Avalanches

Factors involved: Chemical reactions, geography, geological forces, gravitational forces, human activity, ice, plants, rain, snow, temperature, weather conditions, wind

Regions affected: Cities, forests, mountains, towns, valleys

Definition

An avalanche is a large amount of snow, ice, rock, or earth that becomes dislodged and moves rapidly down a sloped surface or over a precipice. Avalanches are generally influenced by one or several natural forces but are increasingly being initiated by human activities. Landslide avalanches are defined as the massive downward and outward movement of some of the material that forms the slope of an incline. Unqualified use of the term “avalanche” in the English language, however, most often refers to a snow avalanche and generally refers to movements big and fast enough to endanger life or property. Avalanche accidents resulting in death, injury, or destruction have increased in direct proportion to the increased popularity of winter recreational activities in mountainous regions.

Science

The term “avalanche” relates to large masses of snow, ice, rock, soil, mud, and/or other materials that descend rapidly down an incline such as a hillside or mountain slope. Precipices, very steep or overhanging areas of earth or rock, are also areas prone to avalanche activity. Landslide avalanches are downward and outward movements of the material that forms the slope of a hillside or mountain. General lay usage of the term “avalanche” often relates to large masses of snow or ice, while the term “landslide” is usually restricted to the movement of rock and soil and includes a broad range of velocities. Slow movements cause gradual damage, such as rupture of buried utility lines, whereas high-velocity avalanches require immediate evacuation of an area to ensure safety.

A landslide avalanche begins when a portion of a hillside weakens progressively to the point where it is no longer able to support the weight of the hillside itself. This weakness may be caused when rainfall or floodwater elevates the overall water content of the slope, thus reducing the sheer strength of the slope materials. Landslides are most common in areas where erosion is constantly wearing away at the local terrain, but they can also be initiated by events such as earthquakes and loud noises. Some landslides move only sporadically—during certain seasons of the year—and may lie dormant for decades, centuries, or millennia; their extremely slow movements may go unnoticed for long periods of time. Slow-moving landslides are distinguished from creep—the slow change of a mountain’s or hill’s dimensions from prolonged exposure to stress or high temperatures—in that they have distinct boundaries and have at least some stable ground.

Natural avalanches can be triggered when additional stressors are provided in the form of the added weight of additional snow, either fresh snowfall or windblown snow, or when the cohesive strength of the snowpack naturally decreases, which serves to weaken the bonds between particles of snow. Artificial avalanches may be triggered when humans, animals, or machinery begin the downslide, due to their contribution of additional stress to the snow. Many avalanches in outdoor recreational areas are triggered by the weight of a single skier or the impact of small masses of snow or ice falling from above. Explosives can also trigger an artificial avalanche, either intentionally or unintentionally. When explosives are detonated to knock down potentially dangerous snow at a prescribed time and location, such as for maintenance of highways or ski areas, the public is temporarily evacuated from the area.

Ground that has remained relatively stable for as little as one hundred years or possibly as long as tens of thousands of years may begin to slide following alteration of the natural slope by human development, such as during grading for roads or building projects on hillsides. Landslide avalanches can also be started by deep cutting into the slope and removal of support necessary for materials higher up the slope, or by overloading the lower part of the slope with the excavated materials. Some have occurred where development has altered groundwater conditions.

Geography

Snow avalanches require a snow layer that has the potential for instability and a sloped surface that is steep enough to enable a slide to continue its downhill momentum once it has started. Slopes with inclines between 25 and 55 degrees represent the broadest range for avalanche danger, but a majority of avalanches originate on inclines between 30 and 45 degrees. Angles above 55 degrees are generally too steep to collect significant amounts of snow, as the snow tends to roll down the hillside very rapidly without accumulating. Slope angles of less than 25 degrees are generally safe, except for the remote possibility of very slow snow avalanches in extremely wet conditions.

When a layer of snow lies on a sloped surface, the constant force of gravity causes it to creep slowly down the slope. When a force imposed on a snow layer is large enough, a failure is triggered somewhere within the snow, thus stimulating the avalanche to begin to move rapidly downhill. There are two distinct types of failures that can occur within the snow prior to an avalanche. When a cohesionless snow layer rests on a slope steeper than its angle of repose, it can cause a loose snow avalanche, which is often also called a point-release avalanche. This can actually be triggered by as little as one grain of snow slipping out of place and dislodging other grains below it, causing a chain reaction that continues to grow in size as the accumulated mass slips down the hill. The point-release avalanche generally appears as an inverted V shape on the snow and is typically limited to only the surface layer of snow cover. In this type of avalanche, the snow has little internal cohesion, no obvious fracture line, and no clear division where the sliding snow separates from the layers underneath.

