idw – Informationsdienst Wissenschaft
Nachrichten, Termine, Experten
Nachrichten, Termine, Experten
idw - Informationsdienst
Wissenschaft
After 1951, the SLF stepped up its research into avalanche protection. The findings led to a move away from walls, the usual defence mechanism until then, towards the snow bridges and snow nets that remain common today, and to a set of guidelines that still serve as a global benchmark.
Winter 1951: in just one season, avalanches killed 98 people and damaged and destroyed around 1,500 buildings and other infrastructure. Many mountain valleys were cut off from the rest of Switzerland. There were well over a thousand damaging avalanches in the country that winter. After the initial shock, it quickly became clear that Switzerland needed to better protect its Alpine population from snow-related hazards. "Federal subsidies for avalanche protection increased substantially after 1951," says Stefan Margreth, head of the Avalanche Protection Measures research group at the WSL Institute for Snow and Avalanche Research (SLF).
This also led to intensive research into the most effective methods of avalanche protection, looking at which measures offer the best protection in which locations, and the issues to be considered when planning and designing them (see the 'History of avalanche defence' box). SLF researchers measured the forces acting on different types of defensive structure as a result of snow pressure and avalanches. They established a dedicated test site on the Dorfberg above Davos, built snow bridges out of wood, aluminium, wire ropes, steel and concrete and compared their effectiveness with the walls and terraces that had been the predominant means of avalanche protection throughout Switzerland until then. All the measures operated on a similar principle, the aim being to prevent snow from becoming detached and forming avalanches. However, the snow bridges turned out to be significantly more effective. "So very soon people started moving away from walls and focusing entirely on the modern, separated supporting structures," says Margreth.
More steel, less stone
Researchers and practitioners continued to compare the different approaches until 1965. "There was a lot of interest from industry as many avalanche defence projects were planned," explains the engineer. In the end, steel won out. Concrete had proved to be vulnerable. If water penetrates into cracks, it bursts the components of the structure when it freezes. Concrete also exhibits brittle behaviour in the event of rockfall. A similar conclusion was reached for aluminium. "The material is simply too soft and too expensive," explains Margreth. "It also retains its metallic sheen and reflects the sunlight." This means that, during the day, aluminium snow bridges can be seen from a long way off and change the look of the landscape more than steel snow bridges or snow nets. Wooden structures, though, can still be found on some slopes, but almost always below the treeline.
The researchers' work had far-reaching consequences. As early as 1955, the SLF published a first, provisional version of its guidelines on snow supporting structures. These guidelines help engineers and industry to plan and design protective structures. In 1961, experts from the SLF drew up the first official guideline Lawinenverbau im Anbruchgebiet ('Defence structures in avalanche starting zones') issued by the Federal Inspectorate for Forestry, now the Federal Office for the Environment. The current version of the guideline, which has been revised several times and is used all over the world from Japan to Iceland, regulates how snow supporting structures are planned and dimensioned. "Particular attention should be paid to three parameters: the maximum possible snow height, the inclination of the slope and the resulting snow pressure," explains Margreth.
Getting snow out of the wind
On a smaller scale, SLF researchers also investigated how snow that is transported by the wind can be deposited in a controlled manner in order to optimise its distribution so that the avalanche danger is as low as possible. Both snow fences and wind deflection systems (kolk crosses) get the snow out of the wind and strategically deposit it in safe places. "Snow fences also help prevent too much snow from accumulating in snow supporting structures," says Margreth. This is important because, once the snowpack reaches the height of the supporting structures, they can lose their effectiveness if any more snow falls.
The avalanche winters of 1999 and 2018 saw a significant reduction in harm to people and damage to buildings compared with previous such winters, thanks to improved avalanche protection measures. In 2018, there were no fatal avalanche accidents at all in villages or on roads. The importance to a mountainous country of the avalanche expertise built up over centuries has now also been recognised by the United Nations: on 29 November 2018, UNESCO, the United Nations Educational, Scientific and Cultural Organization, added avalanche risk management to the Representative List of the Intangible Cultural Heritage of Humanity.
In the next instalment, available from Tuesday, 3 February, we'll learn how the avalanche winter of 1951 marked the launch of hazard maps.
History of avalanche defence
Forests set aside to protect against avalanches are documented in Switzerland as early as the 14th century. From the early 17th century onwards, the first structural measures such as spaltkeile (structural elements rather like log-splitting wedges), deflection walls and large bunker-style basements are recorded. Around 1805, the Kaltwasser avalanche gallery (or snow shed) was built in the Simplon Pass by the French emperor Napoleon to protect his troops. It was not until later in the 19th century that people began to systematically build walls and terraces in avalanche starting zones. Wooden stakes placed in the ground three metres apart were intended to prevent large volumes of snow from sliding downhill when heavily soaked. The chief forestry inspector of the canton of Grisons and later federal forestry inspector Johann Wilhelm Fortunat Coaz brought a structural approach to the subject for the first time. He believed that protection against avalanches was needed on the mountain and not just in the valley, compared different approaches and kept meticulous records. Thanks to him, we know that by 1881 there were 35 avalanche defence structures across the country, rising to 269 by 1910.
https://www.slf.ch/en/news/avalanche-winter-1951-redefines-barrier-technology/
A wall holds back the snow and prevents it from thundering down into the valley as an avalanche...
Source: Ernst Eugster/SLF
...but only as long as it is higher than the snowpack. This picture shows the same spot, but with hi ...
Source: Ernst Eugster/SLF
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