Overlooked Local Sediment Scour Mechanism

A heretofore overlooked mechanism is described for the removal of sediment in the vicinity of a structure, such as a bridge pier, located in erodible sediment and subjected to water flow. The strength of this mechanism depends on, among other things, the ratio of structure width to sediment grain size, b/D ₅₀. The mechanism results from the pressure gradient field imposed on the bed by the mean flow around the structure. The large pressure gradients created by the structure are confined to a relatively narrow region near the structure. The forces due to the pressure gradients acting on the sediment grains near the structure decrease with increasing values of b/D₅₀. This mechanism helps explain the functional dependence of equilibrium local scour depth on b/D₅₀ observed in both laboratory and field tests. It also explains why, in general, physical model local scour tests tend to overpredict prototype scour depths.