In contrast, when snow fails as a cohesive unit, an obvious brittle fracture line appears and an entire layer or slab of snow is set in motion. Because creep formation causes the snow layer to be stretched out along the slope, the fracture releases stored elastic energy. The release of this energy may cause the fracture to spread across an entire slope or basin. Failure may occur deep within the snow layer, allowing a good portion or nearly all the snow to be included in the avalanche. Slab avalanches are often larger and more destructive than point-release avalanches and can continue to slide on weaker layers underneath or actually upon the ground itself.

The specific shape of the slope may reflect the level of avalanche danger, with hazards being highest when snow accumulates on straight, open, and moderately steep slopes. One classic law of avalanches for mountaineers is that they face the least danger while moving on ridges, somewhat more danger while moving on the valley floor, and the most danger when moving directly upon the slope itself. Snow on a convex slope is more prone to avalanches, as it comes under tension because it tends to stretch more tightly over the curve of the hill. When coming down a convex slope, mountaineers may not know how steep the slope is until they pass the curve that temporarily obstructed their view and then discover that they are farther down on the face than is safe. Bowls and cirques (steep-walled basins) have a shape that tends to accumulate snow deposited by the wind. Once an avalanche begins, it most often spreads to the entire face and dumps large quantities of snow into the area below. Couloirs (mountainside gorges) are enticing to climbers because they offer a direct route up a mountain, but they are susceptible to snow movement because they create natural chutes. Forested slopes offer some avalanche protection, but they do not guarantee safety. While slides are less likely to originate within a dense forest, they have been known to crush through even very high-density tree areas. Shattered trees provide clear evidence that a previous avalanche has occurred on the mountainside. A slope that has only bushes and small trees growing on it may indicate that the incline has experienced avalanches so often that the timber is not being given a chance to regrow.

While avalanches can occur anywhere in the world where snow falls on slopes, some countries and regions are prone to such events. In Europe, the Alps—a mountain range stretching through Italy, Austria, Germany, Switzerland, and France—has experienced many devastating avalanches (such as the 1999 Galtür avalanche in Austria). The Andes mountain range in South America has produced avalanches in Peru (notably the 1970 Peruvian earthquake and avalanche). In North America, areas of the Pacific Northwest—particularly Washington State, British Columbia, and Alaska—are most often affected.

Prevention and Preparations

Snow avalanches are among the main hazards facing outdoor winter sports enthusiasts who drive through a mountain pass in an automobile or snowmobile, or hike, climb, snowshoe, hunt, or ski along a mountainside. The relative level of avalanche hazard and the conditions that occur to create the hazard at any given time are relatively easy for a trained professional to identify. The local news media generally report avalanche danger in heavily populated and well-traveled recreational areas. Unfortunately, there currently is no completely valid and reliable way to predict precisely where and when an avalanche will occur. Novice mountaineers can certainly benefit by being able to recognize the formation of different types of snow crystals and hazardous terrain and weather. They should keep in mind that avalanches can sweep even on perfectly level ground for more than 1 mile after the snow has reached the bottom of a slope.

Avalanche hazards can be assessed by examining the snow for new avalanches in the area. Cracks in hard snow may outline an unstable slab as snow settles with the weight of a person moving on it. The sound of a loud thump may indicate that a hard slab is nearly ready to release. Snow stability can be tested by probing with a ski pole to feel for layers of varying solidity or by digging a pit to examine the layers for weakness. Some excellent advice for winter travelers is to always stop to rest or set up camp outside the potential reach of an avalanche.

Avalanche research has consistently shown that approximately 80 percent of avalanches occur during or just after a storm. Avalanche danger escalates when snowfall exceeds a level of 1 inch per hour or an accumulation of 12 inches or more in a single storm. Rapid changes in wind and temperature also significantly increase avalanche danger. Storms that begin with a low ambient temperature and dry snow on the ground and are followed immediately by a rapidly rising temperature are more likely to set off avalanche conditions. Snow that is dry tends to form poor chemical bonds and thus does not possess the strength to support the heavier, wet snow that rapidly accumulates on the surface. Rainstorms or spring weather with warm winds and cloudy nights creates the possibility of a wet snow avalanche and causes a “percolating” effect of the water into the snow.

