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Causes Avalanches occur when the load on the upper snow layers exceeds the bonding forces of a mass of snow (bonding to layer beneath, horizontal internal stability, support from anchors such as rocks and trees, stress support from top or bottom of slope). Contributing factors Determining critical load which would cause a slope avalanche is a complex task involving evaluation of many factors. Some of them are: Terrain Snow The structure of the snowpack determines avalanche danger. Avalanches require a buried weak layer (or instability) and an overlying slab. Unfortunately relations between easily observable properties of snow layers (strength, grain size, grain type, temperature) and avalanche danger are complex and not yet fully understood. Additionally snow cover varies in space and so does stability of snow. Weather Weather determines the evolution of snowpack. The most important factors are heating by solar radiation, radiational cooling, temperature gradients in snow, and snowfall amounts and type. Most avalanches happen during or soon after a storm. Avalanche avoidance Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety is a continuous process, including route selection and examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimise the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings heeded. Never follow in the tracks of others without your own evaluations; snow conditions are almost certain to have changed since they were made. Observe the terrain and note obvious avalanche paths where vegetation is missing or damaged, where there are few surface anchors, and below cornices or ice formations. Avoid travelling below others who might trigger an avalanche. Prevention There are several ways to prevent avalanches and lessen their power and destruction. They are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountains towns, roads and railways. 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 can be used to direct the placement of snow. Snow builds up around the fence, especially the side that faces the prevailing winds. Downwind of the fence, snow buildup is lessened. This is caused by the loss of snow at the fence that would have been deposited and the pickup of the snow that is already there by the wind, which was depleted of snow at the fence. When there is a sufficient density of trees, they can greatly reduce the strength of avalanches. They hold snow in place and when there is an avalanche, the impact of the snow against the trees slows it down. Trees can either be planted or they can be conserved, such as in the building of a ski resort, to reduce the strength of avalanches. Artificial barriers can be very effective in reducing avalanche damage. There are several types. One kind of barrier uses a net strung between poles that are anchored by guy wires in addition to their foundations. These barriers are similar to those used for rockslides. Another type of barrier is a rigid fence like structure and may be constructed of steel, wood or pre-stressed concrete. They usually have gaps between the beams and are built perpendicular to the slope, with reinforcing beams on the downhill side. Rigid barriers are often considered unsightly, especially when many rows must be built. They are also expensive and vulnerable to damage from falling rocks in the warmer months. Finally, there are barriers that stop or deflect avalanches with their weight and strength. These barriers are made out of concrete, rocks or earth. They are usually placed right above the structure, road or railway that they are trying to protect, although they can also be used to channel avalanches into other barriers. Occasionally, mounds of earth are placed in the avalanche's path to slow it down. The group Human survival and avalanche rescue
Search and rescue equipment 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. However, like a seat belt in a vehicle, using the right equipment does not justify exposing yourself to unnecessary risks with the hope that the equipment might save your life when it is needed. Avalanche cords Using an avalanche cord is the oldest form of equipment - mainly used before beacons became available. The principle is simple - an aprox. 10m long, red cord (similar to parachute cord) is attached to the persons belt. While skiing/snowboarding/walking the cord is dragged along behind the person. If the person gets buried in an avalanche, the light cord stays on top of the snow. Due to the color the cord is easily visible for rescue personnel. Typically the cord has iron markings every 1m that indicate the direction and length to the victim. Beacons Beacons - known as "beepers", peeps(pieps), ARVAs (Appareil de Recherche de Victimes en Avalanche, in French), LVS (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche transceivers or under various trade names, are important for every member of the party. They emit a "beep" via 457kHz radio signal in normal use, but can be switched to receive mode to locate a buried victim up to 80 meters away. Analog receivers provide audible beeps that rescuers interpret to estimate distance to a victim. To be effective, beacons require regular practice. Some older models of beepers operated on a different frequency (2.275 kHz) and a group leader should ensure these are no longer in use. Recent digital models also attempt to give visual indications of direction and distance to victims and require less practice to be useful. There are also passive transponder devices that can be inserted into equipment, but they require specialized search equipment that might only be found near an organized sports area. Mobile phones can seriously disrupt the ability of a beacon to receive a transmitting beacon's signal. Phones should be switched off when searching. Probes
