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International audience; The Col du Lac Blanc (French Alps, 2720 m a.s.l., Apes d’Huez ski resort) has been since 1990 a shared research field site dedicated to observation and modelling studies on wind-induced snow transport operated by Snow Research Centre (Centre d’Etudes de la Neige, Centre National de Recherches Météorologiques, Météo-France – CNRS) and the ETNA unit (IRSTEA, Univ. Grenoble Alpes) It is now a part of research Infrastructure OZCAR which is dedicated to Critical Zone study. Its specific toporaphy (pass) allows about 90% of wind conditions to be channelized in the N-S direction, thus acting as a natural wind tunnel. The site is equipped with a dedicated, permanent instrumentation focused on wind and snow transport, standard meteorological observations and more recently, radiations. Meteorological data are shared (Guyomarc’h et al., 2018) (http://doi.osug.fr/public/CRYOBSCLIM_CLB/index.html.) Recent studies have focused on intercomparison of blowing snow sensors (Cierco et al., 2007; Trouvilliez et al., 2015), fine scale processes during blowing snow events (Naaim-Bouvet et al., 2010, 2011, 2013; Nishimura et al., 2014) and the development and evaluation of blowing snow models (Durand et al., 2005; Vionnet et al., 2013, 2014, 2017), including blowing snow scheme which have been implemented in a system for avalanche hazard forecasting (Vionnet et al., 2018).Classical terrestrial laserscans have also been performed at least twice each snow season since 2010, enabling the study of snow erosion/deposition pattern and the validation of numerical models. It also allows to improve a terrain-based parameter for the assessment of snow depths in combination with blowing snow fluxes measurements (Schön et al., 2015, 2018). An automatic Rugged Laser Scan (RLS) covering daily a surface area of 200 m2 was also set up with the aim of understanding the link between snow surface roughness, aerodynamic roughness and snow mass fluxes. Detailed measurements of temperature and humidity in combination with measurements of blowing snow fluxes allowed the study of blowing snow sublimation and its feedbacks on the surface boundary layer. We will present a brief overview of the site's instrumentation and ongoing research related to snow transport in the context of avalanche forecasting.

International audience; The present paper describes preliminary re-analyses of field-based data on past well-documented snow avalanches that have impacted an instrumented tripod structure in one of the paths at Lautaret avalanche test-site, France. The re-analyses done include data on velocity and pressure measurements , as well as new data on flow-depth measurements. The latter data was obtained with the help of re-analyses of pressure signals. The various techniques used and assumptions made are presented and discussed, which allows us to infer how the thickness of the dense flow, and both the velocities and pressures over depth, all change simultaneously over time. The present work pays attention to the gravity-dominated flow regime occurring after the passage of the avalanche front. That regime is characterized by a mean pressure on the tripod structure that is essentially controlled by the flow thickness, unlike the inertia-dominated regime which is deemed to be driven by the square of the flow velocity during the passage of the avalanche front. Moreover, a change in dynamics is well identified during the gravity-dominated flow regime, while moving from the avalanche core to the avalanche tail.

International audience; The present paper describes preliminary re-analyses of field-based data on past well-documented snow avalanches that have impacted an instrumented tripod structure in one of the paths at Lautaret avalanche test-site, France. The re-analyses done include data on velocity and pressure measurements , as well as new data on flow-depth measurements. The latter data was obtained with the help of re-analyses of pressure signals. The various techniques used and assumptions made are presented and discussed, which allows us to infer how the thickness of the dense flow, and both the velocities and pressures over depth, all change simultaneously over time. The present work pays attention to the gravity-dominated flow regime occurring after the passage of the avalanche front. That regime is characterized by a mean pressure on the tripod structure that is essentially controlled by the flow thickness, unlike the inertia-dominated regime which is deemed to be driven by the square of the flow velocity during the passage of the avalanche front. Moreover, a change in dynamics is well identified during the gravity-dominated flow regime, while moving from the avalanche core to the avalanche tail.

