Interaction between hydrodynamic and morphodynamic processes at the confluence of pipe and channel

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Confluences between a pipeline and a channel are commonly found in urban drainage networks. The mechanisms of sediment transport and the coherent structure within the confluence zone remain ambiguous. The hydro-morpho-sedimentary processes at pipe and channel confluences, which are influenced by discharge ratios, are investigated using a laboratory-scale confluence. The pipe current compresses and diverts downstream upon entering the main channel, creating a scour hole that carries sediment downstream. Coarser sediment deposits first, forming the slope of the downstream boundary of the scour hole (D50 = 60.31 μm) and the inner bar (D50 = 62.67 μm). Finer sediments are then deposited in the outer bar (D50 = 53.12 μm), resulting in a bed morphology consisting of the scour hole, outer and inner bars, and corridor. The penetration of the pipe current causes redistribution of the main channel flow, creating two shear layers. The height of the bar is directly related to the intensity of turbulence. Within the range of discharge ratios from 0.06 to 0.09, the shear layers and bars are displaced toward the outer bank by a distance of approximately one pipe diameter (0.05 m). Turbulence in this area is not isotropic, with power-law relations for streamwise and lateral velocity having an exponent of approximately −5/3. Conversely, the separation zone deviates from this pattern. Overall, this study highlights the significant influence of the bed morphology in modifying the flow structure compared to channel confluences and jets into crossflow, which are important in drainage engineering designs and fluvial studies.