Improvement And Application Of Prediction Methods For Flow Structure And Fluvial Processes In Meander Channels

Water flow, sediment transport and fluvial processes in meandering rivers are multidimensional, multiscale and affected by various natural effects and human activities. Due to complicated conditions such as flow in consecutive bends, transport of fine sediments and artificial perturbations, existing simulation methods are faced with many difficulties in predicting such processes. This paper is aimed at improving previous methods in order to better predict flow structure and fluvial processes in meandering rivers. The main improvements are as follows:1) A new formula is developed analytically for the attenuation process of transverse velocity downstream of a bend, which is able to include the impact of transverse slope of water surface in the bend; and this formula is able to include the previous formula which ignores effects of transverse slope in bend flows;2) Based on dimensional analysis and numerical experiments, a multiple-factorformula, which considers several geometrical and hydrodynamic parameters, is established for calculating the attenuation length of transverse velocity downstream of a bend;3) A mud flow and transport model was established in addition to the traditional 3D RANS flow and sediment transport model, which is able to simulate the transport, depositing and gathering process of fine sediments;4) Boundary restriction is applied to an analytical meander model so as to model the large scale planar evolution of meandering river channel under artificial perturbations.The improved methods are applied to both experimental meandering channels and natural meandering rivers, and the following results are obtained:1) The new analytical formula can better predict transverse velocity in the connecting sections between consecutive bends,2) The souces of inaccuracy of previous single-factor formulasfor attenuation length wererevealed; and the new multiple-factor formula successfully estimated the impact of the upper bend on the lower bend in meandering channels, so that the complexity of flow structure in the lower bend can be predicted.3) The capability of 3D RANS models with various turbulence models and wall treatment methods in simulatingflow structure of different complixity in meandering channels is systematically evaluated, providing useful information for its application in natural meandering rivers;4) By taking into consideration the flocculation process of fine sediment, our flow and sediment transport model successfully simulated the flow filed, suspended sediment concentration and river bed deformation in the near-dam area of the Three-Gorges Reservoir;5) The modified analytic meander model is applied for various scenarios involving several flow and sediment parameters as well as the artificial perturbations, to analyze their impact on the large scale planar evolution of meandering rivers. The patterns of evolution of a natural meander reach have been summarized.The improved prediction methods for flow structure and channel evolution of meandering rivers enhanced ability of previous methods in complicated natural situations.