Mechanism Of Channel Pattern Changes And Its Numerical Simulation

The mechanism of channel pattern changes is an important part in the study of fluvial processes and river engineering. Fluvial processes will be substantially affected after the construction of hydropower projects on large rivers, which may lead to channel pattern changes and may have impacts on various aspects, such as flood control, navigation and water diversion, therefore the practical purpose of this study is obvious.In this thesis an improved two dimensional (2-D) mathematical model for water flow, sediment transport and bank erosion is used to simulate fluvial process and channel pattern changes in river channels of the middle Yangtze River reaches, where river banks are very resistant against bank erosion, but point bars and island bars are migrating actively. In this study, the same model was also applied to a conceptual river channel with conditions similar to that of the middle Yangtze River reaches, to study channel pattern changes and the mechanism behind such changes.The secondary flow was taken into account in the 2-D model by adding a dispersion stress term in the momentum conservation equation of water flow. Flow simulation results were compared with published curved-flume measurements in laboratory experiments. The model for bank failure calculation was also improved on the basis of the commonly used method. A non-uniform, non-equilibrium transport model for suspended load and bed load transport was used in the calculations, with better reliability in the results of fluvial processes in natural river channels.A 20-year series of fluvial processes were simulated for the 63km long Yichang - Zhicheng reach just downstream of the Three Gorges Reservoir (TGR), to study the impact of the operation of the TGR. The river banks in the area are strongly resistant to bank erosion, and simulation results demonstrate that although channel bed scours are obvious, the channel pattern will be stable and no channel patter changes will occur.A 6-year series of fluvial processes were simulated using the established model for the 102km long section from Shashi to Shishou in the upper Jingjiang Reach, which is just downstream from Zhicheng, to study possible channel pattern changes at the initial stage of the TGR operation (2006~2012). The results indicate that although wide-spread scour will occur during this period, and both point bars and island bars will migrate actively, especially more severe changes of fluvial processes are observed in the areas of Sanba Tan and Shishou, there will be no essential change in the channel pattern at the present incoming sediment conditions, at least for a short period of time.A 12 km long conceptual river reach, with bed and bank conditions similar to that of the middle Yangtze River reaches, was used to simulate the impact on channel pattern changes by various factors, such as initial channel slope, sediment supply, water discharge, and resistance to bank erosion. Simulated results were discussed, such as plan form of the river channel, longitudinal bed profile, channel cross section, water surface profile and sediment transport in the channel. The simulated results of channel patterns agree well with the classic theory of channel pattern formation. The method in this thesis provided a better way to examine the dynamics behind channel pattern changes.