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Figure 2 Avalanche management is always a team approach Avalanches Gary Podolsky MD
Objectives: 1.Describe the cause and impacts of avalanches 2.Emphasize education strategies for prevention in winter sports enthusiasts 3.Discuss survival techniques and search and rescue in avalanches
An avalanche is an uncontrolled movement of snow, and can happen anywhere with significant snowfall and steep slopes especially in areas where skiing, snowboarding and snowmobiling are done.
They are triggered when stress applied to a snow pack exceeds the amount that snow can absorb. This can happen from either excessive loading or weakening of the snow pack. .
Figure 3 Typical Avalanche plan. B. The three parts of an avalanche path: starting zone, track, and runout zone (Photograph courtesy of Betsy Armstrong.) It is essential for winter sportsmen going to avalanche areas to become avalanche aware. Many skiers and snowboarders travel into avalanche prone areas without appreciating the risk they are at. The average avalanche victim is a late 20's male skilled in winter sport but ignorant of basic mountaineering skills and avalanche awareness. Most victims trigger their own avalanche. Avalanches are natural occurrences and may be anticipated under certain circumstances. Ingredients for avalanches include steep terrains (slopes greater than 30 degrees), smooth slopes, exposure to wind and sun, heavy wind, rain or snow; rapid temperature change, poor bonds or layers in the snow, and prolonged periods of very cold or very warm temperatures.
Types of avalanches include loose release and slab avalanches Loose release (aka "point release " or " sluffs") avalanches start with a small amount of snow that picks up as the collapse spreads down a slope, leaving a fan shaped path.
Figure 4 Loose Avalanche
Figure 5 Slab Avalanche
Slab avalanches occur when slabs of snow are released as the cohesive forces between layers of snow are weakened. Slabs form usually at a 30-50 degree slope but most occur near 38 degrees. Slab avalanches cause most of avalanche deaths. Wind erodes loose snow from windward slopes and deposits it onto downward or leeward slopes into dense cohesive layers. This breaks up the usual crystalline snowflake structure. Broken flakes are then densely packed. Knowing which way the wind blows helps mountaineers recognize the leeward sides of mountains where slabs will tend to accumulate. Slabs will fail or release when the slab exceeds the bonding strength between layers. This can be from a rapid buildup of stress or internal weakening of the supporting layers. When a slab fails at its weakest point it will fracture along a crown line. With very hard slabs this crack will resemble a rifle shot.
Forces involving avalanche triggers The amount of force or action that upsets the snow mass stability (or trigger) will depend on how close to instability the snowmass is. Extra forces may be applied to the snowmass or pre-existent stability may decline. The balance of forces within a snowpack will affect its stability. Compressive forces are greatest at the bottom of a slope or where the slope angle decreases. Tension forces reflect the elastic strength of the pack or layer. In warm weather a layer may be elastic (and accommodate elastic deformation) but at colder temperatures more brittle (more likely to fracture). The tension and force of gravity are greatest at the top of the snowmass and where the slope angle decreases. Snow is able to remain at sloped areas because of the elastic bonding strength of individual crystals resisting gravity.
Friction forces exist between different layers as a result of tension and compressive forces. When the shear strength of the bond between layers is overwhelmed (coefficient of friction) the avalanche will start. Because of the tremendous forces released during an avalanche the snow will behave like a liquid (allowing the victim to "swim") and then rapidly gel to a solid, totally trapping the victim.
Avalanche triggers include the weight of the person , heavy new snow , earthquakes , intense warming from sun, rain, rock falls, rapid cooling , and the collapse of a cornice (a hardened ridge crest). Direct action avalanche triggers occur when a slab fails just after new snowfall where the weight of new snow overwhelms the snowpacks ability to absorb stress. Usually the victim starts this. A snowpack will readjust to stress within 48 hrs after snowfall. Travel during or just after a snowstorm is the most dangerous time.
Indirect action avalanche triggers are when a slab is weakened and destabilized. Very cold temperatures may predispose to formation of ice crystals, "depth hoar" which weakens the cohesiveness of internal layers. Wet and dry avalanches refer to the water content in the snowmass. Wet slides occur more on warm Spring days after new snow or rain has changed the cohesiveness. Dry avalanches move faster (up to 200km/hr) with an accompanying airborne concussive force
Figure 6 Wet Avalanche Water pouring off the Gulliver's avalanche paths during a storm. Above the Milford Road, New Zealand. Photo by Dan Judd, Courtesy of Wayne Carren & Peter Weir
Ice avalanches occur with icefalls or when a glacier tapers down a slope. Extremely Large Avalanches occur in the Himalayas or Andes with snow falling thousands of feet down.
