Study Of One-dimensional Numerical Simulation Technique Of Turbidity Current And Retrogressive Erosion In Reservoir

The phenomena analysis and numerical simulation study of physical processes of turbidity current and retrogressive erosion are helpful to reveal the complex mechanism of water and sediment movement,as well as the law of riverbed scouring and siltation in reservoirs.They also have great engineering application value on reservoir production practice and sediment management.In this paper,the physical process analysis and numerical simulation technology and its applications are studied.In the part of physical process analysis: For turbidity current,the basic characteristics are briefly discussed at first,and the transversal and vertical distribution law of water and sediment factors and the evolution process of turbidity current are briefly analyzed with the measured data of turbidity current in Xiaolangdi reservoir in 2012.Finally,the supplementary data of turbidity current in reservoirs are used to re-calibrate the coefficients in the discriminant formula for submergence of turbidity current.The improved formula is obtained,and then is validated by using flume experiment data.For retrogressive erosion,the typical process based on simple boundary conditions has been generalized,the analysis and elaboration on retrogressive mechanism and scouring law of retrogressive erosion has been made.Based on the measured data of retrogressive erosion in Sanmenxia reservoir from 1974 to 1996,the influencing factors of retrogressive erosion are analyzed,and the accuracy of empirical formulas of sediment transport rate is verified.It is concluded that retrogressive erosion mainly occurs on the steep slope of the silt body,through the force analysis of the bed sand on the inclined bed surface,the discriminant formula of the threshold velocity of sediment considering the inclination angle of the bed surface is obtained.In the part of numerical simulation technology,a one-dimensional mathematical model considering the coupling of water-sediment movement with riverbed scouring and silting has been constructed,which is applicable for both turbidity current and retrogressive erosion.The model considers the flow and sediment exchanges between the main stream and tributaries.In this paper,an improved zero-dimensional reservoir method is proposed to calculate the net flow exchanges in the presence of complex tributaries.The characteristics of the model also include: when applied to calculate turbidity current,the improved discriminant formula for submergence of turbidity current is used to connect the calculation of open flow and turbidity current.The model considers the outflow of turbidity current and its interaction with the specific drop and velocity of its upper water body.The model regards the area and discharge of the cross-section obtained by open flow calculation as conservative physical variables,and the full solution of the factors of the muddy open flow upstream the submerged point,the turbidity current and the upper water body downstream the submerged point can be realized.When applied to calculate retrogressive erosion,it is considered that it belongs to the special evolution phenomenon on the steep slope of the river bed under the muddy open-flow.The range and inclination of the slope are determined according to the topography,and the incipient motion of bed sand is judged by the formula of threshold velocity of sediment considering the inclination of bed surface.In the application part of the model: for turbidity current,the mathematical model is calibrated and validated with the observation data of turbidity current of Xiaolangdi reservoir in 2002,and the complete process of water and sediment movement including the process of turbidity current in the reservoir during the period of water and sediment regulation in 2012 is calculated.Based on the simulation results and the sensitivity analysis of parameters,the characteristics of turbidity current and the law of influences of related physical factors are analyzed.For retrogressive erosion,flume experiment of retrogressive erosion with relatively simple boundary is selected for preliminary simulation,and then the retrogressive erosion process of Sanmenxia reservoir in 1975 is calculated,the characteristics and development law of retrogressive erosion are analyzed.The simulation results show that the model has good accuracy in simulating the turbidity current process in reservoir and the retrogressive erosion test process of flume,and the accuracy of simulating the retrogressive erosion process of reservoir under complex boundary conditions needs to be further improved.Generally speaking,the established model can well describe the process of water and sediment transport and river bed evolution under the existence of turbidity current and retrogressive erosion,and has high engineering and scientific research value.