Planning measures

SLF employees produced one of the first hazard maps of Switzerland, for the community of Wengen, as long ago as 1960. At the time, they were still working on the basis of observed avalanche events and without performing any calculations. During the avalanche period of 1968, when the region of Davos in particular suffered enormous damage and loss of life, numerous avalanches overflowed the boundaries of the existing danger zones. We responded by issuing guidelines governing the production of hazard maps.

During the last two decades, avalanche simulation has emerged as an important tool for danger zoning purposes. Avalanche simulations are used, on the one hand, to quantify avalanche runout zones and, on the other, to evaluate the likely consequences of avalanches so that appropriate protective measures can be planned. It was in this context that we developed the avalanche simulation software RAMMS. It represents the state of the art in avalanche computing. The model automatically calculates the spatial distribution of the flow height, velocity and pressures. In Switzerland, responsibility for producing hazard maps rests with the cantons. The maps are actually created by private engineering offices in collaboration with the cantonal natural hazard agencies. In case of objections, the SLF can be consulted as the senior expert authority. The SLF also advises the cantons and private engineering offices on complex hazard situations.

In the extreme winter of 1999, when the incidence of avalanches was very high, the hazard maps generally proved valuable. Avalanches only seldom flowed further than anticipated. In the wake of the 1999 winter season, many avalanche regions captured the avalanche runout zones in aerial photographs. The availability of this information enabled us to develop our simulation models further, and helped the local authorities to improve their maps. Some 99 % of local authorities with avalanche-prone regions now have a hazard map.

Building activity forbidden in red danger zone

Hazard maps divide the territory into distinct zones depending on the severity of the danger to which they are exposed. Frequency and intensity are the factors which determine the degree and extent of the danger. The individual danger levels are colour-coded red, blue, yellow and white.

Red indicates an area that is exposed to considerable danger. In the event of an avalanche being released, buildings are likely to be destroyed. The compression forces generated by a 300-year avalanche can exceed 30 kN/m². People both inside and outside buildings are endangered. Building zones are not allowed in red areas. For material purposes, the red zone is a forbidden area. In the area coloured blue, rare avalanches are accompanied by only medium compression forces of less than 30 kN/m². People inside buildings are fairly safe, but a danger exists outdoors. New building zones can be approved only after consideration has been given to the conflicting interests. Conditions are to be attached to planning permission. Structural measures must be taken to protect exposed parts of buildings, and evacuation plans have to be produced for the occupants. The danger in the yellow area is low. People are exposed to very little danger, and only slight damage to buildings is to be expected. Yellow is typically used to designate the runout zone of powder avalanches. In the white area, the danger is non-existent or negligible.

Limitations of hazard maps

One problem is that many communities were established in the Alps long before any avalanche hazard maps had been produced. Such places can now be protected only by structural defences