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

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.

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.

Elsevier2016

Analyse des erreurs dues à l'écoulement dans la mesure de débit par la méthode acoustique

The precise flow-rate measurement is a basic problem in the management and the exploitation of a hydraulic power-plant. The efficiency characterisation of an hydraulic machinery or of a water distribution net is linked to the accurate knowledge of the flow-rate. As well as the usual measurement methods, acoustic discharge measurement has become of great importance during the last ten years, specially due to the last development in the field of electronic and signal processing. The acoustic flow measurement uncertainties show that geometry, integration method, influence of the flow field and the interaction between the acoustic transducer and the pipe have an important influence on the measurement accuracy. In this work, the influence of the flow field on the accuracy is of great importance. To estimate the uncertainties of the acoustic discharge measurement for disturbed flow fields, laboratory tests are carried out at the hydraulic machinery laboratory LMH in order to qualify the measurement accuracy depending on the upstream distortion induced by different typical pipe configurations. Furthermore, numerical flow computations with software packages are performed to simulate the turbulent flow profiles at the location of the acoustic flow measurement. These simulations are performed for laboratory and also industrial flow configurations. Comparative discharge measurements for typical disturbed flow fields are performed on a special test rig of LMH between a reference electromagnetic flow meter and an 8 paths in two crossed planes acoustic flow-meter. The accuracy analysis, based upon all laboratory measurements shows that globally the measurement quality is excellent even in the case of a heavily disturbed flow field in the meter section. A specific analysis has to be performed at the flow-rate measurement installation in order to take into account the pipe configuration influence (for example an elbow or a gate), the developing length between the meter section and the disturbance and finally the use of crossed planes. Such analysis allows the reduction of the acoustic measurement uncertainties. The protrusion effect can also introduce an important measurement error in the acoustic measurement. Laboratory measurements in the high speed cavitation tunnel of LMH show clearly the influence of the protrusion effect. In such small dimensions in comparison with the industrial pipes, these effects are amplified and they introduce important errors. The second part of this work is based on the simulation of the velocity profiles at the location of the metering section by numerical flow computations. Comparisons between numerical results and Laser Doppler flow measurements show that the simulation and the flow physics results are in agreement with each other. Based on these convincing results, numerical flow computations are performed for an industrial power-plant in order to obtain an approximation of the flow profile in the metering section. Based on these results, an optimisation of the acoustic path installation is proposed in order to limit the uncertainties due to the flow-field in the pipe. This study is applied to the laboratory configurations and allows one to find the optimum measured orientations of the acoustic paths. Finally, this study also applies to an industrial installation.

EPFL1997

Probabilistic Prediction and Forecast of Daily Suspended Sediment Concentration on the Upper Yangtze River

José Pedro Gamito de Saldanha Calado Matos, Mário Jorge Rodrigues Pereira Da Franca

Sediment transport in suspension can represent more than 90% of a river's total annual flux of sediment. In the case of the Yangtze River, more than 99% of the sediment supplied to the sea is suspended load. Suspended sediment is thus an important component of the total sediment load, with implications for channel dynamics, landscape evolution, ecology, and human-related activities. For hydrological management of large basins such as the Yangtze River, knowledge of the processes governing suspended sediment concentration (SSC) is essential. An analysis of the temporal variation of SSC for the Upper Yangtze basin (defined at Pingshan station) is presented here. For this purpose, a database of 50years of concurrent discharge and SSC measurements, made by the Yangtze River Commission, is used. The analysis is made using a novel probabilistic data-driven technique, the Generalized Pareto Uncertainty (GPU). This technique allows for the testing of several strategies of prediction and forecast applied to a time series of SSC and streamflow. Changing between local or seasonal variables to feed these strategies, we inferred that although the main driver of the SSC transport is flow (as reported by previous authors), sediment storage is also a major control. Furthermore, the maximum necessary time lag for forecasts made with the data is on the order of one week, which provides one indication of the time scale of the local processes of SSC transport in the Upper Yangtze. In this paper, limitations and data requirements of the GPU methodology are also discussed. Plain Language Summary In this manuscript we use a probabilistic technique to study what controls the dynamics of the sediments in suspension in Upper Yangtze River. For that, a database of 50 years of measurements of streamflow and sediments in suspension collected at Pingshan station by the Yangtze River Commission was analyzed with a new technique. Sediments in suspension represent the major portion of total sediment load in most river systems, and the Yangtze River is no exception. Fine sediment dynamics is an important component of many physical, chemical, and biological processes in rivers.

AMER GEOPHYSICAL UNION2018