lecture 10 - avalanches Flashcards
Definition of a snow avalanche
a mass of snow many cubic meters in volume that separates from a snowpack and moves downslope
what is the intensity of a avalanche depend on?
slope steepness, snowpack stability and weather
two types of avalanches travelling
An avalanche travelling as a coherent block
An avalanche that becomes wider as it travels downslope
are avalanches typically seen?
99% are not seen by anyone
snow climatology
amount of snowfall depends on latitude, altitude, and proximity to bodies of water
temp decreases with altitude meaning high mountains have permanent snow cover
2 types of avalanches
- point release avalanche
- slab avalanche
point release avalanche
begins as an initial failure in snowpack after heavy fall
the sliding snow then causes more failures in adjacent snowpack causing the trough to widen
slab avalanches
occurs when snowpack fractures along a week layer parallel to surface
move as cohesive blocks leaving behind a scarp
**most dangerous
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 then dry powdery snow
mass of snow above vegetation level and larger boulder is more likely to slide
slab avalanches require
a buried weak layer such a layer formed from wind or from hoar
weak layers - wind
blowing snow can accumulate on the lee (downwind) slope of mountain (lee side = more risky side)
wind can deposit layers of ice crystals on layer of compacted snow
the boundary between two layers could become a horizon along which failure may occur
weak layers - Hoar
layers composed of hoar have less strength than the rest of the snowpack
can form deep in the snowpack (in air pockets) or on snow surface
changes over time; overlying snow can leave the buried hoar as a weak layer
avalanche motion
often generate clouds of powdered snow due to rapid movement
fastest have been measured 200km/h
some powerful enough to climb opposing slopes
avalanche triggers
occur soon after snowstorms
may occur when daytime heating from the sin warms the upper part of the snowpack
avalanches that cause injuries or fatalities are often triggered by people
some triggered intentionally with explosives
3 avalanche paths
start zone: area where the snowpack first fails
track: area of where avalanche accelerates and reaches maximum velocity
run out zone: area of deceleration and snow deposition
most important factor that influences avalanche formation
the slope angle
most avalanches occurs between what angles
25-60 degrees
angles below _____ degrees does not slide easily
25 degrees
angles above ___ degrees little snow accumulates on the slope
60
the sweet spot of avalanches is between aka highest risk
30-45 degrees
terrain factors
the orientation (direction slope faces)
snow on leeward slopes consist of inter-layered strong and weak layers
smoothness of slope, amount of vegetations and topography
regions at risk
for an avalanche to form a snowpack of that at least 50cm is required
in north america, deep snowpack are most common in the rocky mountains
effects of avalanches
most avalanche deaths occured between 18-1900s
in total 600 people have died (low)
have caused millions of dollars in losses in B.C. each year due to closed highways
damage of forests is evident each year but property damages are minor
chilkoot avalanche
occured in 1898 and remains one of the worst in North American History
avalanche spread over chilkoot trail causing 60 deaths
trail was heavily used during the Klondike gold rush
this trail extends from Alaska to B.C. and is the easiest route through the mountains
