Avalanches

Introduction

An avalanche is a rapid, catastrophic downward movement of a large mass of snow, ice, earth, or rock, often mixed with debris, down a mountain slope. It is one of the most powerful natural hazards in mountainous regions, acting similarly to a landslide but involving snow. In India, they are a severe threat to human life and infrastructure, particularly in the Himalayas, affecting the Indian Army and residents in Jammu & Kashmir, Himachal Pradesh, and Uttarakhand. 

Types of Avalanches

There are avalanches of various types according to their composition and the mode of origin:

• Slab Avalanches (Most Dangerous): A cohesive block of snow is shedded off, and this occurs due to the failure of a weak underlying layer of snow and all the layers above it slide down. Most of them cause death and they have speeds of 100 km/h.
• Loose Snow Avalanches (Sluffs): Form at one point and accumulate additional snow on its way down forming a fan like direction. They tend to be smaller and less harmful as compared to slab avalanches.
• Powder Snow Avalanches: A combination of slab and loose snow which creates a massive cloud of snow powder which travels at extreme speed of up to 300 km/h and causes a lot of destruction due to the shockwaves.
• Wet Snow Avalanches: These occur when the melting snow is released as a result of temperature increase or rain. They are also very destructive as they are so dense and heavy, and move slowly.
• Gliding Avalanches: These are those that take place as the whole body of snow moves over a non-jagged surface (grass or rock), which is typical on the moderate and steep slopes.
• Ice/Debris Avalanches: Huge masses of ice (typically of glaciers) or earth and rock falling down.

Causes of Avalanches

Avalanches are caused by a combination of natural and human-induced factors that destabilize the snowpack: 

Natural Causes

1. Heavy Snowfall: New snow falls fast (e.g.,>1m in 24 hours) and exerts unmatched pressure on old snow, which collapses.
2. Weak Snowpack Layers: Layers that are characterized by low cohesion which can be as a result of rapid temperature variation or due to faceting (formation of granular snow) renders thepack prone to fracturing.
3. Rise in Temperature: Warm weather or high sunlight enhances the melting of snow thereby reducing the bond between layers.
4. Wind Loading: Wind blows the snow on windward slopes and accumulates on the leeward slopes forming unstable and heavy snowy slabs- the wind slabs.
5. Tremors: Tremors can occur in a snowpack and cause massive sliding due to the structural integrity of the snow.

Human Factors

1. Recreational Activities: Skiing, trekking and snowmobiling may serve as a stimulus over unstable slopes, and this explains a high percentage of fatal accidents.
2. Infrastructure Development: Building, deforestation, or vibrations of mining/vehicles in mountainous regions stabilize the slopes.

Preventive & Management Measures

Mitigation requires a combination of structural engineering and non-structural, community-level planning, according to NDMA guidelines: 

Structural Measures (Active Control)

• Avalanche Control Barriers: Steel nets, wooden fences and snow rakes are then used in the starting zone to keep the snow at bay.
• Deflecting Structures: Construction of walls or mounds to deflection path of an avalanche in order to avoid villages and other infrastructure.
• Snow Sheds/Tunnels: This is where roads and railways are covered with fortified structures to facilitate smooth transport all the time.
• Controlled Explosions: Explosives are used to cause small controlled avalanches to avoid the build-up of uncontrollable large

Non-Structural Measures (Passive Control)

• Risk Zonation & Mapping: This involves finding the areas of high risk through satellite images and mapping the hazards in order to limit settlement in these areas.
• Early Warning Systems (EWS): The use of information on snow observatories and weather stations to provide warnings and forecasts. This is managed in India through the Defence Geoinformatics Research Establishment (DGRE).
• Reforestation: Planting of trees in the elevated places so that the snowpack has natural anchoring points.
• Preparedness in the Community and Training: Educational activities on the locals and the tourists, mock drills, and training on rescue measures, such as using beacons, probes, and shovels.

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