Avalanche control or avalanche defense activities reduce the hazard avalanche

Avalanche

An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...

s pose to human life, activity, and property. Avalanche control begins with a risk assessment conducted by surveying for potential avalanche terrain by identifying geographic features such as vegetation patterns, drainages, and seasonal snow distribution that are indicative of avalanches. From the identified avalanche risks, the hazard is assessed by identifying threatened human geographic features such as roads, ski hills, and buildings. Avalanche control programs address the avalanche hazard by formulating prevention and mitigation plans, which are then executed during the winter season. The prevention and mitigation plans combine extensive snow pack observation with three major groups of interventions: active, passive, and social sometimes more narrowly defined as "explosive", "structural", and "awareness", according to the prevalent techniques used. Avalanche control techniques either directly intervene in the evolution of the snow pack, or lessen the effect of an avalanche once it has occurred. For the event of human involvement, avalanche control organizations develop and train exhaustive response and recovery plans.

Risk and Hazard Assessment

Risk assessment geographical surveys for potential avalanche terrain by studying topography, vegetation, and seasonal snow distribution. Hazard assessment geographical surveys for the consequences of avalanche by studying exposure of urbanization, industrialization, transportation, recreational activities, and the distribution of human use of the potential avalanche terrain identified in the risk assessment.

Prevention and Mitigation

Prevention and mitigation begins with observing the snow pack to forecast the risk of avalanche occurrence. The forecast risk then determines the necessary interventions to reduce the hazard posed by an avalanche.

Observation and Forecasting

Snow pack observation studies the layering and distribution of the snow to estimate the instabilities of the snow pack and thus the risk of an avalanche occurring in a particular terrain feature. In areas of heavy human use the snow pack is monitored throughout the winter season to assess its evolution under the prevailing meteorological conditions. In contrast to heavily used avalanche terrain where forecasting is the goal of snow observation, in remote terrain, or terrain that is infrequently visited, snow pack observation elucidates the immediate instabilities of the snow pack.

Active Interventions

Active techniques reduce the risk of an avalanche occurring by promoting the stabilization and settlement of the snow pack through three forms of intervention: disrupting weak layers in the snow pack, increasing the uniformity of the snow pack, and lessening the amount of snow available in snow pack for entrainment in an avalanche; this can be accomplished either by triggering smaller less hazardous avalanches, or by directly influencing the structure of the layering of the snow pack. Active avalanche control can be broadly classified into either mechanical or explosive methods. Mechanical methods are typically used in either remote terrain, smaller terrain, or less hazardous terrain; while explosive methods are used in accessible large high hazard terrain, or terrain with industrial, commercial recreational, urbanized, and transportation usage.

In the smallest terrain features the simplest method of avalanche control is a mechanical intervention that disrupts weak snow layers by directly walking through them, a technique referred to as boot packing. For larger features this method can be extended by mechanized redistribution of snow using large tracked vehicles called snow groomers. These two mechanical interventions can only be safely done as the snow is deposited and before it develops any instabilities. In terrain that can only be sporadically accessed, or in a highly developed snow pack that is too deep for boot packing, ski stabilization techniques are used. The first technique of ski stabilizing is a method of entering a slope called ski cutting. In this method a skier attempts to trigger a small avalanche by breaking the tensile support of the upper snow pack through a quick traverse along the top of the slope, the skier can be belayed on a rope to further protect them from being caught in an avalanche. A snow pack can then be further settled out, or stabilized, by further down slope ski traffic through it. Finally knotted cord can be used to saw through the roots of cornices, causing the cornice to drop onto the snow pack of the slope below. This has the combined effect of reducing the objective hazard posed by the cornice, and providing a large impact force on the snow pack.

Explosive techniques involve the artificial triggering of smaller less destructive avalanches, by detonating charges either above or on the snow surface. The explosives may be deployed by manually hand tossing and lowering, by bombing from a helicopter, or by shelling with a small howitzer

Howitzer

A howitzer is a type of artillery piece characterized by a relatively short barrel and the use of comparatively small propellant charges to propel projectiles at relatively high trajectories, with a steep angle of descent...