Avalanche Awareness Before venturing into avalanche country the most important thing to prepare is having a good understanding of the avalanche risk. Groups should choose a leader and establish group goals, have a pre-agreed rescue plan, be proficient in performing an avalanche search, and be wary of avalanche warning signs. Mountaineers are trained to read the terrain for physical signs of avalanche danger. Snow dumping quickly from tree branches is a sign of very recent snow settlement.
Signs of unstable snow also include cracks shooting underneath skis, a hollow sound of snow while moving over it (a " whoomph" sound). This indicates tension in the snow pack or a hidden weak layer.
Evidence of past or recent avalanches is important and include trees without branches; chutes through forested areas and; steep slopes greater than 30 degrees. Other worrisome geographical features include changes in slope angle; narrow gullies, and leeward slopes.
Avalanche terrain The worst avalanche prone slopes (38 degrees) also coincide with black diamond and double black diamond ski runs. Widely spaced trees are considered more risky than dense clumps of trees. Certain slope angles are also associated with specific types of avalanches . Trees may also be " avalanche flagged" where half of their branches are missing from previous avalanche passage. Aspens and willows are opportunistic and invasive species and their presence may be used as a clue that avalanches are frequent on a slope since more slow growing trees would not be as successful. Buried bushes or small trees may contribute to the weakening of a snow slab even if they are not seen.
Figure 7 Avalanche and slope angle b Avalanche flagged tree
Leeward slopes (facing away from prevailing winds) are more prone to be stacked with snow. South facing slopes receive more sunlight and tend to stabilize more quickly but can also release avalanches when the sun warms them too quickly. Sunballs are snowballs that are released off a slope by warmed snow or small wet sluffs and are signs of destabilization. North facing slopes are slower to stabilize and are prone to the weakening effects of cold temperatures in mid winter. Shady north faces collect ice crystals easily. Terrain traps like confined gullies and bowls are frequent sites of avalanche fatalities. Weather conditions
Avalanche EquipmentBasic avalanche equipment that should be carried by everyone in the backcountry:
• Avalanche transceivers (beacons) are beacons that locate buried victims. Travelers should switch their beacon to "transmit" while in avalanche country. The various types display a signal based on the distance to other beacons, but are not directional. Rescuers using their beacons on reception have to follow a disciplined search pattern in order to find buried victims in time. Proper search drills are necessary to become efficient in this. • Sturdy light shovels (may be collapsible) . They should be strong, lightweight and not bulky. Those made with treated Aluminum or Lexum are good. • Collapsible avalanche probe to help locate victim. • Slope measurement device (clinometer) The clinometer is a card with a free hanging arrow that allows an estimate of a slope. It uses gravity as a plumb line as a reference. This makes a slope evaluation more subjective since a person may be standing on a slope while looking at another. • Compass • First aid winter wilderness survival kit • Hand lens are used to examine snow for the presence of ice crystals Testing Snow stabilityDifferent tests have been devised to grade the risk of an avalanche. None are perfect and must be taken in context with prevailing situations.
Probe with ski pole: Using a ski pole as a probe feel the resistance of the snow. Examine the snow with a hand lens for large crystals, which indicate weaker layers. Digging a snow pitThis is more time consuming but allows you to examine the snow layers directly for hardness and the presence of large crystals.
The snow shovel test involves trying to push forward without prying a block of snow that has been cut out but still attached at its base. Weakness between the layers or in the base will show.
The Rutschblock Test (Swiss Army Ski test) is the best test as this involves checking the stability of a block cut out, measuring one by one-half ski lengths but still attached at its base by the gradual introduction of the skiers weight. After isolating the block, the tester approaches the block from the side. First the tester gently steps on the block with skis. Next, both skis are placed on. If the block falls now do not ski. If stable, then continue the test by jumping once and landing on both feet. If the snow gives here it is still considered fairly unstable. The person next tries two more solid jumps. If it fails now the recommendation is that it might be safe to ski if the overall environment appears to be safe with no other warning signs of avalanche. If the block releases only after many repeated jumps or not at all, it may be considered safe to ski although this is still relative. Obviously even this best test has limitations and the possibility of avalanches cannot be fully excluded.
