Review and future challenges in snow avalanche risk analysis

Background Snow avalanches pose a major threat to alpine communities because they affect safety in villages and on traffic routes. Therefore, dealing with avalanche danger has a long tradition in Alpine countries. In most countries, avalanches contribute only to a small degree to the overall risk of a country. For Switzerland, for example, avalanche risk represents only 2% of all risks (BABS, 2003). Snow avalanche formation, geomorphology and land use planning Snow avalanches are a type of fast-moving mass movement. They can also contain rocks, soil, vegetation or ice. Avalanche size is classified according to its destructive power (McClung and Schaerer, 2006). A medium-sized slab avalanche may involve 10,000 m 3 of snow, equivalent to a mass of about 2,000 tons (snow density 200 kg/m 3 ). Avalanche speeds vary between 50 and 200 km/h for large dry snow avalanches, whereas wet slides are denser and slower (20–100 km/h). If the avalanche path is steep, dry snow avalanches generate a powder cloud. There are different types of snow avalanches (Table 5.1), and in particular two types of release: loose snow avalanches and slab avalanches. Loose snow avalanches start from a point, in a relatively cohesionless surface layer of either dry or wet snow. Initial failure is analogous to the rotational slip of cohesionless sands or soil, but occurs within a small volume ( 3 ) in comparison to much larger initiation volumes in soil slides.