Numerical Simulation Of Beach And Bank Scour And Collapse During Open Discharge Scour Of Reservoirs

Siltation is a common problem in reservoirs on sandy rivers,and low-level operation or open-drain scouring during floods with high sand content is an effective means of reducing siltation and restoring reservoir capacity.During open discharge scouring,the main channel of the reservoir is scoured retrospectively from the front of the dam to the upstream,and the scouring of the bottom of the main channel is accompanied by strong riverbed deformation such as collapse and retreat of the banks of the main channel.Most of the existing mathematical models only simulate the longitudinal deformation processes such as scouring and undercutting of the bottom of the main channel during the open discharge scouring process of the reservoir,and less involve the lateral deformation processes such as the collapse and retreat of the main channel banks.In this paper,based on the analysis of the physical process and mechanism of beach bank collapse,a model for the analysis of the stability of beach bank collapse and the calculation method of the collapse volume during the open discharge flushing of reservoirs is presented,and the beach bank collapse calculation model is embedded in a one-dimensional non-constant suspended mass nonuniform sand mathematical model,and finally a one-dimensional water-sand transport mathematical model that can reflect the deformation process of beach bank collapse during the open discharge flushing of reservoirs is established.This paper provides theoretical and technical support for reservoir operation optimization and reservoir flushing and siltation process prediction.The main research contents and conclusions of this paper are as follows:(1)A one-dimensional water-sand transport mathematical model is established based on the finite volume method for the longitudinal siltation and deformation process of the river channel during the open discharge scour of the reservoir.Based on the existing theoretical research results of sediment mathematical model,this paper constructs a one-dimensional water-sediment transport mathematical model based on the discrete solution of the basic control equations by the finite volume method,which can simulate the longitudinal siltation deformation process of the reservoir channel during the open discharge flushing of the reservoir.The model can effectively solve the simulation problem of cross-critical flow in the open-drain flushing process of reservoirs,taking into account the efficiency and stability of numerical calculation.(2)The physical mechanism of beach-bank collapse during open-drain scouring of reservoirs is analysed and summarised.The deformation process and deformation mechanism of beach-bank scour damage are analysed and summarised based on the morphological characteristics of the channel section formed by open-drain scour of the reservoir,and the intrinsic relationship between bank deformation and stability.The influence mechanism on the overall instability of the bank slope under the action of open-drain scour is summarized,and a method for analysing the stability of beach-bank collapse during open-drain scour of reservoirs and the calculation methods of collapse volume and collapse extent are given.(3)A sediment mathematical model is established for the longitudinal siltation deformation of the river channel and the deformation of the beach-bank collapse during the open-drain flushing of the reservoir.The beach-bank collapse calculation model is embedded in the onedimensional non-constant suspended mass non-uniform sand mathematical model,and a onedimensional water-sand transport mathematical model is constructed to reflect the beach-bank collapse deformation process during the open discharge of the reservoir.The model is based on the calculation results of the aforementioned water-sand transport model,combined with the beach-bank stability calculation method,and takes into account the influence of river scouring factors and periodic soaking factors on beach-bank stability,and can give the flow velocity,water depth and siltation area of each section of the reservoir as a function of time.(4)The model was validated using the actual measurement data of Wang Yao Reservoir.The validation results indicated that the water-sand model and the bank collapse model can simulate the dynamic behaviour of river channel siltation and bank collapse during the process of reservoir open-drain scour more accurately.The average error between the simulated siltation volume of the water-sand model and the measured results is 9.82%,which indicates that the model can better reflect the changing pattern of siltation,confirming the reliability and accuracy of the model.(5)Using the mathematical model of water and sand transport developed in this paper,the reservoir operation mode of the Wang Yao reservoir expansion project was optimised and the reservoir siltation process was predicted,testing the applicability of the model developed in this paper and providing a reference for the determination of the scale of the Wang Yao reservoir expansion project.Firstly,based on the analysis of the historical operation of Wang Yao Reservoir,three different operation modes are proposed.Through the analysis and comparison of the remaining reservoir capacity and siltation volume of the reservoir under different operation methods,a reasonable operation method is proposed as regular open discharge.Secondly,using the regular open discharge operation method recommended in this paper,the predicted sediment flushing process of the reservoir after the completion of the Wang Yao reservoir expansion project was calculated.The results show that the remaining reservoir capacity after 57 years of operation is 23.359 million m3;the multi-year average sand discharge ratio is 72.2%,the accumulated siltation volume is 102.275 million m3,and the multi-year average siltation volume is 1.792 million m3.