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The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique–Application et Recherche or Critical Zone Observatories–Application and Research) is a National Research Infrastructure (RI). OZCAR-RI is a network of instrumented sites, bringing together 21 pre-existing research observatories monitoring different compartments of the zone situated between “the rock and the sky,” the Earth’s skin or critical zone (CZ), over the long term. These observatories are regionally based and have specific initial scientific questions, monitoring strategies, databases, and modeling activities. The diversity of OZCAR-RI observatories and sites is well representative of the heterogeneity of the CZ and of the scientific communities studying it. Despite this diversity, all OZCAR-RI sites share a main overarching mandate, which is to monitor, understand, and predict (“earthcast”) the fluxes of water and matter of the Earth’s near surface and how they will change in response to the “new climatic regime.” The vision for OZCAR strategic development aims at designing an open infrastructure, building a national CZ community able to share a systemic representation of the CZ , and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. OZCAR articulates around: (i) a set of common scientific questions and cross-cutting scientific activities using the wealth of OZCAR-RI observatories, (ii) an ambitious instrumental development program, and (iii) a better interaction between data and models to integrate the different time and spatial scales. Internationally, OZCAR-RI aims at strengthening the CZ community by providing a model of organization for pre-existing observatories and by offering CZ instrumented sites. OZCAR is one of two French mirrors of the European Strategy Forum on Research Infrastructure (eLTER-ESFRI) project.

Chapeau. Le 9 janvier 2018, autour de 7h15, une avalanche d'ampleur significative descend le couloir du Bourgeat. L'écoulement de neige poudreuse, également appelé aérosol, détruit la forêt sur une largeur très importante et franchit le paravalanche du Bourgeat. Une quarantaine de chalets en aval de la digue terminale du paravalanche sont recouverts de neige par l'aérosol, et de nombreux arbres sont détruits et transportés par l'aérosol, ce qui cause les principaux dégâts qui restent fort heureusement uniquement matériels. Lors de sa propagation dans le couloir, l'écoulement dense, de neige froide au départ, se transforme en un écoulement plus lent, de neige humide, et interagit de manière complexe avec le paravalanche qui, in fine, contient bien la masse de neige lourde. Cet article propose une brève analyse qualitative des différentes hypothèses concernant les conditions de déclenchement, de propagation de l'aérosol et des écoulements denses et de leur interaction avec le paravalanche. Il dresse également une liste de questions qui peuvent se poser suite à cet événement.

International audience; Sediment trapping basins are key facilities in flood protection systems of mountain catchments, specifically in torrents prone to debris flows. A better understanding of the processes generating sediment trapping is required to optimize their functioning on both the risk, economic and environmental aspects. A monitoring station composed of a rain gauge, a geophone and a camera has been installed on the Claret torrent (Saint-Julien-Mont-Denis, Maurienne Vallée, Savoie, France). In addition to a classic time-lapse acquisition with one picture per day, a flood monitoring system has been implemented. Over a certain amplitude threshold, the geophone triggered camera acquisition each second. It aimed at obtaining direct observation of debris flow spreading in the retention basin of the open check dams. On June, 14th 2017, the Claret torrent experienced a 10,000 to 15,000 cubic meter debris flow. It reached the open check dam and half-filled the basin in less than 5 minutes. Samples of debris flows were taken a few days after. Grain size diameter measurements by surface counting were also performed on the boulder debris front deposit and on the debris flow body deposit. Data acquired on this debris flow was completed by past observations of debris flows in neighbouring torrents coming down from the same summit (Croix des Têtes, 2,492 m.a.s.l.). Such a direct observation of debris flow spreading in an unconfined area is rare and could be used to better understand open check dam functioning. If sufficient data can be gathered to reconstruct the hydrograph and debris flow features with reasonable uncertainties, this observation could help testing numerical model in a sort of benchmark imperfect approach. This presentation aims at showing the video and the knowledge gained on the debris flow trapping process, but also how we gathered data to reconstruct the event and start building a benchmark case study acknowledging error propagation.

International audience; In alpine terrain, wind-induced snow transport strongly influences the spatial and temporal variability of the snow cover. During their transport, blown snow particles undergo sublimation with an intensity depending on atmospheric conditions (air temperature and humidity). The mass loss due to blowing snow sublimation is a source of uncertainty for the mass balance of the alpine snowpack. Additionally, blowing snow sublimation modifies humidity and temperature in the surface boundary layer. To better quantify these effects in alpine terrain, a dedicated measurement setup has been deployed at the experimental site of Col du Lac Blanc (2720 m a.s.l., French Alps, Cryobs-Clim network) since winter 2015/2016. It consists in three vertical masts measuring the near-surface vertical profiles (0.2-5 m) of wind speed, air temperature and humidity and blowing snow fluxes and size distribution. Observations collected during blowing snow events without concurrent snowfall show only a slight increase in relative humidity (10-20%) and near-surface saturation is never observed. Estimation of blowing snow sublimation rates are then obtained from these measurements. They range between 0 and 5 mmSWE day-1 for blowing snow events without snowfall in agreement with previous studies in different environments (North American prairies, Antarctica). Finally, an estimation of the mass loss due to blowing snow sublimation at our experimental site is proposed for two consecutive winters. Future use of the database collected in this study includes the evaluation of blowing snow models in alpine terrain.

