Despite the existing knowledge concerning the hydrodynamic processes at river junctions, there is still a lack of information regarding the particular case of low width and discharge ratios, which are the typical conditions of mountain river confluences. Aiming at filling this gap, laboratory and numerical experiments were conducted, comparing the results with literature findings. Ten different confluences from 45 • to 90 • were simulated to study the effects of the junction angle on the flow structure, using a numerical code that solves the 3D Reynolds Averaged Navier-Stokes (RANS) equations with the k-turbulence closure model. The results showed that the higher the junction angle, the wider and longer the retardation zone at the upstream junction corner and the separation zone, and the greater the flow deflection at the entrance of the tributary into the post-confluence channel. Furthermore, it was shown that the maximum streamwise velocity does not necessarily increase with the junction angle and that it is not always located in the contraction section.
This study examines the hydrodynamics and morphodynamics of open‐channel confluences characterized by subcritical flow, low flow discharge ratio, a 90° junction angle and dominant sediment supply coming from the tributary. The analysis is based on a novel data set comprised of the three‐dimensional time‐averaged velocity field, turbulence, bed morphology, and water surface levels acquired under controlled conditions in a laboratory setup. Confluence morphology at equilibrium stage was characterized by moderate bed discordance and pronounced bed erosion at the junction. This study suggests an updated conceptual model of flow hydrodynamics in realistic morphologically stable confluences. The near‐bed flow from the main channel proceeds underneath the tributary inflow into the tributary where it engages in the rotational motion of a strong secondary circulation observed within and downstream of the junction. The high‐speed near‐bed flow intensifies the stretching of the secondary cell past the downstream junction corner where it expands and aligns with slope of the bank‐attached bar. It is argued that the post‐confluence secondary cell is a result of flow separation at the tributary‐mouth bar and of the curvature of the shear layer, configuring a Prandtl’s first kind of helical flow.
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