An avalanche is a rapid mass of snow moving down a slope, typically occurring when a cohesive slab breaks loose from a weaker layer beneath it. This slab fracture propagates across the slope, allowing the mass to slide downhill at high speeds. The primary goal of prevention is to reduce the likelihood of this unstable snowpack collapsing and initiating movement. Methods range from individual decision-making in the backcountry to large-scale engineering projects protecting fixed infrastructure.
Personal Prevention Through Terrain Management
Individuals venturing into mountainous terrain play the most direct role in their own safety by carefully managing their route and behavior. Before any trip, a person should consult the local avalanche forecast, which uses a five-level danger scale to communicate current risk based on elevation and slope aspect. Understanding these forecasts allows for informed decisions regarding which terrain should be avoided entirely for the day.
Recognizing observable warning signs is a fundamental aspect of personal prevention while traveling. These signs include recent natural avalanche activity, “whumpfing” sounds indicating a collapse in a deep, weak snow layer, and shooting cracks radiating out from a person’s skis or feet. These observations signal that the snowpack is highly stressed and prone to fracture.
The most effective prevention technique an individual can employ is strategic terrain selection to avoid areas where avalanches are most likely to occur. This means steering clear of slopes steeper than 30 degrees, as this angle provides enough pitch for a slab to slide when triggered. Backcountry travelers should also avoid “terrain traps” like gullies or depressions where avalanche debris can pile up deeply, significantly increasing the burial risk even from a small slide.
Active Control of Unstable Snowpacks
Professional avalanche safety programs, such as those run by ski resorts or highway departments, employ active control measures to stabilize slopes proactively. This process begins with extensive snowpack analysis, often involving digging a snow pit to examine the layers and perform stability tests like the Extended Column Test. These tests reveal the depth, strength, and weakness of buried layers, allowing experts to pinpoint the specific instability that needs to be addressed.
The most common and effective active intervention is artificial triggering, which forces an avalanche to release under controlled conditions before a large, dangerous accumulation can build up. This is primarily achieved through the strategic use of explosives, often delivered remotely via artillery, helicopter drops, or fixed gas exploders like the Gazex system. By detonating a charge above the starting zone, the shock wave imparts a dynamic load that stresses the weak layer, causing a smaller, less destructive slide.
Artificial triggering releases unstable snow on a small scale, thereby “cleaning” the slope and allowing a new, more stable snowpack to form. In areas with lower hazards, or as a follow-up to explosive work, mechanical compaction techniques are utilized. These include “ski cutting,” where an experienced patrol member traverses the top of a slope to intentionally fracture the weak layer, or “boot packing,” where feet compact the snow in high-traffic areas to disrupt the formation of weak layers.
Permanent Structural Mitigation Techniques
Long-term, passive engineering solutions are designed to protect fixed infrastructure like roads, villages, and railways that cannot be easily moved. These structural techniques are highly site-specific and fall into categories based on their function within the avalanche path. Defenses in the starting zone are intended to prevent the avalanche from ever forming by anchoring the snowpack to the slope.
Snow retention structures, such as large steel snow fences or nets, are installed in a series across the upper slopes to physically support the snow cover. These structures prevent the snow from creeping or sliding, which impedes the formation of a cohesive slab and stops a fracture from propagating. By holding the snow in place, they directly eliminate the primary condition needed for a slab avalanche to occur.
Further down the slope, permanent structures are designed to manage the flow and force of a moving avalanche. Diversion structures, like reinforced concrete walls or earthen mounds, are built to deflect the path of a slide away from a protected asset. Additionally, avalanche sheds, or galleries, are reinforced tunnels built directly over roadways or railways, allowing a flowing avalanche to pass harmlessly over the structure and its occupants.