EFFECT OF THE BED LOAD GRADATION ON THE MORPHODYNAMICS OF DISCORDANT CONFLUENCES

Within the fluvial network, river confluences are particular areas characterized by complex hydrodynamic, morphodynamic and sedimentary processes. These processes have been observed to be governed by parameters such as the discharge ratio, the junction angle, the sediment rates and the bed material. This study analyzes the influence of the sediment gradation on the hydro-morphodynamics of open channel confluences, characterized by a bed discordance between the tributary and main channel. For that purpose, experiments are conducted at two different laboratory confluences in which only the gradation of the supplied sediment mixtures is different. This paper presents the results of two of these experiments with a discharge ratio between the tributary and main channel of Qr = Qt/Qm = 0.15. In one experiment, non-uniform sediment mixtures with a mean diameter of d50 = 0.82 mm are supplied to the main channel and tributary. These mixtures are representative of the sediments found in mountain river confluences such as those of the Upper-Rhone River. In the other experiment, a uniform sediment mixture with the same mean diameter (d50 = 0.82 mm) is supplied both to the tributary and main channel. The latter mixture is rather representative of the bed material found in low-land confluences. At equilibrium conditions, i.e. when the outgoing sediment rate is nearly equal to the incoming, bed and water surface topographies are recorded. These measurements show that with non-uniform sediments, the bed morphology at equilibrium presents a high topographic gradient, with a developed bar and scour hole in the main channel, a marked bed discordance and a steep bed-slope in the tributary. In contrast, with uniform sediments, the bed morphology presents attenuated features compared to those observed with non-uniform sediments. Also, with uniform sediments bedforms are observed throughout the flumes, whereas the non-uniform mixtures favor the bed armor.