Geography 2152F/G Lecture 10: Geo 2152 Week 10-Lecture Snow Avalanche
14 Feb 2019
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Geo 2152 – Week 11 Lecture
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Snow Avalanche
• Definition: A mass of snow many cubic meters in volume that separates from a snowpack and flows downslope
o Rocks, soil, ice, and debris can travel in a similar motion; however the term avalanche is generally reserved
for snow.
o The intensity of the hazards is dependent on the snow steepness, snowpack stability and weather
• There are two types:
o An avalanche travelling as a coherent block
o An avalanche that becomes wider as it travels downslope
• It is estimated that over 99% of avalanches are not seen by anyone
• It is likely that over 1 million avalanches large enough to kill a person occur annually in western Canada alone.
Snow Climatology
• 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 at angles of less than 60o
Snow Cover
The probability of a white Christmas takes into account average snow cover. They are updated daily.
Geo 2152 – Week 11 Lecture
2
Types of Avalanches
• Point-release Avalanches
• These begin as an initial failure after a heavy snowfall.
• The sliding snow then causes more failures in adjacent snowpack causing the trough to
widen.
• Slab Avalanches
• These occur when a snowpack fractures along a
weak layer parallel to the surface.
• These avalanches move as a cohesive blocks leaving
behind a scarp.
• They are the most dangerous avalanches.
Avalanche Potential
• New snow that has not been able to bond to the layer below is susceptible to sliding.
• Wet, compacted snow is less likely to slide than dry, powdery snow.
• A mass of snow that is above the vegetation level and above the large boulders is more likely to slide.
Weak Layers
• Slab avalanches require a buried weak layer. Such a layer can form from wind or from hoar.
• Wind
o Blowing snow can accumulate on the lee slope of mountains.
o Wind can deposit a layer of light ice 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 composed of hoar have less strength than the rest of the snowpack.
o Hoar can form deep in the snowpack (in air pockets) or on the snow surface.
o Hoar changes little over time; therefore overlying snow can leave the buried hoar as a weak layer.
Avalanche Motion
• 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.
Avalanche Triggers
• 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 to control them
Avalanche Paths
• Start Zone: The area where the snowpack first fails.
• Track: The area along which the avalanche accelerates and reaches maximum velocity
• Run-out Zone: The area of deceleration and snow deposition.
Document Summary
Snow avalanche: the intensity of the hazards is dependent on the snow steepness, snowpack stability and weather. There are two types: an avalanche travelling as a coherent block, an avalanche that becomes wider as it travels downslope. It is estimated that over 99% of avalanches are not seen by anyone. It is likely that over 1 million avalanches large enough to kill a person occur annually in western canada alone. 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 at angles of less than 60o. The probability of a white christmas takes into account average snow cover. These begin as an initial failure after a heavy snowfall. The sliding snow then causes more failures in adjacent snowpack causing the trough to widen. These occur when a snowpack fractures along a weak layer parallel to the surface.
