Barrières à neige en pente : modélisation physique dans la soufflerie climatique du CSTB à forte vitesse de vent

International audienceIn order to determine the effect of steep slopes on snowdrift generated by snow fences, we have conducted physical modeling experiments in the CSTB (Centre Scientifique et Technique du Bâtiment) cold wind tunnel as part of the European project "Access to Large Facilities". After an overview of previous studies and an accurate description of the drifting snow process inside the experimental chamber, we present the main results obtained. (1) On flat areas, even for high wind speed, the acknowledged results for moderate wind are still valid: the porous snow fence (50%) is the most efficacious and the bottom gap increases the efficacy of the dense snow fence. (2) The steeper the slope is, the less effective all tested snow fences are. Their effectiveness decreases considerably: the snow catch is approximately divided by two for a slope of 10°. (3) Contrary to flat areas, on steep slopes, the "efficacy" is greater for a dense snow fence

Modélisation physique de l'interaction entre obstacles et avalanches de neige poudreuse

International audienceIn order to better understand the interaction between powder snow avalanches and defence structures, we carried out physical experiments on small-scale models. The powder snow avalanche was simulated by a heavy salt solution in a water tank. Quasi two-dimensional and three-dimensional experiments were carried out with different catching dam heights. For the reference avalanche, the velocity just behind the nose in the head was greater than the front velocity. For the 2-D configuration, the ratio Umax/Ufront was as high as 1.6, but it depends on the height. For the 3-D configuration, this ratio differed slightly and was even greater (up to 1.8). The vertical velocity rose to 106% of the front velocity for the 3-D simulation and 74% for the 2-D simulation. The reduction in front velocity due to the presence of dams was an increasing function of the dam height. But this reduction depended on topography: dams were more effective on an open slope avalanche (3-D configuration). The ratio Umax/Ufront was an increasing function of the dam's height and reached a value of 1.9. The obstacle led to a reduction in vertical velocity downstream of the vortex zone

Effect of unsteady wind on drifting snow: first investigations

Wind is not always a steady flow. It can oscillate, producing blasts. However, most of the current numerical models of drifting snow are constrained by one major assumption: forcing winds are steady and uniform. Moreover, very few studies have been done to verify this hypothesis, because of the lack of available instrumentation and measurement difficulties. Therefore, too little is known about the possible role of wind gust in drifting snow. In order to better understand the effect of unsteady winds, we have performed both experiments at the climatic wind tunnel at the CSTB (Centre Scientifique et Technique des Bâtiments) in Nantes, France, and in situ experiments on our experimental high-altitude site, at the Lac Blanc Pass. These experiments were carried out collaboratively with Cemagref (France), Météo-France, and the IFENA (Switzerland). Through the wind tunnel experiments, we found that drifting snow is in a state of permanent disequilibrium in the presence of fluctuating airflows. In addition, the in situ experiments show that the largest drifting snow episodes appear during periods of roughly constant strong wind, whereas a short but strong blast does not produce significant drifting snow.&nbsp;</p> <p style='line-height: 20px;'><b>Key words.</b> Drifting snow, blowing snow, gust, blast, acoustic senso

Two-fluid barotropic models for powder-snow avalanche flows

14 pages, 1 figure. Accepted to Springer series "Notes on Numerical Fluid Mechanics and Multidisciplinary Design". Other authors papers can be downloaded at http://www.lama.univ-savoie.fr/~dutykh/International audienceIn the present study we discuss several modeling issues of powder-snow avalanche flows. We take a two-fluid modeling paradigm. For the sake of simplicity, we will restrict our attention to barotropic equations. We begin the exposition by a compressible model with two velocities for each fluid. However, this model may become non-hyperbolic and thus, represents serious challenges for numerical methods. To overcome these issues, we derive a single velocity model as a result of a relaxation process. This model can be easily shown to be hyperbolic for any reasonable equation of state. Finally, an incompressible limit of this model is derived

Lac Blanc Pass: a natural wind-tunnel for studying drifting snow at 2700ma.s.l

International audienceThe 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

The design of avalanche protection dams : Recent practical and theoretical developments

