Study On The Influence Of Front End Sill On The Hydraulic Characteristics Of Stepped Spillway

With the development of RCC technology,stepped spillway is widely used in high dam discharge structures in recent years because of its advantages of simple construction process and short construction period.However,under large discharge conditions,the upstream section of the stepped chute suffers from severe cavitation and cavitation erosion and poor energy dissipation.Some scholars propose to change the step size,adopt new energy dissipator combined with step spillway,and add aeration step in the transition section to improve the problem of cavitation and poor energy dissipation in the upstream section of the stepped chute.Inspired by the energy dissipation technology of tail sill in stilling basin,this paper applies the energy dissipation technology of tail sill to the upstream section of stepped chute of stepped spillway,and puts forward the measure of front stepped tail sill.Based on the stepped spillway of a high dam in Southwest China,by combining model test with numerical simulation,considering the shape,size,discharge and other parameters of the front tail sill,the influence of the front tail sill on the flow pattern,water surface profile,flow velocity,time average pressure,energy dissipation and other hydraulic characteristics of the stepped spillway is studied.The results are as follows:(1)The flow pattern of traditional stepped spillway and stepped spillway with front tail sill is sliding flow,and no adverse flow pattern is found.(2)The front end sill can lift the water surface of the upstream section of the chute,but the lifting range is not large;It has little influence on the middle and lower reaches of the chute,and the water surface line is gradually stable away from the front tail sill,which is basically parallel to the dam slope.The water surface profile of the chute section of the stepped spillway increases with the increase of the discharge and the size of the tail sill.(3)The appropriate shape of the front tail sill can increase the development space of vortex and reduce the main flow velocity.With the increase of the size of the forerunner,the vortex flow intensifies and the main flow velocity decreases.(4)The front end sill can improve the negative pressure phenomenon of odd steps in the upstream section of the chute and the middle and downstream steps of the chute,but it will increase the negative pressure of even steps and the negative pressure area in the upstream section of the chute.(5)The front end sill can greatly improve the energy dissipation effect of the upstream section of the chute,solve the problem of poor energy dissipation,and slow down the trend of energy dissipation rate decreasing with the increase of discharge.