Avalanches

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Avalanches
Avalanches
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Avalanches were first imagined as giant snowballs
which increased in size from accretion of
underlying snow
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What are avalanches?

• They are rapid downslope movements of snow, ice,
  rock, or soil.
• They can be channelized or unconfined
• may travel as coherent block or disaggregate
  into small particules

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What Drives Avalanches?

• -The driving force is Gravity
• -Gravity is a force that attracts objects toward
  the Earth
• Fgmg
• FgForce of gravity
• mmass
• ggravitational constante

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Avalanche Size
Size Runout Potential Damage Physical Size
1 - Sluff Small snow slide that cannot bury a person, though there is a danger of falling. Unlikely, but possible risk of injury or death to people. length lt50 mvolume lt100 m³
2 - Small Stops within the slope. Could bury, injure or kill a person. length lt100 mvolume lt1,000 m³
3 - Medium Runs to the bottom of the slope. Could bury and destroy a car, damage a truck, destroy small buildings or break trees. length lt1,000 mvolume lt10,000 m³
4 - Large Runs over flat areas (significantly less than 30) of at least 50 m in length, may reach the valley bottom. Could bury and destroy large trucks and trains, large buildings and forested areas. length gt1,000 mvolume gt10,000 m
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Avalanches have severe consequences

• Direct effects
• impact
• Burial
• Most fatalities were people killed while building
  railways (canadian Pacific Railway, in 1886)
• The 8 km Connaught channel was constructed
  beneath the mountains to by pass some of the most
  dangerous avalanche paths.

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Avalanches have severe consequences

• -Traffic delays and economic losses
• (For 100 hours each winter, the Trans Canadian
  Highway in Rogers Pass, British Columbia is
  closed because of avalanches).
• -Property damage
• -Forests damage uprooting, breaking trees

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Avalanches have severe consequences

• Indirect effects
• tsunamis generated if an avalanche enters a lake

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Avalanche Triggering

• Naturally after snow storms or normal daytime
  heating upper part of the snowpack.
• The persons weight increases the shearing force
  in the weak layer triggering failure
• Intentionally with explosives, as part of
  avalanche-control programs

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Avalanche causes
Causes of avalanches Trigger mechanisms
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What is a cornice?
A cornice is an overhanging edge of snow on a
ridge or the crest of a mountain
A cornice of snow about to fall. Cracks in the
snow are visible in area (1). Area (3) fell soon
after this picture was taken, leaving area (2) as
the new edge.
Wikipidia
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Avalanche zones

• a) Starting zone where the snow pack fails
• b) Avalanche track along which the avalanche
  accelerates and achieves its highest velocity
• c) Runout area where the avalanche decelerates
  and snow is deposited

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Avalanche zones
The three parts of an avalanche path starting
zone, track, and runout zone. (Photograph
courtesy of Betsy Armstrong Source NSIDC)  
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a.Starting zone Gravity and slope gradient
shear strength (stay force) internal resistance
to movement or force of cohesion and
friction shear stress (Go force) force causing
movement parallel to slope increases with slope
angle
-At a certain point, gs(go) exceeds the shear
strength(stay), and failure of the mass occurs
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Angle of Repose
-When the go force is equal the stay force the
balance is reached and the angle is 45
degrees. -The angle of repose is at 40 degrees
and is the angle beyond which material will start
to move down a slope.
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When will a slope fail?

• Fs Safety Factor
• Fs (shear strength)/(shear stress)Stay
  force/Go force
• shear strength (stay force)
• internal resistance to movement
• shear stress (Go force)
• force causing movement parallel to slope
• increases with slope angle
• If Fs is less than 1, then the slope is unstable
  and prone to failure

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Slope Angle
-Most avalanches (slabs) are released from slopes
between 30 and 45 degrees -Slopes less than 25
degrees and steeper than 60 degrees have a very
low avalanche risk. -Wet snow slides, can happen
on slopes less than 25 degrees. They contain
liquid water between the grains of snow.
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Avalanche Initiation
1-Point release avalanches (loose snow) initial
failure of a small amount of snow. More snow is
incorporated into the avalanche as it moves
downslope. 2-Slab avalanche begins with
fracturing of the snowpack along a weak layer at
depth. Gravity causes the snowpack to move
downslope with the top moving faster than the
bottom.
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Two weak layers within a slab
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Initial failure - two types
Coherent slab comprising fractured blocks of snow.
Failure at depth
More dangerous
Surface or near-surface
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Loose snow failure
Angle of repose more than 45 degrees
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Slab failure
Coherent slab
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b) Internal structure of the flow
Density and solids concentration gradient
The powder cloud is less dense than the flowing
snow