International audience; Some elements of snow surface roughness, such as ripple or sastrugi, are a direct manifestation of wind erosion and in turn modify the near-surface wind field and consequently the horizontal snow mass fluxes. This leads to a negative feedback between wind strength and surface roughness that must be taken into account in numerical models. Formation of sastrugi, which are elongated metric-scale ridges of wind-packed snow whose longitudinal axis is parallel to the prevailing wind at the time of their formation, is still not well-understood. The first step to provide new information about the formation and evolution of such features is to integrate meteorological data and accurate description of geometrical properties. But the complex and dynamic surface of sastrugi cannot be easily captured by manual measurements (Bellot et al., 2014), which furthermore must be frequent as the formation of new landforms can happen very quickly. That's why the potential of a low-cost time-lapse terrestrial laserscan RLS (Picard et al., 2016) has been investigated during the winter seasons 2015-2016 and 2016-2017 at Col du Lac Blanc in the French Alps. This experimental test site, dedicated to drifting snow studies, and subject to the formation of sastrugi is well-suited for such study : accurate meteorological data, including drifting snow fluxes, are available each 10 minutes. RLS covered a surface area of around 200 m2 for a spatial horizontal resolution of nearly 2 cm and monitored successfully surface roughness once a day during the whole winter seasons. Sastrugi geometrical parameters, such as the frontal area and average height of roughness elements has been extracted from the RLS data and the sastrugi morphometry has be examined over two winter seasons in link with snow fall, drifting snow occurence and intensity and wind speed.

National audience; Le manteau neigeux présente certains éléments de rugosité de surface, tels que les ripples ou les sastrugi, qui sont directement liés à l'érosion éolienne. En retour, ces rugosités modifient l'écoulement du vent à proximité de la surface, et par là même, le flux de neige transporté par le vent (Amory et al., 2016). Cette rétroaction doit donc être prise en compte dans les modèles numériques. Les sastrugi consistent en une irrégularité topographique tranchante du manteau neigeux d'une échelle métrique et dont les arrêtes sont orientées dans la direction des vents dominants au moment de leur formation. Les processus de formation de telles structures restent encore mal identifiés. Pour améliorer la connaissance dans ce domaine, une première étape consiste à combiner données météorologiques et descriptions précises des propriétés géométriques des sastrugi. Mais la complexité des surfaces ne peut pas être facilement appréhendée par de simples mesures manuelles et ce, d'autant plus, que la dynamique de formation de telles structures est très rapide. Un suivi régulier par balayage laser automatique peut donc être une solution. C'est ainsi qu'un laser-scan bas coût mis au point à l'IGE (Picard et al., 2016) a été testé pendant deux saisons hivernales au Col du Lac Blanc (Oisans, 2720 m) dans le cadre du SOERE CRYOBS-CLIM. Ce site expérimental est particulièrement bien adapté pour une telle étude puisqu'il dispose, outre d'une zone où se forment régulièrement des sastrugi, de données météorologiques précises, incluant le flux de particules de neige transportée, et ce toutes les 10 minutes. Le laser-scan a couvert une surface au sol d'environ 200 m² pour une résolution spatiale horizontale de 2 cm et a permis de suivre l'évolution de l'état de surface à raison d'un scan (voir 2) par jour pendant les saisons 2015-2016 et 2016-2017. Les caractéristiques géométriques des sastrugi, comme le maître couple ou encore la hauteur moyenne des rugosités, ont été extraites des modèles numériques de terrain issus du laser-scan et la morphométrie des sastrugi a été examiné à la lumière des données d'intensité des chutes de neige, de vitesses de vent et de flux de particules de neige transportée.

National audience; Le transport de la neige par le vent est une composante importante de l'interaction entre l'atmosphère et la cryosphère. Dans les Alpes, il influence fortement la distribution temporelle et spatiale de la couverture neigeuse. Il s'accompagne également de la sublimation des particules de neige transportées dont l'intensité varie en fonction des conditions atmosphériques (température et humidité de l'air). Cette restitution de la neige à l'atmosphère sous forme de vapeur d'eau est une source d'incertitudes dans le bilan massique du manteau neigeux des régions alpines. La sublimation liée au transport modifie également les conditions atmosphériques dans la couche limite de surface en augmentant l'humidité de la masse d'air et en diminuant sa température. Des observations collectées en Antarctique ont ainsi montré que l'occurrence d'épisodes de transport de neige par le vent s'accompagne de la formation près de la surface d'une couche d'air saturée en vapeur d'eau. En zone alpine, des études de modélisation suggèrent un comportement différent et une augmentation limitée de l'humidité relative liée au mélange turbulent et à l'advection d'air sec. Afin de mieux documenter ces rétroactions en zone alpine, un mât de mesure du profil vertical de température et d'humidité près de la surface a été déployé depuis l'hiver 2015/2016 sur le site expérimental du Col du Lac Blanc (2720 m, massif des Grandes Rousses) appartenant au SOERE CryObs-Clim. Les données de ce mât ont été combinées avec les mesures de vent et des flux de neige transportée collectées sur le site. Les premiers résultats de cette analyse pour des épisodes de transport sans chute de neige simultanée sont présentés dans cette étude. Ils confirment les résultats de modélisation et montrent qu'au Col du Lac Blanc le transport de neige par le vent s'accompagne d'une augmentation limitée de l'humidité relative près de la surface. Cette augmentation peut être reliée à l'intensité du transport de neige par le vent et aux conditions d'humidité et de température de la masse d'air. Cette base de données constitue un nouveau moyen d'évaluation des modèles numériques de transport de neige par le vent en zone alpine.

