EFFECTS OF SOIL-MOISTURE DYNAMICS ON SLOPE FAILURE AT HYRUM RESERVIOR, UTAH

Field observations of shoreline conditions at Hyrum Reservoir, Utah, w ere conducted during the summers of 1991 to 1993. A process of bluff r etreat is described for a multiple-layered bluff environment of sand a nd clay layers. Failure is initiated by wetting and drying of clay sed iments, which produces horizontal cracks within bluff material. These cracks appear to penetrate to a depth of approximately 100-150 mm befo re initiating vertical cracking in the sediments. The vertical cracks are propagated by continued drying of the surface sediment, ultimately leading to failure of the bluff material. The physical dimensions of sediment blocks succumbing to this mechanism range from a few hundred millimetres up to 3 m on a side, with a depth of approximately 100-150 mm. The mechanism described here appears to operate optimally when th e supply of subsurface moisture is abundant and nearly continuous thro ughout the spring and early summer. Reservoir draw-down, large capilla ry fringe effects in the bluff and periodic wetting from upslope undra ined hollows are the dominant moisture controls at this site. Moisture delivery to the face is strongly influenced by anisotropy of saturate d hydraulic conductivity in the alternating clay and sand layers and r elated differences in sediment texture.