Avalanche
An avalanche is a mass of snow that breaks loose and runs down a slope of mountain , or following an imbalance in the snowpack is movement on the slope of a mass of snow. The necessary elements are: snow (quantity, quality), slope (at the start: max and min), the gravity (the engine), a break (the starter). The triggers are:
- internal instability of the snowpack;
- impact or overload point, external to the snowpack.
Summary |
Typology
An avalanche moves in a mountain site where there are three areas:
- Departure: often extended a ridge from which the avalanche may occur where snow has accumulated previously, where the snow mass in motion increases, the trigger types are varied (spontaneous / induced, linear / point);
- flow: along a slope on which the avalanche travels and grows (geometry, speed); types of developments are diverse (densification / dilution, homogeneous / multilayer laminar / turbulent);
- Arrest: extent to which an avalanche stops and settles where the snow mass in motion decreases, and the types of issues are still different (balls / smooth, with or without baseline).
Avalanches can be classified according to their type of flow, then refine this classification by type of release Types of flow This is the default form of flow avalanches, which may relate to any type of snow. These avalanches are a granular flow of snow, which then behaves like a fluid threshold. Their internal friction, which affects their ability to run on very low slopes, varies greatly depending on the quality of the snow mobilized: first, the liquid water content (higher in the snow during melting) increases the internal friction. These avalanches can cause significant damage to buildings due to moving masses of snow, despite their sometimes low speed. Their path follows the line of greatest slope, but is not necessarily easy to predict as a deposit from a previous avalanche can be enough to divert them. To generate an avalanche in aerosol , a "powder", you need a dry snow, without liquid water, very cold and very dense, in quantity at baseline and on the slope (for the resumption of snow), a fast flow (over 20 to 25 m / s) and an impeller suspended snow particles in air (eg topographic projection, a small rocky). The strong turbulence thus created as a spray : a cloud of snow particles (up to 5 to 10 kg / m 3 on average) that behaves like a gas heavy with snow. These spectacular avalanches often occur after episodes of heavy snow, and slide down the slope at high speed (100 to 350 km / h) on a fairly straight trajectory very sensitive to the terrain. They can produce a pressure wave / devastating depression (up to 3 bar overpressure) that sometimes causes significant damage to forests or breaking trees or on the roof of a cabin in the tearing and the based further, almost intact. They are able to cross the valleys and ascend to the heights on the opposite side often impressive. These avalanches are often triggered by skiers and hikers are the ones who make the most of the victims. They sometimes involve wind slab (whose role is often overstated), and in almost all cases a weak layer underlying snow low cohesion (usually the frost depth) which represents the main risk factor. The start is so important an area, and mobilizes large amounts of snow, sometimes in areas far from the initial rupture. If sometimes you can be alerted by the noise of blowing or collapse when stepped on, it is generally not possible to recognize. These plates can be made of hard snow ( cohesive ) or friable (powdery, sometimes very slight). These relate to snow avalanches with little or no cohesion: powder cold type flat faces, melting snow or waterlogged. They are a little safer because of lower amounts of snow mobilized, and less likely to bring the practitioner who triggers as they go below it. Formerly, a distinction was the avalanche along the height of the snowpack involved: avalanche surface (fracture occurs within the snowpack) or avalanche background (the whole mantle slips on the floor). Depending on the trigger, a further distinction: We focus here mainly to trigger an avalanche by a practitioner in the winter mountains: skiing, snowboarding, snowshoeing ... In almost all cases of tripping by a practitioner, the instability of the snowpack is called "plate", with a linear fracture. It is linked to the presence of a stack of layers of snow of different compositions: At the initial state, the upper layer is through its own resistance to the upstream (tension), downstream (compression) and side (shear) but also (perhaps especially) with the shear interface with the underlying layer. It is probably difficult to describe all the avalanche, but a mechanism appears to be a majority of field observations: when a load exceeds the bearing capacity of the weak layer, it collapses in compression, causing shear failure of weak layer (the two failure modes can be more or less mixed). On the surface, one can feel a slump, often accompanied by a characteristic sound ("whump" or "prouf") or cracks visible on the surface. The area covered by this double fracture of the brittle layer depends on the characteristics of the snow surface, which transmit more or less forced into the weak layer as a function of its thickness and rigidity. If the initial rupture exceeds a certain threshold size, it can spread (like a tear in a cloth) over large areas, or in some cases lead to remote triggering. If the slope is sufficient, this reduction in strength of the interface with the underlying layer is enough to upset the balance of the surface layer: the "whump" becomes an avalanche. From the perspective of relevant types of snow, the snow surface can be highly variable, until it has been heavily transformed by the thawing / refreezing, more precisely as: In particular, it is not always need wind to form a plate, most, say soft slabs are made of light powdery snow, very pleasant to ski. One can also find departures slabs of wet snow. The weak layer is much more critical, and involves in most cases of angular grains (flat faces or cups ) of ice covered surface, snow pellets ( sleet ), or in some cases, a crust of frost. The avalanche of exceptional size, involving very large volumes of snow and could do great damage to buildings or forests, are usually the result of an extensive linear departure. Otherwise, point of departure, which may relate in particular to the current snow fall or snow during humidification mass produce much fewer accidents. These relate mainly starts in the hillside above the victim. The power of avalanches is such that they can carry people, animals, trees, rocks, buildings, and in some cases, if the mass of snow is enough at stake, they can block a valley floor and