13th Congress INTERPRAEVENT 2016, Lucerne, CHE, 30-/05/2016 - 02/06/2016; International audience; In steep slope streams, torrential-hazards mainly result from abrupt and massive sediment deposits. Open check dams are regularly used in natural hazard mitigation to trap sedimentand driftwood. A good comprehension of the phenomena that occur in these structures is needed to optimize their design. In this paper, we present new results from small scaleexperiments addressing (i) a validation of water stage - discharge formula proposed in the literature for slit and slot dams; (ii) recommendations in the use of formula dedicated to deposition-thickness-estimation; (iii) geomorphic and hydraulics descriptions that seek to help field practitioners and numerical modelers to better understand what can be observed in labs and in the field and what kind of phenomena should be modeled. A special attention has been paid to highlight the implication of our results in the use of formula and in structure design and maintenance.

International audience; Taconnaz and Lautaret full-scale avalanche test-site and 3935 other French avalanche paths.

International audience; Seasonal variation of the air-snow drag coefficients in coastal Adélie Land.

National audience; Les laves torrentielles et les crues torrentielles à charriage sont des événements exceptionnels qui peuvent mobiliser, en quelques dizaines de minutes, de grandes quantités de sédiments (de 10^3 à 10^5 m^3 dans les environnements alpins). Elles sont ainsi capables de modifier le relief en gérant des exhaussements ou des affouillements pouvant atteindre plusieurs mètres. Ces épisodes de transport solide intense se produisent suite à de fortes précipitations ou à la fonte des neiges. Ils se déclenchent dans certains bassins de tête du réseau hydrographique présentant des prédispositions morphométriques (forte pente du chenal, fort dénivelé spécifique du bassin) et sédimentaires (disponibilité sédimentaire) (thèse de M. Bertrand, 2014). La complexité de ces écoulements de forte pente, encore mal appréhendée, rend les lois de transport solide actuelles difficilement applicables. Pour améliorer la modélisation des mécanismes de transfert sédimentaire dans les bassins amont, une meilleure compréhension des processus d’érosion et de transport est encore nécessaire. Pour y parvenir, une des manières de procéder est d’observer des crues torrentielles (lave/charriage) sur le terrain. A ces fins, l’Irstea de Grenoble effectue le suivi de deux torrents actifs des Alpes Françaises : le Réal (06) et le Manival (38). Des bilans sédimentaires ont ainsi permis de montrer que la majorité des sédiments mobilisés lors des crues torrentielles provient, non pas de la zone de production mais du déstockage d’anciens dépôts situés dans le chenal d’écoulement (thèse de J. Theule, 2012). La présente étude vise à caractériser la re-mobilisation des sédiments disponibles dans le chenal lors des crues torrentielles. Il s’agit pour cela de quantifier, pour une série de crues: le débit, le volume mobilisé, les « respirations » morphologiques occasionnés, les distances de transport effectuées par les cailloux et les blocs rocheux et les contraintes qui leurs ont été appliquées. Dans ce but, des données de monitoring ont été collectées, et un suivi post-événements couplant photogrammétrie et traçage RFID a été mis en place. Depuis fin 2010, les sites du Réal et du Manival sont équipés de stations de mesure (Navratil et al. 2011, 2013) qui enregistrent les précipitations, la hauteur d’écoulement et les vibrations du sol. Elles collectent également des images lors des événements de pluie. Les mesures de vibrations servent à détecter les crues et à estimer la vitesse de propagation des fronts de laves. Combinées à la mesure de hauteur, elles permettent de déduire le débit et le volume mobilisé par les laves. En 2014, les signaux des stations ont été interfacés sur un site internet pour suivre à distance en temps réel l’occurrence des événements. Cela permet de se rendre plus facilement sur le terrain après chaque crue. Le suivi de profils topographiques transversaux permet d’évaluer la dynamique longitudinale de stockage-déstockage des sédiments dans le chenal (Theule et al. 2012). Les changements morphologiques survenus sur certains tronçons sont également mesurés grâce à l’implémentation d’une nouvelle technique de reconstitution 3D à partir de photographies (photogrammétrie SfM), plus pratique et moins coûteuse que l’utilisation d’un laser-scan. Enfin, certains cailloux et blocs rocheux préalablement peints et équipés de marqueurs RFID (Radio Frequency IDentification) sont pistés. Cela permet de mesurer les distances qu’ils ont parcourues et d’estimer les contraintes d’arrachement maximales qu’ils ont subi. Ces mesures ont permis de mettre en place une base de données quantitative caractérisant la dynamique sédimentaire sur deux bassins différents sujets à la fois aux laves torrentielles et au charriage extrême. Ainsi, 19 laves ont été enregistrées entre 2011 et 2014 sur le torrent du Réal. Leur volume varie de quelque centaines de mètre cube à près de 20 000 mètre cube et les distances de transport peuvent atteindre plusieurs kilomètres lors d’un seul événement. Des crues torrentielles à charriage générant plusieurs mètres de dépôt ou d’érosion ont également été mesurées sur le Manival. Des images montrent que lors de ces crues, la formation d’un dépôt peut être précédée d’une phase d’érosion. Dans cette situation, une étude diachronique de la topographie peut être insuffisante et le traçage RFID peut s’avérer pertinent puisqu’il permet de savoir si le caillou est toujours en place sous le dépôt. Finalement, ces données pourront servir de bases au développement de modèles conceptuels et statistiques.

