Influence of Tributary Widening on Confluence Morphodynamics

iv alpine river confluences. Systematic laboratory experiments were performed in a confluence flume where the main channel is 8.5 m long and 0.50 m wide. A 4.9 m long and 0.15 m wide tributary channel is connected at an angle of 90°. Three discharge scenarios were tested for four different geometrical configurations: a reference case (without widening) and three different tributary widenings: the “Small” configuration, (Bw = 0.30 m and Lw = 0.45 m, the “Medium” configuration (Bw = 0.45 m and Lw = 0.45 m) and the “Large” configuration (Bw= 0.45 m and Lw = 0.60 m). Each experiment was conducted under steady flow conditions in the main and tributary channels and with a steady supply of poorly sorted sediments (d50=0.82 mm and d90 = 5.7 mm) at the tributary. There is no sediment transport in the main channel upstream of the confluence. All experiments were conducted until the equilibrium conditions were reached. The duration of the tests varied between 22 and 24 hours. Measurements of the three-dimensional velocity field, turbulence, bed material grain size distribution and morphology as well as observations of the sediment transport in the confluence flume for the reference case revealed that the hydro-morpho-sedimentary processes occurring in alpine confluences differ fundamentally from existing conceptual models of confluence morphodynamics. Therefore, a conceptual model for the main hydraulic and morphological processes occurring in confluences with characteristics similar to those found in alpine environments is proposed. Bed morphology in alpine confluences is characterized by the presence of a significant deposition bar downstream of the confluence. Differences between the water depths in the tributary and in the main channel induce the formation of large bed discordance between the confluent channels. Moreover, no considerable scour hole is observed. Regarding the flow, the tributary momentum input, associated with the presence of the deposition bar resulted in a considerable mass redistribution in the confluence zone as the main flow is deviated towards the outer bank. 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. The two-layer flow plays an important role in dampening the formation of a flow recirculation zone downstream of the confluence. The deposition bar downstream from the confluence reduces the flow area and causes flow acceleration. The sediment supplied by the tributary is mainly sorted and transported on the face of the deposition bar. The sediment transport capacity is further increased by the three-dimensionality of the flow. It is characterized by maximum velocities occurring near the bed and by a considerable increase in turbulent kinetic energy generated in