We have seen lots of videos of skiers desperately trying to survive the dreadful mass of snow falling down a mountainside. Some are lucky to escape, while some are buried alive under tons of snow. This rapid and abrupt flow of snow poses one of the biggest threats in the mountains for property and life. This natural hazard is known as avalanche.
What is an avalanche?
The term “avalanche” comes from “avalantse”, an old French word, which originates from “avaler”, which literally means “to lower or let down.” During an avalanche, a large mass of temporary stationary snow slides down a slope in great speed. This occurs when new snow accumulates on top of a heavier snow layer. Because the topmost layer is not compact, the snow could slide down a mountainside.
At first look, an avalanche is a simple event. But similar to other natural hazards, its dynamics involves a lot of factors.
What causes an avalanche?
An avalanche occurs when there is an over-burden of massive snow pack that is unstable for the slope supporting the material. Determining the extent of over-burden and the critical load is a very complex task that involves the evaluation of such contributing factors as terrain, characteristics and structure of snow, and weather.
Slopes that are steeper than 60 degrees or flatter than 25 degrees usually have a very low risk of avalanche. There is typically less snow accumulation on steep slopes than flatter slopes. Also, snow that accumulates on flat slopes does not slide easily. Here is the rule of thumb: A slope flat enough to support new snow layer but steep enough for skiing has a high risk of avalanche, regardless of the angle.
The characteristics and structure of the snow also predict avalanche danger. An avalanche is likely to occur when the snow pack has a weak or unstable layer below the overlying chunk of cohesive snow. Unfortunately, the link between snow layer properties (temperature, grain type, grain size, strength, etc.) and the dangers of avalanche is very complex.
Weather also affects the formation of the snow pack. Important factors include radiational cooling, heating by the sun, snow types, snowfall amount, and vertical temperature gradients in standing snow. A rapid and significant rise in temperature may cause avalanche.
Strong wind allows for a rapid accumulation of snow, thus increasing the risk of avalanche. Rainstorms and snowstorms also contribute to avalanche danger. In addition, exposure to sunlight can destabilize the upper layer of a snow pack.
How can avalanche be prevented?
Avalanches can be prevented and their impacts can be mitigated. Explosives are often used to induce small avalanches before massive snow pack can accumulate to cause a large-scale and destructive avalanche. Artificial barriers are an effective way to mitigate avalanche damage. These barriers include snow net, snow fence, and barriers made out of rocks or concrete that deflect or stop avalanches with their strength and weight.
