dry ED basins can accept runoff
from stormwater hotspots, but need significant
separation from groundwater when used for
this purpose.
• Damage to existing structures and facilities
– Dry ED basins should be designed to safely
store and/or bypass the overbank flood (Qp25)
and extreme flood (Qf) event to prevent
overflow or failure, which may cause damage
to site structures and facilities.
• Proximity – The following is a list of specific
setback requirements for the location of dry ED
basins:
• Trout Stream – Dry ED basins should not be
used where receiving water temperature is
a concern. In addition, careful consideration
should be given to the potential for perched or
raised groundwater levels.
Coastal Areas
• Poorly Drained Soils – Poorly draining soils do
not inhibit a dry ED basin’s ability to temporarily
store and treat stormwater runoff and
completely drain within 24-72 hours.
• Flat Terrain – The local slope needs to be
relatively flat in order to maintain reasonably
flat side slopes. While there is no minimum
slope requirement, enough elevation drop is
needed from the basin inlet to the basin outlet
to ensure that flow can move through the
facility.
• Shallow Water Table – Except for the case
of hot spot runoff, the only consideration
regarding ground water is that the base of the
dry ED basin should not intersect the ground
water table.
4.6.5 Planning and Design Criteria
Before designing dry ED basins, the following data
is necessary:
• Existing and proposed site, topographic, and
location maps, as well as field reviews.
• Impervious and pervious areas. Other means
may be used to determine land use data.
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site.
• Design data from nearby storm sewer
structures.
• Water surface elevation of nearby water
systems, as well as depth to the seasonally high
groundwater table.
Stormwater Best Management Practices
• Soils – Dry ED basins can be used in nearly
all soils and geology, with minor design
adjustments for regions of karst (i.e., limestone)
topography or in rapidly percolating soils, such
as sand. In these areas, dry ED basins should be
designed with an impermeable liner to prevent
groundwater contamination or sinkhole
The following criteria are to be considered minimum standards for the design of a dry ED basin.
Consult with the local review authority to determine if there are any variations to these criteria or
additional standards that must be met.
4.6.5.1 LOCATION AND LAYOUT
»» 10 feet from building foundations
Dry ED basins are to be located downstream of
»» 10 feet from property lines
other BMPs providing runoff reduction and/or
»» 100 feet from private water supply wells
treatment of the water quality volume (WQv). See
»» 100 feet from open water (measured from
of multiple BMPs in a treatment train.
Subsection 4.1.6 for more information on the use
edge of water)
»» 200 feet from public water supply reservoirs
(measured from edge of water)
»» 1,200 feet from public water supply wells
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4.6.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Dry ED basins are sized to provide storage and
control for multiple rain events, including:
»» Temporary storage of the volume of runoff
required to provide overbank flood (Qp25)
protection (i.e., reduce the post-development
peak flow of the 25-year storm event to the
pre-development rate), and
• Vegetated embankments should be less than
20 feet in height and should have side slopes
no steeper than 2:1 (horizontal to vertical),
although 3:1 is preferred. Riprap-protected
embankments should be no steeper than
2:1. Geotechnical slope stability analysis is
recommended for embankments greater
than 10 feet in height and is mandatory for
embankment slopes steeper than those given
above. All embankments must be designed to
State of Georgia guidelines for dam safety.
»» Control of the 100-year storm (Qf), if
required.
• The depth of the basin should not exceed 10
feet.
»» Storage of the water quality volume (WQv)
»» 24-hour storage of the channel protection
volume (CP v)
• Routing calculations must be used to
demonstrate that the storage volume is
adequate. See Section 3.3 (Storage Design) for
procedures on the design of detention storage.
• Storage volumes greater than 100 acre-feet are
subject to the requirements of the Georgia Safe
Dams Act (Georgia Annotated Code 12-5-370)
unless the facility is excavated to this depth.
• Areas above the normal high water elevations
of the detention facility should be sloped
toward the basin to allow drainage and prevent
standing water. Careful finish grading is
required to avoid creation of upland surface
depressions that may retain runoff. The
bottom area of storage facilities should be
graded toward the outlet to prevent standing
water conditions.
• Designing dry ED basins with a high length to
width ratio (i.e., at least 1.5:1) and incorporating
other design features to maximize the flow
path effectively increases the detention time
in the system by eliminating the potential of
flow to short circuit the basin. Designing basins
with relatively flat side slopes can also help to
lengthen the effective flow path.
• A low-flow or pilot channel across the
facility bottom from the inlet to the outlet
is recommended to convey low flows and
prevent standing water conditions. In order
to prevent stream warming, designers should
place landscaping to provide shade around the
pilot channel and the basin outlet. A minimum
slope of 1% is recommended for grass swales
and 0.5% for armored pilot channels.
• Adequate maintenance access must be
provided for all dry ED basins.
4.6.5.4 PRETREATMENT/INLETS
Stormwater Best Management Practices
4.6.5.2 GENERAL DESIGN
• Inflow channels are to be stabilized with flared
aprons, or the equivalent.
• Pretreatment for a dry ED basin is usually
provided by a sediment forebay. The sediment
forebay should be sized for 0.1 inches per
impervious acre of contributing drainage area.
• Pretreatment may also be provided by using a
grass filter strip, pea gravel diaphragm, or grass
channel. Where filter strips are used, 100% of
the contributing runoff should flow across
the filter strip. Refer to Section 4.9 for design
criteria for a grass channel and Section 4.29 for
vegetated filter strips.
• While there is no minimum slope requirement,
enough elevation drop is needed from the
basin inlet to the basin outlet to ensure that
flow can move through the facility.
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• For a dry ED basin, the outlet structure is sized
for Qp25 control (based upon hydrologic routing
calculations) and can consist of a weir, orifice,
outlet pipe, combination outlet, or other
acceptable control structure. Small outlets that
will be subject to clogging or are difficult to
maintain are not acceptable.
• A dry ED basin has a channel protection
orifice with a minimum diameter of 3 inches
and should be adequately protected from
clogging by an acceptable external trash rack.
The orifice diameter may be reduced to 1
inch if internal orifice protection is used (e.g.,
an overperforated vertical stand pipe with
0.5-inch orifices or slots that are protected
by wirecloth and a stone filtering jacket).
Adjustable gate valves can also be used to
achieve this equivalent diameter.
• Seepage control or anti-seep collars should be
provided for all outlet pipes.
• Riprap, plunge pools or pads, or other energy
dissipaters are to be placed at the end of the
outlet to prevent scouring and erosion. If the basin
discharges to a channel with dry weather flow,
care should be taken to minimize tree clearing
along the downstream channel, and to reestablish
a forested riparian zone in the shortest possible
distance from the dry ED basin.
4.6.5.6 SAFETY FEATURES
• An emergency spillway should be included
in the dry ED basin design to safely pass the
extreme flood flow. The spillway prevents water
levels from overtopping the embankment and
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causing structural damage. The emergency
spillway must be designed to State of Georgia
guidelines for dam safety and must be located
so that downstream structures will not be
impacted by spillway discharges.
• A minimum of 1 foot of freeboard must be
provided, measured from the top of the water
surface elevation for the extreme flood, to the
lowest point of the dam embankment, not
counting the emergency spillway.
• Stormwater should be conveyed to and from
dry ED basins safely and to minimize erosion
potential.
4.6.5.7 LANDSCAPING
• Designers should maintain a vegetated buffer
around dry ED basins, selecting plants within
the detention zone (i.e., the portion of the
basin up to the elevation where stormwater
is detained) that can withstand both wet and
dry periods. The side slopes of dry ED basins
should be relatively flat to reduce safety risks.
• Trees planted on or near the side slopes of the
dry ED basin can intercept and slow rainfall,
reducing its erosive force before hitting the
ground. The trees also transpire soil moisture
within the basin. There is an added benefit of
rainfall storage that is held within the canopy of
the trees and does not reach the ground.
• Plantings should be designed not to conflict
with the current drainage of the basin.
4.6.5.8 CONSTRUCTION CONSIDERATIONS
Construction equipment should be restricted from the
dry ED basin to prevent compaction of native soils.
4.6.5.9 CONSTRUCTION AND MAINTENANCE
COSTS
Construction costs associated with dry extended detention basins range considerably and are
based on a cost per unit area treated. One study
evaluated the cost of all basin systems (Brown
and Schueler, 1997). Adjusting for inflation using a
RSMeans Construction Cost Index, the cost of dry
ED basins can be estimated with the equation:
Stormwater Best Management Practices
4.6.5.5 OUTLET STRUCTURES
C = 22.7V0.760
Where:
C = Total construction cost including
design and permitting cost
V = Total volume required to control the
10-year storm (cubic feet)
Using this equation, typical construction costs are:
$ 76,200 for a 1 acre-foot basin
$ 438,000 for a 10 acre-foot basin
$ 2,530,000 for a 100 acre-foot basin
Dry ED basins are typically less costly than stormwater
(wet) basins, for example, that would provide equivalent flood storage, as less excavation is required.
• All trees should be kept away from any drainage
structures to allow for maintenance access and
repairs as needed.
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(Step 1) Determine if the development site and conditions are appropriate for the use of dry extended detention basins.
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Consider the application and site feasibility criteria in this
chapter. In addition, determine if site conditions are suitable
Complete Step 3 for a water quality (treatment) approach.
for a dry extended detention basin. Create a rough layout
Refer to your local community’s guidelines for any additional
of the dry extended detention basin dimensions taking into
information or specific requirements regarding the use of
consideration existing trees, utility lines, and other obstruc-
either method.
tions.
(Step 3) Calculate the Target Water Quality Volume
(Step 2) Determine the goals and primary function of the dry extended detention basin.
Calculate the Water Quality Volume using the following formula:
Consider whether the dry extended detention basin is intended to:
»» Meet a water quality (treatment) target. See Step 3
to size the BMP utilizing the water quality treatment
approach.
WQV = (1.2) (RV) (A) / 12
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
RV = Volumetric runoff coefficient which can be
found by:
»» Provide a possible solution to a drainage problem
RV = 0.05+0.009(I)
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
Stormwater Best Management Practices
4.6.6 Design Procedures
Where:
there are any additional restrictions and/or surface water or
I = new impervious area of the contributing
watershed requirements that may apply. In addition, consid-
drainage area (%)
er if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
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4.6.7 Inspection and Maintenance Requirements
A flow regulator (or flow splitter diversion structure) should
All best management practices require proper maintenance. Without proper
be supplied to divert the WQv to the dry extended detention
maintenance, BMPs will not function as originally designed and may cease to
basins facility.
function altogether. The design of all BMPs includes considerations for main-
Size low flow orifice, weir, or other device to pass Qwq.
tenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
(Step 5) Determine pretreatment volume.
A sediment forebay is provided at each inlet, unless the inlet
provides less than 10% of the total design storm inflow to the
basin. The forebay should be sized to contain 0.1 inch per
impervious acre of contributing drainage.
(Step 6) Calculate the CPV (1-yr, 24-hour storm), QP25 (25-yr, 24hour storm), and Qf (100-yr, 24-hour storm) flow rates and
Stormwater Best Management Practices
(Step 4) Size flow diversion structure, if needed
volumes.
(Step 7) Design embankment(s) and spillway(s).
Size the emergency spillway, calculate the 100-year water
surface elevation, set the top of the embankment elevation,
and analyze safe passage of the Qf. Set the invert elevation
of the emergency spillway 0.1 foot above the 100-year water
surface elevation.
(Step 8) Investigate potential basin hazard classification.
The design and construction of the dry ED basin may be
required to meet the Georgia Dam Safety standards.
(Step 9) Prepare a site Vegetation and Landscaping Plan.
A vegetation scheme for the dry ED basin should be prepared to indicate how the basin bottom, side slopes and
embankment will be stabilized and established with vegetation. The use of native vegetation is highly recommended for
these facilities.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Dry wells should be designed to completely drain within 24 hours of
the end of a rainfall event.
• There should be at least 2 feet of separation distance between the
bottom of a dry well and the top of the water table.
• Dry wells should be designed with slopes that are as close to flat as
possible to help ensure that stormwater runoff is evenly distributed
throughout the stone reservoir.
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
ADVANTAGES / BENEFITS
• Helps restore pre-development hydrology on development sites
• Reduces post-construction stormwater runoff rates, volumes, and
pollutant loads
• Well-suited for use on urban development sites
DISADVANTAGES / LIMITATIONS
• Can only be used to “receive” runoff from small drainage areas of
2,500 square feet or less
• Should not be used on development sites that have soils with
infiltration rates of less than 0.5 inches per hour
(Source: City of Portland, OR, 2008)
Description: Dry wells are low impact development practices that are located below the surface
of development sites. They consist of shallow
excavations, typically filled with stone, that are
designed to intercept and temporarily store
post-construction stormwater runoff until it infiltrates into the underlying and surrounding soils.
LID/GI Considerations: Use of a dry well decreases the post construction runoff volume from
a site, decreasing the pollutant load as well as
ROUTINE MAINTENANCE REQUIREMENTS
• A dry well and its components shown in Figure 4.7-1 must be
inspected and maintained at least annually.
• Replace gravel when more than 6 inches of sediment has
accumulated.
• Inspect after major storm events to ensure water does not pond for
more than 48 hours. If extended ponding occurs, the gravel may
need to be replaced.
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.7 Dry Wells
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Not recommended
Soils: Dry wells should be considered for use
on development sites where fine sediment (e.g.,
clay, silt) loads will be relatively low, as high
sediment loads will cause them to clog and fail.
Permeable soils with a water table low enough
to provide for the infiltration of stormwater
runoff are recommended.
Other Considerations: Dry wells should not be
located beneath a driveway, parking lot or other
impervious surface.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 100% runoff reduction volume
thermal and erosive impacts to receiving waters.
Dry wells are installed underground, which allows
for multiple uses of development space.
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Stormwater Best Management Practices
4.7.1 General Description
Dry wells (also known as seepage pits and French
drains) are low impact development practices that
are located below the surface of development
sites. They consist of shallow excavations, typically filled with stone, that are designed to intercept
and temporarily store post-construction stormwater runoff until it infiltrates into the underlying
and surrounding soils (Figure 4.7-1). If properly
designed, they can provide significant reductions
in post-construction stormwater runoff rates, volumes, and pollutant loads on development sites.
As infiltration-based low impact development
practices, dry wells are limited to use in areas
where the soils are permeable enough and the
water table is low enough to provide for the infiltration of stormwater runoff. They should only be
considered for use on development sites where
fine sediment (e.g., clay, silt) loads will be relatively low, as high sediment loads will cause them
to clog and fail. In addition, dry wells should be
carefully sited to avoid potential contamination of
water supply aquifers.
The primary concern associated with the design
of a dry well is its storage capacity, which directly
influences its ability to reduce stormwater runoff
Figure 4.7-1: Dry Well
(Source: Maryland Department of the Environment, 2000)
rates, volumes, and pollutant loads. Site planning
and design teams should strive to design dry wells
that can accommodate the stormwater runoff
volume generated by the target runoff reduction
rainfall event (e.g., 85th percentile rainfall event).
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teristics or constraints, site planning and design
teams should consider using dry wells in combination with other runoff reducing low impact
development practices, such as bioretention areas
(Section 4.2) and rainwater harvesting (Section
4.19), to supplement the stormwater management
benefits provided by the dry wells.
4.7.2 Stormwater Management Suitability
• Overbank Flood Protection
Proportionally adjust the post-development
runoff CN to account for the runoff reduction
provided by a dry well when calculating
the overbank peak discharge (Qp25) on a
development site (See Subsection 3.1.7.5).
4.7.3 Pollutant Removal Capabilities
• Extreme Flood Protection
Proportionally adjust the post-development
runoff CN to account for the runoff reduction
provided by a dry well when calculating the
extreme peak discharge (Qf) on a development
site (See Subsection 3.1.7.5).
remove 100% of the Phosphorus, Nitrogen, metals
Dry wells are presumed to remove 100% of the
total suspended solids (TSS) load in typical urban
post-development runoff when sized, designed,
constructed, and maintained in accordance with
the recommended specifications. Dry wells also
(such as Cadmium, Copper, Lead, and Zinc), and
fecal coliform in contributing runoff.
In order to provide the most efficient pollutant
removal, dry wells should be constructed in per-
The Center for Watershed Protection (Hirschman
meable soils of hydrologic soil group A or B. The
et al., 2008) recently documented the ability of
maximum drainage area and site surface slope
dry wells to reduce annual stormwater runoff vol-
should not be exceeded to ensure proper pollut-
umes and pollutant loads on development sites,
ant removal.
Stormwater Best Management Practices
If this cannot be accomplished due to site charac-
as follows:
• Stormwater Runoff Reduction
Subtract 100% of the storage volume provided
by a dry well from the runoff reduction volume
(RRv) conveyed through the dry well.
• Water Quality Protection
If installed as per the recommended design
criteria and properly maintained, 100% total
suspended solids removal will be applied to the
water quality volume (WQv) flowing to the dry
well.
For additional information and data on pollutant
removal capabilities for dry wells, see the National
Pollutant Removal Performance Database (Version 3) available at www.cwp.org and the National
Stormwater Best Management Practices (BMP)
Database at www.bmpdatabase.org.
• Channel Protection
Proportionally adjust the post-development
runoff curve number (CN) to account for the
runoff reduction provided by a dry well when
calculating the channel protection volume
(CP v) on a development site (See Subsection
3.1.7.5).
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Dry wells can be used to treat stormwater runoff
on a wide variety of development sites, including
residential, commercial, and institutional development sites in rural, suburban, and urban areas. Although they are particularly well-suited to receive
rooftop runoff, they can also be used to receive
stormwater runoff from other small drainage
areas, such as local streets and roadways, driveways, small parking areas, and disturbed pervious
areas (e.g., lawns, parks, community open spaces).
When compared with other low impact development practices, dry wells have a moderate construction cost, a moderate maintenance burden,
and require only a small amount of surface area.
General Feasibility
- Suitable for Residential Subdivision Usage – YES
• Flow Path – The length of flow path in
contributing drainage areas should be 150 feet
or less in pervious drainage areas and 75 feet or
less in impervious drainage areas.
• Site Slope – Although dry wells may be used
on development sites with slopes of up to
6%, they should be designed with slopes that
are as close to flat as possible to help ensure
that stormwater runoff is evenly distributed
throughout the stone reservoir.
• Minimum Depth to Water Table – 2 feet
• Minimum Head – 2 feet
• Soils – Dry wells should be designed to
completely drain within 24 hours of the end
of a rainfall event. Consequently, dry wells
generally should not be used on development
sites that have soils with infiltration rates of less
than 0.50 inches per hour (i.e., hydrologic soil
group C and D soils).
Other Constraints / Considerations
• Hot spots – May be used for hot spot runoff
Physical Feasibility – Physical Constraints at
Project Site
• Drainage Area – The size of the contributing
drainage area should be 2,500 square feet or less.
• Space Required – Dry well surface area
requirements vary according to the size of the
contributing drainage area and the infiltration
rate of the soils in which the dry well is located.
In general, dry wells require about 5-10% of the
size of their contributing drainage areas.
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»» 10 feet from building foundations
»» 10 feet from property lines
»» 100 feet from private water supply wells
»» 1,200 feet from public water supply wells
»» 100 feet from septic systems
»» 100 feet from surface waters
»» 400 feet from public water supply surface
waters
• Trout Stream – Use of a dry well reduces
a site’s runoff pollutant load, as well as the
volume and velocity of stormwater runoff.
Therefore, dry wells are an effective BMP for
use where trout streams or other protected
waters may receive stormwater runoff.
Coastal Areas
- Suitable for High Density/Ultra Urban Areas – YES
- Regional Stormwater Control – NO
• Proximity – Dry wells may be used without
restriction, except within:
Stormwater Best Management Practices
4.7.4 Application and Site Feasibility
Criteria
• Damage to existing structures and facilities –
Dry wells should not be used in areas where
their operation may create a risk for basement
flooding, interfere with subsurface sewage
disposal systems, or affect other underground
structures.
• Poorly drained soils, such as hydrologic soil
groups C and D – Limits the ability of dry wells
to reduce stormwater runoff rates, volumes,
and pollutant loads. Dry wells should not be
used on development sites that have soils with
infiltration rates of less than 0.5 inches per
hour (i.e., hydrologic soil group C and D soils).
Use other low impact development practices,
such as rainwater harvesting (Section 4.19), to
“receive” stormwater runoff in these areas.
• Dry wells should be designed so that overflow
drains away from buildings to prevent damage
to building foundations.
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• Flat Terrain – Does not influence the use of dry
wells. In fact, dry wells should be designed with
slopes that are as close to flat as possible.
• Shallow Water Table – In coastal areas it may
be difficult to provide 2 feet of clearance
between the bottom of the dry well and the
top of the water table. This may occasionally
cause stormwater runoff to pond in the bottom
of the dry well. Ensure that the distance from
the bottom of the dry well to the top of the
water table is at least 2 feet. Reduce the depth
of the stone reservoir in dry wells to 18 inches,
if necessary.
• Tidally-influenced drainage system – Does not
influence the use of dry wells.
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4.7.5 Planning and Design Criteria
4.7.5.1 LOCATION AND LAYOUT
Before designing the dry well, the following data
• Dry wells should be located in a lawn or
other disturbed pervious area and should be
designed so that the top of the dry well is
located as close to the surface as possible.
Dry wells should not be located beneath a
driveway, parking lot, or other impervious
surface.
is needed:
• Existing and proposed site, topographic, and
location maps, as well as field reviews
• The proposed site design, including buildings,
parking lots, sidewalks, stairs, handicapped
ramps, and landscaped areas
• Architectural roof plan for rooftop pitches and
downspout locations
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Information about downstream BMPs and
receiving waters
• Design data from nearby storm sewer structures
• Water surface elevation of nearby water
systems as well as the depth to the seasonally
high groundwater table
The following criteria are to be considered
minimum standards for the design of a dry well.
Consult with the local review authority to determine if there are any variations to these criteria or
additional standards that must be followed.
• Although dry wells may be installed on
development sites with slopes of up to 6%,
they should be designed with slopes that
are as close to flat as possible to help ensure
that stormwater runoff is evenly distributed
throughout the stone reservoir.
Stormwater Best Management Practices
• Well drained soils, such as hydrologic soil
groups A and B – Enhance the ability of dry
wells to reduce stormwater runoff rates,
volumes, and pollutant loads, but may allow
stormwater pollutants to reach groundwater
aquifers with greater ease. Rooftop runoff is
relatively clean, so this should not prevent the
use of dry wells, even at stormwater hotspots
and in areas known to provide groundwater
recharge to water supply aquifers. However,
rooftop runoff should not be allowed to
comingle with runoff from other impervious
surfaces in these areas if it will be sent to a dry
well.
• Dry wells should be used on development sites
that have underlying soils with an infiltration
rate of 0.50 inches per hour or greater, as
determined by NRCS soil survey data and
subsequent field testing.
• Although the number of infiltration tests
needed on a development site will ultimately
be determined by the local development
review authority, at least one infiltration test is
recommended for each dry well that will be
used on the development site.
• Since clay lenses or any other restrictive layers
located below the bottom of a dry well will
reduce soil infiltration rates, infiltration testing
should be conducted within any confining
layers that are found within 4 feet of the
bottom of a proposed dry well.
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top of the water table should be at least 2 feet
to prevent nuisance ponding and ensure proper
operation of the dry well.
• If used to receive rooftop runoff, dry wells
should be preceded by a leaf screen installed
4.7.5.2 GENERAL DESIGN
• Dry wells should be used to receive stormwater
runoff from small drainage areas of 2,500
square feet or less. The stormwater runoff rates
and volumes from larger contributing drainage
areas typically become too large to be properly
treated by a dry well.
in the gutter or downspout. This will prevent
leaves and other large debris from clogging the
dry well.
• If used to receive non-rooftop runoff, dry wells
should be preceded by a pea gravel diaphragm
or equivalent level spreader device (e.g.,
concrete sills, curb stops, curbs with sawteeth
cut into them) and a vegetated filter strip that is
designed according to the planning and design
criteria provided in Section 4.29.
• Consideration should be given to the
stormwater runoff rates and volumes generated
by larger storm events (e.g., 25-year, 24-hour
storm event) to help ensure that these larger
storm events are able to safely bypass the dry
well. An overflow, such as a vegetated filter strip
(Section 4.29) or grass channel (Section 4.9),
should be designed to convey the stormwater
runoff generated by these larger storm events
safely out of the dry well.
• The length of the flow path within the
contributing drainage area should be 150 feet
or less for pervious drainage areas and 75
feet or less for impervious drainage areas. In
contributing drainage areas with longer flow
paths, stormwater runoff tends to become
shallow, concentrated flow (Claytor and
Schueler, 1996), which can significantly reduce
the stormwater management benefits that
dry wells can provide. In these situations,
bioretention areas (Section 4.2) and infiltration
practices (Section 4.12) should be used to
“receive” post-construction stormwater runoff.
• Dry wells should be designed to provide
enough storage for the stormwater runoff
volume generated by the target runoff
reduction rainfall event (e.g., 85th percentile
rainfall event).
• Dry wells should be designed to completely
drain within 24 hours of the end of a rainfall
event. Where site characteristics allow, it is
preferable to design dry wells to drain within
12 hours of the end of a rainfall event to help
prevent the formation of nuisance ponding
conditions.
• Broader, shallower dry wells perform more
effectively by distributing stormwater runoff
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over a larger surface area. However, a minimum
depth of 18 inches is recommended for all dry
well designs to prevent them from consuming
a large amount of surface area on development
sites. Whenever practical, the depth of dry
wells should be kept to 36 inches or less.
• Dry wells should be filled with clean, washed
stone. The stone used in the dry well should
be 1.5 to 2.5 inches in diameter, with a void
space of approximately 40% (e.g., GA DOT No.
3 Stone). Unwashed aggregate contaminated
with soil or other fine particulates may not be
used in the dry well. Underlying native soils
should be separated from the dry well stone
by a thin, 2 to 4 inch layer of choker stone (i.e.,
ASTM D 448 size No. 8, 3/8” to 1/8” or ASTM
D 448 size No. 89, 3/8” to 1/16”). The choker
stone should be placed between the dry well
stone and the underlying native soils.
Stormwater Best Management Practices
• The depth from the bottom of a dry well to the
• The top and sides of the dry well should be
lined with a layer of appropriate permeable
filter fabric. The filter fabric should be a nonwoven geotextile with a permeability that is
greater than or equal to the infiltration rate of
the surrounding native soils. The top layer of
the filter fabric should be located 6 inches from
the top of the excavation, with the remaining
space filled with appropriate landscaping. This
top layer serves as a sediment barrier and,
consequently, will need to be replaced over
time. Site planning and design teams should
ensure that the top layer of filter fabric can be
readily separated from the filter fabric used to
line the sides of the dry well.
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4.7.5.5 CONSTRUCTION AND MAINTENANCE
COSTS
4.7.5.8 CONSTRUCTION CONSIDERATIONS
• The cost of a dry well varies based on the size.
Typically, a small residential dry well costs
between $1,350-$1,700.
installed on a development site, site planning
• Larger dry wells can cost between $11,250$16,900.
• If dry wells will be used to receive non-rooftop
runoff, they should only be installed after
their contributing drainage areas have been
completely stabilized. To help prevent dry well
failure, stormwater runoff may be diverted
around the dry well until the contributing
drainage area has been stabilized.
• Costs for downstream infiltration trenches or
regional BMPs can be found in their respective
sections in this manual.
4.7.7.6 SAFETY FEATURES
4.7.5.3 PRETREATMENT/INLETS
Pretreatment and inlet protection need to be
designed to reduce the velocity and energy of
stormwater entering the practice and prevent
Dry wells generally do not require any special
safety features, provided side slopes are maintained at 3:1 or flatter. Fencing of dry wells is not
generally desirable.
scour of the mulch and plantings. Pretreatment
and inlet protection may include splash blocks,
a stone diaphragm, a level spreader, or similar
device.
4.7.5.4 OUTLET STRUCTURES
Outlet structures should be included in the design
of a dry well to ensure that larger storms can be
bypassed without damaging the practice. See
Section 3.4 (Outlet Structures) for more guidance
regarding the proper design and installation of an
outlet structure.
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To help ensure that dry wells are successfully
and design teams should consider the following
recommendations:
• To help prevent soil compaction, heavy
vehicular and foot traffic should be kept out
of dry wells before, during, and immediately
after construction. This can typically be
accomplished by clearly delineating dry wells
on all development plans and, if necessary,
protecting them with temporary construction
fencing.
Stormwater Best Management Practices
• An observation well should be installed in every
dry well. An observation well consists of a 4 to
6 inch perforated PVC (AASHTO M 252) pipe
that extends to the bottom of the dry well. The
observation well can be used to observe the
rate of drawdown within the dry well following
a storm event. It should be installed along
the centerline of the dry well, flush with the
elevation of the surface of the dry well. A visible
floating marker should be provided within the
observation well and the top of the well should
be capped and locked to prevent tampering
and vandalism.
4.7.5.7 LANDSCAPING
• The landscaped area above the surface of a dry
well may be covered with pea gravel (i.e., ASTM
D 448 size No. 8, 3/8” to 1/8”). This pea gravel
layer provides sediment removal and additional
pretreatment upstream of the dry well and
can be easily removed and replaced when it
becomes clogged.
• Alternatively, a dry well may be covered with an
engineered soil mix, such as that prescribed in
Appendix D, and planted with managed turf or
other herbaceous vegetation. This may be an
attractive option when dry wells are placed in
disturbed pervious areas (e.g., lawns, parks, and
community open spaces).
• Excavation for dry wells should be limited
to the width and depth specified in the
development plans. Excavated material should
be placed away from the excavation so as not
to jeopardize the stability of the side walls.
• The native soils along the bottom of the dry
well should be scarified or tilled to a depth
of 3 to 4 inches prior to the placement of the
choker stone and dry well stone.
• The sides of all excavations should be trimmed
of large roots that will hamper the installation
of the permeable filter fabric used to line the
sides and top of the dry well.
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(Step 1) Determine if the development site and conditions are appropriate for the use of a dry well.
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Consider the application and site feasibility criteria in this
chapter. In addition, determine if site conditions are suitable
Complete Step 3A, 3B, and 3C for a runoff reduction ap-
for a dry well. Create a rough layout of the dry wells dimen-
proach, or skip Step 3 and complete Steps 4A and 4B for a
sions taking into consideration existing trees, utility lines, and
water quality (treatment) approach. Refer to your local com-
other obstructions.
munity’s guidelines for any additional information or specific
requirements regarding the use of either method.
(Step 2) Determine the goals and primary function of the dry well.
Consider whether the dry well is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume.
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Stormwater Best Management Practices
4.7.6 Design Procedures
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
RV = Volumetric runoff coefficient which can be
»» Provide a possible solution to a drainage problem
RV = 0.05+0.009(I)
found by:
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
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Splash blocks or level spreaders should be considered to
up the appropriate runoff reduction percentage (or credit)
dissipate concentrated stormwater runoff at the inlet and
provided by the practice:
prevent scour.
Using the RRV calculated above, determine the minimum
(Step 3C) Determine whether the minimum storage volume was met.
When the VP ≥ VPMIN, then the Runoff Reduction require-
Volume of the Practice (VP)
ments are met for this practice. Proceed to Step 5.
(VPMIN) ≥ RRv (target) / (RR%)
When the VP < VPMIN, then the BMP must be sized according
to the WQV treatment method (See Step 4).
Where:
RR% = Runoff Reduction percentage, or credit,
assigned to the specific practice
VPMIN = Minimum storage volume required to
(Step 4A) Calculate the Target Water Quality Volume.
provide Runoff Reduction Target Volume (ft )
Calculate the Water Quality Volume using the following
RRv (target) = Runoff Reduction Target Volume (ft3)
formula:
3
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
WQV = (1.2) (RV) (A) / 12
(Step 3B) Determine the storage volume of the practice and the
Pretreatment Volume
To determine the actual volume provided in the dry well, use
Where:
WQV = Water Quality Volume (ft3)
the following equation:
1.2 = Target rainfall amount to be treated (inches)
VP = (PV + VES (N))
RV = Volumetric runoff coefficient which can be
found by:
Where:
RV = 0.05+0.009(I)
VP = Volume provided (temporary storage)
PV = Ponding Volume
VES = Volume of Engineered Soils
N = Porosity
Where:
I = new impervious area of the contributing drainage area (%)
To determine the porosity, a qualified licensed professional
A = Area draining to this practice (ft2)
should be consulted to determine the proper porosity based
12 = Unit conversion factor (in/ft)
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
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(or flow splitter diversion structure) should be supplied to
the footprint of the dry well practice.
divert the WQv (or RRv) to the dry well.
The peak rate of discharge for the water quality design storm
Size low-flow orifice, weir, or other device to pass Qwq.
is needed for sizing of off-line diversion structures (see Subsection 3.1.7). If designing off-line, follow steps (a) through
(Step 7) Design emergency overflow facilities.
An overflow must be provided to bypass and/or convey
(d) below:
larger flows to the downstream drainage system or stabilized
(a) Using WQv, compute CN.
watercourse. Non-erosive velocities need to be ensured at
(b) Compute time of concentration using the TR-55 method.
the outlet point. The overflow should be sized to safely pass
(c) Determine appropriate unit peak discharge from time of
the peak flows anticipated to reach the practice, up to a 100-
concentration.
year, 24-hour storm event.
(d) Compute Qwq from unit peak discharge, drainage area,
(Step 8) Prepare a site Vegetation and Landscaping Plan.
and WQv.
A landscaping plan for the dry well should be prepared to
To determine the minimum surface area of the dry well, use
indicate how it will be established with vegetation.
the following formula:
See Subsection 4.7.5.7 (Landscaping) and Appendix D for
Stormwater Best Management Practices
(Step 4B) If using the practice for Water Quality treatment, determine
more details.
Af = (WQv) (df) / [ (k) (hf + df) (tf)]
4.7.7 Inspection and Maintenance Requirements
Where:
All best management practices require proper maintenance. Without proper
Af = surface area of ponding area (ft2)
maintenance, BMPs will not function as originally designed and may cease to
WQv = water quality volume (ft )
function altogether. The design of all BMPs includes considerations for main-
df = rock depth (ft)
tenance and maintenance access. For additional information on inspection
k = coefficient of permeability of rock (ft/day)
and maintenance requirements, see Appendix E.
3
hf = average height of water above dry well bed (ft)
tf = design rock drain time (days) (1 day is the
recommended maximum)
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
storm). See Subsection 3.1.7.5 for more information.
(Step 6) Size the flow diversion structure, if needed.
If the contributing drainage area to dry well exceeds the water quality treatment and/or storage capacity, flow regulator
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Longitudinal slopes must be less than 4%
• Bottom width of 2 to 8 feet
• Side slopes 2:1 or flatter; 4:1 recommended
• Convey the 25-year storm event with a minimum of 6 inches of
freeboard
ADVANTAGES / BENEFITS
• Combines stormwater treatment with runoff conveyance system
• Less expensive than curb and gutter
• Reduces runoff velocity
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
DISADVANTAGES / LIMITATIONS
• Higher maintenance than curb and gutter systems
• Cannot be used on steep slopes
• Possible resuspension of sediment
• Potential for odor / mosquitoes (wet swale)
Description: Vegetated open channels that are
explicitly designed and constructed to capture
and treat stormwater runoff within dry or wet cells
MAINTENANCE REQUIREMENTS
• Maintain grass heights of approximately 4 to 6 inches (dry swale)
• Remove sediment from forebay and channel
formed by check dams or other means.
POLLUTANT REMOVAL (DRY SWALE)
LID/GI Consideration: Adaptable to many linear
situations, and often a small BMP used to treat
runoff close to the source.
Land Requirement
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: No
Drainage Area: 5 acres max
Soils: No restrictions
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
Other Considerations:
• Permeable soil layer (dry swale)
• Wetland plants (wet swale)
L=Low M=Moderate H=High
POLLUTANT REMOVAL (WET SWALE)
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IMPLEMENTATION CONSIDERATIONS
Stormwater Best Management Practices
4.8 Dry Enhanced Swales/Wet
Enhanced Swales
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
RUNOFF REDUCTION CREDIT
• Dry Swale: 100% of the runoff reduction
volume provided (no underdrain)
• Dry Swale: 50% of the runoff reduction volume
provided (underdrain)
• Wet Swale: 0% of the runoff reduction volume
provided
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Enhanced swales (also referred to as bioswales,
vegetated open channels, or water quality swales)
are conveyance channels engineered to capture
and treat the water quality volume (WQv) for a
drainage area. They differ from a normal drainage
channel or swale through the incorporation of
• Wet Swale – The wet swale is a vegetated
channel designed to retain water or marshy
conditions that support wetland vegetation.
A high water table or poorly drained soils are
necessary to retain water. The wet swale
essentially acts as a linear shallow wetland
treatment system, where the WQv is retained.
specific features that enhance stormwater pollut-
Dry and wet swales are not to be confused with a
ant removal effectiveness.
filter strip or grass channel, which are not considered acceptable for meeting the TSS removal
Enhanced swales are designed with limited lon-
performance goal by themselves. Ordinary grass
gitudinal slopes to force the flow to be slow and
channels are not engineered to provide the same
shallow, thus allowing for particulates to settle
treatment capability as a well-designed dry swale
and limiting the effects of erosion. Berms and/
with filter media. Filter strips are designed to
or check dams installed perpendicular to the flow
accommodate overland flow rather than channel-
path promote settling and infiltration.
ized flow and can be used as stormwater credits
Enhanced Dry Swale
Stormwater Best Management Practices
4.8.1 General Description
to help reduce the total water quality treatment
There are two primary enhanced swale designs,
volume for a site. Both of these practices may be
the dry swale and the wet swale (or wetland
used for pretreatment or included in a “treatment
channel). Below are descriptions of these two
train” approach where redundant treatment is
designs:
provided. Please see a further discussion of these
• Dry Swale – The dry swale is a vegetated
conveyance channel designed to include a
filter bed of prepared soil that may overlay an
underdrain system. Dry swales are sized to allow
the entire WQv to be filtered or infiltrated through
the bottom of the swale. Because they are dry
most of the time, they are often the preferred
option in residential settings.
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types of BMPs in Sections 4.28 and 4.9, respec-
Enhanced Wet Swale
tively.
Figure 4.8-1 Enhanced Swale Examples
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Enhanced swale systems are designed primarily
for stormwater quality and have only a limited
ability to provide channel protection or to convey
higher flows to other controls.
• Runoff Reduction
Dry swales, with no underdrain, can be
designed to provide 100% of the runoff
reduction volume, if properly maintained. In
order to provide runoff reduction for a dry
enhanced swale that is designed without
an underdrain, a soils test or other reliable
resource must indicate that the ponding area
of the dry swale will drain within 24 – 48 hours.
A dry swale area can also be designed with
capture and detain the full CP v.
• Overbank Flood Protection
Enhanced swales must provide flow diversion
and/or be designed to safely pass overbank
flood flows. Another BMP must be used in
conjunction with an enhanced swale system to
reduce the post-development peak flow of the
25-year storm (Qp25) to pre-development levels
(detention).
at the given site in a series or “treatment train”
approach.
• Total Suspended Solids – 80%
• Total Phosphorus – Dry Swale 50% / Wet Swale
25%
• Total Nitrogen – Dry Swale 50% / Wet Swale
40%
• Fecal Coliform – insufficient data
• Extreme Flood Protection
Enhanced swales must provide flow diversion
and/or be designed to safely pass extreme
storm flows. Another BMP must be used in
conjunction with an enhanced swale system to
reduce the post-development peak flow of the
100-year storm (Qf) if necessary.
an underdrain to provide 50% of the runoff
• Heavy Metals – Dry Swale 40% / Wet Swale
20%
For additional information and data on pollutant removal capabilities for enhanced dry and
wet swales, see the National Pollutant Removal
Performance Database (Version 3) available at
reduction volume, if properly maintained.
www.cwp.org and the National Stormwater Best
Wet enhanced swales do not provide runoff
4.8.3 Pollutant Removal Capabilities
Management Practices (BMP) Database at www.
reduction volume.
Both the dry and wet enhanced swale are pre-
bmpdatabase.org.
• Water Quality
Dry swale systems rely primarily on filtration
through an engineered media to provide
removal of stormwater contaminants. Wet
swales achieve pollutant removal both from
sediment removal and biological removal.
Subsection 4.8.3 provides median pollutant
removal efficiencies that can be used for
planning and design purposes.
• Channel Protection
Generally only the WQv is treated by a dry or
wet swale, and another BMP must be used to
provide CP v extended detention. However, for
some smaller sites, a swale may be designed to
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Stormwater Best Management Practices
4.8.2 Stormwater Management Suitability
sumed to be able to remove 80% of the total
suspended solids load in typical urban post-development runoff when sized, designed, constructed and maintained in accordance with the
recommended specifications. Undersized or
poorly designed swales can reduce TSS removal
performance.
The following design pollutant removal rates are
conservative average pollutant reduction percentages for design purposes derived from sampling
data, modeling and professional judgment. In
a situation where a removal rate is not deemed
sufficient, additional controls may be put in place
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General Feasibility
4.8.5 Planning and Design Criteria
Enhanced swales can be used in a variety of
- Suitable for Residential Subdivision Usage – YES
development types; however, they are primarily
- Suitable for High Density/Ultra Urban Areas – NO
applicable to residential and institutional areas of
- Regional Stormwater Control – NO
The following criteria are to be considered minimum standards for the design of an enhanced
swale system. Consult with the local review
authority to determine if there are any variations
to these criteria or additional standards that must
small, and along roads and highways. Dry swales
Physical Feasibility - Physical Constraints at
be followed.
are mainly used in moderate to large lot residen-
Project Site
tial developments, small impervious areas (parking
• Drainage Area – 5 acres maximum
4.8.5.1 LOCATION AND LAYOUT
• Space Required – Approximately 10 to 20% of
the tributary impervious area
• A dry or wet swale should be sited such that the
topography allows for the design of a channel
with sufficiently mild slope (unless small drop
structures are used) and cross-sectional area to
maintain non-erosive velocities.
low to moderate density where the impervious
cover in the contributing drainage area is relatively
lots and rooftops), and along rural highways. Wet
swales tend to be used for highway runoff applications, small parking areas, and in commercial
developments as part of a landscaped area.
• Site Slope – Typically no more than 4% channel
slope
Because of their relatively large land requirement,
enhanced swales are generally not used in higher
density areas. In addition, wet swales may not be
desirable for some residential applications, due
to the presence of standing and stagnant water,
which may create nuisance odor or mosquito
problems.
The topography and soils of a site will determine
the applicability of the use of one of the two
enhanced swale designs. Overall, the topography
should allow for the design of a swale with suffi-
• Minimum Head – Elevation difference needed
at a site from the inflow to the outflow: 3 to 5
feet for dry swale; 1 foot for wet swale
• Minimum Depth to Water Table – 2 feet
required between the bottom of a dry swale
and the elevation of the seasonally high water
table, if an aquifer or treating a hotspot; wet
swale is below water table or placed in poorly
drained soils
• Soils – Engineered media for dry swale
cient slope and cross-sectional area to maintain
non-erosive velocities. The following criteria
should be evaluated to ensure the suitability of an
enhanced swale for meeting stormwater management objectives on a site or development.
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Stormwater Best Management Practices
4.8.4 Application and Feasibility Criteria
• Enhanced swale systems should have a
contributing drainage area of 5 acres or less.
• Swale siting should also take into account the
location and use of other site features, such
as buffers and undisturbed natural areas, and
should attempt to aesthetically “fit” the facility
into the landscape.
• A wet swale can be used where the water table
is at or near the soil surface, or where there is a
sufficient water balance in poorly drained soils
to support a wetland plant community.
Other Constraints / Considerations
• Aquifer Protection – Exfiltration should not be
allowed in hotspot areas
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4.8.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
Dry Swale
Both types of enhanced swales are designed to
• Channel slopes between 1% and 2% are
recommended unless topography necessitates
a steeper slope, in which case 6- to 12-inch
drop structures can be placed to limit the
energy slope to within the recommended 1 to
2% range. Energy dissipation will be required
below the drops. Spacing between the drops
should not be closer than 50 feet. Depth of the
storage volume at the downstream end should
not exceed 18 inches.
• Dry swale channels are sized to store and
infiltrate the entire water quality volume
(WQv) with less than 18 inches of ponding and
allow for full filtering through the permeable
soil layer. The maximum ponding time is 48
hours, though a 24-hour ponding time is more
desirable.
treat the WQv through a volume-based design,
and to safely pass larger storm flows. Flow enters
the channel through a pretreatment forebay.
Runoff can also enter along the sides of the channel as sheet flow through a flow spreader, such as
a pea gravel trench along the top of the bank.
• Dry Swale
A dry swale system consists of an open
conveyance channel with a filter bed of
permeable soils that may overlay an underdrain
system. Flow passes into and is detained in the
main portion of the channel where it is filtered
through the soil bed. Runoff may be collected
and conveyed by a perforated pipe and gravel
underdrain system to the outlet. Figure 4.8-2
provides a plan view and profile schematic for
the design of a dry swale system.
• Wet Swale
A wet swale or wetland channel consists of
an open conveyance channel which has been
excavated to the water table or to poorly
drained soils. Check dams are used to create
multiple wetland “cells,” which act as miniature
shallow marshes. Figure 4.8-3 provides a plan
view and profile schematic for the design of a
wet swale system.
• Dry and wet swales should have a bottom
width of 2 to 8 feet to ensure adequate
filtration. Wider channels can be designed, but
should contain berms, walls, or a multi-level
cross section to prevent channel braiding or
uncontrolled sub-channel formation.
• Dry and wet swales are parabolic or trapezoidal
in cross-section and are typically designed with
moderate side slopes no greater than 2:1 for
ease of maintenance and side inflow by sheet
flow (4:1 or flatter recommended).
• Dry and wet swales should maintain a
maximum WQv ponding depth of 18 inches
at the end point of the channel. A 12-inch
average depth should be maintained.
• The peak velocity for the 2-year storm must be
non-erosive for the soil and vegetative cover
provided.
• The dry swale consists of a permeable soil
layer of at least 30 inches in depth, above an
underdrain. The soil media should have an
infiltration rate of at least 1 foot per day (1.5 feet
per day maximum) and contain a high level of
organic material to facilitate pollutant removal.
A permeable filter fabric is placed between
the gravel layer and the overlying soil. Where
an underdrain collection system is utilized,
it should be equipped with at least a 4-inch
diameter perforated PVC pipe (AASHTO M 252)
longitudinal underdrain in a gravel layer.
Stormwater Best Management Practices
4.8.5.2 GENERAL DESIGN
• The channel and underdrain excavation should
be limited to the width and depth specified
in the design. The bottom of the excavated
trench shall not be loaded in a way that causes
soil compaction, and shall be scarified prior to
placement of gravel and permeable soil. The
sides of the channel shall be trimmed of all
large roots. The sidewalls shall be uniform with
no voids and scarified prior to backfilling.
• If the system is on-line, channels should be
sized to convey runoff from the overbank flood
event (Qp25) safely with a minimum of 6 inches
of freeboard and without damage to adjacent
property.
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Stormwater Best Management Practices
Figure 4.8-2 Schematic of Dry Swale
(Source: Center for Watershed Protection)
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Figure 4.8-3 Schematic of Wet Swale
(Source: Center for Watershed Protection)
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4.8.5.5 OUTLET STRUCTURES
4.8.5.7 MAINTENANCE ACCESS
• Wet swale channels are sized to retain the
entire water quality volume (WQv) with less
than 18 inches of ponding at the maximum
depth point.
Dry Swale
Adequate access should be provided for all dry
Outlet protection must be used at any discharge
and wet swale systems for inspection and main-
point from a dry swale to prevent scour and
tenance.
• Check dams can be used to achieve multiple
wetland cells. V-notch weirs in the check dams
can be utilized to direct low flow volumes.
downstream erosion. The underdrain system
should discharge to the storm drainage infrastruc4.8.5.8 SAFETY FEATURES
ture or a stable outfall.
Ponding depths should be limited to a maximum
of 18 inches.
Wet Swale
Outlet protection must be used at any discharge
4.8.5.4 PRETREATMENT/INLETS
point from a wet swale to prevent scour and
downstream erosion.
Landscape design should specify proper grass
• Inlets to enhanced swales must be provided
with energy dissipators such as riprap.
• Pretreatment of runoff in both a dry and
wet swale system is typically provided by a
sediment forebay located at the inlet. The
pretreatment volume should be equal to 0.1
inches per impervious acre. This storage is
usually obtained by providing check dams at
pipe inlets and/or driveway crossings.
• Enhanced swale systems that receive direct
concentrated runoff may have a 6-inch drop
to a pea gravel diaphragm flow spreader at the
upstream end of the control.
species and wetland plants based on specific
4.8.5.6 EMERGENCY SPILLWAY
site, soils and hydric conditions present along the
Enhanced swales must be adequately designed
channel. Table 4.8-1 below provides a number of
to safely pass flows that exceed the design storm
grass species that perform well in the stressful en-
flows.
vironment of an open channel BMP. In addition,
wet swales may include other wetland species
(see plant list in Section 5 of Appendix D). Select
plant material capable of salt tolerance in areas
that may include high salt levels.
Table 4.8-1: Common Grass Species for Dry and Wet Enhanced Swales
Common Name
Bermuda Grass
• Vegetated filter strips and gentle side slopes
should be provided along the top of channels
to provide pretreatment for lateral sheet flows.
4.8.5.9 LANDSCAPING
Stormwater Best Management Practices
Wet Swale
Scientific Name
Notes
Cynodon dactylon
Big Bluestem
Andropogon gerardii
Creeping Bentgrass
Agrostis palustris
Not for wet swales
Red Fescue
Festuca rubra
Not for wet swales
Reed Canary Grass
Phalaris arundinacea
Wet swales
Redtop
Agrostis alba
Smooth Brome
Bromus inermis
Switch Grass
Panicum virgatum
Not for wet swales
Note 1: These grasses are sod-forming and can withstand frequent inundation, and are thus ideal for swale or channel environments.
Most are salt-tolerant as well.
Note 2: Where possible, one or more of these grasses should be in the seed mixes.
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Special Downstream Watershed Considerations
wet swales:
• Aquifer Protection – No exfiltration of hotspot
runoff from dry swales; use impermeable liner.
Wet Swale
• Emergent vegetation should be planted, or
wetland soils may be spread on the swale
bottom for seed stock.
• Where wet swales do not intercept the
groundwater table, a water balance calculation
should be performed to ensure an adequate
water budget to support the specified wetland
species. See Subsection 3.1.8 for guidance on
water balance calculations.
4.8.5.11 CONSTRUCTION CONSIDERATIONS
• Construction equipment should be restricted
from the enhanced swale area to prevent
compaction of the native soils.
• A dense and vigorous vegetative cover should
be established over the contributing pervious
drainage areas before runoff can be accepted
into the facility. Otherwise the sediment from
the stormwater runoff will clog the pores in the
planting media and native soils.
Stormwater Best Management Practices
The following information is specific guidance for
4.8.5.10 ADDITIONAL SITE-SPECIFIC DESIGN
CRITERIA AND ISSUES
Physiographic Factors - Local terrain design
constraints
• Low Relief – Reduced need for use of check
dams
• High Relief – Often infeasible if slopes are
greater than 4%
• Karst – No exfiltration of hotspot runoff from
dry swales; use impermeable liner
Soils
No additional criteria
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(Step 1) Determine if the development site and conditions are ap-
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
propriate for the use of an enhanced swale.
of the BMP, described in this section. By considering the
Consider the application and site feasibility criteria in this
primary function, as well as, topographic and soil conditions,
chapter. In addition, determine if site conditions are suitable
the design elements of the practice can be determined (i.e.
for an enhanced swale. Create a rough layout of the en-
planting media, underdrain, inlet/outlet, overflow, etc.).
hanced swale dimensions taking into consideration existing
trees, utility lines, and other obstructions.
Complete Step 3A, 3B, and 3C for a runoff reduction approach, or skip Step 3 and complete Step 4 for a water qual-
(Step 2) Determine the goals and primary function of the enhanced
ity (treatment) approach. Refer to your local community’s
swale and if a dry or wet swale is desired.
guidelines for any additional information or specific require-
Consider whether the enhanced swale is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
ments regarding the use of either method.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Stormwater Best Management Practices
4.8.6 Design Procedures
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Where:
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
P = Target runoff reduction rainfall (inches)
»» Provide a possible solution to a drainage problem
found by:
RRV = Runoff Reduction Target Volume (ft3)
RV = Volumetric runoff coefficient which can be
»» Enhance landscape and provide aesthetic qualities
RV = 0.05+0.009(I)
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
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Provide pretreatment by using a grass filter strip or pea gravel
up the appropriate runoff reduction percentage (or credit)
diaphragm, as needed, (sheet flow), or a grass channel or
provided by the practice:
forebay (concentrated flow). Where filter strips are used,
100% of the runoff should flow across the filter strip. Pre-
Using the RRV calculated above, determine the minimum
treatment may also be desired to reduce flow velocities or
Volume of the Practice (VP)
assist in sediment removal and maintenance. Pretreatment
can include a forebay, weir, or check dam. Splash blocks
(VPMIN) ≥ RRv (target) / (RR%)
or level spreaders should be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour.
Where:
Forebays should be sized to contain 0.1 inches per imperRR% = Runoff Reduction percentage, or credit,
vious acre of contributing drainage. Refer to Section 4.9
assigned to the specific practice
for design criteria for a grass channel and Section 4.29 for
VPMIN = Minimum storage volume required to
vegetated filter strips.
provide Runoff Reduction Target Volume (ft )
3
RRv (target) = Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
(Step 3C) Determine whether the minimum storage volume was met.
When the VP ≥ VPMIN, then the Runoff Reduction require-
(Step 3B) Determine the storage volume of the practice and the
ments are met for this practice. Proceed to Step 5.
Pretreatment Volume
To determine the actual volume provided in the enhanced
When the VP < VPMIN, then the BMP must be sized according
swale, use the following equation:
to the WQV treatment method (See Step 4).
VP = (PV + VES (N))
Where:
VP = Volume provided (temporary storage)
PV = Ponding Volume
VES = Volume of Engineered Soils
N = Porosity
To determine the porosity, a qualified licensed professional
should be consulted to determine the proper porosity based
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
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Determine Swale Dimensions:
Calculate the Water Quality Volume using the following
Size bottom width, depth, length, and slope necessary to
formula:
store VP, with less than 18 inches of ponding at the down-
WQV = (1.2) (RV) (A) / 12
stream end.
»» Slope cannot exceed 4% (1 to 2% recommended)
»» Bottom width should range from 2 to 8 feet
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
RV = Volumetric runoff coefficient which can be
found by:
RV = 0.05+0.009(I)
Where:
I = new impervious area of the contributing
drainage area (%)
A = Site area (total drainage area) (ft2)
12 = Unit conversion factor (in/ft)
Using the WQV calculated above, determine the actual size
and Volume of the Practice (VP) as shown in Step 3B. Note
that VP, calculated using the formula shown in Step 3B,
should be greater than or equal to WQV.
(Step 5) If applicable, calculate the adjusted curve numbers for CPV
(1-yr, 24-hour storm), QP25 (25-yr, 24-hour storm), and Qf
»» Ensure that side slopes are no greater than 2:1 (4:1
recommended)
See Subsection 4.8.5.3 (Physical Specifications / Geometry)
for more details
(Step 7) Compute number of check dams (or similar structures)
required to detain the RRV or WQV, as applicable
Stormwater Best Management Practices
(Step 4) Calculate the Target Water Quality Volume
(Step 8) Calculate draw-down time
Dry swale: Planting soil should pass a maximum rate of 1.5
feet in 24 hours and must completely filter the ponded volume within 48 hours (24 hours preferred).
Wet swale: Must hold the WQv.
(Step 9) Check 2-year and 25-year velocity erosion potential and
freeboard
Check for erosive velocities and modify design as appropriate. Provide 6 inches of freeboard.
(Step 10) Design overflow weir or orifice at downstream berm, headwall, or checkdam.
(100-yr, 24-hour storm). See Subsection 3.1.7.5 for more
information.
(Step 6) Determine the length and channel base width of the
enhanced swale practice and the Pretreatment Volume
required.
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4.8.7 Inspection and Maintenance Requirements
A flow regulator (or flow splitter diversion structure) should
All best management practices require proper maintenance. Without proper
be supplied to divert the WQv (or RRv) to the best manage-
maintenance, BMPs will not function as originally designed and may cease to
ment practice.
function altogether. The design of all BMPs includes considerations for main-
Size low flow orifice, weir, or other device to pass Qwq.
tenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
(Step 12) Size underdrain system
See Subsection 4.2.5.3 (Physical Specifications/Geometry)
Regular inspection and maintenance is critical to the effective operation of
an enhanced swale as designed. Maintenance responsibility for an enhanced
(Step 13) Design emergency overflow
swale should be vested with a responsible authority by means of a legally
An overflow must be provided to bypass and/or convey
binding and enforceable maintenance agreement that is executed as a condi-
larger flows to the downstream drainage system or stabilized
tion of plan approval.
watercourse. Non-erosive velocities need to be ensured at
the outlet point. The overflow should be sized to safely pass
Stormwater Best Management Practices
(Step 11) Size flow diversion structure, if needed
the peak flows anticipated to reach the practice, up to a 100year storm event.
(Step 14) Design inlets, sediment forebay(s), and underdrain system
(dry swale)
See Subsection 4.8.5.4 through 4.8.5.8 for more details.
(Step 15) Prepare Vegetation and Landscaping Plan
A landscaping plan for a dry or wet swale should be prepared
to indicate how the enhanced swale system will be stabilized
and established with vegetation.
See Subsection 4.8.5.9 (Landscaping) and Appendix D for
more details.
See Appendix B-5 for an Enhanced Swale Design Example
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Description: Grass channels designed to en-
KEY CONSIDERATIONS
DESIGN CRITERIA
• Can be used as part of the runoff conveyance system to provide
pretreatment
• Can act to partially infiltrate runoff from small storm events if underlying
soils are pervious
• Less expensive than curb and gutter systems
• Should not be used on slopes greater than 4% (1-2% slopes recommended)
• Potential for bottom erosion and sediment resuspension
• Standing water may not be acceptable in some areas
• Contributing drainage area less than 5 acres
• Minimum residence time of 5 minutes
• Water quality rainfall event flow velocity less than 1.0 ft/s and flow depth
less than 4 inches
• Side slopes are 3:1 or flatter
• Minimum soil infiltration rate of 0.25 in/hr
• Minimum 2-foot clearance from groundwater table
hance water quality through the settling of suspended solids.
LID/GI Considerations: If properly incorporated
into overall site design, grass channels can help
to reduce the impacts of impervious cover and
partially infiltrate runoff with pervious soils. Grass
channels can complement the natural landscape
while providing aesthetic benefits.
ADVANTAGES / BENEFITS
• Lower cost
• Reduces runoff from impervious areas
• Ideal for linear environments (along roadways)
• Stormwater collection and conveyance
• Aesthetic benefits
• Well suited for linear environments, interchanges, and facilities
• May be contained within the roadway right-of-way
DISADVANTAGES / LIMITATIONS
• Cannot achieve the 80% TSS removal target alone; must be used in series
with other BMPs for removal credit
• Limitations for drainage area, flow, velocity, and flow depth
• Design dependent on existing site conditions and topography
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.9 Grass Channel
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes
Other Considerations: Curb and
gutter replacement
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 25% of the RRV conveyed to the
practice (A & B hydrologic soils)
• 10% of the RRV conveyed to the
practice (C & D hydrologic soils)
ROUTINE MAINTENANCE REQUIREMENTS
• Provides access to BMP and appropriate components
• Sediment cleanout, trash and debris removal, revegetation, and repair of
erosion must be completed as necessary to maintain functionality
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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should provide at least 5 minutes of residence
Grass channels, also termed “biofilters,” settle
time. To enhance water quality treatment, grass
suspended solids and other pollutants through
channels must have broader bottoms, lower
filtration, infiltration, and biofiltration. Grass chan-
slopes and denser vegetation than most drainage
nels are also able to help meet runoff velocity
channels. Additional treatment can be provided
targets for the water-quality design storm of small
by placing check-dams across the channel below
drainage areas. Vegetative practices offer a meth-
pipe inflows, and at various other points along the
od to manage pollution while conveying storm-
channel.
water runoff.
Grass channels are well-suited to
a number of applications and land uses, including
surfaces. They can also be used in a variety of
4.9.2 Stormwater Management Suitability
ways including, but not limited to, a single BMP, a
Grass channels, a form of ‘biofilter,’ are trape-
pretreatment to another BMP, or in a “treatment
zoidal or parabolic shaped vegetated channels
train”.
that work to enhance water quality through the
treating runoff from roads and other impervious
settling of suspended solids through filtration,
Grass channels differ from enhanced dry swales in
infiltration, and biofiltration.
that they do not have an engineered filter media
• Runoff Reduction
Grass channels are an effective low impact
development (LID) practices that can be
used in Georgia to reduce post-construction
stormwater runoff and improve water quality.
Like other LID practices, they become more
effective with higher infiltration rates of native
soils. A grass channel can be designed to
provide 25% of the runoff reduction volume
for type A and B hydrologic soils or 10% of
the runoff reduction volume for type C and D
hydrologic soils. Performance is dependent
on vegetation density and contact time for
settling, filtration, and infiltration.
to enhance pollutant removal capabilities and
therefore have a lower pollutant removal rate than
dry enhanced swales. Grass channels are different
from conventional roadside ditches in that they
are designed for water quality purposes and help
increase residence time while decreasing velocities. When properly incorporated into an overall
site design, grass channels can reduce impervious
cover, accent the natural landscape, and provide
aesthetic benefits while removing pollutants.
When designing a grass channel, the two primary
considerations are channel capacity and minimizing erosion. Runoff velocity should not exceed
1.0 foot per second during the peak discharge
associated with the water quality design rainfall
event, and the total length of a grass channel
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• Water Quality
Grass channel can be used to remove a variety
of pollutants from stormwater runoff’ they are
typically used as the pre-treatment component
of a larger “treatment train” to reduce incoming
runoff velocities and filter out particulates.
• Channel Protection
For smaller sites, a grass channel may be
designed to capture the entire channel
protection volume (CP v). Given that a grass
channel facility is typically designed to
completely drain within 48-72 hours, the
requirement of extended detention of the
1-year, 24-hour storm runoff volume will
be met. For larger sites, or where only the
WQv is diverted to the grass channel, another
practice must be used to provide CP v extended
detention.
Stormwater Best Management Practices
4.9.1 General Description
• Overbank Flood Protection
Another practice used in conjunction with a
grass channel will likely be required to reduce
the post-development peak flow of the 25year, 24-hour storm (Qp) to pre-development
levels (detention).
• Extreme Flood Protection
Grass channels must provide flow diversion
and/or be designed to safely pass extreme
storm flows while protecting vegetation.
Credit for the volume of runoff reduced in the
grass channel may be taken in the overbank flood
protection and extreme flood protection calculations. If the practice is designed to provide Runoff
Reduction for Water Quality compliance, then the
practice is given credit for Channel Protection and
Flood Control requirements by allowing the designer to compute an Adjusted CN (see Subsection 3.1.7.5 for more information).
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ity treatment, and the maximum width prevents
Pollutant removal from grass channels is highly
braiding, which is the formation of small channels
variable and depends primarily on the density of
within the channel bottom. The bottom width is
vegetation and contact time for filtration and in-
a dependent variable in the calculation of velocity
filtration. These, in turn, depend on soil and veg-
based on Manning’s equation. If a larger channel
etation type, slope, and contact time. Research
is needed, the use of a compound cross section
on fecal coliform removal has been inconclusive,
is recommended. The depth from the bottom
but suggests that grass channels are generally
of the channel to the groundwater should be at
not considered to be effective BMPs for treating
least 2 feet to prevent a moist channel bottom, or
bacterial loads.
contamination of the groundwater.
For additional information and data on pollutant
removal capabilities for grass channels, see the
General Feasibility
National Pollutant Removal Performance Data-
- Suitable for Residential Subdivision Usage – YES
base (3rd Edition) available at www.cwp.org and
- Suitable for High Density/Ultra Urban Areas – YES
the National Stormwater Best Management Prac-
- Regional Stormwater Control – NO
tices (BMP) Database at www.bmpdatabase.org.
Physical Feasibility - Physical Constraints at
4.9.4 Application and Site Feasibility
Criteria
Grass channels are best suited to treating smaller drainage areas of 5 acres or less. Flow must
enter the practice as sheet flow spread out over
the width (long dimension normal to flow) of the
channel. For longer flow paths, special provision
must be made to ensure design flows spread
Project Site
• Drainage Area – 5 acres or less. If the practice
is used on larger drainage areas, the flows and
volumes through the channel become too
large to allow for filtering and infiltration of
runoff.
or flatter. The maximum flow depth through the
• Runoff Velocities – Most not be erosive.
Maximum flow velocity of 1.0 ft/s or less is
recommended.
Other Constraints / Considerations
Location Requirements – The following is a list of
specific setback requirements for the location of a
vegetated filter strip:
• Grass channels should not be used on soils
that cannot sustain a dense grass cover with
high retardance. Designers should choose a
grass that can withstand relatively high velocity
flows at the entrances of the BMP and both wet
and dry periods. See Appendix D for a list of
appropriate grasses for use in Georgia.
• Grass channels can be used on most soils with
some restrictions on the most impermeable
soils. Grass channels should not be used on
soils with infiltration rates less than 0.25 inches
per hour if infiltration of small runoff flows is
intended.
• Side Slope – Slopes of the channel should be
3:1 or flatter.
• A grass channel should accommodate the peak
flow for the water quality design storm Qwq (see
Subsection 3.1.7).
• Longitudinal Slope - Between 1-4%; slopes
between 1-2% recommended.
• Incorporation of check dams within the
channel will increase retention time.
evenly across the filter strip.
Grass channels should have a side slope of 3:1
elevation of the seasonally high water table.
Stormwater Best Management Practices
4.9.3 Pollutant Removal Capabilities
• Base Width - Between 2-6 feet.
channel should be no more than 4 inches. The
base width of the channel should be 2-6 feet to
ensure a sufficient filtering surface for water qual-
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• Minimum Depth to Water Table – A separation
distance of 1 foot is recommended between
the bottom of the vegetated filter strip and the
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See Section 5.4 (Open Channel Design) for more
information and specifications on the design of
grass channels.
Grass Channels for Pretreatment
A number of other BMPs, including bioretention
areas and infiltration trenches, may utilize a grass
channel as a pretreatment measure. The length
of the grass channel depends on the drainage
area, land use, and channel slope. Table 4.9.1
provides sizing guidance for grass channels for
a 1-acre drainage area. The minimum grassed
channel length should be 20 feet.
Table 4.9-1: Grass Channel Sizing Guidance
Source: Claytor and Schueler, 1996
Parameter
Slope
(max =4%)
Grass channel
minimum length* (feet)
≤ 33% Impervious
Before designing the grass channel, the following
data is necessary:
• Existing and proposed site, topographic and
location maps, and field reviews.
66% Impervious
≥ 67% Impervious
< 2%
> 2%
< 2%
> 2%
< 2%
> 2%
25
40
30
45
35
50
*assumes 2-foot wide bottom width
• Water surface elevation of nearby water
systems as well as the depth to seasonally high
groundwater.
• Infiltration testing of native soils at the
proposed elevation of bottom of grass channel.
• Water surface elevation of nearby water
systems as well as the depth to seasonally high
groundwater.
• Infiltration testing of native soils at the
proposed elevation of bottom of grass channel.
4.9.5 Planning and Design Criteria
Between 34% and
The following criteria are to be considered minimum standards for the design of a grass channel.
Consult with the local review authority to determine if there are any variations to these criteria or
4.9.5.1 LOCATION AND LAYOUT
Grass channels vary based on site constraints
such as proposed and existing infrastructure,
soils, existing vegetation, contributing drainage
area, and utilities. Grass channels are designed for
Stormwater Best Management Practices
• A 5-minute residence time is recommended
for the water quality peak flow. Residence time
may be increased by reducing the slope of the
channel, increasing the wetted perimeter, or
planting a denser grass (raising the Manning’s
n).
intermittent flow and must be allowed to drain
between rain fall events and should not be used
on sites with a continuous flow from groundwater, sump pumps, or other sources. Grass channel
locations should be integrated into the site
planning process, with aesthetic and maintenance
considerations taken into account in their siting
and design.
additional standards that must be met.
• Impervious and pervious areas. Other means
may be used to determine the land use data.
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site.
• Design data from nearby storm sewer
structure(s).
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(Step 1) Determine the goals and primary function of the grass
channel
(Step 3) Calculate the Target Water Quality Volume.
Calculate the Water Quality Volume using the following
formula:
Consider whether the grass channel is intended to:
»» Meet water quality (treatment) target.
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v))
WQV = (1.2) (RV) (A) / 12
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
»» Provide a possible solution to a drainage problem
RV = Volumetric runoff coefficient which can be
Check with local officials and other agencies to determine
found by:
if there are any additional watershed restrictions that may
apply. In addition, consider if the grass channel has any
special site-specific design conditions or criteria. List any
restrictions or other requirements that may apply or affect
RV = 0.05+0.009(I)
Stormwater Best Management Practices
4.9.6 Design Procedures
Where:
the design.
I = new impervious area of the contributing
drainage area (%)
The design of the grass channel should be centered on
the restrictions/requirements, goals, targets, and primary
A = Area draining to the practice (ft2)
function(s) of a grass channel. By considering the primary
12 = Unit conversion factor (in/ft)
function, as well as, topographic and soil conditions, the
design elements of the grass channel can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
(Step 4) Calculate the channel width based on an assumed channel
flow depth:
(Step 2) Determine if the development site and conditions are ap-
W = (n*Q)/(1.49*D5/3*S1/2)
propriate for the use of a grass channel.
Consider the application and site feasibility criteria in this
Where:
chapter. In addition, determine if site conditions are suitable
W = minimum bottom channel width (ft)
for a grass channel. Create a rough layout of the grass chan-
n = Manning’s roughness coefficient
nel dimensions taking into consideration existing trees, utility
Q = peak runoff from the WQV rain event (cfs).
lines, and other obstructions.
See Subsection 3.1.7.2.
D = flow depth (ft)
S = slope (%)
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using a 5-minute (300 seconds) residence time:
V = Q/(W*D)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
(Step 9) Calculate RRv credited
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
L = V X (300 seconds)
up the appropriate runoff reduction percentage (or credit)
provided by the practice:
Where:
RRv (credited) = RRv (RR%)
L = minimum length of channel (ft)
V = velocity through the channel (ft/s)
Where:
(Step 6) Modify base width value and channel slope until the flow
RR% = Runoff Reduction percentage, or credit,
depth is less than 4 inches and the flow velocity is less than
assigned to the specific practice
1 ft/sec.
RRv (credited) = Runoff Reduction Volume credited
Stormwater Best Management Practices
(Step 5) Calculate the minimum length (feet) of the grass channel
by this practice (ft3)
(Step 7) Confirm the channel can pass the local design requirements
RRV = RRV conveyed to the practice
with required freeboard.
(Step 10)If Steps 5 and 6 are met, then downstream calculations can
(Step 8) Calculate the Stormwater Runoff Reduction Volume conveyed to the practice
be performed with adjusted CN using the runoff reduction
volume credited. Refer to Subsection 3.1.7.5.
Calculate the Runoff Reduction Volume using the following
(Step 11) Prepare Vegetation and Landscaping Plan
formula:
A landscaping plan for the grass channel should be prepared
RRV = (P) (RV) (A) / 12
to indicate how it will be established with vegetation.
See Appendix D for more details.
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
4.9.7 Inspection and Maintenance Requirements
RV = Volumetric runoff coefficient which can be
All best management practices require proper maintenance. Without proper
found by:
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for main-
RV = 0.05+0.009(I)
tenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
Where:
I = new impervious area of the contributing
drainage area (%)
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Description: An oil-grit separator is a device
KEY CONSIDERATIONS
DESIGN CRITERIA
• Gravity separators are typically used for drainage areas of less than 5 acres.
• Limit the contributing drainage area to any individual gravity separator to
have 1 acre or less of impervious cover.
• The total wet storage of the gravity separator unit should be at least 400
cubic feet per contributing impervious acre.
• The minimum depth of permanent pools should be 4 feet.
• Horizontal velocity through the separation chamber should be 1-3 ft/min or
less.
• No velocities in the device should exceed the entrance velocity.
• A trash rack should be included in the design to capture floating debris,
preferably near the inlet chamber.
STORMWATER MANAGEMENT
SUITABILITY
ADVANTAGES / BENEFITS
• Well-suited for use on urban development sites, where larger or aboveground BMPs are not an option, or for stormwater retrofit projects
• Can be used as pretreatment for other BMPs
• Can replace a conventional junction or inlet structure
• Multiple inlets can connect to a single unit.
• Some designs require minimal drop between inlet and outlet.
IMPLEMENTATION CONSIDERATIONS
designed to remove suspended solids, oil, grease,
debris, and floatables from stormwater runoff
through gravitational settling, hydrodynamic
separation, and trapping of pollutants. Oil-grit
separators are also called gravity separators or
oil-water separators.
LID/GI Considerations: Gravity oil-grit separators
are not considered low impact development or
green infrastructure. However, for ultra-urban
development projects or stormwater retrofit designs,
oil-grit separators may be one of few design options
for removal of total suspended solids and/or
pollutants from stormwater runoff.
DISADVANTAGES / LIMITATIONS
• Dissolved pollutants are not effectively removed by oil-grit separators.
• Oil-grit separators alone cannot achieve the 80% TSS removal target.
• Frequent maintenance is required.
• Performance is dependent on design and frequency of inspection and
cleanout of unit.
• Some designs may require a confined space entry for inspection,
maintenance, and repairs.
ROUTINE MAINTENANCE REQUIREMENTS
• Maintenance requirements for a proprietary system should be obtained from
the manufacturer.
• Frequency of inspection and maintenance is dependent on land use,
climate, and design of the gravity separator.
• Failure to provide adequate inspection and maintenance can result in the
resuspension of accumulated solids.
• Proper disposal of oil, solids, and floatables removed from the gravity
separator must be ensured.
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
Land Requirement
Stormwater Best Management Practices
4.10 Gravity (Oil-Grit) Separator
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Not recommended
Soils: Gravity oil-grit separator systems can be installed in almost any
soil or terrain.
Other Considerations: Install as an
off-line device unless the separator
can be sized to handle a small drainage area. Also, consider installing a
manhole on the downstream side to
provide easy access for sampling of
effluent.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 0% Runoff Reduction Credit is
provided by this practice.
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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hydrocarbon hotspots, such as gas stations and
Gravity oil-grit separators are hydrodynamic sepa-
areas with high vehicular traffic. However, gravity
ration devices that are designed to remove grit,
separators cannot be used for the removal of
heavy sediments, oil, grease, debris, and floatable
dissolved or emulsified oils and pollutants such as
matter from stormwater runoff through gravi-
coolants, soluble lubricants, glycols and alcohols.
tational settling and trapping. Gravity separator
units contain a permanent pool of water and
Since resuspension of accumulated sediments
typically consist of an inlet chamber, separation
is possible during heavy storm events, gravity
and storage chamber, a bypass chamber, and an
separator units are typically installed off-line.
access port for maintenance purposes (see Figure
Gravity separators are available as prefabricated
4.10-1). Runoff enters the inlet chamber where
proprietary systems from a number of different
heavy sediments and solids drop to the bottom.
commercial vendors.
The flow moves into the main gravity separation
chamber, where further settling of suspended
solids takes place. Oil and grease are skimmed
and stored in a waste oil storage compartment
for future removal. After moving into the outlet
chamber, the clarified runoff is then discharged.
4.10.2 Stormwater Management Suitability
• Runoff Reduction
Gravity oil-grit separators do not provide
stormwater volume runoff reduction. Another
BMP should be used in a treatment train with
gravity oil-grit separators to provide runoff
reduction. See Subsection 4.1.6 for more
information about using BMPs in series.
• Water Quality
If installed as per the recommended design
criteria and properly maintained, 40% total
suspended solids removal will be applied to
the water quality volume (WQv) flowing to the
gravity oil-grit separator. Another BMP should
be used in a treatment train with gravity oil-grit
separators to provide the additional required
water quality treatment. (See Subsection 4.1.6.)
Stormwater Best Management Practices
4.10.1 General Description
The performance of these systems is based primarily on the relatively low solubility of petroleum
products in water and the difference between
their specific gravities. Gravity separators are not
designed to separate other products, such as
solvents, detergents, or dissolved pollutants. The
typical gravity separator unit may be enhanced
with a pretreatment swirl concentrator chamber,
oil draw-off devices that continuously remove the
accumulated light liquids, and flow control valves
regulating the flow rate through the facility.
Gravity separators are often used in commercial,
industrial, and transportation land uses and are
intended primarily as a pretreatment measure
for high-density or ultra-urban sites, or for use in
Figure 4.10-1 Schematic of an Example Gravity (Oil-Grit) Separator
(Source: NVRC, 1992[1])
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4.10.3 Pollutant Removal Capabilities
• Overbank Flood Protection
Gravity oil-grit separators do not provide
overbank flood protection. Another BMP
should be used in a treatment train with gravity
oil-grit separators to provide runoff reduction.
Additionally, the gravity oil-grit separator
should be designed off-line or with a bypass for
higher flows. (See Subsection 3.1.5.)
• Extreme Flood Protection
Gravity oil-grit separators do not provide
extreme flood protection. Another BMP should
be used in a treatment train with gravity oilgrit separators to provide runoff reduction.
Additionally, the gravity oil-grit separator
should be designed off-line or with a bypass for
higher flows. (See Subsection 3.1.5.)
Testing of gravity oil-grit separators has shown
4.10.4 Application and Site Feasibility
Criteria
that they can remove between 40-50% of the TSS
Conventional oil-grit separators contain a per-
loading when used in an off-line configuration
manent pool of water and typically consist of an
(Curran, 1996 and Henry, 1999). Gravity oil-grit
inlet chamber, separation and storage chamber,
separators also provide removal of debris, hydro-
a bypass chamber, and an access port for main-
carbons, trash, and other floatables. They provide
tenance purposes. Runoff enters the inlet cham-
only minimal removal of nutrients and organic
ber where heavy sediments and solids drop to
matter.
the bottom. Then the flow moves into the main
separation chamber, where further settling of
The following design pollutant removal rates are
suspended solids takes place. Oil and grease are
conservative average pollutant reduction percent-
skimmed and stored in a waste oil storage com-
ages for design purposes derived from sampling
partment for future removal. After moving into
data, modeling, and professional judgment.
the outlet chamber, the clarified runoff is then
Stormwater Best Management Practices
• Channel Protection
Gravity oil-grit separators do not provide
channel protection. Another BMP should be
used in a treatment train with gravity oil-grit
separators to provide runoff reduction. (See
Subection 4.1.6.) Additionally, the gravity oilgrit separator should be designed off-line or
with a bypass for higher flows.
discharged to the site’s stormwater conveyance
-- Total Suspended Solids – 40%
system.
-- Total Phosphorus – 5%
-- Total Nitrogen – 5%
-- Fecal Coliform – insufficient data
-- Heavy Metals – insufficient data
A wide variety of separator systems are commercially-available in a variety of layouts, for which
vendors have design data and procedures. Oil-grit
separators are sized based on a design flow rate,
the Water Quality Peak Flow Rate (Qwq). This dif-
Actual field testing data and pollutant removal
fers from how other stormwater BMPs are sized.
rates from an independent source should be obtained before using a proprietary gravity separator
system.
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas – YES
- Regional Stormwater Control – NO
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Project Site
• Minimum Head – 4 feet (The minimum depth
of the permanent pools should be 4 feet.)
• Drainage Area – Gravity oil-grit separators are
typically used for drainage areas less than 5
acres. It is recommended that the contributing
area to any individual gravity separator be
limited to 1 acre or less of impervious cover.
• Soils – Gravity separator systems can be
installed in almost any soil or terrain.
• The total wet storage of the gravity separator
unit should be at least 400 cubic feet per
contributing impervious acre.
Other Constraints/Considerations
• Space Required – Gravity oil-grit separators
are installed underground; therefore, minimal
surface area is required for the device.
• Adequate maintenance access to each
chamber must be provided for inspection and
cleanout of a gravity separator unit.
• Site Slope – Gravity oil-grit separators may be
installed on sites with slopes up to 6%.
• A minimum 20 foot wide maintenance rightof-way or drainage easement shall be provided
for the oil-grit separator from any public or
private road or driveway. The maintenance
access easement shall have a maximum
slope of no more than 15% and shall have a
minimum unobstructed drive path width of
12 feet, appropriately stabilized to withstand
maintenance equipment and vehicles. The
right-of-way shall be located such that
maintenance vehicles and equipment can
access the oil-grit separator.
• Minimum Depth to Water Table – 2 feet
Check with manufacturer recommendations for
additional site design constraints
• Hot spots – Gravity oil-grit separators are wellsuited for hot spot runoff
• Damage to existing structures and facilities:
»» Gravity oil-grit separators may increase the
risk of flooding within a basement, affecting
underground sewage pipes, or cause adverse
effect to other underground structures.
»» Gravity oil-grit separators should be designed
so that overflow drains away from buildings
to avoid causing damage to building
foundations.
• Trout Stream – Gravity oil-grit separators will
not reduce thermal impacts of stormwater
runoff, nor are they effective at removing
soluble pollutants. Therefore, they are not
considered an effective means of protecting
trout streams. However, in urban and highly
developed areas or with a stormwater retrofit,
gravity oil-grit separators may be an effective
BMP for total suspended solids removal and
hydrocarbons.
• Flat Terrain – Flat terrain and low site slopes
do not interfere with the operation of a gravity
oil-grit separator.
• Shallow Water Table – Review manufacturer’s
instructions regarding groundwater elevation.
Anti-flotation calculations may be required
when large open chambers are installed at or
below the water table.
4.10.5 Planning and Design Criteria
Before designing the gravity oil-grit separator, the
following data is necessary:
• Existing and proposed site, topographic, and
location maps, as well as field reviews
Stormwater Best Management Practices
Physical Feasibility – Physical Constraints at
• Impervious and pervious areas. Other means
may be used to determine the land use data
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Design data from nearby storm sewer
structures
• Water surface elevation of nearby water
systems and depth to seasonally high
groundwater
The following criteria are to be considered minimum standards for the design of a gravity oil-grit
separator. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be met.
Coastal Areas
• Poorly Draining Soils – Poorly draining soils do
not inhibit a gravity oil-grit separator’s ability to
temporarily store and treat stormwater runoff.
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Gravity oil-grit separators should be located
upstream or downstream of other BMPs providing runoff reduction, additional treatment of the
water quality volume (WQv), channel protection
volume (CPv), overbank flood protection (QP25),
and extreme flood protection (Qf). See Subsection 4.1.6 for more information on the use of
multiple BMPs in a treatment train.
4.10.5.2 GENERAL DESIGN
• The use of gravity (oil-grit) separators should
be limited to the following applications:
»» Pretreatment for other BMPs
»» High-density, ultra-urban, or other spacelimited development sites
»» Hotspot areas where the control of grit,
floatables, oil, and/or grease are required
• Gravity separators are rate-based devices.
This contrasts with most other stormwater
BMPs, which are sized based on capturing and
treating a specific volume.
• Horizontal velocity through the separation
chamber should be 1-3 ft/min or less. No
velocities in the device should exceed the
entrance velocity.
4.10.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• The design criteria and specifications of a
proprietary gravity separator unit should be
obtained from the manufacturer.
• The separation chamber should provide for
three separate storage volumes:
»» A volume for separated oil storage at the top
of the chamber
4.10.5.4 PRETREATMENT/INLETS
• Gravity oil-grit separators are typically used
for pretreatment in a hotspot area or where
floatable debris and pollutants should be
removed prior to additional treatment.
• Inlets sizing, slope, and invert placement
should be sized based on manufacturer’s
recommendations for flow rate, volume, and
structure size.
»» A volume for settled solids accumulation at
the bottom of the chamber
»» A volume required to give adequate flowthrough detention time for separation of oil
and sediment from the stormwater flow
4.10.5.5 OUTLET STRUCTURES
An important consideration when designing
an oil-grit separator system for a site is how to
bypass large storm events that exceed the design
• Gravity separator units are typically designed
to bypass runoff flows in excess of the design
flow rate. Some designs have built-in high flow
bypass mechanisms. Other designs require a
diversion structure or flow splitter ahead of the
device in the drainage system. An adequate
outfall must be provided.
flow capacity around the separator without dam-
• A trash rack should be included in the design to
capture floating debris, preferably near the inlet
chamber to prevent debris from becoming oil
impregnated.
or resuspension in these instances.
• Ideally, a gravity separator design will provide
an oil draw-off mechanism that empties to a
separate chamber or storage area.
Stormwater Best Management Practices
4.10.5.1 LOCATION AND LAYOUT
aging the unit, exceeding the design flow capacity, or resuspending collected pollutants. Since
resuspension of accumulated sediments and oil
droplets is possible during heavy storm events,
oil-grit separator units are typically installed offline with a bypass to minimize pollutant wash-out
The outlet from the gravity separator needs to
be able to convey stormwater leaving the gravity
separator as well as the bypassed discharge without eroding the surrounding area. Typically the
high flow outlet will discharge at a higher elevation than the low flow outlet.
• Gravity separator units should be watertight to
prevent possible groundwater contamination.
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• The deep inverts, open void sections, and
sometimes larger pipe diameters into and
out of oil-grit separators may present a fall or
entrapment hazard. It is recommended that
gravity oil-grit separators be constructed with
manhole covers and/or grate lids with locking
mechanisms. Structural loading calculations,
such as H-20 loading for traffic areas, should
be performed when sizing and installing gravity
oil-grit separators.
• Some oil-grit separators are considered
confined spaces. Additional training may be
required to perform work inside the units.
4.10.5.8 CONSTRUCTION AND MAINTENANCE
COSTS
• Material and installation costs for gravity oil-grit
separators can vary based on the size, location,
treatment requirements, and manufacturer.
• Typically, gravity oil-grit separator systems
range from approximately $5,000-$6,000 for a
small catch basin or manhole insert type design
to approximately $40,000 for a multi-chamber,
high-volume, high-flow device.
Stormwater Best Management Practices
4.10.5.6 SAFETY FEATURES
4.10.5.7 CONSTRUCTION CONSIDERATIONS
• Contributing drainage areas to the gravity
oil-grit separator should be stabilized with
appropriate erosion and sediment control
devices, such as with temporary or permanent
seeding before runoff can enter a newly
installed-device.
• Newly installed gravity oil-grit separators
should be inspected prior to being placed
in service. Remove sediment and debris that
may have been collected during delivery and
installation.
• A minimum 20-foot wide maintenance rightof-way or drainage easement should be
provided for the oil-grit separator from any
public or private road or driveway.
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In general, site designers should perform the following design procedures
when designing a gravity oil-grit separator.
(Step 1) Determine the goals and primary functions of the gravity
oil-grit separator.
»» A gravity oil-grit separator can be designed to provide
pre-treatment of settled solids, oil, grease, debris, and
floatables.
»» An oil-grit separator may also be used to provide some
treatment of the water quality volume (WQv).
»» Check with local officials and other agencies to
determine if there are any additional watershed
restrictions that may apply. In addition, consider if the
oil-grit separator has any site-specific design conditions
or criteria. List any other requirements that may apply
to or affect the design.
(Step 4) Compute outlet release rate and size outlet.
»» Use manufacturer-recommend design methods to
determine the size, slope, and invert of the device
outlet.
»» Ensure downstream receiving BMPs and/or storm drain
systems can receive the volume and rate of stormwater
flow from the gravity oil-grit separator and/or bypass
system.
»» A hydraulic grade analysis should be performed to
ensure that the receiving stormwater system can accept
the flow, and that the oil-grit separator does not create
a hydraulic head jump that exceeds the elevation of the
upstream system.
Stormwater Best Management Practices
4.10.6 Design Procedures
4.10.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
(Step 2) Determine if the development site and conditions are ap-
maintenance, BMPs will not function as originally designed and may cease to
propriate for the use of a gravity oil-grit separator.
function altogether. The design of all BMPs includes considerations for main-
Consider the application and site feasibility criteria in this
tenance and maintenance access. For additional information on inspection
chapter to determine if site conditions are suitable for a
and maintenance requirements, see Appendix E.
gravity oil-grit separator. Create a rough layout of the device
dimensions taking into consideration existing trees, utility lines, power and telephone poles, roadways, sidewalks,
curbs, and other obstructions.
(Step 3) Compute runoff control volumes and rates.
Oil-grit separators are typically sized based on the Water
Quality peak flow rate (Qwq). See Subsection 3.1.7.2 for more
information on calculating the Qwq. This differs from how
other stormwater BMPs are sized. Refer to manufacturer
instructions and tools to design the gravity oil-grit separator
based on the appropriate design flow rate and volumes.
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Description: Green roofs represent an alternative to
traditional impervious roof surfaces. They typically
consist of underlying waterproofing and drainage
materials and an overlying engineered growing
media that is designed to support plant growth.
Stormwater runoff is captured and temporarily
stored in the engineered growing media, where it is
subjected to the hydrologic processes of evaporation and transpiration with any remaining stormwater conveyed back into the storm drain system. This
allows green roofs to provide measurable reductions in post-construction stormwater runoff rates,
volumes, and pollutant loads on development sites.
LID/GI Considerations: Green roofs are an excellent method for reducing site impervious area,
stormwater runoff volumes, pollutant loads, and
thermal impacts of development. Green roofs
provide outdoor areas and species habitat in developed and highly urbanized areas. The use of green
roofs reduces the amount of ground surface area
required to treat stormwater runoff, maximizing
development space.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Engineered growing media should be a light-weight mix containing about
15% organic material.
• Waterproofing materials should be protected from root penetration by an
impermeable root barrier.
• Green roofs may be installed on rooftops with slopes of up to 25%, but are
not generally recommended for use on rooftops with slopes greater than
10%.
• The use of extensive green roof systems (2”-6” deep growing media) should
be considered prior to the use of more complex and expensive intensive
green roof systems.
• A landscaping plan should be prepared for all green roofs. The landscaping
plan should be reviewed and approved by the local development review
authority prior to construction.
ADVANTAGES / BENEFITS
• Helps reduce post-construction stormwater runoff rates, volumes, and
pollutant loads without consuming valuable land
• Particularly well-suited for use on urban development and redevelopment
sites
• Use of green roofs allows for more development space on a project site.
DISADVANTAGES / LIMITATIONS
• Can be difficult to establish vegetation in the harsh growing conditions found
on rooftops in coastal Georgia.
• The roof structure must be capable of supporting the additional weight (live
and dead load) of a green roof. Additional support, such as trusses, may be
necessary for redevelopment projects to existing structures.
ROUTINE MAINTENANCE REQUIREMENTS
• Inspect green roof for dead or dying vegetation. Dead vegetation should be
removed along with any woody vegetation. Plant replacement vegetation as
needed.
• Inspect waterproof membrane for leaks. Repair as needed.
• Remove invasive vegetation.
• Monitor sediment accumulation and remove periodically.
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.11 Green Roof
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: No
Soils: Planting media should meet
design recommendations.
Other Considerations: Drainage systems for the roof (gutters,
downspouts, etc.) must be capable of
handling large rainfall events.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 60% of the runoff reduction volume
provided
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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Stormwater Best Management Practices
4.11.1 General Description
Green roofs (also known as vegetated roofs or
eco-roofs) represent an alternative to traditional
impervious roof surfaces. They typically consist of
underlying waterproofing and drainage materials
plants
growing media
and an overlying engineered growing media that
is designed to support plant growth (Figure 4.111). Stormwater runoff is captured and temporarily
stored in the engineered growing media, where it
is subjected to the hydrologic processes of evap-
filter fabric
rigid drainage mat
moisture retention geotextile
oration and transpiration, with any excess runoff
conveyed back into the storm drain system. This
allows green roofs to provide measurable reductions in post-construction stormwater runoff
rates, volumes, and pollutant loads on development sites.
root barrier
waterproofing
roof deck
There are two different types of green roof systems: intensive green roof systems and extensive
green roof systems. Intensive green roof systems
(also known as rooftop gardens) have a thick layer
of engineered growing media (i.e., 12-24 inches)
Figure 4.11-1: Components of a Green Roof System
(Source: Carter et al., 2007)
that supports a diverse plant community that may
include trees (Figure 4.11-2). Extensive green roof
systems typically have a much thinner layer of
engineered growing media (i.e., 2-6 inches) that
supports a plant community that is comprised
primarily of drought tolerant vegetation (e.g.,
sedums, succulent plants) (Figure 4.11-3).
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Stormwater Best Management Practices
Figure 4.11-2: Intensive Green Roof System
(Source: City of Portland, OR, 2004)
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Figure 4.11-3: Extensive Green Roof System
(Source: City of Portland, OR, 2004)
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to twice as much as traditional impervious roof
surfaces, are much lighter and less expensive than
intensive green roof systems. Consequently, it is
recommended that the use of extensive green
roof systems be considered prior to the use of
intensive green roof systems.
volume (RRv) conveyed to a green roof may be
reduced by 60%
• Water Quality
If installed as per the recommended design
criteria and properly maintained, 80% total
suspended solids removal will be applied to
the water quality volume (WQv) flowing to the
green roof.
Extensive green roof systems typically contain
multiple layers of roofing materials (Figure 4.11-1),
and are designed to support plant growth while
preventing stormwater runoff from ponding on
the roof surface. Green roof systems are designed
to drain stormwater runoff vertically through the
engineered growing media and then horizontally
through a drainage layer that is sloped towards
an outlet. They are designed to require minimal
long-term maintenance and, if the right plants are
selected to populate the green roof, should not
need supplemental irrigation or fertilization after
an initial vegetation establishment period.
• Channel Protection
No channel protection volume (CP v) storage
is provided by a green roof. Stormwater runoff
generated by the contributing impervious
rooftop area and pervious green roof should
be routed to a downstream regional BMP that
provides storage and treatment of the CP v.
Proportionally adjust the post-development
runoff curve number (CN) to account for the
runoff reduction provided by a green roof
when calculating the CP v for the regional BMP.
See Subsection 3.1.7.5 for more information
about curve number reduction.
sites.
• Overbank Flood Protection
No overbank flood protection volume storage
is provided by a green roof. Proportionally
adjust the post-development runoff CN to
account for the runoff reduction provided by a
green roof when calculating the overbank peak
discharge (Qp25) on a development site. See
Subsection 3.1.7.5 for more information about
curve number reduction.
• Runoff Reduction
Runoff reduction credit can be applied to
the green roof contributing drainage area if
properly designed, installed, and maintained.
As shown in Table 4.1.3-2, the runoff reduction
• Extreme Flood Protection
No extreme flood protection volume storage is
provided by a green roof. Proportionally adjust
the post-development runoff CN to account
for the runoff reduction provided by a simple
4.11.2 Stormwater Management Suitability
The Center for Watershed Protection (Hirschman
et al., 2008) recently documented the ability of
green roofs to reduce annual stormwater runoff
volumes and pollutant loads on development
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green roof for the contributing drainage area
when calculating the extreme peak discharge
(Qf) on a development site. See Subsection
3.1.7.5 for more information about curve
number reduction.
4.11.3 Pollutant Removal Capabilities
Green roofs are presumed to remove 80% of the
total suspended solids (TSS) load in typical urban
post-development runoff when sized, designed,
constructed, and maintained in accordance with
the recommended specifications. Green roofs
remove 50% total Phosphorus and 50% total
Stormwater Best Management Practices
Extensive green roof systems, which can cost up
Nitrogen from stormwater. Green roofs are not
presumed to remove fecal coliform or metals
such as Cadmium, Copper, Lead, and Zinc.
In order to provide the most efficient pollutant
removal, green roofs should be designed according to the criteria provided in this section.
For additional information and data on pollutant
removal capabilities for green roofs, see the National Pollutant Removal Performance Database
(Version 3) available at www.cwp.org and the
National Stormwater Best Management Practices
(BMP) Database at www.bmpdatabase.org.
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• Space Required – Green roofs require 100% of
their contributing drainage areas.
Green roofs can be used on a wide variety of
• Site Slope – Although green roofs may be
installed on rooftops with slopes of up to 25%,
they are not recommended for use on rooftops
with slopes greater than 10%.
development sites in rural, suburban, and urban areas. They are especially well-suited for
use on commercial, institutional, municipal, and
multi-family residential buildings on urban and
suburban development and redevelopment sites.
When compared with other low impact development practices, green roofs have a relatively high
construction cost, a relatively low maintenance
burden, and no additional surface area requirements beyond that which will be covered by the
green roof. Although they can be expensive to install, green roofs are often a component of “green
buildings,” such as those that achieve certification
in the Leadership in Energy and Environmental
Design (LEED) Green Building Rating System.
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas – YES
- Regional Stormwater Control – NO
Physical Feasibility – Physical Constraints at
Project Site
• Drainage Area – Green roofs should only be
used to replace traditional impervious roof
surfaces. They should not be used to treat any
stormwater runoff generated elsewhere on the
development site.
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• Minimum Depth to Water Table – Separation
from the water table is not applicable to a
green roof.
• Minimum Head – 6-12 inches
• Soils – An appropriate engineered growing
media, consisting of approximately 80%
lightweight inorganic material, 15% organic
material, and 5% sand, should be used in green
roof systems.
Other Constraints / Considerations
• Hot spots – May not be used for hot spot
runoff
• Damage to existing structures and facilities
– When designing a green roof, site planning
and design teams must not only consider the
stormwater storage capacity of the green roof,
but also the structural capacity of the rooftop
itself. To support a green roof, a rooftop
must be designed to support an additional
15 to 30 pounds per square foot (psf) of load.
Consequently, a structural engineer or other
qualified professional should be involved with
the design of a green roof to ensure that the
rooftop itself has enough structural capacity to
safely support the green roof system.
• Proximity – Green roofs may be used without
restriction near:
»» Private water supply wells
»» Open water
»» Public water supply reservoirs
»» Public water supply wells
»» Property Lines – Green roofs may be used
near property lines; however, ensure that
stormwater runoff is not redirected onto
an adjacent owner’s property. Stormwater
runoff should remain on the property where
it was generated until it is discharged to
downstream receiving waters, a municipal
stormwater system, or to the preconstruction point of discharge.
Stormwater Best Management Practices
4.11.4 Application and Site Feasibility
Criteria
• Trout Stream – Green roofs help to treat
stormwater for pollutants and reduce the
volume and velocity of runoff. Therefore, green
roofs are an effective BMP for use where trout
streams or other protected waters may receive
stormwater runoff.
Coastal Areas
Green roofs can be used without restriction in
Coastal Georgia, where there is flat terrain, low
site slopes, and shallow water tables.
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Before designing the green roof, the following
data is necessary:
• Architectural roof plan with rooftop pitches and
downspout locations
• The proposed site design, including, buildings,
parking lots, sidewalks, stairs and handicapped
ramps, and landscaped areas for downspout
discharge locations and bypass outfalls
• Information about downstream BMPs and
receiving waters
The following criteria are to be considered minimum standards for the design of a green roof.
Consult with the local review authority to determine if there are any variations to these criteria or
additional standards that must be met.
• During the design of a green roof system, site
planning and design teams should consider
not only the storage capacity of the green roof,
but also the structural capacity of the rooftop
itself. A structural engineer or other qualified
professional should be involved with the design
of a green roof to ensure that the rooftop itself
has enough structural capacity to support the
green roof system.
• Green roofs should not be used to treat any
stormwater runoff generated elsewhere on the
development site.
4.11.5.2 GENERAL DESIGN
• All green roofs should be designed in
accordance with the ASTM International Green
Roof Standards (ASTM, 2005a, ASTM, 2005b,
ASTM, 2005c, ASTM, 2005d, ASTM, 2006).
4.11.5.1 LOCATION AND LAYOUT
• Green roof systems should be designed to
provide enough storage for the stormwater runoff
volume generated by the target runoff reduction
rainfall event (e.g., 85th percentile rainfall event).
The required dimensions of a green roof system
are governed by several factors, including the
hydraulic conductivity and moisture retention
capacity of the engineered growing media and
the porosity of the underlying drainage layer. Site
planning and design teams are encouraged to
consult with green roof manufacturers and/or
materials suppliers to design green roof systems
that provide enough storage for the stormwater
runoff volume generated by the target runoff
reduction rainfall event (e.g., 85th percentile
rainfall event).
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• Supplemental measures, such as battens,
may be needed to ensure stability against
sliding on rooftops with slopes of greater
than 10%.All green roof systems should
include a waterproofing layer that will prevent
stormwater runoff from damaging the
underlying rooftop. Waterproofing materials
typically used in green roof installations include
reinforced thermoplastic and synthetic rubber
membranes.
or chemicals that may leach into postconstruction stormwater runoff should not be
used.
• A drainage layer should be placed between the
root barrier and the engineered growing media.
The drainage layer should consist of synthetic
or inorganic materials (e.g., gravel, recycled
polyethylene) that are capable of both retaining
water and providing efficient drainage when
the layer becomes saturated. The required
depth of the drainage layer will be governed
by the required storage capacity of the green
roof system and by the structural capacity of
the rooftop itself. An appropriate engineered
growing media, consisting of approximately
80% lightweight inorganic materials, 15%
organic matter (e.g., well-aged compost), and
5% sand, should be installed above the drainage
layer. The engineered growing media should
have a maximum water retention capacity of
approximately 30%.
Stormwater Best Management Practices
4.11.5 Planning and Design Criteria
• To prevent clogging within the drainage layer,
the engineered growing media should be
separated from the drainage layer by a layer of
permeable filter fabric. The filter fabric should
be a non-woven geotextile with a permeability
that is greater than or equal to the hydraulic
conductivity of the overlying engineered
growing media.
• The waterproofing layer should be protected
from root penetration by an impermeable,
physical root barrier. Chemical root barriers
or physical root barriers that have been
impregnated with pesticides, metals,
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4.11.5.5 OUTLET STRUCTURES
• To assist in conveying runoff to the building
drainage system, a semi-rigid, plastic geocomposite drain or mat layer should be
included in the design of a green roof. If the
roof is flat, a perforated network may be
necessary to help rainfall drain properly.
• Consideration should be given to the
stormwater runoff rates and volumes generated
by larger storm events (e.g., 25-year, 24-hour
storm event) to design green roofs that are
able to safely convey or bypass these flows. An
overflow system, such as a traditional rooftop
drainage system with inlets set slightly above
the elevation of the surface of the green roof,
should be designed to convey the stormwater
runoff generated by these larger storm events
safely off of the rooftop.
• Extend the roof flashing 6 inches above
engineered growing media and protect by
counter flashing.
4.11.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
The drainage area of a green roof is comprised
of the green roof itself. No additional stormwater
runoff should be allowed to “run on” to the green
An outlet (e.g., scupper and downspout) should
be provided to convey stormwater runoff out of
the drainage layer and off of the rooftop when the
drainage layer becomes saturated.
4.11.5.6 SAFETY FEATURES
• Runoff shall flow through and exit green roof
systems in a safe and non-erosive manner.
Overflow structures should be capable of
passing the 2-year, 24-hour design storm
without inundating the roof.
other qualified professional to identify plants
that will tolerate the harsh growing conditions
found on rooftops in Georgia. Planting
recommendations for green roofs include:
»» Drought- and full sun-tolerant vegetation
that requires minimal irrigation after
establishment
»» Low maintenance vegetation that is selfsustaining and does not require mowing,
trimming or the use of fertilizers, pesticides,
or herbicides
»» Vegetation that is fire resistant and able to
withstand heat, cold, and high winds
• Since sedum and succulent plants possess
many of the characteristics listed above, they
are recommended for use on green roof
systems installed in Georgia. Herbs, forbs,
grasses, and other groundcovers may be used,
but these plants typically have higher watering
and maintenance requirements.
Stormwater Best Management Practices
• The engineered growing media should be
between 4-6 inches deep, unless synthetic
moisture retention materials (e.g., drainage
mat with moisture storage “cups”) are placed
directly beneath the engineered growing
media layer. When synthetic moisture retention
materials are used, a 2 inch deep engineered
growing media layer may be used.
• Methods used to establish vegetative cover on
a green roof should achieve at least 75 percent
vegetative cover one year after installation.
roof with the exception of walking paths or vegetation access ways incorporated into the green
roof design.
4.11.5.4 PRETREATMENT/INLETS
Green roofs are designed to directly receive rainfall. Pretreatment and inlets are not required.
4.11.5.7 LANDSCAPING
• A landscaping plan should be prepared
for all green roofs. The landscaping plan
should be reviewed and approved by the
local development review authority prior to
construction.
• When developing a landscaping plan, site
planning and design teams are encouraged to
consult with a botanist, landscape architect, or
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To help ensure that green roofs are properly
installed on a development site, site planning
and design teams should consider the following
recommendations:
• Care should be given to avoid damage to the
waterproofing membrane during installation of
the green roof. If the integrity of the membrane
is compromised in a manner that may cause
leaks or roof damage, the area should be
identified and repaired. Visually inspect for
damage and test the membrane for water
tightness prior to installation of the engineered
growing media.
4.11.5.9 CONSTRUCTION AND MAINTENANCE
COSTS
• Extensive green roofs can range from roughly
$5-$20 per square foot.
• Intensive green roofs can range from roughly
$20-$80 per square foot.
• Although the cost per square foot of a green
roof is notably higher than a regular roof,
green roofs have been reported to save costs
associated with energy consumption and
increasing the lifespan of the roof.
Stormwater Best Management Practices
4.11.5.8 CONSTRUCTION CONSIDERATIONS
• If the roof is sloped, stabilization measures may
be required before installing the green roof to
prevent soil from sliding down the roof. Some
situations may allow the stabilization measures
to be incorporated into the roof structure.
• Install the green roof according to the
manufacturer’s instructions. Usually the root
barrier layer, walkway, and irrigation system are
installed first.
• To help prevent compaction of the engineered
growing media, heavy foot traffic should be
kept off of green roof surfaces during and after
construction.
• Construction contracts should contain a
replacement warranty that covers at least three
growing seasons to help ensure adequate
growth and survival of the vegetation planted
on a green roof.
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(Step 1) Determine if the development site and conditions are appropriate for the use of a green roof.
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Consider the application and site feasibility criteria in this
chapter. In addition, determine if site conditions are suitable
Complete Step 3A, 3B, and 3C for a runoff reduction ap-
for a green roof. Create a rough layout of the green roof di-
proach, or skip Step 3 and complete Step 4 for a water qual-
mensions taking into consideration existing trees, utility lines,
ity (treatment) approach. Refer to your local community’s
and other obstructions.
guidelines for any additional information or specific requirements regarding the use of either method.
(Step 2) Determine the goals and primary function of the green roof.
Consider whether the green roof is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Stormwater Best Management Practices
4.11.6 Design Procedures
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
RV = Volumetric runoff coefficient which can be
»» Provide a possible solution to a drainage problem
RV = 0.05+0.009(I)
found by:
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
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(Step 3C) Determine whether the minimum storage volume was met.
up the appropriate runoff reduction percentage (or credit)
When the VP ≥ VPMIN, then the Runoff Reduction require-
provided by the practice:
ments are met for this practice. Proceed to Step 5.
Using the RV calculated, determine the minimum Volume of
When the VP < VPMIN, then the BMP must be sized according
the Practice (VP)
to the WQV treatment method (See Step 4).
(VPMIN) ≥ RRv (target) / (RR%)
(Step 4) Calculate the Target Water Quality Volume
Calculate the Water Quality Volume using the following
Where:
formula:
RR% = Runoff Reduction percentage, or credit,
WQV = (1.2) (RV) (A) / 12
assigned to the specific practice
VPMIN = Minimum storage volume required to
provide Runoff Reduction Target Volume (ft3)
Where:
RRv (target) = Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
RV = Volumetric runoff coefficient which can be
(Step 3B) Determine the storage volume of the practice and the
Pretreatment Volume
found by:
To determine the actual volume provided in the green roof,
use the following equation:
RV = 0.05+0.009(I)
VP = (PV + VES (N))
Where:
I = new impervious area of the contributing
Where:
drainage area (%)
VP = Volume provided (temporary storage)
PV = Ponding Volume
A = Site area (total drainage area) (ft2)
VES = Volume of Engineered Soils
12 = Unit conversion factor (in/ft)
N = Porosity
Using the WQV calculated above, determine the actual size
To determine the porosity, a qualified licensed professional
and Volume of the Practice (VP) as shown in Step 3B. Note
should be consulted to determine the proper porosity based
that VP, calculated in Step 3B, should be greater than or
on the engineered soils used. Most soil media has a porosity
equal to WQV.
of 0.25 and gravel a value of 0.40.
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storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
storm). See Subsection 3.1.7.5 for more information.
(Step 6) Prepare Site Vegetation and Landscaping Plan.
Vegetation is critical to the function and appearance of any
green roof. Therefore landscaping plans should be provided
according to the guidance in Subsection 4.11.5.7 (Landscaping) and Appendix D.
4.11.7 Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
Stormwater Best Management Practices
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Pretreatment should be provided upstream of all infiltration practices
• Infiltration practices should be designed to completely drain within 72 hours
of the end of a rainfall event
• Underlying native soils should have an infiltration rate of 0.5 in/hr or more
• The distance from the bottom of an infiltration practice to the top of the
water table should be 2 feet or more
• Facilities include an excavated trench (2-10 foot depth) filled with stone
media (1.5-2.5 inch diameter), as well as pea gravel and sand filter layers
• A pretreatment device is recommended upstream from the practice
• Observation wells are used to monitor percolation and performance of the
practice
• Infiltration practices must not be placed under pavement or concrete
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Description: Infiltration practices, which may
also be classified as a runoff reducing low impact
development practices, are shallow excavations,
typically filled with stone or an engineered soil
mix, that are designed to intercept and temporarily store post-construction stormwater runoff until
it infiltrates into the underlying and surrounding
soils. If properly designed, they can provide significant reductions in post-construction stormwater
runoff rates, volumes and pollutant loads.
LID/GI Considerations: Because infiltration
practices allow more infiltration to the soil surrounding the practice than other BMPs, they are
considered a LID/GI Control. This helps restore a
sites natural hydrology.
