|Shields Jr, Fletcher|
Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2000
Publication Date: N/A
Interpretive Summary: The Missouri River is an important resource for local landowners and government agencies responsible for river management. A September 1995 field reconnaissance study of a 257 km reach of the Upper Missouri River between Fort Peck Dam, Montana and the junction 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 have expressed concern that construction of Fort Peck Dam and subsequent operation of the dam for hydropower generation and flood control have been responsible for triggering changes in flow and sediment regime, causing accelerated land loss from bank caving. Results have shown that although channel deepening has occurred following dam closure, that further bed erosion is unlikely. However, land loss by bank caving is ongoing. The main contributors to this are the channel deepening, maintenance of high flows of prolonged periods, and the shifting of these high flows to winter months. All of these factors are the result of dam operation.
Technical Abstract: The closure of Fort Peck Dam in the 1930's on the Missouri River in eastern Montana initiated a series of changes in channel morphology downstream from the dam that impacted bank stability. These included streambed degradation and the consequent increase in bank heights, and a substantial alteration to the magnitude, frequency, and temporal distribution of flows. A new bank-stability model, developed by the ARS was performed on 17 sites and provided consistent results: The model includes the effects of both matric suction and positive pore-water pressures, confining pressures, and layering. Instability occurs from the loss of matric suction and the generation of positive pore-water pressures. In this semi-arid region, these hydrologic conditions are most likely to occur from the maintenance of moderate and high flows for extended periods, thereby providing a mechanism for saturation of streambanks.