Les dangers objectifs / Objective hazards
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écrit par Wladimir T.

chef de course et instructeur au Club Alpin Suisse

Le 22 février 2013

Les dangers objectifs

On parle de dangers objectifs pour les risques liés au terrain ou à la météo

Les chutes de pierres

C’est le danger le plus grave auquel est exposé le montagnard car la trajectoire des pierres est absolument imprévisible. Les chutes de pierres sont en général provoquées par le dégel; elles se produisent donc le plus souvent lorsque le soleil commence à faire fondre la glace qui scelle les blocs. Les animaux et les hommes peuvent aussi déclencher des chutes de pierres.

Les chutes de glace (séracs, corniches)

Le danger de chute de séracs s’accentue avec l’élévation de la température. Cependant, des chutes de séracs peuvent se produire aussi à n’importe quelle heure du jour et de la nuit, engendrées par la poussée du glacier.

Le danger que représentent les corniches est double. Elles menacent les pentes qu’elles dominent et peuvent à tout moment les balayer en avalanche. Elles peuvent aussi s’effondrer sous le poids du montagnard qui se serait aventuré sur ce balcon de neige. La ligne de rupture d’une corniche est difficile à évaluer.

Les avalanches

Ce phénomène n’est pas réservé à l’hiver ou au printemps. En été aussi il se produit des avalanches, souvent sous la forme de coulées de neige sur les pentes exposées au soleil. Mais le piège le plus insidieux et le plus difficile à déceler est constitué par les plaques à vent. Une grande prudence s’impose dans une course de neige après une période de mauvais temps, surtout si les chutes de neige ont été accompagnées de vent.

Lors de la planification d’une course il serait judicieux de consulter le site du CAS sur les avalanches.

Le SLF donne également des informations pertinentes sur la neige et les avalanches.

Crevasses, ponts de neige

Ce sont les crevasses recouvertes et les ponts de neige qui sont les plus dangereux. En fait, tout glacier recouvert de neige est dangereux. La règle est de ne jamais circuler sur un glacier recouvert de neige sans être encordé.

Il convient également de se méfier des résidus d’avalanche qui recouvrent un torrent. Louis Lachenal (1921-1955), membre de la compagnie des guides de Chamonix, premier vainqueur de l’Annapurna, est décédé suite à une chute dans un torrent recouvert de neige.

Les phénomènes atmosphériques

Les manifestations atmosphériques sont à l’origine des plus grandes tragédies alpines. Ces phénomènes sont:

En haute montagne, les changements de temps peuvent être très rapides et très brutaux. Dans bien des cas, la retraite immédiate sera la solution la plus sage. Si celle-ci est impossible et que le montagnard se trouve « pris au piège » alors, il aura tout le loisir de méditer sur l’importance des prévisions météorologiques. Dans sa lutte contre le mauvais temps, le montagnard devra faire appel à toutes ses ressources physiques et morales. Il verra à quel point sont importants: les prévisions météo, le plan de marche, la tenue d’un horaire et l’équipement.

Le vent

En éliminant le manchon d’air chaud que chaque personne a autour de soit, le vent accentue considérablement les méfaits du froid. Même par beau temps le vent augmente la difficulté de la course. Il compromet l’équilibre, aveugle en soulevant la neige et menace la communication entre les membres de la cordée en emportant les paroles. Il faut, en particulier, craindre le foehn car il accroît les chutes de pierres, de glace et les dangers d’avalanche.

Le froid

Le mauvais temps est presque toujours accompagné d’une baisse importante de la température. Il engendre les gelures et l’hypothermie qui entraîne la mort. Pour s’en défendre, l’importance de l’équipement est primordiale. En cas d’immobilisation forcée et de bivouac il faudra tenir compte de l’humidité et du vent qui ont la particularité d’aggraver les effets du froid.

Le brouillard

Le brouillard peut dérouter le montagnard le plus averti, même dans un terrain qui lui est familier. Le recours au plan de marche et l’utilisation avisée de la boussole et de l’altimètre pourront seuls être de quelques secours. Parfois, mieux vaudra attendre sur place que le brouillard se lève, ou rebrousser chemin en suivant scrupuleusement les traces de montée.

La neige

Elle recouvre les prises et les rend glissantes. Elle donne naissance à des coulées de neige dans les couloirs. Les pieds s’enfoncent et la progression devient pénible. La visibilité diminue. Accompagnée de vent, elle va recouvrir les crevasses de fragiles ponts de neige extrêmement dangereux.

La pluie et la grêle

Elles rendent les surfaces glissantes et donnent naissance dans les couloirs et les cheminées à des cascades qui charrient des pierres.