The manner in which the sun and wind hit a slope can often provide valuable clues regarding potential avalanche danger. In the Northern Hemisphere, slopes that face south receive the most sun. The increased solar heat makes the snow settle and stabilize more quickly than on north-facing slopes. Generally speaking, south-facing slopes are safer in winter, but there are certainly many exceptions to this rule as determined by local factors. South-facing slopes also tend to release their avalanches sooner after a storm. Thus, slides that begin on southern slopes may indicate that slopes facing other directions may soon follow suit. As warmer days arrive near the end of winter, south-facing slopes may actually become more prone to wet snow avalanches, making the north-facing slopes safer. North-facing slopes receive very little or no sun in the winter, so consolidation of the snowpack takes much longer, if it occurs at all. Colder temperatures may create weak layers of snow, thus making northern slopes more likely to slide in midwinter. It is important to note that these guidelines should be reversed for mountainous areas south of the equator.

Windward slopes that face into the snow tend to be safer because they retain less snow—the wind blows it away. The snow that remains tends to become more compact through the blast of the wind. Lee slopes, which face the same direction the wind is blowing, collect snow rapidly during storms and on windy days as the snow blows over from the windward slopes. This results in cornice formation on the lee side of ridges, snow that is deeper and less consolidated, and the formation of wind slabs that can be prone to avalanches. Snow formation often indicates the prevailing wind direction, following the general rule that cornices face the same direction that the wind is blowing.

Attempts have been made to prevent avalanche damage by building artificial supporting structures or transplanting trees within anticipated avalanche zones. The direct impact of an avalanche has been effectively blocked by diversion structures such as dams, sheds, and tunnels in areas where avalanches repeatedly strike. Structural damage can be limited by the construction of various types of fencing and by building splitting wedges, V-shaped masonry walls that are designed to split an avalanche around a structure located behind it. Techniques have been developed to predict avalanche occurrence by analyzing the relationships between meteorological and snow-cover factors, which are often reported through the media. Zones of known or predicted avalanche danger are generally taken into account during commercial development of a mountainous area. The avalanche danger of unstable slope accumulations is often prevented through detonation, from explosives similar to grenades to the sending out of controlled acoustic waves.

Rescue and Relief Efforts

Search and rescue experts recommend that, when individuals know they are about to become caught in an avalanche, they should make as much noise as possible and discard all equipment, including packs and skis. They should try to avoid being swept away by grabbing onto anything stable, such as large rocks or trees. Those who become caught in a slide should attempt to stay on the snow surface by making swimming motions with the arms and legs or by rolling. It is also recommended to attempt to close the mouth in the event that the head begins to fall below the snow surface. If victims anticipate becoming completely buried and no longer moving with the snow, they should attempt to create a breathing space by putting their hands and elbows in front of their faces and inhaling deeply before the snow stops in order to expand the ribs. All available oxygen and energy should be conserved if victims anticipate that rescuers will soon begin making appropriate search and rescue efforts.

Avalanche search and rescue efforts should begin as soon as possible by companions of the victim, who should generally anticipate that there will not be time for professional help to arrive. Despite the shock of the moment, rescue procedures should begin immediately by noting and marking—with an object such as a ski pole—three critical positions on the snow. These positions include the point where the victim was first caught in moving snow, the point where the victim disappeared beneath the snow surface, and the point where the moving surface of the avalanche eventually stopped. Accurately noting these three areas greatly reduces the area that needs to be searched, thus providing an increased chance of a successful search. Rescue beacons, small electronic devices which should be secured to all persons traveling together in a winter excursion party, have proven to be very effective tools in finding buried victims. The beacons can be switched to either transmit or receive signals at a radio frequency that is set to the transmit mode during the initial movement. Searchers who switch their beacons to receive mode immediately after an incident can often locate a buried victim in just a few short minutes. Procedures for avalanche rescue, such as setting up a probe line, have been established by search and rescue organizations and should be reviewed prior to a trip by all persons participating in winter activities within a potential avalanche zone.