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ADVANTAGES / BENEFITS
• Considered a LID-GI control
• Provides for groundwater recharge
• Good for small sites with porous soils
• Helps restore pre-development hydrology on development sites and reduces
post-construction stormwater runoff rates, volumes and pollutant loads
• Can be integrated into development plans as attractive landscaping features
DISADVANTAGES / LIMITATIONS
• Can only be used to manage runoff from relatively small drainage areas of 5
acres or less
• Should not be used to “receive” stormwater runoff that contains high
sediment loads
• Potential for groundwater contamination
• High clogging potential; should not be used on sites with fine-particle soils
(clays or silts) in drainage areas
• Significant setback requirements
• Restrictions in karst areas
• Geotechnical testing required, two borings per practice
•
•
•
•
•
ROUTINE MAINTENANCE REQUIREMENTS
Keep practice free of trash, debris, and dirt
Inspect area for ponding water
If structures become clogged, remove aggregate, wash and replace
Can be susceptible to clogging, so locate in stabilized areas (i.e. not in tree
canopy)
• Keep observation well easily and safely accessible
• Remove sediment from forebay or other pretreatment practice
• Replace pea gravel layer as needed
Land Requirement
Stormwater Best Management Practices
4.12 Infiltration Practices
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Other Considerations: Highly applicable for roadway projects
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 100%- Runoff Reduction Volume
POLLUTANT REMOVAL
Total Suspended Solids
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Metals - Cadmium, Copper,
Lead, and Zinc removal
Pathogens – Fecal Coliform
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tration practices are to be used on a development
Infiltration practices are excavations typically filled
site, great care should be taken to ensure that
with stone to create an underground reservoir
they are adequately designed, carefully installed
for stormwater runoff (see Figure 4.12-1). This
and properly maintained over time. They should
runoff volume gradually exfiltrates through the
only be applied on development sites that have
bottom and sides of the trench into the subsoil
permeable soils (i.e., hydrologic soil group A and
over a three-day period and eventually reaches
B soils) and that have a water table and confining
the water table. By diverting runoff into the soil,
layers (e.g., bedrock, clay lenses) that are located
4.12.2 Stormwater Management Suitability
an infiltration practice not only treats the water
at least two feet below the bottom of the trench
Infiltration practices can be designed for wa-
quality volume, but also helps to preserve the
or basin.
ter quantity, but they are mostly used for water
natural water balance on a site through ground-
also only be used on development sites where
sediment loads can be kept relatively low. It
should be noted that this example is only one
method.
quality, i.e. the removal of stormwater pollutants,
water recharge to preserve baseflow. Due to this
There are two major variations of infiltration prac-
depending upon the native soils. Infiltration prac-
fact, infiltration systems are limited to areas with
tices, namely infiltration trenches and infiltration
tices can provide runoff quantity control, particu-
highly porous soils where the water table and/or
basins. A brief description of each of these design
larly for smaller runoff volumes such as the runoff
bedrock are located well below the bottom of the
variants is provided below:
volume generated by the water quality storm
trench. In addition, infiltration practices must be
• Infiltration Trenches: Infiltration trenches
are excavated trenches filled with stone.
Stormwater runoff is captured and temporarily
stored in the stone reservoir, where it exfiltrates
into the surrounding and underlying native
soils. Infiltration trenches can be used to
manage post-construction stormwater runoff
from contributing drainage areas of up to 2
acres and should only be used on development
sites where sediment loads can be kept
relatively low.
event (1.2 inches). These facilities may sometimes
carefully sited to avoid the potential for groundwater contamination.
Infiltration practices are not intended to trap
sediment and must always be designed with a
sediment forebay and grass channel for concentrated flow or filter strip for sheet flow, or other
appropriate pretreatment measures to prevent
clogging and failure. Due to their high potential
for failure, these facilities must only be considered
for sites where upstream sediment control can be
ensured.
Although infiltration practices can provide significant reductions in post-construction stormwater
runoff rates, volumes and pollutant loads, they
have historically experienced high rates of failure
due to clogging caused by poor design, poor
construction and neglected maintenance. If infilBACK
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Stormwater Best Management Practices
4.12.1 General Discussion
be used to partially or completely meet channel
protection requirements on smaller sites. However, infiltration practices will typically need to
be used in conjunction with another control to
provide channel protection, as well as overbank
flood protection. Infiltration practices need to be
designed and maintained to safely bypass higher
flows.
• Infiltration basins are shallow, landscaped
excavations filled with an engineered soil mix.
They are designed to capture and temporarily
store stormwater runoff in the engineered soil
mix, where it is subjected to the hydrologic
processes of evaporation and transpiration
while infiltrating into the surrounding soils.
They are essentially non-underdrained
bioretention areas (Section 4.2), and should
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• Water Quality
The infiltration practice is an excellent
stormwater treatment practice due to the
variety of pollutant removal mechanisms. Each
of the components of the infiltration practice
is designed to perform a specific function.
The grass filter strip (for sheet flow) or grass
channel or forebay (for concentrated flow)
pre-treatment component reduces incoming
runoff velocity and filters particulates from
the runoff. The planting soil or rock in the
infiltration practice acts as a filtration system,
and clay in the soil provides adsorption sites
for hydrocarbons, heavy metals, nutrients and
other pollutants.
• Channel Protection
For smaller sites, an infiltration practice may be
designed to capture the entire channel protection
volume (CPv). Given that an infiltration practice is
typically designed to completely drain over 48-72
hours, the requirement of extended detention
of the 1-year, 24-hour storm runoff volume will
be met. For larger sites, or where only the WQv.
is diverted to the infiltration practice, another
practice must be used to provide CPv extended
detention.
• Overbank Flood Protection
Although relatively rare, on some development
sites, an infiltration practice can be designed to
attenuate the overbank peak discharge (Qp25).
• Extreme Flood Protection
Although relatively rare, on some development
sites, an infiltration practice can be designed to
attenuate the extreme peak discharge (Qf).
Credit for the volume of runoff reduced in the
infiltration practice may be taken in the overbank
flood protection and extreme flood protection
calculations. If the practice is designed to provide
Runoff Reduction for Water Quality compliance,
then the practice is given credit for Channel
Protection and Flood Control requirements by
allowing the designer to compute an Adjusted CN
(see Subsection 3.1.7.5 for more information).
For additional information and data on pollutant
removal capabilities for infiltration practices, see
the National Pollutant Removal Performance
Database (3rd Edition) available at www.cwp.org
and the National Stormwater Best Management
Practices (BMP) Database at www.bmpdatabase.
org.
4.12.4 Application and Site Feasibility
Criteria
Infiltration practices can be used to manage
post-construction stormwater runoff on development sites in rural, suburban and urban areas
Stormwater Best Management Practices
• Runoff Reduction
Like other LID practices, infiltration practices
become more effective with higher infiltration
rates of native soils. An infiltration practice
can be designed to provide 100% of the runoff
reduction volume, if properly maintained.
In order to provide runoff reduction with an
infiltration practice that is designed without an
underdrain, the infiltration practice should drain
within 72 hours.
where the soils have adequate permeability and
the water table is low enough to provide for the
infiltration of stormwater runoff. They are generally suited for medium-to-high density residential, commercial and institutional developments.
4.12.3 Pollutant Removal Capabilities
Infiltration practices are presumed to be able to
remove 80% of the total suspended solids (TSS)
load in typical urban post-development runoff
when sized, designed, constructed, and maintained in accordance with the recommended
specifications. Infiltration practices can also
Infiltration practices should be considered for use
on development sites where the
• Subsoil is sufficiently permeable to provide a
reasonable infiltration rate.
• Fine sediment (e.g., clay, silt) loads will be
relatively low, as high sediment loads will cause
them to clog and fail.
remove Phosphorus, Nitrogen, metals, and pathogens. An undersized, poorly designed and/or
neglected infiltration practice may have reduced
pollutant removal performance. Proper design
of infiltration practices is critical to ensure that
pollutants are properly removed from stormwater
runoff. Due to clogging issues, Infiltration Practices should not be used for removing sediment
or other coarse material.
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General Feasibility
groundwater contamination that could
- Suitable for Residential Subdivision Usage – YES
potentially contaminate water supply aquifers.
- Suitable for High Density/Ultra Urban Areas – YES
• Impervious areas where there are not high
- Regional Stormwater Control – NO
levels of fine particles (clay/silt soils) in the
runoff.
Physical Feasibility - Physical Constraints at
Infiltration practices can either be used to capture
Project Site
sheet flow or concentrated flow from a drainage
• Drainage Area – Maximum of 5 acres. Although
infiltration practices can be used to manage
stormwater runoff from contributing drainage
areas as large as 5 acres in size, contributing
drainage areas of between 2,500 square feet
and 2 acres are preferred.
area. Due to their relatively narrow shape, infiltration practices can be adapted to many different
types of sites, such as in retrofit situations. Unlike
some other structural stormwater practices,
infiltration practices can easily fit into the margin,
perimeter, or other unused areas of developed
sites.
To protect groundwater from potential contami-
practices should not be used for manufacturing
or industrial sites, where there is a potential for
high concentrations of soluble pollutants and
heavy metals. In addition, infiltration should not
be considered for areas with a high pesticide
concentration. Infiltration practices are also not
suitable in areas with karst geology, unless adequate geotechnical testing by qualified individuals
in accordance with local requirements suggests
otherwise.
The following criteria should be evaluated to
ensure the suitability of an infiltration practice for
meeting stormwater management objectives on
a site.
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• Soils – Infiltration practices should be
designed to completely drain within 72 hours
of the end of a rainfall event. Consequently,
infiltration practices generally should not be
used on development sites that have soils with
infiltration rates of less than 0.5 inches per hour
(i.e., hydrologic soil group C and D soils).
Other Constraints/Considerations
• Space Required – Will vary depending on the
depth of the practice. In general, infiltration
practices require about 5% of the size of their
contributing drainage areas.
• Aquifer Protection – No hotspot runoff
allowed; meet setback requirements in design
• Site Slope – No more than 6% slope (for preconstruction facility footprint). However, they
should be designed with slopes that are as
close to flat as possible.
Coastal Areas
nation, runoff from designated hotspot land uses
or activities must not be infiltrated. Infiltration
• Minimum Depth to Water Table – Two feet
is recommended between the bottom of the
infiltration practice and the elevation of the
seasonally high water table
• Minimum Head – Unless a shallow water
table is found on the development site, all
infiltration practices should be designed to
be at least 36 inches deep. Infiltration basins
may be designed with a maximum ponding
depth of 12 inches, although a ponding depth
of 9 inches is recommended to help prevent
nuisance ponding conditions. Unless a shallow
water table is found on the development site,
all infiltration basin planting beds should be at
least 36 inches deep.
Stormwater Best Management Practices
• The water table is low enough to prevent
criteria.
• Poorly Draining Soils— this condition
minimizes the ability of an infiltration practice
to reduce stormwater runoff rates, volumes,
and pollutant loads. Infiltration practices
should not be used on development sites that
have soils with infiltration rates of less than
0.5 inches per hour (i.e., hydrologic soil group
C and D soils). Another consideration would
be to use other low impact development and
stormwater management practices, such
as rainwater harvesting (Section 4.19) and
underdrained bioretention areas (Section 4.2),
to manage post-construction stormwater
runoff in these areas.
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• Flat Terrain—does not negatively influence
the infiltration practice. In fact, infiltration
practices should be designed with slopes that
are as close to flat as possible.
• Shallow Water Table—it may be difficult to
provide two feet of clearance between the
bottom of the infiltration practice and the top
of the water table, which may occasionally
cause stormwater runoff to pond in the bottom
of the infiltration practice. There are several
potential solutions to this problem:
»» Use stormwater ponds (Section 4.25),
stormwater wetlands (Section 4.26), or grass
channel (Section 4.9), instead of infiltration
practices to intercept and treat stormwater
runoff in these areas.
• Tidally-influenced drainage system—does not
influence the infiltration practice.
4.12.5 Planning and Design Criteria
The following criteria are to be considered minimum standards for the design of an infiltration
practice. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be met.
4.12.5.1 LOCATION AND LAYOUT
• Infiltration practices should be used on
development sites that have underlying soils
with an infiltration rate of 0.5 inches per hour
(in/hr) or greater, as determined by NRCS soil
survey data and subsequent field testing. See
Appendix D for additional information on
the field infiltration test protocol. Note that
soil testing should be approved by the local
development review authority prior to testing.
• Infiltration practices should have a contributing
drainage area of 5 acres or less, with 2 acres or
less being preferred.
»» Reduce the depth of the stone reservoir in
infiltration practices to no less than18 inches.
• There should be at least 24 inches between
the bottom of the infiltration practice and the
elevation of the seasonally high water table.
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• Minimum setback requirements for infiltration
practice facilities (when not specified by local
ordinance or criteria):
»» From a property line – 10 feet
»» From a building foundation – 25 feet
»» Ensure that the distance from the bottom
of the infiltration practice and the top of the
water table is at least 2 feet.
»» Reduce the depth of the planting bed in
infiltration basins to no less than 18 inches.
• Clay lenses, bedrock or other restrictive layers
below the bottom of the trench will reduce
infiltration rates unless excavated.
»» From a private well – 100 feet
»» From a public water supply well – 1,200 feet
»» From a septic system tank/leach field – 100
feet
»» From surface waters – 100 feet
Stormwater Best Management Practices
• Well-Draining Soils—this condition enhances
the ability of infiltration practices to reduce
stormwater runoff rates, volumes and pollutant
loads, but may allow stormwater pollutants
to reach groundwater aquifers with greater
ease. A potential solution is to avoid the use
of infiltration-based stormwater management
practices, including infiltration practices, at
stormwater hotspots and in areas known to
provide groundwater recharge to water supply
aquifers, unless adequate pretreatment is
provided upstream. Another potential solution
is to use bioretention areas (Section 4.2) or
dry enhanced swales (Section 4.6) with liners
and underdrains at stormwater hotspots and in
areas known to provide groundwater recharge
to water supply aquifers.
»» From surface drinking water sources – 400
feet (100 feet for a tributary)
• To reduce the potential for costly maintenance
and/or system reconstruction, it is strongly
recommended that the trench be located in
an open space, with the top of the structure
as close to the ground surface as possible.
Infiltration practices should not be located
beneath paved surfaces, such as parking lots.
• Infiltration practices are designed for
intermittent flow and must be allowed to drain
for reaeration of the surrounding soil between
rainfall events. They must not be used on sites
with a continuous flow from groundwater,
sump pumps, or other sources.
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A well-designed infiltration practice consists of:
1. Excavated a shallow trench or basin
backfilled with sand, coarse stone, and pea
gravel;
2. Appropriate pretreatment measures; and
3. One or more observation well to show how
quickly the trench basin dewaters or to
determine if the practice is clogged.
4.12.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Infiltration practices should be designed to
completely drain within 72 hours of the end of
a rainfall event. Where site characteristics allow,
it is preferable to design infiltration practices
to drain within 48 hours of the end of a rainfall
event to help prevent the formation of nuisance
ponding conditions.
• Infiltration practice depths should be between
3-8 feet, to provide for easier maintenance.
The width of a trench should be less than 25
feet.
• The surface area required is calculated based
on the practice depth, soil infiltration rate,
aggregate void space, and fill time (this will
need to be modeled).
• The bottom slope of an infiltration practice
should be flat across its length and width to
evenly distribute flows, encourage uniform
infiltration through the bottom, and reduce the
risk of clogging.
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• Stone aggregate used in the trench should
be washed, bank-run gravel, 1.5-2.5 inches
in diameter with a void space of about 40%
(GADOT No.3 Stone). Aggregate contaminated
with soil shall not be used. A porosity value
(void space/total volume) of 0.32 should be
used in calculations, unless aggregate specific
data exist.
• An observation well should be installed in
every infiltration practice. An observation
well consists of a 4 to 6 inch perforated
PVC (AASHTO M 252) pipe that extends to
the bottom of the infiltration practice. The
observation well can be used to observe the
rate of drawdown within the infiltration practice
following a storm event. It should be installed
along the centerline of the infiltration practice,
flush with the elevation of the surface of the
infiltration practice. A visible floating marker
should be provided within the observation well
and the top of the well should be capped and
locked to prevent tampering and vandalism.
• Broader, shallower infiltration practices perform
more effectively by distributing stormwater
runoff over a larger surface area. However, a
minimum depth of 36 inches is recommended
for all infiltration practices to prevent them
from consuming a large amount of surface area
on development sites. Whenever practical, the
depth of infiltration practices should be kept to
60 inches or less.
• Underlying native soils should be separated
from the stone reservoir by a thin, 2-4 inch
layer of choker stone (i.e., ASTM D 448 size
No. 8, 3/8” to 1/8” or ASTM D 448 size No. 89,
3/8” to 1/16”) or a 6-inch layer of clean, washed
sand. The choker stone or sand should be
placed between the stone reservoir and the
underlying native soils.
Stormwater Best Management Practices
4.12.5.2 GENERAL DESIGN
• Since clay lenses or any other restrictive layers
located below the bottom of an infiltration
practice will reduce soil infiltration rates,
infiltration testing should be conducted within
any confining layers that are found within 4
feet of the bottom of a proposed infiltration
practice.
• Infiltration practices should be located in an
open pervious area and should be designed so
that the top of the practice is located as close
to the surface as possible. Infiltration practices
should not be located beneath a driveway,
parking lot or other impervious surface.
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»» Using storm drain inlets set slightly above
the elevation of the surface of an infiltration
practice to collect excess stormwater runoff.
This will create some ponding on the surface
of the infiltration practice, but can be used to
safely convey excess stormwater runoff over
the surface of the practice.
»» Using yard drains or storm drain inlets
set at the maximum ponding depth of an
infiltration basin to collect excess stormwater
runoff.
»» Using a spillway with an invert set slightly
above the elevation of maximum ponding
depth to convey stormwater runoff
generated by larger storm events safely out
of an infiltration basin.
»» Placing a perforated pipe (e.g., underdrain)
near the top of the stone reservoir or planting
bed to provide additional conveyance of
stormwater runoff after the infiltration
practice has been filled.
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4.12.5.4 PRETREATMENT/INLETS
4.12.5.7 MAINTENANCE ACCESS
• Pretreatment practices are recommended to be
used in conjunction with an infiltration practice
to prevent clogging and failure.
• Adequate access should be provided to
an infiltration practice for inspection and
maintenance.
• For an infiltration practice receiving sheet
flow from an adjacent drainage area, the
pretreatment system should consist of a
vegetated filter strip (refer to Section 4.29).
• Include access roads and ramps for appropriate
equipment to all applicable components of the
infiltration practice (observation well, forebay,
etc.).
• For concentrated flow, pretreatment should
consist of a sediment forebay, vault, plunge
pool, or similar sedimentation chamber (with
energy dissipaters) sized to a minimum of
10% of the water quality volume (WQv). Exit
velocities from the pretreatment chamber must
be non-erosive for the 2-year, 24-hour design
storm.
• Provide space to safely exit and enter public
roads (if necessary).
4.12.5.5 OUTLET STRUCTURES
4.12.5.9 LANDSCAPING
Outlet structures are not required for infiltration
practices.
4.12.5.6 EMERGENCY SPILLWAY
A non-erosive overflow channel or pipe should
be provided to safely pass flows that exceed the
storage capacity of the infiltration practice to a
stabilized downstream area or watercourse.
4.12.5.8 SAFETY FEATURES
Stormwater Best Management Practices
• Consideration should be given to the
stormwater runoff rates and volumes generated
by larger storm events (e.g., 25-year, 24-hour
storm event) to help ensure that these larger
storm events are able to safely bypass the
infiltration practice. An overflow system should
be designed to convey the stormwater runoff
generated by these larger storm events safely
out of the infiltration practice. Methods that
can be used to accommodate the stormwater
runoff rates and volumes generated by these
larger storm events include:
In general, infiltration practices are not likely to
pose a physical threat to the public and do not
need to be fenced.
• Infiltration practices should fit into and blend
with the surrounding area. Native grasses
are preferable, if compatible. A basin may be
covered with permeable topsoil and planted
with grass in a landscaped area.
• The landscaped area above the surface of an
infiltration practice may also be covered with
pea gravel (i.e., ASTM D 448 size No. 8, 3/8” to
1/8”). This pea gravel layer provides sediment
removal and additional pretreatment upstream
of the infiltration practice and can be easily
removed and replaced when it becomes
clogged.
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• A landscaping plan should be prepared for
all infiltration basins. The landscaping plan
should be reviewed and approved by the
local development review authority prior to
construction.
• Vegetation commonly planted in infiltration
basins includes native trees, shrubs and other
herbaceous vegetation. When developing a
landscaping plan, site planning and design
teams should choose vegetation that will
be able to stabilize soils and tolerate the
stormwater runoff rates and volumes that will
pass through the infiltration basin. Vegetation
used in infiltration basins should also be able
to tolerate both wet and dry conditions. See
Appendix D for a list of grasses and other
plants that are appropriate for use in infiltration
practices installed in the state of Georgia.
• A mulch layer, consisting of 2-4 inches of
fine shredded hardwood mulch or shredded
hardwood chips, should be included on the
surface of an infiltration basin.
• Methods used to establish vegetative cover
within an infiltration basin should achieve
at least 75% vegetative cover one year after
installation.
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• To help prevent soil erosion and sediment loss,
landscaping should be provided immediately
after an infiltration practice has been installed.
Temporary irrigation may be needed to quickly
establish vegetative cover within an infiltration
basin.
• The soils used within infiltration basin planting
beds should be an engineered soil mix that
meets the following specifications:
»» Texture: Sandy loam or loamy sand should
be used.
»» Sand Content: Soils should contain 85%-88%
clean, washed sand.
»» Topsoil Content: Soils should contain 8%12% topsoil.
4.12.5.10 ADDITIONAL SITE-SPECIFIC DESIGN
CRITERIA AND ISSUES
Physiographic Factors - Local terrain design
constraints
»» Low Relief – No additional criteria.
»» High Relief – Maximum site slope of 6%.
»» Karst – Not suitable without adequate
geotechnical testing.
Special Downstream Watershed ConsiderationsNo additional criteria
Stormwater Best Management Practices
• Alternatively, an infiltration practice may be
covered with an engineered soil mix, such as
that prescribed for an infiltration basin, and
planted with managed turf or other herbaceous
vegetation. This may be an attractive option
when infiltration practices are placed in
disturbed pervious areas (e.g., lawns, parks and
community open spaces).
»» Organic Matter Content: Soils should
contain 3%-5% organic matter.
»» Infiltration Rate: Soils should have an
infiltration rate of at least 0.25 inches per
hour (in/hr), although an infiltration rate of
between 1 and 2 in/hr is preferred.
»» Phosphorus Index (P-Index): Soils should
have a P-Index of less than 30.
»» Exchange Capacity (CEC): Soils should have
a CEC that exceeds 10 milliequivalents (meq)
per 100 grams of dry weight.
»» pH: Soils should have a pH of 6-8.
• The organic matter used within an infiltration
basin planting bed should be a well-aged
compost that meets the specifications outlined
in Appendix D.
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(Step 1) Determine if the development site and conditions are ap-
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
propriate for the use of an infiltration practice.
the design elements of the practice can be determined (i.e.
Consider the application and site feasibility criteria in this
planting media, underdrain, inlet/outlet, overflow, etc.)
chapter. In addition, determine if site conditions are suitable
for an infiltration practice. Create a rough layout of the infil-
Complete Step 3A, 3B, and 3C for a runoff reduction ap-
tration practice dimensions taking into consideration existing
proach, or skip Step 3 and complete Steps 4A and 4B for a
trees, utility lines, and other obstructions.
water quality (treatment) approach. Refer to your local community’s guidelines for any additional information or specific
(Step 2) Determine the goals and primary function of the infiltration
requirements regarding the use of either method.
practice.
Consider whether the infiltration practice is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
»» Provide a possible solution to a drainage problem
Stormwater Best Management Practices
4.12.6 Design Procedures
RV = Volumetric runoff coefficient which can be
found by:
RV = 0.05+0.009(I)
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
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Provide pretreatment by using a grass filter strip or pea gravel
up the appropriate runoff reduction percentage (or credit)
diaphragm, as needed, (sheet flow), or a grass channel or
provided by the practice:
forebay (concentrated flow). Where filter strips are used,
100% of the runoff should flow across the filter strip. Pre-
Using the RRV calculated, determine the minimum Volume of
treatment may also be desired to reduce flow velocities or
the Practice (VP)
assist in sediment removal and maintenance. Pretreatment
can include a forebay, weir, or check dam. Splash blocks
(VPMIN) ≥ RRv (target) / (RR%)
or level spreaders should be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour.
Where:
Forebays should be sized to contain 0.1 inches per imperRR% = Runoff Reduction percentage, or credit,
vious acre of contributing drainage. Refer to Section 4.9
assigned to the specific practice
for design criteria for a grass channel and Section 4.29 for
VPMIN = Minimum storage volume required to
vegetated filter strips.
provide Runoff Reduction Target Volume (ft )
3
RRv (target) = Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
(Step 3C) Determine whether the minimum storage volume was met.
When the VP ≥ VPMIN, then the Runoff Reduction require-
(Step 3B) Determine the storage volume of the practice and the
ments are met for this practice. Proceed to Step 5.
Pretreatment Volume
To determine the actual volume provided in the infiltration
When the VP < VPMIN, then the BMP must be sized according
practice, use the following equation:
to the WQV treatment method (See Step 4).
VP = (PV + VES (N))
Where:
VP = Volume provided (temporary storage)
PV = Ponding Volume
VES = Volume of Engineered Soils
N = Porosity
To determine the porosity, a qualified licensed professional
should be consulted to determine the proper porosity based
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
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The required dimensions of an infiltration practice that will
Calculate the Water Quality Volume using the following
be filled with a planting media can be determined using the
formula:
following equation, which is based on Darcy’s Law:
WQV = (1.2) (RV) (A) / 12
Ain = (WQv)(din) / [(kin)(hin + din)(tdrain)]
Where:
Where:
WQV = Water Quality Volume (ft )
Ain = surface area of infiltration basin (ft2)
1.2 = Target rainfall amount to be treated (inches)
WQv = Water Quality Volume, calculated in Step 4 (ft3)
3
RV = Volumetric runoff coefficient which can be
din = depth of infiltration basin planting bed (ft) (use
found by:
3 feet or more, unless a shallow water table is found
on the development site)
RV = 0.05+0.009(I)
kin = coefficient of permeability of infiltration basin
planting bed (ft/day) (use kin = 0.5 ft/day for
Where:
engineered soil mix specified)
I = new impervious area of the contributing drain-
hin = average height of ponded water above infiltration
age area (%)
basin (ft) (use 50% of maximum ponding depth)
A = Site area (total drainage area) (ft )
tdrain = design infiltration basin drain time (days) (use
12 = Unit conversion factor (in/ft)
72 hours or less)
2
Stormwater Best Management Practices
(Step 4A) Calculate the Target Water Quality Volume
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
(Step 4B) If using the practice for Water Quality treatment, determine
the footprint of the infiltration practice and the Pretreat-
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
storm). See Subsection 3.1.7.5 for more information.
ment Volume required.
The peak rate of discharge for the water quality design storm
(Step 6) Size flow diversion structure, if needed.
is needed for sizing of off-line diversion structures (see Sub-
A flow regulator (or flow splitter diversion structure) should
section 3.1.7). If designing off-line, follow steps (a) through
be supplied to divert the WQv (or RRv) to the infiltration prac-
(d):
tice.
.
(a) Using WQv, compute CN
(b) Compute time of concentration using TR-55 method
(Step 7) Size the underdrain system.
See Subsection 4.12.5.3 (Physical Specifications/Geometry)
(c) Determine appropriate unit peak discharge from time of
concentration
(d) Compute Qwq from unit peak discharge, drainage area,
and WQv.
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An overflow must be provided to bypass and/or convey
larger flows to the downstream drainage system or stabilized
watercourse. Non-erosive velocities need to be ensured at
the outlet point. The overflow should be sized to safely pass
the peak flows anticipated to reach the practice, up to a 100year, 24-hour storm event.
(Step 9) Prepare Vegetation and Landscaping Plan.
A landscaping plan for an infiltration practice should be prepared to indicate how it will be established with vegetation.
See Subsection 4.12.5.9 (Landscaping) and Appendix D for
more details.
See Appendix B-4 for an Infiltration Trench Design Example
Stormwater Best Management Practices
(Step 8) Design the emergency overflow system.
4.12.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Must be designed to minimize potential safety risks, potential
property damage, and inconvenience to the facility’s primary
purposes
• Adequate grading and drainage must be provided to allow full use of
facility’s primary purposes following a storm event
• Maximum depth of detention ponding in a parking lot should be 6
inches
• Must have a minimum slope of 0.5% towards the outlet, 1% or greater
is recommended
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
Description: A facility designed primarily for another purpose, such as parking lots and rooftops
that can provide water quantity control through
detention of stormwater runoff.
ADVANTAGES / BENEFITS
• Allows for multiple uses of site areas and reduces the need for
downstream detention facilities
• Used in conjunction with water quality BMPs
• Adequate grading and drainage must be provided to allow full use of
facility’s primary purposes following a storm event
DISADVANTAGES / LIMITATIONS
• Controls for stormwater quantity only – not intended to provide
water quality treatment
• Restrictions in ponding depths
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.13 Multi-Purpose Detention
Areas
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: No
L=Low M=Moderate H=High
LID/GI Consideration: Low land requirement,
adaptable to many situations, and often a BMP
used to treat runoff close to the source.
ROUTINE MAINTENANCE REQUIREMENTS
• Keep practice free of trash, debris, and dirt
• Inspect area for excessive ponding of water
• Remove sediment as needed
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RUNOFF REDUCTION CREDIT
• 0% Runoff Reduction Credit is provided by
this practice
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4.13.2 Design Criteria and Specifications
Parking Lot Storage
Multi-purpose detention facilities are site areas
Location
primarily used for one or more specific activities
• Multi-purpose detention areas can be located
upstream or downstream of other BMPs
providing treatment of the water quality volume
(WQv). See Section 4.1 for more information on
• Parking lot detention can be implemented in
areas where portions of large, paved lots can
be temporarily used for runoff storage without
significantly interfering with normal vehicle and
pedestrian traffic. Parking lot detention can be
created in two ways: by using ponding areas
along sections of raised curbing, or through
depressed areas of pavement at drop inlet
locations.
that are also designed to provide for the temporary storage of stormwater runoff to reduce
downstream water quantity impacts. Example of
multi-purpose detention areas include:
the use of multiple BMPs in a treatment train.
• Parking Lots
• Rooftops
• Sports Fields
• Recessed Plazas
Multi-purpose detention areas are normally dry
between rain events, and by their very nature
must be useable for their primary function the
majority of the time. As such, multi-purpose
detention areas should not be used for extended
detention.
Multi-purpose detention areas are not intended
for water quality treatment and must be used in
a treatment train approach with other BMPs that
provide treatment of the WQv (see Section 4.1).
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‘General Design
• Multi-purpose detention areas may be sized
to temporarily store a portion or all of the
volume of runoff required to provide overbank
flood (Qp25) protection (i.e., reduce the postdevelopment peak flow of the 25-year storm
event to the pre-development rate) and control
the 100-year storm (Qf) if required. Routing
calculations must be used to demonstrate that
the storage volume is adequate. See Section
3.3 (Storage Design) for procedures on the
design of detention storage.
• All multi-purpose detention facilities must be
designed to minimize potential safety risks,
potential property damage, and inconvenience
to the facility’s primary purposes. Emergency
overflows are to be provided for storm
events larger than the design storm. The
overflow must not create an adverse impact
to downstream properties or the conveyance
system.
• The maximum depth of detention ponding in a
parking lot, except at a flow control structure,
should be 6 inches for a 10-year, 24-hour
storm, and 9 inches for a 100-year, 24-hour
storm. The maximum depth of ponding at a
flow control structure is 12 inches for a 100year, 24-hour storm.
Stormwater Best Management Practices
4.13.1 General Description
• The storage area (portion of the parking lot
subject to ponding) must have a minimum
slope of 0.5% towards the outlet to ensure
complete drainage following a storm. A slope
of 1% or greater is recommended.
• Fire lanes used for emergency equipment must
be free of ponding water for runoff events up
to the extreme storm (100-year) event.
• Flows are typically backed up in the parking lot
using a raised inlet.
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Public Plazas
• Rooftops can be used for detention storage as
long as the roof support structure is designed
to address the weight of ponded water and is
sufficiently waterproofed to achieve a minimum
service life of 30 years. All rooftop detention
designs must meet Georgia State Building
Code and local building code requirements.
• In high-density areas, recessed public common
areas such as plazas and pavilions can be
utilized for stormwater detention. These areas
can be designed to flood no more than once
or twice annually, and provide important open
recreation space during the rest of the year.
• The minimum pitch of the roof area subject to
ponding is 0.25 inches per foot.
• The rooftop storage system must include
another mechanism for draining the ponding
area in the event that the primary outlet is
clogged.
• See Section 4.11 for information and guidance
on Green Roof practices.
• Consult the design criteria for the dry detention
basins (see Subsection 4.5.6, Dry Detention
Basins) for the Multi-purpose Detention Basins
sizing and design steps.
4.13.3 Inspection and Maintenance
Requirements
Stormwater Best Management Practices
Rooftop Storage
All best management practices require proper maintenance. Without proper maintenance,
BMPs will not function as originally designed and
Sports Fields
• Athletic facilities such as football fields, soccer
fields, and tracks can be used to provide
stormwater detention. This is accomplished
by constructing berms around the facilities,
which in essence creates very large and
shallow detention basins. Outflow can be
controlled through the use of an overflow weir
or other appropriate control structure. Proper
grading must be performed to ensure complete
drainage of the facility.
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may cease to function altogether. The design of
all BMPs includes considerations for maintenance
and maintenance access. For additional information on inspection and maintenance requirements, see Appendix E.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Minimum head requirement of 5 to 8 feet
• Maximum contributing drainage area of 10 acres
ADVANTAGES / BENEFITS
• Applicable to small drainage areas
• Good for highly impervious areas
• High pollutant removal capability
• Removal of dissolved pollutants is greater than sand filters due to
cation exchange capacity
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
Description: Design variant of the surface sand
filter using organic materials in the filter media.
LID/GI Consideration: BMP used to treat runoff
close to the source.
DISADVANTAGES / LIMITATIONS
• Intended for hotspot, space-limited applications, or for areas
requiring enhanced pollutant removal capability
• High maintenance burden
• Filter may require more frequent maintenance than most of the other
stormwater BMPs
• Severe clogging potential if exposed soil surfaces exist upstream
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Capital Cost
Maintenance Burden
ROUTINE MAINTENANCE REQUIREMENTS
• Inspect for clogging – rake first inch of sand
• Remove sediment from forebay and chamber
Residential Subdivision Use: No
High Density/Ultra-Urban: Yes
Special Considerations: Hotspot areas
• Replace sand filter media as needed
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
POLLUTANT REMOVAL
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may provide partial benefits
Stormwater Best Management Practices
4.14 Organic Filter
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
• 0% of the runoff reduction volume provided
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4.14.2 Pollutant Removal Capabilities
The organic filter is a design variant of the surface
Peat/sand filter systems provide good removal of
sand filter, which uses organic materials such as
bacteria and organic waste metals.
leaf compost or a peat/sand mixture as the filter
The following design pollutant removal rates are
media. The organic material enhances pollutant
conservative average pollutant reduction percent-
removal by providing adsorption of contaminants
ages for design purposes derived from sampling
such as soluble metals, hydrocarbons, and other
data, modeling and professional judgment.
-- Total Suspended Solids – 80%
organic chemicals.
-- Total Phosphorus – 60%
As with the surface sand filter, an organic filter
consists of a pretreatment chamber, and one or
more filter cells. Each filter bed contains a layer
of leaf compost or the peat/sand mixture, fol-
-- Total Nitrogen – 40%
-- Fecal Coliform – 50%
-- Heavy Metals – 75%
lowed by filter fabric and a gravel/perforated pipe
underdrain system. The filter bed and subsoils
can be separated by an impermeable polyliner
or concrete structure to prevent movement into
groundwater.
Organic filters are typically used in high-density
applications, or for areas requiring an enhanced
pollutant removal ability. Maintenance is typically
higher than the surface sand filter facility due to
the potential for clogging. In addition, organic
filter systems have a higher head requirement
than sand filters.
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4.14.3 Design Criteria and Specifications
• Organic filters are typically used on relatively
small sites (up to 10 acres), to minimize
potential clogging.
• The type of peat used in a peat/sand filter
is critically important. Fibric peat in which
undecomposed fibrous organic material is
readily identifiable is the preferred type. Hemic
peat containing more decomposed material
may also be used. Sapric peat made up of
largely decomposed matter should not be used
in an organic filter.
• Typically, organic filters are designed as “offline” systems, meaning that the water quality
volume (WQv) is diverted to the filter facility
through the use of a flow diversion structure
and flow splitter. Stormwater flows greater
than the WQv are diverted to other BMPs or
downstream using a diversion structure or flow
splitter.
Stormwater Best Management Practices
4.14.1 General Description
• Consult the design criteria for the surface sand
filter (see Subsection 4.21.4, Sand Filters) for
the organic filter sizing and design steps.
• The minimum head requirement (elevation
difference needed at a site from the inflow to
the outflow) for an organic filter is 5 to 8 feet.
• Organic filters can utilize a variety of organic
materials as the filtering media. Two typical
media bed configurations are the peat/sand
filter and compost filter (see Figure 4.14-1).
The peat filter includes an 18-inch 50/50 peat/
sand mix over a 6-inch sand layer and can be
optionally covered by 3 inches of topsoil and
vegetation. The compost filter has an 18-inch
compost layer. Both variants utilize a gravel
underdrain system.
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All best management practices require proper maintenance. Without proper maintenance,
BMPs will not function as originally designed and
may cease to function altogether. The design of
all BMPs includes considerations for maintenance
and maintenance access. For additional information on inspection and maintenance requirements, see Appendix E.
Stormwater Best Management Practices
4.14.4 Inspection and Maintenance
equirements
Figure 4.14-1 Schematic of Organic Filter
(Source: Center for Watershed Protection)
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Intended for low traffic areas, or for residential or overflow parking
applications, not ideal for areas with a tree canopy
• Aesthetically pleasing
• Americans with Disabilities Act (ADA) compliant
• Should be a minimum of two feet above the natural water table
• Should be a minimum of 15 feet away from buildings
Description: A pavement surface composed of
structural units with void areas that are filled with
pervious materials such as gravel, sand, or grass
turf. Permeable paver systems are installed over
a gravel base course that provides structural support and stores stormwater runoff that infiltrates
ADVANTAGES / BENEFITS
• Surface flow reduction of peak flows, volume, and stormwater runoff
• High level of pollutant removal
• Aesthetic pleasing options
• Reusable product
• Longer life than traditional pavement
• Decreases impermeable area
LID/GI Consideration: A permeable paver
system provides water quality benefits in addi-
systems should result in a reduction of impervious
area on a site.
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Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes
Other Considerations: Overflow parking,
driveways, & related uses
ROUTINE MAINTENANCE REQUIREMENTS
• High maintenance requirements
• Weed and remove grass out of bricks/blocks as necessary (unless
concrete grid pavers are used)
• Sweep or vacuum the pavers as necessary
tion to groundwater recharge and a reduction in
stormwater volume. The use of permeable paver
Runoff Reduction
Land Requirement
DISADVANTAGES / LIMITATIONS
• High cost compared to conventional pavements
• Potential for high failure rate if not adequately maintained or used in
unstabilized areas
• Geotechnical analysis of soils required
• Ineffective under tree canopy, due to clogging
• Requires specialized knowledge for proper installation
through the system into underlying permeable
soils.
STORMWATER MANAGEMENT SUITABILITY
Stormwater Best Management Practices
4.15 Permeable Paver Systems
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 100% of the runoff reduction credit if an
underdrain is not used
• 75% of the runoff reduction credit if an
upturned underdrain is used
• 50% of the runoff reduction credit if an
underdrain is used
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Modular permeable paver systems are typically
storm event (1.2 inches). These facilities may
Modular permeable paver systems are structural
used in low-traffic areas with low to no tree cov-
sometimes be used to partially or completely
units with regularly inter dispersed void areas used
erage such as:
meet channel protection requirements on smaller
to create a load-bearing pavement surface. The
• Parking pads in parking lots
sites. However, permeable paver systems will typically need to be used in conjunction with another
void areas are filled with pervious materials (gravel) to create a system that can infiltrate storm-
• Overflow parking areas
water runoff. Permeable paver systems provide
• Residential driveways
water quality benefits in addition to groundwater
recharge and a reduction in stormwater volume.
The use of permeable paver systems should result
in a reduction of impervious area on a site.
meable paver systems available from different
systems need to be designed and maintained to
safely bypass higher flows.
• Recreational trails
• Runoff Reduction
Like other LID practices, permeable paver
systems become more effective with higher
infiltration rates of native soils. A permeable
paver system can be designed to provide 100%
of the runoff reduction volume, if properly
maintained.
• Emergency vehicle and fire access lanes
manufacturers, including both pre-cast and mold
A major drawback is the cost and complexity of
in-place concrete blocks, concrete grids, inter-
modular permeable paver systems compared
locking bricks, and plastic mats with hollow rings
to conventional pavements. Permeable paver
or hexagonal cells (see Figure Page 1).
systems require a very high level of construction
workmanship to ensure that they function as de-
Modular permeable paver systems are typically
signed. In addition, there is the difficulty and cost
placed on a gravel (stone aggregate) base course.
of rehabilitating the surfaces when they become
Runoff infiltrates through the permeable paver
clogged. Therefore, consideration of permeable
surface into the gravel base course, which acts
paver systems should include the construction
as a storage reservoir as it exfiltrates runoff to the
and maintenance requirements and costs.
underlying soil. The infiltration rate of the soils in
the subgrade must be adequate to support draw48-72 hours. Special care must be taken during
4.12.2 Stormwater Management Suitability
construction to avoid undue compaction of the
Permeable paver systems can be designed for
underlying soils, which could affect the soils’ infil-
water quantity, but they are mostly used for water
tration capability.
quality, i.e. the removal of stormwater pollutants,
down of the entire runoff capture volume within
as overbank flood protection. Permeable paver
• Residential street parking lanes
• Golf cart and pedestrian paths
There are many different types of modular per-
control to provide channel protection, as well
Stormwater Best Management Practices
4.15.1 General Description
• Water Quality
The permeable paver system is an excellent
stormwater treatment practice due to the
variety of pollutant removal mechanisms.
Each of the components of the permeable
paver system is designed to perform a specific
function. The grass filter strip (for sheet flow)
or grass channel or forebay (for concentrated
flow) pre-treatment component reduces
incoming runoff velocity and filters particulates
from the runoff. The planting soil or rock in
the permeable paver system acts as a filtration
system, and clay in the soil provides adsorption
sites for hydrocarbons, heavy metals, nutrients
and other pollutants.
depending upon the native soils. Permeable paver
systems can provide runoff quantity control,
particularly for smaller runoff volumes such as
the runoff volume generated by the water quality
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• Overbank Flood Protection
Although relatively rare, on some development
sites, a permeable paver system can be
designed to attenuate the overbank peak
discharge (Qp25).
• Extreme Flood Protection
Although relatively rare, on some development
sites, a permeable paver system can be
designed to attenuate the extreme peak
discharge (Qf).
Credit for the volume of runoff reduced in the
permeable paver system may be taken in the
overbank flood protection and extreme flood protection calculations. If the practice is designed to
provide runoff reduction for water quality compliance, then the practice is given credit for channel
protection and flood control requirements by
allowing the designer to compute an Adjusted CN
(see Section 3.1.7.5 for more information).
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4.15.3 Pollutant Removal Capabilities
As they provide for the infiltration of stormwater
runoff, permeable paver systems have a high removal of both soluble and particulate pollutants,
since they become trapped, absorbed or broken
down in the underlying soil layers. Due to the
potential for clogging, permeable paver surfaces
should not be used for the removal of sediment
or other coarse particulate pollutants.
For additional information and data on pollutant
removal capabilities for permeable pavers, see the
National Pollutant Removal Performance Database (3rd Edition) available at www.cwp.org and
the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org.
4.15.4 Design Criteria and Specifications
• Permeable paver systems can be used
where the underlying in-situ subsoils have
an infiltration rate of at least 0.5 inches per
hour. Therefore, permeable paver systems
are not suitable on sites with hydrologic group
D or most group C soils, or soils with a high
(>30%) clay content. During construction and
preparation of the subgrade, special care must
be taken to avoid compaction of soils.
• Permeable paver systems should typically
be used in applications where the pavement
receives tributary runoff only from impervious
areas. The ratio of the contributing impervious
area to the permeable paver surface area
should be no greater than 3:1.
• If runoff is coming from adjacent pervious
areas, it is important that those areas be fully
stabilized to reduce sediment loads and prevent
clogging of the permeable paver surface.
• It is recommended that the subsoil of the
permeable paver systems have a slope of 0%
and the surface have a slope of 0.5% if possible.
• A minimum of 2 feet of clearance is required
between the bottom of the gravel base course
and underlying bedrock or the seasonally high
groundwater table.
• Permeable paver systems should be sited at
least 15 feet down gradient from buildings and
Stormwater Best Management Practices
• Channel Protection
For smaller sites, a permeable paver system
may be designed to capture the entire
channel protection volume (CP v). Given that a
permeable paver system is typically designed
to completely drain over 48-72 hours, the
requirement of extended detention of the
1-year, 24-hour storm runoff volume will be
met. For larger sites, or where only the WQv.
is diverted to the permeable paver system,
another practice must be used to provide CP v
extended detention.
• 100 feet away from drinking water wells.
• An appropriate modular permeable paver
should be selected for the intended
application. A minimum of 40% of the surface
area should consist of open void space. If it is a
load bearing surface, then the pavers should be
able to support the maximum load.
• The permeable paver infill is selected based
upon the intended application and required
infiltration rate. Masonry sand (such as ASTM
C-33 concrete sand or GADOT Fine Aggregate
Size No. 10) has a high infiltration rate (8 in/hr)
and should be used in applications where no
vegetation is desired. A sandy loam soil has a
substantially lower infiltration rate (1 in/hr), but
will provide for growth of a grass ground cover.
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• The gravel base course should be designed to store at a minimum
the water quality volume (WQv). The stone aggregate used should be
washed, bank-run gravel, 1.5 to 2.5 inches in diameter with a void space
of about 40% (GADOT No.3 Stone). Aggregate contaminated with soil
should not be used. A porosity value (void space/total volume) of 0.32
should be used in design calculations.
• The gravel base course must have a minimum depth of 12 inches. The
following equation can be used to determine if the depth of the storage
layer (gravel base course) needs to be greater than the minimum depth:
RRV = A [(p1)(d1)]
• Permeable paver system designs must use some method to convey
larger storm rainfall event flows to the conveyance system. One
option is to use storm drain inlets set slightly above the elevation of the
pavement. This would allow for some ponding above the surface, but
also accepting bypass flows that are too large to be infiltrated by the
permeable paver system. This also helps to address concerns about
handling flows when the paver system is clogged.
• For the purpose of sizing downstream conveyance and structural
control system, permeable paver surface areas can be assumed to
be 40% impervious. In addition, credit can be taken for the runoff
volume infiltrated from other impervious areas using the methodology
described in Section 3.1.
Stormwater Best Management Practices
• A 1-inch top course (filter layer) of sand (ASTM C-33 concrete sand or
GADOT Fine Aggregate Size No. 10) underlain by filter fabric is placed
under the permeable pavers and above the gravel base course.
Where:
RRV - Runoff reduction volume (ft3)
A - area of permeable paver system (ft2)
p1 - porosity of base layer (% void)
d1 - depth of base layer (ft)
Note that this formula works for surfaces with a 0% slope.
• The upper surface of the subgrade should be lined with filter fabric
or an 8-inch layer of sand (ASTM C-33 concrete sand or GADOT Fine
Aggregate Size No. 10) and be completely flat to promote infiltration
across the entire surface.
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(Step 1) Determine if the development site and conditions are ap-
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
propriate for the use of a permeable paver system.
of the BMP, described in this section. By considering the
Consider the application and site feasibility criteria in this
primary function, as well as, topographic and soil conditions,
chapter. In addition, determine if site conditions are suitable
the design elements of the practice can be determined (i.e.
for a permeable paver systems. Create a rough layout of the
planting media, underdrain, inlet/outlet, overflow, etc.).
permeable paver systems dimensions taking into consideration existing trees, utility lines, and other obstructions.
Complete Step 3A, 3B, and 3C for a runoff reduction approach, or skip Step 3 and complete Steps 4A and 4B for a
(Step 2) Determine the goals and primary function of the permeable
water quality (treatment) approach. Refer to your local com-
paver system.
munity’s guidelines for any additional information or specific
Consider whether the permeable paver system is intended
requirements regarding the use of either method.
to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Stormwater Best Management Practices
4.15.5 Design Procedures
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Where:
RRV = Runoff Reduction Target Volume (ft3)
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
P = Target runoff reduction rainfall (inches)
RV = Volumetric runoff coefficient which can be
found by:
»» Provide a possible solution to a drainage problem
RV = 0.05+0.009(I)
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
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Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
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Provide pretreatment by using a grass filter strip or pea gravel
up the appropriate runoff reduction percentage (or credit)
diaphragm, as needed, (sheet flow), or a grass channel or
provided by the practice:
forebay (concentrated flow). Where filter strips are used,
100% of the runoff should flow across the filter strip. Pre-
Using the RRV calculated, determine the minimum Volume of
treatment may also be desired to reduce flow velocities or
the Practice (VP)
assist in sediment removal and maintenance. Pretreatment
can include a forebay, weir, or check dam. Splash blocks
(VPMIN) ≥ RRv (target) / (RR%)
or level spreaders should be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour.
Where:
Forebays should be sized to contain 0.1 inches per imperRR% = Runoff Reduction percentage, or credit,
vious acre of contributing drainage. Refer to Section 4.9
assigned to the specific practice
for design criteria for a grass channel and Section 4.29 for
VPMIN = Minimum storage volume required to
vegetated filter strips.
provide Runoff Reduction Target Volume (ft )
3
RRv (target) = Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
(Step 3C) Determine whether the minimum storage volume was met.
When the VP ≥ VPMIN, then the Runoff Reduction require-
(Step 3B) Determine the storage volume of the practice and the
ments are met for this practice. Proceed to Step 5.
Pretreatment Volume
To determine the actual volume provided in the permeable
When the VP < VPMIN, then the BMP must be sized according
paver system, use the following equation:
to the WQV treatment method (See Step 4).
VP = (PV + VES (N))
Where:
VP = Volume provided (temporary storage)
PV = Ponding Volume
VES = Volume of Engineered Soils
N = Porosity
To determine the porosity, a qualified licensed professional
should be consulted to determine the proper porosity based
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
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Calculate the Water Quality Volume using the following
To determine the minimum surface area of the permeable
paver system, use the following formula:
formula:
Af = (WQv)(dg + des) / [(kg* dg* tg) + (kes* des* tes)]
WQV = (1.2) (RV) (A) / 12
Where:
Af = surface area of permeable paver system (ft2)
Where:
WQV = Water Quality Volume (ft3)
WQv = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
dg = gravel depth
RV = Volumetric runoff coefficient which can be
des = engineered soil layer depth (ft)
found by:
kg = coefficient of permeability for gravel (ft/day)
kes = coefficient of permeability for engineered soil
RV = 0.05+0.009(I)
(ft/day)
tg = drain time of gravel (days)
Where:
tes = drain time of engineered soil (days)
Stormwater Best Management Practices
(Step 4A) Calculate the Target Water Quality Volume
I = new impervious area of the contributing
drainage area (%)
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
A = Site area (total drainage area) (ft2)
storm). See Subsection 3.1.7.5 for more information.
12 = Unit conversion factor (in/ft)
(Step 6) Design system outlets
(Step 4B) If using the practice for Water Quality treatment, determine
Determine which type of outlet design will be used for the per-
the footprint of the infiltration practice and the Pretreat-
meable paver system. There are two types of outlet design that
ment Volume required.
are generally used if infiltration is not possible without additional
The peak rate of discharge for the water quality design storm is
assistance. They are an underdrain system and overflow system.
needed for sizing of off-line diversion structures (see Subsection
The underdrain system should include a series of perforated
3.1.7). If designing off-line, follow steps (a) through (d) below:
pipes that run longitudinal with the pavers to remove additional
.
stormwater runoff that could not otherwise infiltrate into the
(a) Using WQv, compute CN
surrounding soil. An overflow system directs water that cannot
(b) Compute time of concentration using TR-55 method
be infiltrated into the subsoil and moves it to another location,
(c) Determine appropriate unit peak discharge from time of
for instance another BMP or storm sewer system.
concentration
(d) Compute Qwq from unit peak discharge, drainage area,
and WQv.
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Determine the stormwater discharges to the construction
site that could potentially erode and clog the system. Take
the proper steps to stabilize the site and prevent erosion
when construction begins.
(Step 8) Select Permeable Paver System and finalize design
Select the most appropriate paver system based on the specific site conditions. Finalize the design the of the practice.
Make sure that the soil is stabilized by using filter fabric or
other method as determined by the designer.
4.15.6 Inspection and Maintenance Requirements
Stormwater Best Management Practices
(Step 7) Erosion and Sediment Control/Base Protection
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs should include considerations for
maintenance and maintenance access. For additional information on inspection and maintenance requirements, see Appendix E.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Refer to ACI 522 Report on Pervious Concrete for design
considerations
• Construction recommended by National Ready Mixed Concrete
Association (NRMCA) certified personnel as specified in ACI 522.1
• Typically used for low volume auto traffic areas, or for overflow
parking applications
• Not recommended on sites with low permeability soils, wellhead
protection zones, or water supply aquifer recharge areas
• Although not typical, pervious concrete can be designed to
accommodate heavier vehicles under certain circumstances
Description: Pervious concrete is a mixture of
coarse aggregate, Portland cement and water that
allows for rapid infiltration of water and overlays a
stone aggregate reservoir. This reservoir provides temporary storage as runoff infiltrates into
underlying permeable soils and/or out through an
underdrain system.
LID/GI Consideration: Since pervious concrete is
often designed primarily for stormwater quantity, but can also provide runoff quality control
when used as an integral part of the stormwater
management system plan, it can be considered an
LID/GI control.
ADVANTAGES / BENEFITS
• Provides reductions in runoff volume, stormwater runoff, and
impervious area
• Helps minimize size of detention ponds
• Particularly well suited in capturing “first flush” water quality volume
• Reduces standing water on pavement
• May help to reduce stormwater management costs
DISADVANTAGES / LIMITATIONS
• Somewhat higher installation cost than for conventional pavement
• Infiltration testing of existing soils may be required
• Not typically recommended for areas with heavy traffic or trucks.
• Not recommended under tree canopy
ROUTINE MAINTENANCE REQUIREMENTS
• Keep concrete free of trash, debris, and dirt
• Use street sweepers or vacuum trucks to clean pervious concrete as
needed
• Keep grass surrounding the area trimmed and remove grass clippings
from area
• Occasional pressure washing may be necessary
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.16 Pervious Concrete
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Other Considerations: Overflow parking,
driveways, & related uses
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 100% of the runoff reduction credit if an
underdrain is not used
• 75% of the runoff reduction credit if an
upturned underdrain is used
• 50% of the runoff reduction credit if an
underdrain is used
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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gravel and crushed stone. The void spaces in the
Pervious concrete (also referred to as enhanced
stone act as a storage reservoir for runoff.
Stormwater Best Management Practices
4.16.1 General Description
porosity concrete, porous concrete, Portland cement pervious pavement and pervious pavement)
Pervious concrete is often designed primarily for
is a subset of a broader family of pervious pave-
stormwater quantity (i.e. the removal of storm-
ments including porous asphalt, and various kinds
water volume). However, it can provide runoff
of grids and paver systems. Pervious concrete is
quality control, when used as an integral part of
generally regarded to have a greater ability than
the stormwater management system plan. Pervi-
permeable asphalt to maintain its porosity in hot
ous concrete systems can be designed to capture
weather and thus is provided as a limited applica-
and infiltrate the water quality volume (WQv) and
tion control. Please see Figure 4.16-1 for typical
the channel protection volume (CPv) as well as
applications.
providing an infiltration option for overbank protection and extreme flood protection.
Past failures have been attributed to poor design,
inadequate construction techniques, soils with
Modifications or additions to the standard design
low permeability, heavy vehicular traffic and poor
have been used to pass flows and volumes in
maintenance. However, pervious concrete has
excess of the water quality volume, or to increase
experienced increasing success in Georgia due to
storage capacity and treatment. These include
advancements in concrete mix designs, installa-
but are not limited to:
tion techniques and maintenance acceptance. In
• Placing a perforated pipe near the top of the
crushed stone reservoir to pass excess flows
after the reservoir is filled
particular, wider acceptance of the NRMCA Pervious Concrete Contractor certification program (as
required by ACI 522) has led to improved contractor performance
Pervious concrete consists of a specially formulated mixture of Portland cement, uniform,
open graded course aggregate, and water. The
concrete layer has a high permeability, such that
the underlying permeable soil layer allows rapid
percolation of rainwater through the surface and
into the layers beneath. The void space in pervious concrete is in the 15-22% range, as opposed
to 3-5% for conventional pavements. The perme-
• Providing surface detention storage in a parking
lot, adjacent swale, or detention pond with
suitable overflow conveyance
• Connecting the stone reservoir layer to a stone
filled trench
• Adding a sand layer and perforated pipe
beneath the stone layer for filtration of the
water quality volume
Figure 4.16-1 Typical Pervious Concrete System
Applications
(Photos by Bruce Ferguson, Don Wade)
• Placing an underground detention tank or vault
system beneath the layers
able surface is placed over a layer of open-graded
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water infrastructure, which may offset the greater
and accommodate extended detention require-
upfront capital cost of pervious (versus impervi-
ments for the CPv, the minimum drawdown time
ous) concrete.
should be 24-48 hours. Longer drawdown is
acceptable as necessary to infiltrate, bypass or
Like other infiltration controls, pervious concrete
detain and release larger storm events with a
should not be used in areas that experience high
maximum drawdown time of 5 days. Undue com-
rates of wind erosion or in drinking water aquifer
paction, which could affect the soils’ infiltration
recharge areas.
capability, should be avoided.
low-traffic areas such as:
4.16.2 Stormwater Management Suitability
• Parking pads in parking lots
Pervious concrete can provide runoff quantity
Pervious concrete systems are typically used in
• Overflow parking areas
• Residential street parking lanes
control, particularly for smaller runoff volumes
such as the runoff volume generated by the water
quality storm event (1.2 inches). These facilities
• Golf cart and pedestrian paths
• Emergency vehicle and fire access lanes
It is recommended that subsoil slopes should be
equal to or less than 0.5% slope. Surface slopes
should be 6% or less, with 2% maximum slope
preferable. The seasonally high water table or
bedrock should be a minimum of two feet below
the bottom of the pervious concrete, if infiltration
is relied on to drain the stored volume.
Pervious concrete has the positive characteristics
of volume reduction due to infiltration, groundwater recharge, and an ability to blend into the
normal urban landscape relatively unnoticed. It
• Overbank Flood Protection
Although relatively rare, on some development
sites, pervious concrete can be designed to
attenuate the overbank peak discharge (Qp25).
ly need to be used in conjunction with another
• Extreme Flood Protection
Although relatively rare, on some development
sites, pervious concrete can be designed to
attenuate the extreme peak discharge (Qf).
control to provide channel protection, as well as
Credit for the volume of runoff reduced in pervious
overbank flood protection.
concrete may be taken in the overbank flood pro-
• Runoff Reduction
Like other LID practices, pervious concrete
becomes more effective with higher infiltration
rates of native soils. Pervious concrete can
be designed to provide 100% of the runoff
tection and extreme flood protection calculations.
may sometimes be used to partially or completely
• Recreational trails
• Channel Protection
For smaller sites, the pervious concrete may
be designed to capture the entire channel
protection volume (CP v). Given that the storage
volume under the pervious concrete is typically
designed to completely drain over 48-72 hours,
the requirement of extended detention of the
1-year, 24-hour storm runoff volume will be
met. For larger sites, or where only the WQv.
is diverted to the pervious concrete, another
practice must be used to provide CP v extended
detention.
meet channel protection requirements on smaller
sites. However, pervious concrete will typical-
reduction volume, if properly maintained.
• Water Quality
Pervious concrete provides some amount
of pollutant removal, but is not well suited
for removing sediment due to the sediment
potentially clogging the pores within the
pervious concrete.
Stormwater Best Management Practices
In order to meet water quality standards (WQv)
If the practice is designed to provide runoff reduction for water quality compliance, then the practice
is given credit for channel protection and flood
control requirements by allowing the designer to
compute an Adjusted CN (see Subsection 3.1.7.5
for more information).
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The following criteria should be evaluated to
As they provide for the infiltration of stormwater
ensure the suitability of pervious concrete for
runoff, pervious concrete systems have a high
meeting stormwater management objectives on a
removal rate for both soluble and particulate pol-
site or development.
lutants, which become trapped, absorbed or bro-
• Proximity – The following is a list of specific
setback requirements for the location of
pervious concrete:
»» 10 feet from building foundations
»» 100 feet from private water supply wells
ken down in the underlying soil layers. Due to the
potential for clogging, pervious concrete surfaces
General Feasibility
should not be used for the removal of sediment
- Suitable for Residential Subdivision Usage – YES
or other coarse particulate pollutants.
- Suitable for High Density/Ultra Urban Areas – YES
- Regional Stormwater Control – NO
For additional information and data on pollutant
»» 200 feet from public water supply reservoirs
(measured from edge of water)
»» 1,200 feet from public water supply wells
• Trout Stream – Evaluate for stream warming
when an underdrain system is used.
removal capabilities for pervious concrete, see the
National Pollutant Removal Performance Data-
Physical Feasibility - Physical Constraints at
base (2nd Edition) available at www.cwp.org and
Project Site
the national Stormwater Best Management Prac-
• Site Slope – Slopes should be a maximum of
6%
tices (BMP) Database at www.bmpdatabase.org.
Pollutant removal can be improved through
routine vacuuming, sweeping, and high pressure
washing of pervious concrete systems, maintaining a drainage time of at least 24 hours, pretreat-
• Minimum Depth to Water Table – A separation
distance of 2 feet is recommended between
the bottom of the rock storage area of the
pervious concrete and the elevation of the
seasonally high water table.
ing the runoff, having organic material in the
subsoil, and using clean washed aggregate (EPA,
1999).
• Soils – Native soils if they have at least 0.5
inch/hr infiltration ability.
Other Constraints/Considerations
4.16.4 Application and Site Feasibility
Criteria
Pervious concrete is suitable for many types of
development, from single-family residential to
high-density commercial projects. Though not
often used for heavily traveled roads, it is well
• Hot spots – Do not use for hot spot runoff.
• Damage to existing structures and facilities –
Consideration should be given to the impact of
water exfiltrating the pervious concrete.
In addition, careful consideration should be given
to the potential of perched or raised groundwater
Stormwater Best Management Practices
4.16.3 Pollutant Removal Capabilities
levels. Provide adequate distance from building
foundations or use impermeable liner on side of
excavated area nearest to structure.
Challenges and Potential Solutions for Coastal
Areas
»» Poorly Drained Soils—This condition
minimizes the ability of pervious concrete
to reduce stormwater runoff rates and
volumes. One solution would be to include
an underdrain system.
»» Shallow Water Table—This can prevent the
provision of 2 feet of clearance between
the bottom of the pervious concrete and
the top of the water table which may cause
stormwater runoff to pond in the storage
layer of the pervious concrete.
suited for alleys, driveways, and parking lots.
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• Pervious concrete systems can be used
where the underlying in-situ subsoils have
an infiltration rate greater than 0.5 inches
per hour. Infiltration rates for in-situ soils
should be determined through geotechnical
investigation prior to design of the system.
During construction and preparation of the
subgrade, over compaction of soils should
be avoided. Refer to ACI 522 for compaction
recommendations.
• Pervious concrete systems should typically
be used in applications where the pavement
receives tributary runoff only from impervious
areas. It is recommended that the ratio
of tributary impervious area to the area of
the pervious concrete system should be a
maximum of 1:1. Pervious concrete systems
should be sized for a minimum drawdown time
of 24 hours and a maximum drawdown time of
5 days.
• Tributary runoff from adjacent pervious areas is
not recommended. However, if it is necessary
for runoff to come from adjacent pervious
areas, it is important that those areas be fully
stabilized to reduce sediment loads and prevent
clogging of the pervious surface. Pretreatment
using dry or wet enhanced swales or vegetated
filter strips for removal of course sediments is
recommended and continued maintenance of
these areas will be required. (see Sections 4.6
and 4.29)
• A minimum of four feet of clearance is
recommended (may be reduced to two feet
in coastal areas) between the bottom of the
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gravel base course and underlying bedrock or
the seasonally high groundwater table.
• Pervious concrete systems should be sited at
least 10 feet down-gradient from buildings and
100 feet away from drinking water wells.
• To protect groundwater from potential
contamination, runoff from designated hotspot
land uses or activities must not be infiltrated.
Pervious concrete should not be used on
manufacturing or industrial sites, where there
is potential for high concentrations of soluble
pollutants and heavy metals. In addition,
pervious concrete should not be considered
for areas with high pesticide concentrations.
Pervious concrete is also not suitable in
areas with karst geology without adequate
geotechnical testing and approval by qualified
individuals and in accordance with local
requirements.
• Pervious concrete systems can be used
independently or in conjunction with other
stormwater management system components
to effectively infiltrate, bypass, or detain and
time-release all required storm events..
infiltration rate of the sub-grade, so storage
duration of two hours can be used for design
purposes. The total storage volume in a layer
is equal to the percent of void space times the
volume of the layer.
• For the purpose of sizing downstream
conveyance and structural control system,
pervious concrete surface areas can be
estimated as 35% impervious.
• For treatment control, the design volume
should be, at a minimum, equal to the water
quality volume. The water quality storage
volume is contained in the surface layer and
the aggregate reservoir. The storm duration
(fill time) is normally short compared to the
infiltration rate of the sub-grade, so storage
duration of two hours can be used for design
purposes. The total storage volume in a layer
is equal to the percent of void space times the
volume of the layer.
Stormwater Best Management Practices
4.16.5 Planning and Design Criteria
• For the purpose of sizing downstream
conveyance and structural control system,
pervious concrete surface areas can be
estimated as 35% impervious.
• For treatment control, the design volume
should be, at a minimum, equal to the water
quality volume. The water quality storage
volume is contained in the surface layer and
the aggregate reservoir. The storm duration
(fill time) is normally short compared to the
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Stormwater Best Management Practices
• The cross-section typically consists of
two layers, as shown in Figure 4.16-2. The
aggregate reservoir can sometimes be
avoided or minimized if the sub-grade is sandy
and there is adequate time to infiltrate the
necessary runoff volume into the sandy soil
without by-passing the water quality volume. A
description for each of the layers is presented
below:
»» Pervious Concrete Layer – The pervious
concrete layer consists of an open-graded
concrete mixture usually ranging from
depths of 6 – 12 inches depending on
required bearing strength and pavement
design requirements. Pervious concrete
can be assumed to contain 18 percent voids
(porosity = 0.18) for design purposes. Thus,
for example, a 6 inch thick pervious concrete
layer would hold 1.08 inches of rainfall.
Refer to ACI 522R (most current version) for
recommendations on mix proportioning
for pervious concrete. See the GCPA
specifications (referenced) as well.
»» Reservoir Layer – The reservoir gravel base
course consists of washed, bank-run gravel,
1.5 2.5 inches in diameter with a void space
of about 40% (GADOT No.3 Stone). The
depth of this layer depends on the desired
storage volume, which is typically the water
quality volume (WQv) at a minimum. Typical
depths for the reservoir layer range from 2-4
feet. Aggregate contaminated with soil shall
not be used. A porosity value (void space/
total volume) of 0.40 should be used in
calculations unless aggregate-specific data
exist.
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Figure 4.16-2 Pervious Concrete System Section
(Modified From: LAC 2000
»» Filter Fabric – Filter fabric can be used
is certain applications, as site conditions
warrant. General guidance for the use of
filter fabrics is below. Actual use should be
under the guidance of a Georgia licensed
engineer.
»» Geotextiles consisting of permeable
materials should line the sides of the
aggregate base to prevent migration of
adjacent soils into it and subsequent
permeability and storage capacity reduction.
Geotextiles are not recommended under
the aggregate base in an infiltration
design because they can accumulate fine
particulates that inhibit infiltration.
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-- Provide a barrier on the side and bottom
of the aggregate base in a detention
design to prevent infiltration into the
subgrade typically due to soil instability,
the presence of stormwater hotspots, or
potential for groundwater contamination.
Geomembrane barriers reduce the credit
for TSS removal from 85% to 70%.
-- Line the sides of the aggregate base
whenever structure foundations of
conventional pavement are 20 feet or
less from the permeable pavement (to
avoid the risk of structural damage due to
seepage). The use of geomembranes for
this purpose will not reduce credit for TSS
removal in the system.
»» Geogrids may be used on top of subgrade
soils for additional structural support,
especially in very weak, saturated soils. All
manufacturer requirements must be followed
in design and installation of geogrids.
»» Underlying Soil – Pervious concrete systems
cannot be used in fill soils. The underlying
soil should have an infiltration capacity of
at least 0.50 in/hr. as initially determined
from NRCS soil textural classification, and
subsequently confirmed by field geotechnical
tests. The minimum geotechnical testing is
one test hole per 5000 square feet, with a
minimum of two borings per practice (taken
within the proposed limits of the facility).
Test borings are recommended to determine
the soil classification, seasonal high ground
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water table elevation, impervious substrata,
and an initial estimate of permeability. Often
a double-ring infiltrometer test is done
at subgrade elevation to determine the
impermeable layer, and, for safety, one-half
the measured value is allowed for infiltration
calculations.
• The pit excavation should be limited to the
width and depth specified in the design.
Excavated material should be placed away
from the open trench to avoid jeopardizing
the stability of the trench sidewalls. The
bottom of the excavated trench should not
be loaded so as to cause compaction, and
should be scarified prior to placement of
reservoir base material. The sides of the trench
should be trimmed of all large roots. The
sidewalls should be uniform with no voids
and scarified prior to backfilling. All pervious
concrete systems should be protected during
construction and constructed after upstream
areas are stabilized.
• Details of construction of the concrete layer
are beyond the scope of this manual. However,
construction by NRMCA certified personnel,
following the guidelines of ACI 522R and ACI
522.1, is recommended.
Stormwater Best Management Practices
»» Geomembranes consisting of impermeable
materials should be used to accomplish the
following:
• An observation well consisting of perforated
PVC pipe 4-6 inches in diameter should be
placed at the downstream end of the facility
and protected during site construction. The
well should be used to determine actual
infiltration rates for use in final design of the
pervious concrete system.
• A warning sign should be placed at the facility
that states, “Pervious Paving used on this site to
reduce pollution. Do not resurface with nonpervious material. Call (XXX) XXX-XXXX for
more information.”
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(Step 1) Determine if the development site and conditions are ap-
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
propriate for the use of the pervious concrete.
of the BMP, described in this section. By considering the
Consider the application and site feasibility criteria in this
primary function, as well as, topographic and soil conditions,
chapter. In addition, determine if site conditions are suitable
the design elements of the practice can be determined (i.e.
for pervious concrete. Create a rough layout of the pervious
planting media, underdrain, inlet/outlet, overflow, etc.)
concrete dimensions taking into consideration existing trees,
utility lines, and other obstructions.
Complete Step 3A, 3B, and 3C for a runoff reduction approach, or skip Step 3 and complete Steps 4A and 4B for a
(Step 2) Determine the goals and primary function of the pervious
water quality (treatment) approach. Refer to your local com-
concrete.
munity’s guidelines for any additional information or specific
Consider whether the pervious concrete is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
requirements regarding the use of either method.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Stormwater Best Management Practices
4.16.6 Design Procedures
Calculate the Runoff Reduction Volume using the following
formula:
RRV = (P) (RV) (A) / 12
Where:
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
P = Target runoff reduction rainfall (inches)
»» Provide a possible solution to a drainage problem
found by:
RRV = Runoff Reduction Target Volume (ft3)
RV = Volumetric runoff coefficient which can be
»» Enhance landscape and provide aesthetic qualities
RV = 0.05+0.009(I)
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
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100% of the runoff should flow across the filter strip. Pre-
up the appropriate runoff reduction percentage (or credit)
treatment may also be desired to reduce flow velocities or
provided by the practice:
assist in sediment removal and maintenance. Pretreatment
can include a forebay, weir, or check dam. Splash blocks
Using the RRV calculated, determine the minimum Volume of
or level spreaders should be considered to dissipate con-
the Practice (VP)
centrated stormwater runoff at the inlet and prevent scour.
Forebays should be sized to contain 0.1 inches per imper-
(VPMIN) ≥ RRv (target) / (RR%)
vious acre of contributing drainage. Refer to Section 4.9
for design criteria for a grass channel and Section 4.29 for
Where:
vegetated filter strips.
RR% = Runoff Reduction percentage, or credit,
(Step 3C) Determine whether the minimum storage volume was met.
assigned to the specific practice
VPMIN = Minimum storage volume required to
When the VP ≥ VPMIN, then the Runoff Reduction require-
provide Runoff Reduction Target Volume (ft )
ments are met for this practice. Proceed to Step 5.
3
Stormwater Best Management Practices
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
RRv (target) = Runoff Reduction Target Volume (ft3)
When the VP < VPMIN, then the BMP must be sized according
(Step 3B) Determine the storage volume of the practice and the
to the WQV treatment method (See Step 4).
Pretreatment Volume
To determine the actual volume provided in the pervious
concrete, use the following equation:
VP = (VBL) * (N)
Where:
VP = Volume provided (temporary storage)
VBL = Volume of Base Layer
N = Porosity
To determine the porosity, a qualified licensed professional
should be consulted to determine the proper porosity. Most
gravel has a value of 0.40.
Provide pretreatment by using a grass filter strip or pea gravel
diaphragm, as needed, (sheet flow), or a grass channel or
forebay (concentrated flow). Where filter strips are used,
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Calculate the Water Quality Volume using the following
To determine the minimum surface area of the pervious concrete, use the following formula:
formula:
WQV = (1.2) (RV) (A) / 12
Af = (WQv)(dpc + drl)/[(kpc * dpc * tpc)+(krl * drl * trl)]
Where:
Where:
WQV = Water Quality Volume (ft )
Af = surface area of pervious (ft2)
1.2 = Target rainfall amount to be treated (inches)
WQv = Water Quality Volume (ft3)
3
RV = Volumetric runoff coefficient which can be
dpc = pervious concrete depth (ft)
found by:
drl = reservoir layer depth (ft)
kpc = coefficient of permeability for pervious con-
RV = 0.05+0.009(I)
crete (ft/day)
krl = coefficient of permeability for reservoir layer
Where:
(ft/day)
I = new impervious area of the contributing
tpc = drain time of pervious concrete (days)
drainage area (%)
trl = drain time of reservior layer (days)
Stormwater Best Management Practices
(Step 4A) Calculate the Target Water Quality Volume
A = Site area (total drainage area) (ft2)
12 = Unit conversion factor (in/ft)
(Step 5) Size underdrain system (if applicable)
See Subsection 4.2.5.3 (Physical Specifications/Geometry)
(Step 4B) If using the practice for Water Quality treatment, determine
the footprint of the pervious concrete and the Pretreatment
4.16.7 Inspection and Maintenance Requirements
Volume required
All best management practices require proper maintenance. Without proper
The peak rate of discharge for the water quality design storm
maintenance, BMPs will not function as originally designed and may cease to
is needed for sizing of off-line diversion structures (see Sub-
function altogether. The design of all BMPs includes considerations for main-
section 3.1.7). If designing off-line, follow steps (a) through
tenance and maintenance access. For additional information on inspection
(d) below:
and maintenance requirements, see Appendix E.
.
(a) Using WQv, compute CN
(b) Compute time of concentration using TR-55 method
(c) Determine appropriate unit peak discharge from time of
concentration
(d) Compute Qwq from unit peak discharge, drainage area,
and WQv.
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280
KEY CONSIDERATIONS
DESIGN CRITERIA
• Intended for low-traffic areas, or for residential or overflow parking
applications
• Not ideal for areas with a tree canopy or high-traffic flow
• Soil infiltration rate of 0.5 in/hr or greater is required if no
underdrain is present
• Aesthetically pleasing
• Americans with Disabilities Act (ADA) compliant
• Not appropriate as water quality treatment BMP for drainage
discharged from other areas
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
Description: Porous asphalt is asphalt with
reduced sands or fines and larger void spaces,
which allow water to drain through it. Porous
asphalt allows water to infiltrate into the subsoil
below through the paved surface and base layer.
This aggregate base layer acts as a structural layer
and container to temporarily hold stormwater.
LID/GI Consideration: Porous asphalt can be
used to reduce the effective impervious area on
a site, therefore reducing the design volumes and
peak discharges that must be controlled. Porous
asphalt can also eliminate problems with standing water, provide groundwater recharge, control
ADVANTAGES / BENEFITS
• Surface flow reduction of peak flows, volume, and stormwater runoff
• Can be used as a pretreatment for other BMPs for pollutants other
than TSS
• High level of pollutant removal other than TSS
• Decreases impermeable area
DISADVANTAGES / LIMITATIONS
• Potential for high failure rate if not adequately maintained or if used
in unstabilized areas
• Not recommended for areas with sediment-laden runoff that can
clog porous pavement
• Subgrade cannot be over-compacted
• Construction must be sequenced to avoid compaction and clogging
of the pavement.
ROUTINE MAINTENANCE REQUIREMENTS
• Sweep or vacuum the asphalt to increase pavement life and avoid
clogging
• Keep contributing drainage area free of debris and address any areas
of erosion
erosion of streambeds and riverbanks, and facilitate pollutant removal.
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POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.17 Porous Asphalt
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes
Soils: Not recommended for use with hydrologic soils group ‘D’ and ‘C’ without underdrain.
Other Considerations: Overflow parking,
driveways, & related uses
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 100% of the runoff reduction credit if an
underdrain is not used
• 75% of the runoff reduction credit if an
upturned underdrain is used
• 50% of the runoff reduction credit if an
underdrain is used
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Porous asphalt is not recommended and may
Porous asphalt is designed primarily for imper-
Porous asphalt is an asphalt driving surface that
not be approved for use in areas that experience
vious area reduction and the subsequent reduc-
permits the infiltration of water through the pave-
high amounts of traffic volume, heavy loads, or
tion in stormwater treatment volumes and peak
ment and into underlying soils. Porous asphalt can
areas with high amounts of sediment runoff (i.e.
discharges, particularly for smaller storm events.
be used to reduce the effective impervious area
construction areas).
These include:
and peak discharges that must be controlled. This
A major drawback of this BMP is the cost and
will allow a reduction in the cost of other storm-
complexity of porous asphalt compared to con-
• Placing a perforated pipe near the top of the
crushed stone reservoir to pass the excess
flows after the reservoir is filled
water infrastructure, which may offset the greater
ventional pavements. Porous asphalt requires a
placement cost. Porous asphalt can also elimi-
very high level of construction workmanship to
nate problems with standing water, provide for
ensure that it functions as designed. In addition,
groundwater recharge, control erosion of stream-
there is the difficulty and cost of rehabilitating
beds and riverbanks, facilitate pollutant removal,
porous asphalt surfaces should they become
reduce thermal pollution of receiving waters, and
clogged. Therefore, consideration of porous
provide for a more aesthetically pleasing site.
asphalt should include the construction and
on a site, therefore reducing the design volumes
• Connecting the stone reservoir layer to a
stone-filled trench
• Adding a sand layer and perforated pipe
beneath the stone layer for filtration of the
water quality volume
Stormwater Best Management Practices
4.17.1 General Description
maintenance requirements and costs.
Porous asphalt consists of open-graded coarse
aggregate, bonded together by asphalt cement,
with sufficiently interconnected voids to make
it highly permeable to water (see Figure 4.17-1).
Porous asphalt is best applied in areas that experience low amounts of vehicle traffic and have little
to no tree coverage, including:
• Parking pads in parking lots
• Overflow parking areas
• Residential driveways
• Residential street parking lanes
• Recreational trails
• Golf cart and pedestrian paths
• Emergency vehicle and fire access lanes
• Plazas
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Figure 4.17-1
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282
(stone aggregate) base course. Runoff infiltrates
through the porous asphalt into the gravel base
course, which acts as a storage reservoir as it
exfiltrates to the underlying soil. The infiltration
rate of the soils in the subgrade must be adequate
to support drawdown of the entire runoff capture
volume within 24-48 hours. Special care must be
taken during construction to avoid undue compaction of the underlying soils, which could affect
the soils’ infiltration capability.
Another type of porous asphalt is called open-graded friction course (OGFC). OGFC is a thin permeable layer of asphalt that encompasses a support
structure consisting of uniform, coarse aggregate
with minimal fines, and serves as an overlay for conventional asphalt pavements. OGFC has a high void
content that creates permeability, allowing for the
infiltration of stormwater runoff.
4.17.2 Stormwater Management Suitability
Porous asphalt can provide runoff quantity control, particularly for smaller runoff volumes such
as the runoff volume generated by the water
quality storm event (1.2 inches). These facilities
may sometimes be used to partially or completely
meet channel protection requirements on smaller
sites. However, porous asphalt will typically need
to be used in conjunction with another control to
provide channel protection, as well as overbank
flood protection.
• Runoff Reduction
Like other LID practices, porous asphalt
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becomes more effective with higher infiltration
rates of native soils. Porous asphalt can
be designed to provide 100% of the runoff
reduction volume, if properly maintained.
• Water Quality
Porous asphalt provides some amount of
pollutant removal, but is not well suited for
removing sediment due to the sediment
potentially clogging the pores within the
porous asphalt.
• Channel Protection
For smaller sites, the porous asphalt may be
designed to capture the entire channel protection
volume (CPv). Given that the storage volume
under the porous asphalt is typically designed
to completely drain over 48-72 hours, the
requirement of extended detention of the 1-year,
24-hour storm runoff volume will be met. For
larger sites, or where only the WQv. is diverted to
the porous asphalt, another practice must be used
to provide CPv extended detention.is diverted
to the porous asphalt, another practice must be
used to provide CPv extended detention.
• Overbank Flood Protection
Although relatively rare, on some development
sites, porous asphalt can be designed to
attenuate the overbank peak discharge (Qp25).
• Extreme Flood Protection
Although relatively rare, on some development
sites, porous asphalt can be designed to
attenuate the extreme peak discharge (Qf).
Credit for the volume of runoff reduced in porous asphalt may be taken in the overbank flood
protection and extreme flood protection calculations. If the practice is designed to provide runoff
reduction for water quality compliance, then the
practice is given credit for channel protection and
flood control requirements by allowing the designer to compute an Adjusted CN (see Subsection 3.1.7.5 for more information).
4.17.3 Pollutant Removal Capabilities
As it provides for the infiltration of stormwater As
porous asphalt provides infiltration of stormwater
runoff, it has a high removal rate for both soluble and fine particulate pollutants, which can be
trapped, absorbed, or broken down in the underlying soil layers. Due to the potential for clogging,
porous asphalt should not be used for the remov-
Stormwater Best Management Practices
Porous asphalt is typically placed on a gravel
al of sediment or other coarse particulate pollutants. Maintenance efforts and frequency is directly
related to the amount of accumulated or trapped
sediment. OGFC has a TSS removal of 50%. There
is not sufficient data for nutrients, fecal coliform,
or metal removal rates to be determined.
For additional information and data on pollutant
removal capabilities for porous asphalt and OGFC,
see the National Pollutant Removal Performance
Database (3rd Edition) available at www.cwp.org and
the National Stormwater Best Management Practices
(BMP) Database at www.bmpdatabase.org.
4.17.4 Application and Site Feasibility
Criteria
Porous asphalt is suitable for many types of
development, from single-family residential to
high-density commercial projects. Though not
often used for heavily traveled roads, it is well
suited for alleys, driveways, and parking lots.
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283
development.
»» 10 feet from building foundations
4.17.5 Planning and Design Criteria
»» 100 feet from private water supply wells
• The detailed design of porous asphalt is beyond
the scope of this Manual. Refer to The National
Asphalt Pavement Associations’ Porous Asphalt
Pavements for Stormwater Management:
Design, Construction and Maintenance Guide
for detailed design guidance.
»» 200 feet from public water supply reservoirs
(measured from edge of water)
»» 1,200 feet from public water supply wells
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas – YES
• Trout Stream –Evaluate for stream warming
when an underdrain system is used.
- Regional Stormwater Control – NO
In addition, careful consideration should be given
to the potential of perched or raised groundwater
Physical Feasibility - Physical Constraints at
levels. Provide adequate distance from building
Project Site
foundations or use impermeable liner on side of
• Site Slope – Slopes should be a maximum of
20%, 5% preferred
• Minimum Depth to Water Table – A separation
distance of 2 feet is recommended between
the bottom of the rock storage area of the
porous asphalt and the elevation of the
seasonally high water table.
• Soils – Native soils if they have at least 0.5
inch/hr infiltration ability.
Other Constraints / Considerations
• Hot spots – Do not use for hot spot runoff.
• Damage to existing structures and facilities –
Consideration should be given to the impact of
water exfiltrating the porous asphalt.
• Proximity – The following is a list of specific
setback requirements for the location of
porous asphalt:
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excavated area nearest to structure.
Challenges and Potential Solutions for Coastal
Areas
• Poorly Drained Soils— This condition
minimizes the ability of porous asphalt to
reduce stormwater runoff rates and volumes.
One solution would be to include an
underdrain system.
• Shallow Water Table— This can prevent the
provision of 2 feet of clearance between the
bottom of the porous asphalt and the top of
the water table which may cause stormwater
runoff to pond in the storage layer of the
porous asphalt.
• Porous asphalt systems can be used where
the underlying in-situ subsoils have an
infiltration rate of at least 0.5 inches per hour.
During construction and preparation of the
subgrade, special care must be taken to avoid
compaction of the soils.
• Porous asphalt should typically be used in
applications where the pavement receives
tributary runoff only from impervious areas.
It is recommended that the ratio of the
contributing impervious area to the porous
asphalt surface area be no greater than 3:1.
Porous asphalt systems should be sized for a
minimum drawdown time of 24 hours and a
maximum drawdown time of 3 days.
Stormwater Best Management Practices
The following criteria should be evaluated to ensure the suitability of porous asphalt for meeting
stormwater management objectives on a site or
• If runoff is coming from adjacent pervious
areas, it is important that those areas be fully
stabilized to reduce sediment loads and prevent
clogging of the porous asphalt surface.
• It is recommended that the subsoil of the
porous asphalt have a slope of 0% and the
surface have a slope 0.5% or less, if possible.
• A minimum of 2 feet of clearance is required
between the bottom of the gravel base course
and underlying bedrock.
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284
• Porous asphalt can be used independently
or in conjunction with other stormwater
management system components to
effectively infiltrate, bypass, or detain and timerelease all required storm events.
• The surface of the subgrade should be lined
with or an 8-inch layer of sand (ASTM C-33
concrete sand or GADOT Fine Aggregate
Size No. 10) and completely flat to promote
infiltration across the entire surface.
• Porous asphalt must use some method
to convey larger storm event flows to the
conveyance system. One option is to use
storm drain inlets set slightly above the
elevation of the pavement. This would allow
for some ponding above the surface, but would
accept bypass flows that are too large to be
infiltrated by the porous asphalt. This would
also address situations in which the surface
clogs.
• In addition, credit can be taken for the runoff
volume infiltrated from other impervious areas
using the methodology in Section 3.1.
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• For the purpose of sizing downstream
conveyance and structural control system,
porous asphalt surface areas can be estimated
as 35% impervious.
• The cross-section typically consists of two
layers. The aggregate reservoir can sometimes
be avoided or minimized if the sub-grade is
sandy and there is adequate time to infiltrate
the necessary runoff volume into the sandy soil
without by-passing the water quality volume. A
description for each of the layers is presented
below:
»» Porous Asphalt Layer – The porous
asphalt layer consists of a porous mixture
of asphalt. This layer is usually 4-8 inches
deep depending on the required bearing
strength, pavement design requirements, and
manufacturer’s specifications.
licensed engineer.Geotextiles consisting of
permeable materials should line the sides
of the aggregate base to prevent migration
of adjacent soils into it, which may lead to
permeability and storage capacity reduction.
»» Geotextiles consisting of permeable materials
should line the sides of the aggregate base
to prevent migration of adjacent soils into it,
which may lead to permeability and storage
capacity reduction.
Geotextiles are not recommended under
the aggregate base in an infiltration
design because they can accumulate fine
particulates that inhibit infiltration.
Stormwater Best Management Practices
• To protect groundwater from potential
contamination, runoff form designated hotspot
land uses or activities must not be infiltrated.
Porous asphalt should not be used for
manufacturing or industrial sites, where there
is potential for high concentrations of soluble
pollutants and heavy metals. In addition,
porous asphalt is not suitable in areas with karst
geology without adequate geotechnical testing
and approval by qualified individuals and in
accordance with local requirements.
»» Reservoir Layer –The reservoir gravel base
course consists of washed, bank-run gravel,
1.5-2.5 inches in diameter with a void space
of about 40% (GADOT No.3 Stone). The
depth of this layer depends on the desired
storage volume, which is typically the water
quality volume (WQv) at a minimum. Typical
depths for the reservoir layer range from
2-4 feet. Aggregate contaminated with soil
should not be used. A porosity value (void
space/total volume) of 0.40 should be used
in calculations unless aggregate-specific data
exist.
»» Filter Fabric – Filter fabric can be used
in certain applications, as site conditions
warrant. General guidance for the use of
filter fabrics is below. Actual use should
be under the guidance of a Georgia
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»» Underlying Soil – Porous asphalt should not generally be used in fill
soils. The underlying soil should have an infiltration capacity of at least
0.50 in/hr, as initially determined from NRCS soil textural classification,
and subsequently confirmed by field geotechnical tests. The minimum
geotechnical testing is one test hole per 5000 square feet, with a
minimum of two borings per facility (taken within the proposed limits
of the facility). Test borings are recommended to determine the soil
classification, seasonal high groundwater table elevation, and impervious
substrata, to produce an initial estimate of permeability. Often a doublering infiltrometer test is done at subgrade elevation to determine the
impermeable layer.
»» Pit excavation should be limited to the width and depth specified in the
design. Excavated material should be placed away from the open trench
as to not jeopardize the stability of the trench sidewalls. The bottom
of the excavated trench should not be loaded in a way that causes
compaction, but should be scarified prior to placement of reservoir base
material. The sides of the trench should be trimmed of all large roots.
The sidewalls should be uniform with no voids and scarified prior to
backfilling. All porous asphalt systems should be protected during site
construction and constructed after upstream areas have been stabilized.
»» An observation well consisting of perforated PVC pipe 4-6 inches in
diameter should be placed at the downstream end of the practice and
protected during construction. The well should be used to determine
actual infiltration rates for use in final design of the porous asphalt
system.
4.17.6 Design Procedures
(Step 1) Determine if the development site and conditions are appropriate for the use of the porous asphalt.
Consider the application and site feasibility criteria in this
chapter. In addition, determine if site conditions are suitable
for porous asphalt. Create a rough layout of the porous
asphalt dimensions taking into consideration existing trees,
utility lines, and other obstructions.
(Step 2) Determine the goals and primary function of the porous
asphalt.
Consider whether the porous asphalt is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
»» Provide a possible solution to a drainage problem
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
»» A warning sign should be placed at the facility that states, “Porous
pavement used on this site to reduce pollution. Do not resurface with
non-porous material. Call (XXX) XXX-XXXX for more information.”
watershed requirements that may apply. In addition, consid-
»» If OGFC is used, consult the GDOT Manual on Drainage Design for
Highways for design specifications.
other requirements that may apply or affect the design.
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Stormwater Best Management Practices
»» Geogrids may be used on top of subgrade soils for additional structural
support, especially in very weak, saturated soils. All manufacturer
requirements must be followed in design and installation.
er if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
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Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
tions/requirements, goals, targets, and primary function(s)
up the appropriate runoff reduction percentage (or credit)
of the BMP, described in this section. By considering the
provided by the practice:
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
Using the RRV calculated, determine the minimum Volume of
planting media, underdrain, inlet/outlet, overflow, etc.)
the Practice (VP)
(VPMIN) ≥ RRv (target) / (RR%)
Complete Step 3A, 3B, and 3C for a runoff reduction approach, or skip Step 3 and complete Steps 4A and 4B for a
water quality (treatment) approach. Refer to your local com-
Where:
munity’s guidelines for any additional information or specific
RR% = Runoff Reduction percentage, or credit,
requirements regarding the use of either method.
assigned to the specific practice
VPMIN = Minimum storage volume required to
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Calculate the Runoff Reduction Volume using the following
provide Runoff Reduction Target Volume (ft3)
Stormwater Best Management Practices
The design of the BMP should be centered on the restric-
RRv (target) = Runoff Reduction Target Volume (ft3)
formula:
RRV = (P) (RV) (A) / 12
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
RV = Volumetric runoff coefficient which can be
found by:
RV = 0.05+0.009(I)
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
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(Step 3C) Determine whether the minimum storage volume was met.
Pretreatment Volume
When the VP ≥ VPMIN, then the Runoff Reduction require-
To determine the actual volume provided in the porous as-
ments are met for this practice. Proceed to Step 5.
phalt, use the following equation:
When the VP < VPMIN, then the BMP must be sized according
VP = (VBL) * (N)
to the WQV treatment method (See Step 4).
(Step 4A) Calculate the Target Water Quality Volume
Where:
VP = Volume provided (temporary storage)
Calculate the Water Quality Volume using the following
VBL = Volume of Base Layer
formula:
N = Porosity
WQV = (1.2) (RV) (A) / 12
To determine the porosity, a qualified licensed professional
Where:
should be consulted to determine the proper porosity based
on the engineered soils used. Most gravel has a value of
WQV = Water Quality Volume (ft3)
0.40.
1.2 = Target rainfall amount to be treated (inches)
Stormwater Best Management Practices
(Step 3B) Determine the storage volume of the practice and the
RV = Volumetric runoff coefficient which can be
found by:
Provide pretreatment by using a grass filter strip or pea gravel
diaphragm, as needed, (sheet flow), or a grass channel or
forebay (concentrated flow). Where filter strips are used,
RV = 0.05+0.009(I)
100% of the runoff should flow across the filter strip. Pre-
treatment may also be desired to reduce flow velocities or
Where:
assist in sediment removal and maintenance. Pretreatment
I = new impervious area of the contributing
can include a forebay, weir, or check dam. Splash blocks
drainage area (%)
or level spreaders should be considered to dissipate con-
centrated stormwater runoff at the inlet and prevent scour.
A = Site area (total drainage area) (ft2)
Forebays should be sized to contain 0.1 inches per imper-
12 = Unit conversion factor (in/ft)
vious acre of contributing drainage. Refer to Section 4.9
for design criteria for a grass channel and Section 4.29 for
vegetated filter strips.
.
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288
the footprint of the infiltration practice and the Pretreat-
(Step 5) Size underdrain system (if applicable)
See Subsection 4.2.5.3 (Physical Specifications/Geometry)
ment Volume required.
The peak rate of discharge for the water quality design storm
4.17.7 Inspection and Maintenance Requirements
is needed for sizing of off-line diversion structures (see Sub-
All best management practices require proper maintenance. Without proper
section 3.1.7). If designing off-line, follow steps (a) through (d
maintenance, BMPs will not function as originally designed and may cease to
below):
function altogether. The design of all BMPs includes considerations for inspection and maintenance access. For additional information on inspection and
(a) Using WQv, compute CN
(b) Compute time of concentration using TR-55 method
(c) Determine appropriate unit peak discharge from time of
concentration
(d) Compute Qwq from unit peak discharge, drainage area,
maintenance requirements, see Appendix E.
Stormwater Best Management Practices
(Step 4B)If using the practice for Water Quality treatment, determine
and WQv.
To determine the minimum surface area of the pervious concrete, use the following formula:
Af = (WQv)(dpc + drl)/[(kpc * dpc * tpc)+(krl * drl * trl)]
Where:
Af = surface area of pervious (ft2)
WQv = Water Quality Volume (ft3)
dpc = porous asphalt depth (ft)
drl = reservoir layer depth (ft)
kpc = coefficient of permeability for porous asphalt
(ft/day)
krl = coefficient of permeability for reservoir layer
(ft/day)
tpc = drain time of porous asphalt (days)
trl = drain time of reservior layer (days)
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Description: Proprietary systems are manufactured stormwater BMPs and treatment systems
available from commercial vendors; these systems are designed to treat stormwater runoff and/
or provide water quantity control.
LID / GI Consideration: Proprietary systems may
be considered to be low impact development or
green infrastructure practices.
Note: It is the policy of this Manual not to recommend any specific commercial vendors for
proprietary systems. However, this subsection is
being included in order to provide communities
with a rationale for approving the use of a proprietary system or practice in their jurisdictions.
KEY CONSIDERATIONS
DESIGN CRITERIA
• Design criteria, such as drainage area, slope, soils, flow velocity,
storage area, permanent pool depth, and inlet/outlet considerations
are based on manufacturer recommendations.
ADVANTAGES / BENEFITS
• Proprietary systems can be chosen or designed for a site’s specific
stormwater runoff characteristics and/or design constraints.
• Often, proprietary systems are well-suited for use on urban
development sites where larger or above-ground BMPs are not an
option, or for stormwater retrofit projects.
• Can be used as pretreatment for other BMPs
• Can replace a conventional junction or inlet structure
• Some designs require minimal drop between inlet and outlet.
DISADVANTAGES / LIMITATIONS
• Dissolved pollutants may not be effectively removed by proprietary
systems.
• Proprietary systems may not achieve the 80% TSS removal target
alone.
• Performance is dependent on design and maintenance of individual
units.
ROUTINE MAINTENANCE REQUIREMENTS
• Maintenance requirements for a proprietary system should be
obtained from the manufacturer.
• Frequency of inspection and maintenance is dependent on land use,
climatological conditions, and the system’s design.
• Failure to provide adequate inspection and maintenance can result in
the re-suspension of accumulated solids.
• Ensure maintenance access to proprietary systems when designing
each site.
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STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.18 Proprietary Systems
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes
Soils: Proprietary systems can be installed in
almost any structure, soil, or terrain.
Other Considerations: Install as an off-line
device unless the system can be sized to handle a small drainage area.
RUNOFF REDUCTION CREDIT
• Runoff reduction is not typically provided by
proprietary systems.
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There are many types of commercially-available
4.18.2 Guidelines for Using Proprietary Systems
proprietary stormwater BMPs for both water qual-
A proprietary system should have a demonstrated
ity treatment and quantity control. These systems
ability to meet the stormwater management goals
Although local data is preferred, data from other
include:
and uses for which it is intended. This means that
regions can be accepted as long as the design
• Hydrodynamic systems such as gravity and
vortex separators
the system should provide:
1. Independent third-party scientific verification
of the ability of the proprietary system to
meet water quality treatment objectives and/
or to provide water quantity control (channel
or flood protection)
2. A proven record of longevity in the field
3. Proven ability to function in Georgia
conditions (e.g., climate, rainfall patterns, soil
types)
accounts for local conditions.
• Filtration systems
• Catch basin media inserts
• Chemical treatment systems
• Package treatment plants
• Prefabricated detention structures
Many proprietary systems are useful on small
For a propriety system to meet (1) above for water
sites and space-limited areas where there is
quality goals, the following monitoring criteria
not enough land or room for other stormwater
should be met for supporting studies:
treatment alternatives. Proprietary systems can
• Samples should be taken for at least 15 storm
events.
be used as pretreatment in a treatment train.
However, proprietary systems are often more
costly than other alternatives and may have high
maintenance requirements. Perhaps the largest
difficulty in using a proprietary system is the lack
of adequate independent performance data, particularly for use in Georgia conditions. Below are
general guidelines that should be followed before
considering the use of a proprietary commercial
system.
• The study should be independent or
independently verified (i.e., may not be
conducted by the vendor or designer without
third-party verification).
• The study should be conducted in the field, as
opposed to laboratory testing.
• Field monitoring should be conducted using
standard protocols that require proportional
sampling both upstream and downstream of
the device.
• The propriety system or device should have
been in place for at least one year at the time of
monitoring.
Local governments may submit a proprietary
system to further scrutiny based on the performance of similar practices. A poor performance
record or high failure rate is valid justification for
not allowing the use of a proprietary system or
device. Consult your local review authority for
more information regarding the use of proprietary
Stormwater Best Management Practices
4.18.1 General Description
systems.
As an example for a proprietary system evaluation
guideline, the Metropolitan North Georgia Water
Planning District has developed the Post-Construction Stormwater Technology Assessment
Protocol (PCSTAP).
Introduction
These guidelines, however, were not intended to
be testing protocols or procedures for evaluating the performance of a proprietary technology
or product. Furthermore, the lack of consistent
review and evaluation of monitoring and performance data has been a source of frustration for
local governments, vendors and the development
community.
• Concentrations reported in the study should be
flow-weighted.
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Stormwater treatment technologies and products
and approve emerging stormwater treatment
that have been tested according to this protocol
4.18.3 Inspection and Maintenance
Requirements
technologies, a consistent testing protocol and a
can receive consideration to have their results
All best management practices require proper
process for evaluating and accepting proprietary
evaluated and made available publicly on the Met-
maintenance. Without proper maintenance, BMPs
stormwater treatment systems is necessary.
ro Water District website (www.northgeorgiawa-
will not function as originally designed and may
ter.org). The review of vendor data and subse-
cease to function altogether. The design of all
The objective of this protocol is to provide local
quent determinations and public dissemination
BMPs includes considerations for inspection and
governments and other entities with an assess-
is not intended to be an approval process or an
maintenance access. For additional information
ment tool to use, if they so choose, as a starting
endorsement of any product by the Metropolitan
on inspection and maintenance requirements, see
point for evaluating a particular technology’s
North Georgia Water Planning District. Purchas-
Appendix E.
effectiveness in removing pollutants from storm-
ers and users of any technologies or products
water runoff for an intended application and to
presented by a manufacturer or other entity using
compare test results with vendor performance
this protocol should make their own independent
claims.
analyses and evaluations concerning the use-
Stormwater Best Management Practices
As local governments are being asked to review
fulness or value of any stormwater technologies
or combinations of technologies in considering
Purpose
whether to use any particular technology or prod-
This guidance document’s primary purpose is
uct for post-construction stormwater treatment.
to establish a testing protocol and process for
Local governments and other entities are free to
evaluating and reporting on the performance
use this information as part of their process to
and appropriate uses of proprietary stormwater
evaluate the suitability of these technologies or
treatment technologies and systems for address-
products.
ing post-construction stormwater runoff. It is
not intended for use in evaluation of erosion and
Note: The current protocol is only set-up for col-
sedimentation control technologies or products
lecting information on TSS removal. The proto-
for use during construction or land-disturbing
col may be updated in the future to also provide
activities. See the Georgia Soil & Water Conser-
information and evaluation on run-off reduction
vation Commission’s Manual for Sedimentation
volumes.
and Erosion Control in Georgia for information on
assessing E&S technologies.
For the latest protocol and other information, see:
www.northgeorgiawater.org.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Rainwater harvesting systems should be sized based on the size of the
contributing drainage area, local rainfall patterns and projected demand for
harvested rainwater
• Pretreatment should be provided upstream of all rainwater harvesting
systems to prevent leaves and other debris from clogging the system
• Georgia Rainwater Harvesting Guidelines (2009)
(Source: Jones and Hunt, 2008)
Description: Rainwater harvesting is the ancient
stormwater management practice of intercepting, diverting and storing stormwater runoff for
later use. In a typical rainwater harvesting system,
rainfall is collected from a gutter and downspout
system, screened and “washed” and conveyed
ADVANTAGES / BENEFITS
• May reduce water bill
• Helps restore pre-development hydrology on development sites and reduces
post-construction stormwater runoff rates, volumes and pollutant loads
• Can be used on almost any development site
• Reduces demand on public water supplies, which helps protect groundwater
aquifers from drawdown and seawater intrusion
• Allows beneficial reuse of stormwater
DISADVANTAGES / LIMITATIONS
• Stored rainwater should be used on a regular basis to maintain system
storage capacity
• A pump may be required if rainwater harvesting system is below ground
• Not aesthetically pleasing in some cases
• If the system is inside, plumbing codes may be required. See Georgia
Amendments to the International Plumbing Code (latest version)
• Should not be used with tar, gravel, and/or asbestos shingled roofs
into an above- or below-ground storage tank or
cistern. Once captured, stored water may be used
for non-potable indoor or outdoor uses. In some
cases, treatment may be required for indoor uses.
If properly designed, rainwater harvesting systems can significantly reduce post-construction
stormwater runoff rates, volumes and pollutant
loads on development sites. Rainwater harvesting
also helps reduce the demand on public water
supplies, which, in turn, helps protect aquatic
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Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
based on demand
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Other Considerations: Trees near
catchment area may cause clogging in
the system.
L=Low M=Moderate H=High
ROUTINE MAINTENANCE REQUIREMENTS
• Drain and clean out aboveground cistern
• Inspect health of vegetation receiving harvested rainwater to determine
watering needs
• Remove leaves and debris from grated and screened inlet
• Inspect for erosion around the overflow discharge and repair as necessary
• Check for algae growth inside the cistern; if found, treat water to remove the
algae
• Check pumping system to ensure it is working properly
RUNOFF REDUCTION CREDIT
• Varies based on demand
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
resources, such as groundwater aquifers, from
drawdown and seawater intrusion.
STORMWATER MANAGEMENT
SUITABILITY
Stormwater Best Management Practices
4.19 Rainwater Harvesting
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293
development review authority if they are interest-
Rainwater harvesting is the ancient stormwater
ed in using harvested rainwater for non-potable
management practice of intercepting, diverting
indoor uses. If indoor non-potable use is planned,
and storing rainfall for later use. In a typical rain-
filtration followed by disinfection is required, and
water harvesting system (Figure 4.19-1), rainfall is
there are additional system components not pic-
collected from a gutter and downspout system,
tured in Figure 4.19-1.
Stormwater Best Management Practices
4.19.1 General Description
screened, “washed,” and conveyed into an aboveor below-ground storage tank or cistern. Once
Whether it is used to supply water for non-po-
captured stored water may be used for non-pota-
table indoor or outdoor uses, a well-designed
ble indoor or outdoor uses. If properly designed,
rainwater harvesting system typically consists of
rainwater harvesting systems can significantly
five major components (Figure 4.19-2), including
reduce post-construction stormwater runoff
the collection and conveyance system (e.g., gut-
rates, volumes and pollutant loads on develop-
ter and downspout system), pretreatment devices
ment sites.
(e.g., leaf screens, first flush diverters, roof washers), the storage tank or cistern, the overflow pipe
There are two basic types of rainwater harvesting
(which allows excess stormwater runoff to bypass
systems:
the storage tank or cistern) and the distribution
1.
2.
Systems that are used to supply water
for non-potable outdoor uses, such as
landscape irrigation, car and building
washing and firefighting; and
Systems that are used to supply water for
non-potable indoor uses, such as laundry
and toilet flushing.
Figure 4.19-1: Rainwater Harvesting System
(Source: Rupp, 1998)
system (which may or may not require a pump,
depending on site characteristics). When designing a rainwater harvesting system, site planning
and design teams should consider each of these
components, as well as the size of the contributing drainage area, local rainfall patterns and
projected water demand, to determine how large
the cistern or storage tank must be to provide
Rainwater harvesting systems used to supply
enough water for the desired non-potable indoor
water for non-potable indoor uses are more
or outdoor use.
complex and require separate plumbing, pressure
tanks, pumps and backflow preventers. Additionally, the use of harvested rainwater for non-potable indoor uses may be restricted in some areas
of Georgia, due to existing “development rules.”
Developers and their site planning and design
teams are encouraged to consult with the local
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Figure 4.19-2: Major Components of a Rainwater
Harvesting System
(Source: Jones and Hunt, 2008)
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The Center for Watershed Protection (Hirschman
et al., 2008) recently documented the ability of
rainwater harvesting systems to reduce annual
stormwater runoff volumes and pollutant loads on
• Extreme Flood Protection
Proportionally adjust the post-development
runoff CN to account for the runoff reduction
provided by a rainwater harvesting system
when calculating the extreme peak discharge
(Qf) on a development site.
4.19.3 Pollutant Removal Capabilities
The amount of pollutant removal varies and is
based on the type of rainwater harvesting system
used.
For additional information on data and pollutant
removal capabilities for rainwater harvesting sys-
development sites. Consequently, this low impact
development practice has been assigned quanti-
Only 75% of the storage volume provided by a
tems, see the National Pollutant Removal Per-
fiable stormwater management “credits” that can
rainwater harvesting system can be subtracted
formance Database (Version 3) available at www.
be used to help satisfy the stormwater manage-
from the runoff reduction volume (RRv) that is
cwp.org, the National Stormwater Best Manage-
ment criteria presented in this manual:
captured by the system due to the fact that some
ment Practices (BMP) Database at www.bmpda-
of the harvested rainwater may not be used be-
tabase.org, and the Georgia Rainwater Harvesting
tween consecutive storm events.
Guidelines (2009).
• Runoff Reduction
Subtract 75% of the storage volume provided
by a rainwater harvesting system from the
runoff reduction volume (RRv) captured by the
system.
Stormwater Best Management Practices
4.19.2 Stormwater Management Suitability
In order to “receive” stormwater runoff and be
eligible for these “credits,” it is recommended that
rainwater harvesting systems satisfy the planning
• Water Quality
Through the achieved runoff reduction
volume provided by this practice, 100% of the
pollutants associated with the achieved RRV are
removed. Pollutant removal rates are otherwise
not provided for this practice.
and design criteria outlined below.
• Channel Protection
Proportionally adjust the post-development
runoff curve number (CN) to account for
the runoff reduction provided by a rainwater
harvesting system when calculating the
channel protection volume (CP v).
provide Runoff Reduction for Water Quality com-
4.19.4 Application and Site Feasibility
Criteria
The criteria listed in Table 4.19-1 should be
evaluated to determine whether or not a rainwa-
Credit for the volume of runoff reduced in the
ter harvesting system is appropriate for use on
rainwater harvesting system may be taken in the
a development site. It is important to note that
overbank flood protection and extreme flood pro-
rainwater harvesting systems have few constraints
tection calculations. If the practice is designed to
that impede their use on development sites.
pliance, then the practice is given credit for Channel Protection and Flood Control requirements by
allowing the designer to compute an Adjusted CN
(see Subsection 3.1.7.5 for more information).
• Overbank Flood Protection
Proportionally adjust the post-development
runoff CN to account for the runoff reduction
provided by a rainwater harvesting system
when calculating the overbank peak discharge
(Qp25) on a development site.
