Numerical Simulation On Flood Propagation And Sediment Transport With Obstacle Groups

In reality, flood propagation often encounters the obstacle groups such as vegetation and buildings. Both vegetation and buildings can have significant impacts on flow and sediment transport due to drag forces. In this thesis, a complete two-dimensional hydrodynamic and sediment transport model is developed to simulate the flood and sediment movement with obstacle groups. The effects of obstacle groups on flow and sediment are considered by introducing a permeable density coefficient (PDC) in governing equations of flow and sediment transport. The main research results in this paper are summarized as follows:(1) The paper summarizes the resistance of vegetation and buildings on flow during the flood propagation. The resistance of vegetation on unit volume flow can be expressed as while its value can be influenced by vegetations’ characteristics, such as rigid vegetation, flexible vegetation and leaves. The resistance of buildings on unit volume flow is expressed as, while its value varies due to the constructions’ size and shape.(2) The PDC is introduced into the two-dimensional shallow water equations to consider the effect of obstacle groups on flow flux, thus a new two-dimensional hydrodynamic and sediment transport model based on PDC method is developed. The model takes into account the water viscosity, turbulent effect, and interactions between flow, sediment transport, and bed change. In the model, a central-upwind scheme is used to calculate the flow flux and momentum flux across the interface of computational mesh, and then the linear reconstruction of flow variables is adopted to achieve the second-order accuracy in space. An upwind scheme is used to calculate the sediment transport flux A central difference method is used to discretize the bed slope source term to maintain the well-balanced property. The friction source term and resistance term are calculated using a semi-implicit method to ensure the stability of model. The diffusion terms in the model are processed using central difference scheme. The model can guarantee the positivity of water depth when the courant number meets the certain condition, which means no need is required to handle the negative water depth problem.(3) The numerical results of flood on fixed bed indicate that the PDC method can consider the obstacle groups as a whole domain. By giving the same parameters, the mesh size has little influence on the results if the PDC method is used, in comparison with the method that distinguishes the obstacle with high resolution grids. Meanwhile, the computational time of the PDC is only1/50of the method using high resolution grids, which means the PDC method can greatly improve the computational efficiency in the flood simulation with obstacle groups. This efficient method could provide a useful tool for flood risk analyze and disaster prevention decision.(4) The model has been also applied to evaluate the characteristics of sediment transport and bed change during flood propagation with obstacle groups. The influences of riparian vegetation as obstacle groups on dam-break flow and bed change are discussed via numerical simulation. The results indicate that the model can correctly predict the hydrodynamic and morphodynamic behaviors of flood over obstacle groups. By analyzing the influence of different vegetation distribution patterns in the river, the results show that vegetation groups can effectively reduce bed erosion in the upstream, while it also increases water level in the upstream to increase the flood risk. The distribution pattern of riparian vegetation has a great influence on food propagation and sediment transport. Numerical results suggest that the pattern of vegetation along the main channel bank with half width might be the effective scenario for reducing water level, flow velocities and maintaining the ecology stability.