SEEPAGE EROSION FROM DAM-REGULATED FLOW - CASE OF GLEN CANYON DAM, ARIZONA

Seepage erosion-in particular, slope failures (bank slumps, mass wasting)-is prevalent on most sandbars downstream from the Glen Canyon Dam, Ariz. The public is concerned about the loss of the extant biomass and recreational facilities on these sandbars. It is alleged that the operation of the Glen Canyon Dam is responsible for the erosion of the sandbars. In this contribution, a simple, approximate analysis is developed to determine the extent of slope failures due to seepage of bank stored water from transient dam flow. The analysis is intended to assist environmentalists, dam operators, planners, and others to predict zones in which riparian habitat and recreation use will be negatively impacted by fluctuating flows. The affected area of a sandbar was found to be dependent on the range of flows; the rate of rise of river stage; the duration of the peak discharge; and the permeability, friction angle, and cohesion of the soil. Comparison of the predictions of the analysis with field data show good agreement.