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A meteorological and blowing snow data set from the high-elevation experimental site of Col du Lac Blanc (2720 m a.s.l., Grandes Rousses mountain range, French Alps) is presented and detailed in this paper. Emphasis is placed on data relevant to the observations and modelling of wind-induced snow transport in alpine terrain. This process strongly influences the spatial distribution of snow cover in mountainous terrain with consequences for snowpack, hydrological and avalanche hazard forecasting. In situ data consist of wind (speed and direction), snow depth and air temperature measurements (recorded at four automatic weather stations), a database of blowing snow occurrence and measurements of blowing snow fluxes obtained from a vertical profile of snow particle counters (2010–2016). Observations span the period from 1 December to 31 March for each winter season from 2000–2001 to 2015–2016. The time resolution has varied from 15 min until 2014 to 10 min for the last years. Atmospheric data from the meteorological reanalysis are also provided from 1 August 2000 to 1 August 2016. A digital elevation model (DEM) of the study area (1.5 km2) at 1 m resolution is also provided in RGF 93 Lambert 93 coordinates. This data set has been used in the past to develop and evaluate physical parameterizations and numerical models of blowing and drifting snow in alpine terrain. Col du Lac Blanc is also a target site to evaluate meteorological and climate models in alpine terrain. It belongs to the CRYOBS-CLIM observatory (the CRYosphere, an OBServatory of the CLIMate), which is a part of the national research infrastructure OZCAR (Critical Zone Observatories – Application and Research) and have been a Global Cryospheric Watch Cryonet site since 2017. The data are available from the repository of the OSUG data centre https://doi.org/10.17178/CRYOBSCLIM.CLB.all.

This paper presents an assessment of the fragility of a reinforced concrete (RC) element subjected to avalanche loads, and more generally to dynamic pressure fields applied orthogonally to a wall, within a reliability framework. In order to obtain accurate numerical results with supportable computation times, a light and efficient Single-Degree-of-Freedom (SDOF) model describing the mechanical response of the RC element is proposed. The model represents its dynamic mechanical response up to failure. Material non-linearity is taken into account by a moment–curvature approach, which describes the overall bending response. The SDOF model is validated under quasi-static and dynamic loading conditions by comparing its results to alternative approaches based on finite element analysis and the yield line theory. Following this, the deterministic SDOF model is embedded within a reliability framework to evaluate the failure probability as a function of the maximal avalanche pressure reached during the loading. Several reliability methods are implemented and compared, suggesting that non-parametric methods provide significant results at a moderate level of computational burden. The sensitivity to material properties, such as tensile and compressive strengths, steel reinforcement ratio, and wall geometry is investigated. The effect of the avalanche loading rate is also underlined and discussed. Finally, the obtained fragility curves are compared with respect to the few proposals available in the snow avalanche engineering field. This approach is systematic and will prove useful in refining formal and practical risk assessments. It could be applied to other similar natural hazards, which induce dynamic pressure fields onto the element at risk (e.g., mudflows, floods) and where potential inertial effects are expected and for which fragility curves are also lacking.

