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

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.

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

Sediment Suspension Dynamics in Turbulent Unsteady, Depth-Varying Open-Channel Flow over a Gravel Bed

Fereshteh Bagherimiyab Hunkeler

Even though flow in natural rivers and channels is generally unsteady, only a few studies on turbulent structures in unsteady open-channel flows have been carried out. In hydraulic engineering problems, unsteady flow is often approximated with concepts of steady flow, because the treatment of unsteady flow can be difficult. Many variables enter into the mathematical relationship and the differential equations cannot be integrated in closed form, except under very simplified conditions. The limited number of studies of unsteady flow may also be due to the lack of experimental equipment capable of capturing small-scale, unsteady open-channel flow dynamics. Therefore, it is important to know when unsteady flow can be approximated by steady flow concepts and which simplifications are acceptable. In most cases, especially when simulating unsteady flood flow by a steady flow approach, the calculations may not produce reliable results. The mechanism of sediment transport in rivers and open channels is governed by complicated interactions between unsteady accelerating and decelerating turbulent flow, particle motion and bed configuration. Understanding the dynamics of unsteady sediment-laden water flows and characterizing the velocity of suspended particles is essential for enhancing the predictive accuracy of sediment transport and its impact on environmental processes in the water column. In order to simulate fine sediment dynamics over an armored bed in a river during the passage of a flood wave, unsteady accelerating, and decelerating open-channel flow over a movable (but not moving) coarse gravel bed (D50 = 5.5 mm) first without and then with fine sediment were studied. A layer of fine sediment of mean particle size about 120 µm was placed on the coarse gravel bed. The thickness of the fine sediment layer on the gravel bed was varied between 4 mm and 6 mm, but it was found that the thickness of the layer had no effect on the results. Quasi-instantaneous profiles of velocity and sediment concentration were taken simultaneously and co-located. An acoustic Doppler and imaging method, using an Acoustic Doppler Velocity Profiler (ADVP) was combined with an optical method, using Particle Tracking Velocimetry (PTV) for suspended sediment particle tracking. Measurements resolved turbulence scales. Unsteadiness strongly affects the profile shape of velocity and friction velocity, particularly in the final phase of the accelerating range. Flow in the decelerating range approaches steady flow. Systematically higher friction velocities were observed in the accelerating flow than in the decelerating flow for comparable flow depth. This indicates that for the same change of relative submergence, different flow dynamics are generated during accelerating and decelerating flows. For the lowest unsteadiness (90 s hydrograph), the differences between velocity profiles in the accelerating and the decelerating ranges become small, indicating that for this unsteadiness, steady state conditions are approached. During the accelerating flow range, fine sediment suspension from the bed started in bursts and in the final phase of the accelerating flow range, a ripple pattern is rapidly created that remained nearly stationary. Thereafter, vortex shedding produced most of the sediment suspension into the water column in the form of events, making suspension intermittent. Simultaneously, sediment particles rolled along the bed following the ripple structure, thus slowly advancing the ripple pattern in the direction of the flow. However, ripple geometry and ripple shape were not altered by this process, despite the fact that flow velocities changed. Due to the ripple structure, high sediment suspension events continued to occur in bursts during the decelerating flow even though mean flow velocity and friction velocity decreased. The dynamics of sediment suspension observed in this study indicate that mean value concepts cannot be applied in unsteady flow. Fine sediment particles and hydrogen bubbles were used individually and combined as flow tracers in the acoustic measurements. When used individually, hydrogen bubbles provided full depth flow and backscattering information, whereas sediment particles traced only the lower layers of the flow, indicating sediment suspension. When both tracers were combined, hydrogen bubbles could not be distinguished from sediment particles. The intermittency was observed in the backscattering of the acoustic system. The event structure in fine sediment suspension is seen by the PTV method. PTV velocity vectors varied in speed and orientation, but were organized in large coherent packets, mainly in the near-bed layers. They also extended well above the bed, supporting the concept that coherent structure events contribute to sediment suspension over ripples. The two methods provide complementary information. ADVP measurements allow long timeseries analysis, whereas the spatial details seen in the PTV results cannot be resolved in the ADVP measurements.

EPFL2012

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

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

Sebastián Guillén Ludeña

EPFL2015

Sediment Suspension Dynamics in Turbulent Unsteady, Depth-Varying Open-Channel Flow over a Gravel Bed

Fereshteh Bagherimiyab Hunkeler

EPFL2012

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

Sebastián Guillén Ludeña

2015