Le cture 11
• Definition: A mass of snow many cubic metres in volume that separates from a snowpack and flows
• Rocks, soil, ice, and debris can travel in a similar motion; however the term avalanche is generally
reserved for snow.
• The intensity of the hazard is dependent on steepness, snow stability and weather
• There are two types:
o An avalanche travelling as a coherent block. Bigger avalanche
o An avalanche that becomes wider and bigger as it travels downslope (point release avalanche).
• It is estimated that over 99% of avalanches are not noticed by anyone
• There are likely over 1.5 million avalanches (large enough to kill a person) annually in western Canada
• Snowfall accumulation depends on latitude, altitude, and proximity to bodies of water.
• Temperature decreases with altitude therefore high mountains have permanent snow cover.
• Snow accumulates on mountain slopes that are angles of less than 60%
• The probability of a White Christmas takes into account average snow cover
• Snow cover maps are updated daily
Types of Avalanches
• These begin as an initial failure after a heavy snowfall.
• The sliding snow then cause more failures in the adjacent snowpack causing the trough to widen.
• Smaller of the two types of avalanche
• No weak layers
• They occur when a snowpack fractures along a weak layer parallel to the surface
• These avalanches move as cohesive blocks leaving behind a scarp (cliff of hardened snow)
• Slab is breaking off on top of a weak layer of snow (dry, fine snow with cold temps when it fell)
• These are the most dangerous avalanches.
• New snow that has not been able to bond to the layer below is susceptible to sliding.
• Wet, compacted snow is less likely to move than dry, powdery snow.
• A mass of snow that is above the vegetation level and above large boulders is more likely to slide
• Slab avalanches require a buried weak layer. Such a layer can form from wind or from hoar. (ice
o Blowing snow can accumulate on the lee slope (downwind) of mountains.
o Wind can deposit a layer of light snow crystals on a layer of more compacted snow
o The boundary between the two layers could become a horizon along which failure could occur • Hoar
o Layers of hoar have less strength than the rest of the snowpack.
o Hoar can form deep in the snowpack (in air packets) or on the surface
o Hoar changes little over time; therefore overlying snow can leave the buried hoar as a weak
• Rapidly moving avalanches (i.e. speeds of over 35 km/h) often generate clouds of powdered snow.
• Fastest avalanches have been measured at speeds of 200 km/h
• Some avalanches are powerful enough to climb opposing slopes
• Most avalanches occur soon after snowstorms.
• Some may occur when daytime heating from the Sun warms the upper
part of the snowpack.
• Avalanches that cause injuries or fatalities are often triggered by people
• Some avalanches are triggered intentionally with explosives
• Start Zone: The area where the snowpack first fails.
• Track: The area along which the avalanche accelerates and reaches
• Runout Zone: The area of deceleration and snow deposition
• The slope angle is the most important terrain factor for avalanche formation.
• Most avalanches occur at slope angles between 25 and 60.
• At angles below 25°, snow does not easily slide.
• At angles above 60, little snow accumulates on the slope
• The orientation of the slope can also be a factor
• Deposits of snow on leeward slopes can consist of interlayered strong and weak layers.
• Slopes facing the Sun are more prone to daytime avalanches during clear weather. (south side more
prone during sunny days)
• Other factors include the smoothness of the slope, the amount of vegetation, and the topography of the
Regions at risk
• For an avalanche to form, a snowpack of at least 50 cm is typically required.
• Rockies + coast mountains
• In North America, deep snowpacks are most common in the rockies
Effects of Avalanches
• In Canadian history, most avalanche deaths occurred in the late 1800s and early 1900s.
• In total, over 600 people have died from avalanches in Canada
• Avalanches cause millions of dollars in economic losses in B.C. each year due to closed highways
• Damage to forests is evident each year but property damage is relatively minor
• This disaster occurred in 1898 and remains one of the worst avalanches in North American history.
