Study And Application Of A Multi-level And Modular Model For Simulating The Urban Flooding

In recent years,increased extreme weather events have resulted in frequent urban flooding disasters by the effect of global climate change in combination with the rapid urban development.Many cities over our country has been hit by rainstorms of different levels and resulted in “vast sea”.Thus it can be seen that global climate change and the rapid development increase the frequency,intensity and duration of urban rainstorms,in turn,along with high impact and destructive power.Urban rainstorms may directly bring about traffic inconvenience,a lot of death and economic losses,in addition to these direct effects above,it will also bring about a series of subsequent and invisible impacts.Therefore,it is of high practical significance to study the refined model for urban flooding which would contribute to optimize urban drainage system,improve forecast accuracy for heavy rains,strengthen the control and management of urban flooding and reduce social losses.What’s more,the refined model is capable of simulating urban flooding discharge process and provides insight into the movement mechanism of rain-runoff,from this point,deepen research on the refined model has great value in theory.This dissertation focuses on urban flooding and studies the mathematical model based on hydrodynamic methods,the main research work and contributions are as follows:(1)The model concept of “multi-level and modular” are presented in our paper from the views of both two-dimensional(2D)plane and three-dimensional space,thus a multi-level and modular model is built for simulating the urban flooding.According to different underlying surfaces and actual urban flooding path,the presented model integrates four modules which simulate: 1)2D flow routing on communities;2)2D flow routing on road network;3)2D flow routing on river network,and 4)one-dimensional(1D)flow routing on sewer system.This model not only achieves volume exchange between overland flow and underground pipe flow,but also ables to simulate the generation and movement of the urban flooding precisely.(2)The close and complex hydraulic connections among the four modules are analyzed in detail and some computing modes are presented to improve the applicability and reliability of the model.With regard to the coupling between the river network and the underground pipes,we present a pumping water approach that is to set a radius of pumping influence combined with drainage blocks as the drainage coefficient of each unit in the influence area follows normalized distribution.By this method,the drainage coefficient and volume can be achieved for the sewer.system.In addition,when water depth in one cell becomes negative,its value is redistributed among the neighbouring cells.(3)Based on the concept of shock discontinuities,the approximate Riemann solver of Roe used for the computation of inviscid flux functions is derived by characteristic theory and a 2D unsteady flow model is developed with high accuracy.The TVD-MUSCL technique is employed to achieve second-order spatial accuracy and Hancock predictor-corrector method is applied for the time integration.Application to the classical dam break case verifies the efficiency and accuracy of the developed model.(4)The hydraulic characteristics of overland flow on steep slopes are studied from both physical and mathematical model.In the respect of mathematical model,the governing equations of overland flow on slopes with any slope angle are obtained based on the conservation law of mass and momentum.The equations are discretized and solved by the Roe approximate Riemann solver,thus a computing mode for rainfall-runoff is developed with high precision.As for the physical model,16 experimental schemes are designed with a combination of precipitation intensity and slopes and then conducted using rainfall simulation system.The simulated results agree well with the measurements and this great agreement demonstrates the reliability and accuracy of the model,which allows for bed slope and rainfall momentum and can be very useful for relevant research of overland flow over steep slopes.(5)The multi-level and modular model is established for a representative catchment in Tianjin City.Ten rainfall events from the year 2009 to 2015 are simulated successively.The model is verified by comparing the model results with the measurements.The comparisons focus on four aspects: water levels of the Jin River,water depth at feature points,waterlog conditions in two important areas,and water balance.Satisfactory agreements indicate that the model is of capable to process the complex rainfall-runoff problem among communities,road network,sewer system and river network and can used to monitor and forecast urban flooding with accuracy.(6)The multi-level and modular model for Tianjin City is then developed for further testing.The model debugging is performed by using the measured data of flood-prone areas and the model results obtained in Section 5.Furthermore,the model is validated by comparing the model results of water levels with the measurements of different monitoring sites located in river network.The simulated water levels agree well with the measurements.These great agreements demonstrate that the approach presented in our dissertation can not only simulate rainfall-runoff in small urban catchments,but also can be applied to deal with more complex rainfall-runoff problems in larger urban areas.