We used one of the few rockfall models explicitly taking trees into account and compared the results obtained with the 3D simulation model RockyFor with empirical data on tree impacts at three mountain forests in Switzerland. Even though we used model input data with different resolutions at the study sites, RockyFor accurately predicted the spatial distribution of trajectory frequencies at all sites. In contrast, RockyFor underestimated mean impact heights observed on trees at the two sites where high-and medium-resolution input data were available and overestimated them at the site where input data with the lowest resolution data were used. By comparing the results of the simulation scenarios ''current forest cover'' and ''non-forested slope'', we assessed the protective effect of the current stands at all three sites. The number of rocks reaching the bottom parts of the study sites would, on average, almost triple if the ''current forest cover'' were absent.We conclude that RockyFor is able to predict the spatial distribution of rockfall trajectories on forested slopes accurately, based on input data with a resolution of at least 5 m  5 m. With the increasing availability of high-resolution data, it provides a useful tool for assessing the protective effect of mountain forests against rockfall.

Abstract. Snow avalanche hazard mapping has a long tradition in the European Alps. Hazard maps delineate areas of potential avalanche danger and are only available for selected areas where people and significant infrastructure are endangered. They have been created over generations, at specific sites, mainly based on avalanche activity in the past. For a large part of the area (90 % in the case of the canton of Grisons) only strongly generalized hazard indication maps are available (SilvaProtect), not showing impact information such as pressure. This is a problem when new territory with no or an incomplete historical record is to be developed. It is an even larger problem when trying to predict the effects of climate change at the state scale, where the historical record may no longer be valid. To close this gap, we develop an automated approach to generate spatially coherent hazard indication mapping based on a digital elevation model for the canton of Grisons (7105 km2) in the Swiss Alps. We calculate eight different scenarios with return periods ranging from frequent to very rare as well as with and without taking the protective effects of the forest into account, resulting in a total of approximately 2 million individual avalanche simulations. This approach combines the automated delineation of potential release areas, the calculation of release depths and the numerical simulation of the avalanche dynamics. We find that between 47 % (most frequent scenario) and 67 % (most extreme scenario) of the cantonal area can be affected by avalanches. Without forest, approximately 20 % more area would be endangered. This procedure can be applied worldwide, where high-spatial-resolution digital elevation models, detailed information on the forest and data on the snow climate are available, enabling reproducible hazard indication mapping also in regions where no avalanche hazard maps yet exist. This is invaluable for climate change studies. The simulation results are validated with official hazard maps, by assessments of avalanche experts, and by existing avalanche cadastres derived from manual mapping and mapping based on satellite datasets. The results for the canton of Grisons are now operationally applied in the daily hazard assessment work of the authorities. Based on these experiences, the proposed approach can be applied for further mountain regions.

Vom 23. bis 31. August 2017 traten vom Pizzo Cengalo bis nach Bondo in einer aussergewöhnlichen Ereignisabfolge ein Bergsturz, ein Schuttstrom und mehr als zehn Murgänge auf. Bei diesem historischen Ereignis ging der Bergsturz von gut drei Millionen Kubikmeter unmittelbar in einen Schuttstrom über, was bisher weltweit ausserordentlich selten beobachtet wurde. Seit dem Bergsturz werden acht Personen auf einem Bergwanderweg vermisst. Der Schuttstrom und die Murgänge haben insgesamt 500 000 Kubikmeter Schuttmaterial bis nach Bondo geführt. Im vorliegenden Artikel werden Erfahrungen aus Sicht der kantonalen Fachstelle zu Monitoring, Entscheidungsgrundlagen, Gefahrenbeurteilung und Sofortmassnahmen sowie zur Räumung und Wiederinstandstellung geschildert. Im Zentrum steht dabei die Ereignisbewältigung, die nur mit ausserordentlicher Teamleistung zu bewerkstelligen war, sowohl im Bereich Naturgefahren als auch gesamthaft. Die vielfältigen Herausforderungen zeigen, dass das Geschehene durchaus als «Umgang mit Extremereignissen» einzuordnen ist. Abschliessend werden neue Fragen zu den Ursachen des Ereignisses und deren Verkettungen im Hinblick auf mögliche Entwicklungen und Lösungen aufgelistet.

Abstract. Snow avalanche hazard mapping has a long tradition in the European Alps. Hazard maps delineate areas of potential avalanche danger and are only available for selected areas where people and significant infrastructure are endangered. They have been created over generations, at specific sites, mainly based on avalanche activity in the past. For a large part of the area (90 % in the case of the Canton of Grisons) no maps are available. This is a problem when new territory with no or incomplete historical record is to be developed. It is an even larger problem when trying to predict the effects of climate change at the state scale where the historical record may no longer be valid. To close this gap, we develop an automated approach to generate spatial continuous hazard indication mapping based on a digital elevation model for the canton of Grisons (7105 km2) in the Swiss Alps. We calculate eight different scenarios with return periods ranging from frequent to very rare as well as with and without taking the protective effects of the forest into account. This approach combines the automated delineation of potential release areas, the calculation of release depths and the numerical simulation of the avalanche dynamics. This procedure can be applied worldwide, where high spatial resolution digital elevation models, detailed information on the forest and data on the snow climate are available, enabling reproducible hazard indication mapping also in regions where no avalanche hazard maps yet exist. This is invaluable for climate change studies. The simulation results are validated with official hazard maps, by assessments of avalanche experts and by existing avalanche cadastres derived from manual mapping and mapping based on satellite datasets. The results for the canton of Grisons are now operationally applied in the daily hazard assessment work of the authorities. Based on these experiences, the proposed approach can be applied for further mountain regions.