One Dimensional Hydrodynamic Water Quality Model Of Open Channel And Subsurface River With Irregular Cross-section

With the increase of population and the development of industrial and agricultural economy,a large amount of sewage from domestic and industrial and agricultural has an impact on river protection and water conservancy project development in Karst areas.The mixed flow and irregular closed cross-sections of the rivers make conventional open channel models difficult to apply.In order to understand the law of mixed flow movement and pollutant migration and transformation in Karst areas,the paper studies a single set of equation(ASE)description method of mixed flow,proposes symmetric shadow sub-grid technology,establishes a unified one-dimensional hydrodynamic model of mixed flow,develops the Flux-form ELM material transport module,and uses the WASP5 water quality module as the reaction source item,and finally establishes a one-dimensional hydrodynamic water quality model suitable for real open channels and subsurface stream.The main content and results of the paper are as follows:(1)Based on the Saint-Venant equations,a symmetrical shadow sub-grid technique is proposed to describe irregular sections,and a unified hydrodynamic model suitable for mixed flow of open channels alternating sinking channels is established.The semi implicit method is used to discretize the pressure gradient term,the Euler Lagrange method is used to solve the convection term,and the prediction-correction method is used to realize the coupling solution of each branch of the river network.On this basis,the Flux-form ELM is used to solve the one-dimensional convective material transport problem.Based on the EUTRO water quality module in WASP5,a one-dimensional hydrodynamic water quality model of irregular cross-section open channels and subsurface flow is established.The ideal examples are used for testing,the test results of the branched open channel river network of MIKE11 show that the model is suitable for the simulation of open channels and mixed flow hydrodynamic water quality,and can be used to study hydrodynamic characteristics and pollutant transport laws.(2)Based on the measured hydrology and water quality data of the Ganjiang River network in 2007,the hydrodynamic water quality modeling is completed,and the applicability of the model to the plain open channel network and parameter sensitivity analysis are carried out.The river channel roughness is determined by the diversion ratio and the water level.The absolute error of the calculated water level is generally less than20cm after verification,the processes of the water quality indicators DO,COD,BOD and NH₃-N are in good agreement with the measured values and results of the 2D model.The above results show that the model is suitable for the calculation of the hydrodynamic water quality process of the plain open channel river network,and can be used to simulate the water quality change process under the condition of open boundary inversion.(3)Based on the example of the river network of the Ganjiang River,a performance comparison between the model and the MIKE11 software is carried out to verify the stability,calculation accuracy and efficiency of the model.The two models simulate the material transport process,the material transport with source term process,and the water quality adding pollution load process to the unsteady flow of the river network in 2007.In the water quality process simulation,MIKE11(dt=60s)takes 3370s,while the model(dt=120s)only takes 115.125s when the number of computer cores is 4,which is 28 times faster.The results of various working conditions show that when the calculation accuracy of the model in this paper is equivalent to that of MIKE11,the model allows a larger time step,higher operating efficiency,and wider application range.(4)Taking the proposed Guizhou Jiayan Reservoir as the research object,the simulation application and prediction analysis of the mixed flow of the model in the Karst areas are carried out.Based on the measured hydrological data in 2000,the regional hydrodynamic modeling is completed,and the flow velocity changes along the longitudinal direction of the open channel alternating sinking channel and the diversion ratio between the underground flow and the artificial tunnel are analyzed.Using adjacent water quality index data and existing parameter coefficients,the typical water quality scenarios of open channels alternating underground channels and the series of annual water quality change processes are simulated and predicted,and the reference change process and law of water quality indicators are obtained.Comprehensively results show that the model can be used to study the hydrodynamic characteristics and water quality migration laws in Karst areas,and can provide design basis and technical support for engineering construction.