Avalanche

An alud on Mount Everest in May 2006.

The result of an avalanche in Kenai Fjords National Park (Alaska).

Path produced by the fall of an alud on the north side of the Pui de Linya (Catalonia).

An avalanche, also known as an avalanche, is the displacement of a layer of snow on a sloping area that moves downhill with a speed that can vary.

Part of the substrate and vegetation cover of the slope can also be incorporated, usually caused either by the amount of snow that accumulates in the starting area of the avalanche, that is to say by overload, which generates a collapse within snowpack, usually at the level of a weak layer; In addition, there must be a slope angle between 25° and 45° preferably; It can also be generated by people, skiers, mountaineers, climbers, snowmobiles, snowcats, and animals such as guanacos, and everything is related to the overload of the snow cover. On the other hand, rains on the snow cover can be the cause of an avalanche, and there can also be avalanches caused or induced with explosives, which are classified as artificial avalanches.

Classes

Start of a dust snow alud

Avalancha caused by a guanaco.

Panoramic photography of the avalanche caused by the guanaco.

Photographic sequence of the guanaco avalanche.

Rescue the guanaco.

The guanaco, rescued.

There are three types of avalanches:

• Alud aerosol. It usually comes from a point and drags more and more snow. This alud starts when the pes of the snow succumbs to the force of gravity. This happens mainly during or after strong snowfalls (from 25 centimeters thick) and at low temperatures (less than -5 °C), especially when it accumulates on a smooth (mantle) basis (due to rain, frost or melting snow). This snow, dry and light, mixes with air and forms an aerosol that lowers the slope at a speed of 100 to 300 km/h. It could occur at any time of the year.
• Wet snow Alud. This type of alud occurs in the slopes very exposed to the Sun (when the snow melts) when temperatures rise mainly during the spring. They slide slowly (20 to 60 km/h). The factors that activate it are the air or the hot wind, the Sun and the overweight. They usually occur in slope south-facing and may leave the soil uncovered due to its erosive power.
• Plate alud. This type of alud is most common. A plate is a compact snow surface that is detached from the rest of the snow mantle and slides over the ground or on the existing snow. They usually originate to open slopes by the passage of a skier. These plates can be immense and leave a very visible fracture limit.

All avalanches can have serious consequences, but plate avalanches have two factors that make them very dangerous. First: they can be caused simply by the passage of a skier. And, secondly, they are the most common in the Pyrenees.

There is also the Mixed plate avalanche. An example is June 17, 2005, around 12:10 p.m. m., a guanaco descends from the top of the mountain and causes the avalanche (see image sequences, the guanaco avalanche); This was in the Andes Mountains, on the Argentine side, in the sector near the Mina Veladero camp.

The area where the avalanche started has a slope of 37° and an average slope angle of 31°.

Classes by magnitude

There are two types of avalanches:

• Surface alud: where only part of the nival mantle is mobilized.
• Background: where it is mobilized suddenly and violently all eroding the substrate of the slope, transporting and depositing these materials at the point where the angle of it with respect to the horizontal, allows it.

In addition, the movement or dynamics of an avalanche can be characterized by the speed it can develop (m/s); the height of its flow (m) and the impact pressures that can be generated (kPa); there is the Avalanche Atlas published by Unesco in 1981, where avalanches are classified by their morphology, by their origin, by their shape etc

Europe

Canada

The Canadian classification for measuring avalanches is based on the consequences it may have. Normally avalanches are categorized as medium size.

United States

Impact pressure of an avalanche

It is important to take into account the pressure of the impact of an avalanche on the infrastructures destined to contain them or on the constructions or structures that have to be in an avalanche area, such as electrical towers.

The force of the impact will be proportional to the speed of the avalanche squared and to the density of the flow of the material that forms the avalanche. This impact pressure force can range from one to 1,000 kPa.

Classification of avalanche risks

In winter mountain activities, avalanches cannot be avoided if it is not with a correct knowledge of the environment and a continuous evaluation of the terrain, they are one of the objective dangers in the practice of mountaineering and unique in ski mountaineering.

Everywhere where these two sports are practiced, "snow bulletins" are published (with data obtained daily by taking measurements on the ground) where the risk of avalanches in a certain area is assessed, according to a "ladder of danger".

Europe

This scale was approved in April 1993 to replace the previous criteria of the different national systems and establish homogeneous ones throughout Europe. It was last updated in May 2003.

