QUANTIFYING REDUCTIONS OF MASS-FAILURE FREQUENCY AND SEDIMENT LOADINGS FROM STREAMBANKS USING TOE PROTECTION AND OTHER MEANS: LAKE TAHOE, UNITED STATES

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). 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. A typical high-flow annual hydrograph was selected for analysis. Bank-material strength data were collected for each layer as were species-specific root-reinforcement values. The effects of the first flow event on bank-toe erosion were simulated using an excess shear-stress approach. The resulting geometry was then exported into the bank-stability submodel to test for the relative stability of the bank under peak flow and draw-down conditions. In this way, BSTEM was used iteratively for all flow events for both existing and mitigated conditions. On average, 13.6% of the material was eroded by hydraulic shear, the remainder by mass failures, which occurred about five times over the simulation period. Simulations with 1.0 m-high rock-toe protection showed a dramatic reduction in streambank erosion (69-100%). Failure frequency for the simulation period was reduced in most cases to a single episode. Thus, an almost 90% reduction in streambank loadings was achieved by virtually eliminating the erosion of only 14% of the material that was entrained by hydraulic forces. Consequently, simulations show average load reductions of about an order of magnitude. 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.