DIFFERENT TYPES OF AVALANCHES
The stability of the mantle of snow varies in accordance with its evolution. Indeed, as soon as snow is in the ground, and sometimes before, it begins to turn in accordance with a physical phenomenon and the weather conditions. These modifications of size and shape, dragging different physical and mechanical properties, are due to the action of wind, to the vertical distribution of snow temperature (warming up, rain), etc. The result is a stratified mantle of snow, composed of different layers of snow. In accordance with these successive layers features and their evolution, the mantle of snow become steady or unsteady, creating avalanches in this last case or making easy their activation.
An avalanche is a large mass of snow that moves rapidly down a mountain slope, sweeping and grinding everything in its path. Schematically, we count three types of avalanche characterize each by the snow implication in the initial movement: the power avalanches, the slab avalanches and the wet shot avalanches. But very often the reality is complex and, during its way down, an
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Powder avalanches These generally start from a single point and carry more and more snow (snowball effect) They release when the weight of snow succumbs to the strength of the gravity. It especially arrives after strong snowfall (from 3.9 inches), especially at the time of the accumulation on a smooth coat (due to rain, frost, melting). This snow, very light, mixes itself to air while forming a spray that descends the slopes by a speed from 100 to 300 km/h (62 to 186 mph). Its density is often lower than 200 kg/m3. This spray pushes air before it while creating a shock wave (the "cui") that pulls all on its passage. Snow arrives then and, when it meets an obstacle, it compacts himself in real concrete. These avalanches are the most devastating; they can cause enormous damages on dwellings, roads and forests… |
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Slab avalanches This kind of avalanche is the most frequent. A slab is a compact snow surface that detaches itself from the rest of the snow cover and slip on the ground or on the previous snow. These slabs can be immense and let a very fracture limit. The initial rupture interests a snow of good cohesion; its density is from 200 to 400 kg/m3. The risks are especially important when some compact snow puts down on the soft or less dense snow. The factors triggering are the sun, wind or people overcharge. A variety of slabs, called wind slabs, are formed under the action of wind after a snowfall. Broken by wind, crystals are reduced in fine particles that while putting down on the ground take quickly a good cohesion. That also explains the formation of ledges at the neighborhoods of the crests. |
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Wet shot avalanches This type of avalanche occurs by elevated temperatures and in spring, in the very sunny slopes (during the snow melting). Snow became very heavy due to its elevated capacity of water: its density goes from 350 to 500 kg/m3 about. The wet shot avalanches are generally dispersed, numerous and not very important. They flow out slowly (20 to 60 km/h - 12 to 37 mph). The triggering factors are the hot air and wind, the sun and the overcharge. These avalanches have big power of erosion and, for the most important ones, a big devastating power. The deposits, sometimes several meters thick, are constituted for formless blocks of very dense snow. It's not rare to find some of the rests at the bottom of the passageway, whereas the spring is well advanced. |
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