The European avalanche danger scale consists of five degrees depending on the stability of the snow cover and the probability of triggering avalanches (from 1 to 5, 0 means there is no risk), this scale is used almost all over the world

Hazard level	Stability of the ny mantle	Icon	Probability to trigger alludes
1 – weak	Nivous mantle is generally well stabilized		In general it is only possible to trigger alludes on steep slopes or especially unfavourable ground (1) and because of heavy overloads (2). Spontaneously they can only trigger small threads or alludes.
2 – Limited	On some steep slopes the nest mantle is only moderately stabilized; on the rest of the slopes it is generally well stabilized.		It is possible to trigger alludes above all by strong overloads (2), especially on propitiated steep slopes (1). It is very unlikely that large alludes will spontaneously be unleashed.
3 – Notable	On many steep slopes the nest mantle is between moderately and weakly stabilized.		It is possible to trigger alludes, even by weak overloads (2), especially on propitiated steep slopes (1). In some cases, medium-sized alludes are possible and, in isolated cases, large, spontaneously triggered.
4 – Strong	In most steep slopes the mantle is weakly stabilized.		The triggering of alludes is likely even through weak overloads (2), on many steep slopes. In some cases many medium-sized, often large, alludes are expected to be spontaneously triggered.
5 – Very strong	Nivous mantle is generally very unstable.		Many large alludes are expected, often very large, spontaneously triggered, even on slopes only moderately tilted

• (1) Areas propitious to alludes are described in more detail in the alludes danger newsletters (altitude, orientation, type of land, etc.).
  • − Little or moderately tilted land: slopes with a tilt less than 30o.
  • − In steep slopes: slopes with a slope greater than 30°.
  • − Very tilted or extreme land: slopes of more than 40 degrees of inclination and especially unfavourable terrain due to its profile, proximity to ridges or the low roughness of the underlying soil surface.
• (2) Overloads:
  • − Weak: a single skier or surfer, moving smoothly and without falling. A group of people who respect the security distance (minimum 10 m). A racket.
  • − Strong: two or more skiers, surfers etc. without respecting the safety distance. Pisanieve machines or other vehicles that circulate on snow, explosives. Occasionally, a single hiker or climber.

North America

In the United States of America and Canada, the avalanche danger ladder has been in force since the late 1990s. Descriptions vary depending on the country.

Causes of avalanche formation

Avalanches are caused by the lack of homogeneity of the snow cover and by the existence, between the limits of physically different layers, of an agent that facilitates the sliding of one of them over another underlying one. It happens, for example, that the snow that has just fallen or accumulated by the wind does not become cohesive to the surface of the pre-existing layer.

In other cases, the rain soaks a recent layer, which then slides under its own weight, if the slope allows it. Rainwater can also infiltrate between two layers of snow and then act as a lubricant that allows the upper layer to slide over the lower one. The same can happen if water penetrates snow and ground, making it slippery.

Ambient temperature changes are also very important. In particular, a significant increase in temperature reduces the cohesion of the snow, which is why avalanches are more likely in the afternoon than in the morning, especially on those slopes that have been exposed to the sun's rays during the hottest hours of the year. day. Sometimes the sudden heating by the morning sun is enough to cause avalanches on the steep slopes facing east.

The conditions of the soil that supports the snow can also be decisive: clayey terrain and, therefore, slippery; smooth, humid or icy soils, convex slope or excessive slope.

Whatever the circumstances favorable to an avalanche, it can be triggered by a minimal cause although capable of overcoming the little cohesion that the mass of snow retained: a noise, the detachment of a rock or a block of ice, or the simple passage of a skier through the unstable layer.

Avalanches or avalanches can be triggered naturally, 24 hours a day, there is no fixed rule regarding the hours of the day; also when there are intense snowfalls, a greater instability of the snow cover is generated and therefore an avalanche can be generated; When snow falls on an old layer of snow deposited on the slope or mountain, there is a greater chance of generating an avalanche, since the same deposited snow acts as a slide for new snow. On the contrary, if we have a slope without snow or dry with the earth substrate, the avalanche is less likely to be generated, since there is a part of the new or precipitated snow that melts when taking the soil or earth from the slope.

Snow avalanches occur when support is lost. This loss of support and the consequent avalanche can be spontaneous or caused by man.

Spontaneous avalanches

They are produced by an overaccumulation of snow or by a change in the conditions of excessive snow.

The over accumulations occur in times of significant snowfall and are normally new powder snow. This category would include the continuous purges that occur in the steepest places on the mountains, on the crests and in the steepest channels. right, where the snow hardly finds support and therefore empties very often.

Spontaneous avalanches can also be caused by a change in conditions that kept too much snow stable.

Increases in temperature and rain are the most frequent alterations and affect snow of any quality: powder snow because it increases in relative weight on the surface to the point that the lower layers cannot support it. The large sheets of fanned snow because they break at a given line, from which the sheet slides down. Spring or wet snow because it increases its fluidity and slides over other layers of hardened, untransformed snow, and even over the same soil, then called "bottom avalanche".

Among the spontaneous avalanches, the most dangerous are the powder snow ones: they are triggered at a fairly high point on the mountain and thus they descend increasing their volume and the speed of descent can reach 200 km/h. The combination of mass and speed make them enormously destructive, large trees and entire forests are uprooted by the push of the snow, rocks of any size are torn from their beds and pushed down. It is common that when a powder snow avalanche reaches the bottom of a valley that is not too wide, it covers rivers and torrents and comes up (by inertia) the opposite slope. It is very unlikely that people or animals hit by a powder snow avalanche could survive. However, as these avalanches occur regularly in the same places, often evident, it is avoidable to stay in the affected area.