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Harvesting System on a Development Site
Site Characteristic
Criteria
Drainage Area
No restrictions
Area Required
Varies according to the size of the contributing drainage area and the dimensions
of the rain tank or cistern used to store the harvested rainwater.
Slope
No restrictions, although placing rainwater harvesting systems at higher elevations may reduce or eliminate pumping requirements.
Minimum Head
N/A
Minimum Depth to Water Table
N/A
Water Table
N/A
Soils
N/A
Site Applicability
Rainwater harvesting systems can be used on a
4.19.5 Planning and Design Criteria
wide variety of development sites in rural, subur-
The following criteria should be considered minimum standards for the design of a rainwater harvesting system. Consult with the local authority
to determine if there are any variations to these
criteria or additional standards that must be met.
ban and urban areas. They are especially well suited for use on commercial, institutional, municipal
and multi-family residential buildings on urban
and suburban development and redevelopment
sites. When compared with other low impact development practices, rainwater harvesting systems
have a moderate construction cost, a relatively
high maintenance burden and require a relatively
small amount of surface area. Although they can
be expensive to install, rainwater harvesting systems are often a component of “green buildings,”
(LEED) Green Building Rating System. Refer to the
Georgia Rainwater Harvesting Guidelines (2009)
document for a detailed discussion of beneficial
reuse and associated rule and requirements of
harvested rainwater.
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• Staff working on larger sites may use vehicles
to transport stormwater storage containers
to specific sites for watering outside of the
immediate vicinity of the collection area.
• Rain barrels (i.e., small storage tanks capable
of storing less than 100 gallons of stormwater
runoff) rarely provide enough storage
capacity to accommodate the stormwater
runoff volume generated by the target runoff
reduction rainfall event. Consequently, they
should not be used as part of a rainwater
harvesting system, except on small drainage
areas of 2,500 square feet or less in size.
4.19.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
Rainwater harvesting systems should be designed to provide at least enough storage for
4.19.5.1 LOCATION AND LAYOUT
the stormwater runoff volume generated by the
Rainwater harvesting systems may be installed on
target runoff reduction rainfall event (e.g., 85th
nearly any development site. However, placing
percentile rainfall event). The required size of a
storage tanks or cisterns at higher elevations may
rainwater harvesting system is governed by sev-
reduce or eliminate pumping requirements
eral factors, including the size of the contributing
drainage area, local rainfall patterns and projected
such as those that achieve certification in the
Leadership in Energy and Environmental Design
use gravity to feed watering hoses through a
tap and spigot arrangement.
Stormwater Best Management Practices
Table 4.19-1: Factors to Consider When Evaluating the Overall Feasibility of Using a Rainwater
demand for harvested rainwater. Site planning
4.19.5.2 GENERAL DESIGN
and design teams should calculate the projected
• Distribution systems may be gravity fed or
may include a pump to provide the energy
necessary to convey harvested rainwater
from the storage tank to its final destination.
Rainwater harvesting systems used to provide
water for non-potable outdoor uses typically
water demand and then conduct water balance
calculations, based on the size of the contributing
drainage area and local precipitation data, to size
a rainwater harvesting system.
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provided by North Carolina State University
(NCSU, 2008) at: http://www.bae.ncsu.edu/
topic/waterharvesting, can be used to design a
rainwater harvesting system, provided that the
precipitation data being used in the model reflects
local rainfall patterns and distributions and has
been approved by the local development review
authority prior to use.
4.19.5.4 PRETREATMENT/INLETS
Pretreatment is needed to remove debris, dust,
leaves and other materials that accumulate on
rooftops, and may cause clogging within a rainwater harvesting system. Pretreatment devices
that may be used include leaf screens, roof washers and first-flush diverters, each of which are
described briefly below:
• Leaf Screens: Leaf screens are mesh screens
installed either in the gutter or downspout
that are used to remove leaves and other large
debris from rooftop runoff. Leaf screens must
be regularly cleaned to work effectively. If
not regularly maintained, they can become
clogged and prevent rainwater from flowing
into the storage tank.
• First Flush Diverters: First flush diverters direct
the initial pulse of stormwater runoff away from
the storage tank and into an adjacent pervious
area. While leaf screens effectively remove
larger debris such as leaves and twigs from
harvested rainwater, first flush diverters can be
used to remove smaller contaminants such as
dust, pollen and animal feces.
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• Roof Washers: Roof washers are placed just
ahead of storage tanks and are used to filter
small debris from the harvested rainwater.
Roof washers consist of a small tank, usually
between 25-50 gallons in size, with leaf
strainers and filters with openings as small as
30 microns (TWDB, 2005). The filter functions
to remove very small particulate matter from
harvested rainwater. All roof washers must be
cleaned on a regular basis. Without regular
maintenance, they may not only become
clogged and prevent rainwater from entering
the storage tank, but may become breeding
grounds for bacteria and other pathogens.
Rooftop drainage systems (e.g., gutter and downspout systems) should be designed as they would
• All overflow pipes should be directed away
from buildings to prevent damage to building
foundations
4.19.5.6 SAFETY FEATURES
• Storage tanks (also known as cisterns) are the
most important and often the most expensive
component of a rainwater harvesting system.
Storage tanks can be constructed from a
variety of materials, including wood, plastic,
fiberglass, concrete, and galvanized metal. Site
planning and design teams should choose an
appropriate cistern for the intended application
and should ensure that it has been sealed with
a water safe, non-toxic substance.
Stormwater Best Management Practices
A rainwater harvesting model, such as the one
be for a building designed without a rainwater
harvesting system. Drainage system components
leading to the cistern should have a minimum
slope of 2% to ensure that harvested rainwater is
actually conveyed into the storage tank.
4.19.5.5 OUTLET STRUCTURES
• An overflow pipe should be provided to allow
stormwater runoff to bypass the storage tank
or cistern when it reaches its storage capacity.
The overflow pipe should have a conveyance
capacity that is equal to or greater than that
of the inflow pipe and should direct excess
stormwater runoff to another low impact
development practice, such as a vegetated filter
strip (Section 4.29), grass channel (Section 4.9)
or bioretention areas (Section 4.2).
• All storage tanks should be opaque or
otherwise protected from direct sunlight to
inhibit algae growth. They should also be
screened to discourage mosquito breeding and
reproduction, but should be accessible for use,
cleaning, inspection and maintenance.
• The quality of harvested rainwater will vary
according to the rooftop material. For example,
water harvested from certain types of rooftops,
such as those constructed of asphalt, tar, gravel
and treated wood shingles, should be avoided
or only be used for non-potable outdoor uses,
as these materials may leach toxic compounds
into stormwater runoff.
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Landscaping requirements for the rainwater
harvesting system is generally minimal. Rainwater harvesting systems typically drain to planted
vegetation as a method of watering. Vegetation
should be non-invasive and native plants. During
dry periods, additional watering may be necessary
to keep plants healthy. Check with local jurisdictions for any restrictions on using harvested
rainwater.
4.19.5.8 CONSTRUCTION CONSIDERATIONS
To help ensure that rainwater harvesting systems
are successfully installed on a development site,
Stormwater Best Management Practices
4.19.5.7 LANDSCAPING
site planning and design teams should consider
the following recommendations:
• Rainwater harvesting systems may be installed
on development and redevelopment sites after
building rooftops and their drainage systems
(e.g., gutter and downspout systems) have been
constructed.
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(Step 1) Determine if the development site and conditions are ap-
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
propriate for the use of rainwater harvesting
Consider the application and site feasibility criteria in this
If the rainwater harvesting system will be used to supply wa-
chapter. In addition, determine if site conditions are suit-
ter for toilet and/or urinal flushing, provide filtering and treat-
able for rainwater harvesting. Create a rough layout of the
ment in accordance with the Georgia Rainwater Harvesting
rainwater harvesting dimensions taking into consideration
Guidelines (2009) and the plumbing features in accordance
existing trees, utility lines, and other obstructions.
with the Georgia Amendments to the International Plumbing
Code (latest version).
(Step 2) Determine the goals and primary function of rainwater harvesting.
Complete Step 3A, 3B, and 3C for a runoff reduction ap-
Consider whether the rainwater harvesting is intended to:
»» Meet a runoff reduction* target. For information on the
sizing of a BMP utilizing the runoff reduction approach,
see Step 3A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
proach. Refer to your local community’s guidelines for any
additional information or specific requirements regarding the
use of either method.
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
Calculate the Runoff Reduction Volume using the following
formula:
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
»» Provide a possible solution to a drainage problem
RRV = (P) (RV) (A) / 12
Where:
RRV = Runoff Reduction Target Volume (ft3)
»» Enhance landscape and provide aesthetic qualities
P = Target runoff reduction rainfall (inches)
Check with local officials and other agencies to determine if
RV = Volumetric runoff coefficient which can be
there are any additional restrictions and/or surface water or
found by:
watershed requirements that may apply. In addition, consider if the best management practice has any special site-spe-
RV = 0.05+0.009(I)
cific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
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Stormwater Best Management Practices
4.19.6 Design Procedures
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
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(Step 5) Determine the number and size of the systems
practice should be determined by the local jurisdiction. As
Based on the volume calculated in Step 4, determine site
guidance, if the demand for the harvested rainwater volume
constrains, cistern costs, and appropriate size of the rainwa-
(VPDemand) is ≥ VP and VP ≥ VPMIN, then the RR% should 100%.
ter harvesting system.
Using the RRV calculated above, determine the minimum
Volume of the Practice (VP)
(Step 6) Size the overflow structure
Calculate the peak roof runoff rate to each downspout for
the 2-, 10-, and 100-year storm events. Size the overflow for
(VPMIN) ≥ RRv (target) / (RR%)
each cistern so that it can pass the larger storm events. The
overflow should be designed in such a way that the dis-
Where:
charge does not cause any erosion or scour.
RR% = Runoff Reduction percentage, or credit,
(Step 7) Select and size treatment components, as needed, when the
assigned to the specific practice
VPMIN = Minimum storage volume required to
harvested rainwater will be used to supply water for toilet
provide Runoff Reduction Target Volume (ft )
and/or urinal flushing.
3
Stormwater Best Management Practices
The runoff reduction percentage (or credit) provided by the
RRv (target) = Runoff Reduction Target Volume (ft3)
(Step 3B) Determine the storage volume of the practice and the
4.19.7 Maintenance Requirements
Pretreatment Volume
All best management practices require proper maintenance. Without proper
The actual volume provided (VP) for rainwater harvesting is
maintenance, BMPs will not function as originally designed and may cease to
equal to the size of the rainwater harvesting tank.
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
(Step 3C) Determine whether the minimum storage volume was met
and maintenance requirements, see Appendix E.
When VPDemand ≥ VPMIN and and VP ≥ VPMIN, then the Runoff
Reduction requirements are met.
(Step 4) Determine the drainage area from the roof and required RRV
to each downspout
Assess roof to determine the percentage of the entire roof
and the specific area of the roof that drains to each downspout. Use this area along with the RRV calculation in Step 3
to estimate the volume for each downspout.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Ideally, channel slopes should be less than 10%; however, RSC designs
can be adapted for slopes exceeding 10%.
• Natural drainage patterns should be matched as closely as possible.
• Stabilize the outfall from the RSC to receiving waters.
• RSCs can be designed to receive runoff from up to 50 acres, however
typical drainage areas range from 10-30 acres.
• The storage volume of the pools should be large enough and include
storage volume above the seasonally high groundwater table so that
the practice is not inundated by groundwater.
• Pools should drain to their design (ponding) levels within 72 hours
from the end of a storm event.
• The outlet of the storage facility should be sloped to prevent
conditions that promote standing water.
• RSC should not be used to treat runoff from hot spots.
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Description: Regenerative stormwater conveyance (RSC) provides treatment, infiltration and
conveyance through the combination of sand,
wood chips, native vegetation, riffles (with either
cobble rocks or boulders), and shallow pools.
RSCs are designed to convey water while minimizing the effects of erosion.
LID/GI Considerations: By repairing and restoring eroded and damaged drainage channels by
ADVANTAGES / BENEFITS
• When designed correctly, RSCs are safe and aesthetically pleasing and
may potentially increase the natural value of the site.
• RSC systems provide high total suspended solids and soluble
pollutant removal rates.
• Effective at restoring highly eroded areas such as outfalls, ditches,
and channels.
DISADVANTAGES / LIMITATIONS
• RSC is a new type of BMP and significant data is not available making
performance of the BMP uncertain.
• RSCs are unfamiliar to most designers due to the recent use of this
practice.
• Design of RSC is an iterative practice.
• Construction of an RSC system can be time and labor intensive.
volume treatment for total suspended solids dissolved pollutants, while also providing mitigation
of thermal impacts of stormwater runoff.
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Not recommended
Roadway Projects: Yes
Soils: RSC systems can be installed in hydrologic soil group A, B, C, or D soils.
Other Considerations: Vegetation is essential
to the function of RSC systems. The use of
native plants is recommended.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
mimicking natural drainage patterns, regenerative
stormwater conveyance provide water quality
Stormwater Best Management Practices
4.20 Regenerative Stormwater
Conveyance
ROUTINE MAINTENANCE REQUIREMENTS
• There are intensive maintenance requirements during establishment
of the practice
• Check for erosion or “end-cutting” of weirs and riffle structures and
stable water levels in pools.
• Remove invasive vegetation.
• Bare or eroding areas in the contributing drainage area or around the
RSC channel should be immediately stabilized.
• Remove and replace dead plants.
• Once every 2 to 3 years, remove accumulated sediment in pools.
• Prune and weed the practice four times per year.
• 0% Runoff Reduction Credit is provided by
this practice
POLLUTANT REMOVAL
Total Suspended Solids
Nutrients - Total Phosphorus / Total
Nitrogen removal
Metals - Cadmium, Copper, Lead, and
Zinc removal
Pathogens – Fecal Coliform
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Regenerative stormwater conveyance (RSC)
systems are BMPs that are designed to restore incised and eroded channels, ditches, and intermittent (ephemeral) streams. They are constructed
with a series of shallow pools, riffles, cascades,
weirs, and outfalls that dissipate stormwater
runoff energy and allow for temporary ponding,
internal storage, and infiltration. The temporary
ponding area of a RSC provides settling time
for total suspended solids. The wood chip/sand
Figure 4.20-1 Typical RSC Profile
(Source: Anne Arundel County, Maryland, June 2009)
layer provides filtration as well as an environment
conducive to the growth of microorganisms that
degrade hydrocarbons and organic material. Both
woody and herbaceous plants in the ponding area
provide vegetative uptake of runoff and pollutants, and also serve to stabilize the surrounding
soils. See Figure 4.20-1 for a design schematic of
a regenerative stormwater conveyance system.
For additional information about the design, construction and pollutant removal provided by RSC
systems, visit Anne Arundel County: http://www.
4.20.2 Stormwater Management Suitability
• Runoff Reduction
RRSC systems are not designed to provide
runoff reduction. The substrate provides storage
and gradual release, especially of ‘first flush’
events. RSC systems must be installed as part of
a treatment train with other BMPs that provide
runoff reduction, if it is desired.
aacounty.org/DPW/Watershed/StepPoolStormConveyance.cfm.
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• Water Quality
If installed as per the recommended design
criteria and properly maintained, 80% total
suspended solids removal can be applied to
the water quality volume (WQv) flowing to
the RSC system. In addition, RSC is effective
at removing 70% of the total phosphorus
and nitrogen found in the water. RSC is not
effective at removing metal or pathogens from
the water.
Stormwater Best Management Practices
4.20.1 General Description
• Channel Protection
RSC systems do not provide channel
protection. Another BMP should be used in a
treatment train with RSC systems to provide
channel protection or runoff reduction. (See
Subsection 4.1.6.)
• Overbank Flood Protection
RSC systems do not provide overbank flood
protection. Another BMP should be used in a
treatment train with RSC systems to provide
overbank flood protection or runoff reduction.
• Extreme Flood Protection
RSC systems do not provide extreme flood
protection. Another BMP should be used in a
treatment train with RSC systems to provide
extreme flood protection or runoff reduction.
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- Regional Stormwater Control – NO
Regenerative stormwater conveyance systems
-- 10 feet from property lines
are presumed to be able to remove 80% of the
Physical Feasibility – Physical Constraints at
total suspended solids (TSS) load in typical urban
Project Site
post-development runoff when sized, designed,
• Drainage Area – 50 acres or less, but typically
10-30 acres
constructed, and maintained in accordance with
the recommended specifications. Other pollutants that RSC systems can remove include Phosphorus and Nitrogen. Metals, including Cadmium,
Copper, Lead, and Zinc, and Pathogens, such as
Fecal Coliform, are not presumed to be removed
by a RSC system. See Table 4.1.2-3 for pollutant
removal rates.
4.20.4 Application and Site Feasibility
Criteria
Regenerative stormwater conveyance systems
are usually used to retrofit or repair an existing
channel. They can be designed to receive stormwater runoff from up to 50 acres, usually highly
• Site Slope – Drainage channel slopes should
be 10% or less.
• Minimum Depth to Water Table – Shallow
ponding areas should include storage volume
above the seasonally high groundwater table
to allow for temporary ponding in a majority
of the pools and storage of the water quality
volume.
• Soils – RSC systems can be installed in
hydrologic soil group A, B, C or D soils.
construction projects and roadway designs when
Other Constraints/Considerations
site conditions allow. Although an RSC system
• Hot spots – RSC systems should not be used
for hot spot runoff.
runoff, they are not considered for control of
the CPv, Qp25, and Qf. Designers should consider
the importance of native herbaceous and woody
vegetation in the function of an RSC system, and
ensure that the development area can support the
-- 100 feet from private water supply wells
-- 200 feet from public water supply
reservoirs (measured from edge of water)
-- 1,200 feet from public water supply wells
• Space Required – A rough rule of thumb is that
RSC systems comprise approximately 0.5–3%
of the contributing drainage area.
impervious. They can also be designed for new
can receive relatively high volume and rates of
-- 10 feet from building foundations
• Trout Stream –The ponding and settling
functions provided by RSC systems allow for a
reduction of the thermal impacts and pollutant
loads of runoff from highly urbanized areas.
Coastal Areas
• Poorly Drained Soils and Shallow Water Table
— RSC systems may be installed in any soil
type and where there is a shallow water table
as long as the shallow pools of an RSC system
drain to the designed ponding levels within 72
hours of a rain event.
Stormwater Best Management Practices
4.20.3 Pollutant Removal Capabilities
• Flat Terrain – Adequate slope from inlet to
outfall of the RSC system must be provided to
generate flow.
• Damage to existing structures and facilities
– Ensure that runoff through the RSC system
is conveyed in a safe, non-erosive manner to
minimize damage to existing structures and
facilities.
necessary landscaping.
General Feasibility
- Suitable for Residential Subdivision Usage – YES
• Proximity – The following is a list of specific
setback requirements for the location of a
regenerative stormwater conveyance system:
- Suitable for High Density/Ultra Urban Areas – NO
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4.20.5.1 LOCATION AND LAYOUT
Before designing the regenerative stormwater
RSC systems are best used to restore ecologi-
conveyance system, assess site conditions to
cal functions to an existing eroded ditch, outfall,
determine its applicability. Check with the local
channel, or ephemeral stream. RSC systems
stormwater authority to ensure that RSC is appli-
are designed for intermittent flow and must be
cable to the particular site. The following informa-
allowed to drain and reaerate between rainfall
tion is needed prior to initiating design:
events.
• Existing and proposed site and location maps
and field reviews
• Topographic map with 1-foot minimum
contour intervals
• Existing and proposed impervious and pervious
areas
• Dimensions and profiles of existing drainage
channels for retrofit projects
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Design data from nearby storm sewer
structures
• Average and 10 and 100 year water surface
elevation of nearby water systems and depth to
the seasonally high groundwater
• Infiltration testing of native soils at the
proposed bottom elevation of the RSC system
4.20.5.2 GENERAL DESIGN
• A RSC consists of the following:
1. A sequence of pools, riffles, and cascades to
assist in treating, detaining, and conveying
storm flow.
2. Organic/mulch layer to protect planting
media.
3. Native soils to infiltrate the treated runoff,
(see description of infiltration trenches,
Section 4.12, for infiltration criteria).
4. A grade control structure and settling pool
should be used if the slope of the channel is
greater than 5%.
• A RSC design may include some of the
following:
»» Pretreatment maybe required to keep
sediment and large debris out of the practice.
»» A series of riffle cobbles and boulders,
shallow pools, and sand and wood chips.
4.20.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Recommended total length of grade control
structures and pools is less than 10 feet.
• The invert of the upstream elevation of the
grade control structure should be 1 foot higher
than the elevation of the downstream grade
control structure.
• The width of the grade control structure should
be 8-20 times the depth of the grade control
structure. 10 feet preferred.
• The West Virginia design manual recommends
using the following equation to determine the
length of the grade control structure for RSC:
Stormwater Best Management Practices
4.20.5 Planning and Design Criteria
Where:
LGCS = Length of grade control structure
Lpool=Length of pool
LRSC Path=Length of the RSC flow path
∆E=Change in elevation of the RSC
practice.
The following criteria are to be considered
minimum standards for the design of a regenerative stormwater conveyance system. Consult with
the local review authority to determine if there
are any variations to these criteria or additional
standards that must be met.
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Table 4.20-1: Cobble Diameter Based on Flow Velocity
Cobble Diameter, inches
Allowable velocity (ft/s)
• Cascades should have a maximum slope of
2H:1V, a maximum vertical drop of 5 feet, and
followed by three pools instead of the usual
one.
4
5.8
5
6.4
6
6.9
7
7.4
• Pool widths should be greater than the width of
the grade control structure.
8
7.9
9
8.4
10
8.8
11
9.2
12
9.6
15
10.4
• Sand layer should be a mixture of sand and
wood chips with a ratio of 4:1. This layer should
run along the length of the RSC system. To
stabilize the sand layer, 1 foot of bank-run
gravel should be placed below the sand layer.
A 1 foot layer of gravel should be placed on top
of the sand layer to stabilize the grade control
structures. Maximum width of the sand bed is
14 feet.
• The velocity of the water going through the
pool should be less than 4 ft/s.
• The mulch layer should consist of 3 to 4 inches
of triple-shredded hardwood mulch. This
provides additional benefits such as removing
sediment and metals and retaining soil
moisture.
• Footer boulders should be inserted 6 inches
lower than the invert of the pool.
• If the native soils are not suitable for planting,
then an engineered soil mix should be provided
that meets the following specifications:
»» Texture: Sandy loam or loamy sand
»» Infiltration Rate: Soils should have an
infiltration rate of at least 0.50 inches per
hour (in/hr), although an infiltration rate of
between 2 and 4 in/hr is preferred
»» Sand Content: Soils should contain 35%-60%
clean, washed sand
»» Phosphorus Index (P-Index): Soils should
have a P-Index of less than 30
»» Topsoil Content: Soils should contain 20%30% topsoil
»» Exchange Capacity (CEC): Soils should have
a CEC that exceeds 10 milliequivalents (meq)
per 100 grams of dry weight
»» Organic Matter Content: Soils should
contain 10%-25% organic matter
»» Clay: Soils should contain less than 15%
Stormwater Best Management Practices
• Four inches should be the maximum depth of
flow going over the grade control structure.
»» pH: Soils should have a pH of 6-8
»» For additional information on the soils for a
Regenerative Stormwater Conveyance, refer
to Appendix D.
• Flow velocity going through the RSC should
be less than the maximum allowable velocity
for the cobble size that was selected, use Table
4.20-1 to size the cobble stones based on the
velocity of flow.
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A grass filter strip or channel can be used for
pretreatment. The length of the grass channel
or width of the grass filter strip depends on the
drainage area, land use, and channel slope. Design guidance on grass channels for pretreatment
can be found in Section 4.9 (Grass Channel) and
filter strips can be found in Section 4.29 (Vegetated Filter Strip).
• The RSC should be vegetated to resemble a
terrestrial forest ecosystem, with a mature
tree canopy, subcanopy of understory trees,
shrub layer, and herbaceous ground cover.
Three species each of trees, shrubs, and grass/
herbaceous species should be planted to avoid
creating a monoculture.
• Woody vegetation should not be specified at
inflow locations.
• Plants should be installed prior to mulch.
4.20.5.5 OUTLET STRUCTURES
The outlet of the RSC should end with an outlet
pool with a grade control structure just upstream
of the outlet pool. The outlet pool elevation
should match the existing grade.
• Choose plants based on factors such as
whether they are native or not, resistance
to drought and inundation, cost, aesthetics,
maintenance, etc. Planting recommendations
for RSCs are as follows:
• A dense and vigorous vegetative cover should
be established over the contributing pervious
drainage areas before runoff can be accepted
into the facility. Otherwise sediment from the
stormwater runoff will clog the pores in the
planting media and native soils.
»» Vegetation should be selected based on a
specified zone of hydric tolerance.
RSC generally does not require any special safety
features, and fencing the RSC facility is not gener-
»» A selection of trees with an understory of
shrubs and herbaceous materials should be
provided.
ally desired.
• Landscaping is critical to the performance
and function of the RSC; the vegetation filters
and transpires runoff and the root systems
encourage infiltration.
• Construction equipment should be restricted
from the regenerative stormwater conveyance
system to prevent compaction of the native
soils.
»» Native plant species should be specified over
non-native species.
4.20.5.6 SAFETY FEATURES
4.20.5.7 LANDSCAPING
4.20.5.8 CONSTRUCTION CONSIDERATIONS
Stormwater Best Management Practices
4.20.5.4 PRETREATMENT/INLETS
Additional information and guidance on the appropriate woody and herbaceous species appropriate for RSC in Georgia, and their planting and
establishment, can be found in
.
• Vegetation should be selected to match the
look and maintenance effort desired by the
local community and those responsible for
maintaining the facility.
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(Step 1) Determine if the development site and conditions are appropriate for the use of a RSC.
(Step 3) Calculate the Target Water Quality Volume
Calculate the Water Quality Volume using the following
formula:
Consider the application and site feasibility criteria in this
WQV = (1.2) (RV) (A) / 12
chapter. In addition, determine if site conditions are suitable
for a RSC. Create a rough layout of the RSC dimensions
taking into consideration existing trees, utility lines, and other
Where:
WQV = Water Quality Volume (ft3)
obstructions.
1.2 = Target rainfall amount to be treated (inches)
RV = Volumetric runoff coefficient which can be
(Step 2) Determine the goals and primary function of the RSC.
found by:
Consider whether the RSC is intended to:
»» Meet the water quality (treatment) target. For
information on the sizing of the BMP utilizing the water
quality treatment approach, see Step 3.
»» Provide a possible solution to a drainage problem
»» Enhance landscape and provide aesthetic qualities
RV = 0.05+0.009(I)
Stormwater Best Management Practices
4.20.6 Design Procedures
Where:
I = new impervious area of the contributing
drainage area (%)
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
A = Area draining to the practice (ft2)
watershed requirements that may apply. In addition, consid-
12 = Unit conversion factor (in/ft)
er if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
Using the WQV calculated above, determine the actual size
and Volume of the Practice (VP) as shown in Step 4. Note
that VPTOTAL, calculated in Step 4, must be greater than or
equal to WQV.
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Complete Step 3 for a water quality (treatment) approach.
Refer to your local community’s guidelines for any additional
information or specific requirements regarding the use of
this method.
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treatment Volume.
The total volume provided (VP) is calculated by the following
equation:
VPTOTAL = VPSAND + Σ VPPOOLS
Where:
VPPOOLS = Volume provided in the pools throughout
the RSC system
VPOOL = Volume of a single storage pool
(Step 5) Determine whether the WQv was met.
When the VPTotal ≥ WQv, then proceed to Step 6.
When the VPTotal < WQv, then proceed to Step 7.
Where:
VPTOTAL = Total volume provided by the RSC system
VPSAND = Volume provided in the sand layer
VPPOOLS = Volume provided in the pools throughout the RSC system
To determine the volume provided for the sand layer in the
RSC, use the following equation:
VPSAND = (VSB)* (N)
(Step 6) Design grade control structure, pools, and cascades as described in Subsection 4.20.5. Design and Planning Criteria.
(Step 7) Prepare Vegetation and Landscaping Plan.
A landscaping plan for a dry or wet swale should be prepared
to indicate how the RSC system will be stabilized and estab-
Stormwater Best Management Practices
(Step 4) Determine the storage volume of the practice and the Pre-
lished with vegetation.
See Subsection 4.20.5.7 (Landscaping) and Appendix D for
more details.
Where:
VPSAND = Volume provided in sand layer
VSB = Volume of Sand Bed
N = Porosity
4.20.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
To determine the porosity, a qualified licensed professional
function altogether. The design of all BMPs includes considerations for main-
should be consulted to determine the proper porosity based
tenance and maintenance access. For additional information on inspection
on the sand used. Most sand has a porosity value of 0.40.
and maintenance requirements, see Appendix E.
To determine the volume provided for the shallow pools in
the RSC, use the following equation:
VPPOOLS = (VPOOL_1)+ (VPOOL_2)+ (VPOOL_3)+…
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Typically requires 2 to 6 feet of head, depending on the type of sand
filter
• Maximum contributing drainage area of 10 acres for surface sand
filter; 2 acres for perimeter sand filter
• Sand filter media with underdrain system
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
ADVANTAGES / BENEFITS
• Applicable to small drainage areas
• Good for highly impervious areas
• Good retrofit capability
Extreme Flood Protection
suitable for this practice
may provide partial benefits
Description: Multi-chamber structure designed to
treat stormwater runoff through filtration, using a
sediment forebay, a sand bed as its primary filter
DISADVANTAGES / LIMITATIONS
• High maintenance burden
• Not recommended for areas with high sediment content in
stormwater or clay/silt runoff areas
• Relatively costly
• Possible odor problems
media and, typically, an underdrain collection
LID/GI Consideration: Sand filters have low land
MAINTENANCE REQUIREMENTS
• Inspect for clogging – rake first inch of sand
• Remove sediment from forebay and chamber
• Replace sand filter media as needed
requirement and may be incorporated to comple-
POLLUTANT REMOVAL
system.
ment the natural landscape.
Total Suspended Solids
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Metals - Cadmium, Copper,
Lead, and Zinc removal
Pathogens – Fecal Coliform
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.21 Sand Filters
Capital Cost
Maintenance Burden
Residential Subdivision Use: No
High Density/Ultra-Urban: Yes
Drainage Area: 2-10 acres max.
Soils: No restrictions
Other Considerations: Typically needs
to be combined with other controls to
provide water quantity control
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 0% of the runoff reduction volume
provided
• Accepts Hotspot Runoff: Yes (requires
impermeable liner)
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Sand filters (also referred to as filtration basins)
are BMPs that capture and temporarily store
stormwater runoff and pass it through a filter
bed of sand. Most sand filter systems consist of
two-chamber structures. The first chamber is
a sediment forebay or sedimentation chamber,
which removes floatables and heavy sediments.
The second is the filtration chamber, which removes additional pollutants by filtering the runoff
through a sand bed. The filtered runoff is typically
collected and returned to the conveyance system,
though it can also be partially or fully exfiltrated
into the surrounding soil in areas with porous
soils.
• Perimeter Sand Filter – The perimeter sand
filter is an enclosed filter system typically
constructed just below grade in a vault
along the edge of an impervious area such
as a parking lot. The system consists of a
sedimentation chamber and a sand bed filter.
Runoff flows into the structure through a series
of inlet grates located along the top of the
control.
A third design variant, the underground sand filter,
is intended primarily for extremely space limited
Surface Sand Filter
and highly dense areas and is thus only considered when local communities allow. Underground
sand filters require additional planning for access,
maintenance, and incorporation with the storm-
Stormwater Best Management Practices
4.21.1 General Description
water management plan.
Because they have few site constraints beside
head requirements, sand filters can be used
on development sites where the use of other
structural controls may be precluded. However,
sand filter systems can be relatively expensive to
construct and maintain.
There are two primary sand filter system designs,
Perimeter Sand Filter
the surface sand filter and the perimeter sand filter. Below are descriptions of these filter systems:
Figure 4.21-1 Sand Filter Examples
• Surface Sand Filter – The surface sand filter
is a ground-level open air structure that
consists of a pretreatment sediment forebay
and a filter bed chamber. This system can
treat drainage areas up to 10 acres in size and
is most commonly located off-line. Surface
sand filters can be designed as an excavation
with earthen embankments or as a concrete or
block structure.
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Sand filter systems are designed primarily as
off-line systems for stormwater quality (i.e., the
removal of stormwater pollutants) and will typically need to be used in conjunction with another
BMP to provide downstream channel protection,
overbank flood protection, and extreme flood
protection, if required. However, under certain
circumstances, filters can provide limited runoff
quantity control, particularly for smaller storm
events. If used for smaller drainage areas, or used
to provide limited quantity control, sand filters
could be used as an on-line system provided
proper design planning for erosion and scour is
considered.
• Runoff Reduction Volume
Another BMP should be used in a treatment
train with sand filters to provide runoff
reduction as they are not designed to provide
RRV as a stand-alone BMP.
• Water Quality
In sand filter systems, stormwater pollutants
are removed through a combination of
gravitational settling, filtration and adsorption.
The filtration process effectively removes
suspended solids and particulates, biochemical
oxygen demand (BOD), fecal coliform bacteria,
and other pollutants. Surface sand filters with
a grass cover have additional opportunities for
bacterial decomposition as well as vegetation
uptake of pollutants, particularly nutrients.
Subsection 4.21.3 provides median pollutant
removal efficiencies that can be used for
planning and design purposes.
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• Channel Protection
For smaller sites, a sand filter may be designed
to capture the entire channel protection
volume CP v in either an off- or on-line
configuration. Given that a sand filter system
is typically designed to completely drain over
40 hours, the time requirement of extended
detention of the 1-year, 24-hour storm runoff
volume will be met. For larger sites –or–
where only the WQv is diverted to the sand filter
facility, another structural control must be used
to provide CP v extended detention.
• Overbank Flood Protection
Another BMP must be used in conjunction
with a sand filter system to reduce the postdevelopment peak flow of the 25-year, 24hour storm (Qp) to pre-development levels
(detention).
The following design pollutant removal rates are
conservative average pollutant reduction percentages for design purposes derived from sampling
data, modeling and professional judgment. In
a situation where a removal rate is not deemed
sufficient, additional controls may be put in place
at the given site in a series or “treatment train”
approach.
-- Total Suspended Solids – 80%
-- Total Phosphorus – 50%
-- Total Nitrogen – 25%
-- Fecal Coliform – 40%
-- Heavy Metals – 50%
Stormwater Best Management Practices
4.21.2 Stormwater Management Suitability
For additional information and data on pollutant removal capabilities for sand filters, see the
National Pollutant Removal Performance Database (Version 3) available at www.cwp.org and the
• Extreme Flood Protection
Sand filter facilities must provide flow diversion
and/or be designed to safely pass extreme
storm flows and protect the filter bed and
facility.
National Stormwater Best Management Practices
(BMP) Database at www.bmpdatabase.org.
4.21.3 Pollutant Removal Capabilities
Both the surface and perimeter sand filters are
presumed to be able to remove 80% of the total
suspended solids load in typical urban post-development runoff when sized, designed, constructed
and maintained in accordance with the recommended specifications. Undersized or poorly
designed sand filters can reduce TSS removal
performance.
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General Feasibility
Other Constraints/Considerations
- Suitable for Residential Subdivision Usage – NO
Sand filter systems are well suited for highly
- Suitable for High Density/Ultra Urban Areas – YES
• Aquifer Protection – Do not allow exfiltration
of filtered hotspot runoff into groundwater
impervious areas where land available for BMPs is
- Regional Stormwater Control – NO
• Landscaping -
limited. Sand filters should primarily be considered for new construction or retrofit opportunities
for commercial, industrial, and institutional areas
Physical Feasibility - Physical Constraints at
where the sediment load is relatively low, such
Project Site
as: parking lots, driveways, loading docks, gas
• Drainage Area – 10 acres maximum for surface
sand filter; 2 acres maximum for perimeter
sand filter
stations, garages, airport runways/taxiways, and
storage yards. Sand filters may also be feasible
and appropriate in some multi-family or higher
density residential developments.
To avoid rapid clogging and failure of the filter
media, the use of sand filters should be avoided in
areas with less than 50% impervious cover, or high
sediment yield sites with clay/silt soils. Special
attention should also be given to the topsoil and
sod layers, if being used (See Figure 4.21-2).
The following basic criteria should be evaluated
to ensure the suitability of a sand filter facility for
meeting stormwater management objectives on a
site or development.
• Space Required – Function of available head
at site
• Site Slope – No more than 6% slope across
filter location
• Minimum Head – Elevation difference needed
at a site from the inflow to the outflow: 5 feet
for surface sand filters; 2 to 3 feet for perimeter
sand filters
❏❏Check with your local review authority to
see if the planning of a grass cover or turf
over a sand filter is allowed.
❏❏Do not plant trees or provide shade within
15 feet of filtering area or where leaf litter
will collect and clog filtering area.
❏❏Do not locate plants to block maintenance
access to the facility.
❏❏Sod areas with heavy flows that are not
stabilized with erosion control mats.
❏❏Divert flows temporarily from seeded areas
until stabilized.
❏❏Planting on any area requiring a filter fabric
should include material selected with care
to insure that no tap roots will penetrate
the filter fabric.
Stormwater Best Management Practices
4.21.4 Application and Site Feasibility
Criteria
• Minimum Depth to Water Table – For a surface
sand filter with exfiltration (earthen structure),
2 feet are required between the bottom of the
sand filter and the elevation of the seasonally
high water table
• Soils – No restrictions; Group “A” soils generally
required to allow exfiltration (for surface sand
filter earthen structure)
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The following criteria are to be considered minimum standards for the design of a sand filter
facility. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be followed.
4.21.5.1 LOCATION AND LAYOUT
• Surface sand filters should have a contributing
drainage area of 10 acres or less. The
maximum drainage area for a perimeter sand
filter is 2 acres.
Stormwater Best Management Practices
4.21.5 Planning and Design Criteria
• Sand filter systems are generally applied
to land uses with a high percentage of
impervious surfaces. Sites with less than 50%
imperviousness or high clay/silt sediment loads
must not use a sand filter without adequate
pretreatment due to potential clogging and
failure of the filter bed. Any disturbed areas
within the sand filter facility drainage area
should be identified and stabilized. Filtration
controls should only be constructed after the
construction site is stabilized.
• Surface sand filters are generally used in an
off-line configuration where the water quality
volume (WQv) is diverted to the filter facility
through the use of a flow diversion structure
and flow splitter. Stormwater flows greater
than the WQv are diverted to other controls or
downstream using a diversion structure or flow
splitter.
Figure 4.21-2 Typical Sand Filter Media Cross Sections
(Source: GDOT Drainage Manual, 2014)
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• Sand filter systems are designed for intermittent
flow and must be allowed to drain and reaerate
between rainfall events. They should not be
used on sites with a continuous flow from
groundwater, sump pumps, or other sources.
4.21.5.2 GENERAL DESIGN
• Surface Sand Filter
A surface sand filter facility consists of a
two-chamber open-air structure, which is
located at ground-level. The first chamber
is the sediment forebay (a.k.a sedimentation
chamber) while the second chamber
houses the sand filter bed. Flow enters the
sedimentation chamber where settling of larger
sediment particles occurs. Runoff is then
discharged from the sedimentation chamber
through a perforated standpipe into the
filtration chamber. After passing though the
filter bed, stormwater runoff is collected by a
perforated pipe and gravel underdrain system.
Figure 4.21-3 provides plan view and profile
schematics of a surface sand filter.
• Perimeter Sand Filter
A perimeter sand filter facility is a vault
structure located just below grade level.
Runoff enters the device through inlet
grates along the top of the structure into the
sedimentation chamber. Runoff is discharged
from the sedimentation chamber through a
weir into the filtration chamber. After passing
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though the filter bed, runoff is collected by a
perforated pipe and gravel underdrain system.
Figure 4.21-4 provides plan view and profile
schematics of a perimeter sand filter.
• Underground Sand Filter
An underground sand filter is in an
underground vault structure designed for highdensity land use or ultra-urban applications
where there is not enough space for a surface
sand filter or other BMP. Due to its location
below the surface, underground sand filters
have a high maintenance burden and should
only be used where adequate inspection and
maintenance can be ensured. Figure 4.21-5
provides plan view and profile schematics of an
underground sand filter.
4.21.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Surface Sand Filter
»» The entire treatment system (including the
sedimentation chamber) must temporarily
hold at least 75% of the WQv prior to filtration.
Figure 4.21-6 illustrates the distribution of
the treatment volume (0.75 WQv) among
the various components of the surface sand
filter, including
-- Vs – volume within the sedimentation
basin
-- As – the surface area of the sedimentation
basin
-- Af – surface area of the filter media
-- hs – height of water in the sedimentation
basin
-- hf – average height of water above the
filter media
-- df – depth of filter media
»» The sedimentation chamber must be sized to
at least 25% of the computed WQv and have
a length-to-width ratio of at least 2:1. Inlet
and outlet structures should be located at
opposite ends of the chamber.
Stormwater Best Management Practices
• Perimeter sand filters are typically sited along
the edge, or perimeter, of an impervious area
such as a parking lot.
»» The filter area is sized based on the principles
of Darcy’s Law. A coefficient of permeability
(k) of 3.5 ft/day for sand should be used. The
filter bed is typically designed to completely
drain in 40 hours or less.
»» The filter media consists of an 18-inch layer
of clean washed medium sand (meeting
ASTM C-33 concrete sand or GDOT Fine
Aggregate Size No. 10) on top of the
underdrain system. Three inches of topsoil
are placed over the sand bed. Permeable
filter fabric is placed both above and below
the sand bed to prevent clogging of the sand
filter and the underdrain system. Figure
4.21-2 illustrates a typical media cross
section.
-- Vf – volume within the voids in the filter
bed
-- Vf-temp – temporary volume stored above
the filter bed
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Stormwater Best Management Practices
Figure 4.21-3 Schematic of Surface Sand Filter
(Source: Center for Watershed Protection)
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Figure 4.21-4 Schematic of Perimeter Sand Filter
(Source: Center for Watershed Protection)
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Stormwater Best Management Practices
Figure 4.21-5 Schematic of Underground Sand Filter
(Source: Center for Watershed Protection
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Figure 4.21-6 Surface Sand Filter Volumes
Source: GDOT Drainage Manual, 2014
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»» The structure of the surface sand filter may
be constructed of impermeable media
such as concrete, or through the use of
excavations and earthen embankments.
When constructed with earthen walls/
embankments, filter fabric should be
used to line the bottom and side slopes
of the structures before installation of the
underdrain system and filter media.
• Perimeter Sand Filter
»» The entire treatment system (including the
sedimentation chamber) must temporarily
hold at least 75% of the WQv prior to
filtration. Figure 4.21-7 illustrates the
distribution of the treatment volume (0.75
WQv) among the various components of the
perimeter sand filter, including:
-- Vw – wet pool volume within the
sedimentation basin
-- Vf – volume within the voids in the filter
bed
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-- Vtemp – temporary volume stored above the
filter bed
Aggregate contaminated with soil shall not
be used.
-- As – the surface area of the sedimentation
basin
-- Af – surface area of the filter media
-- hf – average height of water above the filter
media (1/2 htemp)
-- df – depth of filter media
»» The sedimentation chamber must be sized to
at least 50% of the computed WQv.
4.21.5 PRETREATMENT/INLETS
• Pretreatment of runoff in a sand filter system is
provided by the sedimentation chamber.
• Inlets to surface sand filters are to be provided
with energy dissipators. Exit velocities from the
sedimentation chamber must be nonerosive.
»» The filter area is sized based on the principles
of Darcy’s Law. A coefficient of permeability
(k) of 3.5 ft/day for sand should be used. The
filter bed is typically designed to completely
drain in 40 hours or less.
• Figure 4.21-8 shows a typical inlet pipe from
the sedimentation basin to the filter media
basin for the surface sand filter.
»» The filter media should consist of a 12- to
18-inch layer of clean washed medium sand
(meeting ASTM C-33 concrete sand or GDOT
Fine Aggregate Size No. 10) on top of the
underdrain system. Figure 4.21-2 illustrates a
typical media cross section.
4.21.5.5 OUTLET STRUCTURES
»» The perimeter sand filter is typically equipped
with a minimum 4 inch perforated PVC
pipe (AASHTO M 252) underdrain in a gravel
layer. The underdrain must have a minimum
grade of 1/8 inch per foot (1% slope). Holes
should be 3/8-inch diameter and spaced
approximately 6 inches on center. A
permeable filter fabric should be placed
between the gravel layer and the filter media.
Gravel should be clean washed aggregate
with a maximum diameter of 3.5 inches and
a minimum diameter of 1.5 inches with a
void space of about 40% (GDOT No.3 Stone).
Stormwater Best Management Practices
»» The filter bed is typically equipped with
a minimum 6-inch perforated PVC pipe
(AASHTO M 252) underdrain in a gravel
layer. The underdrain must have a minimum
grade of 1/8-inch per foot (1% slope). Holes
should be 3/8-inch diameter and spaced
approximately 6 inches on center. Gravel
should be clean washed aggregate with
a maximum diameter of 3.5 inches and a
minimum diameter of 1.5 inches with a void
space of about 40% (GDOT No.3 Stone).
Aggregate contaminated with soil shall not
be used.
Outlet pipe is to be provided from the underdrain
system to the facility discharge. Due to the slow
rate of filtration, outlet protection is generally
unnecessary (except for emergency overflows
and spillways).
4.21.5.6 EMERGENCY SPILLWAY
An emergency or bypass spillway must be included in the surface sand filter to safely pass flows
that exceed the design storm flows. The spillway
prevents filter water levels from overtopping the
embankment and causing structural damage. The
emergency spillway should be located so that
downstream buildings and structures will not be
impacted by spillway discharges.
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Figure 4.21-7 Perimeter Sand Filter Volumes
(Source: GDOT Drainage Manual, 2014)
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Figure 4.21-8 Surface Sand Filter Perforated Stand-Pipe
(Source: GDOT Drainage Manual, 2014)
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Adequate access must be provided for all sand
4.21.5.10 ADDITIONAL SITE-SPECIFIC DESIGN
CRITERIA AND ISSUES
filter systems for inspection and maintenance,
including the appropriate equipment and vehicles.
Physiographic Factors - Local terrain design
Access grates to the filter bed need to be included
constraints
in a perimeter sand filter design. Facility designs
• Low Relief – Use of surface sand filter may be
limited by low head
must enable maintenance personnel to easily
replace upper layers of the filter media.
4.21.5.8 SAFETY FEATURES
Surface sand filter facilities can be fenced to pre-
• High Relief – Filter bed surface must be level
• Karst – Use polyliner or impermeable
membrane to seal bottom of earthen surface
sand filter or use watertight structure
vent access. Inlet and access grates to perimeter
sand filters may be locked.
Soils
Stormwater Best Management Practices
4.21.5.7 MAINTENANCE ACCESS
No restrictions
4.21.5.9 LANDSCAPING
Surface sand filters can be designed with a grass
cover to aid in pollutant removal and prevent
clogging. The grass should be capable of withstanding frequent periods of inundation and
drought. The sand filter is covered with permeable
topsoil and planted with grass in a landscaped
area. Properly planted, these facilities can be
designed to blend into natural surroundings. Vegetated filter strips and buffers should fit into and
Special Downstream Watershed Considerations
• Trout Stream – Evaluate for stream warming;
use shorter drain time (24 hours) or consider
increasing media filter depth
• Aquifer Protection – Use polyliner or
impermeable membrane to seal bottom of
earthen surface sand filter or use watertight
structure; no exfiltration of filter runoff into
groundwater
blend with surrounding area. Native grasses are
preferable, if compatible.
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(Step 1) Determine if the development site and conditions are appropriate for the use of a sand filter.
(Step 3) Calculate the Target Water Quality Volume
Calculate the Runoff Reduction Volume using the following
formula:
Consider the application and site feasibility criteria in this
WQV = (1.2) (RV) (A) / 12
chapter. In addition, determine if site conditions are suitable
for a sand filter. Create a rough layout of the sand filter dimensions taking into consideration existing trees, utility lines,
Where:
WQV = Water Quality Volume (ft3)
and other obstructions.
1.2 = Target rainfall amount to be treated (inches))
RV = Volumetric runoff coefficient which can be
(Step 2) Determine the goals and primary function of the sand filter.
found by:
Consider whether the sand filter is intended to:
»» Meet a water quality (treatment) target. See Step 3
to size the BMP utilizing the water quality treatment
approach.
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
RV = 0.05+0.009(I)
Stormwater Best Management Practices
4.21.6 Design Procedures
Where:
I = new impervious area of the contributing
drainage area (%)
»» Provide a possible solution to a drainage problem
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
A = Area draining to the practice (ft2)
12 = Unit conversion factor (in/ft)
(Step 4) Compute WQv peak discharge (Qwq)
The peak rate of discharge for the water quality design storm is
needed for sizing of off-line diversion structures (see Subsection
3.1.7). If designing off-line, follow steps (a) through (d) below:
(a) Using WQv, compute CN
(b) Compute time of concentration using TR-55 method
(c) Determine appropriate unit peak discharge from time of
concentration
(d) Compute Qwq from unit peak discharge, drainage area, and WQv
Complete Step 3 for a water quality (treatment) approach.
Refer to your local community’s guidelines for any additional
information or specific requirements regarding the use of
either method.
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(Step 7) Size sedimentation chamber.
A flow regulator (or flow splitter diversion structure) should
Surface sand filter: The sedimentation chamber should
be supplied to divert the WQv to the sand filter facility.
be sized to at least 25% of the computed WQv and have a
Size low flow orifice, weir, or other device to pass Qwq.
length-to-width ratio of 2:1. The Camp-Hazen equation is
used to compute the required surface area:
(Step 6) Size filtration basin chamber.
As = – (Qo/w) * Ln (1-E)
The filter area is sized using the following equation (based on
Darcy’s Law):
Where:
Af = (WQv)(df) / [(k)(hf + df) (tf)]
As = sedimentation basin surface area (ft2)
Qo = rate of outflow = the WQv over a 24-hour
Where:
period (ft3/s)
Af = surface area of filter bed (ft )
w = particle settling velocity (ft/sec)
df = filter bed depth (typically 18 in, no more than 24 in)
E = trap efficiency
2
Stormwater Best Management Practices
(Step 5) Size flow diversion structure, if needed.
k = coefficient of permeability of filter media
(ft/day) use 3.5 ft/day for sand)
hf = average height of water above filter bed (ft)
(1/2 hmax, which varies based on site but hmax
is typically ≤ 6 feet)
tf = design filter bed drain time (days), (1.67 days or
40 hours is recommended maximum)
Set preliminary dimensions of filtration basin chamber.
Assuming:
»» 90% sediment trap efficiency (0.9)
»» Particle settling velocity (ft/sec) = 0.0033 ft/sec for
imperviousness ≥ 75%
»» Particle settling velocity (ft/sec) = 0.0004 ft/sec for
imperviousness < 75%
»» Average of 24 hour holding period
See Subsection 4.21.5.3 (Physical Specifications/Geometry)
for filter media specifications.
Then:
As = (0.066) (WQv) ft2 for I < 75%
As = (0.0081) (WQv) ft2 for I > 75%
Set preliminary dimensions of sedimentation chamber.
Perimeter sand filter: The sedimentation chamber should
be sized to at least 50% of the computed WQv. Use same
approach as for surface sand filter.
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Vmin = 0.75 * WQv
(Step 9) Compute storage volumes within entire facility and sedimentation chamber orifice size.
Surface sand filter:
Vmin = 0.75 WQv = Vs + Vf + Vf-temp
1. Compute Vf = water volume within filter bed/gravel/
pipe = Af * df * n, where: n = porosity = 0.4 for most
applications
2. Compute Vf-temp = temporary storage volume above the
filter bed = 2 * hf * Af
3. Compute Vs = volume within sediment chamber = Vmin
- Vf - Vf-temp
4. Compute hs = height in sedimentation chamber = Vs/As
5. Ensure hs and hf fit available head and other dimensions
still fit – change as necessary in design iterations until
all site dimensions fit.
6. Size orifice from sediment chamber to filter chamber to
release Vs within 24-hours at average release rate with
0.5 hs as average head.
7. Design outlet structure with perforations allowing for a
safety factor of 10.
8. Size distribution chamber to spread flow over filtration
media – level spreader weir or orifices.
Perimeter sand filter:
1. Compute Vf = water volume within filter bed/gravel/pipe
= A f * df * n
2. Where: n = porosity = 0.4 for most applications
3. Compute Vw = wet pool storage volume As * 2 feet
minimum
4. Compute Vtemp = temporary storage volume = Vmin – (Vf + Vw)
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5. Compute htemp = temporary storage height = Vtemp / (Af + As)
6. Ensure htemp ≥ 2 * hf, otherwise decrease hf and recompute. Ensure dimensions fit available head and area
– change as necessary in design iterations until all site
dimensions fit.
7. Size distribution slots from sediment chamber to filter
chamber.
(Step 10) Design inlets, pretreatment facilities, underdrain system,
and outlet structures
See Subsection 4.21.5.4 through 4.21.5.8 for more details.
(Step 11) Compute overflow weir sizes.
Surface sand filter:
1. Size overflow weir at elevation hs in sedimentation
chamber (above perforated stand pipe) to handle
surcharge of flow through filter system from 25-year
storm (see example).
2. Plan inlet protection for overflow from sedimentation
chamber and size overflow weir at elevation hf in
filtration chamber (above perforated stand pipe) to
handle surcharge of flow through filter system from 25year storm (see example).
Stormwater Best Management Practices
(Step 8) Compute Vmin
Perimeter sand filter: Size overflow weir at end of sedimentation chamber to handle excess inflow, set at WQv elevation.
See Appendix B-3 for a Sand Filter Design Example
4.21.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Ideal for use in pervious areas that have been disturbed by clearing,
grading and other land disturbing activities
• Methods used for site reforestation/revegetation should achieve at
least 75% vegetative cover one year after installation
• Reforested/revegetated areas should be protected in perpetuity as
secondary conservation areas
ADVANTAGES / BENEFITS
• Applicable to small drainage areas
• Good for highly impervious areas
• Good retrofit capability
Description: Site reforestation/revegetation refers
to the process of planting trees, shrubs and other
tions. The process can be used to help establish
mature native plant communities (e.g., forests) in
LID/GI Consideration: Restoring sites back to
their pre-developed conditions improves their
Water Quality
1
Channel Protection
1
Overbank Flood Protection
1
Extreme Flood Protection
suitable for this practice
may provide partial benefits
DISADVANTAGES / LIMITATIONS
• High maintenance burden
• Not recommended for areas with high sediment content in
stormwater or clay/silt runoff areas
• Relatively costly
• Possible odor problems
MAINTENANCE REQUIREMENTS
• Inspect for clogging – rake first inch of sand
• Remove sediment from forebay and chamber
• Replace sand filter media as needed
pervious areas that have been disturbed by clearing, grading and other land disturbing activities.
Runoff Reduction
1
1
native vegetation in disturbed pervious areas to
restore them to their pre-development condi-
STORMWATER MANAGEMENT SUITABILITY
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.22 Site Reforestation/Revegetation
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
POLLUTANT REMOVAL
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
• 0% of the runoff reduction volume
provided (see note 1)
ability to reduce post-construction stormwater
runoff rates, volumes and pollutant loads. The
process can also be used to provide restored habitat for high priority plant and animal species.
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1 = helps restore pre-development hydrology, which implicitly reduces post-construction stormwater runoff rates, volumes
and pollutant loads. Runoff reduction credit is given to other BMPs that use soil restoration to improve hydrologic soil groups.
See guidance below for calculating RRV and other reductions.
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another and with other primary and secondary
Site reforestation/revegetation refers to the
conservation areas through the use of nature
process of planting trees, shrubs and other native
trails, bike trails and other “greenway” areas.
Stormwater Best Management Practices
4.22.1 General Description
vegetation in disturbed pervious areas to restore
them to their pre-development conditions (Figure
mature native plant communities (e.g., forests)
4.22.2 Stormwater Management Suitability
in pervious areas that have been disturbed by
The Center for Watershed Protection (Hirschman
clearing, grading and other land disturbing activ-
et al., 2008) documented the ability of the site re-
ities. Mature plant communities intercept rainfall,
forestation/revegetation process to reduce annual
increase evaporation and transpiration rates, slow
stormwater runoff volumes and pollutant loads on
and filter stormwater runoff and help improve
development sites. Consequently, this low impact
soil porosity and infiltration rates (Cappiella et al.,
development practice can be used to help satisfy
2006a), which leads to reduced post-construction
the reduce runoff volume and provide water qual-
stormwater runoff rates, volumes and pollutant
ity improvements:
4.22-1). The process can be used to help establish
loads. The site reforestation/revegetation process
can also be used to provide restored habitat for
high priority plant and animal species.
Areas that have been reforested or revegetated
should be maintained in an undisturbed, natural
state over time. These areas should be designated
as conservation areas and protected in perpetuity
through a legally enforceable conservation instrument (e.g., conservation easement, deed restriction). If properly maintained over time, these areas
can help improve aesthetics on development
sites, provide passive recreational opportunities
and create valuable habitat for high priority plant
and animal species.
• Runoff Reduction
Site reforestation/revegetation is an effective
low impact development (LID) practice that
can reduce post-construction stormwater
runoff and improve water quality. When used
to improve site areas and create conservation
Figure 4.22-1: Active Replanting of Native Trees
in a Disturbed Pervious Area
(Source: Center for Watershed Protection)
amenities; runoff reduction, lower postdeveloped flow rates, and lower discharge
of any reforested/revegetated areas from the
velocities are all benefits of reforestation or
total site area and re-calculate the water quality
revegetation. Subtract 50% of any reforested/
volume (WQv) that applies to the development
revegetated areas from the total site area and
site.
re-calculate the runoff reduction volume (RRv)
that applies to the development site.
• Channel Protection
Assume that the post-development hydrologic
• Water Quality Protection
conditions of any reforested/revegetated areas
Site reforestation and/or revegetation
are equivalent to those of a similar cover type
To help create contiguous, interconnected green
helps restore pre-development hydrology,
(e.g., meadow, brush, woods) in fair condition.
infrastructure corridors on development sites,
which implicitly reduces post-construction
site planning and design teams should strive to
stormwater runoff rates in addition to runoff
connect reforested or revegetated areas with one
volumes and pollutant loads. Subtract 50%
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324
• Extreme Flood Protection
Assume that the post-development hydrologic
conditions of any reforested/revegetated areas
are equivalent to those of a similar cover type
(e.g., meadow, brush, woods) in fair condition.
Reforested/revegetated areas can only be
assumed to be in “fair” hydrologic condition
due to the fact that it will take many years for
them to mature and provide full stormwater
management benefits.
Stormwater Management Suitability when combined with Soil Restoration
• Channel Protection
Assume that the post-development hydrologic
conditions of any restored and reforested/
revegetated areas are equivalent to those of a
similar cover type (e.g., meadow, brush, woods)
in good condition.
Site Applicability
• Overbank Flood Protection
Assume that the post-development hydrologic
conditions of any restored and reforested/
revegetated areas are equivalent to those of a
similar cover type (e.g., meadow, brush, woods)
in good condition.
sites in rural and suburban areas. When compared
• Extreme Flood Protection
Assume that the post-development hydrologic
conditions of any restored and reforested/
revegetated areas are equivalent to those of a
similar cover type (e.g., meadow, brush, woods)
Although it may be difficult to apply in urban
areas, due to space constraints, site reforestation/
revegetation can be used on a wide variety of
development sites, including residential, commercial, industrial and institutional development
with other low impact development practices, it
has a moderate construction cost, a relatively low
maintenance burden and requires no additional
surface area beyond that which will undergo the
reforestation/revegetation process. It is ideal for
use in pervious areas that have been disturbed
by clearing, grading and other land disturbing
Stormwater Best Management Practices
• Overbank Flood Protection
Assume that the post-development hydrologic
conditions of any reforested/revegetated areas
are equivalent to those of a similar cover type
(e.g., meadow, brush, woods) in fair condition.
activities.
in good condition.
If site reforestation/revegetation can be combined
with soil restoration (Section 4.23) on a development site, the following stormwater management
benefits and incentives are available to help satisfy
the requirements presented in this manual:
• Stormwater Runoff Reduction
Subtract 100% of any restored and reforested/
revegetated areas from the total site area and
re-calculate the runoff reduction volume (RRv)
that applies to the development site.
• Water Quality Protection
Subtract 100% of any restored and reforested/
revegetated areas from the total site area and
re-calculate the water quality volume (WQv)
that applies to the development site.
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4.22.3 Applications and Site Feasibility
Criteria
The criteria listed in Table 4.22-1 should be evaluated to determine whether or not site reforestation/revegetation is appropriate for use on a
development site.
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas – YES
- Regional Stormwater Control – NO
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Revegetation on a Development Site
Site Characteristic
Criteria
Drainage Area
N/A
Area Required
Reforested/revegetated areas should at least be 10,000 square feet in size in order
to be eligible for the stormwater management “credits” assigned to this low impact
development practice (2008, Center for Watershed Protection – Runoff Reduction
Technical Memo).
Slope
Maximum 25% in the disturbed pervious area to be reforested/ revegetated.
Minimum Head
N/A
Minimum Depth to
Water Table
No restrictions
Soils
Soils need to be capable of sustaining the vegetation proposed which will require
significant amendments in most cases.
4.22.4 Planning and Design Criteria
Landscaping
It is recommended that the reforestation/re-
• A soil test should be performed to determine
what type of vegetation can be supported
by the soils in the area to be reforested/
revegetated and/or what soil amendments will
be required.
vegetation process used on a development site
meet all of the following criteria to be eligible for
the stormwater management “credits” described
above:
General Planning and Design
• Reforested/revegetated areas should have a
contiguous area of 10,000 square feet or more.
• Reforested/revegetated areas should not
be disturbed after construction (except for
disturbances associated with landscaping or
removal of invasive vegetation).
• Reforested/revegetated areas should be
protected, in perpetuity, from the direct
impacts of the land development process by
a legally enforceable conservation instrument
(e.g., conservation easement, deed restriction).
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• Methods used for site reforestation/
revegetation should achieve at least 75
percent vegetative cover one year after
installation.
• A long-term vegetation management plan
should be developed for all reforested/
revegetated areas. The plan should clearly
specify how the area will be maintained
in an undisturbed, natural state over time.
Plan should include method for watering
during plant establishment period of one
to two years. Turf management is not
considered to be an acceptable form of
vegetation management. Consequently, only
reforested/revegetated areas that remain in
an undisturbed, natural state are eligible for
this “credit” (i.e., pervious areas consisting of
managed turf are not eligible for this “credit”).
Stormwater Best Management Practices
Table 4.22-1: Factors to Consider When Evaluating the Overall Feasibility of Using Site Reforestation/
• A landscaping plan should be prepared
by a qualified licensed professional for all
reforested/revegetated areas. The landscaping
plan should be reviewed and approved by the
local development review authority prior to
construction.
• Landscaping commonly used in site
reforestation/revegetation efforts includes
native trees, shrubs and other herbaceous
vegetation. Because the goal of the site
reforestation/revegetation process is to
establish a mature native plant community
(e.g., forest), managed turf cannot be used to
landscape reforested/revegetated areas.
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326
• Construction contracts should contain a
To help ensure that the site reforestation/reveg-
replacement warranty that covers at least three
etation process is successfully completed on a
growing seasons to help ensure adequate
development site, site planning and design teams
growth and survival of the vegetation planted
should consider the following recommendations:
on the reforested/revegetated area.
Document the condition of the reforested/revegetated area before, during and after the completion of the site reforestation/revegetation process.
• Document the condition of the reforested/
4.22.6 Inspection and Maintenance
Requirements
revegetated area before, during and after
All best management practices require prop-
the completion of the site reforestation/
er maintenance. Without proper maintenance,
revegetation process.
BMPs will not function as originally designed and
• To help prevent soil compaction, heavy
vehicular and foot traffic should be kept out of
all reforested/revegetated areas before, during
and after construction. This can typically be
accomplished by clearly delineating reforested/
may cease to function altogether. The design of
all BMPs includes considerations for maintenance
Stormwater Best Management Practices
4.22.5 Construction Considerations
and maintenance access. For additional information on inspection and maintenance requirements, see Appendix E.
revegetated areas on all development plans
and, if necessary, protecting them with
temporary construction fencing.
• Simple erosion and sediment control measures,
such as temporary seeding and erosion
control mats, should be used on reforested/
revegetated areas. If the reforested/revegetated
areas will “receive” any stormwater runoff
from other portions of the development site,
measures should be taken (e.g., silt fence,
temporary diversion berm) to prevent it from
compromising the reforestation/ revegetation
effort.
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327
KEY CONSIDERATIONS
DESIGN CRITERIA
• Ideal for use in pervious areas that have been disturbed by clearing,
grading and other land disturbing activities
• To properly restore disturbed pervious areas, soil amendments
should be added to existing soils to a minimum depth of 18 inches
until an organic matter content of 8% to 12% is obtained. Depths
greater than 18” should be amended when shrubs or trees are being
installed.
• Restored pervious areas should be protected from future land
disturbing activities
STORMWATER MANAGEMENT SUITABILITY
1
Runoff Reduction
1
Water Quality
1
Channel Protection
1
Overbank Flood Protection
1
Extreme Flood Protection
suitable for this practice
ADVANTAGES / BENEFITS
• Helps restore pre-development hydrology on development sites and
reduces post-construction stormwater runoff rates, volumes and
pollutant loads
• Promotes plant growth and improves plant health, which helps
reduce stormwater runoff rates, volumes and pollutant loads
(Source: http://www.towncountryltd.com)
Description: Soil restoration refers to the process
of tilling and adding compost and other amendments to soils to restore them to their pre-de-
DISADVANTAGES / LIMITATIONS
• Should not be used on areas that have slopes of greater than 10%
• To help prevent soil erosion, landscaping should be installed
immediately after the soil restoration process is complete
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.23 Soil Restoration
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Yes
L=Low M=Moderate H=High
velopment conditions, which improves their
ability to reduce post-construction stormwater
runoff rates, volumes and pollutant loads. The soil
restoration process can be used to improve the
hydrologic conditions of pervious areas that have
been disturbed by clearing, grading and other
RUNOFF REDUCTION CREDIT
• 0% of the runoff reduction volume
provided (see note 1)
1 = helps restore pre-development hydrology, which implicitly reduces post-construction stormwater runoff rates, volumes
and pollutant loads. See guidance below for calculating RRV and other reductions.
land disturbing activities.
LID/GI Consideration: Increases the reduction in
stormwater runoff rates, volumes and pollutant
loads provided by other low impact development
practices.
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Stormwater Best Management Practices
4.23.1 General Description
Soil restoration refers to the process of tilling
and adding compost and other amendments to
soils to restore them to their pre-development
conditions. It is ideal for use on lawns and other
pervious areas that have been disturbed by clearing, grading and other land disturbing activities.
Organic compost (Figure 4.23-1) and other
amendments can be tilled into soils in these areas
to help create healthier, uncompacted soil matrices that have enough organic matter to support
a diverse community of native trees, shrubs and
other herbaceous plants.
Soil restoration can also be used to increase the
stormwater management benefits provided by
other low impact development practices, such
as site reforestation/revegetation (Section 4.22),
vegetated filter strips (Section 4.29), grass channels (Section 4.9) and simple downspout disconnection (Section 4.4), on sites that have soils with
low permeabilities (i.e., hydrologic soil group C or
D soils). The soil restoration process can be used
to help increase soil porosity and improve soil
infiltration rates on these sites, which improves
the ability of these and other low impact development practices to reduce post-construction
stormwater runoff rates, volumes and pollutant
loads.
4.23.2 Stormwater Management Suitability
The Center for Watershed Protection (Hirschman
et al., 2008) documented the ability of the soil
restoration process to reduce annual stormwater
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Figure 4.23-1: Organic Compost
(Source: http://www.organicgardeninfo.com)
opment sites. Consequently, this low impact
development practice can be used to help satisfy
the stormwater requirements presented in this
manual. Consider including soil restoration in
all other stormwater management BMPs where
applicable.
• Runoff Reduction
Soil restoration is one of the most effective
low impact development (LID) practices that
can be combined with other BMPs to reduce
post-construction stormwater runoff and
improve runoff quality. Like other LID practices,
soil restoration becomes more effective the
higher the infiltration rate increases. When
used to improve native soils when paired with
another BMP, runoff reduction percentages can
increase from 15 to 25 percent. Subtract 50%
of any restored pervious areas from the total
site area and re-calculate the runoff reduction
volume (RRv) that applies to the development
site.
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329
• Channel Protection
Soil restoration helps restore pre-development
hydrology, which implicitly reduces postconstruction stormwater runoff rates in
addition to runoff volumes and pollutant loads.
Assume that the post-development hydrologic
conditions of any restored pervious areas are
equivalent to those of open space (e.g., lawns,
parks, golf courses) in good condition
• Overbank Flood Protection
Soil restoration helps restore pre-development
hydrology, which implicitly reduces postconstruction stormwater runoff rates in
addition to runoff volumes and pollutant loads.
Assume that the post-development hydrologic
conditions of any restored pervious areas are
equivalent to those of open space (e.g., lawns,
parks, golf courses) in good condition.
• Extreme Flood Protection
Soil restoration helps restore pre-development
hydrology, which implicitly reduces postconstruction stormwater runoff rates in
addition to runoff volumes and pollutant loads.
Assume that the post-development hydrologic
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conditions of any restored pervious areas are
equivalent to those of open space (e.g., lawns,
parks, golf courses) in good condition.
4.23.3 Applications and Site Feasibility
Criteria
The criteria listed in Table 4.23-1 should be evalu-
In order to be eligible for these “credits,” it is
ated to determine whether or not soil restoration
recommended that restored pervious areas satisfy
is appropriate for use on a development site.
the planning and design criteria outlined below.
General Feasibility
If any type of vegetation other than managed turf
- Suitable for Residential Subdivision Usage – YES
can be planted on a restored pervious area, site
- Suitable for High Density/Ultra Urban Areas – YES
planning and design teams are encouraged to
- Regional Stormwater Control – NO
combine soil restoration with site reforestation/
revegetation (Section 4.22) to further reduce
Site Applicability
post-construction stormwater runoff rates, vol-
Soil restoration can be used on a wide variety of
umes and pollutant loads.
development sites, including residential, commer-
Stormwater Best Management Practices
• Water Quality Protection
Due to the soil amendments themselves, in
addition to the runoff reduction benefits, soil
restoration inherently improves water quality.
Depending on the organic compounds and
other amendments added, nutrient uptake
and other pollutant removal processes can
be achieved. Subtract 50% of any restored
pervious areas from the total site area and recalculate the water quality volume (WQv) that
applies to the development site.
cial, industrial and institutional development sites
When soil restoration is used to enhance the
in rural, suburban and urban areas. When com-
performance of other low impact development
pared with other low impact development practic-
practices (e.g., site reforestation/revegetation,
es, it has a moderate construction cost, a relatively
vegetated filter strips, grass channels), it may be
low maintenance burden and requires no addition-
“credited” by improving the soils characterization
al surface area beyond that which will undergo the
for purposes of determining the runoff reduction
soil restoration process. It is ideal for use in per-
percentage.
vious areas that have been disturbed by clearing,
grading and other land disturbing activities.
Table 4.23-1: Factors to Consider When Evaluating the Overall Feasibility of Using Soil Restoration on a
Development Site
Site Characteristic
Criteria
Drainage Area
N/A
Area Required
No restrictions
Slope
Maximum 10% in the disturbed pervious area to be restored.
Minimum Head
N/A
Minimum Depth to
Water Table
A minimum separation distance of 18 inches is recommended between the surface of a restored pervious area and the top of the water table. Depths greater than 18” should be amended when shrubs or trees are installed.
Soils
Pervious areas that have soils with low permeabilities (i.e. hydrologic soil group C or D soils) or
that have been disturbed by land disturbing activities are good candidates for soil restoration.
Areas that have permeable soils (i.e. hydrologic soil group A or B soils) and that have not been
disturbed by land disturbing activities do not need to be restored.
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330
It is recommended that the soil restoration process used on a development site meet all of the
following criteria to be eligible for the stormwater
management “credits” described above:
General Planning and Design
• To avoid damaging existing root systems, soil
restoration should not be performed in areas
that fall within the drip line of existing trees.
• Compost should be incorporated into existing
soils, using a rototiller or similar equipment,
to a depth of 18 inches and at an application
rate necessary to obtain a final average organic
matter content of 8%-12%. Required application
rates can be determined using a compost
calculator, such as the one provided on the
following website: http://www.soilsforsalmon.
org/resources.htm. Other calculations are
available online.
• Only well-aged composts that have been
composted for a period of at least one year
should be used to amend existing soils.
Composts should be stable and show no signs
of further decomposition.
• Composts used to amend existing soils should
meet the following specifications (most
compost suppliers will be able to provide this
information):
»» Bulk Density: Composts should have an “asis” bulk density of 40-50 pounds per cubic
foot (lb/cf). In composts that have a moisture
content of 40%-60%, this equates to a bulk
density range of 450-800 pounds per cubic
yard (lb/cy), by dry weight.
»» Carbon to Nitrogen (C:N) Ratio: Composts
should have a C:N Ratio of less than 25:1.
»» pH: Composts should have a pH of 6-8.
»» Cation Exchange Capacity (CEC): Composts
should have a CEC that exceeds 50
milliequivalents (meq) per 100 grams of dry
weight.
»» Foreign Material Content: Composts should
contain less than 0.5% foreign materials (e.g.,
glass, plastic), by weight.
»» Pesticide Content: Composts should be
pesticide free.
• The use of biosolids (except Class A biosolids)
and composted animal manure to amend
existing soils is not recommended.
Landscaping
• Vegetation commonly planted on restored
pervious areas includes turf, shrubs, trees
and other herbaceous vegetation. Although
managed turf is most commonly used, site
planning and design teams are encouraged
to use trees, shrubs and/or other native
vegetation to help establish mature native plant
communities (e.g., forests) in restored pervious
areas.
• Methods used to establish vegetative cover
within a restored pervious area should achieve
at least 75 percent vegetative cover one year
after installation.
Stormwater Best Management Practices
4.23.4 Planning and Design Criteria
• To help prevent soil erosion and sediment loss,
landscaping should be installed immediately
after the soil restoration process is complete.
Temporary irrigation may be needed to quickly
establish vegetative cover on a restored
pervious area.
• It is recommended that composts used to
amend existing soils be provided by a member
of the U.S. Composting Seal of Testing
Assurance program. Additional information
on the Seal of Testing Assurance program is
available on the following website: http://www.
compostingcouncil.org.
»» Organic Content Matter: Composts should
contain 35%-65% organic matter.
»» Moisture Content: Composts should have a
moisture content of 40%-60%.
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VOL 2
331
Additional Resources
To help ensure that the soil restoration process
4.23.6 Inspection and Maintenance
Requirements
is successfully completed on a development site,
All best management practices require prop-
sources for Implementing Soil Quality and Depth
site planning and design teams should consider
er maintenance. Without proper maintenance,
BMP T5.13 in Washington Department of Ecology
the following recommendations:
BMPs will not function as originally designed and
(WDOE) Stormwater Management Manual for
• To help minimize compaction, heavy
vehicular and foot traffic should be kept
out of all restored pervious areas during
and after construction. This can typically
be accomplished by clearly delineating soil
restoration areas on all development plans and,
if necessary, protecting them with temporary
construction fencing.
may cease to function altogether. The design of
Western Washington. Public Works Department.
all BMPs includes considerations for maintenance
Snohomish County, WA. Available Online: http://
and maintenance access. For additional infor-
www.soilsforsalmon.org/resources.htm.
• Simple erosion and sediment control measures,
such as temporary seeding and erosion control
mats, should be used on restored pervious
areas that exceed 2,500 square feet in size. If
the restored pervious areas will “receive” any
stormwater runoff from other portions of the
development site, measures should be taken
(e.g., silt fence, temporary diversion berm)
to prevent it from compromising the soil
restoration effort.
• Test pits or a rod penetrometer can be used
to verify that soil amendments have reached a
depth of 18 inches.
Stenn, H. 2007. Building Soil: Guidelines and Re-
mation on inspection and maintenance requirements, see Appendix E.
Washington Department of Ecology (WDOE).
2005. “BMP T5.13: Post-Construction Soil Quality
and Depth.” Stormwater Management Manual for
Western Washington. Volume 5: Runoff Treatment
BMPs. Washington Department of Ecology. Water
Stormwater Best Management Practices
4.23.5 Construction Considerations
Quality Program. Available Online: http://www.
ecy.wa.gov/programs/wq/stormwater/manual.
html.
Pennsylvania Department of Environmental Protection (PA DEP). 2006. “BMP 6.7.3: Soil Amendment and Restoration.” Pennsylvania Stormwater
Best Management Practices Manual. Pennsylvania
Department of Environmental Protection. Bureau
of Watershed Management. Available Online:
http://www.depweb.state.pa.us/watershedmgmt/
site/default.asp.
• Construction contracts should contain a
replacement warranty that covers at least three
growing seasons to help ensure adequate
growth and survival of the vegetation planted
on a restored pervious area.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Stormwater planters should be designed to completely drain within
24 hours of the end of a rainfall event.
• A maximum ponding depth of 6 inches is recommended within
stormwater planters to help prevent nuisance ponding conditions.
• Unless a shallow water table is found on the development site,
stormwater planter planting beds should be at least 2 feet deep.
(Source: Center for Watershed Protection)
ADVANTAGES / BENEFITS
• Helps restore pre-development hydrology on development sites and
reduces post-construction stormwater runoff rates, volumes, and
pollutant loads
• Can be integrated into development plans as attractive landscaping
features
• Particularly well-suited for urban development sites
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Description: Stormwater planters are landscape
planter boxes that are specially designed to receive
DISADVANTAGES / LIMITATIONS
• Can only be used to “receive” runoff from small drainage areas of
2,500 square feet or less
post-construction stormwater runoff. They consist
of planter boxes that are equipped with waterproof liners, filled with an engineered soil mix, and
planted with trees, shrubs, and other herbaceous
vegetation. Stormwater planters are designed to
capture and temporarily store stormwater runoff
in the engineered soil mix, where runoff is subject
to the hydrologic processes of evaporation and
ROUTINE MAINTENANCE REQUIREMENTS
• Water to promote plant growth and survival.
• Inspect stormwater planters following rainfall events. Plant
replacement vegetation in any eroded areas.
• Prune and weed stormwater planter.
• Remove accumulated trash and debris.
transpiration before being conveyed back into the
storm drain system through an underdrain.
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: In urban areas, check
clear and safety zones for guidance
Soils: The soils used within stormwater
planter planting beds should be an engineered soil mix.
L=Low M=Moderate H=High
POLLUTANT REMOVAL
Total Suspended Solids
Stormwater Best Management Practices
4.24 Stormwater Planters/Tree
Boxes
Metals - Cadmium, Copper,
Lead, and Zinc removal
Pathogens – Fecal Coliform
RUNOFF REDUCTION CREDIT
• 50% of the runoff reduction volume
provided
LID/GI Considerations: Use of a stormwater planter provides measurable reductions in post-construction stormwater runoff rates, volumes, and
pollutant loads on development sites. Stormwater
planters and/or tree boxes can take the place of
traditional landscaped areas, thus minimizing development space required to treat stormwater runoff.
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Stormwater planters are small, underdrained bioretention areas (Section 4.2) that are designed to fit
within landscape planter boxes (Figure 4.24-1); they
consist of landscape planter boxes equipped with
waterproof liners, filled with an engineered soil mix
and planted with trees, shrubs, and other herbaceous
vegetation. Stormwater planters are designed to
capture and temporarily store stormwater runoff in
the engineered soil mix, where runoff is subject to the
hydrologic processes of evaporation and transpiration before being conveyed back into the storm drain
system through an underdrain. This allows stormwater planters to provide measurable reductions in
Figure 4.24-1: Various Stormwater Planters
(Sources: Center for Watershed Protection, City of Portland, OR, 2008)
Stormwater Best Management Practices
4.24.1 General Description
post-construction stormwater runoff rates, volumes,
and pollutant loads.
A primary concern associated with the design of
a stormwater planter (Figure 4.24-2) is its storage
capacity, which directly influences its ability to reduce
stormwater runoff rates, volumes, and pollutant
loads. Site planning and design teams should strive
to design stormwater planters that can accommodate the stormwater runoff volume generated by the
target runoff reduction rainfall event (e.g., 85th percentile rainfall event). If this cannot be accomplished,
due to site characteristics or constraints, site planning
and design teams should consider using stormwater
planters in combination with other runoff reducing
low impact development practices, such as dry wells
(Section 4.7) and rainwater harvesting (Section 4.19),
to supplement the stormwater management benefits
provided by the planters.
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Figure 4.24-2: Stormwater Planters
(Source: City of Portland, OR, 2004)
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The Center for Watershed Protection (Hirschman et
provided by a stormwater planter when
calculating the extreme peak discharge (Qf) on
a development site (see Subsection 3.1.7.5).
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas –
YES
al., 2008) recently documented the ability of storm-
- Regional Stormwater Control – NO
water planters to reduce annual stormwater runoff
4.24.3 Pollutant Removal Capabilities
volumes and pollutant loads on development sites.
Stormwater planters are presumed to remove
• Stormwater Runoff Reduction
Subtract 50% of the storage volume provided
by a stormwater planter from the runoff
reduction volume (RRV) conveyed through the
stormwater planter.
80% of the total suspended solids (TSS) load in
Physical Feasibility – Physical Constraints at
typical urban post-development runoff when
Project Site
sized, designed, constructed, and maintained in
• Drainage Area – The size of the contributing
drainage area should be 2,500 square feet or
less.
• Water Quality Protection
If installed as per the recommended design
criteria and properly maintained, 80% total
suspended solids removal will be applied to
the water quality volume (WQV) flowing to the
stormwater planter.
• Channel Protection
Proportionally adjust the post-development
runoff curve number (CN) to account for the
runoff reduction provided by a stormwater
planter when calculating the channel
protection volume (CPV) on a development site
(see Subsection 3.1.7.5).
• Overbank Flood Protection
Proportionally adjust the post-development
runoff CN to account for the runoff reduction
provided by a stormwater planter when
calculating the overbank peak discharge (Qp25)
on a development site (see Subsection 3.1.7.5).
• Extreme Flood Protection
Proportionally adjust the post-development
runoff CN to account for the runoff reduction
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accordance with the recommended specifications. Stormwater planters also remove 60% of
the Phosphorus and Nitrogen, and 80% of fecal
coliform. Stormwater planters are not presumed
to remove metals such as Cadmium, Copper,
Lead, and Zinc in contributing runoff.
4.24.4 Application and Site Feasibility
Criteria
Stormwater planters are typically used on commercial, institutional, and industrial development
sites. Because they can be constructed immediately adjacent to buildings and other structures,
they are ideal for use in urban areas. Although
they are well-suited to receive rooftop runoff,
they can also be used to treat stormwater runoff
from other small impervious and pervious drainage areas, such as sidewalks, plazas, and small
parking lots (Figure 4.24-1). When compared with
• Flow path – The length of flow path in
contributing drainage areas should be 150 feet
or less in pervious drainage areas and 75 feet or
less in impervious drainage areas.
Stormwater Best Management Practices
4.24.2 Stormwater Management Suitability
• Space Required – Stormwater planter surface
area requirements vary according to the size
of the contributing drainage area. In general,
stormwater planters require about 5% of the
size of their contributing drainage areas.
• Site Slope – Although stormwater planters may
be used on development sites with slopes of up
to 6%, they should be designed with slopes that
are as close to flat as possible to help ensure
that stormwater runoff is evenly distributed
over the planting bed.
• Minimum Depth to Water Table – 2 feet
other low impact development practices, stormwater planters have a moderate construction
• Minimum Head – 2 feet
cost and maintenance burden, while requiring a
relatively small amount of surface area.
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of a rainfall event.
effective BMP for use where trout streams or
other protected waters may receive stormwater
from being conveyed through a stormwater
planter, particularly during high tide.
runoff.
Other Constraints/Considerations
Coastal Areas
4.24.5 Planning and Design Criteria
• Hot spots – Because they are constructed
with waterproof liners and under/overdrains,
stormwater planters may be used to treat hot
spot runoff.
• Poorly Draining Soils – Since stormwater
planters are equipped with waterproof liners
and underdrains, the presence of poorly
draining soils does not influence their use.
Before designing a stormwater planter, the fol-
• Damage to existing structures and facilities
– Stormwater planters should not be used
in areas where their operation may create
a risk for basement flooding, interfere with
subsurface sewage disposal systems, or
negatively affect other underground structures.
Stormwater planters should be designed so that
overflow drains away from buildings to prevent
damage to building foundations.
• Well-Draining Soils – Since stormwater
planters are equipped with waterproof liners
and underdrains, the presence of well-draining
soils does not influence their use.
• Proposed site design, including buildings,
parking lots, sidewalks, stairs, handicapped
ramps, and landscaped areas
• Proximity – Stormwater planters may be used
without restriction in areas that are:
»» 10 feet from property lines
»» 100 feet from private water supply wells
»» 1,200 feet from public water supply wells
»» 100 feet from septic systems
»» 100 feet from surface waters
»» 400 feet from public water supply surface
waters
• Trout Stream – Use of a stormwater planter
reduces a site’s runoff pollutant load, as well
as the volume and velocity of stormwater
runoff, without significantly warming the
water. Therefore, stormwater planters are an
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• Flat Terrain – It may be difficult to provide
adequate drainage on flat terrain, which
may cause stormwater runoff to pond in the
stormwater planter for extended periods of
time. Ensure that the underdrain will allow
the stormwater planter to drain completely
within 24 hours of the end of a rainfall event to
prevent nuisance ponding conditions.
• Shallow Water Table – It may be difficult
to provide 2 feet of clearance between the
bottom of the stormwater planter and the
top of the water table, which may cause
stormwater runoff to pond in the stormwater
planter. Designers can address this issue by
reducing the depth of the planting bed to 18
inches or reducing the distance between the
bottom of the stormwater planter and top of
the water table to 12 inches and providing an
adequately sized underdrain.
lowing data is necessary:
• Existing and proposed site, topographic, and
location maps, as well as field reviews
• Architectural roof plan for rooftop pitches and
downspout locations
Stormwater Best Management Practices
• Soils – Stormwater planters should be designed
to completely drain within 24 hours of the end
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Information about downstream BMPs and
receiving waters
• Design data from nearby storm sewer
structures
• Water surface elevation of nearby water
systems and depth to seasonally high
groundwater table
The following criteria are to be considered minimum standards for the design of a stormwater
planter. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be met.
• Tidally-influenced drainage system – Tides
may occasionally prevent stormwater runoff
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• Stormwater planters should be used to receive
stormwater runoff from small drainage areas
of 2,500 square feet or less. The stormwater
runoff rates and volumes from larger
contributing drainage areas typically become
too large to be properly treated by stormwater
planters.
• The length of the flow path within the
contributing drainage area should be 150 feet
or less for pervious drainage areas and 75
feet or less for impervious drainage areas. In
contributing drainage areas with longer flow
paths bioretention areas (Section 4.2) should be
used to receive post-construction stormwater
runoff.
• Stormwater planters should be designed to
provide enough storage for the stormwater
runoff volume generated by the target runoff
reduction rainfall event (e.g., 85th percentile
rainfall event).
• A minimum width, measured from inside wall
to inside wall, of 18 inches is recommended for
all stormwater planters.
• Although stormwater planters may be used
on development sites with slopes up to 6%,
they should be designed with slopes that are
as close to flat as possible to help ensure that
stormwater runoff is evenly distributed over
the planting bed. Multiple planters can be
constructed at varying elevations.
• Unless a shallow water table is found on
the development site, the distance from
the bottom of a stormwater planter to the
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top of the water table should be at least 2
feet. If a shallow water table is found on the
development site, the distance from the
bottom of a stormwater planter to the top of
the water table may be reduced to 12 inches.
4.24.5.2 GENERAL DESIGN
• Stormwater planters should be designed to
completely drain within 24 hours of the end of a
rainfall event. Where site characteristics allow, it is
preferable to design stormwater planters to drain
within 12 hours of the end of a rainfall event to
help prevent nuisance ponding conditions.
• Stormwater planters may be designed with
a maximum ponding depth of 12 inches,
although a ponding depth of 6 inches is
recommended to help prevent nuisance
ponding conditions.
• A minimum of 2 inches of freeboard should
be provided between the elevation of the
maximum ponding depth and the top of the
planter box.
• Unless a shallow water table is found on the
development site, all stormwater planter
planting beds should be at least 24 inches
deep. If a shallow water table is found on the
development site, the depth of the planting bed
may be reduced to 18 inches.
• The soils used within stormwater planter beds
should be an engineered soil mix that meets
the following specifications:
»» Texture: Sandy loam or loamy sand should
be used.
»» Sand Content: Soils should contain 85-88%
clean, washed sand.
»» Topsoil Content: Soils should contain 8-12%
topsoil.
»» Organic Matter Content: Soils should
contain 3-5% organic matter.
»» Infiltration Rate: Soils should have an
infiltration rate of at least 0.25 inches per
hour (in/hr), although an infiltration rate of
between 1-2 in/hr is preferred.
»» Phosphorus Index (P-Index): Soils should
have a P-Index of less than 30.
»» Exchange Capacity (CEC): Soils should have
a CEC that exceeds 10 milliequivalents (meq)
per 100 grams of dry weight.
Stormwater Best Management Practices
4.24.5.1 LOCATION AND LAYOUT
»» pH: Soils should have a pH of 6-8.
• The organic matter used within a stormwater
planter bed should be a well-aged compost
that meets the specifications outlined in
Appendix D.
• All stormwater planters should be equipped
with a waterproof liner to prevent damage
to building foundations and other adjacent
impervious surfaces. Waterproof liners should
be 30 mil (0.030 inch) polyvinylchloride (PVC)
or equivalent.
• Stormwater planters should be constructed
of stone, concrete, brick, or another durable
material. Chemically treated wood that can
leach toxic chemicals and contaminate
stormwater runoff should not be used to
construct a stormwater planter.
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• If permeable filter fabric is used, the filter fabric
should be a non-woven geotextile with a
permeability that is greater than or equal to the
hydraulic conductivity of the overlying planting
bed.
• Consideration should be given to the
stormwater runoff rates and volumes generated
by larger storm events (e.g., 25-year, 24hour storm event) to help ensure that these
larger storm events are able to safely bypass
stormwater planters. An overflow system, such
as an overdrain with an invert set slightly above
the elevation of maximum ponding depth,
should be designed to convey the stormwater
runoff generated by these larger storm events
safely out of the stormwater planter.
4.24.5.3 PRETREATMENT/INLETS
4.24.5.6 LANDSCAPING
• If used to receive non-rooftop runoff, consider
preceding the stormwater planters with a pea
gravel diaphragm or equivalent level spreader
device (e.g., concrete sills, curb stops, curbs
with “sawteeth” cut into them) to intercept
stormwater runoff and distribute it evenly, as
overland sheet flow, across the stormwater
planter.
• A landscaping plan should be prepared for all
stormwater planters. The landscaping plan
should be reviewed and approved by the
local development review authority prior to
construction.
4.24.5.4 OUTLET STRUCTURES
• An overflow system, such as an overdrain with
an invert set slightly above the elevation of
maximum ponding depth, should be designed
to convey the stormwater runoff generated
by larger storm events safely out of the
stormwater planter.
• If designed to receive rooftop runoff,
stormwater planters should be constructed
with bypass structures and/or piping to ensure
that stormwater runoff from larger storm
events can be safely passed without damaging
the practice or nearby property.
4.24.7.5 SAFETY FEATURES
Except for stable outlet and bypass structures,
stormwater planters generally do not require any
special safety features. Fencing of stormwater
planters is not generally desirable.
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• Vegetation commonly planted in stormwater
planters includes native trees, shrubs, and other
herbaceous vegetation. When developing a
landscaping plan, site planning and design
teams should choose vegetation that will
be able to stabilize soils and tolerate the
stormwater runoff rates and volumes that
will pass through the stormwater planter.
Vegetation used in stormwater planters should
also be able to tolerate both wet and dry
conditions. See Appendix D for a list of grasses
and other plants that are appropriate for use in
stormwater planters in the State of Georgia.
Stormwater Best Management Practices
• Stormwater planters should be equipped with
an underdrain consisting of a 4-inch perforated
PVC (AASHTO M 252) pipe bedded in a 6-inch
layer of clean, washed stone. The pipe should
have 3/8-inch perforations, spaced 6 inches
on center, and a minimum slope of 0.5%. The
clean, washed stone should be ASTM D448
size No. 57 stone (i.e., 1-1/2 to 1/2 inches in
diameter) and should be separated from the
planting bed by a layer of permeable filter
fabric or a thin, 2-4-inch layer of choker stone
(i.e., ASTM D 448 size No. 8, 3/8” to 1/8” or
ASTM D 448 size No. 89, 3/8” to 1/16”).
• A mulch layer, consisting of 2-4 inches of
fine shredded hardwood mulch or hardwood
chips, should be included on the surface of the
stormwater planter.
• Methods used to establish vegetative cover
within a stormwater planter should achieve
at least 75% vegetative cover one year after
installation.
• To help prevent soil erosion and sediment loss,
landscaping should be provided immediately
after the stormwater planter has been installed.
Temporary irrigation may be needed to quickly
establish vegetative cover within a stormwater
planter.
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To help ensure that stormwater planters are successfully installed, site planning and design teams
should consider the following recommendations:
• If stormwater planters will be used to receive
non-rooftop runoff, they should only be
installed after their contributing drainage
areas have been completely stabilized. To help
prevent stormwater planter failure, stormwater
runoff may be diverted around the stormwater
planter until the contributing drainage area has
been stabilized.
• To help prevent soil compaction, heavy
vehicular and foot traffic should be kept out
of stormwater planters before, during, and
immediately after construction. This can
typically be accomplished by clearly delineating
stormwater planters on all development
plans and, if necessary, protecting them with
temporary construction fencing.
4.24.5.8 CONSTRUCTION AND MAINTENANCE
COSTS
• The initial cost of a stormwater planter
averages around $8 per square foot; however,
the overall cost will vary depending on the type
and size of vegetation and planters used.
• Maintenance costs average around $400-$500
per year for a 500-square-foot planter. These
also vary depending on size and plant choice.
Stormwater Best Management Practices
4.24.5.7 CONSTRUCTION CONSIDERATIONS
• Excavation for stormwater planters should be
limited to the width and depth specified in the
development plans. Excavated material should
be placed away from the excavation so as not
to jeopardize the stability of the side walls.
• Native soils along the bottom of the
stormwater planter should be scarified or tilled
to a depth of 3-4 inches prior to the placement
of the choker stone and stormwater planter
stone.
• The sides of all excavations should be trimmed
of large roots that will hamper the installation
of the permeable filter fabric used to line the
sides and top of the stormwater planter.
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(Step 1) Determine if the development site and conditions are ap-
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
propriate for the use of a stormwater planter or tree box.
Consider the application and site feasibility criteria in this chap-
Complete Step 3A, 3B, and 3C for a runoff reduction ap-
ter. In addition, determine if site conditions are suitable for a
proach, or skip Step 3 and complete Steps 4A and 4B for a
stormwater planter or tree box. Create a rough layout of the
water quality (treatment) approach. Refer to your local com-
stormwater planters or tree box dimensions taking into consid-
munity’s guidelines for any additional information or specific
eration existing trees, utility lines, and other obstructions.
requirements regarding the use of either method.
(Step 2) Determine the goals and primary function of the stormwa-
(Step 3A) Calculate the Stormwater Runoff Reduction Target Volume
ter planters or tree box.
Calculate the Runoff Reduction Volume using the following
Consider whether the stormwater planters or tree box is
formula:
intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target. For information on the sizing of a
BMP utilizing the runoff reduction approach, see Step
3A. For information on the sizing of the BMP utilizing
the water quality treatment approach, see Step 4A.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
»» Provide a possible solution to a drainage problem
RRV = (P) (RV) (A) / 12
Stormwater Best Management Practices
4.24.6 Design Procedures
Where:
RRV = Runoff Reduction Target Volume (ft3)
P = Target runoff reduction rainfall (inches)
RV = Volumetric runoff coefficient which can be
found by:
RV = 0.05+0.009(I)
»» Enhance landscape and provide aesthetic qualities
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, look
up the appropriate runoff reduction percentage (or credit)
provided by the practice:
primary function, as well as, topographic and soil conditions,
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assist in sediment removal and maintenance. Pretreatment
Volume of the Practice (VP)
can include a forebay, weir, or check dam. Splash blocks
or level spreaders should be considered to dissipate con-
(VPMIN) ≥ RRv (target) / (RR%)
centrated stormwater runoff at the inlet and prevent scour.
Forebays should be sized to contain 0.1 inches per imper-
Where:
vious acre of contributing drainage. Refer to Section 4.9
RR% = Runoff Reduction percentage, or credit,
for design criteria for a grass channel and Section 4.29 for
assigned to the specific practice
vegetated filter strips.
VPMIN = Minimum storage volume required to
provide Runoff Reduction Target Volume (ft3)
(Step 3C) Determine whether the minimum storage volume was met
RRv (target) = Runoff Reduction Target Volume (ft3)
When the VP ≥ VPMIN, then the Runoff Reduction requirements are met for this practice. Proceed to Step 5.
(Step 3B) Determine the storage volume of the practice and the
Pretreatment Volume
When the VP < VPMIN, then the BMP must be sized according
To determine the actual volume provided in the stormwater
to the WQV treatment method (See Step 4).
Stormwater Best Management Practices
Using the RRV calculated above, determine the minimum
planter or tree box, use the following equation:
(Step 4A) Calculate the Target Water Quality Volume
VP = (PV + VES (N))
Calculate the Water Quality Volume using the following formula:
Where:
VP = Volume provided (temporary storage)
WQV = (1.2) (RV) (A) / 12
PV = Ponding Volume
VES = Volume of Engineered Soils
Where:
N = Porosity
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
To determine the porosity, a qualified licensed professional
RV = Volumetric runoff coefficient which can be
should be consulted to determine the proper porosity based
found by:
on the engineered soils used. Most soil media has a porosity
of 0.25 and gravel a value of 0.40.
RV = 0.05+0.009(I)
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Provide pretreatment by using a grass filter strip or pea gravel
Where:
diaphragm, as needed, (sheet flow), or a grass channel or
I = new impervious area of the contributing
forebay (concentrated flow). Where filter strips are used,
drainage area (%)
100% of the runoff should flow across the filter strip. Pre-
A = Area draining to this practice (ft2)
treatment may also be desired to reduce flow velocities or
12 = Unit conversion factor (in/ft)
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(Step 6) Size flow diversion structure, if needed
the footprint of the stormwater planter practice
If the contributing drainage area to stormwater planter ex-
The peak rate of discharge for the water quality design storm
ceeds the water quality treatment and/or storage capacity, a
is needed for sizing of off-line diversion structures (see Sub-
flow regulator (or flow splitter diversion structure) should be
section 3.1.7). If designing off-line, follow steps (a) through
supplied to divert the WQv (or RRv) to the stormwater planter.
(d) below:
(Step 7) Design emergency overflow system
.
(a) Using WQv, compute CN
An overflow system, such as an overdrain with an invert set
(b) Compute time of concentration using TR-55 method
slightly above the elevation of maximum ponding depth,
(c) Determine appropriate unit peak discharge from time of
must be provided to bypass and/or convey larger flows to
concentration
the downstream drainage system or stabilized watercourse.
(d) Compute Qwq from unit peak discharge, drainage area,
Non-erosive velocities need to be ensured at the outlet
and WQv.
point. The overflow should be sized to safely pass the peak
flows anticipated to reach the practice, up to a 100-year, 24-
To determine the minimum surface area of the stormwater
Stormwater Best Management Practices
(Step 4B) If using the practice for Water Quality treatment, determine
hour storm event.
planter, use the following formula:
(Step 8) Prepare site Vegetation and Landscaping Plan
Af = (WQv) (df) / [ (k) (hf + df)(tf)]
A landscaping plan for the stormwater planter should be prepared to indicate how it will be established with vegetation.
Where:
Af = surface area of ponding area (ft2)
See Subsection 4.24.5.6 (Landscaping) and Appendix D for
WQv = water quality volume (ft )
more details.
3
df = media depth (ft)
k = coefficient of permeability of planting media
(ft/day) (use 1 ft/day for silt-loam if engineered soils
4.24.7 Maintenance Requirements
is being used)
All best management practices require proper maintenance. Without proper
hf = average height of water above stormwater
maintenance, BMPs will not function as originally designed and may cease to
planter bed (ft)
function altogether. The design of all BMPs includes considerations for main-
tf = design planting media drain time (days) (1 day is
tenance and maintenance access. For additional information on inspection
recommended maximum)
and maintenance requirements, see Appendix E.
(Step 5) Calculate the adjusted curve numbers for CPV (1-yr, 24-hour
storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr, 24-hour
storm). See Subsection 3.1.7.5 for more information
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Minimum contributing drainage area of 25 acres; 10 acres for
micropool extended detention pond
• A sediment forebay or equivalent upstream pretreatment facility
should be provided.
• Minimum length-to-width ratio for the pond is 1.5:1
• Depth of the permanent pool should not exceed 8 feet.
• Side slopes to the pond should not exceed 3:1 (h:v) without safety
precautions or if mowing is anticipated and should terminate on a
safety bench
STORMWATER MANAGEMENT SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
Description: Constructed stormwater retention
basins that have a permanent pool (or micropool).
Runoff from each rain event is detained and treated
in the pool primarily through settling and biological
uptake mechanisms.
LID/GI Considerations: Stormwater ponds are not
generally considered low impact development or
green infrastructure best management practices
(BMPs).
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ADVANTAGES / BENEFITS
• Moderate to high removal rate of urban pollutants
• High community acceptance
• Opportunities for wildlife habitat
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
DISADVANTAGES / LIMITATIONS
• Potential for thermal impacts/downstream warming
• Dam height restrictions for high-relief areas
• Pond drainage can be problematic for low-relief terrain
• Cost of dredging and disposal
• Dredging operations may require a stream buffer variance
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Not recommended
Roadway Projects: Yes
MAINTENANCE REQUIREMENTS
• Remove debris from inlet and outlet structures.
• Maintain side slopes and remove invasive vegetation.
• Monitor sediment accumulation and remove it periodically.
Soils: Underlying soils of hydrologic group
“C” or “D” should be adequate to maintain
a permanent pool. Hydrologic group “A”
soils generally require a pond liner; group
“B” soils may require infiltration testing.
POLLUTANT REMOVAL
L=Low M=Moderate H=High
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
Stormwater Best Management Practices
4.25 Stormwater Ponds
RUNOFF REDUCTION CREDIT
• 0% of the runoff reduction volume
provided by this practice.
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There are several variants of stormwater pond de-
Stormwater ponds (also referred to as retention
sign, the most common of which include the wet
ponds, wet ponds, or wet extended detention
pond, the wet extended detention pond, and the
ponds) are constructed stormwater retention ba-
micropool extended detention pond. In addition,
sins that have a permanent (dead storage) pool of
multiple stormwater ponds can be placed in series
water throughout the year. They can be created
or parallel to increase performance or meet site
by excavating an existing natural depression or
design constraints. Below are descriptions of each
through the construction of embankments.
design variant:
In a stormwater pond, runoff from each rain
event is detained and treated in the pool through
gravitational settling and biological uptake until it
is displaced by runoff from the next storm.
The permanent pool also serves to protect deposited sediments from resuspension. Above the
permanent pool level, additional temporary storage (live storage) is provided for runoff quantity
control. The upper stages of a stormwater pond
are designed to provide extended detention of
the 1-year, 24-hour rainfall event for downstream
channel protection, as well as normal detention
of larger storm events (25-year, 24-hour and, optionally, the 100-year, 24-hour storm event).
Stormwater ponds are among the most cost-effective and widely used stormwater practices. A
well-designed and landscaped pond can be an
aesthetic feature on a development site when
planned, located, implemented, and maintained
properly. However, ponds only work as designed
until the deign sediment volume is reached. At
that point, the sediment must be removed to
maintain treatment performance and aquatic resource, channel and extreme flood protection.
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• Wet Pond – Wet ponds are stormwater basins
constructed with a permanent (dead storage)
pool of water equal to the water quality
volume. Stormwater runoff displaces the water
already present in the pool. Temporary storage
(live storage) can be provided above the
permanent pool elevation for larger flows.
• Multiple Pond Systems – Multiple pond
systems consist of constructed facilities
that provide water quality and quantity
volume storage in two or more cells. The
additional cells can create longer pollutant
removal pathways and improved downstream
protection.
Figure 4.25-1 on the next page shows several examples of stormwater pond variants.
Stormwater Best Management Practices
4.25.1 General Description
• Wet Extended Detention (ED) Pond – A
wet extended detention pond is a wet pond
where the water quality volume is split evenly
between the permanent pool and extended
detention (ED) storage provided above the
permanent pool. During storm events, water
is detained above the permanent pool and
released over 24 hours. This design has similar
pollutant removal to a traditional wet pond, but
consumes less space.
• Micropool Extended Detention (ED) Pond –
The micropool extended detention pond is
a variation of the wet ED pond where only a
small “micropool” is maintained at the outlet to
the pond. The outlet structure is sized to detain
the water quality volume for 24 hours. The
micropool prevents resuspension of previously
settled sediments and also prevents clogging of
the low flow orifice.
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Wet ED Pond
Micropool ED Pond
Wet ED Pond
Stormwater Best Management Practices
Wet Pool
Figure 4.25-1 Stormwater Pond Examples
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Stormwater Best Management Practices
Figures 4.25-2 through 4.25-5 provide plan view and
profile view schematics for the design of a wet pond,
wet extended detention pond, micropool extended
detention pond, and multiple pond system.
Figure 4.25-2 Schematic of Wet Pond
(Source: Center for Watershed Protection)
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Figure 4.25-3 Schematic of Wet Extended Detention Pond
(Source: Center for Watershed Protection)
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Stormwater Best Management Practices
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Figure 4.25-4 Schematic of Micropool Extended Detention Pond
Figure 4.25-5 Schematic of Multiple Pond System
(Source: Center for Watershed Protection)
(Source: Center for Watershed Protection)
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Stormwater ponds are designed to control both
stormwater quantity and quality. Thus, a stormwater pond can be used to address most of the
unified stormwater sizing criteria for a given
drainage area.
• Runoff Reduction
Stormwater ponds provide negligible
stormwater volume runoff reduction. Another
BMP should be used in a treatment train with
stormwater ponds to provide runoff reduction.
• Water Quality
Ponds treat incoming stormwater runoff
by physical, biological, and chemical
processes. The primary removal mechanism
is gravitational settling of particulates, organic
matter, metals, bacteria, and organics as
stormwater runoff resides in the pond. Another
mechanism for pollutant removal is uptake
by algae and wetland plants in the permanent
pool—particularly of nutrients. Volatilization
and chemical activity also work to break down
and eliminate a number of other stormwater
contaminants, such as hydrocarbons.
• Channel Protection
A portion of the storage volume above the
permanent pool in a stormwater pond can
be used to provide control of the channel
protection volume (CP v). This is accomplished
by releasing the 1-year, 24-hour storm runoff
volume over 24 hours (extended detention).
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• Overbank Flood Protection
A stormwater pond can also provide storage
above the permanent pool to reduce the
post-development peak flow of the 25-year,
24-hour storm (Qp25) to pre-development levels
(detention).
-- Total Suspended Solids – 80%
• Extreme Flood Protection
In situations where it is required, stormwater
ponds can also be used to provide detention
to control the 100-year, 24-hour storm peak
flow (Qf). Where this is not required, the pond
structure is designed to safely pass extreme
storm flows.
-- Heavy Metals – 50%
4.25.3 Pollutant Removal Capabilities
-- Total Phosphorus – 50%
-- Total Nitrogen – 30%
-- Fecal Coliform – 70% (if no resident
waterfowl population present)
For additional information and data on pollutant
removal capabilities for stormwater ponds, see the
National Pollutant Removal Performance Database
(Version 3) available at www.cwp.org and the National Stormwater Best Management Practices (BMP)
Database at www.bmpdatabase.org.
All of the stormwater pond design variants are
4.25.4 Application and Site Feasibility
Criteria
presumed to remove 80% of the total suspended
Stormwater ponds are generally applicable to
solids load in typical urban post-development
most types of new development and redevel-
runoff when sized, designed, constructed, and
opment, and can be used in both residential and
maintained in accordance with the recommend-
nonresidential areas. Ponds can also be used in
ed specifications. Undersized or poorly designed
retrofit situations.
Stormwater Best Management Practices
4.25.2 Stormwater Management Suitability
ponds can reduce TSS removal performance, as
can excess sediment.
The following criteria should be evaluated to
ensure the suitability of a stormwater pond for
The following pollutant removal rates are con-
meeting stormwater management objectives on a
servative average pollutant reduction percent-
site or development.
ages for design purposes derived from sampling
data, modeling, and professional judgment. In
a situation where a removal rate is not deemed
General Feasibility
sufficient, additional controls may be put in place
- Suitable for Residential Subdivision Usage – YES
at the given site in a series or “treatment train”
- Suitable for High Density/Ultra Urban Areas – NO
approach.
- Regional Stormwater Control – YES
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Other Constraints/Considerations
Project Site
• Hot spots – Stormwater ponds can accept
runoff from stormwater hot spots. Reduce
potential groundwater contamination by
preventing infiltration of hot spot runoff.
Pretreatment should be provided for hot spot
runoff, as well as a 2-4-foot separation distance
from the water table.
• Drainage Area – A minimum of 25 acres
is needed for a wet pond or wet ED pond
to maintain a permanent pool, or 10 acres
minimum for a micropool ED pond. A smaller
drainage area may be acceptable with an
adequate water balance and anti-clogging
device.
• Site Slope – There should not be more than
15% slope across the drainage area to the pond.
• Damage to existing structures and facilities
– SStormwater ponds should be designed to
safely store and/or bypass the overbank flood
(Qp25) and extreme flood (Qf) storms to prevent
overflow or failure, which may cause damage
to site structures and facilities.
• Minimum Head – 6-8 feet of elevation
difference needed on-site from the inflow to
the outflow
• Minimum setback requirements for stormwater
pond facilities (when not specified by local
ordinance or criteria):
• Space Required – Approximately 2-3% of the
tributary drainage area
• Minimum Depth to Water Table – If used on a
site with an underlying water supply aquifer, or
when treating a hot spot, a separation distance
of 2 feet is required between the bottom of the
pond and the elevation of the seasonally high
water table.
