Effect Of Debris Accumulation On Local Scour Characteristics Of Bridge Piers

In recent years,extreme precipitation events have increased worldwide,and over-alert floods have occurred frequently in small and medium-sized rivers and large rivers.During the flood evolution,the trunks and branches on both sides of the river are easily taken away by the water flow to form debris accumulation.When the debris accumulation move to the bridge,the cross section of the river is severely compressed,which affects the flow characteristics around the pier,aggravates the local scour of the river bed,and seriously threatens the safety of the bridge foundation.In this paper,the flow characteristics around the pier and the local scour characteristics of the riverbed under the influence of debris deposits are analyzed by means of theoretical analysis,flume experiment and FLOW-3D numerical simulation,so as to strengthen the understanding of the influence of debris accumulation on the local scour of bridges and provide strong support for bridge protection.The main research contents and results are as follows:(1)The local scour characteristics of piers affected by debris accumulation under clear water scouring conditions were studied by flume scouring experiments.The results show that compared with the case without debris accumulation,the existence of debris accumulation aggravates the local scour of bridge piers,and the maximum scour depth can reach 7.3cm,with an increase of 58.7%.The study of three different shapes of debris accumulation shows that the larger the water blocking area,the larger the range and maximum depth of the local scour pit of the pier.(2)Through the flume erosion test,the influence of the location of debris accumulation on the local scour of the pier was studied when the debris accumulation was at three different positions:free surface,middle water depth and near the bottom of the riverbed.The research shows that the maximum depth of the local scour pit of the pier increases linearly with the increase of the H_b/H of the debris accumulation position,that is,when the debris accumulation is on the water surface H_b/H=0,the impact on the local scour of the pier is minimal;when the debris accumulation is close to the bottom of the riverbed H_b/H=0.8,it has the greatest influence on the local scour,and the maximum depth of local scour can reach 29.5%.(3)Based on the governing equations of water flow,sediment transport equations and river bed deformation equations,a three-dimensional local scour numerical model of flow-sediment-pier-debris interaction is established.The reliability of the numerical model is verified by flume experimental data such as cross-section velocity,scour pit shape and maximum depth.The results show that the velocity distribution and scour pit shape are basically consistent with the experimental results.(4)The flow structure around the pier under the influence of debris accumulation obtained by numerical simulation is analyzed,and the mechanism of aggravating local scour is analyzed.The results show that under the influence of debris accumulation,there is still the influence of submerged flow and vortex near the pier.The accumulation compressed the cross-sectional area of water flow along the depth direction.As the debris accumulation position was closer to the bottom of the riverbed,the intensity of the submergence flow was greater,the maximum increase of the submergence velocity could reach about 10.6%,and the maximum scour depth was greater.The flow structure around the pier under the influence of debris accumulation obtained by numerical simulation is analyzed,and the mechanism of aggravating local scour is analyzed.The results show that under the influence of debris accumulation,there is still the influence of submerged flow and vortex near the pier.The debris accumulation compresse the cross-sectional area of water flow along the depth direction.The closer the debris accumulation is to the bottom of the riverbed,the greater the intensity of the submerged flow.