, recoilless rifle

Recoilless rifle

A recoilless rifle or recoilless gun is a lightweight weapon that fires a heavier projectile than would be practical to fire from a recoiling weapon of comparable size. Technically, only devices that use a rifled barrel are recoilless rifles. Smoothbore variants are recoilless guns...

, or air gun

Air gun

An air gun is a rifle , pistol , or shotgun that fires projectiles by means of compressed air or other gas, in contrast to a firearm, which burns a propellant. Most air guns use metallic projectiles as ammunition. Air guns that only use plastic projectiles are classified as airsoft...

. In balancing the hazard to personnel with the effectiveness of the deployment method at accessing and triggering avalanche terrain, each method has its drawbacks and advantages. Among the newest methods, strategically placed remote controlled installations that generate an air blast by detonating a fuel-air explosive

Fuel-air explosive

A fuel-air explosive device consists of a container of fuel and two separate explosive charges. After the munition is dropped or fired, the first explosive charge bursts open the container at a predetermined height and disperses the fuel in a cloud that mixes with atmospheric oxygen...

 above the snow pack in an avalanche starting zone, offer fast and effective response to avalanche control decisions while minimizing the risk to avalanche control personnel; a feature especially important for avalanche control in transportation corridors. For example the Avalanche Towers (Sprengmast) installed in Switzerland, Austria, and Norway use solar powered launchers to deploy charges from a magazine containing 12 radio controlled charges. The magazines can be transported, loaded, and removed from the towers by helicopter, without the need for a flight assistant, or on site personnel.

Explosive control has proved to be effective in areas with easy access to avalanche starting areas and where minor avalanches can be tolerated. It is mostly unacceptable, however, in areas with human residence and where even a small probability of a larger avalanche is unacceptable.

Passive Interventions

Passive techniques slow, stop, divert, or prevent snow from moving; either completely or to enough of an extent that the destructive forces are significantly lessened. Passive techniques involve constructing structure

Structure

Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...

s and modifying terrain for purposes classified as:

• Snow retention structures (snow racks, avalanche snow bridges, snow nets), used in the upper path of probable avalanche paths
• Avalanche barriers: The main part of the avalanche barriers is based on a high tensile strength steel wire mesh, extending across the slope and reaching to the surface of the snow. The supporting effect created by the retaining surface prevents possible creeping within the snow cover and sliding of the snow cover on the terrain surface. Breaking-away of avalanches is thus prevented at the starting zone, while occurring snow movements are restricted to the extent that they remain harmless. The forces resulting from the snow pressure are absorbed by the snow nets and carried off over the swivel posts and anchor ropes into the anchor points.
• Snow redistribution structures (wind baffles, snow fence
  Snow fence
  A snow fence is a structure, similar to a sand fence, that forces drifting snow to accumulate in a desired place. They are primarily employed to minimize the amount of snowdrift on roadways and railways. Farmers and ranchers may use temporary snow fences to create large drifts in basins for a...
  
  s)
• Snow deflection structures used to deflect and confine the moving snow within the avalanche track. They should not deflect the avalanche sharply, because in the latter case they may be easily overrun by snow.
• Snow retardation structures (e.g. snow breakers), mostly used in small-slope parts of the avalanche track, to enhance the natural retardation
• Snow catchment structures
• Direct protection of important objects and structures, e.g., by snow shed
  Snow shed
  An avalanche snow bridge or simply snow bridge is a type of rigid snow-supporting structure for avalanche control . Avalanche bridges can be made of steel, prestressed concrete frames, or timber....
  
  s (avalanche sheds).