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

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.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; In mountainous areas, drifting snow influences the spatial distribution of the snow cover and consequently snowpack stability and avalanche risk. When comparing models with in situ-measurements, it is first necessary to identify snow-drift events with and without concurrent falling snow. In Antarctica, the mass balance of the Antarctic ice sheet is a key parameter of sea level rise, which can be moderated by accumulation change. However, in the coastal areas where katabatic winds are strong and frequent, it is necessary to identify and separate blowing snow and precipitation. It was shown that size distribution of blowing snow particles given by a snow particle counter at a specific height can be approximated by a two-parameter gamma probability function and that a bi-modal distribution could appear at high elevation when snow fall occurs (Nishimura and Nemoto, 2005). In such way, it could be possible to evaluate precipitation. But depending on the height, the size of snow fall particle and the wind speed it is not so easy to distinguish between both types of events. In such case, the analysis of snow flux and mean diameter according to wind speed allows to separate blowing snow event with and without precipitation. It gives better results in predicting the timing of precipitation than done by precipitation gauges. A simpler photoelectric counter such as designed by Wenglor could also give some interesting results. These conclusions are supported by field campaigns conducted in French Alps and in Antarctica (coastal Adélie Land) during last winters.

International audience; Snow avalanches threaten mountain communities worldwide: avalanches affect not only snow sport tourists but people can also be caught in inhabited areas and on roads, as reminded by the extraordinary winter of 1998/99 in the Alps. It means that we have to increase local population awareness of avalanche hazard, risk assessment and mitigation. In this case, it is interesting to focus not only on avalanche release, as it is generally done for snow sport tourists but also on avalanche dynamics and interaction with structures. In the laboratory, powder avalanches are often modeled by density currents (salt water in pure water) and dense avalanches are modeled by granular avalanches. Similitude requirements are fulfilled to estimate real pressure, velocity, run-out distance from the small-scale model. But even if small-scale models are used in the laboratory, their dimensions are too large to be removed from the lab to exhibition halls, schools or meeting rooms. With water, salt, grains, aquarium and wood panels it is still possible to reproduce laboratory experiments physically based at a smaller scale to explain qualitatively the dynamics of powder and dense avalanches and to show the effectiveness or ineffectiveness of protective measures such as dams and retarding mounds. Examples of such experiments and devices, which can be reproduced everywhere, will be described in this paper, offering the children a unique opportunity to release the avalanche by their own and to play with grains and water. Such an initiative is thus always a success.

Le transport de neige par le vent, phénomène courant dans les régions froides et ventées, qu'elles soient polaires ou montagneuses, entraîne la formation de congères, augmente le risque d'avalanche et limite la visibilité. Que ce soit au niveau de la prévision ou de la prévention du phénomène, une meilleure connaissance des quantités de neige transportée est fondamentale. Cet article fait le point sur les différentes technologies disponibles, mécaniques, acoustiques ou optiques. Il base ses conclusions sur des essais effectués en soufflerie et in situ à 2 700 m d'altitude, sur le site du Col du Lac Blanc, dédié à l'étude du transport de neige par le vent. Si les capteurs acoustiques ne s'avèrent pas aussi prometteurs qu'attendu, les capteurs optiques permettent d'obtenir des données précises. La zone de mesures reste cependant dans ce cas de petite taille. Le couplage de la mesure avec une approche théorique reste donc indispensable. / Wind-transported snow is a common phenomenon in cold windy areas such as mountainous and polar regions. The resulting snowdrifts often cause problems for infrastructure and road maintenance and contribute significantly to the loading of the avalanche release area. It also reduces visibility. It is very important to better determine drifting snow fluxes in the framework of forecast or mitigation. Therefore this article reviews the different available technologies such as mechanical traps, acoustic or optical sensors. It bases its conclusion on essays carried out in wind-tunnel and on in situ experimental site at Col du Lac Blanc which is dedicated to the study of drifting snow. If acoustic sensors are not so promising as expected, optical sensors give accurate results. Nevertheless the measurement area is small-sized. That is why the coupling with a theoretical approach is still indispensable.