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Avalanche

This article refers to the natural event known as an avalanche. For other uses, see below:


An avalanche is caused when a build up of snow is released down a slope, and is one of the major dangers faced in the mountains in Winter. An avalanche is an example of a gravity current consisting of granular material.

In an avalanche, lots of material or mixtures of different types of material fall or slide rapidly under the force of gravity. Avalanches are often classified by what they are made of, for example snow, ice, rock or soil avalanches. A mixture of these would be called a debris avalanche.

Contents

Causes

Avalanches occur when the load on the upper snow layers excedes bonding forces (bonding to layer beneath, support from anchors such as rocks and trees, stress support from top or bottom of slope).

Critical load may be exceeded naturally by adding new snow or by rapid loading, by falling ice, cornices and similar means. Avalanches are also triggered by humans - because of the additonal weight, kicks during skiing (e.g. during jumps) or intentionaly by explosives, slope-cuts and other means.

More than 90% of avalanche victims worldwide are due to avalanches triggered by victim or someone in the victims party.

Contributing factors

Determining critical load which would cause a slope avalanche is a complex task involving evaluation of many factors. Some of them are:

Terrain

  • Steepness - slopes under 25 degrees and over 60 degrees have a low avalanche risk. Distribution of avalanches by slope has a sharp peak between 35 to 45 degrees. That peak lies for example at 38 degrees in the Alps. Unfortunately, slopes with the most dangerous steepness are favourite for skiing.
  • Direction - snowpack evolution is influenced by solar heating and wind. In medium latitudes on the northern hemisphere, more accidents happen on shady slopes with northern and north-eastern aspects. Slopes sheltered from the wind tend to gather more snow.
  • Profile - convex slopes are statistically more dangerous than concave. Reasons lie partly in human behaviour.
  • Surface - base avalanches are more common on slopes covered with grass surfaces than slopes with dwarf pines.

Snow

Structure of the snowpack determines avalanche danger. Unfortunately relations between easily observable properties of snow layers (strength, grain size, grain type, temperature) and avalanche danger are complex and not yet fully understood. Additionaly snow cover varies in space and so does stability of snow.

Weather

Weather determines the evolution of snowpack. The most important factors are heating by solar radiation, radiational cooling, temperature gradients in snow and snowfall.

  • Temperature - if the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and re-freezing of water in the snow stabilizes the snow crystals. Temperatures rising significantly over freezing point may cause the whole slope to avalanche, especially in spring. Persistent low temperatures cause the snow to not gain stability from the freeze-thaw action.
  • Wind - anything more than a gentle wind can contribute to rapid build up of snow on sheltered slopes, while the wind pressure can also stabilize slopes.
  • Heavy snowfall - cause instability, both through the additional weight, and because the snow has insufficient time to bond.
  • Rain - in short-term causes instability

Avalanche avoidance

Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety is a continous process, including route selection and examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimise the risk

The group

  • Traversing - is to be avoided to minimise cutting across the slope.
  • Risk exposure - minimise the number of people on the slope. Ideally one person should pass over the slope into an avalanche protected area before the next one leaves protective cover.
  • Group size - the party should be large enough to perform a rescue, but additional people will increase the disturbance to the slope.

Human survival and avalanche rescue

Even small avalanches are a serious danger to life, even with properly trained and equipped companions who avoid the avalanche.

Research carried out in Italy based on 422 buried skiers indicates how the chances of survival drop

  • very rapidly from 92% within 15 minutes to only 30% after 35 minutes (victims die of suffocation)
  • and near zero after two hours (victims die of hypothermia)

- Nature vol 368 p21. Consequently it is vital that everyone surviving an avalanche is used in an immediate search and rescue operation, rather than sending for help.

(Historically, the chances of survival were estimated at 85% within 15 minutes, :50% within 30 minutes, 20% within one hour).

Equipment

Chances of being found and rescued are increased by using standard avalanche equipment.

  • Beacons - known as ARVAs, avalanche transceivers or under trade names including Ortovox http://www.ortovox.com/ and Pieps are important for every member of the party. They emit a 457kHz radio signal in normal use, but can be switched to receive mode to locate a buried victim. To be effective, beacons require a significant amount of practice.
  • Probes - can be extended to probe into the snow to locate the exact location of a victim at several yards / metres in depth.
  • Shovels - essential for digging through the snow to the victim.

Other rescue devices are proposed, developed and used, such as Avalanche Ball http://www.lawinenball.at/ , Avalung vest http://www.avalung.com and avalanche airbags.

European avalanche risk table

In Europe, the avalanche risk is widely rated on the following scale.

Risk Level Snow Stability Avalance Risk
1 - Low Snow is very stable on most slopes. Avalanches are unlikely except when heavy loads are applied on a few steep slopes. Any spontaneous avalances will be small.
2 - Limited On some steep slopes the snow is only moderately stable. Elshewhere it is very stable. Avalanches may occur especially on a few generally identified steep slopes when heavy loads are applied. Large spontaneous avalanches are not expected.
3 - Medium On many slopes the snow is only moderately or weakly stable. Avalanches are possible on many slopes even if only light loads are applied. On some slopes, medium or even fairly large spontaneous avalanches may occur.
4 - High On most steep slopes the snow is not very stable. Avalanches are likely on many slopes even if only light loads are applied. In some places, many medium or fairly large spontaneous avalanches are likely.
5 - Very High The snow is generally unstable. Even on gentle slopes, many large spontaneous avalanches are likely to occur.

See also

External links

  • Avalanche.Org http://www.avalanche.org
  • Cyberspace Snow and Avalanche Center http://www.csac.org/News/
  • Avalanche Avoidance and Route Planning http://www.pistehors.com/articles/avalanche/avoidance/


References

  • D. McClung, P. Shaerer: The avalanche handbook, The Mountaineers 1993, ISBN 0-89886-364-3
Last updated: 02-06-2005 20:24:33
Last updated: 02-11-2005 17:47:38