Incorporating Bank-Toe Erosion by Hydraulic Shear into a Bank-Stability Model: Missouri River, Eastern Montana
Andrew Simon, Eddy J. Langendoen, Robert Thomas
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.
Fort Peck Dam was constructed on the Upper Missouri River between 1933 and 1940. Closure in 1937 radically modified the downstream regime. Ecologic and geomorphic function of the Missouri River, with the natural cycle of intermittent, short duration, high flows during spring were replaced by a system of controlled, long duration, moderately high flows during the winter. This regime, combined with the sediment- depleted nature of the discharge, led to downstream incision. Work by Simon et al. (1999a) showed that bank instability along the Missouri River is promoted by a combination of channel incision, fluvial undercutting and increases in pore-water pressure in the bank due to the modified flow regime. During long- duration flow releases, water infiltrates into the riverbank eliminating matric suction that enhances soil strength and promoting positive pore-water pressures. Infiltration is enhanced compared with natural events since high stages are maintained for a longer duration. When flow is lowered in regulated rivers this has traditionally occurred at a rapid rate, resulting in a loss of confining pressure that is often more rapid than the dissipation of pore-water pressure from drainage. Such drawdown conditions often result in bank instability and mass failures, with associated problems including loss of farmland and damage to water-supply inlets.
According to the simulated flow-release plan, flows of 216 m /s are increased by 38.3 m3/s/day for 12 days to 675 m /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 F < 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.
Keywords: bank-stability, toe erosion, dams
Simon is a Research Geologist and Langendoen is a Research Hydraulic Engineer, both at the USDA-ARS, National Sedimentation Laboratory, Oxford, MS 38655. E-mail: email@example.com. Thomas is a doctoral student in the Department of Geography, University of Leeds, UK.
The objective of this study was to determine the potential impact of a synthetic spring release from Fort Peck Dam, MT on bank erosion of five downstream sites in eastern Montana. The study reach extends from River Mile 1762, below Fort Peck Dam, downstream to River Mile 1589. Five sites were chosen: River Mile 1762 (Milk River), River Mile 1716 (Pipal), River Mile 1676 (Woods Peninsula), River Mile 1624 (Tveit-