| Under the combined action of multiple hazard factors with natural-social attributes,the frequency and intensity of rainstorm accumulation have become higher and higher,and the scope of impact has become larger and larger.Faced with the increasingly severe flood disasters,the study of the storm accumulation model has become an increasingly important non-engineering measure in our national defense disaster mitigation measures.Constructing a mathematical model of hydrodynamic accumulation of rainstorms,and deeply studying the hydrodynamic mechanism of surface convergence and pipe network convergence during the accumulation process,which can more accurately and accurately reflect the actual water inundation and flooding process,and can provide early warning monitoring for water accumulation.Scientific and effective technical support is provided for the establishment of flood control information system and municipal temporary drainage planning.Based on the hydrodynamic method,this paper studies the hydrodynamic mechanism of each confluence process during the rainstorm accumulation process,and constructs a two-dimensional coupled stormwater hydrodynamic mathematical model.The main research work and achievements are as follows:(1)A two-dimensional hydrodynamic model for surface confluence is established.The bottom slope term is processed by the unit-center approximation method,and the two-dimensional shallow water equation is improved by using the water depth and the bottom elevation instead of the water depth variable,so that the two-dimensional surface confluence model could still ensure the overall water balance and the balance between the convective flux and the source of bottom slope without any additional momentum and flux correction.Meanwhile,the impact of urban terrain features on the surface confluence process is fully considered,an improved model of the sloping bottom unit based on the relationship between water level and volume is presented,and a meshing method based on the terrain features is proposed,so these methods could effectively solve the numerical calculation problems caused by small scale terrain and various surface features.After that,under the framework of Godunov finite volume method,the convective numerical flux is calculated by adopting HLLC approximate Riemann operator,and MUSCL-Hancock prediction correction method with second-order accuracy in time is using to discretize the governing equations,combining the limit function to ensure the high resolution of the model,so these methods could effectively avoid the phenomenon of non-physical false numerical oscillations generated near the discontinuity or large gradient solution.Moreover,the water exchange calculation mode of different units and channels is proposed,it includes the calculation method for the response radius of the forced drainage of the pump station,so the discontinuity problem at the cell interface generated by the actual terrain and surface features distribution could be effectively solved.In addition,the rigidity problem caused by frictional resistance term under extremely irregular terrain conditions is analyzed and proved theoretically,and the semi-implicit method is applying to solve it,so the direction of flow velocity component could be ensured to be constant,and the non-physical false large flow rate caused by the small water depth could be avoided,thus the stability of numerical calculation could be ensured.(2)The two-dimensional surface confluence model is verified and applied.Six calculating examples(or project)are used to verify the numerical calculation capabilities of harmony,adaptability,shock capture and computational stability,complex terrain processing and conservation,high-speed water flow discontinuity processing and convergence,and calculation accuracy.Meanwhile,the model is adopted to simulate the rainstorm waterlogging in the southern plain of Tianjin,the processing capacity in plain with large area and complex terrain and surface features is validated,the water balance processing capability and simulation accuracy are also validated,thus the model could be further verified and applied.(3)A one-dimensional fluid dynamic model of pipe network confluence is established.The governing equations for one-dimensional non-constant non-pressure flow and one-dimensional non-constant pressurized flow are deduced in detail,and three main methods of solving the transitional flow problem at home and abroad were evaluated,thus a new transitional flow simulation method that could be applied in extreme conditions,be more suitable for multiple flow regimes and accord with the physical significance is proposed,namely the two-phase flow pressure method.By modifying and adjusting the mass and momentum equations,the governing equations of non-pressure flow and pressurized flow are unified,and then Preissmann four-point implicit difference method is adopted to discretize the unified governing equations.Meanwhile,a propulsion algorithm of pipe node water level which could avoid solving large sparse matrices is proposed,this method could solve the pipe network system stably and efficiently.(4)The one-dimensional pipe network confluence model is verified.Six calculating examples(or project)are used to verify the numerical calculation capabilities of complex open channel water flow processing,pressure flow processing and calculation accuracy,dry and wet alternating and non-pressure-pressurized flow processing,reliability,calculation accuracy,transitional flow processing.Meanwhile,the model is used to simulate the community drainage,the reliability and simulation accuracy are validated,thus the model could be further verified and applied.(5)A one-dimensional two-dimensional coupled hydrodynamic model is constructed.Based on the two-dimensional surface confluence model and the one-dimensional pipe network confluence model,for the horizontal direction coupling calculation,the water exchange in the forward connection and the lateral connection process is calculated by the mutual boundary method and the weir flow method respectively.Aiming at the problems and shortcomings in the calculation of vertical direction coupling,based on the advantages and application scope of the weir flow method and the orifice flow method,an improved water exchange calculation and determination method,namely the weir-orifice flow determination method,is proposed.Thus,the method is used to calculate the vertical direction coupling.Based on the above horizontal and vertical coupling calculation methods,a one-dimensional two-dimensional coupled hydrodynamic model is finally constructed.(6)The one-and two-dimensional coupled model is verified and applied.Two calculating examples(or project)are used to verify the numerical calculation capabilities of forward and lateral connection processing,refined processing and simulation accuracy.The calculation results under the conditions of forward connection and lateral connection are reasonable and reliable,and the phenomenon law is in good agreement with the previous research results,indicating that the model can ensure the overall water balance of the model.The simulation results of street flooding are consistent with the verification data.Well,the calculation results are basically in line with historical survey data in the overall trend,and the application effect is good,which can accurately reflect the actual flood evolution and flood area accumulation,and provide a theoretical basis for flood control and disaster mitigation in flood storage and detention areas.The application results are good,which can accurately reflect the actual street flooding process and provide a theoretical basis for flood control and disaster reduction of key road sections. |
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