A single intervention may fulfill the needs of multiple classes of purpose, for example Avalanche dam

Avalanche dam

Avalanche dams are a type of avalanche control structures used for protection of inhabited areas, roads, electric lens, etc., from avalanches. The two major types are deflection and catchment dams...

s, ditch

Ditch

A ditch is usually defined as a small to moderate depression created to channel water.In Anglo-Saxon, the word dïc already existed and was pronounced 'deek' in northern England and 'deetch' in the south. The origins of the word lie in digging a trench and forming the upcast soil into a bank...

es, earth mounds, and terraces

Terrace (agriculture)

Terraces are used in farming to cultivate sloped land. Graduated terrace steps are commonly used to farm on hilly or mountainous terrain. Terraced fields decrease erosion and surface runoff, and are effective for growing crops requiring much water, such as rice...

 are used for deflection, retardation, and catchment. Other passive methods include:

• reforestation
  Reforestation
  Reforestation is the natural or intentional restocking of existing forests and woodlands that have been depleted, usually through deforestation....
  
  , up the natural tree line — forests serve all the functions of artificial avalanche defenses: retention, redistribution, retardation and catchment
• snow cave
  Snow cave
  A snow cave is a shelter constructed in snow by certain animals in the wild, human mountain climbers, winter recreational enthusiasts, and winter survivalists. It has thermal properties similar to an Igloo and is particularly effective at providing protection from wind as well as low temperatures...
  
  s, as well as recessed, dug out, and snow walled quinzhee
  Quinzhee
  A quinzhee or quinzee is a shelter made by hollowing out a pile of settled snow. This is in contrast to an igloo, which is made from blocks of hard snow...
  
  s and bivouac shelter
  Bivouac shelter
  A bivouac traditionally refers to a military encampment made with tents or improvised shelters, usually without shelter or protection from enemy fire or such a site where a camp may be built. It is also commonly used to describe a variety of improvised camp sites such as those used in scouting and...
  
  s are used to temporarily protect bivouacking climbers and skiers by providing them with breathing space in the event of burial by avalanches.
• Architectural streamlining and wedge shaping buildings, such as those found in the historic high mountain villages of the Alps

Social Interventions

To mitigate the hazard of avalanches, social interventions reduce the incidence

Incidence (epidemiology)

Incidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.Incidence proportion is the...

 and prevalence

Prevalence

In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...

 of human avalanche involvement by modifying the behavior of people, so that their use of avalanche terrain is adapted to prevent their involvement in avalanches. Avalanche control organizations accomplish this by targeting awareness and education programs at communities that frequent avalanche terrain. Surveys of avalanche accidents have observed that most avalanches that involve people are caused by people, and of those victims many were unaware of the risk of avalanche occurrence. To address this observation, introductory awareness and education programs provide instruction in the avoidance of hazardous avalanche involvement through the recognition of avalanche terrain, the observation of snow pack instabilities, and the identification of human activities that cause avalanches. Avalanche control organizations also publicly disseminate forecasts, bulletins, warnings, and reports of avalanche activity to assist communities of avalanche terrain users.

Response and Recovery

Avalanche control organizations plan for, and respond, to avalanches. Typical responses span from clearing transportation corridors of avalanche debris, to repairing industrial and recreational facilities, to search, rescue, and recovery. To improve the outcome of human avalanche involvement avalanche control organizations offer training and education to both professionals and recreational amateurs in avalanche preparedness.

Professional Preparedness

Professional responses to avalanches are targeted at avalanches involving the general unprepared public. When avalanches are forecast to occur, avalanche terrain to which the general unprepared public is exposed will be closed, and after the avalanches have occurred the area is cleared of debris, and repaired. When unexpected avalanches occur that involve the general unprepared public, avalanche control organizations respond with large professionally organized search teams involving probe lines, and trained search and rescue dogs.

Amateur Preparedness

Recreational response to avalanches involves the rapid formation of an ad hoc search and rescue team. The ad hoc search and rescue teams rely on all the participants having prepared for a potential avalanche by carrying the correct search and rescue equipment, and undergoing the appropriate training.

External links

• National Avalanche Center of the United States Forest Service
  United States Forest Service
  The United States Forest Service is an agency of the United States Department of Agriculture that administers the nation's 155 national forests and 20 national grasslands, which encompass...