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How do avalanches affect pylons and other sensitive infrastructure? Using detailed simulations, SLF researcher Michael Kohler has shown that the compressibility of snow initially reduces avalanche pressure, but that at high speeds this buffer suddenly fails.
Avalanches encounter all kinds of obstacles on their downward path, including electricity and aerial lift pylons. Particularly in remote areas, sensitive infrastructure in the form of slender objects often stands in their way. Michael Kohler, a researcher at the WSL Institute for Snow and Avalanche Research (SLF), is investigating what happens when flowing snow comes up against such an obstacle. "Of course, efforts are made not to place critical infrastructure in avalanche paths, but sometimes this can't be avoided." Engineers need to know how much pressure an avalanche exerts on objects such as aerial lift pylons so that they can reinforce them accordingly.
In the future, the hope is that such calculations will be more realistic and accurate. Until now, researchers have often treated avalanche snow as incompressible in their computational models. This implies that the snow masses do not compact and therefore collide with the obstacle at full force. "That's obviously a simplification. Anyone who's ever made a snowball knows how tightly cold, dry snow in particular can be compressed," says Kohler.
It is precisely this kind of cold, dry snow that forms the basis for the very fast and therefore dangerous powder snow avalanches. If such snow hits an obstacle at only moderate speed, its compressibility acts like the crumple zone in a car. "As the snow compacts in front of the obstacle, it can greatly reduce the force of the avalanche," explains Kohler. The maximum damage is thus actually less than previously assumed in the calculation models.
A bolt from the blue
However, if the avalanche becomes very fast-moving, everything changes. "Above a certain speed, our simulations show a surprising increase in pressure for loose, weakly bonded snow," says Kohler. He explains this by describing how forces and disturbances propagate in liquid media.
Kohler compares it to a boat on a calm lake. When the boat travels slowly, it pushes a wave ahead of it and the water 'senses' that something is coming. A water flea in front of the bow would be warned about the boat by the waves and could get out of the way in time. But if the boat travels faster than the waves, the water surface in front of the bow remains smooth – the flea notices nothing and is caught by surprise.
The situation is similar with very fast avalanches. The very loose material cannot pass on the information about the impending impact quickly enough and hits the obstacle with full force instead of flowing around it and cushioning the impact. "I found that paradoxical at first," says Kohler. Instead, he had expected the pressure to decrease as a matter of course. However, his findings show that avalanches do not form an effective crumple zone at high speeds and that the avalanche transfers its entire kinetic energy to the obstacle. "This means that it strikes like a bolt from the blue."
Numerical laboratory
To systematically investigate these phenomena, the scientist has created a model on his computer, which he calls a 'numerical laboratory'. He explains: "In a real laboratory, you can't easily increase the internal friction of a material by 30% or change other parameters, but on a computer you can." In this lab, Kohler unleashes virtual snow masses onto virtual aerial lift and electricity pylons. The advantage over complex experiments is that, if he changes one value, all the others remain constant. This enables him to determine what effects the mechanical properties of snow, such as its internal friction, have on avalanche pressure.
The computer can take several hours to complete a single pass. After all, it is simulating the behaviour of up to five million virtual snow clumps. "Avalanches have significantly more, but to investigate local effects at the obstacle, this is sufficient," says Kohler.
Michael Kohler
michael.kohler@slf.ch
+41 81 4170 283
Avalanches regularly cause damage to aerial lifts. In his numerical laboratory, SLF researcher Micha ...
Copyright: SLF
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Umwelt / Ökologie
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Avalanches regularly cause damage to aerial lifts. In his numerical laboratory, SLF researcher Micha ...
Copyright: SLF
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