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 m and Lw = 0.45 m) and the "Large" configuration (Bw = 0.45 m and Lw = 0.60 m). Each experiment was conducted under steady flow conditions in the main and tributary channels and with a steady supply of poorly sorted sediments (d50 = 0.82 mm and d90 = 5.7 mm) at the tributary. There is no sediment transport in the main channel upstream of the confluence. All experiments were conducted until the equilibrium conditions were reached. The duration of the tests varied between 22 and 24 hours. Measurements of the three-dimensional velocity field, turbulence, bed material grain size distribution and morphology as well as observations of the sediment transport in the confluence flume for the reference case revealed that the hydro-morpho-sedimentary processes occurring in alpine confluences differ fundamentally from existing conceptual models of confluence morphodynamics. Therefore, a conceptual model for the main hydraulic and morphological processes occurring in confluences with characteristics similar to those found in alpine environments is proposed. Bed morphology in alpine confluences is characterized by the presence of a significant deposition bar downstream of the confluence. Differences between the water depths in the tributary and in the main channel induce the formation of large bed discordance between the confluent channels. Moreover, no considerable scour hole is observed. Regarding the flow, the tributary momentum input, associated with the presence of the deposition bar resulted in a considerable mass redistribution in the confluence zone as the main flow is deviated towards the outer bank. The main channel flow is hardly hindered by the tributary in the lower part of the water column, giving rise to a two-layer flow structure at the tributary mouth. The two-layer flow plays an important role in dampening the formation of a flow recirculation zone downstream of the confluence. The deposition bar downstream from the confluence reduces the flow area and causes flow acceleration. The sediment supplied by the tributary is mainly sorted and transported on the face of the deposition bar. The sediment transport capacity is further increased by the three-dimensionality of the flow. It is characterized by maximum velocities occurring near the bed and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary. Laboratory experiments revealed that the local widening of tributaries creates a pronounced heterogeneity in the sediment substrate, flow velocities and flow depths, without having any adverse effects on flood safety in the confluence zone. Although the local tributary widening allows a reduction in the confluence angle, it locally amplifies the hydro-morpho-sedimentary processes in the confluence zone. This is due to the reduction of the effective flow area in the local tributary widening, resulting in locally increased tributary velocities and momentum flux. This reduction of the effective flow area occurs due to a general rise of the bed elevation and by a lateral constriction of the flow induced by a zone of flow stagnation at the upstream confluence corner. Flow coming from the tributary remains in the upper part of the water column in the main river and it is considerably more directed outwards than flow in the lower part of the water column coming from the main channel. The increased tributary velocities lead to increased bed discordance and a higher tributary penetration in the confluence zone. Despite the different morphodynamic responses at the widened zones depending on the geometry and the discharge ratio, the local tributary widening always enhance the variability of flow depths, bed constitution and flow velocities without causing adverse effects on the morphodynamics of the tributary and main channels in equilibrium conditions. Furthermore, the lateral freedom obtained by the local widening associated to the different combinations of the main and tributary discharge events further allows the formation of different bed forms, which contributes to the improvement the lateral connectivity of regulated networks. Therefore, local tributary widening can be considered as an efficient solution for increasing the ecological potential of fluvial systems without reducing the conveyance capacity of a given network.