• 2 types of snow avalanche (a spectrum exists)
• flow avalanches
• airborne powder snow avalanches

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Avalanche flow structure

• Note the head, body, and tail of the flow
• a lower dense portion which is highly hazardous
  and destructive
• an overlying more dilute portionalso can be
  hazardous and destructive, since it is turbulent

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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 flow

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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
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Mixed flow and powder avalanche
At velocities above 35km/h flow avalanches
generate a cloud of powdered snow. The powder
cloud is much less dense than the flowing mass.
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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
• powder avalanches may develop from flow
  avalanches, but the mechanisms are not well
  understood

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Powder avalanche note frontal zone of higher
density, low-density cloud behind front
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Fully-developed powder avalanche due to cascading
down near-vertical cliffs
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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 obstacle

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Some Canadian statistics

• Activity 1959-74 74-89
• Fatalities recreational 33
  97
• activities
• Fatalities buildings, 53
  6
• roads, worksites

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Some interesting statistics from the Canadian
Avalanche Association
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Types of snow and slopes prone to failure
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Lee-side avalanche with cornice above
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Survival
Burried victims die of suffocation, hypothermia,
injuries.
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Some U.S. statistics
Fatalities
Property damage (thousands of dollars)
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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)

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HAZARD MAPS, Alta, Utah Avalanche Risk can be
estimated by determining the distribution,
frequency, and sizes of avalanches in given area.
Avalanche frequency is described in terms of a
recurrence interval
Note lack of vegetation, which could help
dissipate avalanches
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Engineering works

• Reforestation
• to stabilize slopes and snow
• Highways
• locate to avoid avalanche tracks
• use of defense structures deflectors, mounds,
  benches with dams

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Avalanche avoidance
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Use of defense structures
Starting zones defenses to support the snowpack
and prevent large avalanges (famous in Europe)
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Starting zone defenses
Terracing in avalanche starting zones

• To help reduce avalanches from forming
• use of terraces
• use of supporting structures

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Supporting structures
Prevent large avalanges from starting, they are
very expensive and used only where people and
property are at risk.
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Details of supporting structures
Only practical for protecting inhabited
structures, busy roads and critical
infrastructure (not commonly used in North
America)
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Some specific examples of mitigation attempts

• Deflectors
• Used in the track and Run-out zones
• Deflect avalanches from building or parallel to
  roads.

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Arresters

• Arresters are used to slow or stop avalanches
• need adequate height if too low, flow can
  accelerate above barrier, increasing damage

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Splitters

• These are placed directly in front of a single
  object
• They redirect and divert the avalanche flow
  around the structure

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Use of splitters on ski slopes
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Mounds

• These are used to retard flowing snow at the end
  of the runout zone
• Slow avalanches and reduce their run-out.

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Detail of mounds
Become buried early in the winter, avalanches
travel over them
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Snow sheds

• These sheds allow the avalanche to pass over the
  structure
• Run over the roads
• and railway

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Snow sheds
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Avalanche Safety

• 1-Is the slope to be crossed prone to avalanches?
• .using terrain factors as slope angle
• 2-Is the snowpack unstable?
• Using public bulletins, recent avalanches in the
  area, weather observations.
• 3-What are the consequences to be caught in an
  avalanche in this terrain? If an avalanche
  occurs
• Large or small? A slab or point release? Wet or
  dry snow?
• Could I be swept over a cliff or into trees or
  boulders?

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How to avoid an avalanche?

• 1-check the surface of the snow. Avalanches are
  common on slopes with smooth surfaces, such as
  grass or smooth rock than on treed or rough
  slopes.
• 2-Climbing under cornices should be avoided
  especially during snowstorms. In general convex
  slopes are more dangerous than uniform or concave
  slopes.

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How to avoid an avalanche

• 3- check for snow an avalanche reports. If there
  are any warnings, make sure to avoid the areas.
  Never ski off marked slopes. If snow build-up has
  been heavy, avoid going out.
• 4-If you're caught in an avalanche, try to ski
  toward the side of the moving mass of snow, keep
  your mouth closed to avoid choking. To stay on
  top of the avalanche, make swimming motions. When
  the avalanche starts to slow down, thrust your
  hand above the surface.

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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. Seattle, The Mountaineers.

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Avalanches - web

• Canada http//www.avalanche.ca/
• USA http//www.americanavalancheassociation.org/
  
• North America http//www.avalanche.org/