• Soils – Underlying soils of hydrologic group
C or D should be adequate to maintain a
permanent pool. Most group A soils and
some group B soils will require a pond liner.
Evaluation of soils should be based upon an
actual subsurface analysis and permeability
tests.
»» Property line – 10 feet
»» Private well – 100 feet; if a well is downgradient from a hot spot land use then the
minimum setback is 250 feet
• High-relief – Embankment heights are
restricted.
• Karst – Requires clay or polyliner to sustain a
permanent pool of water and protect aquifers.
Ponding depth is limited. Geotechnical tests
may be required.
• Swimming Area / Shellfish – Design for geese
prevention. Provide 48-hour ED for maximum
coliform die-off.
• Aquifer Protection – Reduce potential
groundwater contamination by preventing
infiltration of hot spot runoff. A liner may be
required for type A and B soils. Pretreat hot
spots and provide 2-4-foot separation distance
from the groundwater table.
Stormwater Best Management Practices
Physical Feasibility – Physical Constraints at
Coastal Areas
• Poorly Draining Soils – Poorly draining soils
do not inhibit a stormwater pond’s ability to
temporarily store and treat stormwater runoff.
»» Septic system tank/leach field – 50 feet
• Trout Streams – Consideration should be
given to the thermal influence of stormwater
pond outflows on downstream trout waters.
A micropool ED pond is the best pond
alternative, but wet ponds and wet ED ponds
can be designed off-line and under shade to
minimize their thermal impact. Limit WQv-ED to
12 hours.
• Flat Terrain – Consider stormwater wetlands
as an alternative stormwater management
(Section 4.26) practice in areas with flat terrains
and a shallow water table.
• Low-relief – Maximum normal pool depth
is limited, so providing a pond drain can be
problematic.
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4.25.5 Planning and Design Criteria
Before designing the stormwater pond, the following data is necessary:
• Existing and proposed site, topographic, and
location maps, as well as field reviews
• Impervious and pervious areas and other
means to determine the land use data as
needed
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Design data for nearby storm sewer structures
• Water surface elevation of nearby water
systems and the depth to the seasonally high
groundwater table
• Desired sediment storage volume and cleanout
frequency
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The following criteria are to be considered
minimum standards for the design of a stormwater pond facility. Consult with the local review
authority to determine if there are any variations
to these criteria or additional standards that must
be met.
• Stormwater ponds cannot be located within
a stream or any other navigable waters of the
U.S., including wetlands, without obtaining a
Section 404 permit under the Clean Water Act,
and any other applicable state permit.
• All utilities should be located outside of the
pond/basin site.
4.25.5.1 LOCATION AND LAYOUT
• Stormwater ponds should have a minimum
contributing drainage area of 25 acres for a wet
pond or wet ED pond to maintain a permanent
pool. For a micropool ED pond, the minimum
drainage area is 10 acres. A smaller drainage
area can be considered when water availability
can be confirmed (such as from a groundwater
source or in areas with a high water table). In
these cases a water balance may be performed.
Ensure that an appropriate anti-clogging device
is provided for the pond outlet.
• A stormwater pond should be sited such that
the site topography allows for maximum runoff
storage at minimum excavation or construction
costs. Pond siting should also take into account
the location and use of other site features,
such as buffers and undisturbed natural areas,
and should attempt to aesthetically “fit” the
facility into the landscape. Bedrock close to the
surface may prevent excavation.
4.25.5.2 GENERAL DESIGN
• A well-designed stormwater pond consists of
the following components.
1. One or more permanent pools of water
2. An overlying zone in which runoff control
volumes are stored
3. Shallow littoral zone (aquatic bench) along
the edge of the permanent pool that acts as
a biological filter
Stormwater Best Management Practices
• Shallow Water Table – If used on a site with
an underlying water supply aquifer or when
treating a hot spot, a separation distance of
2 feet is required between the bottom of the
pond and the elevation of the seasonally high
water table. Otherwise, the elevation of the
stormwater pond bottom can be below the
seasonally high water table, as this will help to
maintain a permanent pool within the BMP.
• In addition, all stormwater pond designs should
include a sediment forebay at their inflow
to allow heavier sediments to drop out of
suspension before runoff enters the permanent
pool.
• Additional pond design features include an
emergency spillway, maintenance access,
safety bench, pond buffer, and appropriate
native landscaping.
• Stormwater ponds should not be located on
steep (>15%) or unstable slopes.
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In general, pond designs are unique for each site
and application. However, there are a number of
geometric ratios and limiting depths for ponds
that must be observed for adequate pollutant removal, ease of maintenance, and improved safety.
• Permanent pool volume is typically sized as
follows:
»» Standard wet ponds: 100% of the water
quality treatment volume (1.0 WQv)
»» Wet ED ponds: 50% of the water quality
treatment volume (0.5 WQv)
»» Micropool ED ponds: Volume should be
approximately 0.1 inch per impervious acre
draining to the pond.
• Proper geometric design is essential to prevent
hydraulic short-circuiting (unequal distribution
of inflow), which results in failure of the pond
to achieve adequate levels of pollutant removal.
The minimum length-to-width ratio for the
permanent pool shape is 1.5:1, and should
ideally be greater than 3:1 to avoid shortcircuiting. In addition, ponds should be wedgeshaped when possible so that flow enters the
pond and gradually spreads out, improving
the sedimentation process. Baffles, pond
shaping, and/or islands can be added within the
permanent pool to increase the flow path.
• Maximum depth of the permanent pool
generally should not exceed 8 feet to avoid
stratification and anoxic conditions. Minimum
depth for the pond bottom should be 3-4 feet.
Deeper depths near the outlet will yield cooler
bottom water discharges that may help to
mitigate downstream thermal effects.
• Side slopes to the pond usually should not
exceed 3:1 (h:v) without safety precautions or if
mowing is anticipated and should terminate on
a safety bench (see Figure 4.25-6). The safety
bench requirement may be waived if slopes are
4:1 or flatter.
• The perimeter of all deep pool areas (4 feet or
deeper) should be surrounded by two benches:
safety and aquatic. For larger ponds, a safety
bench extends approximately 15 feet outward
from the normal water edge to the toe of the
pond side slope. The maximum slope of the
safety bench should be 6%. An aquatic bench
extends inward from the normal pool edge (15
feet on average) and has a maximum depth of
18 inches below the normal pool water surface
elevation (see Figure 4.25-6).
Stormwater Best Management Practices
4.25.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• The contours and shape of the permanent pool
should be irregular to provide a more natural
landscaping effect.
Figure 4.25-6 Typical Stormwater Pond Geometry Criteria
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• Each pond should have a sediment forebay
or equivalent upstream pretreatment. A
sediment forebay is designed to remove
incoming sediment from the stormwater flow
prior to dispersal in a larger permanent pool.
The forebay should consist of a separate cell,
formed by an acceptable barrier. A forebay is
to be provided at each inlet, unless the inlet
provides less than 10% of the total design
storm inflow to the pond. In some design
configurations, the pretreatment volume may
be located within the permanent pool.
• The forebay is sized to contain 0.1 inches per
impervious acre of contributing drainage and
should be 4-6 feet deep. The pretreatment
storage volume is part of the total WQv
requirement and may be subtracted from WQv
for permanent pool sizing.
• Forebays should contain a fixed vertical
sediment depth marker to measure sediment
deposition over time. The bottom of the
forebay may be hardened (using concrete,
paver blocks, etc.) to make sediment removal
easier.
• Inflow channels are to be stabilized with flared
aprons, or the equivalent. Inlet pipes to the
pond can be partially submerged. Exit velocities
from the forebay must be non-erosive.
4.25.5.5 OUTLET STRUCTURES
• Flow control from a stormwater pond is
typically accomplished with the use of a
concrete or corrugated metal riser and barrel.
The riser is a vertical pipe or inlet structure
that is attached to the base of the pond with
a watertight connection. The outlet barrel is
a horizontal pipe attached to the riser that
conveys flow under the embankment (see
Figure 4.25-7). The riser should be located
within the embankment for maintenance
access, safety, and aesthetics.
• A number of outlets at varying depths in the
riser provide internal flow control for routing
of the water quality, channel protection, and
overbank flood protection runoff volumes. The
number of orifices can vary and is usually a
function of pond design.
Stormwater Best Management Practices
4.25.5.4 PRETREATMENT/INLETS
»» For example, a wet pond riser configuration
is typically comprised of a channel protection
outlet (usually an orifice) and overbank flood
protection outlet (often a slot or weir). The
channel protection orifice is sized to release
the channel protection storage volume
over a 24-hour period (12-hour extended
detention may be warranted in some cold
water stream basins). Since the water quality
volume is fully contained in the permanent
pool, no orifice sizing is necessary for this
volume. As runoff from a water quality event
enters the wet pond, it simply displaces that
same volume through the channel protection
orifice. Thus an off-line wet pond providing
only water quality treatment can use a simple
overflow weir as the outlet structure.
Figure 4.25-7 Typical Pond Outlet Structure
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»» A number of outlets at varying depths in the
riser provide internal flow control for routing
of the water quality, channel protection, and
overbank flood protection runoff volumes.
The number of orifices can vary and is
usually a function of pond design.
• The water quality outlet (for a wet ED or
micropool ED pond) and channel protection
outlet should be fitted with adjustable gate
valves or other mechanism(s) that can be used
to adjust detention time.
• Higher flows (overbank and extreme flood
protection) pass through openings or slots
protected by trash racks further up on the riser.
• After entering the riser, flow is conveyed
through the barrel and discharged
downstream. Anti-seep collars should be
installed on the outlet barrel to reduce the
potential for pipe failure.
• Riprap, plunge pools or pads, or other energy
dissipators should be placed at the outlet of
the barrel to prevent scouring and erosion. If a
pond daylights to a channel with dry weather
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flow, care should be taken to minimize tree
clearing along the downstream channel, and
to reestablish a forested riparian zone in the
shortest possible distance.
• Each pond must have a bottom drain pipe with
an adjustable valve that can completely or
partially drain the pond within 24 hours. This
requirement may be waived for coastal areas,
where positive drainage is difficult to achieve
due to very low relief.
• The pond drain should be sized one pipe size
larger than the calculated design diameter. The
drain valve is typically a hand-wheel activated
knife or gate valve. Valve controls should be
located inside of the riser at a point where they
will not normally be inundated and can be
operated in a safe manner.
See the design procedures in Subsection 4.25.6 as
well as Section 3.3 (Storage Design) and
Section 3.4 (Outlet Structures) for additional information and specifications on pond routing and
• A minimum of 1 foot of freeboard must be
provided, measured from the top of the water
surface elevation for the extreme flood to the
lowest point of the dam embankment, not
counting the emergency spillway.
4.25.5.7 MAINTENANCE ACCESS
• A maintenance right-of-way or easement must
be provided to a pond from a public or private
road. Maintenance access should be at least 12
feet wide, have a maximum slope of no more
than 15%, and be appropriately stabilized to
support maintenance equipment and vehicles.
Stormwater Best Management Practices
»» Flow control from a stormwater pond is
typically accomplished with the use of a
concrete or corrugated metal riser and barrel.
The riser is a vertical pipe or inlet structure
that is attached to the base of the pond with
a watertight connection. The outlet barrel is
a horizontal pipe attached to the riser that
conveys flow under the embankment (see
Figure 4.25-7). The riser should be located
within the embankment for maintenance
access, safety, and aesthetics.
• The maintenance access must extend to the
forebay, safety bench, riser, and outlet and,
to the extent feasible, be designed to allow
vehicles to turn around.
• Access to the riser should be provided by
lockable manhole covers, and manhole steps
within easy reach of valves and other controls.
outlet works.
4.25.5.6 EMERGENCY SPILLWAYS
• An emergency spillway should be included in
the stormwater pond design to safely pass the
extreme flood flow. The spillway prevents pond
water levels from overtopping the embankment
and causing structural damage. The
emergency spillway must be located so that
downstream structures will not be impacted by
spillway discharges.
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• Based on pond embankment height and pond
storage volume, it may have to be designed to
State of Georgia rules for dam safety. Check
the latest rules and corresponding laws.
• Fencing of ponds is not generally desirable, but
may be required by the local review authority.
A preferred method is to manage the contours
of the pond through the inclusion of a safety
bench (see above) to eliminate drop-offs and
reduce the potential for accidental drowning. In
addition, the safety bench may be landscaped
to deter access to the pool.
• The principal spillway opening should not
permit access by small children. Ponds with
endwalls above pipe outfalls greater than 48
inches in diameter should be fenced to prevent
access. Warning signs should be posted near
the pond to prohibit swimming and fishing in
the facility.
4.25.5.9 LANDSCAPING
• Aquatic vegetation can play an important role
in pollutant removal in a stormwater pond,
including enhancing the appearance of the
pond, stabilizing side slopes, serving as wildlife
habitat, and temporarily concealing unsightly
trash and debris. Therefore, wetland plants
should be encouraged in a pond design,
along the aquatic bench (fringe wetlands),
safety bench, and side slopes (ED ponds), and
within shallow areas of the pool itself. The
best elevations for establishing wetland plants,
either through transplantation or volunteer
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colonization, are within 6 inches (plus or
minus) of the normal pool elevation. Additional
information on establishing wetland vegetation
and appropriate wetland species for Georgia
can be found in Appendix D (Planting and Soil
Guidance).
• Fish such as Gambusia can be stocked in a
pond to aid in mosquito prevention.
• A fountain or solar-powered aerator may
be used for oxygenation of water in the
permanent pool.
• Woody vegetation may not be planted on the
embankment or allowed to grow within 15 feet
of the toe of the embankment or 25 feet from
the principal spillway structure.
• Compatible multi-objective use of stormwater
ponds is strongly encouraged to leverage
additional benefits.
• A pond buffer should be provided that extends
25 feet outward from the maximum water
surface elevation of the pond. The pond buffer
should be contiguous with other buffer areas
that are required by existing regulations (e.g.,
stream buffers) or that are part of the overall
stormwater management concept plan. No
structures should be located within the buffer,
and an additional setback to permanent
structures may be provided.
4.25.5.10 CONSTRUCTION CONSIDERATIONS
• Construction equipment should be restricted
from the stormwater pond area to prevent
compaction of the native soils.
Stormwater Best Management Practices
4.25.5.8 SAFETY FEATURES
• Contributing drainage areas should be properly
stabilized with the appropriate erosion and
sediment controls or permanent seeding
before allowing stormwater runoff to drain to
the stormwater pond.
• Existing trees should be preserved in the
buffer area during construction. It is desirable
to locate forest conservation areas adjacent
to ponds. To discourage resident geese
populations, the buffer can be planted with
trees, shrubs and native ground covers.
• The soils of a pond buffer are often severely
compacted during construction to ensure
stability. The density of these compacted
soils is so great that it effectively prevents
root penetration and, therefore, may lead
to premature mortality or loss of vigor.
Consequently, it is advisable to excavate large,
deep holes around the proposed planting sites
and backfill these with uncompacted topsoil.
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A recent study (Brown and Schueler, 1997) estimated that construction costs for stormwater
ponds could be calculated with the following
equation:
C = 24.5V0.705
Where:
C = Construction cost and
V = Volume in the pond to include the
10-year storm (ft3).
Using this equation, typical construction costs are:
Stormwater Best Management Practices
4.25.5.11 CONSTRUCTION AND MAINTENANCE
COSTS
• $45,700 for a 1 acre-foot facility
• $232,000 for a 10 acre-foot facility
• $1,170,000 for a 100 acre-foot facility
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(Step 1) Determine if the development site and conditions are ap-
Complete Step 3 for a water quality (treatment) approach.
Refer to your local community’s guidelines for any additional
propriate for the use of a stormwater pond.
information or specific requirements regarding the use of
Consider the application and site feasibility criteria in this
either method.
chapter. In addition, determine if site conditions are suitable
for a stormwater pond. Create a rough layout of the storm-
(Step 3) Calculate the Target Water Quality Volume
water pond dimensions taking into consideration existing
Calculate the Water Quality Volume using the following
trees, utility lines, and other obstructions.
formula:
WQV = (1.2) (RV) (A) / 12
(Step 2) Determine the goals and primary function of the stormwater pond.
Consider whether the stormwater pond is intended to:
»» Meet a water quality (treatment) target. See Step 3
to size the BMP utilizing the water quality treatment
approach.
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CP v)
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
Stormwater Best Management Practices
4.25.6 Design Procedures
RV = Volumetric runoff coefficient which can be
found by:
RV = 0.05+0.009(I)
»» Provide a possible solution to a drainage problem
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
(Step 4) Determine the pretreatment volume.
A sediment forebay should be provided at each inlet, unless
the inlet provides less than 10% of the total design storm
inflow to the pond. The forebay should be sized to contain
0.1 inches per impervious acre of contributing drainage and
should be 4-6 feet deep. The forebay storage volume counts
toward the total WQv requirement and may be subtracted
from the WQv for subsequent calculations.
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volume).
tion is used (i.e., an over-perforated vertical stand pipe with
½-inch orifices or slots that are protected by wirecloth and a
»» Wet Pond: Size permanent pool volume to 1.0 WQv
stone filtering jacket). Adjustable gate valves can also be used
»» Wet ED Pond: Size permanent pool volume to 0.5 WQv
to achieve this equivalent diameter.
and extended detention volume to 0.5 WQv
»» Micropool ED Pond: Size permanent pool volume to
25-30% of WQv and e extended detention volume to
remainder of WQv
Wet ED Pond and Micropool ED Pond: Based on the elevations established in Step 6 for the extended detention portion of the water quality volume, the water quality orifice is
sized to release this extended detention volume in 24 hours.
The water quality orifice should have a minimum diame-
(Step 6) Determine the pond location and preliminary geometry.
ter of 3 inches and be adequately protected from clogging
Conduct pond grading and determine storage volume avail-
by an acceptable external trash rack. A reverse slope pipe
able for the permanent pool (and water quality extended
attached to the riser, with its inlet submerged 1 foot below
detention volume as appropriate).
the elevation of the permanent pool, is a recommended de-
This step involves initially grading the pond (establishing
sign. Adjustable gate valves can also be used to achieve this
contours) and determining the elevation-storage relationship
equivalent diameter. The CPv elevation is then determined
for the pond.
»» Include safety and aquatic benches
from the stage-storage relationship. The invert of the chan-
»» Set WQv permanent pool elevation (and WQv-ED elevation
for wet ED and micropool ED ponds)
Stormwater Best Management Practices
(Step 5) Determine permanent pool volume (and water quality ED
nel protection orifice is located at the water quality extended
detention elevation, and the orifice is sized to release the
channel protection storage volume over a 24-hour period
(12-hour extended detention may be warranted in some cold
(Step 7) Compute extended detention orifice release rate(s) and
size(s), and establish the CPv elevation.
Wet Pond: The CPv elevation is determined from the
stage-storage relationship and the orifice is then sized to
release the channel protection storage volume over a 24hour period (12-hour extended detention may be warranted
in some cold water stream basins). The channel protection
orifice should have a minimum diameter of 3 inches and
should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the
riser, with its inlet submerged 1 foot below the elevation of
the permanent pool, is a recommended design. The orifice
diameter may be reduced to 1 inch if internal orifice protec-
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water streams).
(Step 8) Calculate the Qp25 release rate and water surface elevation.
Set up a stage-storage-discharge relationship for the control
structure for the extended detention orifice(s) and the 25year, 24-hour rainfall event.
(Step 9) Design embankment(s) and spillway(s).
To size the emergency spillway, calculate the 100-year,
24-hour storm water surface elevation. Set the top of the
embankment elevation at least one foot higher, and analyze
safe passage of the Extreme Flood Volume (Qf).
At final design, provide safe passage for the 100-year, 24hour rainfall event.
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4.25.7 Inspection and Maintenance Requirements
The design and construction of stormwater management
All best management practices require proper maintenance. Without proper
ponds are required to follow the latest version of the State of
maintenance, BMPs will not function as originally designed and may cease to
Georgia dam safety rules.
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
(Step 11) Design inlets, sediment forebay(s), outlet structures, main-
and maintenance requirements, see Appendix E.
tenance access, and safety features.
See Subsection 4.25.5.4 to 4.25.5.8 for more details.
Additional Resources
(Step 12)Prepare site Vegetation and Landscaping Plan.
Brown, W., and T. Schueler. 1997. The Economics of Stormwater BMPs in the
A landscaping plan for a stormwater pond and its buffer
Mid-Atlantic Region. Prepared for the Chesapeake Research Consortium,
should be prepared to indicate how aquatic and terrestrial
Edgewater, MD, by the Center for Watershed Protection, Ellicott City, MD.
areas will be stabilized and established with vegetation.
Stormwater Best Management Practices
(Step 10) Investigate potential pond hazard classification.
See Subsection 4.25.5.9 (Landscaping) and Appendix D for
more details.
See Appendix B-1 for a Stormwater Pond Design Example
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Minimum contributing drainage area of 25 acres; 5 acres for pocket wetland
• Two design variations (level 1 and level 2) to achieve different pollutant
removal rates
• Outflow hydrograph should mimic the existing conditions hydrograph,
where applicable
Description: Constructed wetland systems used
for stormwater management. Runoff volume is
both stored and treated in the wetland facility
which consists of a shallow impoundment with
a permanent pool designed to mimic natural
wetlands.
LID/GI Considerations: Wetlands should not be
designed close to the source of runoff as LID dictates because it is not practical or cost effective.
However wetlands employ several LID/GI characteristics, for example mimicking natural systems
and providing infiltration and evapotranspiration.
Runoff Reduction
Water Quality
Channel Protection
• Design should include water balance analysis and landscaping
Overbank Flood Protection
ADVANTAGES / BENEFITS
• Good nutrient removal for level 2
• Provides natural wildlife habitat
• Relatively low maintenance costs
• Provides moderate to high removal of many of the pollutants of concern
typically contained in post-construction stormwater runoff
• Ideal for use in flat terrain and in areas with high groundwater
Extreme Flood Protection
DISADVANTAGES / LIMITATIONS
• Requires large land area
• Needs continuous baseflow for viable wetland
• Sediment regulation is critical to sustain wetlands
• Provides minimal reduction of post-construction stormwater runoff
volumes
• More costly than some BMPs
• Difficulties in maintaining the permanent pool may arise
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Capital Cost
Maintenance Burden:
Shallow Wetland
ED Shallow Wetland
Pocket Wetland
Pond/Wetland
MAINTENANCE REQUIREMENTS
• Plant replacement vegetation in any eroded areas.
• Remove invasive vegetation
• Monitor sediment accumulation and remove periodically
• A valve will be required to dewater the wetland
Residential Subdivision Use: Yes
High Density/Ultra-Urban: No
Drainage Area: Minimum contributing
drainage area of 25 acres; 5 acres for
pocket wetland
Roadway Projects: Not applicable
POLLUTANT REMOVAL
Level 1
Soils: Hydrologic group ‘A’ and ‘B’
soils may require a liner (not relevant
for pocket wetland)
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
Level 2
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STORMWATER MANAGEMENT
SUITABILITY
Stormwater Best Management Practices
4.26 Stormwater Wetlands
Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• Level 1: 0% - Runoff Reduction Credit
• Level 2: 0% - Runoff Reduction Credit
• Accepts Hotspot Runoff: Yes, 2 feet of
separation distance required to water
table
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Stormwater wetlands (also referred to as constructed
Shallow Wetland
Shallow ED Wetland
wetlands) are constructed shallow marsh systems
that are designed to both treat urban stormwater and
control runoff volumes (see Figure 4.26-1 for examples). As stormwater runoff flows through the wetland facility, pollutant removal is achieved through
settling and uptake by marsh vegetation.
Wetlands are among the most effective stormwater
practices in terms of pollutant removal and also offer
aesthetic value and wildlife habitat. Constructed
stormwater wetlands differ from natural wetland
systems in that they are engineered facilities designed
Newly Constructed Shallow Wetland
Pocket Wetland
Stormwater Best Management Practices
4.26.1 General Description
specifically for the purpose of treating stormwater
runoff and typically have less biodiversity than natural
wetlands both in terms of plant and animal life. However, as with natural wetlands, stormwater wetlands
require a continuous base flow or a high water table
to support aquatic vegetation.
Wetlands are divided into two levels. Level 1 stormwater wetlands can be used to meet water quality
volume, channel protection volume, and the 25-year,
24-hour storm event. A riser is used to create the
pool and a small orifice is placed in the riser above
Figure 4.26-1 Stormwater Wetland Examples
the bottom of the wetland to create a shallow permanent pool. This allows the wetland to store additional runoff for a short period of time. Storm events
that are greater than the design volume can be
released through the top of the riser and /or through
an emergency spill way channel.
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360
meet water quality requirements. They cannot be
used for extended detention, so the outlet structure should be simplified. Level 2 wetlands can be
installed parallel to wet detention ponds to meet
detention requirements and help maintain the
wetland permanent pool level.
There are several design variations of the stormwater wetland, each differing in the relative
amounts of shallow and deep water, and dry storage above the wetland. These include shallow
wetland, extended detention shallow wetlands,
pond/wetland systems, and pocket wetlands. Below are descriptions of each design variant:
• Shallow Wetland (Level 2) – In the shallow
wetland design, most of the water quality
treatment volume is in the relatively shallow
high marsh or low marsh depths. The only
deep portions of the shallow wetland design
are the forebay at the inlet to the wetland, and
the micropool at the outlet. One disadvantage
of this design is that, since the pool is very
shallow, a relatively large amount of land is
typically needed to store the water quality
volume.
• Extended Detention (ED) Shallow Wetland
(Level 1)– The extended detention (ED) shallow
wetland design is the same as the shallow
wetland; however, part of the water quality
treatment volume is provided as extended
detention above the surface of the marsh and
released over a period of 24 hours. This design
can treat a greater volume of stormwater in a
smaller space than the shallow wetland design.
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In the extended detention wetland option,
plants that can tolerate both wet and dry
periods need to be used in the ED zone.
• Pond/Wetland Systems (Level 1) – The pond/
wetland system has two separate cells: a wet
pond and a shallow marsh. The wet pond traps
sediment and reduces runoff velocities prior
to entry into the wetland, where stormwater
flows receive additional treatment. Less land
is required for a pond/wetland system than for
the shallow wetland or the ED shallow wetland
systems.
• Pocket Wetland (Level 2) – A pocket wetland is
intended for drainage areas of 5-10 acres and
typically requires excavation down to the water
table for a reliable water source to support the
wetland system.
Certain types of wetlands, such as submerged
gravel wetland systems are not recommended
for general use to meet stormwater management
goals due to limited performance data. They
may be applicable in special or retrofit situations
where there are severe limitations on what can be
implemented. Please see a further discussion in
Section 4..27.
4.26.2 Stormwater Management Suitability
• Runoff Reduction
None. Although stormwater wetlands provide
moderate to high removal of many of the
pollutants of concern typically contained in
post-construction stormwater runoff, recent
research shows that they provide little, if any,
reduction of post-construction stormwater
runoff volumes (Hirschman et al., 2008,
Strecker et al., 2004).
• Water Quality
Pollutants are removed from stormwater runoff
in a wetland through uptake by vegetation
and algae, filtering, and gravitational settling in
the slow moving marsh flow. Other pollutant
removal mechanisms are also at work in a
stormwater wetland, including chemical and
biological decomposition, and volatilization.
Subsection 4.26.3 provides median pollutant
removal efficiencies that can be used for
planning and design purposes.
Stormwater Best Management Practices
Level 2 stormwater wetlands are mainly used to
• Channel Protection
The storage volume above the permanent
pool/water surface level in a stormwater
wetland is used to provide control of the
channel protection volume (CP v) by releasing
the 1-year, 24-hour storm runoff volume
over 24 hours (extended detention). It is best
to do this with minimum vertical water level
fluctuation, as extreme fluctuation may stress
vegetation.
Similar to stormwater ponds, stormwater wetlands
are designed to control both stormwater quantity
and quality. Thus, a stormwater wetland can be used
to address all of the unified stormwater sizing criteria
for a given drainage area.
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Database (3rd Edition) available at www.cwp.org
and the National Stormwater Best Management
Practices (BMP) Database at www.bmpdatabase.
org.
4.26.4 Application and Site Feasibility
Criteria
Stormwater wetlands are generally applicable to
most types of new development and redevelop-
• Extreme Flood Protection
In situations where it is required, stormwater
wetlands can also be used to provide detention
to control the 100-year, 24-hour storm
peak flow (Qf). Where Qf peak control is
not required, a stormwater wetland must be
designed to safely pass extreme storm flows.
ment, and can be utilized in both residential and
nonresidential areas. However, due to the large
land requirements, wetlands may not be practical
in higher density areas.
The following criteria should be evaluated to
ensure the suitability of a stormwater wetland for
meeting stormwater management objectives on a
4.26.3 Pollutant Removal Capabilities
site or development.
The pollution removal rates below may be used
for design purposes. It should be noted that for
General Feasibility
this BMP there are two different types of Storm-
- Suitable for Residential Subdivision Usage – YES
water Wetlands that can be constructed. Level
- Suitable for High Density/Ultra Urban Areas –
1 wetlands are a more traditional design and can
be used to meet WQV, CPv, QP25, and Qf require-
Land requirements may preclude use
- Regional Stormwater Control – YES
ments. Level 2, on the other hand, has a different
design configuration that includes modifications
after nearly 20 years of research. Both of these
Physical Feasibility - Physical Constraints at
levels will be explained in greater detail in this
Project Site
section.
For additional information and data on pollutant
removal capabilities for stormwater wetlands,
see the National Pollutant Removal Performance
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• Drainage Area – A minimum of 25 acres
and a positive water balance are needed to
maintain wetland conditions; 5 acres for pocket
wetlands
• Space Required – Approximately 3 to 5% of the
tributary drainage area
• Site Slope – There should be no more than
8% slope across the wetland site for level 1
wetlands. Level 2 wetlands should be generally
flat.
• Minimum Head – Elevation difference needed
at a site from the inflow to the outflow: 3 to 5
feet; 2 to 3 feet for pocket wetland
• Minimum Depth to Water Table – If used on a
site with an underlying water supply aquifer or
when treating a hotspot, a separation distance
of 2 feet is recommended between the
bottom of the wetland and the elevation of the
seasonally high water table; pocket wetlands
are typically below the water table.
Stormwater Best Management Practices
• Overbank Flood Protection
A stormwater wetland can also provide storage
above the permanent pool/water surface level
to reduce the post-development peak flow of
the 25-year storm (Qp25) to pre-development
levels (detention). If a wetland facility is not
used for overbank flood protection, it should
be designed as an off-line system to pass
higher flows around rather than through the
wetland system.
• Soils – Permeable soils are not well-suited for
a constructed stormwater wetland without a
high water table. Underlying soils of hydrologic
group “C” or “D” should be adequate to
maintain wetland conditions. Most group “A”
soils and some group “B” soils will require a
liner. Evaluation of soils should be based upon
a subsurface analysis and permeability tests.
• Setback requirements:
»» Property lines - 10 feet (this is for site
development projects only)
»» Private wells - 100 feet; if well is
downgradient from a hotspot land use then
the minimum setback is 250 feet
»» Septic system tank/leach field - 50 feet
»» Airports - 5 miles
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• Trout Streams – Consideration should be given to the thermal influence of stormwater wetland outflows on downstream trout waters.
• Coastal Areas - Several site characteristics commonly encountered in coastal Georgia may present challenges to site planning and design teams interested in
using stormwater wetlands to manage post-construction stormwater runoff on a development site. Table 4.26-1 identifies these common site characteristics
and describes how they influence the use of stormwater wetlands on development sites. The table also provides site planning and design teams with some ideas
about how they can work around these potential constraints.
Table 4.26-1: Challenges Associated with Using Stormwater Wetlands in Coastal Georgia
Site Characteristic
How it Influences the Use of Stormwater Wetlands
Poorly draining soils, such
as hydrologic soil group C
and D soils
•
Since stormwater wetlands are designed to have a permanent water surface, the presence of poorly drained soils
does not influence their use on development sites. In fact,
the presence of poorly draining soils may help maintain a
permanent water surface within a stormwater wetland.
Well draining soils, such as
hydrologic soil group A and
B soils
•
May be difficult to maintain a permanent water surface
within a stormwater wetland.
May allow stormwater pollutants to reach groundwater
aquifers with greater ease.
•
Potential Solutions
•
•
•
Install a liner to maintain a permanent water surface.
At stormwater hotspots and in areas known to provide groundwater recharge to
water supply aquifers, install a liner to prevent pollutants from reaching underlying groundwater aquifers.
In areas that are not considered to be stormwater hotspots and areas that do
not provide groundwater recharge to water supply aquifers, use non-underdrained bioretention areas (Section 4.2) and infiltration practices (Section 4.12) to
significantly reduce stormwater runoff volumes.
Flat terrain
•
Makes it difficult, if not impossible, to provide a drain at
the bottom of a stormwater wetland.
•
Eliminate the use of drains, if necessary.
Shallow water table
•
Makes it easier to maintain a permanent water surface
within a stormwater wetland
May allow stormwater pollutants to reach groundwater
aquifers with greater ease.
•
Excavation below the water table to create a stormwater wetland is acceptable,
but any storage volume found below the water table should not be counted when
determining the total storage volume provided by the stormwater wetland.
At stormwater hotspots and in areas known to provide groundwater recharge to
water supply aquifers, install a liner to prevent pollutants from reaching underlying groundwater aquifers.
Use bioretention areas (Section 4.2) and infiltration practices (Section 4.12) with liners
and underdrains to intercept and treat stormwater runoff at stormwater hotspots
and in areas known to provide groundwater recharge to water supply aquifers.
•
•
•
Tidally-influenced drainage
system
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•
May occasionally prevent stormwater runoff from being
conveyed through a stormwater wetland, particularly
during high tide.
•
•
Stormwater Best Management Practices
Other Constraints/Considerations
Maximize the use of low impact development practices in these areas to reduce
stormwater runoff rates, volumes and pollutant loads.
Consider the use of bubbler aeration and proper fish stocking to maintain nutrient
cycling and healthy oxygen levels in stormwater wetlands located in these areas.
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The following criteria are to be considered minimum standards for the design of a stormwater
wetland facility. Consult with the local review
authority to determine if there are any variations
to these criteria or additional standards that must
be met.
4.26.5.1 LOCATION AND LAYOUT
• A continuous base flow or high water table is
required to support wetland vegetation. A water
balance must be performed to demonstrate that
a stormwater wetland can withstand a 30-day
drought at summer evaporation rates without
completely drawing down.
• Wetland siting should also take into account
the location and use of other site features
such as natural depressions, buffers, and
undisturbed natural areas, and should attempt
to aesthetically “fit” the facility into the
landscape. Bedrock close to the surface may
prevent excavation.
• Stormwater wetlands cannot be located
within navigable waters of the U.S., including
wetlands, without obtaining a Section 404
permit under the Clean Water Act, and any
other applicable State permit. In some isolated
cases, a wetlands permit may be granted to
convert an existing degraded wetland in the
context of local watershed restoration efforts.
• If a wetland facility is not used for overbank
flood protection, it should be designed as an
off-line system to bypass higher flows rather
than passing them through the wetland system.
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• All utilities should be located outside of the
wetland site.
• Additional pond design features include an
emergency spillway, maintenance access,
safety bench, wetland buffer, and appropriate
wetland vegetation and native landscaping.
4.26.5.2 GENERAL DESIGN
• A well-designed stormwater wetland consists
of:
1. Shallow marsh areas of varying depths with
wetland vegetation,
2. Permanent micropool(s)
3. An overlying zone where runoff control
volumes are stored.
Figures 4.26-2 through 4.26-5 in Subsection
4.26.8 provide plan view and profile schematics
for the design of a shallow wetland, ED shallow
wetland, pond/wetland system, and pocket wetland.
• Pond/wetland systems also include a
stormwater pond facility (see Section
4.25, Stormwater Ponds, for pond design
information).
Stormwater Best Management Practices
4.26.5 Planning and Design Criteria
• In addition, all wetland designs must include
a sediment forebay at the inflow to the facility
to allow heavier sediments to drop out of
suspension before the runoff enters the
wetland marsh.
Table 4.26-2 Approximate Level 1 and 2 Dimensional Information for Various Wetland Zones
Wetland Zone
Criteria
Level 1 Design
Level 2 Design
Deep Pools
Depth
-18” to -72”
-18" to -48"
% of Total Volume
20%
25%
Low Marsh
High Marsh
Low Land
Depth
-6” to -18”
N/A
% of Total Volume
20%
N/A
Depth
-6” to 0”
-6" to +6"
% of Total Volume
10%
75%
Depth
0"+
N/A
% of Total Volume
50%
N/A
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Table 4.26-3 Design Criteria for Level 1 and 2 Stormwater Wetlands
Criteria
Level 1
Level 2
WQV
Use rainfall depth of 1.2
Use rainfall depth of 1.2
Deep pools
2 (forebay and outlet)
3 (forebay, middle, outlet)
design of a stormwater wetland that must be
Wetland side slopes (max)
3:1
5:1
observed for adequate pollutant removal, ease of
Slope profile
8% across the site
Should be generally flat, use several cells if needed; use a maximum drop of 1' between cells
Normal flow path (distance from
inlet to oulet)
1:1
1.5:1
Normal flow path (distance from
inlet to outlet)
1:1
1.5:1
Dry weather flow path
Not required
5:1
Vegetation
Can use herbaceous only
Include woody vegetation (trees
and shrubs)
four basic depth zones are:
Average wetland depth
Can be >1
Should be < 1
• Deepwater zone
Includes the outlet micropool and deepwater
channels through the wetland facility. This
zone supports little emergent wetland
vegetation, but may support submerged or
floating vegetation.
Extended detention
Limit to 1' vertically
Not allowed
In general, wetland designs are unique for each
site and application. However, there are a number
of geometric ratios and limiting depths for the
maintenance, and improved safety.
The stormwater wetland should be designed with
the recommended proportion of “depth zones.”
Each of the four wetland design variants has
depth zone allocations that are given as a percentage of the stormwater wetland surface area.
Target allocations are found in Table 4.26-2. The
• Low marsh zone
This zone is suitable for the growth of several
emergent wetland plant species.
• High marsh zone
This zone will support a greater density and
diversity of wetland species than the low marsh
zone. The high marsh zone should have a
higher surface area to volume ratio than the
low marsh zone.
• Semi-wet zone
This zone includes areas above the permanent
pool that are inundated during larger storm
events; it supports a number of species that
can survive flooding.
Stormwater Best Management Practices
4.26.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
It is recommended that a stream restoration
specialist be consulted for additional guidance on
these items.
• Micro-topology is important to Level 2 wetland
designs. Planting peninsulas are the preferred
method, but the following list can be used to
enhance micro-topology:
»» Snags
»» Inverted root wads
»» Tree peninsulas
»» Coir fiber islands
»» Internal pools
»» Cobble sand weirs
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Stormwater Best Management Practices
4.26.5.4 PRETREATMENT/INLETS
• Sediment regulation is critical to sustain
stormwater wetlands. A wetland facility
should have a sediment forebay or equivalent
upstream pretreatment. A sediment forebay is
designed to remove incoming sediment from
the stormwater flow prior to dispersal into
the wetland. The forebay should consist of a
separate cell, formed by an acceptable barrier.
A forebay is to be provided at each inlet, unless
the inlet provides less than 10% of the total
design storm inflow to the wetland facility.
• The forebay is sized to contain 0.1 inches per
impervious acre of contributing drainage and
should be 4 to 6 feet deep. The pretreatment
storage volume is part of the total WQv
requirement and may be subtracted from WQv
for wetland storage sizing.
• A fixed vertical sediment depth marker shall be
installed in the forebay to measure sediment
deposition over time. The bottom of the
forebay may be hardened (e.g., using concrete,
paver blocks, etc.) to make sediment removal
easier.
• Inflow channels are to be stabilized with flared
riprap aprons, or the equivalent. Inlet pipes
to the pond can be partially submerged. Exit
velocities from the forebay must be nonerosive.
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Figure 4.26-6 Typical Wetland Facility Outlet Structure
4.26.5.5 OUTLET STRUCTURES
• Flow control from a stormwater wetland
is typically accomplished with the use of a
concrete or corrugated metal riser and barrel.
The riser is a vertical pipe or inlet structure that
is attached to the base of the micropool with
a watertight connection. The outlet barrel
is a horizontal pipe attached to the riser that
conveys flow under the embankment (see
Figure 4.26-6) The riser should be located
within the embankment for maintenance
access, safety and aesthetics.
• A number of outlets at varying depths in the
riser provide internal flow control for routing
of the water quality, channel protection, and
overbank flood protection runoff volumes. The
number of orifices can vary and is usually a
function of the pond design.
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a 24-hour period (12-hour extended detention
figuration is typically comprised of a channel
may be warranted in some cold water streams).
protection outlet (usually an orifice) and overbank
flood protection outlet (often a slot or weir). The
Alternative hydraulic control methods to an orifice
channel protection orifice is sized to release the
can be used and include the use of a broad-crest-
channel protection storage volume over a 24-
ed rectangular, V-notch, proportional weir, or an
hour period (12-hour extended detention may
outlet pipe protected by a hood that extends at
be warranted in some cold water streams). Since
least 12 inches below the normal pool.
the water quality volume is fully contained in the
• The water quality outlet and channel protection
outlet should be fitted with adjustable gate
valves or other mechanism that can be used to
adjust detention time.
permanent pool, no orifice sizing is necessary for
this volume. As runoff from a water quality event
enters the wet pond, it simply displaces that same
volume through the channel protection orifice. Thus an off-line shallow or pocket wetland
providing only water quality treatment can use a
simple overflow weir as the outlet structure.
In the case of an extended detention (ED) shallow
wetland, there is generally a need for an additional
outlet (usually an orifice) that is sized to pass the
extended detention water quality volume that is
surcharged on top of the permanent pool. Flow
will first pass through this orifice, which is sized to
release the water quality ED volume in 24 hours.
The preferred design is a reverse slope pipe attached to the riser, with its inlet submerged 1 foot
below the elevation of the permanent pool to
prevent floatables from clogging the pipe and to
avoid discharging warmer water at the surface of
the pond. The next outlet is sized for the release
of the channel protection storage volume. The
outlet (often an orifice) invert is located at the
maximum elevation associated with the extended
detention water quality volume and is sized to release the channel protection storage volume over
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• Higher flows (overbank and extreme flood
protection) flows pass through openings or
slots protected by trash racks further up on the
riser.
• After entering the riser, flow is conveyed
through the barrel and is discharged
downstream. Anti-seep collars should be
installed on the outlet barrel to reduce the
potential for pipe failure.
• Riprap, plunge pools or pads, or other energy
dissipators are to be placed at the outlet of the
barrel to prevent scouring and erosion. If a
wetland facility daylights to a channel with dry
weather flow, care should be taken to minimize
tree clearing along the downstream channel,
and to reestablish a forested riparian zone in
the shortest possible distance. See Section 5.5
(Energy Dissipation Design) for more guidance.
• The wetland facility must have a bottom
drain pipe located in the micropool with an
adjustable valve that can completely or partially
dewater the wetland within 24 hours. (This
requirement may be waived for coastal areas,
where positive drainage is difficult to achieve
due to very low relief)
• The wetland drain should be sized one
pipe size greater than the calculated design
diameter. The drain valve is typically a
handwheel activated knife or gate valve. Valve
controls shall be located inside of the riser
at a point where they (a) will not normally be
inundated and (b) can be operated in a safe
manner.
See the design procedures in Subsection 4.26.6 as
well as Section 3.3 (Storage Design) and Section
3.4 (Outlet Structures) for additional information
Stormwater Best Management Practices
For shallow and pocket wetlands, the riser con-
and specifications on pond routing and outlet
works.
4.26.5.6 EMERGENCY SPILLWAY
• An emergency spillway shall be included in
the stormwater wetland design to safely pass
flows that exceed the design storm flows.
The spillway prevents the wetland’s water
levels from overtopping the embankment and
causing structural damage. The emergency
spillway must be located so that downstream
structures will not be impacted by spillway
discharges.
• A minimum of 1 foot of freeboard must be
provided, measured from the top of the water
surface elevation for the extreme flood to the
lowest point of the dam embankment, not
counting the emergency spillway.
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4.26.5.9 LANDSCAPING
• A maintenance right of way or easement
must be provided to the wetland facility
from a public or private road. Maintenance
access should be at least 12 feet wide, have
a maximum slope of no more than 15%,
and be appropriately stabilized to withstand
maintenance equipment and vehicles.
• A landscaping plan should be provided that
indicates the methods used to establish
and maintain wetland coverage. Minimum
elements of a plan include: delineation of
landscaping zones, selection of corresponding
plant species, planting plan, sequence
for preparing wetland bed (including soil
amendments, if needed) and sources of plant
material.
• The maintenance access must extend to the
forebay, safety bench, riser, and outlet and,
to the extent feasible, be designed to allow
vehicles to turn around.
• Access to the riser is to be provided by lockable
manhole covers, and manhole steps within
easy reach of valves and other controls.
4.26.5.8 SAFETY FEATURES
• All embankments and spillways must be
designed to State of Georgia guidelines for dam
safety.
• Fencing of wetlands is not generally desirable,
but may be required by the local review
authority. A preferred method is to manage
the contours of deep pool areas through the
inclusion of a safety bench (see above) to
eliminate dropoffs and reduce the potential for
accidental drowning.
• The principal spillway opening should not
permit access by small children, and endwalls
above pipe outfalls greater than 48 inches in
diameter should be fenced to prevent a hazard.
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• Landscaping zones include low marsh, high
marsh, and semi-wet zones. The low marsh
zone ranges from 6 to 18 inches below the
normal pool. This zone is suitable for the
growth of several emergent plant species.
The high marsh zone ranges from 6 inches
below the pool up to the normal pool. This
zone will support greater density and diversity
of emergent wetland plant species. The high
marsh zone should have a higher surface area
to volume ratio than the low marsh zone. The
semi-wet zone refers to those areas above
the permanent pool that are inundated on an
irregular basis and can be expected to support
wetland plants.
with an additional 15-foot setback to structures.
The wetland buffer should be contiguous with
other buffer areas that are required by existing
regulations (e.g., stream buffers) or that are
part of the overall stormwater management
concept plan. No structures should be located
within the buffer, and an additional setback to
permanent structures may be provided.
• Existing trees should be preserved in the
buffer area during construction. It is desirable
to locate forest conservation areas adjacent
to ponds. To discourage resident geese
populations, the buffer can be planted with
trees, shrubs and native ground covers.
Stormwater Best Management Practices
4.26.5.7 MAINTENANCE ACCESS
• The soils of a wetland buffer are often severely
compacted during the construction process
to ensure stability. The density of these
compacted soils is so great that it effectively
prevents root penetration and therefore may
lead to premature mortality or loss of vigor.
Consequently, large and deep holes should be
excavated around the proposed planting sites
and backfill these with uncompacted topsoil.
Guidance on establishing wetland vegetation can
be found in Appendix D (Planting and Soil Guid-
• The landscaping plan should provide elements
that promote greater wildlife use within the
wetland and buffers.
ance).
• Woody vegetation may not be planted on the
embankment or allowed to grow within 15 feet
of the toe of the embankment and 25 feet from
the principal spillway structure.
• A wetland buffer shall extend 25 feet outward
from the maximum water surface elevation,
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CRITERIA AND ISSUES
• Physiographic Factors - Local terrain design
constraints
»» Low Relief – Providing wetland drain can be
problematic
»» High Relief – Embankment heights restricted
»» Karst – Requires poly or clay liner to
sustain a permanent pool of water and
protect aquifers; limits on ponding depth;
geotechnical tests may be required
• Soils
»» Hydrologic group “A” soils and some group
“B” soils may require liner (not relevant for
pocket wetland)
Stormwater Best Management Practices
4.26.5.10 ADDITIONAL SITE SPECIFIC DESIGN
• Special Downstream Watershed
Considerations
»» Trout Stream – Design wetland offline and
provide shading to reduce thermal impact;
limit WQv-ED to 12 hours
»» Aquifer Protection – Prevent possible
groundwater contamination by preventing
infiltration of hotspot runoff. May require
liner for type “A” soils; pretreat hotspots; 2 to
4 foot separation distance from water table.
»» Swimming Area/Shellfish – Design for geese
prevention (see Appendix D); provide 48hour ED for maximum coliform die-off.
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369
(Step 1) Determine if the development site and conditions are ap-
Complete Step 3 for a water quality (treatment) approach.
Refer to your local community’s guidelines for any additional
propriate for the use of a stormwater wetland.
information or specific requirements regarding the use of
Consider the application and site feasibility criteria in this
either method.
chapter. In addition, determine if site conditions are suitable for a stormwater wetland. Create a rough layout of the
(Step 3) Calculate the Target Water Quality Volume
stormwater wetland dimensions taking into consideration
Calculate the Water Quality Volume using the following for-
existing trees, utility lines, and other obstructions.
mula:
(Step 2) Determine the goals and primary function of the stormwa-
WQV = (1.2) (RV) (A) / 12
ter wetland.
Consider whether the stormwater wetland is intended to:
Where:
»» Meet a water quality (treatment) target. See Step 3
WQV = Water Quality Volume (ft3)
to size the BMP utilizing the water quality treatment
1.2 = Target rainfall amount to be treated (inches)
approach.
RV = Volumetric runoff coefficient which can be
»» Be “oversized” to include partial credit for storage
capacity for other stormwater requirements (Channel
Protection Volume (CPv)
»» Provide a possible solution to a drainage problem
Stormwater Best Management Practices
4.26.6 Design Procedures
found by:
RV = 0.05+0.009(I)
Where:
Check with local officials and other agencies to determine if
I = new impervious area of the contributing
there are any additional restrictions and/or surface water or
drainage area (%)
watershed requirements that may apply. In addition, consid-
er if the best management practice has any special site-spe-
A = Area draining to this practice (ft2)
cific design conditions or criteria. List any restrictions or
12 = Unit conversion factor (in/ft)
other requirements that may apply or affect the design.
(Step 4) Determine the pretreatment (forebay volume)
The design of the BMP should be centered on the restric-
Size the forebay. The forebay storage volume counts toward
tions/requirements, goals, targets, and primary function(s)
the total runoff reduction volume and may be subtracted
of the BMP, described in this section. By considering the
for the following calculation. Note that the forebay volume
primary function, as well as, topographic and soil conditions,
should be at least 10% of the total water quality volume.
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
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Check water surface elevations for all design storm events to
ensure the stormwater wetland can pass these flows safely.
An overflow must be provided to bypass and/or convey larger flows to the downstream drainage system or stabilized
watercourse. Non-erosive velocities need to be ensured
at the outlet point. The overflow should be sized to safely
pass the peak flows anticipated to reach the practice, up to a
100-year storm event.
(Step 6) Prepare Site Vegetation and Landscaping Plan
A landscaping plan for the submerged gravel wetland should
be prepared to indicate how it will be established with vegetation. See Subsection 4.26.5 (Landscaping) and Appendix D
Stormwater Best Management Practices
(Step 5) Determine the inlet/outlet design and emergency overflow
for more details.
4.26.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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Stormwater Best Management Practices
4.26.8 Example Schematics
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Figure 4.26-2 Schematic of Shallow Wetland
Figure 4.26-3 Schematic of Extended Detention Shallow Wetland
(Source: Center for Watershed Protection)
(Source: Center for Watershed Protection)
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Stormwater Best Management Practices
4.26.8-2 Example Schematics
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Figure 4.26-4 Schematic of Pond/Wetland System
Figure 4.26-5 Schematic of Pocket Wetland
(Source: Center for Watershed Protection)
(Source: Center for Watershed Protection)
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373
KEY CONSIDERATIONS
DESIGN CRITERIA
• Submerged gravel wetlands should be designed as off-line systems sized to
handle only water quality volume.
• Submerged gravel wetland systems need sufficient drainage area to
maintain vegetation.
• The local slope should be relatively flat (<4%).
• Elevation drop from the inlet to the outlet is required to ensure that
hydraulic conveyance by gravity is feasible (generally about 2-5 feet).
• All submerged gravel wetland designs should include a sediment forebay
or equivalent pretreatment measures to prevent sediment and debris from
entering and clogging the gravel bed.
• Unless they receive hot spot runoff, submerged gravel wetland systems can
be allowed to intersect the groundwater table.
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable or this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Description: One or more cells filled with
crushed rock designed to support wetland plants.
ADVANTAGES / BENEFITS
• High total suspended solids removal
• High removal rate of urban pollutants
• Useful in space-limited applications
• Can be located in low-permeability soils with a high groundwater table
Stormwater flows subsurface through the root
zone of the constructed wetland, where pollutant
removal takes place.
LID/GI Considerations: Submerged gravel wetlands are designed with a small footprint area and
provide high total suspended solids and pollutant
removal rates for highly impervious areas.
DISADVANTAGES / LIMITATIONS
• High maintenance requirements
• Drainage through the wetland can be problematic for low-relief terrain.
ROUTINE MAINTENANCE REQUIREMENTS
• Ensure that inlets and outlets for each submerged gravel wetland cell are
free from debris and not clogged.
• Check for sediment buildup in the gravel bed.
• If sediment buildup is preventing flow through the wetland, remove
gravel and sediment from the cell, replace it with clean gravel, and replant
vegetation.
Land Requirement
Stormwater Best Management Practices
4.27 Submerged Gravel Wetlands
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: No
Soils: Submerged gravel wetlands can
be used in almost all soils and geology,
but HSG C or D soils are preferred to
maintain submerged flow.
Other Considerations: Extra space is
recommended for pretreatment and
to keep the practice from clogging
due to sediment and debris.
L=Low M=Moderate H=High
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
RUNOFF REDUCTION CREDIT
• 0% Runoff Reduction Credit is
provided by this practice.
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The submerged gravel wetland system consists
of one or more treatment cells that are filled with
crushed rock or gravel and designed to allow
stormwater to flow subsurface through the root
zone of the constructed wetland. The outlet from
each cell is set at an elevation to keep the rock
or gravel submerged. Wetland plants are rooted
in the media, where they can directly take up
pollutants. In addition, algae and microbes thrive
on the surface area of the rocks. In particular, the
anaerobic conditions on the bottom of the filter
can foster the denitrification process. Although
Figure 4.27-1 Schematic of Submerged Gravel Wetland System
widely used for wastewater treatment in recent
(Source: Center for Watershed Protection; Roux Associates Inc.)
Stormwater Best Management Practices
4.27.1 General Description
years, only a handful of submerged gravel wetland
systems have been designed to treat stormwater.
Mimicking the pollutant removal ability of nature,
this BMP relies on the pollutant-stripping ability of plants and soils to remove pollutants from
runoff. Typical schematics for a submerged gravel
wetland are shown in Figure 4.27-1 and Figure
4.27-2.
Figure 4.27-2 Schematic of Submerged Gravel Wetland System
(Sources: Center for Watershed Protection; Roux Associates Inc.)
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• Runoff Reduction
Submerged gravel wetlands provide minimal
stormwater volume runoff reduction. Another
BMP should be used in a treatment train with
submerged gravel wetlands to provide runoff
reduction.
• Water Quality
If installed as per the recommended design
criteria and properly maintained, 80% total
suspended solids removal will be applied to
the water quality volume (WQv) flowing to the
submerged gravel wetland.
• Extreme Flood Protection
Submerged gravel wetlands do not provide
extreme flood protection. Another BMP should
be used in a treatment train with submerged
gravel wetlands to provide extreme flood
protection or runoff reduction. Additionally, the
submerged gravel wetlands should be designed
off-line or with a bypass for higher flows. (See
Subsection 3.1.5.)
4.27.4 Application and Site Feasibility
Criteria
The submerged gravel wetland system is similar to a
regular stormwater wetland; however, it is filled with
crushed rock or gravel and designed to allow stormwater to flow through the root zone of the constructed wetland. The outlet from each cell is set at
an elevation to keep the rock or gravel submerged.
Wetland plants are rooted in the rock media, where
they can directly take up pollutants. In addition,
4.27.3 Pollutant Removal Capabilities
algae and bacteria grow in the rock media and
provide an additional avenue for pollutant removal
The pollution removal efficiency of the sub-
through biological uptake. Mimicking the pollutant
merged gravel wetland is similar to a typical
removal ability of nature, this structural control relies
wetland. Recent data show a TSS removal rate in
• Channel Protection
Submerged gravel wetlands do not provide
channel protection. Another BMP should be
used in a treatment train with submerged
gravel wetlands to provide channel protection
or runoff reduction. (See Subsection 4.1.6.)
Additionally, the submerged gravel wetlands
should be designed off-line or with a bypass for
higher flows.
on the pollutant-stripping ability of plants and bac-
excess of the 80% goal. This reflects the settling
teria to remove pollutants from runoff.
The following pollutant removal rates are conser-
General Feasibility
• Overbank Flood Protection
Submerged gravel wetlands do not provide
overbank flood protection. Another BMP
should be used in a treatment train with
submerged gravel wetlands to provide
overbank flood protection or runoff reduction.
Additionally, the submerged gravel wetlands
should be designed off-line or with a bypass for
higher flows. (See Subsection 3.1.5.)
vative average pollutant reduction percentages
- Suitable for Residential Subdivision Usage – YES
for design purposes derived from sampling data,
- Suitable for High Density/Ultra Urban Areas – YES
modeling, and professional judgment.
-- Total Suspended Solids – 80%
- Regional Stormwater Control – NO
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Stormwater Best Management Practices
4.27.2 Stormwater Management Suitability
environment of the gravel media. These systems
also exhibit removals of about 50% TP, 20% TN,
The following criteria should be evaluated to ensure
and 50% Zn. The growth of algae and microbes
the suitability of submerged gravel wetlands for
among the gravel media has been determined to
meeting stormwater management objectives on a
be the primary removal mechanism of the sub-
site.
merged gravel wetland.
-- Total Phosphorus – 50%
-- Total Nitrogen – 20%
-- Fecal Coliform – 70%
-- Heavy Metals – 50%
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impermeable liner to prevent groundwater
Coastal Areas
Project Site
contamination and sinkhole formation.
• Poorly Draining Soils, such as hydrologic
soil groups C and D — Since they would
normally be equipped with waterproof liners,
the presence of poorly draining soils does
not influence the use of submerged gravel
wetlands on development sites.
• Drainage Area – In general, submerged
gravel wetlands should be used on sites with
a minimum drainage area of 1 acre to ensure
Other Constraints/Considerations
submerged flow conditions. The maximum
• Hot spots – Submerged gravel wetlands
without a liner should not be used to treat hot
spots that generate higher concentrations of
hydrocarbons, trace metals, or toxicants than
are found in typical stormwater runoff, which
could contaminate groundwater.
drainage area for a submerged gravel wetland
is 5 acres.
• Space Required – Additional space is
recommended for pretreatment measures to
prevent sediment and debris from entering and
clogging the gravel bed.
• Site Slope – The local slope should be
relatively flat (<4%). While there is no minimum
slope requirement, there does need to be
enough elevation drop from the inlet to the
outlet to ensure that hydraulic conveyance by
gravity is feasible (generally about 2-5 feet).
• Minimum Depth to Water Table – Unless they
receive hot spot runoff, submerged gravel
wetland systems can be allowed to intersect
the groundwater table. If a submerged gravel
wetland receives hot spot runoff and has an
underlying water supply aquifer, a liner and
a separation distance of 2 feet is required
between the bottom of the gravel and the
elevation of the seasonally high water table to
prevent groundwater contamination.
• Soils – Submerged gravel wetlands can be
used in almost all soils and geology, with minor
design adjustments for regions of karst (i.e.,
limestone) topography or in rapidly percolating
soils such as sand. In these areas, submerged
gravel wetlands should be designed with an
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• Proximity – The following is a list of specific
setback requirements for the location of a
submerged gravel wetland:
»» 10 feet from building foundations
»» 10 feet from property lines
»» 100 feet from private water supply wells
»» 100 feet from open water (measured from
edge of water)
»» 200 feet from public water supply reservoirs
(measured from edge of water)
»» 1,200 feet from public water supply wells
• Well-draining soils, such as hydrologic soil
group A and B – Since they are equipped with
waterproof liners, the presence of well-draining
soils does not influence the use of submerged
gravel wetlands on development sites.
• Flat Terrain – The presence of flat terrain does
not preclude the use of a submerged gravel
wetland.
Stormwater Best Management Practices
Physical Feasibility – Physical Constraints at
• Shallow Water Table – Except in the case of
hot spot runoff, the base of the submerged
gravel wetland can intersect the groundwater
table.
• Tidally-influenced drainage system – Saltwater
intrusion of the submerged gravel wetland
should not be allowed.
• Trout Stream – In cold water streams,
submerged gravel wetlands should be designed
to detain stormwater for a relatively short
time (i.e., less than twelve hours) to minimize
the potential amount of stream warming that
occurs in the practice.
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4.27.5.2 GENERAL DESIGN
A perforated pipe (4-6-inch diameter preferred) at
Maximum slope of access easement should
be 15%, and the driveway path should be at
least 12 feet wide. This driveway path should
be able to support maintenance vehicles and
• Impervious and pervious areas and other
means used to determine the land use data
the base of the gravel layer allows for flow-through
equipment.
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
stable outfall at non-erosive velocities.
• Existing and proposed site, topographic, and
location maps, as well as field reviews
• Design data from nearby storm sewer
structures
• Water surface elevation of nearby water
systems as well as the depth to seasonally high
groundwater
Pretreated stormwater enters via piped or overland
flow and discharges into the gravel-filled chamber.
conditions and maintains a constant water surface
elevation. Discharges that exceed the WQv exit to a
4.27.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
el wetland. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be met.
4.27.5.1 LOCATION AND LAYOUT
• Forebays should be designed to remove
sediment from stormwater runoff prior to
entering the submerged gravel wetland.
• Provide storage for 75% of the submerged
gravel wetland drainage area WQv. Maximum
ponding depth should not be higher than the
level of wetland vegetation.
• The forebay should be a separated from the
wetland by a barrier and located at each inlet to
the gravel wetland, except if the inlet supplies
10% or less of the inflow to the gravel wetland.
• Use the porosity of the gravel media when
calculating submerged gravel wetland storage
volumes.
• The forebay should be sized to contain 0.1
inches of impervious area of the WQv. This
volume may be subtracted from the WQv for
wetland storage sizing. A device to measure
vertical sediment depth maybe may be installed
to measure sediment accumulation over time.
Submerged gravel wetlands are generally applied
to land uses with a high percentage of impervious
• The gravel layer should be 4 feet or less.
surfaces. Submerged gravel wetlands should be
• A flow splitter may be required to redirect the
WQv to the submerged gravel wetland.
located upstream or downstream of other BMPs
providing runoff reduction, channel protection
volume (CPv), overbank flood protection (Qp25)
and extreme flood protection (Qf). See Subsection 4.1.6 for more information on the use of
multiple BMPs in a treatment train.
• Optional earth berms may separate multiple
treatment cells.
• Observation wells should consist of 6-inch
diameter perforated pipe and be at least 6
inches above grade.
• Maintenance right-of-way or drainage
easement should be at least 20 feet wide.
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• A sediment forebay or pretreatment is
important for controlling and the amount of
sediment entering a submerged gravel wetland.
• Gravel layer should be 18-48 inches thick of
clean washed uniformly graded material with a
porosity of 40%.
The following criteria are to be considered minimum standards for the design of a submerged grav-
4.27.5.4 PRETREATMENT/INLETS
Stormwater Best Management Practices
4.27.5 Planning and Design Criteria
4.27.5.5 OUTLET STRUCTURES
• For a submerged gravel wetland, the outlet structure
can consist of a weir, orifice, outlet pipe, combination
outlet, or other acceptable control structure.
• Small outlets that will be subject to clogging or
are difficult to maintain are not acceptable.
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4.27.5.8 CONSTRUCTION CONSIDERATIONS
• A minimum of 6 inches of freeboard must be
provided, measured from the top of the water
surface elevation for the water quality volume,
to the lowest point of the ground surface
elevation, not counting the outlet weir.
• Construction equipment should be restricted
from the submerged gravel wetland to prevent
compaction of the native soils.
• Stormwater should be conveyed to and from
submerged gravel wetlands safely and to
minimize erosion potential.
4.27.5.7 LANDSCAPING
• It is recommended that native (local) wetland
plant stock is used for establishing vegetation.
• Stabilize disturbed areas prior to runoff entering
the constructed wetland.
• Protect the location of a submerged gravel
wetland during construction. Divert surface
runoff from the practice during grading. Any
conveyance infrastructure should be blocked.
• Use wide-tracked equipment to minimize
disturbance and compaction. If it is necessary
to pump water during construction, discharge
filtered water to a stable outlet.
Stormwater Best Management Practices
4.27.5.6 SAFETY FEATURES
• A minimum of three types of wetland species
should be provided.
• Mulch or topsoil may be placed on top of the
rock media to establish vegetation. Note that
using rock media to establish the wetland may
require specific planting stock.
4.27.5.9 CONSTRUCTION AND MAINTENANCE
COSTS
• An estimate for submerged gravel wetland
construction costs is $5-6 per square foot. This
includes sediment forebays.
• Regular inspection and maintenance may be
necessary until vegetation within the wetland
is established. It may be necessary to replace
some of the plants.
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Complete Step 3 for a water quality (treatment) approach.
Refer to your local community’s guidelines for any additional
(Step 1) Determine if the development site and conditions are appropriate for the use of a submerged gravel wetland.
information or specific requirements regarding the use of
Consider the application and site feasibility criteria in this
either method.
chapter. In addition, determine if site conditions are suitable
for a submerged gravel wetland. Create a rough layout of
(Step 3) Calculate the Target Water Quality Volume
the submerged gravel wetland dimensions taking into con-
Calculate the Water Quality Volume using the following for-
sideration existing trees, utility lines, and other obstructions.
mula:
(Step 2) Determine the goals and primary function of the submerged
WQV = (1.2) (RV) (A) / 12
gravel wetland.
Consider whether the submerged gravel wetland is intended
Where:
to:
»» Meet a water quality (treatment) target. See Step 3
to size the BMP utilizing the water quality treatment
approach.
WQV = Water Quality Volume (ft3)
»» Provide a possible solution to a drainage problem
Check with local officials and other agencies to determine if
there are any additional restrictions and/or surface water or
watershed requirements that may apply. In addition, consider if the best management practice has any special site-specific design conditions or criteria. List any restrictions or
other requirements that may apply or affect the design.
The design of the BMP should be centered on the restrictions/requirements, goals, targets, and primary function(s)
of the BMP, described in this section. By considering the
primary function, as well as, topographic and soil conditions,
the design elements of the practice can be determined (i.e.
planting media, underdrain, inlet/outlet, overflow, etc.)
1.2 = Target rainfall amount to be treated (inches)
Stormwater Best Management Practices
4.27.6 Design Procedures
RV = Volumetric runoff coefficient which can be
found by:
RV = 0.05+0.009(I)
Where:
I = new impervious area of the contributing
drainage area (%)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
(Step 4) Size flow diversion structure, if needed
If the contributing drainage area to the submerged gravel
wetland exceeds the water quality treatment and/or storage
capacity, a flow regulator (or flow splitter diversion structure) should be supplied to divert the WQv to the submerged
gravel wetland.
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An overflow, such as an overdrain with an invert set slightly
above the elevation of maximum ponding depth, must be
provided to bypass and/or convey larger flows to the downstream drainage system or stabilized watercourse. Non-erosive velocities need to be ensured at the outlet point. The
overflow should be sized to safely pass the peak flows anticipated to reach the practice.
(Step 6) Prepare Site Vegetation and Landscaping Plan
A landscaping plan for the submerged gravel wetland should
be prepared to indicate how it will be established with vegetation. See Subsection 4.27.5.7 (Landscaping) and Appendix D
for more details.
Stormwater Best Management Practices
(Step 5) Design stable outfall(s).
4.27.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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Description: Detention storage located in un-
KEY CONSIDERATIONS
DESIGN CRITERIA
• The maximum contributing drainage area to be served by a single
underground detention vault or tank is 25 acres.
• Detention vaults should be constructed with a minimum 3,000 psi structural
reinforced concrete.
• All construction joints must be provided with water stops.
• Cast-in-place wall sections must be designed as retaining walls.
• The maximum depth from finished grade to the vault invert should be 20
feet.
• The minimum pipe diameter for underground detention tanks is 36 inches.
• Underground detention vaults and tanks must meet structural requirements
for overburden support and traffic loading, if appropriate.
• Riprap, plunge pools or pads, or other energy dissipators are to be placed at
the end of the outlet to prevent scouring and erosion.
• A high flow bypass should be included in the underground detention system
design to safely pass the extreme flood flow.
derground tanks or vaults designed to provide
water quantity control through detention and/or
extended detention of stormwater runoff.
LID/GI Considerations: Underground detention
facilities do not provide runoff reduction or water
quality treatment and are not generally considered to be low impact development or green
infrastructure
ADVANTAGES / BENEFITS
• Ideal for highly urbanized areas where land is limited
• Can be used for stormwater quantity control downstream of other runoff
reducing or water quality treating BMPs
• Some designs require minimal drop between inlet and outlet
DISADVANTAGES / LIMITATIONS
• Not designed to provide storm water quality benefits
• Underground installation may make these systems difficult to maintain.
• Performance dependent on design and frequency of inspection and
cleanout of unit
• Some designs may require a confined space entry for maintenance and
repairs.
ROUTINE MAINTENANCE REQUIREMENTS
• Adequate maintenance access must be provided for all underground
detention systems.
• Remove any trash, debris, and sediment buildup in the underground vaults
or tanks.
• Perform structural repairs to inlet and outlets, as needed.
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
suitable for this practice
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.28 Underground Detention
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: Yes
Roadway Projects: Not recommended
Soils: Geotechnical testing for the
structural load bearing capacity of
subsurface soils may be required prior
to underground detention installation.
Other Considerations: Install a
manhole on the downstream side to
provide easy access for sampling of
effluent.
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
• 0% Runoff Reduction Credit is
provided by this practice.
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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Underground detention vaults and tanks are not intended for water quality
Detention vaults are box-shaped underground stormwater storage facilities
treatment and must be used in a treatment train approach with other BMPs
typically constructed with reinforced concrete. Detention tanks are under-
that provide treatment of the WQv (see Subsection 4.1.6). This will prevent the
ground storage facilities typically constructed with large diameter metal or
underground vault or tank from becoming clogged with trash or sediment
plastic pipe. Both serve as an alternative to surface detention for stormwater
and significantly reduces the maintenance requirements for an underground
quantity control, particularly for space-limited areas where there is not ade-
detention system.
quate land for a dry detention basin or multi-purpose detention area.
Prefabricated concrete vaults are available for commercial vendors. In addiBoth underground vaults and tanks can provide channel protection through
tion, several pipe manufacturers have developed packaged detention systems.
extended detention of the channel protection volume (CPv) and overbank
Figures 4.28-1 and Figure 4.28-2 show example design schematics for under-
flood Qp25 (and in some cases protection for the extreme flood Qf). Basic stor-
ground detention systems.
age design and routing methods are the same as for detention basins except
that a bypass for high flows is typically included.
Figure 4.28-1 Example Underground Detention Tank System
(Source: WDE, 2000)
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Stormwater Best Management Practices
4.28.1 General Description
Figure 4.28-2 Example Underground Detention Tank System
(Source: WDE, 2000)
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4.28.3 Pollutant Removal Capabilities
Physical Feasibility – Physical Constraints at
Underground detention does not provide measur-
Project Site
• Runoff Reduction
Underground detention provides negligible
stormwater volume runoff reduction unless
modified to include an infiltration component.
Another BMP should be used in a treatment
train with underground detention to provide
runoff reduction. See Subsection 4.1.6 for more
information about using BMPs in series.
able removal of total suspended solids, nutrient,
• Drainage Area – The maximum contributing
drainage area to be served by a single
underground detention vault or tank is 25
acres.
• Water Quality
Underground detention provides minimal
water quality volume (WQv) treatment. Another
BMP should be used in a treatment train
with underground detention to provide WQv
treatment. See Subsection 4.1.6 for more
information about using BMPs in series.
• Channel Protection
Underground detention can be sized to store
the channel protection volume (CP v) and to
completely drain over 24-72 hours, meeting
the requirement of extended detention of the
1-year, 24-hour storm runoff volume.
• Overbank Flood Protection
Underground detention is intended to provide
overbank flood protection (peak flow reduction
of the 25-year, 24-hour storm, Qp25).
• Extreme Flood Protection
Underground detention can be designed to
control the extreme flood (100-year, 24-hour
rainfall event peak flow, Qf).
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metals, or organic matter. For additional information and data on pollutant removal capabilities
for bioretention areas, see the National Pollutant
Removal Performance Database (2nd Edition)
available at www.cwp.org and the National
Stormwater Best Management Practices (BMP)
Database at www.bmpdatabase.org.
4.28.4 Application and Site Feasibility
Criteria
Underground detention systems are sized to
• Space Required – Underground detention
is installed underground; therefore, minimal
surface area is required for the facility.
• Adequate maintenance access to each
chamber must be provided for inspection and
cleanout of underground detention units.
• Site Slope – Underground detention may be
installed on sites with slopes up to 15%.
Stormwater Best Management Practices
4.28.2 Stormwater Management Suitability
provide extended detention of the channel
protection volume over 24 hours and temporary
• Minimum Depth to Water Table – 2 feet
storage of the runoff volume required to provide
• Minimum Head – 4-8 feet
overbank flood (Qp25) protection (i.e., reduce the
post-development peak flow of the 25-year, 24hour storm event to the pre-development rate).
Due to the storage volume required, underground
detention vaults and tanks are typically not used
to control the 100-year storm (Qf) except for very
• Soils – Structural load bearing capacity of
subsurface soils must be adequate to support
the detention device and stormwater runoff.
• Check with manufacturer recommendations
for additional site design constraints.
small drainage areas (<1 acre).
General Feasibility
- Suitable for Residential Subdivision Usage – YES
- Suitable for High Density/Ultra Urban Areas – YES
- Regional Stormwater Control – YES
- Roadway Project – NO
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4.28.5 Planning and Design Criteria
4.28.5.2 GENERAL DESIGN
• Hot spots – Underground detention is wellsuited for hot spot runoff.
Before designing the underground detention sys-
• The maximum contributing drainage area to be
served by a single underground detention vault
or tank is 25 acres.
• Damage to existing structures and facilities:
»» Underground detention should not be used
in areas where their operation may create
a risk for basement flooding, interfere with
subsurface sewage disposal systems, or
affect other underground structures.
»» Underground detention should be designed
so that overflow drains away from buildings
to prevent damage to building foundations.
• Trout Stream – Underground detention will
not reduce thermal impacts of stormwater
runoff, suspended solids, or soluble pollutants
impacts. Therefore, they are not considered an
effective means of protecting trout streams.
tem, the following data is necessary:
• Existing and proposed site, topographic, and
location maps, as well as field reviews
• Impervious and pervious areas and other
means used to determine the land use data
• Roadway and drainage profiles, cross sections,
utility plans, and soil report for the site
• Design data from nearby storm sewer
structures
• Water surface elevation of nearby water
systems as well as the depth to seasonally high
groundwater table
The following criteria are to be considered minimum standards for the design of an underground
detention system. Consult with the local review
authority to determine if there are any variations
Coastal Areas
to these criteria or additional standards that must
• Poorly Draining Soils — Poorly draining soils do
not inhibit an underground detention facility’s
ability to temporarily store and treat stormwater
runoff.
be met.
• Flat Terrain — Flat terrain and low site
slopes do not interfere with the operation of
underground detention.
• Shallow Water Table — Review manufacturer’s
instructions regarding groundwater elevation.
Anti-flotation calculations may be required
when large open chambers are installed at or
below the water table.
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4.28.5.1 LOCATION AND LAYOUT
Underground detention systems should be
located downstream of other BMPs providing
• Routing calculations must be used to
demonstrate that the storage volume is
adequate. See Section 3.3 (Storage Design) for
procedures on the design of detention storage.
• Detention Vaults: Minimum 3,000 psi
structural reinforced concrete may be used for
underground detention vaults. All construction
joints must be provided with water stops. Castin-place wall sections must be designed as
retaining walls.
Stormwater Best Management Practices
Other Constraints / Considerations
• Underground detention vaults and tanks must
meet structural requirements for overburden
support and traffic loading, if appropriate.
• Adequate maintenance access must be
provided for all underground detention
systems. Access must be provided over the
inlet pipe and outflow structure. Access
openings can consist of a standard frame,
grate, and solid cover, or a removable panel.
• Vaults with widths of 10 feet or less should have
removable lids.
runoff reduction and/or treatment of the water
quality volume (WQv). See Subsection 4.1.6 for
more information on the use of multiple BMPs in
a treatment train.
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• Detention Tanks: The minimum pipe diameter
for underground detention tanks is 36 inches.
• The maximum depth from finished grade to the
vault invert should be 20 feet.
4.28.5.5 OUTLET STRUCTURES
4.28.5.7 CONSTRUCTION CONSIDERATIONS
• For overbank flood protection, an additional
outlet is sized for Qp25 control (based
upon hydrologic routing calculations) and
can consist of a weir, orifice, outlet pipe,
combination outlet, or other acceptable control
structure. See Section 3.4 (Outlet Structures)
for more information on the design of outlet
works.
• Newly installed underground detention should
be inspected prior to being placed in service.
Remove sediment and debris that may have
collected in the system during delivery and
installation.
4.28.5.4 PRETREATMENT/INLETS
• A separate sediment sump or vault chamber
sized to 0.1 inches times the impervious acres
of contributing drainage should be provided at
the inlet for underground detention systems
that are in a treatment train with off-line water
quality treatment BMPs.
• Riprap, plunge pools or pads, or other energy
dissipators should be placed at the end of the
outlet to prevent scouring and erosion.
• A high-flow bypass should be included in the
underground detention system design to safely
pass the extreme flood flow (Qf).
• For CP v control, a low-flow orifice capable of
releasing the channel protection volume over
4.28.5.6 SAFETY FEATURES
• 24 hours must be provided. The channel
protection orifice should have a minimum
diameter of 3 inches and be adequately
protected from clogging by an acceptable
external trash rack. The orifice diameter may be
reduced to 1 inch if internal orifice protection
is used (i.e., an overperforated vertical stand
pipe with 0.5-inch orifices or slots that are
protected by wirecloth and a stone filtering
jacket). Adjustable gate valves can also be used
to achieve an equivalent diameter.
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• Maintenance activities for an underground
detention device may require a confined space
entry.
• A minimum 20-foot wide maintenance rightof-way or drainage easement should be
provided for the underground detention.
4.28.5.8 CONSTRUCTION AND MAINTENANCE
COSTS
• Material and installation costs for underground
detention systems and vaults can vary based on
the size, location, treatment requirements, and
manufacturer.
Stormwater Best Management Practices
4.28.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Typically, underground detention systems can
range from approximately $12,000 for a small
pipe and manifold system to over $60,000 for
a multiple-chamber, high-volume, high-flow
device.
• Vaults that are greater than 4 feet deep should
be equipped with a safety ladder.
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(Step 5) Determine pretreatment volume..
In general, site designers should perform the following design procedures
A separate sediment sump or vault chamber sized to 0.1
when designing underground detention.
inches times the impervious acres of contributing drainage
(Step 1) Determine the goals and primary functions of the underground detention.
»» Underground detention can be designed to provide 24-
area should be provided at the inlet for underground detention systems that are in a treatment train with off-line water
quality treatment BMPs.
hour detention of the channel protection volume (CPv),
and provide Overbank Flood (Qp25) and Extreme Flood
(Qf) protection.
»» Check with local officials and other agencies to
determine if there are any additional watershed
restrictions that may apply. In addition, consider if the
underground detention has any special site-specific
design conditions or criteria. List any other requirements
that may apply to or affect the design.
(Step 6) Calculate CPv release rates and water surface elevations.
Set up a stage-storage-discharge relationship for the control
structure for the 1-year, 24-hour rainfall event orifice. Size
and determine the invert elevation of the CPv orifice to ensure that the channel protection volume is stored for at least
24 hours within the underground detention facility.
Stormwater Best Management Practices
4.28.6 Design Procedures
(Step 7) Calculate Qp25 and Qf release rates and water surface elevations.
Set up a stage-storage-discharge relationship for the control
(Step 2) Determine if the development site and conditions are ap-
structure for the 25- and 100-year, 24-hour storm orifices.
propriate for the use of underground detention.
Consider the application and site feasibility criteria in this
chapter to determine if site conditions are suitable for underground detention. Create a rough layout of the proposed
underground detention facility taking into consideration
existing trees, utility lines, and other obstructions.
(Step 3) Determine underground detention location and preliminary
geometry.
4.28.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
Ensure that there is adequate site area for installation of
the underground detention facility, including maintenance
access to the vault.
(Step 4) Compute runoff control volumes and rates.
Calculate CPv, Qp25, and Qf, in accordance with the guidance
presented in Subsection 3.1.5.
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KEY CONSIDERATIONS
DESIGN CRITERIA
• Runoff from an adjacent impervious area must be evenly distributed across
the filter strip as sheet flow
• Can be used as part of the runoff conveyance system to provide
pretreatment
• Slopes should be between 2-6%
• Both the top and toe of the slope should be as flat as possible to encourage
sheet flow and prevent erosion.
• Uniform grading across filter strip to encourage sheet flow and prevent
STORMWATER MANAGEMENT
SUITABILITY
Runoff Reduction
Water Quality
Channel Protection
Overbank Flood Protection
Extreme Flood Protection
concentrated flows
suitable for this practice
Description: Vegetated filter strips are uniformly
graded and densely vegetated sections of land,
engineered and designed to treat runoff from and
remove pollutants through vegetative filtering and
ADVANTAGES / BENEFITS
• Can provide groundwater recharge
• Reasonably low construction cost, effort, and changes to existing
landscaping
• Works well for mitigating highway runoff pollution
• Works well alone or in series
• Adaptable to a variety of site conditions
• Flexible in design and layout
• Lower cost alternative
infiltration.
LID/GI Considerations: The main means for filtration through vegetated filter strip is where there
is permeable soil. These practices work well as a
pretreatment and when used with other types of
structural stormwater BMPs.
DISADVANTAGES / LIMITATIONS
• Cannot alone achieve the 80% TSS removal target
• Large land requirement
• Requires periodic repair, regrading, and sediment removal to prevent
channelization
• Vulnerable to erosion and concentrated flow
• Provides less runoff reduction than most BMPs
may provide partial benefits
IMPLEMENTATION CONSIDERATIONS
Land Requirement
Stormwater Best Management Practices
4.29 Vegetated Filter Strip
Capital Cost
Maintenance Burden
Residential Subdivision Use: Yes
High Density/Ultra-Urban: No
Roadway Projects: Yes
Other Considerations: Use in a buffer
system and to treat runoff from pervious areas
L=Low M=Moderate H=High
RUNOFF REDUCTION CREDIT
ROUTINE MAINTENANCE REQUIREMENTS
• Mow grass to a height to maintain a dense vegetative cover
• Inspect for invasive species and remove as needed
• Inspect pea gravel diaphragm for clogging and remove sediment buildup
• Inspect vegetation for rills and gullies. Seed or sod bare areas.
• Sheet flow onto filter strips can be difficult to maintain, resulting in
reconcentration of flow.
• Remove trash, debris, sediment, and dead grass.
• Reseed or resod as needed.
• 50% of the RRv conveyed to the
practice (A & B hydrologic soils)
• 25% of the RRv conveyed to the
practice (C & D hydrologic soils)
POLLUTANT REMOVAL
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Total Suspended Solids
Metals - Cadmium, Copper,
Lead, and Zinc removal
Nutrients - Total Phosphorus
/ Total Nitrogen removal
Pathogens – Fecal Coliform
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to treat stormwater runoff. Filter strips must be
Filter strips are uniformly graded and densely veg-
designed to withstand the full range of storm
etated sections of land, engineered and designed
events without eroding.
to treat runoff and remove pollutants through
vegetative filtering and infiltration. Filter strips
highways, roof downspouts, very small parking
4.29.2 Stormwater Management Suitability
lots, and pervious surfaces. They are also ide-
Filter strips are typically built for flowing moving
al components of the “outer zone” of a stream
perpendicular to and away from a roadway or
buffer, or as pretreatment for another structural
parking lot. They must be designed to withstand a
stormwater practice. Filter strips can serve as a
full range of storm events without eroding.
buffer between incompatible land uses, be land-
• Runoff Reduction
Vegetated filter strips are an effective low
impact development (LID) practice that can be
used in Georgia to reduce post-construction
stormwater runoff and improve stormwater
runoff quality. Like other LID practices, they
become even more effective the higher the
infiltration rate of the native soils. A vegetated
filter strip can be designed to provide 50% of
the runoff reduction volume for type A and B
hydrologic soils or 25% of the runoff reduction
volume for type C and D hydrologic soils.
Performance is dependent on vegetation
density and contact time for settling, filtration,
are best suited to treating runoff from roads and
scaped in an aesthetically pleasing way, and provide groundwater recharge in areas with pervious
soils. Filter strips are often used as a stormwater
site design credit (see Subsection 2.2.4 for more
information).
Filter strips rely on the use of vegetation to slow
runoff velocities and filter out sediment and other
pollutants from urban stormwater. There can also
be a significant reduction in runoff volume for
smaller flows that infiltrate pervious soils beneath
the filter strip. To be effective, sheet flow must
be maintained across the entire filter strip. Once
runoff flow concentrates, it effectively short-circuits the filter strip and reduces any water quality
benefits. Therefore, a flow spreader must normally be included in the filter strip design.
There are two different filter strip designs: a simple filter strip and a design that includes a permeable berm at the bottom. The presence of the
berm increases contact time with the runoff, thus
reducing the overall width of filter strip required
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and infiltration. .
• Channel Protection
For smaller sites, a vegetated filter strip may
be designed to capture the entire channel
protection volume (CP v). Given that a
vegetated filter strip is typically designed
to completely drain over 48-72 hours, the
requirement of extended detention of the
1-year, 24-hour rainfall event runoff volume will
be met. For larger sites, or where only the WQv
is diverted to the vegetated filter strip, another
practice must be used to provide CP v extended
detention.
• Overbank Flood Protection
Another practice in conjunction with a
vegetated filter strip will likely be required to
reduce the post-development peak flow of the
25-year storm (Qp) to pre-development levels
Stormwater Best Management Practices
4.29.1 General Description
(detention).
• Extreme Flood Protection
Vegetated filter strips must provide flow
diversion and/or be designed to safely pass
extreme storm flows (Qf) and protect the
vegetation.
Credit for the volume of runoff reduced in the
vegetated filter strip may be taken in the overbank
flood protection and extreme flood protection
• Water Quality
Use of vegetated filter strip is a stormwater
treatment practice that can remove a variety
of pollutants through several removal
mechanisms. Vegetated filter strips are
typically used as a pre-treatment component
to reduce incoming runoff velocity, filter
particulates, and uptake pollutants from the
calculations. If the practice is designed to provide
runoff reduction for water quality compliance,
then the practice is given credit for channel
protection and flood control requirements by
allowing the designer to compute an Adjusted CN
(see Subsection 3.1.7.5 for more information).
runoff.
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Because of design constraints, vegetated filter
Other Constraints/Considerations
Pollutant removal from filter strips is highly vari-
strips generally have a maximum drainage area
able and depends primarily on density of vegeta-
of 5 acres, but 2 areas are preferred. Filter strips
tion and contact time for filtration and infiltration.
should be designed for slopes between 2-6%.
• Location requirements – The following is a
list of specific setback requirements for the
location of a vegetated filter strip:
These, in turn, depend on soil and vegetation
Greater slopes than this would encourage the
type, slope, and presence of sheet flow. Research
formation of concentrated flow. Flatter slopes
on fecal coliform removal has been inconclusive,
would encourage standing water. The sheet flow
but suggests that filter strips are generally not
depth through the filter strip should be no more
effective BMPs for treating bacterial loads.
than 2 inches.
For additional information and data on pollutant
removal capabilities for vegetated filter strips,
General Feasibility
see the National Pollutant Removal Performance
- Suitable for Residential Subdivision Usage – YES
Database (2nd Edition) available at www.cwp.org
- Suitable for High Density/Ultra Urban Areas – NO
and the National Stormwater Best Management
- Regional Stormwater Control – NO
Practices (BMP) Database at www.bmpdatabase.
- Roadway Projects – YES
org.
Physical Feasibility - Physical Constraints at
4.29.4 Application and Site Feasibility
Criteria
Vegetated filter strips are best suited to treat
smaller drainage areas. Flow must enter the filter
strip as sheet flow spread out over the width
(long dimension normal to flow) of the strip,
generally no deeper than 1 to 2 inches. As a rule,
flow concentrates within a maximum of 75 feet
for impervious surfaces, and 150 feet for pervious surfaces (CWP, 1996). For longer flow paths,
special provision must be made to ensure design
flows spread evenly across the filter strip.
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Project Site
• Drainage Area – 5 acres or less, 2 acres
preferred.
»» Filter strips should be constructed outside
the natural stream buffer area whenever
possible to maintain a more natural buffer
along the streambank.
»» Filter strips should not be used on soils that
cannot sustain a dense grass cover with
high retardance. Designers should choose
a grass that can withstand relatively high
velocity flows at the entrances, and both wet
and dry periods. See Appendix D for a list of
appropriate grasses for use in Georgia.
Stormwater Best Management Practices
4.29.3 Pollutant Removal Capabilities
»» Pedestrian traffic across the filter strip
should be limited through channeling onto
sidewalks.
»» The filter strip should be at least 15 feet long
(25 feet preferred) to provide filtration and
contact time for water quality treatment.
100 feet the recommended maximum strip
length.
• Space Required – Rough rule of thumb ratio
of drainage area to filter strip surface area
required is 10:1.
• Site Slope – Slopes should be between 2-6%
(perpendicular to the roadway).
• Minimum Depth to Water Table – A separation
distance of 1-2 feet is recommended between
the bottom of the vegetated filter strip and the
elevation of the seasonally high water table.
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reaerate between rainfall events. They should
Before designing the vegetated filter strip, the
not be used on sites with a continuous flow from
following data is necessary:
groundwater, sump pumps, or other sources.
• Existing and proposed site, topographic and
location maps, and field reviews
strip width (perpendicular to flow path) is found
using the Manning’s equation:
Vegetated filter strip locations should be integrated into the site planning process and aesthetic
and maintenance considerations should be taken
• Field measured topography or digital terrain
model (DTM)
• Aerial site photographs
• Drainage basin characteristics (slope, shape,
size, soils, and land use)
• Preliminary plans including plan view, roadway
and drainage profiles, cross sections, utility
plans, and soil report
• Environmental constraints
• Design data of nearby structures (storm sewer
as-built information)
• Additional survey information
The following criteria are considered to be minimum standards for the design of a vegetated
filter strip. Consult with the local review authority
to determine if there are any variations to these
criteria or additional standards that must be met.
4.29.5.1 LOCATION AND LAYOUT
Vegetated filter strips location and layout areas
based on site constraints such as proposed and
existing infrastructure, soils, existing vegetation,
contributing drainage area, and utilities. Vegetated filter strips systems are designed for intermittent flow and must be allowed to drain and
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into account in their siting and design. Elevations
must be carefully worked out to ensure that the
(Equation 4.29.1)
Where:
desired runoff flow enters the facility as sheet flow
q = discharge per foot of width of filter
with no more than the maximum design depth
strip (cfs/ft)
and velocity.
Y = allowable depth of flow (inches;
maximum flow depth of 2 inches)
S = slope of filter strip (percent)
4.29.5.2 GENERAL DESIGN
n = Manning’s “n” roughness coefficient
• Filter strips should be integrated within site
designs.
(n=0.15 for medium grass, 0.25 for dense
• An effective flow spreader may use a pea gravel
diaphragm at the top of the slope (ASTM D
448 size no. 6, 1/8” to 3/8”). The pea gravel
diaphragm (a small trench running along the
top of the filter strip) serves two purposes.
First, it acts as a pretreatment device, settling
out sediment particles before they reach the
practice. Second it acts as a level spreader,
maintaining sheet flow as runoff flows over
the filter strip. Other types of flow spreaders
include a concrete sill, curb stops, or curb and
gutter with “sawteeth” cut into it.
da-type grass)
• Ensure that flows in excess of the design
flow move across or around the strip without
damaging it. Often a bypass channel or
overflow spillway with protected channel
section is designed to handle higher flows.
Stormwater Best Management Practices
4.29.5 Planning and Design Criteria
grass, and 0.35 for very dense Bermu-
• The minimum length of a filter strip is:
QWQ
W fMIN =
q
(Equation 4.29.2)
Where:
WfMIN = minimum filter strip width
perpendicular to flow (feet)
QWQ = water quality volume peak flow
(ft3/s)
q = discharge per foot of width of filter
strip (cfs/ft)
• Maximum discharge loading per foot of filter
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391
• Size filter strip (parallel to flow path) for a
contact time of 5 minutes minimum
• The equation for filter length is based on the
SCS TR55 travel time equation (SCS, 1986):
Table 4.29-1 Vegetated Filter Strip Sizing Guidance
(Source: Claytor and Schueler, 1996)
Parameter
Impervious Areas
35
75
75
100
Filter strip minimum length (feet)
15
25
12
18
Filter Strips with Berm
• Size outlet pipes to ensure that the bermed
area drains within 24 hours.
(Equation 4.29.3)
Where:
Lf = length of filter strip parallel to flow
path (ft)
Tt = travel time through filter strip
(minutes)
P2-24 = 2-year, 24-hour rainfall depth
(inches)
S = slope of filter strip (percent)
n = Manning’s “n” roughness coefficient
• Specify grasses resistant to frequent inundation
within the shallow ponding limits.
• Berm material should be composed of sand,
gravel, and sandy loam to encourage grass
cover (Sand: ASTM C-33 fine aggregate
concrete sand 0.02”-0.04”, Gravel: AASHTO
M-43 ½” to 1”).
• Size filter strip to contain the WQv within the
wedge of water backed up behind the berm.
• Maximum berm height is 12 inches.
(n=0.15 for medium grass, 0.25 for dense
grass, and 0.35 for very dense Bermuda-type grass)
Pervious Areas (lawns, etc)
Maximum inflow approach length (feet)
4.29.5.3 PHYSICAL SPECIFICATIONS/GEOMETRY
• Both the top and toe of the slope should be as
flat as possible to encourage sheet flow and
prevent erosion.
• A minimum strip length of 15 feet should be
used (25 feet is preferred).
Stormwater Best Management Practices
Filter without Berm
• A maximum strip length of 100 feet is
recommended.
• Slopes should be between 2-6% (perpendicular
to the roadway).
• Acceptable velocities for filter strips should be
less than 4 feet per second for grass and less
than 1 foot per second for native herbaceous
vegetation.
Filter Strips for Pretreatment
• A number of other structural controls, including
vegetated filter strips and infiltration trenches,
may utilize a filter strip as a pretreatment
measure. The required length of the filter strip
depends on the drainage area, imperviousness,
and the filter strip slope. Table 4.29-1 provides
sizing guidance for bioretention filter strips for
pretreatment.
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392
Stormwater Best Management Practices
Figure 4.29-1 Typical Vegetated Filter Strip
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393
(Step 1) Determine the goals and primary function of the vegetated
filter strips.
Consider whether vegetated filter strip is intended to:
»» Meet a runoff reduction* target or water quality
(treatment) target.
*Note that minimum infiltration rates of the surrounding
native soils must be acceptable and suitable when used
in runoff reduction applications.
(Step 3) Calculate the Target Water Quality Volume
Calculate the Water Quality Volume using the following formula:
WQV = (1.2) (RV) (A) / 12
Where:
WQV = Water Quality Volume (ft3)
1.2 = Target rainfall amount to be treated (inches)
»» Be “oversized” to include partial credit for storage
capacity for Channel Protection Volume (CPv).
RV = Volumetric runoff coefficient which can be
»» Provide a possible solution to a drainage problem.
»» Enhance landscape and provide aesthetic qualities.
RV = 0.05+0.009(I)
found by:
Check with local officials and other agencies to determine if
Where:
there are any additional watershed restrictions that may ap-
I = new impervious area of the contributing
ply. In addition, consider if the vegetated filter strip has any
drainage area (%)
special site-specific design conditions or criteria. List any
other requirements that may apply to or affect the design.
A = Area draining to this practice (ft2)
Stormwater Best Management Practices
4.29.6 Design Procedures
12 = Unit conversion factor (in/ft)
The design of the vegetated filter strip should be centered
on the restrictions/requirements, goals, targets, and primary function(s) of a vegetated filter strip. By considering the
(Step 4) Determine the maximum discharge loading per foot of filter
strip.
primary function, as well as, topographic and soil conditions, the design elements of the vegetated filter strip can
be determined (i.e. planting media, underdrain, inlet/outlet,
overflow, etc.).
Where:
(Step 2) Determine if the development site and conditions are appropriate for the use of a vegetated filter strip
q = discharge per foot of width of filter strip (cfs/
Consider the application and site feasibility criteria in this
ft) based on WQv storm event
S = slope of filter strip (percent)
chapter to determine if site conditions are suitable for a
n = Manning’s “n” roughness coefficient
vegetated filter strip. Create a rough layout of the vegetated
(n=0.15 for medium grass, 0.25 for dense grass, and
filter strip dimensions taking into consideration existing trees,
0.35 for very dense Bermuda-type grass)
utility lines, and other obstructions.
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394
filter strip.
Where:
Lf = length of filter strip parallel to flow path (ft)
Tt = travel time through filter strip (minutes)
P2-24 = 2-year, 24-hour rainfall depth (feet)
S = slope of filter strip (percent)
n = Manning’s “n” roughness coefficient
(Equation 4.29.5)
(n=0.15 for medium grass, 0.25 for dense grass, and
0.35 for very dense Bermuda-type grass)
Where:
WfMIN = min. filter strip width perpendicular to flow (ft)
(Step 9) To calculate the RRV credited for the practice (sized for
QWQ=water quality volume peak flow (ft3/s)
WQV), Steps 5, 6, 7 and 8 have to be met, then proceed to
q=discharge per foot of width of filter strip (cfs/ft)
Step 10. Otherwise proceed to Step 13.
(Step 6) Calculate the depth of flow of the stormwater runoff across
the buffer to be sure it is <1 inch.
(Step 10) Calculate the Stormwater Runoff Reduction Volume con-
Stormwater Best Management Practices
(Step 5) Determine the minimum width (perpendicular to flow) of a
veyed to the practice.
Calculate the Runoff Reduction Volume using the following
D = (1.04*q0.6*n0.6)/S0.3
formula:
RRV = (P) (RV) (A) / 12
Where:
D = depth of flow (ft)
Where:
(Step 7) Calculate the velocity of the stormwater runoff across the
RRV = Runoff Reduction Target Volume (ft3)
buffer to be sure it is < 2.0 fps.
P = Target runoff reduction rainfall (inches)
RV = Volumetric runoff coefficient which can be
V = QWQ/(D* WfMIN)
found by:
(Step 8) Size filter strip (parallel to flow path) for a contact time of at
RV = 0.05+0.009(I)
least 5 minutes.
Where:
I = new impervious area of the contributing
drainage area (%)
(Equation 4.29.4)
A = Area draining to this practice (ft2)
12 = Unit conversion factor (in/ft)
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395
Using Table 4.1.3-2 - BMP Runoff Reduction Credits, lookup
the appropriate runoff reduction percentage (or credit)
provided by the practice:
RRv (credited) = RRv(RR%)
Where:
RR% = Runoff Reduction percentage, or credit,
assigned to the specific practice
RRv (credited) = Runoff Reduction Volume provided
by this practice (ft3)
RRV = RRV conveyed to the practice
Stormwater Best Management Practices
(Step 11) Calculate RRv credited.
(Step 12) Calculate the adjusted curve numbers for CPV (1-yr, 24hour storm), QP25 (25-yr, 24-hour storm), and Qf (100-yr,
24-hour storm). See Subection 3.1.7.5 for more information
(Step 13) Prepare site Vegetation and Landscaping Plan
A landscaping plan for the vegetated filter strip should be
prepared to indicate how it will be established with vegetation.
See section Appendix D for more details.
4.29.7 Inspection and Maintenance Requirements
All best management practices require proper maintenance. Without proper
maintenance, BMPs will not function as originally designed and may cease to
function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection
and maintenance requirements, see Appendix E.
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396
US Department of Agriculture. Forest Service.
Center for Watershed Protection, Ellicott City,
Anne Arundel County. 2012. Step Pool Storm
Northeastern Area. State and Private Forestry.
MD.
Conveyance (SPSC) Systems. Anne Arundel
Newtown Square, PA.
Oregon Department of Transportation. Appendix
County, Maryland.
Chesapeake Stormwater Network. 2015. About.
ASTM International. 2005. Standard Practice
E – Bioslopes. GDOT, Salem, Oregon.
Chesapeake Stormwater Network, Ellicott City, MD.
Pennsylvania Department of Environmental
for Determination of Deadloads and Live Loads
Associated with Green Roof Systems. Standard
Green Building Alliance. 2013. Stormwater Plant-
Protection. 2006. Pennsylvania Stormwater Best
E2397.05. ASTM International. West Consho-
ers. Green Building Alliance. Pittsburgh, PA.
Management Practices Manual. Pennsylvania DEP.
Georgia Department of Transportation. 2014.
Schueler, T., D. Hirschman, M. Novotney, and J.
ASTM International. 2006. Standard Guide for Se-
Manual on Drainage Design for Highways. Atlanta,
Zielinski. 2007. Urban Stormwater Retrofit Prac-
lection, Installation and Maintenance of Plants for
GA.
tices. Manual 3: Urban Subwatershed Restoration
hocken, PA.
Manual Series. Center for Watershed Protection
Green Roof Systems. Standard E2400-06. ASTM
International. West Conshohocken, PA.
Knox County, Tennessee. 2008. Knox County
Stormwater Best Management Practices
References
(CWP). Ellicott City, MD.
Tennessee Stormwater Management Manual.
Atlanta Regional Commission (ARC). 2001. Geor-
Knox County, Tennessee.
2014. National Menu of Stormwater Best Man-
gia Stormwater Management Manual, Volume 2.
Atlanta Regional Commission, Atlanta, GA.
Bureau of Environmental Services. 2008. Portland
United States Environmental Protection Agency.
Maryland Department of the Environment (MDE).
agement Practices. United States Environmental
2009. Environmental Site Design, Chapter 5. Balti-
Protection Agency. Washington, D.C.
more, Maryland.
Washington State Department of Transportation.
Stormwater Management Manual. City of PortMinnesota Pollution Control Agency (MPCA).
2006. Technology Evaluation and Engineering
2006. Minnesota Stormwater Manual. Minnesota
Report. Washington State Department of Trans-
Pollution Control Agency.
portation (WSDOT).
serving and Planting Trees at Development Sites.
NCHRP. 2013. Synthesis 444: Pollutant Load
West Virginia Department of Environmental Pro-
NA-TP-01-06. US Department of Agriculture.
Reductions for Total Maximum Daily Loads for
tection. 2015. West Virginia Stormwater Manage-
Forest Service. Northeastern Area. State and Pri-
Highways. National Cooperative Highway Re-
ment and Design Guidance Manual. West Virginia
vate Forestry. Newtown Square, PA.
search Program, Transportation Research Board,
Department of Environmental Protection.
land, OR.
Cappiella, K., T. Schueler and T. Wright. 2006.
Urban Watershed Forestry Manual. Part 2: Con-
National Research Council.
Cappiella, K., T. Schueler, J. Tomlinson and T.
Wright. 2006. Urban Watershed Forestry Manual.
Novotney, M., P. Sturm, C. Swann, and J. Tasillo.
Part 3: Urban Tree Planting Guide. NA-TP-01-06.
2008. Downspout Disconnection in the City of
Baltimore, Maryland. City of Baltimore, Maryland.
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5. Stormwater Drainage System Design
roadway gutters, roadside ditches, small channels
inlets and storm drain pipe systems (Section 5.2);
and swales, and small underground pipe systems
culverts (Section 5.3); vegetated and lined open
which collect stormwater runoff and transport it
channels (Section 5.4); and energy dissipation de-
5.1.1 Stormwater Drainage System
Design
to structural control facilities, pervious areas and/
vices for outlet protection (Section 5.5). The rest
or the major drainage system (i.e., natural water-
of this section covers important considerations to
ways, large man-made conduits, and large water
keep in mind in the planning and design of storm-
5.1.1.1 INTRODUCTION
impoundments).
water drainage facilities.
Stormwater drainage design is an integral component of both site and overall stormwater manage-
Paths taken by runoff from very large storms are
ment design. Good drainage design must strive to
called major systems. The major system (de-
5.1.1.3 CHECKLIST FOR DRAINAGE PLANNING
maintain compatibility and minimize interference
signed for the less frequent storm up to the 100-
AND DESIGN
with existing drainage patterns; control flooding
yr level) consists of natural waterways, large man-
The following is a general procedure for drainage
of property, structures and roadways for design
made conduits, and large water impoundments.
system design on a development site.
flood events; and minimize potential environmen-
In addition, the major system includes some less
tal impacts on stormwater runoff.
obvious drainageways such as overload relief
swales and infrequent temporary ponding areas.
Stormwater collection systems must be designed
The major system includes not only the trunk line
to provide adequate surface drainage while at
system that receives the water from the minor
the same time meeting other stormwater man-
system, but also the natural backup system which
agement goals such as runoff reduction, water
functions in case of overflow from or failure of
quality, streambank channel protection, habitat
the minor system. Overland relief must not flood
protection and groundwater recharge.
or damage houses, buildings or other property.
The major/minor concept may be described as
5.1.1.2 DRAINAGE SYSTEM COMPONENTS
a ‘system within a system’ for it comprises two
In every location there are two stormwater drain-
distinct but conjunctive drainage networks. The
age systems, the minor system and the major
major and minor systems are closely interrelat-
system. Three considerations largely shape the
ed, and their design needs to be done in tandem
design of these systems: flooding, public safety
and in conjunction with the design of structural
and water quality.
stormwater controls and the overall stormwater
management concept and plan.
The minor drainage system is designed to remove stormwater from areas such as streets and
This chapter is intended to provide design criteria
sidewalks for public safety reasons. The minor
and guidance on several drainage system com-
drainage system consists of inlets, street and
ponents, including street and roadway gutters,
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1.
Stormwater Drainage System Design
5.1 Stormwater Drainage Design Overview
Analyze topography
a. Check off-site drainage pattern. Where is
water coming onto the site? Where is water
leaving the site?
b. Check on-site topography for surface runoff
and storage, and infiltration
1. Determine runoff pattern; high points,
ridges, valleys, streams, and swales.
Where is the water going?
2. Overlay the grading plan and
indicate watershed areas; calculate
square footage (acreage), points of
concentration, low points, etc.
c. Check potential drainage outlets and
methods
1. On-site (structural control, receiving
water)
2. Off-site (highway, storm drain, receiving
water, regional control)
3. Natural drainage system (swales)
4. Existing drainage system (drain pipe)
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3.
4.
Analyze other site conditions.
a. Land use and physical obstructions such
as walks, drives, parking, patios, landscape
edging, fencing, grassed area, landscaped
area, tree roots, etc.
b. Soil type determines the amount of water
that can be absorbed by the soil.
c. Vegetative cover will determine the amount
of slope possible without erosion.
Analyze areas for probable location of
drainage structures and facilities.
Identify the type and size of drainage system
components that are required. Design the
drainage system and integrate with the
overall stormwater management system and
plan.
5.1.2 Key Issues in Stormwater Drainage Design
5.1.2.1 INTRODUCTION
The traditional design of stormwater drainage systems has been to collect and convey stormwater
runoff as rapidly as possible to a suitable location
where it can be discharged. This Manual takes a
different approach wherein the design methodologies and concepts of drainage design are to be
integrated with the objectives for water quantity
and quality control in the stormwater management minimum standards. This means that:
• Stormwater conveyance systems are to
remove water efficiently enough to meet
flood protection criteria and level of service
requirements, and
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• These systems are to complement the ability
of the site design and structural stormwater
controls to mitigate the major impacts of urban
development.
The following are some of the key issues in integrating water quantity and quality control consid-
occurs during these periods, the risk to life and
property could be significantly increased.
• In establishing the layout of stormwater
networks, it is essential to ensure that flows
will not discharge onto private property during
flows up to the major system design capacity.
eration in stormwater drainage design.
5.1.2.3 STREET AND ROADWAY GUTTERS
5.1.2.2 GENERAL DRAINAGE DESIGN CONSIDERATIONS
• Stormwater systems should be planned and
designed so as to generally conform to natural
drainage patterns and discharge to natural
drainage paths within a drainage basin. These
natural drainage paths should be modified as
necessary to contain and safely convey the
peak flows generated by the development.
• Runoff must be discharged in a manner that
will not cause adverse impacts on downstream
properties or stormwater systems. In general,
runoff from development sites within a
drainage basin should be discharged at the
existing natural drainage outlet or outlets. If
the developer wishes to change discharge
points he or she must demonstrate that the
change will not have any adverse impacts on
downstream properties or stormwater systems.
• It is important to ensure that the combined
minor and major system can handle blockages
and flows in excess of the design capacity to
minimize the likelihood of nuisance flooding
or damage to private properties. If failure
of minor systems and/or major structures
• Gutters are efficient flow conveyance
structures. This is not always an advantage if
removal of pollutants and reduction of runoff
is an objective. Therefore, impervious surfaces
should be disconnected hydrologically where
possible and runoff should be allowed to flow
across pervious surfaces or through grass
channels. Gutters should be used only after
other options have been investigated and
only after runoff has had as much chance
as possible to infiltrate and filter through
vegetated areas.
Stormwater Drainage System Design
2.
• It may be possible not to use gutters at all, or
to modify them to channel runoff to off-road
pervious areas or open channels. For example,
curb opening type designs take roadway runoff
to smaller feeder grass channels. Care should
be taken not to create erosion problems in
off-road areas. Protection during construction,
establishment of strong stands of grass, and
active maintenance may be necessary in some
areas.
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5.1.2.5 STORM DRAIN PIPE SYSTEMS (STORM
SEWERS)
Figure 5.1-1 Alternate Roadway Section without Gutters
(Source: Prince George’s County, MD, 1999)
• Use road cross sections that include grass
channels or swales instead of gutters to
provide for pollution reduction and reduce
the impervious area required. Figure
5.1-1 illustrates a roadway cross section
that eliminates gutters for residential
neighborhoods. Flow can also be directed
to center median strips in divided roadway
designs. To protect the edge of pavement,
ribbons of concrete can be used along the
outer edges of asphalt roads.
