Transport de la neige par le vent sur un site de montagne : mesures et modélisation numérique à l'échelle d'un couloir

Drifting snow is a serious problem, since it produces snowdrift on roads, and it is responsible of most avalanches in mountains, by creating wind slabs. In order to fight against this phenomena, the Etna unit of Cemagref, and the Snow Studies Center of Météo France has been working on snow drift for ten years, on the experimental site of Lac Blanc Pass. This site was developed thanks to the financial support of the Rhone-Alps Region, and with the logistic support of SATA (Alpe d'Huez ski resort company). It is a kind of natural cold wind tunnel, with dominant winds from the north to the south. During the last two years, we have installed on this site many sensors such as anemometers, snowdrift acoustic sensors, and snow poles to measure the height of snow. The aim was to create a data basis used to perform our knowledge concerning drifting snow, and to test the numerical model of drifting snow developed by the team (Naaim M., Naaim-Bouvet F., Martinez H., 1996). This paper describes the results of the last two years. In particular, we present the analysis of the data of the drifting snow acoustic sensor, which allow us to determine threshold velocities, and to establish preliminary relations between drifting snow flows and the signal on the sensor. Then, thanks to an analysis of gust factors, both for the wind and for the snowdrift, we will show that most important drifting snow episodes occur during a period of relatively high regular wind, and not during period of high wind gusts. Lastly, we present a comparison of the results coming from our numerical model, and the in situ snow depth, for a drifting snow event. Le transport de la neige par le vent pose de nombreux problèmes notamment au niveau de la viabilité hivernale ou de la formation de plaque à vent en montagne, responsable de la plupart des avalanches. Pour lutter contre ce phénomène, la division ETNA du Cemagref de Grenoble et le Centre d'Etude de la Neige de Météo-France étudient le transport de la neige par le vent sur le site expérimental du Col du Lac Blanc depuis une dizaine d'années. Ce site, développé grâce au support financier de la région Rhône Alpes, et au soutien logistique de la SATA (Station de ski de l'Alpe d'Huez), se comporte comme une soufflerie climatique naturelle orientée nord - sud. Ces deux dernières années, ce site a été fortement instrumenté (anémomètres, réseaux de perches pour mesurer la hauteur de neige, détecteurs acoustiques de transport, _), afin de créer une base d