Hydro‐Morphodynamics of an Open‐Channel Confluence With Bed Discordance at Dynamic Equilibrium

Abstract

AbstractThis 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.