Scour of gravel beds at bridge piers and abutments

Abstract

Results of an experimental investigation into local scour at piers and abutments in uniform gravels under a clear water scour condition are presented. Two types of pier, namely circular and square, and two types of short abutment (abutment length/upstream flow depth < 1, for short abutments), namely wing-wall and vertical-wall, were tested. Although scour at piers and abutments has been studied widely in sand beds, it remains almost unexplored in gravel beds. Therefore, different gravel sizes, ranging from 4·1 mm to 14·25 mm, were used in this experimental study. The influence of gravel size on equilibrium scour depth at piers and abutments is prominent. From the experimental results, it is observed that the equilibrium scour depth increases with decrease in gravel size and with increase in pier width and abutment length. The variations of equilibrium scour depth with gravel size for both pier and abutment scour depart considerably from the variations of equilibrium scour depth with sand size. Consequently, for scour at piers and abutments, the resulting sediment size factors for gravels, evaluated by fitting envelope curves, are significantly different from the existing sediment size factor for sands. Combining pier and abutment scour data, an integrated approach to estimating the equilibrium scour depth at a bridge foundation is proposed. Scaling of distorted experimental model results to prototype is also discussed.