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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. [ABSTRACT FROM AUTHOR]

Local scouring around a bridge pier poses a severe threat to the safety of the bridge. A better understanding of flow features around the bridge piers is necessary for accurate prediction of the scour depth. The ridges of the finite-time Lyapunov exponent, called Lagrangian coherent structures (LCSs), were used to extract the flow features around a circular bridge pier with an evolving scour hole. The velocity field required for the LCSs computation was obtained using a three-dimensional Reynolds-averaged Navier–Stokes simulation. The simulation results were validated with the published experimental and numerical findings. The computed LCS stretching field extracted all the flow features around the bridge pier that were previously reported in the literature. In addition, the LCSs extracted the region of flow acceleration on both sides of the pier. The forward LCSs upstream of the pier extracted a particle trapping region, providing insight into the volume of fluid converting into the downflow. They extracted anchor-like structures inside the scour hole upstream of the pier. The analysis of velocity variations along the width and depth of the flow domain revealed that a change in the velocity profile is triggering the formation of LCS. The behavior of non-inertial particles released and integrated into the flow field revealed the significance of LCSs in particle transport. Using the LCS method, the study extracted the flow features that were difficult to extract with traditional flow visualization methods. [ABSTRACT FROM AUTHOR]