Avalanche

Typology

The following types of avalanche are distinguished:

Types of flow

Avalanches of running snow

It is about the form of flow per defect of the avalanches, which can thus relate to any type of snow. These avalanches constitute a granular flow of snow, which behaves then like a fluid with threshold. Their internal friction, which conditions their capacity to be run out on very weak slopes, varies largely according to the quality of mobilized snow: initially, the water content liquid (more important in snows in the course of cast iron) increases internal friction. These avalanches can cause extensive damage with the buildings because of the masses of snow moving, in spite of their sometimes low speed. Their trajectory follows the line of greater slope, but is not therefore very easy to envisage, because a deposit of a preceding avalanche can be enough to deviate them.

Avalanches in aerosol

To generate an aerosol, one needs a not very dense snow which quickly runs out (more 20-25 m/s). The passage above rock bars constitutes a worsening factor, while contributing to put the particles of snow suspended in the air. When turbulence is sufficient to maintain these suspended particles, it is formed a Aérosol, a cloud of fine particles of snow (5 with 10 kg/m ³) which behaves like a gas weighed down by snow. These spectacular avalanches often occur after abundant snowfalls fresh, and descend the slope at high speed very, of 100 with 350 km/h, on a rectilinear trajectory not very sensitive to the configuration of the ground. They produce a wave of pressure/depression devastator (until 3 bar of overpressure) which can cause extensive damage in the main forests by breaking the trees, or to tear off the roof of a country cottage and to further rest it almost intact. They are able to cross valleys to go up on the opposite slope.

Types of release

Avalanches of plates

These avalanches, often started by skiers or hikers, which is those make the most victims (80%). They often imply gusts of wind, and in almost all the cases a fragile layer subjacent of snow with low cohesion (white frost of surface, frosts of depth, more rarely crust of smooth regelation). The departure is done then on an important surface, and mobilizes very great quantities of snow, in zones sometimes far away from the initial rupture. So sometimes one can be alerted by noises of soufflement or of collapse when one walks above, it is generally not possible to recognize them a priori. These plates can be made up of hard snow (Cohésive) or friable (duster, sometimes very light).

Avalanches with specific departure

These avalanches relates to snows with little or not cohesion: standard cold duster plane faces, or snows of cast iron mouthful of water. They are a little less dangerous because of the smallest mobilized quantities of snow, and are less likely to carry the practitioner who starts them because they leave below him.

Formerly, one distinguished the avalanches according to the height from the snowy coat implied: surface avalanche (the rupture takes place within the snowy coat) or basic avalanche (the whole of the coat slips on the ground).

According to the starting factor, one can also distinguish:

the spontaneous avalanches , due to the natural evolution of the snowy coat,
the accidental avalanches , involuntarily caused by an human activity,
the artificial avalanches , started with the explosive (or more and more seldom with skis) to make safe a zone at the risk by avoiding the too important snow drifts.

Release

One will be interested here primarily in release of an avalanche by a practitioner of the winter mountain: raquettist, skier, surfer…

In the near total of the cases of releases by a practitioner, the instability of the snowy coat is known as “of plate”, it is related to the presence of a stacking of layers of snow of various compositions:

a layer not very coherent, known as “fragile layer”, acting as layer of unhooking while behaving a little with the manner of an house of cards (during the rupture) then of a travelator (during the flow),
one (or several) surface layers which constitute the principal mass which will descend the slope.

At the initial state, the roadbase is due thanks to its own resistance to the upstream (traction), the downstream (compression) and on the sides (shearing), but also (even especially) thanks to the shear strength of the interface with the subjacent layer.

It is undoubtedly difficult to describe all the avalanches, but a mechanism seems to correspond to a majority of observations of terrain : when an overload exceeds the capacity of bearing of the fragile layer, this one crumbles in compression, involving the rupture in shearing of the fragile layer (the two modes of rupture can be more or less mixed). On the surface, one can feel a depression, often accompanied by a characteristic noise (“whump” or “prouf”) or visible cracks on the surface. The surface concerned with this double rupture of the fragile layer is function of the characteristics of the surface snow, which will more or less transmit the constraints to the fragile layer according to its thickness and of its rigidity. If the initial rupture exceeds a certain threshold in surface, it can be propagated (like a tear in a fabric) on great extents, in certain cases to even give place to remote releases. If the slope is sufficient, this reduction in resistances of the interface with the subjacent layer is enough to break the balance of the surface layer: the “whump” becomes an avalanche then.

From the point of view of the types of snow concerned, surface snow can be very variable more precisely so much, as long as it was not strongly transformed by the thaw/regelation:

that it is capable of retransmettre constraints to the fragile layer;
that it is sufficiently fragile not to hold in balance without the shear strength of the fragile layer.

