Flow and sediment dynamics in bed discordant channel confluences

Confluences between small steep tributaries with dominant supply of poorly sorted sediment and larger main channels with dominant flow supply, which are characterized by a pronounced bed discordance, have not yet been considered in the literature. The hydro-morpho-sedimentary processes in such confluences are not well described by existing conceptual models of confluence dynamics. Examples of such confluences on the Upper Rhone River, Switzerland, served as prototype for the reported laboratory experiments. Based on detailed measurements of the morphology, the sediment size, the sediment transport, and the three-dimensional flow field in a laboratory experiment, Leite Ribeiro, et al. (submitted) have proposed a conceptual model for the hydro-morpho-sedimentary processes in the investigated type of confluences. According to this model, the pronounced bed discordance is essentially due to the difference between the low flow depth in the steep tributary and the higher flow depth in the main channel. The tributary flow penetrates into the main channel mainly in the upper part of the water column. Due to the bed discordance, 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 at the tributary mouth. In confluences with dominant sediment supply from the tributary, the development of a deposition bar downstream of the confluence reduces the flow area and causes a flow acceleration that contributes to the required increase in sediment transport capacity. The sediment supplied by the tributary is mainly sorted and transported on the sloping face of the depositional bar. The sediment transport capacity is further increased by the three-dimensionality of the flow, which is characterized by maximum velocities occurring near the bed. The present paper reports data on the morphology and flow visualizations for two additional hydrological scenario, characterized by different flow ratios, but an identical sediment supply in the tributary. An increase in the ratio of tributary discharge to main-channel discharge led to an attenuation of the morphological features in the confluence zone: (i) the bed discordance was less pronounced; (ii) the tributary penetration was reduced; (iii) the volume of sediment accumulated in the depositional bar just downstream of the confluence decreased. Leite Ribeiro et al. „s (submitted) conceptual model is able to explain these morphological adaptations, which lends credit to the robustness of their model.