uwo geo 2152 - snow avalanches

• Snow Avalanche


  A mass of snow many cubic metres in volume that separates from a snowpack and moves downslope
• Rocks, soil, ice, and debris can travel in a similar motion; however, the term avalanche is generally reserved for snow
• Intensity of an avalanche

  • Dependent on slope steepness, snowpack stability, and weather
• types of avalanches

  • An avalanche travelling as a coherent block
  • An avalanche that becomes wider as it travels downslope - less severe
• 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

  The amount of snowfall in an area depends on its latitude, altitude, and proximity to bodies of water
• Temperature decreases with altitude therefore high mountains have permanent snow cover
• BC and Upper Ontario and Quebec
• Snow Cover

  The probability of a White Christmas considers the probability of snow cover on December 25
• Point-Release Avalanche


  • It begins as an initial failure in the snowpack after a heavy snowfall
  • The sliding snow causes more failures in the adjacent snowpack causing the trough to widen
  • Smaller to wider as it moves - less severe
• Slab Avalanche


  • It occurs when a snowpack fractures along a weak layer parallel to the surface
  • These avalanches move as a cohesive block leaving behind a scarp - cliff
  • They are the most dangerous avalanches
• Types of snow


  • Sluff - sliding loose snow
  • Slab - large sections crack off and crash down
• • Over 60 degrees too steep
  • Under 25 degrees
• Danger Zone is between 30-45 degrees - angles in which people like to ski
• 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 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 (ice crystals - wind takes it and blows it onto one side of the mountain)
• Wind


  1. Blowing snow can accumulate on the lee slope of mountains

  2. Wind can deposit a layer of light crystals on a layer of more compacted snow

  3. The boundary between the two layers could become a horizon along which failure could occur
• Hoar


  • Layers composed of hoar have less strength than the rest of the snowpack
  • Hoar can form deep in the snowpack (in air pockets) or on the snow surface
  • Hoar changes little over time; therefore, overlying snow can leave the buried hoar as a weak layer
• Avalanche Motion


  • Rapidly moving avalanches (ie speeds of over 35 km/h) often generate clouds of powdered snow
  • The fastest avalanches have been measured at speeds near 200km/hr
  • Some avalanches are powerful enough to climb opposing slopes
• Avalanche Triggers


  • Most avalanches occur soon after snowstorms
  • Some may occur when the daytime heating from the Sun warns the upper part of the snowpack
  • Avalanches that cause injuries or fatalities are often triggered by people
  • Some avalanches are triggered intentionally with explosives
• Avalanche Paths


  • Start zone - The area where the snowpack first fails - where failure tends to occur
  • Track - The area where the avalanche accelerates and reaches maximum velocity
  • Run-out Zone - The area of deceleration and snow deposition
• Terrain Factors


  • The slope angle is the most important terrain factor that influences avalanche formation
  • Most avalanches occur at slope angles between 25 degrees and 60 degrees
  • At angles below 25 degrees, snow does not easily slide
  • At angles above 60 degrees, little snow accumulates on the slope
  • The orientation (the direction that a slope faces) of the slope can also be a factor - south and east slope are most prone - southeast - sun is south and ice crystals is east
  • Snow deposited on leeward slopes can consist of interlayered strong and weak layers
  • Slopes facing the Sun are more prone to daytime avalanches during clear weather
  • Other factors include the smoothness of the slope, the amount of vegetation, and the topography of the slope itself
• Regions at Risk


  • For an avalanche to form, a snowpack of at least 50 cm is typically required
  • Mountain areas and coast areas - BC, Yukon and Alberta
  • In North America, deep snowpacks are most common in the Rocky Mountains
• In Canadian history, most avalanche deaths occurred in the late 1800s and early 1900s
• Gold Rush and the creation of the railroad through Alberta and BC
• In total, over 600 people have died from avalanches in Canada
• Avalanches cause millions of dollars in economic losses in BC each year due to closed highways
• Damage to forests is evident each year but property damage is relatively minor
• Chilkoot Avalanche


  • This disaster occurred in 1898 and remains one of the worst avalanches in North American history
  • An avalanche spread over the Chilkoot Trail causing 60 deaths
  • The trail was heavily used at that time during the Klondike Gold Rush
  • The Chilkoot Trail extends from Alaska to BC and is the easiest route through the mountains
• 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 snowpack
• Natural Service Functions


  • Like landslides, avalanches act as an ecological disturbance
  • This may increase local plant and animal diversity
  • Avalanches maintain open areas otherwise forested regions
  • This can serve as an important habitat zone for certain plants and animals
• Human Interactions with Avalanches

  • Avalanches only become a hazard when humans encroach on areas that are prone to them
  • As tourism and recreation have increased in the Rockies and the Alps, deaths from avalanches have increased
• Minimizing Avalanche Risk


  1. Risk is greatly reduced when buildings, roads, and other infrastructure are located away from known avalanche paths

  2. Hazard maps provide planners with locations of the highest risk areas

  3. Buildings in hazardous areas within a specific recurrence interval may require special engineering - This may include reinforced walls or deflection structures

  4. In avalanche start zones, fences or nets can be installed to stabilize the snowpack

  5. Mounds and berms can be used to slow and deflect avalanches away from populated areas

  6. Splitting wedges on the sides of the buildings can force an avalanche around the structure

  7. Avalanche sheds allow avalanches to travel over roads or railways without disruption to traffic

  8. Controlled triggers are used to force avalanches to occur to prevent build-up of the snowpack - This is performed using explosives
• Avalanche Forecasting


  • Locations of past avalanches
  • Strength and stability tests
  • Snowpack observations
  • Weather
• Strength and Stability Tests

  • Compression Test - A vertical force placed on top of the snowpack to detect weak layers
  • Shovel Test - A column of snow is isolated and then a force is applied on the uphill side
  • Rutschblock Test - A skier pushes and jumps on a column of snow to determine the cohesion of a snowpack
• Avalanche Safety


  • Before travelling in an avalanche prone area, it is important to check the current danger level as well as any public bulletins
  • Knowledge of slope angles and the terrain is also necessary
• Avaluator


  • The Canadian Avalanche Centre has the developed the Avaluator
  • It is a chart designed to warn travellers of the risk of an avalanche in an area
• Avalanche Rescue


  • 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 due of a combination of suffocation and hypothermia
  • Less than 10% of victims survive in more than 1.5 m in snow
  • The best chance of survival depends on an effective immediate 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 survival aids
• Avalanche Survival Aids


  • Avalanche Cord - A 10 m rope that drags behind the person while skiing, snowboarding, or snowshoeing
  • Avalanche Transceiver - A portable device that emits of radio signal to assist in finding the location of a victim
  • Avalanche Dogs - They can quickly detect human scent rising through the snow and can quickly cover large areas