The mountain cryosphere of mainland Europe is recognized to have important impacts on a range of environmental processes. In this paper, we provide an overview on the current knowledge on snow, glacier, and permafrost processes, as well as their past, current, and future evolution. We additionally provide an assessment of current cryosphere research in Europe and point to the different domains requiring further research. Emphasis is given to our understanding of climate–cryosphere interactions, cryosphere controls on physical and biological mountain systems, and related impacts. By the end of the century, Europe's mountain cryosphere will have changed to an extent that will impact the landscape, the hydrological regimes, the water resources, and the infrastructure. The impacts will not remain confined to the mountain area but also affect the downstream lowlands, entailing a wide range of socioeconomical consequences. European mountains will have a completely different visual appearance, in which low- and mid-range-altitude glaciers will have disappeared and even large valley glaciers will have experienced significant retreat and mass loss. Due to increased air temperatures and related shifts from solid to liquid precipitation, seasonal snow lines will be found at much higher altitudes, and the snow season will be much shorter than today. These changes in snow and ice melt will cause a shift in the timing of discharge maxima, as well as a transition of runoff regimes from glacial to nival and from nival to pluvial. This will entail significant impacts on the seasonality of high-altitude water availability, with consequences for water storage and management in reservoirs for drinking water, irrigation, and hydropower production. Whereas an upward shift of the tree line and expansion of vegetation can be expected into current periglacial areas, the disappearance of permafrost at lower altitudes and its warming at higher elevations will likely result in mass movements and process chains beyond historical experience. Future cryospheric research has the responsibility not only to foster awareness of these expected changes and to develop targeted strategies to precisely quantify their magnitude and rate of occurrence but also to help in the development of approaches to adapt to these changes and to mitigate their consequences. Major joint efforts are required in the domain of cryospheric monitoring, which will require coordination in terms of data availability and quality. In particular, we recognize the quantification of high-altitude precipitation as a key source of uncertainty in projections of future changes. Improvements in numerical modeling and a better understanding of process chains affecting high-altitude mass movements are the two further fields that – in our view – future cryospheric research should focus on.

Changement climatique : quelle stratégie d'adaptation pour les territoires de montagnes ? Avant-Propos

Rapid and large deformations of snow are mainly controlled by grain rearrangements, which occur through the failure of cohesive bonds and the creation of new contacts. We exploit a granular description of snow to develop a discrete element model based on the full 3-D microstructure captured by microtomography. The model assumes that snow is composed of rigid grains interacting through localized contacts accounting for cohesion and friction. The geometry of the grains and of the intergranular bonding system are explicitly defined from microtomographic data using geometrical criteria based on curvature and contiguity. Single grains are represented as rigid clumps of spheres. The model is applied to different snow samples subjected to confined compression tests. A detailed sensitivity analysis shows that artifacts introduced by the modeling approach and the influence of numerical parameters are limited compared to variations due to the geometry of the microstructure. The model shows that the compression behavior of snow is mainly controlled by the density of the samples, but that deviations from a pure density parameterization are not insignificant during the first phase of deformation. In particular, the model correctly predicts that, for a given density, faceted crystals are less resistant to compression than rounded grains or decomposed snow. For larger compression strains, no clear differences between snow types are observed.

Dry-snow slab avalanches are generally caused by a sequence of fracture processes, including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually increased our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically based statistical model of the slab–WL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by slab elasticity and possible tensile failure of the slab. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and trigger a tensile failure within the slab; (2) partial-slope release if fracture arrest and slab tensile failure occur due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as often claimed in the literature.

Snowpack weak layers may fail due to excess stresses of various natures, caused by snowfall, skiers, explosions or strong ground motion due to earthquakes, and lead to snow avalanches. This research presents a numerical model describing the failure of "sandwich" snow samples subjected to shaking. The finite element model treats weak layers as interfaces with variable mechanical parameters. This approach is validated by reproducing cyclic loading snow fracture experiments. The model evaluation revealed that the Mohr–Coulomb failure criterion, governed by cohesion and friction angle, was adequate to describe the experiments. The model showed the complex, non-homogeneous stress evolution within the snow samples and especially the importance of tension on fracture initiation at the edges of the weak layer, caused by dynamic stresses due to shaking. Accordingly, a simplified analytical solution, ignoring the inhomogeneity of tangential and normal stresses along the failure plane, may incorrectly estimate the shear strength of the weak layers. The values for "best fit" cohesion and friction angle were ≈1.6 kPa and 22.5–60°. These may constitute valuable first approximations in mechanical models used for avalanche forecasting.

Natural sintering in ice is a fundamental process determining mechanical properties of various ice forms. According to the literature, limited data are available about the complex subjects of snow sintering and bond formation. Here, through cold laboratory mechanical tests with a new shear apparatus we demonstrate time-dependent effects of isothermal sintering on interface strengthening at various normal pressures. Measurements showed that interfacial strength evolved rapidly, conforming to a power law (mean exponent ≈ 0.21); higher pressure corresponded to higher initial strength and sintering rates. Our findings are consistent with observations on homogeneous snow, provide unique records essential for slope stability models and indicate the significant importance of normal load on data interpretation.