This book discusses the design of dams and other protective measures in the run-out zones of wet- and dry-snow avalanches. It summarises recent theoretical developments and the results of field and laboratory studies, combining them with traditional design guidelines and principles to formulate design recommendations. Not discussed are hazard zoning, land use planning, evacuations, supporting structures in starting zones, snow fences in catchment areas, and other safety measures outside the run-out zone. Reinforcement of individual buildings also falls outside the scope of the book, as do protective measures against landslides and slushflows.European Comissio

Classification and comparison of snow fences for the protection of transport infrastructures

Blowing snow or sand transport generates serious problems such as transport infrastructures buried under snow or sand in many parts of the world. Some of the most important problems that snow and sand storms can cause include drivers getting trapped on the roads, traffic being held up indefinitely, accidents occurring and populations being isolated. Snow fences provide a solution to this problem as they can hold back the snow, preventing displacement and wind-induced drifting. In this way, they reduce these problems on transport infrastructures and improve visibility, providing safer driving conditions. In this review, a classification is proposed of snow fences based on three basic types: earth, structural and living snow fences. Among the structural ones, non-porous and porous snow fences are distinguished. The different possibilities in terms of the placement of snow fences are also analyzed. Finally, different types of snow fences have been compared under design, construction and operation criteria. This review can provide initial guidelines for technicians to choose the best snow fence for blizzard conditions

De l'usage de la modélisation numérique des avalanches de neige pour le zonage du risque

By cold weather, most of the biggest snow-avalanches, which are the most extended and have the most destructive effects, are due to high-level precipitation triggered storms. Spatial repartition of snow is modified by the transportation of snow by the wind in 80% of the case at the beginning of the snow-avalanche. As the zoning is essentially about the biggest snow-avalanche, modelling has been restricted to snow-avalanche mobilising fresh snow without cohesion . The modelling process treats the snow-avalanche flow from its beginning to its end. Snow-avalanche is divided into two layers: thick layer and with suspension layer. Two ways of modelling are possible in order to take into account the snow-avalanche risk. The first approach is based on the shape analysis of the avalanche corridor, an analysis of the vegetal clad and a survey to the inhabitants. This way of processing was the only means to make cartography of the hazards. For the last decades, the growing numerical modelling of avalanches has allowed the conception of a new approach based on the use of numerical models. Thanks to geographical information systems, the first two approaches were associated. This integrated approach, which is illustrated in this paper merges the naturalist observations with results from models in order to draw the best solution to the problem of zoning. The corridor of Bourgeat (74, France) illustrates this process.Les avalanches majeures, qui ont les plus grandes extensions et les effets destructeurs les plus importants se produisent, pour la majorité d'entre elles, à la suite de tempêtes qui occasionnent d'importantes précipitations par temps froid. La répartition spatiale de la neige dans la zone de départ est dans 80% des cas, modifiée par le transport de la neige par le vent. Comme le zonage concerne essentiellement l'avalanche majeure, nous nous sommes restreints pour notre modélisation aux avalanches mobilisant de la neige fraîche sans cohésion. La modélisation proposée traite de l'écoulement de l'avalanche depuis son déclenchement jusqu'à son arrêt. L'avalanche est décomposée en deux couches : une couche dense et une couche de suspension. Afin de prendre en compte le risque d'avalanches en zone de montagne, deux approches sont utilisées. La première est basée sur l'analyse morphologique du couloir complètée par une analyse du couvert végétal et une enquête auprès des habitants. Cette procédure a été pendant longtemps la seule voie possible pour cartographier l'aléa. Ces dernières décennies, le fort développement en matière de modélisation numérique des avalanches a permis d'envisager une deuxième approche basée sur l'utilisation des modèles numériques. Grâce à l'exploitation des apports des systèmes d'informations géographiques, une approche intégrée couplant les deux premières, est alors envisagée. Cette méthodologie, qui est présentée dans cet article, cherche à faire coupler observations naturalistes et résultats des modèles pour apporter une meilleure réponse au problème de zonage. Cette démarche est illustrée par le cas du couloir du Bourgeat (74, France)

