Week 10 Flashcards
what is a 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… BUT, the term avalanche is generally reserved for snow
what determines the intensity of an avalanche?
slope steepness
snowpack
stability
weather
describe the prevalence of avalanches
estimated that over 99% of avalanches aren’t seen by anyone
it is likely that over 1 million avalanches large enough to kill a person occur annually in Western Canada alone
what factors affect the amount of snowfall in an area?
latitude
altitude
proximity to bodies of water
what are the types of avalanches?
point release
—- begins as an initial failure in the snowpack after a heavy snowfall
—- the sliding snow then causes more failure in the adjacent snowpacks causing the trough to widen
slab
—- when a snowpack fractures along a weak layer parallel to the surface
—- these avalanches move as cohesive blocks leaving scarp
—- they are the most dangerous avalanches
describe avalanche potential
new snow that not been able to bond to the layer below – susceptible sliding ????
wet, compacted snow is less likely to slide than powder snow
a mass of snow that’s above the vegetation level and above large boulders is more likely to slide
what is a weak layer? how is a weak layer obtained?
slab avalanches require a buried weak layer. such a layer can form from wind or from hoar
wind
—- blowing snow can accumulate on the leeward (downward slope but sheltered from wind) slope of mountains
—- wind can deposit a layer of light ice crystals on a layer of more compacted snow
—- the boundary between the 2 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
describe the motion of avalanches. what are the fastest speeds measured?
rapidly moving avalanches (speeds of over 35 km/h) often generate clouds of powdered snow
some avalanches are powerful enough to climb opposing slopes
the fastest avalanches have been measured at speeds near 200 km/h
what triggers avalanches?
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
describe the general path of an avalanche
start zone
—- the area where the snowpack first fails
track
—- the area where the avalanche accelerates and reaches max velocity
run-out zone
—- the area of deceleration and snow deposition
what are some terrain factors that influence avalanche formation?
slope angles
—- most avalanches occur at slope angles between 25 and 60 degrees
—- at angles below 25 degrees, snow doesn’t slide easily
—- at angles above 60 degrees, little snow accumulates on the slope
orientation
—- this is the direction that the slope faces
—- snow deposited on leeward (downward slope sheltered from the wind) 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 amt of vegetation, and the topography of the slope itself
regarding snowpacks, why is North America more prone to avalanches?
deep snowpacks are most common in the Rocky Mountains of North American
note that, for an avalanche to form, a snowpack of at least 50cm is typically required
describe the effects of avalanches in regards to death tolls for Canada
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
describe the effects of avalanches economically
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
describe the Chilkoot Avalanche
occurred in 1898 and remains one of the worst avalanches in North American history
it spread over the Chilkoot Trail, causing 60 deaths
the trail was heavily used during the Klondike Gold Rush
the chilkoot trail extends from Alaska to BC and is the easiest route thru the mountains
how are avalanches linked to other natural hazards?
avalanches can be caused by earthquakes
climate change may increase winter snowfall in some areas and increase severity of winter stormes
some areas will experience more thaws in winter, enhancing the instability of the snowpack
what are the natural service functions of avalanches?
like 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
how have avalanches become hazardous?
avalanches only become a hazard when humans enroach on areas that are prone to them
as tourism and recreation have increased in the Rockies and the Alps, deaths from avalanches have increased
how can the risk from avalanches be minimized?
risk is greatly reduced when buildings, roads, and other infrastructure are located away from known avalanche paths
hazard maps provide planners with locations of the lightest risk areas
buildings in hazardous areas within a specific recurrence interval may require special engineering
—– this may include reinforced walls or deflection structures
—– in avalanche star zones, fences or nets can be installed to stabilize the snowpack
mounds and berms can be used to slow and deflect avalanches away from populated areas
—– splitting wedges on the sides of buildings can force an avalanche around the structure
avalanche sheds allow avalanches to travel over roads or railways without disruption to traffic
controlled triggers are used to force avalanches to occur to prevent build-up of the snowpacl
—– this is performed using explosives
what is avalanche forecasting based on?
locations of past avalanches
strength and stability tests
snowpack observations
weather
how is the strength and stability test performed?
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
—- 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
what should you do before travelling to an avalanche prone area?
check the current danger level and any public bulletins
knowledge of slope angles and the terrain is also necessary
what is an avaluator?
developed by the Canadian Avalanche Centre
a chart designed to warn travellers of the risk of an avalanche in an area
how do people die from avalanches?
the motion of the snow itself kills about 25% of avalanche victims
buried victims die of a combination of suffocation and hypothermia
