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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; The transport of particles by the wind takes place in different natural contexts: sand in deserts and coastal areas or snow in cold regions. For spherical, dry and non-cohesive sand, the physics of aeolian transport is quite well understood. In contrast, much less is known about aeolian transport over wet sand beds which is relevant to sandy coastal areas. Although moisture effects have been the focus of numerous studies, there exist significant discrepancies in experimental or model predictions and reliable and quantitative results for aeolian sand transport in moist environments are lacking. In this experimental work, we conducted well-controlled wind tunnel experiments with wet sand beds ranging from 0.5 to 3% of moisture content at different wind speeds. The experiment al challenge is to control the homogeneity of the sand-water mixture and to minimize water evaporation during the experiments. To this end, the mixture of a 0.2mm natural sand and water was achieved in a concrete mixer and then left in a sealed container over 24 hours before it is spread evenly over the whole 7m length of the tunnel. Importantly, the air entering the tunnel is fully saturated of water vapor with the help of ultrasonic fog makers. We investigated two different configurations of transport: (i) the first one with zero upwind particle flux and (ii) the second with a finite upwind flux. The first configuration allows us to assess the static threshold of transport while the second one provides information about the dynamic threshold. For each experiment which typically lasts one or two minutes, the mass flow rate is obtained both via sand trap measurements and particle imaging. Preliminary results confirm that the presence of moisture within the bed increases the static threshold of transport but by a much greater factor than previously reported in the literature. For example with 3% moisture content, we found a threshold wind speed of 25m/s compared to the 8m/s given in the literature (see Fig. 1a). In contrast, the dynamic threshold is significantly lower and is close to that obtained for dry sand. Additionally, experiments with a finite upwind mass flux lead to mass flow rates comparable with those obtained for dry sand. At moderate wind speed (5m/s), the mass flow rate over 3% wet sand bed is surprisingly equivalent to that found for dry sand while at higher wind speed (7 and 9 m/s), it is smaller (see Fig. 1b). These results suggest that the saturated value of transport over a wet sand bed is the same as over dry sand but that the saturation length is much longer than that in dry conditions. We strongly suspect that with a longer tunnel, the mass flow rate would have reached the same value as for dry sand at any speed. These new results clearly rise the issues of saturated state of transport over wet sand beds and the sensitivity of the saturation length to moisture content.

International audience; In alpine terrain, blowing snow events strongly affect the local evolution of the avalanche danger and must be taken into account by avalanche hazard forecasters. This study presents the implementation and the evaluation of the blowing snow scheme Sytron into the operational chain for avalanche hazard forecasting (named S2M) used in the main French mountain ranges. S2M-Sytron provides information on blowing snow occurrence and intensity per 300-m elevation bands and aspects for several regions of the French mountains. The wind forcing is provided by the meteorological analysis system SAFRAN. S2M-Sytron was evaluated for winter 2015/16 at 11 automatic stations measuring wind speed and blowing snow fluxes in the French Alps. The system detects 55% of blowing snow days with less than 10% of false alarms. S2M-Sytron captures the occurrence of blowing snow events with and without concurrent snowfall. Improvements are obtained when considering an updated parameterization for the properties of falling snow which reduces the threshold velocity for freshly fallen snow. Using observed wind speed instead of SAFRAN wind speed to drive Sytron shows further improvements at stations where SAFRAN wind speed differs from the observations due to local topographic features. Overall, S2M-Sytron provides a regional blowing snow assessment but cannot fully reproduce the local intensity of blowing snow events.

International audience; The present study uses a numerical avalanche propagation model based on depth-averaged equations supplemented with the Voellmy snow rheology. The effectiveness of the complex avalanche protection system currently built in the run-out zone of Taconnaz avalanche path, in the French Alps in Chamonix Mont-Blanc valley, is evaluated. This work was motivated by the fact that the protection structure initially planned to mitigate a centennial reference avalanche event could not be fully achieved primarily due to some budgetary constraints. A number of numerical simulations were conducted, considering different avalanche scenarios in terms of both the volume and the Froude number of the incoming flows at the entrance of the current avalanche protection system. Particular attention was paid to the residual volumes that were able to overtop the 25-m-high catching dam settled at the downstream end of the avalanche protection system, thus quantifying the overall effectiveness of the current protection system for the different scenarios considered.