Le verglas

Sous l’effet du froid l’eau de ruissellement se transforme en verglas. La pellicule de glace est si fine qu’elle est invisible, avec pour conséquence le risque d’être surpris. Le verglas peut se produire également lorsque les gouttes de pluie sont en surfusion, c’est à dire encore liquides à une température inférieure à zéro degré. Le choc sur la roche rompt l’équilibre et l’eau se transforme instantanément en glace.

Les chutes de pierres

C’est le danger le plus grave auquel est exposé le montagnard car la trajectoire des pierres est absolument imprévisible. Les chutes de pierres sont en général provoquées par le dégel; elles se produisent donc le plus souvent lorsque le soleil commence à faire fondre la glace qui scelle les blocs. Les animaux et les hommes peuvent aussi déclencher des chutes de pierres.

L’orage et la foudre

Ce sont des manifestations atmosphériques que le montagnard craint à juste titre. L’orage se déchaîne le plus souvent dans l’après-midi. Les signes précurseurs sont connus: formation de cumulo-nimbus sombres (nuages en forme d’enclume), levée d’un fort vent et ionisation de l’air. Ce dernier phénomène est particulièrement impressionnant. L’air s’emplit de crépitements, la pointe du piolet émet un son qui ressemble au bourdonnement d’une abeille, les cheveux peuvent se dresser sur la tête comme tirés par une main invisible, dans l’obscurité des aigrettes bleutées scintillent aux cheveux et aux dentelures rocheuses.

La foudre peut frapper à tout instant. Pour éviter un coup direct, le montagnard doit quitter les sommets et les crêtes vers des zones plus plates. Ne pas courir. Descendre d’au moins 30 mètres. Si possible se réfugier sur un replat, à proximité d’un point élevé qui servira de paratonnerre.
En pratique, le ressaut devrait dominer de 5 à 10 fois la hauteur de la personne. Celle-ci doit se tenir écarté de la paroi d’une distance égale à sa propre hauteur pour éviter d’être victime des courants de terre.

Ces risques indirects sont d’ailleurs ceux auxquels les montagnards sont le plus exposés. Se tenir éloigné des parois d’au moins 1.5 mètres, qu’il s’agisse d’un rocher vertical, d’un plafond ou d’un fond de grotte. En forêt s’éloigner le plus possible des troncs. S’écarter des pylônes des lignes à haute tension, et de toute structure métallique. Eviter les zones humides.

 

 

La position assise, genoux relevés et pieds joints est la meilleure. Une corde roulée, placée entre le corps et le sol, augmente encore la protection. Enfin, une commotion même légère, peut faire lâcher prise et entraîner une chute grave. Un auto-assurage est vivement conseillé.

Ne pas se placer dos contre la paroi. Se mettre à l’écart, en adoptant la position assise, genoux relevés et pieds joints.

Eviter les trous peu profonds et ne pas se tenir à l’entrée des grottes, le courant de terre peut sauter la dépression et traverser le corps.

En groupe, maintenir un écart de 3 mètres minimum entre chaque membre.

Remplacer ses vêtements humides de sueur par des vêtements secs et enfiler une veste imperméable.

Écarter piolet, crampons et autres objets métalliques. Éteindre le téléphone portable et la radio.

Cet article de Montagne-Magazine donne des informations utiles sur le comportement à adopter en cas d’orage en montagne.

Dans la forme la plus grave, le foudroyé est inconscient,
avec arrêt cardiaque et respiratoire.
Il faut aussitôt entreprendre une réanimation !

Objective hazards

Objective hazards are risks associated with the terrain or the weather.

Falling rocks

This is the most serious danger facing mountaineers, as the trajectory of the rocks is absolutely unpredictable. Rockfalls are generally triggered by the thawing of the ice, so they usually occur when the sun begins to melt the ice that seals the blocks. Animals and humans can also trigger rockfalls.

Ice falls (séracs, corniches)

The danger of falling seracs increases as the temperature rises. However, seracs can also fall at any time of the day or night as a result of the glacier’s thrust.

The danger posed by cornices is twofold. They threaten the slopes they dominate, and can sweep them away in an avalanche at any moment. They can also collapse under the weight of a mountaineer who has ventured onto this snow balcony. The breaking line of a cornice is difficult to assess.

Avalanches

This phenomenon is not confined to winter or spring. Avalanches also occur in summer, often in the form of snowflows on slopes exposed to the sun. But the most insidious trap, and the hardest to detect, is wind slabs. Great care must be taken on a snow run after a period of bad weather, especially if the snowfall has been accompanied by wind.

When planning a trip, it’s a good idea to consult the SAC avalanche website.

The SLF also provides relevant information on snow and avalanches.

Crevasses, snow bridges

Covered crevasses and snow bridges are the most dangerous. In fact, any snow-covered glacier is dangerous. The rule is never to travel on a snow-covered glacier without being roped up.

