3-D simulations of flows around bridge piers at a river bend

In order to calculate the local scour depth around the bridge piers which are to be constructed in a river bend, and with existing bridges in both the immediate upstream and downstream sides, numerical simulations are carried out to predict the now field, under different conditions including the design high water discharge. The new bridge's site being 12km upstream of the river mouth, the tide effect should also be considered. In the model, roughness coefficients are given considering the flood plain's surface conditions. The quasi 3-D model is first calibrated with results from aerial photographs of the river reach under study, as well as the water level measured simultaneously at a section 1.5km to the upstream, both of which being taken during a food event. The photographs, if carefully taken and properly analyzed, could render useful information on the surface velocity vectors, turbulence structures, and the bedforms. After the model's verification, it was then used to predict the flow field in the river reach with or without the new bridge with various combinations of discharge and tide level. From the obtained results it is observed that the velocity variations in the transverse direction, caused by the new bridge piers, may lead to erosion outside the local scour area during floods. With numerical results of the flow field, the local scour depth around the new bridge piers are estimated with three representative formulae.