Abstract

River confluences are key sites in river systems where, when the difference in depth between the two incoming streams can give rise to a distortion of the mixing layer and to upwelling of the deeper channel fluid in the shallower channel. This has been shown to enhance mixing between the two rivers. In the present study, the relation between the confluence bed geometry and the mixing characteristics is studied using a three-dimensional numerical model (PHOENICS software). Curvilinear coordinates and modified k-f closure model are used to investigate the flow structure for a laboratory confluence model and a field stream confluence. A scalar tracer with different concentrations was introduced in the main and the tributary channels of the simulated numerical model to examine the mixing rates in the region downstream of the junction. The tracer is subject to both convection and turbulent diffusion. The results displayed the enhanced mixing due to the amplified secondary flow pattern triggered by the effect of bed discordance. Numerical simulations at two flow conditions of the natural confluence with a similar bed discordance in the tributary also confirmed that a rapid mixing occurs due to the effect of bed discordance