Evolution of Local Pier-Scour Depth with Dune Migration in Subcritical Flow Conditions

Bridge-pier scour in alluvial rivers often occurs in live-bed conditions with dune migration. The scour depth attains its deepest value when the dune trough reaches the pier, followed by depth reduction when the crest arrives. Therefore, the instantaneous scour depth may be larger than its time-average counterpart. Consequently, accurate prediction of the instantaneous scour depth is pivotal to the safety of the bridge. Although published methods may be used to estimate the temporal variations of pier-scour depth, only few have focused on the live condition. This study presents a prediction method to simulate the temporal evolution of live-bed pier-scour depth. The erosion rate related to the combined effect of the local vortex system and sediment transport into the scour hole associated with the passage of dunes are examined to evaluate the pier-scouring rate. Temporal variations and computed equilibrium time-average scour depth are compared with published data with satisfactory agreement.