There are, however, exceptions to this rule. Thus, in January 1996 in the Pyrenees there were numerous large powder snow avalanches that devastated unusual areas. Also in the Alps, where they also caused serious loss of life and facilities.

Spontaneous plate avalanches and wet snow avalanches are much less dangerous, since their range is much smaller, as well as their volume and speed.

Provoked avalanches

The fall of an alud to the Grisons (1810)

Normally —in the case of avalanches caused by powder snow— the crisis occurs due to overload, when one or more people or animals step on a fairly steep snow-covered surface. Its weight causes the equilibrium prior to the overload to be exceeded and the mass of snow collapses, first below the place of passage, but immediately also above it, since it loses its lower support. The beings that have caused the avalanche are inevitably swept away and their survival unlikely.

In addition to the thickness of snow and the degree of the slope, there are other factors to evaluate when crossing exposed slopes: the most important is the existence of several layers of snow, which may not be cohesive among themselves, due to different densities and also at different temperatures, giving rise in the latter case to what is called «thermal fracture».

There are natural but infrequent overloads: the fall of snow sills is the most classic.

The same passage of people, almost always on skis and uphill, can cut the surface of the snow sheets formed by the wind. If the slope is sufficient, the plate will slide down as in the case of spontaneous rupture. Naturally, in this case, plate avalanches are dangerous, since they will drag the skier who causes it and those around him. They are the case of most frequent provoked avalanches. On condition of a faster rescue than the time of suffocation or hypothermia, survival is possible.

Avalanches caused by spring snow, wet and heavy, which are frequently caused by downhill skiers, are the least dangerous: their speed is very slow and their range too, which makes it possible and easy to avoid their trajectory.

Zones

Alud

Identifying the avalanche zones allows mapping the affected areas that can be used in territorial planning. There are three:

• Output zone: is where the snow gets moving and starts to accelerate along the slope (about 30o–40o or more), where it gains mass constantly.
• Travel area: is the area where the alud keeps a speed more or less constant. The median slope is between about 20° and 30°.
• Arrival area: is where the alud stands. The slope is usually less than 20 degrees.

Safety recommendations in avalanche areas

1. Avalanches can occur at any time of the day.

2. During the storm there is the greatest instability of the snow cover.

3. The snow overload of the slope can generate avalanches.

4. The wind is the greatest sculptor of avalanches, because it transports the snow to the starting area.

5. Avalanches are destructive, as they normally have significant impact pressure (kPa).

6. The energy of the avalanches, we must know them in approximate terms versus their potential.

7. The rain helps the generation of avalanches.

8. Crossings must be made with skis, in post to be a group.

9. Avoid the bottom of ravines, when the snow cover is unstable.

10. Avoid outings with recent snowfall.

11. Consider in the safety analysis the direction and speed of the wind.

Notable avalanches

Two avalanches occurred in March 1910 in the Selkirk Mountains on the Canada-US border; On March 1, the Wellington Avalanche killed 96 people in the state of Washington, United States. Three days later, 62 railroad workers were killed in the Rogers Pass avalanche in British Columbia, Canada.

During World War I, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the Alps on the Austrian-Italian front, many of which were caused by artillery fire. Around 10,000 men, from both sides, died in avalanches in December 1916.

In the Northern Hemisphere winter of 1950-1951, approximately 649 avalanches were recorded in the three-month period in the Alps in Austria, France, Switzerland, Italy, and Germany. This series of avalanches killed around 265 people, and that period was called the winter of terror.

In 1990 at Lenin Peak, present-day Kyrgyzstan, a mountain climbing camp was swept away when an earthquake caused a large avalanche to hit the camp. 43 climbers were killed.

In 1993, the Bayburt Üzengili avalanche killed 60 people in Üzengili in Bayburt Province, Turkey.

In 1993 in Montroc, France, an avalanche carrying 300,000 cubic meters of snow slid down a 30° slope, reaching a speed of around 100 km/h. It killed 12 people in their chalets, burying them under 100,000 tons of snow under a 5m-thick layer. The mayor of Chamonix was convicted of manslaughter for failing to evacuate the area, although the sentence was suspended.

The small Austrian town of Galtür was hit in 1999 by the Galtür avalanche. The town was thought to be in a safe area, but the avalanche was exceptionally large and entered the town. 31 people died.

On December 1, 2000, the Glory Bowl Avalanche formed on Mount Glory. Mount Glory is in the Teton Range in Wyoming, United States. Joel Roof was amateur snowboarding, and he caused that avalanche in that bowl-shaped environment. Roof was dragged nearly 600 m to the base of the mountain and could not be successfully rescued.

A review of the deadliest avalanches or avalanches is compiled in Worldatlas.

Avalanches in fiction

Avalanches have been a resource used in many fictional stories set in snowy environments, often for humorous purposes. An example would be in the Disney movie Mulan.

In these cases it is very common for an avalanche to occur as a consequence of having produced too loud a noise, as happens in Tintin in Tibet, or in the chapter "Mountain of Madness" from The Simpsons.