THE AVALANCHE HAZARD

transcript

2 news clips from 1999, 7.17 mins

Avalanche

A Definition:A Definition: A mass movement of snow and ice(and/or A mass movement of snow and ice(and/or

regolith) down a slope due to structural regolith) down a slope due to structural weakness in the snow cover on that slope, weakness in the snow cover on that slope, under the pull of gravity.under the pull of gravity.

Avalanches!

Avalanches are common phenomena Avalanches are common phenomena throughout mountain areas.throughout mountain areas.

People are increasingly active in mountain People are increasingly active in mountain areas : recreation and leisure tourism; areas : recreation and leisure tourism; infrastructure to support tourism; pressure infrastructure to support tourism; pressure on more easily inhabited land.on more easily inhabited land.

Therefore increased RISK of interaction, Therefore increased RISK of interaction, and increased hazard.and increased hazard.

What are the main characteristics of avalanches?

Distribution over space.Distribution over space. Location : same places each time therefore Location : same places each time therefore

relatively predictable.relatively predictable. Frequency: changing through time?Frequency: changing through time? Speed of onset/warning time : little Speed of onset/warning time : little

warning.warning. Relative to other hazards?Relative to other hazards?

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

A spectrum of hazards (Smith 1992)

Impacts

Main Main direct direct impacts are deaths, impacts are deaths, injuries, and economic losses to injuries, and economic losses to communities affected.communities affected.

IndirectIndirect impacts include wider impacts include wider economic losses to tourism industry, economic losses to tourism industry, transport delays, and wider spatial transport delays, and wider spatial economic losses.economic losses.

FATALITIES IN EUROPE AND NORTH AMERICA

21 2815 12 17 19

43 39 3649

120 115

North America

Europe

AVALANCHE TYPES

Powder snow avalanchesPowder snow avalanches

Wet snow avalanchesWet snow avalanches

Slab avalanchesSlab avalanches

Powder snow avalanches No warning.No warning. Up to 200mphUp to 200mph Up to 50 tonnes/m Up to 50 tonnes/m 22

forceforce Occur at any time Occur at any time

in season.in season. Example :Galtur, Example :Galtur,

Austria.Austria.

Wet snow avalanches

Usually late in season.Usually late in season. Slow moving (5-15mph).Slow moving (5-15mph). Considerable weight of wet snow Considerable weight of wet snow

(up a million tonnes).(up a million tonnes). Example : Odda, Norway.Example : Odda, Norway.

Slab avalanches Slab avalanches are most common Slab avalanches are most common

occurrence.occurrence. Often started by human error.Often started by human error. Most frequent cause of death Most frequent cause of death

amongst skiers, snowboarders, and amongst skiers, snowboarders, and power-skiers.power-skiers.

Speeds up to 100mph.Speeds up to 100mph.

Origins of avalanches

WeatherWeather : snowfall is essential : snowfall is essential ingredient.ingredient.

Slope Slope : more than 30: more than 30oo and less than and less than 4545oo for starting an avalanche. for starting an avalanche.

Changes in the snow-pack.Changes in the snow-pack.

Contributing factors

Heavy rainfall on snow (more likely in Heavy rainfall on snow (more likely in Scotland than the Alps!)Scotland than the Alps!)

Deforestation - reducing slope stabilityDeforestation - reducing slope stability Vibrations - skiers,more dangerous earth Vibrations - skiers,more dangerous earth

movementsmovements Long cold winters then heavy snow falls in Long cold winters then heavy snow falls in

spring. i.e. slip plane created.spring. i.e. slip plane created.

Changes in the snow-pack

Temperature changes in the air can Temperature changes in the air can bring about :bring about :

PartialPartial melting melting in the snow-pack. in the snow-pack. FreezingFreezing of melt-water in snow. of melt-water in snow. ChangesChanges in snow crystal in snow crystal shapes and shapes and

sizes.sizes. ChangesChanges in the in the strengthstrength of the snow of the snow

layers.layers.

Chamonix & Galtur, 18mins

Snow-pack stress Snow lying on a slope is subject to stress : Snow lying on a slope is subject to stress : gravitygravity is pulling it downwards is pulling it downwards adhesion and weightadhesion and weight is keeping it “stuck” is keeping it “stuck”

to the slope.to the slope. Often these stresses are delicately balanced, Often these stresses are delicately balanced,

and a slight change can bring about failure, and a slight change can bring about failure, particularly where the snow-pack is particularly where the snow-pack is under under tension.tension.

Reconstruction of Galtur, 1.41 mins

What was important? Speed 200mph : No warning timeSpeed 200mph : No warning time Weight : three quarters of a million tonnes.Weight : three quarters of a million tonnes. Force : 50 tonnes per square metre, enough Force : 50 tonnes per square metre, enough

to demolish a house.to demolish a house. Over 30 people killed, in village, not on ski Over 30 people killed, in village, not on ski

slopes.slopes. Why was village constructed in avalanche Why was village constructed in avalanche

area?area? What can be done to prevent avalanches What can be done to prevent avalanches

like this?like this?

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

Physical structures used to prevent avalanches (Smith 1992)

Responses at Galtur, 1.50 mins

THE AVALANCHE HAZARD

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