Equilibrium Clear-Water Scour around an Abutment

Based on the flow-continuity equation, scour geometry, and a generalized form of the power-law formula for flow resistance in an alluvial channel, a semiempirical analysis of equilibrium local clear-water scour at an abutment is presented. The method is based on the premise that the flow obstruction and subsequent increases in bed shear stress due to projection of the abutment into the flow are responsible for the scouring action around the structure. The proposed scour-depth equation is derived for an abutment placed perpendicular to the flow direction and involves only the approach flow quantities, the median sediment size, and the projecting length of the abutment. A total of 252 data on clear-water maximum scour depth, from the present study and other sources, were analyzed and compared with the developed relationship. The formula is extended to include the effects of abutment shapes and nonuniform sediment mixtures. For mixtures an effective sediment size, d50a, that corresponds to the critical armor layer in the scour hole is used. In general, comparisons of computed and measured maximum equilibrium scour depths indicate that the predictions are satisfactory and the approach is reasonable.