The avalanche cycle of December 2008 in the eastern part of the southern French Alps: cross comparison of meteorological and avalanche data

Abnormal temporal clustering of avalanche events are generally named as avalanche cycles. Quantifying their magnitude implies using an appropriate definition of a cycle and finding right spatial and temporal scales to estimate the magnitude/frequency relationship. An intense cycle occurred in December 2008 in the eastern part of the Southern French Alps during southerly atmospheric fluxes that progressively evolved into an easterly return, causing abundant snowfalls. The accumulations of new snow, with local increase of drifting snow, favoured the release of dry snow avalanches and the development of powder clouds. Some of the events showed very long runouts and exceeded the historical limits registered in the French avalanche map. Buildings have been attained and destroyed, and traffic roads have been blocked during several days. The aim of this paper is to precisely delimitate and describe this avalanche cycle. For instance, it is shown how the return period concept can be used at different spatial scales, not only for snowfalls, but also for the events themselves. Two data sources are predominantly considered: the nivometeorological data collected by Meteo France and the EPA database. This data has been supplemented by results from numerical modelling of the snow cover that are used for assessing snow stability at the massif scale.