With the recent increase in wildfire activity in the southeast Australia, an apparent gap has eme... more With the recent increase in wildfire activity in the southeast Australia, an apparent gap has emerged in relation to our understanding of how the landscape responds to fire and the role of extreme erosion events in the region. Numerous reports of `flash floods", "mud torrents' and "landslides" in fire affected areas have only recently been recognised as significant events that
The overland flow pathway between hillslopes and streams remains a critical, yet poorly represent... more The overland flow pathway between hillslopes and streams remains a critical, yet poorly represented transport pathway linking hillslope constituent generation and catchment exports via the stream network. The poor representation of this runoff and pollutant pathway largely results from the mismatch in scales between the pervasive point-scale representations of infiltration (eg Green and Ampt) within hydrologic and erosion models, and
Post fire debris flows and other extreme erosion events result from the spatial and temporal inte... more Post fire debris flows and other extreme erosion events result from the spatial and temporal intersection of burnt areas and rainfall events of sufficient magnitude to initiate these erosion processes. Both the size and frequency of fires and storms can be considered as random variables. The spatial and temporal properties of burnt areas and storms affect the probability of an
Wildfires can reduce infiltration capacity of hillslopes by causing (i) extreme soil drying, (ii)... more Wildfires can reduce infiltration capacity of hillslopes by causing (i) extreme soil drying, (ii) increased water repellency and (iii) reduced soil structure. High severity wildfire often results in a non-repellent layer of loose ash and burned soil overlying a water repellent soil matrix. In these conditions the hydraulic parameters vary across discrete layers in the soil profile, making the infiltration process difficult to measure and model. The difficulty is often exacerbated by the discrepancy between actual infiltration processes and the assumptions that underlie commonly used infiltration models, most of which stem from controlled laboratory experiments or agricultural environments, where soils are homogeneous and less variable in space and time than forest soils. This study uses a simple two-layered infiltration model consisting of surface storage (H), macropore flow (Kmac) and matrix flow (Kmat) in order to identify and analyze spatial–temporal infiltration patterns in fore...
In this study soil water repellency was monitored seasonally following prescribed burns to better... more In this study soil water repellency was monitored seasonally following prescribed burns to better understand the effects of fire severity on soil hydrological properties. The study sites consisted of dry eucalypt forest with clay loam soil. This forest type is common in south-eastern Australia and is frequently burnt by both wildfires and prescribed burns. A feature of the soil at
The infiltration capacity of burnt forest soil acts as an important control on the susceptibility... more The infiltration capacity of burnt forest soil acts as an important control on the susceptibility of catchments to post-fire erosion events. The effect of water repellency on infiltration depends on a number of factors including i) the degree of water repellency, ii) its distribution within the soil profile, iii) its distribution across infiltrating hillslopes and iv) the availability of macropores
ABSTRACT Immediately after wildfire there is an abundant supply of non-cohesive ash, soil and gra... more ABSTRACT Immediately after wildfire there is an abundant supply of non-cohesive ash, soil and gravel which is easily entrained by overland flow. Under these conditions the sediment flux on hillslopes can be assumed to be equal to the transport capacity of the flow. However, the supply of material is finite and at some point the hillslope could shift towards a system where entrainment is restricted by armouring and soil cohesion. In this study we test the notion that burnt hillslopes can be represented as a two-layered system of non-cohesive and cohesive soils. Using a combination of i) shear vane measurements, ii) confined hillslope flow experiments and iii) a laboratory flume, we demonstrate how erosion on burnt hillslopes primarily takes place in a distinct layer of non-cohesive soil with erosion properties that are very different to the underlying soil matrix. Shear vane measurements were taken at 5 soil depths at more than 50 points along transects in order to quantify the depth and spatial distribution of non-cohesive soil in two small (0.5 ha) and steep (30 deg) convergent basins (SE Australia) that were burnt at high severity. The measurements showed that the recently burnt hillslopes were mantled with non-cohesive soil to an average depth of 18mm and 20mm at the two sites which were situated in different geologic terrain but in similar eucalyptus dominated forests. In the hillslope flow experiments, the rapid entrainment of non-cohesive material resulted in very high sediment concentration (50-60% by volume) in the initial surge from the test area. During the flow experiments the sediment concentration decreased exponentially with time until the erosion rate reached a steady state reflecting the erodibility of the underlying cohesive soil. The formation of shallow rills and the presence of large clasts (>16cm) within the test area resulted in incomplete removal of the non-cohesive material at shear stress < 50 Ncm-2. At shear stress > 50 Ncm-2 all material was removed, and the erosion depth at the end of the experiments was equal to the depth of non-cohesive material measured using the shear vane. In a separate set of experiments, a laboratory flume was used to measure the erodibility at different soil depths using soil cores that were burnt at moderate to high severity. Unlike the field based flow experiments, the erodibility measurements of non-cohesive soils in the flume were not restricted by the transport capacity of the flow. Results from the flume experiments showed a two order of magnitude decrease in erodibility within the top 2cm of the soil profile for soil cores from both chaparral and coniferous forests (western US). In summary, these results indicate that a majority of hillslope sediment may be generated from a relatively shallow layer of non-cohesive and highly erodible material. The depth of this material may be an important property that can help determine the post-fire erosion and debris flow potential, particularly in systems where other sources of sediment are limited. The study confirms that erodibility of burnt soil shows strong variation with depth and that the assumption of a constant erodibility factor may lead to misrepresentation of important processes.
