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

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

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

Flow resistance for a varying density of obstacles on smooth and rough beds

High-return-period floods involve complex flows over large floodplains covered with different types of resistive elements, from bed roughness to large emerging obstacles. In order to characterize the flow resistance in such configurations, experiments are conducted in a flume with emergent aligned obstacles on smooth and rough beds. Under uniform flow conditions, the drag force is measured by a hydrodynamic force measurement device, thereby obtaining the relative contribution of the drag and bed friction forces to the total flow resistance. The results show a strong correlation between the reference surface ratio (λf), i.e., the obstacle frontal area divided by the cell bed surface, and the relative contribution of the drag to the flow resistance, with negligible influence of bed roughness and shallowness. Equal contributions of bed roughness and form drag to flow resistance occur for λf≈0.02. After showing that the drag coefficient is roughly constant and the bed friction is consistent with Darcy-Weisbach's head-loss coefficient, a Chezy-type equation is proposed to predict the mean uniform velocity U as a function of the flow and land occupation parameters.This project was founded by the French National Research Agency project FLOWRES (ANR-14-CE03-0010)

Extreme flood flow in an increasingly urbanized floodplain: an experimental approach

During extreme flood events, the land occupation in floodplains becomes a major issue as it highly influences the flow resistance and the hydrodynamic processes. The present work aims at identifying the land occupation (number of emerging obstacles) increment on the evolution of the relative magnitude of resistance forces acting on the flow. The resistance forces are those corresponding to the bed friction and to the drag induced by the obstacles. Firstly, we characterize the transition from a flow governed mostly by bed-friction forces to a flow governed mostly by drag forces. Secondly, we quantify the evolution of the bed friction and obstacle drag resistance coefficients as the number of obstacles increases.This project is funded by the French ANR: FLOWRES project (grant ANR-14-CE03-0010). Authors are grateful to C. Marmounier for his participation to the building and design of the hydrodynamic balance

Factores sociodemográficos y gineco-obstétricos asociados a preeclampsia en el marco de la pandemia COVID-19 en mujeres gestantes del servicio de gineco- obstetricia del Hospital María Auxiliadora durante el periodo 2020 - 2021

La preeclampsia afecta al 3-8% de las gestantes y sigue siendo una de las principales causas de morbimortalidad neonatal y materna a corto y largo plazo. Objetivo: Determinar los Factores Sociodemográficos y Gineco-obstétricos asociados a Preeclampsia en el marco de la Pandemia Covid-19 en mujeres gestantes del servicio de Gineco-Obstetricia del Hospital María Auxiliadora durante el periodo 2020 - 2021. Métodos: El diseño de la investigación es de casos y controles. El tamaño de la muestra resultó en 699 gestantes, 233 con Preeclampsia y 466 sin esta patología. Los datos se obtuvieron mediante la recolección de historias clínicas y se utilizó el programa STATA v.14 para el análisis, y cuadros de frecuencia utilizando odds ratio con sus respectivos intervalos de confianza al 95%. Resultados: En el análisis bivariado, tener una prueba positiva para Covid-19 mostró asociación con Preeclampsia (OR:1.98, IC 95%: 1.16-3.41, p=0.013). En el análisis multivariado los factores asociados a Preeclampsia fueron: valor elevado de RDW (ORa:2.35, IC 95%: 1.91-2.89, p<0.001, con exceso de peso (ORa:19.75, IC 95%: 10.62-36.49, p<0.001), controles prenatales <6 (ORa:2.07, IC 95%:1.27-3.37, p=0.004), ser multíparas (ORa:2.23, IC:95%: 1.19-4.16,p=0.011), mujeres con hijos recién nacidos con muy bajo peso (ORa:6.82, IC95%: 1.09-42.68, p=0.04), y no trabajar (ORa: 0.49, IC 95%: 0.29-0.84, p=0.009). Conclusiones: Se identificó una asociación significativa entre los factores sociodemográficos y gineco-obstétricos con el desarrollo de Preeclampsia en el marco de la pandemia COVID-19. El RDW elevado, el exceso de peso, controles prenatales inadecuados, multiparidad y parámetros antropométricos del recién nacido son factores de riesgo para preeclampsia. El no trabajar resultó ser un factor protector

Hydromorphodynamic effects of the width ratio and local tributary widening on discordant confluences