Figure 8 Rutschblock test Safe Travel in Avalanche country• Avoid steep terrain greater than 30 degrees. • Avoid trigger points. • Use safe terrain to your advantage like dense forests and broad ridge tops while avoiding confined gullies, leeward slopes and areas of steep slope changes. • Consider the possibility of avalanches and have all transceivers set to transmit. • Have a leader and a follow through plan if an accident occurs. • Travel one at a time with everyone watching the lead person. Remember even if one person passes it still might not be safe. • Move quickly towards areas of safety and don't linger. • Wait 48 hrs if possible after a snowstorm before traveling. • Tighten zippers, gloves and loose clothing. If wearing a pack release the waist so it can be removed quickly. Remove ski pole straps and loosen the bindings on skis and snowboards. If caught by an avalanche they can hinder survival. Surviving an AvalancheAs soon as the snow starts to move each person must move. Either escape to the side or grab a tree. Attempt to keep upright but going straight down in an attempt to outrun the avalanche will not work. It is best to get to the side. People should shout, (to alert others), then immediately close their mouth to avoid inhaling snow. Next, if knocked off their feet people should try to swim. Discard people packs and equipment including ski poles and skis. If pushed forward, attempting a breaststroke will help. If feet first, the person shall try to roll onto the back and attempt to skull with arms and legs similar to treading water.
When the flow seems to be slowing, the victim should be alert that the snow will start to harden in the next 1-2 seconds. This is the best time to reach for the surface and thrust forward with swimming motions. Getting a head, leg or, arm to the surface will help improve survival, as this will aid rescuers. Use one hand to clear a space around the mouth. Those who fight the hardest while being buried do the best . A victim may be dead from the physical force alone and survivors must be carefully checked for signs of severe trauma including neck injuries. Victims who have not had lethal injuries may succumb unless rescued quickly so every effort must be made to ensure a fast rescue.
If the last seen point is known, mark it immediately. Usually it is safe to run onto the snow surface (unless another slab is still hanging above where the fracture occurred). Investigations should be thorough. Snow should be kicked or turned to look for clues. Using probes will help. Often victims are found in uphill sides of trees and rocks or bends in the slope where the snow slows.
Receiver searchesRescuers must set their beacons to "receive". There may be different search patterns. People should take an outdoor wilderness survival course to properly learn this skill. A grid search uses the transceiver to find the strongest signal, then turn 90 degrees and repeat, narrowing the distance to the victim(s). A tangential search allows the operator to turn towards the signal as it gets stronger. If enough people are available some can dig at prospective sites while others continue their grid search. Having one member elected for monitoring for a second avalanche may be helpful if risk is felt to be significant. In the first 15 minutes after more victims are found alive then dead (87%), in the next 15 minutes equal care found alive or dead 50%, and after 30 minutes the mortality is much higher. Some people have survived for several hours, especially if they found refuge in a pocket formed by debris or buildings. Victims should also try to yell to us as this could help their chances of survival.
Figure 9 Transceiver and Search patterns- grid and tangential patterns
Self-Rescue devices: New devices like the Avalung and the ABS (avalanche air bag system) could also help survival.
Figure 10 A. Avalung. B. ABS System demonstrated The avalung is a vest device that allows victims to breath through a larger air space provided by the vest prolonging avoidance of asphyxiation.
The ABS attempts to provide immediate floatation for those caught in an avalanche, preventing burial and increasing survival. Pulling a ripcord that releases compressed nitrogen gas inflating two 75-liter air bags, which provide buoyancy and protection from trauma, triggers the air bag system. In 27 documented cases where the air bag was deployed all 27 survived (14 were buried, 9 partially buried and 4 were buried but the air bag was exposed allowing for quick rescue).
Conclusion
Natural forces we cannot control form avalanches. Avalanches constantly occur in the wilderness but only become problems when they involve human activity. Respecting the danger and reacting quickly is the best way to avoid problems.
This talk only briefly discusses practical techniques but is no substitution for an avalanche course with transceiver demonstrations to learn these survival skills adequately.
Several commercial devices have been mentioned and more details can be found at their websites. All photos used have been credited with their original photographers or with Avalanche awareness.
References :
• Wilderness Medicine 4 th Edition (Paul Auerback) • Avalanche Awareness (John Moynie) 1998 Falcon Publishing, ISBN 1-56044-670-6 • Cyberspace snow and Avalanche Center, www.csac.org • West side Avalanche Network www.avalanche.org • www.avalung.com • www.abssystem.com • Nova online http://www.pbs.org/wgbh/nova/avalanche/ • Avalanche Awareness http://nsidc.org/snow/avalanche/ "Information on avalanches obtained from the Internet on-line information page, "Avalanche Awareness," (http://nsidc.org/snow/avalanche/) maintained by the National Snow and Ice Data Center, University of Colorado, Boulder [Sept 6 2005]."
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