Analysis of avalanche risk factors in backcountry terrain based on usage frequency and accident data in Switzerland

Recreational activities in snow-covered mountain- ous terrain in the backcountry account for the vast majority of avalanche accidents. Studies analyzing avalanche risk mostly rely on accident statistics without considering exposure (or the elements at risk), i.e., how many, when and where people are recreating, as data on recreational activity in the winter mountains are scarce. To fill this gap, we explored volun- teered geographic information on two social media moun- taineering websites - bergportal.ch and camptocamp.org. Based on these data, we present a spatiotemporal pattern of winter backcountry touring activity in the Swiss Alps and compare this with accident statistics. Geographically, activ- ity was concentrated in Alpine regions relatively close to the main Swiss population centers in the west and north. In con- trast, accidents occurred equally often in the less-frequented inner-alpine regions. Weekends, weather and avalanche con- ditions influenced the number of recreationists, while the odds to be involved in a severe avalanche accident did not depend on weekends or weather conditions. However, the likelihood of being involved in an accident increased with increasing avalanche danger level, but also with a more un- favorable snowpack containing persistent weak layers (also referred to as an old snow problem). In fact, the most criti- cal situation for backcountry recreationists and professionals occurred on days and in regions when both the avalanche danger was critical and when the snowpack contained per- sistent weak layers. The frequently occurring geographical pattern of a more unfavorable snowpack structure also ex- plains the relatively high proportion of accidents in the less- frequented inner-alpine regions. These results have practical implications: avalanche forecasters should clearly communi- cate the avalanche danger and the avalanche problem to the backcountry user, particularly if persistent weak layers are of