Bank Stability Analysis for the Upper Missouri River.

Changes in the operation of Fort Peck and Garrison dams on the Upper Missouri River in Montana and North Dakota have been proposed as a means to enhance fisheries and wildlife interests, while continuing to fulfill essential flood-water storage and hydropower functions. Concern has been raised that the proposed changes in regulated flow regime may lead to accelerated rates of bank erosion. To investigate the possible impacts of the proposed regulated flow regime on fluvial bank erosion and bank stability with respect to mass failure, a field and modeling study was undertaken. Modeling studies were conducted using data at 18 study sites selected to represent the range of conditions encountered in the field. Contemporary September 1995 conditions of bank stability with respect to mass failure, identification of problem reaches, and identification of dominant erosion processes were characterized using stream reconnaissance. Based on field observations, 57 of banks in the Fort Peck dam reach and 41 of banks in the Garrison dam reach exhibit evidence of mass-wasting at the present time September 1995. The short-term 5 years impact on bank stability with respect to mass failure is analyzed by simulating changes in key bank hydrological parameters. Excess bank pore water pressures and hydrostatic confining pressures generated under the proposed flow regime are found to be indiscernible from those under the existing flow regime. Hence, short-term impacts on bank stability with respect to mass failure are negligible. The long-term up to 50 years impacts on near-bank bed scour and fluvial bank erosion, and bank stability with respect to mass failure are analyzed by extrapolating historical trends of channel adjustment into the future under the existing flow regime.