The science of surviving an avalanche

Avalanches are deadly dangerous, but there is much we can do to avoid them...

When I tell people I am doing a PhD on avalanches, I find that most people either find it interesting, or not really. Those interested to know more are mostly keen skiers, and the first question they ask is always ‘Oh, so you can help discover how to survive an avalanche?’

Although this is not quite what my research is about - being mathematical, all my work was on theoretical avalanches - this is a good question. Avalanches kill about a hundred people per year in the Alps alone, and free-skiers (those adventurers going off-piste) must be actively careful to avoid getting caught in an avalanche.

To understand how to stop an avalanche from killing us, we need to understand how they happen in the first place.

Avalanches basically occur when a mass of snow can no longer be held on a slope. While snow accumulates on the side of a mountain, friction with the ground holds the snow on the slope.

But a number of factors can cause whole snow structures to collapse in a fantastic and unstoppable natural phenomenon.

The coordinator of the Scottish Avalanche Information Service and Senior Forecaster for the Northern Cairngorms Area, Mark Diggins, listed six problems that contribute to the risk of avalanches occurring in a region. The six problems are (1) wind, (2) new snow, (3) wet snow (4) gliding snow, (5) a persistent layer, and (6) a cornice.

The wind cause grains to break and produces a compact layer of small grains.

New snow is problematic because it can load the slope until friction cannot hold the weight of the old plus the new snow, and at that point it all slides away.

Wet snow caused by warming or rain, “will make the snow turn into a sort of slurry,” said Diggins. Wet water will act like a viscous fluid that cannot be held by friction on the slope.

Gliding snow is the name given to patches of snow that slowly creep down the mountainside separating from the snow around it and forming big cracks.

Diggins defined a persistent layer as “where you have a layer of grains or crystals underneath the surface.” The crystals are produced by changing weather and temperature conditions. “You actually have no idea that the persistent layer exists, but it may be getting weaker and weaker”.

Finally, cornices are a problem specific to the UK. Diggins explained, “a cornice is are a structure where snow overhangs a slope, and if it collapses a cornice can trigger an avalanche.”

However, Diggins adds that “in almost all cases, it is the skiers themselves who trigger avalanches unknowingly.”`

Jim McElwaine, professor of Geology at the University of Durham explained that “a trigger is a sudden disturbance that overloads the weak persistent layer and causes it to fail, starting the avalanche.”

Interestingly, McElwaine added that groups of men are more likely to ignore warning signs, and the more women are in a group, the less likely it is that they will be involved in an avalanche. He speculated this might be due to social pressures that make men less likely to admit fear and be cautious.

Once the avalanche stops, even if the victim is not fatally hit by the debris carried by the avalanche, there is no way to predict where they will end.

Mark Diggins, whose job it is to provide the information on which hikers and skiers judge the risk of avalanches, said that in his experience survival is extremely unlikely, particularly in Scotland where avalanches are likely to pick up large rocks and debris from the ground, making them particularly dangerous.

“Statistics show that after ten minutes you've got a 10% chance of survival. It's absolutely critical then that the people on the surface have equipment that enables them to find somebody that's buried.”

An essential piece of equipment is a transceiver. Diggins explained that while skiing, the transceivers transmit a signal, but if someone gets buried, the people on the surface switch theirs to ‘receive’, so they can find the missing person. “They will take you to an area of probably about a metre square or so on the surface.”

For the exact location, it is important that everyone in the group carries a probe. “These are three metres long. So you push those into the snow, and they're actually quite sensitive so you can tell when you've got something that's yielding.”

Finally, a shovel, to dig the missing person out once they have been located.

Additionally, avalanche airbags can offer an extra bit of help. Skiers and hikers can carry vests or backpacks with airbags that can be pumped by pulling a cord.

McElwaine explained that the airbags serve a double purpose. First, it protects the wearer from the impact of the rocks and debris carried by the avalanche, and secondly it helps the wearers chances of being lifted and carried close to the surface of the avalanche.

The physical mechanism, by which this occurs is called granular segregation. Jim explained that “it's the same reason that if you have a box of cereal and you shake it around, then the larger lumps of cereal end up on the top; and this is through an effect called kinetic sieving.”

“If you have large particle with lots of small particles moving around it, they can fall through the gaps in between, and they effectively force the large particle up to the top.”

“And in an avalanche, if you put an airbag on, you are acting like a very large particle in amongst these smaller snow grains or blocks of snow, so you've got a very much higher chance of ending up on the top of the snow, where you are easier to find.”

However, no amount of equipment will guarantee survival. Both Diggins and McElwaine stress that the only way to guarantee surviving an avalanche, is by not getting caught in one by being smart and careful when choosing when and where to go off-piste.

But deadly as they are, avalanches form essential parts of some ecosystems.

Christopher Rixen is a researcher at the Institute for Snow and Avalanche Research in Davos, Switzerland, where he studies the impact of avalanches on biodiversity. He told us that in the avalanche tracks where he focuses his studies in the Swiss Alps, he found three times as many plant species than outside the tracks.

Rixen explained that this is due to avalanches carrying away the taller and more competitive types of plants, such as trees, leaving room and sunlight for a larger number of smaller species to grow.

In addition, Rixen told us that on the tracks, they have found altitude specialists, that is, species that usually grow at high elevation, because the avalanches act as disseminators by carrying seeds and parts of plants from high altitudes down the slopes of mountains.

And a wide variety of plants means a healthy and varied diet for certain animal species such as bears and caribou, who risk roaming the avalanche tracks for food.

Rixen told us of studies that show that in the Rocky Mountains “10 to 15% of mountain caribou feeding in these avalanche tracks might in the end become victims of avalanches themselves.”

But they in turn become the source of food for scavengers that live in such areas, such as the wolverine.

Entire ecosystems are able to coexist with avalanches - surely we could do so too in a safely manner. They are one of nature's unstoppable forces, but by the sound of it, there is much we can do to avoid the risk of falling victim of one.