High-quality snow-avalanche hazard indication maps for large areas

What is it about?

We present a new method for creating hazard indication maps for snow avalanches over large areas. In contrast to earlier and contemporary approaches, the release probability of avalanches is estimated through a simple, yet physics-based stability criterion using meteorological and forest data. This allows elimination of potential release areas from which avalanches release so seldom that they are considered a negligible residual hazard in the land-use regulations of mountainous European countries. In release areas where avalanches are found to occur more frequently than the limit set by the regulations, the run-out of the largest events within these limits is calculated with a fast, state-of-the-art 2-dimensional flow model. finally, the run-out areas of all avalanche paths within the study area are superposed to form the final hazard indication map. The model has been extensively validated by avalanche experts and was found to give highly plausible results for avalanches with return period up to 1000 years in very diverse areas of Norway—without human intervention or parameter adjustments. The new maps are available for all of continental Norway and replace the earlier generation of maps from 2010. Those had been found to be far too pessimistic in warm, maritime areas with dense forests while they were much too optimistic in many cases in regions with continental climate. The new approach has its shortcomings, too. Chief among them are the neglect of powder-snow avalanches, which can endanger larger areas than indicated in the map in some cases, and of drifting snow, which can increase the release probability in wind-exposed release areas in continental climate by an order of magnitude. These problems are being worked on.

Featured Image

Photo by Krzysztof Kowalik on Unsplash

Photo by Krzysztof Kowalik on Unsplash

Why is it important?

Hazard indication maps are a valuable tool for land-use planning. This is especially true in regions with low population density, where the elaboration of detailed hazard maps by experts would be too time-consuming and expensive. However, the hazard must be assessed realistically—too "pessimistic" maps can hamper the economic development of settlements in the mountains severely, but too "optimistic" maps put human lives at risk. For many gravity-driven natural hazards, the local climate is a decisive factor for the degree and extent of hazard. In addition, forests can have a strong influence on where and how often such events occur. In Norway, experience has shown that the earlier, purely topographic approach was insufficient. Our new approach is novel because it estimates—for the first time—the release probability for each potential release area and thus allows to get a better picture of how exposed settlements or roads are. We plan to extend NAKSIN to a fully probabilistic tool for avalanche hazard mapping, which will give information not only on how far avalanches with a fixed return period will flow, but also on how often any given point in the mapped area will be hit by an avalanche. This opens the door for creating well-founded risk maps, which can then be used to manage the hazard in an objective and optimized way.