Engineering Evaluation of Channel Stability on the Upper Missouri River, Montana

A study of a 160 mile reach of the Upper Missouri River between Fort Peck Dam, Montana and the confluence of the Yellowstone River has indicated that approximately 50% of the river banks in this reach exhibit evidence of recent geotechnical failure and instability. Riparian landowners and users, organized as the Coordinated Resource Management (CRM) group, have expressed concern that construction of Fort Peck Dam and subsequent operation of the dam for hydropower generation and flood-control functions may have been responsible for triggering changes in flow and sediment regime compared to pre-dam conditions. The CRM group perceives that these changes may have been responsible for triggering accelerated bank erosion and loss of riparian property. Concerns over the magnitude of bank-erosion problems prompted an interagency request from the NRCS and the Army Corps of Engineers, Omaha District, for NSL scientists to conduct an investigation of shear-strength properties, mechanisms of bank failure, ice effects, and relative changes in bank stability and channel morphology since closure of Ft. Peck Dam in 1937. Field data on shear strength, unit weight and moisture content, and channel morphology, were collected in 1996 and 1997.

In-situ shear-strength determinations of 10 of the principle soil series along this reach of river provided effective cohesion and friction-angle data by which to assess bank stability conditions under a variety of soil moisture and tension/desiccation cracking conditions. For all units in all soil series, mean effective cohesion and friction angle are 16.6 kPa and 24.9?, respectively. Effective cohesive strength ranged from 0.0 kPa in sands and desiccated clays to 45.3 kPa in moist clays. Matric suction was found to contribute as much as 100% of the apparent cohesion. Bank-stability charts were developed from shear-strength and bank profile data to estimate stable-bank configurations. Vertical face bank angles would need to be reduced to about 30-40? to reduce failure frequency. Toe erosion of failed blocks and cracking of soils from tension, desiccation, and probably internal pressure from introduced water and probably ice in winter months, were identified as major contributors to bank-stability problems.

To evaluate bank-stability conditions and channel migration before and after the closure of Ft. Peck dam, analysis of field data and of historical maps and aerial photographs are used. Ice effects are also to be analyzed in terms of bank erosion from direct impact, rafting, interstitial expansion and freeze-thaw, and erosive effects of flowing water beneath the ice cover.

Point of Contact:

Dr. Andrew Simon, Retired