Snow avalanches are a threat to many kinds of elements (human beings, communication axes, structures, etc.) in mountain regions. For risk evaluation, the vulnerability assessment of civil engineering structures such as buildings and dwellings exposed to avalanches still needs to be improved. This paper presents an approach to determine the fragility curves associated with reinforced concrete (RC) structures loaded by typical avalanche pressures and provides quantitative results for different geometrical configurations. First, several mechanical limit states of the RC wall are defined using classical engineering approaches (Eurocode 2), and the pressure of structure collapse is calculated from the usual yield line theory. Next, the fragility curve is evaluated as a function of avalanche loading using a Monte Carlo approach, and sensitivity studies (Sobol indices) are conducted to estimate the respective weight of the RC wall model inputs. Finally, fragility curves and relevant indicators such a their mean and fragility range are proposed for the different structure boundary conditions analyzed. The influence of the input distributions on the fragility curves is investigated. This shows the wider fragility range and/or the slight shift in the median that has to be considered when a possible slight change in mean/standard deviation/inter-variable correlation and/or the non-Gaussian nature of the input distributions is accounted for.

This paper deals with the assessment of physical vulnerability of civil engineering structures to snow avalanche loadings. In this case, the vulnerability of the element at risk is defined by its damage level expressed on a scale from 0 (no damage) to 1 (total destruction). The vulnerability of a building depends on its structure and flow features (geometry, mechanical properties, type of avalanche, topography, etc.). This makes it difficult to obtain vulnerability relations. Most existing vulnerability relations have been built from field observations. This approach suffers from the scarcity of well documented events. Moreover, the back analysis is based on both rough descriptions of the avalanche and the structure. To overcome this problem, numerical simulations of reinforced concrete structures loaded by snow avalanches are carried out. Numerical simulations allow to study, in controlled conditions, the structure behavior under snow avalanche loading. The structure is modeled in 3-D by the finite element method (FEM). The elasto-plasticity framework is used to represent the mechanical behavior of both materials (concrete and steel bars) and the transient feature of the avalanche loading is taken into account in the simulation. Considering a reference structure, several simulation campaigns are conducted in order to assess its snow avalanches vulnerability. Thus, a damage index is defined and is based on global and local parameters of the structure. The influence of the geometrical features of the structure, the compressive strength of the concrete, the density of steel inside the composite material and the maximum impact pressure on the damage index are studied and analyzed. These simulations allow establishing the vulnerability as a function of the impact pressure and the structure features. The derived vulnerability functions could be used for risk analysis in a snow avalanche context.

For snow avalanches, passive defense structures are generally designed by considering high return period events. In this paper, taking inspiration from other natural hazards, an alternative method based on the maximization of the economic benefit of the defense structure is proposed. A general Bayesian framework is described first. Special attention is given to the problem of taking the poor local information into account in the decision-making process. Therefore, simplifying assumptions are made. The avalanche hazard is represented by a Peak Over Threshold (POT) model. The influence of the dam is quantified in terms of runout distance reduction with a simple relation derived from small-scale experiments using granular media. The costs corresponding to dam construction and the damage to the element at risk are roughly evaluated for each dam height-hazard value pair, with damage evaluation corresponding to the maximal expected loss. Both the classical and the Bayesian risk functions can then be computed analytically. The results are illustrated with a case study from the French avalanche database. A sensitivity analysis is performed and modelling assumptions are discussed in addition to possible further developments.

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. Key words. Drifting snow, blowing snow, gust, blast, acoustic sensor

In 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.

An experimental investigation with dry granular flows passing over an obstacle down a rough inclined channel has been performed. The aim is to improve our understanding of the interaction between dense snow avalanches and defence structures. Specific attention was directed to the study of the zone of influence upstream from the obstacle, linked to the formation of a dead zone. The dead zone length L was systematically measured as a function of the obstacle height H and the channel inclination θ, for several discharges. In a whole range of channel inclinations, all the data are shown to collapse into a single curve when properly scaled. The scaling is based on the introduction of a theoretical deposit length (depending on H, θ and the internal friction angle of the material, φ) and a Froude number of the flow depending on the obstacle height.

