3D numerical simulation of local scouring under hydrographs

Owing to the interaction between three-dimensional (3D) flow pattern and bed material, the process of local scouring around bridge piers is complex. Simply using the empirical scour formula may be insufficient for the evaluation of pier scour under the unsteady flow condition. In the present study, the evolution of scour depth under unsteady flow has been simulated using a 3D numerical model. The model solves 3D Navier–Stokes equations incorporated with the sediment continuity equation to take into account the interaction between flow and sediment. The simulated scour-depth evolution coincides with the measured one quite well under the steady flow condition. For the unsteady hydrograph, a compounded method is proposed for simulations of scour-depth evolution as well as for scour hole development. The results show the simulated bed elevation contours are in good agreement in front of the pier, while the simulated final scour depth is slightly lower than the measured data by 15%. In addition, it is found that the simulated scour depth at the end of the rising limb is 74–79% of the final scour depth, which implies that the recession period of the hydrograph plays a less effective role than the rising limb in the unsteady scouring process.