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 met hod for approximating the effects of water surface elevation patterns and a renormalization group modified form of the k-epsilon turbulence model. The model was tested by comparison with laboratory measurements of an asymmetr ic tributary junction. This suggests that although the model is unable to r eproduce the quantitative detail (notably upwelling velocity magnitudes) of the flow structures as measured in laboratory experiments, statistically s ignificant aspects of the experimental observations are reproduced. The mod el is used to (1) describe and explain the characteristic flow structures t hat form in a confluence with one of the tributaries angled at 45 degrees, both with and without an elevation difference Cbed discordance) in the angl ed tributary; and (2) investigate the relative importance of junction angle s (30 degrees, 45 degrees, and 60 degrees), bed discordance, and ratio of m ean velocities in the tributary channels upon flow structures. This shows t hat bed discordance significantly enhances secondary circulation because of the effects of flow separation in the lee of the bed step, which significa ntly increases lateral pressure gradients at the bed and reduces water surf ace superelevation in the center of the tributary and water surface depress ion at the downstream junction corner. Extension to consideration of a numb er of junction angles, levels of bed discordance, and velocity ratios sugge sts that a small (10%) reduction in tributary depth can significantly incre ase the intensity of secondary circulation, albeit in a relatively localize d manner. Simulations involving a numerical tracer illustrate the importanc e 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.