Role of Bed Discordance at Asymmetrical River Confluences

This paper studies laboratory open-channel confluences using a 3D, elliptic solution of the Reynolds-averaged Navier-Stokes equations, including a method for approximating the effects of water surface elevation patterns and a renormalization group modified form of the k-ε turbulence model. The model was tested by comparison with laboratory measurements of an asymmetric tributary junction. This suggests that although the model is unable to reproduce the quantitative detail (notably upwelling velocity magnitudes) of the flow structures as measured in laboratory experiments, statistically significant aspects of the experimental observations are reproduced. The model is used to (1) describe and explain the characteristic flow structures that form in a confluence with one of the tributaries angled at 45°, both with and without an elevation difference (bed discordance) in the angled tributary; and (2) investigate the relative importance of junction angles (30°, 45°, and 60°), bed discordance, and ratio of mean velocities in the tributary channels upon flow structures. This shows that bed discordance significantly enhances secondary circulation because of the effects of flow separation in the lee of the bed step, which significantly increases lateral pressure gradients at the bed and reduces water surface superelevation in the center of the tributary and water surface depression at the downstream junction corner. Extension to consideration of a number of junction angles, levels of bed discordance, and velocity ratios suggests that a small (10%) reduction in tributary depth can significantly increase the intensity of secondary circulation, albeit in a relatively localized manner. Simulations involving a numerical tracer illustrate the importance of bed discordance for mixing between the two flows and question the use of simple 2D parameterizations of mixing processes that do not consider bed discordance when the latter is present.