Avalanches

Avalanches are masses of snow, ice, and rocks that fall rapidly down a mountainside. It is a cohesive slab of snow lying upon a weaker layer of snow in the snowpack that fractures and slides down a steep slope when triggered and usually accelerate rapidly and grow in mass and volume as they entrain more snow.

While avalanches are sudden, the warning signs are almost always numerous before they let loose. Yet in 90 percent of avalanche incidents, the snow slides are triggered by the victim or someone in the victim's party. Avalanches kill more than 150 people worldwide each year. Most are snowmobilers, skiers, and snowboarders.

Types of Avalanches:

• Slab avalanches: form frequently in snow that has been deposited, or redeposited by wind.
  • Has a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces of the snow exceed its strength but sometimes only with gradual widening (loose snow avalanche).
  • They have the characteristic appearance of a block (slab) of snow cut out from its surroundings by fractures.
  • Slabs can vary in thickness from a few centimeters to three meters.
  • Slab avalanches account for around 90% of avalanche-related fatalities in backcountry users.
• Powder snow avalanches: form turbulent suspension currents known as powder snow avalanches or mixed avalanches. A type of gravity current that moves fast enough, some of the snow may mix with the air.
  • These consist of a powder cloud, which overlies a dense avalanche.
  • They can form from any type of snow or initiation mechanism, but usually occur with fresh dry powder.
  • They can exceed speeds of 300 kilometers per hour (190 mph), and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even uphill for short distances.
• Wet snow avalanches: are a low velocity suspension of snow and water, with the flow confined to the track surface.
  • The low speed of travel is due to the friction between the sliding surface of the track and the water saturated flow. Despite the low speed of travel (~10–40 km/h), wet snow avalanches are capable of generating powerful destructive forces, due to the large mass and density.
  • The body of the flow of a wet snow avalanche can plough through soft snow, and can scour boulders, earth, trees, and other vegetation; leaving exposed and often scored ground in the avalanche track.
  • Wet snow avalanches can be initiated from either loose snow releases, or slab releases, and only occur in snow packs that are water saturated and isothermally equilibrated to the melting point of water.
  • The isothermal characteristic of wet snow avalanches has led to the secondary term of isothermal slides found in the literature (for example in Daffern, 1999, page 93).
  • At temperate latitudes wet snow avalanches are frequently associated with climatic avalanche cycles at the end of the winter season, when there is significant daytime warming.
• Ice Avalanche: occurs when a large piece of ice, such as from a serac or calving glacier, falls onto ice (such as the Khumbu Icefall), triggering a movement of broken ice chunks.
  • The resulting movement is more analogous to a rockfall or a landslide than a snow avalanche.
  • They are typically very difficult to predict and almost impossible to mitigate.

Many avalanches are small slides of dry powdery snow that move as a formless mass. These "sluffs" account for a tiny fraction of the death and destruction wrought by their bigger, more organized cousins.

Disastrous avalanches occur when massive slabs of snow break loose from a mountainside and shatter like broken glass as they race downhill. These moving masses can reach speeds of 80 miles (130 kilometers) per hour within about five seconds. Victims caught in these events seldom escape.

Avalanches are most common during and in the 24 hours right after a storm that dumps 12 inches (30 centimeters) or more of fresh snow. The quick pileup overloads the underlying snowpack, which causes a weak layer beneath the slab to fracture. The layers are an archive of winter weather: Big dumps, drought, rain, a hard freeze, and more snow. How the layers bond often determines how easily one will weaken and cause a slide.

Avalanches can only occur in a standing snowpack. Storminess, temperature, wind, slope steepness and orientation (the direction it faces), terrain, vegetation, and general snowpack conditions are all factors that influence whether and how a slope avalanches. Different combinations of these factors create low, moderate, considerable, and high avalanche hazards.

Notable avalanches

1. Wikipedia - Avalanche (“Notable Avalanches” listed here.)
2. World Facts - Deadliest Avalanches in History
3. Wikipedia - List of avalanches by death toll