Avalanche mixte de neige sèche considérée comme un écoulement granulaire

Extrait de documentProposed modelling deals with the avalanche flow since its beginning to its stop. An avalanche is divided into two parts. The lower layer, which is the most concentrated, behaves like a dense flow. According to shearing rate and volume concentration, its flow is described either by the frictional model of Mohr-Coulomb, or by an inertial model that is drawn from the kinetic theory of granular environments. This part of the model was ratified after calculating some experimental measures in reduced model at the Col du Lac Blanc. The outer layer has a low concentration. It reports the development of an aerosol. The flow is turbulent biphasic and energy dissipation is dominated by the interstitial flow turbulence. The formation and the development of the aerosol result from erosion at the edge of the dense layer in movement. This interface is analysed with the saltation theory. Mass flux that is taken is proportional with the difference between constraints, which are exerted by the aerosol, and the resistance against the pulling out of particles. A stocking model that is operating when the turbulence decreases completes erosion modelling.La modélisation proposée traite de l'écoulement de l'avalanche depuis son déclenchement jusqu'à son arrêt. L'avalanche est décomposée en deux couches. La couche inférieure, la plus concentrée, se comporte comme un écoulement dense : en fonction du taux de cisaillement et de la concentration volumique, l'écoulement est décrit soit par le modèle frictionnel de Mohr-Coulomb, soit par un modèle inertiel issu de la théorie cinétique des milieux granulaires. Cette partie du modèle a été validée sur des mesures expérimentales en modèle réduit réalisées au Col du Lac Blanc. La couche supérieure de faible concentration, rend compte du développement d'un aérosol ; l'écoulement y est turbulent diphasique et la dissipation d'énergie dominée par la turbulence du fluide interstitiel. La formation et le développement de l'aérosol résultent de l'érosion au sommet de la couche dense en mouvement. Cette interface est traitée par la théorie de la saltation. Le flux de masse prélevée est proportionnel à l'écart entre les contraintes exercées par l'aérosol et la résistance à l'arrachement des particules. Le modèle d'érosion est complété par un modèle de dépôt, qui opère lorsque la turbulence diminue. Ces deux modèles d'érosion et de dépôt ont été calibrés par des essais en soufflerie réalisés notamment à l'occasion des travaux de recherche conduits dans le domaine du transport de la neige par le vent. Enfin le modèle global bi-couches a été utilisé pour reproduire des évènements d'avalanches réelles

Drifting snow modelling in an alpine context

International audienceWind-transported snow is a common phenomenon in cold windy areas such as mountainous and polar regions. 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. In this context, a numerical simulation of drifting snow would be very helpful. The NEMO numerical model of drifting snow is based on a physical model for saltation and turbulent diffusion.The saltation layer is described by its height, concentration and two turbulent friction velocities, one for the solid phase and one for the gaseous phase. The suspension layer is described by mass and momentum conservation equations. These equations were formulated both for the solid phase and the gaseous phase. The interaction between these two phases was taken into account by an equation based on the drag force of a particle in a turbulent flow. Turbulence was modeled by the k-ε model, in which a reduction in the turbulence with the concentration was introduced. The exchange between the saltation layer and the snow cover was described by an erosion and deposition model. The mesh was adapted to the temporal change of the drift. The model needs a set of input parameters including fall velocity, threshold shear velocity, shear velocity, mass concentration and roughness, which can be accurately measured in the case of cohesionless particles blown in a wind tunnel. NEMO has been tested by comparing leeward and windward drift equilibrium obtained with sand or sawdust in a wind tunnel near a small-scale snow fence with all these input parameters known; moreover these parameters remained constant throughout the experiment. However, the computation results were often less conclusive when compared to the results of field experiments conducted at Col du Lac Blanc, in the French Alps at 2700 m elevation. It is true that the fully coupled wind and snowdrift model was not used in this case because of the time required for calculation and that additional assumptions were made. However, it is probable that the most important source of uncertainty stems from the lack of accurate evaluations of the input parameters needed for the numerical model. Thus, some parameters were estimated from semiempirical relationships obtained with data probably collected under different conditions than those encountered in the Alps in terms of topography and snow types. That is why it is necessary to go back to the field to better determine the required field data in an alpine context : blowing snow acoustic instruments (Flowcapt and Snow Particle counter), a 10-m mast with six anemometers, two temperature sensors and a depth sensor were set up on our experimental site at Col du Lac Blanc (2700 m) in the French Alps. Data obtained during the last several winters will be presented and compared with semiempirical formulae used in the NEMO numerical model