【作者】 假冬冬；

【导师】 邵学军；

【作者基本信息】 清华大学 ， 水利工程， 2010， 博士

【摘要】 冲积河道的自然演变过程中,不仅有垂向上的河床冲淤变化,在平面上也有显著的横向摆动特征。河道横向摆动不仅给防洪、航运、取水等工程带来严重影响,它同时也是蜿蜒河道河型演变研究中的一大难点。因此,研究非均质河岸河道平面摆动的过程机理及其三维数值模拟方法具有重要的理论和实际意义。论文以长江中游荆江典型河段河道摆动的三维模拟为应用目标,对河道摆动的三维数值模拟动力学方法进行了研究。其中河岸崩塌是冲积河流平面变形的重要体现形式,论文基于河岸崩塌力学机理,推导建立了考虑相邻土体影响的粘性河岸及二元结构河岸坍塌力学模式,弥补了已有模式仅考虑断面二维力学因子的不足;建立了全三维水沙数学模型,并基于非正交网格提出局部网格可动技术处理由崩岸引起的河道摆动过程,将河岸崩塌力学模式与三维水沙模型有效结合,构建了河道摆动的三维数值模型。模型不仅能模拟出河道沿垂向的冲淤变化,而且还能模拟出由非均质河岸崩塌引起的河道横向摆动过程;模型可反映河岸变形对水沙输移计算的影响,亦能反映坍塌粘性土体对河床的掩护作用,为复杂河道平面河势变化的研究提供了一有效技术手段。本数值模型首先应用于模拟室内概化河槽的平面河势演变过程,并探讨了不同水沙条件下,河道演变的复杂响应过程。之后将模型用于模拟长江中游荆江典型河段的河道摆动过程,与实测结果符合良好;以此为基础,初步模拟分析了三峡水库运用后水沙条件变化对荆江不同河段平面河势演变的影响,结果表明非均质河岸抗冲性的差异对河道的河势演变过程存在显著影响。基于河道平面变形幅度一致的原则,提出了等效造槽流量计算方法,成功运用于下荆江石首河段的摆动模拟过程中;此外还探讨了考虑河岸坍塌的二、三维水沙数值模型按计算域进行嵌套的长河段模拟方法,并模拟了上荆江沙市-新厂河段的平面河势演变过程。计算结果表明两种方法均可较大地提高模型计算效率,这对促进河道摆动三维模拟方法的发展及应用具有重要意义。更多还原

【Abstract】 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.更多还原