• 60 people were killed as the avalanche spread over the Chilkoot Trail • The trail was heavily used at that time by people heading to Yukon during the Klondike Gold Rush.
• The Chilkoot Trail extends from Alaska to B.C. And is the easiest route through the mountains
Linkages to Other Natural Hazards
• Avalanches can be caused by earthquakes.
• Climate change may increase winter snowfall in some areas and increase severity of winter storms.
• Some areas will experience more thaws in winter enhancing the instability of the slope
Natural Service Functions
• Similar to landslides, avalanches act as an ecological disturbance.
• This may increase local plant and animal diversity.
• Avalanches maintain open areas in otherwise forested regions
• This can serve as an important habitat zone for certain plants and animals
Human Interaction with Avalanches
• Avalanches only become a hazard when
humans encroach on areas that are prone
• As tourism and recreation have increased
in the Rockies and the Alps, deaths from
avalanches have increased.
Minimizing Avalanche Risk
• Risk is greatly reduced when buildings, roads, and
other infrastructure are located outside
• Hazard maps provide planners with the locations of these areas.
• Splitting wedges on the sides of buildings can force an
avalanche around the structure. (individual building)
• Mounds and berms can be used to slow and deflect avalanches away from populated areas (entire
• Avalanche sheds allow avalanches to travel over roads or railways without disruption to traffic.
• Controlled trigger are used to force avalanches to occur in order to prevent build up of the snowpack
• This is performed through the use of explosives.
• Forecasting is based on:
o Occurrences of avalanches in the past
o Strength and stability test
o Snowpack observations
Strength and Stability Tests
• There are 3 major tests used to assess a snowpack.
• Compression Test: A vertical force is placed on the top of the snowpack to detect weak layers.
• Shovel Test: It assesses the strength by isolating a column of snow and applying force on the uphill
• Rutschblock Test: A skier pushes and jumps on a column of snow to detect cohesion of the snowpack
Avalanche Safety • Before travelling in avalanche prone areas, it is important to check for any public bulletins as well as
the current danger level.
• Knowledge of slope angles and the terrain is also necessary
• The Canadian Avalanche Centre has developed the Avaluator.
• This is a chart designed to warn travellers o the risk of avalanche in an area
• The motion of the snow itself kills about 25% of avalanche victims.
• Survival depends on the length of time the person is buried and the burial depth
• Over 90% survive if rescued within 15 minutes, 30% within 35 minutes, and 0% within 2 hours.
• Buried victims die of a combination of suffocation and hypothermia
• Less than 10% of victims survive burial in more than 1.5 m in snow.
• The best chance of survival depends on effective search by other members of the group, rather than
waiting for help.
• Chances of finding a buried victim increase when everyone in the group carries standard avalanche
Avalanche Survival Aids
• Avalanche Cord: A 10 m rope that drags behind a person while skiing, snowboarding or snowshoeing
(every meter marked with yellow strip to help determine how deeply buried the person is)
• Avalanche Transceiver: A portable device that emits a radio signal to assist in finding the location of
• Avalanche Dogs: They can detect human scent rising through the snow and can quickly cover large
• Of all hazards, large scale diseases affect human populations most directly.
• These hazards are different than others since they only impact people rather than property.
• Comunicable diseases are the leading cause of mortality in less developed countries
• Outbreak: A simultaneous, related occurrence of several cases
• Epidemic: An uncontrolled outbreak of communicable (contagious) disease.
• Pandemic: International or widetravelling simultaneous epidemics of the same condition
• Epidemiology: The study of distribution and determinants of health related events in the human
• Agent: The actual cause of the disease (ie. Virus, bacteria, toxin, fungus, etc)
• Host: The infected individual Disease in Less Developed Countries
• They are the direct result of poor hygiene and are indirectly related to socioeconomic conditions.
• Spread of disease is enhanced by the limited preventative programs
• The water system is a common source for disease agents. Over 60% of people in less developed
countries have limited access to clean water.
Spread of Disease
What conditions will lead to a rapid spread of disea