Research On The Influence Of Non-Equal Length Spur Dike Group On Local Scour Characteristics Of Bends

As a common hydraulic structure,spur dikes are often used to control the river regime in curved rivers,so that the mainstream deviates from the embankment to protect it from erosion.If the design layout of spur dike group is improper,the foundation of spur dike may be emptied under the severe erosion of water flow,resulting in local or overall instability and collapse of spur dike,endangering the safety of spur dike project and river channel.In this paper,the non-equal length spur dike group is formed by changing the river width narrowing rate,which further optimizes the flow characteristics and local scour characteristics of the spur dike group.Through theoretical analysis,flume experiment and numerical simulation,the flow field and local scour of the first spur dike river width narrowing rate change,the secondary spur dike river width narrowing rate change and their joint change in the non-equal-length spur dike group were studied.The main research contents and results are as follows:（1）When the non-equal-length spur dike group is arranged in the bend,three different ranges of recirculation zones are formed after the first spur dike,the secondary spur dike and the tail spur dike.The maximum turbulent kinetic energy and the maximum recirculation zone are located behind the tail spur dike.With the increase of river width narrowing rateα₁of the first spur dike,the range of the recirculation zone and the maximum turbulent kinetic energy of the first spur dike and the tail spur dike gradually increase.With the increase of river width narrowing rateα₂of the secondary spur dike or the simultaneous increase ofα₁andα₂,the range of the head spur dike and the secondary spur dike recirculation zone increases,and the range of the tail spur dike recirculation zone and the maximum turbulent kinetic energy decrease.（2）The hydrodynamic axis at the inlet of the bend is more and more inclined to the convex bank side with the increase of the width narrowing rate of the spur dike.Whenα₁orα₂is 0.25,the hydrodynamic axis almost fits the bank wall,which aggravates the erosion of the convex bank and is not conducive to the protection of the convex bank.Whenα₁=0.10,α₂=0.20 andα₃=0.25,the hydrodynamic axis of the non-equal-length spur dike group is not only picked off the concave bank,but also not close to the convex bank,which is more conducive to shore protection.（3）With the increase ofα₁,the scour range of riverbed and the scour depth of dam head near the first dam increase,and the scour depth of dam head of secondary dam and tail dam decrease.With the increase ofα₂,the scour depth of the tail spur dike head decreases,and the scour change of the first spur dike head is not obvious.Whenα₂is 0.20and 0.25,the second spur dike is not affected by the cover effect of the first spur dike,and the pick flow begins to occur,and the water erosion occurs at the head of the spur dike.With the increase ofα₁andα₂,the scour depth of the first spur dike head increases.The scour depth of tail spur dike head decreases;sediment deposition occurred at the head of the secondary dam under the cover of the first spur dike.（4）For the maximum flow velocity,the maximum bed shear stress and the maximum scour depth,the non-equal-length spur dike group is smaller than the equal-length spur dike group under all working conditions,and the non-equal-length spur dike group withα₁=0.10,α₂=0.20 andα₃=0.25 is the optimal working condition.The maximum flow velocity is 0.439 m/s,which is 8.16%lower than that of the equal-length spur dike group.The maximum bed shear stress is 0.465pa,which is 18.42%lower than that of the equal-length spur dike group.The maximum scour depth is 3.36cm,which is 17.65%lower than that of the equal-length spur dike group.