Erosion Tests On Rock From Spillway Channel At Lewisville Dam PDF Download

Studies of erosion in unlined emergency spillways channels excavated in rock have demonstrated that erosional processes operating in these spillway channels are similar to those operating in natural stream channels and that rock discontinuities are critical factors in the initiation and extent of erosion. Knickpoint formation and headcutting are typical erosion processes. Fractures, faults, joints, dip orientation, igneous contacts, veins, bedding planes, unconformities, bed pinch outs, and facies changes are types of structural or stratigraphic discontinuities which must be characterized if present. Analysis of 14 Soil Conservation Service and 2 ACE dams which have experienced emergency spillway flow revealed that the extent of erosion at these sites could be categorized in terms of volumetric and horizontal erosion rankings. These ranking parameters provide insight as to the seriousness of the erosion threat at a particular dam and may be used to give priority for remediation. The ranking parameters were more closely correlatable with the geometry of the spillway channel than with the hydraulics of the spillway flow event. Preliminary laboratory studies demonstrated that maximum undermining and erosion of a stratified, two-layer system was a function of the ratio of water depth to knickpoint height and the venting condition of the waterfall. Maximum erosion did not occur at peak discharge but occurred when the discharge passed through windows or thresholds on the rising and falling limbs of the hydrograph which were, in turn, controlled by the ratio (above) and venting.