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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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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! • 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.
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?
AVALANCHE FREQUENCY and DISTRIBUTION • 1 million per year.
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.
AVALANCHE TYPES • Powder snow avalanches • Wet snow avalanches • Slab avalanches
Powder snow avalanches • No warning. • Up to 200mph • Up to 50 tonnes/m 2 force • Occur at any time in season. • Example :Galtur, Austria.
Wet snow avalanches • Usually late in season. • Slow moving (5-15mph). • Considerable weight of wet snow (up a million tonnes). • Example : Odda, Norway.
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.
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.
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.
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.
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.
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?
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