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  1. 1. Avalanche
  2. 2. <ul><li>Snow avalanches are a natural process, occurring </li></ul><ul><li>perhaps 1,000,000 times per year, world-wide. Snow on an </li></ul><ul><li>incline adjusts to the pull of gravity. The vast majority of </li></ul><ul><li>these slides are not a problem, because an avalanche, in </li></ul><ul><li>and of itself, is not a hazard. A person (or a person's stuff) </li></ul><ul><li>has to get involved in order for there to be a problem. </li></ul><ul><li>The fact is, that avalanches don't drop from the peaks onto </li></ul><ul><li>the heads of unsuspecting innocents with the </li></ul><ul><li>unpredictability of a plummeting meteorite. 95% of people </li></ul><ul><li>who are caught in avalanches are caught by a slide that </li></ul><ul><li>was triggered by themselves or a member of their party. If </li></ul><ul><li>it is our behavior that is creating the hazard, then we can </li></ul><ul><li>change our behavior to avoid problems. </li></ul>
  3. 3. <ul><li>A rapid flow of snow down a slope, from either natural triggers or human activity. </li></ul><ul><li>Occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. </li></ul><ul><li>Entrain ice, rocks, trees, and other material on the slope </li></ul><ul><li>Initiated in snow, are primarily composed of flowing snow, and are distinct from mudslides , rock slides , rock avalanches , and serac collapses from an icefall . </li></ul><ul><li>In mountainous terrain avalanches are among the most serious objective hazards to life and property </li></ul>
  4. 4. Occurance <ul><li>Snow is deposited in successive layers as the winter </li></ul><ul><li>progresses. These layers may have dissimilar physical </li></ul><ul><li>properties and an avalanche occurs when one layer slides </li></ul><ul><li>on another (Surface Avalanche), or the whole snow cover </li></ul><ul><li>slides on the ground (Full-Depth). An avalanche may be </li></ul><ul><li>Dry or Wet,according to whether free water is present in </li></ul><ul><li>the snow. It may be of Loose Snow, when the avalanche </li></ul><ul><li>starts at a single point or a Slab Avalanche which occurs </li></ul><ul><li>when an area of more cohesive snow separates from the </li></ul><ul><li>surrounding snow and slides out. In practice, any snow slide </li></ul><ul><li>big enough to carry a person down is important. </li></ul>
  5. 5. Causes <ul><li>Terrain affects avalanche occurrence and development </li></ul><ul><li>through three factors: </li></ul><ul><li>1. Terrain affects the evolution of the snow pack by determining the meteorological exposure of the snow pack. </li></ul><ul><li>2. Terrain affects the stability of the snow pack, through the geometry and ground composition of the slope. </li></ul><ul><li>3. The down slope features of the terrain affects the path and consequences of a flowing avalanche. </li></ul>
  6. 6. <ul><li>Avalanches are classified by their morphological(structure) </li></ul><ul><li>characteristics, and are rated by either their destructive </li></ul><ul><li>potential, or the mass of the downward flowing snow. Some </li></ul><ul><li>of the morphological characteristics used to classify </li></ul><ul><li>avalanches include </li></ul><ul><li>the type of snow involved, </li></ul><ul><li>the nature of the failure, </li></ul><ul><li>the sliding surface, </li></ul><ul><li>the propagation mechanism of the failure, </li></ul><ul><li>the trigger of the avalanche, </li></ul><ul><li>the slope angle, </li></ul><ul><li>direction, and </li></ul><ul><li>elevation. </li></ul>
  7. 7. <ul><li>Avalanche size, mass, and destructive potential are </li></ul><ul><li>rated on a logarithmic scale , typically made up of 4 to 7 </li></ul><ul><li>categories, with the precise definition of the categories </li></ul><ul><li>depending on the observation system or forecast region. </li></ul><ul><li>All avalanches have common elements as : </li></ul><ul><li>A trigger which causes the avalanche, </li></ul><ul><li>A start zone from which the avalanche originates, </li></ul><ul><li>A slide path along which the avalanche flows, </li></ul><ul><li>A run out where the avalanche comes to rest, and </li></ul><ul><li>A debris deposit which is the accumulated mass of the avalanched snow once it has come to rest. </li></ul><ul><li>A failure layer that propagates the failure and </li></ul><ul><li>The bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same. </li></ul>
  8. 8. Classification <ul><li>The nature of the failure of the snow pack is used to </li></ul><ul><li>morphologically classify the avalanche. </li></ul><ul><li>Slab avalanches - are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab </li></ul><ul><li>Loose snow, are generated when a </li></ul><ul><li>stress causes a shear </li></ul><ul><li>Point release failure in a weak interface, </li></ul><ul><li>either within the </li></ul><ul><li>Isothermal avalanches snow pack, or at the base </li></ul>
