Defensive structures prevent the formation of avalanches. In other circumstances, when an avalanche is released, it can be diverted or intercepted by a dam. Other means of protection against avalanches include physical structures for buildings and snow sheds. We conduct research and offer advice on structural avalanche protection.
In Switzerland, defensive structures are the most important and most prevalent form of physical avalanche protection. They protect the territory over a wide area below the site by stabilising the snowpack with barriers anchored in the ground. The first defensive structures were walled terraces, erected back in the 19th century. A series of major disasters in the winter of 1950/51 prompted a major realignment of avalanche protection activities. Within a short time, walls and terraces were replaced by lean and much more effective structures made of steel, aluminium, wood, cables or concrete. Nowadays, steel snow bridges and flexible snow nets consisting of steel cables are being used. The defensive structures erected in Switzerland span a total distance of around 500 km. Where possible, the protection they afford is enhanced by afforestation. In the late winter, when the snowpack is especially deep and heavy, a defensive structure can be exposed to a load of as much as 40 metric tons. The cost of safeguarding an area of one hectare is around one million Swiss francs. In collaboration with the Federal Office for the Environment (FOEN), we conduct type testing of new defensive structures. We have been assisting both practitioners and the relevant industrial actors with advice and guidelines ever since the early days.
Defensive measures in the path and runout zone of released avalanches are restricted to protecting buildings and other assets. An avalanche can be arrested and brought to a standstill by retention dams and retarding mounds. In order to stop an avalanche completely, depending on its speed, a dam may need to be more than 20 m tall. Deflecting dams divert the snow masses to areas where they cannot cause any destruction. Compared with defensive structures, dams are often less expensive to construct, but they occupy a lot of land and permanently change the landscape. Many dams have a dual function: they protect against avalanches in wintertime, and against flooding and debris flows once the snow has melted.
Snow sheds and protection for buildings
Also known as avalanche galleries or tunnels, snow sheds are the classic structures for protecting transportation routes. Avalanches either overflow the shed or deposit their snow on the roof without adversely affecting the traffic. If they are sufficiently long, snow sheds afford very considerable protection. Protecting buildings ranks among the oldest means of defence against avalanches. It is an efficient method of reducing the risk to which people and property are exposed. Among the typical measures for protecting buildings are wall reinforcement, the erection of a solid structure (Spaltkeil), rather like a log splitting wedge, to break the avalanche, and a building design (Ebenhöch) in which the roof seamlessly merges with the terrain or an embankment.