• Two avalanches occurred in March 1910 in the Cascade and Selkirk Mountain ranges;
  • On March 1 the Wellington avalanche killed 96 in Washington State, United States.
  • Three days later 62 railroad workers were killed in the Rogers Pass avalanche in British Columbia, Canada.
  • Seattle Times - ‘The white horror’: 107 years ago today, Stevens Pass saw deadliest avalanche in US history. by: Evan Bush. 1 Mar 2017
    • Ninety-six people were killed when an avalanche tore down a mountainside near Stevens Pass and buried two snowbound trains.
• During World War I, an estimated 40,000 to 80,000 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.
  • White Friday/Alpine Front Line avalanches, Marmolada, Italy, December 1916 - Some 10,000 Italian and Austrian soldiers, lost their lives in avalanches in December 1916. Some witnesses claim that the avalanches were purposefully triggered by the activities of soldiers on both sides in efforts to destroy their respective oppositions' forces. Heavy snowfall in the winter of 1916 had further catalyzed the possibility of avalanches in the region. On December 13th, the first avalanche, involving around 100,000 tons of ice, snow, and rocks, plunged down Mount Marmolada into the barracks of the Austrian soldiers lying directly in its path. Though 200 soldiers survived, 300 others died in this accident. Within the next few weeks, many other avalanches struck the area, with disturbingly high frequencies of snowfalls claiming several more thousands of lives.
• Winter of Terror, Austro-Swiss Alps, 1950-1951 - Approximately 649 avalanches were recorded in a three-month period throughout the Alps in Austria, France, Switzerland, Italy and Germany. This series of avalanches killed around 265 people and was termed the Winter of Terror.
• A mountain climbing camp on Lenin Peak, in what is now Kyrgyzstan, was wiped out in 1990 when an earthquake triggered a large avalanche that overran the camp. Forty-three climbers were killed.
• In 1993, the Bayburt Üzengili avalanche killed 60 individuals in Üzengili in the province of Bayburt, Turkey.
• A large avalanche in Montroc, France, in 1999, 300,000 cubic metres of snow slid on a 30° slope, achieving a speed in the region of 100 km/h (62 mph). It killed 12 people in their chalets under 100,000 tons of snow, 5 meters (16 feet) deep. The mayor of Chamonix was convicted of second-degree murder for not evacuating the area, but received a suspended sentence.
• 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. Thirty-one people died.
• December 1, 2000, the Glory Bowl Avalanche formed on Mt. Glory which is located within the Teton Mountain Range in Wyoming, United States. Joel Roof was snowboarding recreationally in this backcountry, bowl-shaped run and triggered the avalanche. He was carried nearly 2,000 feet to the base of the mountain and was not successfully rescued.
• Lahual Valley, India, March 6, 1979 (254 deaths) - The avalanche that battered the villages in the Lahual Valley claimed the lives of around 254 villagers in the region. A period of intense snowstorms were believed to have triggered the avalanches, which buried the valley under almost 6 meters of snow.
• 1970 Huascarán-Ancash, Peru, May/June 1970 (20,000 deaths) - known as the Ancash Earthquake, or the Great Peruvian Earthquake. The earthquake triggered an avalanche that alone claimed the lives of almost 20,000 people, making it the deadliest avalanche in the recorded history of humankind. The epicenter of the earthquake was located 21 miles off the coast of Peru in the Pacific Ocean, and the Peruvian regions of Ancash and La Libertad were the worst affected in this disaster. A massive avalanche struck the towns of Yungay and Ranrahirca when the earthquake destabilized the northern walls of Mount Huascarán. A large chunk of ice and snow, 910 meters wide and 1.6 kilometers long, sped down the mountain at speeds of 280 to 335 kilometers per hour. As it moved, it completely devastated all that came in its path, with its massive volumes of ice, water, mud, and rock alike.
• 1962 Huascaran Avalanche, Peru, January 1962 (4,000 deaths) - The deadly avalanche covered a distance of 9.5 miles in only 7 mins and completely buried the towns of Ranrahirca and Huarascucho under 40 feet of snow, and continued on in its killing spree until it reached he Santa River. There, it blocked the river flow, subsequently causing massive flooding in the nearby areas. Over 4,000 people lost their lives in the disaster, with many bodies remaining undiscovered for long periods, still buried under several feet of snow. A large number of farm animals and millions of dollars of crops were also lost in the disaster.

How does an avalanche kill you?

• Most commonly, avalanches kill you through trauma - broken bones, internal bleeding etc. You're being thrown off cliffs, bounced off rocks, crushed and hit by bits of snow and ice.
  • Assuming they survive the slide itself, and have come to a stop, they won't be able to move. The snow will set hard around them like concrete. They can't dig themselves out.  If they're lucky they will have some sort of air pocket in front of their face, which is why we're told (though how easy this would be is questionable) to try and put a hand in front of the mouth to stop it being filled with snow.
• Another cause would be suffocation or asphyxiation. Typically, if you’re buried and not recovered within a certain amount of time, you will not have enough air to survive. As they breathe, this air pocket will gradually be replaced with the CO2 they expel, which will be what kills them. In general, you have around 18 minutes before their chance of survival becomes very very small.
• Burial itself is the last cause, though most rare, simply being crushed by the weight of so much snow on top of you.

Notables dying in an avalanche

1. Wikipedia - List of skiing death
2. Ranker - Famous Skiing Deaths: Celebs Who Died in Skiing accidents
3. People - Nine Perish as a Fatal Avalanche Turns Heli-Skiing into a skiing Hell.1 Apr 1991

• Mieczyslaw Karlowicz (1876-1909) - Polish composer & conductor.
• Barbara Henneberger (1940-1964) - Alpine ski racer & Olympic West Germany medalist; (Wikipedia - Barbara Henneberger)
• Prins van Oranje-Nassau Johan-Friso Bernhard Christiaan David van Oranje-Nassau (van Amsberg) (Johan Friso) - ( 1968-2013) - Dutch prince; buried in avalanche, dug out, unconscious, resuscitated, but in coma, died 1-1/2 year later. (PRI - 12 celebrity ski accidents. by Emily Lodish, 6 Jan 2014)

References & Additional Reading

• National Geographic - Avalanches
• Wikipedia - Avalanche
• National Snow & Ice Data Center- Snow avalanches
• Wilderness & Environmental Medicine 18, 293 297 (2007) - Cause of Death in Avalanche Fatalities. by: Scott E. McIntosh, MD; Colin K. Grissom, MD; Christopher R. Olivares, MD; Han S. Kim, PhD, MSPH; Bruce Tremper, MS Original Research. From the Division of Emergency Medicine, University of Utah, Salt Lake City, UT (Drs McIntosh, Olivares, and Kim); LDS Hospital, Salt Lake City, UT (Dr Grissom); Utah Avalanche Center, Salt Lake City, UT (Mr Tremper).
• Why Files - Avalanches unveiled

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