Avalanche control

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 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 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.

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.

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