In particular, it is not need for wind to form a plate ; the majority, known as friable plates , are made light powder snows, very pleasant to ski. One can also find departures in plates of wet snow.

The fragile layer is much more determining, and utilizes in the majority of the cases of the angular grains (plane faces or goblet S), of the Givre of surface covered, rolled snow (Grésil) or, in certain cases, a crust of regelation.

One treated here the case of the departure linéaire ; the specific starting cases, which can in particular relate to fresh snow in the course of fall or of snow in the course of massive humidification, generate much less accidents. Those relate to mainly departures on the slopes which overhang the victims. As for them, the exceptional avalanches, implying volumes of snow very important and likely to make large damage with the buildings or the forests, are generally the fact of a very wide linear departure.

Protection

The power of the avalanches is such as they can carry trees, rocks, buildings; in certain cases, if the mass of snow concerned is sufficient, they can block a bottom of valley and constitute a temporary Barrage natural on a river.

Each year, out of many people are killed by avalanches at the time of excursions in mountain, of which a good number are largely tested: a great expertise, if it decreases the level of risk notably, does not cancel it unfortunately. But right now, of the simple tools for evaluation of the risk are available like NivoTest de Bolognesi or the method 3x3 of Munter.

The expertise being necessary, but not sufficient, the prevention thus passes by the use of a material of help effective and controlled , which makes it possible to minimize the consequences of carrying and especially of the burial. adapted behaviors will be able to also reduce the risk, by decreasing the starting probability (important spacing within a group) or by minimizing the number of victims (only one person at the same time in the danger zones, the others await it in protected areas).

It also happens that avalanches occur in inhabited areas, causing true Catastrophe S by destroying houses and by burying under a snowdrift their victims. The prevention is done then in terms of town and country planning, while initially avoiding building in the zones at the risk, or by setting up Paravalanche S (racks or forests fixing snow in the starting zones, turn deviating the avalanche towards not inhabited zones…), or by managing the evacuation of the inhabitants of the zones at the risk during the periods of very strong risk of avalanche.

Survival after burial

The chances of survival, according to the duration of burial of the person in an avalanche, are approximately of:

91% between 0 and 18 minutes;
34% between 18 and 35 minutes;
20% between 35 and 120 minutes;
7% after 140 minutes, the curve being asymptotic.

That does not take account of the damage possibly undergone by the person carried by the avalanche - according to the sources, 10 to 20% of the victims died with the stop of the avalanche.

It is thus crucial, once the victim is buried, to adopt a strategy making it possible to release it before the fateful fifteen minutes. In particular, it is illusory to count for this phase on the organized helps, which cannot arrive before this time of fifteen minutes: research must be made by the practitioners themselves.

Seek victims of avalanches

The most effective method currently for research of the victims of avalanche is the use of the ARVA which makes it possible to locate the hidden people carrying the apparatus in emission mode. Then the use of a Sonde allows a precise localization of the victim and sometimes the determination of its position. It any more but does not remain to dig, which often takes the most time!

The time assigned with these three phases of help is of fifteen minutes approximately, which implies that it are carried out with a very great effectiveness.

The trio ARVA - shovel Sonde thus constitutes the basic equipment of all Freeride R, and is not used for nothing without a regular drive helps.

Other apparatuses were developed with an aim of increasing the chances of survival of the victims, thus Airbag ABS is it interesting insofar as it avoids mainly the burial (cf statistics of the manufacturer). One can also quote Avalung, which makes it possible to avoid the risks of suffocation to a buried victim. However, a weakness common to these two apparatuses is to require an action of the victim to implement them at the time of the departure of the avalanche.

European scale of risk of avalanche

The European scale identifies five levels of risk (from 1 to 5, not existing risk 0) based on the stressing and the geographical extension of the instability of the snowy coat. It applies on a solid mass scale without reference of slope or hour.

See too

the Effect of avalanche, in electronic
Avalanches of Mont Blanc, nickname of the Mont Blanc Hockey Club
the Catex, tool allowing to voluntarily start avalanches

External bonds

File Futura-Sciences on the avalanches
File Avalanche-Net on snow and the avalanches
National association for the Study of Snow and Avalanches (ANENA)
Presentation of the principles and the results of the programmes of observation of the avalanches (MEDD-CEMAGREF-ONF)
Chart of Localization of the Phenomena of Avalanches (CLPA)
Permanent Investigation into Avalanches (EPA)
Hot sites with the Avalanches (SSA)
federal Institute for the study of snow and avalanches (SLF/IFENA)
ASARC, University of Calgary
ALPdidact: Assosciation for the formation snows and avalanches
environmental Hydraulic research laboratory of FPSL
: database of real avalanches (RISK limited liability company)

Simple: Avalanche

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