Lagrangian coherent structures around a bridge pier with scour hole

Accurate prediction of scour depth is essential for the safety of the bridge. The downflow upstream of the pier plays a significant role in scour hole formation. The present study used Lagrangian coherent structures (LCSs) to derive an expression to estimate the force exerted by the downflow on the sediment bed. The LCSs extracted a trapping region upstream of the pier, which trapped the fluid, which was then converted into downflow. The expressions derived in this Letter can be used to improve the efficiency of scour depth prediction equations.