Study On Mutual Influence Of Local Scour Of Pier And Downstream Submerged Dam

The interaction between the bridge pier and the river flow leads to local scour at the bridge pier,which causes the instability of the bridge substructure.Usually,local scour of bridge piers can be prevented by the construction of river regulation structures such as sand bars,submerged dams and low weirs,but the construction of regulation structures on the river will also cause local scour and lead to structural instability and failure.At present,there is no clear provision on the distance between bridge and submerged dam in the relevant codes.In the past few decades,scholars have extensively studied local scour of bridge piers and downstream local scour of low-weir structures such as submerged dams on river courses.However,there are relatively few studies on the influence of hydrodynamic interaction between bridge piers and submerged dams on their respective local scour.The research methods mainly focus on theoretical analysis and physical model tests.Based on Flow-3D software and Wang scour test as the prototype,this paper adopts large eddy simulation(LES)combined with sediment scour model to conduct three-dimensional numerical simulation research on the local clear water scour process of cylindrical pier and downstream submersible dam.Firstly,a mathematical model of scour of a single cylinder pier was established to understand the flow pattern,turbulence kinetic energy distribution and vortex body distribution around the pier,and to analyze the development process of local scour of the pier.Secondly,by changing the distance between the piers and the submerged dam and the height of the riverbed exposed by the submerged dam,the paper analyzes the diachronic variation of the local scour pits around the piers and the submerged dam,the diachronic variation of the velocity distribution and the distribution of the shear stress of the riverbed,and discusses the influence of these factors on the local scour of the pier-submerged dam.Research shows that:(1)The close arrangement between the pier and the submersible dam can restrain the sediment transmission from the scour pit of the pier to the downstream,which effectively prevents scour of the pier,but siltation is formed in front of the dam,and the flow around the upstream pier strengthens the water flow over the dam top,causing stronger drop to impact the bed surface of the submersible dam,thus forming a larger scour depth downstream of the submersible dam.With the increase of the distance between the pier and the submersible dam,the inhibition effect of the submersible dam on the pier gradually decreases,the enhanced flow effect of the submersible dam over the dam gradually weakens,the local scour pit depth around the pier gradually deepens,and the scour depth at the downstream of the submersible dam gradually decreases.When L= 6.0D(D is the pier diameter),the scour pattern around the pier is basically close to that of the single pier.The sediment in the scour pit of bridge piers is transported to the downstream of the submersible dam to supplement the sediment from the upstream,which causes slow deposition of the scour pit downstream of the submersible dam.(2)The higher the height of the submerged dam downstream of the pier,the greater the height of the front backwater of the dam,the smaller the specific drop of the water surface around the pier,the lower the velocity around the pier,and the smaller the local scour depth of the pier.However,the high backwater of the dam also strengthens the drop impact,which causes the scour of the downstream of the submerged dam to intensify.(3)Considering the influence of the distance L between the bridge pier and the submerged dam and the height H of the submerged dam higher than the riverbed surface,the concept of effective slope is put forward to select the distance between the bridge pier and the submerged dam and the height of the submerged dam.Considering the calculation conditions in this paper,the effective dam height spacing ratio is 0.893.