Influence of Tributary Widening on Confluence Morphodynamics

In alpine regions of Europe, river training works were typically the reason for the transformation of wide and braided rivers into linear river systems with a lack of structural diversity, i.e. gravel banks, islands, woody debris, riffles or pools. These interventions considerably impoverished river ecosystems. From the end of the 20th century, "river rehabilitation" has been a concept commonly used by environmental professionals and river management authorities. The rehabilitation purpose is to recover the vital space required for the rivers that were degraded by human interventions and to link the sustainable use of rivers and wetlands with human well-being. Furthermore the flood safety has to be adapted to the higher hydrological risk resulting from increased urbanization. A good understanding of the flow dynamics, sediment transport patterns and of the development of the bed morphology is essential to successfully accomplish river rehabilitation projects. Alpine confluences are typically characterized by steep gravel-bed streams carrying large sediment loads, which often connect asymmetrically at large angles with the main river. Such zones present important challenges, not only for flood protection but also for rehabilitation works. Current knowledge of river channel confluences, mainly based on lowland confluences is not applicable to alpine conditions. The morphodynamics of confluences have been experimentally investigated with special attention to the potential of local tributary widening in the framework of confluence rehabilitation projects. Local widening of the tributary in the confluence zone aims to increase the heterogeneity in sediment substrate, flow depth and flow velocity, which is favourable for in-stream habitat (e.g. aquatic invertebrates, fish, and vegetation) and for the connectivity between the main river and the tributary. Zones of quiescent water (flow stagnation or flow recirculation zones) may play an important role as refuges during flood events. Moreover, a local tributary widening can create a riparian zone which favours the diversity of plants and animals (e.g. birds, mammals, insects, amphibians). Obviously, river rehabilitation by means of local tributary widening is only feasible if it has negligible adverse effects on the flood safety of the confluence zones. The experimental set-up and the test configurations are based on the analysis of the Upper Rhone River, in Switzerland, which can be considered as representative of regulated 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