July 12, 2012

expert reaction to French avalanche

An avalanche in the French Alps claimed the lives of 9 climbers including 2 Britons. Information about the mechanics behind avalanches was provided by experts.

Dr Jim McElwaine, senior research associate at the University of Cambridge, said:

“The avalanche occurred low down on the North Face of Mont Maudit near the col over to Mont Blanc du Tacul. I have actually climbed and skied this slope many times. It was a very small avalanche and came to on relatively flat terrain. There is a large fracture line visible indicating that it is a slab avalanche; these occur very infrequently in the summer. They are caused by the wind blowing snow and redepositing. It forms a hard brittle slab that can be up to 2 metres in thickness. If there is a weak layer of snow underneath, or a weak bond to an ice layer, then the situation is very dangerous.

“These wind-slab avalanches have, killed the largest number of people in the Alps even though they are not the biggest. This is because they are usually triggered by the climbers or skiers themselves. Imagine a sheet of glass lying on sand, if you walk into the middle it may hold, but if you exert too much force the glass will shatter and you will be carried down in the middle of the avalanche and probably killed.

“An avalanche of this size will not have caused a powder cloud and probably travelled no faster than 40 mph. It was probably triggered by the climbers, but may also have been triggered by a serac breaking or climbers higher on the slope. A serac fall is less likely because of the time of day 05:25 and they usually occur after the sun has been on a slope for several hours.

“This is the standard route up Mont Blanc which sees 10,000s of passages a year. The climbers were near the bottom of the face, probably descending at this time of day, and they would have reached
the safety of the col in a few more minutes. There is no indication that they did anything remotely reckless, merely that they were incredibly unlucky.

“It is very unlikely that any survivors will be found. 50% of people die within the first 20 minutes and the chance of life continues to fall rapidly. Many people are killed by the trauma of the avalanche
and massive internal bleeding or shock. Those who survive initially are usually killed by their own breath. As they breathe out water vapour it freezes around them until they can no longer obtain oxygen and they suffocate.”

Dr Nathalie Vriend, NERC Research Fellow at the University of Cambridge, has provided the following background information about avalanches, said:

“Large snow avalanches usually consist of a dense layer close to ground (approx 2 – 3m height) and a powder cloud above this layer which can reach heights of up to 100 meters.

“Large snow avalanches travel fast — our field measurements in Switzerland indicate a top speed of more than 45 m/s, that is more than 100 miles/hour or 160 km/hour!

“There are on average 150 avalanche deaths per year in the entire world.

“Without additional survival gear, 50% of the avalanche victims that are buried in a large avalanche suffocate within 20 minutes.

“A commercial device that is available to backcountry skiers is the Avalung. When a person is buried, they are initially able to breath in air through the snow. However, the warm exhaled air melts the snow and forms a frozen layer around the face. This causes suffocation within a short time-frame. An Avalung consist of a “diving”-mouthpiece that, when somebody inserts it directly in their mouth after they are caught in an avalanche, redirects the exhaled air via a tube to the back of the buried person. That extends the time that a buried person survives in the snow.

“Another commercial device available is the ‘Avalanche Airbag’. By pulling a cord on your backpack, you are inflating two big airbags on each side of you. This creates upwards buoyancy in the snow avalanche, as suddenly you are a larger particle in the flow. This is very similar to the fact that the large particles in a cereal box are always at the top of the bag!

“Avalanches are very hard to predict. A few conditions need to be satisfied — for example fresh snowfall on top of a weaker snow layer, and a local slope angle between 30 and 50 degrees — but one never knows exactly when a slope fails.

“In ski-areas, as a precaution, they use explosives to create avalanches overnight. Skiers, snow mobile riders or mountaineers can create avalanches accidentally by disturbing the snow.

“It can be that the first few members of a group cross a dangerous section safely, while the later members are caught in an avalanche.”