Hydro‐Thermodynamic Processes at a Large Confluence Under Reservoir Regulation

Confluences are ubiquitous and important components in river systems, and their hydrodynamic complexity attracts increasing attention. The flow pattern in a confluence is influenced by the planform geometry, momentum, discharge ratio and bed concordance/discordance. In recent years, flow density is found to be a significant factor in natural confluence flow patterns, while the generated hydrodynamic processes that affects mixing within confluences are not fully known, especially in large rivers under heavy human activities. In this study, we investigate the hydro‐thermodynamic processes at a major confluence in the Yangtze River using a three‐dimensional numerical model. The water level in the confluence zone is regulated seasonally by the Three Gorges Reservoir, that is, the zone is within the reservoir's fluctuating backwater reach. The results show that reservoir regulation alters flow patterns in the confluence zone significantly in both horizontal and vertical planes, including shrinking re‐circulation, complicated cross‐sectional vortexes, as well as weakened riverine mixing capacity. The density difference leads to changes in the hydro‐thermodynamic processes, especially in combination with the reservoir regulation. When the tributary water temperature is high, the generated buoyancy strengthens the secondary flow in the mainstream, which further enhances the lateral transport capacity and accelerates the water mixing. More importantly, it is found that the combined effect of reservoir regulation and thermal buoyancy alters the flow structure considerably in the confluence zone, and modifies the water transport mode in the mainstream. Thus, a dimensionless number S is defined to represent the lateral water transport capacity.