Science time - Avalanches
Saturday, July 19 2008 | Comments (0)
Avalanche guards inspect conditions.
Jamie and Adam had a chance to cause an avalanche, but they don't all start with 100 pounds of TNT. Thanks to Encyclopedia Brittanica Online and Wikipedia, we'll explore a real avalanche.
An avalanche is sudden and rapid flow of snow, often mixed with air and water, down a mountainside. Avalanches are one of the biggest dangers to people and property in mountainous areas. There are also rock avalanches, but we need to cool off in these hot summer months, so let's skip those for now.
There are several types of snow avalanches.
Loose snow avalanches occur when the weight of the snowpack exceeds the strength within it - these are most common on steeper terrain. In fresh, loose snow it usually begins at a point near the top and then gradually widens down the slope, gathering more snow on the way down, forming a teardrop shape to those below, if they aren't close enough to see a wall shape.
The avalanche the guys were responsible for a slab avalanche. Slab avalanches are responsible for about 90 percent of avalanche-related fatalities, which is why the ski patrol set off controlled avalanches. If they cause it to happen, they can make sure the area is clear. Slab avalanches occur when there is a strong, stiff layer of snow known as a slab. The snow is deposited by the wind on a lee slope. A fracture forms in a weak layer then very rapidly expands - these cracks can be hundreds of feet long and several thick. The avalanche starts moving almost instantaneously.
The third type is a slush avalanche, which occurs when the snowpack becomes saturated by water.
As avalanches move down the slope they gather snow from the snowpack and grow in size. The snow may also mix with the air and form a powder cloud. An avalanche with a powder cloud is known as a powder snow avalanche.
The powder cloud is a turbulent suspension of snow particles that flows as a gravity current. Powder snow avalanches are the largest type and can exceed 150 miles per hour. They can pull up to 10,000,000 tons of snow.
All avalanches are caused by an over-burden of snow that is too massive and unstable for the slope to support. Determining the amount of over-burden which is likely to cause an avalanche is a complex task involving the evaluation of a number of factors.
Terrain Slopes flatter than 25 degrees or steeper than 60 degrees typically have a low risk of avalanche - snow does not accumulate significantly on steep slopes.
Human triggered avalanches have the greatest incidence when the slope's angle is between 35 and 45 degrees. The rule of thumb is: a slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche. Avalanche risk increases with use.
Convex slopes are more dangerous than concave slopes. The primary factor contributing to the increased avalanche danger on convex slopes is a disparity between the tensile strength of snow layers and their compressive strength.
Another factor affecting the incidence of avalanches is the ground underneath the snow.
The structure of the snowpack is a strong predictor of avalanche danger. For an avalanche to occur, it is necessary that a snowpack have a weak layer below the surface and an overlying slab of snow. Unfortunately, it is very difficult to predict if and when an avalanche will occur.
Weather also influences the evolution of snowpack formation. The most important factors are heating by the sun, radiation cooling, vertical temperature gradients in standing snow, snowfall amounts, and snow types.
If the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and refreezing of water in the snow strengthens the snowpack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point, may cause a slope to avalanche, especially in spring.
Persistent cold temperatures prevent the snow from stabilizing; long cold spells may contribute to the formation of depth hoar, a condition where there is a pronounced temperature gradient, from top to bottom, within the snow. When the temperature gradient becomes sufficiently strong, thin layers of faceted grains may form above or below embedded crusts, allowing slippage to occur.
Snowstorms and rainstorms are important contributors to avalanche danger as well. Heavy snowfall may cause instability in the existing snowpack. In the short-term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snowpack. Once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snowpack together. Most avalanches happen during or soon after a storm.
Daytime exposure to sunlight can rapidly destabilize the upper layers of a snowpack.
When an avalanche occurs any slab present begins to fragment into increasingly smaller tumbling fragments. If the fragments become small enough the avalanche takes on the characteristics of a fluid. Fine particles can become airborne and, separated from the bulk of the avalanche, traveling a greater distance as a powder snow avalanche.
Attempts to model avalanche behavior date from the early 20th century, notably the work of Professor Lagotala in preparation for the 1924 Winter Olympics in Chamonix.
His method was developed by A. Voellmy and popularised following the publication in 1955 of his Ober die Zerstorunskraft von Lawinen (On the Destructive Force of Avalanches).
Voellmy used a simple empirical formula based on Bernoulli's principle, treating an avalanche as a sliding block of snow moving with a force that was proportional to the square of the speed of its flow. He and others subsequently derived other formulas that take other factors into account. Since the 1990s many more sophisticated models have been developed.
Due to the complexity of the subject, winter traveling in the back country (off-piste) is never 100 percent safe.
Explosives are used extensively to prevent avalanches, especially at ski resorts where other methods are often impractical. Explosive charges are used to trigger small avalanches before enough snow can build up to cause a large avalanche. Snow fences and light walls are installed to direct snow.
Artificial barriers can be very effective in reducing avalanche damage. There are several types. However, they are expensive and vulnerable to damage from falling rocks in the warmer months.
Avalanches are the largest cause of death and damage in mountainous regions.
Even small avalanches are a serious danger to life, even with properly trained and equipped companions who know how to avoid the avalanche. Between 55 and 65 percent of victims buried in the open are killed, and only 80 percent of the victims remaining on the surface survive.
In some cases avalanche victims are not located until spring thaw melts the snow, or even years later when objects emerge from a glacier.
Chances of a buried victim being found alive and rescued are increased when everyone in a group is carrying and using standard avalanche equipment, and have trained in how to use it. A beacon, shovel and probe is considered the minimum equipment to carry when exposing yourself to avalanche danger.
Other rescue devices are proposed, developed and used, such as avalanche balls, vests and airbags, based on statistics that most deaths are due to suffocation.
Victims caught in an avalanche are advised to try to ski or board toward the side of the avalanche until they fall, then to jettison their equipment and attempt swimming motions. As the snow comes to rest an attempt should be made to preserve an air-space in front of the mouth, and try to thrust an arm, leg or object above the surface.
So, what kind of damage can these cause?
A large avalanche in Montroc, France, in 1999, killed 12 people, burying them under 100,000 tons of snow, 15 feet deep.
The small Austrian village of Galtür was hit by the Galtür avalanche in 1999. The village was thought to be in a safe zone but the avalanche was exceptionally large and flowed into the village killing 31.
On May 31, 1970 the Ancash earthquake caused a large avalanche from Huascaran, resulting in the destruction of the town of Yungay, Peru and the death of at least 18,000 people.
As we learned in this week's episode, avalanches were also common in WWI. Approximately 50,000 soldiers died as a result of avalanches during the mountain campaign in the Alps at the Austrian-Italian front, many of which were caused by artillery fire.
Avalanche prediction is nearly impossible; forecasters can only assert the conditions, terrain and relative likelihood of slides with the help of detailed weather reports and from localized snowpack observation.
The moral would be, pay attention to warning flags, know where you're traveling and, for God's sake, leave the dynamite at home.
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