Abstract: In this study, a laboratory experiment was conducted to examine the behaviors of flow scour around one bridge pier and two bridge piers under different flow conditions. The SFM method was used to achieve three-dimensional reconstruction of the scour pit topography; A PIV system was adopted to measure the two-dimensional flow fields. We also present a quantitative analysis of the coupling relationship between the flow structure and scour topography. The results show (1) Three-dimensional reconstruction of scour topography can be well completed by the SFM method. The scour pit is an irregular inverted cone shape, with the largest depth in front and both sides of the model and a hump shaped sand dune behind it. (2) As flow intensity increases, the depth and diameter of the scour pit are increased linearly toward a parabolic shape, and its area and volume are also increased. The sand dune behind the pit grows in length and height, and its tail fin angle decreases. (3) The deep scour pit can be related to the downcutting by large flow shear. Large-scale streamwise vortices from both sides of the bridge pier extend to the downstream, promoting the formation of inclined grooves behind the pit. The shed vortices are weakened along the flow, and the sediment carrying capacity is reduced, resulting in sediment accumulation downstream of the pier.

Key words: flow around bridge pier, local scour, three-dimensional reconstruction, water-sediment coupling