Abstract
Abstract. Understanding wet avalanche intensity and the role of past environmental changes on wet avalanche occurrence is a main concern especially in the context of a warming climate and accelerated environmental mutations. Avalanches are closely related to fast cryosphere changes and may cause major threats to human society. Here, we used the sedimentary archive of the Alpine Lake Lauvitel (Lac du Lauvitel; western French Alps) to establish the first long-term avalanche record in this Alpine region. For this purpose, we used a novel CT-scan methodology that allows the precise identification of coarse material – from sand to pebble – transported to the lake and embedded within the finer continuous sedimentation. We identified a total of 166 deposits over the last 3300 yr cal. BP. In parallel, a detailed pollen analysis gave an independent record of environmental changes. Based on modern observation, lake monitoring, seismic investigations and sedimentological evidences, coarse material deposits were attributed to wet avalanche events. Our results highlight the effect of vegetation cover on the avalanche hazard while a period of strong frequency increase occurred after 780 yr cal. BP. In Lake Lauvitel, this period corresponds to a major forest clearance induced by the rise of human land use. Climate forcing on the avalanche hazard was investigated before and after the vegetation shift. On a multicentennial scale, wet avalanches preferably occur during periods of larger glacier extent, in which higher winter precipitation probably generates a sufficiently thick snow cover. On a sub-centennial scale, avalanches are more frequent during periods of relative warming, resulting in a destabilization of the same snow cover in spring season. Our results highlight as well the role of forest cover in mitigating wet snow avalanches' occurrence. In the context of predicted warmer temperatures, this study raises the question of whether a wet avalanche hazard increase may be expected in the near future especially at higher altitudes.
Highlights
Avalanche hazard is one of the major threats to human societies in mountain environments, and in the context of warmer temperatures, avalanche occurrence patterns will likely be modified by a reduction of dry snowpack and an increase in wet snowpack (Castebrunet et al, 2014)
Regional models are predicting an increase in wet avalanche activity for the coming decades, mostly related to an earlier occurrence in the spring season and an avalanche risk expected at higher altitude (Lazar and Williams, 2008; Castebrunet et al, 2014)
We focus on wet avalanches, considered as dense flows characterized by wet snow in which liquid water content is around 3 vol %
Summary
Avalanche hazard is one of the major threats to human societies in mountain environments, and in the context of warmer temperatures, avalanche occurrence patterns will likely be modified by a reduction of dry snowpack and an increase in wet snowpack (Castebrunet et al, 2014). Complex meteorological conditions over a period of days to several weeks control the snowpack stabilization, and occurrence of wet avalanches highly depends on temperature conditions because they are more likely to occur when the average daily temperature exceeds 0 ◦C (Baggi and Schweizer, 2009). Regional models are predicting an increase in wet avalanche activity for the coming decades, mostly related to an earlier occurrence in the spring season and an avalanche risk expected at higher altitude (Lazar and Williams, 2008; Castebrunet et al, 2014). Fouinat et al.: Wet avalanches: long-term evolution in the Western Alps
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