Hydrodynamic processes, sediment erosion mechanisms, and Reynolds-number-induced scale effects in an open channel bend of strong curvature with flat bathymetry

Sharply curved open channel flow with a flat bed is investigated with eddy-resolving numerical simulations that complement laboratory experiments. The focus is on the role of coherent flow structures, how these structures contribute to shear stresses and the capacity of the flow to pick up sediment at the boundaries, and on changes resulting from increasing the Reynolds number between typical values for laboratory model studies and for field conditions. In sharply curved bends, secondary flow leads to a transverse component of the bed shear stress that is of comparable magnitude as the streamwise component. Just downstream of the bend entrance, the locus of highest velocities migrates outward and separates from the inner bank. A highly energetic thin shear layer containing large-scale eddies develops at the interface between the core of high streamwise velocities and the retarded fluid moving close to the inner bank. Highly energetic Streamwise-Oriented Vortices (SOVs) develop in the zone of retarded flow. Turbulence, the boundary shear stress, and the sediment pickup capacity are considerably increased by the SOVs and the large-scale eddies inside the shear layer. These large-scale turbulent structures are amplified and become more coherent with increasing Reynolds number. The results indicate that flow processes in scaled laboratory flumes and natural rivers are qualitatively similar, although some quantitative Reynolds-number-induced scale effects exist. The paper also discusses application of several improved methods to estimate mean sediment pickup rates for flow in sharply curved bends. Such methods try to account in an approximate way for the effects of large-scale turbulence in numerical simulations that do not resolve these structures. Key Points Reynolds number affects coherent structures and boundary stresses Streamwise oriented vortical cells affect distribution of boundary stress Sediment entrainment is underpredicted if effect of large eddies is neglected