Impact of Density Gradients and Momentum Ratios on Streamwise Circulation at River Confluences

Abstract Mixing processes in confluences influence concentrations of water quality parameters such as suspended sediment loads and water chemistry in the post‐confluent reach. Thus, improving our understanding of confluence flow structures and mixing is key to predicting the impact of the downstream propagation of pollutants and nutrients. Parameters such as density difference and momentum ratio significantly influence the flow field and mixing processes. This study uses eddy‐resolved numerical simulations to investigate the impact of these parameters on the formation of density‐driven streamwise‐oriented vortices (density‐driven SOVs) and mixing in a symmetric experimental confluence. Three values of with varying magnitudes of the densimetric Froude number , representing were considered in the numerical simulations. A clear pattern for the dominant streamwise circulation (circulation caused by the sign) with changes in and is observed. Therefore, an equation is proposed to predict the dominant streamwise circulation as a function of , , and the distance from the apex. Moreover, the results suggest that the streamwise circulation and mixing rate are positively correlated with (i.e., decreased ). The findings indicate that mixing is promoted by increased and reduced (thus increasing the ). The results reveal that mixing in density‐driven confluences is not solely driven by density‐driven SOVs, but also by other coexisting structures, such as vertically oriented Kelvin‐Helmholtz instabilities.