A method for estimating unsteady flow structures of the large river confluence

The confluence of rivers is a crucial node for the transport of materials within river networks, and its complex hydrodynamic structure makes it difficult to predict the mixing processes that occur here. Due to limitations in measurement techniques, previous studies of mixing dynamics, especially field experiments, have primarily focused on the analysis of time-averaged flow structures. However, recent field observations have revealed that the spatial competition between the two converging flows at the confluence may generate large-scale periodic flow fluctuations. This makes it challenging to accurately describe the mixing dynamics at the confluence using time-averaged flow structure analysis. To investigate the impact of large-scale flow fluctuations on flow structure, we conducted field experiments at the confluence of the Yangtze River and Poyang Lake in October 2022 (Momentum flux ratio = 0.01). We used a vessel based ADCP measurement with predefined meshes to obtain instantaneous flow fields and time series of flow velocity across the cross-section. Reliable flow velocity fluctuations were obtained through a regularization method based on physical constraints. The measurements captured significant periodic flow fluctuations (T~1000s), with spatial competition between the two flows leading to a “flapping” of the mixing interface and secondary flows. Additionally, due to density differences between the two flows, when the denser flow dominated, underflow was predominant in the secondary flows, while surface flow dominated when the less dense flow was dominant. This study can help improve the prediction of mixing dynamics at river confluences and provide new insights for field measurements of non-steady flow structures.