What merred the excitement of watching the snowfall ? Certainly, the Avalanche. The fear of avalanche looms large in mountain regions. Occuring due to heavy snowfall, a snow avalanche basically comprises a large mass of rock debris and snow that moves rapidly down a mountain slope and sweeps everything in it’s path. Increasing movement of people ( involved in through sports, tourism, vigilance and pilgrimage ) through the avalanche terrain increases the snow avalanche fatalities. An avalanche path is a fixed locality within which avalanche moves. Avalanche paths may cover a vertical distance of only 50m or may fall the full length of a mountain sidewith a vertical drop of 2000m or much more.Each avalanche has its own starting zone, track zone and run out zone. The starting zone is the location where the unstable snow falled and began to move. All avalanches are caused by an over-burden of material, typically snowpack, that is too massive and unstable for the slope that supports it. Determining the critical load, the amount of over-burden which is likely to cause an avalanche, is a complex task involving the evaluation of a number of factors.
An avalanche is the downward motion of a large mass of snow on a slope with velocity as high as 60 km/hour. It may contain rocks, soil, ice or trees with an impact pressure of 50 ton/square metre. Avalanche affect people directly by causing injury or death, or by detaining them. Avalanches also cause immence damage to property. Transportation, Construction, Tourism are the chief industries, badly affected by avalanches.
Various types of snow avalanches may occur. “Loose snow avalanches” start at or near the surface and they usually involve only surface or near surface snow. Loose snow avalanches start at a single area or point and spread out as they move down the slope in a triangular pattern as more snow is pushed down the slope Loose snow avalanches occur when the weight of the snowpack exceeds the shear strength within it, and are most common on steeper terrain. Loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. On the other hand, the “slab avalanche” is usually more dangerous. Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, stiff layer of snow known as a slab. These are usually formed when snow is deposited by the wind on a lee slope. When the slab fails, the fracture, in a weak layer, very rapidly propagates so that a large area, that can be hundreds of metres in extent and several metres thick, starts moving almost instantaneously. Slab avalanche comprises a block of snow, usually rectangular in shape, that is entirely cut out by propagating fractures in the snow. Another kind of avalanche is “slush avalanche”. occurs when the snowpack becomes saturated by water. These tend to also start and spread out from a point. As avalanches move down the slope they may entrain snow from the snowpack and grow in size. The snow may also mix with the air and form a powder cloud. An avalanche with a powder cloud is known as a powder snow avalanche. The powder cloud is a turbulent suspension of snow particles that flows as a gravity current. Powder snow avalanches are the largest avalanches and can exceed 300 km/h and 10,000,000 tonnes of snow, they can flow for long distance along flat valley bottoms and even up hill for short distances.
Avalanche Terrain
Slopes flatter than 25 degrees or steeper than 60 degrees typically have a low risk of avalanche. Snow does not accumulate significantly on steep slopes; also, snow does not flow easily on flat slopes. Human triggered avalanches have the greatest incidence when the snow’s angle of repose is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. An avalanche area is a location with one or more avalanche paths. An avalanche path is a fixed locality within which avalanches move. Avalanche paths may cover a vertical distance of only 50m or may fall the full length of a mountain side with a vertical drop of 2000m or more.within an avalanche path, smaller avalanches may start and stop at various places. Each avalanche has its own starting zone, track zone and run out zone. The starting zone or the zone of origin is the location where the unstable snow failed and begun to move. The fracture line of a slab avalanche and the initiation point of a loose snow avalanche define the upper limit of the starting zone for each avalanche. The track zone or the zone of transition is the slope below the starting zone that connects the starting zone with the zone where debris collects ( run out zone ). Avalanche speed attains its maximum value in the track zone. The run out zone or the zone of deposition is the area where deceleration is rapid and debris is deposited, as the avalanche finally stops here.
Slopes flatter than 25 degrees or steeper than 60 degrees typically have a low risk of avalanche. Snow does not accumulate significantly on steep slopes; also, snow does not flow easily on flat slopes. Human triggered avalanches have the greatest incidence when the snow’s angle of repose is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. The four variables that influence snowpack evolution and composition are temperature, precipitation, solar radiation, and wind. In the mid-latitudes of the Northern Hemisphere, more avalanches occur on shady slopes with northern and north-eastern exposures. However, when the human triggered incidence of avalanches are normalized to mid-latitude rates of recreational use, no significant difference in hazard for a given exposure direction can be found. The snowpack on slopes with southern exposures are strongly influenced by sunshine; daily cycles of surface thawing and refreezing create a crust that may tend to stabilize an otherwise unstable snowpack, but the crust, once it has been fractured, may detach itself from the underlying layers of snow, slide, and promote the generation of an avalanche.
Prevent The Avalanche
Accidants and damages due to avalanches can be prevented either by controlling the avalanches, by regulating the presence of personsin that area or by placing structures out of avalanche paths. Avalanche protection may be divided into temporary and permanent measures. Temporary measures or active measures are short-term measures that are applied for short periods when avalanches are expected to occur. Their advantages are flexibility and low cost, but they require a continuous evaluation of avalanche hazards and the application of safety measures. Such protective measures include avalanche warning, change of road / tract alignment to avoid danger zones, temporary closure of the road. evacuation from the danger areas, artificial release of avalanches prior to their developing into a catastrophic avalanche waves etc. Permanent or Passive or long term measures usually require expenses for engineering works which perform without the need for a daily hazard evaluation. These protective measures are related to numerous avalanche situation. In this way, extreme situations can be taken into account and the measures are, as a rule, carried out or prepared long before the emergence of the danger. Such protective measures include the avoiding of permanent construction in the avalanche affected zones, allowing of only minimum construction in the danger zone that can withstand the avalanche forces, construction of permanent avalanche control structures and afforestation on the avalanche slopes etc.
Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety is a continuous process, including route selection and examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimize the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings heeded. Never follow in the tracks of others without your own evaluations; snow conditions are almost certain to have changed since they were made. Observe the terrain and note obvious avalanche paths where vegetation is missing or damaged, where there are few surface anchors, and below cornices or ice formations. Avoid traveling below others who might trigger an avalanche. Artificial barriers can be very effective in reducing avalanche damage. There are several types. One kind of barrier (snow net) uses a net strung between poles that are anchored by guy wires in addition to their foundations. These barriers are similar to those used for rockslides. Another type of barrier is a rigid fence like structure (snow fence) and may be constructed of steel, wood or pre-stressed concrete. They usually have gaps between the beams and are built perpendicular to the slope, with reinforcing beams on the downhill side. Rigid barriers are often considered unsightly, especially when many rows must be built. They are also expensive and vulnerable to damage from falling rocks in the warmer months. Finally, there are barriers that stop or deflect avalanches with their weight and strength. These barriers are made out of concrete, rocks or earth. They are usually placed right above the structure, road or railway that they are trying to protect, although they can also be used to channel avalanches into other barriers. Occasionally, earth mounds are placed in the avalanche’s path to slow it down.
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