A method for assessing the feasibility of air-bubble screens to reduce morphological gradients in open-channel bends

Within the fluvial network, confluences are particular areas characterized by great ecological value where flow dynamics and bed morphology are much influenced by local patterns. The aim of this article is to describe the influence of the convergence angle on the morphology and hydrodynamics at river channel confluences, where the tributary bed level is higher than the main channel bed (discordant bed). For that purpose, experiments were carried out in a laboratory flume running three discharge ratio scenarios for two different convergence angles (70 and 90 degrees). The tests were run until equilibrium was reached, i.e. when the outgoing solid discharge was equal or larger than 90% of the incoming. Once the bed topography remained stable, bed and water level surfaces were measured. As a result of these tests, and based on the performed measurements, the convergence angle is identified as an important parameter that influences the main-channel bed morphology features and the water level by modifying the shape and position of the main morphological and flow features. Also, the influence of the discharge ratio (Qr = Qt / Qm) on these modifications is observed and evaluated.

Influence of the convergence angle on the morphodynamics of discordant bed confluences

Sebastián Guillén Ludeña, António Heleno Cardoso, Anton Schleiss

TPU2013

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

Sebastián Guillén Ludeña

River confluences in which the tributary supplies the dominant sediment load, and the flow discharge is abundantly provided by the main river, are typically observed in mountain-river basins. The existent knowledge on the hydrodynamic, morphodynamic and sedimentary processes involved in mountain-river confluences is sparse, since most of the studies on confluence dynamics focus on low land confluences. In this context, the present research study aims to deepen the knowledge on the hydro-morphodynamics and sedimentology of mountain-river confluences inspired in those of the Upper-Rhone river basin. For that purpose twelve laboratory experiments were conducted in two experimental facilities: one located at the Laboratory of Hydraulic Constructions of the École Polytechnique Fédérale de Lausanne (LCH-EPFL), and the other one located at the Instituto Superior Técnico de Lisboa (IST-UL). The experimental setup covered a wide range of configurations including three unit-discharge ratios, two junction angles, two sediment mixtures with different gradation coefficients, and two width ratios. The experiments were performed under movable bed conditions and with continuous sediment supply to both flumes, which represents a novelty in the study of this type of confluence. Systematic surveys of bed topography and water surface were recorded at different instants during the experiments and at equilibrium condition. Additionally and also at equilibrium, point velocities were measured and the spatial and grain size distribution of the bed sediments were analyzed. These measurements allowed the analysis of the influence of the discharge ratio, junction angle, sediment gradation and width ratio on the hydro-morphodynamics of open channel confluences. The bed morphology and hydrodynamics displayed some common features that were influenced by the variation of the tested parameters. The most significant morphological features were a marked discordance between the bed elevations of the tributary and main channel, a steep bed slope in the tributary, a bank-attached bar along the inner bank of the main channel and a scour hole that extended from the tributary mouth flanking the bar. The hydrodynamics were mainly characterized by a double flow deflection to the outer bank of the main channel by the tributary inflow and the bank-attached bar, flow acceleration downstream of the confluence, a shear layer that separated each confluent flows, and eventually supercritical flow regime in the tributary depending on the parameter setup. In summary, this research study presents a broad analysis of the hydrodynamic, morphodynamic and sedimentary processes involved in confluences characterized by low discharge ratios, and where the dominant sediment load is supplied by the tributary. This analysis contributes to widen the existent knowledge on the dynamics of this type of confluence and constitutes a benchmark for further studies, as well as a valuable tool for restoration projects.

EPFL2015

Evolution of the hydromorphodynamics of mountain river confluences for varying discharge ratios and junction angles

Sebastián Guillén Ludeña, António Heleno Cardoso, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

Mountain river confluences are characterized by narrow and steep tributaries that supply abundant sediment load to a main channel that, in turn, provides the dominant flow discharge. In addition, bed sediments consist of poorly sorted mixtures that promote bed armoring. The knowledge of the hydrodynamics and morphodynamics of mountain river confluences is sparse because most of the existent studies on confluence dynamics focus on lowland confluences. This study aims at examining the influence of the junction angle (a) and discharge ratio (Q(r) = Q(t)/Q(m)) on flow dynamics and bed morphology of mountain river confluences. This study presents the results of six laboratory experiments in which three discharge ratios were tested (Q(r) = Q(t)/Q(m) = 0.11, 0.15, 0.23) with two different junction angles (alpha = 90 degrees and 70 degrees). The experiments were conducted under movable bed conditions and with continuous sediment supply to both flumes. Measurements consisted of systematic bed topography and water surface surveys performed at different instants during the experiments and at equilibrium, i.e., when the outgoing sediment rate coincided with the incoming and bed topography reached a steady state. The results show that the discharge ratio and the junction angle parameters are major controls of the dynamics of mountain river confluences. Also, the evolution of bed morphology and flow dynamics for varying junction angles and discharge ratios present some patterns that contrast with those reported for lowland confluences. Among these patterns are the different flow regimes adopted by the tributary for different junction angles and the decrease of the height of the bank -attached bar for increasing discharge ratios. Moreover, results show that the abundant sediment load of the tributary plays a major role on the dynamics of this type of confluence. This load resulted in a marked bed discordance that, in turn, influenced flow dynamics and bed morphology of the confluence. (C) 2015 Elsevier B.V. All rights reserved.