Experimental Study On The Influence Of Different Riverbed Topography On Local Scour Of Downstream Piers

In the natural river,the riverbed is uneven,which can be generalized into flat riverbed,concave riverbed and convex riverbed.Topography of riverbed will cause different hydraulic characteristics of the river and affect local scour of the downstream pier,which may endanger the safety of the bridge.The existing research results focus on the local scour of piers under flat riverbed.The research on the influence of concave riverbed and convex riverbed on the local scour of downstream piers is not perfect.By setting flat riverbed,concave riverbed and convex riverbed in the laboratory water tank,this paper carries out the local scouring test of pier.The test considers the factors such as riverbed terrain size and velocity to discuss the influence of water flow under different terrain.In the test,concave riverbed terrain model and convex riverbed terrain model are arranged in the flume.By measuring the local scour depth,scouring pit width and flow velocity around the pier under different terrain.the test analyzes the effects of flow velocity,relative height of riverbed terrain and pier position on the downstream pier local scour.The test results show that the riverbed topography has an obvious influence on the flow and local scouring characteristics of downstream piers.Compared with the pier in the flat riverbed,the convex riverbed will shrink the overflow section and increase the velocity gradient at the downstream pier.The convex riverbed will also aggravate the general scour of the riverbed and affect the shape and depth of the scour pit.The concave riverbed will reduce the velocity gradient at the downstream pier and the average velocity at the downstream section and increase the velocity near the surface of the riverbed,making the pier scouring depth greater than that of the flat riverbed.With the increase of the relative height of the riverbed topography,the change of velocity gradient and scouring depth at the pier is more obvious.The farther away from the concave or convex riverbed,the smaller the impact of riverbed terrain on the local scouring of downstream piers.Based on the experimental data,the empirical equation of scouring depth of downstream pier considering the topographic factors of riverbed is obtained and analyzed.A numerical simulation is established by using FLOW-3D,which is based on RNGκ-ε.The flow characteristics of the flow field around the downstream pier in different terrain are analyzed and compared with the experimental results,which proves the feasibility of the simulation.The results show that the numerical simulation is in good agreement with the experimental results.