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Hydro-morphodynamics of open-channel confluences with low discharge ratio and dominant tributary sediment supply

River confluences are not only hot spots of diversity but also main elements for the connectivity in water catchments. In the past century many of the Alpine rivers in Europe as well as their tributaries have been channelized. Especially the confluences have often been transferred to rigid channels. Even sills were installed between the tributary and the main river. The result is that the connectivity of the river system was severely interrupted. In order to restore these confluences, its hydro-morphosedimentary processes have to be known. Alpine confluences have steep tributaries with a high supply of coarse sediment, which strongly influences the hydro-morpho-sedimentary processes. They differ fundamentally from existing conceptual models of confluence dynamics mainly developed on lowland river confluences. In his research project Mr. Dr. Sebastian Guillen conducted for the first time a systematic experimental study on hydro-morphodynamic processes in alpine confluences with simultaneous sediment supply in the tributary and in the main channel. The candidate studied systematically the influence of two junction angles, namely 90° and 70°, as well as two different grain size distributions, with high and low gradation coefficients and the same median diameter. It could be observed that with continuous sediment supply into the main channel, deeper scour along the bank-attached bar was observed since the armouring of the bed was not easily possible. Dr. Guillen could also highlight that increasing discharge ratios produce deeper scour at the tributary mouth. With non-uniform sediments, the size of the bank-attached bar decreased with increasing discharge ratio. On the other hand the bank-attached bar was larger with increasing discharge ratio when uniform sediments were used in the tests. Furthermore Dr. Guillen demonstrated that lower junction angle (α = 70°) facilitates the penetration of the tributary-mouth bar and the delivery of tributary sediment load into the main channel. This deeper penetration of the tributary-mouth bar with α = 70° reduced the erosion at the tributary mouth and enlarged the bank-attached bar compared to α = 90°. When using uniform sediments in the experiments it could be also shown that the bankattached bar and the scour hole are strongly under-reproduced as compared to natural confluences, where a wide grain size distribution is normally predominant.

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