Effect of bed elevation discordance in the main river on the confluence hydrodynamics in 90o straight-channels’ confluences

A role of the difference in bed elevations in the main-river (ΔzMR) at its entrance to a 90º straight-channels' confluence is studied numerically using a 3D finite-volume model. Three characteristic hydrological scenarios with DR = {0.250, 0.583, 0.750} are analysed in four confluence layouts having ΔzMR / hd [0.0, 0.50] (where hd stands for the flow depth in the deeper canal at the confluence). It is found that: 1) a portion of the tribu-tary flow in the bottom layers is entrained by the bed-step, 2) the w- velocity has the same order of magnitude as Vxy for ΔzMR ≥ 0.25hd and DR > 0.5, 3) the opposite bank is endangered by erosion due to strong upward motion along this bank, 4) additional shear layer develops on the outer side of the distorted umax-core and 5) the presence of bed-step favours bed-load transport both in and far downstream of the confluence