Avalanches Avalanche s

Avalanches
Avalanche
s
Avalanches were first imagined as giant
snowballs which increased in size from
accretion of underlying snow
What are avalanches?
❚ They are flows which move under the
influence of gravity
❚ They can be channelized or
unconfined
❚ In this sense, they are similar to
pyroclastic flows, debris flows, etc.
Avalanches have severe
consequences
❚ Direct effects:
❙ impact
❙ burial
❚ Indirect effects:
❙ tsunamis generated if an avalanche
enters a lake
Avalanche zones
❚ a) Starting zone: where an
avalanche is initiated
❚ b) Avalanche track: where it goes
❚ c) Runout area: where it dissipates
a) Starting zones of avalanches
❚ Fs = Safety Factor
❚ Fs = (shear strength)/(shear stress)
❚ shear strength: internal resistance to
movement
❚ shear stress: force causing movement parallel
to slope; increases with slope angle
❚ If F is less than 1, then the slope is unstable
Shear stress
On a slope, gravity has two
components, one at right angles
to the slope (gp), and the other
parallel to the slope (gs)
As the slope angle increases, gp
decreases while gs increases
At a certain point, gs exceeds
the shear strength, and failure
of the mass occurs
Layering of snow
Two weak layers within a slab
Initial failure - two types
Failure at
depth
Surface or near-surface
More
dangerous
Loose snow failure
Slab
failure
b) Internal structure of the flow
Density and solids concentration gradient
❚ 2 types of snow avalanche (a spectrum
exists):
❘ flow avalanches
Avalanche flow structure
❚ Note the head, body, and tail of the
flow
❚ Vertical and lateral gradients in
solids (i.e., snow) concentration and
in density:
❘ a lower dense portion which is highly
hazardous and destructive
Flow avalanches
❚ Velocities up to 216 km/hr (60 m/s)
❚ Flow heights 5-10 meters
❚ Collisions of particles - granular flow
❚ Initially tends to slide as a rigid
body (similar to a landslide)…
❚ …but rapidly breaks up into smaller
particles and becomes a granular
Interior of the flow
-There is a high-density core near the base of the flow
-In this zone, particles collide, resulting in friction and
producing heat
-When the avalanche flow stops, freezing can occur,
making the deposit very hard
-sets like concrete
High-density core
Mixed flow and powder avalanche
Airborne powder snow
avalanches
❚ Velocities can exceed 360 km/hr (100
m/s)
❚ Flow thicknesses may exceed 100
meters
❚ Essentially a highly dilute density
current flowing down an incline:
❘ partial entrainment of underlying snow by
turbulent, erosive flow
❘ dense core small or absent
Velocity
Newly-fallen snow depth
Powder avalanche: note
frontal zone of higher
density, low-density cloud
behind front
Fully-developed
powder
avalanche due
to cascading
down nearvertical cliffs
c) Runout area
❚ Powder snow avalanches flow around
obstacles, while flow avalanches do
not
❚ When powder snow avalanches hit a
barrier, the lower dense portion of
the flow is stopped, while the more
dilute cloud behaves like a fluid
which can flow around or over the
Some Canadian statistics
Activity
❚
1959-74
89
❚ Fatalities: recreational
97
❚ activities
33
❚ Fatalities: buildings,
53
74-
Some interesting statistics from the
Canadian Avalanche Association
Avalanche causes
Causes of avalanches
Trigger mechanisms
Types of snow and slopes
prone to failure
Lee-side
avalanche
with cornice
above
Survival
Some U.S. statistics
Fatalities
Property damage
(thousands of dollars)
Mitigation
❚ Avoid steep slopes, gullies
❚ Close high-hazard areas to reduce
risk and vulnerability
❚ Set off explosive charges to
artificially induce avalanches and
remove the source material
(unstable snow)
HAZARD MAPS, Alta, Utah: note the
very fine line between zones of high
hazard, potential hazard, and no hazard
Note lack of vegetation, which could
help dissipate avalanches
Engineering works
❚ Reforestation:
❘ to stabilize slopes and snow
❚ Highways:
❘ locate to avoid avalanche tracks
❘ use of defense structures: deflectors,
mounds, benches with dams
Avalanche
avoidance
Use of defense structures
Starting zone defenses
Terracing in avalanche starting zones
❚ To help reduce
avalanches from
forming:
❚ use of terraces
❚ use of
supporting
structures
Supporting structures
Details of supporting
structures
Some specific examples of
mitigation attempts
❚ Deflectors:
❚ must be gradual,
otherwise the
avalanche will
overflow the
deflector
Arresters
❚ Arresters are
used to slow or
stop avalanches
❚ need adequate
height; if too low,
flow can
accelerate
above barrier,
increasing
damage
Splitters
❚ These are placed
directly in front of
a single object
❚ They redirect and
divert the
avalanche flow
around the
structure
Use of
splitters on
ski slopes
Mounds
❚ These are used to
retard flowing
snow at the end of
the runout zone
Detail of mounds
Snow sheds
❚ These sheds allow
the avalanche to
pass over the
structure
Note long
dikes to
prevent
spreading
Snow sheds
Current El Niño conditions
Global winter impacts from El Niño
Temperature
anomalies from El
Niño, Winter 1998
Precipitation
anomalies from El
Niño, Winter 1998
Avalanches - readings
❚
Committee on Ground Failure Hazards Mitigation Research, 1990.
Snow avalanches and mitigation in the United States. Washington,
National Academy Press.
❚
Fredston, J., 2005. Snowstruck: In the Grip of Avalanches. New
York, Harcourt.
❚
Fredston, J., and D. Fesler, 1994. Snow sense: a guide to evaluating
snow avalanche hazard. Anchorage, Alaska Mountain Safety
Center, Inc.
❚
International Commission on Snow and Ice of the International
Association of Hydrological Sciences. 1981. Avalanche atlas :
illustrated international avalanche classification. Paris, Unesco.
❚
McClung, D., and P. Schaerer, 1993. The avalanche handbook.
Avalanches - web
❚ Canada: http://www.avalanche.ca/
❚ USA: http://www.americanavalancheassociation.org/
❚ North America: http://www.avalanche.org/