Quantifying Existing and Potential Reductions in Sediment Loads from Streambanks

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.