Avalanche

 Avalanche

The snow avalanche, a common occurrence in snow covered mountainous regions, is a slide of snow mass down a mountainside. This is a rapid downslope movement of a large detached mass of snow, ice, and associated debris such as rocks and vegetation

• A large avalanche can run for many kilometres, and result in massive destruction of forests and anything else that comes in its way. 
• There are two basic types of avalanches, loose snow avalanches and slab avalanches.
• Loose snow avalanches form in snow masses with little internal cohesion among the individual snow crystals. When such snow lies in a state of unstable equilibrium on a slope steeper than its natural angle of repose, a slight disturbance sets progressively more and more snow in downhill motion.
• Slab avalanches originate in snow with sufficient internal cohesion to enable a snow layer
• A slab avalanche breaks free along a characteristic fracture line 
• A slab release may take place across an entire mountainside
• Slab avalanches are often dangerous, unpredictable in behaviour, and account for most of the damage.
• Avalanches composed of dry snow are called powder snow avalanches
• A wind blast can achieve high velocities, to inflict heavy destruction well beyond the normal bounds of the avalanche path.
• Avalanches form as soon as the force of gravity on the snow cover exceeds its mechanical strength.
• Formation of an avalanches needs 
• a steep slope, 
• snow cover, 
• a weak layer in the snow cover, and 
• a trigger to initiate movement.
• The forces generated by moderate or large avalanches can damage or destroy most man-made structures
• The debris from small avalanches is enough to block a highway or rail-road.
• Where avalanches cross highways, passing vehicles can be swept away and destroyed, killing their occupants

There are two basic methods of anticipating an avalanche hazard. 

1. Examination of the snow cover structure for patterns of weakness, particularly those leading to slab avalanches. 
2. Analysis of the meteorological factors affecting snow depositions. 
3. Rising temperature during a storm accompanied by rising new snow density tends to cause avalanche while falling temperatures have the opposite effect. 
4. New snow precipitation intensity is a significant factor, as it represents the rate at which the slopes are being overloaded.
5. Wet snow avalanches are generated by the intrusion of percolating water (rain or snow melt) in the snow cover. The rapid rise in temperature quickly alters snow behaviour
6. The most extensive wet snow avalanche occurs during winter rains
7. The major inputs for forecasting of snow avalanche are:
1. Snow cover
2. Terrain and 
3. Atmospheric parameters 
8. In order to gather the latest terrain information on avalanche-prone areas, snow cover and atmospheric parameters is gathered by
1. Optical [MODIS, AVHRR, AWiFS, WiFS, LISS-III, PAN, Cartographic Satellite (CARTOSAT), IKONOS, Quickbird] and 
2. Microwave (AMSR-E, SSM/I, Radarsat, ENVISAT) imagery is used
9. Snow avalanches have long posed a threat to the local populations of the Himalayan and Trans-Himalayan mountains. 
10. Land use intensification due to population growth, new transportation routes, defence related activities and tourism are continuously raising this level of risk.
11. The risks due to avalanche can be reduced if appropriate structural controls are employed. 
12. Methods of avalanche control include structural terrain modification to deflect the sliding snow away from the fixed facilities to be protected, or to actually prevent the avalanche release, and the planned release of small snow slides with explosives before snow accumulation increases their destructive potential to unmanageable proportions. 
13. Engineering structures for the control of snow avalanches are of the following four types:
1. Supporting structures to prevent avalanches from initiating, or to retard movement before it gains momentum. For example:
1. Massive earth or stone walls and terraces
2. Rigid structures 
3. Flexible supporting structures called ‘snow nets’ 
2. Deflecting and retarding structures in the run-out zone to keep the moving snow of an avalanche away from structures in critical locations
3. Retarding structures usually stop all but large, dry snow avalanches
4. Direct protection structures like avalanche sheds or shelters are roofs over roads or  railroads that allow avalanches to cross the road/railroad without interrupting or threatening the traffic.
14. In India, the responsibility of dealing with the different aspects of avalanches rests with the Border Roads Organisation (BRO)