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Physical Causes Lesson 3 Avalanche

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Physical Causes Lesson 3   Avalanche Physical Causes Lesson 3 Avalanche Presentation Transcript

  • Avalanches as hazards
  • Where? • Avalanches are common in mountainous areas of the world, especially in Arctic and temperate regions • Frequency is variable according to local climatic and topographic conditions e.g. Alps, Scandinavian countries, US and Canadian Rockies • Their distribution is becoming more widespread as human activity (recreation and tourism) increases. Specialist infrastructure to support such tourism is increasing.
  • What? An avalanche is …. A rapid movement of snow down a slope as a result of structural weakness in the snow cover Three main forms: loose snow, slab, slush Slab avalanches are potentially the most dangerous as large masses of snow are involved Snow builds up in layers following each snowfall event As snow layers thicken the density in lower layers increases as air is squeezed out to form firn. In some snow packs the snow becomes less dense in places as ice and snow crystals grow and the voids between them are enlarged – this forms a weak layer in the snow Thawing and refreezing between different snowfalls
  • 3 types of avalanche hazard Powder snow avalanches • these can occur with little warning • At any time in the season • Speeds up to 300km / hr • Force up to 50 tonnes per m2
  • Wet snow avalanches • Usually occur later in the season • Movement rate 8 – 25km / hr • Carry a considerable weight of snow – up to 1 million tonnes.
  • Slab avalanches • The most commonly occurring type • Often started by human error • Speeds up 10 150km/hr • Avalanches: A fatal attraction
  • Why? - causes … • Weather - Large avalanches are common above the snow line in regions of heavy snowfall, rapid temperature changes which cause the snow to melt or where rain falls on to the snow. • Snow layers - Air temperature changes can bring about partial melting of the snow pack or changes in snow crystal shapes and sizes. Changes in the snow pack. There is a balance between the tensional forces keeping snow on the slope and the forces trying to move it. • Slope – more than 30o and less than 45o for an avalanche to occur the slope must be steep enough to allow the snow to slide Avalanches tend to recur at the same sites and can be detected by breaks of slope, eroded sections, damaged vegetation and debris mounds in the run out zone. Spring is the season when most avalanches are likely to occur
  • Velocity of avalanche depends upon: • Slope angle • Density of the snow mass • The shearing resistance at the base of the side • Total length of the downward path
  • Avalanches as hazards • The impact of an avalanche is significantly increased in areas with high population density in vulnerable locations. Avalanches can represent a major hazard • As settlements are expanding in to mountainous regions and tourist development increases, people are becoming vulnerable • Communications, powerlines and transport routes are at risk • Death and injury from avalanches are the results of impacts which fracture limbs, suffocation, hypothermia, exhaustion, frostbite and shock • Buildings and structures are at risk from impact pressures of up to 100 tonnes / m2
  • Modifying the event Most management techniques aim to retain the snow on the mountainside or to deflect the avalanche. Techniques are expensive and can be unsightly Measures taken to control avalanches include: Artificially triggering avalanches using explosive charges or shelling at safe times and before the snow accumulates to dangerous levels (especially prior to the start of the tourist season). This is expensive Planting trees along known avalanche paths (deforestation has increased the incidence of avalanches in some places e.g. Austria in the last 200 years)
  • Modifying vulnerability • Warning systems These use forecasts (for day to day management) and prediction (hazard land zoning) Forecasts rely on snow stability tests and meteorological data 2. Land use planning / Avalanche zoning – This identifies three zones – high potential, moderate and no hazard. Zones are based on: – topographic maps – Field observations – Long term records – Run out distance Method is expensive and requires time investment In France there is no construction in ‘red’ zones – high hazard ‘blue’ zones (moderate hazard) mean special building – construction codes, such as reinforced concrete and now windows on up-slope sides Use of warnings to restrict access to vulnerable slopes Educating skiers in risk assessment and likely occurrence of events
  • Case study - Montroc http://www.edu4hazards.org/avalanche.html Twelve die in Austrian avalanches Galtur 1999 http://www.bbc.co.uk/science/horizon/1999/avalanche_scrip Holiday hell for British skiers World: Europe Ski resort hit again Dec 1999Twelve die in Austrian avalanches
  • Plenary • What is the difference between forecasting and prediction as applied to avalanche management? • How is new technology helping in the management of avalanches?
  • http://www.edu4hazards.org/avalanche.html • http://en.wikipedia.org/wiki/Avalanche • http://www.avalanche.org/ • http://www.sais.gov.uk/ • http://library.thinkquest.org/TQ0313197/picture • http://green.nationalgeographic.com/environm