Incorporating Bank-toe Erosion by Hydraulic Shear into a Bank-stability Model

Bank-stability concerns along the Missouri River, eastern Montana are heightened by a simulated change in flow releases from Fort Peck Dam to improve habitat conditions for Pallid Sturgeon. The effects of the simulated flow releases on streambank pore-pressures and bank-toe erosion needs to be evaluated to properly model bank-stability. The Bank-Stability Model used incorporates pore-water pressure distributions, layering, confining pressures, reinforcement effects of riparian vegetation and complex bank geometries to solve for the factor of safety. To increase the applicability and accuracy of the model for use in predicting critical conditions, the hydraulic effects of bank-toe erosion have been added. According to the simulated flow-release plan, flows of 216 m3/s are increased by 38.3 m3/s/day for 12 days to 675 m3/s, held for 60 days and decreased for 12 days back to 216 m3/s. Results show the important contribution of bank-toe erodibility in controlling mass failure. Banks at River Miles 1624, 1676 and 1716 attain Fs < 1.0 indicating imminent failure. These sites contain less resistant sandy-silt material at the bank toe, and experienced simulated undercutting up to 3m. More resistant cohesive, clay bank toes at River Miles 1589 and 1762 were undercut only 0.2 m and remained stable