Experimental investigation of the evolution of bed topography in an asymmetrical confluence

This study assessed the time evolution of the bed morphology of six experiments performed by Bombar and Cardoso (Geomorphology 367:107329, 2020. https://doi.org/10.1016/j.geomorph.2020.107329) on mobile-bed confluences. These experiments were characterized by a unique junction angle, a fixed plan layout of the converging channels, constant water discharges and water discharge ratio, and six combinations of the converging sediment transport rates. The end aspect ratio of both channels was sufficiently large to avoid strong wall effects, and the flow was always subcritical and rough turbulent. The bed sediment was a uniform non-ripple forming sand. Detail analysis of the time evolution of the bed morphology showed that balanced fluxes of sediments through the boundaries of the system were a sufficient condition of equilibrium within the range of variables covered by the study. The most important morphological features of the confluence were discussed for several time instants, including those referring to the equilibrium phase. The equilibrium features essentially complied with the morpho-dynamic models reported in the literature for mobile-bed confluences, meaning that the time evolution and the end (equilibrium) values of the bed morphologies reported in this paper can be used to validate CFD models thought of as predictive tools of confluence morpho-dynamics. Two experiments are, indeed, ideal for this purpose since they converged to static equilibria, and wrong predictions cannot be attributed to oscillations of the bed, whereas those models may only be able to predict the average, equilibrium values of the bed features of experiments run with sediment supply into the main channel and characterized by the movement of bed forms.