International audience; Generally snow avalanches consist a dense-flow layer at the bottom and a powder snow cloud on top. Snow particle size and speed are key parameters to describe the turbulent condition in the powder cloud, however, the information on the particles were not well investigated. In this study, we observed powder snow avalanches using a snow particle counter (SPC) to measure the particle size and speed. The SPC is an optical device consisting a laser diode and photodiode; a pulse signal proportional to its diameter is generated resulting from a snow particle passing through the sensing volume. In general use, the signals are sent to a transducer and divided into 32 size classes based on particle diameter to observe the snow particle size distribution and mass flux at 1-s intervals. In this study, the direct output signal from the transducer was also acquired at a high frequency to obtain the original pulse signal produced by each snow particle. Then the speed of each particle can be calculated using the peak of the pulse, which corresponds to particle diameter and the duration over which the particle passes through the sampling area. We also employed an ultrasonic anemometer to measure air flow speed. Both sensors were installed at the Col du Lautaret Pass in the French Alps. The results of the particle size and speed distribution were then compared with airflow movement in the powder cloud. The ratio of the particle and airflow speeds changed by the particle size distribution and the distance from the dense-flow layer.

The SedAlp project contributes to an integrated management of sediment transport in Alpine basins. It is directed to an effective reduction of sediment-related risk while promoting the enhancement of riverine ecosystems and reducing the impacts of hydropower plants (balancing the implementation of EU Directives and national law e.g. RES, WFD, NGP etc.). SedAlp includes pilot actions in various representative Alpine river basins of five countries and contributes in particular to monitor sediment and wood transfer in a large set of Alpine catchments. A major goal of the project was to understand spatial and temporal variability of processes and to provide planning, warning and predictive tools but also sediment and wood management recommendations for protecting people. Furthermore the project includes strategy policy development and implementation actions for the improvement of sediment continuity in Alpine river basins. SedAlp regards the geological and climatic variability across the Alps that generate complex patterns of sediment transfer, whereas management conflicts are similar. To reach the ambitious goals of the project, it is essential to understand the key processes of sediment and wood transport. Workpackage 5 with the title "sediment transport monitoring" was focused to enhance the knowledge about these transport processes by conducting the first standardized transnational monitoring of sediment and wood transport in Alpine basins. WP5 concentrated on monitoring and assessing fluxes of sediment and wood at the local scale. Measured fluxes are strongly depending on the supply from the catchment and on the connectivity to source areas, analysed in WP4. The output of WP5 feeds WP6 which studied the interferences between structures and sediment/wood fluxes and forms the basis for WP7 which linked the catchment with channel processes. Finally, the results of WP5 contribute to policy recommendations in WP8. Within WP5, monitoring activities on 28 different pilot sites in Austria, Germany, France, Italy and Slovenia have been conducted between 2012 and 2015. The monitored transport processes covered woody debris, debris flow, bedload and suspended load. The different pilot sites showed a great diversity in catchment size, channel slope and hydraulic parameters and covered the whole bandwidth from small, high alpine catchments (e.g. Strimm: 5 km² catchment size, 2427 m elevation a.s.l) to large catchments in the alpine valleys (e.g. Drau/Dellach: 2131 km² catchment size, 600 m elevation a.s.l). With the results of the monitoring activities, a database using different parameters regarding the transport process has been established and reflects the big diversity of the different pilot sites. The coordinated sediment transport monitoring outlined the links between the various processes responsible for sediment delivery at catchment scale and stresses out the need for a closer integration between the monitoring of various sediment transport processes in Alpine headwaters. Due to the wide range of different measurement methods used within the project, one focus of this work package was to standardize the measurement procedures and harmonize data analysis. This led to the 1st milestone "Protocol on standardized data collection methods in sediment transport monitoring for transboundary exchange". These protocols were intended to describe the used monitoring techniques and data processing methods for debris flows, wood transport and bedload transport. Furthermore, the protocols work also as guidelines to assist in choosing the appropriate monitoring method for supporting prospective monitoring efforts. 4 An important aspect of the project was to understand spatial and temporal variability of the monitored processes. The observed appearance of clockwise and counter clockwise hysteresis effects between sediment and discharge, showed the significant role of the location of the active sediment source in the temporal variability of sediment transport. Analysis of the spatial distribution showed an evolvement of the bedload transport width with increasing discharge which provides vital information for an improved planning of river related measures. To determine these process related variability, the use of appropriate monitoring methods as well as their right application is of great importance. Indirect monitoring methods (e.g. geophone devices) pointed out their potential in the automatic and continuous detection of these transport processes. The presented data reflect a high spatial and temporal variability in the occurrence of bedload, suspended load debris flow and wood transport and reveal significant consequences for the practical usage of monitoring methods, data and application. Many river engineering tasks require detailed information about the extent of sediment transport and wood mobility, which are provided by sediment and wood transport relations and equations. The 3rd milestone "First set of practically applicable bedload/wood transport relations and models" gives an overview about the most common transport relations and formulas. Furthermore it presents the difficulties and challenges in the application of these relations and shows the last developments in improving transport equations. The comparison between measured and calculated specific bedload transport rates showed substantial differences in the derived results. For the practical use of these relations it is strongly recommended to select, calibrate and validate the sediment and wood transport equations using monitored field data. The evaluation of river restoration projects showed the need of an improved process understanding between sediment transport an engineering measures. The functionality and sustainability of river restoration measures are, beside the hydrologic and hydraulic conditions, mainly depending on the superior sediment regime and thus the sediment input into the reach. By increase or decrease of the mean sediment input, the hydraulic and thus morphological conditions of the reach need to rearrange to the given input by e.g. lateral and vertical adjustments of the river bed or by changing the river type.

