Research On Numerical Simulation Technology Of Urban Floods

In the context of global climate change, with the rapid development of socio-economic and continuous improvement of urbanization level of China, urban flood disasters occur more frequently, and economic losses and social impact is much more serious than these in the past. Numerical simulation of urban floods has very important significance in rational and scientifically planning and designing of urban drainage system, raising the city’s flood disaster control and management level and effectively reducing the loss of life and property caused by the urban floods; however, there is an overall lack of systemic and deep research on the numerical simulation technology of urban flood simulation in China, it is difficult to provide effective supports for urban flood management. Thus, to research numerical simulation of urban flooding has very important practical significance and application value.Taking the urban flood as the object, the simulation methods of urban floods based on hydrodynamic theory were studied in this thesis, the main work of the thesis includes:(1) A one-dimensional(1D) free surface-pressurized flow model is proposed and established. Preissmann four-point linear implicit scheme is used to discrete the governing equations of free surface-pressurized flow, and then a one-dimensional model that can handle both the free surface flow and pressurized flow is established. To improve the computational efficiency of the model and to avoid solving large sparse matrix, the junction-point water stage prediction and correction method after some improvements, which was initially proposed for open channel flow, is extended to the simulation of pressurized flow and the flow that free surface flow and pressurized flow coexists. The proposed one-dimensional free surface-pressurized flow model is validated by a set of different examples form different aspects, and the results show the model has the ability to handle actual flood, open channel flow and pressure flow, loop pipe network and tree pipe network with good accuracy.(2) A two-dimensional(2D) hydrodynamic model is proposed and established. A Godunov-type finite volume model with second-order accuracy in both spatial and temporal for two-dimensional shallow water flows on unstructured grid is proposed, while the MUSCL scheme is applied to the variable reconstruction. An implicit dual time-stepping method is applied to the model to improve the run-time efficiency and stability in dealing with the urban floods. The capabilities of the present model to achieve good convergence to steady state, to capture the discontinuous flows and to handle complex shallow water flows involving wetting and drying over highly irregular terrain were verified by applying it to a set of benchmark cases. The results show that the model can handle complex flows, real topography and city building dense area accurately and efficiently, and the good performance of the model also indicated a greatly improvement in stability and efficiency compared with the explicit scheme.(3) A coupled one-dimensional and two-dimensional numerical model for urban floods is proposed and developed. In lateral mode, the exchange flow between the 1D model and 2D models is computed by the an empirical weir formula, while in frontal mode, 1D model and 2D model provide boundary conditions for each other. A new method of calculating the vertical exchange water is proposed and applied in the coupled 1D and 2D model. Two horizontal and one vertical linking examples were used to test the coupled model, and the results showed that the model has good accuracy and meet the requirements of water conservation. The coupled model proposed and established in this thesis is further tested by applying it to two real application cases including a dam-break case, which is to analyze and assess the risk of dam-break flood of Minzhi reservoir in Shenzhen City, and a case of urban rainstorm simulation in Xinhepu Community in Guangzhou City. The reasonable results show that the coupled model has the capability of handling the real topography and problems, thus having a good potential for real urban flood simulation.