River training works performed in the last couple of centuries constrained the natural dynamics of channel networks in locations that include the confluences between tributaries and main channels. As a result, the dynamics of these confluences are currently characterized by homogeneous flow depths, flow velocities, and morphologic conditions, which are associated with impoverished ecosystems. The widening of river reaches is seen as a useful measure for river restoration, as it enhances the heterogeneity in flow depths, flow velocities, sediment transport, and bed substrates. The purpose of this study is to analyze the effects of local widening of the tributary mouth as well as the effects of the ratio between the width of the tributary and that of the main channel on the flow dynamics and bed morphology of river confluences. For that purpose, 12 experiments were conducted in a 70° laboratory confluence. In these experiments, three unit-discharge ratios were tested (q r = 0.37, 0.50, and 0.77) with two width ratios and two tributary configurations. The unit-discharge ratio is defined as the unit discharge in the tributary divided by that of the main channel, measured upstream of the confluence. The width ratio, which is defined as the width of the tributary divided by that of the main channel, was modified by changing the width of the main channel from 0.50 to 1.00 m (corresponding to B r = 0.30 and 0.15 respectively). The tributary configurations consisted of (i) a straight reach with a constant width (the so-called reference configuration) and (ii) a straight reach with a local widening at the downstream end (the so-called widened configuration). During the experiments, a uniform sediment mixture was continuously supplied to both channels. This experimental setup is novel among existing experimental studies on confluence dynamics, as it addresses new confluence configurations and includes a continuous sediment supply to both channels. The experiments were run until the outgoing sediment rate was nearly the same as the incoming; i.e., equilibrium had been achieved. The bed topography and water surface were then recorded in both channels. The results reveal that the width ratio and the locally widened tributary reach influence the dynamics of the confluence. The different width ratios influenced the size of the bank-attached bar at equilibrium, which was wider and longer for B r = 0.15 than for B r = 0.30. Other morphological differences were observed at equilibrium for the different width ratios, such as deeper scour holes and increased penetration of the tributary into the main channel. These differences were attributed to the different values of the ratio between the unit momentum-flux of the tributary and that of the main channel that were noted at equilibrium for the different width ratios. The local widening of the downstream reach of the tributary significantly enhanced the heterogeneity in flow depth, flow velocity, and bed morphology within the widened reach. This heterogeneity contrasts with the homogeneity observed in the tributary without widening (reference configuration). Additionally, the effects of the local tributary widening were limited to the tributary, with minor or negligible effects on the main channel.This study was supported by the Portuguese National Funding Agency for Science, Research and Technology (FCT) and the Laboratory of Hydraulic Construction (LCH) at EPFL in the framework of the Joint Doctoral Initiative IST-EPFL (SFRH/BD/51453/2011)

Three dimensional flow field at confluent fixed-bed open channels

River confluences are known for their marked 3D patterns and, due to the complexity of local flow features and bed morphology, they require particular consideration. Although there are studies on the flow structure in junctions of two streams, amore detailed description of the flow patterns is still required. The objective of this work is to contribute to such description at river confluence for concordant bed, where both main and tributary channel beds have the same elevation. For that purpose, a set of experiments with fixed bed under quasi-uniform approach flow conditions was carried out. The experimental facility included a 12 m long and 1 m wide main channel, and a 4.5 m long and 0.15 m wide tributary channel. The confluence angle was 70° so as to represent common natural conditions and the discharge ratio between discharges in the tributary and in the main channel was kept constant. An Acoustic Doppler Velocimeter (ADV) with a side-looking configuration was used to measure the three components of flow velocity on a dense grid of measuring points, providing the accurate characterization of the mean flow and second-order velocity moments. The acquired data set constitutes the most refined characterization of river confluence flow structures known to date, due to the adopted temporal and spatial discretization, and may serve for the validation of 3D numerical models of complex confluent flows. Flow dynamics zones typical of confluent beds are present in the studied case. The separation zone limits the effective lateral flow cross-section, and, hence, results in the added acceleration of the mainstream flow near the downstream junction corner. The upward orientated vertical velocities of high magnitude were observed all along the separation zone

Three-dimensional flow structure at fixed 70° open-channel confluence with bed discordance

This experimental study provides a characterization of the flow field created by bed discordance at a fixed 70∘ open-channel confluence on the basis of detailed free surface topography and three-component acoustic Doppler velocimetry measurements, which are contrasted with the results for a concordant bed junction. Due to bed discordance, the jet-like flow from the tributary is bent downstream to be aligned with the main channel axis. Underneath the tributary backward-facing step, separation occurs, producing negative vorticity along the main channel axis. A strong secondary motion downstream of the tributary junction is related to flow separation at the step and to flow release from the jet-like flow induced by a difference in the curvature of jet-like motion. A quantification of the terms in the equation of conservation of the longitudinal vorticity shows that the vortex stretching terms are sufficient to explain the development of the helical motion, which is thus classified as a secondary motion of Prandtl's first kind.Olga Birjukova Canelas received a grant from Universidade Técnica de Lisboa (UTL) from January 2013 to February 2014 and a PhD grant [SFRH/BD/52482/2014] from the Portuguese Foundation for Science and Technology (FCT) as a part of the H2Doc Doctoral Programme. The work with ADV instrumentation and the three-axis traverse system was partially funded by FEDER, program COMPETE, and by national funds through FCT [project PTDC/ECM/118775/2010]

Morphodynamic differences induced by different confluence angles in widen confluences

River widening is commonly used as a solution for the rehabilitation of rivers with the main purposes of allowing the river to adjust to its natural dynamics and to improve riparian and instream habitat for flora and fauna, by increasing the heterogeneity in substrate, flow and morphology. Local widening may also be used for confluences rehabilitation as, for many of them, the heterogeneity and fragmentation of the river system were severely affected by previous human interventions. Local tributary widening amplifies the morphodynamic processes, enhancing the diversity in sediment substrate, flow velocities and flow depths. In addition, widening should neither affect aspects such as water level in the tributary nor in the main channel, which is an important consideration for flood safety. The aim of this paper is to analyze the differences in terms of bed morphology and flow dynamics induced by different confluence angles in widen channel confluences