Impact

The impact pressures resulting from high-speed avalanches and landslides can completely destroy or harm human and animal life and property. About one-third of avalanche victims die from the impact; the remaining two-thirds die from suffocation and hypothermia. Movement of snow and other debris is most destructive when it is able to generate extremely high speeds. Small to medium avalanches can hit with impact pressures of 1 to 5 tons per square meter, which is generally enough force to damage or destroy wood-frame structures. Larger avalanches can generate forces that can exceed 100 tons per square meter, which is easily enough to uproot mature forested areas and destroy large concrete structures.

Measurements have shown that highly turbulent dry snow or dry powder creates avalanche speeds averaging 115 to 148 feet (35 to 45 meters) per second, with some velocities being clocked as high as 223 to 279 feet (68 to 85 meters) per second. These high speeds are possible only in dry powder avalanches because these avalanches incorporate large amounts of air within the moving snow, thus serving to reduce internal frictional forces. Wet snow avalanches comprise liquid or snow that is very dense, which creates less turbulent movement once the slide begins. With a reduction in turbulence, a more flowing type of motion is generated, and speeds are generally reduced to approximately 66 to 98 feet (20 to 30 meters) per second.

Persons who do not live in mountainous regions might mistakenly believe that damage caused by an avalanche is minimal when compared to the destruction caused by other environmental hazards such as tornadoes and floods. However, the frequency of accidents resulting in destruction, injury, or death has risen tremendously in direct proportion to the increased popularity of winter recreational activities in mountainous areas. An estimated 150 to 200 avalanche-related deaths occur per year, but it should be noted that these avalanche data are systematically and accurately recorded mainly in developed countries in North America, Europe, and northern Asia.

Historical Overview

Considered one of the greatest military commanders in the history of the world, Hannibal and his North African army were no match for the natural forces unleashed in 218 b.c.e. when an avalanche descended upon his invading army of thirty-eight thousand soldiers, eight thousand horsemen, and thirty-seven elephants. The rapidly moving snowmass, which wreaked havoc at Col de la Traversette pass in the Italian Alps and claimed nearly 40 percent of Hannibal’s fighting force, dealt the general one of the most devastating losses of his entire military career. The historic and horrendous tragedy experienced by Hannibal was also one of the first documented avalanches in European history.

Like so many before and since, Hannibal either was unaware of the dangerous physical environment created by the heavy snows or chose to ignore the danger. Thousands of avalanches occur annually worldwide, but most cause little damage. Each year, however, avalanches consistently claim about 150 lives and cause millions in property losses.

The European Alps, where the lives of Hannibal’s troops were claimed, have been the site of the most deadly avalanches in recorded history, although the greatest number occur in the much more sparsely populated Himalayas, Andes, and Alaska. During World War I an estimated 40,000 to 80,000 soldiers were killed and maimed in the Alps by avalanches caused by the sounds and explosions of combat.

Such massive loss of life, however, is not representative of avalanche disasters. In a more typical year the country of Switzerland, for example, has an average avalanche death rate of fewer than 25. An 1892 avalanche that destroyed the Swiss resort towns of St. Gervais and La Fayet, killing 140 residents and tourists, was considered a fairly deadly and unusual occurrence. As one of the most avalanche-prone nations, Switzerland committed considerable resources after the early 1900s to identify ways to avoid the loss of property and life. The leading avalanche research center in Europe is the Swiss Federal Snow and Avalanche Research Unit, which takes considerable pride in its successful Avalanche Warning System. Similar research programs are located in the United States, Japan, and other countries.

Despite the best efforts and intentions of warning systems, avalanches continue to claim the lives of unsuspecting victims each year. Even the most highly trained and skilled scientists are often vulnerable to the deadly force of avalanches. Many of the individuals killed by the 1980 Mount St. Helens volcanic explosion and the resulting 250-mile-per-hour avalanche, the largest in recorded history, were scientists on location to study the volcano. In March 1997, a park geologist and a volunteer were killed by an avalanche while working on a project to monitor Yellowstone National Park's geothermal features.

As a result of years of research and data collection, however, scientists have identified ways of diminishing the potential for destruction and loss of life resulting from avalanches. Federal, state, regional, and local governments may coordinate efforts to detect and identify potentially unstable snow-covered slopes by monitoring weather conditions; geographic data available through photogrammetry, the science and art of deducing the physical dimensions of objects from measurements on photographs; and satellite imagery.

Bibliography

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