Avalanche control or avalanche defense activities reduce the hazard avalanches 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 most prevalent technique used in each. 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 for 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 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, recoilless rifle, or air gun. 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 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 there is even a small probability of a larger avalanche.
Permanent techniques slow, stop, divert, or prevent snow from moving; either completely or to enough of an extent that the destructive forces are significantly lessened. Permanent techniques involve constructing structures 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 guard devices (used to increase snow retention on roofs).
- Snow redistribution structures (wind baffles, snow fences)
- 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 sheds (avalanche sheds).
A single intervention may fulfill the needs of multiple classes of purpose, for example Avalanche dams, ditches, earth mounds, and terraces are used for deflection, retardation, and catchment. Other passive methods include:
- reforestation, up the natural tree line — forests serve all the functions of artificial avalanche defenses: retention, redistribution, retardation and catchment
- snow caves, as well as recessed, dug out, and snow walled quinzhees and bivouac shelters 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..
To mitigate the hazard of avalanches, social interventions reduce the incidence and prevalence 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 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.
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.
- "Mitigation and Land Use - Avalanches", Colorado Geological Survey
- Jaedicke, Christian; Naaim-Bouvet, Florence; Granig, Matthias (2004) "Wind-tunnel study of snow-drift around avalanche defense structures", Annals of Glaciology, vol. 38, p.325-330
- Michael Falser: Historische Lawinenschutzlandschaften: eine Aufgabe für die Kulturlandschafts- und Denkmalpflege In: kunsttexte 3/2010, unter: http://edoc.hu-berlin.de/kunsttexte/2010-3/falser-michael-1/PDF/falser.pdf