Avalanche

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Avalanche
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Avalanche a mass of snow ( sometimes ice and
rock) that moves rapidly down a steep mountain
slope.
A typical dry snow avalanche travels 60-80 mph
and can reach these speeds within five seconds
after it fractures.
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A large avalanche in North America might release
300,000 cubic yards of snow. That's the
equivalent of 20 football fields filled 10 feet
deep with snow. Large avalanches are often
naturally released whereas smaller ones are more
commonly human caused.
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Anatomy of an Avalanche
The starting zone is where unstable snow can
fracture from the surrounding snowcover and begin
to slide. Typical starting zones are higher up on
slopes, but not always. The avalanche track is
the path or channel that an avalanche follows as
it goes downhill. The runout zone is where the
snow and debris finally come to a stop. This
deposition zone is where the snow and debris pile
the highest. It is also where a victim will most
likely be buried.
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• What causes an avalanche?
• An avalanche occurs when the stress (from
  gravity) trying to pull the snow downhill exceeds
  the strength (from bonds between snow grains) of
  the snow cover.
• There are four ingredients necessary for
  avalanche
• a steep slope
• a snow cover
• a weak layer in the snow cover
• a trigger

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1. Steep slope Slope angle Most avalanches
occur on slopes between 30 and 45?, but can occur
on any slope angle given the right conditions.
Slope orientation Most avalanches run on slopes
facing north, east, and northeast. Because the
sun is at such a low angle, particularly during
the winter, a colder and deeper snowpack
develops. Slopes that are under shadow
throughout most of the day are suspect because
the snowpack remains cooler, without much of the
melting and bonding that can make the snow layers
stronger.
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2. Snow Cover Perhaps the most significant
factor is how the snowpack has developed over the
season.

• The snowpack changes
• During the course of the winter season,
• Throughout the course of a single day.
• Each layer in the snowpack is built up under
  different weather conditions and will bond
  differently to the layers below.

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Snowflakes, or snow crystals, within the snowpack
eventually become more rounded due to
melting/re-freezing and settlement allowing them
to compress and (generally) form stronger bonds.
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3. Weak Layers In between snows, the temperature
may rise and melt the exposed surface layers,
which when they re-freeze create a smoother, less
stable surface for the next snowfall. Failure
is much more likely to occur during or after the
next few snowfalls. Rain between snows creates
a slicker surface as well, and can weaken the
bonds between snow layers.
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A type of snow called depth hoar (a course,
grainy form of snow crystal) is often the culprit
behind avalanches. Because of its granular
structure, similar to dry sand, depth hoar bonds
poorly and creates a very weak layer in the
snowpack. The weather conditions necessary to
produce depth hoar most often occur very early in
the season, and these weak layers are buried
under subsequent snows.
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• Avalanches are most likely to run either during
  or immediately after a storm.
• The extra weight of new snow can cause a slab to
  break off and fall down the slope.
• Rules of thumb
• Snowfall amounts of one foot or more create the
  most hazardous situations, producing avalanches
  that are often large enough to block highways and
  cause major destruction.
• Amounts of six to twelve inches pose some threat,
  particularly to skiers.
• Amounts less than six inches seldom produce
  avalanches.

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On the other hand, light snowfalls and
consistently cold temperatures help strengthen
the snowpack and make it more resistant to
avalanche. Weak layers deep in the snowpack can
cause avalanches even if the surface layers are
strong or well bonded
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• 4. Triggers
• Natural Triggers
• Snow
• Wind-loading
• Rain
• Rapid temperature increase
• Artificial Triggers
• Skiers
• Snowboarders
• Snowmobiles
• Animals
• Explosives

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• In 90 percent of all avalanche accidents, the
  victim or someone in the victims party triggers
  the slide.
• Avalanches are the only natural hazard commonly
  triggered by the victim.

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There are a number of types of avalanches
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Slab avalanche Definition A mass of snow, ice
and possibly other material caused when a large
slab of snow breaks free from the layers beneath.
Context A slab avalanche is the most deadly.
The weight of a skier is enough to break a slab

• Slab avalanches are the most common and most
  deadly avalanches.
• Since 1950, 235 people in the U.S. have been
  killed in slab avalanches.

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3-27-2004 Wet slab avalanche in Tuckerman
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Cornice Fall Avalanche Definition Cornices are
snow structures formed by wind drifting snow onto
the lee (downwind) side of an obstacle, such as a
ridgeline. The weight of a falling cornice breaks
into hundreds of pieces and forms its own
avalanche. Context Cornice fall fatalities are
a significant problem in big mountains, they
often break off farther back than expected
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Drift sticks out over retaining wall 4.5 and was
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form
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Days Fork avalanche caused by skier kicking off
cornice
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Powder or Loose Snow Avalanche Definition A mass
of loosely packed snow that begins with a piece
of falling rock or ice. Small loose snow
avalanches are called "sluffs". Context The
largest and most destructive is a powder
avalanche. A piece of falling ice or rock starts
a mass of loose snow sliding down the mountain.
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Avalanche on a warm spring day, Wasatch Range
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Ice Avalanche Definition Falling blocks of ice
create an avalanche, which often entrains snow
below it or triggers slabs. Context Glaciers
flowing over cliffs form icefalls and most common
in big mountains.
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Wet avalanche Definition A flow of snow, ice,
rock, and other material that occurs as a result
of thawing. Context These occur mostly late in
the snow season when the snowpack is deep and the
thaw is just beginning.
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Avalanche Prediction Avalanches do not strike
without warning. They happen in particular
places due to specific combinations of snow and
weather conditions. There are almost always
obvious signs that these conditions exist.
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• Avalanche chute a downslope hillside pathway
  along which avalanches repeatedly fall.
• Characterized by
• Large vertical swaths of trees missing from a
  slope
• A large pile-up of snow and debris at the bottom
  of the slope

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Avalanche Chute, Centennial Mountains, Red Rock
Wildlife Refuge MT May 200
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Avalanche Mitigation
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Little Cottonwood Canyon
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Oops! Artillery Overshoot Lands in Neighborhood
PLEASANT GROVE, Utah - A 20-pound artillery
shell loaded with too much gunpowder overshot its
Provo Canyon avalanche target Wednesday by about
three miles, exploding in the back yard of a
couple and piercing three homes with shrapnel.
Remarkably, no one was injured. Now the Utah
Department of Transportation is scrambling to
understand what went wrong and make sure it never
happens again. 2005
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San Juan Mts
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San Juan Mts
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