ABSTRACT:

The Glen Canyon Dam, located in the north-eastern corner of Arizona, is operated to satisfy peak power demands of neighboring states. The discharge regime of the dam is such that the river stage in the Colorado River downstream of the dam varies diurnally as much as 4 m in some sections. Many sand bars, scattered along the 402 km downstream stretch of the Colorado River, are eroding. These sand bars support a large and diverse riparian habitat, and are often used for public recreation. The fluctuations of the river stage, due to the operation of the Glen Canyon Dam, are claimed to be responsible for the destruction of these sand bars and, as a result, the loss of the extant bio-mass and the reduction of recreational facilities. Three interrelated erosion mechanisms (seepage, traction and wave induced) were identified. Under typical hydraulic conditions, one of these mechanisms is dominant at a given sand bar. For most of the sand bars examined in this study, the dominant erosion mechanism is soil loss due to seepage. Seepage driven erosion is responsible for rilling, formation of rivulets, slope failures (bank slumps, mass wasting), piping and tunneling. In this contribution, an analysis is developed to determine the equilibrium slope below which slope failures (mass wasting, bank failures) due to seepage of bank stored water would be unlikely. The sediments enclosed by the equilibrium seepage slope (range 11o - 14o for the sediments downstream of the Glen Canyon Dam) and the maximum slope angle (range from 26o to 32o ) will be subjected to ongoing aggradation and erosion. Whenever the hydraulic and hydrologic conditions are favorable for accretion, any sediment deposition on a slope angle greater than the equilibrium slope will subsequently be eroded by seepage, especially mass wasting, under fluctuating flows. The equilibrium slope depends on the soil type and the local hydraulic conditions. Field data from ground surveys and time lapsed photography generally support the analysis described in this paper.