This paper presents the SPH (Smoothed Particles Hydrodynamics) numerical method adapted to complex rheology and free surface flow. It has been developed to simulate the local effect of a simple obstacle on a granular flow. We have introduced this specific rheology to the classical formalism of the method and thanks to experimental devices, we were able to validate the results. Two viscosity values have been simultaneously computed to simulate "plugs" and "dead zone" with the same code. First, some experiments have been done on a simple inclined slope to show the accuracy of the numerical results. We have fixed the mass flow rate to see the variations of the flow depth according to the channel slope. Then we put a weir to block the flow and we analysed the dependence between the obstacle height and the length of influence upstream from the obstacle. After having shown that numerical results were consistent, we have studied speed profiles and pressure impact on the structure. Also results with any topography will be presented. This will have a great interest to study real flow over natural topography while using the model for decision help.

The experience of calibrating Eglit’s avalanche model using data about the avalanche travel distance lengths and velocity data from the Zailiyskiy Alatau range (Kazakhstan) is discussed. Empirical equations are proposed that describe the dependence of the friction coefficients on the avalanche volume and type.

In 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.

During the winters of 1999 and 2000 large avalanches occurred in the ski resort of Las Leñas (Los Andes, Mendoza, Argentina). On 8 September 1999 an avalanche of new, dry snow ran over a path with a 1000 m vertical drop. On 30 June and on 1 July 2000 five avalanches of similar vertical drop, which start with new snow, entrained very wet snow during their descent, and evolved into dense snow avalanches. To use the MN2D dynamics model correctly, calibration of model parameters is necessary. Also, no previous works with the use of dynamics models exist in South America. The events used to calibrate the model occurred during the winters of 1999 and 2000 and are a good sample of the kind of avalanches which can occur in this area of the Andes range. By considering the slope morphology and topography, the snow and meteorological conditions and the results of the model simulations, it was estimated that these avalanches were not extreme events with a return period greater than one hundred years. This implies that, in natural conditions, bigger, extreme avalanches could happen. In this work, the MN2D dynamics model is calibrated with two different avalanches of the same magnitude: dry and wet. The importance of the topographic data in the simulation is evaluated. It is concluded that MN2D dynamics model can be used to simulate dry extreme avalanches in Argentinean Andes but not to simulate extreme wet avalanches, which are much more sensitive to the topography.

Based on wind tunnel measurements at the CSTB (Jules Verne) facility in Nantes and based on field observations at the SLF experimental site Versuchsfeld Weissfluhjoch, two acoustic wind drift sensors are evaluated against different mechanical snow traps and one optical snow particle counter. The focus of the work is the suitability of the acoustic sensors for applications such as avalanche warning and research. Although the acoustic sensors have not yet reached the accuracy required for typical research applications, they can, however, be useful for snow drift monitoring to help avalanche forecasters. The main problem of the acoustic sensors is a difficult calibration that has to take into account the variable snow properties. Further difficulties arise from snow fall and high wind speeds. However, the sensor is robust and can be operated remotely under harsh conditions. It is emphasized that due to the lack of an accurate reference method for snow drift measurements, all sensors play a role in improving and evaluating snow drift models. Finally, current operational snow drift models and snow drift sensors are compared with respect to their usefulness as an aid for avalanche warning. While drift sensors always make a point measurement, the models are able to give a more representative drift index that is valid for a larger area. Therefore, models have the potential to replace difficult observations such as snow drift in operational applications. Current models on snow drift are either only applicable in flat terrain, are still too complex for an operational application (Lehning et al., 2000b), or offer only limited information on snow drift, such as the SNOWPACK drift index (Lehning et al., 2000a). On the other hand, snow drift is also difficult to measure. While mechanical traps (Mellor 1960; Budd et al., 1966) are probably still the best reference, they require more or less continuous manual operation and are thus not suitable for remote locations or long-term monitoring. Optical sensors (Schmidt, 1977; Brown and Pomeroy, 1989; Sato and Kimura, 1993) have been very successful for research applications, but suffer from the fact that they give a single flux value at one specific height. In addition, they have not been used, to our knowledge, for long-term monitoring applications or at remote sites. New developments of acoustic sensors have taken place recently (Chritin et al., 1999; Font et al., 1998). Jaedicke (2001) gives examples of possible applications of acoustic snow drift sensors. He emphasizes the advantages of acoustic sensors for snow drift monitoring at remote locations, but could not present any evaluation of the accuracy of the measurements. We present a complete evaluation of the new acoustic sensors for snow drift and discuss their applications for research or avalanche warning. We compare the suitability of sensors for operational applications.