5.1.2.4 INLETS AND DRAINS
• Inlets should be located to maximize overland
flow path, take advantage of pervious areas,
and seek to maximize vegetative filtering and
infiltration. For example, it might be possible
to design a parking lot so that water flows into
vegetated areas prior to entering the nearest
inlet.
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• Inlet location should not compromise safety
or aesthetics. It should not allow for standing
water in areas of vehicular or pedestrian
traffic, but should take advantage of natural
depression storage where possible.
• Inlets should be located to serve as overflows
for structural stormwater controls. For
example, a bioretention device in a commercial
area could be designed to overflow to a catch
basin for larger storm events.
• The choice of inlet type should match its
intended use. A sumped inlet may be more
effective supporting water quality objectives.
• Use several smaller inlets instead of one large
• The use of better site design practices (and
corresponding site design credits) should be
considered to reduce the overall length of a
piped stormwater conveyance system.
Stormwater Drainage System Design
the other surface drains may pick up the
water.
3. Improve aesthetics. Several smaller drains will
be less obvious than one large drain.
4. Spacing smaller drain inlets will give surface
runoff a better chance of reaching the drain.
Water will have farther to travel to reach one
large drain inlet.
• Shorter and smaller conveyances can be
designed to carry runoff to nearby holding
areas, natural conservation areas, or filter strips
(with spreaders at the end of the pipe).
• Ensure that storms in excess of pipe design
flows can be safely conveyed through a
development without damaging structures or
flooding major roadways. This is often done
through design of both a major and minor
drainage system. The minor (piped) system
carries the mid-frequency design flows while
larger runoff events may flow across lots
and along streets as long as it will not cause
property damage or impact public safety.
inlet in order to:
1. Prevent erosion on steep landscapes by
intercepting water before it accumulates too
much volume and velocity.
2. Provide a safety factor. If a drain inlet clogs,
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Stormwater Drainage System Design
5.1.2.6 CULVERTS
• Culverts can serve double duty as flow
retarding structures in grass channel design.
Care should be taken to design them as storage
control structures if depths exceed several feet,
and to ensure safety during flows.
• Improved inlet designs can absorb considerable
slope and energy for steeper sloped designs,
thus helping to protect channels.
5.1.2.7 OPEN CHANNELS
• Open channels provide opportunities for
reduction of flow peaks and pollution loads.
They may be designed as wet or dry enhanced
swales or grass channels.
• Channels can be designed with natural
meanders improving both aesthetics and
pollution removal through increase of contact
time.
• Grass channels generally provide better habitat
than hardened channel sections, though
studies have shown that riprap interstices
provide significant habitat as well. Velocities
should be carefully checked at design flows
and the outer banks at bends should be
specifically designed for increased shear stress.
• Compound sections can be developed that
carry the annual flow in the lower section
and higher flows above them. Figure 5.1-2
illustrates a compound section that carries the
2-year and 10-year flows within banks. This
reduces channel erosion at lower flows, and
meandering, self-forming low flow channels
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Figure 5.1-2 Compound Channel
that attack banks. The shelf in the compound
section should have a minimum 1:12 slope to
ensure drainage.
• Flow control structures can be placed in the
channels to increase residence time. Higher
flows should be calculated using a channel
slope that goes from the top of the cross piece
to the next one if it is significantly different
from the channel bottom for normal depth
calculations. Channel slope stability can
also be ensured through the use of grade
control structures that can serve as pollution
reduction enhancements if they are set above
the channel bottom. Regular maintenance is
necessary to remove sediment and keep the
channels from aggrading and losing capacity
for larger flows.
5.1.2.8 ENERGY DISSIPATORS
• Energy dissipaters should be designed to return
flows to non-eroding velocities to protect
downstream channels.
• Care must be taken during construction that
design criteria are followed exactly. The
designs presented in this Manual have been
carefully developed through model and
full-scale tests. Each part of the criteria is
important to the proper function.
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Listed below are the design storm recommendations for various stormwater drainage system
structures and property subject to flooding,
emergency access, and road replacement
costs)
components to be designed and constructed
5.2 Minor Drainage System Design
5.2.1 Overview
in accordance with the minimum stormwater
Open Channel Design
management standards. Some jurisdictions may
Open channels include all channels, swales, etc.
5.2.1.1 INTRODUCTION
require the design of both a minor and major
• 25-year design storm
Minor stormwater drainage systems, also known
stormwater conveyance system, sized for two
different storm frequencies. Please consult your
local review authority to determine the local
requirements. It is recommended that the full
build-out conditions be used to calculate flows
for the design storm frequencies below.
as convenience systems, quickly remove runoff
Channels may be designed with multiple stages
(e.g., a low flow channel section containing the
2-year to 5-year flows, and a high flow section
that contains the design discharge) to improve
stability and better mimic natural channel dimen-
Storm Drainage Systems
sions. Where flow easements can be obtained
Includes storm drainage systems and pipes that
and structures kept clear, overbank areas may
do not convey runoff under public roadways,
also be designed as part of a conveyance system
sometimes called lateral closed systems.
wherein floodplain areas are designed for storage
• 10- to 25-year design storm (for pipe and
culvert design)
• 10- to 25-year design storm (for inlet design)
• 50-year design storm (for sumped inlets, unless
overflow facilities are provided)
and/or conveyance of larger storms.
Energy Dissipation Design
Includes all outlet protection facilities.
• 25-year design storm
Check Storm
Roadway Culvert Design
Cross drainage facilities that transport storm runoff under roadways.
• 25- to 100-year design storm, or in accordance
with GDOT requirements, whichever is
more stringent. (Criteria to be taken into
consideration when selecting design flow
include roadway type, depth of flow over road,
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Used to estimate the runoff that is routed through
the drainage system and stormwater manage-
from areas such as streets and sidewalks for public safety purposes. The minor drainage system
consists of inlets, street and roadway gutters,
roadside ditches, small channels and swales, and
small underground pipe systems which collect
Stormwater Drainage System Design
5.1.3 Design Storm Recommendations
stormwater runoff and transport it to structural
control facilities, pervious areas and/or the major
drainage system (i.e., natural waterways, large
man-made conduits, and large water impoundments).
This section is intended to provide criteria and
guidance for the design of minor drainage system
components including:
• Street and roadway gutters
• Stormwater inlets
• Storm drain pipe systems
ment facilities to determine the effects on the
facilities, adjacent property, floodplain encroach-
Ditch, channel and swale design criteria and guid-
ment and downstream areas.
ance are covered in Section 5.4, Open Channel
• 100-year design storm, or as required by the
Georgia Safe Dams Act.
Design.
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5.2.2 Symbols and Definitions
cation of Manning’s Equation. Inlet capacity calculations for grate, curb and
combination inlets are based on information contained in HEC-12 (USDOT,
To provide consistency within this section as well as throughout this Manual,
FHWA, 1984). Storm drain system design is based on the use of the Rational
the symbols listed in Table 5.2-1 will be used. These symbols were selected
Formula.
because of their wide use. In some cases, the same symbol is used in existing
publications for more than one definition. Where this occurs in this section,
5.2.1.2 GENERAL CRITERIA
Design Frequency
See Section 5.1 or the local review authority for design storm requirements for
the sizing of minor storm drainage system components.
Flow Spread Limits
Catch basins shall be spaced so that the spread in the street for the 25-year
design flow shall not exceed the following, as measured from the face of the
curb:
• 8 feet if the street is classified as a Collector or Arterial street (for 2-lane
streets spread may extend to one-half of the travel lane; for 4-lane streets
spread may extend across one travel lane)
• 16 feet at any given section, but in no case greater than 10 feet on one side
of the street, if the street is classified as a Local or Sub-Collector street
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the symbol will be defined where it occurs in the text or equations.
Table 5.2-1 Symbols and Definitions
Symbol
Definition
Units
a
A
d or D
D
EO
g
h
H
K
L or LT
L
n
Gutter depression
Area of cross section
Depth of gutter flow at the curb line
Diameter of pipe
Ratio of frontal flow to total gutter flow Qw/Q
Acceleration due to gravity (32.2 ft/s2)
Height of curb opening inlet
Head Loss
Loss coefficient
Length of curb opening inlet
Pipe length
Roughness coefficient in the modified Manning’s formula for
triangular gutter flow
Perimeter of grate opening, neglecting bars and side against curb
Rate of discharge in guttter
Intercepted flow
Gutter capacity above the depressed section
Cross Slope - Traverse Slope
Longitudinal slope of pavement
Friction slope
Depression section slope
Top width of water surface (spread on pavement)
Spread above depressed section
Velocity of flow
Width of depression for curb opening inlets
T/d, reciprocal of the the cross sope
in
ft2
ft
ft
ft/s2
ft
ft
ft
ft
-
P
Q
Qi
Qs
S or SX
S or SL
Sf
SW
T
TS
V
W
Z
Stormwater Drainage System Design
Procedures for performing gutter flow calculations are based on a modifi-
ft
cfs
cfs
cfs
ft/ft
ft/ft
ft/ft
ft/ft
ft
ft
ft/s
ft
-
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5.2.3.3 MANNING’S N TABLE
Effective drainage of street and roadway pavements is essential to the mainte-
Table 5.2-2 Manning’s n Values for Street and Pavement Gutters
nance of the roadway service level and to traffic safety. Water on the pave-
Type of Gutter or Pavement
ment can interrupt traffic flow, reduce skid resistance, increase potential for
Concrete gutter, trowled finish
0.012
hydroplaning, limit visibility due to splash and spray, and cause difficulty in
Ashphalt pavement:
Smooth Texture
Rough Texture
0.013
0.016
Concrete gutter with asphalt pavement:
Smooth
Rough
0.013
0.015
Concrete pavement:
Float Finish
Broom Finish
0.014
0.016
For gutters with small slopes, where sediment may accumulate, increase above values of n by
0.002
steering a vehicle when the front wheels encounter puddles. Surface drainage is a function of transverse and longitudinal pavement slope, pavement
roughness, inlet spacing, and inlet capacity. The design of these elements is
dependent on storm frequency and the allowable spread of stormwater on the
pavement surface.
This section presents design guidance for gutter flow hydraulics originally
published in HEC-12, Drainage of Highway Pavements and AASHTO’s Model
Drainage Manual.
Range of Manning’s n
Stormwater Drainage System Design
5.2.3 Street and Roadway Gutters
Note: Estimates are by the Federal Highway Administration
Source: USDOT, FHWA, HDS-3 (1961)
5.2.3.1 FORMULA
The following form of Manning’s Equation should be used to evaluate gutter
flow hydraulics:
5.2.3.4 UNIFORM CROSS SLOPE
The nomograph in Figure 5.2-1 is used with the following procedures to find
Q = [0.56 / n] Sx5/3 S1/2 T8/3
(5.2.1)
Where:
Q= gutter flow rate, cfs
Sx =pavement cross slope, ft/ft
n = Manning’s roughness coefficient
S = longitudinal slope, ft/ft
T = width of flow or spread, ft
gutter capacity for uniform cross slopes:
Condition 1: Find spread, given gutter flow.
(Step 1) Determine input parameters, including longitudinal slope (S),
cross slope (Sx), gutter flow (Q), and Manning’s n.
(Step 2) Draw a line between the S and Sx scales and note where it
intersects the turning line.
(Step 3) Draw a line between the intersection point from Step 2 and
the appropriate gutter flow value on the capacity scale. If
5.2.3.2 NOMOGRAPH
Figure 5.2-1 is a nomograph for solving Equation 5.2.1. Manning’s n values
for various pavement surfaces are presented in Table 5.2-2 below.
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Manning’s n is 0.016, use Q from Step 1; if not, use the product of Q and n.
(Step 4) Read the value of the spread (T) at the intersection of the line
from Step 3 and the spread scale.
VOL 2
404
(Step 3) Calculate the ratios Qw/Q or Eo and Sw/Sx and use Figure 5.2-
(Step 1) Determine input parameters, including longitudinal slope (S),
cross slope (Sx), spread (T), and Manning’s n.
(Step 2) Draw a line between the S and Sx scales and note where it
2 to find an appropriate value of W/T.
(Step 4) Calculate the spread (T) by dividing the depressed section
width (W) by the value of W/T from Step 3.
(Step 5) Find the spread above the depressed section (Ts) by subtract-
intersects the turning line.
(Step 3) Draw a line between the intersection point from Step 2 and
the appropriate value on the T scale. Read the value of Q or
Qn from the intersection of that line on the capacity scale.
(Step 4) For Manning’s n values of 0.016, the gutter capacity (Q) from
ing W from the value of T obtained in Step 4.
(Step 6) Use the value of Ts from Step 5 along with Manning’s n, S,
and Sx to find the actual value of Qs from Figure 5.2-1.
(Step 7) Compare the value of Qs from Step 6 to the trial value from
Step 3 is selected. For other Manning’s n values, the gutter
Step 1. If values are not comparable, select a new value of
capacity times n (Qn) is selected from Step 3 and divided by
Qs and return to Step 1.
the appropriate n value to give the gutter capacity.
Condition 2: Find gutter flow, given spread.
Stormwater Drainage System Design
Condition 2: Find gutter flow, given spread.
(Step 1) Determine input parameters, including spread (T), spread
5.2.3.5 COMPOSITE GUTTER SECTIONS
above the depressed section (Ts), cross slope (Sx), longitudi-
Figure 5.2-2 in combination with Figure 5.2-1 can be used to find the flow in
a gutter with width (W) less than the total spread (T). Such calculations are
generally used for evaluating composite gutter sections or frontal flow for
grate inlets.
nal slope (S), depressed section slope (Sw), depressed section
width (W), Manning’s n, and depth of gutter flow (d).
(Step 2) Use Figure 5.2-1 to determine the capacity of the gutter
section above the depressed section (Qs). Use the procedure
Figure 5.2-3 provides a direct solution of gutter flow in a composite gutter section. The flow rate at a given spread or the spread at a known flow rate can be
found from this figure. Figure 5.2-3 involves a complex graphical solution of the
for uniform cross slopes, substituting Ts for T.
(Step 3) Calculate the ratios W/T and Sw/Sx, and, from Figure 5.2-2,
find the appropriate value of Eo (the ratio of Qw/Q).
(Step 4) Calculate the total gutter flow using the equation:
equation for flow in a composite gutter section. Typical of graphical solutions,
extreme care in using the figure is necessary to obtain accurate results.
Q = Qs / (1 - Eo)
(5.2.3)
Condition 1: Find spread, given gutter flow.
Where:
(Step 1) Determine input parameters, including longitudinal slope (S),
cross slope (Sx), depressed section slope (Sw), depressed section width (W), Manning’s n, gutter flow (Q), and a trial value
of gutter capacity above the depressed section (Qs).
(Step 2) Calculate the gutter flow in W (Qw), using the equation:
Qw = Q – Q s
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(5.2.2)
Q = gutter flow rate, cfs
Qs = flow capacity of the gutter section above the
depressed section, cfs
Eo = ratio of frontal flow to total gutter flow (Qw/Q)
(Step 5) Calculate the gutter flow in width (W), using Equation 5.2.2.
VOL 2
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Stormwater Drainage System Design
Figure 5.2-2 Ratio of Frontal Flow to Total Gutter Flow
(Source: AASHTO Model Drainage Manual, 2005)
Figure 5.2-1 Flow in Triangular Gutter Sections
(Source: AASHTO Model Drainage Manual, 2005)
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406
(2)
and Sw/Sx = 0.0833/0.03 = 2.78
Example 1
Given:
Find:
(4)
Calculate the gutter flow in
width, W, using Equation 5.2.2:
Use Figure 5.2-2 to find
T = 8 ft
Sx = 0.025 ft/ft
n = 0.015
S = 0.01 ft/ft
(a)
Flow in gutter at design spread
(b)
Flow in width (W = 2 ft) adjacent
Calculate W/T = 1.5/6 = 0.25
Qw = 7.5 - 2.7 = 4.8 cfs
Eo = 0.64
(3)
Calculate the gutter flow using
Equation 5.4.2.3:
Q = 2.7/(1 - 0.64) = 7.5 cfs
to the curb
Solution:
(a)
(b)
From Figure 5.2-1, Qn = 0.03
Q = Qn/n = 0.03/0.015 = 2.0 cfs
T = 8 - 2 = 6 ft
(Qn)2 = 0.014 (Figure 5.2-1)
Stormwater Drainage System Design
5.2.3.6 EXAMPLES
(flow in 6-foot width outside of
width (W))
Q = 0.014/0.015 = 0.9 cfs
Qw = 2.0 - 0.9 = 1.1 cfs
Flow in the first 2 ft adjacent to the curb is 1.1 cfs
and 0.9 cfs in the remainder of the gutter.
Example 2
Given:
Ts = 6 - 1.5 = 4.5 ft
T = 6 ft
Sw = 0.0833 ft/ft
W = 1.5 ft
Sx = 0.03 ft/ft
n = 0.014
S = 0.04 ft/ft
Find:
Flow in the composite gutter
Solution:
(1)
Use Figure 5.2-1 to find the gut-
ter section capacity above the
depressed section.
Qsn = 0.038
Qs = 0.038/0.014 = 2.7 cfs
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Figure 5.2-3 Flow in Composite Gutter Sections
(Source: AASHTO Model Drainage Manual, 1991)
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407
Where significant ponding can occur, in locations
roughness. The depth of water next to the curb
Inlets are drainage structures used to collect
such as underpasses and in sag vertical curves in
is the major factor in the interception capacity of
surface water through grate or curb open-ings
depressed sections, it is good engineering prac-
both gutter inlets and curb opening inlets. At low
and convey it to storm drains or direct outlet to
tice to place flanking inlets on each side of the in-
velocities, all of the water flowing in the section of
culverts. Grate inlets subject to traffic should be
let at the low point in the sag. The flanking inlets
gutter occupied by the grate, called frontal flow,
bicycle safe and be load-bearing adequate. Ap-
should be placed so that they will limit spread on
is intercepted by grate inlets, and a small portion
propriate frames should be provided.
low gradient approaches to the level point and act
of the flow along the length of the grate, termed
in relief of the inlet at the low point if it should be-
side flow, is intercepted. On steep slopes, only
come clogged or if the design spread is exceeded.
a portion of the frontal flow will be intercepted
Inlets used for the drainage of highway surfaces
if the velocity is high or the grate is short and
can be divided into three major classes:
1.
2.
3.
Grate Inlets – These inlets include grate
inlets consisting of an opening in the gutter
covered by one or more grates, and slotted
inlets consisting of a pipe cut along the
longitudinal axis with a grate or spacer bars
to form slot openings.
Curb-Opening Inlets – These inlets are
vertical openings in the curb covered by a
top slab.
Combination Inlets – These inlets usually
consist of both a curb-opening inlet and
a grate inlet placed in a side-by-side
configuration, but the curb opening may be
located in part upstream of the grate.
The design of grate inlets will be discussed in Sec-
splash-over occurs. For grates less than 2 feet
tion 5.2.5, curb inlet design in Section 5.2.6, and
long, intercepted flow is small.
combination inlets in Section 5.2.7.
Stormwater Drainage System Design
5.2.4 Stormwater Inlets
A parallel bar grate is the most efficient type of
gutter inlet; however, when crossbars are added
5.2.5 Grate Inlet Design
for bicycle safety, the efficiency is greatly reduced.
Where bicycle traffic is a design consideration, the
5.2.5.1 GRATE INLETS ON GRADE
curved vane grate and the tilt bar grate are rec-
The capacity of an inlet depends upon its geom-
ommended for both their hydraulic capacity and
etry and the cross slope, longitudinal slope, total
bicycle safety features. They also handle debris
gutter flow, depth of flow and pavement
better than other grate inlets but the vanes of the
grate must be turned in the proper direction.
Table 5.2-3 Grate Debris Handling Efficiencies
Rank
Grate
Longitudinal Slope
(0.005)
(0.04)
1
CV - 3-1/4 - 4-1/4
46
61
Inlets may be classified as being on a continuous
2
30 - 3-1/4 - 4
44
55
grade or in a sump. The term “continuous grade”
3
45 - 3-1/4 - 4
43
48
refers to an inlet located on the street with a con-
4
P - 1-7/8
32
32
tinuous slope past the inlet with water entering
5
P - 1-7/8 - 4
18
28
from one direction. The “sump” condition exists
6
45 - 2-1/4 - 4
16
23
when the inlet is located at a low point and water
7
Recticuline
12
16
enters from both directions.
8
P - 1-1/8
9
20
Source: “Drainage of Highway Pavements” (HEC-12), Federal Highway Administration, 1984.
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VOL 2
408
This ratio is equivalent to frontal flow interception efficiency. Figure 5.2-4
efficiency rankings of grate inlets from laboratory tests in which an attempt
provides a solution of equation 5.2.6, which takes into account grate length,
was made to qualita-tively simulate field conditions. Table 5.2-3 presents the
bar configuration and gutter velocity at which splash-over occurs. The gutter
results of debris handling efficiencies of several grates.
velocity needed to use Figure 5.2-4 is total gutter flow divided by the area
of flow. The ratio of side flow intercepted to total side flow, Rs, or side flow
The ratio of frontal flow to total gutter flow, Eo, for straight cross slope is ex-
interception efficiency, is expressed by:
pressed by the following equation:
Eo = Qw/Q = 1 - (1 - W/T)
2.67
Rs = 1 / [1 + (0.15V1.8/SxL2.3)]
(5.2.7)
(5.2.4)
Where:
Where:
L = length of the grate, ft
SX = pavement cross slope, ft/ft
Q = total gutter flow, cfs
Qw = flow in width W, cfs
W = width of depressed gutter or grate, ft
Stormwater Drainage System Design
Where debris is a problem, consideration should be given to debris handling
Figure 5.2-5 provides a solution to equation 5.2.7.
T = total spread of water in the gutter, ft
The efficiency, E, of a grate is expressed as:
Figure 5.2-2 provides a graphical solution of Eo for either depressed gutter
sections or straight cross slopes. The ratio of side flow, Qs, to total gutter flow
E = RfEo + Rs(1 -Eo)
(5.2.8)
is:
The interception capacity of a grate inlet on grade is equal to the efficiency of
Qs/Q = 1 - Qw/Q = 1 - Eo
(5.2.5)
The ratio of frontal flow intercepted to total frontal flow, Rf, is expressed by the
the grate multiplied by the total gutter flow:
Qi = EQ = Q[RfEo + Rs(1 - Eo)]
(5.2.9)
following equation:
Rf = 1 - 0.09 (V - V0)
(5.2.6)
Where:
V = velocity of flow in the gutter, ft/s
(using Q from Figure 5.2-1)
Vo = gutter velocity where splash-over first occurs, ft/s
(from Figure 5.2-4)
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409
Given:
W = 2 ft
T = 8 ft
Sx = 0.025 ft/ft
S = 0.01 ft/ft
Eo = 0.69
Q = 3.0 cfs
V = 3.1 ft/s
Gutter depression = 2 in
Find:
Interception capacity of:
(1) a curved vane grate, and
(2) a reticuline grate 2-ft long and 2-ft wide
Stormwater Drainage System Design
The following example illustrates the use of this procedure.
Solution:
From Figure 5.2-4 for Curved Vane Grate, Rf = 1.0
From Figure 5.2-4 for Reticuline Grate, Rf = 1.0
From Figure 5.2-5 Rs = 0.1 for both grates
From Equation 5.2.9:
Qi = 3.0[1.0 X 0.69 + 0.1(1 - 0.69)] = 2.2 cfs
For this example, the interception capacity of a curved vane grate is the same
as that for a reticuline grate for the sited conditions.
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410
Stormwater Drainage System Design
Figure 5.2-5 Grate Inlet Side Flow Interception Efficiency
(Source: HEC-12, 1984)
Figure 5.2-4 Grate Inlet Frontal Flow Interception Efficiency
(Source: HEC-12, 1984)
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411
Stormwater Drainage System Design
5.2.5.2 GRATE INLETS IN SAG
A grate inlet in a sag operates as a weir up to a certain depth, depending
on the bar configuration and size of the grate, and as an orifice at greater
depths. For a standard gutter inlet grate, weir operation continues to a
depth of about 0.4 feet above the top of grate and when depth of water
exceeds about 1.4 feet, the grate begins to operate as an orifice. Between
depths of about 0.4 feet and about 1.4 feet, a transition from weir to orifice
flow occurs.
The capacity of grate inlets operating as a weir is:
Qi = CPd1.5
(5.2.10)
Where:
P = perimeter of grate excluding bar widths and the
side against the curb, ft
C = 3.0
d = depth of water above grate, ft
and as an orifice is:
Qi = CA(2gd)0.5
(5.2.11)
Where:
C = 0.67 orifice coefficient
A = clear opening area of the grate, ft2
g = 32.2 ft/s2
Figure 5.2-6 is a plot of equations 5.2.10 and 5.2.11 for various grate
sizes. The effects of grate size on the depth at which a grate operates as
an orifice is apparent from the chart. Transition from weir to orifice flow
results in interception capacity less than that computed by either weir or the
orifice equation. This capacity can be approximated by drawing in a curve
Figure 5.2-6 Grate Inlet Capacity in Sag Conditions
(Source: HEC-12, 1984)
between the lines representing the perimeter and net area of the grate to be
used. The following example illustrates the use of this figure.
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412
Given:
Assuming that the installation chosen to meet design conditions is a double 2
A symmetrical sag vertical curve with equal bypass from
x 3 ft grate, for 50% clogged conditions: P = 1 + 6 + 1 = 8 ft
inlets upgrade of the low point; allow for 50% clogging of
the grate.
For 25-year flow: d = 0.5 ft (from Figure 5.2-6)
The American Society of State Highway and Transportation Officials (AASHTO)
Qb = 3.6 cfs
geometric policy recommends a gradient of 0.3% within 50 ft of the level point
Q = 8 cfs, 25-year storm
in a sag vertical curve.
T = 10 ft, design
Sx = 0.05 ft/ft
Check T at S = 0.003 for the design and check flow (from Figure 5.2-1):
d = TSx = 0.5 ft
Q = 3.6 cfs,
Find:
Grate size for design Q. Check spread at S = 0.003 on
T = 8.2 ft (25-year storm)
approaches to the low point.
Stormwater Drainage System Design
Example
Thus a double 2 x 3-ft grate 50% clogged is adequate to intercept the deSolution: From Figure 5.2-6, a grate must have a perimeter of 8 ft to
intercept 8 cfs at a depth of 0.5 ft.
sign flow at a spread that does not exceed design spread, and spread on the
approaches to the low point will not exceed design spread. However, the
tendency of grate inlets to clog completely warrants consideration of a com-
Some assumptions must be made regarding the nature of the clogging in or-
bination inlet, or curb-opening inlet in a sag where ponding can occur, and
der to com-pute the capacity of a partially clogged grate. If the area of a grate
flanking inlets on the low gradient approaches.
is 50% covered by debris so that the debris-covered portion does not contribute to interception, the effective perimeter will be reduced by a lesser amount
than 50%. For example if a 2-ft x 4-ft grate is clogged so that the effective
width is 1 ft, then the perimeter, P = 1 + 4 + 1 = 6 ft, rather than 8 ft, the total
perimeter, or 4 ft, half of the total perimeter. The area of the opening would
be reduced by 50% and the perimeter by 25%.
Therefore, assuming 50% clogging along the length of the grate, a 4 x 4, a 2 x
6, or a 3 x 5 grate would meet requirements of an 8-ft perimeter 50% clogged.
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413
It is apparent from examination of Figure 5.2-7 that the length of curb opening required for total interception can be significantly reduced by increasing
5.2.6.1 CURB INLETS ON GRADE
the cross slope or the equivalent cross slope. The equivalent cross slope can
Following is a discussion of the procedures for the design of curb inlets on
be increased by use of a continuously depressed gutter section or a locally
grade. Curb-opening inlets are effective in the drainage of highway pave-
depressed gutter section.
ments where flow depth at the curb is sufficient for the inlet to perform
efficiently. Curb openings are relatively free of clogging tendencies and offer
little interference to traffic operation. They are a viable alternative to grates in
many locations where grates would be in traffic lanes or would be hazardous
for pedestrians or bicyclists.
The length of curb-opening inlet required for total interception of gutter flow
on a pave-ment section with a straight cross slope is determined using Figure
Stormwater Drainage System Design
5.2.6 Curb Inlet Design
5.2-7. The efficiency of curb-opening inlets shorter than the length required
for total interception is determined using Figure 5.2-8.
The length of inlet required for total interception by depressed curb-opening
inlets or curb-openings in depressed gutter sections can be found by the use
of an equivalent cross slope, Se, in the following equation:
Se = Sx + SwEo
(5.2.12)
Where:
SX = pavement cross slope, ft/ft
Sw= gutter cross slope = (a/12W), ft/ft
a = gutter depression, in
W = gutter width, ft
Eo = ratio of flow in the depressed section to total gutter flow
Figure 5.2-7 Curb-Opening and Slotted Drain Inlet Length for Total Interception
(Source: HEC-12, 1984)
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VOL 2
414
labeled n and S. Locate the value for Manning’s n and longitudinal slope and draw a line connecting these points and
extend this line to the first turning line.
(Step 3) Locate the value for the cross slope (or equivalent cross
slope) and draw a line from the point on the first turning line
through the cross slope value and extend this line to the
second turning line.
(Step 4) Using the far right vertical line labeled Q locate the gutter
flow rate. Draw a line from this value to the point on the
Stormwater Drainage System Design
(Step 2) Enter Figure 5.2-7 using the two vertical lines on the left side
second turning line. Read the length re-quired from the
vertical line labeled LT.
(Step 5) If the curb-opening inlet is shorter than the value obtained in
Step 4, Figure 5.2-8 can be used to calculate the efficiency.
Enter the x-axis with the L/LT ratio and draw a vertical line
upward to the E curve. From the point of intersection, draw
a line horizontally to the intersection with the y-axis and read
the efficiency value.
Figure 5.2-8 Curb-Opening and Slotted Drain Inlet Interception Efficiency
(Source: HEC-12, 1984)
Design Steps
Steps for using Figures 5.2-7 and 5.2-8 in the design of curb inlets on grade
are given below.
(Step 1) Determine the following input parameters:
Cross slope = Sx (ft/ft)
Longitudinal slope = S (ft/ft)
Gutter flow rate = Q (cfs)
Manning’s n = n
Example
Given:
Sx = 0.03 ft/ft
n = 0.016
Q = 5 cfs
S = 0.035 ft/ft
Sw = 0.083 (a = 2 in, W = 2 ft)
Find:
(1) Qi for a 10-ft curb-opening inlet
(2) Qi for a depressed 10-ft curb-opening inlet with a = 2 in,
W = 2 ft, T = 8 ft (Figure 5.2-1)
Spread of water on pavement = T (ft) from Figure 5.2-1
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VOL 2
415
The capacity of curb-opening inlets in a sump location with a vertical orifice
(1) From Figure 5.2-7, LT = 41 ft, L/LT = 10/41 = 0.24
opening but without any depression can be determined from Figure 5.2-10.
From Figure 5.2-8, E = 0.39, Qi = EQ = 0.39 x 5 = 2 cfs
The capacity of curb-opening inlets in a sump location with other than vertical
(2) Qn = 5.0 x 0.016 = 0.08 cfs
orifice openings can be determined by using Figure 5.2-11.
Sw/Sx = (0.03 + 0.083)/0.03 = 3.77
Design Steps
T/W = 3.5 (from Figure 5.2-3)
Steps for using Figures 5.2-9, 5.2-10, and 5.2-11 in the design of curb-opening
T = 3.5 x 2 = 7 ft
inlets in sump locations are given below.
W/T = 2/7 = 0.29 ft
Eo = 0.72 (from Figure 5.2-2)
(Step 1) Determine the following input parameters:
Cross slope = Sx (ft/ft)
Therefore, Se = Sx + SwEo = 0.03 + 0.083(0.72) = 0.09
Spread of water on pavement = T (ft) from Figure 5.2-1
Gutter flow rate = Q (cfs) or dimensions of curb-opening
From Figure 5.2-7, LT = 23 ft, L/LT = 10/23 = 0.4
From Figure 5.2-8, E = 0.64, Qi = 0.64 x 5 = 3.2 cfs
Dimensions of depression if any [a (in) and W (ft)]
The depressed curb-opening inlet will intercept 1.6 times the flow intercepted
by the undepressed curb opening and over 60% of the total flow.
Stormwater Drainage System Design
Solution:
inlet [L (ft) and H (in)]
(Step 2) To determine discharge given the other input parameters,
select the appropriate figure (5.2-9, 5.2-10, or 5.2-11 depending on whether the inlet is in a de-pression and if the
orifice opening is vertical).
5.2.6.2 CURB INLETS IN SUMP
For the design of a curb-opening inlet in a sump location, the inlet operates as
(Step 3) To determine the discharge (Q), given the water depth (d),
a weir to depths equal to the curb opening height and as an orifice at depths
locate the water depth value on the y-axis and draw a hori-
greater than 1.4 times the opening height. At depths between 1.0 and 1.4
zontal line to the appropriate perimeter (p), height (h), length
times the opening height, flow is in a transition stage.
(L), or width x length (hL) line. At this intersection draw a
vertical line down to the x-axis and read the discharge value.
The capacity of curb-opening inlets in a sump location can be determined
from Figure 5.2-9, which accounts for the operation of the inlet as a weir and
(Step 4) To determine the water depth given the discharge, use the
as an orifice at depths greater than 1.4h. This figure is applicable to depressed
procedure described in Step 3 except enter the figure at the
curb-opening inlets and the depth at the inlet includes any gutter depression.
value for the discharge on the x-axis.
The height (h) in the figure assumes a vertical orifice opening (see sketch
on Figure 5.2-9). The weir portion of Figure 5.2-9 is valid for a depressed
curb-opening inlet when d ≤ (h + a/12).
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Stormwater Drainage System Design
Figure 5.2-10 Curb-Opening Inlet Capacity in Sump Locations
(Source: AASHTO Model Drainage Manual, 2005)
Figure 5.2-9 Depressed Curb-Opening Inlet Capacity in Sump Locations
(Source: AASHTO Model Drainage Manual, 2005)
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Stormwater Drainage System Design
Example
Given:
Curb-opening inlet in a sump location
L = 5 ft
h = 5 in
(1)
Undepressed curb opening
Sx = 0.05 ft/ft
T = 8 ft
(2)
Depressed curb opening
Sx = 0.05 ft/ft
a = 2 in
W = 2 ft
T = 8 ft
Find:
Discharge Qi
Solution:
(1)
d = TSx = 8 x 0.05 = 0.4 ft
d<h
From Figure 5.2-10, Qi = 3.8 cfs
(2)
d = 0.4 ft
h + a/12 = (5 + 2/12)/12 = 0.43 ft
Since d < 0.43 the weir portion of Figure 5.2-9 is applicable
(lower portion of the figure).
P = L + 1.8W = 5 + 3.6 = 8.6 ft
Figure 5.2-11 Curb-Opening Inlet Orifice Capacity for
Inclined and Vertical Orifice Throats
(Source: AASHTO Model Drainage Manual, 2005)
From Figure 5.2-9, Qi = 5 cfs
At d = 0.4 ft, the depressed curb-opening inlet has about 30% more capacity
than an inlet without depression.
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of natural drainageways and/or vegetated open
channels is not feasible.
5.2.7.1 COMBINATION INLETS ON GRADE
clogged combination inlet with the curb opening
5.2.8.2 GENERAL DESIGN PROCEDURE
located adjacent to the grate is approximately equal
The design of storm drain systems generally fol-
to the capacity of the grate inlet alone. Thus ca-
lows these steps:
(Step 1) Determine inlet location and spacing as
outlined earlier in this section.
inlet and the design procedures should be followed
based on the use of Figures 5.2-4, 5.2-5 and 5.2-6.
5.2.7.2 COMBINATION INLETS IN SUMP
All debris carried by stormwater runoff that is not intercepted by upstream inlets will be con-centrated at
the inlet located at the low point, or sump. Because
this will increase the probability of clogging for grated
inlets, it is generally appropriate to estimate the capacity of a combination inlet at a sump by neglecting
the grate inlet capacity. Assuming complete clogging
of the grate, Figures 5.2-9, 5.2-10, and 5.2-11 for
curb-opening inlets should be used for design.
5.2.8 Storm Drain Pipe Systems
5.2.8.1 INTRODUCTION
Storm drain pipe systems, also known as storm
sewers, are pipe conveyances used in the minor
stormwater drainage system for transporting
runoff from roadway and other inlets to outfalls
at structural stormwater controls and receiving
waters. Pipe drain systems are suitable mainly for
medium to high-density residential and commercial/industrial development where the use
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adjustments are needed to the final design.
It should be recognized that the rate of discharge to
On a continuous grade, the capacity of an un-
pacity is computed by neglecting the curb opening
(Step 5) Examine assumptions to determine if any
(Step 2) Prepare a tentative plan layout of the
storm sewer drainage system including:
a. Location of storm drains
b. Direction of flow
c. Location of manholes
d. Location of existing facilities such as water,
gas, or underground cables
(Step 3) Determine drainage areas and compute
runoff using the Rational Method
(Step 4) After the tentative locations of inlets, drain
pipes, and outfalls (including tailwaters)
have been determined and the inlets
sized, compute of the rate of discharge
to be carried by each storm drain pipe
and determine the size and gradient of
pipe required to care for this discharge.
This is done by proceeding in steps from
upstream of a line to downstream to the
point at which the line connects with other
lines or the outfall, whichever is applicable.
The discharge for a run is calculated, the
pipe serving that discharge is sized, and
the process is repeated for the next run
downstream. The storm drain system
design computation form (Figure 5.2-12)
can be used to summarize hydrologic,
hydraulic and design computations.
be carried by any particular section of storm drain
pipe is not necessarily the sum of the inlet design
discharge rates of all inlets above that section of
pipe, but as a general rule is somewhat less than
this total. It is useful to understand that the time of
concentration is most influential and as the time
of concentration grows larger, the proper rainfall
intensity to be used in the design grows smaller.
Stormwater Drainage System Design
5.2.7 Combination Inlets
5.2.8.3 DESIGN CRITERIA
Storm drain pipe systems should conform to the
following criteria:
• For ordinary conditions, storm drain pipes should
be sized on the assumption that they will flow full or
practically full under the design discharge but will
not be placed under pressure head. The Manning
Formula is recommended for capacity calculations.
• The maximum hydraulic gradient should not
produce a velocity that exceeds 15 ft/s.
• The minimum desirable physical slope should
be 0.5% or the slope that will produce a
velocity of 2.5 feet per second when the storm
sewer is flowing full, whichever is greater.
• If the potential water surface elevation exceeds
1 foot below ground elevation for the design
flow, the top of the pipe, or the gutter flow line,
whichever is lowest, adjustments are needed
in the system to reduce the elevation of the
hydraulic grade line.
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Stormwater Drainage System Design
Figure 5.2-12 Storm Drain System Computation Form
(Source: AASHTO Model Drainage Manual, 2005)
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The Manning’s equation can be written to determine friction losses for storm
drain pipes as:
Formulas for Gravity and Pressure Flow
Hf = [2.87 n2V2L]/[S4/3]
The most widely used formula for determining the hydraulic capacity of storm
(5.2.17)
drain pipes for gravity and pressure flows is the Manning’s Formula, expressed
Hf = [29 n2V2L]/[(R4/3) (2g)]
by the following equation:
V = [1.486 R2/3S1/2]/n
(5.2.13)
(5.2.18)
Where:
Hf = total head loss due to friction, ft
Where:
V = mean velocity of flow, ft/s
n = Manning’s roughness coefficient
R = the hydraulic radius, ft -defined as the area of flow
D = diameter of pipe, ft
divided by the wetted flow surface or wetted
L = length of pipe, ft
perimeter (A/WP)
V = mean velocity, ft/s
S = the slope of hydraulic grade line, ft/ft
R = hydraulic radius, ft
n = Manning’s roughness coefficient
g = acceleration of gravity = 32.2 ft/sec2
In terms of discharge, the above formula becomes:
Q = [1.486 AR2/3S1/2]/n
(5.2.14)
Where:
Stormwater Drainage System Design
5.2.8.4 CAPACITY CALCULATIONS
5.2.8.5 NOMOGRAPHS AND TABLE
The nomograph solution of Manning’s formula for full flow in circular storm drain
pipes is shown in Figures 5.2-13, 5.2-14, and 5.2-15. Figure 5.2-16 has been provided to solve the Manning’s equation for partially full flow in storm drains.
Q = rate of flow, cfs
A = cross sectional area of flow, ft2
For pipes flowing full, the above equations become:
V = [0.590 D2/3S1/2]/n
(5.2.15)
Q = [0.463 D8/3S1/2]/n
(5.2.16)
Where:
D = diameter of pipe, ft
5.2.8.6 HYDRAULIC GRADE LINES
All head losses in a storm sewer system are considered in computing the
hydraulic grade line to determine the water surface elevations, under design
conditions in the various inlets, catch basins, manholes, junction boxes, etc.
Hydraulic control is a set water surface elevation from which the hydraulic calculations are begun. All hydraulic controls along the alignment are established. If the
control is at a main line upstream inlet (inlet control), the hydraulic grade line is the
water surface elevation minus the entrance loss minus the difference in velocity head.
If the control is at the outlet, the water surface is the outlet pipe hydraulic grade line.
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in Column 5 and enter the friction loss (Hf) in Column 7. On
The head losses are calculated beginning from the control point upstream
curved alignments, calculate curve losses by using the for-
to the first junction and the procedure is repeated for the next junction. The
mula Hc = 0.002 (∆)(Vo2/2g), where ∆ = angle of curvature in
computation for an outlet control may be tabulated on Figure 5.2-17 using the
degrees and add to the friction loss.
following procedure:
(Step 8) Enter in Column 8 the velocity of the flow (Vo) of the outflow pipe.
(Step 1) Enter in Column 1 the station for the junction immediately
upstream of the outflow pipe. Hydraulic grade line computations begin at the outfall and are worked upstream taking
(Step 9) Enter in Column 9 the contraction loss (Ho) by using the
formula:
each junction into consideration.
Ho = [0.25 Vo2)]/2g, where g = 32.2 ft/s2
(Step 2) Enter in Column 2 the outlet water surface elevation if the
outlet will be sub-merged during the design storm or 0.8
(Step 10)Enter in Column 10 the design discharge (Qi) for each pipe
diameter plus invert elevation of the outflow pipe, whichever
flowing into the junction. Neglect lateral pipes with inflows
is greater.
of less than 10% of the mainline outflow. Inflow must be
Stormwater Drainage System Design
Design Procedure - Outlet Control
adjusted to the mainline outflow duration time before a
(Step 3) Enter in Column 3 the diameter (Do) of the outflow pipe.
(Step 4) Enter in Column 4 the design discharge (Qo) for the outflow
pipe.
comparison is made.
(Step 11) Enter in Column 11 the velocity of flow (Vi) for each pipe
flowing into the junction (for exception see Step 10).
(Step 5) Enter in Column 5 the length (Lo) of the outflow pipe.
(Step 12)Enter in Column 12 the product of Qi x Vi for each inflowing pipe. When several pipes inflow into a junction, the line
(Step 6) Enter in Column 6 the friction slope (Sf) in ft/ft of the outflow
pipe. This can be determined by using the following formu-
producing the greatest Qi x Vi product is the one that should
be used for expansion loss calculations.
la:
(Step 13)Enter in Column 13 the controlling expansion loss (Hi) using
Sf = (Q )/K
2
(5.2.19)
the formula:
Hi = [0.35 (V12)]/2g
Where:
Sf = friction slope
K = [1.486 AR2/3]/n
(Step 14)Enter in Column 14 the angle of skew of each inflowing pipe
to the outflow pipe (for exception, see Step 10).
(Step 7) Multiply the friction slope (Sf) in Column 6 by the length (Lo)
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using the formula H = [KVi )]/2g where K = the bend loss
2
(Step 22)Repeat the procedure starting with Step 1 for the next junction upstream.
coefficient corresponding to the various angles of skew of
the inflowing pipes.
(Step 23)At last upstream entrance, add V12/2g to get upstream water
surface elevation.
(Step 16)Enter in Column 16 the total head loss (Ht) by summing the
values in Column 9 (Ho), Column 13 (Hi), and Column 15 (H∆).
(Step 17) If the junction incorporates adjusted surface inflow of 10% or
more of the mainline outflow, i.e., drop inlet, increase Ht by
30% and enter the adjusted Ht in Column 17.
(Step 18)If the junction incorporates full diameter inlet shaping, such
Stormwater Drainage System Design
(Step 15) Enter in Column 15 the greatest bend loss (H ) calculated by
as standard man-holes, reduce the value of Ht by 50% and
enter the adjusted value in Column 18.
(Step 19)Enter in Column 19 the FINAL H, the sum of Hf and Ht, where
Ht is the final adjusted value of the Ht.
(Step 20)Enter in Column 20 the sum of the elevation in Column 2
and the Final H in Column 19. This elevation is the potential
water surface elevation for the junction under design conditions.
(Step 21)Enter in Column 21 the rim elevation or the gutter flow line,
whichever is lowest, of the junction under consideration in
Column 20. If the potential water surface elevation exceeds
1 foot below ground elevation for the design flow, the top of
the pipe or the gutter flow line, whichever is lowest, adjustments are needed in the system to reduce the elevation of
the Hydraulic Grade Line (H.G.L.).
Figure 5.2-13 Nomograph for Solution of Manning’s Formula for
Flow in Storm Sewers
(Source: AASHTO Model Drainage Manual, 2005)
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Stormwater Drainage System Design
Figure 5.2-14 Nomograph for Computing Required Size of Circular Drain,
Figure 5.2-15 Concrete Pipe Flow Nomograph
Flowing Full n = 0.013 or 0.015
(Source: AASHTO Model Drainage Manual, 2005)
(Source: AASHTO Model Drainage Manual, 2005)
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Stormwater Drainage System Design
V = Average of mean velocity in feet per second
Q = Discharge of pipe or channel in cubic feet per second
S = Slope of hydraulic grade line
Figure 5.2-16 Values of Various Elements of Circular Section
for Various Depths of Flow
(Source: AASHTO Model Drainage Manual, 2005)
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Stormwater Drainage System Design
Figure 5.2-17 Hydraulic Grade Line Computation Form
(Source: AASHTO Model Drainage Manual, 2005)
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All storm drains should be designed such that velocities of flow will not be
less than 2.5 feet per second at design flow or lower, with a minimum slope
of 0.5%. For very flat flow lines the general practice is to design components
so that flow velocities will increase progressively throughout the length of
the pipe system. Upper reaches of a storm drain system should have flatter
slopes than slopes of lower reaches. Progressively increasing slopes keep
solids moving toward the outlet and deter settling of particles due to steadily
increasing flow streams.
The minimum slopes are calculated by the modified Manning’s formula:
S = [(nV)2]/[2.208R4/3]
(5.2.20)
Stormwater Drainage System Design
5.2.8.7 MINIMUM GRADE
Where:
S = the slope of the hydraulic grade line, ft/ft
n = Manning’s roughness coefficient
V = mean velocity of flow, ft/s
R = hydraulic radius, ft (area dived by wetted perimeter)
5.2.8.8 STORM DRAIN STORAGE
If downstream drainage facilities are undersized for the design flow, a structural stormwater control may be needed to reduce the possibility of flooding.
The required storage volume can also be provided by using larger than needed
storm drain pipe sizes and restrictors to control the release rates at manholes
and/or junction boxes in the storm drain system. The same design criteria for
sizing structural control storage facilities are used to determine the storage
volume required in the system (see Section 3.3 for more information).
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5.3.3.1 FREQUENCY FLOOD
5.3.3.2 VELOCITY LIMITATIONS
See Section 5.1 or the local review authority for
Both minimum and maximum velocities should be
5.3.1 Overview
design storm requirements for the sizing of cul-
considered when designing a culvert. The maxi-
verts.
mum velocity should be consistent with channel
A culvert is a short, closed (covered) conduit that
conveys stormwater runoff under an embankment, usually a roadway. The primary purpose of
a culvert is to convey surface water, but properly
designed it may also be used to restrict flow and
reduce downstream peak flows. In addition to the
hydraulic function, a culvert must also support the
embankment and/or roadway, and protect traffic
stability requirements at the culvert outlet. The
The 100-year frequency storm shall be routed
maximum allowable velocity for corrugated metal
through all culverts to be sure building struc-
pipe is 15 feet per second. There is no specified
tures (e.g., houses, commercial buildings) are not
maximum allowable velocity for reinforced con-
flooded or increased damage does not occur to
crete pipe, but outlet protection shall be provided
the highway or adjacent property for this design
where discharge velocities will cause erosion prob-
event.
lems. To ensure self-cleaning during partial depth
flow, a minimum velocity of 2.5 feet per second,
and adjacent property owners from flood hazards
for the 2-year flow, when the culvert is flowing
to the extent practicable.
Most culvert design is empirical and relies on
nomographs and “cookbook procedures.” The
Stormwater Drainage System Design
5.3 Culvert Design
partially full is required.
Table 5.3-1 Symbols and Definitions
Symbol
Definition
Units
of culvert design criteria and procedures.
A
Area of cross section of flow
ft2
B
Barrel width
ft
Cd
Overtopping discharge coefficient
-
5.3.2 Symbols and Definitions
D
Culvert Diameter or barrel depth
in or ft
d
Depth of flow
ft
dc
Critical depth of flow
ft
du
Uniform depth of flow
ft
g
Acceleration of gravity
ft/s2
Hf
hO
HW
Ke
L
N
Q
S
TW
V
VC
Depth of pool or head, above the faction section of invert
Height of hydraulic grade line above outlet invert
ft
ft
ft
ft
cfs
ft/ft
ft
ft/s
ft/s
purpose of the section is to provide an overview
To provide consistency within this section as well
as throughout this Manual the symbols listed in
Table 5.3-1 will be used. These symbols were
selected because of their wide use.
5.3.3 Design Criteria
The design of a culvert should take into account
many different engineering and technical aspects
at the culvert site and adjacent areas. The following design criteria should be considered for all
culvert designs as applicable.
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Headwater depth above invert of culvert (depth from inlet invert to upstream total energy grade line)
Inlet loss coefficient
Length of culvert
Number of barrels
Rate of discharge
Slope of culvert
Tailwater depth above invert of culvert
Mean velocity of flow
Critical velocity
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The following criteria related to headwater should
Headwalls, endwalls, slope paving or other means
be considered:
of anchoring to provide buoyancy protection
• The allowable headwater is the depth of water
that can be ponded at the upstream end of
the culvert during the design flood, which will
be limited by one or more of the following
constraints or conditions:
should be considered for all flexible culverts.
5.3.3.4 LENGTH AND SLOPE
The culvert length and slope should be chosen
to approximate existing topography and, to the
degree practicable, the culvert invert should be
aligned with the channel bottom and the skew
angle of the stream, and the culvert entrance
should match the geometry of the roadway embankment. The maximum slope using concrete
pipe is 10% and for CMP is 14% before pipe-restraining methods must be taken. Maximum drop
in a drainage structure is 10 feet.
1. Headwater be nondamaging to upstream
property
2. Ponding depth be no greater than the low
point in the road grade
3. Ponding depth be no greater than the
elevation where flow diverts around the
culvert
4. Elevations established to delineate floodplain
zoning
5. 18-inch (or applicable) freeboard requirements
• Either the headwater should be set to produce
acceptable velocities, or stabiliza-tion or
energy dissipation should be provided where
these velocities are ex-ceeded.
• In general, the constraint that gives the lowest
allowable headwater elevation establishes the
criteria for the hydraulic calculations.
• Other site-specific design considerations
should be addressed as required.
5.3.3.7 TAILWATER CONSIDERATIONS
Stormwater Drainage System Design
5.3.3.3 BUOYANCY PROTECTION
The hydraulic conditions downstream of the
culvert site must be evaluated to determine a
tailwater depth for a range of discharge. At times
there may be a need for calculating backwater
curves to establish the tailwater conditions. The
• The following HW/D criteria:
5.3.3.5 DEBRIS CONTROL
In designing debris control structures it is recommended that the Hydraulic Engineering Circular No. 9 entitled Debris Control Structures be
consulted.
5.3.3.6 HEADWATER LIMITATIONS
Headwater is water above the culvert invert at the entrance end of the culvert. The allowable headwater
elevation is that elevation above which damage may
be caused to adjacent property and/or the roadway
and is determined from an evaluation of land use
upstream of the culvert and the proposed or existing
1. For drainage facilities with cross-sectional
area equal to or less than 30 ft2, HW/D
should be equal to or less than 1.5
2. For drainage facilities with cross-sectional
area greater than 30 ft2, HW/D should be
equal to or less than 1.2
• The headwater should be checked for the
100-year flood to ensure compliance with
flood plain management criteria and for most
facilities the culvert should be sized to maintain
flood-free conditions on major thoroughfares
with 18-inch freeboard at the low-point of the
road.
following conditions must be considered:
• If the culvert outlet is operating with a free
outfall, the critical depth and equival-ent
hydraulic grade line should be determined.
• For culverts that discharge to an open channel,
the stage-discharge curve for the channel must
be determined. See Section 5.4, Open Channel
Design.
• If an upstream culvert outlet is located near
a downstream culvert inlet, the head-water
elevation of the downstream culvert may
establish the design tailwater depth for the
upstream culvert.
roadway elevation. It is this allowable headwater
depth that is the primary basis for sizing a culvert.
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• The maximum acceptable outlet velocity
should be identified (see Subsection 5.4.3).
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This apron should extend at least one pipe diam-
5.3.3.15 CULVERT SIZES
eter upstream from the entrance, and the top of
The minimum allowable pipe diameter shall be 18”.
the apron should not protrude above the normal
streambed elevation.
5.3.3.16 WEEP HOLES
Weep holes are sometimes used to relieve uplift
5.3.3.8 STORAGE
5.3.3.11 WINGWALLS AND APRONS
pressure. Filter materials should be used in con-
If storage is being assumed or will occur upstream
Wingwalls are used where the side slopes of the
junction with the weep holes in order to intercept
of the culvert, refer to Subsection 5.3.4.6 regard-
channel adjacent to the entrance are unstable or
the flow and prevent the formation of piping
ing storage routing as part of the culvert design.
where the culvert is skewed to the normal chan-
channels. The filter materials should be designed
nel flow.
as an underdrain filter so as not to become
clogged and so that piping cannot occur through
5.3.3.9 CULVERT INLETS
the pervious material and the weep hole.
Hydraulic efficiency and cost can be significantly
5.3.3.12 IMPROVED INLETS
affected by inlet conditions. The inlet coefficient
Where inlet conditions control the amount of flow
Ke, is a measure of the hydraulic efficiency of the
that can pass through the culvert, improved inlets
5.3.3.17 OUTLET PROTECTION
inlet, with lower values indicating greater efficien-
can greatly increase the hydraulic performance of
See Section 5.5 for information on the design of
cy. Recommended inlet coefficients are given in
the culvert.
outlet protection. Outlet protection should be
Stormwater Drainage System Design
• If the culvert discharges to a lake, pond, or
other major water body, the expected high
water elevation of the particular water body
may establish the culvert tailwater.
provided for the 25-year storm.
Table 5.3-2.
5.3.3.13 MATERIAL SELECTION
5.3.3.10 INLETS WITH HEADWALLS
Reinforced concrete pipe (RCP) is generally rec-
5.3.3.18 EROSION AND SEDIMENT CONTROL
Headwalls may be used for a variety of reasons,
ommended for use (1) under a roadway, (2) when
Erosion and sediment control shall be in accor-
including increasing the efficiency of the inlet,
pipe slopes are less than 1%, or (3) for all flowing
dance with the latest approved Soil Erosion and
providing embankment stability, providing em-
streams. RCP and any other Georgia Department
Sediment Control Ordinance for the municipality.
bankment protection against erosion, providing
of Transportation approved pipe material may
See also the Manual for Erosion and Sediment
protection from buoyancy, and shortening the
be used as allowed by local regulations. Table
Control in Georgia for design standards and de-
length of the required structure. Headwalls are
5.3-3 gives recommended Manning’s n values for
tails related to erosion and sediment control.
required for all metal culverts and where buoy-
different materials.
ancy protection is necessary. If high headwater
depths are to be encountered, or the approach
velocity in the channel will cause scour, a short
channel apron should be provided at the toe of
the headwall.
5.3.3.19 ENVIRONMENTAL CONSIDERATIONS
5.3.3.14 CULVERT SKEWS
Where compatible with good hydraulic engineering,
Culvert skews shall not exceed 45 degrees as
a site should be selected that will permit the culvert
measured from a line perpendicular to the road-
to be constructed to cause the least impact on the
way centerline without approval.
stream or wetlands. This selection must consider the
entire site, including any necessary lead channels.
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Table 5.3-3 Manning’s n Values for Culverts
Type of Structure and Design of Entrance
Coefficient Ke
Pipe, Concrete
Projecting from fill, socket end (grove-end)
Projecting from fill, square cut end
Headwall or headwall and wingwalls
Socket end of pipe (grove-end)
Square-edge
Rounded [radius = 1/12(D)]
Mitered to conform to fill slope
*End-Section conforming to fill slope
Beveled edges, 33.7° or 45° bevels
Side- or slope-tapered inlet
0.2
0.5
0.2
0.5
0.2
0.7
0.5
0.2
0.2
Pipe, or Pipe-Arch, Corrugated Metal
1
Projecting from fill (no headwall)
Headwall or headwall and wingwalls square-edge
Mitered to fill slope, paved or unpaved slope
*End-section conforming to fill slope
Beveled edges, 33.7° or 45° bevels
Side- or slope-tapered inlet
0.9
0.5
0.7
0.5
0.2
0.2
Box, Reinforced Concrete
Headwall parallel to embankment (no wingwalls)
Square-edged on 3 edges
Rounded on 3 edges to radius of [1/12(D)] or beveled edges on 3 sides
Wingwalls at 30° to 75° to barrel
Square-edged at crown
Crown edge rounded to radius of [1/12(D)] or beveled top edge
Wingwalls at 10° or 25° to barrel
Square-edged at crown
Wingwalls parallel (extension of sides)
Square-edged at crown
Side- or slope-tapered inlet
Type of Conduit
Wall & Joint Description
Manning’s n
Concrete Pipe
Smooth
0.010 - 0.011
Concrete Box
Good joints, smooth finished walls
Poor joints, rough, unfinished walls
0.012
0.015
2 2/3- by 1/2 inch corrugations
Corrugated Metal Pipes 5- by 1-inch corrugations
3- by 1-inch corrugations
and Boxes Annular
6- by 2-inch structural plate
Corrugations
9- by 2-1/2 inch structural plate
0.022 - 0.027
0.025 - 0.026
0.027 - 0.028
0.033 - 0.035
0.033 - 0.037
Corrugated Metal Pipes, 2 2/3-by 1/2-inch corrugated 24-inch plate width
6-by 1-inch
Helical Corrugations
0.011 - 0.023
0.022 - 0.025
Spiral Rib Metal Pipe
3/4 by 3/4 in recesses at 12 inch spacing, good joints
0.012 - 0.013
High Density Polyethylene (HDPE)
Corrugated Smooth Liner
Corrugated
0.009 - 0.015
0.018 - 0.025
Polyvinyl Chloride (PVC)
Stormwater Drainage System Design
Table 5.3-2 Inlet Coefficients
0.009 - 0.011
Source: HDS No. 5, 2012
0.5
0.2
Note: For further information concerning Manning n values for selected culverts consult Hydraulic
Design of Highway Culverts, Federal Highway Administration, HDS No. 5, page B.6 (208).
0.4
0.2
0.5
0.7
0.2
1
Although laboratory tests have not been completed on Ke values for High-Density Polyethylene (HDPE) pipes, the Ke
values for corrugated metal pipes are recommended for HDPE pipes
* Note: End Section conforming to fill slope, made of either metal or concrete, are the sections commonly available from
manufacturers. From limited hydraulic tests, they are equivalent in operation to a headwall in both inlet and outlet control.
Source: HDS No. 5, 2012, C.6 (216)
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Stormwater Drainage System Design
5.3.4 Design Procedures
5.3.4.1 TYPES OF FLOW CONTROL
There are two types of flow conditions for culverts that are based upon the
location of the control section and the critical flow depth:
»» Inlet Control – Inlet control occurs when the culvert barrel is capable
of conveying more flow that the inlet will accept. This typically happens
when a culvert is operating on a steep slope. The control section of
a culvert is located just inside the entrance. Critical depth occurs at
or near this location, and the flow regime immediately downstream is
supercritical.
»» Outlet Control – Outlet control flow occurs when the culvert barrel is
not capable of conveying as much flow as the inlet opening will accept.
The control section for outlet control flow in a culvert is located at the
barrel exit or further downstream. Either subcritical or pressure flow
exists in the culvert barrel under these conditions.
Figure 5.3-1(b) Typical Outlet Control Flow Conditions
Proper culvert design and analysis requires checking for both inlet and outlet
control to determine which will govern particular culvert designs. For more
information on inlet and outlet control, see the FHWA Hydraulic Design of
Highway Culverts, HDS-5, 2012.
5.3.4.2 PROCEDURES
(Adapted from: HDS-5, 2012)
There are two procedures for designing culverts: manual use of inlet and
outlet control nomographs, and the use computer programs such as HY8. It
is recommended that the HY8 computer model or equivalent be used for culvert design. The computer software package HYDRAIN, which includes HY8,
uses the theoretical basis from the nomographs to size culverts. In addition,
this software can evaluate improved inlets, route hydrographs, consider road
overtopping, and evaluate outlet streambed scour. By using water surface
profiles, this procedure is more accurate in predicting backwater effects and
outlet scour.
5.3.4.3 NOMOGRAPHS
The use of culvert design nomographs requires a trial and error solution. NoFigure 5.3-1(a) Typical Inlet Control Flow Section
(Adapted from: HDS-5, 2012)
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mograph solutions provide reliable designs for many applications. It should be
remembered that velocity, hydrograph routing, roadway overtopping, and
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Stormwater Drainage System Design
outlet scour require additional, separate computations beyond what can be
obtained from the nomographs. Figures 5.3-2(a) and (b) show examples of
an inlet control and outlet control nomograph for the design of concrete pipe
culverts. For other culvert designs, refer to the complete set of nomographs
in Appendix C.
Figure 5.3-2(a) Headwater Depth for Concrete Pipe Culvert with Inlet Control
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Figure 5.3-2(b) Head for Concrete Pipe Culverts Flowing Full
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433
Where:
The following design procedure requires the use of inlet and outlet nomo-
ho = ½ (critical depth + D), or tailwater depth,
graphs.
whichever is greater
L = culvert length
(Step 1) List design data:
S = culvert slope
Q = discharge (cfs)
L = culvert length (ft)
S = culvert slope (ft/ft)
TW= tailwater depth (ft)
(Step 4) Compare the computed headwaters and use the higher HW
V = velocity for trial diameter (ft/s)
nomograph to determine if the culvert is under inlet or outlet
Ke= inlet loss coefficient
control.
HW= allowable headwater depth for the design storm (ft)
• If inlet control governs, then the design is complete and no
further analysis is required.
(Step 2) Determine trail culvert size by assuming a trial velocity 3 to 5
ft/s and computing the culvert area, A = Q/V. Determine the
culvert diameter (inches).
(Step 3) Find the actual HW for the trial size culvert for both inlet and
outlet control.
• For inlet control, enter inlet control nomograph with D and
Q and find HW/D for the proper entrance type.
• Compute HW and, if too large or too small, try another
culvert size before computing HW for outlet control.
• If outlet control governs and the HW is unacceptable, select
a larger trial size and find another HW with the outlet control
nomographs. Since the smaller size of culvert had been
selected for allowable HW by the inlet control nomographs,
the inlet control for the larger pipe need not be checked.
Stormwater Drainage System Design
5.3.4.4 DESIGN PROCEDURE
(Step 5) Calculate exit velocity and if erosion problems might be
expected, refer to Section 5.5 for appropriate energy dissipation designs.
• For outlet control enter the outlet control nomograph with
the culvert length, entrance loss coefficient, and trial culvert
diameter.
5.3.4.5 PERFORMANCE CURVES - ROADWAY OVERTOPPING
• To compute HW, connect the length scale for the type of
entrance condition and culvert diameter scale with a straight
line, pivot on the turning line, and draw a straight line from
the design discharge through the turning point to the head
loss scale H. Compute the headwater elevation HW from
the equation:
interest for that particular culvert design. A graph is then plotted of headwater
A performance curve for any culvert can be obtained from the nomographs
by repeating the steps outlined above for a range of discharges that are of
versus discharge with sufficient points so that a curve can be drawn through
the range of interest. These curves are applicable through a range of headwater, velocities, and scour depths versus discharges for a length and type of
culvert. Usually charts with length intervals of 25 to 50 feet are satisfactory
for design purposes. Such computations are made much easier by the use of
computer programs.
HW = H + ho LS
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(5.3.1)
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434
Note: See Figure 5.3-3 for guidance in determining a value for Cd. For more
performance curve showing the culvert flow as well as the flow across the
information on calculating overtopping flow rates see pages 39 - 42 in HDS
roadway is a useful analysis tool. Rather than using a trial and error procedure
No. 5.
to determine the flow division between the overtopping flow and the culvert
flow, an overall performance curve can be developed.
(Step 4) Add the culvert flow and the roadway overtopping flow at
the corresponding headwater elevations to obtain the overall
The overall performance curve can be determined as follows:
culvert performance curve.
(Step 1) Select a range of flow rates and determine the corresponding headwater elevations for the culvert flow alone. The flow
rates should fall above and below the design discharge and
cover the entire flow range of interest. Both inlet and outlet
control headwaters should be calculated.
Stormwater Drainage System Design
To complete the culvert design, roadway overtopping should be analyzed. A
(Step 2) Combine the inlet and outlet control performance curves to
define a single performance curve for the culvert.
(Step 3) When the culvert headwater elevations exceed the roadway
crest elevation, overtopping will begin. Calculate the equivalent upstream water surface depth above the roadway (crest
of weir) for each selected flow rate. Use these water surface
depths and equation 5.3.2 to calculate flow rates across the
roadway.
Q = CdL(HW)1.5
(5.3.2)
Where:
Q = overtopping flow rate (ft3/s)
Cd = overtopping discharge coefficient
L = length of roadway (ft)
HW = upstream depth, measured from the roadway
crest to the water surface upstream of the weir
drawdown (ft)
Figure 5.3-3 Discharge Coefficients for Roadway Overtopping
(Source: HDS No. 5, 2012)
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435
5.3.5.3 EXAMPLE DATA
A significant storage capacity behind a highway
Input Data
»» Discharge for 2 yr flood = 35 cfs
embankment attenuates a flood hydrograph.
Because of the reduction of the peak discharge
»» Discharge for 25 yr flood = 70 cfs
associated with this attenuation, the required ca-
»» Length of culvert = 100 ft
considerably. If significant storage is anticipated
routing the design hydrographs through the cul-
»» Natural channel invert elevations - inlet =
15.50 ft, outlet = 14.30 ft
vert to determine the discharge and stage behind
»» Culvert slope = 0.012 ft/ft
behind a culvert, the design should be checked by
the culvert. See subsection 5.3.7 and Section 3.3
»» Tailwater depth is the normal depth in
downstream channel
ing procedures are outlined in Hydraulic Design
of Highway Culverts, Section V - Storage Routing,
»» Entrance type = Groove end with headwall
HDS No. 5, Federal Highway Administration.
5.3.5.4 COMPUTATIONS
1.
5.3.5 Culvert Design Example
5.3.5. INTRODUCTION
The following example problem illustrates the
3.
the nomographs.
5.3.5.2 EXAMPLE
which were determined by physical limitations at
the culvert site and hydraulic procedures de-
Assume a culvert velocity of 5 ft/s. Required
flow area = 70 cfs/5 ft/s = 14 ft2 (for the 25 yr
recurrence flood).
2.
Size a culvert given the following example data,
Using the same procedures outlined in steps
4 and 5 the following results were obtained.
»» 42-inch culvert – HW = 4.13 ft
»» 36-inch culvert – HW = 4.98 ft
»» Select a 36-inch culvert to check for outlet
control.
»» Tailwater depth for 25 yr discharge = 3.5 ft
for more information on routing. Additional rout-
procedures to be used in designing culverts using
5.
»» Allowable Hw for 25 yr discharge = 5.25 ft
pacity of the culvert, and its size, may be reduced
Note: Storage should be taken into consideration
only if the storage area will remain available for
the life of the culvert as a result of purchase of
ownership or right-of-way or an easement has
been acquired.
ft is considerably less than 5.25 try a small
culvert.
The corresponding culvert diameter is about
48 in. This can be calculated by using the
formula for area of a circle: Area = (3.14D2)/4
or D = (Area times 4/3.14)0.5. Therefore: D =
((14 sq ft x 4)/3.14)0.5 x 12 in/ft) = 50.7 in
A grooved end culvert with a headwall is
selected for the design. Using the inlet
control nomograph (Figure 5.3-1), with a pipe
diameter of 48 inches and a discharge of 70
cfs; read a HW/D value of 0.93.
4.
The depth of headwater (HW) is (0.93)
x (4) = 3.72 ft, which is less than the
allowable headwater of 5.25 ft. Since 3.72
6.
The culvert is checked for outlet control by
using Figure 5.3-2.
Stormwater Drainage System Design
5.3.4.6 STORAGE ROUTING
»» With an entrance loss coefficient Ke of
0.20, a culvert length of 100 ft, and a pipe
diameter of 36 in., an H value of 2.8 ft is
determined. The headwater for outlet control
is computed by the equation: HW = H + ho
LS
»» Compute ho
-- ho = Tw or ½ (critical depth in culvert + D),
whichever is greater.
-- ho = 3.5 ft or ho = ½ (2.7 + 3.0) = 2.85 ft
»» Therefore: ho = 3.5 ft
»» The headwater depth for outlet control is:
HW = H + ho LS = 2.8 + 3.5 (100) x (0.012)
= 5.10 ft
scribed elsewhere in this handbook.
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436
Since HW for inlet outlet (5.10 ft) is greater
than the HW for inlet control (4.98 ft), outlet
control governs the culvert design. Thus, the
maximum headwater expected for a 25 year
recurrence flood is 5.10 ft, which is less than
the allowable headwater of 5.25 ft.
8.
Estimate outlet exit velocity. Since this
culvert is on outlet control and discharges
into an open channel downstream with
tailwater above culvert, the culvert will be
flowing full at the flow depth in the channel.
Using the design peak discharge of 70
cfs and the area of a 36-inch or 3.0-foot
diameter culvert the exit velocity will be:
Stormwater Drainage System Design
7.
»» Q = VA
»» Therefore: V = 70 / (3.14(3.0)2)/4 = 9.9 ft/s
»» With this high velocity, consideration should
be given to provide an energy dissipator at
the culvert outlet. See Section 5.5 (Energy
Dissipation Design).
9.
Check for minimum velocity using the 2-year
flow of 35 cfs.
»» Therefore: V = 35 / (3.14(3.0)2/4 = 5.0 ft/s >
minimum of 2.5 - OK
10. The 100-year flow should be routed through
the culvert to determine if any flooding
problems will be associated with this flood.
Figure 5.3-4 provides a convenient form to organize culvert design calculations.
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437
Stormwater Drainage System Design
Figure 5.3-4 Culvert Design Calculation Form
(Source: HDS No. 5, 2012)
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438
»» D - Height of box culvert or diameter of pipe
culvert
5.3.6.4 DESIGN FIGURE LIMITS
research results from culvert models with bar-
5.3.6.1 INTRODUCTION
»» Hf - Depth of pool or head, above the face
section of invert
Improved inlets include inlet geometry refinements beyond those normally used in conventional culvert design practice. Several degrees
»» N - Number of barrels
»» Q - Design discharge
of improvements are possible, including bev-
The improved inlet design figures are based on
rel width, B, to depth, D, ratios of from 0.5:1 to
2:1. For box culverts with more than one barrel,
the figures are used in the same manner as for a
single barrel, except that the bevels must be sized
on the basis of the total clear opening rather than
el-edged, side-tapered, and slope-tapered inlets.
Those designers inter-ested in using side and
5.3.6.3 DESIGN PROCEDURE
slope tapered inlets should consult the detailed
The figures for bevel-edged inlets are used for
design criteria and example designs outlined in
design in the same manner as the con-ventional
the U. S. Department of Transportation publica-
inlet design nomographs discussed earlier. Note
tion Hydraulic Engineering Circular No. 5 entitled,
that Charts 10, 11, and 12 in Appendix C apply
Hydraulic Design of Highway Culverts.
only to bevels having either a 33o angle (1.5:1) or
a 45o angle (1:1).
5.3.6.2 DESIGN FIGURES
For box culverts the dimensions of the bevels to
Four inlet control figures for culverts with beveled
be used are based on the culvert dimensions. The
edges are included in Appendix C.
top bevel dimension is determined by multiplying
on individual barrel size.
For example, in a double 8 ft by 8 ft box culvert:
»» Top Bevel is proportioned based on the
height of 8 feet which results in a bevel of
4 in. for the 1:1 bevel and 8 in. for the 1.5:1
bevel.
Stormwater Drainage System Design
5.3.6 Design Procedures for Beveled-Edged Inlets
»» Side Bevel is proportioned based on the clear
width of 16 feet, which results in a bevel of
8 in. for the 1:1 bevel and 16 in. for the 1.5:1
bevel.
the height of the culvert by a factor. The side
Chart
Page
Use for :
bevel dimensions are determined by multiply-
3
A-3
circular pipe culverts with
ing the width of the culvert by a factor. For a 1:1
5.3.6.5 MULTIBARREL INSTALLATIONS
bevel, the factor is 0.5 inch/ft. For a 1.5:1 bevel
For multibarrel installations exceeding a 3:1 width
the factor is 1 inch/ft. For example, the minimum
to depth ratio, the side bevels become excessively
wingwalls)
bevel dimensions for a 8 ft x 6 ft box culvert with
large when proportioned on the basis of the total
11
A-11
skewed headwalls
1:1 bevels would be:
clear width. For these structures, it is recom-
12
A-12
wingwalls with flare angles of 18
beveled rings
10
A-10
90o headwalls (same for 90o
to 45 degrees
mended that the side bevel be sized in propor»» Top Bevel = d = 6 ft x 0.5 inch/ft = 3 inches
tion to the total clear width, B, or three times the
»» Side Bevel = b = 8 ft x 0.5 inch/ft = 4 inches
height, whichever is smaller.
The following symbols are used in these figures:
»» B - Width of culvert barrel or diameter of
pipe culvert
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For a 1.5:1 bevel computations would result in d =
6 and b = 8 inches.
The top bevel dimension should always be based
on the culvert height.
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5.3.7 Flood Routing and Culvert Design
mediate walls of multibarrel installations is not
however, an engineer should be familiar with the
culvert flood routing design process.
as important to the hydraulic performance of a
5.3.7.1 INTRODUCTION
culvert as the edge condition of the top and sides.
Flood routing through a culvert is a practice that
A multiple trial and error procedure is required for
Therefore, the edges of these walls may be square,
evaluates the effect of temporary upstream pond-
culvert flood routing. In general:
rounded with a radius of one half their thickness,
ing caused by the culvert’s backwater. By not
chamfered, or beveled. The intermediate walls
considering flood routing it is possible that the
may also project from the face and slope down-
findings from culvert analyses will be conservative.
ward to the channel bottom to help direct debris
If the selected allowable headwater is accepted
through the culvert.
without flood routing, then costly overdesign of
both the culvert and outlet protection may result,
Multibarrel pipe culverts should be designed as a
depending on the amount of temporary storage
series of single barrel installations since each pipe
involved. However, if storage is used in the design
requires a separate bevel.
of culverts, consideration should be given to:
5.3.6.6 SKEWED INLETS
It is recommended that Chart 11 (see Appendix C)
for skewed inlets not be used for multiple barrel
installations, as the intermediate wall could cause
an extreme contraction in the downstream barrels.
This would result in underdesign due to a greatly
reduced capacity. Skewed inlets (at an angle with
• The total area of flooding,
• The average time that bankfull stage is
exceeded for the design flood up to 48 hours
in rural areas or 6 hours in urban areas, and
• Ensuring that the storage area will remain
available for the life of the culvert through the
purchase of right-of-way or easement.
the centerline of the stream) should be avoided
whenever possible and should not be used with
side or slope tapered inlets. It is important to align
culverts with streams in order to avoid erosion
problems associated with changing the direction of
the natural stream flow.
5.3.7.2 DESIGN PROCEDURE
(Step 1) A trial culvert(s) is selected
(Step 2) A trial discharge for a particular
hydrograph time increment (selected
time increment to estimate discharge
from the design hydrograph) is selected
Stormwater Drainage System Design
The shape of the upstream edge of the inter-
(Step 3) Flood routing computations are made
with successive trial discharges until the
flood routing equation is satisfied
(Step 4) The hydraulic findings are compared to
the selected site criteria
(Step 5) If the selected site criteria are satisfied,
then a trial discharge for the next time
increment is selected and this procedure
is repeated; if not, a new trial culvert
is selected and the entire procedure is
repeated.
The design procedure for flood routing through
a culvert is the same as for reservoir routing. The
site data and roadway geometry are obtained and
the hydrology analysis completed to include estimating a hydrograph. Once this essential information is available, the culvert can be designed.
Flood routing through a culvert can be time
consuming. It is recommended that a computer
program be used to perform routing calculations;
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440
and provides water quality benefits (see
Chapter 4 for more details on using
enhanced swales and grass channels for
runoff reduction and water quality purposes).
5.4.1 Overview
5.4.1.1 INTRODUCTION
»» Conditions under which vegetation may not
be acceptable include but are not limited to:
Open channel systems and their design are an
integral part of stormwater drainage design,
particularly for development sites utilizing better
-- High velocities
site design practices and open channel structural
-- Standing or continuously flowing water
controls. Open channels include drainage ditch-
-- Lack of regular maintenance necessary
to prevent growth of taller or woody
vegetation
es, grass channels, dry and wet enhanced swales,
riprap channels and concrete-lined channels.
-- Lack of nutrients and inadequate topsoil
The purpose of this section is to provide an
3.
Rigid Linings – Rigid linings are generally
constructed of concrete and used where
high flow capacity is required. Higher
velocities, however, create the potential
for scour at channel lining transitions and
channel headcutting.
Stormwater Drainage System Design
5.4 Open Channel Design
-- Excessive shade
overview of open channel design criteria and
methods, including the use of channel design
»» Proper seeding, mulching and soil
preparation are required during construction
to assure establishment of healthy
vegetation.
nomographs.
5.4.1.2 OPEN CHANNEL TYPES
The three main classifications of open channel
types according to channel linings are vegetated,
flexible and rigid. Vegetated linings include grass
with mulch, sod and lapped sod, and wetland
channels. Riprap and some forms of flexible
man-made linings or gabions are examples of
flexible linings, while rigid linings are generally
concrete or rigid block.
1.
Vegetative Linings – Vegetation, where
practical, is the most desirable lining for an
artificial channel. It stabilizes the channel
body, consolidates the soil mass of the bed,
checks erosion on the channel surface,
provides habitat, provides runoff reduction,
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2.
Flexible Linings – Rock riprap, including
rubble, is the most common type of flexible
lining for channels. It presents a rough
surface that can dissipate energy and
mitigate increases in erosive velocity. These
linings are usually less expensive than rigid
linings and have self-healing qualities that
reduce maintenance. However, they may
require the use of a filter fabric depending
on the underlying soils, and the growth of
grass and weeds may present maintenance
problems.
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Table 5.4-1 Symbols and Definitions
To provide consistency within this section as well
Symbol
Definition
Units
as throughout this Manual, the symbols listed in
α
A
Energy coefficient
-
Cross-sectional area
ft2
b
Bottom width
ft
for more than one definition. Where this occurs
Cg
Specific weight correction factor
-
D or d
Depth of flow
ft
in this section, the symbol will be defined where it
d
Stone Diameter
ft
occurs in the text or equations.
delta d
Superelevation of the water surface profile
ft
dx
Diamter of stone for which x percent, by weight, of the gradation is finer
ft
E
Fr
g
hloss
K
ke
KT
L
Lp
n
P
Q
R
RC
Specific energy
Froude Number
Acceleration of gravity
Head loss
Channel conveyance
Eddy head loss coefficient
Trapezoidal open channel conveyance factor
Length of channel
Length of downstream protection
Manning’s roughness coefficient
Wetted Perimeter
Discharge rate
Hydraulic radius of flow
ft
32.2 ft/s2
ft
ft
ft
ft
ft
cfs
ft
Mean radius of the bend
Slope
Specific weight of stone
Top width of water surface
Velocity of flow
Stone weight
Critical depth
Normal depth
Critical flow section factor
ft
ft/ft
lbs/ft3
ft
ft/s
lbs
ft
ft
-
Table 5.4-1 will be used. These symbols were selected because of their wide use. In some cases,
the same symbol is used in existing publications
S
SWS
T
V or v
w
yC
yn
z
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Stormwater Drainage System Design
5.4.2 Symbols and Definitions
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442
5.4.3.2 VELOCITY LIMITATIONS
The final design of artificial open channels should be consistent with the velocity
5.4.3.1 GENERAL CRITERIA
limitations for the selected channel lining. Maximum velocity values for selected lining
The following criteria should be followed for open channel design:
categories are presented in Table 5.4-2. Seeding and mulch should only be used
when the design value does not exceed the allowable value for bare soil. Velocity lim-
• Channels with bottom widths greater than 10 feet shall be designed with a
minimum bottom cross slope of 12 to 1, or with compound cross sections.
• Channel side slopes shall be stable throughout the entire length and side
slope shall depend on the channel material. A maximum of 2:1 should be
used for channel side slopes, unless otherwise justified by calculations.
Roadside ditches should have a maximum side slope of 3:1.
• Trapezoidal or parabolic cross sections are preferred over triangular shapes.
• For vegetative channels, design stability should be determined using low
vegetative retardance conditions (Class D) and for design capacity higher
vegetative retardance conditions (Class C) should be used.
• For vegetative channels, flow velocities within the channel should not
exceed the maximum permissible velocities given in Tables 5.4-2 and 5.4-3.
• If relocation of a stream channel is unavoidable, the cross-sectional
shape, meander, pattern, roughness, sediment transport, and slope should
conform to the existing conditions insofar as practicable. Some means of
energy dissipation may be necessary when existing conditions cannot be
duplicated.
• Streambank stabilization should be provided, when appropriate, as a result
of any stream disturbance such as encroachment and should include both
upstream and downstream banks as well as the local site.
• Open channel drainage systems are sized to handle a 25-year design storm.
The 100-year design storm should be routed through the channel system
to determine if the 100-year plus applicable building elevation restrictions
are exceeded, structures are flooded, or flood damages increased.
itations for vegetative linings are reported in Table 5.4-3. Vegetative lining calculations
are presented in Section 5.4.7 and riprap procedures are presented in Section 5.4.8.
Table 5.4-2 Maximum Velocities for Comparing Lining Materials
Material
Maximum Velocity (ft/s)
Sand
Silt
Firm Loam
Fine Gravel
Stiff Clay
Graded Loam or Silt to Cobbles
Coarse Gravel
Shales and Hard Pans
2.0
3.5
3.5
5.0
5.0
5.0
6.0
6.0
Stormwater Drainage System Design
5.4.3 Design Criteria
Source: AASHTO Model Drainage Manual, 2005
Table 5.4-3 Maximum Velocities for Vegetative Channel Linings
Vegetation Type
Bermuda Grass
Bahia
Tall fescue grass mixtures3
Kentucky bluegrass
Buffalo Grass
Grass Mixture
Sericea lespedeza, Weeping lovegrass Alfalfa
Annuals5
Sod
Lapped Sod
Slope Range (%)1
Maximum Velocity2 (ft/s)
0->10
5
4
4
6
5
4
4
3
3
3
4
5
0-10
0-5
5-10
>10
0-51
5-10
0-54
0-5
Do not use on slopes steeper than 10%, except for side-slope in combination channel.
Use velocities exceeding 5 ft/s only where good stands can be maintained.
3
Mixures of Tall Fescue, Bahia, and/or Bermuda
4
Do not use on slopes steeper than 5%, except for side-slope in combination channel.
5
Annuals - used on mild slopes or as temporary protection until permanent covers are established.
1
2
Source: Manual for Erosion and Sediment Control in Georgia
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VOL 2
443
nel flow. The Federal Highway Administration has
tions can be calculated from geometric dimen-
The Manning’s n value is an important variable in
prepared numerous design charts to aid in the
sions. Irregular channel cross sections (i.e., those
open channel flow computations. Variation in this
design of rectangular, trapezoidal and triangular
with a narrow deep main channel and a wide
variable can significantly affect discharge, depth,
open channel cross sections. In addition, design
shallow overbank channel) must be subdivided
and velocity estimates. Since Manning’s n values
charts for grass-lined channels have been devel-
into segments so that the flow can be computed
depend on many different physical characteris-
oped. These charts and instructions for their use
separately for the main channel and overbank
tics of natural and man-made channels, care and
are given in Appendix C.
portions. This same process of subdivision may
be used when different parts of the channel cross
good engineering judgment must be exercised in
the selection process.
Recommended Manning’s n values for artificial
channels with rigid, unlined, temporary, and
riprap linings are given in Table 5.4-4. Recom-
section have different roughness coeff-icients.
5.4.5.2 MANNING’S EQUATION
When computing the hydraulic radius of the
Manning’s Equation, presented in three forms be-
subsections, the water depth common to the two
low, is recommended for evaluating uniform flow
adjacent subsections is not counted as wetted
conditions in open channels:
perimeter.
mended values for vegetative linings should be
v = (1.49/n) R2/3 S1/2
determined using Figure 5.4-1, which provides a
and the product of velocity and hydraulic radius
for several vegetative retardance classifications
(see Table 5.4-6). Figure 5.4-1 is used iteratively
Manning’s values for natural channels that are
either excavated or dredged and natural are given
in Table 5.4-5. For natural channels, Manning’s
n values should be estimated using experienced
judgment and information presented in publications such as the Guide for Selecting Manning’s
Roughness Coefficients for Natural Channels and
Flood Plains, FHWA-TS-84-204, 1984.
(5.4.1)
5.4.5.4 DIRECT SOLUTIONS
graphical relationship between Manning’s n values
as described in Section 5.4.6. Recommended
Stormwater Drainage System Design
5.4.4 Manning’s Values
Q = (1.49/n) A R2/3 S1/2
(5.4.2)
S = [Qn/(1.49 A R2/3)]2
(5.4.3)
When the hydraulic radius, cross-sectional area,
and roughness coefficient and slope are known,
discharge can be calculated directly from equation 5.4.2. The slope can be calculated using
Where:
v = average channel velocity (ft/s)
Q = discharge rate for design conditions (cfs)
n = Manning’s roughness coefficient
A = cross-sectional area (ft2)
R = hydraulic radius A/P (ft)
P = wetted perimeter (ft)
S = slope of the energy grade line (ft/ft)
equation 5.4.3 when the discharge, roughness
coefficient, area, and hydraulic radius are known.
Nomographs for obtaining direct solutions to
Manning’s Equation are presented in Figures
5.4-2 and 5.4-3. Figure 5.4-2 provides a general solution for the velocity form of Manning’s
Equation, while Figure 5.4-3 provides a solution
of Manning’s Equation for trapezoidal channels.
For prismatic channels, in the absence of backwater
5.4.5 Uniform Flow Calculations
conditions, the slope of the energy grade line, water
surface and channel bottom are assumed to be equal.
5.4.5.1 DESIGN CHARTS
5.4.5.3 GEOMETRIC RELATIONSHIPS
Following is a discussion of the equations that can
Area, wetted perimeter, hydraulic radius, and
be used for the design and analysis of open chan-
channel top width for standard channel cross sec-
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VOL 2
444
Depth Ranges
0.5-2.0 ft
Category
Lining Type
0-0.5 ft
Rigid
Concrete
Grouted Riprap
Stone Masonry
Soil Cement
Asphalt
0.015
0.040
0.042
0.025
0.018
0.013
0.030
0.032
0.022
0.016
0.013
0.028
0.030
0.020
0.016
Unlined
Bare Soil
Rock Cut
0.023
0.045
0.020
0.035
0.020
0.025
Temporary*
Woven Paper Net
Jute Net
Fiberglass Roving
Straw with Net
Curled Wood Mat
Synthetic Mat
0.016
0.028
0.028
0.065
0.066
0.036
0.015
0.022
0.022
0.033
0.035
0.025
0.015
0.019
0.019
0.025
0.028
0.021
Gravel Riprap
1-inch D50
2-inch D50
6-inch D50
12-inch D50
0.044
0.066
0.104
-
0.033
0.041
0.069
0.078
0.030
0.034
0.035
0.040
Rock Riprap
>2.0 ft
Stormwater Drainage System Design
Table 5.4-4 Manning’s Roughness Coefficients (n) for Artificial Channels
Note: Values listed are representative values for the respective depth ranges. Manning’s roughness
coefficients, n, vary with the flow depth.
*Some “temporary” linings become permanent when buried.
Source: HEC-15, 2005.
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VOL 2
445
Stormwater Drainage System Design
Figure 5.4-1 Manning’s n Values for Vegetated Channels
(Source: USDA, TP-61, 1947)
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VOL 2
446
Type of Channel and Description
EXCAVATED OR DREDGED
a. Earth, straight and uniform
1. Clean, recently completed
2. Clean, after weathering
3. Gravel, uniform sector, clean
b. Earth, winding and sluggish
1. No vegetation
2. Grass, some weeds
3. Dense weeds/plants in deep channels
4. Earth bottom and rubble sides
5. Stony bottom and weedy sides
6. Cobble bottom and clean sides
c. Dragline-excavated or dredged
1. No vegetation
2. Light brush on banks
d. Rock cuts
1. Smooth and uniform
2. Jagged and irregular
e. Channels not maintained, weeds and brush uncut
1. Dense weeds, high as flow depth
2. Clean bottom, brush on sides
3. Same, highest stage of flow
4. Dense brush, high stage
NATURAL STREAMS
Minor Streams (top width at flood stage < 100 ft)
a. Steams on Plain
1. Clean, straight, full stage, no rifts or deep pools
2. Same as above, but more stones and weeds
3. Clean, winding, some pools and shoals
4. Same as above, but some weeds and some stones
5. Same as above, lower stages, more ineffective
slopes and sections
6. Same as 4, but more stones
7. Sluggish reaches, weedy, deep pools
8. Very weedy reaches, deep pools, or floodways
with heavy stand of timber and underbrush
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Minimum
Normal
Maximum
0.016
0.018
0.022
0.022
0.018
0.022
0.025
0.027
0.020
0.025
0.030
0.033
0.023
0.025
0.030
0.025
0.025
0.030
0.025
0.030
0.035
0.030
0.035
0.040
0.030
0.033
0.040
0.035
0.045
0.050
0.025
0.035
0.028
0.050
0.033
0.060
0.025
0.035
0.035
0.040
0.040
0.050
0.050
0.040
0.045
0.080
0.080
0.050
0.070
0.100
0.120
0.080
0.110
0.140
0.025
0.030
0.033
0.035
0.030
0.035
0.040
0.045
0.033
0.040
0.045
0.050
0.040
0.048
0.055
0.045
0.050
0.050
0.070
0.060
0.080
0.075
0.100
0.150
Type of Channel and Description
Minimum
Normal
Maximum
0.030
0.040
0.040
0.050
0.050
0.070
0.025
0.030
0.030
0.035
0.035
0.050
0.020
0.025
0.030
0.030
0.035
0.040
0.040
0.045
0.050
0.035
0.035
0.040
0.045
0.070
0.050
0.050
0.060
0.070
0.100
0.070
0.060
0.080
0.110
0.160
0.110
0.030
0.050
0.150
0.040
0.060
0.200
0.050
0.080
0.080
0.100
0.120
0.100
0.120
0.160
0.025
0.035
-
0.060
0.100
b. Mountain streams, no vegetation in channel, banks
usually steep, trees and brush along banks submerged
at high stages
1. Bottom: gravels, cobbles, few boulders
2. Bottom: cobbles with large boulders
FLOODPLAINS
a. Pasture, no brush
1. Short grass
2. High grass
b. Cultivated area
1. No crop
2. Mature row crops
3. Mature field crops
c. Brush
1. Scattered brush, heavy weeds
2. Light brush and trees, in winter
3. Light brush and trees, in summer
4. Medium to dense brush, in winter
5. Medium to dense brush, in summer
d. Trees
1. Dense willows, summer, straight
2. Cleared land, tree stumps, no sprouts
3. Same as above, but with heavy growth of spouts
4. Heavy stand of timber, a few down trees, little
undergrowth, flood stage below branches
5. Same as above, but with flood stage reaching
branches
Stormwater Drainage System Design
Table 5.4-5 Uniform Flow Values of Roughness Coefficient n
Major streams (top width at flood stage > 100 ft). The n value is less than that for minor streams of similar description,
because banks offers less effective resistance.
a. Regular section with no boulders or brush
b. Irregular and rough section
Source: HEC-15, 2005
VOL 2
447
(Step 4) Extend the line from Step 3 to the velocity scale to obtain the
Table 5.4-6 Classification of Vegetal Covers as to Degrees of Retardance
Cover
Condition
A
Weeping Lovegrass
Yellow Bluestem Ischaemum
Kudzu
Bermuda Grass
Native grass mixture:
little bluestem, bluestem, blue
gamma, other short and long
stem midwest lovegrass
Weeping lovegrass
Laspedeza sericea
Alfalfa
Weeping lovegrass
Kudzu
Blue gamma
Excellent stand, tall (average 30”)
Excellent stand, tall (average 36”)
Very dense growth, uncut
Good stand, tall (average 12”)
Crabgrass
Bermuda Grass
Common lespedeza
Grass-legume mixture:
summer (orchard grass redtop,
Italian ryegrass, and common
lespedeza
Centipede Grass
Fair stand, uncut (10 - 48”)
Good stand, mowed (average 6”)
Good stand, uncut (average 11”)
Bermuda grass
Common lespedeza
Buffalo grass
Grass-legume mixture:
fall, spring (orchard grass, redtop,
Italian ryegrass, and common
lespedeza
Lespedeza serices
Good stand, cut to 2.5”
Excellent stand, uncut (average 4.5”)
Good stand, uncut (3 - 6”)
Bermuda grass
Bermuda grass
Good stand, cut to 1.5”
Burned stubble
B
C
D
E
Good stand, unmowed
Good stand, tall (average 24”)
Good stand, not woody, tall (average 19”)
Good stand, uncut (average 11”)
Good stand, unmowed (average 13”)
Dense growth, uncut
Good stand, uncut (average 13”)
velocity in ft/s.
Trapezoidal Solution Nomograph
The trapezoidal channel nomograph solution to Manning’s Equation in Figure
5.4-3 can be used to find the depth of flow if the design discharge is known or
the design discharge if the depth of flow is known.
Determine input data, including slope in ft/ft, Manning’s n value, bottom width
in ft, and side slope in ft/ft.
1.
Good stand, uncut (6 - 8”)
Very dense cover (average 6”)
Given Q, find d.
a. Given the design discharge, find the product of Q times n, connect a line
from the slope scale to the Qn scale, and find the point of intersection on
the turning line.
Stormwater Drainage System Design
Retardance
b. Connect a line from the turning point from Step 2a to the b scale and find
the intersection with the z = 0 scale.
c. Project horizontally from the point located in Step 2b to the appropriate z
value and find the value of d/b.
Good stand, uncut (4 - 5”)
d. Multiply the value of d/b obtained in Step 2c by the bottom width b to
find the depth of uniform flow, d.
After cutting to 2” (very good before cutting)
Note: Covers classified have been tested in experimental channels. Covers were green and generally uniform.
Source: HEC-15, 2005
General Solution Nomograph
The following steps are used for the general solution nomograph in Figure
5.4-2:
(Step 1) Determine open channel data, including slope in ft/ft, hydraulic radius in ft, and Manning’s n value.
(Step 2) Connect a line between the Manning’s n scale and slope
scale and note the point of intersection on the turning line.
(Step 3) Connect a line from the hydraulic radius to the point of
2.
Given d, find Q
a. Given the depth of flow, find the ratio d divided by b and project a
horizontal line from the d/b ratio at the appropriate side slope, z, to the z
= 0 scale.
b. Connect a line from the point located in Step 3a to the b scale and find
the intersection with the turning line.
c. Connect a line from the point located in Step 3b to the slope scale and
find the intersection with the Qn scale.
d. Divide the value of Qn obtained in Step 3c by the n value to find the
design discharge, Q.
intersection obtained in Step 2.
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Stormwater Drainage System Design
Figure 5.4-2 Nomograph for the Solution of Manning’s Equation
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Figure 5.4-3 Solution of Manning’s Equation for Trapezoidal Channels
VOL 2
449
Where:
A trial and error procedure for solving Manning’s Equation is used to compute
K T = trapezoidal open channel conveyance factor
the normal depth of flow in a uniform channel when the channel shape, slope,
Q = discharge rate for design conditions (cfs)
roughness, and design discharge are known. For purposes of the trial and
n = Manning’s roughness coefficient
error process, Manning’s Equation can be arranged as:
b = bottom width (ft)
S = slope of the energy grade line (ft/ft)
AR2/3 = (Qn)/(1.49 S1/2)
(5.4.4)
(Step 3) Enter the x-axis of Figure 5.4-4 with the value of KT calcu-
Where:
lated in Step 2 and draw a line vertically to the curve corre-
A = cross-sectional area (ft)
sponding to the appropriate z value from Step 1.
R = hydraulic radius (ft)
Q = discharge rate for design conditions (cfs)
(Step 4) From the point of intersection obtained in Step 3, draw a
n = Manning’s roughness coefficient
horizontal line to the y-axis and read the value of the normal
S = slope of the energy grade line (ft/ft)
depth of flow over the bottom width, d/b.
To determine the normal depth of flow in a channel by the trial and error
process, trial values of depth are used to determine A, P, and R for the given
channel cross section. Trial values of AR
2/3
Stormwater Drainage System Design
5.4.5.5 TRIAL AND ERROR SOLUTIONS
(Step 5) Multiply the d/b value from Step 4 by b to obtain the normal
depth of flow.
are computed until the equality of
equation 5.4.4 is satisfied such that the design flow is conveyed for the slope
Note: If bends are considered, refer to equation 5.4.11
and selected channel cross section.
Graphical procedures for simplifying trial and error solutions are presented in
Figure 5.4-4 for trapezoidal channels.
(Step 1) Determine input data, including design discharge, Q, Manning’s n value, channel bottom width, b, channel slope, S,
and channel side slope, z.
(Step 2) Calculate the trapezoidal conveyance factor using the equation:
K T = (Qn)/(b8/3S1/2)
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(5.4.5)
VOL 2
450
Stormwater Drainage System Design
Figure 5.4-4 Trapezoidal Channel Capacity Chart
(Source: Nashville Storm Water Management Manual, 1988)
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VOL 2
451
5.4-5) can be used to simplify trial and error criti-
Where:
cal depth calculations. The following equation is
Fr = Froude number (dimensionless)
5.4.6.1 BACKGROUND
used to determine critical depth with the critical
v = velocity of flow (ft/s)
In the design of open channels, it is important to cal-
flow section factor, Z:
g = acceleration of gravity (32.2 ft/sec2)
culate the critical depth in order to determine if the
A = cross-sectional area of flow (ft2)
Z = Q/(g )
0.5
flow in the channel will be subcritical or supercritical.
If the flow is subcritical it is relatively easy to handle
(5.4.7)
T = top width of flow (ft)
Where:
the flow through channel transitions because the
Z = critical flow section factor
If Fr is greater than 1.0, flow is supercritical; if it is
flows are tranquil and wave action is minimal. In sub-
Q = discharge rate for design conditions (cfs)
under 1.0, flow is subcritical. Fr is 1.0 for critical
critical flow, the depth at any point is influenced by a
g = acceleration due to gravity (32.2 ft/sec2)
flow conditions.
downstream control, which may be either the critical
depth or the water surface elevation in a pond or
The following guidelines are given for evaluating
larger downstream channel. In supercritical flow, the
critical flow conditions of open channel flow:
depth of flow at any point is influenced by a control
1.
upstream, usually critical depth. In addition, the flows
have relatively shallow depths and high velocities.
Critical depth depends only on the discharge rate
and channel geometry. The general equation for
determining critical depth is expressed as:
Q2/g = A3/T
(5.4.6)
Where:
Q = discharge rate for design conditions (cfs)
g = acceleration due to gravity (32.2 ft/sec2)
A = cross-sectional area (ft2)
T = top width of water surface (ft)
Note: A trial and error procedure is needed to
solve equation 5.4-6.
2.
3.
4.
A normal depth of uniform flow within about
10% of critical depth is unstable and should
be avoided in design, if possible.
If the velocity head is less than one-half the
mean depth of flow, the flow is subcritical.
If the velocity head is equal to one-half the
mean depth of flow, the flow is critical.
If the velocity head is greater than one-half the
mean depth of flow, the flow is supercritical.
Semi-empirical equations (as presented in Table
5.4-7) or section factors (as presented in Figure
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Channel Type1
Semi-Empirical Equations2
for Estimating Critical Depth
Range of
Applicability
1. Rectangular3
dC = [Q2/(gb2)]1/3
N/A
2. Trapezoidal3
0.1 < 0.5522 Q/
dC = 0.81[Q2/gz0.75b1.25)]0.27 - b/30z 2.5
b < 0.4
For 0.5522 Q/b2.5
< 0.1, use rectangular channel
3. Triangular3
dC = [(2Q2)/(gz2)]1/5
N/A
4. Circular
dC = 0.325(Q/D)2/3 + 0.083D
0.3 < dC/D < 0.9
3
(A3/T) = (Q2/g)
5. General3
Where:
dC = critical depth (ft)
Note: The head is the height of water above any
point, plane or datum of reference. The velocity
head in flowing water is calculated as the velocity squared divided by 2 times the gravitational
constant (V2/2g).
N/A
Q = design discharge (cfs)
G = acceleration due to gravity (32.2 ft/s2)
b = bottom width of channel (ft)
z = side slopes of a channel (horizontal to vertical)
D = diameter of a circular conduit (ft)
A = cross-sectional area of flow (ft2)
The Froude number, Fr, calculated by the following equation, is useful for evaluating the type of
5.4.6.2 SEMI-EMPIRICAL EQUATIONS
Table 5.4-7 Critical Depth Equations for Uniform Flow
in Selected Channel Cross Sections
Stormwater Drainage System Design
5.4.6 Critical Flow Calculations
flow conditions in an open channel:
Fr = v/(gA/T)0.5
(5.4.8)
T = top width of water surface (ft)
See Figure 5.4-5 for channel sketches
1
Assumes uniform flow with the kinetic energy coefficient equal to 1.0
Reference: French (1985)
2
3
Reference: USDOT, FHWA, HDS-4 (1965)
Reference: Brater and King (1976)
4
5
VOL 2
452
Stormwater Drainage System Design
Figure 5.4-5 Open Channel Geometric Relationships for Various Cross Sections
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VOL 2
453
Where:
R = hydraulic radius of flow (ft)
5.4.7.1 INTRODUCTION
vR = value obtained from Figure 5.4-1 in Step 3
A two-part procedure is recommended for final design of temporary and
vm = maximum velocity from Step 2 (ft/s)
vegetative channel linings. Part 1, the design stability component, involves
determining channel dimensions for low vegetative retardance conditions, using Class D as defined in Table 5.4-6. Part 2, the design capacity component,
(Step 5) Use the following form of Manning’s Equation to calculate
the value of vR:
involves determining the depth increase necessary to maintain capacity for
higher vegetative retardance conditions, using Class C as defined in Table 5.4-
vR = (1.49 R5/3 S1/2)/n
(5.4.10)
6. If temporary lining is to be used during construction, vegetative retardance
Class E should be used for the design stability calculations.
Where:
vR = calculated value of vR product
If the channel slope exceeds 10%, or a combination of channel linings will be
R = hydraulic radius value from Step 4 (ft)
used, additional procedures can be found in HEC-15 (USDOT, FHWA, 2005)
S = channel bottom slope (ft/ft)
and HEC-14 (USDOT, FHWA, 2012).
n = Manning’s n value assumed in Step 3
Stormwater Drainage System Design
5.4.7 Vegetative Design
(Step 6) Compare the vR product value obtained in Step 5 to the
5.4.7.2 DESIGN STABILITY
The following are the steps for design stability calculations:
(Step 1) Determine appropriate design variables, including discharge, Q,
bottom slope, S, cross section parameters, and vegetation type.
(Step 2) Use Table 5.4-3 to assign a maximum velocity, vm based on
vegetation type and slope range.
(Step 3) Assume a value of n and determine the corresponding value
of vR from the n versus vR curves in Figure 5.4-1. Use retardance Class D for permanent vege-tation and E for temporary construction.
value obtained from Figure 5.4-1 for the assumed n value in
Step 3. If the values are not rea-sonably close, return to Step
3 and repeat the calculations using a new assumed n value.
(Step 7) For trapezoidal channels, find the flow depth using Figures
5.4-3 or 5.4-4, as described in Section 5.4.4. The depth of
flow for other channel shapes can be evaluated using the
trial and error procedure described in Section 5.4.5.
(Step 8) If bends are considered, calculate the length of downstream
protection, Lp, for the bend, using Figure 5.4-6. Provide additional protection, such as gravel or riprap in the bend and
extending downstream for length, Lp.
(Step 4) Calculate the hydraulic radius using the equation:
R = (vR)/vm
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(5.4.9)
VOL 2
454
Where:
The following are the steps for design capacity calculations:
∆d = superelevation of the water surface profile due
(Step 1) Assume a depth of flow greater than the value from Step 7
to the bend (ft)
v = average velocity from Step 6 (ft/s)
above and compute the waterway area and hydraulic radius
T = top width of flow (ft)
(see Figure 5.4-5 for equations).
g = acceleration of gravity (32.2 ft/sec2)
Rc = mean radius of the bend (ft)
(Step 2) Divide the design flow rate, obtained using appropriate procedures from Chapter 2, by the waterway area from Step 1 to
Note: Add freeboard consistent with the calculated ∆d.
find the velocity.
(Step 3) Multiply the velocity from Step 2 by the hydraulic radius from
Step 1 to find the value of vR.
Stormwater Drainage System Design
5.4.7.3 DESIGN CAPACITY
(Step 4) Use Figure 5.4-1 to find a Manning’s n value for retardance
Class C based on the vR value from Step 3.
(Step 5) Use Manning’s Equation (equation 5.4.1) or Figure 5.4-2 to
find the velocity using the hydraulic radius from Step 1, Manning’s n value from Step 4, and appropriate bottom slope.
(Step 6) Compare the velocity values from Steps 2 and 5. If the values are not reasonably close, return to Step 1 and repeat the
calculations.
(Step 7) Add an appropriate freeboard to the final depth from Step 6.
Generally, 20% is adequate.
(Step 8) If bends are considered, calculate superelevation of the water surface profile at the bend using the equation:
∆d = (v2T)/(gRc)
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(5.4.11)
Figure 5.4-6 Protection Length, Lp, Downstream of Channel Bend
VOL 2
455
Where:
n = Manning’s roughness coefficient for
5.4.8.1 ASSUMPTIONS
stone riprap
The following procedure is based on results and
d50 = diameter of stone for which 50%,
analysis of laboratory and field data (Maynord,
by weight, of the gradation is finer (ft)
1987; Reese, 1984; Reese, 1988). This procedure
applies to riprap place-ment in both natural and
prismatic channels and has the following assumptions and limitations:
• Minimum riprap thickness equal to d100
• The value of d85/d15 less than 4.6
• Froude number less than 1.2
• Side slopes up to 2:1
• A safety factor of 1.2
• Maximum velocity less than 18 feet per second
(Step 2) If rock is to be placed at the outside of a
bend, multiply the velocity determined
in Step 1 by the bend correction
coefficient, Cb, given in Figure 5.4-7 for
either a natural or prismatic channel.
This requires determining the channel
top width, T, just upstream from the
bend and the centerline bend radius, Rb.
(Step 3) If the specific weight of the stone varies
significantly from 165 pounds per cubic
foot, multiply the velocity from Step 1 or
2 (as appropriate) by the specific weight
correction coefficient, Cg, from Figure
5.4-8.
If significant turbulence is caused by boundary
irregularities, such as obstructions or structures,
this procedure is not applicable.
(Step 5) Determine available riprap gradations.
A well graded riprap is preferable
to uniform size or gap graded. The
diameter of the largest stone, d100,
should not be more than 1.5 times the
d50 size. Blanket thickness should be
greater than or equal to d100 except as
noted below. Sufficient fines (below d15)
should be available to fill the voids in the
larger rock sizes. The stone weight for
a selected stone size can be calculated
from the equation:
W = 0.5236 SWs d3
(5.4.14)
Stormwater Drainage System Design
5.4.8 Riprap Design
Where:
W = stone weight (lbs)
d = selected stone diameter (ft)
SWs = specific weight of stone (lbs/ft3)
Filter fabric or a filter stone layer should be used
to prevent turbulence or groundwater seepage
(Step 4) Determine the required minimum d30
value from Figure 5.4-9, or from the
equation:
5.4.8.2 PROCEDURE
or to serve as a foundation for unconsolidated
material. Layer thickness should be increased by
d30/D = 0.193 Fr
2.5
Following are the steps in the procedure for riprap
from removing bank material through the stone
(5.4.13)
50% for underwater placement.
design:
Where:
(Step 1) Determine the average velocity in the
main channel for the design condition.
Manning’s n values for riprap can be
calculated from the equation:
n = 0.0395 (d50)1/6
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(5.4.12)
d30 = diameter of stone for which 30%,
by weight, of the gradation is finer (ft)
D = depth of flow above stone (ft)
Fr = Froude number (see equation 5.4.8),
dimensionless
v = mean velocity above the stone (ft/s)
VOL 2
456
(Step 7) Perform preliminary design, ensuring
that adequate transition is provided
to natural materials both up and
downstream to avoid flanking and that
toe protection is provided to avoid riprap
undermining.
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Figure 5.4-7 Riprap Lining Bend Correction Coefficient
Stormwater Drainage System Design
(Step 6) If d85/d15 is between 2.0 and 2.3 and a
smaller d30 size is desired, a thickness
greater than d100 can be used to offset
the smaller d30 size. Figure 5.4-10
can be used to make an approximate
adjustment using the ratio of d30 sizes.
Enter the y-axis with the ratio of the
desired d30 size to the standard d30 size
and find the thickness ratio increase
on the x-axis. Other minor gradation
deficiencies may be compensated for by
increasing the stone blanket thickness.
