Effect of the sediment discharge on the equilibrium bed morphology of movable bed open-channel confluences

The objective of this study was to characterize the influence of sediment discharge on the morphology of movable bed, asymmetric, open-channel confluences. For this purpose, fifteen experiments were performed on rough turbulent flows through an experimental confluence, by running a single combination of confluent discharges and using a single bed material, with three different sediment loads in the main channel combined with five different sediment loads in the tributary channel. The experiments were run until equilibrium was reached. Physical interpretations of the influence of the sediment amount fed into the system on the most important variables and parameters of the confluent flows and confluence morphology were provided. Morphological variables considered included the height of the tributary mouth bar, the depth of the scour hole located in the vicinity of the tributary mouth, the deviation angle of the tributary flow and the dimensions of the bank-attached bar. The hydrodynamic parameters of the confluent flows studied included the unit momentum flux, the unit stream power or the ratio of confluent unit momentum fluxes. The crucial importance of the sediment load entering the system was highlighted since it was documented that i) both channels upstream of the confluence adapted their respective bed slope, flow depth and average flow velocity to respond to the independent sand amounts entering the system; ii) the characteristic dimensions and shape of the most important confluence features changed consistently with the imposed changes on the sediment discharge ratio and concentration; iii) for a given water discharge ratio, the unit momentum flux ratio depended on the sediment amounts flowing through the confluence instead of being an independent parameter. (C) 2020 Published by Elsevier B.V.