Monitoring of bedload in river beds with an hydrophone: first trials of signal analyses

River engineeringInnovative field and laboratory instrumentatio

Drag produced by waves trapped at a density interface in non-hydrostatic flow over an axisymmetric hill

Linear non-hydrostatic theory is used to evaluate the drag produced by 3D trapped lee waves forced by an axisymmetric hill at a density interface. These waves occur at atmospheric temperature inversions, for example at the top of the boundary layer, and contribute to low-level drag possibly misrepresented as turbulent form drag in large-scale numerical models. Unlike in 2D waves, the drag has contributions from a continuous range of wavenumbers forced by the topography, because the waves can vary their angle of incidence to match the resonance condition. This leads to non-zero drag for Froude numbers (Fr) both 1, and a drag maximum typically for Fr slightly below 1, with lower magnitude than in hydrostatic conditions due to wave dispersion. These features are in good agreement with laboratory experiments using two axisymmetric obstacles, particularly for the lower obstacle, if the effects of a rigid lid above the upper layer and friction are taken into account. Quantitative agreement is less satisfactory for the higher obstacle, as flow nonlinearity increases. However, even in that case the model still largely outperforms both 3D hydrostatic and 2D non-hydrostatic theories, emphasizing the importance of both 3D and non-hydrostatic effects. The associated wave signatures are dominated by transverse waves for Fr lower than at the drag maximum, a dispersive ‘Kelvin ship wave’ pattern near the maximum, and divergent waves for Fr beyond the maximum. The minimum elevation at the density interface depression existing immediately downstream of the obstacle is significantly correlated with the drag magnitude

Mean Velocity Predictions in Vegetated Flows

Vegetation plays an important role in influencing the hydrodynamic behavior, ecological equilibrium and environmental characteristics of water bodies. Several previous models have been developed, to predict hydraulic conditions in vegetated rivers, but only few are actually used in practice. In This paper six analytic model derived for submerged vegetation are compared and evaluate: Klopstra et al. (1997); Stone and Shen (2002); Van velzen (2003); Baptist et al. (2007); Huthoff et al. (2007) and Yang and Choi (2010). The evaluation of the flow formulas is based on the comparison with experimental data from literature using the criteria of deviation. Most descriptors show a good performance for predicting the mean velocity for rigid vegetation. However, the flow formulas proposed by Klopstra et al. (1997) and Huthoff et al. (2007) show the best fit to experimental data. Only for experiments with law density, these models indicate an underestimation. Velocity predicted for flexible vegetation by the six models is less accurate than the prediction in the case of rigid vegetation

Exploring the role of wave drag in the stable stratified oceanic and atmospheric bottom boundary layer in the cnrs-toulouse (cnrm-game) large stratified water flume

This paper reports on a laboratory experiment in the CNRM-GAME (Toulouse) stratified water flume of a stably stratified boundary layer, in order to quantify the momentum transfer due to orographically induced gravity waves by gently undulating hills in a boundary layer flow. In a stratified fluid, a surface corrugation is towed with different speeds to cover a range of Froude numbers. PIV measurements are used to quantify the flow field which is divided in a mean flow, a wave component and turbulent component. In addition wave drag divergence over the boundary layer is investigated. The experimental results aim to improve formulations for turbulent heat and momentum transfer for use in numerical weather prediction, climate models and ocean model

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

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

Low power main memory configuration and tasks allocation

International audienc

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