Le vent et l'impact d'un mouvement gravitaire rapide dans une retenue d'altitude, produisent des vagues dont la nature et l'amplitude dépendent des grandeurs de l'impactant (vent ou mouvement gravitaire rapide) et de la profondeur d'eau. Dans cet article, nous avons donné les principales lois d'échelle qui permettent d'évaluer le volume d'eau déplacé lors de l'interaction et l'onde hydrodynamique qui en résulte, en insistant non seulement sur l'amplitude des vagues mais aussi sur leurs natures. Le vent et les mouvements gravitaires lents produisent des ondes sinusoïdales alors que les mouvements gravitaires rapides produisent des ondes plus raides de type tsunami. L'extension des retenues d'altitude est limitée et les vagues produites rencontrent rapidement le parement du barrage (ou une berge). Elles génèrent une surélévation, dite run-up, dont l'amplitude dépend des caractéristiques de l'onde incidente et de la pente de la berge. Nous avons rassemblé suffisamment de données expérimentales et développé une analyse théorique pour montrer les similitudes et les différences qu'il y a entre les ondes sinusoïdales et les ondes solitaires en termes de run-up. Nous avons enfin choisi des formules simples et établi un abaque permettant à tout ingénieur de faire un premier diagnostic.

La carte de localisation des phénomènes d'avalanche (CLPA) a été créée en 1971 en réaction à l'avalanche meurtrière de Val d'Isère (février 1970, trente-neuf morts). Il s'agit d'inventorier et de mémoriser l'existence de zones où des avalanches se sont produites dans le passé et ainsi conserver une mémoire précise des limites extrêmes atteintes par ces avalanches. La CLPA s'est rapidement avérée indispensable à la mise en place des projets d'aménagement dans les territoires de montagne. À la suite de l'avalanche, tout aussi catastrophique, de Chamonix au hameau de Montroc en février 1999, la mission de retour d'expérience a recommandé « la valorisation réciproque de l'EPA (enquête permanente sur les avalanches) et de la CLPA intégrées dans un système d'information contenant les informations sur les avalanches et les informations sur les autres risques naturels en montagne ». Le ministère chargé de l'environnement a en conséquence décidé de rénover et poursuivre la CLPA, et en a confié la mission au Cemagref, avec la participation de l'Office national des forêts. La rénovation a principalement porté sur l'achèvement de la numérisation de la carte et des fiches signalétiques des témoignages, la rédaction de notices par massif synthétisant les principales informations sur les avalanches, la mise en place d'une mise à jour permanente de la carte et la mise en ligne de toutes les informations sur le site internet www.avalanches.fr. Les données contenues dans cette base représentent un potentiel réel pour la recherche. Le Cemagref a pour ambition de les exploiter en les couplant à des outils calibrés de modélisation numérique. Les paramètres de calage seront traités afin de dégager des lois statistiques qui pourraient être utilisées pour des sites non documentés.

