Nathaniel Bergman

Despite that many places around the world in general, and North America in particular, were glaciated during the last ice age, relatively little is known about rivers that evolved over glaciated landscapes once they deglaciated. They are commonly categorized as alluvial with a glacial legacy, and often described as simple gravel-bed or sand-bed rivers. Alternatively, they are classified as bedrock rivers because the glacial deposits have been eroded and the underlying rock is exposed. However, the glacial history of these rivers is important and my research shows that classification for them is "semi-alluvial". This work shows that classification is important, not only for scientific accuracy but for river management that must be based on a sound understanding of river form and process. Poor understanding can be costly (i.e. restoration and management that do not achieve their goals; failed infrastructure), leading to wasted resources and inefficient functioning of the riv...

ABSTRACT Nahal (stream in Hebrew) Oz Reservoir is located is located in southwestern Israel, on the semiarid fringe between the southeastern Mediterranean Sea coast and the northwestern Negev Desert, near the Israeli villages (kibbutz) of Nahal Oz, Alumim and Sa'ad and just east of the northern Gaza Strip. The reservoir was completed in the year 2000 (cost of $5 million by the Jewish National Fund, JNF) and it is one of 7 terminal reservoirs system carrying the reclaimed water of heavily populated central Israel which were designed to enhance the irrigation of cultivated lands during the summer. The Nahal Oz Reservoir virtually dissects the natural headwaters of the Nahal Yare'akh catchment. The reservoir’s continuous filling started on January 15, 2001 until the failure when water level was 5 m below the head of the western dike. Nahal Yare'akh is a small first order catchment with an area of 10 km2. Its natural headwaters is an aeolianite (kurkar) ridge - 102 m a.s.l. and it flows to the northwest into Nahal Hanoun stream within the Gaza Strip and from thereon to the much larger Nahal Shikma (744 km2), which drains the southern Judea Mountains and northwestern Negev towards the Mediterranean Sea. The streams in this region are all ephemeral and some are blocked by coastal sand dunes and aeolianite ridges oriented parallel to the coastline. The channels are mainly incised within a Late Pleistocene loess cover. The Shikma Reservoir serves as the terminus point for the entire catchment, originally constructed for capturing floodwater in 1958 (initial volume of 2.8*106 m3 and later enlarged to 4.2*106 m3), was filled with sediments within a few years due to the very high annual sediment yield typical of semi-arid areas and erodible loess. On March 26, 2001, part of the western dike of the Nahal Oz Reservoir collapsed and generated a flow release of 3.5*106 m3 of secondary irrigation water. Three main construction faults caused the collapse of the dike: (a) The location of the dike over the partially infilled channel, (b) lack of soil compaction and moisture of the dike and (c) tears in the HDPE cover. The seepage through possible cracks in the dike led to differential settlement. As the reservoir water level receded, the reservoir’s western foundations base close to the outlet to Nahal Yare'akh sunk more than 1 m, creating heterogeneous depressions. These were caused by large underground burrows and tree regrowth within the exposed breach section. Most of the water drained directly into the Nahal Yare'akh gully since the western dike that was breached was just above it. Some of the escaping water were deflected from the main channel northward into a gully tributary of Nahal Yare'akh and returned to the main stem several hundred meters downstream. The weather during that day was sunny and therefore the flood was totally unexpected and had no external causes. The damage from the flood was only to public and private property, infrastructure and fields. No fatalities or injuries occurred due to the daylight time occurrence and sufficient warning time for people downstream to avoid the flood course and evacuate quickly those that were in harms way. Post-flood mapping documented the geomorphic and hydraulic response of the small loess channel to the flood. High water marks (HWMs) and organic drift were mapped with GPS to reconstruct the water surface slope of the flood. Peak discharge was determined using the slope-area method below the reservoir and at the reach end, step-backwater HEC-RAS 1D model and a comparison to literature equations based on the breach parameters. The HEC-RAS model simulation was run with 22 post-flood cross-sections that were extracted from 1:2500 scale 1 m contour map that was prepared after the dam breached and coupled with the high water marks described above. Rectified high-resolution aerial photographs aided in locating specific areas that experienced significant flooding and scour. Eye-witness reports of local residents, photos and professional engineering personnel completed the real-time visual scenario which was later verified in the field. Scour depth was extracted from the difference between the pre and post-flood topography of the river bed and compared to bed scour prediction formulae. The major geomorphic impact was scour and channel widening along the initial 2 km downstream of the reservoir. The increase in cross-sectional area was about 60% and had an estimated 170,000 m3 of sediment bank, floodplain and channel erosion. Calculated peak shear stress and stream power along this section are estimated at 300 Nm-2 and 900 wm-2, respectively. The peak discharge at the end of this segment (1.32 km2) was estimated at >800 m3s-1 indicating only minor attenuation along this section. The floodwater uprooted the seasonal grasses shrub vegetation and young eucalypt trees. Only thick-trunked deeply rooted eucalypt trees were capable of withstanding the water…

he Nahal Oz Reservoir - in the coastal, semi-arid southwestern Israel was designed to enhance local irrigation of crops using reclaimed sewage water during the dry summer months. On March 2001, part of the western dike of the reservoir was breached and generated a flow release of 3.5*106 m3 of secondary irrigation water that was channeled down the 1st order ephemeral loess stream (Nahal Yare'akh). The consequent 12-hour flood surge, with an estimated peak discharge of 1000 m3s-1, inflicted severe loess erosion, agricultural, property and infrastructural damage downstream ($6 million). Post-flood mapping documented the geomorphic response to the flood which included channel scour and widening along the initial 2 km downstream of the reservoir where a spillway channel was formed. The increase in the cross-sectional area was about 60% and had an estimated 170,000 m3 of sediment bed, bank and floodplain erosion. Calculated maximum shear stress and stream power along this section are...