VOL 2
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Stormwater Drainage System Design
Figure 5.4-8 Riprap Lining Specific Weight Correction Coefficient
(Source: Nashville Storm Water Management Manual, 1988)
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Figure 5.4-9 Riprap Lining d30 Stone Size –
Function of Mean Velocity and Depth
VOL 2
458
3.
Use equation 5.4.9 to calculate the value of
R: R = 5.4/4 = 1.35 ft
--
z=3
4.
Use equation 5.4.10 to calculate the value
of vR:
--
d = 1 ft
--
v = 3.9 ft/s (equation 5.4.1)
vR = [1.49 (1.35)5/3 (0.015)1/2]/0.035 = 8.60
--
Fr = 0.76 (equation 5.4.8)
--
Flow is subcritical
Select:
-- b = 10 ft, such that R is approximately 0.80 ft
Example 1 -- Direct Solution of Manning’s Equation
Use Manning’s Equation to find the velocity, v, for
an open channel with a hydraulic radius value of
0.6 ft, an n value of 0.020, and slope of 0.003 ft/ft.
Solve using Figure 5.4-2:
1.
2.
5.
Connect a line between the slope scale at
0.003 and the roughness scale at 0.020 and
note the intersection point on the turning
line.
Connect a line between that intersection
point and the hydraulic radius scale at 0.6
ft and read the velocity of 2.9 ft/s from the
velocity scale.
Since the vR value calculated in Step 4 is
higher than the value obtained from Step 2,
a higher n value is required and calculations
are repeated. The results from each trial of
Design capacity calculations for this channel are
presented in Example 3 below.
calculations are presented below:
Example 3 -- Grassed Channel Design Capacity
Assumed
vR
R
vR
n Value
(Fig. 5.4-1)
(eq. 5.4.9)
(eq. 5.4.10)
0.035
5.40
1.35
8.60
0.038
3.8
0.95
4.41
0.039
3.4
0.85
3.57
0.040
3.2
0.80
3.15
Example 2 -- Grassed Channel Design Stability
Use a 10-ft bottom width and 3:1 side-slopes for
the trapezoidal channel sized in Example 2 and
find the depth of flow for retardance Class C.
1.
at a bottom slope of 0.015 ft/ft. Find the channel
Select n = 0.040 for stability criteria.
dimensions required for design stability criteria
1.
2.
From Table 5.4-3, the maximum velocity, vm,
for a grass mixture with a bottom slope less
than 5% is 4 ft/s.
Assume an n value of 0.035 and find the
value of vR from Figure 5.4-1, vR = 5.4
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Assume a depth of 1.0 ft and calculate the
following (see Figure 5.4-5):
-- A = (b + zd) d = [10 + (3) (1) ] (1) = 13.0 sq ft
-- R = {[b + zd] d}/{b + [2d(1 + z2)0.5]} =
{[10+(3)(1)](1)}/{10+[(2)(1)(1+32)0.5]}
A trapezoidal channel is required to carry 50 cfs
(retardance Class D) for a grass mixture.
Stormwater Drainage System Design
5.4.9 Uniform Flow - Example Problems
-- R = 0.796 ft
6.
Use Figure 5.4-3 to select channel
dimensions for a trapezoidal shape with 3:1
side slopes.
-- Qn = (50) (0.040) = 2.0
2.
Find the velocity: v = Q/A = 50/13.0 = 3.85 ft/s
3.
Find the value of vR: vR = (3.85) (0.796) =
3.06
4.
Using the vR product from Step 3, find
Manning’s n from Figure 5.4-1 for retardance
Class C (n = 0.047)
S = 0.015
-- For b = 10 ft, d = (10) (0.098) = 0.98 ft
b = 8 ft, d = (8) (0.14) = 1.12 ft
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459
Select a depth of 1.1 with an n value of 0.048
for design capacity requirements. Add at
least 0.2 ft for freeboard to give a design
depth of 1.3 ft. Design data for the trapezoidal channel are summarized as follows:
-- Vegetation lining = grass mixture, vm = 4
ft/s
-- Q = 50 cfs
-- b = 10 ft, d = 1.3 ft, z = 3, S = 0.015
-- Top width = (10) + (2) (3) (1.3) = 17.8 ft
Stormwater Drainage System Design
7.
-- n (stability) = 0.040, d = 1.0 ft, v = 3.9 ft/s,
Froude number = 0.76 (equation 5.4.8)
-- n (capacity) = 0.048, d = 1.1 ft, v = 3.45
ft/s, Froude number = 0.64 (equation
5.4.8)
Figure 5.4-10 Riprap Lining Thickness Adjustment for d85/d15 = 1.0 to 2.3
(Source: Maynord, 1987)
5.
Use Figure 5.4-2 or equation 5.4.1 to find
the velocity for S = 0.015, R = 0.796, and n =
0.047: v = 3.34 ft/s
6.
Since 3.34 ft/s is less than 3.85 ft/s, a
higher depth is required and calculations
are repeated. Results from each trial of
calculations are presented below:
Example 4 -- Riprap Design
A natural channel has an average bankfull channel velocity of 8 ft per second with a top width of
20 ft and a bend radius of 50 ft. The depth over
Assumed
Area (ft)
R (ft)
Depth (ft)
Velocity Q/A
vR
(ft/sec)
Manning’s n
Velocity
(Fig. 5.4-3)
(Eq. 5.4.11)
1.0
13.00
0.796
3.85
3.06
0.047
3.34
1.05
13.81
0.830
3.62
3.00
0.047
3.39
1.1
14.63
0.863
3.42
2.95
0.048
3.45
1.2
16.32
0.928
3.06
2.84
0.049
3.54
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the toe of the outer bank is 5 ft. Available stone
weight is 170 lbs/ft3. Stone placement is on a side
slope of 2:1 (horizontal:vertical). Determine riprap
size at the outside of the bend.
VOL 2
460
Use 8 ft/s as the design velocity, because the
reach is short and the bend is not protected.
5.4.10 Gradually Varied Flow
apart for ditches or streams and 500 feet apart for
The most common occurrence of gradually
floodplains, unless the channel is very regular.
varied flow in storm drainage is the backwater
2.
Determine the bend correction coefficient
for the ratio of Rb/T = 50/20 = 2.5. From
Figure 5.4-7, Cb = 1.55. The adjusted
effective velocity is (8) (1.55) = 12.4 ft/s.
created by culverts, storm sewer inlets, or channel
constrictions. For these conditions, the flow
depth will be greater than normal depth in the
channel and the water surface profile should be
5.4.11 Rectangular, Triangular, and
Trapezoidal Open Channel Design
Figures
computed using backwater techniques.
3.
4.
5.
6.
Determine the correction coefficient for the
specific weight of 170 lbs from Figure 5.4-8
as 0.98. The adjusted effective velocity is
(12.4) (0.98) = 12.15 ft/s.
Determine minimum d30 from Figure 5.4-9 or
equation 5.4.13 as about 10 inches.
Use a gradation with a minimum d30 size of
10 inches.
(Optional) Another gradation is available
with a d30 of 8 inches. The ratio of desired
to standard stone size is 8/10 or 0.8. From
Figure 5.4-10, this gradation would be
acceptable if the blanket thickness was
increased from the original d100 (diameter of
the largest stone) thickness by 35% (a ratio of
1.35 on the horizontal axis).
5.4.11.1 INTRODUCTION
Many computer programs are available for com-
The Federal Highway Administration has prepared
putation of backwater curves. The most general
numerous design figures to aid in the design of
and widely used programs are, HEC-RAS, devel-
open channels. Copies of these figures, a brief
oped by the U.S. Army Corps of Engineers and
description of their use, and several example de-
Bridge Waterways Analysis Model (WSPRO) de-
sign problems are presented. For design condi-
veloped for the Federal Highway Administration.
tions not covered by the figures, a trial and error
These programs can be used to compute water
solution of Manning’s Equation must be used.
surface profiles for both natural and artificial
channels.
For prismatic channels, the backwater calculation can
be computed manually using the direct step method.
For an irregular nonuniform channel, the standard
step method is recommended, although it is a more
tedious and iterative process. The use of HEC-RAS is
recommended for standard step calculations.
Cross sections for water surface profile calculations should be normal to the direction of
7.
Perform preliminary design. Make sure that
the stone is carried up and downstream far
enough to ensure stability of the channel
and that the toe will not be undermined. The
downstream length of protection for channel
bends can be determined using Figure 5.4-6.
flood flow. The number of sections required
will depend on the irregularity of the stream and
flood plain. In general, a cross section should
be obtained at each location where there are
significant changes in stream width, shape, or
vegetal patterns. Sections should usually be no
more than 4 to 5 channel widths apart or 100 feet
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Stormwater Drainage System Design
1.
5.4.11.2 DESCRIPTION OF FIGURES
Figures given in Appendix C are for the direct
solution of the Manning’s Equation for various
sized open channels with rectangular, triangular, and trapezoidal cross sections. Each figure
(except for the triangular cross section) is prepared for a channel of given bottom width and a
particular value of Manning’s n.
The figures for rectangular and trapezoidal cross
section channels (Appendix C) are used the same
way. The abscissa scale of discharge in cubic feet
per second (cfs), and the ordinate scale is velocity in
feet per second (ft/s). Both scales are logarithmic.
Superimposed on the logarithmic grid are steeply
inclined lines representing depth (ft), and slightly
inclined lines representing channel slope (ft/ft).
VOL 2
461
The procedure is reversed to determine the
flow conditions. Auxiliary abscissa and ordinate
discharge at a given depth of flow. Critical depth,
scales are provided for use with other values of
slope, and velocity for a given discharge can be
n and are explained in the example problems. In
read on the appropriate scale at the intersection
the figures, interpolations may be made not only
of the critical curve and a vertical line through the
on the ordinate and abscissa scales but between
discharge.
to intersect the slope line S = .0006, and
from the depth lines read dn = 3.7 ft.
3.
Move horizontally from the same intersection
and read the normal velocity, V = 3.2 ft/s, on
the ordinate scale.
4.
The intersection lies below the critical curve,
and the flow is therefore in the subcritical
range.
the inclined lines representing depth and slope.
Auxiliary scales, labeled Qn (abscissa) and Vn
The chart for a triangular cross section (Appen-
(ordinate), are provided so the figures can be
dix C) is in nomograph form. It may be used for
used for values of n other than those for which
street sections with a vertical (or nearly vertical)
the charts were basically prepared. To use these
curb face. The nomograph also may be used
scales, multiply the discharge by the value of n
for shallow V shaped sections by following the
and use the Qn and Vn scales instead of the Q
Given:
instructions on the chart.
and V scales, except for computation of critical
A trapezoidal channel with 2:1 side slopes and a 4
depth or critical velocity. To obtain normal veloc-
ft bottom width, with n = 0.030, 0.2% slope (S =
ity V from a value on the Vn scale, divide the value
0.002), discharging 50 cfs.
5.4.11.3 INSTRUCTIONS FOR RECTANGULAR
AND TRAPEZOIDAL FIGURES
Figures in Appendix C provide a solution of the
by n. The following examples will illustrate these
points.
vided the flow is not affected by backwater and
Example Design Problem 1
the channel has a length sufficient to establish
Given:
uniform flow.
A rectangular concrete channel 5 ft wide with n =
0.015, .06 percent slope (S = .0006), discharging
For a given slope and channel cross section, when
Find:
Depth, velocity, type flow.
Manning equation for flow in open channels of
uniform slope, cross section, and roughness, pro-
Example Design Problem 2
Procedure:
1.
Select the trapezoidal figure for b = 4 ft (see
Figure 5.4-12).
2.
From 50 cfs on the Q scale, move vertically
to intersect the slope line S = 0.002 and from
the depth lines read dn = 2.2 ft.
3.
Move horizontally from the same intersection
and read the normal velocity, V = 2.75 ft/s,
on the ordinate scale. The intersection
lies below the critical curve,\; the flow is
therefore subcritical.
60 cfs.
n is 0.015 for rectangular channels or 0.03 for
trapezoidal channels, the depth and velocity of
Find:
uniform flow may be read directly from the figure
Depth, velocity, and type of flow
for that size channel. The initial step is to locate
the intersection of a vertical line through the discharge (abscissa) and the appropriate slope line.
At this intersection, the depth of flow is read from
Procedure:
1.
From Appendix C select the rectangular
figure for a 5-ft width (Figure 5.4-11).
2.
From 60 cfs on the Q scale, move vertically
the depth lines, and the mean velocity is read on
the ordinate scale.
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Stormwater Drainage System Design
A heavy dashed line on each figure shows critical
VOL 2
462
Given:
A rectangular cement rubble masonry channel 5 ft wide, with n = 0.025, 0.5%
slope (S = 0.005), discharging 80 cfs.
Find:
Depth velocity and type of flow
Procedure:
1.
Select the rectangular figure for a 5 ft width (Figure 5.4-13).
2.
Multiply Q by n to obtain Qn: 80 x 0.025 = 2.0.
3.
From 2.0 on the Qn scale, move vertically to intersect the slope line, S =
0.005, and at the intersection read dn = 3.1 ft.
4.
Move horizontally from the intersection and read Vn = .13, then Vn/n =
0.13/0.025 = 5.2 ft/s.
5.
Critical depth and critical velocity are independent of the value of n so
their values can be read at the intersection of the critical curve with a
vertical line through the discharge. For 80 cfs, on Figure 5.4-13, dc = 2.0
ft and Vc = 7.9 ft/s. The normal velocity, 5.2 ft/s (from step 4), is less than
the critical velocity, and the flow is therefore subcritical. It will also be
noted that the normal depth, 3.0 ft, is greater than the critical depth, 2.0
ft, which also indicates subcritical flow.
6.
To determine the critical slope for Q = 80 cfs and n = 0.025, start at the
intersection of the critical curve and a vertical line through the discharge,
Q = 80 cfs, finding dc (2.0 ft) at this point. Follow along this dc line to
its intersection with a vertical line through Qn = 2.0 (step 2), at this
intersection read the slope value Sc = 0.015.
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Stormwater Drainage System Design
Example Design Problem 3
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463
Stormwater Drainage System Design
Figure 5.4-11 Example Nomograph #1
(Source: FHWA)
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464
Stormwater Drainage System Design
Figure 5.4-12 Example Nomograph #2
(Source: FHWA)
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465
Stormwater Drainage System Design
Figure 5.4-13 Example Nomograph #3
(Source: FHWA)
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VOL 2
466
general design criteria, instructions on how to use
Superimposed on the logarithmic grid are lines for
The Manning equation can be used to determine
the figures, and several example design problems.
velocity in feet per second and lines for depth in
the capacity of a grass lined channel, but the val-
For design conditions not covered by the figures,
feet. A dashed line shows the position of critical
ue of n varies with the type of grass, development
a trial and error solution of the Manning equation
flow.
of the grass cover, depth, and velocity of flow.
must be used.
The variable value of n complicates the solution
of flow must be estimated and the Manning equa-
5.4.11.5 DESCRIPTION OF FIGURES
5.4.11.6 INSTRUCTIONS FOR GRASSED CHANNEL FIGURES
tion solved using the n value that corresponds
The figures in Appendix C are designed for use in
The grassed channel figures provide a solution of
to the estimated depth and velocity. The trial
the direct solution of the Manning equation for
the Manning equation for flow in open grassed
solution provides better estimates of the depth
various channel sections lined with grass. The
channels of uniform slope and cross section. The
and velocity for a new value of n and the equation
figures are similar in appearance and use to those
flow should not be affected by backwater and the
is again solved. The procedure is repeated until a
for trapezoidal cross sections described earlier.
channel should have length sufficient to establish
depth is found that carries the design discharge.
However, their construction is much more diffi-
uniform flow. The figures are sufficiently accurate
cult because the roughness coefficient (n) chang-
for design of drainage channels of fairly uniform
To prevent excessive erosion, the velocity of flow
es as higher velocities and/or greater depths
cross section and slope, but are not appropriate
in a grass lined channel must be kept below some
change the condition of the grass. The effect of
for irregular natural channels.
maximum value (referred to as permissible veloci-
velocity and depth of flow on n is evaluated by the
ty). The permissible velocity in a grass lined chan-
product of velocity and hydraulic radius V times R.
The design of grassed channels requires two
nel depends upon the type of grass, condition of
The variation of Manning’s n with the retardance
operations:
the grass cover, texture of the soil comprising the
(Table 5.4-6) and the product V times R is shown
1.
channel bed, channel slope, and to some extent
in Figure 5.4-1. As indicated in Table 5.4-6,
the size and shape of the drainage channel. To
retardance varies with the height of the grass and
Selecting a section that has the capacity to
carry the design discharge on the available
slope
guard against overtopping, the channel capacity
the condition of the stand. Both of these factors
2.
should be computed for taller grass than is ex-
depend upon the type of grass, planting condi-
pected to be maintained, while the velocity used
tions, and maintenance practices. Table 5.4-6 is
Checking the velocity in the channel to
ensure that the grass lining will not be
eroded.
to check the adequacy of the protection should
used to determine retardance classification.
of the Manning equation. The depth and velocity
Because the retardance of the channel is largely
be computed assuming a lower grass height than
The grassed channel figures each have two
beyond the control of the designer, it is good
graphs, the upper graph for retardance Class D
practice to compute the channel capacity using
To aid in the design of grassed channels, the Fed-
and the lower graph for retardance Class C. The
retardance Class C and the velocity using retar-
eral Highway Administration has prepared numer-
figures are plotted with discharge in cubic feet per
dance Class D. The calculated velocity should
ous design figures. Copies of these figures are
second on the abscissa and slope in feet per foot
then be checked against the permissible velocities
in Appendix C. Following is a brief description of
on the ordinate. Both scales are logarithmic.
listed in Tables 5.4-2 and 5.4-3.
will likely be maintained.
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Stormwater Drainage System Design
5.4.11.4 GRASSED CHANNEL FIGURES
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467
Example Design Problem 1
Example Design Problem 2
Given:
Given:
A trapezoidal channel in easily eroded soil, lined
The channel and discharge of Example 1.
steps:
(Step 1) Select the channel cross section to be
used and find the appropriate figure.
with a grass mixture with 4:1 side slopes, and a
4 ft bottom width on slope of 0.02 ft per foot
Find:
(Step 2) Enter the lower graph (for retardance
Class C) on the figure with the design
discharge value on the abscissa and
move vertically to the value of the
slope on the ordinate scale. As this
intersection, read the normal velocity
and normal depth and note the position
of the critical curve. If the intersection
point is below the critical curve, the flow
is subcritical; if it is above, the flow is
supercritical.
(S=.02), discharging 20 cfs.
The maximum grade on which the 20 cfs could
(Step 3) To check the velocity developed against
the permissible velocities (Tables 5.4-2
and 5.4-3), enter the upper graph on
the same figure and repeat Step 2. Then
compare the computed velocity with the
velocity permissible for the type of grass,
channel slope, and erosion resistance
of the soil. If the computed velocity is
less, the design is acceptable. If not,
a different channel section must be
2.
Enter the lower graph with Q = 20 cfs, and
move vertically to the line for S=0.02. At
this intersection read dn = 1.0 ft, and normal
velocity Vn 2.6 ft/s.
3.
The velocity for checking the adequacy of
the grass cover should be obtained from the
upper graph, for retardance Class D. Using
the same procedure as in step 2, the velocity
is found to be 3.0 ft/s. This is about threequarters of that listed as permissible, 4.0 ft/s
in Table 5.4-3.
selected and the process repeated.
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safely be carried
Find:
Depth, velocity, type of flow, and adequacy of
Procedure:
grass to prevent erosion
With an increase in slope (but still less than 5%),
the allowable velocity is estimated to be 4 ft/s
Procedure:
(see Table 5.4-3). On the upper graph of Figure
1.
5.4-15 for short grass, the intersection of the 20
From Appendix C-4, select the FHWA
channel design figure for 4:1 side slopes (see
Figure 5.4-15).
Stormwater Drainage System Design
The use of the figures is explained in the following
cfs line and the 4 ft/s line indicates a slope of 3.7%
and a depth of 0.73 ft.
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468
Stormwater Drainage System Design
Figure 5.4-14 Example Nomograph #4
(Source: FHWA)
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469
Stormwater Drainage System Design
Figure 5.4-15 Example Nomograph #5
(Source: FHWA)
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470
5.5.1.3 RECOMMENDED ENERGY DISSIPATORS
5.5.1 Overview
tors provide sufficient protection at a reasonable cost:
For many designs, the following outlet protection devices and energy dissipa• Riprap apron
5.5.1.1 INTRODUCTION
The outlets of pipes and lined channels are points of critical erosion poten-
• Riprap outlet basins
tial. Stormwater that is transported through man-made conveyance systems
• Baffled outlets
at design capacity generally reaches a velocity that exceeds the capacity of
the receiving channel or area to resist erosion. To prevent scour at stormwater outlets, protect the outlet structure and minimize the potential for
downstream erosion, a flow transition structure is needed to absorb the initial
impact of flow and reduce the speed of the flow to a non-erosive velocity.
This section focuses on the design on these measures. The reader is referred
to the Federal Highway Administration Hydraulic Engineering Circular No. 14
entitled, Hydraulic Design of Energy Dissipators for Culverts and Channels, for
the design procedures of other energy dissipators.
Stormwater Drainage System Design
5.5 Energy Dissipation Design
Energy dissipators are engineered devices such as rip-rap aprons or concrete
baffles placed at the outlet of stormwater conveyances for the purpose of
reducing the velocity, energy and turbulence of the discharged flow.
5.5.1.2 GENERAL CRITERIA
• Erosion problems at culvert, pipe and engineered channel outlets are
common. Determination of the flow conditions, scour potential, and
channel erosion resistance shall be standard procedure for all designs.
• Energy dissipators shall be employed whenever the velocity of flows leaving
a stormwater management facility exceeds the erosion velocity of the
downstream area channel system.
• Energy dissipator designs will vary based on discharge specifics and
tailwater conditions.
• Outlet structures should provide uniform redistribution or spreading of the
flow without excessive separation and turbulence.
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Table 5.5-1 Symbols and Definitions
Symbol
Definition
Units
A
D
d50
dW
Fr
g
hS
L
La
LB
LS
PI
Q
SV
t
tC
TW
VL
VO
VO
VS
WO
WS
ye
yO
Cross-sectional area
Height of box culvert
Size of riprap
Culvert width
Froude Number
Acceleration of gravity
Depth of dissipator pool
Length
Riprap apron length
Overall length of basin
Length of dissipator pool
Plasticity index
Rate of discharge
Saturated shear strength
Time of scour
Critical tractive shear stress
Tailwater depth
Velocity L feet from brink
Normal velocity at brink
Outlet mean velocity
Volume of dissipator pool
Diameter or width of culvert
Width of dissipator pool
Hydraulic depth at brink
Normal flow depth at brink
ft2
ft
ft
ft
ft/s2
ft
ft
ft
ft
ft
cfs
lbs/in2
min.
lbs/in2
ft
ft/s
ft/s
ft/s
ft3
ft
ft
ft
ft
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471
2.
When outlet protection facilities are selected, appropriate design flow
conditions and site-specific factors affecting erosion and scour potential,
construction cost, and long-term durability should be considered.
3.
If outlet protection is not provided, energy dissipation will occur through
formation of a local scourhole. A cutoff wall will be needed at the
discharge outlet to prevent structural undermining. The wall depth
should be slightly greater than the computed scourhole depth, hs. The
scourhole should then be stabilized. If the scourhole is of such size that
it will present maintenance, safety, or aesthetic problems, other outlet
protection will be needed.
4.
Evaluate the downstream channel stability and provide appropriate
erosion protection if channel degradation is expected to occur. Figure
5.5-1 provides the riprap size recommended for use downstream of
energy dissipators.
To provide consistency within this section as well as throughout this Manual,
the symbols listed in Table 5.5-1 will be used. These symbols were selected
because of their wide use. In some cases, the same symbol is used in existing
publications for more than one definition. Where this occurs in this section,
the symbol will be defined where it occurs in the text or equations.
5.5.3 Design Guidelines
1.
If outlet protection is required, choose an appropriate type. Suggested
outlet protection facilities and applicable flow conditions (based on
Froude number and dissipation velocity) are described below:
»» Riprap aprons may be used when the outlet Froude number (Fr) is
less than or equal to 2.5. In general, riprap aprons prove economical
for transitions from culverts to overland sheet flow at terminal
outlets, but may also be used for transitions from culvert sections to
stable channel sections. Stability of the surface at the termination of
the apron should be considered.
Stormwater Drainage System Design
5.5.2 Symbols and Definitions
»» Riprap outlet basins may also be used when the outlet Fr is less than
or equal to 2.5. They are generally used for transitions from culverts
to stable channels. Since riprap outlet basins function by creating
a hydraulic jump to dissipate energy, performance is impacted by
tailwater conditions.
»» Baffled outlets have been used with outlet velocities up to 50 feet
per second. Practical application typically requires an outlet Fr
between 1 and 9. Baffled outlets may be used at both terminal outlet
and channel outlet transitions. They function by dissipating energy
through impact and turbulence and are not significantly affected by
tailwater conditions.
Figure 5.5-1 Riprap Size for Use Downstream of Energy Dissipator
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(Source: Searcy, 1967)
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conditions exist and the curves in Figure
5.5-3 should be used.
5.5.4.1 DESCRIPTION
A riprap-lined apron is a commonly used practice
for energy dissipation because of its relatively
low cost and ease of installation. A flat riprap
apron can be used to prevent erosion at the
transition from a pipe or box culvert outlet to a
natural channel. Protection is provided primarily
by having sufficient length and flare to dissipate
(Step 2) Determine the correct apron length
and median riprap diameter, d50, using
the appropriate curves from Figures
5.5-2 and 5.5-3. If tailwater conditions
are uncertain, find the values for both
minimum and maximum conditions and
size the apron as shown in Figure 5.5-4.
energy by expanding the flow. Riprap aprons are
appropriate when the culvert outlet Fr is less than
or equal to 2.5.
5.5.4.2 DESIGN PROCEDURE
The procedure presented in this section is taken
from USDA, SCS (1975). Two sets of curves, one
for minimum and one for maximum tailwater
conditions, are used to determine the apron size
and the median riprap diameter, d50. If tailwater
conditions are unknown, or if both minimum
and maximum conditions may occur, the apron
should be designed to meet criteria for both.
Although the design curves are based on round
pipes flowing full, they can be used for partially
full pipes and box culverts. The design procedure
consists of the following steps:
(Step 1) If possible, determine tailwater
conditions for the channel. If tailwater
is less than one-half the discharge
flow depth (pipe diameter if flowing
full), minimum tailwater conditions
exist and the curves in Figure 5.5-2
apply. Otherwise, maximum tailwater
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»» For pipes flowing full:
Use the depth of flow, d, which equals
the pipe diameter, in feet, and design
discharge, in cfs, to obtain the apron
length, La, and median riprap diameter,
d50, from the appropriate curves.
the intersection of the d and v curves,
then find the riprap median diameter,
d50, from the scale on the right. From
the lower d and v intersection point,
move vertically to the upper curve until
intersecting the curve equal to the flow
depth, d. Find the minimum apron
length, La, using the scale on the left.
(Step 3) If tailwater conditions are uncertain, the
median riprap diameter should be the
larger of the values for minimum and
maximum conditions. The dimensions
of the apron will be as shown in Figure
5.5-4. This will provide protection under
either of the tailwater conditions.
Stormwater Drainage System Design
5.5.4 Riprap Aprons
»» For pipes flowing partially full:
Use the depth of flow, d, in feet, and
velocity, v, in ft/s. On the lower portion
of the appropriate figure, find the
intersection of the d and v curves, then
find the riprap median diameter, d50,
from the scale on the right. From the
lower d and v intersection point, move
vertically to the upper curves until
intersecting the curve for the correct
flow depth, d. Find the minimum apron
length, La, from the scale on the left.
»» For box culverts:
Use the depth of flow, d, in feet, and
velocity, v, in feet/second. On the lower
portion of the appropriate figure, find
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Figure 5.5-2 Design of Riprap Apron under Minimum Tailwater Conditions
(Source: USDA, SCS, 1975)
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474
Stormwater Drainage System Design
Figure 5.5-3 Design of Riprap Apron under Maximum Tailwater Conditions
(Source: USDA, SCS, 1975)
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475
1.
La is the length of the riprap apron.
2.
D = 1.5 times the maximum stone diameter,
but not less than 6”.
3.
In a well-defined channel, extend the apron
up to the channel banks to an elevation of 6”
above the maximum tailwater depth or to the
top of the bank, whichever is less.
4.
A filter blanket or filter fabric should be
installed between the riprap and soil
foundation.
5.5.4.3 DESIGN CONSIDERATIONS
The following items should be considered during
• If there is a well-defined channel, the apron
length should be extended as necessary so that
the downstream apron width is equal to the
channel width. The sidewalls of the channel
should not be steeper than 2:1.
• If the ground slope downstream of the apron
is steep, channel erosion may occur. The
apron should be extended as necessary until
the slope is gentle enough to prevent further
erosion.
• The potential for vandalism should be
considered if the rock is easy to carry. If
vandalism is a possibility, the rock size must
be increased or the rocks held in place using
concrete or grout.
Stormwater Drainage System Design
Notes on Figure 5.5-4:
riprap apron design:
• The maximum stone diameter should be 1.5
times the median riprap diameter. dmax = 1.5
x d50 , d50 = the median stone size in a wellgraded riprap apron.
• The riprap thickness should be 1.5 times
the maximum stone diameter or 6 inches,
whichever is greater. Apron thickness = 1.5 x
dmax (Apron thickness may be reduced to 1.5
x d50 when an appropriate filter fabric is used
under the apron.)
Figure 5.5-4 Riprap Apron
(Source: Manual for Erosion and Sediment Control in Georgia, 2014)
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• The apron width at the discharge outlet should
be at least equal to the pipe diameter or culvert
width, dw. Riprap should extend up both sides
of the apron and around the end of the pipe or
culvert at the discharge outlet at a maximum
slope of 2:1 and a height not less than the pipe
diameter or culvert height, and should taper to
the flat surface at the end of the apron.
5.5.4.4 EXAMPLE DESIGNS
Example 1 Riprap Apron Design for Minimum
Tailwater Conditions
A flow of 280 cfs discharges from a 66-in pipe
with a tailwater of 2 ft above the pipe invert. Find
the required design dimensions for a riprap apron.
1.
Minimum tailwater conditions = 0.5 do, do =
66 in = 5.5 ft; therefore, 0.5 do = 2.75 ft.
2.
Since TW = 2 ft, use Figure 5.5-2 for
minimum tailwater conditions.
3.
Figure 5.5-2, the apron length, La, and
median stone size, d50, are 38 ft and 1.2 ft,
respectively.
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476
5.
6.
The downstream apron width equals the
apron length plus the pipe diameter:
-- W = d + La = 5.5 + 38 = 43.5 ft
Maximum riprap diameter is 1.5 times the
median stone size:
-- 1.5 (d50) = 1.5 (1.2) = 1.8 ft
(Riprap depth = 1.5 (dmax) = 1.5 (1.8) = 2.7 ft.
5.5.5 Riprap Basins
5.5.5.1 DESCRIPTION
Another method to reduce the exit velocities
from stormwater outlets is through the use of a
riprap basin. A riprap outlet basin is a preshaped
scourhole lined with riprap that functions as an
energy dissipator by forming a hydraulic jump.
Example 2 Riprap Apron Design for Maximum
5.5.5.2 BASIN FEATURES
Tailwater Conditions
General details of the basin recommended in
this section are shown in Figure 5.5 5. Principal
A concrete box culvert 5.5 ft high and 10 ft wide
features of the basin are:
conveys a flow of 600 cfs at a depth of 5.0 ft.
Tailwater depth is 5.0 ft above the culvert outlet
invert. Find the design dimensions for a riprap
apron.
1.
Compute 0.5 do = 0.5 (5.0) = 2.5 ft.
2.
Since TW = 5.0 ft is greater than 2.5 ft,
use Figure 5.5-3 for maximum tailwater
conditions.
--
3.
v = Q/A = [600/(5) (10)] = 12 ft/s
On Figure 5.5-3, at the intersection of the
curve, do = 60 in and v = 12 ft/s, d50 = 0.4 ft.
Reading up to the intersection with d = 60 in,
find La = 40 ft.
4.
Apron width downstream = dw + 0.4 La = 10
+ 0.4 (40) = 26 ft.
5.
Maximum stone diameter = 1.5 d50 = 1.5 (0.4)
= 0.6 ft.
6.
Riprap depth = 1.5 dmax = 1.5 (0.6) = 0.9 ft.
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• The basin is preshaped and lined with riprap of
median size (d50).
• The floor of the riprap basin is constructed at
an elevation of hs below the culvert invert. The
dimension hs is the approximate depth of scour
that would occur in a thick pad of riprap of size
d50 if subjected to design discharge. The ratio
of hs to d50 of the material should be between
2 and 4.
• The length of the energy dissipating pool is
10 x hs or 3 x Wo, whichever is larger. The
overall length of the basin is 15 x hs or 4 x Wo,
whichever is larger.
5.5.5.3 DESIGN PROCEDURE
The following procedure should be used for the
design of riprap basins.
(Step 1) Estimate the flow properties at the brink
(outlet) of the culvert. Establish the
outlet invert elevation such that TW/yo ≤
0.75 for the design discharge.
(Step 2) For subcritical flow conditions (culvert set
on mild or horizontal slope) use Figure
5.5-6 or Figure 5.5-7 to obtain yo/D, then
obtain Vo by dividing Q by the wetted area
associated with yo. D is the height of a box
culvert. If the culvert is on a steep slope,
Vo will be the normal velocity obtained
by using the Manning equation for
appropriate slope, section, and discharge.
Stormwater Drainage System Design
4.
(Step 3) For channel protection, compute the
Froude number for brink conditions with
ye = (A/2)1.5. Select d50/ye appropriate for
locally available riprap (usually the most
satisfactory results will be obtained if
0.25 < d50/ye < 0.45). Obtain hs/ye from
Figure 5.5-8, and check to see that 2 <
hs/d50 < 4. Recycle computations if hs/
d50 falls out of this range.
(Step 4) Size basin as shown in Figure 5.5-5.
(Step 5) Where allowable dissipator exit velocity
is specified:
»» Determine the average normal flow
depth in the natural channel for the
design discharge.
»» Extend the length of the energy basin
(if necessary) so that the width of the
energy basin at section A A, Figure 5.5-5,
times the average normal flow depth
in the natural channel is approximately
equal to the design discharge divided by
the specified exit velocity.
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Stormwater Drainage System Design
(Step 6) In the exit region of the basin, the walls
and apron of the basin should be warped
(or transitioned) so that the cross section
of the basin at the exit conforms to the
cross section of the natural channel.
Abrupt transition of surfaces should be
avoided to minimize separation zones
and resultant eddies.
(Step 7) If high tailwater is a possibility and
erosion protection is necessary for the
downstream channel, the following
design procedure is suggested:
»» Design a conventional basin for low
tailwater conditions in accordance with
the instructions above.
»» Estimate centerline velocity at a series
of downstream cross sections using the
information shown in Figure 5.5-9.
»» Shape downstream channel and size
riprap using Figure 5.5-1 and the stream
velocities obtained above.
Material, construction techniques, and design
details for riprap should be in accordance with
specifications in the Federal Highway publication HEC No. 11 entitled Use of Riprap For Bank
Protection.
Figure 5.5-5 Details of Riprap Outlet Basin
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(Source: HEC-14, 2012)
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Stormwater Drainage System Design
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Figure 5.5-6 Dimensionless Rating Curves for the Outlets
of Rectangular Culverts on Horizontal and Mild Slopes
Figure 5.5-7 Dimensionless Rating Curves for the Outlets
of Circular Culverts on Horizontal and Mild Slopes
(Source: USDOT, FHWA, HEC-14, 2012)
(Source: USDOT, FHWA, HEC-14, 2012)
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Stormwater Drainage System Design
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Figure 5.5-8 Relative Depth of Scour Hole Versus Froude Number
at Brink of Culvert with Relative Size of Riprap as a Third Variable
(Source: USDOT, FHWA, HEC-14, 2012)
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Stormwater Drainage System Design
Figure 5.5-9 Distribution of Centerline Velocity for Flow
from Submerged Outlets to Be Used for Predicting Channel Velocities
Downstream from Culvert Outlet Where High Tailwater Prevails
(Source: USDOT, FHWA, HEC-14, 2012)
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5.5.5.5 EXAMPLE DESIGNS
Riprap basin design should include consideration of the following:
Following are some example problems to illustrate the design procedures
• The dimensions of a scourhole in a basin constructed with angular rock
can be approximately the same as the dimensions of a scourhole in a basin
constructed of rounded material when rock size and other variables are
similar.
outlined.
• When the ratio of tailwater depth to brink depth, TW/yo, is less than 0.75
and the ratio of scour depth to size of riprap, hs/d50, is greater than 2.0, the
scourhole should function very efficiently as an energy dissipator. The
concentrated flow at the culvert brink plunges into the hole, a jump forms
against the downstream extremity of the scourhole, and flow is generally
well dispersed leaving the basin.
• The mound of material formed on the bed downstream of the scourhole
contributes to the dissipation of energy and reduces the size of the
scourhole; that is, if the mound from a stable scoured basin is removed and
the basin is again subjected to design flow, the scourhole will enlarge.
• For high tailwater basins (TW/yo greater than 0.75), the high velocity core
of water emerging from the culvert retains its jet-like character as it passes
through the basin and diffuses similarly to a concentrated jet diffusing in
a large body of water. As a result, the scourhole is much shallower and
generally longer. Consequently, riprap may be required for the channel
downstream of the rock-lined basin.
• It should be recognized that there is a potential for limited degradation
to the floor of the dissipator pool for rare event discharges. With the
protection afforded by the 2(d50) thickness of riprap, the heavy layer
of riprap adjacent to the roadway prism, and the apron riprap in the
downstream portion of the basin, such damage should be superficial.
• See Standards in the in FHWA HEC No. 11 for details on riprap materials and
use of filter fabric.
• Stability of the surface at the outlet of a basin should be considered using
the methods for open channel flow as outlined in Section 5.4, Open
Channel Design.
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Example 1
Given:
Box culvert- 8 ft by 6 ft
Design Discharge Q = 800 cfs
Supercritical flow in culvert
Normal flow depth = brink depth
Yo = 4 ft Tailwater depth
TW = 2.8 ft
Find:
Riprap basin dimensions for these conditions
Stormwater Drainage System Design
5.5.5.4 DESIGN CONSIDERATIONS
Solution:
Definition of terms in Steps 1 through 5 can be found in Figures 5.5-5 and 5.5-8.
1.
yo = ye for rectangular section; therefore, with yo given as 4 ft, ye = 4 ft.
2.
Vo = Q/A = 800/(4 x 8) = 25 ft/s
3.
Froude Number = Fr = V/(g x ye)0.5
(g = 32.3 ft/s2)
Fr = 25/(32.2 x 4)0.5 = 2.20 < 2.5 O.K.
4.
TW/ye = 2.8/4.0 = 0.7 Therefore, TW/ye < 0.75 OK
5.
Try d50/ye = 0.45, d50 = 0.45 x 4 = 1.80 ft
From Figure 5.5-8, hs/ye = 1.6, hs = 4 x 1.6 = 6.4 ft
hs/d50 = 6.4/1.8 = 3.6 ft, 2 < hs/d50 < 4 OK
6.
L s = 10 x hs = 10 x 6.4 = 64 ft (L s = length of energy dissipator pool); L s
min = 3 x Wo = 3 x 8 = 24 ft; therefore, use L s = 64 ft
.
LB = 15 x hs= 15 x 6.4 = 96 ft (LB = overall length of riprap basin); LB min =
4 x Wo = 4 x 8 = 32 ft; therefore, use LB = 96 ft
7.
Thickness of riprap: On the approach = 3 x d50 = 3 x 1.8 = 5.4 ft
Remainder = 2 x d50 = 2 x 1.8 = 3.6 ft
Other basin dimensions designed according to details shown in Figure
5.5-5.
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482
the basin. Protection must extend at least 135 ft downstream from the
Given:
culvert brink. Channel should be shaped and riprap should be installed in
accordance with details shown in the HEC No. 11 publication.
Same design data as Example 1 except:
-- Tailwater depth TW = 4.2 ft
Example 3
-- Downstream channel can tolerate only 7 ft/s discharge
Given:
Find:
Riprap basin dimensions for these conditions
6-ft diameter CMC
Design discharge Q = 135 cfs
Slope channel So = 0.004
Manning’s n = 0.024
Solutions:
Normal velocity is 5.9 ft/s
Flow is subcritical
Note- High tailwater depth, TW/yo = 4.2/4 = 1.05 > 0.75
Tailwater depth TW = 2.0 ft
Normal depth in pipe for Q = 135 cfs is 4.5 ft
1.
From Example 1: d50 = 1.8 ft, hs = 6.4 ft, L s = 64 ft, LB = 96 ft.
Find:
2.
Design riprap for downstream channel. Use Figure 5.5-9 for estimating
average velocity along the channel. Compute equivalent circular
diameter De for brink area from:
-- A = 3.14De2/4 = yo x Wo = 4 x 8 = 32 ft2
Riprap basin dimensions for these conditions.
Solution:
1.
-- De = ((32 x 4)/3.14)0.5 = 6.4 ft
.
Set up the following table:
Q/D2.5= 135/62.5= 1.53
TW/D = 2.0/6 = 0.33
*L/Wo is on a logarithmic scale so interpolations must be done logarithmically.
Riprap should be at least the size shown but can be larger. As a practical
v1
Rock Size
(ft/s)
d50 (ft)
L/De
L (ft)
VL/VO
(Assume) De = Wo
(Compute)
(Fig. 5.5-9)
10
64
0.59
14.7
1.4
15*
96
0.37
9.0
0.6
20
128
0.30
7.5
0.4
21
135
0.28
7.0
0.4
(Fig. 5.5-1)
consideration, the channel can be lined with the same size rock used for
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Determine yo and Vo
From Figure 5.5-7, yo/D = 0.45
-- Vo = 25 ft/s (From Example 1)
3.
Stormwater Drainage System Design
Example 2
yo = .45 x 6 = 2.7 ft
TW/yo = 2.0/2.7 = 0.74 TW/yo < 0.75 O.K.
Determine Brink Area (A) for yo/D = 0.45
From Uniform Flow in Circular Sections Table (from Section 5.3)
For yo/D = d/D = 0.45
A/D2 = 0.3428; therefore, A = 0.3428 x 62 = 12.3 ft2
Vo = Q/A = 135/12.3 = 11.0 ft/s
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Stormwater Drainage System Design
2.
For Froude number calculations at brink conditions,
ye = (A/2)1/2 = (12.3/2)1/2 = 2.48 ft
3.
Froude number = Fr = Vo/(32.2 x ye)1/2 = 11/(32.2 x 2.48)1/2 =
1.23 < 2.5 OK
4.
For most satisfactory results 0.25 < d50/ye < 0.45
Try d50/ye = 0.25
d50 = 0.25 x 2.48 = 0.62 ft
From Figure 5.5-8, hs/ye = 0.75; therefore, hs = 0.75 x 2.48 =
1.86 ft
Uniform Flow in Circular Sections Flowing Partly Full
(From Section 5.3)
Check: hs/d50 = 1.86/0.62 = 3, 2 < hs/d50 < 4
5.
OK
Ls = 10 x hs = 10 x 1.86 = 18.6 ft or L s = 3 x Wo = 3 x 6 = 18 ft;
therefore, use L s = 18.6 ft
LB = 15 x hs = 15 x 1.86 = 27.9 ft or LB = 4 x Wo = 4 x 6 = 24 ft;
therefore, use LB = 27.9 ft
d50 = 0.62 ft or use d50 = 8 in
Other basin dimensions should be designed in accordance with details
shown on Figure 5.5-5. Figure 5.5-10 is provided as a convenient form
to organize and present the results of riprap basin designs.
Note: When using the design procedure outlined in this section, it is
recognized that there is some chance of limited degradation of the
floor of the dissipator pool for rare event discharges. With the protection afforded by the 3 x d50 thickness of riprap on the approach and
the 2 x d50 thickness of riprap on the basin floor and the apron in the
downstream portion of the basin, the damage should be superficial.
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Figure 5.5-10 Riprap Basin Design Form
(Source: USDOT, FHWA, HEC-14, 2012)
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Stormwater Drainage System Design
5.5.6 Baffled Outlets
5.5.6.1 DESCRIPTION
The baffled outlet (also known as the Impact Basin - USBR Type VI) is a boxlike
structure with a vertical hanging baffle and an end sill, as shown in Figure 5.511. Energy is dissipated primarily through the impact of the water striking the
baffle and, to a lesser extent, through the resulting turbulence. This type of
outlet protection has been used with outlet velocities up to 50 feet per second
and with Froude numbers from 1 to 9. Tailwater depth is not required for adequate energy dissipation, but a tailwater will help smooth the outlet flow.
5.5.6.2 DESIGN PROCEDURE
The following design procedure is based on physical modeling studies summarized from the U.S. Department of Interior (1978). The dimensions of a
baffled outlet as shown in Figure 5.5-11 should be calculated as follows:
(Step 1) Determine input parameters, including:
»» h = Energy head to be dissipated, in ft (can be approximated as the
difference between channel invert elevations at the inlet and outlet)
»» Q = Design discharge (cfs)
»» v = Theoretical velocity (ft/s = 2gh)
»» A = Q/v = Flow area (ft2)
Figure 5.5-11 Schematic of Baffled Outlet
»» d = A0.5= Representative flow depth entering the basin (ft) assumes
square jet
Where:
»» Fr = v/(gd)0.5 = Froude number, dimensionless
W = minimum basin width (ft)
(Step 2) Calculate the minimum basin width, W, in ft, using the following
equation.
W/d = 2.88Fr0.566 or W = 2.88dFr0.566
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(Source: U.S. Dept. of the Interior, 1978)
(5.5.2)
d = depth of incoming flow (ft)
Fr = v/(gd)0.5 = Froude number, dimensionless
(Step 3) Calculate the other basin dimensions as shown in Figure 5.5-11,
as a function of W. Construction drawings for selected widths are
available from the U.S. Department of the Interior (1978).
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485
3.
(Step 5) Calculate the baffled outlet invert elevation based on expected
tailwater. The maximum distance between expected tailwater
elevation and the invert should be b + f or some flow will go over the
baffle with no energy dissipation. If the tailwater is known and fairly
controlled, the baffled outlet invert should be a distance, b/2 + f,
below the calculated tailwater elevation. If tailwater is uncontrolled,
the baffled outlet invert should be a distance, f, below the downstream channel invert.
5.
d = (A)0.5 = (4.8 ft2)0.5 = 2.12 ft
4.
Use 13 ft as the design width.
6.
-- e = 1/12 (W) = 1.08 ft, use e = 1 ft, 1 in
-- H = 3/4 (W) = 9.75 ft, use H = 9 ft, 9 in
-- a = 1/2 (W) = 6.5 ft, use a = 6 ft, 6 in
-- b = 3/8 (W) = 4.88 ft, use b = 4 ft, 11 in
-- c = 1/2 (W) = 6.5 ft, use c = 6 ft, 6 in
Baffle opening dimensions would be calculated as shown in Figure 5.511.
and possible surging flow conditions.
7.
5.5.6.3 EXAMPLE DESIGN
.
v = (2gh)0.5 = [2(32.2 ft/sec2)(15 ft)]0.5 = 31.1 ft/s
This is less than 50 ft/s, so a baffled outlet is suitable.
2.
Determine the flow area using the theoretical velocity as follows:
Basin invert should be at b/2 + f below tailwater, or
(4 ft, 11 in)/2 + 2 ft, 2 in = 4.73 ft
A cross-drainage pipe structure has a design flow rate of 150 cfs, a head, h, of
Compute the theoretical velocity from
Compute the remaining basin dimensions (as shown in Figure 5.5-11):
-- L = 4/3 (W) = 17.3 ft, use L = 17 ft, 4 in
-- f = 1/6 (W) = 2.17 ft, use f = 2 ft, 2 in
(Step 8) If it is possible that both the upstream and downstream ends of the
pipe will be submerged, provide an air vent approximately 1/6 the
pipe diameter near the upstream end to prevent pressure fluctuations
1.
Determine the basin width using equation 5.5.2 with the Froude number
from Step 4.
W = 2.88 dFr0.566 = 2.88 (2.12) (3.8)0.566 = 13.0 ft (minimum)
(Step 7) If the entrance pipe slopes steeply downward, the outlet pipe should
be turned horizontal for at least 3 ft before entering the baffled
outlet.
face. Find the baffled outlet basin dimensions and inlet pipe requirements.
Compute the Froude number using the results from Steps 1 and 3.
Fr = v/(gd)0.5 = 31.1 ft/sec/[(32.2 ft/sec2)(2.12 ft)]0.5 = 3.8
(Step 6) Calculate the outlet pipe diameter entering the basin assuming a
velocity of 12 ft/s flowing full.
15 ft from invert of pipe, and a tailwater depth, TW, of 3 ft above ground sur-
Compute the flow depth using the area from Step 2.
Stormwater Drainage System Design
(Step 4) Calculate required protection for the transition from the baffled
outlet to the natural channel based on the outlet width. A riprap
apron should be added of width W, length W (or a 5-foot minimum),
and depth f (W/6). The side slopes should be 1.5:1, and median rock
diameter should be at least W/20.
Use 4 ft 8 in; therefore, invert should be 2 ft, 8 in below ground surface.
8.
The riprap transition from the baffled outlet to the natural channel should
be 13 ft long by 13 ft wide by 2 ft, 2 in deep (W x W x f). Median rock
diameter should be of diameter W/20, or about 8 in.
9.
Inlet pipe diameter should be sized for an inlet velocity of about 12 ft/s.
(3.14d)2 /4 = Q/v; d = [(4Q)/3.14v)]0.5 = [(4(150 cfs)/3.14(12 ft/sec)]0.5 = 3.99 ft
Use 48-in pipe. If a vent is required, it should be about 1/6 of the pipe
diameter or 8 in.
A = Q/v = 150 cfs/31.1 ft/sec = 4.8 ft2
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VOL 2
486
American Association of State Highway and
Federal Highway Administration, 1978. Hydraulics of
Peterska, A. J., 1978. Hydraulic Design of Still-
Bridge Waterways. Hydraulic Design Series No. 1.
ing Basins and Energy Dissipators. Engineering
Monograph No. 25. U. S. Department of Interior,
Transportation Officials, 1982. Highway Drainage
Guidelines.
Federal Highway Administration, 1996. Urban
Bureau of Reclamation. Washington, DC.
Drainage Design Manual. Hydraulic Engineering
American Association of State Highway and
Circular No. 22.
Prince George’s County, MD, 1999. Low-Impact
Development Design Strategies,
Federal Highway Administration, 1967. Use of
An Integrated Design Approach.
Transportation Officials, 1981 and 1998. Model
Drainage Manual.
Riprap for Bank Protection. Hydraulic EngineerChow, V. T., ed., 1959. Open Channel Hydraulics.
ing Circular No. 11.
Reese, A. J., 1984. Riprap Sizing, Four Methods.
In Proceedings of ASCE Conference on Water
McGraw Hill Book Co. New York.
French, R. H., 1985. Open Channel Hydraulics.
for Resource Development, Hydraulics Division,
McGraw Hill Book Co. New York.
ASCE. David L. Schreiber, ed.
Harza Engineering Company, 1972. Storm Drain-
Reese, A. J., 1988. Nomographic Riprap Design.
age Design Manual. Prepared for the Erie and
Miscellaneous Paper HL 88-2. Vicksburg, Missis-
Niagara Counties Regional Planning Bd. Harza
sippi: U. S. Army Engineers, Waterways Experiment
Engineering Company, Grand Island, NY.
Station.
HYDRAIN Culvert Computer Program (HY8).
Searcy, James K., 1967. Use of Riprap for Bank
Federal Highway Administration, 1971. De-
Available from McTrans Software, University of
Protection. Federal Highway Administration.
bris-Control Structures. Hydraulic Engineering
Florida, 512 Weil Hall, Gainesville, Florida 32611.
Debo, Thomas N. and Andrew J. Reese, 1995.
Municipal Storm Water Management. Lewis
Publishers.
Federal Highway Administration, 1989. Bridge
Waterways Analysis Model (WSPRO), Users Manu-
Stormwater Drainage System Design
References
al, FHWA IP-89-027.
U. S. Department of Interior, 1983. Design of
Circular No. 9.
Maynord, S. T., 1987. Stable Riprap Size for Open
Federal Highway Administration, 1987. HY8 Culvert Analysis Microcomputer Program Applications
Small Canal Structures.
Channel Flows. Ph.D. Dissertation. Colorado
State University, Fort Collins, CO.
U.S. Department of Interior, Bureau of Reclamation, 1978. Design of Small Canal Structures.
Guide. Hydraulic Microcomputer Program HY8.
Morris, J. R., 1984. A Method of Estimating FloodFederal Highway Administration, 1983. Hydraulic
Design of Energy Dissipators for Culverts and
way Setback Limits in Areas of Approximate Study.
Channels. Hydraulic Engineering Circular No. 14.
on Urban Hydrology, Hydraulics and Sediment
In Proceedings of 1984 International Symposium
Control. Lexington, Kentucky: University of Ken-
Federal Highway Administration, 2012. Hydraulic
tucky.
Design of Highway Culverts. Hydraulic Design
Series No. 5.
BACK
TO
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VOL 2
487
Highway Administration, 1973. Design Charts For
Open Channel Flow. Hydraulic Design Series No.
3. Washington, DC.
U. S. Department of Transportation, Federal Highway Administration, 1986. Design of Stable Channels with Flexible Linings. Hydraulic Engineering
Circular No. 15. Washington, DC.
U.S. Department of Transportation, Federal Highway Administration, 1984. Drainage of Highway
Pavements. Hydraulic Engineering Circular No.
Stormwater Drainage System Design
U. S. Department of Transportation, Federal
12.
U. S. Department of Transportation, Federal Highway Administration, 1984. Guide for Selecting
Manning’s Roughness Coefficients For Natural
Channels and Flood Plains. FHWA-TS-84-204.
Washington, DC.
U. S. Department of Transportation, Federal
Highway Administration, 1983. Hydraulic Design
of Energy Dissipators for Culverts and Channels.
Hydraulic Engineering Circular No. 14. Washington, DC.
Wright-McLaughlin Engineers, 1969. Urban Storm
Drainage Criteria Manual, Vol. 2. Prepared for
the Denver Regional Council of Governments.
Wright-McLaughlin Engineers, Denver, CO
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VOL 2
488
Appendix A: Rainfall Tables for Georgia
for the State of Georgia on their website:
http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=ga
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Appendix A: Rainfall Tables for Georgia
The National Oceanic and Atmoshpheric Administration provides rainfall tables
VOL 2
489
Appendix B: Best Management Practices Design Examples
Appendix B: Best Management Practice Design Examples
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VOL 2
490
Appendix B: Best Management Practice Design Examples
Appendix B-1: Stormwater
Pond Design Example
The following design example is for a wet
extended detention (ED) stormwater pond
Figure 1. Peachtree Meadows Site Plan
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VOL 2
491
Other site screening aspects listed in Section
stormwater facility to meet the water quality
4.25 were assessed and a pond was found to be
requirements of the site through the water quality
suitable.
treatment (method 4) requirement. In general,
the primary function of stormwater ponds is to
provide water quality treatment (TSS removal) and
detention. Stormwater ponds do not contribute
to a runoff reduction goal.
Computation of Preliminary Stormwater Storage
Volumes and Peak Discharges
The layout of the Peachtree Meadows subdivision
is shown on the previous page. This example assumes that the local community has adopted the
unified stormwater sizing criteria requirements.
STEP 1 - CONFIRM LOCAL DESIGN CRITERIA
AND APPLICABILITY
There are no additional requirements for this site.
STEP 3 - COMPUTE RUNOFF CONTROL VOLUMES FROM THE UNIFIED STORMWATER
SIZING CRITERIA
More detailed hydrologic calculations will be
required during the design step – these numbers
THE USE OF A STORMWATER POND
Site Specific Data:
The site area and drainage area to the pond is
38.0 acres (38 ac > min. 25 ac required). Existing
ground at the pond outlet is 919 MSL. Soil boring
observations reveal that the seasonally high water
table is at elevation 918. The underlying soils
are SC (sandy clay) and are suitable for earthen
embankments and to support a wet pond without
Condition
Runoff
Pre-developed
Post-developed
Q1-yr
inches
0.70
1.42
Q5-yr
cfs
22
67
Q25-yr
cfs
104
192
Q100-yr
cfs
148
251
COMPUTE CHANNEL PROTECTION VOLUME,
(CPV)
For stream channel protection, provide 24 hours
are preliminary.
of extended detention for the 1-year event.
Compute Water Quality Volume (WQV)
In order to determine a preliminary estimate
• Compute Runoff Coefficient, Rv
of storage volume for channel protection and
Rv
overbank flood control, it will be necessary to
= 0.05 + (I) (0.009)
perform hydrologic calculations using approved
= 0.05 + (36.3) (0.009) = 0.38
methodologies. This example uses the NRCS
TR-55 methodology presented in Section 3.1.5
• Compute WQv
to determine pre- and post-development peak
WQv = (1.2”) (Rv) (A) / 12
discharges for the 1-yr, 25-yr, and 100-yr 24-hour
= (1.2”) (0.38) (1,655,280 ft2) (1ft/12in)
= 62,900 ft3 (1.44 ac-ft)
STEP 2 - DETERMINE IF THE DEVELOPMENT
SITE AND CONDITIONS ARE APPROPRIATE FOR
Perform Preliminary Hydrologic Calculations
return frequency storms.
Appendix B: Best Management Practice Design Examples
This example focuses on the design of a wet
Develop Site Hydrologic Input Parameters
Per Figures 2 and 3. Note that any hydrologic
models using NRCS TR-55 procedures, such as
TR-20, HEC-HMS, or other software platforms,
can be used to perform preliminary hydrologic
calculations.
Condition
Pre-developed
Post-developed
Area
ac
CN
TC
hrs
38
38
65
78
0.32
0.17
a liner. The stream invert at the adjacent stream is
at elevation 916.
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VOL 2
492
DETERMINE OVERBANK FLOOD PROTECTION VOLUME, (QP25)
Storage Volume
• For a Qin of 192 cfs, and an allowable Qout of 104 cfs, and a runoff volume of
520,364 cubic feet (11.95 ac-ft) the Vs necessary for 25-year control is 3.33
ac-ft, under a developed CN of 78. Note that 6.5 inches of rain fall during
this event.
See Section 3.1.5
Initial abstraction (Ia) for CN of 78 is 0.564: [Ia = (200/CN - 2)]
• While the TR-55 short-cut method reports to incorporate multiple stage
structures, experience has shown that an additional 10-15% storage is
required when multiple levels of extended detention are provided inclusive
with the 25-year storm. So, for preliminary sizing purposes add 15% to the
Ia/P = (0.564)/ 3.4 inches = 0.17
Tc = 0.17 hours
qu = 800 csm/in (From Figure 3.1.5-6)
Knowing qu and T (extended detention time), find qo/qi. For a Type II rainfall
distribution.
required volume for the 25-year storm. Qp-25 = 3.33 × 1.15 = 3.8 ac-ft.
ANALYZE SAFE PASSAGE OF 100 YEAR DESIGN STORM (Qf)
Peak outflow discharge/peak inflow discharge (qo/qi) = 0.022 (From Figure
3.3.5-1)
At final design, provide safe passage for the 100-year event, or detain it, depending on downstream conditions and local policy. Based on field observation and review of local requirements no control of the 100-year storm is
Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi) - 0.804(qo/qi) (Equation 3.3.9)
2
3
Where Vs equals channel protection storage (CPv) and Vr equals the volume of
runoff in inches.
Vs/Vr = 0.65
Using Equation 3.3.10, calculate VS…
Therefore, Vs = CPv = 0.65(1.42”)(1/12)(38ac) = 2.9 ac-ft (126,324
cubic feet)
necessary. If it were storage estimates would have been made similar to the
Qp Volume in the previous sub-step.
Table 1 Summary of General Storage Requirements for Peachtree Meadows
Symbol
Control Volume
WQv
Water Quality
CPv
Channel Protection
Qp25
Qf
Overbank Flood
Protection
Extreme Flood
Protection
Volume Required
(ac-ft)
1.44
Appendix B: Best Management Practice Design Examples
Utilize NRCS TR-55 approach to Compute Channel Protection
Notes
2.9
Average ED release rate is
1.46 cfs over 24 hours
3.33
3.8 ac-ft will be provided
NA
Provide safe passage for the
100-year event in final design
Define the average CPv-ED Release Rate
The above volume, 2.9 ac-ft (126,324 ft3), is to be released over 24 hours.
(126,324 ft3) / (24 hrs × 3,600 sec/hr) = 1.46 cfs
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VOL 2
493
Job:
Drainage Area Name:
Cover Description
P’tree Meadows
Pre-Developed Conditions
Soil Name
meadow (good cond.)
meadow (good cond.)
wood (good cond.)
Time of Concentration
2-Yr 24 Hr Rainfall = 4.1 In
Sheet Flow
Shallow Flow
EWB
3-Jan-00
Group
A,B,C,D?
CN from
TABLE 2.1-5.1
Area
(in acres)
C
B
B
71
58
55
22.80 Ac.
9.20 Ac.
6.00 Ac.
Area Subtotals:
38.00 Ac.
Surface Cover/
Cross Section
Manning ‘n’/
Wetted Per
Flow Length/
Avg Velocity
Slope/
Tt (Hrs)
dense grass
‘n’ = 0.24
150 Ft.
2.50%
0.27 Hrs
500 Ft.
3.23 F.P.S.
4.00%
0.04 Hrs.
unpaved
Appendix B: Best Management Practice Design Examples
Figure 2 Peachtree Meadows Pre-Development Conditions
Peak Discharge Summary
Channel Flow
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Total Area in Acres =
Weighted CN =
Time of Concentration =
Pond Factor =
38.00 Ac.
65
0.31 Hrs.
1
Total Sheet
Flow =
0.27 Hrs.
Storm
Precipitation
(P) inches
Runoff
(Q)
Qp,
Peak Discharge
Total Storm
Volumes
1 Year
2 Year
5 Year
10 Year
25 Year
50 Year
100 Year
3.4 In.
4.1 In.
4.8 In.
5.5 In.
6.5 In.
7.2 In.
7.9 In.
0.7 In.
1.1 In.
1.5 In.
2.0 In.
2.7 In.
3.3 In.
3.8 In.
21.9 CFS
37.2 CFS
54 CFS
74 CFS
101 CFS
124 CFS
147 CFS
93,771 Cu. Ft.
148,313 Cu. Ft
209,936 Cu. Ft.
277,081 Cu. Ft.
373,288 Cu. Ft.
449,409 Cu. Ft.
528,261 Cu. Ft.
Total Shallow
Total Channel
Flow =
Flow =
0.04 Hrs.
0.00 Hrs.
RAINFALL TYPE II
VOL 2
494
Job:
Drainage Area Name:
P’tree Meadows
Post-Developed Conditions
EWB
3-Jan-00
Group
A,B,C,D?
CN from
TABLE 2.1-5.1
Area
(in acres)
open space
open space
woods (good cond.)
C
B
B
74
61
55
13.00 Ac.
5.20 Ac.
6.00 Ac.
impervious area
impervious area
C
B
98
98
7.90 Ac.
5.90 Ac.
Cover Description
Soil Name
Area:
Time of Concentration
2-Yr 24 Hr Rainfall = 4.1 In
Sheet Flow
Shallow
Channel
Hydraulic Radius
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38.00 Ac.
Surface Cover/
Cross Section
Manning ‘n’/
Wetted Per
Flow Length/
Avg Velocity
Slope/
Tt (Hrs)
short grass
‘n’ = 0.15
100 Ft.
2.50%
0.13 Hrs.
300 Ft.
2.87 F.P.S.
2.00%
0.03 Hrs.
600 Ft.
16.21 F.P.S.
2.00%
0.01 Hrs.
paved
X-S estimated
‘n’ = 0.013
WP estimated
Total Area in Acres =
Weighted CN =
Time of Concentration =
Pond Factor =
38.00 Ac.
78
0.17 Hrs.
1
Total Sheet
Flow =
0.13 Hrs.
Storm
Precipitation
(P) inches
Runoff
(Q)
Qp,
Peak Discharge
Total Storm
Volumes
1 Year
2 Year
5 Year
10 Year
25 Year
50 Year
100 Year
3.4 In.
4.1 In.
4.8 In.
5.5 In.
6.5 In.
7.2 In.
7.9 In.
1.4 In.
2.0 In.
2.5 In.
3.2 In.
4.0 In.
4.7 In.
5.3 In.
67.1 CFS
95.8 CFS
127 CFS
159 CFS
202 CFS
234 CFS
267 CFS
198,988 Cu. Ft.
269,103 Cu. Ft.
350,750 Cu. Ft.
435,668 Cu. Ft.
552,584 Cu. Ft.
642,337 Cu. Ft.
733,444 Cu. Ft.
Appendix B: Best Management Practice Design Examples
Figure 3 Peachtree Meadows Post-Development Conditions
Peak Discharge Summary
Total Shallow
Total Channel
Flow =
Flow =
0.03 Hrs.
0.01 Hrs.
RAINFALL TYPE
VOL 2
495
Size wet forebay to treat 0.1”/impervious acre. (13.8 ac) (0.1”) (1’/12”) = 0.12 ac-ft
(forebay volume is included in WQv as part of permanent pool volume)
STEP 5 - DETERMINE PERMANENT POOL VOLUME (AND WATER QUALITY
ED VOLUME)
Size permanent pool volume to contain 50% of WQv:
0.5 × (1.44 ac-ft) = 0.72 ac-ft. (includes 0.12 ac-ft of forebay volume)
Size ED volume to contain 50% of WQv: 0.5 × (1.44 ac-ft) = 0.72 ac-ft
Note: This design approach assumes that all of the ED volume will be in the
pond at once. While this will not be the case, since there is a discharge during
the early stages of storms, this conservative approach allows for ED control over
a wider range of storms, not just the target rainfall.
STEP 6 - DETERMINE POND LOCATION AND PRELIMINARY GEOMETRY.
CONDUCT POND GRADING AND DETERMINE STORAGE AVAILABLE FOR
PERMANENT POOL AND WATER QUALITY EXTENDED DETENTION
Appendix B: Best Management Practice Design Examples
STEP 4 - DETERMINE PRETREATMENT VOLUME
This step involves initially grading the pond (establishing contours) and determining the elevation-storage relationship for the pond. Storage must be provided for the permanent pool (including sediment forebays), extended detention
(WQv-ED), CPv-ED, and 25-year storm, plus sufficient additional storage to pass
the 100-year storm with minimum freeboard. An elevation-storage table and
curve is prepared using the average area method for computing volumes. See
Figure 4 for pond location on site, Figure 5 grading and Figure 6 for Elevation-Storage Data.
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VOL 2
496
Appendix B: Best Management Practice Design Examples
Figure 4. Pond Location on Site
VOL 2
497
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Appendix B: Best Management Practice Design Examples
Figure 5. Plan View of Pond Grading (Not to Scale)
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VOL 2
498
Average
Area
Average Area
ft^2
(ft2)
920.0
920.0
921.0
921.0
923.0
923.0
924.0
924.0
925.0
925.0
925.5
925.5
926.0
926.0
926.5
926.5
927.0
927.0
927.5
927.5
928.0
928.0
928.5
928.5
929.0
929.0
929.5
929.5
930.0
930.0
930.5
930.5
931.0
931.0
931.5
931.5
932.0
932.0
932.5
932.5
933.0
933.0
933.5
933.5
934.0
934.0
935.0
7838
11450 7838
1453811450
1507514538
1665515075
17118 16655
2100017118
2500021000
3000025000
3600030000
3800036000
4100038000
4300041000
4500043000
4700045000
47000
49000
49000
52000
52000
55000
55000
58000
58000
61000
61000
65000
65000
69000
69000
74000
935.0
74000
Depth
Volume
Depth Volume
ft
ft^3
(ft)
(ft3)
1
2 1
1 2
1 1
0.5 1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
1
7838
7838
22900
22900
14538
14538
15075
15075
8328
8328
8559
8559
10500
10500
12500
12500
15000
15000
18000
18000
19000
19000
20500
20500
21500
21500
22500
22500
23500
23500
24500
24500
26000
26000
27500
27500
29000
29000
30500
30500
32500
32500
34500
34500
74000
74000
Cumulative
Cumulative
Cumulative
Volume
Volume
(ft3)
ft^3
Cumulative
Volume
Volume
(ac-ft)
ac-ft
7838
307387838
30738
45275
45275
60350
60350
68678
68678
77236
77236
87736
87736
100236
100236
115236
115236
133236
133236
152236
152236
172736
172736
194236
194236
216736
216736
240236
240236
264736
264736
290736
290736
318236
318236
347236
347236
377736
377736
410236
410236
444736
444736
518736
518736
0.18
0.71 0.18
1.04 0.71
1.39 1.04
1.58 1.39
1.77 1.58
2.01 1.77
2.30 2.01
2.65 2.30
3.06 2.65
3.49 3.06
3.97 3.49
4.46 3.97
4.98 4.46
5.52 4.98
5.52
6.08
6.08
6.67
6.67
7.31
7.31
7.97
7.97
8.67
8.67
9.42
9.42
10.21
10.21
11.91
11.91
Volume Above
Volume
Above
Permanent Pool
Permanent
(ac-ft) Pool
ac-ft
0
0.35
0.54
0.73
0.97
1.26
1.61
2.02
2.46
2.93
3.42
3.94
4.48
5.04
5.64
6.27
6.93
7.63
8.38
9.17
10.87
0
0.35
0.54
0.73
0.97
1.26
1.61
2.02
2.46
2.93
3.42
3.94
4.48
5.04
5.64
6.27
6.93
7.63
8.38
9.17
10.87
Elevation [MSL-ft]
Storage Above Permanent Pool
Appendix B: Best Management Practice Design Examples
Elevation
Elevation
MSL
(MSL)
936.0
935.0
934.0
933.0
932.0
931.0
930.0
929.0
928.0
927.0
926.0
925.0
924.0
0.0
1.2
2.3
3.5
4.7
5.8
7.0
8.1
9.3
10.5
11.6
Storage [Ac-ft]
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Figure 6. Storage-Elevation Table/ Curve
VOL 2
499
Compute the stage-discharge equation for the 3.7” dia. WQv orifice
• The pond bottom is set at elevation 920.0.
• QWQv-ED = CA(2gh)0.5 = (0.6) (0.075 ft2) [((2)(32.2 ft/s2))0.5] (h0.5),
• Provide gravity flow to allow for pond drain, set riser invert at 919.5
• QWQv-ED = (0.36) h0.5, where: h = wsel - 924.16
• Set barrel outlet elevation at 919.0.
(Note: account for one half of orifice diameter when calculating head)
Set water surface and other elevations
STEP 7 - COMPUTE EXTENDED DETENTION ORIFICE RELEASE RATE(S) AND
SIZE(S), AND ESTABLISH CPV ELEVATION
• Required permanent pool volume = 50% of WQv = 0.72 ac-ft. From the
elevation-storage table, read elevation 924.0 (1.04 ac-ft > 0.72 ac-ft) site
can accommodate it and it allows a small safety factor for fine sediment
accumulation - OK
• Forebay volume provided in two pools with avg. vol. = 0.08 ac-ft each (0.16
ac-ft > 0.12 ac-ft) OK
• Required extended detention volume (WQv-ED)= 0.72 ac-ft. From the
elevation-storage table (volume above permanent pool), read elevation 926.0
(0.73 ac-ft > 0.72 ac-ft) OK. Set ED wsel = 926.0
Note: Total storage at elevation 926.0 = 1.77 ac-ft (greater than required WQv
Set the CPv pool elevation
• Required CP v storage = 2.9 ac-ft (see Table 1).
• From the elevation-storage table, read elevation 929 (this includes the WQv).
• Set CP v wsel = 929
Size CPv orifice
• Size to release average of 1.46 cfs.
»» Average WQv-ED orifice release rate is 0.66 cfs, based on average head of
3.34’ (926 – 924.16 + (929 – 926)/2)
»» CP v-ED orifice release = 1.46 -0.66 = 0.80 cfs
of 1.44 ac-ft)
• Head = (929 - 926.0)/2 = 1.5’
Compute the required WQv-ED orifice diameter to release 0.72 ac-ft over 24
hours
• Avg. ED release rate = (0.72 ac-ft)(43,560 ft2/ac)/(24 hr)(3600 sec/hr) = 0.36
cfs
• Average head = (926.0 - 924.0)/ 2 = 1.0’
• Use orifice equation to compute cross-sectional area and diameter
»» Q = CA(2gh)0.5, for Q=0.36 cfs h = 1.0 ft; C = 0.6 = discharge coefficient
solve for A
Use orifice equation to compute cross-sectional area and diameter
• Q = CA(2gh)0.5, for h = 1.5’
»» A = 0.80 cfs / [(0.6)((2)(32.2’/s2)(1.5’))0.5]
»» A = 0.14 ft2, A =πd2 / 4;
»» dia. = 0.42 ft = 5.0”
• Use 6” pipe with 6” gate valve to achieve equivalent diameter
Compute the stage-discharge equation for the 5.0” dia. CPv orifice
• QCPv-ED = CA(2gh)0.5 = (0.6) (0.14 ft2) [((2) (32.2’/s2))0.5] (h0.5),
»» A = 0.36 cfs / [(0.6)((2)32.2 ft/s2)(1.0 ft))0.5] A = 0.075 ft2, A =πd2 / 4;
dia. = 0.31 ft = 3.7”
• QCPv-ED = (0.67) (h0.5), where: h = wsel - 926.21
»» Use 4” pipe with 4” gate valve to achieve equivalent diameter
(Note: account for one half of orifice diameter when calculating head)
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Appendix B: Best Management Practice Design Examples
Set basic elevations for pond structures
VOL 2
500
In order to calculate the 25 year release rate and water surface elevation, the
designer must set up a stage-storage-discharge relationship for the control
structure for each of the low flow release pipes (WQv-ED and CPv-ED) plus the
25 year storm.
• Q = 0.6 (42.0ft2) [(64.4)(0.75)]0.5 = 175 cfs > 104 cfs, so use weir equation
Q25 = (3.1) (28’) H3/2 Q25 = (86.8) H3/2, where H = wsel – 929.0
Size barrel to release approximately 104 cfs at elevation 930.1
• Check inlet condition: (use Section 5.3 culvert charts)
»» Hw = 930.1-919.5 = 10.6 ft
Develop basic data and information
»» Try 33” diameter RCP, Using Figure 5.3-1a
• The 25 year pre-developed peak discharge = 104 cfs,
»» Hw / D = 10.6/2.75 = 3.85,
• The post developed inflow = 192 cfs, from Table 1,
• From previous estimate Qp-25 = 3.33 ac-ft. Adding 15% to account for ED
storage yields a preliminary volume of 3.8 ac-ft.
Discharge = 88 cfs
• Check outlet condition:
• Q = a [(2gH)/(1+km+kpL)]0.5
where:Q = discharge in cfs
• From elevation-storage table (Figure 6), read elevation 930.1.
a = pipe cross sectional area in ft2
Size 25 year slot to release 104 cfs at elevation 930.1.
g = acceleration of gravity in ft/sec2
H = head differential (wsel - downstream centerline of pipe or tailwater elev.)
• @ wsel 930.1:
km = coefficient of minor losses (use 1.0)
»» WQv-ED orifice releases 0.88 cfs,
»» CP v-ED orifice releases 1.32 cfs, therefore;
»» Allowable Qp-25 = 104 cfs - (.88 + 1.32) = 101.8 cfs, say 102 cfs.
• Max head = (930.1 – 929) = 1.1’
• Use weir equation to compute slot length
»» Q = CLH
3/2
kp = pipe friction loss coef. (= 5087n2/d4/3, d in “, n is Manning’s n)
L = pipe length in ft
»» H = 930.1 - (919.0 + 1.38) = 9.72’
Appendix B: Best Management Practice Design Examples
STEP 8 - CALCULATE QP25 (25-YEAR STORM) RELEASE RATE AND WATER
SURFACE ELEVATION
»» for 33” RCP, 70 feet long:
»» Q = 7.1 [(64.4) (9.72) / 1+1+(.007) (70)]0.5 = 112.6 cfs
»» 88 cfs < 112.6 cfs, so barrel is inlet controlled.
Note: pipe will control flow before high stage inlet reaches max head.
»» L = 102 cfs / (3.1) (1.13/2) = 28 ft
• Use four 7ft x 1.5 ft slots for 25-year release (opening should be slightly larger
than needed so as to have the barrel control before slot goes from weir flow
to orifice flow).
Check orifice equation using cross-sectional area of opening
Complete stage-storage-discharge summary (Figure 7) up to preliminary 25year wsel (930.1) and route 25 year post-developed condition inflow using computer software. Pond routing computes 25-year wsel at 930.8 with discharge =
92.4 cfs.
• Q = CA(2gh)0.5, for h = 0.75’ (For orifice equation, h is from midpoint of slot)
• A = 4 (7.0’) (1.5’) = 42.0ft2
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VOL 2
501
924.0
925.0
925.5
926.0
926.5
927.0
927.5
928.0
928.5
929.0
929.5
930.0
930.1
930.5
931.0
931.5
932.0
932.5
933.0
0.00
0.35
0.54
0.73
0.97
1.26
1.61
2.02
2.46
2.93
3.42
3.94
4.10
4.48
5.04
5.64
6.27
6.93
7.63
Low Flow
CPv-ED
WQv-ED
3.7” eq dia 5.0” eq. dia
H
Q
H
Q
ft
cfs
ft
cfs
0
0.8
1.3
1.8
2.3
2.8
3.3
3.8
4.3
4.8
5.3
5.8
5.9
6.3
-
0
0.33
0.42
0.49
0.55
0.61
0.66
0.71
0.75
0.79
0.83
0.87
0.88
0.91
-
0
0.3
0.8
1.3
1.8
2.3
2.8
3.3
3.8
3.9
4.3
-
0
0.36
0.60
0.76
0.90
1.01
1.12
1.22
1.30
1.32
1.39
-
Riser
High Stage Slot
Orifice
Weir
H
Q
H
Q
ft
cfs
ft
cfs
N/A
-
0.75
-
175
-
0.0
0.5
1.0
1.1
1.5
-
0.0
30.7
86.8
100.1
159.5
-
Per Chapter 391-3-8.04, Dam safety rules do not apply to artificial barriers that
Barrel
Inlet
H
ft
10.6
11.0
11.5
12.0
12.5
13.0
13.5
Q
cfs
88
90
92.5
95
97
100
101.7
Pipe
H
ft
9.7
10.1
10.6
11.1
11.6
12.1
12.6
Q
cfs
112.6
114.9
117.7
120.4
123.1
125.7
128.3
Emergency
Total
Spillway Discharge
H
ft
0.0
0.5
1.0
1.5
2.0
Q
cfs
Q
cfs
0.0
24.0
79.0
154.0
252.0
0.00
0.33
0.42
0.49
0.91
1.20
1.42
1.60
1.76
1.91
32.7
89.0
90.2
92.3
92.5
119.0
176.0
253.5
353.7
Figure 7 Stage-Storage-Discharge Summary
are:
• Classified as a Category II Dam – dams where improper operation or dam
failure would not expect to result in probable loss of human life
• Not in excess of 6 feet in height regardless of storage capacity, or which has
a storage capacity at maximum water storage elevation not in excess of 15
acre-feet, regardless of height.
Check pond classification: Height = 931 -919 = 12’, equals assumed embankment height, Pond will remain Category II or lower.
As reported in Table 1, the preliminary maximum storage volume required is
about 3.33 acre-feet, which is substantially less than the 15 acre-feet exempt
Note: Adequate outfall protection must be provided in the form of a riprap
limit. Therefore, for initial design considerations, no additional dam safety
channel, plunge pool, or combination to ensure non-erosive velocities.
requirements will apply. Once final design elevations and storage volumes have
been determined, a final check for dam rules exemption should be made by the
STEP 9 - DESIGN EMBANKMENT(S) AND SPILLWAY(S)
The 25-year wsel is at 930.8. Set the emergency spillway at elevation 931.0 and
use design information and criteria Earth Spillways (not included in this manual)
• Q100 inflow = 251 cfs.
designer.
STEP 11 - DESIGN INLETS, SEDIMENT FOREBAY(S), OUTLET STRUCTURES,
MAINTENANCE ACCESS, AND SAFETY FEATURES.
Table 2 Summary of Controls Provided
• Try 34’ wide vegetated emergency spillway with 3:1 side slopes.
Control
Element
• @ elevation 932.6, H = 1.5’, Emergency spillway, QES = 172 cfs. Primary
spillway, QPS 100 cfs
Units
• QES + QPS = 272 cfs, will be able to safely convey Qf = 251. (use computer
routing for exact elevations and discharges).
Forebay
• 100 year wsel = 931.7, say 932, so set top of embankment with 1 foot of
freeboard at elevation 933.
STEP 10 - INVESTIGATE POTENTIAL POND HAZARD CLASSIFICATION
Refer to Georgia Department of Natural Resources Rules for Dam Safety in
Appendix H to establish preliminary classification of embankment and whether
special design criteria need to be met.
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Type/Size
of Control
Storage
Provided
Elevation Discharge Remarks
Acre-feet
MSL
0.86
924.0
0
part of WQV
submerged
berm
0.12
924.0
0
included in permanent
pool volume
Water Quality
Extended
Detention
(WQV-ED)
4” pipe, sized to
3.7” equivalent
diameter
0.72
926.0
0.36
part of WQV above
perm. pool, discharge
is average release rate
over 24 hours
Channel
Protection
(CPv-ED)
6” pipe sized to
5.0” equivalent
diameter
2.9
929.0
3.8
930.8
92.4
6.3
931.7
141
Permanent Pool
Overbank Flood Four 7’ x 1.5’
Protection (Qp25) slots on a 8’ x 8’
riser, 36”barrel.
Extreme Flood
34’ wide earth
Protection
spillway
(Qf-100)
Appendix B: Best Management Practice Design Examples
Elevation Storage
(MSL)
(ac-ft)
cfs
1.46
See Figure 8 for profile through principal spillway of the facility.
See Figure 9 for a schematic of the riser.
volume above perm.
pool, discharge is
average release rate
over 24 hours
volume above perm.
pool
volume above perm.
pool
VOL 2
502
Appendix B: Best Management Practice Design Examples
STEP 12 - PREPARE VEGETATION AND LANDSCAPING PLAN
Figure 8. Profile of Principle Spillway
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503
Appendix B: Best Management Practice Design Examples
Figure 9. Schematic of Riser Detail
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VOL 2
504
Appendix B: Best Management Practice Design Examples
Appendix B-2: Bioretention
Area Design Example
Figure 1. Etowah Recreation Center Site Plan
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VOL 2
505
STEP 2 - DETERMINE IF THE DEVELOPMENT SITE AND CONDITIONS ARE
quality requirements of the site. Channel protection and overbank flood control
APPROPRIATE FOR THE USE OF A BIORETENTION AREA.
are not addressed in this example other than quantification of preliminary stor-
Existing ground elevation at the facility location is 922.0 feet, mean sea level.
age volume and peak discharge requirements. It is assumed that the designer
Soil boring observations reveal that the seasonally high water table is at 913.0
can refer to the previous pond example in order to extrapolate the necessary in-
feet and underlying soil is silt loam (ML). Adjacent creek invert is at 912.0 feet.
formation to determine and design the required storage and outlet structures to
meet these criteria. In general, the primary function of bioretention is to provide
storm attenuation. As such, flows in excess of the water quality volume are
STEP 3 - COMPUTE RUNOFF CONTROL TARGET VOLUMES FROM THE
UNIFIED STORMWATER SIZING CRITERIA (EITHER METHOD 3 OR 4 AS
DESCRIBED IN THE DESIGN STEPS)
typically routed to bypass the facility or pass through the facility. Where quantity
Method 3) Compute Runoff Reduction Volume (RRV):
water quality through runoff reduction or treatment (TSS removal) and not large
control is required, the bypassed flows can be routed to conventional detention
basins (or some other facility such as underground storage vaults). Under some
conditions, channel protection storage can be provided by bioretention facilities. Due to the Class C soils identified on-site, an underdrain will be provided in
this practice that will provide a runoff reduction volume credit of 50%.
Computation of Preliminary Stormwater Storage Volumes and Peak Discharges
The layout of the Etowah Recreation Center is shown in Figure 1. This example
assumes that the local community has adopted the unified stormwater sizing
criteria requirements.
• Compute Runoff Coefficient, Rv
RV = 0.05 + (63.3) (0.009) = 0.62
• Compute the target Runoff Reduction Volume (RRV):
RRV
= (1.0”) (RV) (A) / 12
= (1.0”) (0.62) (130,680 ft2) (1ft/12in)
= 6,752 ft3
• Using the RRV calculated above, Compute the minimum volume of the
practice (VPMIN):
(VPMIN)
≥ RRV (target) / (RR%)
STEP 1 – CONFIRM LOCAL DESIGN REQUIREMENTS AND DETERMINE THE
GOAL AND FUNCTION OF THE BMP
It was determined by the local municipality that this best management practice
Note: The Volume Provided (VP) of this practice must be a minimum of 13,504 ft3.
Appendix B: Best Management Practice Design Examples
This example focuses on the design of a bioretention facility to meet the water
= (6,752 ft3) / (50%)
= 13,504 ft3
would be designed by the runoff reduction volume calculation approach (described as Step 3 in Section 4.2)
The total designed volume of the practice must be provided to retain or remove
the stormwater volume associated with the 1.0 inch storm event (this examples
target runoff reduction). Runoff reduction credit will then be utilized by the
designer through an adjusted curve number calculations.
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• Calculate the amount of Runoff Reduction (RRV provided) by the bioretention
area with an underdrain. This information will be needed in the adjusted
curve number calculation, Compute the RRV provided:
RRV (provided) = (RR%) (VP)
= (50%) (13,504 ft3) *
= 6,752 ft3 *
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506
R = RRV (provided) / Basin Area
For stream channel protection, provide 24 hours of extended detention for the
= (6,752 ft3) / A
1-year event.
= 6,752 ft3 / 130,680 ft2 (12 in / 1 ft) = 0.62 inches
(3.4 − 0.2S ) 2
1.56 =
(3.4 + 0.8S)
In order to determine a preliminary estimate of storage volume for channel protection and overbank flood control, it will be necessary to perform hydrologic
calculations using approved methodologies. This example uses the NRCS TR-55
Solve for “S” to back calculate CN: S = 2.49
methodology presented in Section 3.1.5 to determine pre- and post-development peak discharges for the 1-yr, 25-yr, and 100-yr 24-hour return frequency
S = 1000/CN-10: therefore, CN = 80.1
storms.
• Hydrologic output based on the given information provided
Condition
Pre-Developed
Post-Developed
* Post-Developed
(Adjusted CN for RRV)
CN
70
88
1-yr 80.1
25-yr 82.4
Q1-year
inches
cfs
0.95
2.6
2.18
8.2
Q25-year
inches
cfs
3.21
9.8
5.12
18.5
Q100-year
inches
cfs
4.38
13.5
6.48
23.2
• Utilize modified TR-55 approach to compute channel protection storage
volume (based on adjusted CN value calculated above)
Initial abstraction (Ia) for CN of 80.1 is 0.5: [Ia = (200/CN - 2)]
Ia/P = (0.50)/ 3.4 inches = 0.15
1.56
5.9
4.50
16.7
5.86
21.5
Tc = 0.20 hours
100-yr 82.8
qu = 770 csm/in (From Figure 3.1.5-6)
• *Adjusted Curve Number Procedure for Peak Flow Reduction of CPV (Section
3.1.7.5)
Given Q = 2.18 in. and P = 3.4 in., Find “R” and “S” to back
Knowing qu and T (extended detention time), find qo/qI for a Type II rainfall distri-
Appendix B: Best Management Practice Design Examples
Compute Stream Channel Protection Volume (CPv):
bution.
Peak outflow discharge/peak inflow discharge (qo/qi) = 0.023
calculate an adjusted CN
(From Figure 3.3.5-1)
2
(P − 0.2S )
(P + 0.8S)
Modified
Equation 3.1.5)
Q-R =
For a Type II rainfall distribution,
Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi)2 - 0.804(qo/qi)3
(Equation 3.3.9)
Retention storage (expressed in inches) for this basin is calculated by
the following formula:
Where Vs equals channel protection storage (CPv) and Vr equals the volume of
runoff in inches.
Vs/Vr = 0.65
Using Equation 3.3.10, calculate VS…
Therefore, Vs = CPv = 0.65 (1.56 in) (3 ac) (43,560 ft2 / ac) (1 ft / 12 in)
= 11,043 ft3
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VOL 2
507
For a Qin of 16.7 cfs, and an allowable Qout of 9.8 cfs, the Vs necessary for 25-
STEP 6 - DETERMINE SIZE OF BIORETENTION PONDING / FILTER AREA
BASED ON VPMIN
year control is 10,942 ft3, under an adjusted post-developed CN of 82.4. Note
VPMIN = PV + (VES)(N)
that 6.5 inches of rainfall occurs during this event.
Analyze for Safe Passage of 100 Year Design Storm (Qf):
Where: VPMIN
= Volume Provided
At final design, prove that discharge conveyance channel is adequate to con-
(Calculated above, 13,504 ft3)
vey the 100-year event and discharge to receiving waters, or handle it with a
PV
= Ponding Volume (Ponding depth typically 9 inches)
peak flow control structure, typically the same one used for the overbank flood
VES
= Volume of Engineered Soils
protection control.
Table 1 Summary of General Design Information for Etowah Recreation Center
Symbol
Control Volume
RRV
Runoff Reduction
WQV
CPV
Qp25
(Media depth typically 36 inches)
= Porosity of engineered soils, typically 0.25
13,504 ft3 = (Surface Area x 0.75 ft) + (Surface Area x 3 ft x 0.25)
6,752 cf provided (100%)
Water Quality
8,102
Not used in this example
Channel Protection
Overbank Flood
Protection
11,043
10,942
N/A
N
Notes
6,752
Extreme Flood
Protection
Qf
Volume Required
(cubic feet)
13,504 ft3 = (Surface Area x 1.5 ft)
9,003 ft2 = Surface Area
Provide safe passage for the
100-year event in final design
STEP 4 - COMPUTE WQV PEAK DISCHARGE (QWQ)
STEP 5 - SIZE FLOW DIVERSION STRUCTURE, IF NEEDED
Bioretention areas can be either on or off-line. On-line facilities are generally
sized to receive, but not necessarily treat, the 25-year event. Off-line facilities
STEP 7 - SET DIMENSIONS OF FACILITY
Assume a roughly 2 to 1 rectangular shape. Given a filter area requirement of
9,003 sq ft, say facility is roughly 67’ by 135’ for constructability.
Appendix B: Best Management Practice Design Examples
Determine Overbank Flood Protection Volume (Qp25) based on the adjusted CN:
STEP 8 - DESIGN CONVEYANCE TO FACILITY (OFF-LINE SYSTEMS)
This facility is not designed as an off-line system.
are designed to receive a more or less exact flow rate through a weir, channel,
manhole, “flow splitter”, etc. This facility is situated to receive direct runoff from
grass areas and parking lot curb openings and piping for the 25-year event (16.7
cfs – adjusted flow rate), and no special flow diversion structure is incorporated.
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VOL 2
508
Solve for L: L = Q / [(C) (H3/2)] or (16.7 cfs) / [(2.65) (.5)1.5] = 17.8’ (say 18’)
Base underdrain design on 10% of the Surface Area or 900 sq ft. Using 6” perforated plastic pipes surrounded by a three-foot-wide gravel bed, 10’ on center (o.c.).
Outlet protection in the form of riprap or a plunge pool/stilling basin should be
See Figure 2.
provided to ensure non-erosive velocities.
(900 sq ft)/3’ per foot of underdrain = 300’ of perforated underdrain
STEP 11 - PREPARE VEGETATION AND LANDSCAPING PLAN
Choose plants based on factors such as whether native or not, resistance to
STEP 10 - DESIGN EMERGENCY OVERFLOW
drought and inundation, cost, aesthetics, maintenance, etc. Select species loca-
The parking area, curb and gutter is sized to convey the 25-year event to the facili-
tions (i.e., on center planting distances) so species will not “shade out” one anoth-
ty. Should filtering rates become reduced due to facility age or poor maintenance,
er. Do not plant trees and shrubs with extensive root systems near pipe work. A
an overflow weir is provided to pass the 25-year event. Size this weir with 6” of
potential plant list is presented in Appendix D.
head, using the weir equation.
Q = CLH3/2
Where C = 2.65 (smooth crested grass weir)
Q = 16.7 cfs
H = 6”
Appendix B: Best Management Practice Design Examples
STEP 9 - SIZE UNDERDRAIN AREA
Figure 2. Typical Section of Bioretention Facility
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VOL 2
509
Appendix B: Best Management Practice Design Examples
Appendix B-3: Sand Filter Design Example
Figure 1. Georgia Pines Community Center Site Plan
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VOL 2
510
STEP 3 - COMPUTE RUNOFF CONTROL VOLUMES FROM THE UNIFIED
quality treatment (TSS removal) requirements of the site. Channel protection
STORMWATER SIZING CRITERIA
and overbank flood control is not addressed in this example other than quan-
Method 4) Compute Water Quality Volume (WQv):
tification of preliminary storage volume and peak discharge requirements. It
• Compute Runoff Coefficient, Rv
is assumed that the designer can refer to the previous pond example in order
Rv = 0.05 + (63.3) (0.009) = 0.62
to extrapolate the necessary information to determine and design the required
storage and outlet structures to meet these criteria. In general, the primary
• Compute WQv
function of sand filters is to provide water quality treatment and not large storm
WQv = (1.2”) (Rv) (A) / 12
attenuation. As such, flows in excess of the water quality volume are typically
= (1.2”) (0.62) (130,680 ft2) (1ft/12in)
routed to bypass the facility. Where quantity control is required, the bypassed
= 8,102 ft3
flows can be routed to conventional detention basins (or some other facility
such as underground storage vaults). Sand filters do not contribute to a runoff
reduction goal.
Compute Stream Channel Protection Volume, (CPv):
For stream channel protection, provide 24 hours of extended detention for the
1-year event.
Computation of Preliminary Stormwater Storage Volumes and Peak Discharges
The layout of the Georgia Pines Community Center is shown in Figure 1. This
In order to determine a preliminary estimate of storage volume for channel pro-
example assumes that the local community has adopted the unified stormwater
tection and overbank flood control, it will be necessary to perform hydrologic
sizing criteria requirements.
calculations using approved methodologies. This example uses the NRCS TR-55
STEP 1 - CONFIRM LOCAL DESIGN CRITERIA AND APPLICABILITY.
There are no additional requirements for this site.
STEP 2 - DETERMINE IF THE DEVELOPMENT SITE AND CONDITIONS ARE
APPROPRIATE FOR THE USE OF A SURFACE SAND FILTER.
Site Specific Data:
The site area and drainage area to the sand filter is 3.0 acres (3 ac < 10 ac max.)
Existing ground elevation at the facility location is 22.0 feet, mean sea level. Soil
boring observations reveal that the seasonally high water table is at 13.0 feet.
Adjacent creek invert is at 12.0.
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methodology presented in Section 3.1.5 to determine pre- and post-development peak discharges for the 1-yr, 25-yr, and 100-yr 24-hour return frequency
Appendix B: Best Management Practice Design Examples
This example focuses on the design of a surface sand filter to meet the water
storms.
• Hydrologic output based on the given information provided
Note that any hydrologic models using NRCS TR-55 procedures, such as TR-20,
HEC-HMS, or other software platforms, can be used to perform preliminary
hydrologic calculations.
Condition
CN
cfs
0.7
Q25-year
cfs
6.4
Q100-year
cfs
9.7
5.0
14.6
18.9
Q1-year
Pre-Developed
57
inches
0.45
Post-Developed
83
1.94
VOL 2
511
vey the 100-year event and discharge to receiving waters, or handle it with a
peak flow control structure, typically the same one used for the overbank flood
Initial abstraction (Ia) for CN of 83 is 0.41: (TR-55) [Ia = (200/CN - 2)]
protection control.
Ia/P = (0.41)/ 3.6 inches = 0.11
Table 1 Summary of General Design Information for Georgia Pines Community Center
Symbol Control Volume
Volume Required
Notes
(cubic feet)
Tc = 0.15 hours
qu = 605 csm/in (From Figure 3.1.5-7)
Knowing qu and T (extended detention time), find qo/qi for a Type III rainfall
WQV
Water Quality
8,102
CPV
Channel Protection
13,521
Qp25
Overbank Flood
Protection
40,981
Qf
Extreme Flood
Protection
distribution.
Peak outflow discharge/peak inflow discharge (qo/qi) = 0.03
N/A
Provide safe passage for the
100-year event in final design
(From Figure 3.3.5-1)
STEP 4 - COMPUTE WQV PEAK DISCHARGE (QWQ) & HEAD
Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi) 2 - 0.804(qo/qi)3
• Water Quality Volume:
(Equation 3.3.9)
WQv previously determined to be 8,102 cubic feet.
Where Vs equals channel protection storage (CPv) and Vr equals the
• Determine available head (See Figure 3)
volume of runoff in inches.
Low point at parking lot is 23.5. Subtract 2’ to pass Q25 discharge (21.5) and
Vs/Vr = 0.64
a half foot for channel to facility (21.0). Low point at stream invert is 12.0. Set
Using Equation 3.3.10, calculate VS…
Appendix B: Best Management Practice Design Examples
• Utilize modified TR-55 approach to compute channel protection storage
volume
outfall underdrain pipe 2’ above stream invert and add 0.5’ to this value for drain
Therefore, Vs = CPv = 0.64(1.94”)(1/12)(3 ac) (43,560 ft / ac) = 13,521 ft
2
3
(14.5). Add to this value 8” for the gravel blanket over the underdrains, and 18”
for the sand bed (16.67). The total available head is 21.0 - 16.67 or 4.33 feet.
• Define the average ED Release Rate
The above volume, 13,521 ft3, is to be released over 24 hours.
(13,521 ft3) / (24 hrs × 3,600 sec/hr) = 0.16 cfs
Determine Overbank Flood Protection Volume (Qp25):
For a Qin of 14.6 cfs, and an allowable Qout of 6.4 cfs, the Vs necessary for 25year control is 40,981 ft3, under a developed CN of 83. Note that 7.9 inches of
rain fall during this event.
Analyze for Safe Passage of 100 Year Design Storm (Qf):
Therefore, the average depth, hf, is (hf) = 4.33’ / 2, and hf = 2.17’.
The peak rate of discharge for the water quality design storm is needed for the
sizing of off-line diversion structures, such as sand filters and grass channels.
Conventional NRCS TR-55 methods have been found to underestimate the
volume and rate of runoff for rainfall events less than 2”. This discrepancy in
estimating runoff and discharge rates can lead to situations where a significant
amount of runoff by-passes the filtering treatment practice due to an inadequately sized diversion structure and leads to the design of undersized bypass
channels.
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events.
• Using the water quality volume (WQv), a corresponding Curve Number (CN) is
computed utilizing the following equation (See Section 3.1.7.2):
• Read initial abstraction (Ia), compute Ia/P (Ia/P = 0.09)
• Read the unit peak discharge (qu) for appropriate tc (qu = 625)
• Using the water quality volume (WQv), compute the water quality peak
discharge (Qwq)
CN = 1000/[10 + 5P +10QWV - 10(QWV ² + 1.25 QWV P)½]
Qwq = qu*A*WQV
where
P = rainfall, in inches (use 1.2” for the Water Quality Storm)
and
QWV = runoff volume, in inches (equal to 1.2 x RV = 0.744)
where
Qwq = the peak discharge, in cfs
qu = the unit peak discharge, in cfs/mi²/inch
A = drainage area, in square miles
• Once a CN is computed (CN = 95), the time of concentration (tc) is computed
WQv = Water Quality Volume, in watershed inches
• Using the computed CN, tc and drainage area (A), in acres; the peak discharge
(Qwq) for the Water Quality Storm is computed (based on the procedures
identified in Section 3.1.5 (Type III for this example).
For this example, the steps are as follows:
CONCRETE FLUME
LOW
POINT
23.5
Compute modified CN for 1.2” rainfall
P = 1.2”
Q = 1.2 Rv = 1.2 * 0.62 = 0.74”
CN
= 1000/[10+5P+10Q-10(Q2+1.25*Q*P)½]
= 1000/[10+5*1.2+10*0.74-10(0.742+1.25*0.74*1.2)½]
= 95.01
Use CN = 95
21.0
Appendix B: Best Management Practice Design Examples
The following equation can be used to estimate peak discharges for small storm
For CN = 95 and the Tc = 0.15 hours, compute the Qp for a 1.2” storm. With the
CN = 95, a 1.2” storm will produce 0.74” of runoff. Ia = 0.105, therefore Ia/P =
AVAILABLE
EXISTING
0.105/1.2 = 0.088. From Section 3.1.5.7 qu = 625 csm/in, and therefore Qwq =
HEAD = 4.33‘
GRADE
(625 csm/in) (3.0 ac/640ac/sq mi.) (0.74”) = 2.2 cfs.
16.67
18"
FILTER BED
14.5
14.0
WT = 13.0
STREAM
INVERT = 12.0
PVC OUTLET PIPE
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Figure 3. Available Head Diagram
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513
Size a low flow orifice to pass 2.2 cfs with 1.5’ of head using the Orifice equa25' Y R . OV E R F LO W
tion.
E LE V AT ION = 23.0
Q = CA(2gh)1/2 ; 2.2 cfs = (0.6) (A) [(2) (32.2 ft/s2) (1.5’)]1/2
INF LOW
21.0 OV E R F LOW
P IP E
W E IR E LE V AT ION
A = 0.37 sq ft = πd2/4: d = 0.7’ or 8.5”; use 9 inches
Size the 25-year overflow as follows: the 25-year wsel is set at 23.0. Use a
concrete weir to pass the 25-year flow (14.6 cfs) into a grassed overflow chan-
1.5'
nel using the Weir equation. Assume 2’ of head to pass this event. Overflow
channel should be designed to provide sufficient energy dissipation (e.g., riprap,
plunge pool, etc.) so that there will be non-erosive velocities.
9" OR IF IC E
INV . = 19.13
Q = CLH
3/2
14.6 = 3.1 (L) (2’)1.5
L = 1.66’; use L = 2’-0” which sets flow diversion chamber dimension.
Weir wall elev. = 21.0. Set low flow invert at 21.0 [1.5’ + (0.5*9”*1ft/12”)] = 19.13.
STEP 6 - SIZE FILTRATION BED CHAMBER (SEE FIGURE 6):
From Darcy’s Law: Af = WQv (df) / [k (hf + df) (tf)]
where df = 18”
k = 3.5 ft/day
T O S E DIME NT AT ION
C HAMB E R
Figure 4. Flow Diversion Structure
STEP 7- SIZE SEDIMENTATION CHAMBER
From Camp-Hazen equation, for I < 75%: As = 0.066 (WQv)
As = 0.066 (8,102 cubic ft) or 535 sq ft
given a width of 17 feet, the length will be 535’/17’ or 31.5 feet
(use 17’x32’)
STEP 8 - COMPUTE VMIN
Vmin = 0.75 (WQv) or 0.75 (8,102 cubic feet) = 6,077 cubic feet
hf = 2.17’
STEP 9. COMPUTE STORAGE VOLUMES WITHIN ENTIRE FACILITY AND
tf = 40 hours
SEDIMENTATION CHAMBER ORIFICE SIZE:
Af = (8,102 cubic feet) (1.5’) / [3.5 (2.17’ + 1.5’) (40hr/24hr/day)]
Appendix B: Best Management Practice Design Examples
2'
STEP 5 - SIZE FLOW DIVERSION STRUCTURE (SEE FIGURE 4):
Volume within filter bed (Vf): Vf = Af (df) (n); n = 0.4 for sand
Vf = (578 sq ft) (1.5’) (0.4) = 347 cubic feet
Af = 567.7 sq ft; using a 2:1 ratio, say filter is 17’ by 34’ (= 578 sq ft)
Temporary storage above filter bed (Vf-temp): Vf-temp = 2hfAf
Vf-temp = 2 (2.17’) (578 sq ft) = 2,509 cubic feet
Compute remaining volume for sedimentation chamber (Vs):
Vs = Vmin - [ Vf + Vf-temp] or 6,077 - [347 + 2,509] = 3,221 cubic feet
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514
chamber should be between 2 and 4 feet in length and same width as filter bed.
Flow distribution to the filter bed can be achieved either with a weir or multiple
(3,221 cubic ft)/(17’ x 32’) = 5.9’ which is larger than the head
orifices at constant elevation. See Figure 7 for stand pipe details.
available (4.33’); increase the size of the settling chamber, using 4.33
as the design height;
(3,221 cubic ft)/4.33’ = 744 sq ft; 744’/17’ yields a length of 43.8 feet
(say 44’)
New sedimentation chamber dimensions are 17’ by 44’
With adequate preparation of the bottom of the settling chamber (rototil earth,
place gravel, then surge stone), the bottom can infiltrate water into the substrate. The runoff will enter the groundwater directly without treatment. The
stone will eventually clog without protection from settling solids, so use a
removable geotextile to facilitate maintenance. Note that there is 2.17’ of freeboard between bottom of recharge filter and water table.
Provide perforated standpipe with orifice sized to release volume (within sedimentation basin) over a 24 hr period (see Figure 6). Average release rate equals
3,221 ft3/24 hr = 0.04 cfs
Equivalent orifice size can be calculated using orifice equation:
Q = CA(2gh)1/2 , where h is average head, or 4.33’/2 = 2.17’.
0.04 cfs = 0.6*A*(2*32.2 ft/s2*2.17 ft)1/2
A = 0.005 ft2 = πD2/4: therefore equivalent orifice diameter equals 1”.
Recommended design is to cap stand pipe with low flow orifice sized for 24 hr
detention. Over-perforate pipe by a safety factor of 10 to account for clogging.
Note that the size and number of perforations will depend on the release rate
STEP 10 -DESIGN INLETS, PRETREATMENT FACILITIES, UNDERDRAIN SYSTEM, AND OUTLET STRUCTURES
STEP 11 - COMPUTE OVERFLOW WEIR SIZES
Assume overflow that needs to be handled is equivalent to the 9” orifice discharge under a head of 3.5 ft (i.e., the head in the diversion chamber associated
with the 25-year peak discharge).
Q = CA(2gh)½
Q = 0.6(0.44 ft2)[(2)(32.2 ft/s2)(3.5 ft)]½
Q = 3.96 cfs, say 4.0 cfs
For the overflow from the sediment chamber to the filter bed, size to pass 4 cfs.
Weir equation: Q = CLh3/2, assume a maximum allowable head of 0.5’
4.0 = 3.1 * L * (0.5 ft) 3/2
L = 3.65 ft, Use L = 3.75 ft.
Appendix B: Best Management Practice Design Examples
Compute height in sedimentation chamber (hs): hs = Vs/As
Similarly, for the overflow from the filtration chamber to the outlet of the facility,
size to pass 4.0 cfs.
Weir equation: Q = CLh3/2, assume a maximum allowable head of 0.5’
4.0 = 3.1 * L * (0.5 ft) 3/2
L = 3.65 ft, Use L = 3.75 ft.
Adequate outlet protection and energy dissipation (e.g., riprap, plunge pool, etc.)
should be provided for the downstream overflow channel.
needed to achieve 24 hr detention. A multiple orifice stage-discharge relation
needs to be developed for the proposed perforation configuration. Stand pipe
should discharge into a flow distribution chamber prior to filter bed. Distribution
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515
Appendix B: Best Management Practice Design Examples
Figure 5. Surface Sand Filter Site Plan
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516
Appendix B: Best Management Practice Design Examples
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Figure 6. Plan and Profile of Surface Sand Filter
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Appendix B: Best Management Practice Design Examples
Figure 7. Perforated Stand Pipe Detail
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518
Appendix B: Best Management Practice Design Examples
Appendix B-4: Infiltration
Trench Design Example
Figure 1. Georgia Pines Community Center Site Plan
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519
reduction goals and water quality requirements of the site. Channel protection
Table 1 Infiltration Feasibility
Criteria
Status
Infiltration rate (fc) greater than or equal to 0.5
inches/hour.
Infiltration rate is 1.0 inches/hour. OK.
is assumed that the designer can refer to the previous pond example in order
Soils have a clay content of less than 20% and a
silt/clay content of less than 40%.
Sandy Loam meets both criteria.
to extrapolate the necessary information to determine and design the required
Infiltration cannot be located on slopes greater
than 6% or in fill soils.
Slope is <1%; not fill soils. OK.
Hotspot runoff should not be infiltrated.
Not a hotspot land use. OK.
Infiltration is prohibited in karst topography.
Not in karst. OK.
The bottom of the infiltration facility must be
separated by at least two feet vertically from the
seasonally high water table.
Elevation of seasonally high water table: 13’
Elevation of BMP location: 20’.
The difference is 7’.
Thus, the trench can be up to 5’ deep. OK.
Infiltration facilities must be located 100 feet
horizontally from any water supply well.
No water supply wells nearby. OK.
and overbank flood control is not addressed in this example other than quantification of preliminary storage volume and peak discharge requirements. It
storage and outlet structures to meet these criteria. In general, the primary
function of infiltration trenches is to provide water quality treatment and/or runoff reduction and groundwater recharge, not large storm attenuation. As such,
flows in excess of the water quality volume are typically routed to bypass the
facility. Where quantity control is required, the bypassed flows can be routed
to conventional detention basins (or some other facility such as underground
storage vaults). The layout of the Georgia Pines Community Center is shown in
Figure 1. Due to the Class B soils identified on-site, the infiltration trench will be
Maximum contributing area generally less than 2 3 acres. OK.
or 5 acres. (Optional)
designed for runoff reduction. The runoff reduction credit given for infiltration
Setback 25 feet down-gradient from structures.
trenches is 100%.
Computation of Preliminary Stormwater Storage Volumes and Peak Discharges
The layout of the Georgia Pines Community Center is shown in Figure 1.
STEP 1 - CONFIRM LOCAL DESIGN CRITERIA AND APPLICABILITY
It was determined by the local municipality that this best management practice
would be designed by the runoff reduction volume calculation approach (described as Step 3 in Section 4.12)
Fifty feet straight-line distance between the
parking lot and the tree line. OK if the trench is
25’ wide or narrower.
