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THE AVALANCHE HAZARD

THE AVALANCHE HAZARD. 2 news clips from 1999, 7.17 mins. Avalanche. A Definition: A mass movement of snow and ice(and/or regolith) down a slope due to structural weakness in the snow cover on that slope, under the pull of gravity. Avalanches!.

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THE AVALANCHE HAZARD

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  1. THE AVALANCHE HAZARD

  2. 2 news clips from 1999, 7.17 mins

  3. Avalanche • A Definition: • A mass movement of snow and ice(and/or regolith) down a slope due to structural weakness in the snow cover on that slope, under the pull of gravity.

  4. Avalanches! • Avalanches are common phenomena throughout mountain areas. • People are increasingly active in mountain areas : recreation and leisure tourism; infrastructure to support tourism; pressure on more easily inhabited land. • Therefore increased RISK of interaction, and increased hazard.

  5. What are the main characteristics of avalanches? • Distribution over space. • Location : same places each time therefore relatively predictable. • Frequency: changing through time? • Speed of onset/warning time : little warning. • Relative to other hazards?

  6. AVALANCHE FREQUENCY and DISTRIBUTION • 1 million per year.

  7. A spectrum of hazards (Smith 1992)

  8. Impacts • Main direct impacts are deaths, injuries, and economic losses to communities affected. • Indirect impacts include wider economic losses to tourism industry, transport delays, and wider spatial economic losses.

  9. FATALITIES IN EUROPE AND NORTH AMERICA

  10. AVALANCHE TYPES • Powder snow avalanches • Wet snow avalanches • Slab avalanches

  11. Powder snow avalanches • No warning. • Up to 200mph • Up to 50 tonnes/m 2 force • Occur at any time in season. • Example :Galtur, Austria.

  12. Wet snow avalanches • Usually late in season. • Slow moving (5-15mph). • Considerable weight of wet snow (up a million tonnes). • Example : Odda, Norway.

  13. Slab avalanches • Slab avalanches are most common occurrence. • Often started by human error. • Most frequent cause of death amongst skiers, snowboarders, and power-skiers. • Speeds up to 100mph.

  14. Origins of avalanches • Weather : snowfall is essential ingredient. • Slope : more than 30o and less than 45o for starting an avalanche. • Changes in the snow-pack.

  15. Contributing factors • Heavy rainfall on snow (more likely in Scotland than the Alps!) • Deforestation - reducing slope stability • Vibrations - skiers,more dangerous earth movements • Long cold winters then heavy snow falls in spring. i.e. slip plane created.

  16. Changes in the snow-pack • Temperature changes in the air can bring about : • Partial melting in the snow-pack. • Freezing of melt-water in snow. • Changes in snow crystal shapes and sizes. • Changes in the strength of the snow layers.

  17. Chamonix & Galtur, 18mins

  18. Snow-pack stress • Snow lying on a slope is subject to stress : • gravity is pulling it downwards • adhesion and weight is keeping it “stuck” to the slope. • Often these stresses are delicately balanced, and a slight change can bring about failure, particularly where the snow-pack is under tension.

  19. Reconstruction of Galtur, 1.41 mins

  20. What was important? • Speed 200mph : No warning time • Weight : three quarters of a million tonnes. • Force : 50 tonnes per square metre, enough to demolish a house. • Over 30 people killed, in village, not on ski slopes. • Why was village constructed in avalanche area? • What can be done to prevent avalanches like this?

  21. What can be done? • Stop avalanches before they start. • Slow them down. • Build only in avalanche free areas. • Protect buildings. • Protect roads, railways, power lines etc. • Artificial avalanche control. • Warnings

  22. Physical structures used to prevent avalanches (Smith 1992)

  23. Responses at Galtur, 1.50 mins

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