Research On Simulation Of Flow And Sediment Transport In Levee-Breach By Overtopping

Dike engineering has long been used to protect people from flood disaster, simultaneously, once dike that plays a major role in restricting flow is destroyed by extreme flood synthesis upstream, the loss will be huge. As a consequence, there is desperate demand for making a flood control plan to cover the risk of building dike engineering, including the possibility of dike burst at the Large-discharge flood process. Due to the limited measurement instruments, numerical simulation is an important tool for simulating the process of dike burst.Numerical simulation of dike burst consists of two parts. One is the process of flow spilling over the breach, the other is the evolution of dike breach caused by flow erosion. The interaction of the two parts codetermines the evolution of flood waves in the downstream of the breach. As the complexity of flow pattern, including supercritical flow、subcritical flow and critical flow at the same time, routine numerical methods stultifies the whole process of calculating, so it is required to select or establish appropriate numerical methods. The occur and evolution of dike burst is a more complex process, before the description of the process by numerical model, research on mechanism and process of dike burst is necessary. Consequently, starting with the mechanism dike burst, the physical model of dike burst deepen the understanding of dike burst process and the experimental data can serve as the basis for testing the numerical model. Content of this dissertation include the following parts:(1) Exposit the origin, background and significance of dike burst. On the basis of predecessors’ work, the main research contents and key problems to solve are decided.(2) Mechanism of dike burst is the foundation for numerical simulation of the dike burst process. For the sake of model being established in the correct mechanism, the mechanism of three most important dike destruction modes, that are overtopping failure for dike, river water scour and piping, is discussed. Base on this, the difference and common ground of various kinds of dike destruction modes and process is evaluated and summarized, as well as the numerous influence factors of dike burst process. At last, the evolution process of dike burst flood and the evaluation methodology are also analyzed.(3) A 2-D numerical model for dike breach simulation is established based on the differential conservation law form of non-linear shallow water equations, which uses WENO sheme containning three cells to calculate the conserved variables at the interface and Roe method to calculate the flux at the interface. In order to keep the precision of the model, three steps Runge-Kutta method is used for solving source terms, and the adaptive method for time step is applied for time evolvement. The model is verified by compared the numerical result with the analysed result and it shows that the two results are indentical. The result of the numerical model shows high accurancy and with essentially non-oscillatory, so it can be used in calculating the the flow evolution due to dam-break. Finally the model is applied in two practicle situations, one is predicting the dam-break flow evolvement in dry river bed; the other is to simulate the flow in the river and in the flood storage and detention basins at the same time when dike burst happens, in which the flow field and water level around the burst are presented.(4)Based on the mechanism, the simulation of evolution of dike breach is divided into longitudinal deformation and lateral widening. The processing method of the focal problems, such as 2-D sediment carrying capacity formulae and graded sediment transport rate, is decided in the simulation of longitudinal deformation by flow-sand dynamic models which are used currently. The simulation of breach lateral widening use the experience of channel lateral widening underpinning the mechanical approach, and dike destruction mode according with the phenomenon of the physical model is decided so as to reflect the breach growth process correctly.(5)First, data analysis and processing of physical models in which the evolution of dike-burst flood waves and dike breach is shown can serve as the basis for testing the numerical model. Next, on the basis of discoverable phenomena and laws of dike burst experiment in channel bends, some parameters and processing methods are fixed and improved. Finally, it is demonstrated that the numerical model established in the dissertation which is validated in channel bends can reflect the phenomenon of the dike burst process basically.(6)Existing problems in the numerical simulation should be summarized and discussed, and further detailed research is ongoing.