STEP 2 - DETERMINE IF THE DEVELOPMENT SITE AND CONDITIONS ARE
APPROPRIATE FOR THE USE OF AN INFILTRATION TRENCH
Site Specific Data:
Table 2 presents site-specific data, such as soil type, percolation rate, and slope,
for consideration in the design of the infiltration trench.
Table 2 Site Specific Data
Criteria
Value
the stormwater volume associated with the 1.0 inch storm event (this examples
Soil
Sandy Loam
target runoff reduction). Runoff reduction credit will then be utilized by the
Percolation Rate
1”/hour
Ground Elevation at BMP
20’
Seasonally High Water Table
13’
Stream Invert
12’
Soil slopes
<1%
The total designed volume of the practice must be provided to retain or remove
designer through an adjusted curve number calculations.
Table 1, below, summarizes the requirements that need to be met to successfully implement infiltration practices. On this site, infiltration is feasible, with
restrictions on the depth and width of the trench.
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Appendix B: Best Management Practice Design Examples
This example focuses on the design of an infiltration trench to meet the runoff
VOL 2
520
methodology presented in Section 3.1.5 to determine pre- and post-develop-
Method 3) Compute Runoff Reduction Volume (RRV):
• Hydrologic output based on the given information provided
• Compute Runoff Coefficient, Rv
RV = 0.05 + (63.3) (0.009) = 0.62
• Compute the target Runoff Reduction Volume (RRV):
RRV = (1.0”) (RV) (A) / 12
ment peak discharges for the 1-yr, 25-yr, and 100-yr 24-hour return frequency
storms.
Condition
CN
Pre-Developed
Post-Developed
57
83
* Post-Developed
(Adjusted CN for RRV)
1-yr
74.2
25-yr 78
100-yr 78
Q1-year
inches
cfs
0.45
0.7
1.94
5.0
1.32
Q25-year
inches
cfs
2.93
6.4
5.88
14.6
3.3
5.26
13.4
Q100-year
inches
cfs
4.34
9.7
7.71
18.9
7.09
17.7
= (1.0”) (0.62) (130,680 ft2) (1ft/12in)
= 6,752 ft3
• *Adjusted Curve Number Procedure for Peak Flow Reduction of CPV (Section
3.1.7.5)
• Using the RRV calculated above, Compute the minimum volume of the
practice (VPMIN):
Given Q = 1.94 in. and P = 3.6 in., Find “R” and “S” to back calculate an
adjusted CN
(VPMIN) ≥ RRV (target) / (RR%)
(P − 0.2S ) 2
(P + 0.8S)
(Modified
Equation 3.1.5)
Q-R =
= (6,752 ft3) / (100%)
= 6,752 ft3
• Calculate the amount of Runoff Reduction (RRV provided) by the infiltration
practice. This information will be needed in the adjusted curve number
calculation, Compute the RRV provided:
RRV (provided) = (RR%) (VP)
Retention storage (expressed in inches) for this basin is calculated by
Appendix B: Best Management Practice Design Examples
STEP 3 - COMPUTE RUNOFF CONTROL TARGET VOLUMES FROM THE
UNIFIED STORMWATER SIZING CRITERIA (EITHER METHOD 3 OR 4 AS
DESCRIBED IN THE DESIGN STEPS)
the following formula:
R = RRV (provided) / Basin Area
= (6,752 ft3) / A
= (100%) (6,752 ft )*
= 6,752 ft3 *
3
Compute Stream Channel Protection Volume (CPv):
For stream channel protection, provide 24 hours of extended detention for the
= 6,752 ft3 / 130,680 ft2 (12 in / 1 ft) = 0.62 inches
(3.6 − 0.2S ) 2
1.32 =
(3.6 + 0.8S)
1-year event.
Solve for “S” to back calculate CN: S = 3.48
In order to determine a preliminary estimate of storage volume for channel pro-
S = 1000/CN-10: therefore, CN = 74.2
tection and overbank flood control, it will be necessary to perform hydrologic
calculations using approved methodologies. This example uses the NRCS TR-55
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volume (based on adjusted CN value calculated above)
Initial abstraction (Ia) for CN of 74.2 is 0.7: [Ia = (200/CN - 2)]
Ia/P = (0.70)/ 3.6 inches = 0.19
Tc = 0.15 hours
qu = 575 csm/in (From Figure 3.1.5-7)
Knowing qu and T (extended detention time), find qo/qI for a Type II rainfall
distribution.
Peak outflow discharge/peak inflow discharge (qo/qi) = 0.035
(From Figure 3.3.5-1)
Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi) 2 - 0.804(qo/qi) 3
(Equation 3.3.9)
Runoff Reduction
6,752
100% provided
WQV
Water Quality
8,102
Not required in this example
CPV
9,056
Qp25
Channel Protection
Overbank Flood
Protection
Qf
Extreme Flood
Protection
22,175
N/A
Provide safe passage for the
100-year event in final design
STEP 4 - COMPUTE WQV PEAK DISCHARGE (QWQ)
STEP 5 - SIZE FLOW DIVERSION STRUCTURE, IF NEEDED
to receive, but not necessarily treat, the 25-year event. Off-line facilities are
designed to receive a more or less exact flow rate through a weir, channel, manhole, “flow splitter”, etc. This facility is situated to receive direct runoff from grass
areas and parking lot curb openings and piping for the 25-year event (14.6 cfs –
unadjusted flow rate), and no special flow diversion structure is incorporated.
Where Vs equals channel protection storage (CPv) and Vr equals the
volume of runoff in inches.
STEP 6 - SIZE THE INFILTRATION TRENCH BASED ON VPMIN
VPMIN = PV + (VG)(N)
Using Equation 3.3.10, calculate VS…
Therefore, Vs = CPv = 0.63 (1.32 in) (3 ac) (43,560 ft2 / ac) (1 ft / 12 in
= 9,056 ft3
Determine Overbank Flood Protection Volume (Qp25) based on the adjusted CN:
For a Qin of 13.4 cfs, and an allowable Qout of 6.4 cfs, the Vs necessary for 25year control is 22,175 ft3, under an adjusted developed CN of 78. Note that 7.9
inches of rain fall during this event.
Analyze for Safe Passage of 100 Year Design Storm (Qf):
At final design, prove that discharge conveyance channel is adequate to convey the
100-year event and discharge to receiving waters, or handle it with a peak flow control structure, typically the same one used for the overbank flood protection control.
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RRV
Infiltration areas can be either on or off-line. On-line facilities are generally sized
For a Type III rainfall distribution,
Vs/Vr = 0.63
Table 3 Summary of General Design Information for Georgia Pines Community Center
Symbol Control Volume
Volume Required
Notes
(cubic feet)
Where: VPMIN
PV
= Volume Provided (Calculated above, 6,752 ft3)
= Ponding Volume (Ponding depth typically 9 inches)
VG
= Volume of Gravel (depth typically 2-10 feet)
= Porosity of gravel, typically 0.4
N
Appendix B: Best Management Practice Design Examples
• Utilize modified TR-55 approach to compute channel protection storage
Note: Assume a maximum depth value of 5 feet, and work through iterations as
needed to fit the practice to the available space within the site. Recall that Table
1 indicates the practice cannot exceed 5 feet due to the proximity of the water
table.
6,752 ft3 = (Surface Area x 0.75 ft) + (Surface Area x 5 ft x 0.4)
6,752 ft3 = (Surface Area x 2.75 ft)
2,456 ft2 = Surface Area
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522
Since the width can be no greater than 25’ (see Table 1; feasibility), determine
the length:
L = 2,456 ft2 / 25 ft
• Assume a trapezoidal channel with 10’ channel bottom, 5H:1V side slopes,
and a Manning’s n value of 0.30. Use a nomograph to size the swale; assume
a 0.5% slope.
• Use a peak discharge of 1.71 cfs (Peak flow for WQv, or 8102 ft3)
L = 98.25 feet
• Compute velocity: V=0.99 ft/s
Say this facility is roughly 25’ wide x 98.25’ long x 5’ deep
• To retain the RRv (6,752 ft3 ) for 5 minutes, the length would be 300 feet.
STEP 8 - SIZE PRETREATMENT VOLUME AND DESIGN PRETREATMENT
MEASURES
As rule of thumb, size pretreatment to treat 10% of the WQv. Therefore, treat
8,102 × 0.10 = 810.2 ft3.
• Since the swale needs to treat the 10% of the water quality volume minus the
treatment provided by the plunge pool and the gravel layer, or 479.2 ft3, the
length should be pro-rated to reflect this reduction.
Therefore, adjust length:
For pretreatment, use a pea gravel filter layer with filter fabric, a plunge pool and
L= (300 ft)(479.2 ft3/8,102 ft3) =17.7 feet. Use 20 feet.
a grass channel.
Pea Gravel Filter
STEP 8 - DESIGN SPILLWAY(S)
The pea gravel filter layer covers the entire trench with 2” (see Figure 2). Assum-
Adequate stormwater outfalls should be provided for the overflow associated
ing a porosity of 0.32, the water quality treatment in the pea gravel filter layer is:
with the 25-year and larger design storm events, ensuring non-erosive velocities
on the down slope.
WQfilter= (0.32)(2”)(1 ft/12 inches)(2,456 ft ) = 131 ft
2
3
Appendix B: Best Management Practice Design Examples
STEP 7 - SET DIMENSIONS OF FACILITY
Plunge Pools
Use a 10’X10’ plunge pool with average depths of 2’.
Total WQpool= (10’ x 10’)(2’) = 200 ft3
Grass Channel
Thus, the grass channel needs to treat at least (810.2 – 131 - 200) ft3 = 479.2 ft3
Use a Manning’s equation nomograph or software to size the swale.
The channel should treat 479.2 ft3
Figure 2. Infiltration Trench Cross Section
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Appendix B: Best Management Practice Design Examples
Appendix B-5: Enhanced Swale
Design Example
Base Data
Site Area = Total Drainage Area (A) = 3.0 ac (130,680ft²)
Impervious Area = 1.9 ac; or 1 = 1.9/3.0 = 63.3%
Soils Type “C”
1-yr, 24-hour Rainfall “P” = 3.4 inches
25-yr, 24-hour Rainfall “P” = 6.5 inches
100-yr, 24-hour Rainfall “P” = 7.9 inches
Hydrologic Data
Pre
CN
70
tc
.39
Post
88
.20
Figure 1. Etowah Recreation Center Site Plan
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flood control is not addressed in this example other than quantification of
Existing ground elevation at the facility location is 922.0 feet, mean sea level.
preliminary storage volume and peak discharge requirements. It is assumed that
Soil boring observations reveal that the seasonally high water table is at 913.0
the designer can refer to the previous pond example in order to extrapolate the
feet and underlying soils are silt loams (ML). Adjacent creek invert is at 912.0
necessary information to determine and design the required storage and outlet
feet.
structures to meet these criteria. In general, the primary function of dry swales
is to provide water quality treatment and groundwater recharge and not large
Two swales will be designed to carry flow to the existing stream, one around
storm attenuation. As such, flows in excess of the water quality volume are
each side of the development.
typically routed to bypass the facility. Where quantity control is required, the
.
STEP 3 - COMPUTE RUNOFF CONTROL TARGET VOLUMES FROM THE
UNIFIED STORMWATER SIZING CRITERIA (EITHER METHOD 3 OR 4 AS DESCRIBED IN THE DESIGN STEPS)
bypassed flows can be routed to conventional detention basins (or some other
facility such as underground storage vaults). Due to the Class C soils identified
on-site, an underdrain will be provided in this practice that will provide a runoff
reduction volume credit of 50%.
Computation of Preliminary Stormwater Storage Volumes and Peak Discharges
The layout of the Etowah Recreation Center is shown in Figure 1. This example
assumes that the local community has adopted the unified stormwater sizing
criteria requirements.
Method 4) Compute Water Quality Volume (WQV):
• Compute Runoff Coefficient, Rv
RV = 0.05 + (63.3) (0.009) = 0.62
• Compute Water Quality Volume (WQV):
WQV = (1.2”) (RV) (A) / 12
= (1.2”) (0.62) (130,680 ft2) (1ft/12in)
STEP 1 - CONFIRM LOCAL DESIGN REQUIREMENTS AND DETERMINE THE
GOAL AND FUNCTION OF THE BMP
It was determined by the local municipality that this best management practices
would be designed by the water quality volume calculation approach (described
as Step 4 in Section 4.8)
The total designed volume of the practice must be provided to remove 80% TSS
from the 1.2 inch storm event. Runoff reduction credit may still be utilized by
the designer through an adjusted curve number calculations.
Appendix B: Best Management Practice Design Examples
quality treatment requirements of the site. Channel protection and overbank
STEP 2 - DETERMINE IF THE DEVELOPMENT SITE AND CONDITIONS ARE
APPROPRIATE FOR THE USE OF AN ENHANCED DRY SWALE SYSTEM
This example focuses on the design of a dry enhanced swale to meet the water
= 8,102 ft3
Note: The Volume Provided (VP) of this practice must be a minimum of 8,102 ft3.
• Even though the water quality volume calculation approach was used in this
design, some amount of Runoff Reduction (RRV provided) is provided by the
enhanced dry swale with an underdrain. This information will be needed in
the adjusted curve number calculation, Compute the RRV provided:
RRV (provided) = (RR%) (VP)
= (50%) (8,102 ft3) *
= 4,051 ft3 *
There is also a local requirement that the 25-year storm is contained within the
top of banks of all channels, including these enhanced swale controls.
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Solve for “S” to back calculate CN: S = 1.98
For stream channel protection, provide 24 hours of extended detention for the
S = 1000/CN-10: therefore, CN = 83.5
1-year event.
In order to determine a preliminary estimate of storage volume for channel pro-
• Utilize modified TR-55 approach to compute channel protection storage
volume (based on adjusted CN value calculated above)
tection and overbank flood control, it will be necessary to perform hydrologic
Initial abstraction (Ia) for CN of 83.5 is 0.4: [Ia = (200/CN - 2)]
calculations using approved methodologies. This example uses the NRCS TR-55
methodology presented in Section 3.1.5 to determine pre- and post-develop-
Ia/P = (0.40)/ 3.4 inches = 0.12
ment peak discharges for the 1-yr, 25-yr, and 100-yr 24-hour return frequency
Tc = 0.20 hours
storms.
qu = 800 csm/in (From Figure 3.1.5-6)
• Hydrologic output based on the given information provided
Condition
CN
Pre-Developed
Post-Developed
70
88
* Post-Developed
(Adjusted CN for RRV)
1-yr 83.5
25-yr 84.7
Q1-year
inches
cfs
0.95
2.6
2.18
Q25-year
inches
cfs
3.21
9.8
8.2
5.12
18.5
Q100-year
inches
cfs
4.38
13.5
6.48
bution.
23.2
Peak outflow discharge/peak inflow discharge (qo/qi) = 0.023
1.81
6.9
4.75
17.5
6.11
22.2
(From Figure 3.3.5-1)
100-yr 84.9
• *Adjusted Curve Number Procedure for Peak Flow Reduction of CPV (Section
3.1.7.5)
Given Q = 2.18 in. and P = 3.4 in., Find “R” and “S” to back calculate an
adjusted CN
2
(P − 0.2S )
(P + 0.8S)
(Modified
Equation 3.1.5)
Q-R =
Retention storage (expressed in inches) for this basin is calculated by
the following formula:
Knowing qu and T (extended detention time), find qo/qI for a Type II rainfall distri-
For a Type II rainfall distribution,
Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi) 2 - 0.804(qo/qi) 3
Appendix B: Best Management Practice Design Examples
Compute Stream Channel Protection Volume (CPv):
(Equation 3.3.9)
Where Vs equals channel protection storage (CPv) and Vr equals the
volume of runoff in inches.
Vs/Vr = 0.65
Using Equation 3.3.10, calculate VS…
Therefore, Vs = CPv = 0.65 (1.81 in) (3 ac) (43,560 ft2 / ac) (1 ft / 12 in)
= 12,812 ft3
R = RRV (provided) / Basin Area
= [(RR%)(VP)] / A
= 4,051 ft3/130,680 ft2 (12 in / 1 ft) = 0.37 inches
(3.4 − 0.2S ) 2
1.81 =
(3.4 + 0.8S)
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Appendix B: Best Management Practice Design Examples
Determine Overbank Flood Protection Volume (Qp25) based on the adjusted CN:
For a Qin of 17.5 cfs, and an allowable Qout of 9.8 cfs, the Vs necessary for 25-year
control is 13,498 ft3, under an adjusted post-developed CN of 84.7. Note that
6.5 inches of rainfall occurs during this event.
Analyze for Safe Passage of 100 Year Design Storm (Qf):
At final design, prove that discharge conveyance channel is adequate to convey the 100-year event and discharge to receiving waters, or handle it with a
peak flow control structure, typically the same one used for the overbank flood
protection control.
Table 1 Summary of General Design Information for Etowah Recreation Center
Symbol
Control Volume
Volume Required
(cubic feet)
RRV
Runoff Reduction
6,752
4,051 cf provided (60%)
WQV
Water Quality
8,102
Swale sized for this volume
CPV
Channel Protection
12,812
Qp25
Overbank Flood
Protection
13,498
Qf
Extreme Flood
Protection
N/A
Notes
Provide safe passage for the
100-year event in final design
STEP 4 - DETERMINE PRETREATMENT VOLUME AND STORAGE VOLUME
Size two shallow forebays at the head of the swales equal to 0.05” per impervious acre of drainage (each) (Note, total recommended pretreatment requirement is 0.1”/imp acre). (1.9 ac) (0.05”) (1ft/12”) (43,560 sq ft/ac) = 344.9 ft3 for
each forebay.
Figure 2. Enhanced Dry Swale Site Plan
Use a 2’ deep pea gravel drain at the head of the swale to provide erosion protection and to assist in the distribution of the inflow. There will be no side inflow
nor need for pea gravel diaphragm along the sides.
Required: bottom width, depth, length, and slope necessary to store required
volume with less than 18” of ponding (see Figure 2 for representative site plan).
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Using a engineered soil porosity of 0.25, the maximum volume able to be pro-
WQV depth of 18”. Control for this swale will be a shallow concrete wall with
vided of the practice can be calculated as follows:
a low flow orifice, trash rack located per Figures 2 and 3. Per the site plan, we
have about 1,400’ of swale available, if the swale is put in with two tails. The
VPMAX = 8,680 ft3 + (21,000 ft3 x 0.25) = 13,930 ft3
outlet control will be set at the existing invert minus three feet (922.0 - 3.0 =
919.0). The existing uphill invert for the northwest fork is 924.0 (length of 500’),
13,930 cubic feet > Minimum required volume of 8,102 ft3; OK
the invert for the northeast fork is 928.0 (at a length of 900’).
To reduce the footprint of the enhanced swale area, or media portion of the
Slope of northwest fork is (924 - 919)/500’ = 0.01 or 1.0%
swale, the new minimum required length could be back-calculated by using the
Slope of northeast fork is (928 - 919)/900’ = 0.01 or 1.0%
minimum required volume above:
Minimum slope is 1.0 % [okay]
(Min. required volume) / (sum of the cross-sectional areas) = Min. length of BMP
For an enhanced swale, use the following equation to calculate volume provid-
8,102 ft3 / (6.2 ft2 + (15 ft2 x 0.25)) = 815 Linear Feet (Revision to Figure 2)
ed (VP):
VP = (PV + VES (N))
New lengths of the swales would be the following:
Where:
Northwest fork: 290 feet
VP
=
Volume provided (temporary storage)
PV
=
Ponding Volume
VES =
N
=
Volume of Engineered Soils
Northeast fork: 525 feet
VP = 8,102 ft3
RRV(Provided) = (RR%)(VP) = (50%)(8,102 ft3) = 4,051 ft3
Appendix B: Best Management Practice Design Examples
Assume a trapezoidal channel with an average WQV depth of 9” and a maximum
Porosity (typically 0.25 for soil media)
To calculate the ponding volume (PV), use the trapezoidal section with a bottom
width of 6’, an average depth of 9”, 3:1 side slopes, compute a cross-sectional
area of (6’) (0.75’) + (0.75’) (2.25’) = 6.2 ft2 (see Figure 4). Using this cross-sectional area, multiply by the provided length to find the total ponding volume:
(6.2 ft2) x (1,400 ft) = 8,680 ft3
To calculate the volume of engineered soil media (VES), use the base width and
depth of media and compute a cross-sectional area of (6’) (2.5’) = 15 ft2 (See
Figure 4). Using this cross-sectional area, multiply by the provided length to find
the total volume of engineered soils: (15 ft2)(1,400 ft) = 21,000 ft3
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Appendix B: Best Management Practice Design Examples
STEP 5 - COMPUTE NUMBER OF CHECK DAMS (OR SIMILAR STRUCTURE)
REQUIRED TO DETAIN WQV (SEE FIGURE 4)
For the northwest fork, 290 ft @ 1.0% slope, and maximum depth at 18”, place
checkdams at: 1.5’/0.01 = 150’
place at 150’, 1 required
For the northeast fork, 525 ft @ 1.0% slope, and maximum 18” depth, place
checkdams at 1.5’/0.01 = 150’
place at 150’, 3 required
Figure 3 Control Structure at End of Swale
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In order to ensure that the swale will draw down within 24 hours, the planting soil
will need to pass a maximum rate of 1.5’ in 24 hours (k = 1.5’ per day). Provide 6”
perforated underdrain pipe and gravel system below soil bed (see Figure 4)
Appendix B: Best Management Practice Design Examples
STEP 6 - CALCULATE DRAW-DOWN TIME
Figure 4 Trapezoidal Dry Swale Section
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STEP 8 - DESIGN LOW FLOW ORIFICE AT DOWNSTREAM HEADWALL AND
Given the local requirements to contain the 25-year flow within banks with
CHECKDAMS (SEE FIGURE 3)
freeboard. In this example only the 25-year flow will be checked assuming that
Design orifice to pass 8,102 cubic feet in 6 hours.
lower flows will be handled. The 25-year flow is 17.1 cfs, assume that 30% goes
through northwestern swale (5.1 cfs) and 70% goes through the northeastern
8,102 cubic feet/ [(6 hours) (3600 sec/hour)] = 0.38 cfs
swale (12.0 cfs). Design for the larger amount (12.0 cfs). From a separate com-
Use Orifice equation:
Q = CA(2gh)1/2
puter analysis using an open channel flow calculator, with a slope of 1.0%, the
25-year velocity will be 2.8 feet-per-second at a depth of 0.60 feet, provide an
Assume h = 1.5’
additional 0.5’ of freeboard above top of checkdams or about 1.1’ (total channel
A = (0.38 cfs) / [(0.6) ((2) (32.2 ft/s2) (1.5’))1/2]
depth = 2.6’).
A = 0.06 sq ft, dia = 0.29 feet or 3.4” use 3” - 3.5” orifice
Find 25-year overflow weir length required: (weir eq. Q= CLH3/2),
where C = 3.1, Q25 = 17.1 cfs, H =1.1;
Provide 3” v-notch slot in each check dam
Rearranging the equation yields:
L = 17.1 cfs/ (3.1*1.11.5) = 4.8’ Use 5 ft
STEP 9 - PREPARE VEGETATION AND LANDSCAPING PLAN
Appendix B: Best Management Practice Design Examples
STEP 7 - CHECK 2-YEAR AND 25-YEAR FLOWS FOR VELOCITY EROSION
POTENTIAL AND FREEBOARD
Figure 5 Profile of Northwest Fork Dry Swale
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Appendix C: Nomographs & Design Aids
Appendix C: Nomographs & Design Aids
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C-1 Culvert Design Charts and Nomographs
1991.
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CHART 1
Appendix C: Nomographs & Design Aids
All of the figures in this section are from the AASHTO Model Drainage Manual,
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C-2 Open Channel Design Figures
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Appendix D: Planting & Soil Guidance
The proper selection of plants and soil composition specifications is a critical
aspect to the function and success of many stormwater BMPs.
Plants: Plants reduce erosion, increase pollutant removal, reduce runoff velocity, enhance infiltration, control access and contribute to wildlife habitat. Plants
can also improve the appearance of stormwater facilities.
Soil: The soil composition of many stormwater BMPs is vital to their relative
success or failure. Properly specified, mixed and placed soil and/or planting
media aids in infiltration and natural detention as well as plant health. The
purpose of this Appendix is to provide guidance on plant selection and planting media for stormwater facilities. This appendix is divided into the following
Appendix D: Planting & Soil Guidance
Introduction
sections:
Section 1: General Planting Guidance
Section 2: Site Characteristics and Soil - discusses the physical site factors
and considerations involved in selecting plant material for stormwater facility
landscaping.
Section 3: Plant Selection for Stormwater Facilities - reviews key factors to
consider in selecting plant material for stormwater landscaping, including hardiness, physiographic regions, inundation tolerance, and other factors.
Section 4: Specific Landscaping Criteria for BMPs
Section 5: Plant List for Stormwater BMPs
Section 6: Minimum Requirements for Landscape Plans
Section 7: Plant Establishment and Maintenance
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Below are general guidelines that should be
followed in the planting of any stormwater control or conveyance facility. Native plant species
should be specified over exotic or foreign species
because they are well adapted to local soil and
climactic conditions.
• A variety of species should be utilized for the
best performance of the stormwater BMP and
for survivability of the plants.
• Trees, shrubs and grasses should be placed and
spaced according to their mature size.
• Turf areas should be minimized or eliminated
because of the maintenance (mowing,
pesticides and fertilizers) required after
installation. Recent research indicates
that disturbed soils and managed turf also
impact stormwater quality (Law et al, 2008).
Groundcovers can be utilized as an alternative.
• Trees and shrubs should not be planted within
15 feet of the toe of the slope of a dam or from
a non-clogging, low flow orifice.
• Tree species with tap roots that seek water
(e.g., willow and poplar), should be avoided
within 50 feet of sewer or water pipes or
manmade drainage structures that continually
hold water.
• Woody vegetation should not be planted on
an embankment and herbaceous embankment
plantings should be limited to 10” in height
to allow inspectors to look for integrity
compromising burrowing rodents.
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• Evergreen trees and trees which produce
relatively little leaf-fall (once per year) are
preferred in areas draining to a detention
practice.
• Site characteristics (e.g. views to be preserved,
buffer, screens, slopes, existing soil) and plant
characteristics, including plant maintenance
should be considered when selecting plant
material. Larger plant material should not
block views at entrances, exits, or difficult road
curves.
• Access for maintenance and blocking of access
to pedestrians should both be considered.
Appendix D: Planting & Soil Guidance
Section 1: General Planting
Guidance
• Drought tolerant species are recommended.
• Plants should be purchased from a source
near the installation site for the best chance of
survival.
• Existing vegetation should be considered
before siting of stormwater BMP and preserved
when possible.
• Water tolerances of existing vegetation should
be analyzed.
• Water availability should be considered
(see Section 7: Plant Establishment and
Maintenance)
• Educational signage should be provided
near stormwater BMPs to help educate the
public and preserve/identify existing natural
vegetation.
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A development site’s characteristics often will
help to determine which plant materials and
planting methods the site designer should select
and will help improve plant establishment. Primary site considerations include:
Utilizing On-Site Soils: A soil test will help to
there is no universal ASTM standard for planting
determine if on-site soils can be utilized for your
media. Links are provided below for different
BMP. There are drawbacks to utilizing on-site soils
planting media.
which include the staging area required to mix
• See guidance for bioretention media
composition: http://stormwater.pca.state.
mn.us/index.php/Design_criteria_for_
bioretention
soils, weather conditions making the blending
process difficult, availability of clean organic content and control over the performance of the soil
created which is particularly important for bioret-
1.
Soil Characteristics
2.
Drainage
can be utilized, recognize that both weight and
3.
Slope
volume of soil media components are routinely
4.
Orientation
Soils provide nutrients for plant growth, filtration
of pollutants as well as the storage/release of
storm flows which makes soils a critical piece of a
successful stormwater BMP.
Soil Tests: To determine the characteristics of soil
on your site, samples of on-site soils should be
analyzed by experienced and qualified individuals
who can explain the results and provide information on any soil amendments that are required
for the intended stormwater BMP. For information about testing your own soil and to order a
soil test kit you can contact your local extension
service (UGA-Cooperative Extension Office here:
http://extension.uga.edu/about/county/index.
cfm or click here: http://aesl.ces.uga.edu/soiltest123/Georgia.htm to order a soil test kit). The
test should indicate soil PH, content and texture.
Where poor soils can’t be amended, seed mixes
and plant material must be selected to establish
ground cover as quickly as possible.
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ention area soils. If it is decided that on-site soils
specified and stay consistent between the two.
Utilizing a Manufactured Soil Media: Several
companies in Georgia are now manufacturing soil
media for BMP’s and can create mixes that meet
the nutrient levels and textures that you require.
The manufacturer should be able to provide a
• http://www.lowimpactdevelopment.org/epa03/
biospec_left.htm
• North Carolina State University Stormwater
Engineering Group Soil Media Mix: https://
www.bae.ncsu.edu/topic/bioretention/designsoil.html
Appendix D: Planting & Soil Guidance
Section 2: Site Characteristics
and Soil
• Page 367 / G1 http://www.sandiegocounty.
gov/content/dam/sdc/dpw/WATERSHED_
PROTECTION_PROGRAM/susmppdf/lid_
handbook_2014sm.pdf
specification for their mix and to show the soil
components, installation instructions, and infiltration rates (if applicable).
Soil Performance: Different planting media with
different ratios of organic content, sand/clay content, permeability and pH will perform in different
ways. Higher organic content is better for plants
but can cause nutrient export. Higher sand content will increase permeability but can decrease
the ability of the soil to hold proper nutrients to
sustain plant life. Increasing or decreasing the
clay in the planting media can slow or speed
infiltration. An analysis of desired plant material,
specific site conditions and desired function of
the proposed BMP will need to be performed to
determine the mix that is best suited. At this time
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Infiltration Testing Requirements
Tiered Testing Approach
The purpose of green infrastructure (GI) or green
Infiltration tests shall not be conducted in the rain
A tiered approach to infiltration testing recogniz-
management practices (GMPs) is to store, treat
or within 48 hours of significant rainfall events
es the importance of accurate in situ conditions
and infiltrate stormwater into the soil, mimicking
(greater than 0.5 inches), or when the tempera-
while screening out sites unsuitable for infiltration
natural systems. Subsurface conditions are key in
ture is below freezing.
practices and thereby reducing soil investigation
and testing costs. The following tiers include:
assessing the feasibility of infiltration in the design
of GI. Infiltration capacity testing and design of
Infiltration testing performed; including testing
best management practices (BMPs) that rely on
procedures followed, shall be documented and
infiltration to treat the stormwater runoff shall fol-
submitted (as required) to the governing agency.
low the specifications summarized in this Section.
While the Natural Resources Conservation Service
Infiltration testing shall be conducted by a
(NRCS) soil classification of the site is encouraged
qualified professional and plans including infiltra-
as part of a desktop analysis to gain familiari-
tion testing results must be certified by a profes-
ty with potential native soil conditions, it is not
sional engineer or professional geologist. How-
adequate justification for infiltration testing results
ever, homeowners may perform their own simple
and cannot be substituted for infiltration testing
infiltration tests to site rain gardens in the proper
using infiltrometers, test pits or other infiltration
location.
1.
Feasibility Analysis
2.
Conceptual Design Testing
Minimum testing recommendations for each tier
are summarized in Table D-1, on the following
page.
Appendix D: Planting & Soil Guidance
D.2.1 Infiltration Testing
testing methods.
Portions of this Section present testing methods
at the bottom of an excavation. It is the testing
Background and Desktop Analysis
personnel’s responsibility to be aware of and take
A desktop analysis of soils data, topography, the
proper health and safety precautions for activities
location of streams, waterbodies, existing/pre-
in an excavation. See the U.S. Occupational Health
vious land uses, and structures is encouraged
and Safety Administration (OSHA) for guidelines
to identify potential BMP locations and types.
and requirements (www.osha.gov).
Existing or previous soil investigation or lab data
may also be used to support preliminary siting of
BMPs and infiltration testing. While NRCS soil
classification of the site is encouraged as part of a
desktop analysis to gain familiarity with potential
native soil conditions, it is not adequate justification for infiltration testing results and cannot be
substituted for infiltration testing using infiltrometers or test pits.
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A minimum of one single-ring infiltrometer
7.
Appendix D: Planting & Soil Guidance
Tier 1: Feasibility Analysis
Record the infiltration rate as the decrease in
depth of water per hour (inches/hour).
test must initially be performed on site.
Where the feasibility analysis does not meet minSingle-Ring Infiltrometer Infiltration Test
imum infiltration criteria, the designer may prefer
This test method utilizes perforated 200 mm to
the use of an underdrain rather than continue
250 mm (8-inch to 10-inch) plastic or metal can-
with further testing.
isters with bottom, set in coarse drainage sand,
to minimize disturbance to in-place soils and to
Where the feasibility analysis meets the minimum
prevent siltation of the test hole during testing.
infiltration criteria, the test pits are necessary for
1.
Holes in the test canister should be 3 mm
(1/8 inch) diameter and spaced on 25 mm (1
inch) centers.
2.
Excavate a test hole to the depth of the
infiltration plane, or the bottom of the BMP
and approximately 25 mm (1 inch) larger
diameter and approximately 25 mm (1 inch
deeper than the dimensions of a test canister.
If the depth of testing is greater than 18”, it
may be necessary to excavate a shallow test
pit to conduct testing.
3.
Check that the sides of the test hole are not
smooth, but scarified.
4.
Place coarse drainage sand in the bottom
of the hole and place the canister firmly into
the hole. The bottom of the hole should be
uncompacted.
5.
Backfill the space around the canister with
soil and tamp the soil into place.
6.
Fill canister with water and allow to drain
completely or to soak the surrounding soils
for a minimum of one hour, whichever
occurs first. Re-fill the canister and measure
the rate at which the water level drops.
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conducting infiltration testing per Table D-1 to
further verify site information characteristics.
Table D-1 Testing/Design Considerations
BMP Type
Tier 1: Feasibility
Analysisection
Tier 2: Conceptual Design Testing
For initial yields equal to or
greater than 0.5”/hr
For initial yields less
than 0.5”/hr
Linear practices
1 single-ring
(i.e., bioswales,
infiltrometer test
interconnected tree boxes, per site
infiltration trenches, etc.)
1 single-ring infiltrometer test
Underdrain required
and 1 test pit per 400 linear feet
(minimum 1 infiltration test per
test pit) of BMP practice
Non-linear practices
(rain gardens, basins, etc.)
1 single-ring infiltrometer test
and 1 test pit per 400 square
feet of practice area (minimum
1 infiltration test per test pit)
1 single-ring
infiltrometer test
per site
Underdrain required
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Soil samples may be collected at various
horizons for additional analysis at the
designer’s discretion.
Construction Equipment and Minimizing Com-
After testing is complete, re-fill test pit with
original native soils and stake the location of
from construction equipment. Care should be
overall soil conditions at a particular location
on the site. Multiple test pit observations can be
the test pit.
BMP and especially at the plane of infiltration so
5.
Test Pit Infiltration Test
This test method consists of a trench or pit that
allows visual observation of the soil horizons and
6.
made for a relatively low cost and in a short time
paction
Soils should not be compromised by compaction
taken to minimize soil compaction throughout the
that infiltration rates of native soils are not impact-
period. The use of soil borings shall not be substi-
Other Infiltration Testing and Verification Meth-
ed. Acceptable excavation methods at infiltration
tuted for test pits. Test pits (see Figure D-1) allow
ods
practices include hand labor with shovels or the use
in-situ visual observation of soil conditions, where
Other infiltration testing standards that are ac-
of an excavator such as a backhoe or trackhoe (lo-
soil borings do not. Soil borings are encouraged
ceptable include ASTM D3385—09 Standard Test
cated outside the perimeter or footprint of the
to supplement data collection, but cannot be
Method for Infiltration Rate of Soils in Field Using
practice). Heavy equipment should never be used
substituted for infiltrometer or test pits.
Double Ring Infiltrometer.
over the footprint of existing or planned infiltration
1.
2.
3.
4.
Dig a backhoe-excavated trench/pit, 2-1/2
to 3 feet wide, to the proposed depth of
the infiltration plane of the practice, or
until bedrock or fully saturated conditions
are encountered. Safe test pit entry should
always be observed. A test pit should never
be accessed if it is not safe to do so. OSHA
regulations should always be observed.
Appendix D: Planting & Soil Guidance
Tier 2: Conceptual Design Testing
practices. Prior to site disturbance, the perimeter of
Several simple infiltration tests do exist for
the practice should be partitioned off with temporary
homeowners and small sites to determine if a rain
fencing/tape to keep heavy equipment from cross-
garden or small bioretention area can be sited in
ing the perimeter throughout time of active con-
a specific place. One example of a simple test can
struction. In cases where the BMP is sufficiently large
be found here: http://www.phillywatersheds.org/
that equipment must enter it, methods proposed to
whats_in_it_for_you/residents/infiltration-test.
limit and restore compacted soil must be approved
in advance.
Document soil profile (soil horizons, soil
texture and color and depth below ground
surface, depth to water table, depth to
bedrock, etc).
Verification methods such as soil borings may be
used to verify site conditions where final locations
Long-term Infiltration Rates
of BMPs are adjusted and do not fall within the
Infiltration rates may decrease over time due to
original testing location. Test results must verify
settlement of filter media, compaction, or accu-
Based on observed field conditions, the
qualified professional should consider
modifying the proposed infiltration plane
of the practice and adjust infiltration testing
locations as necessary.
that the soil conditions are the same as those
mulation of sediment in the practice. To sustain
from the original test results.
infiltration rates long-term, it is important that
Perform Single-Ring Infiltrometer test (above)
at depth of infiltration plane of the proposed
practice.
tivities that could compact soils where BMPs are
a maintenance plan is in place. Regular mainteDesigners should also consider construction
nance should be conducted to optimize operating
access and staging during the design process. Ac-
infiltration rates.
sited should be avoided. Where site constraints
make this unavoidable, the designer shall compensate accordingly in the design of the BMP.
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sand, silt, and clay in the soil. The structure of a
soil is influenced by soil texture and also by the
combination of small soil particles into larger
particles. The amount of aggregation in a soil
is strongly influenced by the amount of organic
matter present. Soils are made up of four basic ingredients: mineral elements, pore space,
organic matter and other items consisting mainly
of living organisms including fungi, bacteria, and
nematodes. One classification of soils is based
upon the mineral part of soil and consists of four
sizes of particles. Clay particles are the smallest,
Appendix D: Planting & Soil Guidance
Soil texture: is determined by the percentage of
followed by silt, sand, and gravel. The USDA has
devised another system of classifying soil particles. In this system soil is divided into seven
Figure D-1: Test Pit
categories: clay, silt, and five sizes of sand.
D.2.2 Compaction, Construction and
Soil
D.2.3 Soil Characteristics
Soil Permeability: Soil permeability is an im-
The ability of the soil to store and release water
portant design factor in stormwater BMPs. It is
Areas that have recently been involved in con-
and to provide plant establishment and plant
advantageous and sometimes necessary to have
struction as well as some native soils can become
growth can be limited by a number of different
high permeability in-situ soils for systems where
compacted so that plant roots cannot penetrate
soil characteristics such as:
infiltration may be desired (e.g. bioretention, infil-
the soil. Seeds lying on the surface of compacted
• Soil texture
tration practices, etc.). It is also advantageous and
Soils should be loosened to a minimum depth of
• Soil Permeability
in-situ soil for systems where permanent ponded
four inches, preferably to a six-inch depth. Hard
• pH -- whether acid, neutral, or alkali
soils can be washed away or be eaten by birds.
soils may require a deeper depth. Loosening soils
will improve seed contact with the soil, provide
greater germination rates, and allow the roots to
penetrate into the soil. Sod and other plantings
will also benefit from loosened soil.
• Nutrient levels -- nitrogen, phosphorus,
potassium
• Minerals -- such as chelated iron, lime
sometimes necessary to have low permeability
water is required (e.g. stormwater wetlands, wet
detention basins, etc.). In some BMP systems (e.g.
sand filters, bioretention, etc.), high permeability
media is required within the BMP, but since relatively small quantities are typically required, suitable soils can be imported to a site if necessary.
• Salinity
• Toxicity
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Organic Content/Compost: Vegetation cannot
survive without the proper amount of organic matter. Organic content requirements vary
based on the site, the BMP and plants that are
being selected. Organic content affects pollutant removal rates: higher organic content filters
more pollutants than lower organic content but
depending on the sources of the organic amendment, it can raise phosphorus levels which can
become a contaminat. Compost utilized to meet
the organic matter content within planting media
should be a well-decomposed, stable, weedfree organic matter source derived from waste
materials including yard debris, wood wastes or
other organic materials, not including manure or
biosolids. Compost shall have a dark brown color
and a soil-like odor. Compost that is exhibiting a
sour or putrid smell, contains recognizable grass
or leaves, or is hot (120 degrees Fahrenheit) upon
delivery is not acceptable. Examples of acceptable
material include pine bark fines and leaf compost.
Topsoil: Topsoil provides organic matter, nutrients, and microbes beneficial to the plant material. This also allows the stabilizing plant materials
during a heavy storm. It is important to under-
On or off-site topsoil should be obtained from
stand the composition of the topsoil that is being
sources that are naturally well-drained sites where
utilized. For example, the clay content of topsoil
topsoil occurs at least 4 inches (100 mm) deep,
affects the infiltration rate of the planting media.
not from bogs, or marshes; and that do not con-
Also, the organic content in topsoil must be taken
tain undesirable organisms; disease-causing plant
into consideration when calculating the total or-
pathogens; or obnoxious weeds and invasive
ganic content in a bioretention planting media.
plants including, but not limited to, quackgrass,
Johnsongrass, poison ivy, nutsedge, nimblewill,
Whenever possible, the topsoil on a site should
Canada thistle, bindweed, bentgrass, wild garlic,
be stripped and stockpiled prior to any grading
ground ivy, perennial sorrel, and bromegrass.
activities, After final grading, stockpiled topsoil
should be spread to a depth of six inches (or more
depending on the size of the plant material to
Appendix D: Planting & Soil Guidance
pH: Soil pH affects the acidity of a soil. A pH of
7 is considered to be neutral. A pH less than 7 is
acidic and solutions with a pH greater than 7 are
basic. Different trees, shrubs and grasses thrive in
different pH levels and the availability of nutrients
is optimal at a pH between 6.5 and 7.5.
be sustained) over the entire area to be planted.
If stockpiled topsoil is insufficient, off-site soils
and amendments may need to be brought in. If
topsoil has been stockpiled in deep mounds for a
long period of time, it is desirable to test the soil
for pH as well as microbial activity. If the microbial activity has been destroyed, it may be necessary
to inoculate the soil after application.
Topsoil should conform to ASTM D5268 – 13:
Total Sample Compositional Category:
Deleterious materials
(rock, gravel, slag, cinder, roots, sod)
Organic material
Sand content
Silt and clay content
pH
Percentage by Mass
5 max material passing the No. 10 (2 mm) sieve
2 to 20
20 to 60
35 to 70
5 to 7
Source: (ASTM D5268-13, Standard Specification for Topsoil Used for Landscaping Purposes,
ASTM International, West Conshohocken, PA, 2013, www.astm.org)
to become established faster, while the roots are
able to penetrate deeper and stabilize the soil,
making it less likely that the plants will wash out
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Soil moisture and drainage have a direct bearing
on the plant species and communities that can be
supported on a site. Factors such as soil texture,
topography, groundwater levels and climatic patterns all influence soil drainage and the amount of
❏❏Dry
Areas where water drains rapidly through
the soil. Soils are usually coarse, sandy,
rocky or shallow. Slopes are often steep
and exposed to sun and wind. Water runs
off quickly and does not remain in the soil.
moisture gradients. The following categories can
be used to describe the drainage properties of
soils on a site:
❏❏Flooded
Areas where standing water is present
most of the growing season.
❏❏Wet
Areas where standing water is present
most of the growing season, except during
times of drought. Wet areas are found
at the edges of ponds, rivers, streams,
ditches, and low spots. Wet conditions
exist on poorly drained soils, often with a
high clay content.
❏❏Moist
Areas where the soil is damp.
Occasionally, the soil is saturated and
drains slowly. These areas usually are at
slightly higher elevations than wet sites.
Moist conditions may exist in sheltered
areas protected from sun and wind.
❏❏Well-drained
Areas where rain water drains readily,
and puddles do not last long. Moisture is
available to plants most of the growing
season. Soils usually are medium textures
with enough sand and silt particles to allow
water to drain through the soil.
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ticularly important in wetlands because they allow
plants that would otherwise die while flooded to
escape inundation.
In wetland plant establishment, ground surface
slope interacts with the site hydrology to de-
water in the soil. Identifying the topography and
drainage of the site will help determine potential
areas without such features. Raised sites are par-
D.2.5 Slope
termine water depths for specific areas within
the site. Depth and duration of inundation are
The degree of slope can also limit its suitability for
principal factors in the zonation of wetland plant
certain types of plants. Plant establishment and
species. A given change in water levels will
growth requires stable substrates for anchoring
expose a relatively small area on a steep slope in
root systems and preserving propagules such as
comparison with a much larger area exposed on
seeds and plant fragments, and slope is a primary
a gradual or flat slope. Narrow planting zones will
factor in determining substrate stability. Estab-
be delineated on steep slopes for species tolerant
lishing plants directly on or below eroding slopes
of specific hydrologic conditions, whereas gradual
is not possible for most species. In such instances,
slopes enable the use of wider planting zones.
Appendix D: Planting & Soil Guidance
D.2.4 Drainage
plant species capable of rapid spread and anchoring soils should be selected or bioengineering
techniques should be used to aid the establishment of a plant cover.
In addition, soils on steep slopes generally drain
more rapidly than those on gradual slopes. This
means that the soils may remain saturated longer
on gradual slopes. If soils on gradual slopes are
classified as poorly drained, care should be taken
that plant species are selected that are tolerant of
saturation.
D.2.6 Orientation
Slope exposure should be considered for its effect
on plants. A southern-facing slope receives more
sun and is warmer and drier, while the opposite
is true of a northern slope. Eastern- and western-facing slopes are intermediate, receiving
morning and afternoon sun, respectively. Western-facing slopes tending to be drier and receive
more wind.
Site topography also affects maintenance of plant
species diversity. Small irregularities in the ground
surface (e.g., depressions, etc.) are common in
natural systems. More species are found in areas
with many micro-topographic features than in
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635
D.3.1 Hardiness Zones
Hardiness zones are based on historical average
annual minimum temperatures recorded in an
area. A site’s location in relation to plant hardiness zones is important to consider first because
plants differ in their ability to withstand very cold
winters. This does not imply that plants are not
affected by summer temperatures. Given that
Georgia summers can be very hot, heat tolerance
is also a characteristic that should be considered
Appendix D: Planting & Soil Guidance
Section 3: Plant Selection for
Stormwater Facilities
in plant selection.
It is best to recommend plants known to thrive in
specific hardiness zones. The plant list included
at the end of this appendix identifies the hardiness
zones for each species listed as a general planting
guide. It should be noted, however, that certain
site factors can create microclimates or environmental conditions which permit the growth
of plants not listed as hardy for that zone. By
investigating numerous references and based on
personal experience, a designer should be able to
confidently recommend plants that will survive in
microclimates.
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Figure D-2: USDA Plant Hardiness Zones in Georgia
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636
There are five physiographic provinces in Georgia that describe distinct geographic regions in
the state with similar physical and environmental
conditions (Figure F-2). These physiographic
• Growth rate.
• Site conditions (e.g., wind direction and
intensity, street lighting, type and quantity of
pollutants contained within stormwater runoff,
etc.).
provinces include, from northwest to southeast,
Appalachian Plateau, Ridge and Valley, Blue Ridge,
Piedmont and Coastal Plain (subdivided into
upper and lower regions). Each physiographic
region is defined by unique geological strata,
soil type, drainage patterns, moisture content,
temperature and degree of slope which often
dictate the predominant vegetation. Because the
predominant vegetation has evolved to live in
these specific conditions it is important to under-
• Soil moisture and drought tolerance.
• Tolerance of periodic inundation.
• Sediment and organic matter build-up.
• Potential for outlet structure clogging (e.g., root
structure).
• Maintenance requirements.
many resources available to guide designers in the
selection of plant material for stormwater BMPs.
Knowledgeable landscape architects, wetland
scientists, urban foresters, and nursery suppliers
provide valuable information for considering specific conditions for successful plant establishment
and accounting for the variable nature of stormwater hydrology.
D.3.3.1 AVAILABILITY AND COST
Often overlooked in plant selection is the availability from wholesalers and the cost of the plant
material. There are many plants listed in landscape books that are not readily available from the
• Wildlife use
nurseries. Without knowledge of what is avail-
will reside. The five physiographic regions are
Plants need to provide aesthetics/ability to meet
able, time spent researching and finding the one
described here: http://www.georgiawildlife.com/
both landscape and stormwater BMP require-
plant that meets all the needs will be wasted, if it
node/1704 with an associated map: http://www1.
ments. In urban or suburban settings, a plant’s
is not available from the growers. It may require
gadnr.org/cwcs/PDF/ga_eco_l3_pg.pdf.
aesthetic interest may be of greater importance.
shipping, therefore, making it more costly than
Residents living next to a stormwater system may
the budget may allow. Some planting require-
desire that the facility be appealing or interest-
ments, however, may require a special effort to
D.3.3 Other Considerations in Plant
Selection
ing to look at throughout the year. Aesthetics is
find the specific plant that fulfills the needs of the
an important factor to consider in the design of
site and the function of the plant in the landscape.
The landscape planting design must include
these systems. Failure to consider the aesthetic
Plants should be sourced from similar geographic
elements that ensure plant survival and overall
appeal of a facility to the surrounding residents
regions to optimize survivability.
stormwater BMP functional success. Plant se-
may result in reduced value to nearby lots. Care-
lection is a complex task, involving matching the
ful attention to the design and planting of a BMP
plant’s physiological characteristics with a site’s
can result in maintained or increased values of a
particular environmental conditions. The follow-
property.
stand the region in which your stormwater facility
Appendix D: Planting & Soil Guidance
D.3.2 Physiographic Provinces
ing factors should be considered:
• Full grown plant size and shape (e.g., limbs
growing into power lines, maintenance access
impediment).
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Individual plants often have physiological characteristics difficult to convey in a general list. It
is necessary to investigate specific information
to ensure successful plant selection. There are
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The six zones in the next section describe the
This Manual encourages the use of native plants in
different conditions encountered in stormwater
stormwater management facilities since they are
management facilities. Every facility does not
best suited to thrive and can provide benefits to
necessarily reflect all of these zones. The hydro-
pollinators and other native fauna. Unfortunately
logic zones designate the degree of tolerance the
native plants may not always be available in quan-
plant exhibits to differing degrees of inundation by
tity from local nurseries. Therefore, non-invasive
water. Each zone has its own set of plant selec-
naturalized plants that can survive under difficult
tion criteria based on the hydrology of the zone,
conditions may be a useful alternative. Great care
the stormwater functions required of the plant
should be taken, however, when introducing plant
and the desired landscape effect.
species so as not to create a situation where they
may become invasive and take over adjacent
natural plant communities. The Georgia Exotic
Appendix D: Planting & Soil Guidance
D.3.3.2 NATIVE VERSUS NONNATIVE SPECIES
Pest Plant Council keeps a list of species that are
invasive or could become invasive: http://www.
gaeppc.org/list/.
D.3.3.3 MOISTURE STATUS
In landscaping stormwater management facilities,
hydrology plays a large role in determining which
species will survive in a given location.
For areas that are to be planted within a stormwater management facility it is necessary to
determine what type of hydrologic zones will be
created.
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Table D-2 Hydrolic Zones
Zone #
Zone Description
Hydrologic Conditions
Zone 1
Deep Water Pool
1-6 feet depth (permanent pool)
Zone 2
Shallow Water Bench
Normal pool elevation to 1 foot depth
basins and wetland areas designed to control and
Zone 3
Shoreline Fringe
Regularly inundated
treat stormwater runoff. Aquatic vegetation plays
Zone 4
Riparian Fringe
Periodically inundated
Zone 5
Floodplain Terrace
Infrequently inundated
Zone 6
Upland Slopes
Seldom or never inundated
D.4.1 Stormwater Ponds and Wetlands
Stormwater ponds and wetlands are engineered
an important role in pollutant removal in both
stormwater ponds and wetlands. In addition, vegetation can enhance the appearance of a pond
or wetland, stabilize side slopes, serve as wildlife
habitat, and can temporarily conceal unsightly
trash and debris.
Within a stormwater pond or wetland, there are
various hydrologic zones as shown in Table D-2
that must be considered in plant selection. These
hydrologic zones designate the degree of tolerance a plant must have to differing degrees of
inundation by water. Hydrologic conditions in an
area may fluctuate in unpredictable ways; thus the
use of plants capable of tolerating wide varieties
of hydrologic conditions greatly increases the
successful establishment of a planting. Plants
suited for specific hydrologic conditions may
perish when those conditions change, exposing
the soil, and therefore, increasing the chance for
erosion. Each of the hydrologic zones is described in more detail below along with examples
of appropriate plant species.
Zone 1: Deep Water Area (1- 6 Feet)
Ponds and wetlands both have deep pool areas
that comprise Zone 1. These pools range from
one to six feet in depth, and are best colonized by
submergent plants, if at all.
This pondscaping zone is not routinely planted
for several reasons. First, the availability of plant
materials that can survive and grow in this zone is
limited, and it is also feared that plants could clog
the stormwater facility outlet structure. In many
cases, these plants will gradually become established through natural recolonization (e.g., transport of plant fragments from other ponds via the
feet and legs of waterfowl). If submerged plant
Appendix D: Planting & Soil Guidance
Section 4: Specific Landscaping
Criteria for Structural Stormwater Controls
❏❏Plant material must be able to withstand
constant inundation of water of one foot
or greater in depth.
❏❏Plants may be submerged partially or
entirely.
❏❏Plants should be able to enhance pollutant
uptake.
❏❏Plants may provide food and cover for
waterfowl, desirable insects, and other
aquatic life.
Some suggested emergent or submergent species
include, but are not limited to: Water Lily, Deepwater Duck Potato, Spatterdock, Wild Celery and
Redhead Grass.
material is commercially available and clogging
concerns are addressed, this area can be planted.
The function of the planting is to reduce resedimentation and improve oxidation while creating a
greater aquatic habitat.
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Zone 3: Shoreline Fringe (Regularly Inundated)
1 Foot)
Zone 3 encompasses the shoreline of a pond or
Zone 2 includes all areas that are inundated below
wetland, and extends vertically about one foot
the normal pool to a depth of one foot, and is the
in elevation from the normal pool. This zone
primary area where emergent plants will grow in
includes the safety bench of a pond, and may also
stormwater wetlands. Zone 2 also coincides with
be periodically inundated if storm events are sub-
the aquatic bench found in stormwater ponds.
ject to extended detention. This zone occurs in a
This zone offers ideal conditions for the growth
wet pond or shallow marsh and can be the most
of many emergent wetland species. These areas
difficult to establish since plants must be able
may be located at the edge of the pond or on low
to withstand inundation of water during storms,
mounds of earth located below the surface of
when wind might blow water into the area, or the
the water within the pond. When planted, Zone 2
occasional drought during the summer. In order
can be an important habitat for many aquatic and
to stabilize the soil in this zone, Zone 3 must have
nonaquatic animals, creating a diverse food chain.
a vigorous cover.
❏❏Plants should stabilize the shoreline to
minimize erosion caused by wave and wind
action or water fluctuation.
❏❏Plant material must be able to withstand
occasional inundation of water. Plants
will be partially submerged partially at this
time.
❏❏Plant material should, whenever possible,
shade the shoreline, especially the
southern exposure. This will help to
reduce the water temperature.
❏❏Plants should be able to enhance pollutant
uptake.
❏❏Plants may provide food and cover for
waterfowl, songbirds, and wildlife. Plants
could also be selected and located to
control overpopulation of waterfowl.
❏❏Plants should be located to reduce human
access, where there are potential hazards,
but should not block the maintenance
access.
❏❏Plants should have very low maintenance
This food chain includes predators, allowing a
natural regulation of mosquito populations, thereby reducing the need for insecticidal applications.
❏❏Plant material must be able to withstand
constant inundation of water to depths
between six inches and one foot deep.
❏❏Plants will be partially submerged.
❏❏Plants should be able to enhance pollutant
uptake.
❏❏Plants may provide food and cover for
waterfowl, desirable insects and other
aquatic life.
Common emergent wetland plant species used
for stormwater wetlands and on the aquatic
benches of stormwater ponds include, but are not
limited to: Arrowhead/Duck Potato, Soft Rush,
various Sedges, Softstem Bulrush, Cattail, Switchgrass, Southern Blue-Flag Iris, Swamp Hibiscus,
Swamp Lily, Pickerelweed, Pond Cypress and
various Asters.
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requirements, since they may be difficult or
impossible to reach.
❏❏Plants should be resistant to disease and
other problems which require chemical
applications (since chemical application is
not advised in stormwater ponds).
Many of the emergent wetland plants that perform well in Zone 2 also thrive in Zone 3. Some
other species that do well include Broom Grass,
Upland Sea-Oats, Dwarf Tickseed, various Ferns,
Hawthorns. If shading is needed along the shoreline, the following tree species are suggested:
Boxelder, Ash, Willow, Red Maples and Willow
Appendix D: Planting & Soil Guidance
Zone 2: Shallow Water Bench (Normal Pool To
Oak.
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640
Zone 5: Floodplain Terrace (Infrequently Inun-
Zone 6: Upland Slopes (Seldom or Never Inun-
Zone 4 extends from one to four feet in elevation
dated)
dated)
above the normal pool. Plants in this zone are
Zone 5 is periodically inundated by flood waters
The last zone extends above the maximum 100
subject to periodic inundation after storms, and
that quickly recede in a day or less. Operational-
year water surface elevation, and often includes
may experience saturated or partly saturated soil
ly, Zone 5 extends from the maximum two year
the outer buffer of a pond or wetland. Unlike
inundation. Nearly all of the temporary extended
or CPv water surface elevation up to the 25 or
other zones, this upland area may have sidewalks,
detention (ED) storage area is included within this
100 year maximum water surface elevation. Key
bike paths, retaining walls, and maintenance ac-
zone.
❏❏Plants must be able to withstand periodic
inundation of water after storms, as well
as occasional drought during the warm
summer months.
❏❏Plants should stabilize the ground from
erosion caused by run-off.
❏❏Plants should shade the low flow channel
to reduce the pool warming whenever
possible.
❏❏Plants should be able to enhance pollutant
uptake.
❏❏Plant material should have very low
maintenance, since they may be difficult or
impossible to access.
❏❏Plants may provide food and cover for
waterfowl, songbirds and wildlife. Plants
may also be selected and located to
control overpopulation of waterfowl.
❏❏Plants should be located to reduce
pedestrian access to the deeper pools.
landscaping objectives for Zone 5 are to stabilize
cess roads. Care should be taken to locate plants
the steep slopes characteristic of this zone, and
so they will not overgrow these routes or create
establish a low maintenance, natural vegetation.
❏❏Plant material should be able to withstand
occasional but brief inundation during
storms, although typical moisture
conditions may be moist, slightly wet, or
even swing entirely to drought conditions
during the dry weather periods.
❏❏Plants should stabilize the basin slopes
from erosion.
❏❏Ground cover should be very low
maintenance, since they may be difficult to
access on steep slopes or if the frequency
of mowing is limited. A dense tree cover
may help reduce maintenance and
discourage resident geese.
❏❏Plants may provide food and cover for
waterfowl, songbirds, and wildlife.
❏❏Placement of plant material in Zone 5 is
often critical, as it often creates a visual
focal point and provides structure and
shade for a greater variety of plants.
hiding places that might make the area unsafe.
❏❏Plant material is capable of surviving the
particular conditions of the site. Thus, it is
not necessary to select plant material that
will tolerate any inundation. Rather, plant
selections should be made based on soil
condition, light, and function within the
landscape.
❏❏Ground covers should emphasize
infrequent mowing to reduce the cost of
maintaining this landscape.
❏❏Placement of plants in Zone 6 is important
since they are often used to create a visual
focal point, frame a desirable view, screen
undesirable views, serve as a buffer, or
provide shade to allow a greater variety of
plant materials. Particular attention should
be paid to seasonal color and texture of
these plantings.
Some frequently used plant species in Zone
4 include Broom Grass, Yellow Indian Grass,
Appendix D: Planting & Soil Guidance
Zone 4: Riparian Fringe (Periodically Inundated)
Some frequently used plant species in Zone 6
include most ornamentals (as long as soils drain
Ironweed, Joe Pye Weed, Lilies, Flatsedge, Native
Some commonly planted species in Zone 5
well, many wildflowers or native grasses, Linden,
Hollies, Forsythia, Lovegrass, Hawthorn and Sugar
include many wildflowers or native grasses, many
False Cypress, Magnolia, most Spruce, Mountain
Maples.
Fescues, many Viburnums, Witch Hazel, Blueber-
Ash and most Pine.
ry, American Holly, American Elderberry and Red
Oak.
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VOL 2
641
Specific plants appropriate for wetlands are listed in the Plant List in Section 5.
Appendix D: Planting & Soil Guidance
The figures below (Figures D-3 to D-6) illustrate a sample design of plantings within a constructed wetland:
Figure D-3: Plan View of Hydrologic Zones around Stormwater Wet ED Pond
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VOL 2
Figure D-4: Plan View of Hydrologic Zones around Stormwater ED Shallow Wetland
642
Water Lily, Deep Water Duck Potato, Spatterdock, Wild Celery, Redhead Grass
0 to 12 inch depth below normal pool elevation
Arrowhead/Duck Potato, Soft Rush, various Sedges, Softstem Bulrush, Cattail, Switchgrass, Southern Blue Flag Iris, Swamp Hibiscus,
Swamp Lily, Pickerelweed, Pond Cypress, various Asters
0 to 12 inch elevation above normal pool elevation
Various species from above, Broom Grass, Upland Sea-Oats, Dwarf Tickseed, various Ferns, Hawthorns, Boxelder, Ash, Willow,
Red Maple, Willow Oak
1 to 4 foot elevation above normal pool elevation
Broom Grass, Yellow Indian Grass, Ironweed, Joe Pye Weed, various Lilies, Flatsedge, Hollies, Lovegrass, Hawthorn, Sugar Maple
CPv to Qp25 or Qf water surface elevation
Many wildflowers or native grasses, many Fescues, many Viburnums, Witch Hazel, Blueberry, American Holly, American Elderberry,
Red Oak
Appendix D: Planting & Soil Guidance
12 to 36 inch depth below normal pool elevation
Qf water surface elevation and above
Many ornamentals as long as soils drain well, many wildflowers or native grasses, Linden, False Cypress, Magnolia, most Spruce,
Mountain Ash, most Pine
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Figure D-5 Legend of Hydrologic Zones Around Stormwater Facilities
Figure D-6: Section of Typical Shallow ED Wetland
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643
Plant material selection should be based on the goal of simulating a terrestrial
Bioretention areas are structural stormwater controls that capture and treat
forested community of native species. Bioretention simulates an ecosystem
runoff using soils and vegetation in shallow basins or landscaped areas. Land-
consisting of an upland-oriented community dominated by trees, but having
scaping is therefore critical to the performance and function of these facilities.
a distinct community, or sub-canopy, of understory trees, shrubs and her-
Below are guidelines for soil characteristics, mulching, and plant selection for
baceous materials. The intent is to establish a diverse, dense plant cover to
bioretention areas.
treat stormwater runoff and withstand urban stresses from insect and disease
infestations, drought, temperature, wind, and exposure.
D.4.2.1 PLANTING MEDIA CHARACTERISTICS
The proper selection and installation of plant materials is key to a successful
Specifying the correct soils is critical in order to achieve stormwater objec-
system. There are essentially three zones within a bioretention facility (Figure
tives and plant health. Soils must balance three primary design objectives: 1)
D-7). The lowest elevation supports plant species adapted to standing and
High enough infiltration rates to meet surface water draw down requirements,
fluctuating water levels. The middle elevation supports a slightly drier group of
2) infiltration rates that are not so high that they preclude pollutant removal
plants, but still tolerates fluctuating water levels. The outer edge is the highest
function of soils and 3) soil composition that supports plant establishment and
elevation and generally supports plants adapted to dryer conditions. A sample
long-term health.
of appropriate plant materials for bioretention facilities is included in Section 5.
Appendix D: Planting & Soil Guidance
D.4.2 Bioretention Areas
If the native soils cannot suffice for the planting media used within the bioretention area planting beds, then an engineered planting media should be
provided that meets the specifications in the BMP write up. Keep in mind that
increasing sand content increases permeability in the media but lowers phosphorus levels in the soil which hurts plant survivability.
D.4.2.2 MULCH LAYER
The mulch layer plays an important role in the performance of the bioretention system. The mulch layer helps maintain soil moisture and avoids surface
sealing which reduces permeability. Mulch helps prevent erosion, and provides
a micro-environment suitable for soil biota at the mulch/soil interface. It also
serves as a pretreatment layer, trapping the finer sediments which remain suspended after the primary pretreatment. The mulch layer should be standard
Figure D-7: Planting Zones for Bioretention Facilities
landscape style, single or double, shredded hardwood mulch or chips. The
mulch layer should be well aged (stockpiled or stored for at least 12 months),
uniform in color, and free of other materials, such as weed seeds, soil, roots,
etc. The mulch should be applied to a maximum depth of three inches. Grass
clippings should not be used as a mulch material.
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VOL 2
644
but should follow the general principals described
below. The objective is to have a system that
resembles a random and natural plant layout,
while maintaining optimal conditions for plant
establishment and growth. A sample bioretention
planting plan is presented in Figure D-8.
❏❏Trees provide aesthetic and performance
benefits in bioretention areas. When
including trees, refer to the following
guidelines:
-- Provide sufficient landscape width (a rule
of thumb is 8’ min.)
❏❏A canopy should be established with an
understory of shrubs and herbaceous
materials.
❏❏Woody vegetation should not be specified
in the vicinity of inflow locations.
❏❏Urban stressors (e.g., wind, sun, exposure,
insect and disease infestation, drought)
should be considered when laying out the
planting plan.
❏❏Noxious weeds should not be specified.
❏❏Aesthetics and visual characteristics should
be a prime consideration.
❏❏Traffic and safety issues must be
considered.
❏❏Existing and proposed utilities must be
identified and considered.
See plant list in Section 5 for species appropriate
to bioretention areas.
-- Locate trees on the side slopes, not in
areas that pond
Appendix D: Planting & Soil Guidance
The layout of plant material should be flexible,
-- Select trees that will tolerate wet soils.
-- Locate trees with invasive roots away from
perforated pipes.
-- Consider behavior and maintenance of
plant material. Deciduous plant material
that will drop leaves can impair the
function of a bioretention area if not
located properly or maintained.
❏❏Native plant species should be specified.
❏❏Appropriate vegetation should be selected
based on the zone of hydric tolerance
❏❏Species layout should generally be random
and natural.
❏❏The tree-to-shrub ratio should be 2:1 to
3:1.
❏❏Plants should be placed at regular intervals
based on their mature size to replicate a
natural forest.
❏❏Woody vegetation should not be specified
at inflow locations.
Figure D-8: Sample Bioretention Area Planting Plan
(Source: VDCR, 1999)
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VOL 2
645
The following plant list is not representative of every plant that can be utilized
in a stormwater bmp, but is meant as a starting point for native plant material.
Many factors must be analyzed before selecting plant material types including
sun/shade tolerance, flowering and/or fruiting habits, shape and full grown
size of species, and local availability.
Many additional online and printed resources exist, including the following:
• Georgia Native Plant Society: http://gnps.org/indexes/Plant_Gallery_Index.
php
• UGA Extension Native Plants for Georgia:
Appendix D: Planting & Soil Guidance
Section 5: Plant Selection for Stormwater Facilities
»» Trees, Shrubs and Woody Vines: http://extension.uga.edu/publications/
files/pdf/B%20987_8.PDF
»» Grasses and Sedges: http://extension.uga.edu/publications/files/pdf/B%20
987-4_1.PDF
»» Ferns: http://extension.uga.edu/publications/detail.cfm?number=B987-2
»» Wildflowers: http://extension.uga.edu/publications/files/pdf/B%209873_4.PDF
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VOL 2
646
TREES
Scientific Name
Common Name
Habit
Acer negundo
Acer rubrum
Asimina triloba
Betula nigra
C Carya aquatica
Carpinus caroliniana
Carya cordiformus
Carya illinoensis
Carya laciniosa
Celtis laevigata
Chemaecyparis thyoides
Cornus drummondii
Crataegus spp.
Diospyros virginiana
Fraxinus caroliniana
Fraxinus pennsylvanica
Fraxinus profonda
Gordonia laisianthus
Juniperus silicicola
Juniperus virginiana
Liquidamber styraciflua
Liriodendron tulipifera
Boxelder
Red Maple
Common Pawpaw
River Birch
Water Hickory
American Hornbeam
Bitternut Hickory
Pecan
Shellbark Hickory
Sugarberry
Atlantic White Cedar
Rough-Leaf Dogwood
Hawthornes
Common Persimmon
Carolina Ash
Green Ash
Pumpkin Ash
Loblolly Bay
Southern Red Cedar
Eastern Red Cedar
Sweetgum
Yellow Poplar
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Magnolia virginiana
Sweetbay
Tree
Morus rubra
Myrica cerifera
Nyssa aquatica
Nyssa ogeche
Red Mulberry
Southern Bayberry
Water Tupelo
Ogeechee Tupelo
Black Gum/
Swamp Tupelo
Slash Pine
Spruce Pine
Pond Pine
Loblolly Pine
American Sycamore
Tree
Tree
Tree
Tree
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Evergreen
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Evergreen
Evergreen
Evergreen
Deciduous
Deciduous
SemiEvergreen
Deciduous
Evergreen
Deciduous
Deciduous
Nyssa sylvatica
Pinus elliottii
Pinus glabra
Pinus serotina
Pinus taeda
Platanus occidentalis
H ZONE*
Hardiness
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
3,4,5
3,4,5
3,4,5
2,3,4,5
3,4
4,5
3,4,5
3,4,5
2,3,4,5
3,4,5
2,3,4,5
3,4,5
4,5
4,5
3,4,5
3,4,5
3,4,5
3,4
3,4,5
3,4,5
3,4,5
3,4,5
Native
3,4,5
USDA Zone 5-10
Native
Native
Native
Native
4,5
3,4,5
3,4,5
3,4,5
USDA Zone 4-8
USDA Zone 7-10
USDA Zone 6-9
USDA Zone 7a-9b
Tree
Deciduous Native
3,4,5
USDA Zone 4b-9
Tree
Tree
Tree
Tree
Tree
Evergreen
Evergreen
Evergreen
Evergreen
Deciduous
4,5
4,5
4,5
5
3,4,5
USDA Zone 7-11
USDA Zone 8,9
USDA Zone 7-9
USDA Zone 6b-9
USDA Zone 4-9
Native
Native
Native
Native
Native
USDA Zone 2-10
USDA Zone 3-9
USDA Zone 5-9
USDA Zone 4-9
USDA Zone 4-8
USDA Zone 3-9
USDA Zone 4-9
USDA Zone 5b-9a
USDA Zone 5-8
USDA Zone 6-9
USDA Zone 4-8
USDA Zone 5-8
USDA Zone 5-8
USDA Zone 4-9
USDA Zone 4-8
USDA Zone 3-9a
USDA Zone 5-9
USDA Zone 6-9
USDA Zone 8a-10b
USDA Zone 2-9
USDA Zone 5b-10a
USDA Zone 4-9
Appendix D: Planting & Soil Guidance
Georgia Native Plant List
*Hydrologic Zone for Stormwater Pond or Wetland
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VOL 2
647
TREES
Scientific Name
Common Name
Habit
Populus deltoides
Populus heterophylla
Ptelea trifoliata
Quercus bicolor
Quercus laurifolia
Quercus lyrata
Quercus michiauxii
Quercus nigra
Quercus pagoda
Quercus palustris
Quercus phellos
Quercus shumardii
Salix caroliniana
Salix nigra
Taxodium distichum
Eastern Cottonwood
Swamp Cottonwood
Wafer Ash
Swamp White Oak
Laurel Oak
Overcup Oak
Swamp Chestnut Oak
Water Oak
Cherrybark Oak
Pin Oak
Willow Oak
Shumard Oak
Coastal Plain Willow
Black Willow
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Tree
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
4,5
3,4,5
5
3,4,5
4,5
3,4,5
4,5
4,5
4,5
4,5
3,4,5
4,5
3,4,5
3,4,5
USDA Zone 3-9
USDA Zone 7-8
USDA Zone 4-9
USDA Zone 3-8
USDA Zone 7-9
USDA Zone 5-9
USDA Zone 6-8
USDA Zone 5-9
USDA Zone 4-8
USDA Zone 4-8
USDA Zone 5-9
USDA Zone 5-9
USDA Zone 7-8
USDA Zone 2-8
Baldcypress
Tree
Deciduous Native
2,3,4
USDA Zone 4-9
Pondcypress
Tree
Deciduous Native
2,3,4
USDA Zone 4-9
American Elm
Slippery Elm
Bottlebrush Buckeye
Red Buckeye
Hazel Alder
Red Chokeberry
Common Buttonbush
Eastern Burning Bush
Fothergilla
Witch Hazel
Common
St. Johns Wort
Inkberry
Winterberry
Decidious Holly
Creeping Juniper
Spicebush
Tree
Tree
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
Deciduous
3,4,5
3,4,5
3,4,5
3,4,5
3,4,5
3,4,5
2,3,4
4,5
4,5
3,4,5
USDA Zone 2-9
USDA Zone 3-9
USDA Zone 4-8
USDA Zone 4-8
USDA Zone 5-8
USDA Zone 4-9
USDA Zone 5-9
USDA Zone 3-7
USDA Zone 5-8
USDA Zone 3-8
Shrub
Deciduous Native
4,5
USDA Zone 5-9
Shrub
Shrub
Shrub
Shrub
Shrub
Evergreen
Deciduous
Deciduous
Evergreen
Deciduous
3,4,5
2,3,4
3,4,5
5
3,4,5
USDA Zone 4-9
USDA Zone 3-9
USDA Zone 5-9
USDA Zone 3-9
USDA Zone 4-9
var. distichum
Taxodium distichum
SHRUBS
var. nutans
Ulmus americana
Ulmus rubra
Aesculus pariviflora
Aesculus pavia
Alnus serrulata
Aronia arbutifolia
Cephalanthus occidentalis
Euonymus atropurpuresu
Fothergilla gardenii
Hamemelis virginiana
Hypericum densiflorum
Ilex glabra
Ilex verticillata
Illex decidua
Juniperus horizontalis
Lindera benzoin
H ZONE*
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Hardiness
Appendix D: Planting & Soil Guidance
Georgia Native Plant List (continued)
*Hydrologic Zone for Stormwater Pond or Wetland
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648
GRASSES/HERBACEOUS
Scientific Name
Common Name
Habit
Andropogon glomeratus
Andropogon virginicus
Chasmanthium latifolium
Leersia oryzoides
Panicum virgatum
Sorghastrum nutans
Osmunda cinnamomea
Osmunda regalis
Woodwardia virginica
Bushy Broom Grass
Broom Grass
Upland Sea-Oats
Rice Cut Grass
Switchgrass
Yellow Indian Grass
Cinnamon Fern
Royal Fern
Virginia Chain Fern
Grass
Grass
Grass
Grass
Grass
Grass
Fern
Fern
Fern
Carex spp.
Carex Sedges
Cyperus odoratus
Juncus effusus
Scirpus californicus
Scirpus cyperinus
Scirpus validus
Canna flaccida
Coreopsis leavenworthii
Coreopsis tinctoria
Crinum americanum
Eleocharis cellulosa
Eleocharis interstincta
Eupatorium fistolosum
Eupatorium perpurea
Helianthus angustifolius
Hibiscus coccinieus
Iris louisiana
Iris virginica
Liatris spicata
Lobelia cardinalis
Peltandra virginica
Polygonum
hydropiperoides
Pontederia cordata
H ZONE*
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Hardiness
Native
Native
Native
Native
Native
Native
Native
Native
Native
3
4
3
2
2
4
3
3
2
USDA Zone 5-9
USDA Zone 5-8
USDA Zone 3-8
USDA Zone 3a-9b
USDA Zone 5-9
USDA Zone 5-9
USDA Zone 2-10
USDA Zone 3-9
USDA Zone 3-10
Sedge
Use only
Native
2
Varies
Flat Sedge
Soft Rush
Giant Bulrush
Woolgrass
Softstem Bulrush
Golden Canna
Tickseed
Dwarf Tickseed
Swamp Lily
Coastal Spikerush
Jonited Spikerush
Joe Pye Weed
Joe Pye Weed
Swamp Sunflower
Swamp Hibiscus
Louisiana Iris
Southern Blue-Flag
Spiked Gayfeather
Cardinal Flower
Green Arum
Sedge
Sedge
Sedge
Sedge
Sedge
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
2
2
2
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
2
2
2
2
3
3
2
USDA Zone 7-11
USDA Zone 4-9
USDA Zone 6-9
USDA Zone 4-8
USDA Zone 3-9
USDA Zone 8-11
USDA Zone 8-11
USDA Zone 3-11
USDA Zone 7-11
USDA Zone 8-11
USDA Zone 8-10
USDA Zone 4-8
USDA Zone 4-9
USDA Zone 6-9
USDA Zone 6-9
USDA Zone 5-9
USDA Zone 5-9
USDA Zone 3-8
USDA Zone 3-9
USDA Zone 5-9
Smartweed
Perennial Perennial
Native
2
USDA Zone 3-10
Pickerelweed
Perennial Perennial
Native
2
USDA Zone 3-10
2
2
2
3
2
2
2
4
Appendix D: Planting & Soil Guidance
Georgia Native Plant List (continued)
*Hydrologic Zone for Stormwater Pond or Wetland
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649
Scientific Name
Common Name
Habit
Pontederia lanceolata
Rudbeckia hirta
Rudbeckia laciniata
Sagittaria lancifolia
Sagittaria latifolia
Saururus cernuus
Scirpus americanus
Thalia geniculata
Typha latifolia
Vernonia gigantea
Pickerelweed
Black-eyed Susan
Greenhead Coneflower
Lance-leaf Arrowhead
Duck Potato
Lizard’s Tail
Three-square
Alligator Flag
Broadleaf Cattail
Ironweed
Nuphar luteum
Water Lily
Nymphaea mexicana
Yellow Water Lily
Nymphaea odorata
Fragrant Water Lily
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Water
Lily
Water
Lily
Water
Lily
H ZONE*
Hardiness
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Native
Native
Native
Native
Native
Native
Native
Native
Native
Native
2
4
4
2
2
2
2
2
2
4
USDA Zone 3-10
USDA Zone 3-9
USDA Zone 3-9
USDA Zone 5-10
USDA Zone 5-10
USDA Zone 3-9
USDA Zone 3-9
USDA Zone 7-9
USDA Zone 3-10
USDA Zone 5-8
Perennial
Native
1
USDA Zone 4-10
Perennial
Native
1
USDA Zone 3-11
Perennial
Native
1
USDA Zone 3-11
Appendix D: Planting & Soil Guidance
GRASSES/HERBACEOUS
Georgia Native Plant List (continued)
*Hydrologic Zone for Stormwater Pond or Wetland
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5.
Soil media specifications. If topsoil is
specified, indicate the topsoil stockpile
location, including source of the topsoil if
imported to the site.
6.
Construction notes with sequencing, soil
and plant installation instructions, and initial
maintenance requirements.
7.
A description of the landscape contractor’s
responsibilities.
8.
A minimum two-year warranty period
stipulating requirements for plant survival/
replacement.
Landscape plans must be prepared by a qualified
design professional. They must include the following items, at a minimum:
1.
Landscape plan sheet − A scaled
construction drawing (typically at 1” = 20’)
to accurately locate and represent the
plant material used within the BMP facility.
Representation of plant material should be
to scale and depicted at the mature width or
spread.
2.
A key that identifies all plant material used
in the planting plan. The symbols used to
identify the plants will correlate with the
plant schedule. Plant groupings on the
drawing are usually shown by an identifying
symbol and the number of plants in that
particular group.
3.
List any other necessary information
to communicate special construction
requirements, materials, or methods such as
specific plants that must be field located or
approved by the designer and size or form
matching of an important plant grouping.
4.
Plant list/table − This must include scientific
name, common name, quantity, nursery
container size, container type (e.g., bare
root, b&b, plug, container, etc.), appropriate
planting season, and other information in
accordance with the BMP facility-specific
planting section and landscape industry
standards.
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Appendix D: Planting & Soil Guidance
Section 6: Minimum Requirements for Landscape Plans
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651
Establishment
Slope stabilization methods (such as planted erosion control mats or fiber rolls) should be utilized
for slopes susceptible to washout. Erosion control
mats and fabrics should also be utilized to protect
channels that are susceptible to washing out.
Flows should be diverted temporarily from seeded
areas until they are stabilized. Aquatic and safety
benches should be stabilized with emergent wetland plants and wet seed mixes
Irrigation
Planting design should minimize the need for a
permanent irrigation system, however, irrigation
is an important aspect of any landscape establishment. New plantings need two to three years
of irrigation to become established but this varies
by location and seasonal conditions. Temporary
irrigation systems, hand watering or alternative
methods of irrigation for landscape establishment should be specified. After that period,
native plants will need little to no supplemental
irrigation. Where permanent irrigation systems
are utilized, they should include a weather-based
controller to avoid watering during wet weather.
Because bioretention soils are formulated to infiltrate, irrigation application rates must be properly designed to avoid overwatering and prevent
potential discharges via underdrains.
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Staking
amendments at any time before, during, and after
Provide extra support to trees, especially in high
construction and on a long-term basis. Instead, a
wind areas. They should be securely staked during
compost top dressing or application of compost
establishment and inspected once or twice a year
tea can be used to introduce nutrients and bene-
and following storm events. Stakes should be
ficial microorganisms to the soil.
removed as soon as they are no longer needed to
Apply compost mulch once per year in spring
stabilize the tree (between one and two years).
or fall or spray apply compost tea once per year
between March and June.
Weeding
Weeds compete with plants for nutrients, water
Plant Replacement
and sunlight. They should be regularly removed,
At the end of the first year and again at the end of
with their roots, by hand pulling or with manual
the two-year warranty period, all plants that do
pincer-type weeding tools. Care should be given
not survive must be replaced to avoid spreading
to avoid unnecessary compaction of soils while
disease, establishment of weeds in bare areas and
weeding.
reduced LID function. Before replacing with the
Appendix D: Planting & Soil Guidance
Section 7: Plant Establishment
and Maintenance
same species, determine if another species may
Leaf litter and trimmings present during mainte-
be better suited to the conditions.
nance should be removed from BMP rather than
left to decompose because nitrogen levels can be
affected and can change the function of the BMP.
Mulching
Compost Mulch (1” - 2”) should be applied to
specified areas to retain moisture, prevent erosion
and suppress weed growth. Reapply annually as
the mulch breaks down. Use a compost mulch
and avoid bark mulches that can float during
storm events.
Fertilization
The design for plantings shall minimize the
need for herbicides, fertilizers, pesticides, or soil
VOL 2
652
Hodler, T.W. and H.A. Schretter. 1986. The Atlas of
dix B and C, Chesapeake Bay Consortium, Silver
Georgia. University of Georgia Press, Athens.
Spring, MD.
Law, N.L., K. Cappiella and M.E. Novotney. 2008.
The Pennsylvania State University, College of Ag-
“The Need to Address Both Impervious and Pervi-
riculture, Cooperative Extension Service, File No.
Clausen, Ruth Rogers and Ekstrom, Nicolas, H.,
ous Surfaces in Urban Watershed and Stormwater
IVC9 10M386, U. Ed. 85-439 and File No. IVC9
1989. Perennials for American Gardens, Random
Management.” Journal of Hydrologic Engineering.
10M587 U.Ed. 86-356, Weed Identification, The
House, New York, NY.
14(4): 305-308.
Pennsylvania State University, College of Agricul-
Art, Henry W., 1986. A Garden of Wildflowers, 101
Native Species and How to Grow Them, Storey
Communications, Inc., Pownal, VT.
ture, Cooperative Extension Service, University
Dirr, Michael A., 1990. Manual of Woody Land-
Miles, Bebe, 1996. Wildflower Perennials for Your
scape Plants, Their Identification, Ornamental
Garden, A Detailed Guide to Years of Bloom from
Characteristics, Culture, Propagation, and Uses,
America’s Native Heritage, Stackpole Books, Me-
Tiner, Ralph W. Jr., April 1988. Field Guide to
4th Edition, Stipes Publishing Company, Cham-
chanicsburg, PA.
Non-Tidal Wetland Identification, U.S. Fish and
Park, PA.
Appendix D: Planting & Soil Guidance
References
Wildlife Service, Maryland Department of Natural
paign, IL.
Newcomb, Lawrence, 1977. Newcomb’s WildEngineering Technology Associates Inc. and
flower Guide, Little Brown and Company, Boston,
Biohabitats, Inc. (ETA&B), 1993. Design Manual for
MA.
Resources Maryland Geological
U.S. Army Corps of Engineers, Wetlands Research
Program (WRP), 1993. Baseline Site Assessments
Use of Bioretention in Stormwater Management,
Prince Georges County Dept. of Environmental
Schueler, Thomas R., July 1987. Controlling
for Wetland Vegetation Establishment. WRP
Resources, Upper Marlboro, MD.
Urban Runoff: A Practical Manual for Planning and
Technical Note VN-EV-2.1, August 1993.
Designing Urban BMP’s, Department of EnviGarber, M.P. and Moorhead, D.J., 1999. Selection,
ronmental Programs Metropolitan Washington
https://www.casqa.org/sites/default/files/down-
Production and Establishment of Wetland Trees
Council of Governments, Metropolitan Informa-
loads/central_coast_bioretention_plant_guid-
and Shrubs. University of Georgia, College of
tion Center, Washington, DC.
ance_print.pdf
Warnell School of Forest Resources Cooperative
Schueler, Thomas R., October 1996. Design of
http://www.sandiegocounty.gov/content/dam/
Extension Service.
Stormwater Wetland Systems: Guidelines for Cre-
sdc/dpw/WATERSHED_PROTECTION_PRO-
ating Diverse and Effective Stormwater Wetland
GRAM/susmppdf/lid_appendix_g_bioretention_
Georgia Wildlife Web: http://fishesofgeorgia.uga.
Systems in the Mid-Atlantic Region, Department
soil_specification.pdf
edu/gawildlife/index.php
of Environmental Programs Metropolitan Wash-
Agricultural & Environmental Sciences & Daniel B.
Greenlee, John, (photographed by Derek Fell)
ington Council of Governments, Metropolitan
Special thanks to the N.C. Department of Environ-
Information Center, Washington, D.C.
ment and Natural Resources’ Stormwater Permitting Program for assistance in developing content
1992. The Encyclopedia of Ornamental Grasses,
How to Grow and Use Over 250 Beautiful and
Schueler, Thomas R. and Claytor, Richard A., 1997.
Versatile Plants, Rodale Press, Emmas, PA.
Design of Stormwater Filtering Systems: Appen-
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for this appendix.
VOL 2
653
Appendix E: Best Management Practice Operations & Maintenance
Appendix E: Best Management Practice Operations & Maintenance
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Operations & Maintenance
Guidance Document
Georgia Stormwater Management Manual
Appendix E
Sponsored by:
Atlanta Regional Commission
Georgia Environmental Protection Division
Produced by:
AECOM
September 2015
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Operations & Maintenance Guidance Document
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Operations & Maintenance Guidance Document
Table of Contents
Introduction............................................................................................................................................ 1
Why are BMPs important? .................................................................................................................. 1
Importance of Inspection .................................................................................................................... 3
Maintenance Agreements ................................................................................................................... 4
General Maintenance.......................................................................................................................... 4
Vegetation Maintenance ..................................................................................................................... 5
When to Call a Professional................................................................................................................. 5
Additional Resources........................................................................................................................... 5
Bioretention Areas .................................................................................................................................. 7
Bioslopes .............................................................................................................................................. 13
Downspout Disconnects........................................................................................................................ 17
Dry Detention Basins............................................................................................................................. 21
Dry Enhanced Swales/Wet Enhanced Swales......................................................................................... 25
Dry Extended Detention Basins ............................................................................................................. 31
Dry Wells .............................................................................................................................................. 35
Grass Channel ....................................................................................................................................... 39
Gravity (Oil-Grit) Separators.................................................................................................................. 43
Green Roofs .......................................................................................................................................... 47
Infiltration Practice ............................................................................................................................... 51
Multi-Purpose Detention Basins............................................................................................................ 55
Organic Filter ........................................................................................................................................ 59
Permeable Bricks/Blocks ....................................................................................................................... 65
Pervious Concrete................................................................................................................................. 71
Porous Asphalt...................................................................................................................................... 75
Proprietary Systems .............................................................................................................................. 79
Rainwater Harvesting............................................................................................................................ 83
Regenerative Stormwater Conveyance.................................................................................................. 87
Sand Filters ........................................................................................................................................... 91
Site Restoration/Revegetation .............................................................................................................. 97
Soil Restoration................................................................................................................................... 101
Stormwater Planters/Tree Boxes......................................................................................................... 105
Operations & Maintenance Guidance Document
Stormwater Ponds .............................................................................................................................. 109
Stormwater Wetland........................................................................................................................... 115
Submerged Gravel Wetland ................................................................................................................ 121
Underground Detention...................................................................................................................... 127
Vegetated Filter Strips......................................................................................................................... 131
Operations & Maintenance Guidance Document
Introduction
The purpose of this Operation and Maintenance (O&M) Guidance Document is to define a Stormwater Best
Management Practice (also called a BMP or a practice), explain the importance of BMPs, show the
components of a typical BMP, and offer direction and information to keep the practice operational.
BMPs are structural practices designed to store or treat stormwater runoff to prevent or reduce pollution
from entering surface waters in the State of Georgia. They improve water quality by treating, detaining,
and retaining stormwater runoff. Detaining water is accomplished by creating a basin that holds the water
for a short period of time to allow some of the water to exfiltrate into the ground and the remainder of the
water to release slowly over a period of time. Retaining stormwater is similar to detaining stormwater;
however, the difference is the length of time the water is held. Retaining the water extends the period of
time the water is held.
In order for a BMP to work properly, it must be maintained. BMPs generally require annual inspections, but
more frequent routine inspections, such as after major storm events, may be required based on the site
conditions, past maintenance issues, or risk associated with safety due to non-performance of a structure.
The key to the long-term success of a BMP is routine inspection and maintenance.
Why are BMPs important?
When an area is being developed, the property or portions of the property often change from grassed or
wooded areas to paved areas. Grassed or wooded areas are pervious, which means that rainwater can
infiltrate into the ground. Paved areas, on the other hand, are impervious, which means that rainwater
cannot infiltrate into the ground. Because impervious areas cannot infiltrate water, increasing the amount
of impervious area during development results in higher volumes of stormwater runoff. This can cause
flooding and stormwater pollution, if not controlled through BMPs or other stormwater control measures.
BMPs designed to retain or infiltrate stormwater help recharge the ground water and create a pervious
area for the stormwater to infiltrate in the ground that has otherwise been altered by development. BMPs
can also help reduce erosion and habitat loss in streams caused by excessive runoff and can reduce
flooding and potentially make areas that are prone to flooding safer.
In addition, BMPs improve the quality of stormwater runoff from developed areas by removing pollutants
that can contaminate the surrounding streams, rivers, lakes, etc. which, in turn, may contaminate our
drinking water and food. Building BMPs in new developments, small business parks, or an individual
residential lot or residential subdivisions, provides opportunities to remove the pollutants generated by the
development. Example stormwater pollutants include sediment, excess nutrients, trash, fecal coliform, and
metals.
An operational BMP will include some variation of the following components as shown in Figure 1.
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Operations & Maintenance Guidance Document
Inlet Structure
Main Treatment
Emergency Overflow
Pretreatment
Outlet Structure
Figure 1 – Components of a BMP
The purpose and function of the main components of a BMP are described below:
2
Inlet structure – This component brings water into the practice.
The picture to the right shows an example of an inlet structure.
Another example of an inlet structure would be a catch basin.
Pretreatment – Pretreatment is designed to act as the first layer
of protection for the main treatment area. Protection is provided
by removing debris and coarse sediment, which reduces the
frequency of clogging in the main treatment area. The pretreatment area is designed to be
somewhat sacrificial so that it can be cleaned (or even replaced)
before the main treatment area of the practice. This provides
two maintenance benefits: ease of maintenance and less cost to
maintain. Because of this, maintenance on this section is
critical. The picture to the left shows a forebay, a type of
pretreatment device. Other types of pretreatment devices
include filter strips or grassy areas, grass channels, or rock lined
plunge pools.
Main treatment – The main treatment area is where
the majority of the stormwater treatment takes place
by removing sediment, nutrients, pollutants, etc. It is
also the area where stormwater is contained, either
through detention or retention, so that the water can
be discharged at a controlled rate. Therefore, it is
important that this section is routinely inspected and
maintained to ensure the practice is functioning
properly. The picture to the right shows an example of the main treatment area of a dry enhanced
swale. Main treatment areas treat stormwater runoff through different methods including
vegetated conveyance, infiltration, filtration, and settling. For example, the main treatment area of
a pond treats stormwater runoff primarily through settling, and the main treatment area of a sand
filter treats runoff through filtration. Specific maintenance concerns within a treatment area are
Operations & Maintenance Guidance Document
based on the method of treatment in a BMP. Examples of specific maintenance concerns for each
treatment method include the following:
o Vegetated Conveyance – Erosion
o Infiltration – Media clogging and clogging the underdrain
o Filtration – Media clogging and clogging the underdrain
o Settling – Excessive sediment, embankment failure, and debris or other issues at the outlet
structure.
Emergency overflow – An emergency overflow is
necessary for rain events that are larger than the practice was
designed to treat. This component will keep the area surrounding
the practice from flooding by allowing water to continue to flow
into a nearby drainage system or water body. Usually an
emergency overflow is an elevated grass or paved channel that
provides a way for stormwater to leave the BMP in an extreme
rain event. It should be noted that other types of emergency
overflow exist, and sometimes the emergency overflow and
outflow structure can be combined.
Outlet structure –The outlet structure allows treated water
to exit the practice. It is important that this component has
regular maintenance because if the outlet structure is
clogged, flooding will occur within the practice. The picture
on the right shows an example of an outlet structure. Other
examples of outlet structures include open pipes and
underdrains.
Importance of Inspection
Once the BMP is built, routine inspection is very important to keep the practice working properly and catch
potential problems before they become major problems (such as financial problems, legal problems, or
both). Another benefit of routine inspection is it allows you to see the area surrounding the site and
observe possible pollutants. For example, inspecting a BMP provides the opportunity to discover an
unstable or eroding area upstream of the BMP that may be providing excessive sediment to the BMP,
which could clog the practice quickly.
Items to check during routine inspections include, but are not limited to, the following:
Structural problems
Excessive ponding
Unhealthy or undesirable vegetation
Erosion
Stability of the surrounding ground
Clogging in the inlet or outlet structures or practice (from sediment, debris, or animals)
Deterioration of pipes (or observation wells)
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Operations & Maintenance Guidance Document
New pollutant sources
Infiltration rate by completing soil testing
Monitoring water levels in observation wells
Some BMPs require an underground system, making inspection difficult to conduct. Generally these
underground systems can be inspected by looking in the observation well. Sometimes, however,
maintenance requires an individual who is certified in Occupational Safety and Health Administration
(OSHA) confined space entry. Should there be a situation where a safety concern arises, the inspection
should stop and the safety concern addressed. Once the concern is addressed, the inspection can continue.
Signs indicating a potential maintenance problem with the underground system include the following:
Ponding water or water remaining in the observation well longer than the design time
Excessive sediment built up
Damage to the structure through compaction or settling
Maintenance Agreements
Oftentimes BMPs are covered by a maintenance agreement between the owner and local city or county or
other jurisdiction. Be sure to follow these maintenance requirements for the practice. This guidance
document provides helpful maintenance tips; however, the maintenance performed on the practice has to
meet the written standards and specifications in the maintenance agreement.
General Maintenance
Proper maintenance of each BMP is important to make sure
the components of the BMP are operating and functioning
the way the practice was designed to work. In other words,
if the structure is not properly working, this could lead to the
release of sediment, debris, and potential pollutants to a
receiving water. Generally, maintenance for each practice
includes:
Removing built up sediment, debris, or trash within
the practice
Removing debris from the inflow and outflow structure of the practice
Implementing erosion and sediment control practices on portions of the BMP where vegetation is
missing or in poor condition, replace vegetation
Inspecting the BMPs regularly to ensure the structural integrity and functionality of the BMP
Replacing the filter media (as needed)
Before and after photos are recommended as proof that maintenance has been performed.
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Operations & Maintenance Guidance Document
Vegetation Maintenance
Many BMPs include vegetation within or around the practice.
Vegetation is an important part of the practice and aids
infiltration and filtration. In addition, vegetation keeps the soil
from eroding and washing into nearby drainage systems and
water bodies. Finally, planting vegetation gives the area an
additional aesthetic value. General vegetation maintenance
includes:
Irrigating and weeding during the first few months to
establish the vegetation
Maintaining the vegetation to ensure the health and abundance of native species and plantings
Mowing, trimming, or pruning annually to prevent unwanted plants from growing in the practice
Removing grass clippings or dead leaves from the practice to prevent clogging
Minimize use of fertilizers and herbicides
When to Call a Professional
It is unlikely that a lawn care (or similar) company will know how to properly inspect or maintain a BMP.
Therefore, a qualified licensed professional is recommended to perform inspections and maintain the
practice. Sources for potential assistance include the following:
Local stormwater authority
Professional Engineer
Landscape architect
Extension service office
If it is decided that a licensed professional is not required to perform routine inspection and maintenance,
there are times where one will be necessary for major problems. Examples of when to call a licensed
professional include, but are not limited to, the following:
Significant damage to the structure
Significant sediment build-up
Excessive ponding of water
Abnormal odor
Signs of water seepage on the downstream side of a dam
Excessive erosion
Signs of pollutants other than sediment, such as chemical spills
Additional Resources
Although most BMPs are similar in that they improve water quality or control quantity, they differ through
site conditions, purpose, and function. Even two of the same type of BMP may require different
maintenance due to site conditions. BMPS are designed to remove pollutants from stormwater which
means they must be cleaned out periodically or they will cease to function. Since each BMP is unique there
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Operations & Maintenance Guidance Document
is no “one size fits all” approach to maintaining them. Some BMPs may also have more complicated
maintenance requirements that require special skills, tools, or equipment. When in doubt about the
proper way to test and maintain a BMP the owner should seek additional assistance.
The following is a list of suggested resources for inspection and maintenance of a BMP:
Georgia Stormwater Management Manual, Volumes 1 and 2
Georgia Department of Transportation Stormwater System Inspection and Maintenance Manual
Louisville Kentucky Metropolitan Sewer District Design Manual (Chapter 18.7)
City of Philadelphia Green Stormwater Infrastructure Maintenance Manual
North Carolina Division of Energy, Mineral and Land Resources Stormwater BMP Manual and BMP
Forms
West Virginia Stormwater Manual
Post Construction Manual Managing Stormwater in Your Community
Local stormwater authority
Local Cooperative Extension Service office
Manufacturer’s Guidelines (if applicable)
Within individual sections of this document, there may be references to certain tests such as soil infiltration
and pH testing. It is recommended that testing for infiltration or pH be performed periodically on certain
BMPs. In those cases it is recommended that a professional is consulted to ensure that the proper test is
being performed in accordance with accepted procedures. The following is a list of suggestions for finding a
professional:
6
Local stormwater authority
Local Cooperative Extension Service office
Local Professional Engineer
US Department of Agriculture Natural Resources Conservation Service
Operations & Maintenance Guidance Document
Bioretention Areas
A bioretention area is a shallow stormwater basin
or landscaped area with well-draining soils,
generally composed of sand, fines, and organic
matter, and vegetation to capture and treat
stormwater runoff. The basin or main treatment
area of the bioretention area includes plants to
aid in the filtration and infiltration of the
stormwater flowing through the practice. An
underdrain may be placed in the bioretention
area to collect runoff that has filtered through
the soil layers and pipe it to the storm sewer
system or a nearby water body.
There are some common problems to be aware of when maintaining a bioretention area. They include,
but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Establishing vegetation within the bioretention area
Clogging the underdrain (if applicable)
Mosquitoes breeding in the practice
Ant mounds
Maintaining the proper pH levels for plants
Pruning and weeding to maintain appearance
Routine maintenance should be performed on the bioretention areas to ensure that the structure is
functioning properly. Note that during the first year the bioretention area is built, maintenance may be
required at a higher frequency to ensure the proper establishment of vegetation in the practice.
In addition to routine maintenance, bioretention areas have seasonal and intermittent maintenance
requirements. For example, the following are maintenance activities and concerns specific to winter
months. Planting material should be trimmed during the winter, when the plants are dormant. In the
event of snow, ensure that snow does not pile up in the bioretention area. Accumulated snow adds
additional weight and may compact the bioretention area soil, which would reduce its infiltration
capacity. In addition, check to make sure that the materials used to de-ice the surrounding areas stay out
of the practice to avoid clogging and further pollution.
Bioretention areas should be inspected after a large rainstorm. Keep drainage paths, both to and from
the BMP, clean so that the water can properly infiltrate into the ground. Note that it might take longer
for the water to infiltrate into the ground during the winter months and early spring. Mulch the practice
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Operations & Maintenance Guidance Document
as needed to keep a thickness of 3-4 inches. Shredded hardwood mulch is preferred, and care should be
taken to keep the mulch from piling on the stems of the plants. For more information on vegetation in
bioretention areas, see Appendix D: Planting and Soil Guidance.
If the bioretention area is not draining properly, check for clogging of the inflow and outflow structures
as well as the infiltration rate of the soil media. If the soil is not draining properly, it could be clogged or
over-compacted. In a bioretention area, the media is likely to become clogged at the mulch or upper
layer of the soil first. If the media is clogged or over-compacted, then the media should be replaced.
Potential sources of excessive sediment that could clog the media include ant mounds and unstable soil
upstream of the practice. Possible sources of compaction are vehicles, such as tractors, traveling through
the practice. If the practice includes an underdrain, a structural repair or cleanout to unclog the
underdrain may be necessary.
In order to keep the water that exits the bioretention area clean, fertilizers should only be used sparingly
during the establishment of the practice. Once the vegetation in the practice has been established,
fertilizers should not be used. While vegetation in the bioretention area is important, the primary
purpose of a bioretention area is to act as a water quality device and introducing fertilizers into the
bioretention area introduces nutrients such as phosphorus and nitrogen that can pollute downstream
waters. In addition, bioretention areas should already be a nutrient rich environment that does not
require fertilization. To control animal nuisances and invasive species, pesticides (including herbicides,
fungicides, insecticides, or nematode control agents) should be used sparingly and only if necessary.
If designed correctly, there is no danger of bioretention areas becoming a breeding ground for
mosquitoes. A mosquito egg requires 24-48 hours to hatch. In addition, it takes 10-14 more days for the
larvae to develop and become an adult. By having a bioretention area that drains properly, it is unlikely
that a bioretention area would provide a habitat that could become a breeding area for mosquitoes.
Should the bioretention area become a breeding ground for mosquitoes, the problem is likely with the
soil media or the overflow structure which may need to be addressed.
The table below shows a schedule for when different maintenance activities should be performed on the
bioretention area.
Bioretention Area Typical Routine Maintenance Activities and Schedule
Activity
Schedule
8
Prune and weed to maintain appearance.
Dissipate flow when erosion is evident.
Remove trash and debris.
Remove sediment and debris from inlets and outlets.
Remove and replace dead or damaged plants.
Mow around the bioretention area as necessary, ensuring grass clippings are
not placed in the practice.
Observe infiltration rates after rain events. Bioretention areas should have no
standing water within 24 hours of a storm event.
Inspect for evidence of animal activity.
As needed or 4 times
during growing season
Operations & Maintenance Guidance Document
Activity
Inspect for erosion, rills, or gullies and repair.
Inspect filter strip/grass channel for erosion or gullying, if applicable. Re-seed
or sod as necessary.
Inspect trees and shrubs to evaluate their health, and remove and replace any
dead or severely diseased vegetation.
Obtain a mulch depth of at least 3 to 4 inches should be inspected and
obtained. Additional mulch should be added as necessary.
Trim planting material.
Inspect for snow accumulation.
Test the planting soils for pH levels. Consult with a qualified licensed
Professional to determine and maintain the proper pH levels.
Replace/repair inlets, outlets, scour protection or other structures as needed.
Implement plant maintenance plan to trim and divide perennials to prevent
overcrowding and stress.
Check soil infiltration rates to ensure the bioretention area soil is draining the
water at a proper rate. Re-aerate or replace soil and mulch layers as needed
to achieve infiltration rate of at least 0.5 inches per hour.
Schedule
Semi-annually in spring
and fall
As needed or during
winter months
Annually
2 to 3 years
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Operations & Maintenance Guidance Document
Bioretention Area
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment.
Weir – area is free of trash, debris, and
sediment is less than 25% of the total depth
of the weir.
Filter Strip or Grass Channels – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Rock Lined Plunge Pools – area is free of
trash debris and sediment. Rock thickness in
pool is adequate.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
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Operations & Maintenance Guidance Document
Bioretention Area
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc).
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Mulching depth of 3-4 inches is maintained.
Comment on mulch depth.
Native plants were used in the practice
according to the planting plan.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Results
Overall condition of Bioretention Area:
Additional Comments
Notes: *If a specific maintenance item was not checked, please check N/A and explain why in the appropriate
comment box.
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Operations & Maintenance Guidance Document
Bioslopes
Bioslopes are linear BMPs with a permeable media
that allows stormwater runoff to infiltrate and filter
through the practice before exiting through an
underdrain. High flows bypass the bioslope in the
form of sheet flow running over the bioslope.
Generally, a filter strip is placed before the bioslope
for pretreatment where it captures sediment and
debris and prevents premature clogging of the
bioslope. If the space available for the bioslope is
limited, a grass shoulder or pea gravel diaphragm
may be used as an alternate method of
pretreatment.
There are some common problems to be aware of when maintaining a bioslope. They include, but are
not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure as well as the underdrain
Undesirable vegetation
Erosion
Mowing the grass filter strip
Compaction
Typically bioslopes are indistinguishable from the surrounding areas, so it is recommended that GPS coordinates of the bioslope location be obtained and the BMP be staked with markers. If markers are
used, they should be placed at both ends at the toe of the slope and every 50 feet.
Routine inspection and maintenance should be performed on the bioslope to ensure that the practice is
functioning properly. Generally maintenance will consist of removing debris and trash that could
accumulate on the practice and cause clogging. Other routine maintenance includes mowing the
bioslope and removing grass clippings. Mowing and landscaping crews should be alerted not to access
the bioslope during wet conditions to avoid damaging or rutting the area.
Inspect the bioslope after a large rainstorm. Keep drainage paths (both to and from the BMP) clean to
promote sheet flow and allow stormwater runoff to be routed in the intended direction.
In addition to routine maintenance, bioslopes have seasonal and intermittent maintenance
requirements. For example, during the winter months, the bioslope should be inspected after a snow
event (this is specific to northern areas of Georgia). Accumulated snow adds additional weight and may
compact the media, which would reduce its infiltration capacity. In addition, check to make sure that
the materials used to de-ice the surrounding areas stay out of the practice to avoid clogging and further
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Operations & Maintenance Guidance Document
pollution. Also, note that it might take longer for the water to infiltrate into the ground during the
winter months and early spring.
If the bioslope is not draining properly, it may be necessary to repair or unclog the underdrain as well as
the inflow and outflow structures. Another possible reason the bioslope is not draining properly could
be due to clogged or over-compacted bioslope media. If the mix becomes clogged or over-compacted,
then it should be replaced. The degree of required media removal and replacement can vary depending
on the characteristics of the contributing drainage area and the consistency of regular maintenance. For
example, it is likely that removal and replacement of the top two to five inches of media will be
necessary every three to five years for low sediment applications. Media replacement may be needed
more often for areas of high sediment yield or high oil and grease.
The table below shows a schedule for when different maintenance activities should be performed on
the bioslopes.
Bioslope Typical Routine Maintenance Activities and Schedule
Activity
14
Clear debris in inlets and outlets.
Mow and stabilize the area surrounding the bioslope. Remove grass
clippings.
Ensure that activities in the drainage area minimize oil/grease and
sediment entry to the system.
Remove trash and debris.
Stabilize eroded areas on the bioslope.
Ensure that flow is not bypassing the facility.
Ensure that no noticeable odors are detected outside the facility.
Mow the bioslope grass using a retractable arm mower to avoid
compaction. Grass height should be mowed to a height of 6 to 15
inches. Remove grass clippings.
Ensure that gravel spreader or other structural elements of the bioslope
are in good condition and free of debris.
Test the permeability of the bioslope media using a hydraulic
conductivity test. Replace the media as needed.
Flow test the cleanouts to look for signs indicating the underdrain
system is clogged.
Evaluate sediment accumulation and remove once it reaches or
exceeds a depth of 3 inches.
Schedule
As needed or 4 times
during growing season
Monthly
Annually
Operations & Maintenance Guidance Document
Bioslope
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet is mowed and grass
clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet.
Water is going through the bioslope (i.e. no
evidence of water going around the BMP).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment
Area is free of trash debris and sediment.
Area has been mowed and grass clippings
are removed. Grass seems healthy and there
are no bare areas or dying grass.
No evidence of erosion or gullies.
Area is free of undesirable vegetation.
No standing water in the practice.
No sediment accumulation within in the
pretreatment area.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
No evidence of erosion of gullies within the
bioslope.
No evidence of long-term ponding or
standing water in the practice (examples
include: stains, odors, mosquito larvae, etc).
Practice seems to be working properly. No
settling around the structure. Comment on
overall condition of practice.
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Operations & Maintenance Guidance Document
Bioslope
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
No undesirable vegetation within the
bioslope.
Area has been mowed at a height of 6-15
inches. Grass clippings are removed.
Cleanout caps for underdrain are not
damaged or missing.
Flow testing has been performed on
bioslope to determine if underdrain is
clogged.
Observation well has no signs of standing
water.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet pipe is not damaged or clogged and is
in good condition.
Results
Overall condition of Bioslope:
Additional Comments
Notes: *If a specific maintenance item was not checked, please check N/A and explain why in the appropriate
comment box.
