Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake

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.

Automated summary

Mixing processes downstream of river confluences have significant impacts on the ecology and environmental management of river networks. This study investigates the mixing dynamics at the confluence between the Yangtze River and the Poyang Lake in China. Results show that the mixing dynamics are influenced by various factors, including the momentum flux ratio, secondary flow, and the lock-exchange flow associated with density differences. Slow mixing is observed during high-flow conditions, while rapid mixing is observed during low-flow conditions. Additionally, the lock-exchange flow enhances mixing when the Yangtze River waters have higher temperature.