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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #219997
Title: Quantifying Reductions of Mass-Failure Frequency and Sediment Loadings from Streambanks Using Toe Protection and Other Means: Lake Tahoe, USA

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

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2008
Publication Date: 2/1/2009
Citation: Simon, A., Bankhead, N.L., Mahacek, V., Langendoen, E.J. 2009. Quantifying Reductions of Mass-Failure Frequency and Sediment Loadings from Streambanks Using Toe Protection and Other Means: Lake Tahoe, USA. Journal of the American Water Resources Association. 45(1)1-17.

Interpretive Summary: Erosion of streambanks by failures is an important process in the adjustment of channels, and also provides much of the sediment in unstable streams. Bank failures occur by a combination of two processes. First, erosion of material from the base of the bank undercuts the bank and second, the weight of the material from above then leads to a failure. Although banks have been protected in various ways in the past, little data is available to show how effective these measures are, and what the likely reductions in sediment loadings are. To look at the effect of different measures, different solutions were modeled using a Bank-Stability and Toe-Erosion Model (BSTEM) developed by the USDA-ARS, National Sedimentation Laboratory. Two sites were selected from each of 3 watersheds known to supply the highest amounts of sediment from streambanks to Lake Tahoe. Flow events from the year 1995 were selected for simulations because it was a year of high flows. In addition, the rain-on-snow event of January 1-2 1997 was added to simulations to represent the worst case conditions. Data was collected in the field for bank material types at each of the six sites, using a borehole shear-test device and species-specific root-reinforcement values were used. The effects of the first flow event were simulated using an excess shear-stress approach to examine the amount of erosion from the base of the bank and the change in shape of the bank. The new bank-shape was then used to assess the stability of the streambank at the peak of the flow event and afterwards when the flow level had gone back down. This process was repeated for each flow event to see how bank shape and stability changed over time. With no bank protection, total streambank erosion ranged from 472 m3 to 5260 m3 of which 35 m3 to 900 m3 were fine grained (silts and clays). On average, 13.6% of the material was eroded by flow from the base of the bank, the remainder by bank failures, which occurred about 5 times over the simulation period. The simulations with 1.0 m-high rock protection at the base of the bank showed a dramatic reduction in average, total and fine-grained streambank erosion (87%; std. error = 4.2%). The actual reduction in sediment added to the stream varied from 69 to 100%. In most cases the addition of streambank protection reduced the number of failures to a single event which occurred after the rain-on-snow event of January 2007. Results stress the importance of protecting the bank toe-region at the base of the bank from eroding and steepening the streambank.

Technical Abstract: Streambank erosion by mass-failure processes represents an important form of channel adjustment and a significant source of sediment in disturbed streams. Mass failures regularly occur by a combination of hydraulic processes that undercut bank toes and geotechnical processes that cause bank collapse by gravity. Little if any quantitative information is available on the effectiveness of bank treatments on reducing erosion. To evaluate potential reduction in sediment loadings emanating from streambanks, the hydraulic and geotechnical processes responsible for mass failure were simulated under existing and mitigated conditions using a Bank-Stability and Toe-Erosion Model (BSTEM) developed by the USDA-ARS, National Sedimentation Laboratory. Two critical erosion sites were selected from each of the three watersheds known to contribute the greatest amounts of fine sediment by streambank processes in the Lake Tahoe Basin. The 1995 annual hydrograph plus the rain-on-snow event of January 1-2, 1997 was selected because it was typical of a high-flow year. Bank-material strength data was collected at the six sites for each layer using a borehole shear-test device and species-specific root-reinforcement values were applied based on root distributions using a fiber-bundle model. The effects of the first flow event were simulated using an excess shear-stress approach in the toe-erosion sub model to determine the amount of hydraulic erosion and the change in geometry in the bank-toe-region. The new 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 stone-toe protection. Under existing conditions, total streambank erosion by hydraulic and geotechnical processes ranged from 472 m3 to 5260 m3 of which 35 m3 to 900 m3 were fine grained (silts and clays). On average, 13.6% of the material was eroded by hydraulic shear, the remainder by mass failures, which occurred about 5 times over the simulation period. The iterative simulations with 1.0 m-high rock-toe protection showed a dramatic reduction in average, total and fine-grained streambank erosion (87%; std. error = 4.2%). The actual load-reduction range was from 69 to 100%. 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 loadings was realized by virtually eliminating the erosion of only 14% of the material that was entrained by hydraulic forces. As a consequence, simulations show average load reductions of about an order of magnitude (2070m3 to 127 m3 for total erosion; 292m3 to 21.2 m3 for fines). Results stress the critical importance of protecting the bank toe-region from steepening by hydraulic forces that would otherwise entrain previously-failed and in situ bank materials, thereby allowing the upper bank to flatten (by failure) to a stable slope.