International audience; Knowledge of snow particle speeds is necessary for deepening our understanding of the internal structures of drifting snow. In this study, we utilized a snow particle counter (SPC) developed to observe snow particle size distributions and snow mass flux. Using high-frequency signals from the SPC transducer, we obtained the sizes of individual particles and their durations in the sampling area. Measurements were first conducted in the field, with more precise measurements being obtained in a boundary layer established in a cold wind tunnel. The obtained results were compared with the results of a numerical analysis. Data on snow particle speeds, vertical velocity profiles, and their dependence on wind speed obtained in the field and in the wind tunnel experiments were in good agreement: both snow particle speed and wind speed increased with height, and the former was always 1 to 2m s(-1) less than the latter below a height of 1 m. Thus, we succeeded in obtaining snow particle speeds in drifting snow, as well as revealing the dependence of particle speed on both grain size and wind speed. The results were verified by similar trends observed using random flight simulations. However, the difference between the particle speed and the wind speed in the simulations was much greater than that observed under real conditions. Snow transport by wind is an aeolian process. Thus, the findings presented here should be also applicable to other geophysical processes relating to the aeolian transport of particles, such as blown sand and soil.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; The investigation of the spatial variability of snow depth in high alpine areas is an important topic in snow hydrology, glacier and avalanche research and the transport of snow by wind is an important process for the distribution of snow in mountainous regions. That's why, for 25 years IRSTEA (previously Cemagref) and Météo France (Centre for the Study of Snow) have joined together in studying drifting snow at Col du Lac Blanc 2700 m a.s.l. near the Alpe d'Huez ski resort in the French Alps. Initially, the site was mainly equipped with conventional meteorological stations and a network of snow poles, in order to test numerical models of drifting snow Sytron (CEN) and NEMO (Cemagref). These models are complementary in terms of spatial and temporal scales: outputs of Sytron model will form the inputs of NEMO model. Then new sensors and technologies appeared which allow to develop new knowledge dealing with thresholds velocity according to morphological features of snow grains, snow flux profiles including parameters such as fall velocity and Schmidt number, histograms of particle widths, aerodynamic roughness, gust factors. More recently, the coupled snowpack/ atmosphere model Meso-NH/Crocus has been evaluated at the experimental site. At the same time, some tested sensors have been deployed in Adelie Land in Antarctica, where blowing snow accounts for a major component of the surface mass balance. Japanese and Austrian research teams have been accomodated at Lac Blanc Pass and new foreign teams are welcome. Initial observations continue. That's why Lac Blanc Pass is also a climatological reference for 25 years at 2700 m. Data are available.