International audience; OZCAR (Critical Zone Observatories - Application and Research) is a national Research Infrastructure dedicated to providing an integrated understanding of the Earth's surface processes ranging from the subsurface to the lower atmosphere and from high mountains to coastal areas. It includes over 100 highly instrumented sites distributed among 21 observatories for long-term measurements of biological, chemical and physical parameters of groundwater, river water, glaciers, soils, and wetlands in France and overseas. Based on long-term observations at the landscape scale, OZCAR sites address specific environmental question of local societal relevance but all share the same overarching goal of better predicting the response of the Critical Zone to perturbations across a range of timescales. Diversity of OZCAR observation sites allows exploring laterally and vertically all compartments of the CZ from mountains to costal areas. This enables data acquisition on water, soils, agricultural practices and exchanges with the atmosphere. Thus, OZCAR proposes a great variety of models that ultimately will help reproducing observations and predict the evolution of the CZ. Through its wealth of environmental data portal and modelling platerforms, OZCAR aims at advising policy makers and stakeholers on the water, soil, and biodiversity resource. Recently, OZCAR integrated together with the French LTSER network (Zones Ateliers) the pan-European eLTER-ESFRI project.

National audience; Cet article fait une analyse en retour brève de l'avalanche de neige de grande ampleur survenue le 9 janvier 2018 et qui a rempli en grande partie le paravalanche.

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; 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; The SOERE CRYOBS-Clim aims at gathering monitoring strategies and observations performed on mountain glaciers, polar ice-sheets, seasonal snow cover and mountain permafrost in different regions (European Alps, tropical Andes, Himalayas, Antarctica,Svalbard).It addresses major societal issues such as water resources, ice-related hazards, atmospheric processes, avalanches, sea level rise, global climate change, …The observation strategy is based on in-situ (automated and manual) and remote (photogrammetry, LiDAR, satellite) monitoring of snow and ice related variables, which are conducted at different spatial scales (from tens of square meters to tens of square kilometers) and different time scales (from hourly to decadal) depending on the observation purpose. Many variables of interest are challenging to measure, requiringthe developments of innovative technology.An overview of data, new technologies and examples of significant results obtained in he framework of CRYOBS-CLIM will be given in the presentation.

International audience; The SOERE CRYOBS-Clim aims at gathering monitoring strategies and observations performed on mountain glaciers, polar ice-sheets, seasonal snow cover and mountain permafrost in different regions (European Alps, tropical Andes, Himalayas, Antarctica, Svalbard). The monitoring and research topics consist in documenting and studying: i) Ice, water, and vapor mass fluxes (precipitation, snow transport, melt ...) mainly at the interface with the atmosphere. ii) Radiative and turbulent energy fluxes between the atmosphere and the surface, and the thermodynamic variables of the boundary layer. iii) The internal state of subsurface systems (temperature, conductivity, density, liquid water content, etc.) and the resulting continental water mass storage (water resource, sea-level). iv) Ice dynamics It addresses major societal issues such as water resources, ice-related hazards, atmospheric processes, avalanches, sea level rise and ocean circulation, global climate change. Significant examples obtained in the framework of CRYOBS-CLIM will be given in the presentation : water ressources in the Andes, blowing snow forecast related to avalanche hazard, glacial destabilization (including rock glaciers) in the French Alps...

International audience; Draix-Bleone observatory is located in the French South Alps in a highly erodible badland area. It was created in 1983 to study hydrology and erosion processes in mountain and its focus has been extended since then to critical zone processes, including interactions between physical, chemical and biological components. In this poster we present recent results and innovative methods ranging from detailed measurements and modeling of sediment transport and soil moisture, chemical and physical denudation rates, chemical tracing of water fluxes, to the interactions between plant traits anddiversity, soil formation and stability, and interactions between vegetation cover and erosion.

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.

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.

International audience

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.

