Three-dimensional Numerical Simulation Of Lateral Migration Of Alluvial Channels With Composite Banks

The morphological changes in alluvial rivers include both longitudinal bed deformation and lateral migration, which result in a series of engineering, environmental and socio-economic problems in terms of flood defense and navigation. In particular, lateral migration of alluvial channels is also one of the most important problems in the study of channel pattern changes. Prediction of channel lateral migration based on mechanism of composite bank failure therefore has both theoretical and engineering significance.In order to simulate lateral migration in the Jingjiang section of the middle Yangtze River, a three-dimensional (3-D) dynamic numerical method of meander migration is developed in this dissertation. Bank erosion is an important and general form of lateral migration in alluvial rivers. A method for simulating the erosion of composite banks is established based on the mechanism of bank failure. Both cohesive and non-cohesive bank material in the different layers are considered. The bank erosion module also includes other factors affecting the rate of bank erosion, such as the longitudinal length of failed bank, the thickness of each layer in the double-layer structure. A 3-D mathematical model is then developed using a 3-D flow and sediment transport module and the bank erosion module. In this study, a locally-adaptive grid system based on non-orthogonal grids is proposed and applied to calculate lateral migration of river channel due to bank erosion. This 3-D model is able to calculate both the vertical bed deformation and the lateral migration of alluvial channels, and the effects of bank failure are integrated into calculations of flow and sediment transport in this model. The erosion-resisting effect of cohesive material from the top layer of failed bank is also considered. The above features made this model a powerful tool for the study of morphological changes in alluvial channels.The 3-D model is first applied to simulate the complex response of laboratory channels to changes in flow and sediment conditions. Secondly, meander migration in typical river bends on the middle Yangtze River is calculated by this 3-D model, with good agreements with observations. Different responses to changes in flow and sediment inlet conditions caused by the filling of the Three Gorges Reservoir are numerically simulated for different sections of the Jinajiang River, according to differences in bank compositions. The calculated results imply that the erodbility of river banks play an important role in channel planform changes.Additionally, in order to improve the calculation efficiency in modeling morphological changes, a method is proposed to evaluate the equivalent dominant discharge, defined as a constant discharge that can create the same amount of bank erosion in an alluvial channel as that created by natural runoff processes during the same period of time. A method to integrate 2-D model and 3-D model is also established to calculate long river sections. Simulation results of fluvial processes in the Jingjiang section of the middle Yangtze River demonstrate that the calculation efficiency can be improved significantly, and the 3-D morphological model can be applied to simulate natural river problems with an acceptable computation time by the improved methods.