EROSION OF RIVER SANDBARS BY DIURNAL STAGE FLUCTUATIONS IN THE COLORADO RIVER IN THE MARBLE AND GRAND CANYONS: FULL-SCALE LABORATORY EXPERIMENTS

This research examines the mass failure and seepage erosion of sandbars due to rapid fluctuations in river stage using a full-scale laboratory model. Hydroelectric dams operated to provide electricity at peak demand produce rapid river stage fluctuations. During decreasing river stage, the groundwater table becomes higher than the river stage, increasing pore water pressures and exfiltrating groundwater. This can cause seepage erosion and mass failures in the banks and bars. In the Colorado River in the Marble and Grand Canyons, maximal downramp and upramp rates have been imposed on the Glen Canyon Dam operations. Our experiments research the efficacy of these discharge ramp rate restrictions to reduce sandbar erosion. The laboratory model consists of a two-dimensional sandbar face (8 m long, 2.5 m high and 0.5 m wide). Multiple experiments were conducted in a range of slopes, varying from 12 degrees to 26 degrees. An analysis of historical and current ramp rates at 47 locations along the river provided the basis of laboratory downramp rates in the range from 0.1 to 0.6 m h(-1). Results show that bank stability is reached at a slope of approximately 14 degrees. The erosion of intermediate slopes (18 degrees - 22 degrees) is controlled by seepage erosion, whereas the erosion of steep slopes (26 degrees) is governed by mass failures. Erosion rates per diurnal cycle do not depend on ramp rates, but they increase with sandbar steepness. Therefore, steep sandbar faces would rapidly erode by mass failure and seepage erosion to shallower stable slopes in the absence of other erosion processes, regardless of dam discharge ramp rates. Our experiments only address seepage erosion and mass failure; increasing the daily magnitude and/or duration of peak discharge may increase the erosion of bars by turbulent sediment transport. Copyright (c) 2012 John Wiley & Sons, Ltd.