Loi de frottement pour écoulement sur lit de gravier : prise en compte de la pente

In presence of bedload transport it is difficult to measure the average velocity of the flow. As the sediment discharge increases, the use of a point gauge becomes difficult, even impossible when slopes become steep (1 to 10%). In this paper, we describe a velocity measurement technique based on image analyses. This method, inspired by the well known salt-velocity technique, appeared to be simple and reliable. Velocity measurements are used to investigate the flow resistance. Instead of considering dispersion of data around one unique logarithmic law, we propose an original slope dependent law.Une loi de frottement est proposée pour les écoulement sur lit de gravier. Plutôt que de considérer une loi logarithmique unique, une loi est proposée comme fonction de la pente géométrique pour tenir compte des effets du transport sédimentaire sur la résistance totale à l'écoulement

An experimental investigation of mechanisms involved in bed load sheet production and migration.

International audienceField measurements indicate that, in gravel bed rivers, bed load may not be a one-to-one response to shear stress but may instead fluctuate a great deal over time for a given flow condition. Both in flume and field experiments, these fluctuations were often associated with migration of low-relief bed forms called bed load sheets. Whereas several studies have described bed load sheets as a consequence of grain sorting, little is known about the mechanisms responsible for their production and migration. These were investigated in flume experiments. A set of 20 experiments was conducted under constant feeding rate conditions, with mixtures of different uniform sediments and for slopes varying from 0.8 to 9%. Except for runs performed in high flow conditions, we observed periodic bed load sheet production and migration associated with fluctuations of bed slope, bed state (bed fining and paving), and bed load. Observations allowed us to conclude that bed load sheets resulted from very efficient vertical and longitudinal grain sorting that produced periodic local bed aggradation and erosion clearly observed in the upstream section of the flume. Fractional transport rates were measured in one run. Combined with the results of experiments previously conducted by authors with uniform sediments, this experiment showed that the highest (peak) solid discharges were essentially caused by the much greater mobility of the coarser gravels when transported within bed load sheets. A scenario is proposed for the mechanisms involved in bed load sheet production and migration

Spectral variations of underwater river sounds

International audiencePassive acoustic monitoring of the self-generated noise of particle impacts has been shown to be correlated to bedload flux and bedload size. However, few studies have concentrated on the role of acoustic wave propagation in a river. For the first time, the river environment is modeled as a Pekeris waveguide, where a wave number integration technique is used to predict the transformation of sounds through their propagation paths. Focusing on the distance of a hydrophone from the channel bed and cutting off the low frequencies produced by impacts between gravel particles, we demonstrate that acoustic propagation modifies the spectral content of bedload-generated sound. Acoustic signals analyzed with the proposed model are interpreted by comparison to Helley–Smith bedload data obtained during flood conditions on the large gravel-bedded Arc-en-Maurienne River, France.This study shows that careful attention to acoustic propagation effects is required when estimating bedload grain size distribution with hydrophones in rivers, especially for rivers with slopes higher than 1%. Bedload monitoring with a hydrophone is particularly appropriate for large gravel-bed rivers – especially so during large floods, when in situ sampling is difficult or impractical and the impact of acoustic propagation is weaker relative to the self-generated noise of bedload impacts

Passive acoustic monitoring of large gravel bed rivers

International audienc

Premières de charriage dans une rivière à gravier alpine

International audienceIn June 2011, a first attempt for measuring bedload transport in the main channel of an alpine river, the Arc en Maurienne River, was undertaken contemporaneously deploying bedload samplers and hydrophones. The Ehrenberger type bedload basket sampler was unstable at high velocities. Samples were successively taken using a large (152 mm) Helley-Smith type bedload sampler. The considerable temporal variability of bedload flux based on H-S sampling contradicts the generally steady hydrophone signals. The position of the sampler on the river bed may have likely been uneven due to the presence of boulders, thereby decreasing sampler catch in some instances. Apparently when the sampler was evenly set on the riverbed, its efficiency decreased due to the presence of organic matter and large concentration of suspended sediments. A simple hydraulic model was applied to estimate the variation of water depth, velocity and bedload discharge with water discharge, yielding consistent results compared to measured water depths and velocities. The calculated bedload discharge using classical formulae is, however, larger than the average monitored, confirming the relatively low efficiency of the bedload sampler

Passive acoustic measurement of bedload grain size distribution using self-generated noise

