Influence of Downstream Channel Width on Flow Features in a T-Shaped Open-Channel Confluence

Open-channel confluences are important junctions in natural and human-made channel networks. Different controls of the confluence hydrodynamics have already been studied intensively, often in schematized geometrical configurations. The T-shaped planform with branches having rectangular cross-sections of equal width is a popular geometry in lab experiments and numerical modeling. However, limited research has been conducted concerning the influence on the flow features of widening or narrowing the downstream branch in such a confluence. Therefore, four geometrical cases with a different downstream channel width were studied with large eddy simulations, while keeping the tributary-dominant discharge ratio, the width of the upstream branches, and the tailwater depth constant. The effect of increasing the downstream-to-upstream branch width ratio was analyzed with regard to the water surface shape, the stagnation zone location, the tributary inflow angles at the interface with the junction, the separation zone dimensions, the flow acceleration toward the vena contracta, the secondary flow patterns and intensity, the turbulent kinetic energy in the shear layers, the bed shear stresses and the backwater effects in the upstream branches. When comparing the numerically predicted upstream-to-downstream water depth ratios with those predicted by the single analytical model applicable to confluences with unequal channel widths, it is found that both models capture the reduction of the upstream-to-downstream water depth ratios when widening the downstream branch, but benchmark cases show that the numerical model predicts the correct relative magnitude of the water depth ratios in the main channel and the tributary channel, contrary to the analytical model.