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Surveillance and anticipation

Avalanche control

A source of pleasure but also of mortal danger, snow is an unstable, ever-changing substance and something that should never be taken for granted.
In ski resorts, protecting thousands of skiers from this sword of Damocles, in other words, an avalanche that can so suddenly wreak havoc, is a major challenge for professionals.

Unfortunately, the snowpack is not simply a vast and immobile white mantle. To start with, its composition is far from stable. That would be too easy! Snow accumulates over the course of the season and is transformed as soon as it hits the ground depending on the weather conditions, the wind, and the cooling or warming of the air. As a result, the snowpack becomes stratified, a bit like a lasagne made up of many layers that link together each new fall of snow. Depending on the nature of the different layers, the pack is more or less stable, able to trigger avalanches or at least facilitating their release.
What is an avalanche?
In short, it’s a sudden, more or less large mass of snow that flows down a more or less long incline, more or less rapidly. In reality, however, it’s far more complex. There are three main types of avalanche.
A powder snow avalanche
This avalanche of recent snow is generally triggered almost immediately after a significant snowfall. The snow is powdery and has changed little, and its density is no more than 200kg/m3. A skier can easily trigger an avalanche, even a few days after the fall. The flow is like a dense fluid or the contents of an aerosol, with very high speeds (up to 300 km/h), and it can cause considerable damage. This type of avalanche is all the more dangerous as the powdery aspect doesn’t look as if it’s about to give way so it lulls skiers into a false sense of security.
A slab avalanche
This type of avalanche starts off with a fracture which can sometimes be seen from far away. The fracture spreads very quickly. The instability of the slabs is due to the presence of a fragile under-layer, and the delicate balance is endangered by the slightest surcharge, even as light as the weight of a skier.

A wet snow avalanche
These avalanches occur during warmer weather conditions, usually in spring. The thawing snow, full of excess water, can reach volumes of up to 500 kg/m3. They tend to move relatively slowly, from 20 to 60 km/h, but their capacity to do damage is colossal because of the huge weight per m3. Snow deposits can sometimes reach several metres in thickness…
These sleeping monsters need to be monitored constantly to ensure they’re not about to suddenly wake up.
Intense observation and anticipation
In France, ski patrols manage the 140 observation posts installed at between 1500 and 2000 metres in altitude. Twice a day, they relay information about the wind, its speed, its direction, the snow’s thickness, its quality, and the total height of the snow. The snowpack is regularly measured using probes to assess its stability. Automatic stations installed at between 2000 and 3000 metres in altitude relay information from the highest altitudes. The data is gathered in nine centres, providing the input for avalanche forecasts.
In the resorts, avalanche management is the winter’s biggest challenge. Ensuring safe skiing for thousands of holiday makers every day is an ongoing task as the snowpack is constantly changing and things can alter very quickly that nothing can be taken for granted.
In France each resort draws up an avalanche control programme (the PIDA), which it then submits to the prefecture. The programme indicates all the pistes and all the sectors where the different bomb experts will intervene, whether manually, on skis, or using automatic devices. Each intervention must be conducted within the framework of the avalanche control programme, which was first introduced in 1975 by the ANENA (national association for the study of snow and avalanches).
Every year, more than 2500 ski patrollers work in the French ski resorts. While not all of them take part in avalanche control directly, they are all more or less involved in the daily observation and monitoring work.
Avalanche control: from DIY to modern methods
At first, avalanches were artificially released in ski resorts in a pretty haphazard way. The ski patrollers would ski to the areas where the snow had accumulated, obviously staying above the hazardous area. While still in use, this dangerous method remains extremely risky. The snow cover may take a person’s weight at 8 in the morning, but two hours later, the fluttering of a butterfly's wing could be enough to release the avalanche!
The most effective means is to provoke an artificial avalanche with the help of explosives. This type of method triggers relatively small flows of snow, thus preventing a more spontaneous and much bigger avalanche.
Explosives in the back-pack: the basic method
A team of ski patrollers, who generally work in pairs, go to the area in question and detonate sticks of dynamite, strategically placed directly on the snow. The explosion is carried out remotely using an electric impulse or with the help of a slow-burning fuse (a bit like that of a banger except that this fuse is waterproof). Although still in use, this very old method is becoming increasingly rare, first because the amount of explosives is not always enough to trigger an avalanche, and second because it’s obviously dangerous for the snow safety experts.