You should also be wary of avalanche residue covering a torrent. Louis Lachenal (1921-1955), a member of the Compagnie des Guides de Chamonix and the first winner of the Annapurna, died after falling into a snow-covered torrent.

Atmospheric phenomena

Atmospheric phenomena are at the root of some of the Alps’ greatest tragedies. These phenomena are:

Wind

Cold

Fog

Snow

Rain

Ice

falling rocks

Thunderstorms and lightning

In the high mountains, weather changes can be very rapid and brutal. In many cases, immediate retreat will be the wisest solution. If this is impossible and the mountaineer finds himself « trapped », then he will have plenty of time to reflect on the importance of weather forecasts.

In his battle against bad weather, the mountaineer will have to call on all his physical and moral resources. He will see just how important the weather forecast, the route plan, keeping to a timetable and the equipment are.

The wind

By removing the sleeve of warm air that everyone has around them, the wind considerably accentuates the effects of the cold.

Even in fine weather, the wind increases the difficulty of the race. It compromises your balance, blinds you by blowing up the snow and threatens communication between the members of the roped party by taking away their words.

In particular, you need to be wary of the foehn wind, which increases the risk of rock and ice falls and the danger of avalanches.

The cold

Bad weather is almost always accompanied by a significant drop in temperature. This can lead to frostbite and hypothermia, which can be fatal.
To protect against this, the importance of equipment is paramount. In the event of forced immobilisation or bivouac, you need to take account of the humidity and wind, which have the particularity of aggravating the effects of the cold.

The fog

Fog can confuse the most experienced mountaineer, even in familiar terrain. A walking plan and the wise use of a compass and altimeter are the only things that can help.

Sometimes it’s better to wait for the fog to lift, or to turn back and follow the tracks back up carefully.

The snow

It covers the holds and makes them slippery. It gives rise to snow slides in the corridors. Feet sink and progress becomes difficult. Visibility diminishes. Accompanied by wind, it covers the crevasses with fragile, extremely dangerous snow bridges.

Rain and hail

They make surfaces slippery and create waterfalls in corridors and chimneys that carry stones.

The ice

Under the effect of the cold, run-off water turns to ice. The film of ice is so thin that it is invisible, so there is a risk of being caught out.

Freezing rain can also occur when raindrops are supercooled, i.e. still liquid at sub-zero temperatures. The impact on the rock breaks the equilibrium and the water is instantly transformed into ice.

Falling rocks

This is the most serious danger facing mountaineers, as the trajectory of the rocks is absolutely unpredictable. Rockfalls are generally triggered by the thawing of the ice, so they usually occur when the sun begins to melt the ice that seals the blocks. Animals and humans can also trigger rockfalls.

Storm and lightning

These are atmospheric events that mountain dwellers rightly fear. Thunderstorms usually break out in the afternoon. The warning signs are well known: the formation of dark cumulonimbus clouds (anvil-shaped clouds), strong winds and ionisation of the air.
This last phenomenon is particularly impressive. The air fills with crackling sounds, the tip of an ice axe emits a sound that resembles the buzzing of a bee, hair can stand on end as if pulled by an invisible hand, and in the darkness bluish egrets sparkle in the hair and on the jagged rocks..

Lightning can strike at any time. To avoid a direct hit, mountaineers should leave the peaks and ridges for flatter areas. Do not run.
Descend at least 30 metres. If possible, take refuge on a flat area close to a high point that can be used as a lightning rod.
In practice, the rise should be 5 to 10 times the person’s height. The person should keep away from the wall by a distance equal to their own height to avoid falling victim to earth currents.

These are the indirect risks to which mountaineers are most exposed. Keep at least 1.5 metres away from walls, whether they are vertical rocks, ceilings or cave floors. In the forest, keep as far away as possible from tree trunks.
Keep away from high-voltage pylons and all metal structures. Avoid damp areas.

 

 

Sitting with knees up and feet together is the best position. A rolled rope, placed between the body and the ground, further increases protection. Finally, even a slight concussion can cause you to let go and lead to a serious fall.
Self-belay is strongly recommended.

Do not stand with your back against the wall. Move out of the way, adopting a sitting position with knees up and feet together.

Avoid shallow holes and do not stand at the entrance to caves, as the earth current can jump the depression and pass through the body.

In groups, maintain a minimum distance of 3 metres between each member.

Replace sweaty clothes with dry ones and put on a waterproof jacket.

Put away ice axes, crampons and other metal objects. Switch off mobile phones and radios.

This article from Montagne-Magazime provides useful information on what to do in the event of a storm in the mountains.

In the most severe form, the victim is unconscious,
with cardiac and respiratory arrest.
Resuscitation must be started immediately !

Vues : 109

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