International audience; Wind-transported snow is a common phenomenon in French Alps, creating snowdrifts and contributing significantly to the loading of avalanche release areas. The wind erodes snow from high wind speed areas and deposits it in low wind speed areas. The resulting snowdrifts often cause problems for infrastructure and road maintenance and contribute significantly to the loading of the avalanche release area. Numerical blowing snow model can be a useful tool to investigate this phenomena but they need input parameters such as size distribution of snow particles (Vionnet et al., 2013), Schmidt number and terminal snow particles velocity (Michaux et al., 2001) (Naaim-Bouvet et al., 2000) (Naaim-Bouvet et al., 2008). Some studies have already been conducted to address the size distribution of snow particles at a given height (Budd (1966), Schmidt (1982), Nishimura and Nemoto (2005), Gordon and Taylor (2009)). Such data could depend on topography and snow type and all of these studies have been conducted under different conditions than those encountered in the Alps. Consequently, the present study was carried out at the Lac Blanc Pass (2700 m), an experimental site in the French Alps, using three snow particles counter set up at different heights. Such optical devices are able to detect particles between 20 and 500 μm in mean radius size particle, divides them into 32 classes. In the main cases and as usual, the size distribution of snow particles is represented by a gamma density function. The Schmidt number, the shape parameter and the mean particle diameter were studied as function of height, friction velocity and the results were compared with previous studies.

International audience; Heterogeneity of snow cover in high mountain area may increase avalanche hazard. In Antarctica, drifting snow plays an important role in the surface mass balance and therefore on the sea level rise. It is therefore necessary to evaluate blowing snow in the field in terms of snow fluxes and occurrence, and, if possible, using to automatic measurements. We present here two sensors based on the same measurements techniques, using the optical beam obstruction method: the Snow Particle Counter (SPC) and the Wenglor YH08PCT8 sensor. The SPC has been widely tested and used in the past and is considered the reference in the present study. The Wenglor YH08PCT8 sensor is a relatively simple, inexpensive commercial counter used in industry which recognizes extremely small parts, holes, slots and notches. It has been tested for 4-5 years as a sensor for a eolian sand and snow transport. The present study focused on the performance of the Wenglor sensor in the context of blowing snow measurement. During two consecutive winters from 2011 to 2013, we set up Wenglor sensors and SPCs at Lac Blanc Pass (French Alps 2800 m) and tested Wenglor sensors in cold room. The results presented here will focus on Wenglor sensor's performance and limitations for uses in high mountain area and cold regions.

International audience; An ultrasonic anemometer and a snow particle counter were newly installed in the avalanche test site locating at the Col du Lautaret pass, France. Simultaneous measurements of both wind and snow particles in the powder snow cloud were made to investigate the powder part of the avalanche in 2013 winter. The particle speed and diameter were calculated from the high-frequency recording of direct SPC output. The particle speed showed lower values than the wind speed. Relatively large particles were observed around the avalanche front and the size of the particles was slightly decreased in the following flow. The particle size distribution was well estimated by the aerodynamic theory except around the avalanche front.

International audience; In high elevation areas drifting snow influences the snowpack spatial distribution and consequently its stability. The amount of snow drifted from the windward side and accumulated on the leeward side is not only a function of wind conditions but it also depends on the snowpack characteristic. One of the aim of the project MAP3 "Monitoring for the Avalanche Prevision, Prediction and Protection" - P.O. ALCOTRA Italy/France (Alps) 2007-2013 is to better understand the influence of snowdrift on the snow spatial distribution in the leeward side of a ridge, where avalanches naturally or artificially occur. To achieve this aim, in Summer 2012 the Seehore avalanche test site in Aosta Valley (Italy) was implemented with a snow and weather station installed on the ridge at 2570m asl in order to measure wind speed and direction, snow depth and drifted snow at different heights above the ground. In case of avalanche release, snowpack properties and release features are recorded by field surveys. Laser scan measurements and photogrammetry are also performed to get information about the spatial distribution of the snowpack, with a special focus on the release zone. This paper presents the design and the installation of the system and the first results of the measurements. As the experimental device was fully operational only for four months during winter 2012- 2013, in this paper we focus on two specific events: 1) snowdrift event without snowfall, 2) snowdrift event with concurrent snowfall before an artificially released avalanche in the study site. More work is planned for the future, in particular concerning the avalanche release and snowpack features in respect to snowdrift conditions. This new experimental test site presents a great potentiality to advance in this research topic, due to its small size and relatively easy logistic in the data recording.

