EFFECTS OF PLANNED FLOW REGIME MODIFICATIONS ON THE STABILITY OF THE BANKS OF THE MISSOURI RIVER DOWNSTREAM OF FORT PECK DAM, MONTANA

Authors: COLLISON, ANDREW - UNIVERSITY OF MISSISSIPPI; LAYZELL, TONY - UNIVERSITY OF MISSISSIPPI; Simon, Andrew

Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 2/15/2002
Publication Date: 2/15/2002
Citation: Collison, A., Layzell, T., Simon, A. 2002. Effects of planned flow regime modifications on the stability of the banks of the missouri river downstream of fort peck dam, montana. USDA-ARS National Sedimentation Laboratory Research Report. No. 24. 29 pp.

Interpretive Summary: Placing dams on large rivers changes the pattern of river flow downstream (the 'flow regime') in a way that is very different from 'natural' or unregulated rivers. Whereas in unregulated rivers flow consists of generally low levels interrupted by occasional large, but short-lived floods, in regulated rivers flows can be held up to high levels for long period of time, for example during the winter to generate hydropower. These changes have been shown to have undesirable effects on the riverbanks downstream of dams; during sustained high flows the banks become saturated and then slump when the flow level drops. In addition the high flows can increase the erosion rate at the toe of the bank, making it steeper and again encouraging slumping. Because of concern about these processes in the past, the National Sedimentation Laboratory (NSL) has been asked to look at a proposed change in the way water is released from Fort Peck Dam on the Missouri River in Montana. The proposal is for water to be released at high levels for a period of sixty days, then dropped back to a lower level, in order to trigger fish spawning. To predict the impact we have used to computer models developed by the NSL; a bank toe erosion model and a bank stability model (the models are available on the NSL web site http://www.sedlab.olemiss.edu/cwp_unit/bank.html. The models have been applied to five field sites on the Missouri River, to consider a wide range of typical conditions. Our findings are that the proposed flow regime has undesirable effects on some sites. Although most banks are able to remain stable in their present forms, toe erosion on banks with weak layers will lead those sites to become unstable and eventually slump.

Technical Abstract: As part of the re-writing of the U.S. Army Corps of Engineers Master Manual for the Missouri River, various flow release scenarios are being evaluated. This Technical Report evaluates the geomorphic impact of one proposed change; a high flow release from Fort Peck Dam to stimulate the spawning of Pallid Sturgeon. Previous studies have raised the potential for sustained high flows to destabilize the riverbanks downstream due to bank saturation and subsequent draw-down, and toe erosion. The methodology involved converting flow to stage at each of five test sites using local rating curves, using the stage v. time curves in conjunction with in-situ measurements of erodibility and critical shear stress to predict bank and bank toe erosion rates, simulating pore-water pressure changes in a finite element hydrology model and simulating the stability of the banks in a limit equilibrium stability model, the ARS Bank Stability Model http://www.sedlab.olemiss.edu/cwp_unit/bank.html. The results indicate that the proposed flow release has the potential to trigger bank failure at three of five sites simulated. Bank failure is not triggered directly by saturation and draw-down; the proposed stage changes allow enough time for pore-water pressure to dissipate on the falling limb of the hydrograph. However, the sustained high stage exerts considerable boundary shear stress that is greater than the critical shear stresses of some of the bank toe materials. Failure is therefore triggered when bank toe erosion undercuts or over steepens the banks.