Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake. Issue 11 (1st November 2022)

• Record Type: Journal Article
• Authors: Xu, Lei; Yuan, Saiyu; Tang, Hongwu; Qiu, Jiajian; Xiao, Yang; Whittaker, Colin; Gualtieri, Carlo
• Is Part Of: Water resources research. Volume 58:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-01
• Subjects: mixing dynamics -- confluence -- Yangtze River -- Poyang Lake -- density effects -- ADCP; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2022WR032195
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: Mixing processes downstream of river confluences impacts the ecology and the related environmental management of river networks. A clear understanding of such processes is challenging, especially for confluences having width‐to‐depth ratios larger than 100, due to the limited available field data. In this study, four field surveys based on hydro‐acoustic and conductivity measurements were conducted near the confluence between the Yangtze River and the Poyang Lake, which are the largest river and freshwater lake in China, respectively. It was found that mixing dynamics at the confluence were controlled by a complex interaction among the momentum flux ratio, secondary flow and the lock‐exchange flow associated to the density contrast between the two tributaries. Slow mixing was observed during high‐flow conditions that generated dual counter‐rotating secondary cells, with the downwelling flow acting as a barrier in the post‐confluence channel. In contrast, more rapid mixing was observed during low‐flow conditions when only a single channel‐scale secondary flow was identified. The mixing processes were also affected by the lock‐exchange flow associated to the density difference between the two confluent flows. Such lock‐exchange enhanced mixing when the Yangtze River waters had higher temperature, that is, lower density than that of the Poyang Lake. In low flow condition, the penetration of the much larger momentum flux of Yangtze River created a "two‐layers" structure with the contribution of the density difference, which further enhanced the curvature‐induced helicity. The findings from the present study improve our current understanding of mixing dynamics in large river confluences. Plain Language Summary: The confluence between the Yangtze River and the Poyang Lake, which are the largest river and the largest freshwater lake in China, respectively, is one of the largest on the Earth. Understanding this mixing processes at such large‐scale river confluence is significantly important for local flood control and aquatic ecology management, but even challenging due to the lack of detailed field data. Four field surveys were conducted to investigate hydrodynamics, water quality and mixing at the confluence of the Yangtze River and Poyang Lake. The effect of momentum and density difference between the two rivers and large‐scale secondary flow on mixing processes was identified. The results of the present study indicated that such density difference can produce different patterns of vertical stratification which are related to the mixing between the Yangtze River and the Poyang Lake. Key Points: Four field surveys on the mixing process downstream of a large river confluence were conducted Different mixing patterns were distinguished based on conductivity and hydro‐acoustic measurements Mixing dynamics around the confluence were controlled by the momentum flux ratio, secondary flow and the lock‐exchange