Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/9/2003
Publication Date: 6/15/2003
Citation: SIMON, A., LANGENDOEN, E.J., COLLISON, A., LAYZELL, A. INCORPORATING BANK-TOE EROSION BY HYDRAULIC SHEAR INTO A BANK-STABILITY MODEL: MISSOURI RIVER, EASTERN MONTANA. PROCEEDINGS, EWRI-ASCE, WORLD WATER & ENVIRONMENTAL RESOURCES CONGRESS. 2003. 11 P. CD-ROM. Interpretive Summary: The United States Army Corps of Engineers is considering plans to restore some aspects of natural flow to the Missouri River in response to environmental concerns. One proposal is to simulate a spring release every three years downstream of Fort Peck Dam, Montana to trigger migration and spawning by Pallid Sturgeon. The plan is to raise flows from a base flow level over 12 days, maintain the flow for between 6 and 36 days (depending on water temperature) and then return the flow to the original base level over 12 days. Although flow releases are increasingly popular with the public because they are seen as restoring natural function to rivers, those living downstream understandably view them with more skepticism. The downstream impact of flow releases is a politically contentious matter and often raises concerns amongst landowners who fear their land will be adversely affected or lost. Accurate prediction of flood release impact is, therefore, needed so impacts can be assessed and remedial measures or anticipated if appropriate. To analyze the potential impacts numerical models developed by the ARS that can predict bank erosion by water and gravity are used. Results indicate that about one half of the studied sites would be unstable under the proposed flow, but for different reasons. Some are made unstable by water entering and weakening the streambank while others become unstable because of erosion at the base of the bank. The work highlights the need to account for all of the controlling processes infiltration, erosion at the base and changes in material strength that lead to bank failure. Simpler modeling approaches run the risk of overestimating bank stability.
Technical Abstract: The effects of proposed flow releases on streambank pore-pressures and bank-toe erosion needs to be evaluated to properly model bank-stability. The Bank-Stability Model 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 the hydraulic effects of bank-toe erosion have been added. Upper-bank stability is often a function of the degree of fluvial undercutting that occurs during rises in stage when the bank toe becomes submerged and steepened. Recent field research on erosion of in situ cohesive streambeds and bank toes with a submerged jet-test device provides a means of calculating bank-toe erosion. Inputs for the bank-toe erosion routine are:(1) a rectangular-shaped hydrograph of specified height and duration, (2) bed slope, (3) flow depth, (4) bank geometry, and (5) erodibility for all bank layers and failed debris. Erosion is simulated normal to the submerged bank surface and the resulting bank geometry serves as input into the bank-stability model. According to the proposed 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 according. Results show the important contribution of bank-toe erodibility in controlling mass failure. All sites indicate a destabilizing influence during the lowering of stage after 60 days due to seepage, and loss of matric suction and confining pressure. Banks at river miles 1624, 1676 and 1716 are unstable. 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.