With the recent increase in wildfire activity in the southeast Australia, an apparent gap has eme... more With the recent increase in wildfire activity in the southeast Australia, an apparent gap has emerged in relation to our understanding of how the landscape responds to fire and the role of extreme erosion events in the region. Numerous reports of `flash floods", "mud torrents' and "landslides" in fire affected areas have only recently been recognised as significant events that
The overland flow pathway between hillslopes and streams remains a critical, yet poorly represent... more The overland flow pathway between hillslopes and streams remains a critical, yet poorly represented transport pathway linking hillslope constituent generation and catchment exports via the stream network. The poor representation of this runoff and pollutant pathway largely results from the mismatch in scales between the pervasive point-scale representations of infiltration (eg Green and Ampt) within hydrologic and erosion models, and
Post fire debris flows and other extreme erosion events result from the spatial and temporal inte... more Post fire debris flows and other extreme erosion events result from the spatial and temporal intersection of burnt areas and rainfall events of sufficient magnitude to initiate these erosion processes. Both the size and frequency of fires and storms can be considered as random variables. The spatial and temporal properties of burnt areas and storms affect the probability of an
Wildfires can reduce infiltration capacity of hillslopes by causing (i) extreme soil drying, (ii)... more Wildfires can reduce infiltration capacity of hillslopes by causing (i) extreme soil drying, (ii) increased water repellency and (iii) reduced soil structure. High severity wildfire often results in a non-repellent layer of loose ash and burned soil overlying a water repellent soil matrix. In these conditions the hydraulic parameters vary across discrete layers in the soil profile, making the infiltration process difficult to measure and model. The difficulty is often exacerbated by the discrepancy between actual infiltration processes and the assumptions that underlie commonly used infiltration models, most of which stem from controlled laboratory experiments or agricultural environments, where soils are homogeneous and less variable in space and time than forest soils. This study uses a simple two-layered infiltration model consisting of surface storage (H), macropore flow (Kmac) and matrix flow (Kmat) in order to identify and analyze spatial–temporal infiltration patterns in fore...
In this study soil water repellency was monitored seasonally following prescribed burns to better... more In this study soil water repellency was monitored seasonally following prescribed burns to better understand the effects of fire severity on soil hydrological properties. The study sites consisted of dry eucalypt forest with clay loam soil. This forest type is common in south-eastern Australia and is frequently burnt by both wildfires and prescribed burns. A feature of the soil at
The infiltration capacity of burnt forest soil acts as an important control on the susceptibility... more The infiltration capacity of burnt forest soil acts as an important control on the susceptibility of catchments to post-fire erosion events. The effect of water repellency on infiltration depends on a number of factors including i) the degree of water repellency, ii) its distribution within the soil profile, iii) its distribution across infiltrating hillslopes and iv) the availability of macropores
ABSTRACT Immediately after wildfire there is an abundant supply of non-cohesive ash, soil and gra... more ABSTRACT Immediately after wildfire there is an abundant supply of non-cohesive ash, soil and gravel which is easily entrained by overland flow. Under these conditions the sediment flux on hillslopes can be assumed to be equal to the transport capacity of the flow. However, the supply of material is finite and at some point the hillslope could shift towards a system where entrainment is restricted by armouring and soil cohesion. In this study we test the notion that burnt hillslopes can be represented as a two-layered system of non-cohesive and cohesive soils. Using a combination of i) shear vane measurements, ii) confined hillslope flow experiments and iii) a laboratory flume, we demonstrate how erosion on burnt hillslopes primarily takes place in a distinct layer of non-cohesive soil with erosion properties that are very different to the underlying soil matrix. Shear vane measurements were taken at 5 soil depths at more than 50 points along transects in order to quantify the depth and spatial distribution of non-cohesive soil in two small (0.5 ha) and steep (30 deg) convergent basins (SE Australia) that were burnt at high severity. The measurements showed that the recently burnt hillslopes were mantled with non-cohesive soil to an average depth of 18mm and 20mm at the two sites which were situated in different geologic terrain but in similar eucalyptus dominated forests. In the hillslope flow experiments, the rapid entrainment of non-cohesive material resulted in very high sediment concentration (50-60% by volume) in the initial surge from the test area. During the flow experiments the sediment concentration decreased exponentially with time until the erosion rate reached a steady state reflecting the erodibility of the underlying cohesive soil. The formation of shallow rills and the presence of large clasts (>16cm) within the test area resulted in incomplete removal of the non-cohesive material at shear stress < 50 Ncm-2. At shear stress > 50 Ncm-2 all material was removed, and the erosion depth at the end of the experiments was equal to the depth of non-cohesive material measured using the shear vane. In a separate set of experiments, a laboratory flume was used to measure the erodibility at different soil depths using soil cores that were burnt at moderate to high severity. Unlike the field based flow experiments, the erodibility measurements of non-cohesive soils in the flume were not restricted by the transport capacity of the flow. Results from the flume experiments showed a two order of magnitude decrease in erodibility within the top 2cm of the soil profile for soil cores from both chaparral and coniferous forests (western US). In summary, these results indicate that a majority of hillslope sediment may be generated from a relatively shallow layer of non-cohesive and highly erodible material. The depth of this material may be an important property that can help determine the post-fire erosion and debris flow potential, particularly in systems where other sources of sediment are limited. The study confirms that erodibility of burnt soil shows strong variation with depth and that the assumption of a constant erodibility factor may lead to misrepresentation of important processes.
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