Monitoring sediment transport processes in rivers is of particular interest to engineers and scientists to assess the stability of rivers and hydraulic structures. Various methods for sediment transport process description were proposed using conventional or surrogate measurement techniques. This paper addresses the topic of the passive acoustic monitoring of bedload transport in rivers and especially the estimation of the bedload grain size distribution from self-generated noise. It discusses the feasibility of linking the acoustic signal spectrum shape to bedload grain sizes involved in elastic impacts with the river bed treated as a massive slab. Bedload grain size distribution is estimated by a regularized algebraic inversion scheme fed with the power spectrum density of river noise estimated from one hydrophone. The inversion methodology relies upon a physical model that predicts the acoustic field generated by the collision between rigid bodies. Here we proposed an analytic model of the acoustic energy spectrum generated by the impacts between a sphere and a slab. The proposed model computes the power spectral density of bedload noise using a linear system of analytic energy spectra weighted by the grain size distribution. The algebraic system of equations is then solved by least square optimization and solution regularization methods. The result of inversion leads directly to the estimation of the bedload grain size distribution. The inversion method was applied to real acoustic data from passive acoustics experiments realized on the Isère River, in France. The inversion of in situ measured spectra reveals good estimations of grain size distribution, fairly close to what was estimated by physical sampling instruments. These results illustrate the potential of the hydrophone technique to be used as a standalone method that could ensure high spatial and temporal resolution measurements for sediment transport in rivers

Passive acoustic monitoring of bed load discharge in a large gravel bed river

International audienceSurrogate technologies to monitor bed load discharge have been developed to supplement and ultimately take over traditional direct methods. Our research deals with passive acoustic monitoring of bed load flux using a hydrophone continuously deployed near a river bed. This passive acoustic technology senses any acoustic waves propagated in the river environment and particularly the sound due to interparticle collisions emitted during bed load movement. A data set has been acquired in the large Alpine gravel-bedded Drau River. Analysis of the short-term frequency response of acoustic signals allows us to determine the origin of recorded noises and to consider their frequency variations. Results are compared with ancillary field data of water depth and bed load transport inferred from the signals of a geophone array. Hydrophone and geophone signals are well correlated. Thanks to the large network of deployed geophones, analysis of the spatial resolution of hydrophone measurements shows that the sensor is sensitive to bed load motion not only locally but over distances of 5–10 m (10–20% of river width). Our results are promising in terms of the potential use of hydrophones for monitoring bed load transport in large gravel bed rivers: acoustic signals represent a large river bed area, rather than being local; hydrophones can be installed in large floods; they can be deployed at a low cost and provide continuous monitoring at high temporal resolution

Vigicrues Flash, un service automatique d'avertissement pour les crues rapides

National audienceAbout 22 000 km of the main rivers of continental France are supervised by the French governmental service called Vigilance Crues. Despite this large supervision, many rivers are not integrated in Vigilance Crues because of their rapid response time and lack of flow measurements. In march 2017, Vigicrues opened a new service complementary to the Vigilance Crues: Vigicrues Flash. This warning service is intended for public authorities and crisis managers, who can subscribe on line . The Vigicrues Flash system assesses the risk of a flood in the next hours, with an expected anticipation close to the response time of the watershed. It relies on an event-based hydrological model developed by IRSTEA, called GRD, which is fed with radar-based rainfall. The production of the effective rainfall is distributed, while the routing is global. Two other models with daily time step initialize GRD every day. The regionalized calibration of the GRD model aims to successfully detect the exceedance of flood return period thresholds. Two return periods have been chosen for minor flood and major flood. The calculations are made on numerous strategic points along the streams with a maximum interval of 4km between two points. At each point, the discharge values corresponding to the two return periods have been estimated based on a long time series of GRD modelled discharge. In real time, calculations are updated every 15 minutes. When a threshold is exceeded, a warning is directly sent to subscribers by text message, e-mail and phone call. The subscriber is directed to the flood map (available on the website) to know precisely which streams would exceed the flood thresholds. Since the system relies on a simplified hydrological model, a selection of watersheds is necessary to meet specific requirements, like a minor influence of snow and dams, a time response over 1h30, and satisfying radar-based rainfall measurements. A first selection of eligible watersheds has been made based on the objective criteria of the SHYREG project. This selection has been reviewed by the local flood forecasting services and public authorities involved in flood management. Finally, about 30,000km of streams are eligible to Vigicrues Flash in continental France, corresponding to 10,328 eligible municipalities. After one year of operation, the users underlined that the service provided added value for flood risk mitigation.Les cours d'eau surveillés par l'État dans le cadre de la vigilance crues représentent un linéaire d'environ 22 000 km. Les cours d'eau à réaction rapides ne peuvent intégrer le réseau surveillé dans le cadre de la vigilance crues. Un service complémentaire a donc été mis en place depuis mars 2017 par le réseau Vigicrues : Vigicrues Flash. Ce service s'adresse aux collectivités et aux gestionnaires de crise, qui peuvent s'y abonner, afin de recevoir des avertissements sur le risque de crue rapide sur leur territoire, et de consulter en ligne la cartographie des avertissements. Ce service repose sur un modèle hydrologique alimenté par les pluies radar observées. Le modèle a été paramétré de manière régionalisée, pour une utilisation en milieu non jaugé. À partir de chroniques de débits modélisés, deux périodes de retour de crue ont été choisies afin de rendre compte d'un risque de crue forte et d'un risque de crue très forte. Les calculs sont rafraîchis toutes les 15 min, et lors d'un dépassement de seuil, un message est envoyé à l'abonné par SMS, appel téléphonique et courriel. Les bassins éligibles au service répondent à plusieurs critères permettant une modélisation fiable. À l'issue de la sélection, environ 30 000 km de cours d'eau ont été retenus sur la France continentale, représentant 10 328 communes éligibles au service. Après un an de mise en service, les premiers retours des utilisateurs montrent une plus-value de ce système dans le cadre de la gestion du risque de crue