International audience; This paper describes the instrumentation related to velocity and pressure measurements recently set up in the avalanche protection system of Taconnaz. Previous studies on the impact pressure stemming from snow avalanches on the basis of experimental measures revealed very complex loading spectra. More particularly, open questions relative to the incident flow regime and the flow-obstacle geometry, and to how the mean pressure and fluctuations can be influenced, remain challenging issues. The Taconnaz avalanche path (Chamonix, France) is characterized by remarkable dimensions. The passive protection system has been recently improved in the framework of an expertise conducted by Irstea (formerly Cemagref). The new defence structures, retarding mounds in particular, have been equipped with sensors in order to measure the local velocity in the vicinity of those obstacles as well as the impact pressure stemming from snow avalanches, in the framework of the Alcotra DYNAVAL Interreg project. The first measurements have been obtained from the avalanche that occurred at the end of December, 2010.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; Wind and the associated snow drift are dominating factors determining the snow distribution and accumulation in alpine areas, resulting in a in high spatial variability of snow depth that is difficult to evaluate and quantify. In our study, we compare the results of a terrain-based parameter to snow-depth distribution and redistribution data obtained by high-accuracy TLS (Terrestrial Laser Scan) and to the results of wind-field simulations. Our results are from the test site at the Col du Lac Blanc, close to the ski resort of Alpe d’Huez in the French Alps, an area particularly suited for our study due to its constant wind direction, the availability of wind and precipitation data from a meteorological station and data from multiple TLS scan campaigns. We first spatially identify areas of good and poor correlation between terrain-based parameter and measured data, and identify common characteristics for these areas. Second, we present the differences in the results of the terrain-parameter based on a) summer terrain and b) snow-covered surface in mid-winter, and show how this affects the spatial correlation with the measured snow depth data. Third, we present comparisons of the terrain-based parameter to wind-field simulations with ARPS (Advanced Regional Prediction System), a 3-dimensional atmospheric model for simulating microscale airflows. The results of our study are another step towards improving the terrain-based parameter’s ability to quantitatively describe snow redistribution.

International audience; The Surface Mass Balance (SMB) of the Antarctic ice sheet is probably the only important negative contribution to the sea-level rise. Net erosion of snow by the wind may contribute significantly to the SMB of the Antarctic costal zone. However, there are very few field observations to confirm this hypothesis and to evaluate numerical models developed for this purpose. Adélie Land, located in East Antarctica, is one of the windiest places in the world in term of mean wind speed at the coast. Furthermore, the frequency of the blowing snow events, determined by visual observations, is very high. That is why a field campaign was launched in January 2009 to acquire new modelevaluation- oriented observations in the framework of the European project ICE2SEA and with the logistical support of the French polar Institute (IPEV). Three automatic weather and snow stations, including acoustic sensors for the aeolian transport of snow named FlowCapt, have been deployed in Adélie Land. The stations locations are distinct ranging from 1 to 100 km inland. One of them is a 7 mmast with 6 levels of anemometers and thermo-hygrometers. Thus, the campaign can assess, inter alias, transport events periods, transport frequencies, snow quantities transported, threshold friction velocities and the ratio between small and large fluxes events. Those results can be use in the evaluations of the regional climate models.

International audience; Snow particle speeds are one of the key issues in order to deepen our understanding for the internal structures of blowing snow. In this study, we utilized the snow particle counter (SPC) developed to observe the snow particle size distribution and the mass flux every one second. We have recorded the direct output signal from the transducer with high frequency, and obtained the particle size and the duration of time when the individual particle passed the sampling area. Then, utilizing two data the particle speeds are calculated. Firstly, measurements were carried out at Lac Blanc Pass in French Alps. Then, more precise measurements were conducted at the cold wind tunnel in NIED, which is 14 m long and its working section is 1 m x 1 m. Snow particle speed, its distribution with height and the relation to the wind speeds showed good agreement; the snow particle speed and the wind speed increased with height, and the former was always smaller than the latter below 1 m. In this manner, we succeeded to obtain the reliable snow particle speeds in the blowing snow for the first time and revealed its grain size dependence and the relation to the wind speeds. Similar trends were also shown with the random flight calculations; particle speeds are lower than the air speed. However, the differences in wind and particle speeds amounted larger in the simulation.

International audience; Présentation orale faite lors de la conférence Davos Atmosphere and Cryosphere Assembly DACA-13.

International audience; Présentation orale lors du séminaire Ice2sea.

International audience

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; In order to make progress in avalanche research and to improve avalanche risk forecasting, at least two elements must be taken into account: the investigation of the spatial variability of snow depth and the study of meteorological conditions during blowing snow events in high alpine areas. Numerical models (NEMO, Sytron 2 & 3) have been developed to simulate the redistribution of the snow pack during those events. More recently, we tested the ability of a meso-scale atmospheric model to explicitly simulate wind-induced snow transport in alpine terrain. The characteristics and mechanisms of blowing snow events must be verified and the results of the models can then be validated through the use of in-situ measurements. Over the last two years we have successfully performed intensive measurement campaigns at our experimental site (Col du Lac Blanc, French Alps, 2,700 m asl). We followed the evolution of snow depth over an area of 2x1 km² around our experimental site using data from a Terrestrial Laser Scanner. In addition, in-situ measurements were collected during blowing snow events: - Vertical profile of snow fluxes using mechanical traps and Snow Particles Counters (SPC), - Vertical profile of wind velocity, air temperature and relative humidity on a meteorological mast, - Wind speed and air temperature at three automatic weather stations. These measurements represent a whole dataset suitable for the evaluation of models that simulate wind-induced snow transport in alpine terrain. We present in this paper the first results of this in-situ study.

