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Disaster of Avalanche

The Horrible Face of Natural Disasters

According to Wikpedia

A natural disaster is the consequence of the combination of a natural hazard (a physical event e.g. volcanic eruption, earthquake, landslide) and human activities. Human vulnerability, caused by the lack of appropriate emergency management, leads to financial, structural, and human losses. The resulting loss depend on the capacity of the population to support or resist the disaster, their resilience. This understanding is concentrated in the formulation: "disasters occur when hazards meet vulnerability". A natural hazard will hence never result in a natural disaster in areas without vulnerability, e.g. strong earthquakes in uninhabited areas. The term natural has consequently been disputed because the events simply are not hazards or disasters without human involvement. The degree of potential loss can also depend on the nature of the hazard itself, ranging from wildfires, which threaten individual buildings, to impact events, which have the potential to end civilization.

Avalanches: Cause

An avalanche is a very large slide of rapidly moving granular material, most commonly snow, down a mountainside caused by a build up of material. When a mass of material exceeds the static friction threshold, a cascading effect takes place and accumulates more material as it travels down the mountainside and causes massive, widespread destruction. There are many different types of avalanches including snow, ice, rock and soil.

There are three main factors that contribute to causing an avalanche. If the steepness of the terrain is between 35 to 45 degrees, is shady, has a convex shape and has a rock or slab base with little vegetation the chance of an avalanche is extremely high. Weather is another main factor where everything from temperature to wind and rain can loosen the material pack and cause an avalanche. For a snow avalanche, the snow itself can contribute to the probability of an avalanche. If there is a large amount of new, unbounded snow with little compaction and a large crystal size, the snow can cause an avalanche all by itself.

Avalanches: Steps to take

Steps can be taken to prevent avalanches. Minimizing the number of people on a slope helps reduce the stress on the surface; although for safety reasons one should never be on a slope alone. Traversing, or cutting across a slope, should be kept to a minimum as traffic will help to compromise the surface strength. Finally, always be able to recognize danger and consider an alternative route to prevent starting an avalanche.

 

 

 

 

Contributing factors

Determining critical load which would cause a slope avalanche is a complex task involving evaluation of many factors. Some of them are:

Terrain

  • Steepness — slopes under 25 degrees and over 60 degrees typically have a low avalanche risk because of the angle of repose for snow. Snow does not accumulate significantly on steep slopes and does not easily flow on flat slopes. Distribution of avalanches by slope has a sharp peak between 35 to 45 degrees. That peak hazard lies at around 38 degrees. Unfortunately, slopes with the most dangerous steepness are favored for skiing.
  • Direction — The three primary variables that influence snowpack evolution are temperature, precipitation and wind. In medium latitudes of the Northern Hemisphere, more accidents occur on shady slopes with northern and north-eastern aspects. Slopes in the lee of the wind accumulate more snow, presenting locally deep areas and windslabs. Cornices also accumulate on the downwind side of ridges, and can contribute to avalanche danger.
  • Profile — convex slopes are statistically more dangerous than concave. The reasons lie partly in human behavior, and the tensile strength of snow layers versus the compression strength.
  • Surface — Full-depth avalanches are more common on slopes with smooth ground cover such as grass or rock slab. Vegetation cover is important for anchoring the snowpack; however in certain snowpack's boulders or buried vegetation may create weak areas within the snowpack.

Snow

The structure of the snowpack determines avalanche danger. Avalanches require a buried weak layer (or instability) and an overlying slab. Unfortunately relations between easily observable properties of snow layers (strength, grain size, grain type, temperature) and avalanche danger are complex and not yet fully understood. Additionally snow cover varies in space and so does stability of snow.

  • New snow — New snow has not had time to bond with the layers below, especially if it is light and powdery.
  • Snow depth — Snow that is above the layer of boulders and plants on the slope has none of these natural objects to help anchor it to the slope, and is therefore more dangerous. Naturally this is just the type of snow needed for snow sports such as skiing.
  • Snow crystal size — Generally speaking, the larger the crystal, the weaker it is.
  • Snow compaction — Compacted snow is less likely to move than the light powdery layers.

Weather

Weather determines the evolution of snowpack. The most important factors are heating by solar radiation, radiational cooling, temperature gradients in snow, and snowfall amounts and type. Most avalanches happen during or soon after a storm.

  • Temperature — If the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and re-freezing of water in the snow strengthens the snowpack during the freeze cycle and weakens it in the thaw cycle. Temperatures rising significantly over the freezing point may cause the whole slope to avalanche, especially in spring. Persistent cold temperatures cause the snow to not gain stability and may contribute to formation of depth hoar, where there is a high temperature gradient within the snow. Thin layers of "faceted grains" may form above or below crusts when temperature gradients become strong through the crust.
  • Wind — anything more than a gentle wind can contribute to rapid build up of snow on sheltered slopes (downwind), while the wind pressure can also stabilize other slopes. "Wind slab" is a particularly fragile brittle structure — heavily loaded, poorly bonded. Even on a clear day, wind can quickly shift snow-load to the snow pack. This can occur two ways, by top-loading, where wind deposits snow parallel to the fall-line, or through cross-loading, which occurs when the wind deposits snow perpendicular to the fall-line of a slope. When wind blows over the top of a mountain, the leeward, or downwind, side of the mountain experiences top-loading. When the wind blows over a ridge that leads up the mountain for example, the leeward side of the ridge experiences cross-loading. Cross-loaded wind-slabs are usually more difficult to spot and also tend to be less stable than top-loaded wind-slabs, and are therefore much more dangerous.
  • Heavy snowfall — Heavy snowfall may cause instability, both through the additional weight, and because the snow has insufficient time to bond.
  • Rain — In the short-term causes instability through additional load and possible lubrication of lower layers. Avalanche also occurs if the upper snow layer is moved. Rain reduces friction in the snowpack.

 

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