Two-dimensional Numerical Model For Flood Propagation Over Fixed And Movable Bed

Flood disasters are the most severe natural disaster in China. Normally, lanslide, bed erosion and sediment transport process are often associated with the flood event, which threatens human life and property safety. Thus, the prevention and management of flood disasters are of great importance in practice.The major purpose of this study is to develop reliable and stable2D flood models over fixed and movable bed, which can better understand the hydrodynamic characteristics of the flood over complex topography and the bed deformation, and to give technical supports to the flood control and disaster mitigation in our country. The major work can be summarized as follows:1. Based on the rectangular grid, a two-dimensional hydrodynamic model using finite volume method is developed to simulate flood propagation over complicated topography. A central upwind scheme is used to calculate the flux at the interface of computational mesh and the linear reconstruction of flow variables is adopted to achieve the second-order accuracy in space. Meanwhile, the wetting and drying treatment is improved by using non-negative depth and local bed elevation modification to avoid the problems of negative depth and mass non-conservation. The bed slope source term and friction term are respectively discretized using a central difference method and a semi-implicit method to ensure the well-balanced property and stability of the model. The negative water depth is the key reason to cause the instability of numerical model when simulating flood over, irregular terrain. The established model can guarantee the positivity of water depth when the Courant number is kept less than0.25. Since no need to treat negative water depth problem, the developed model is more robust and stable in comparison with the most of existing models. The model is validated against several cases and the results show that the model can accurately capture shock wave, handle wetting and drying boundary, and is very stable in computing supercritical flow, subcritical flow, and mixing flow over complicated topograpy.2. Based on the unstructured triangular grids, a two-dimensional hydrodynamic model using finite volume method is developed to simulate flood propagation over complicated topography with complex geometries. A central upwind scheme is used to calculate the flux at the interface and second-order accuracy in space is achieved by using limiter techniques. The local bed modification method is adopted to modify flow variables at interface to simulate wetting and drying. The established model can both preserve the well-balanced property and guarantee the positivity of water depth in the computations. The developed model is tested against several cases and results show that the model ccan capture the moving fronts, wetting and drying, and time histories of water depth and discharge with a good accuracy for the flood over complicated topography with complex geometries. The developed model has been proved to be capable of capturing shock wave and preserving mass conservation, which can be used to simulate flood over complicated topography with structures such as rails, roads, buildings, etc.3. Based on the established flood model with rectangular grid and finite volume method, a two-dimensional model coupling flow and sediment transport for dam-break flow over mobiled bed is developed to study the unsteady flow due to dam break and overtopping breaching, sediment transport and bed deformation. The model applies non-equilibrium total-load sediment transport equation to calculate the sediment transport and bed change. The coupled model considers the impacts of sediment density variaions and bed deformation on the mass and momentum of flow. The central upwind scheme is used to calculate the flux at the interface of computational mesh and the linear reconstruction of flow variables is adopted to achieve the second-order accuracy in space, which is the first use in solving coupled flood and sediment transport model. The bed slope source term and friction term are respectively discretized using a central difference method and a semi-implicit method. The improved weting and drying treament assures the positivity of water depth in the computations even if the bed elevation is changed due to erosion and deposition. The model is tested using three experimental dam-break flows over movable beds. The calculated spatial and temporal variations of water surfaces and bed elevation are generally in good agreement with the measured data, which shows that the developed is capable of simulating and predicting flood propagation, sediment transport and bed changes during dam break processes.