SEEPAGE-INDUCED SLOPE FAILURES ON SANDBARS IN GRAND-CANYON

The effect of fluctuating discharge from Glen Canyon Dam on downstream sandbars is of significant concern to dam operators, environmentalists, and the public. In this contribution, the observations of seepage-related erosion caused by fluctuations in dam discharge are presented. A finite-element model embracing Blot's coupled stress-pore water pressure theory is used to study seepage-induced slope failures of sandbars in the Grand Canyon. In addition, a simple model based on seepage parallel to slope in an infinite, homogeneous, cohesionless soil was used to determine the limiting stable seepage slope. In this paper, it is shown that this limiting stable seepage slope becomes a predefined failure plane. Sand deposited above this stable seepage slope will eventually fail along the predefined plane from gravitational forces, high pore-water pressure and seepage forces. Field data from an instrumented sandbar in the Grand Canyon subjected to the fluctuating discharge from Glen Canyon Dam are compared with the predictions from the simple model, the finite-element model, and conventional slope-stability analyses.