Snow cover on a slope tends to slide down the slope because of gravity. Conditions affecting stability include the gravitational force, component of the snow and resisting forces such as the frictional resistance of the slope or the anchoring effect of shrubs. In general, avalanches are caused when this balance is lost and when the forces exceed the resistance. Avalanches are rarely observed closely since they normally occur during a short time period of one or two minutes. Major causes of avalanches can be classified into fixed (prime factors) and variablefactors (exciting factors) such as weather conditions and the weight of the snow cover. Avalanches occur when these factors are combined. The types and scale of avalanches can differ depending on the combination of these various factors and their scale. Major prime factors and exciting factors are shown in the following table.
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Types |
Characteristics |
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Dry, Loose Surface
–Layer, Snow Avalanche |
These often occur in
low atmospheric temperature or during snowfalls. This type of avalanche is
caused mainly by small masses of snow falling from snow cornices, tree
branches or exposed rock. Dry snow moves down in loose layer. |
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Dry, Slab Surface
–Layer, Snow Avalanche |
These often occur when
new snow with a depth of more than 10cm falls over existing snow cover during
low atmospheric temperatures. Avalanches flow rapidly taking form of loose
snow powder and often reach several kilometres down the foot of the mountain causing
serious disasters. |
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Dry, Slab full-Depth
Snow Avalanche |
Avalanche occurring in
areas with low temperatures can have different mechanisms. In regions of
relatively high temperatures, this type of avalanche occurs extensively when
the weight of large quantity of snow falls quickly over existing snow
deposits on slope at low temperatures. In cold regions, snow layers near the
ground tend to become collapsible and can slide in a full depth if severe
cold weather has continued for a long time. The dry new surface snow layers
tend to slide in the form of snow powder and often reach further down the
foot of the mountain. |
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Wet, Loose Surface
Layer, Snow Avalanche |
These can be caused when
a 20 to 30 cm layer of new snow layer starts to move and takes a wedge-shaped
from and reduces in width. This avalanche flows smoothly as it advances. |
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Wet, Slab Surface
Layer Snow Avalanche |
These can occur when
temperature rises in fair weather after a snowfall when the slab snow surface
layer contains water. Avalanches do not take from of snow powder but move in
a smooth flow. |
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Wet, Slab Full-depth
Snow Avalanche |
These can be caused when
snow starts to melt in the early spring season and can also result if
temperatures rise the winter season. It can occur either on a rainy day or on
a warm day. These will not take from of snow powder and move in a smooth
flow. This type of avalanche often causes serious disasters |
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Characteristic Features of
Avalanche Movement |
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Type |
Characteristics |
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Flow Avalanche |
This
type avalanche often reaches a depth of several tens of meters, taking from
of snow powder moving at a high speed. These most often occur during
snowfalls at low temperature |
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Mixed Avalanche |
This
type of avalanche appears to move as a flow of water over the snow surface.
These are seen as full- depth avalanche occurring when atmospheric
temperature increases. Powder
type and flow avalanches occurring in combination. This type avalanche can
occur quickly when large quantity to snow falls over unstable snow cover |
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Major Causes of Snow
Avalanches |
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Causes & Factors |
Characteristics |
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Fixe/PrimeFactors |
Topographic factors |
v Inclination of slope v Shape of slope v Location (ridge line or toe of
slope) v Orientation of slope |
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Vegetation factors |
v Vegetation cover and height of
trees v Vegetation cover and its thickness |
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Variable/Exciting Factors |
Weather factors |
v Depth of snow cover v Depth of snowfall v Wind velocity v Atmospheric and snow temperatures |
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Other factors |
v Increase in weight of snow cover
because of snow dropping from cornices or snow covers v Vibrations such as earthquake or
the sound of gunfire |
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Factors Used for Classification
of Avalanches |
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Type |
Characteristics |
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Loose snow avalanche |
Avalanches that flow
rapidly, spreading widely from a point normally small in scale |
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Slab avalanche |
Avalanches that start to
move suddenly over wide areas, normally large in scale. |
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Dry snow avalanche |
Avalanches that contain
no water. |
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Wet snow avalanche |
Avalanches that contain
water. |
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Surface layer
avalanche |
Slip surface exists
within the snow cover |
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Full- depth avalanche |
Slip surface occurs on
the ground surface |
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Typical examples of damage to roads caused by avalanches(The scale of damage can differ depending on the scale and
type of avalanche). v Traffic blocked by snow deposited
on road surface. v Roads damaged by avalanches. v Road structures, such as retaining
walls, overturned. v Structures damaged by an avalanche
during construction of roads occur most frequently. |
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Estimation of Hazardous Slopes: The following actions are
appropriate when avalanche hazard prone slopes are identified: v Advice residents of avalanche risk
areas using published maps. v Forestation programmes for areas
where there is risk of avalanches. v Trap avalanches by control
measures. v Dispose avalanche potential snow
packs by artificial triggering. v Predict occurrence of avalanches
through stability analysis and issue warnings as and when necessary. v Guide residents to emergency
evacuation shelters. |
1. Avalanch Control Measures
Avalanche control measures can roughly be classified into hardware and software types. Hardware measures are for the purpose of preventing avalanches or for blocking or deflecting avalanches with protective structures. Software measures provide safety by eliminating the probability of avalanches by removing snow deposits on slope with blasting and by predicting the occurrence of avalanches and recommending evacuation from hazardous areas.
