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!
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.
A spectrum of hazards (Smith 1992)
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.
FATALITIES IN EUROPE AND NORTH
AMERICA
200
183
180
164
160
133
90
100
69
80
21
28
15
20
12
97
91
17
19
26
43
38
39
49
36
21
0
Year
1997-98
1996-97
1995-96
1994-95
1993-94
1992-93
1991-92
1990-91
1989-90
1988-89
1987-88
0
1986-87
North America
Europe
73
60
40
120 115
106
105
120
1985-86
Fatalities
140
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.
Chamonix &
Galtur, 18mins
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.

Reconstruction of Galtur,
1.41 mins
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

Physical structures used to prevent avalanches (Smith 1992)
Responses at Galtur,
1.50 mins