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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]

Large-eddy simulations (LES) were used to simulate the flow features around a circular pier with evolving scour hole. The bathymetry corresponding to four stages of scour hole evolution, published in the previous literature, was used in the flow domain to recreate the flow at a particular stage of scour hole development. The growth of coherent structures and their interactions in a developing scour hole were investigated. The formation of secondary vortices inside the scour hole and their interactions with the primary horseshoe vortex (HV) made the HV system highly dynamic. High-level pressure fluctuations were observed inside the core of HV. The simulation captured the bimodal aperiodic oscillations in the HV at all stages of scour hole development. The mechanisms responsible for the formation of scour hole behind the pier were discussed. High levels of mean and instantaneous bed shear stress values were observed beneath the horseshoe vortex. These values were found to have decreased with an increase in the scour hole depth. The LES also captured the detachment of vortices from the legs of HV and their convection in the pier downstream. [ABSTRACT FROM AUTHOR]