Avalanche: Types, Causes and Mitigation

Avalanches a common natural as well as man-made disasters common in mountainous regions. During an avalanche, a mass of snow, rock, ice, soil, and other material slides swiftly down a mountainside. It can cause heavy causalities and change the geography of the region substantially. Read here to know more.

Recently, an avalanche in Sikkim’s Nathu la Pass claimed the lives of several tourists in India.

An avalanche can be a natural disaster or triggered by human activities as well.

Avalanches of rocks or soil are often called landslides. Snowslides, the most common kind of avalanche, can sweep downhill faster than the fastest skier.

What is an avalanche?

When an unstable snow mass detaches from a slope, a snow avalanche starts. As it descends, the snow accelerates, creating a river of snow and an ice cloud that rises far into the air.

As it rushes downward, the mass scoops up even more snow. The weight of a massive, fully-grown avalanche can reach one million tonnes. It has a top speed of 320 kilometers per hour.

Most avalanches, like the ones that claimed people’s lives in Sikkim recently, are started by recent precipitation.

• The kind of precipitation, the makeup of the snow that has already fallen, and the slope’s steepness are a few of the variables that affect the physics at play.
• Gravity, the friction that keeps the snow in a pack, and shear, which causes a piece of snow to break off, are the major factors at work.

The starting zone, the avalanche track, and the runout zone are the three basic components of an avalanche. It erupts from the starting zone.

• That area of the stope is often the most unstable and is located higher up the mountain.
• The avalanche follows its natural route downhill after it begins to slide, continuing along the avalanche track. Large clearings or tree chutes missing from an avalanche offer hints as to the route of the slide.
• The runout zone, where snow and debris accumulate, is where the avalanche eventually comes to an end.

Types

Avalanches come in a variety of forms, including-

• Rock avalanches are made up of enormous chunks of broken rock. These are also known as landslides.
• Ice avalanches frequently happen around a glacier.
• Debris avalanches contain a variety of unconsolidated materials, such as loose stones and soil.

There are two main types of snow avalanches- sluffs and slabs.

• Sluff avalanches occur when the weak layer of a snowpack is on top. A sluff is a small slide of dry, powdery snow that moves as a formless mass. Sluffs are much less dangerous than slab avalanches.
• A slab avalanche occurs when the weak layer lies lower down in a snowpack. This layer is covered with other layers of compressed snow. When the avalanche is triggered, the weak layer breaks off, pulling all the layers on top of it down the slope. These layers tumble and fall in a giant block, or slab.

Avalanches can be as small as a minor shifting of loose snow or as large as the displacement of massive slabs of snow.

• A slab avalanche’s mass of descending snow may move at a pace of 130 km/h (80 mph), capable of obliterating small towns and forests in its path.

What are the causes of avalanches?

Avalanches occur as layers in a snowpack slide off.

• A snowpack is simply layers of snow that build up in an area, such as the side of a mountain.
• In winter, repeated snowfalls build a snowpack dozens of meters thick.
• The layers vary in thickness and texture, and the bonds between the layers of a snowpack may be weak.

There are numerous natural and man-made factors responsible for an avalanche.

• Heavy Snowfall: When a high rate of snowfall occurs, it leads to snow accumulation on the mountain slopes. This triggers the weaker layer of snow in the snowpack of unstable areas of the mountain causing avalanches.
• Wind Direction: The direction of the wind determines the patterns of the snowfall as well as snow accumulation on the mountain slopes.
• Layering of Snow: The gradual snowfall creates a layer-by-layer accumulation of snow that creates a hypersensitive snowpack. A small trigger can cause these layers of snow to fall.
• Steeper Slopes: An avalanche is also caused by the influence of gravity. If the snow gets accumulates on the slopes of the mountain, then it is prone to rush down the slopes at greater speeds.
• High Temperatures: Temperature is one of the important factors for the avalanche because of high temperatures the surface layer of the snowpack gets melted. The accumulated snow will become highly susceptible to sliding down.
• Earthquakes: It is one of the important factors that trigger the layer of accumulated snowpack because earthquakes generate seismic waves that cause the ground to vibrate.
• Developmental activities: During the developmental activities, the terrain vehicles in regions with unstable layers of snow can dislodge the layers from the surface and cause them to slide down under gravity.
• Deforestation: Clearing trees and forests causes the soil to become loose, hence during snowfall the ground does not have the strength to hold the heavy snow. This causes landslides or avalanches of rocks and soil.
• Winter Sports Activities: At times, excessive pressure is transferred to snow layers during winter sports activities like skis and sledding. Excess of these activities can cause the topmost loose layer of snow to slip and eventually cause a greater catastrophe.

Mitigation

At present, it is difficult for scientists to predict with certainty when and where avalanches will happen. However, they can estimate hazard levels by checking the snowpack, temperature, and wind conditions.

Certain measures like avalanche detention walls, rakes, diversion structures, etc, can be employed in areas prone to avalanches.

At some ski areas, patrols use explosives to set off avalanches. Or they may blast hazardous slopes with a cannon to shake loose any large, new accumulations of snow.

Conclusion

Avalanches strike suddenly and can be deadly, and are one of the most powerful events in nature.

• In 1970, a massive avalanche of rocks and ice destroyed the town of Yungay, Peru, killing 18,000 people.
• During World War I, more than 60,000 Italian and Austrian troops died in avalanches while fighting in snowy mountain passes in the Alps.
• Avalanches killed more soldiers in World War I than poison gas did.

In India, the National disaster management authority (NDMA) brings out mitigation and survival guidelines for avalanches. All major ski regions in the country post warnings based on local knowledge.

­-Article written by Swathi Satish