Numerical simulation of three-dimensional flow field and local scour at bridge crossings

Local scour process at bridge crossings involves a three-dimensional flow field together with sediment transport. The basic mechanism of local scour is the formation of turbulence secondary motions and vortices induced at the base of the structures. In order to simulate such a complex three-dimensional flow field in the vicinity of a structure, a stochastic turbulence-closure model has been implemented into the numerical model, CCHE3D, developed by the Center for Computational Hydroscience and Engineering at the University of Mississippi. The anisotropic three-dimensional flow fields around bridge piers and abutments have successfully been obtained. A formula for predicting sediment transport capacity for local scour is proposed. For deriving this formula, the factors that reflect the secondary flow motions generated by the three-dimensional flow model are adopted to modify the sediment transport capacity formula that was developed originally for estimating general scour. The proposed sediment transport capacity formula is in a general form. It can also be applied to predict general scour. The present study also includes some investigations on sediment incipient movement in local scour. The critical velocity near the structures was determined through two sets of the laboratory experiments conducted in the FHWA's hydraulic laboratory. Finally, a series of cases selected from the reported laboratory experiments has been used for the model's verification and sensitivity studies. The results reported in the present study show that numerical simulations are in good agreement with the data obtained by physical modeling.