Sur la base de l'analyse du comportement réel de structures impactées par des avalanches, cet article propose une synthèse accessible des principes relatifs au comportement des ouvrages sollicités de manière dynamique. Améliorer la connaissance de la pression exercée par une avalanche sur un ouvrage est en effet essentiel pour concevoir des structures résistantes. L'expérimentation in situ sur des ouvrages-capteurs exposés aux avalanches est une approche originale et complémentaire par rapport aux travaux dérivés de la mécanique des fluides. Des principes généraux de conception et de réalisation d'ouvrages rigides soumis à des impacts sont présentés. Enfin, des propositions et des perspectives opérationnelles sont avancées pour améliorer la prise en compte des actions dynamiques et mieux gérer les risques.

Cet article vise à montrer comment la connaissance scientifique récemment développée dans le domaine des avalanches de neige peut être utilisée pour l'étude d'un cas réel en ingénierie paravalanche. Il concerne le site de Taconnaz pour lequel un dispositif de protection existe déjà mais doit être maintenant amélioré. Dans un premier temps, une analyse morphologique du site est proposée et les données d'avalanches historiques sont analysées. Dans un second temps, chaque événement recueilli est rétro-analysé à l'aide d'un modèle numérique d'écoulements denses, et, pour chaque avalanche, les paramètres d'entrée dans la zone d'arrêt en amont des structures de protection sont définis. Dans un troisième temps, une analyse statistique de ces paramètres nous permet de caractériser les scénarii centennaux. Dans un quatrième temps, les modélisations physique et numérique des avalanches denses intéragissant avec les ouvrages de protection sont combinées afin de dimensionner le système de défense le plus efficace. Finalement, les modélisations physique et numérique de l'interaction de l'avalanche en aérosol avec le système de défense sont conduites pour montrer que les améliorations proposées n'accentuent pas le risque résiduel dû à l'écoulement de neige poudreuse en aval des ouvrages.

Pour la prévention des risques naturels en montagne, les notions d'aléa de référence et de période de retour sont souvent utilisées de manière peu académique. Cet article se propose de faire le point sur les méthodes existantes de prédétermination et de calcul de risque dans le cas des avalanches. Après un rappel des méthodes traditionnelles d'ingénierie ainsi que de leurs avantages et limites, les méthodes statistiques-dynamiques sont introduites. Celles-ci rendent possible l'utilisation rigoureuse de la période de retour en tant que fonction univoque de la distance d'arrêt. Elles permettent également d'évaluer l'ensemble des scenarii de référence correspondant à n'importe quelle période de retour. Enfin, dès lors que les conséquences dommageables sont évaluées en fonction de l'intensité de l'aléa, un calcul de risque conforme à la théorie de la décision peut être mené pour le zonage et le dimensionnement d'ouvrages de protection. Le cadre de travail proposé est illustré par un cas d'étude issu de l'enquête permanente sur les avalanches (EPA).

La prévention des risques naturels en montagne, des avalanches aux coulées de boue, passe par des simulations numériques de plus en plus poussées. Ces dernières mettent en ouvre des lois de comportement de matériaux complexes, comme les fluides à seuil ou les écoulements granulaires, ainsi que des méthodes numériques délicates. Ces simulations débouchent, souvent au bout de plusieurs heures voire de plusieurs jours de calcul, sur d'importants volumes de données. La dernière étape du processus consiste à communiquer de tels résultats au public ou aux politiques. Considérant que les outils d'aide à la décision en matière de risques naturels en montagne sont encore d'une accessibilité trop limitée pour les non-initiés, nous avons initié un projet dont la finalité est de tirer parti de la génération d'images virtuelles, afin d'améliorer le niveau de communication possible des outils de simulation existants. En pratique, le but de cette action est de développer un ensemble d'outils permettant d'aller de la simulation d'un risque naturel en montagne jusqu'à sa reproduction graphique visuellement réaliste. Après une rapide présentation des modèles mis en oeuvre, nous détaillons les travaux effectués pour dans un premier temps mettre au point des structures de maillage de calcul « adaptatifs » visant à rendre plus rapides et plus précis les modèles de calcul, dans un deuxième temps valider les modèles par confrontation à des événements réels, et dans un troisième temps mettre au point des outils d' « habillage « des coulées permettant un rendu visuel réaliste.