  9. 9. Cont… <ul><li>When the failure occurs at the base they are known as full depth avalanches. </li></ul><ul><li>Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage. </li></ul>
  10. 10. SLAB AVALANCHES <ul><li>Crown fracture at the top of the start zone, </li></ul><ul><li>Flank fractures on the sides of the start zones, and </li></ul><ul><li>Shallow staunch fracture at the bottom of the start zone. </li></ul><ul><li>The crown and flank fractures are vertical walls in the snow </li></ul><ul><li>delineating the snow that was entrained in the avalanche </li></ul><ul><li>from the snow that remained on the slope. Slab avalanches </li></ul><ul><li>account for around 90% of avalanche-related fatalities, and </li></ul><ul><li>occur when there is a strong, cohesive layer of snow known </li></ul><ul><li>as a slab. These are usually formed when falling snow is </li></ul><ul><li>deposited by the wind on a lee slope, or when loose ground </li></ul><ul><li>snow is transported elsewhere. When there is a failure in a </li></ul><ul><li>weak layer, a fracture very rapidly propagates so that a </li></ul><ul><li>large area, that can be hundreds of meters in extent and </li></ul><ul><li>several meters thick, starts moving almost instantaneously. </li></ul>
  11. 11. 2.LOOSE SNOW AVALANCHES <ul><li>Occur in freshly fallen snow that has a lower density and </li></ul><ul><li>are most common on steeper terrain. </li></ul><ul><li>In fresh, loose snow the release is usually at a point and </li></ul><ul><li>the avalanche then gradually widens down the slope as </li></ul><ul><li>more snow is entrained, usually forming a teardrop </li></ul><ul><li>appearance. </li></ul>
  12. 12. ISOTHERMAL AVALANCHE- (wet snow avalanche), <ul><li>Occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes. </li></ul>
  13. 13. Powder snow avalanches <ul><li>Is a powder cloud that forms when an avalanche </li></ul><ul><li>accelerates over an abrupt change in slope, such as a cliff </li></ul><ul><li>band, causing the snow to mix with air. This turbulent </li></ul><ul><li>suspension of snow particles then flows as a gravity current </li></ul><ul><li>the largest and most powerful of among avalanches – </li></ul><ul><li>exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; </li></ul><ul><li>travels long distances along flat valley bottoms and even up hill for short distances. </li></ul>
  14. 14. <ul><li>The information provided on temperature, wind speed and </li></ul><ul><li>direction often enables useful predictions to be made before </li></ul><ul><li>leaving home.For instance, if a SW wind of 25mph is </li></ul><ul><li>indicated with freezing temperatures and snow known to be </li></ul><ul><li>lying, then it may be assumed that some avalanche hazard </li></ul><ul><li>will be building on NE - facing slopes </li></ul>
  15. 15. PREVENTION <ul><li>1. Explosives – Ski Resorts </li></ul><ul><li>2. Fences </li></ul><ul><li>3. Trees </li></ul><ul><li>4. Earth Mounds </li></ul><ul><li>5. Snow Nets </li></ul>
  16. 16. SAFETY BASICS <ul><li>Terrain Management - Practice safe route finding skills,Distinctions between geographic areas, elevations, slope aspects and slope angles are approximate and transition zones between dangers exist,be aware of changing conditions. </li></ul><ul><li>2. Carry avalanche rescue gear. </li></ul><ul><li>3. Group Management </li></ul><ul><li>4. Leadership </li></ul><ul><li>5. Risk Factor Awareness </li></ul>
  17. 17. AVALANCHE RESCUE <ul><li>Search and rescue equipment </li></ul><ul><li>Avalanche cords </li></ul><ul><li>Probes </li></ul><ul><li>Beacons -&quot;beepers&quot;/peeps/ avalanche transceivers </li></ul><ul><li>Shovels - digging snow pits as part of evaluating the snow pack </li></ul><ul><li>Avalung - consists of a mouth piece, a flap valve, an exhaust pipe, and an air collector </li></ul><ul><li>Other Equipments - Emergency Position-Indicating Radio Beacon (EPIRB) or Personal Locating Beacons (PLBs) containing the Global Positioning System (GPS). </li></ul>
  18. 18. <ul><li>Avalanches are formed by a combination of 3 things that together are known as the &quot; Avalanche Triangle&quot;. These 3 ingredients may be present in one location but absent 10 feet away. The 3 legs of the triangle are Snowpack, Terrain and Weather . </li></ul><ul><li>Avalanche victims are almost exclusively backcountry recreationists--snowmobilers, climbers, snowboarders, snowshoers, skiers and hikers. Snowmobilers lead the list with twice the number of fatalities as any other activity. </li></ul><ul><li>Eg. Switzerland's Alps, Utah's LaSals, Nepal's Himalayas </li></ul>