| The relation between the Yangtze River and the Dongting Lake is very complex. With the construction and usage of the Three Gorges Project (TGP) and the other reservoirs upstream, the river-lake relation will become more complex. The range of the river-lake network is so large and there are so many complex branches inside it. So,1D network model has been carried on research in a general way in the past. However, the hydrodynamic characteristics are short of an indepth research, such as storage and discharge relation, flood routing characteristics in the river-lake network. Furthermore, the3D flow character at confluence section of the Yangtze River and Dongting Lake is obvious, and the flow and sediment movement characteristics are complex. Therefore, a2D hydrodynamic mathematical model of the whole river-lake network and a2D&3D compound model of flow and sediment at confluence section have been built in the thesis. Several aspects of the research have been carried out as follows.(1) The mesh generation method suitable for simulation of large scope of river-lake network is discussed. And a new unstructured mixed mesh method is put forward. First, the quadrangle grids are used for long and narrow channel, and the triangle grids are used for other regions. And then, a whole unstructured mixed mesh is generated by connecting the two kinds of grids. The mesh generation method has so many advantages. On the one hand, the mesh can satisfy the simulation requirement in long distance river reach and large area. On the other hand, many unnecessary grids are saved in order to reduce the calculation time. The method has been used successfully in the Yangtze River and Dongting Lake network. Results show that the mesh can accommodate the complex boundary condition, and the mesh quality is very good.(2)With the unstructured mixed mesh, a2D flow model has been built in the Yangtze River reach from Yichang to Huangshi and the whole Dongting Lake. The model has been verified by field data. According to the Dongting Lake planning, many hydraulic structures such as weir and gates will be built in the flood channels, the model takes weirs and gates as inner boundary in order not to affect the whole calculation. In consideration of the existence of dry river-bed in Dongting Lake, the dry wet boundary treatment method is used in the model. In order to verify the model’s adaptability to dry river bed, a flood routing simulation in Jingjiang flood diversion and storage area has been carried out, and results show that the model can simulate the flood routing in dry river bed very well.(3) From the macroscopic perspective, a whole2D flow model has been built in the area from Yichang to Huangshi river reach of the Yangtze River and the whole Dongting Lake. The model has been verified with field data. The key technology of compound model is discussed in the thesis. First, there is the same planar mesh in the2D model and3D model, it is easy to model connecting. Second, the shallow3D grids are replaced by2D grids, it is beneficial to model calculation stability. Third, the same computing method has been used in the2D model and3D model, it is beneficial to improved model’s overall performance.(4) The model interface has been developed in the thesis with many programming language such as FORTRAN, VB, VC and OpenGL. It concludes the pre-processing, model calculation, the post-processing, database, and other modules, the calculated data picture can be embed into the3D aerial landscape with the Google earth software, and many kinds of view function, such as zoom, aero view, roaming, and dynamic demo can be realized in the software synchronously. As an example, a dynamic demo of calculated water depth and concentration at confluence section of river and lake has been show in Google earth software.(5)Taking1998flood process as calculated condition, a flood routing simulation has been carried out with the overall2D flow model. Calculation results show that:the longitudinal slope of the low Jingjiang river is less than upper Jingjiang River, the slope during rising period is larger than falling period, and the slope downstream of Tiaoguan changes little with flood rising or falling, because of being affected by the Dongting Lake’s confluence.(6) Taking1998flood process as calculated condition, the calculation about the flood routine influence of TGP in the river-lake area has also been carried out. Results show that:with the TGP’s coming into use, the peak flood in the middle Yangtze River is cut down, the peak flood of diversion from three outfalls in Jingjiang River is reduced, too. The flood water level of hydrometric stations in Yangtze River and Dongting Lake is lowered, and the water level can be reduced to45meters without using Jingjiang flood diversion and storage area after Chenglingji flood control scheduling. The flood control situation is remitted.(7) Statistics of discharge rate at the confluence has been carried out, and the relation between discharge rate and converged discharge has been given.With the2D and3D composite model, a flow field simulation has been carried out at confluence section of the Yangtze River and Dongting Lake in condition of different confluence ratio. Results show that:the flow is concentrated in main channel in condition of low water level, and the upper level flow inundates the bottomland in condition of high water level. When the Yangtze River flow is main, the transverse circulation at confluence section of the Yangtze River and lake is clockwise, otherwise is anticlockwise.(8) With the2D and3D composite model, the riverbed erosion and deposition simulation has been carried out after TGP’s use to2018, and results show that:the riverbed in the channel has been eroded and the bottomland has been deposited by sediment in the initial stage usage of TGP. The Overall river regime changes little, but riverbed near concave bank has been scoured distinctly and Shoreline has been eroded back. |
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