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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; Within the Operational programme 'Italy - France (Alps - ALCOTRA) 'Project DynAval - Dynamique des avalanches: départ et interactions écoulement/obstacles three test sites were built or developed in France and Italy. The goal of the paper is to present the experimental devices and typical pressure generated by avalanches in these 3 sites differing mainly by their scale. (a) Seehore test site is located in Aosta Valley on Monte Rosa Massif. The slope, with an altitude difference of about 300 m (from 2300 to 2570 m a.s.l.), has a mean dip of about 38°. The site is instrumented with a steel obstacle on which load cells and other devices are mounted in order to measure the effects of impact of the avalanche. Surveys are made before and after each artificial event: snow density, front velocity, erosion and deposition are measured and photogrammetric and laser-scanner views are taken. (b) Col du Lautaret test site is located near the Lautaret pass (2058 m a.s.l) between Cerces and Ecrins range. Different avalanche paths are located on the south-east slope of Chaillol Mountain (2600 m a.s.l.). Small to medium avalanches occur at a sufficient frequency (up to 3 or 4 each winter). Avalanche flows are generally dense, wet or dry, with sometimes a small but fast powder cloud (or saltation layer). The dense part is usually less than one meter thick. The run-out distance is 500 to 800 m with an average gradient of 36°. Typical released volume is about 5000 m3 and front velocity can reach 30 m/s. Instrumentation includes a 3 m-high mast recording pressure and velocity each 20 cm, and a one square meter plate integrating the pressure all over the flow height. A high speed photogrammetric system is able to measure the avalanche front velocity. (c) The Taconnaz avalanche path is located in the Arves valley, close to Mont Blanc in France. The Taconnaz path is 7 km long, has a mean slope of 25° and a mean width of 300400 m. A defence structure system made of breaking mounds and dams was designed in 2009 based on a 100-year return period event of 1.6 Mm3 volume. In 2010, velocity and pressure sensors were set up on the breaking mounds in order to improve our knowledge of the interaction between avalanches and breaking dams.