Morphodynamics in river confluences

Within the fluvial network, confluences are considered particular areas in what concerns ecological connectivity, flood safety and water quality. These characteristics were degraded in many cases, by previous channelization works, which resulted in rivers with quasi-homogeneous hydro and morphodynamic conditions. To rehabilitate such impoverished ecosystems, it is essential to deepen the knowledge about the confluence morphodynamic processes. These respond to complex and three-dimensional patterns, which are influenced by parameters such as discharge and momentum flux ratios, confluence angle, sediment grain size distribution, and bulk geometry. In addition, the existence of bed discordance between the main channel and the tributary affects the main hydro-morphodynamic features in the confluence. This study analyzes the effects caused on the confluence morphodynamics by discharge and momentum ratios, confluence angle, grain size distribution, ratio of tributary to main-channel width (Bt / Bm), and local widening of the tributary mouth. For that purpose 24 experimental tests, divided in 8 sets, were carried out in two laboratory confluences. Three discharge ratios (Qr = 0.11; 0.15; 0.23) were tested in two different confluences in which the ratio of tributary to main-channel width was Bt / Bm = 0.30 for the sets 1 to 6, and it was reduced to 0.15 by doubling the width of the main channel for the sets 7 and 8. In the first two sets, the confluence angle was 90º whereas for the rest of the experiments the angle was 70º. All the experiments were run under mobile bed condition by supplying sediment into both channels at constant but different rates for each flume. For the sets 1 to 4, the supplied sediment were composed by two different poorly shorted mixtures of sand and gravel with high gradation coefficients (σ = 3.50 and 4.15), whereas for the sets 5 to 8 a more uniform sand (σ = 1.40) was supplied in both channels. Bed topography and water level were registered periodically during the tests duration and the bed grain size distribution was analyzed at equilibrium. Differences in bed morphology and hydrodynamics together with patterns of spatial distribution of bed sediment are summarized and discussed in this study.