Short-term local scour

Local scour downstream of an apron with a swept-out hydraulic jump is shown to develop very rapidly, that is in less than 1% of the time to reach the ultimate scour depth. The short-term scour although not as deep as the long-term scour, occurs much closer to the apron. The limiting short-term scour depth is found to be related to flow regime or type of hydraulic jump that dominates the flow in the scour hole. The deepest shortterm scour was associated with the plunging jump (B-jump) and the adverse jump regimes. A modification of the B-jump equation of Hager and Bretz is proposed to estimate the limiting depth of short-term local scour.