Measurements on human-triggered avalanches from the Swiss Alps

ABSTRACT: Ten years of avalanche incidents data from the Swiss Alps have been analysed in order to find characteristics of human triggered avalanches. We were particularly interested in avalanche release and snowpack patterns. In the 10 years (1987-88 to 1996-97) the average number of avalanche f talities in the Swiss Alps was 23 per year. More than 90 % of the victims were caught during recreational activities in the free (not controlled) terrain. Of these, 90 %, were killed by an avalanche that was triggered by themselves or by another member of their group. The recreationist nearly exclusively triggered dry snow slab avalanches. The slab detached of a human triggered slab avalanche is (median values given) 50 m wide, and 80 m long; the overall avalanche length is 150 m. The fracture depth is 45 cm, and the inclination in the starting zone is 38°. The analysis of the 90 profiles available did support most of the mainly unstructured knowledge used in stability evaluation based on snow profiles. The slab failure is only in 38 % at the boundary between storm snow and old snow. The other failur s are due to weak layers or interfaces within the old snowpack. A weak layer was found in 42 % of the cases. In all other cases the failure was between to adjacent layers, a so-called interface failure. The median rutschblock score was 3 (weighting). The thin (1 cm) weak layer is usually soft, found between one or two harder layers (above and below) and consists primarily of large crystals ( 2 mm) with plane faces: surface hoar, faceted crystals and depth hoar