Numerical Simulation Of Urban Flooding Based On Unstructured Grid And Study On Flooding Mitigation Scheme

Since the 21 st century,the accelerated urbanization and industrialization,as well as the frequent extreme rainfall events fulfilled by the tendency of global climate change,have caused many cities and regions in China’s economic centers to suffer from severe flooding disasters,which have brought great losses to China’s economic development and people’s normal life.Therefore,numerical simulation of urban flooding,prediction of flooding and formulation of prevention and control plans are the major problems that need to be solved urgently to deal with flooding disasters,and the key basic link of which lies in the accurate and comprehensive analysis and simulation of the flooding process.Based on this,this paper takes the main campus of Chang’an University as the main research area,and analyzes the flooding formation factors by integrating the recent flooding disasters in the research area,and clarifies that the main causes of flooding in the research area are topographic and meteorological factors.The study area’s coupled surface and pipe network model,manhole pipe model,and surface runoff model were all built.A thorough evaluation of the current drainage capacity of the pipe network and surface flooding in the campus was carried out using the numerical model of urban flooding created in Fortran.Based on the results of the flooding risk assessment,three retrofitting schemes were designed based on the shortterm emergency concept,long-term sustainable concept and green concept,and the improvement of the drainage capacity of the pipe network and the flooding mitigation effect of different schemes under four rainfall recurrence periods were exhaustively examined.The following are the study’s main findings.(1)Summarize the land use information and drainage network of the study area,construct a Fortran language based waterlogging model based on the basic terrain,hydrology,and network data of the study area,and rate and verify the model parameters based on the measured network flow data and surface inundation situation.The research results show that the Nash efficiency coefficient RNS is greater than 0.80,the relative error value REp of the peak flow is less than 10%,and the absolute error AEr of the peak arrival time is close to 0,which ensures the accuracy and reliability of the waterlogging model.(2)The model was used to simulate the drainage capacity and surface inundation of the pipe network in the campus.According to the analysis of the results,the number of drainage pipe overflow nodes increases along with the length of the pipe filling,the length of the pipe filling,and the level of surface inundation.These changes are all correlated with the amount of rainfall return time.In the study area,the pipe network’s length with a 1-year rainfall return period is 93.55%.Based on statistical analyses of surface flooding during various rainfall return periods,the risk level of flooding is divided for the campus,and it is found that the campus is mainly a medium and low risk area,and there is no high risk area.(3)Based on the results of waterlogging risk assessment,taking into account the economic benefits of the old campus and the feasibility of engineering implementation,three drainage and waterlogging control plans have been formulated,namely short-term plan,long-term plan,and green plan.According to the simulation results,the current drainage capacity of the pipeline network is analyzed and compared from multiple perspectives such as the number of overflow nodes in the pipeline section,pipeline fullness,and surface inundation depth.The reduction rates of overflow nodes in the pipeline are 43.59%～83.33% and 66.67%～100%,respectively.The maximum water depth reduction rates of overflow nodes in the pipeline reach 5.96%～15.02%and 9.04%～20.56%,respectively;The reduction in pipeline filling length reaches 4.25%～100%and 48.53%～100%,with the highest reduction rates of pipeline filling reaching 48.11% and83.50%;The proportion of submerged grids in the total grid is less than 10%,with a maximum grid reduction rate of 100% and a maximum water level of 0.308 m,with a reduction rate of47.08%.