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- Title
Hydro‐Thermodynamic Processes at a Large Confluence Under Reservoir Regulation.
- Authors
Shi, Lidi; Sun, Jian; Lin, Binliang; Liu, Zhaowei; Zuo, Xinyu
- Abstract
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. Plain Language Summary: Confluences are common components of all riverine systems. Existing studies mostly focus on the specific confluence hydrodynamics influenced by confluence planform geometry, momentum and discharge ratio and bed concordance/discordance, but there are few research on the density flow, which could alter the fluvial processes that affect mixing within confluences, especially combined with the impacts of reservoir regulation. In this study we established a three‐dimensional hydro‐thermodynamic model for a large confluence zone, considering both the temperature difference, the seasonal reservoir regulation and quantified all these influencing factors on the hydrodynamic processes. The results indicate that the confluence area is affected by the reservoir regulation, leading to the disappearance of the re‐circulation zone, the enhanced development of the cross‐sectional vortexes, and the weakening of the lateral mixing. These variations are further altered with the tributary water temperature excess, the secondary flow is strengthened, which enhances the lateral water transport capacity and accelerates the lateral mixing, while the vertical mixing is inhibited. Moreover, a dimensionless number S is defined to evaluate the transport capacity of the secondary flow and the longitudinal primary flow. These findings can provide a useful reference for the water mixing in other reservoir confluences. Key Points: Reservoir regulation has altered the flow pattern of a large confluence significantly in both the horizontal plane and cross‐sectionsThe tributary‐mainstream temperature difference changes hydro‐thermodynamic processes, especially when combined with dam operationA dimensionless number S is defined to represent the lateral water transport capacity and mode in this confluence
- Subjects
RESERVOIRS; FLUVIAL geomorphology; DAMS; WATER temperature; DIMENSIONLESS numbers; WATERSHEDS; THREE-dimensional modeling
- Publication
Water Resources Research, 2022, Vol 58, Issue 12, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2022WR033315