Three-dimensional flow structure, morphodynamics, suspended sediment, and thermal mixing at an asymmetrical river confluence of a straight tributary and curving main channel

Abstract

Past field studies of confluence dynamics have focused mainly on experimental or small stream confluences and confluences of large rivers. Few studies have explored fluvial processes and forms at confluences of medium-size rivers, especially those where river planform varies from a straight alignment. This study examines changes in flow structure, bed morphology, sediment suspension, and mixing with changing incoming flow conditions at a river confluence where a straight tributary joins a curving main channel at the upstream end of a bend. Data include five sets of field measurements of three-dimensional velocity components, bed topography, acoustic backscatter intensity, and water-surface temperatures at the confluence of the curving Wabash River and its straight tributary, the Embarras River. Results show that, in contrast to large river confluences, channel-scale helical motion is a prominent feature of flow structure at this confluence. Dual counter-rotating helical cells, separated by a distinct shear layer, develop over a region of scour within the confluence and contribute to maintenance of scour by sweeping sediment away from this region. Helical motion develops through channel curvature and flow deflection within the confluence. The location and geometry of the scour hole are subject to change depending on the relative momentum fluxes of the two rivers with the zone of scour shifting outward and enlarging as the momentum flux of the tributary increases relative to the momentum flux of the main river. Patterns of backscatter intensity, used here as a surrogate for suspended sediment concentration, confirm that near-bed suspended sediment moves around the scour hole. The thermal mixing interface between the two rivers corresponds to the position of the shear layer, and thermal mixing at the surface within the immediate vicinity of the confluence is limited. The hydrodynamics, morphodynamics, and patterns of sediment suspension and thermal mixing at this river confluence reflect a combination of confluence and bend dynamics.