Everyday life in many mountain towns would be completely impossible without avalanche control. Major roads and rail links would have to be shut down every winter and for most of the spring and fall as well. Recreational skiing, snowboarding, snowmobiling, and climbing would be out of the question. Not only would they be dangerous to do, you couldn't even get to mountain resorts in the first place.
Avalanche control consists of three parts: a risk assessment based on terrain and type of snow cover, a hazard assessment which identifies threatened structures, and actions taken to address the hazard. Assessments are made based on ongoing observation of unstable snowpacks, especially in heavy use areas. Actions can be passive or active, ranging all the way from marking some backcountry as a no-go area to dynamiting unstable snowpacks.
Avalanches, like lightning, tend to strike in the same places repeatedly. Fortunately, that means that it's easy to predict the most dangerous avalanche-prone areas.
The perfect slope for an avalanche is a bowl or chute with a slope between 35 and 40 degrees. That's because at smaller angles, gravity usually isn't enough by itself to dislodge poorly bonded snow against the other forces holding it in place, while at steeper angles, a poorly bonded layer of snow never has a chance to build up.
Once it's dislodged, an avalanche follows the easiest way down. The main paths of previous avalanches are bare of large trees. The base of common avalanche paths are usually covered with snow and rubble from previous avalanches. Over time, even the mountain rocks start wearing away along the path.
Finally, risk assessment looks at snowfall and temperature. Avalanches usually start when a trigger releases a layer of freshly fallen snow on top of a layer of poorly bonded snow. The record of snowfall and temperatures gives a good indication of which areas are probably unstable. In safer areas, snow cores can be drilled out so that the layers of snow can be examined directly.
Even with all this information, risk assessment is not perfect. High-risk places are easy to identify. Other places, not so much. No one knows exactly when unstable snow is going to let loose and become an avalanche. Even after an avalanche has passed, there could still be unstable layers deeper down or higher up. It's not unusual for one avalanche to be followed by another. Where there's a known risk of a one-two punch, rescue efforts for an avalanche won't be sent out until the region can be made safe.
Hazard is assessed based on the damage an avalanche could do to human structures and people in the area. This includes everything from transportation and utility links to permanent buildings and towns. It also takes into account whether people are likely to be in the area.
In large parts of the mountain backcountry, there aren't any human structures. They're also too far away from inhabited areas for most recreational skiers and snowmobilers to go. In these places, avalanche-prone slopes are pretty much left to themselves.
At the other end of the spectrum, ski resorts and mountain towns are often built in avalanche-prone areas. Even if the risk from the slopes is low, the hazard is very high.
Active interventions all change the risk of an avalanche by altering the type and amount of snow so that it is no longer likely to cause a destructive avalanche. Sometimes that snow is packed down so hard that it can't slip loose anymore. Sometimes small avalanches are triggered deliberately so that snow can't build up enough to cause larger, more destructive avalanches.
Avalanches take place when a poorly bonded layer of snow slips and falls away from the rest of the snowpack. Mechanically packing down the snow disrupts these weak layers and makes the snowpack more uniform. This is most commonly done by special tracked machines called snow groomers. However, snow grooming only works if it can be done immediately while the snow is falling, before it can settle into layers. It's used most often on resort ski slopes.
In less accessible areas, slopes can be stabilized by using ski stabilization techniques. These cause many small avalanches which remove unstable layers of snow. Ski stabilization techniques are always done starting at the top of a slope and working down, so that the skiers who are setting off the avalanches do not get caught in them.
At the top, the projecting cornice has to be cut off and dropped, so that it triggers a small avalanche instead of a larger one. Ski cutting along the top of the slope deliberately sets off many small avalanches below the skier. Skiers keep working their way down the slope, settling out the unstable snow by traversing the slope back and forth. If the slope is very unstable, the skier doing the original cut can be belayed on a rope.
The most famous active intervention to reduce avalanche risk is to dynamite parts of the slope. As with ski stabilization, the purpose of explosives is to release unstable snow before it can build up. Explosive methods are usually used in high-hazard areas which can tolerate small avalanches but not big ones.
There are many ways to deploy the explosives. Usually explosives are shot or dropped into the snow surface from a distance, but they can also be placed manually. Explosives can also be detonated to create an air blast above the snow surface. Some established ski areas have permanent unmanned installations above the avalanche starting zones where fuel-air explosions can be triggered by remote control. Some of these installations are even powered by solar power!
Most passive interventions are structures which block or divert avalanches before they can cause damage. These structures can be rigid or flexible. They can interrupt the avalanche path to keep snow from going any further or block the avalanche before it really gets started. All passive intervention structures are built of steel, prestressed concrete, and sometimes nylon to be strong and durable. They have to be, if they're going to hold back that much heavy snow.
Snow retention structures such as snow racks and avalanche nets interrupt the avalanche before it can get going, so they are always installed on the upper parts of common avalanche paths. Although all snow retention structures act the same way, flexible avalanche nets are cheaper and easier to install than rigid snow racks. They are also more resistant to damage. However, avalanche nets are more difficult to anchor than snow racks.
Avalanche snow can be diverted or redistributed by snow bridges, snow sheds, and snow dams. Snow bridges are installed near the top of avalanche runs to limit the size of the avalanche. Snow sheds divert snow so that it passes above roads and railroad tracks rather than through them. Because they're not holding back the snow, they can be built of wood.
Unlike most snow diversion structures, snow dams are built right into the terrain, using existing rocks and earth reinforced with concrete. Snow dams redirect snow near the bottom of avalanche runs to keep it away from buildings and roads. Sometimes snow dams can also be found along the sides of avalanche runs. They have to be carefully placed so that the snow does not overrun them and make them useless.
A series of parallel avalanche barriers is sometimes used all the way down a slope to keep avalanches from building up at any point along that slope. Avalanche nets and avalanche fences are often used this way. Building terraces into the slope surface or planting trees also stabilizes snow and keeps avalanches from forming. However, the lines of trees have to go all the way up the slope to the treeline. Otherwise, they'll snap like matchsticks when an avalanche hits them.
In avalanche-prone parts of the Alps, many old houses are built in the shape of wedges, with the pointy end facing upslope. That way, the force of the avalanche is directed around the house without battering against it.
Most avalanches which cause injury or death are triggered by the people who are caught in them. Avalanche education is the best way to avoid triggering an avalanche in the first place.
When skiing the backcountry, use ridgelines whenever possible. Be especially wary of chutes and bowls. Practice group management whenever traversing a suspect area, so that each member of the group individually clears the area before the next one starts. Use ski stabilization techniques.
Pay attention to the local forecast. Avalanche control organizations issue bulletins and warnings when slopes are unstable. They also assign avalanche ratings to each region based on the current avalanche risk, ranging from low risk to extreme. When venturing away from groomed slopes, there is always a risk of avalanche during winter. The chance of an avalanche is always higher when a person is there to trigger it.
Avoid the backcountry during avalanche weather. Slab avalanches, the most dangerous kind, are caused when a weak layer of snow deep under the current snowfall gives way under the new weight of snow. These layers of poorly bonded snow are caused by repeated freeze-thaw cycles, especially when followed by heavy snowfall. They can also be triggered by repeated snowstorms over a short period of time, or when wind has caused snow to pile up in a bowl. If any of these weather conditions exists, avoid slopes which have not been groomed.
If a resort has closed off a slope or prohibited access to parts of the backcountry, don't go around the posted warnings. The local ski staff know which slopes are prone to unstable snow buildup and avalanches. Respect their judgement. It may just save your life.