On the effect of bed elevation discordance in confluences with unequal channel widths

Abstract

The paper studies the effect of bed elevation discordance on the hydrodynamics in fixed-bed open channel confluences where the tributary channel is narrower than the main channel and where the flow is subcritical. The study is inspired by two facts: 1) that the main river is wider than the tributary in majority of cases and 2) that the bed elevation discordance is a usual morphological feature in alluvial river confluences. To this aim nine confluence layouts with three channel width ratios BT / BMR = {1.00, 0.75, 0.50} and three extents of bed elevation discordance ΔzT / hd = {0.10, 0.25, 0.50} are analysed under the three possible hydrological scenarios defined by the discharge ratio values DR = QMR / Qtot ={0.250, 0.583, 0.750}. It is found that: 1) the tributary flow acts as a jet when DR = 0.250 regardless of the BT / BMR and ΔzT / hd values, 2) the recirculation zone with several vortices exists throughout the flow depth when DR = 0.250 and BT < BMR, 3) the primary vortex is shifted farther downstream and the secondary one is inclined and attached to the boundary streamline, 4) the strong shear exists both in the horizontal and vertical planes and 5) the shear layer is developed on each side of the inclined recirculation zone.