Morphodynamics in river confluences

Within the fluvial network, confluences are considered particular areas in what concerns ecological connectivity, flood safety and water quality. These characteristics were degraded in many cases, by previous channelization works, which resulted in rivers with quasi-homogeneous hydro and morphodynamic conditions. To rehabilitate such impoverished ecosystems, it is essential to deepen the knowledge about the confluence morphodynamic processes. These respond to complex and three-dimensional patterns, which are influenced by parameters such as discharge and momentum flux ratios, confluence angle, sediment grain size distribution, and bulk geometry. In addition, the existence of bed discordance between the main channel and the tributary affects the main hydro-morphodynamic features in the confluence. This study analyzes the effects caused on the confluence morphodynamics by discharge and momentum ratios, confluence angle, grain size distribution, ratio of tributary to main-channel width (Bt / Bm), and local widening of the tributary mouth. For that purpose 24 experimental tests, divided in 8 sets, were carried out in two laboratory confluences. Three discharge ratios (Qr = 0.11; 0.15; 0.23) were tested in two different confluences in which the ratio of tributary to main-channel width was Bt / Bm = 0.30 for the sets 1 to 6, and it was reduced to 0.15 by doubling the width of the main channel for the sets 7 and 8. In the first two sets, the confluence angle was 90º whereas for the rest of the experiments the angle was 70º. All the experiments were run under mobile bed condition by supplying sediment into both channels at constant but different rates for each flume. For the sets 1 to 4, the supplied sediment were composed by two different poorly shorted mixtures of sand and gravel with high gradation coefficients (σ = 3.50 and 4.15), whereas for the sets 5 to 8 a more uniform sand (σ = 1.40) was supplied in both channels. Bed topography and water level were registered periodically during the tests duration and the bed grain size distribution was analyzed at equilibrium. Differences in bed morphology and hydrodynamics together with patterns of spatial distribution of bed sediment are summarized and discussed in this study.

Hydro-morphodynamic differences induced by different angles in an experimental discordant confluence

Sebastián Guillén Ludeña

Confluences, within the fluvial network, are considered particular areas with great ecological value, where flow dynamics and bed morphology are complex and highly three-dimensional. Several parameters influence the morphodynamics and hydrodynamics of river confluences including discharge and momentum ratios, angle of confluence and sediment transport. Confluences with low discharge and momentum ratios, where narrow steep tributaries with high sediment load join a wide low-gradient main-channel that provides the dominant discharge, are often observed in the Upper-Rhone river in Switzerland. Few existing studies have looked at the hydro-morphodynamics of this type of river confluences considering sediment discharge in both confluent channels. This paper aims to analyze the influence of the confluence angle on the flow dynamics and bed morphology of this type of confluences in an experimental facility under movable bed conditions. For that purpose, two experiments were carried out in a laboratory confluence with low discharge and momentum ratios (Qr = 0.11 and Mr = 0.16 respectively), and with two different confluence angles (α = 90° and 70°). During the experiments, two distinct poorly shorted sediment mixtures were supplied into the tributary and into the main channel at constant but different rates for each flume. The experiments were run until equilibrium in bed morphology was reached. During the experiments bed topography surveys were systematically recorded, and water surface was measured at equilibrium. These measures illustrate the bed topography evolution during the experiments, and the confluence hydrodynamics at equilibrium. The results show that different confluence angles resulted in different flow regimes and bed elevations in the tributary. Also, the distinct angle configurations yielded different water surface elevations in the main channel upstream of the confluence.

2015

Influence of Tributary Widening on Confluence Morphodynamics

Marcelo Leite Ribeiro

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.

EPFL2011

Hydromorphological implications of local tributary widening for river rehabilitation

Koen Blanckaert, Marcelo Leite Ribeiro, André Roy, Anton Schleiss

The hydromorphological implications of the local widening of a tributary where it enters a confluence were investigated in a laboratory setting that is representative of the 20 major confluences on the channelized Upper Rhone River. Although local tributary widening reduces the confluence angle, it amplifies the hydromorphosedimentary processes in the confluence hydrodynamic zone (CHZ), because local widening reduces the effective flow area, causing increased tributary velocities and momentum flux. The reduction in effective flow area is caused by an increase in bed elevation and by lateral constriction of the flow induced by flow stagnation at the upstream corner of the confluence. The increased tributary velocities amplify the two-layer flow structure in the CHZ. Flow originating from the tributary is confined to the upper part of the water column and is more markedly directed outward than flow in the lower part of the water column originating from the main channel. A shear layer characterized by increased turbulence activity develops at the interface between the two flow layers. The increased tributary velocities enhance bed discordance, the penetration of the tributary into the CHZ and the channel bed gradients in the postconfluence channel. The results indicate that local tributary widening can enhance heterogeneity in sediment substrate, flow velocities and flow depths. Widening may therefore enhance local habitat and improve the connectivity of the tributary to the main river network. This may, in turn, provide favorable conditions for the improvement and reestablishment of ecological river functions, without having adverse impact on flood safety.