Critical velocities for local scour around twin piers in tandem

Experiments of the local scour around twin piers are carried out under steady clear-water conditions, including 95 tests to observe the influence of the pier spacing and the flow velocity on the local scour characteristics of the twin piers. It is shown that the start of the transition region is synchronous with the sediment transport from the upstream scour hole to the downstream one. The equations for the critical velocities are derived to quantify the velocity range of each of four different scour regions. Finally, a prediction formula of the downstream pier scour depth in the radical-deviation region is established.