[Departement_IRSTEA]EAUX [TR1_IRSTEA]RIVAGE [Departement_IRSTEA]EAUX [TR1_IRSTEA]RIVAGE; International audience; Snow accumulation in alpine terrain is controlled by three main processes that act at different spatial scales: (i) orographic snowfall, (ii) preferential deposition of snowfall, and (iii) wind-induced snow transport of deposited snow. The relative importance of these processes largely remains uncertain at small scale (10-100 m). This study presents how high-resolution coupled snowpack/atmosphere simulations help quantifying the effects of these processes. The simulation system consists of the detailed snowpack model Crocus and the atmospheric model Meso-NH used in Large Eddy Simulation mode. Dedicated routines allow the coupled system to explicitly simulate wind-induced snow transport. Our case study is a snowfall event that occurred in February 2011 in the French Alps. Three nested domains at 450, 150 and 50 m grid spacing allow the model to simulate the complex 3D precipitation and wind fields down to fine scale. We firstly assess the ability of the coupled model to reproduce meteorological conditions during the event (wind speed and direction, snowfall amount, and blowing snow fluxes). The spatial variability of snowfall and snow accumulation is then considered. At 50 m grid spacing, snowfall presents local maxima associated with the formation of rimed snow aggregates and graupel in regions of sustained updrafts. Variograms show that the resultant spatial variability of snowfall is lower than the variability of snow accumulation when considering snow transport. Despite an overestimation of simulated blowing fluxes, our results suggest that wind-induced snow transport is the main source of spatial variability of snow accumulation in our case study.

International audience; It is well known by avalanche forecasters that wind plays a determinant role in shaping the small-scale distribution of snow and strongly influences the evolution of the avalanche danger. That is why the Snow Research Center (Météo-France) has focused, over the last years, its efforts to incorporate a parameterization of the wind effects on the snowpack in its numerical modeling chain for avalanche hazard forecasting. It simulates snow redistribution between the windward and the leeward side of virtual crests and takes into account the modification of particle morphology and the sublimation of a part of the redistributed snow amount. This drifting snow scheme has been implemented in the French operational modeling chain for avalanche hazard forecasting (S2M) for the last winter season (2015-2016). New diagnosis are proposed to the avalanche forecasters including data on wind (speed and direction) and intensity of blowing snow (on a 4-level scale) for the last 24 hours and the next 48 hours. An evaluation of the interest and relevance of this application by the forecasters in the French Alps and Pyrenees is in progress. In this paper we describe the drifting snow scheme and the information provided to the avalanche forecasters. Then we evaluate the ability of this scheme to simulate the daily occurrence of drifting snow at several observation points equipped with FlowCapt sensors measuring drifting snow fluxes. We also carry out the evaluation at the experimental site of Col du Lac Blanc (2700m a.s.l) where drifting snow events are carefully monitored along the winter season.

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; Main features of debris flows and generated hazards.

National audience; An overview of phenomena occurring in case of flood in mountain streams.

Description of the main features of a debris flow event occurred in 2005 in the Mont-Blanc range, France., Description de la lave torrentielle du 22 août 2005 sur le Nant d'Armancette.

National audience; Aussi fascinante qu’elle puisse être, la montagne n’en est pas moins dangereuse. Avalanches, risques glaciaires, crues et laves torrentielles, mouvements de pente et chutes de blocs font partie du vécu de tout habitant ou pratiquant de la montagne. Le développement économique, dont le tourisme qui attire en montagne des populations peu conscientes des dangers, accroît l’exposition aux risques. La catastrophe du camping du Grand-Bornand et ses 23 victimes en 1987, ou encore l’avalanche de Montroc en 1999 qui causa la mort de 12 personnes, en sont de frappantes illustrations. En toile de fond de l’ouvrage se profilent également le problème du changement climatique et le défi qu’il constitue pour les territoires de montagne plus sensibles aux variations de température que le reste du territoire national métropolitain. Cet ouvrage apporte au lecteur, simple amateur ou passionné de montagne, du grand public au décideur, une meilleure connaissance de ces phénomènes, des techniques de protection pour s’en prémunir, des recherches en cours et de leurs limites… Il a été rédigé par les meilleurs spécialistes du domaine afin de renforcer la prise de conscience des risques et des actions de prévention par les différents acteurs de la montagne. Schémas pédagogiques, photos prises sur le vif et témoignages en font un ouvrage de référence sur les risques naturels dans tous les massifs montagneux européens.

National audience; Une fois que l'on a imaginé comment mesurer le transport de neige par le vent, que l'on a réalisé des prototypes, que l'on a éprouvé ces instruments en soufflerie, que l'on a conçu des protocoles de mesure, il faut passer à l'épreuve de vérité : tester les systèmes en conditions réelles. Rien ne remplace alors les campagnes d'essais et de mise au point sur le terrain. mais il s'avère bien compliqué de tester des systèmes dédiés à l'étude et à la prévision des blizzards sans installations adéquates. C'est pour cette raison que le laboratoire du col du Lac blanc, en Isère, a été mis en place. Son but est de permettre la validation d'instruments ou de modèles et, de façon générale, d'aider à élargir les connaissances sur le phénomène du transport de neige par le vent.

