DEFINITION

An avalanche (also called a snowslide or snowslip) is a rapid flow of snow (a crystalline form of H2O) down a sloping surface. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche).

INTRODUCTION

Avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current. The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches.

Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger the failure in the snowpack and avalanches. A popular myth is that avalanches can be triggered by loud noise or shouting, but the pressure from sound is orders of magnitude too small to trigger an avalanche.

Although primarily composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks, trees, and other material on the slope, and are distinct from mudslides, & rock slides, collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack.

Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds. A large, fully developed avalanche can weigh as much as a million tons. It can travel faster than 320 kilometers per hour (200 miles per hour).

Fig:- avalanche

AVALANCHES

FORMATION AND TYPE

Most avalanches occur spontaneously during storms under increased load due to snowfall.

The second largest cause of formation of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation.

Other natural causes include rain, earthquakes, rock fall and icefall. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work.

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.

A snow avalanche begins when an unstable mass of snow breaks away from a slope. The snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles that rises high into the air. 

The bonds between the layers of a snowpack may be weak. Melted snow that refreezes may cause a slick coating of ice to form on the surface of a layer.

During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery. Added weight or vibration can easily send the top layers of a snowpack hurtling downhill.

PARTS OF AVALANCHES

A) STARTING ZONE

The area near or at the top of an avalanche path where unstable snow breaks loose from the snow-cover and starts to slide. The starting zone of a particular slab avalanche can be outlined by the flank, stauch, and crown walls. The starting zone of an avalanche path includes the collective outline of all potential avalanche fracture surfaces (slab and loose-snow avalanches).

B) TRACK ZONE

The middle part of an avalanche path below the starting zone and above the zone. Within the track the highest speed will be attained. Deposition of debris will be minimal and typically limited to areas around trees, rocks, or other obstacles. The slope angle of the track does not need to be as great as that in the starting zone. They are usually over 15 degrees and more commonly 20-25 degrees. Tracks can be divided into channeled flow tracks (gullies, couloirs, etc) and un channeled or unconfined flow tracks (open slopes).

C) RUNOUT ZONE

The part of an avalanche path where deceleration is rapid, debris is deposited, and the avalanche stops.

FACTORS

1. Temperature: Solar radiation can penetrate the surface and effectively reduce the stiffness of the upper layers. Changing slab properties directly affect snow instability in many ways. With surface warming we mean that in the surface layers of the snowpack (i.e., in the upper layers of the slab) snow temperature increases. The temperature increase is due to a net energy flux directed into the snowpack which indicates an energy gain. The low thermal conductivity will cause cooling to take more time than warming by penetrating shortwave radiation.

2. Wind speed: After precipitation, wind is the next most important weather variable affecting avalanche potential. It is fundamental to slab development and determines where wind-transported snow accumulates. Wind speed determines the amount and rate of wind loading. Winds between 20 and 60 mph are optimal for transporting snow, while those less than 20 mph can only transport very low-density snow (< 5%) on the ground.

3. Direction of wind: The wind direction piling snow which can overhang a mountain. Wind direction indicates which slope aspects are susceptible to snow being deposited from wind loading. When wind blows over a ridge top and transports snow from the windward to leeward side, it’s known as top-loading. Wind blowing parallel to a ridgeline may dump wind-deposited snow on avalanche paths. This cross-loading may be difficult to detect or monitor without observations from mid-slope and valley anemometers.

4. Vegetation: It decreases the speed of avalanches and the chances of slab avalanches get decreased.

5. Snowpack characteristics: - The structure of snow pack is a strong predictor of avalanche danger. For avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below the surface and an overlying slab of snow.

6. Terrain: Slopes flatter than 25 degrees stepper than 60 degrees typically have a low risk of avalanche. Snow does not accumulate on steep slopes also does not flow easily on flat slopes.

7. Weather:- It also influences the evolution of snowpack formation. A wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Even on clear day wind can quickly shift the snow load on slope.

On the basis of position of weak layer categories of avalanche

a) Loose snow avalancheA loose snow avalanche is an avalanche formed in snow with little internal cohesion among individual snow crystals. Usually very few fatalities occur from loose snow avalanches, as the avalanches have a tendency to break beneath the person and are usually small even having a path as small as a few centimeters, and as a result are sometimes called "harmless sloughs" that usually at most cause the person to merely fall.However based on the terrain loose snow avalanches can grow large, and have been known to carry people off a cliff and into a crevass or bury them in a gully, and even completely destroy houses and other similar buildings. Ideal conditions for a loose snow avalanche are steep slope angles of 40 degrees and more, persistent sub-zero temperatures and low humidity, moderate to heavy snowfall, and also in an area where winds are very light or are not affecting the density of the snow. This produces light, fluffy snow that is hard or unable to pack.

Fig:- loose snow avalanche

B) Slab avalanche

A snow slab is basically a cohesive layer of snow sitting on a weaker layer. The danger with slabs is that a large mass of snow can slide down a slope burying a skier. Slabs can be divided into hard and soft although the boundary is something of a continuum. Soft slabs are comprised of fresh snow, usually with less than 30% water. Over time soft slabs can transform into hard slabs as crystals bind together. Hard slabs can also be formed under the action of wind where the fine crystal structures are broken down under a

sintering action. The classic wind slab that forms on lee (sheltered) slopes often close to ridgelines and cols. Soft slab avalanches can form anywhere on a slope where there is sufficient snowfall and are undetectable from powder snow. They are usually small slides which break directly under a skier or snowboarder but are never the less deadly, especially where there is a flat runout, terrain trap, trees or cliffs.Slab avalanches are responsible for around 75% of fatalities and are triggered by the victim or a member of his group in 90% of cases. They are therefore the most important type of avalanche as far as backcountry safety is concerned and also avoidable by route planning and safe travel procedures. They are most common in the winter - January and February in France but can occur even in the mid-summer on high mountains such as Mont-Blanc.

Prevention

Active preventative measures reduce the likelihood and size of avalanches by disrupting the structure of the snowpack, while passive measures reinforce and stabilize the snowpack in situ.

Passive preventive systems such as snow fences and light walls can be used to direct the placement of snow.  Explosives are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snowpack, and removing overburden that can result in larger avalanches. 

Mitigation

To mitigate the effect of avalanches the construction of 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.

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.