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Operations & Maintenance Guidance Document
Downspout Disconnects
A downspout disconnect is a method to spread rooftop runoff
from individual downspouts across lawns and other pervious
areas, where it is slowed, filtered and allowed to infiltrate into
the native soils. Downspout disconnects can be used in
conjunction with other BMPs such as bioretention areas,
enhanced swales, and vegetated filter strips. If the downspout
disconnect is used in conjunction with another BMP, then that
BMP will need to be inspected and maintained as well.
Common problems that can occur when maintaining a
downspout disconnect include, but are not limited to, the
following:
Clogged gutters or downspouts
Loose gutters or downspouts
Water not draining away from buildings
Poorly draining soils
Poorly functioning splash blocks
Cracks within the downspout extension
Routine maintenance should be performed on the downspout disconnects to ensure that the practice is
properly functioning to ensure that a water problem is not created for neighbors. Ensure that there is no
erosion occurring at the base of the downspout. Other specific maintenance items depend on where the
downspout has been disconnected from the storm or sanitary sewer and where the flow has been
redirected.
Inspect the downspout disconnect after a large rainstorm. Make sure that there is no evidence of a leak
in the gutters or downspouts, and ensure that rooftop runoff is directed through the system. During the
winter months, downspouts should be inspected for cracks caused by water freezing in the downspout.
The table below shows a schedule for when different maintenance activities should be performed
routine maintenance activities typically associated with downspout disconnects.
Downspout Disconnect Typical Routine Maintenance Activities and Schedule
Activity
Pervious areas located below simple downspout disconnections
should be watered to promote plant growth and survival.
Inspect the pervious areas located below simple downspout
disconnections following rainfall events. Plant replacement
vegetation in any eroded areas.
Schedule
As Needed
(Following Construction)
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Operations & Maintenance Guidance Document
18
Activity
Inspect pervious area located below simple downspout
disconnection. Maintain vegetation (e.g., mow, prune, trim) as
needed.
Remove accumulated trash and debris in pervious area located
below the simple downspout disconnection.
Inspect gutters and downspouts. Remove any accumulated
leaves or debris.
Inspect the pervious areas located below simple downspout
disconnections for erosion and the formation of rills and gullies.
Plant replacement vegetation in any eroded areas.
Inspect the pervious areas located below simple downspout
disconnections for dead or dying vegetation. Plant replacement
vegetation as needed.
Schedule
Regularly
(Monthly)
Annually
(Semi-Annually During First Year)
Operations & Maintenance Guidance Document
Downspout Disconnects
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
Access to the site is adequately maintained
for inspection and maintenance.
Gutters are clean. No sediment, debris, or
trash to clog the system.
Downspouts are properly fastened to convey
water from the roof.
Downspouts are free of trash, debris, and
sediment and conveying water properly.
No evidence of leaks at joints or other
components of downspouts.
Erosion control mats are present on site to
prevent erosion on pervious area below
downspout disconnects.
Area is clean (trash, debris, grass clippings,
etc. removed).
Vegetation is in place and is healthy. No bare
or dying areas.
Unwanted vegetation is trimmed and
removed.
No evidence of erosion, scour, or flooding.
Results
Overall condition of Downspout
Disconnects:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Dry Detention Basins
A dry detention basin is a storage basin designed to
provide water quantity control through detention
of stormwater runoff. The purpose of detention is
to allow some of the water to exfiltrate into the
ground and the remainder of the water to release
slowly over a period of time to reduce downstream
water quantity impacts. Dry detention basins are
designed to completely drain following a storm
event and are normally dry between rain events.
They provide limited pollutant removal benefits and
are not intended for water quality treatment alone.
There are some common problems to be aware of when maintaining a dry detention basin. They include,
but are not limited to, the following:
Sediment build-up
Trash, litter, and debris accumulation
Clogging and structural repairs in the inlet and outlet structures
Establishing vegetation within the dry detention basin
Erosion
Mowers compacting and rutting the basin bottom
Mosquitoes breeding in the practice
Ant mounds
Routine maintenance should be performed on the dry detention basins to ensure that the structure is
properly functioning. Note that during the first year the dry detention basin is built, maintenance may be
required at a higher frequency to ensure the proper establishment of vegetation in the practice. In the
event of snow, check to make sure that the materials used to de-ice the surrounding areas stay out of the
practice to avoid clogging and further pollution.
Dry detention basins should be inspected after a large rainstorm. Keep drainage paths, both to and from
the BMP, clean so that the water can properly infiltrate into the ground. Note that it might take longer
for the water to infiltrate into the ground during the winter months and early spring. If the dry detention
basin is not draining properly, check for clogging of the inflow and outflow structures.
If the forebay or dry detention basin has received a significant amount of sediment over a period of time,
then the sediment at the bottom of the forebay or dry detention basin may need to be removed.
Accumulated sediment in the practice decreases the available storage volume and affects the basin’s
ability to function as it was designed.
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Operations & Maintenance Guidance Document
If designed and maintained correctly, dry detention basins should not become a breeding ground for
mosquitoes. A mosquito egg requires 24-48 hours to hatch. In addition, it takes 10-14 more days for the
egg to develop and become an adult. By having a dry detention basin that drains properly, it is unlikely
that a dry detention basin would provide a habitat that could become a breeding area for mosquitoes.
Should the dry detention basin become a breeding ground for mosquitoes, the problem is likely with the
overflow structure which may need to be addressed.
The table below shows a schedule for when different maintenance activities should be performed on the
dry detention basins.
Dry Detention Basin Typical Routine Maintenance Activities and Schedule
Activity
22
Remove debris from basin surface to minimize outlet clogging and
improve aesthetics.
Note erosion of detention basin banks or bottom
Inspect for damage to the embankment.
Monitor for sediment accumulation in the facility and forebay.
Examine to ensure that inlet and outlet devices are free of debris and
operational.
Remove sediment buildup.
Repair and revegetate undercut and/or eroded areas.
Perform structural repairs to inlet and outlets.
Repair undercut or eroded areas.
Mow side slopes.
Seed or sod to restore dead or damaged ground cover.
Mow to limit unwanted vegetation.
Litter/ Debris Removal.
Schedule
Annually and following
significant storm events
As needed based on
inspection
Routine
Operations & Maintenance Guidance Document
Dry Detention Basin
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Inlet pipe is in good condition and is not
clogged.
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (forebay)
Area is free of trash, debris, and sediment.
Sediment accumulation is less than 50% of
the forebay volume.
No undesirable vegetation within the
forebay. Weeds are removed to prevent
clogging.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc.).
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Operations & Maintenance Guidance Document
Dry Detention Basin
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Basin seems to be working properly. No
settling around the basin. Comment on
overall condition of basin.
Vegetation within and around practice is
maintained. Grass clippings are removed.
Sediment accumulation within dry detention
basin is less than 3 inches.
No standing water within the basin.
No evidence of use of fertilizer on grass
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
No shrubs or trees growing on embankment.
No signs of seepage on the downstream
face.
No signs of animal activity.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
All moveable components are operational.
Results
Overall condition of Dry Detention Basin:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Dry Enhanced Swales/Wet Enhanced Swales
An enhanced swale is a vegetated open channel
designed to capture and treat stormwater
runoff within dry or wet cells formed by check
dams or other means. Enhanced swales are
generally shallow, wide, and vegetated to help
slow and filter stormwater runoff.
There are two different types of enhanced
swales. The first is a dry swale which includes a
filter bed of prepared soil that overlays an
underdrain system. They are designed to let
stormwater be filtered or infiltrated through the
bottom of the swale. Because they are dry most of the time, they are often the preferred option in
residential settings. The second type of enhanced swale is a wet swale. Wet swales are designed to
retain water or marshy conditions that support wetland vegetation. Because this practice is meant to
retain water, they are generally used in areas with a high water table or poorly drained soils. Wet
swales achieve pollutant removal both from sediment accumulation and biological removal.
There are some common problems to be aware of when maintaining an enhanced swale. They include,
but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Establishing vegetation
Clogging in the underdrain (if applicable)
Mosquitoes breeding in the practice
Ant mounds
Maintaining the proper pH levels for plants
Pruning and weeding to maintain appearance
Routine inspection and maintenance should be performed on the dry or wet enhanced swale to ensure
that the practice is properly functioning. Note that during the first year the enhanced swale is built,
maintenance may be required at a higher frequency to ensure the proper establishment of vegetation in
the practice. For more information on vegetation within a swale, see Appendix D: Planting and Soil
Guidance. Enhanced swales should be inspected after a large rainstorm. Keep drainage paths, both to
and from the BMP, clean so that the water can properly flow in and out of the practice.
In addition to routine maintenance, dry or wet enhanced swales have seasonal and intermittent
maintenance requirements. For example, during the winter months, the enhanced swale should be
inspected after a snow event (this is specific to northern areas of Georgia). Accumulated snow adds
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Operations & Maintenance Guidance Document
additional weight and may compact the dry enhanced swale soil, which would reduce its infiltration
capacity. In addition, check to make sure that the materials used to de-ice the surrounding areas stay
out of the practice to avoid clogging and further pollution. Note that it might take longer for the water
to infiltrate into the ground during the winter months and early spring.
If the dry enhanced swale is not draining properly, check for clogging in the inflow and outflow
structures. Another consideration would be the permeable soil layer, which could be clogged or overcompacted. In a dry enhanced swale, the media is likely to become clogged at the upper layer of the soil
first. Potential sources of excessive sediment that could clog the media include ant mounds and
unstable soil upstream of the practice. Possible sources of compaction are vehicles, such as tractors,
traveling through the practice. If the media is clogged or over-compacted, then the media should be
replaced. If the practice includes an underdrain, a structural repair or cleanout to unclog the underdrain
may be necessary.
In order to keep the water that exits the dry or wet enhanced swale clean, fertilizers should only be used
sparingly during the establishment of the practice. Once the vegetation in the practice has been
established, fertilizers should not be used. While vegetation in the enhanced swale is important, the
primary purpose of an enhanced swale is to act as a water quality device, and introducing fertilizers into
the enhanced swale introduces nutrients such as phosphorus and nitrogen that can pollute downstream
waters. In addition, enhanced swales should already be nutrient rich environments that do not require
fertilization. To control animal nuisances and invasive species, pesticides (including herbicides,
fungicides, insecticides, or nematode control agents) should be used sparingly and only if necessary.
If designed and maintained correctly, there is no danger of dry enhanced swales becoming a breeding
ground for mosquitoes. A mosquito egg requires 24-48 hours to hatch. In addition, it takes 10-14 more
days for the egg to develop and become an adult. By having a dry enhanced swale that drains properly
(within 24-48 hours), it is unlikely that a dry enhanced swale would provide a habitat that could become
a breeding area for mosquitoes. Should the dry enhanced swale become a breeding ground for
mosquitoes, the problem is likely with the soil media or the overflow structure which may need to be
addressed.
The table below shows a schedule for when different maintenance activities should be performed on an
enhanced swale.
Enhanced Swale Typical Routine Maintenance Activities and Schedule
Activity
26
Prune and weed to maintain appearance.
Dissipate flow when erosion is evident.
Remove trash and debris.
Remove sediment and debris from inlets and outlets.
Remove sediment build-up within the bottom of the swale once it has
accumulated to 25% of the original design volume.
Remove and replace dead or damaged plants.
Schedule
As needed or 4 times
during growing season
Operations & Maintenance Guidance Document
Activity
Mow the dry enhanced swale as necessary to maintain a grass height of 4-6
inches, ensuring grass clippings are not placed in the practice.
Observe infiltration rates after rain events. Dry enhanced swales should have
no standing water within 48 hours of a storm event (though 24 hours is more
desirable).
Inspect for evidence of animal activity.
Inspect for erosion, rills, or gullies and repair.
Replant wetland species (for wet swale) if not sufficiently established.
Test the planting soils for pH levels. Consult with a qualified licensed
Professional to determine and maintain the proper pH levels.
Inspect pea gravel diaphragm for clogging.
Trim planting material.
Inspect for snow accumulation.
Replace/repair inlets, outlets, scour protection or other structures as needed.
Implement plant maintenance plan to trim and divide perennials to prevent
overcrowding and stress.
Check soil infiltration rates to ensure the dry enhanced swale soil is draining
the water at a proper rate. Roto-till or cultivate the surface of the sand/soil
bed of dry swales if the swale does not draw down within 48 hours.
Schedule
Annually
(Semi-annually the first
year)
As needed or during
winter months
2 to 3 years
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Operations & Maintenance Guidance Document
Dry Enhanced Swale/Wet Enhanced Swale
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed (for dry
enhanced swale).
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment.
Weir – area is free of trash, debris, and
sediment is less than 25% of the total depth
of the weir.
Filter Strip or Grass Channels – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Rock Lined Plunge Pools – area is free of
trash debris and sediment. Rock thickness in
pool is adequate.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
For dry enhanced swale, no evidence of
long-term ponding or standing water in the
ponding area of the practice (examples
include: stains, odors, mosquito larvae, etc).
Plants were used in the practice according to
the planting plan.
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Operations & Maintenance Guidance Document
Dry Enhanced Swale/Wet Enhanced Swale
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
No evidence of undesirable vegetation.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
No evidence of erosion around the sides of
the check dam.
Cleanout caps are in place and in good
condition (for dry enhanced swale).
The underdrain appears to be unclogged
evidenced by water exiting the practice
freely (for dry enhanced swale).
Pea gravel diaphragm or other flow spreader
is clean and working properly.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Results
Overall condition of Enhanced Swale:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Operations & Maintenance Guidance Document
Dry Extended Detention Basins
A dry extended detention basin provides
temporary storage of stormwater runoff to
control the peak rate of runoff by allowing the
stored water to release slowly over a period of
time. This practice is mostly used to control
water quantity, although some water quality
benefits can be obtained by the settling of
floatables and sediment. This extended version
of a dry detention basin is designed to maximize
water quality benefits.
There are some common problems to be aware of when maintaining a dry extended detention basin.
They include, but are not limited to, the following:
Sediment build-up
Trash, litter, and debris accumulation
Clogging in the inlet and outlet structures
Erosion
Structural repairs to inlets and outlets
Mowers compacting and rutting the basin bottom
Clogging in the emergency spillway
Mosquitoes breeding in the practice
Routine maintenance should be performed on dry extended detention basins to ensure that the
structure is properly functioning. Note that during the first year the dry extended detention basin is
built, maintenance may be required at a higher frequency to ensure the proper establishment of
vegetation in the practice. In the event of snow, check to make sure that the materials used to de-ice
the surrounding areas stay out of the practice to avoid clogging and further pollution.
Inspect the dry extended detention basin after a large rainstorm. Keep drainage paths (both to and from
the BMP) clean so that the water can properly flow into the basin. If the dry extended detention basin is
not draining properly, check for clogging of the inflow and outflow structures.
If the forebay or dry detention basin has received a significant amount of sediment over a period of
time, then the sediment at the bottom of the forebay or dry detention basin may need to be removed.
Accumulated sediment in the practice decreases the available storage volume and affects the basin’s
ability to function as it was designed.
The table on the next page shows a schedule for when different maintenance activities should be
performed on the dry extended detention basin.
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Operations & Maintenance Guidance Document
Dry Extended Detention Basin Typical Routine Maintenance Activities and Schedule
Activity
32
Remove trash, sediment, and debris from forebay and inlet and outlet
structures.
Mow the embankment and maintenance access. Periodically mow
along maintenance rights-of-ways and the embankment. Remove grass
clippings.
Repair and re-vegetate eroded areas.
Remove and dispose of vegetation that may hinder the operation of the
pond.
Perform structural repairs to pond, outlet structures, embankments,
control gates, valves, or other mechanical devices.
Remove sediment when volume of pond is significantly reduced.
Schedule
Monthly or as needed
As needed
As needed (roughly
every 20-50 years, but
will vary based on the
characteristics of the
drainage area and
amount of sediment
entering the practice)
Operations & Maintenance Guidance Document
Dry Extended Detention Basin
Condition
Inspection Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet/Outlet Structure
Drainage ways to and from the practice is
free of trash, debris, large branches, etc.
Area around the inlet/outlet structure is
mowed and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet/outlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
No signs of significant sediment
accumulation.
Concrete is in good condition. No signs of
cracks.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Vegetation seems healthy. No signs of bare
spots or dead vegetation.
No signs of undesirable vegetation growth.
No signs of excessive sedimentation.
No signs of pollution draining into the
practice (oil sheens, discolored or unnatural
water, odor, etc.).
Embankment and Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No signs of animal activity in embankment.
No signs of seepage on downstream side of
embankment.
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Operations & Maintenance Guidance Document
Dry Extended Detention Basin
Inspection Item
Condition
Good
Marginal
Poor
N/A*
Comment
No signs structural deformation of
embankment.
No obstructions in spillway.
Results
Overall condition of Dry Extended Detention
Basin:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Operations & Maintenance Guidance Document
Dry Wells
Dry wells are located below the surface of development sites,
and consist of shallow excavations, typically filled with stone,
that are designed to intercept and temporarily store postconstruction stormwater runoff until it infiltrates into the
underlying and surrounding soils. If properly designed, they
can provide significant reductions in post-construction
stormwater runoff rates, volumes, and pollutant loads on
development sites.
There are some common problems to be aware of when
maintaining a dry well. They include, but are not limited to,
the following:
Sediment build-up
Clogging in the gutters, pipes, and downspouts
Routine inspection and maintenance should be performed on the dry wells to ensure that the structure
is functioning properly. Dry wells should be inspected after a large rainstorm. Keep gutters, pipes, and
downspouts draining to the dry well clean and free of trash and debris. Every dry well should include an
observation well to observe the draw down time of the dry well following a storm event. This is
important to determine if clogging is occurring within the dry well.
If water is not draining to the dry well properly, check for clogging in the gutters, pipes, and
downspouts. If the dry well is not draining properly the filter fabric may be clogged. The filter fabric lines
the top and sides of the dry well. In addition, if the soil is not draining properly, the soil may be overcompacted. In a dry well, the media is likely to become clogged at the upper layer of the soil first. If the
media is clogged or over-compacted, then the filter fabric and media should be replaced.
The table below shows a schedule for when different maintenance activities should be performed on
the dry well.
Dry Well Typical Routine Maintenance Activities and Schedule
Activity
Schedule
If applicable, water to promote plant growth and survival within
landscaped area over the top of the dry well.
If applicable, inspect vegetative cover on the surface of the dry
well following rainfall events. Plant replacement vegetation in any
eroded areas.
As Needed
If applicable, inspect gutters and downspouts. Remove any
accumulated leaves or debris.
Inspect dry well following rainfall events. Check observation well to
Annually
(Semi-Annually During
First Year)
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Operations & Maintenance Guidance Document
Activity
36
ensure that complete drawdown has occurred within 24 hours
after the end of a rainfall event. Failure to drawdown within this
timeframe may indicate dry well failure.
If applicable, inspect pretreatment devices for sediment
accumulation. Remove accumulated trash and debris.
Inspect top layer of filter fabric for sediment accumulation.
Remove and replace if clogged.
Perform total rehabilitation of the dry well, removing dry well
stone and excavating to expose clean soil on the sides and bottom
of the well.
Schedule
Upon Failure
Operations & Maintenance Guidance Document
Dry Well
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean around the practice (trash,
debris, grass clippings, etc. removed).
Gutters, pipes, and downspouts to the dry
well are free of trash, debris, leaves, etc.
No evidence of structural deficiencies or
settling around the structure.
Main treatment area is free of trash, debris,
and sediment.
Sediment has not accumulated and clogged
filter fabric.
Preatreatment is in place if dry well does not
receive roof top runoff. Pretreatment is in
good condition.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc).
The observation well is capped and locked
when not in use.
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Results
Overall condition of Dry Well:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Operations & Maintenance Guidance Document
Grass Channel
Grass channels are vegetated open channels
designed to enhance water quality by settling
suspended solids through filtration, infiltration, and
biofiltration. This practice offers a method to
manage pollution while also conveying stormwater
runoff. Grass channels are well suited to a number
of applications and land uses, including treating
runoff from roads and highways and pervious
surfaces. Grass channels are broad and shallow
channels that are generally positioned parallel to
roadways or other impervious areas. They can also
be used as a single BMP, a pretreatment to another
BMP, or as a link between other BMPs.
There are some common problems to be aware of when maintaining a grass channel. They include, but
are not limited to, the following:
Trash, litter, and debris accumulation
Watering the practice during dry periods
Establishing vegetation within the grass channel
Clogging in the inlet and outlet pipes
Ant mounds
Erosion
Routine inspection and maintenance should be performed on the grass channels to ensure that the
practice is functioning properly. Routine maintenance tasks include removing trash from the grass
channel and ensuring that grass clippings and other debris are removed from the channel.
In order to keep the water that exits the grass channel clean, fertilizers should only be used sparingly
during the establishment of the practice. Once the vegetation in the practice has been established,
fertilizers should not be used. While vegetation in the grass channel is important, a primary purpose of a
grass channel is to act as a water quality device and introducing fertilizers into the grass channel
introduces nutrients such as phosphorus and nitrogen that can pollute downstream waters. To control
animal nuisances and invasive species, pesticides (including herbicides, fungicides, insecticides, or
nematode control agents) should be used sparingly and only if necessary.
The table on the following page show routine maintenance activities typically associated with grass
channels.
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Operations & Maintenance Guidance Document
Grass Channel Typical Routine Maintenance Activities and Schedule
Activity
40
Mow grass to maintain a height of 3 to 4 inches. Remove grass
clippings.
Repair eroded or bare spots.
Remove accumulated sediment, trash, and debris.
Water the practice during dry condition while vegetation is establishing.
Inspect grass alongside slopes for erosion and formation of rills or
gullies and correct.
Remove sediment from bottom of channel once sediment is 25% of the
original design volume.
Remove trash and debris accumulated in the inflow forebay.
Inspect and correct erosion problems in the sand/soil bed of dry swales.
Based on inspection, plant an alternative grass species if the original
grass cover has not been successfully established.
Inspect pea gravel diaphragm for clogging and correct the problem.
Schedule
As needed
Annually (Semiannually the first year
and then annually
thereafter)
Operations & Maintenance Guidance Document
Grass Channel
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet is mowed and grass
clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet.
No signs of clogging or damage around the
inlet.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment.
Filter Strip or Grass Channels – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
No evidence of erosion in the practice.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc).
No undesirable vegetation located within
the practice.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
blackened roots, etc.).
Grass within and around practice is
maintained at the proper height (3-4 inches).
Grass clippings are removed.
Grass cover seems healthy with no bare
spots or dying grass.
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Operations & Maintenance Guidance Document
Grass Channel
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
No accumulating sediment within the grass
channel.
Outlet
Outlet is free of trash, debris, and sediment.
No evidence of erosion, scour, or flooding.
Results
Overall condition of Grass Channel:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Gravity (Oil-Grit) Separators
Gravity (oil-grit) separators are designed to treat
stormwater runoff by removing settleable solids, oil
and grease, debris and floatables from stormwater
runoff through gravitational settling and trapping of
pollutants. Typically these systems are underground
and installed at inlet structures. Gravity (oil-grit)
separators come in different shapes and sizes ranging
from small to large systems that have multiple
chambers that use gravity to separate sediment,
floatables, and oil/grease from stormwater runoff.
There are some common problems to be aware of when maintaining gravity (oil-grit) separators. They
include, but are not limited to, the following:
Clogging in the inlet and outlet structure
Sediment and oil/grease build-up
Inability to remove dissolved or emulsified oils and pollutants such as coolants, soluble
lubricants, glycols and alcohols
Routine inspection and maintenance should be performed on the gravity separator to ensure that the
structure is functioning properly. Typical maintenance will include removing accumulated sediment and
pressure washing the system to remove blockage. Additional maintenance may be necessary if a spill
occurs upstream of the system and drains into the practice. The contributing drainage areas should be
maintained to limit the amount of trash and debris that enter the practice.
Gravity (oil-grit) separators should be inspected after a large rainstorm. It may be necessary to make
repairs to the inlets, outlets, and other structural components. Check with the manufacturer’s guidelines
for recommended maintenance on the system. In addition, it is required that a maintenance plan be
developed and implemented.
The table below shows a schedule for when different maintenance activities should be performed on
gravity (oil-grit) separators.
Gravity (Oil-Grit) Separators Typical Routine Maintenance Activities and Schedule
Activity
Keep contributing drainage area free of trash, chunks of sediment, and debris.
Cleanout if spill occurs and enters the system.
Repair structural components.
Check to make sure practice is draining properly.
Schedule
As needed (quarterly or
after a large rain storm
event)
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Operations & Maintenance Guidance Document
Activity
44
Check maintenance plan and/or manufacturer’s guidelines for additional
maintenance needs.
Check system to make sure no blockage or significant sediment accumulation
is occurring in the system.
Cleanout system with vacuum or boom trucks.
Remove sediment and oil from chambers
Schedule
Quarterly
Annually
Operations & Maintenance Guidance Document
Gravity (Oil-Grit) Separator
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Contributing drainage area is clean (trash,
debris, grass clippings, etc. removed).
Inlet and outlet pipes are clean; stormwater
can enter and exit the practice without being
blocked.
Overflow structure is in good condition and
clean.
Maintenance is being performed according
to manufacturer’s guidelines.
Maintenance is being performed according
to maintenance plan.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Results
Overall condition of Gravity (Oil-Grit)
Separator:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Green Roofs
Green roofs represent an alternative to
traditional impervious roof surfaces.
They typically consist of underlying
waterproofing and drainage materials
and an overlying engineered growing
media that is designed to support plant
growth. Stormwater runoff is captured
and temporarily stored in the
engineered growing media, where it is
subjected to evaporation and
transpiration before being conveyed back into the storm drain system. This allows green roofs to
provide measurable reductions in post-construction stormwater runoff rates, volumes and pollutant
loads on development sites. There are two different types of green roof systems, intensive and
extensive. Intensive green roofs have a thick layer of soil, can support a diverse plant community, and
may include trees. Extensive green roofs have a much thinner layer of soil that is comprised primarily of
drought tolerant vegetation. Plants chosen for a green roof should be compatible for warmer
temperatures found on rooftops.
There are some common problems to be aware of when maintaining a green roof. They include, but are
not limited to, the following:
Clogging in the outlet structure
Establishing vegetation within the green roof
Clogging the drainage layer
Maintaining the proper pH levels for plants
Pruning and weeding to maintain appearance and prevent roots from potentially compromising
the waterproof membrane
Routine inspection maintenance should be performed on green roofs to ensure that the system is
functioning properly. Note that during the first year the green roof is built, inspection and maintenance
will be required at a higher frequency to ensure the proper establishment of vegetation in the practice.
Frequent watering and weed germination during establishment is key to maintaining a healthy green
roof and preventing more long-term maintenance problems. For more information on green roof
vegetation, see Appendix D: Planting and Soil Guidance.
Green roofs should be inspected after a large rainstorm. Keep drainage paths, both to and from the
BMP, clean so that the water can properly flow to the plants and keep the vegetation healthy. Impaired
drainage can cause damage to the roofing system and add structural loads beyond the building’s design
limits; this could lead to structural failure. Note that it might take longer for the water to infiltrate into
the system during the winter months and early spring.
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Operations & Maintenance Guidance Document
In order to keep the water that exits the green roof clean, fertilizers should be used only be used
sparingly and during the establishment of the practice. Once the vegetation in the practice has been
established, fertilizers should not be used. While vegetation in the green roof is important, the primary
purpose of a green roof is to act as a water quality device and introducing fertilizers into the green roof
introduces nutrients such as phosphorus and nitrogen that can pollute downstream waters. In addition,
after initial vegetation establishment, green roofs should already be a nutrient rich environment that
does not require fertilization. To control animal nuisances and invasive species, pesticides (including
herbicides, fungicides, insecticides, or nematode control agents) should be used sparingly and only if
necessary.
The table below shows a schedule for when different maintenance activities should be performed on a
green roof.
Green Roofs Typical Routine Maintenance Activities and Schedule
Activity
Schedule
48
Water to promote plant growth and survival.
Inspect green roof for dead or dying vegetation. Dead vegetation should
be removed along with any woody vegetation. Plant replacement
vegetation as needed.
Mow and remove grass clippings.
Remove trash, debris, and other pollutants from the rooftop
Observe infiltration rates after rain events, the roof should drain in 24
hours of rain event.
Inspect waterproof membrane for leaks. Repair as needed.
Inspect outflow and overflow areas for trash, debris, and sediment
accumulation. Remove any accumulated sediment or debris.
Inspect vegetation for signs of stress. If vegetation begins showing
signs of stress, including drought, flooding, disease, nutrient deficiency
or insect attack, treat the problem or replace the vegetation.
Weed and prune vegetation.
Test the planting soils for pH levels. Consult with a qualified licensed
Professional to determine and maintain the proper pH levels.
As Needed
Semi-Annually
(Quarterly During First
Year)
Annually
Operations & Maintenance Guidance Document
Green Roof
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet and outlet pipes are free of trash,
debris, etc.
Inspect waterproof membrane.
No signs of structural deficiency or settling.
Comment on overall condition of roof.
Water can flow freely in the drainage routes,
no obstructions.
Native plants were used in the practice
according to the landscaping plan. Plants
seem to be in good condition. Comment on
condition of plants.
No unwanted vegetation in the practice.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
No evidence of long-term ponding or
standing water (examples include: stains,
odors, mosquito larvae, etc).
Results
Overall condition of Green Roof:
Additional Comments
Notes: *If a specific maintenance item was not checked, please check N/A and explain why in the appropriate
comment box.
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Operations & Maintenance Guidance Document
Infiltration Practice
An infiltration practice is a shallow excavation, typically filled
with stone or an engineered soil mix, which is designed to
temporarily hold stormwater runoff until it infiltrates into the
surrounding soils. Infiltration practices are able to reduce
stormwater quantity, recharge the groundwater, and reduce
pollutant loads.
There are some common problems to be aware of when
maintaining infiltration practices. They include, but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Clogging the underdrain (if applicable)
Mosquitoes breeding in the practice
Routine maintenance should be performed on infiltration practices to ensure that the practice is
functioning properly. Infiltration practices should be inspected after a large rainstorm. Keep drainage
paths, both to and from the BMP, clean so that the water can properly infiltrate into the ground. Note
that it might take longer for the water to infiltrate into the ground during the winter months and early
spring.
In order to limit the sediment that enters the infiltration practice, infiltration practices should always be
designed with adequate pretreatment (e.g., vegetated filter strip, sediment forebay). Routine
maintenance of the pretreatment device, such as removing accumulated sediment, trash, and debris,
decreases the amount of maintenance required on the infiltration practice as well as its likelihood of
clogging and failing. Infiltration trenches can have either exposed aggregate at the surface of the
practice which provides sediment removal and additional pretreatment upstream of the infiltration
trench and can be easily removed and replaced when it becomes clogged.
If the infiltration practice is not draining properly, check for clogging of the inflow structure or
underdrain. To help ensure that larger storm events are able to safely bypass the infiltration practice a
perforated pipe (e.g., underdrain) is sometimes placed near the top of the stone reservoir or planting
bed. This provides additional conveyance of stormwater runoff after the infiltration trench or basin has
filled. Another consideration is the infiltration rate of the soil media. If the soil is not draining properly,
the filter fabric could be clogged or the soil could be clogged or over-compacted. In an infiltration
practice, the filter fabric is likely to be clogged along the top and sides of the infiltration practice. If the
filter fabric becomes clogged, the practices will need to be dug up, cleaned, and the fabric replaced. The
media is likely to become clogged at the upper layer of the soil first. If the media is clogged or overcompacted, then the media should be replaced. Potential sources of excessive sediment that could clog
the media include ant mounds and unstable soil upstream of the practice. Possible sources of
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Operations & Maintenance Guidance Document
compaction are tractors or maintenance vehicles traveling through the practice. If the practice includes
an underdrain, a structural repair or cleanout to unclog the underdrain may be necessary.
If designed and maintained correctly, there is no danger of infiltration practices becoming a breeding
ground for mosquitoes. A mosquito egg requires 24-48 hours to hatch. In addition, it takes 10-14 more
days for the egg to develop and become an adult. By having an infiltration practice that drains properly,
it is unlikely that it would provide a habitat that could become a breeding area for mosquitoes. Should
the infiltration practices become a breeding ground for mosquitoes, the problem is likely with the soil
media or the overflow structure which may need to be addressed.
The table below shows a schedule for when different maintenance activities should be performed on
the infiltration practice.
Infiltration Practice Typical Routine Maintenance Activities and Schedule
Maintenance Activity
52
Inspect to ensure that contributing drainage area and
infiltration practice are clear of sediment, trash and debris.
Remove any accumulated sediment and debris.
Ensure that the contributing drainage area is stabilized.
Plant replacement vegetation as needed.
Check observation well to ensure that infiltration practice
is properly dewatering after storm events.
Inspect pretreatment devices for sediment accumulation.
Remove accumulated sediment, trash and debris.
Inspect top layer of filter fabric and pea gravel or
landscaping for sediment accumulation. Remove and
replace if clogged.
Inspect the practice for damage, paying particular
attention to inlets, outlets and overflow spillways. Repair
or replace any damaged components as needed.
Inspect the practice following rainfall events (specifically
large rainfall events). Check observation well to ensure
that complete drawdown has occurred within 72 hours
after the end of a rainfall event. Failure to drawdown
within this timeframe may indicate infiltration practice
failure.
Remove aggregate and install clean, washed trench
aggregate
It may be necessary to replace piping, filter fabric, etc.
Schedule
Monthly
Semi-Annually during first year
and Annually thereafter
Upon Failure
Operations & Maintenance Guidance Document
Infiltration Practice
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc. Drainage ways are in good
condition.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Diversion structure (high flow bypass
structure or underdrain) is free of trash,
debris, or sediment. Comment on overall
condition of diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment.
Forebay – No undesirable vegetation.
Forebay – No signs of erosion, rills, or gullies.
Erosion protection is present on site.
Forebay – No signs of standing water.
Filter Strip– area is free of trash debris and
sediment. Area has been mowed and grass
clippings are removed. No evidence of
erosion or sediment accumulation.
Filter Strip – No signs of unhealthy grass,
bare or dying grass. Grass height is
maintained to a height of 6 – 15 inches.
Filter Strip– No signs of erosion, rills, or
gullies. Erosion protection is present on site.
Filter Strip – No undesirable vegetation.
Filter Strip – No signs of standing water
(examples include: stains, odors, mosquito
larvae, etc).
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Operations & Maintenance Guidance Document
Infiltration Practice
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
No signs of ponding water more than 48
hours after a rain storm event (examples
include: stains, odors, mosquito larvae, etc).
No undesirable vegetation growing within
the practice.
Native plants were used in the practice
according to the landscaping plan.
Observation well is capped and locked when
not in use
Flow testing has been performed on
infiltration practice to determine if
underdrain is clogged.
Emergency Overflow and Outlet Structure
Area is free of trash, debris, and sediment.
No evidence of erosion, scour, or flooding
around the structure.
No signs of sediment accumulation.
Grass height of 6 – 15 inches is maintained.
Results
Overall condition of Infiltration Practice:
Additional Comments
Notes: *If a specific maintenance item was not checked, please check N/A and explain why in the appropriate
comment box.
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Operations & Maintenance Guidance Document
Multi-Purpose Detention Basins
Multi-purpose detention basins are facilities
designed primarily for another purpose, such as a
parking lot or roof top, that also provide water
quantity control through detention of stormwater
runoff. The temporary storage provided by multipurpose detention basins reduces downstream
water quantity impacts. Multi-purpose detention
areas are normally dry between rain events, and by
their very nature must be useable for their primary
function the majority of the time.
There are some common problems to be aware of when maintaining a multi-purpose detention basin.
They include, but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structures
Establishing vegetation within the multi-purpose detention basin
Erosion
Structural repairs to inlets and outlets
Clogging in the emergency spillway
Routine inspection and maintenance should be performed on a multi-purpose detention basin to ensure
that the structure is properly functioning. In addition to routine maintenance, multi-purpose detention
basins may have seasonal and intermittent maintenance requirements. For example, if vegetation is
included in the practice, trim the planting material during the winter, when the plants are dormant.
The table below shows routine maintenance activities typically associated with multi-purpose detention
basins.
Multi-Purpose Detention Basin Typical Routine Maintenance Activities and Schedule
Activity
Remove debris from ponding area to minimize outlet clogging and
improve aesthetics.
Remove sediment buildup.
Repair and revegetate eroded areas.
Perform structural repairs to inlet and outlets.
Perform additional maintenance activities specific to the type of facility.
Schedule
Annually and following
significant storm events
As needed based on
inspection
As required
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Multi-Purpose Detention Basin
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure and Pretreatment
Drainage ways (overland flow, pipes or
preteatment) to the practice are free of
trash, debris, large branches, etc.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc).
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Emergency Overflow and Outlet Structure
Area is free of trash, debris, and sediment.
No evidence of erosion, scour, or flooding
around the structure.
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Operations & Maintenance Guidance Document
Multi-Purpose Detention Basin
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Results
Overall condition of Multi-Purpose
Detention Basin:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Operations & Maintenance Guidance Document
Organic Filter
Organic filters, a design variant of the surface sand
filter with organic materials as the filter media,
are multi-chamber structures designed to treat
stormwater runoff through filtration. An organic
filter consists of a pretreatment chamber, and one
or more filter cells. Each filter bed contains a
layer of leaf compost or a peat/sand mixture,
followed by an underdrain system. Maintenance
frequency on organic filters is typically high due to
clogging.
Common problems to be aware of when maintaining an organic filter include, but are not limited to, the
following:
Sediment build-up
Clogging in the inlet and outlet structure
Clogging the underdrain
Mosquitoes breeding in the practice
Ant mounds
Routine inspection and maintenance should be performed on the organic filters to ensure that the
structure is functioning properly. Note that if the organic filter includes topsoil and vegetation,
maintenance may be required at a higher frequency during the first year the organic filter is built to
ensure the proper establishment of vegetation in the practice.
Inspect the organic filter after a large rainstorm. Keep drainage paths (both to and from the BMP) clean
so that the water can properly infiltrate into the ground. If the organic filter is not draining properly,
check for clogging at the inflow and outflow structures as well as the infiltration rate of the filter bed. In
an organic filter, the filter bed is likely to become clogged at the upper layer of the filter (top 2-3 inches)
and will need to be removed and replaced. If the filter becomes clogged or over-compacted, then the
media should be replaced. In order to determine if maintenance is necessary, a record should be kept of
the dewatering time for an organic filter. Typically the filter bed is designed to drain in 40 hours or less,
if it the practice takes longer to drain, maintenance may be required for the practice.
For organic filters with vegetation, to keep the water that exits the organic filter clean, fertilizers should
only be used sparingly during the establishment of the practice. Once the vegetation in the practice has
been established, fertilizers should not be used. While vegetation in the organic filter is important, the
primary purpose of an organic filter is to act as a water quality device and introducing fertilizers into the
organic filter introduces nutrients such as phosphorus and nitrogen that can pollute downstream
waters. In addition, organic filters should already be a nutrient rich environment that does not require
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Operations & Maintenance Guidance Document
fertilization. To control animal nuisances and invasive species, pesticides (including herbicides,
fungicides, insecticides, or nematode control agents) should be used sparingly and only if necessary.
Potential sources of excessive sediment that could clog the media include ant mounds and unstable soil
upstream of the practice. Possible sources of compaction are maintenance vehicles traveling through
the practice. If the underdrain does not work properly, a structural repair or cleanout to unclog the
underdrain may be necessary.
In the event of snow, ensure that the snow does not pile up in the organic filter. Accumulated snow
adds additional weight and may compact the organic filter soil, which would reduce its infiltration
capacity. In addition, check to make sure that the materials used to de-ice the surrounding areas stay
out of the practice to avoid clogging and further pollution.
If designed and maintained correctly, there is no danger of organic filters becoming a breeding ground
for mosquitoes. A mosquito egg requires 24-48 hours to hatch. In addition, it takes 10-14 more days for
the egg to develop and become an adult. By having an organic filter that drains properly, it is unlikely
that an organic filter would provide a habitat that could become a breeding area for mosquitoes. Should
the organic filter become a breeding ground for mosquitoes, the problem is likely with the soil media or
the overflow structure which may need to be addressed. This is not applicable to the perimeter organic
filter which has a permanent pool.
The table below shows a schedule for when different maintenance activities should be performed on
the organic filter.
Organic Filter Typical Routine Maintenance Activities and Schedule
Activity
Schedule
60
Check to see that the filter bed is clean of sediment, and the sediment
chamber is not more than 50% full or 6 inches, whichever is less, of
sediment (also check after moderate and major storms). Remove
sediment as necessary.
Make sure that there is no evidence of deterioration, spalling or cracking
of concrete.
Inspect grates (perimeter Organic Filter).
Inspect inlets, outlets and overflow spillway to ensure good condition
and no evidence of erosion.
Repair or replace any damaged structural parts.
Stabilize any eroded areas.
Ensure that flow is not bypassing the practice.
Ensure that no noticeable odors are detected outside the BMP.
Ensure that contributing area, organic filter, inlets and outlets are clear
of trash and debris.
Ensure that the contributing area is stabilized and mowed, with clippings
removed.
Prune and weed to maintain appearance, if applicable.
Ensure that activities in the drainage area minimize oil/grease and
sediment entry to the system.
If permanent water level is present (perimeter Organic Filter), ensure
that the chamber does not leak, and normal pool level is retained.
Monthly
As needed or 4 times
during growing season
Operations & Maintenance Guidance Document
Activity
If filter bed is clogged or partially clogged, manual manipulation of the
surface layer of sand may be required. Remove the top few inches of
sand, roto-till, or otherwise cultivate the surface, and replace media with
sand meeting the design specifications.
Replace any filter fabric that has become clogged.
Remove and replace the top 2-3 inches of sand in the filter.
Schedule
Annually
Every 3-5 years or as
needed
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Organic Filter
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Area is free of signs of erosion.
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment. Area is free of undesirable
vegetation.
Sedimentation Chamber – area is free of
trash, debris, and sediment.
Perforated stand-pipe is free of trash, debris,
and sediment. Surrounding vegetation is
trimmed back so that there is no potential to
restrict flow. Pipe is in good working order.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
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Organic Filter
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Undesirable vegetation within and around
practice is trimmed and removed.
Significant sediment accumulation is not
occurring within the filter bed.
Grass cover is healthy and there are no bare
areas or dying grass.
No evidence of leaks at joints or other
components of the practice.
Underdrain cleanout caps are not missing or
damaged.
Observation well does not have standing
water.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
No evidence of animal activity.
No evidence of seepage on the downstream
face of the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet structure does not appear to be
blocked.
Results
Overall condition of Organic Filter:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Permeable Bricks/Blocks
Permeable bricks/blocks are pavers with void areas areas
between the bricks or blocks that are generally filled with
pervious materials such as small pieces of gravel, or top soil if a
grid is used. Beneath the bricks/blocks is a base layer of
aggregate that acts as a holding area for stormwater runoff
while still providing structural support for the road. This
practice provides enough structural support so that cars can
drive over them or they can be used in parking lots. Permeable
brick/blocks are not recommended in areas with high traffic
volume or heavy trucks. These systems provide water quality
benefits in addition to groundwater recharge and a reduction in
stormwater volume.
There are some common problems to be aware of when
maintaining permeable bricks/blocks. They include, but are not
limited to, the following:
Sediment build-up and clogging between bricks/blocks
Settling
Bricks/blocks cracking or splitting
There are four basic types of permeable bricks/blocks that are used. They are bricks, concrete blocks,
concrete grid, and articulated concrete block. The concrete grid can be filled with grass or gravel.
Routine maintenance should be performed on the permeable bricks/blocks to ensure that the structure
is functioning properly. Permeable bricks/blocks should be cleaned with a street vacuum or low pressure
washer to remove debris and sediment monthly, or as needed, and all vegetation between bricks/blocks
should be mowed and clippings removed to reduce clogging. Cleaning the bricks will help keep the water
permeating through the pavers. After cleaning, the bricks/blocks may need to be filled in with additional
aggregate or top soil to replace anything that may have been removed during cleaning.
In addition to routine maintenance, permeable bricks/blocks have seasonal and intermittent
maintenance requirements. In the winter months permeable bricks/blocks can be plowed similarly to
any other unpaved road by lifting the blade a few inches above the road or by using a beveled plow.
Deicing materials such as sand, ash, or salt should be avoided if possible. They can potentially harm the
bricks/blocks and may cause clogging. Non-toxic, organic deicers are recommended.
Permeable bricks/blocks should be inspected after a large rainstorm. Keep drainage paths, both to and
from the BMP, clean so that the water can properly infiltrate into the ground. Note that it might take
longer for the water to permeate into the ground during the winter months and early spring.
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Operations & Maintenance Guidance Document
If the permeable bricks/blocks are not draining properly, check for clogging between the bricks or blocks
or at the upper layer of the aggregate, directly below the bricks/blocks. If clogging occurs, then the
stones between the blocks/bricks should be replaced. In addition, the top layer of soil under the
bricks/blocks may also need to be cleaned and replaced. Some areas of the blocks/bricks may need
additional maintenance due to potential sources of clogging which include unstable soil upstream of the
practice, leaves from trees, low points in blocks/bricks, trash, and debris from vehicle traffic. Another
reason for the bricks/blocks not draining properly is settling. If major settling occurs, then the
bricks/blocks should be removed, cleaned, and replaced.
Permeable bricks/blocks may also include an underdrain. If the practice includes an underdrain,
additional maintenance will be required. Periodic testing will need to be done on the system to make
sure that the underdrain is not clogged. This is done by pouring water into cleanout and observing how
the water exits the practice. The observation well should be checked to make sure water is draining out
of the practice.
The table below shows a schedule for when different maintenance activities should be performed on
the permeable bricks/blocks.
Permeable Bricks/Blocks Typical Routine Maintenance Activities and Schedule
Activity
66
Keep the permeable bricks/blocks free of trash, debris, and sediment.
Make sure that there is no standing water in the bricks/blocks between
storms.
Remove weeds and grass growing between the bricks/blocks (unless
concrete grid pavers are being used).
Mow grass within the bricks/blocks (only for concrete grid with grass)
Mow / trim grass or vegetation near the bricks/blocks and remove
clippings from area.
Visually inspect the bricks/blocks after large storms to ensure the
overflow drainage system is working.
Inspect the bricks/blocks for damage and repair.
Vacuum sweep permeable brick/block surface to keep free of
sediment.
After cleaning, additional aggregate may need to be added between
the pavers. Replace aggregate between pavers as necessary.
Keep the contributing drainage area and surface of the bricks/blocks
clear of debris, trash, and sediment.
Ensure that the areas surrounding the practice are stabilized and
mowed, remove grass clippings.
Schedule
Monthly during warm
weather
As needed, based on
inspection
Operations & Maintenance Guidance Document
Activity
If the pavers are installed in an area that is subject to high amounts of
sediment, leaves, or low point (i.e. large trucks traveling on the
bricks/blocks daily) additional cleaning may be necessary.
Replace any joint material that has eroded or washed away.
Observe the system during a rain event.
Areas should be routinely inspected for settling and loss of water flow
through the system.
Organic deicers may be used to melt ice and snow.
Snow plows may be used when necessary under the following
conditions:
o The edges of the plows are beveled.
o The blade of the snow plow is raised 1-2 inches.
o The snow plow is equipped with snow shoes which allow the blade
to glide across uneven surfaces.
Inspect the surface for deterioration or breaking into fragments.
Flush the underdrain system to check for clogging (if applicable).
Remove the permeable bricks/blocks; include the top and base layers
of the practice. Clean bricks/blocks and base aggregate, and replace as
needed.
Schedule
Semi-annually in Spring
and Fall
As needed in winter
Annually
Upon failure
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Permeable Bricks/Blocks
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
leaves, etc. removed).
Area around the practice is mowed and
grass clippings are removed. No signs of bare
or dead grass.
No evidence of gullies, rills, or erosion
around the practice.
Water is permeating the bricks/blocks (i.e.
no evidence of water going around the
practice).
Bricks/blocks are structurally sound. No
signs of cracks or splitting.
Aggregate between the bricks/blocks is
reasonable.
No evidence of long-term ponding or
standing water in the practice.
Grass in the concrete grid is healthy, no dead
grass or bare spots.
Grass in the concrete grid is mowed and
grass clippings are removed.
Structure seems to be working properly. No
signs of the bricks/blocks settling. Comment
on overall condition of bricks/blocks.
Vegetation within and around practice is
maintained. Grass clippings are removed.
No exposed soil near the bricks/blocks that
could cause sediment accumulation within
the practice.
Cleanout caps are present and not missing (if
applicable).
The underdrain system has been flushed
properly and there is no sign of clogging (if
applicable).
Results
Overall condition of Permeable
Bricks/Blocks:
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Operations & Maintenance Guidance Document
Permeable Bricks/Blocks
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Pervious Concrete
Pervious concrete is a special mixture with a high
void space that allows water to infiltrate into the
subsoil through the pavement surface and base
layers. This aggregate base layer acts as both a
structural layer and a container to temporarily hold
stormwater runoff until it can infiltrate into the
subsoil or drainage system.
There are some common problems to be aware of
when maintaining pervious concrete. They include,
but are not limited to, the following:
Sediment build-up on surface
Settling
Cracking
Routine maintenance should be performed on pervious concrete to ensure that the area is functioning
properly. Pervious concrete should be observed monthly to ensure the practice is functioning properly.
Maintenance activities including street vacuum or low pressure washer to remove debris and sediment
should be conducted at least annually, or as needed.
In addition to routine maintenance, pervious concrete has seasonal and intermittent maintenance
requirements. In the winter months pervious concrete can be plowed, however, the snow plow should
be equipped with snow shoes which can allow the blade to glide across uneven surfaces. Deicing
materials such as sand, ash, salt, or other products should be avoided if possible. They will harm the
concrete and other materials and may cause clogging. Organic deicers are recommended.
Pervious concrete should be inspected after a large rainstorm. Keep drainage paths, both to and from
the BMP, clean so that the water can properly infiltrate through the concrete and into the ground. Note
that it might take longer for the water to infiltrate into the ground during the winter months and early
spring.
If the pervious concrete is not draining properly, check for clogging at the top of the concrete. If clogging
occurs, then the concrete should be cleaned by vacuuming or jet washing the area. Potential sources of
clogging include unstable soil, leaves from trees, trash, and debris from vehicle traffic. The concrete
could also not be draining properly due to settling or structural failure. If this happens, then the
concrete should be removed and replaced. Settling or structural failure is most likely to occur in areas
with high volumes of traffic or in areas with heavy traffic, such as large trucks.
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Operations & Maintenance Guidance Document
The surface of the concrete should be inspected for deterioration. If the concrete fails, then the
concrete should be resurfaced. Pervious concrete is intended for areas of low traffic; constant traffic and
heavy equipment will cause the pavement to deteriorate more quickly.
Pervious concrete may also include an underdrain or a trench outlet. If the practice includes an
underdrain or a trench, additional maintenance will be required. Periodic testing may be necessary to
make sure that the underdrain or trench is not clogged. The underdrain or trench can be tested by
pouring water into cleanout and observing how the water exits the practice. The observation well for
the underdrain should be checked to make sure water is draining out of the practice.
The table below shows routine maintenance activities typically associated with pervious concrete.
Pervious Concrete Typical Routine Maintenance Activities and Schedule
Maintenance Activity
Ensure that contributing area, facility, inlets and outlets are clear of
debris.
Ensure that the contributing area is stabilized and mowed, with clippings
removed.
Remove trash and debris.
Check to ensure that the pavement surface is not clogging (also check
after moderate and major storms).
Ensure that activities in the drainage area minimize oil/grease and
sediment entry to the system.
Make sure that there is no evidence of deterioration or cracking of the
concrete.
Inspect inlets, outlets and overflow spillway to ensure good condition
and no evidence of erosion.
Repair or replace any damage to the asphalt.
Ensure that flow is not bypassing the facility.
Vacuum sweep pervious concrete surface followed by high pressure
hosing to keep pores free of sediment.
Flush the underdrain system and check for signs of clogging.
Utilize organic de-icers on the pavement surface
72
Schedule
As needed
Monthly
Annually or based on
inspection
During temperatures
below freezing
Operations & Maintenance Guidance Document
Pervious Concrete
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance. No signs of
bare or dead grass.
Area is clean (trash, debris, grass clippings,
etc. removed).
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
No evidence of long-term ponding or
standing water in the practice (examples
include: stains, odors, etc).
Structure seems to be working properly. No
signs of concrete settling or cracking.
Comment on overall condition of concrete.
Vegetation around practice is maintained.
Grass clippings are removed.
No exposed soil near the concrete.
Cleanout caps are present and not missing (if
applicable).
The underdrain system or trench has been
flushed properly and there is no sign of
clogging.
Results
Overall condition of Pervious Concrete:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Porous Asphalt
In general, porous asphalt is asphalt with
reduced sands or fines and larger void spaces
to allow water to drain through it. Porous
asphalt allows water to infiltrate into the
subsoil through the paved surface and base
layer. This base, aggregate layer acts as both a
structural layer and container to temporarily
hold water. Porous asphalt is generally used
instead on sidewalks or bicycle paths or roads
with low traffic volumes.
There are some common problems to be aware of when maintaining porous asphalt. They include, but
are not limited to, the following:
Sediment build-up on surface
Settling
Cracking
Routine maintenance should be performed on porous asphalt to ensure that the area is functioning
properly. Porous asphalt should be observed monthly to ensure the practice is functioning properly.
Maintenance activities including vacuuming and jet washing should be conducted at least annually, or as
needed.
In addition to routine maintenance, porous asphalt has seasonal and intermittent maintenance
requirements. In the winter months porous asphalt can be plowed, however, the snow plow should be
equipped with snow shoes which allow the blade to glide across uneven surfaces. Deicing materials such
as sand, ash, or salt should be avoided if possible because they may harm the asphalt and aggregate and
may cause clogging. Non-toxic, organic deicers are recommended.
Porous asphalt should be inspected after a large rainstorm. Keep drainage paths, both to and from the
BMP, clean so that the water can properly infiltrate into the ground. Note that it might take longer for
the water to infiltrate into the ground during the winter months and early spring.
If the porous asphalt is not draining properly, check for clogging at the top of the asphalt. If clogging
occurs, then the asphalt should be cleaned by vacuuming or low pressure washer the area. Potential
sources of clogging include upstream unstable soil, leaves from trees, trash, and debris from vehicle
traffic. Asphalt could also not be draining properly due to settling or structural failure. If this happens,
then the asphalt should be removed and replaced. Settling or structural failure is most likely to occur in
areas with high volumes of traffic or in areas with heavy traffic, such as large trucks.
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The surface of both types of porous asphalt should be inspected for deterioration. If the pavement fails,
the asphalt should be resurfaced. Potholes, though uncommon, can be patched using standard
measures. If the damaged area is 10% or more of the total area, consult a qualified licensed Professional
Engineer for repair.
Porous asphalt may also include an underdrain. If the practice includes an underdrain, additional
maintenance will be required. Periodic testing will be necessary to make sure that the underdrain is not
clogged. This is done by pouring water into cleanout and observing how the water exits the practice. The
observation well should be checked to make sure water is draining out of the practice.
The table below shows routine maintenance activities typically associated with porous asphalt.
Porous Asphalt Typical Routine Maintenance Activities and Schedule
Activity
Ensure that contributing area, facility, inlets and outlets are clear of
debris.
Ensure that the contributing area is stabilized and mowed, with clippings
removed.
Remove trash and debris.
Check to ensure that the pavement surface is not clogging (also check
after moderate and major storms).
Ensure that activities in the drainage area minimize oil/grease and
sediment entry to the system.
Make sure that there is no evidence of deterioration, spalling or cracking
of asphalt.
Inspect inlets, outlets and overflow spillway to ensure good condition
and no evidence of erosion.
Repair or replace any damage to the asphalt.
Ensure that flow is not bypassing the facility.
Schedule
As needed
Monthly
Vacuum sweep pervious concrete surface followed by high pressure
hosing to keep pores free of sediment.
Flush the underdrain system and check for signs of clogging.
Annually or based on
inspection
Utilize organic de-icers on the pavement surface
During temperatures
below freezing
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Porous Asphalt
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Filter Strip (if applicable) – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Asphalt is structurally sound. No signs of
cracks or raveling (disintegration of material
from surface down).
No evidence of long-term ponding or
standing water in the practice.
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Vegetation around practice is maintained.
Grass clippings are removed.
No exposed soil near the asphalt.
Cleanout caps are present and not missing.
The underdrain system has been flushed
properly and there is no sign of clogging (if
applicable).
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Results
Overall condition of Porous Asphalt:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain what and why in the appropriate
comment box.
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Proprietary Systems
Proprietary systems are control systems available from commercial vendors designed to treat
stormwater runoff and/or provide water quantity control. Typically these systems are underground and
installed at inlet structures. There are many types of proprietary stormwater structural controls to
provide water quality treatment and quantity control. These systems come in different shapes and sizes
ranging from small systems that use a swirling vortex to a large system that has multiple chambers to
separate sediment, floatables, and oil/grease from the stormwater runoff.
There are some common problems to be aware of when maintaining propriety systems. They include,
but are not limited to, the following:
Sediment and oil/grease build-up
Clogging in the inlet and outlet structure
Inability to remove dissolved pollutants
Must be maintained routinely so that system does not become a potential source of pollutants
Routine inspection and maintenance should be performed on the proprietary systems to ensure that the
structure is functioning properly. Typical maintenance will include removing accumulated sediment and
pressure washing the system to remove blockage. It is important that the accumulated sediment and
water from cleaning the proprietary system be collected and disposed of properly. This is important to
keep the ground surrounding the system clean to avoid clogging.
Additional maintenance may be necessary if a spill occurs upstream of the system and drains into the
practice. The contributing drainage area should be maintained to limit the amount of trash and debris
that enters the practice.
Proprietary systems should be inspected after a large rainstorm. It may be necessary to make repairs to
the inlets, outlets, and other structural components. Check the manufacturer’s guidelines for
recommended maintenance on the system. In addition, it is required that a maintenance plan is
developed and implemented.
The table below shows a schedule for when different maintenance activities should be performed on
proprietary systems.
Proprietary Systems Typical Routine Maintenance Activities and Schedule
Activity
Check to make sure practice is draining properly.
Keep contributing drainage area free of trash, chunks of sediment, and debris.
Cleanout if spill occurs and enters the system.
Repair structural components.
Schedule
After a large rain storm
event or as needed
As needed
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Operations & Maintenance Guidance Document
Activity
80
Check maintenance plan and/or manufacturer’s guidelines for additional
maintenance needs.
Check system to make sure no blockage or significant sediment accumulation
is occurring in the system.
Cleanout system with vacuum or boom trucks.
Remove sediment and oil from chambers
Schedule
Quarterly
Annually
Operations & Maintenance Guidance Document
Proprietary System
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Contributing drainage area is clean (trash,
debris, grass clippings, etc. removed).
Inlet and outlet pipes are clean; stormwater
can enter and exit the practice without being
blocked.
Overflow structure is in good condition and
clean.
Maintenance is being performed according
to manufacturer’s guidelines.
Maintenance is being performed according
to the maintenance plan.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Results
Overall condition of Proprietary System:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Rainwater Harvesting
Rainwater harvesting is a common stormwater management
practice used to catch rainfall and store it to be used later.
Gutters and downspout systems are typically used to collect
the water from roof tops. Rainwater harvesting systems can
be either above or below the ground. Once captured in the
storage tank, the water may be used for non-potable indoor
and outdoor uses. If properly designed, rainwater harvesting
systems can significantly reduce post-construction
stormwater runoff rates, volumes and pollutant loads on
development sites. Rainwater harvesting also helps reduce
the demand on public water supplies, which in turn helps
protect aquatic resources, such as groundwater aquifers,
from drawdown and seawater intrusion.
There are some common problems to be aware of when
maintaining a rainwater harvesting system. They include, but are not limited to, the following:
Sediment build-up in the system
Wear and tear on pumping equipment (if applicable)
Clogging in the gutters or downspout connections
Algae growing in the rainwater system
Routine maintenance should be performed on the rainwater harvesting system. A well-designed
rainwater harvesting system typically consists of five major components, including the collection and
conveyance system (e.g., gutter and downspout system), pretreatment devices (e.g., leaf screens, first
flush diverters, roof washers), the storage tank or cistern, the overflow pipe (which allows excess
stormwater runoff to bypass the storage tank or cistern) and the distribution system (which may or may
not require a pump, depending on site characteristics). Each of these components should be inspected
and maintained.
Generally, maintenance should be performed in the spring and fall. Before the first significant freeze,
downspouts should be disconnected, and the rainwater harvesting system should be completely
drained. Other maintenance includes checking the system to make sure algae is not growing in the
system. Check the elements of the unit and make repairs or replace broken parts as necessary. Any
vegetation that receives accumulated water from the system should be checked for signs of stress.
Replace the plants as necessary.
The table below shows a schedule for when different maintenance activities should be performed on
the rainwater harvesting system.
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Rainwater Harvesting System Typical Routine Maintenance Activities and Schedule
Activity
84
Disconnect rainwater harvesting system from roof downspouts
(this may not be necessary for all areas of the state of Georgia).
Drain aboveground cisterns and clean for winter.
Connect rainwater harvesting system to roof downspouts.
Empty harvesting rainwater system periodically by watering
vegetation.
Examine vegetation for health/distress and determine if additional
watering needs are necessary.
Inspect storage tank screens and pretreatment devices. Clean as
needed.
Inspect gutters and downspouts. Remove any accumulated leaves
or debris.
Clean storage tank screens.
Inspect pretreatment devices for sediment accumulation. Remove
accumulated trash and debris.
Inspect for tight connection at inlet and drain valve.
Verify pumping system is properly working.
Keep pipe clear of obstructions.
Inspect storage tank for algal blooms. Treat as necessary.
Inspect overflow areas for erosion and the formation of rills and
gullies. Plant replacement vegetation in any eroded areas.
Check system for sediment. Clean out the tank when the sediment
is more than 5% of the volume in the cistern.
Schedule
Late Fall (Before major
freeze)
Early Spring (After last
major freeze)
Regularly during above
freezing temperatures
Semi-annually in spring
and fall
Annually
Operations & Maintenance Guidance Document
Rainwater Harvesting
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Gutters and downspouts are free of trash,
debris, etc.
Leaf screens are clean and in good condition.
First flush diverter is working properly and in
good condition (if applicable).
Roof washer is working properly and in good
condition (if applicable).
Cistern inlet and downspout fits tightly.
Cistern tank is clean and free of sediment.
Cistern is free of indication of algal blooms.
Plants being watered from the rainwater
harvesting system seem to be healthy and in
good condition. Comment on condition of
plants.
No signs of the overflow valve leaking
(stains, dampness).
Cistern is in good condition structurally, no
signs of cracking or leaking.
Performance of pump matches pumping
details (if applicable).
Results
Overall condition of Rainwater Harvesting:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Regenerative Stormwater Conveyance
A regenerative stormwater conveyance (RSC) is a
practice that provides treatment, infiltration, and
conveyance to stormwater runoff through a
combination of pools, riffles (with either cobble rocks or
boulders), native vegetation, an underlying sand layer,
and wood chips.
There are some common problems to be aware of when
maintaining a RSC. They include, but are not limited to,
the following:
Establishing vegetation within the RSC area
Ant mounds
Pruning and weeding to maintain appearance
Deterioration of riprap
Routine maintenance should be performed on RSC to ensure that the system is functioning properly.
Note that during the first year the RSC is built, maintenance may be required at a higher frequency to
ensure the proper establishment of vegetation in the practice.
A RSC should be inspected after a large rainstorm, especially during the first six months after
establishment. Keep drainage paths, both to and from the BMP, clean so that the water can properly
infiltrate into the ground.
Spot fertilization may be required during the first two months to establish vegetation. After that period,
however, fertilizers should not be used. While vegetation in the RSC is important, a primary purpose of a
RSC is to act as a water quality device, and introducing fertilizers into the RSC introduces nutrients such
as phosphorus and nitrogen that can pollute downstream waters. To control animal nuisances and
invasive species, pesticides (including herbicides, fungicides, insecticides, or nematode control agents)
should be used sparingly and only if necessary.
The table on the following page shows a schedule for when different maintenance activities should be
performed on the RSC area.
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Regenerative Storm Conveyance Typical Maintenance Activities and Schedule
Activity
Schedule
Upon establishment
Inspect two times after establishment for first 6 months after storms that
exceed ½ inch of rain.
Repair any erosion, rills, or gullies that may form in the practice
Conduct any needed repairs or stabilization
Repair areas with bare or dead grass in the contributing drainage area
or around the RSC.
Watering and spot fertilization may be necessary during first 2 months
to establish vegetation.
Remove and replace dead, damaged, or diseased plants.
Prune and weed vegetation.
Remove trash, sediment, and debris.
Four times per year
Add additional plants to maintain needed vegetation density.
Remove and replace any dead, damaged, or diseased plants.
Repair any eroded areas.
Make sure weirs, riffles, and pools are in structurally good condition and
that the practice has stable water levels.
Prune trees and shrubs (when they are dormant).
Remove any invasive species.
Remove any sediment accumulation in pretreatment area and inflow
points.
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Remove accumulated sediment in pools
Repair damage to weirs, riffles, pools, or other structural components.
As Needed
Annually
Once every 2 to 3 years
Operations & Maintenance Guidance Document
Regenerative Stormwater Conveyance
Inspection Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Native plants were used in the practice
according to the planning plan.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
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Regenerative Stormwater Conveyance
Inspection Item
Condition
Good
Marginal
Poor
N/A*
Comment
Results
Overall condition of RSC:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Sand Filters
Sand filters are multi-chamber structures designed to
treat stormwater runoff through filtration, using a
sediment forebay, a sand bed as its primary filter media,
and typically an underdrain system. Sand filters can be
designed in many ways; however, there are three
primary sand filter system designs, surface sand filter,
perimeter sand filter, and underground sand filter. A
surface sand filter is a ground-level open air structure
that consists of a pretreatment sediment forebay and a
filter bed chamber. A perimeter sand filter is an enclosed system typically just below the ground in a
vault along the edge of an impervious area such as a parking lot. Finally, an underground sand filter is for
areas with limited space and high density areas and should only be considered where local communities
allow this practice. Because underground sand filters require additional planning, maintenance and
incorporation with the stormwater management plan, coordinate with the local community for specific
maintenance concerns. Maintenance frequency on sand filters is typically high due to clogging.
There are some common problems to be aware of when maintaining a sand filter. They include, but are
not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Clogging the underdrain
Mosquitoes breeding in the practice
Ant mounds
Routine inspection and maintenance should be performed on the sand filters to ensure that the
structure is functioning properly. Note that if the sand filter include a grass cover or vegetation,
maintenance may be required at a higher frequency during the first year the sand filter is built to ensure
the proper establishment of grass cover or vegetation in the practice. For more information on
vegetation in a sand filter, see Appendix D: Planting and Soil Guidance.
Inspect the sand filter after a large rainstorm. Keep drainage paths (both to and from the BMP) clean so
that the water can properly infiltrate into the ground. If the sand filter is not draining properly, check for
clogging at the inflow and outflow structures as well as the infiltration rate of the filter bed. In a sand
filter, the filter bed is likely to become clogged at the upper layer of the filter (top 2-3 inches) and will
need to be removed and replaced. If the filter becomes clogged or over-compacted, then the media
should be replaced. In order to determine if maintenance is necessary, a record should be kept of the
dewatering time for a sand filter. Note that sand filters are typically designed to completely drain over
40 hours.
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Potential sources of excessive sediment that could clog the media include ant mounds and unstable soil
upstream of the practice. Possible sources of compaction are maintenance vehicles traveling through
the practice. If the underdrain does not work properly, a structural repair or cleanout to unclog the
underdrain may be necessary.
In the event of snow, ensure that the snow does not pile up in the sand filter. Accumulated snow adds
additional weight and may compact the sand filter, which would reduce its infiltration capacity. In
addition, check to make sure that the materials used to de-ice the surrounding areas stay out of the
practice to avoid clogging and further pollution.
If designed and maintained correctly, there is no danger of sand filters becoming a breeding ground for
mosquitoes. A mosquito egg requires 24-48 hours to hatch. In addition, it takes 10-14 more days for the
larvae to develop and become an adult. By having a sand filter that drains properly, it is unlikely that a
sand filter would provide a habitat that could become a breeding area for mosquitoes. Should the sand
filter become a breeding ground for mosquitoes, the problem is likely with the sand media or the
overflow structure which may need to be addressed. This is for surface sand filters, where there is open
water.
The table below shows a schedule for when different maintenance activities should be performed on
the sand filter.
Sand Filter Typical Routine Maintenance Activities and Schedule
Activity
92
Check to see that the filter bed is clean of sediment, and the sediment
chamber is not more than 50% full or 6 inches, whichever is less, of
sediment. Remove sediment as necessary.
Make sure that there is no evidence of deterioration, spalling or cracking
of concrete.
Inspect grates (perimeter sand filter).
Inspect inlets, outlets and overflow spillway to ensure good condition
and no evidence of erosion.
Repair or replace any damaged structural parts.
Stabilize any eroded areas.
Ensure that flow is not bypassing the BMP.
Ensure that no noticeable odors are detected outside the practice.
Ensure that contributing area, sand filter, inlets and outlets are clear of
debris.
Prune and weed to maintain appearance (if applicable).
Ensure that the contributing area is stabilized and mowed, with clippings
removed.
Ensure that activities in the drainage area minimize oil/grease and
sediment entry to the system.
If permanent water level is present (perimeter sand filter), ensure that
the chamber does not leak, and normal pool level is retained.
Schedule
Monthly
As needed or 4 times
during growing season
Operations & Maintenance Guidance Document
Activity
If filter bed is clogged or partially clogged, manual manipulation of the
surface layer of sand may be required. Remove the top few inches of
sand, roto-till or otherwise cultivate the surface, and replace media with
sand meeting the design specifications.
Replace any filter fabric that has become clogged.
Remove and replace the top 2-3 inches of sand in the filter.
Schedule
Annually
Every 3-5 years or as
needed
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Sand Filter
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Area is free of signs of erosion.
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment. Area is free of undesirable
vegetation.
Sedimentation Chamber – area is free of
trash, debris, and sediment.
Perforated stand-pipe is free of trash, debris,
and sediment. Surrounding vegetation is
trimmed back so that there is no potential to
restrict flow. Pipe is in good working order.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples: stains, odors, mosquito
larvae, etc).
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Sand Filter
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Sand Filter seems to be working properly. No
settling around the practice. Comment on
overall condition of structure.
Undesirable vegetation within and around
practice is trimmed and removed.
Significant sediment accumulation is not
occurring within the filter bed.
Grass cover is healthy and there are no bare
areas or dying grass.
No evidence of leaks at joints or other
components of the practice.
Underdrain cleanout caps are not missing or
damaged.
Observation well does not have standing
water.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
No evidence of animal activity.
No evidence of seepage on the downstream
face of the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet structure does not appear to be
blocked.
Results
Overall condition of Sand Filter:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Site Reforestation/Revegetation
Site reforestation/revegetation is a process of
planting trees, shrubs and other native vegetation in
disturbed pervious areas to restore the area to predevelopment or better conditions. The process can
be used to establish mature native plant
communities, such as forests, in pervious areas that
have been disturbed by clearing, grading and other
land disturbing activities. These plant communities
intercept rainfall and slow and filter the stormwater
runoff to improve infiltration in the ground. This in turn can reduce the total amount of stormwater
runoff and pollutant loads leaving the site. Areas that have been reforested or revegetated should be
maintained in an undisturbed, natural state over time. These areas must be designated as conservation
areas and protected in perpetuity through a legally enforceable conservation instrument (e.g.,
conservation easement, deed restriction).
There are some common problems to be aware of when maintaining a site reforestation/revegetation
area. They include, but are not limited to, the following:
Establishing vegetation within the area
Watering the practice
Erosion
Routine inspection and maintenance should be performed on the reforestation/revegetation site to
ensure that the practice is functioning properly. Note that during the first year this process is
implemented, maintenance may be required at a higher frequency to ensure the vegetation is properly
established. Once the vegetation is established, very little maintenance is typically needed. For more
information on vegetation, see Appendix D: Planting and Soil Guidance.
In order to keep the stormwater runoff that exits the site reforestation/revegetation area clean,
fertilizers should only be used sparingly during the establishment of the practice. Once the vegetation in
the practice has been established, fertilizers should not be used. While vegetation growth in the site
reforestation/revegetation area is important, the primary purpose of this process is to act as both a way
to filter and infiltrate stormwater. Introducing fertilizers into the site reforestation/revegetation area
introduces nutrients such as phosphorus and nitrogen that can pollute downstream waters. To control
animal nuisances and invasive species, pesticides (including herbicides, fungicides, insecticides, or
nematode control agents) should be used sparingly and only if necessary.
The table on the following page shows routine maintenance activities typically associated with site
reforestation/revegetation areas.
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Site Reforestation/Revegetation Routine Maintenance Activities and Schedule
Activity
98
Schedule
Water to promote plant establishment, growth, and survival.
Inspect reforested/revegetated area following rainfall events. Plant
replacement vegetation in any eroded areas.
Remove sediment from practice.
Revegetate if eroded.
As needed
(Following Construction)
Inspect reforested/revegetated area for erosion. Plant replacement
vegetation in any eroded areas.
Inspect reforested/revegetated area for dead or dying vegetation.
Plant replacement vegetation as needed.
Prune and care for individual trees and shrubs as needed.
Annually
(Semi-Annually During First
Year)
Operations & Maintenance Guidance Document
Site Reforestation/Revegetation
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
No evidence of gullies, rills, or excessive
erosion.
Area is free of trash, debris, and sediment.
No evidence of long-term ponding or
standing water (examples include: stains,
odors, mosquito larvae, etc).
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Native plants were used in the practice
according to the planting plan.
No evidence of excessive use of fertilizer on
plants (fertilizer crusting on the surface of
the soil, tips of leaves turning brown or
yellow, blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Results
Overall condition of Site
Reforestation/Revegetation area:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Soil Restoration
Soil restoration is the process of tilling and adding
compost and other amendments to soils to restore them
to their pre-development conditions. This improves the
soil’s ability to reduce post-construction stormwater
runoff rates, volumes and pollutant loads. This process is
ideal for areas that have been disturbed by clearing,
grading and other land disturbing activities. This process
is generally used in conjunction with other practices
including, but not limited to, vegetated filter strips, grass
channels, and simple downspout disconnections.
Restored pervious areas require some maintenance during the first few months following construction,
but typically require very little maintenance after that.
In order to keep the water that exits the soil restoration area clean, fertilizers should be used sparingly
during the establishment of the practice. Once the vegetation in the practice has been established,
fertilizers should not be used. While vegetation growth in the soil restoration area is important,
introducing fertilizers into the soil restoration area introduces nutrients such as phosphorus and
nitrogen that can pollute downstream waters. To control animal nuisances and invasive species,
pesticides (including herbicides, fungicides, insecticides, or nematode control agents) should be used
sparingly and only if necessary.
The table below shows routine maintenance activities typically associated with soil restoration areas.
Soil Restoration Typical Routine Maintenance Activities and Schedule
Activity
Water to promote plant growth and survival.
Inspect restored pervious area following rainfall events. Plant
replacement vegetation in any eroded areas.
Inspect restored pervious area for erosion. Plant replacement
vegetation in any eroded areas.
Inspect restored pervious area for dead or dying vegetation. Plant
replacement vegetation as needed.
Schedule
As Needed
(Following Construction)
Annually
(Semi-Annually During First
Year)
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Soil Restoration
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
No evidence of gullies, rills, or excessive
erosion.
No evidence of long-term ponding or
standing water (examples include: stains,
odors, mosquito larvae, etc).
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Native plants were used in the practice
according to the landscaping plan.
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Results
Overall condition of Soil Restoration:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Stormwater Planters/Tree Boxes
Stormwater planters are similar to bioretention areas in
their design purpose to detain, filter, and infiltrate
stormwater. In addition stormwater planters utilize
native or non-invasive flowers, shrubs, and trees to
provide aesthetic qualities to the site. Planters and tree
boxes receive stormwater from a variety of sources such
as, roof tops and downspouts and runoff from streets.
There are some common problems when maintaining a
stormwater planter. They include, but are not limited to,
the following:
Sediment build-up
Clogging in the downspouts (only applicable of stormwater planters receive water from
downspouts)
Establishing vegetation within the stormwater planter
Clogging the underdrain
Maintaining the proper pH levels for plants
Routine maintenance should be performed on stormwater planters to ensure that the structure is
functioning properly. Note that during the first year the stormwater planter is built, maintenance may
be required at a higher frequency to ensure the proper establishment of vegetation in the practice.
In addition to routine maintenance, stormwater planters have seasonal and intermittent maintenance
requirements. For example, the following are maintenance activities and concerns specific to winter
months. Planting material should be trimmed during the winter, when the plants are dormant. In the
event of snow, ensure that snow does not pile up in the stormwater planter. Accumulated snow adds
additional weight and may compact the stormwater planter soil, which would reduce its infiltration
capacity. In addition, check to make sure that the materials used to de-ice the surrounding areas stay
out of the practice to avoid clogging and further pollution.
Stormwater planters should be inspected after a large rainstorm. Mulch the practice as needed to keep
a thickness of 2-4 inches. Shredded hardwood mulch is preferred, and care should be taken to keep the
mulch from piling on the stems of the plants. For more information on vegetation in stormwater
planters, see Appendix D: Planting and Soil Guidance.
In order to keep the water that exits the stormwater planter clean, fertilizers should only be used
sparingly during the establishment of the practice. Once the vegetation in the practice has been
established, fertilizers should not be used. While vegetation in the stormwater planter is important, the
primary purpose of a stormwater planter is to act as a water quality device and introducing fertilizers
into the stormwater planter introduces nutrients such as phosphorus and nitrogen that can pollute
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downstream waters. In addition, stormwater planters should already be a nutrient rich environment
that does not require fertilization. To control animal nuisances and invasive species, pesticides
(including herbicides, fungicides, insecticides, or nematode control agents) should be used sparingly and
only if necessary.
Invasive species should be kept out of the stormwater planters and the overall health of the plants
should be maintained. If periodic observations indicate the presence of contaminants, the soil and
mulch in the plants should be tested to avoid the build-up of pollutants that may harm the vegetation.
The table below shows a schedule for when different maintenance activities should be performed on a
stormwater planter.
Stormwater Planters/Tree Boxes Typical Routine Maintenance Activities and
Schedule
Activity
106
Water to promote plant growth and survival.
Inspect stormwater planter following rainfall events to ensure
the planter is working properly.
Prune and weed stormwater planter.
Remove accumulated trash and debris.
Remove and replace dead or damaged plants. Plant replacement
vegetation as needed.
Observe infiltration rates after rain events. Planters should have
no standing water within 24 hours.
Inspect inflow and outflow areas for sediment accumulation.
Remove any accumulated sediment or debris.
Inspect stormwater planter for erosion and the formation of rills
and gullies. Plant replacement vegetation in any eroded areas.
Replace mulch.
Test the planting soils for pH levels. Consult with a qualified
licensed Professional to determine and maintain the proper pH
levels.
Implement plant maintenance plan to trim and divide perennials
to prevent overcrowding and stress.
Check soil infiltration rates to ensure the soil is draining the
water at a proper rate. Re-aerate or replace soil and mulch
layers as needed to achieve infiltration rate of at least 0.25
inches per hour (1-2 in/hr preferred).
Schedule
As Needed
(Following Construction)
As needed, or 4 times during
growing season
Semi-annually in spring and
fall
2 to 3 years
Operations & Maintenance Guidance Document
Stormwater Planter (Tree Box)
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comments
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Pipes to the practice are free of trash,
debris, large branches, etc.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Level spreader is in good condition with no
trash, debris, or sediment accumulation
(applicable if planters do not receive rooftop
runoff).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc).
Structure seems to be working properly. No
settling around the structure. Comment on
overall condition of structure.
Vegetation within the practice is maintained
per landscaping plan.
Mulching depth of 2-4 inches is maintained.
Comment on mulch depth.
Plants used in the practice are consistent
with the requirements in the landscaping
plan.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
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Stormwater Planter (Tree Box)
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comments
Plants seem to be healthy and in good
condition. Comment on condition of plants.
The underdrain has been flushed and there
is no indication the underdrain system is
clogged.
Cleanout caps for underdrain is present and
in good condition.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Results
Overall condition of Stormwater Planter:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Stormwater Ponds
A stormwater pond is a constructed, shallow
stormwater retention basin or landscaped area
with a permanent pool of water. Stormwater
runoff collected in the pool is treated through
settling. In addition, the aquatic bench (fringe
wetlands), safety bench, side slopes, and shallow
areas of the pond include plants to aid in the
filtration and infiltration of the stormwater
runoff flowing through the practice.
There are some common problems to be aware of when maintaining a stormwater pond. They include,
but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Establishing vegetation within the stormwater pond
Pruning and weeding to maintain appearance
Eutrophic conditions indicated by excessive algae growth or fish kills
Creating a mosquito habitat
Routine inspection and maintenance should be performed on stormwater ponds to ensure that the
structure is functioning properly. Note that during the first year the stormwater pond is built,
maintenance may be required at a higher frequency to ensure the proper establishment of vegetation in
the practice. For more information on vegetation in stormwater ponds, see Appendix D: Planting and
Soil Guidance.
In addition to routine maintenance, stormwater ponds have seasonal and intermittent maintenance
requirements. During the winter months, the stormwater pond should be inspected after a snow event
(this is specific to northern areas of Georgia) to make sure that the materials used to de-ice the
surrounding areas stay out of the practice to avoid further pollution. In addition, planting material
should be trimmed during the winter, when the plants are dormant.
Inspect the stormwater pond after a large rainstorm. Keep drainage paths (both to and from the BMP)
clean so that the water can properly flow into the stormwater pond. If the stormwater pond is not
draining properly, check for clogging in the inflow and outflow structures.
If the forebay or stormwater pond has received a significant amount of sediment over a period of time,
then the sediment at the bottom of the forebay or pond may need to be removed. Accumulated
sediment in the practice decreases the available storage volume and affects the pond’s ability to
function as it was designed. A sediment marker should be placed in the forebay to determine when
sediment removal is required. It important to note that sediment excavated from stormwater ponds
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Operations & Maintenance Guidance Document
that does not receive stormwater runoff from stormwater hotspots are typically not considered to be
toxic and can be safely disposed through either land application or landfilling. Stormwater hotspots are
areas that produce higher concentrations of metals, hydrocarbons, or other pollutants than normally
found in urban runoff. Examples of operations performed in potential stormwater hotspots include
vehicle maintenance and repair, vehicle washing, landscaping/grounds care, and outdoor material and
product storage. Check with the local development review authority to identify any additional
constraints on the disposal of sediments excavated from stormwater ponds.
Periodic mowing of the pond buffer is only required along maintenance right-of-way and the
embankment. The remaining buffer can be managed as a meadow (mowing every other year) or a
forest.
In order to keep the water that exits the stormwater pond clean, fertilizers should be used sparingly
during establishment. Once the vegetation in the practice has been established, fertlizers should not be
used. While vegetation in the stormwater pond is important, the primary purpose of a stormwater pond
is to act as a water quantity and quality device, and introducing fertilizers into the stormwater pond
introduces nutrients such as phosphorus and nitrogen that can pollute downstream waters. In addition,
stormwater ponds should already be nutrient rich environments that do not require fertilization. To
control animal nuisances and invasive species, pesticides (including herbicides, fungicides, insecticides,
or nematode control agents) should be used sparingly and only if necessary.
Stormwater ponds create a challenge for controlling mosquitos, because some types of vegetation, such
as cattails, can create an environment that allows mosquitoes to breed both in the pond and along the
shoreline. Keeping the practice free of trash will help the practice from becoming a mosquito habitat.
Another method to control mosquitoes is to place fish, such as the mosquitofish (Gambusia affinis), in
the pond to help with controlling the mosquitoes. Animals such as dragonflies, diving beetles, birds, and
bats may aid on controlling mosquitoes, however it is likely that additional measures, such as chemicals,
may be required to control the mosquitoes (using chemicals should be a last resort). Keeping the pond
at a depth of four feet or greater can aid in mosquito control by limiting vegetation growing around the
pond. If mosquitoes begin to pose a problem, consult a qualified professional.
Pond dam inspection and maintenance is also very important. The pond dam should be inspected for
seepage and structural integrity. Look for saturated soil, sediment deposits, and flowing water at the
base of an earthen dam and on the rear face of the dam. On concrete dams, look for seepage, cracks,
leaks and rust stains, or bulges. If any signs of seepage are found, consult a Professional Engineer. Pests
such as burrowing animals and fire ants can pose a major threat to dam safety. Fire ant tunnels and
animal burrows can weaken the dam structure and create an undesired water pathway through the
dam. In addition, tree roots are another source of potential damage and failure. Woody vegetation may
not be planted on the embankment or allowed to grow within 15 feet of the toe of the embankment
and 25 feet from the principal spillway structure. If you have a large dam that is subject to regulations by
the state, other maintenance items may be required. Please consult a Professional Engineer for
additional guidance.
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Ponds can be an attractive nuisance, so security and safety should be taken into consideration. Fencing
requirements are at the discretion of the local government. If security measures such as a fence and
gate are present, ensure that they are functional and locked.
It is important that the embankment for a pond be inspected regularly for trees and animal activity.
Trees growing on the top or sides of the embankment should be removed. The roots of trees grow into
the embankment and will weaken the structure of the embankment by creating passage ways that allow
water to flow through the embankment. Trees that are blown over or damaged by storms can loosen or
remove soil which weakens the strength of the embankment. In the same way animals can burrow holes
weakening the structure of the embankment. These holes act as a passage way for the water to travel
through the embankment, increasing the potential for the embankment to fail.
Geese are attracted to open water, clean lines of sight, and grass. They can become a nuisance to
stormwater ponds if they are causing damage to plants or the banks, or if they are ‘loading’ the pond
with nutrients and bacteria. Geese can be discouraged from using a stormwater pond by planting the
buffer with shrubs and native ground covers or installing an aquatic shelf, but ensure that access points
are maintained.
The table below shows a schedule for when different maintenance activities should be performed on a
stormwater pond.
Stormwater Ponds Typical Routine Maintenance Activities and Schedule
Activity
Inspect inlets, outlets and overflow spillway to ensure good
condition and no evidence of erosion.
Clean and remove debris from inlet and outlet structures.
Mow side slopes.
Inspect pond dam for structural integrity.
Remove trash from the area around the pond.
If wetland components are included, inspect for invasive
vegetation.
Inspect for damage, paying particular attention to the control
structure.
Check for signs of eutrophic conditions (e.g., algal blooms and
fish kills).
Note signs of hydrocarbon build-up (e.g., an oil sheen), and
remove appropriately.
Monitor for sediment accumulation in the facility and forebay.
Check all control gates, valves, or other mechanical devices.
Repair undercut or eroded areas.
Perform wetland plant management and harvesting.
Remove sediment from the forebay.
Schedule
Monthly
Semiannual Inspection
Annual Inspection
As Needed
Annually (if needed)
5 to 7 years or after 50% of
the total forebay capacity has
been lost
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Operations & Maintenance Guidance Document
Activity
Monitor sediment accumulations, and remove sediment when the
pool volume has become reduced significantly, or the pond
becomes eutrophic.
(Source: WMI, 1997)
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Schedule
10 to 20 years or after 25% of
the permanent pool volume
has been lost
Operations & Maintenance Guidance Document
Stormwater Pond
Maintenance Item
Condition
Good Marginal
Poor
General Inspection
N/A*
Comment
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Inlet pipe is in good condition, and water is
going through the structure (i.e. no evidence
of water going around the structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment.
Filter Strip or Grass Channels – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Rock Lined Plunge Pools – area is free of
trash debris and sediment. Rock thickness in
pool is adequate.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No algal growth along or within the pond.
Native plants were used in the practice
according to the planting plan. No
undesirable vegetation.
Practice seems to be working properly. No
settling around the stormwater pond.
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Stormwater Pond
Maintenance Item
Good
Condition
Marginal
Poor
N/A*
Comment
Comment on overall condition of
stormwater pond.
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
No significant sediment accumulation within
the practice.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, flooding, or
animal activity around the structure.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet structure does not appear to be
blocked.
No evidence of animal activity.
No evidence of seepage on the downstream
face.
Results
Overall condition of Stormwater Pond:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Stormwater Wetland
Stormwater wetlands are constructed wetland systems
built for stormwater management purposes. They
typically consist of a combination of open water, shallow
marsh and semi-wet areas that are located just above
the permanent water surface. As stormwater runoff
flows through a wetland, it is treated, primarily through
gravitational settling and biological uptake.
There are some common problems to be aware of when maintaining a stormwater wetland. They
include, but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Establishing vegetation within the wetland area
Maintaining the proper pH levels for plants
Pruning and weeding to maintain appearance
Mosquitoes breeding in the practice
Routine maintenance should be performed on the stormwater wetlands to ensure that the structure is
properly functioning. Note that during the first year the stormwater wetland is built, maintenance may
be required at a higher frequency to ensure the proper establishment of vegetation in the practice. For
more information on stormwater wetland vegetation, see Appendix D: Planting and Soil Guidance.
Regular inspection and maintenance is crucial to the success of the wetland as an effective stormwater
management practice.
In addition to routine maintenance, stormwater wetlands have seasonal and intermittent maintenance
requirements. During the winter months, the stormwater pond should be inspected after a snow event
(this is specific to northern areas of Georgia) to make sure that the materials used to de-ice the
surrounding areas stay out of the practice to avoid further pollution. In addition, planting material
should be trimmed during the winter, when the plants are dormant.
Inspect the stormwater wetland after large rainstorm events. Keep drainage paths (both to and from the
BMP) clean so that the water can properly flow into the stormwater wetland. If the stormwater wetland
is not draining properly, check for clogging in the inflow and outflow structures.
If the forebay or stormwater wetland has received a significant amount of sediment over a period of
time, then the sediment at the bottom of the forebay or wetland may need to be removed.
Accumulated sediment in the practice decreases the available storage volume and affects the wetland’s
ability to function as it was designed. It important to note that sediment excavated from stormwater
wetlands that do not receive stormwater runoff from stormwater hotspots are typically not considered
to be toxic and can be safely disposed through either land application or landfilling. Stormwater
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Operations & Maintenance Guidance Document
hotspots are areas that produce higher concentrations of metals, hydrocarbons, or other pollutants than
normally found in urban runoff. Examples of operations performed in potential stormwater hotspots
include vehicle maintenance and repair, vehicle washing, landscaping/grounds care, and outdoor
material and product storage. Check with the local development review authority to identify any
additional constraints on the disposal of sediments excavated from stormwater wetlands.
In order to keep the water that exits the stormwater wetland clean, fertilizers should be used sparingly
around the wetland. Once the vegetation in the practice has been established, fertilizers should not be
used. While vegetation in the stormwater wetland is important, the primary purpose of a stormwater
wetland is to act as a water quantity and quality device and introducing fertilizers into the stormwater
wetland introduces nutrients such as phosphorus and nitrogen that can pollute downstream waters. In
addition, stormwater wetlands should already be a nutrient rich environment that does not require
fertilization. To control animal nuisances and invasive species, pesticides (including herbicides,
fungicides, insecticides, or nematode control agents) should be used sparingly and only if necessary.
It is important that the embankment for a wetland be inspected regularly for trees and animal activity.
Trees growing on the top or sides of the embankment should be removed. The roots of trees grow into
the embankment and will weaken the structure of the embankment by creating passage ways that allow
water to flow through the embankment. Trees that are blown over or damaged by storms can loosen or
remove soil which weakens the strength of the embankment. In the same way animals can burrow
holes weakening the structure of the embankment. These holes act as a passage way for the water to
travel through the embankment, increasing the potential for the embankment to fail.
Stormwater wetlands create a challenge for controlling mosquitos, because some types of vegetation,
such as cattails, can create an environment that allows mosquitoes to breed both in the pond and along
the shoreline. Keeping the practice free of trash will help the practice from becoming a mosquito
habitat. Another method to control mosquitoes is to place fish, such as the mosquitofish (Gambusia
affinis), in the wetland to help with controlling the mosquitoes. Animals such as dragonflies, diving
beetles, birds, and bats may aid on controlling mosquitoes, however it is likely that additional measures,
such as chemicals, may be required to control the mosquitoes (using chemicals should be a last resort).
Keeping the wetland at a depth of four feet or greater can aid in controlling mosquitoes by limiting
vegetation growing around the wetland. If mosquitoes begin to pose a problem, consult a qualified
professional.
The table below shows a schedule for when different maintenance activities should be performed on a
stormwater wetland.
Stormwater Wetland Typical Routine Maintenance Activities and Schedule
Activity
Schedule
116
Water side slopes and buffers to promote plant growth and
survival.
Inspect wetland, side slopes and buffers following major
storm events. Plant replacement vegetation in any eroded
areas.
As Needed
(Following Construction)
Operations & Maintenance Guidance Document
Activity
Examine to ensure that inlet and outlet devices are free of
sediment and debris and are operational.
Inspect wetland, side slopes and buffers for dead or dying
vegetation. Plant replacement vegetation as needed.
Inspect wetland, side slopes and buffers for invasive
vegetation and remove as needed.
Inspect wetland, side slopes and buffers for erosion. Plant
replacement vegetation in any eroded areas.
Monitor wetland vegetation and perform replacement
planting as necessary.
Harvest wetland plants that have been “choked out” by
sediment build-up.
Inspect for damage, paying particular attention to the
control structure and side slopes. Repair as necessary.
Examine stability of the original depth zones and
microtopographical features (i.e., shallow areas with minor
ridges that increase water quality, provide flood storage,
and enhance the development of a more diverse vegetative
community).
Inspect side slopes for erosion and undercutting and repair
as needed.
Check for signs of eutrophic conditions (e.g., excessive
algal growth).
Check for signs of hydrocarbon accumulation (e.g., oil
sheens) and remove appropriately.
Monitor sediment markers for sediment accumulation in
forebays and permanent pools.
Check all control gates, valves and other mechanical
devices.
Schedule
Monthly
Semi-Annually
(Quarterly During First Year)
Annually
Remove sediment, trash, and debris from inlets/forebay.
5 years or after 50% of the total
forebay storage capacity has been
lost
Monitor sediment accumulation in the wetland and remove
sediment when the permanent pool volume has become
reduced significantly, plants are “choked” with sediment, or
the wetland becomes eutrophic.
10 plus years or after 25% of the
wetland storage volume has been
lost
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Stormwater Wetland
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Inlet pipe is in good condition, and water is
going through the structure (i.e. no evidence
of water going around the structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment. Sediment accumulation in forebay
is less than 50% of the storage capacity.
Filter Strip or Grass Channels – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Rock Lined Plunge Pools – area is free of
trash debris and sediment. Rock thickness in
pool is adequate.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No algal growth along or within the wetland.
Native plants were used in the practice
according to the planting plan. No
undesirable vegetation.
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Stormwater Wetland
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Wetland seems to be working properly. No
settling around the practice. Comment on
overall condition.
No significant sediment accumulation within
the practice.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, flooding, or
animal activity around the structure. No
evidence of seepage on the downstream
face.
No evidence of unwanted vegetation and
vegetation is in good condition.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet structure does not appear to be
blocked.
Results
Overall condition of Stormwater Wetland:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Operations & Maintenance Guidance Document
Submerged Gravel Wetlands
The submerged gravel wetland system is similar to
a regular stormwater wetland; however, it is filled
with crushed rock or gravel and designed to allow
stormwater to flow through the root zone of the
constructed wetland. The outlet from each cell is
set at an elevation to keep the rock or gravel
submerged. Wetland plants are rooted in the
media, where they can directly take up pollutants.
In addition, algae and microbes thrive on the
surface area of the rocks. Mimicking the pollutant
removal ability of nature, this structural control relies on the pollutant-stripping ability of plants and
soils to remove pollutants from runoff.
There are some common problems to be aware of when maintaining a submerged gravel wetland. They
include, but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Establishing vegetation within the wetland area
Maintaining the proper pH levels for plants
Pruning and weeding to maintain appearance
Mosquitoes breeding in the practice
Routine maintenance should be performed on the submerged gravel wetlands to ensure that the
structure is properly functioning. Note that during the first year the submerged gravel wetland is built,
maintenance may be required at a higher frequency to ensure the proper establishment of vegetation in
the practice. For more information on vegetation in submerged gravel wetlands, see Appendix D:
Planting and Soil Guidance. Regular inspection and maintenance is crucial to the success of the wetland
as an effective stormwater management practice.
In addition to routine maintenance, submerged gravel wetlands have seasonal and intermittent
maintenance requirements. During the winter months, the stormwater pond should be inspected after
a snow event (this is specific to northern areas of Georgia) to make sure that the materials used to deice the surrounding areas stay out of the practice to avoid further pollution. In addition, planting
material should be trimmed during the winter, when the plants are dormant.
Inspect the submerged gravel wetland after large rainstorm events. Keep drainage paths (both to and
from the BMP) clean so that the water can properly flow into the submerged gravel wetland. If the
submerged gravel wetland is not draining properly, check for clogging in the inflow and outflow
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structures. If sediment buildup is preventing flow through the wetland, remove gravel and sediment
from cell. Replace with clean gravel and replant vegetation.
If the forebay or submerged gravel wetland has received a significant amount of sediment over a period
of time, then the sediment at the bottom of the forebay or gravel wetland may need to be removed. It is
important to note that sediment excavated from submerged gravel wetlands that do not receive
stormwater runoff from stormwater hotspots are typically not considered toxic and can be safely
disposed through either land application or landfilling. Stormwater hotspots are areas that produce
higher concentrations of metals, hydrocarbons, or other pollutants than normally found in urban runoff.
Examples of operations performed in potential stormwater hotspots include vehicle maintenance and
repair, vehicle washing, landscaping/grounds care, and outdoor material and product storage. Check
with the local development review authority to identify any additional constraints on the disposal of
sediments excavated from submerged gravel wetlands.
In order to keep the water that exits the submerged gravel wetland clean, fertilizers should be used
sparingly during the establishment of the practice. Once the vegetation in the practice has been
established, fertilizer should not be used. While vegetation in the submerged gravel wetland is
important, the primary purpose of a submerged gravel wetland is to act as a water quantity and quality
device and introducing fertilizers into the submerged gravel wetland introduces nutrients such as
phosphorus and nitrogen that can pollute downstream waters. In addition, submerged gravel wetlands
should already be a nutrient rich environment that does not require fertilization. To control animal
nuisances and invasive species, pesticides (including herbicides, fungicides, insecticides, or nematode
control agents) should be used sparingly and only if necessary.
It is important that the embankment of a submerged gravel wetland be inspected regularly for trees and
animal activity. Trees growing on the top or sides of the embankment should be removed. The roots of
trees grow into the embankment and will weaken the structure of the embankment by creating passage
ways that allow water to flow through the embankment. Trees that are blown over or damaged by
storms can loosen or remove soil which weakens the strength of the embankment. In the same way
animals can burrow holes weakening the structure of the embankment. These holes act as a passage
way for the water to travel through the embankment, increasing the potential for the embankment to
fail.
The table below shows a schedule for when different maintenance activities should be performed on a
submerged gravel wetland.
Submerged Gravel Wetlands Typical Routine Maintenance Activities and Schedule
122
Activity
Schedule
Ensure that inlets and outlets to each submerged gravel wetland cell
are free from debris and not clogged.
Remove any accumulated sediment and debris from inlet and outlet
structures.
Monthly
Operations & Maintenance Guidance Document
Activity
Inspect wetland, side slopes and buffers for erosion. Replace vegetation
in eroded areas.
Inspect wetland, side slopes and buffers for dead or dying vegetation.
Replace vegetation as needed.
Inspect wetland, side slopes and buffers for invasive vegetation and
remove as needed.
Inspect for damage to the embankment and inlet/outlet structures.
Repair as necessary.
Monitor for sediment accumulation in the facility.
Examine to ensure that inlet and outlet devices are free of sediment and
debris and operational.
Inspect side slopes for erosion and undercutting and repair as needed.
Check for signs of eutrophic conditions (e.g., excessive algal growth).
Check for signs of hydrocarbon accumulation and remove appropriately.
Monitor sediment markers for sediment accumulation in forebays and
permanent pools.
Check all control gates, valves and other mechanical devices.
Water side slopes and buffers to promote plant growth and survival.
Inspect wetland, side slopes, strucutres, and buffers following rainfall
events. Plant replacement vegetation in any eroded areas.
Remove sediment, trash, and debris from inlets/forebay.
Monitor sediment accumulation in the wetland and remove sediment
when the permanent pool volume has become reduced significantly,
plants are “choked” with sediment, sediment buildup is preventing flow
through the wetland, or the wetland becomes eutrophic. Replace with
clean gravel and replant vegetation.
Schedule
Semi-Annually
(Quarterly During First
Year)
Annually
As Needed
5 years or after 50% of
the total forebay
storage capacity has
been lost
10 plus years or after
25% of the wetland
storage volume has
been lost
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Submerged Gravel Wetlands
Inspection Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet structure is mowed
and grass clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet structure.
Water is going through structure (i.e. no
evidence of water going around the
structure).
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Pretreatment (choose one)
Forebay – area is free of trash, debris, and
sediment.
Filter Strip or Grass Channels – area is free of
trash debris and sediment. Area has been
mowed and grass clippings are removed. No
evidence of erosion.
Rock Lined Plunge Pools – area is free of
trash debris and sediment. Rock thickness in
pool is adequate.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Erosion protection is present on site (i.e. turf
reinforcement mats). Comment on types of
erosion protection and evaluate condition.
No algal growth along or within the wetland.
Native plants were used in the practice
according to the landscaping plan. No
undesirable vegetation.
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Submerged Gravel Wetlands
Inspection Item
Condition
Good
Marginal
Poor
N/A*
Comment
Vegetation within and around practice is
maintained per landscaping plan. Grass
clippings are removed.
Wetland seems to be working properly. No
settling around the practice. Comment on
overall condition.
No significant sediment accumulation within
the practice.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
Plants seem to be healthy and in good
condition. Comment on condition of plants.
Emergency Overflow
Emergency overflow is free of trash, debris,
and sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet Structure
Outlet structure is free of trash, debris, and
sediment.
No evidence of erosion, scour, or flooding
around the structure.
Outlet structure does not appear to be
blocked.
Results
Overall condition of Submerged Gravel
Wetland:
Additional Comments
Notes: * If a specific maintenance item was not checked, please check N/A and explain why in the
appropriate comment box.
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Underground Detention
Underground detention is detention
storage located in underground tanks or
vaults designed to provide water quantity
control through temporary storage of
stormwater runoff. In addition they can
improve water quality by removing heavy
amounts of sediment.
There are some common problems to be aware of when maintaining an underground detention area.
They include, but are not limited to, the following:
Sediment build-up
Clogging in the inlet and outlet structure
Requirement to have Occupational Safety and Health Administration (OSHA) confined space
entry training
Routine maintenance should be performed on the underground detention areas to ensure that the
structure is properly functioning. Routine maintenance includes the removal of debris from inlet and
outlet structures and cleaning sediment built up inside the structure. Because this is an underground
system, inspection and maintenance may be difficult to conduct. Generally these underground systems
can be inspected by looking in an access opening. Sometimes, however, maintenance requires an
individual who is certified in OSHA confined space entry. Should there be a situation where a safety
concern arises, the inspection should stop and the safety concern addressed. Once the concern is
addressed, the inspection can continue.
Inspect the underground detention area after a large rainstorm. If the underground detention area is
not draining properly, check the inlet and outlet structures to make sure they are not clogged.
Sediment should be removed from the practice by either a vacuum or boom. If the system is accepting
water that flowed from a hazardous facility, the sediment may need to be disposed of by other means.
Check with the local government to identify any additional constraints on the disposal of sediments
excavated from underground detention.
The table on the following page shows a schedule for when different maintenance activities should be
performed on a submerged gravel wetland.
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Operations & Maintenance Guidance Document
Underground Detention Typical Routine Maintenance Activities and Schedule
Activity
128
Remove any trash/debris and sediment buildup in the underground
trash racks, vaults or tanks.
Check drainage areas for trash, erosion, and debris.
Clean underground detention if hazardous or foreign substances are
spilled in the contributing drainage area.
Perform structural repairs to inlet and outlets.
Follow manufacturer’s guidelines and develop/adjust plan for the
underground detention.
Clean out underground detentions with vacuum or boom trucks.
Clean sediment or oil chambers
Schedule
As needed
Annually
Operations & Maintenance Guidance Document
Underground Detention
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet Structure and Pretreatment
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Inlet structure is in good condition. No signs
of cracks or leaks.
Diversion structure (high flow bypass
structure or other) is free of trash, debris, or
sediment. Comment on overall condition of
diversion structure and list type.
Inlet pipe fits tightly to the underground
detention.
Inlet has protection to prevent clogging with
leaves or other debris and has fine mesh for
mosquito control.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
Structure seems to be working properly. No
signs of settling, leaking, or cracking.
Comment on overall condition of structure.
Emergency Overflow and Outlet Structure
Area is free of trash, debris, and sediment.
Overflow valve appears to be in good
condition and show no signs of leaking.
Results
Overall condition of Underground Detention:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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Operations & Maintenance Guidance Document
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Operations & Maintenance Guidance Document
Vegetated Filter Strips
Vegetated filter strips are uniformly
graded and densely vegetated sections
of land, designed to treat runoff and
remove pollutants through vegetative
filtering and infiltration. Vegetated
filter strips are best suited to treating
runoff from roads and highways, roof
downspouts, very small parking lots,
and pervious surfaces. These filter
strips may be constructed with turf, meadow grasses, or other dense vegetation. They are also ideal
components of the "outer zone" of a stream buffer, or as pretreatment for another structural
stormwater control. Filter strips can serve as a buffer between incompatible land uses, be landscaped
to be aesthetically pleasing, and provide groundwater recharge in areas with pervious soils.
There are some common problems to be aware of when maintaining a vegetated filter strip. They
include, but are not limited to, the following:
Sediment build-up
Clogging in the pea gravel diaphragm or other flow spreader
Establishing vegetation within the vegetated filter strip
Ant mounds
Erosion
Concentrated flow
Routine maintenance should be performed on the vegetated filter strips to ensure that the practice is
functioning properly. Note that during the first year the vegetated filter strip is built, maintenance may
be required at a higher frequency to ensure the proper establishment of grass and vegetation in the
practice. Upon establishment, grass should be routinely cut and vegetation trimmed, as necessary, to
maintain a grass height of 3-12 inches or 6-15 inches along a roadway. Other routine maintenance
includes removing trash from the vegetated filter strip and ensuring that grass clippings and other debris
are removed from the filter strip.
Vegetated filter strips should be inspected after a large rainstorm. Keep drainage paths, both to and
from the BMP, clean to promote sheet flow so that the water can be filtered by the BMP.
If the vegetated filter strip is not draining properly, check for clogging in the inlet and outlet structures.
Also, consider if the filter strip has a sufficient slope or if there are obstructions within the filter strip
that may cause inhibit the flow of water. If the practice includes a permeable berm, a structural repair or
cleanout to unclog the outlet pipe may be necessary.
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Operations & Maintenance Guidance Document
In order to keep the water that exits the vegetated filter strip clean, fertilizers should be used sparingly
during the establishment of the practice. Once the vegetation in the practice has been established,
fertilizers should not be used. While vegetation in the vegetated filter strip is important, a primary
purpose of a vegetated filter strip is to act as a water quality device and introducing fertilizers into the
vegetated filter strip introduces nutrients such as phosphorus and nitrogen that can pollute downstream
waters. To control animal nuisances and invasive species, pesticides (including herbicides, fungicides,
insecticides, or nematode control agents) should be used sparingly and only if necessary.
The table below shows a schedule for when different maintenance activities should be performed on a
vegetated filter strip.
Vegetated Filter Strips Typical Routine Maintenance Activities and Schedule
Activity
132
Mow grass to a height to maintain a dense vegetative cover. It is
recommended that the height of grass is 3-12 inches and 6-15 inches
along a roadway. Remove any grass clippings
Keep the practice clean and remove all trash, sediment, and debris.
Reseed any eroded or bare spots.
Water the practice during dry conditions of vegetation establishment.
Inspect vegetated filter strip for signs of erosion, and repair the strip as
needed.
Inspect for invasive species and remove as needed.
Inspect pea gravel diaphragm for clogging and remove built-up
sediment.
Inspect vegetation for rills and gullies. Seed or sod bare areas.
Inspect to ensure that grass has established. If not, replace with an
alternative species.
Schedule
As needed
Annual Inspection
Operations & Maintenance Guidance Document
Vegetated Filter Strip
Maintenance Item
Condition
Good
Marginal
Poor
N/A*
Comment
General Inspection
Access to the site is adequately maintained
for inspection and maintenance.
Area is clean (trash, debris, grass clippings,
etc. removed).
Inlet
Drainage ways (overland flow or pipes) to
the practice are free of trash, debris, large
branches, etc.
Area around the inlet is mowed and grass
clippings are removed.
No evidence of gullies, rills, or excessive
erosion around the inlet.
Water is going through the filter (i.e. no
evidence of water going around the filter).
Diversion structure (high flow bypass
channel or overflow spillway) is free of trash,
debris, or sediment. Comment on overall
condition of diversion structure and list type.
Pretreatment (choose one)
Area is free of trash, debris, and sediment.
No signs of erosion, rills, or gullies.
Pea gravel diaphragm or other level or flow
spreader – No cracks or structural damage in
concrete trough.
Main Treatment
Main treatment area is free of trash, debris,
and sediment.
No signs of erosion, rills, or gullies.
No evidence of long-term ponding or
standing water in the ponding area of the
practice (examples include: stains, odors,
mosquito larvae, etc).
Practice seems to be working properly.
No areas of unhealthy grass or bare areas.
No unwanted or invasive vegetation.
No evidence of use of fertilizer on plants
(fertilizer crusting on the surface of the soil,
tips of leaves turning brown or yellow,
blackened roots, etc.).
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Operations & Maintenance Guidance Document
Vegetated Filter Strip
Condition
Maintenance Item
Good
Marginal
Poor
N/A*
Comment
Grass is kept at the proper mowing height,
3-12 inches and 6-15 inches along the
roadway. Grass clippings are removed.
No signs of accumulated sediment.
Outlet Structure
Outlet is free of trash, debris, and sediment.
No evidence of erosion, scour, or flooding.
Results
Overall condition of Vegetated Filter Strip:
Additional Comments
Notes: *If a specific maintenance item was not checked, please explain why in the appropriate comment box.
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