Flow and sediment dynamics in channel confluences

Confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large, low‐gradient main channel commonly occur in nature, but they have not yet been investigated. Measurements of the three‐dimensional velocity field, turbulence, sediment transport, bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the Upper Rhone River, Switzerland. The difference between the low‐flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone. Due to this bed discordance, the tributary flow penetrates into the main channel mainly in the upper part of the water column, whereas the main‐channel flow is hardly hindered by the tributary in the lower part of the water column, giving rise to a two‐layer flow structure in the confluence zone. In confluences with high supply of coarse sediment from the tributary, the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity. The sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near‐bed flow originating from the main channel. The sediment transport capacity is further increased by the three‐dimensionality of the flow, which is characterized by maximum velocities occurring near the bed, and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary. A conceptual model is proposed for the hydro‐morpho‐sedimentary processes, and compared to existing conceptual models for confluences with different characteristics.