The Fate of Failed Bank Material and Implications for Lateral Retreat: Lake Tahoe Basin

The ability to deterministically predict the critical conditions for streambank failure in alluvial materials has improved markedly in recent years. Analytic tools such as the Bank-Stability and Toe-Erosion Model (BSTEM) account for a broad range of controlling processes and factors including hydraulic erosion of the bank toe, positive and negative pore-water pressures, layers of varying geotechnical resistance and root reinforcement. When failure is predicted, the failed mass is assumed to be transported away from the section by the flow, either as a single mass or as dispersed aggregates. Field observations indicate, however, that in cases where cohesive strength is high, either due to the effective cohesion of the soil skeleton or due to dense mats of fine roots, the failed block comes to rest in the vicinity of the bank toe. In this case, the resistance of the bank-toe region to hydraulic scour may be increased markedly and resistance to geotechnical failure may also be increased by buttressing. Conversely, deposition of blocks at bank toes may cause flow acceleration and scour landward of the block, resulting in further undercutting of the bank mass. Failure to account for these processes can lead to errors in predicting of rates of failure frequency, lateral retreat and streambank loadings.Once deposited at the bank toe, failed blocks can be eroded by hydraulic forces either as a mass and/or by erosion of aggregates comprising the block. Field research on the nature of hydraulic resistance and block erosion has been conducted along selected reaches of the Upper Truckee River (UTR) and Trout Creek, Lake Tahoe Basin, California. Block materials are generally characterized by lower apparent cohesive strength than their in situ counterparts due to the lower values of matric suction owing to their proximity to the water surface. Still, submerged jet-test device conducted in root-permeated blocks show critical shear stresses one to two orders of magnitude greater (10 - 30 Pa) than the non root-permeated materials (0.3 - 1.7 Pa). This effect is particularly enhanced along the top surface of the block where the highest critical shear stresses obtained. This is also the location where the above-ground biomass impacts flow resistance, causing a reduction in the shear stress applied to the block. Entrainment of entire blocks does occur, requiring shear stresses from 50 to 120 Pa, using a modified Shields criteria. Based on measurements of block dimensions taken in the summers of 2008 and 2009 along the UTR study reach, although the number of blocks doubled from 13 to 26, hydraulic erosion resulted in a 32% reduction in median block size and a 20% reduction in total block volume. The field experiments conducted here illustrate root-permeated blocks provide for greater hydraulic resistance than in situ bank-toe materials. With bank-toe erosion playing a vital role in streambank stability, these blocks tend to reduce lateral retreat of the bank. Further field tests are being conducted in other regions along with flume studies of the impact of blocks on flow acceleration and shear-stress generation landward of the failed block.