THE USE OF GEOMORPHOLOGY, GEOPHYSICS, HYDROGEOLOGY, GEOTECHNICAL MODELING, AND SEDIMENT TRANSPORT MODELING TO DETERMINE THE CAUSE OF BANK EROSION ON THE SNAKE RIVER IN HELLS CANYON

Bank failure on the Snake River in Hells Canyon is threatening an archeological site. Therefore, a project was undertaken to identify the causes and mechanisms of bank failure at the Tin Shed Site. Land degradation problems, like riverbank failure, seldom result from one isolated cause, and correct diagnosis of the factors causing bank failure is crucial prior to selection of remediation measures. Consequently, a multi-disciplinary investigation was undertaken.

The geomorphic investigation detailed landforms, river morphology, and flow data. Geophysics were used to characterize the bank materials via ground penetrating radar and electrical resistivity imaging. The bank materials were further characterized by measuring the insitu hydraulic conductivity. Core sediment samples were also taken for particle size and direct shear data for use in the geotechnical model. SLIDE software was used to evaluate geotechnical failures given varying river elevations, rates of hydropower reductions and pore water pressures. Sediment transport was evaluated using incipient motion calculations and the Erodibility Index, which calculates the stream power threshold at which a particle will be eroded. Stream power within the river was obtained from MIKE21C model results.

The investigation revealed that the principal mode of failure originates from an increased risk of geotechnical failure as the bank face becomes steeper due to erosion at high, relatively infrequent discharges. These relatively high discharges are also the process by which most of the sediment is removed from the site. The removal process is exacerbated by the flow patterns in the river that are created by a large alluvial fan downstream of the site. Although groundwater piping plays a role in the bank stability, it was not thought to be a significant contributor to the large-scale erosion that is occurring.