CATEX
Catex is an abbreviation for ‘explosive-carrying cables’. This is a rudimentary ski lift that runs above the areas to be secured. The ski patroller clips the charge to a cable and then pulls it into position above the hazardous area. There’s always some danger linked to handling explosives but at least the professional does not need to go into the unstable area on skis, and the explosion is triggered from further away.
GAZEX
The Gazex (abbreviation for gas explosion) is the best way to set off an avalanche from a distance and the least dangerous as you can do it from an office while sitting safely in front of a computer. The idea is relatively simple. It involves setting off an explosive mixture of oxygen and propane gas inside an expansion chamber. These chambers are installed permanently in high risk areas, and are basically large bent pipes turned downwards towards the ground. This explosion is more effective than the Catex and creates a shock wave that directly releases the target snowpack as well as adjacent snow. The propane and oxygen comes from a small central gas unit called a Gazex shelter that the ski patrollers periodically change.

The Avalex
In this system, an artificial avalanche is again released from a distance but this time with the help of hydrogen and oxygen-filled balloons from a base above the area to be secured. The method works well as long as there’s no wind.
Manual firing from the sky
In this method, a helicopter hovers over the areas to be secured while a snow safety expert throws out the charge (attached to a rope). This manual hand tossing technique can also be carried out from a ski lift or a cable car. The main disadvantage is that the method is still somewhat imprecise and only snow safety experts who know the area perfectly well can conduct this type of mission, which nonetheless remains extremely delicate.

The Avalancheur
The avalancheur is nothing less than a compressed air canon which is aimed at the slopes exposed to risk of avalanche. In Russian resorts in the Caucasus, like Tcheget near Mont Elbrouz, for instance, they actually use real artillery fire. In France, it’s a little less drastic. The French avalancheur is a long glass fibre tube in which the snow safety expert introduces an arrow carrying an explosive mixture. Pressurized nitrogen can thus be launched with extreme precision onto the slope that needs to be secured via the explosive arrow.
Avalancheurs, Gazex, Catex, Avalex and manual firing are all tried and tested methods used by resorts to control avalanches. However, avalanche control is also a personal matter and every precaution should be taken before venturing off-piste.
Precautions and risk management
ENSA, the french national school of alpine skiing, has drawn up guidelines including five basic precautions:
1- Check out the regional avalanche bulletin on the radio (i.e. météo France) and check with the local professionals before venturing out
2- Respect the official and other guidelines provided by professionals and ski patrols services
3- Make sure you have all the indispensable basic equipment on you: i.e. shovel, beacon and probe, and know how to use them
4- Learn to recognise the different types of snow and the snowpack
5- Adopt a rigorous risk management procedure

These five basic rules are already invaluable tools for deciding where to go, and whether it’s wise to leave or not.
If an avalanche occurs, certain measures on the ground can help to reduce the risk. Avalanche prevention also involves adopting the ‘10 golden rules’ set out by the ENSA:

- Keep a certain distance between each person in a group
- Choose a safe route
- Organise meeting points at regular intervals in the protected areas
- Keep within sight and hearing of all the other skiers
- The speed and mode of advancing must be adapted to the conditions and the level of the least expert
- Follow the single trail
- Avoid critical areas
- Never go beyond a discontinuity in the slope without first stopping to assess the situation and analyse the lower level
- Never ski above or below other skiers
- As soon as any warning signs appear, be even more wary and careful
Decision-making tools: to go or not to go?
For adepts, there are a certain number of methods that can really hep you to decide whether to venture off piste or not.
The Munter method is the one that inspired the others, and is based on calculating the residual risk. We recommend that you check out the website www.anena.org and read the article on decision-making methods.

While avalanche prevention on the roads and in ski resorts is a matter for professionals, as soon as you step outside the ski area, everyone needs to take precautions. Most accidents are not down to bad luck but rather to a series of human mistakes and failure to respect the guidelines. Avalanche control also means having the humility to admit that the natural environment is greater than you, and being curious enough to learn about snow and how it changes while remaining cautious, a wise person who observes and thinks before acting.
For more information, you might like to read snow management .
Texte et photos: E. Beallet
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