One-dimensional Numerical Simulation Study Of Scour At Drop Sill

There exist many excessive sand excavation situations in the reaches near the bridges in China, which results in massive bridge collapses directly or indirectly and affects the safety and normal operation of the bridges seriously. Thus, it is necessary for bridge protection and river training to study the development of retrogressive erosion at drop sill and the impact of sand pit on bridge safety. The drop sill in this paper is the simplification of the upstream slope of the sand pit.In this paper, a discrete method combined with finite volume method and finite difference method is proposed, and a one-dimensional numerical model of scour at drop sill is established to simulate complex mixture flow. This model adopts the finite volume method with Godunov scheme to discretize the shallow water equation and the finite difference method with backward difference scheme to discretize the riverbed deformation equation. In finite volume method, flux at interface is evaluated by the HLL Riemann solver, the second-order scheme in time and space is ensured by MUSCL-Hancock method, while the stability is controlled by CFL condition.The harmony, applicability and accuracy of this model is verified by simulating still water, hydraulic drop and scour at drop sill. Scour development at drop sill is calculated in different conditions, such asdischarge, waterfall height and slope. The calculated and experimental data suggests that the scour depth and erosion rate increases as the discharge and waterfall height grows, while the slope of drop sill has little influence on the process and result of scour.Based on the conditions of experiment, the development of scour at drop sill from initiative state to the equilibrium state is simulated. It infers that in the earlier stage the scour depth grows rapidly and the sediment transport rate reaches the maximum, while in the later stage the former slows down and the latter decreases to a stable value. Erosion rate increases as local sedimnet transport rate raises, and the increment speed grows with discharge. Retrogressive erosion length grows mainly in the initial stage, and is positive correlated with discharge and waterfall height. Based on the conditions of Hutuo River, the influence of discharge, relative waterfall height and relative pit depth on retrogressive erosion length and equilibrium slope is measured. The result infers that retrogressive erosion length has positive correlations with these three factors, while equilibrium slope decreases withdischarge raises but increases asthe other two factors grow. When deciding the forbidding area of sand mining in the downstream of the bridge, both the size of the bridge and the discharge of the river should be taken into consideration, while the depth and size of the sand pit shoule also be controlled.In order to study the impact of flood characteristics on scour of sand pit in seasonal rivers, the moving boundary processing module is added to the one-dimensional numerical model of scour at drop sill to simulate flood routing and scour development at drop sill in dry bed. It shows that when the distance from knickpoint increases, the scour depth and sediment transport rates decrease, while the time scour begins and the time sediment transport rate reaches the maximum delay; scour occurs mainly in the rising limb. When the peak discharge grows, the maximum scour depth, sediment transport rate and length of retrogressive erosion increase. When the flood wave type coefficientincreases, the erosion rate raises but the final scour depth declines, the maximum transport rate increases, while retrogressive erosion length decreases.