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Influence of tributary widening on confluence morphodynamics: Communication (Laboratoire de constructions hydrauliques, Ecole polytechnique fédérale de Lausanne)

Confluences are the nodes of the fluvial network. They are typically characterized by highly heterogeneous conditions that are favorable for a sound fluvial ecosystem, and play a key role in the connectivity of the river system. During the past centuries, many rivers and their tributaries have been channelized and regulated, often resulting in quasi-homogeneous flow and morphologic conditions, deficient flood safety, reduced natural dynamics, impoverished ecological value, and reduced connectivity. Marcelo Leite Ribeiro’s PhD research investigated the potential of river rehabilitation by means of a local widening in the tributary of channelized confluences in an alpine environment. Such confluences are characterized by a dominant discharge in the low-gradient main channel, a dominant supply of poorly graded sediment in the steep tributary, and a pronounced bed discordance at the tributary mouth. Due to considerably different confluence characteristics, none of the existing conceptual models can represent the hydro-morphosedimentary processes in this type of confluences. Dr. Leite Ribeiro’s research was performed in a laboratory set-up that is representative of the 20 major confluences on the Rhône River upstream of Lake Geneva, Switzerland. Measurements of the three-dimensional velocity field, turbulence, sediment transport, morphology and bed material characteristics in a wide range of configurations of the local tributary widening and under various hydraulic conditions provided the following results: - A conceptual model has been proposed for the hydro-morpho-sedimentary processes in the investigated type of channelized alpine confluences. This conceptual model explains the adaptation to changing hydraulic conditions and to the local widening of the tributary. - Without having adverse effects on flood safety, the local widening of the tributary was found to considerably enhance the hydraulic and morphologic heterogeneity, which enhances the ecological value. - The validity of this result for all investigated widening configurations indicates that relatively small interventions conditioned by local constraints can considerably contribute to the rehabilitation of confluences in strongly anthropologized environements.

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