The benefits of three simple modifications to the design of a Birkbeck bedload slot-sampling system that has been continuously operating in Nahal Eshtemoa, Israel, since the early 1990s are demonstrated. The modifications include the deployment of a removable slot cover which delays the accumulation of sediment, so allowing sampling at late stages of a flood and, in conjunction with other samplers, extending the period of sampling during a flood wave; inclusion of a slot the size of which is adjustable so that that the probability of sampling the largest clast sizes in transit as bedload can be increased post-installation, once knowledge is gained about the bedload grain-size distribution; and a sampler side-wall door that allows stratification and textural changes within the accumulated bedload to be identified, so pro-moting intelligent sampling of the deposit for grain-size determination. Results from seven flash-floods are presented and discussed, with recommendations for bedloa...

... N. Bergman et al. Copyright © 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 32, 317–328 (2007) DOI: 10.1002/esp or partial channel width (Leopold and Emmett, 1976); in-stream weighing devices that continuously sample a fraction of the channel width (Ashida ...

134 Erosion oml Sediment Transport Measurement in Rivers: Technological and Methodological Advances (Pnx-eedings of the Oslo Workshop. June 20 () 2). lAHS Publ. 283. 2003. The continuous monitoring of bed load flux in various fluvial environments JONATHAN B. LARONNE1, ...

he Nahal Oz Reservoir - in the coastal, semi-arid southwestern Israel was designed to enhance local irrigation of crops using reclaimed sewage water during the dry summer months. On March 2001, part of the western dike of the reservoir was breached and generated a flow release of 3.5*106 m3 of secondary irrigation water that was channeled down the 1st order ephemeral loess stream (Nahal Yare'akh). The consequent 12-hour flood surge, with an estimated peak discharge of 1000 m3s-1, inflicted severe loess erosion, agricultural, property and infrastructural damage downstream ($6 million). Post-flood mapping documented the geomorphic response to the flood which included channel scour and widening along the initial 2 km downstream of the reservoir where a spillway channel was formed. The increase in the cross-sectional area was about 60% and had an estimated 170,000 m3 of sediment bed, bank and floodplain erosion. Calculated maximum shear stress and stream power along this section are...

Systematic bed material measurements of 15 macro-bedforms in the West Walker River in eastern California demonstrate that the channel is enriched with fine sediment (< 2 mm). Along 15 km of the stream system, bed material varies from sand up to giant boulders in the upstream canyon exit from the Sierra Nevada, fines downstream in Antelope Valley to cobbles and fines which the latter become the dominant sedimentary fraction once leaving this valley downstream to Smith and Mason Valleys. The stream network is heavily impacted by irrigation diversions and off-channel storage into Topaz Lake reservoir and the manipulated hydrology affects the bed material distribution by hampering the river’s competence to transport the coarser fractions, especially during high flows for the period of the spring freshet. As a result, the intervening agricultural valleys downstream suffer from continuous river aggradation by the overwhelming amounts of fines which are transported at a spectrum of disc...

Streams incised into till are common in southern Ontario, and other regions of North America, in which rivers are incised into thick and extensive glacial deposits. These streams are partially alluvial rivers but are also soft-bed analogues for some types of bedrock channels. Recent theories of alluvial (gravel) cover, gravel transport and bed erosion derived for bedrock channel may be adapted for analysis of fluvial processes in till-bed channels. This requires some initial observation and measurement of till-bed channel characteristics including spatial extent, thickness and grain size characteristics of alluvial gravel cover, mechanisms of till erosion, composition of till and the role of till-derived gravel in alluviation processes, and the sources of gravel supply to channels. At the study site in Medway Creek, till is about 6% gravel (by volume) and erosion also yields rounded till clasts 40-300mm in diameter that form part of the alluvial cover. Exposed till occurs in patches...

An unusual 240 mm rainstorm and consequent major flood (peak discharge 110 m3s-1) with an estimated recurrence interval of about 50 years occurred on Dec. 31, 1998 in Nahal Me'arot, an ungauged 25 km2 coastal ephemeral stream in Mt. Carmel, NW Israel. The flood caused morphological modifications along the channel, including intensive stripping of the channel bed and floodplain along several segments and deposition of large gravel bars downstream of these segments. One of such segments served as a study reach; composed of a massive, coarse, 200 m long gravel-bar formed downstream of a segment (150 m long) that was deeply scoured. The bar’s stratigraphy consisted of a very coarse gravel lag, well sorted on the surface overlying much finer sediment characterized by sandy layers mixed with gravel among different types of facies. The difference between its volume and the scoured reach upstream was only 20% (1270 m3 compared to 1010 m3) signifying that scour and aggradation occurred i...

The Dead Sea, a hyper-saline terminal lake, has undergone substantial lake elevation changes due to climatic change since the last ice age. However, anthropogenic water abstraction and its allocation during the past 50 years have led to various accelerated geomorphic processes and in turn physical environmental change. The current low level of the lake is of major concern to the region not only due to a loss of a unique natural resource but also due to the high economical cost and the impact these environmental changes pose. Obviously, a viable solution to stop the lake level degradation will be very costly, technologically complex and will require major efforts from the involved parties. Three alternatives are offered to address the Dead Sea degradation problem; each has its own merits and flaws. The “Zero Option” is to simply not take any action, continue the present situation as is and let the lake reach a geochemical equilibrium which will occur once the brine will no longer eva...