Shovels When an avalanche stops, the deceleration normally compresses the snow to a hard mass. Shovels are essential for digging through the snow to the victim, as the deposit is too dense to dig with hands or skis. A large scoop and sturdy handle are important. Shovels are also useful for digging snow pits as part of evaluating the snowpack for hidden hazards, such as weak layers supporting large loads. Other devices 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. There are also passive signalling devices that can be carried or inserted into sports equipment, but they require specialized search equipment which might only be found near an organized sports area. When considering any of theses devices, one should consider that if the group does not recover the avalanche victim within 15 minutes, the chance of survival rapidly decreases. Reliance on technology to summon outside help is used with the knowledge that those responding will likely be performing a body recovery. Any group that wants to survive must be capable of self-rescue. More back-country adventurers are also carrying EPIRBs (Emergency Position-Indicating Radio Beacons) containing GPS. This device can quickly notify search and rescue of an emergency and the general location (within 100 yards), but only if the person with the EPIRB has survived the avalanche and can activate the device manually. With modern mobile phone developments, an emergency GPS transmitter may also become more widely available (again, for use by a rescuer, because a victim may be unconscious or completely immobilised beneath dense snow). Prior to the arrival of beacons, some brave skiers carried a 50-ft ball of nylon cord, tied to the skier, with direction and distance markings attached at intervals. When traversing a dangerous area, the ball would be dropped or thrown and the unravelled string would trail behind. If caught in an avalanche, the hope was that a segment of the string would be quickly found and indicate the distance and direction to the victim. Although it will be very inefficient, some rescue equipment can also be hastily improvised: ski poles can become short probes, skis or snowboards can be used as shovels. A first aid kit and equipment will also be useful for assisting survivors who may have cuts, broken bones, or other injuries, in addition to hypothermia. Witnesses as rescuers Survival time is short, if a victim is buried. There is no time to waste before starting a search, and many people have died because the surviving witnesses failed to do even the simplest search. Witnesses to an avalanche that engulfs people are frequently limited to those in the party involved in the avalanche. Those not caught should try to note the locations where the avalanched person or people were seen. This is such an important priority it should be discussed before initially entering an avalanche area. Once the avalanche has stopped, and there is no danger of secondary slides, these points should be marked with objects for reference. Survivors should then be counted to see who may be lost. If the area is safe to enter, a visual search of the likely burial areas should begin (along a downslope trajectory from the marked points last seen). Some victims are buried partially or shallowly and can be located quickly by making a visual scan of the avalanche debris and pulling out any clothing or equipment found. It may be attached to someone buried. Alert others if a radio is available, especially if help is nearby, but do NOT waste valuable resources by sending a searcher for help at this point. Switch transceivers to receive mode and check them. Select likely burial areas and search them, listening for beeps (or voices), expanding to other areas of the avalanche, always looking and listening for other clues (movement, equipment, body parts). Probe randomly in probable burial areas. Mark any points where signal was received or equipment found. Only after the first 15 minutes of searching should consideration be given to sending someone for help. Continue scanning and probing near marked clues and other likely burial areas. After 30-60 minutes, consider sending a searcher to get more help, as it is more likely than not that any remaining victims have not survived. Line probes are arranged in most likely burial areas and marked as searched. Continue searching and probing the area until it is no longer feasible or reasonable to continue. Avoid contaminating the scent of the avalanche area with urine, food, spit, blood, etc, in case search dogs arrive. The areas where buried victims are most likely to be found are: below the marked point last seen, along the line of flow of the avalanche, around trees and rocks or other obstacles, near the bottom runout of the debris, along edges of the avalanche track, and in low spots where the snow may collect (gullies, crevasses, creeks, ditches along roads, etc). Although less likely, other areas should not be ignored if initial searches are not fruitful. Once a buried victim is found and his or her head is freed, perform first aid (airway, breathing, circulation/pulse, arterial bleeding, spinal injuries, fractures, shock, hypothermia, internal injuries, etc), according to local law and custom. Victims Victims caught in an avalanche are advised to try to ski or board towards 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, assuming you are still conscious. If it is possible to move once the snow stops enlarge the air space, but minimise movement to maximise the oxygen supply. Warm breath may soon cause a mask of ice to glaze over the snow in your face, sealing it against further air. European avalanche risk table In Europe, the avalanche risk is widely rated on the following scale, which was adopted in April 1993 to replace the earlier non-standard national schemes. Descriptions were last updated in May 2003 to enhance uniformity. * 1 Stability: 2 additional load: Gradient: European avalanche size table Avalanche size: North American Avalanche Danger Scale In the United States and Canada, the following avalanche danger scale is used. See also | |||||||||||||
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