Impact of Density Gradients on the Secondary Flow Structure of a River Confluence

Abstract

AbstractA small gradient in the densities (Δρ) of two rivers in Canada was recently shown to develop coherent streamwise orientated vortices (SOVs) in their confluence. Here we use eddy‐resolved numerical modeling to examine how the magnitude and direction of Δρ affect these secondary flow structures. At equal density, lone anticlockwise SOVs are predicted near the surface, a possibility supported herein by recent aerial observations of such SOVs at the confluence. When a Δρ is considered, a front from the denser channel always slides underneath the lighter channel independent of whether the dense front pushes into the fast (Coaticook) or slow (Massawippi) tributary. When the fast Coaticook is denser, coherent clockwise rotating SOVs tend to form. However, when the slow Massawippi is denser, interfacial instabilities are generated as the fast flow of the Coaticook shears overtop the dense front. Thus, the tributary's velocity opposing the dense front's propagation modifies secondary flow characteristics. Importantly, both these cases would have the same densimetric Froude number (FD) if defined using an average velocity of the confluence (e.g., that of the downstream channel), yet this single FD value cannot account for the differences observed between both. This ambiguity inherent to current conventions for calculating FD places into question its use as an adequate predictor of density‐driven secondary flow at confluences.