TEMPORAL VARIATION OF SCOUR AROUND CIRCULAR BRIDGE PIERS

Determination of scour depth is needed for economical design of bridge pier foundation. Currently, determination of design scour depth is mainly based on use of relationships for maximum scour depth in steady flow along with the design discharge. Computations have revealed that time taken by the design discharge to scour to its full potential is generally larger than the time for which it runs. Hence, the computation of temporal variation of scour depth should form the basis of the design. Experiments are conducted on temporal variation of scour around circular bridge piers placed in uniform, nonuniform, and stratified beds under steady and unsteady clear-water flows. Considering the primary vortex in front of thc pier to be the prime agent causing scour, a procedure is developed for computing the temporal variation of scour depth under these conditions. Since the maximum scour depth is the scour depth at large time, the procedure is logically extended to obtain an expression for the same. Sediment nonuniformity and stratification are shown to have a significant effect on scour depth. The effect of these elements as well as that of unsteadiness of flow on scour depth are studied and taken into account in the proposed method of scour calculations.