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.

National audience; Potential damage processes generated by floods of mountain streams.

National audience; Le transport de neige par le vent, phénomène courant dans les régions froides et ventées, est problématique : il entraîne notamment la formation de congères, réduit la visibilité et accroît le risque d'avalanche en montagne. On sait que la connaissance des quantités de neige transportée est indispensable à la prévention de ces phénomènes. Mais comment évaluer ces quantités ? Faut-il attraper, compter ou encore écouter les flocons de neige soufflés par le vent ?

National audience; S’il suffit de retourner une boule à neige pour simuler grossièrement une chute de neige, la secouer frénétiquement ne vous permettra pas de reproduire une tempête. Car, qui dit tempête, dit vent et impossible de simuler du vent sans ventilateur…

National audience; Chaque hiver, on voit fleurir dans les médias articles et reportages sur la neige et les avalanches. C’est l'occasion de rafraîchir ses connaissances dans le domaine et d'intégrer les messages de prévention. C'est également la saison des idées reçues (ou non) sur le sujet.

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.

International audience; Several CFD snow drift models were proposed in the literature. These models include various assumptions and scaling laws representing the mass exchange, the transport and diffusion of particles and the feedback on the air flow. The validations of such existing models were conducted over a partial set of data. This is one of the reasons why we carried out a series of experiments using the low temperature wind tunnel at Shinjo Branch of Snow and Ice Studies (CES : Cryospheric Environment Simulator), NIED in Shinjo, Yamagata Prefecture Japan. The first experiments we undertook allowed to characterize the used snow regarding the wind action. We placed an ultra-sonic anemometer and a snow particle counter at the end of the wind tunnel, and by increasing gradually the wind velocity until the snow entrainment start and we continued until attaining a significant snow fluxes and gradually decreasing the velocity until the transport vanished. We determined the threshold friction velocity. This procedure was conducted twice to evaluate the uncertainty of the determination. Secondly, and for three wind tunnel velocity (5.5 m/s, 7 m/s and 9 m/s) and using a snow depth sensor we measured the mass exchange rate at the base along the flow each 0.5 m and afterwards, we conducted the same experiments by measuring the basal wind velocity and mass fluxes each 1 m from the ground. Finally and at the end of the wind tunnel, where the steady state is clearly attained and using an ultrasonic anemometer and a snow particle counter, we explored the vertical velocity and mass flux evolution. These data are analyzed to explore the macroscopic scaling law relating the mass exchange to the air flow velocity and the mass fluxes and the relation between the horizontal mass flux and the mean velocity. Since these data cover a significant range of flow velocity and include the determination of grain characteristics, the threshold velocity, the erosion fluxes, the air flow and snow fluxes at the interface between the air and the snow at rest, along the flow in the unsteady phase and the vertical snow fluxes and wind velocity profiles in the steady phase, it is a good opportunity to use these data to conduct an international benchmark between available snow drift CFD numerical 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; The influence of an obstacle on the dynamics of a finite-volume density current modelling a powder-snow avalanche was investigated. A constant volume of a dyed salt solution reproduced the small-scale aerosol flowing down an inclined channel immersed in a water tank. Reference tests in the absence of the obstacle characterized the dynamics parameters of the flow and then the influence of two types of obstacles on these parameters was studied. Both of the obstacles represent a catching dam, one with a vertical uphill face (OBS1), and the second one with an inclined uphill face (OBS2). A high resolution acoustic velocimeter allows measurements on the 3D flow velocity. For the reference avalanche, it was shown that the maximum velocity norm can be up to 18% greater than the maximum horizontal contribution (parallel to the slope). In terms of protection effectiveness, laboratory tests showed that a catching dam with the upstream face vertical to the slope is more efficient than a dam with an inclined upstream face. In presence of OBS2 the flow does not hit the obstacle but it rather passes smoothly over it, without any visible detachment from the surface. The ramp effect is remarkable and the avalanche reaches in a shorter time a given point downstream from the obstacle. On the contrary, in the OBS1 configuration, the incoming flow hits the vertical wall and bursts. The flow is subjected to a strong deflection with the formation of a vertical jet.

[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.