EQUILIBRIUM CLEAR-WATER SCOUR AROUND AN ABUTMENT

Based on the flow-continuity equation, scour geometry, and a generaliz ed form of the power-law formula for dow resistance in an alluvial cha nnel, a semiempirical analysis of equilibrium local clear-water scour at an abutment is presented. The method is based on the premise that t he flow obstruction and subsequent increases in bed shear stress due t o projection of the abutment into the Row are responsible for the scou ring action around the structure. The proposed scout-depth equation is derived for an abutment placed perpendicular to the flow direction an d involves only the approach flow quantities, the median sediment size , and the projecting length of the abutment. A total of 252 data on cl ear-water maximum scour depth, from the present study and other source s, were analyzed and compared with the developed relationship. The for mula is extended to include the effects of abutment shapes and nonunif orm sediment mixtures. For mixtures an effective sediment size, d(50a) , that corresponds to the critical armor layer in the scour hole is us ed. In general, comparisons of computed and measured maximum equilibri um scour depths indicate that the predictions are satisfactory and the approach is reasonable.