Local Scour Characteristics Of Step-shaped Spur Dike With Numerical Simulation

Step-shaped spur dike are widely used in waterway regulation projects,but the research on the local scour characteristics is rarely involved.It is of great theoretical significance to study the local scour characteristics of step-shaped spur dike,which can provide technical support for waterway regulation engineering design and optimization.By combining numerical simulation with theoretical analysis,this paper studies the variation law of the scour hole with single factor and discusses the influence law of unsteady flow on the scour hole morphology and dimensions.The main research results are as follows:（1）Under the condition of clear water constant flow,it takes the same time for the single and step-shaped spur dike to reach the maximum depth under the non-submerged conditions,and the final shape of the bed surface is basically the same.But the scale of the reflux zone,the negative flow velocity of the reflux and the scale of the scour hole are all larger than the step-shaped spur dike.（2）The local scour of spur dike can be divided into three stages:initial section,development section and balance section.The scour hole morphology scale develops rapidly in the initial section and completes 40%of the maximum depth;In the development section,the scour hole morphology scale growth rate slows down;When the curve of the depth of the scour with the duration of the scour is gradually flat,the bed surface scour basically reaches the equilibrium state.In general,the depth of the scour hole increases with time in a basically logarithmic pattern.（3）The influence of Froude number Fr,the height of primary spur dike H_d1,the length of secondary spur dike L_d2,the size of bed sand d₅₀on the shape scale of local scour hole were analyzed.Defined the relative height of primary spur dikeψ=H_d1/H,the relative length of secondary spur dikeε=L_d2/B,the top submerged plane of the primary spur dike are step layers,and the relative length of the step layer is the ratio of the length of the step layer along the channel width to the L_d1.Whenψ=0.13,the influence of secondary dam on the flow and sediment movement is dominant,and the maximum depth is reduced by 45.34%compared with the single spur dike.In addition,the step-shaped spur dike can reduce the maximum depth to about 10%.If the water retaining area of the step-shaped spur dike is the same,the longer of relative length of step layer is,the larger of the maximum depth is.When the height of step-shaped spur dike is fixed,if the relative length of step layer is longer and the water retaining area is smaller,or if the relative length of step layer is shorter and the water retaining area is larger,the maximum depth of step-shaped spuer dike is about 2%～8%higher than the single spur dike.（4）Based on the results of numerical simulation,through the correlation analysis of two variables,dimension analysis and optimization combination.A formula for calculating the maximum scour depth of non-submerged step-shaped spur dike under constant flow was established.（5）The Markov model was used to simulate the natural flow process randomly,the simulated flow process was generalized to the typical unsteady flow process,the average flow rate,scour duration,total incoming water in the period of time of the unsteady flow is the same as constant flow.Two typical generalized processes with different initial scour flow rates are used as the flow boundary conditions of the numerical mode l,i.e.the wave valley flow and the wave peak flow.（6）Because of the different development of bed surface erosion and siltation,the flow field at the same flow rate is different,and the difference of the flow process from the trough to the crest is stronger than the wave peak flow.（7）During the wave valley flow,the scale and the morphology of scour hole increases linearly,and the development of the puddles began to slow down before the peak flow arrived,but the depth was still growing at a high speed.The maximum width and length of the scour hole in the process of the wave valley flow reach 2L_d1and9.19L_d1respectively.（8）During the wave peak flow,the scour hole is formed rapidly and the depth increase linearly.Because of the decrease of the flow rate in fall perm,the long-stripe siltation area behind the spur dike and the collapse of the side wall of the scour hole,the length of the scour hole increases first,then steadily and then increases.The area of the scour hole increases linearly during the rising period,and TTS length growth rate decreases gradually.The maximum width of the scour hole reaches 2.1L_d1,the length reaches 11.3L_d1at the end of the time.（9）Under the condition of clear water unsteady flow,the law of local scour evolution is different from that of constant flow.Siltation occurs in the upstream slow flow area,and erosion occurs in the downstream reflux flow area,which results in a small amount of siltation in the reflux area by moving bed sand toward the reverse flow direction.The length of the scour hole reaches 2～2.5 times at constant flow,and the maximum depth increases by 25.6%～54.41%.