form a dam on a natural temporary watercourses. Each year many people are killed by avalanches, most often during the off-piste skiing or hiking in the mountains. A large winter mountain experience significantly decreases the level of risk, but unfortunately never cancels. However simple tools for risk assessment are already available as NivoTest Bolognesi or the 3x3 method of Werner Munter. The expertise is necessary but not sufficient, therefore prevention is the use of emergency equipment and effective control, which minimizes the consequences of the payload and especially the burial. Appropriate behavior may also reduce the risk by reducing the probability of departure (large spacing within a group) or minimizing the number of victims (one person at a time in dangerous areas, others are waiting in protected areas). There are also cases of avalanches occur in populated areas, causing real disaster , destroying houses and burying under a heap of snow victims. Prevention is then in terms of planning, avoiding first building in risk areas (zoning), or by erecting avalanche (racks or forest setting snow in the starting areas, screwdrivers, deflecting Avalanche to areas not inhabited ...), or by handling the evacuation of residents of areas at risk during periods of very high risk of avalanche. The chances of survival, depending on the duration of burial of the person in an avalanche are about: It does not take into account possible damage suffered by the person away by the avalanche. According to sources, 10-20% of the victims died in the avalanche stopped. It is therefore crucial, once the victim is buried, to adopt a strategy to reach the quarter before the fateful hour. Notably, it is unrealistic to rely on this phase relief efforts, which can not arrive before that time a quarter of an hour: the research must be done by the practitioners themselves. The most effective method currently searching for avalanche victims is the use of the transceiver , which we must now call transceiver, detector avalanche victims, who can locate people buried wearing the device in transmit mode. Then using a probe allows accurate location of the victim and sometimes determining its position. With the shovel out of the bag, it remains only to dig, which often takes more time. The time allotted to these three phases of relief is a quarter of an hour, which means that it be conducted with great efficiency. The trio beacon -shovel- probe and constitutes the basic equipment of any freerider and is useless without regular training in rescue operations: not only looking for transceivers , but also looking at cutting the probe and shovel, while managing other aspects of aid: avoid an accident, alert the emergency medical care for treating victims once dsensevelies ... Other devices have been developed to increase the chances of survival, ABS and the airbag is it interesting in that it largely avoids the burying . One can also cite Avalung, which avoids the risk of suffocation in a buried victim. However, a weakness common to these two devices is that it requires action by the victim for their implementation at the start of the avalanche. The European scale identifies five risk levels (from 1 to 5, with 0 risk does not exist) based on stress and the geographical extension of the unstable snowpack. It applies to a massive scale regardless of slope or time. A new North American avalanche risk will help users of the backcountry to make better decisions based on risk when they access avalanche terrain. The new scale improves the clarity and usefulness of methods of communication to the public regarding the danger and the risk of avalanche. The scale is a five-level warning that indicates the probability that an avalanche occurs, its size and scope of the situation and recommend measures to those visiting the backcountry. In France, Cemagref (unit torrential erosion, snow and avalanches in Grenoble) and NFB (agencies and food services field in the mountains) are responsible for the Department for Environment, both operational arrangements observation of avalanches in the 11 departments of the Alps and the Pyrenees: Conducted according to fixed procedures, these two devices can have lots of homogeneous data and systematic. The LFS data are used primarily for the frequency analysis of events, while the CLPA is used to study the spatial properties of phenomena. Their information is public and used as input data for the most objective analysis of the hazard or risk of avalanches, especially for urban planning in general or regional planning. A classification of sensitive sites inhabited avalanches, on the whole of France, allows a rapid assessment of avalanche risk in areas over 1400. Their data are presented and available on the portal . The CLPA is also available in town hall. The Catex and Gazex , can voluntarily trigger avalanches. There are also " guns " tires capable of launching explosive arrows to several hundred meters , . In Switzerland , it employs shooting rocket tube to artificially trigger avalanches and the mine launcher . Avalanches of snow flowing
Avalanche aerosol
Types of trip
Avalanche Plate
Avalanches in starting spot
Other criteria
Mechanical triggering
Protection
Survival after burial
Search for avalanche victims
Scale European avalanche risk
Risk Index Snowpack Signage (flag) Probability of occurrence 1 - Poor The snowpack is well bonded in most steep. 
Yellow Triggering is generally possible only with high overhead and few very steep slopes.
Only cast or small avalanches can occur spontaneously. 2 - Limited In some steep slopes, the snowpack is only moderately stable.
Moreover, it is well bonded.
Yellow Triggering possible with high overhead and the steep slopes usually identified.
Of natural avalanches major are not waiting. 3 - Marked In many steep slopes, the snowpack is only moderately or weakly bonded 
Checkered yellow / black Triggering possible, sometimes even with low overhead and many slopes.
In some situations, a few natural avalanches medium and occasionally large, are possible. 4 - Fort The snowpack is weakly bonded in most steep slopes.
Checkered yellow / black Triggering probable even with low loads on many steep slopes.
In some situations, many natural avalanches medium and occasionally large, are expected. The backcountry is to avoid 5 - Very Strong The instability of the snowpack is widespread. 
Black Numerous large natural avalanches are likely, even in moderately steep terrain. Avalanche when they're there, go in all directions, the off-track is to avoid North American Avalanche Danger
The two devices avalanche observation in France
artificial triggering of avalanches
References
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