International audience; Braided river patterns and sediment supply interactions are significant. Small-scale braided rivers were developed in a rectangular flume with adjustable slope to investigate these relationships and to gain insight into the effect of grain sorting on bedform formation and migration. The 1.20-m-wide by 4.5-m-long flume was filled with fine sand, which ranged from 0.5mm to 1.5 mm, with a median grain size of about 1 mm. The sediment feed rate and water discharge were maintained constant. The initial bed was a flat surface with a 1.7% slope. The mean bed load discharge was calculated from the weight of output sediments. The Moiré method was chosen to measure the entire bed topography. The experiment produced bedforms and braided patterns. Equilibrium was reached with a constant number of moving bars. The Moiré method can be used for a precise study bed topography changes and bedform migration.

International audience; Braided river patterns and sediment supply interactions are significant. Small-scale braided rivers were studied in a rectangular flume with an adjustable slope to investigate these relationships and to gain insight into the effect of grain sorting on bedform formation and migration. We used a 1.20-m-wide and 4.5-m-long flume and a mixture of fine and coarse sand ranging from 0.5 mm to 1.5 mm and from 1.5 to 3 mm (with median sizes 1 and 2 mm, respectively). The sediment feed rate and water discharge were maintained constant. The initial bed was flat with a 3% slope. The mean bed load discharge was calculated by weighing output sediments. The experiment produced bedforms and braided patterns. Equilibrium was reached with a constant number of moving bars.

International audience; Cet article présente l’instrumentation relative aux mesures automatiques de vitesse et de pression récemment mise en place dans le dispositif paravalanche de Taconnaz. Tout d’abord l’article analyse le problème de la pression d'impact des avalanches de neige sur la base des mesures expérimentales existantes qui révèlent des spectres d'effort très complexes. En particulier, les questions ouvertes relatives au régime d'écoulement incident et à la géométrie "écoulement-obstacle", et à leurs effets sur la pression moyenne et les fluctuations, sont discutées. Le couloir d’avalanche de Taconnaz (Chamonix, France), caractérisé par des dimensions exceptionnelles, est alors présenté. Le dispositif de protection passive a été récemment amélioré suite à un travail d’expertise mené par Irstea (anciennement Cemagref). Les ouvrages nouvellement implantés – tas freineurs en particulier – ont été équipés pour mesurer les vitesses locales de l’avalanche au voisinage de ces ouvrages ainsi que les pressions exercées, et ce dans le cadre du projet Interreg Alcotra DYNAVAL. Les premières mesures qui sont présentées ont été obtenues lors de l’avalanche survenue à la fin du mois de décembre 2010.

International audience; In the framework of the Alcotra DYNAVAL Interreg project, this experimental study aims at investigating the dynamical behaviour of a powder snow avalanche impacting an obstacle. Tests have been realised in a water tank where a salty water solution (rho=1.2kg m-3) flows down in a channel and impacts an obstacle at a distance d=1m from the releasing gate. The set-up geometry reproduces a simplified small-scale model of the real avalanche site of Taconnaz (Chamonix, France). A high-resolution acoustic velocimeter allows measurements on the 3D flow velocity. By measuring the velocity just upstream and downstream of the obstacle, it is possible to determine the influence of the obstacle on the flow. In a lack of suitable sensors, the pressure exerted on the obstacle is calculated using the classical formula P=1/2rhoU2. Then, density values are required. A new method to measure the flow density is advanced and preliminary results are presented.

International audience; The investigation of the spatial variability of snow depth in high alpine areas is an important topic in snow hydrology, glacier and avalanche research. In a highly structured alpine terrain, the snow accumulation is highly dependent on orography during the winter. The dependence is not only on elevation, aspect and slope at the point of interest but also on all features in the surroundings which influence the near-surface wind field. The locally convergent and divergent flow patterns cause snow redistribution. Not only is previously deposited snow influenced, but also snow precipitation is non-uniformly deposited because of the wind. The current study presents the combination of two approaches to measure parameters of drifting and blowing snow - in particular the flux, to determine the amount of snow particles within the wind flow and the resultant erosion and deposition behavior of those particles using terrestrial laser scanning by creation of snow height maps. To measure erosion and deposition behavior and flux of different events such as snow drift without snowfall and snow drift with additional snowfall allowed us determining the impact of snow particle concentration to the measured process. We present the results of the terrestrial laser scanning campaign at the experimental site, Col du Lac Blanc (2700ma.s.l.), French Alps and we discuss how the particle flux influence the deposition behaviour at different location within the test site.

International audience; This study describes the instrumentation and the obtained experimental data related to velocity and pressure measurements set up in the avalanche protection system of Taconnaz in 2009. First, the problem related to the impact pressure stemming from snow avalanches is discussed on the basis of experimental measures revealing very complex loading spectra. More particularly, open questions related to the incident flow regime and the "flow-obstacle" geometry, and to how the mean pressure and fluctuations can be influenced, are discussed. Then, the Taconnaz avalanche path (Chamonix, France), characterized by huge dimensions, is introduced. In the framework of the Alcotra DYNAVAL Interreg project, the new defence structures –retarding mounds in particular- have been equipped with sensors in order to measure the local velocity in the vicinity of those obstacles as well as the impact pressure stemming from snow avalanches. The first measurements, obtained from the avalanche that occurred at the end of December, 2010, are presented.