On the combined effect of bed elevation discordances in the main-river and the tributary in 90° straight-channel confluences

Abstract

The role of the difference in bed elevations in the main-river (ΔzMR) and the tributary (ΔzT) at their entrance to a 90° straight-channel confluence is studied numerically using a 3D finite-volume model. Three characteristic hydrological scenarios with DR = QMR/Qtot = {0.250, 0.583, 0.750} (where QMR and Qtot are flow discharges in the main-river upstream and downstream of the confluence, respectively) are analysed in nine confluence layouts having ΔzMR/hd ε[0.10, 0.50] and ΔzT/hd ε[0.10, 0.50] (where hd stands for the flow depth in the deeper channel at the confluence). It is found that the presence of the bed-step in the main river: 1) does not affect the flow and bed shear stress distributions for DR = 0.250, 2) causes inclination of the recirculation zone and reduces its size for DR > 0.5, 3) favours conditions for the bed-load transport both in and far downstream of the confluence when DR ≠ 0.750 regardless of the ΔzT-value.