Vigicrues Flash, un service automatique d'avertissement pour les crues rapides

National audienceAbout 22 000 km of the main rivers of continental France are supervised by the French governmental service called Vigilance Crues. Despite this large supervision, many rivers are not integrated in Vigilance Crues because of their rapid response time and lack of flow measurements. In march 2017, Vigicrues opened a new service complementary to the Vigilance Crues: Vigicrues Flash. This warning service is intended for public authorities and crisis managers, who can subscribe on line . The Vigicrues Flash system assesses the risk of a flood in the next hours, with an expected anticipation close to the response time of the watershed. It relies on an event-based hydrological model developed by IRSTEA, called GRD, which is fed with radar-based rainfall. The production of the effective rainfall is distributed, while the routing is global. Two other models with daily time step initialize GRD every day. The regionalized calibration of the GRD model aims to successfully detect the exceedance of flood return period thresholds. Two return periods have been chosen for minor flood and major flood. The calculations are made on numerous strategic points along the streams with a maximum interval of 4km between two points. At each point, the discharge values corresponding to the two return periods have been estimated based on a long time series of GRD modelled discharge. In real time, calculations are updated every 15 minutes. When a threshold is exceeded, a warning is directly sent to subscribers by text message, e-mail and phone call. The subscriber is directed to the flood map (available on the website) to know precisely which streams would exceed the flood thresholds. Since the system relies on a simplified hydrological model, a selection of watersheds is necessary to meet specific requirements, like a minor influence of snow and dams, a time response over 1h30, and satisfying radar-based rainfall measurements. A first selection of eligible watersheds has been made based on the objective criteria of the SHYREG project. This selection has been reviewed by the local flood forecasting services and public authorities involved in flood management. Finally, about 30,000km of streams are eligible to Vigicrues Flash in continental France, corresponding to 10,328 eligible municipalities. After one year of operation, the users underlined that the service provided added value for flood risk mitigation.Les cours d'eau surveillés par l'État dans le cadre de la vigilance crues représentent un linéaire d'environ 22 000 km. Les cours d'eau à réaction rapides ne peuvent intégrer le réseau surveillé dans le cadre de la vigilance crues. Un service complémentaire a donc été mis en place depuis mars 2017 par le réseau Vigicrues : Vigicrues Flash. Ce service s'adresse aux collectivités et aux gestionnaires de crise, qui peuvent s'y abonner, afin de recevoir des avertissements sur le risque de crue rapide sur leur territoire, et de consulter en ligne la cartographie des avertissements. Ce service repose sur un modèle hydrologique alimenté par les pluies radar observées. Le modèle a été paramétré de manière régionalisée, pour une utilisation en milieu non jaugé. À partir de chroniques de débits modélisés, deux périodes de retour de crue ont été choisies afin de rendre compte d'un risque de crue forte et d'un risque de crue très forte. Les calculs sont rafraîchis toutes les 15 min, et lors d'un dépassement de seuil, un message est envoyé à l'abonné par SMS, appel téléphonique et courriel. Les bassins éligibles au service répondent à plusieurs critères permettant une modélisation fiable. À l'issue de la sélection, environ 30 000 km de cours d'eau ont été retenus sur la France continentale, représentant 10 328 communes éligibles au service. Après un an de mise en service, les premiers retours des utilisateurs montrent une plus-value de ce système dans le cadre de la gestion du risque de crue