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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #224257
Title: Quantifying Existing and Potential Reductions in Sediment Loads from Streambanks

Author
item Simon, Andrew
item Bankhead, Natasha
item MAHACEK, VIRGINIA - VALLEY-MOUNTAIN CONSULTIN
item Langendoen, Eddy

Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/15/2008
Publication Date: 5/12/2008
Citation: Simon, A., Bankhead, N.L., Mahacek, V., Langendoen, E.J. 2008. Quantifying Existing and Potential Reductions in Sediment Loads from Streambanks. Environmental and Water Resources Institute World Congress Proceedings. Honolulu, HI, May, 2008. 10 p. (CD-ROM)

Interpretive Summary: Lake Tahoe is a beautiful natural resource whose clarity is threatened by high loadings of fine sediment (silt and clay). A previous study has determined that about 25% of this fine sediment emanates from streambank erosion, predominantly from three streams: Upper Truckee River, and Blackwood and Ward Creeks. This study was initiated to determine the potential reductions in fine-sediment loadings coming from streambanks that could be achieved using certain mitigation measures. A numerical model of bank stability developed by the National Sedimentation Laboratory was used to simulated streambank conditions under existing and mitigated conditions. The model has the ability to simulate the effects of erosion of surface materials by flow as well as the collapse of a streambank by gravity. The model was used to simulate the effects of a series of flow events and resulting bank instability using a typical high flow year for two sites in each of the three basins. The amount of material eroded during and after each flow event was summed to obtain an annual rate of streambank erosion under existing conditions. Results showed that of the total amount of bank erosion, only about 14% occurred by flowing water, the rest by bank collapse. The analysis was then repeated for conditions where rock was simulated at the base of the bank to protect it from erosion by flowing water. The placement of rock was shown to virtually eliminate erosion at the base of the bank and, therefore, reduce the frequency and amount of erosion by bank collapse by about 87%. This represents a significant reduction in fine-sediment loadings.

Technical Abstract: Streambank erosion by mass-failure processes represents an important form of channel adjustment and a significant source of sediment in disturbed streams. Little if any quantitative information is available on the effectiveness of bank treatments on reducing erosion. To evaluate existing streambank-derived sediment loads and the potential reduction in sediment loadings emanating from streambanks afforded by remediation measures, the hydraulic and geotechnical processes responsible for mass failure were simulated under existing and remediated conditions using the Bank-Stability and Toe-Erosion Model (BSTEM) developed by the USDA-ARS, National Sedimentation Laboratory. Two sites were selected from each of the three watersheds known to contribute the greatest amounts of streambank-derived fine sediment in the Lake Tahoe Basin. The 1995 annual stage hydrographs supplemented by the large rain-on-snow event of January 1-2, 1997 were discretized into individual events to be used with surveyed channel slope data to calculate boundary shear stress for the toe-erosion sub-model. An excess shear-stress approach was first utilized to simulate the extent of toe erosion. The updated geometry was then exported into the bank-stability sub-model to test for the relative stability of the bank under peak flow and drawdown conditions. In this way, BSTEM was used iteratively for all flow events for both existing conditions and with riprap toe protection. Volumes of material eroded by hydraulic and geotechnical processes were tracked for each event and summed to make comparisons between existing and remediated conditions. Under existing conditions, total streambank erosion by hydraulic and geotechnical processes ranged from 472 m3 to 5260 m3. On average, 13.6% of the material was eroded by hydraulic shear, the remainder by mass failures. Iterative simulations with 1.0 m-high riprap toe protection showed a dramatic reduction in mean, total and fine-grained streambank erosion (87%; std. error = 4.2%). Failure frequency for the simulation period was reduced in most cases to a single episode, which generally coincided with recession of the January 1-2, 1997 rain-on-snow event. Thus, an almost 90% reduction in streambank loads was realized by virtually eliminating the erosion of only 14% of the material that was entrained by hydraulic forces. As a consequence, average load reductions were about an order of magnitude. Results stress the critical importance of protecting the bank toe-region from steepening by hydraulic forces. Iterative simulations using bank-top vegetation showed about a 50% reduction in loads.