Avalanche Home
Table of Contents
Types of Avalanches:
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.
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.
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.
Cornice Fall Avalanche
- Definition: Cornices are snow structures
formed by wind drifing 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.
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.
Glide Avalanche
- Definition: A type of wet avalanche,
they occur when the entire snowpack slowly slides
as a unit on the ground (similar to a glacier).
- Context: Glide avalanches are a
slow process which usually occur over several days,
weeks or even months. Caused by melt water libricating
the ground and allowing the overlying snowpack to
slowly "glide" downhill.
Slush Avalanche
- Definition: A sluch avalanche occurs
when a cold dry snowpack suddenly becomes saturated
with water. It then catastrophically looses strength
and the resulting slush often runs long distances
on gentle terrain.
- Context: Common in northern latitudes
such as Alaska and other high latitiude permafrost
regions.
Return
to Avalanche Table of Contents
Avalanche
Causes:
What factors cause an avalanche to
occur?
How frequently to avalanches occur?
- Avalanches are most likely to occur
when danger increases with major snowstorms and periods
of thaw. Thousands of avalanches are reported to Avalanche
Centers in the U.S. in an average winter. More than
80% of these fall during or just after large snowstorms.
The most avalanche-prone months are, in order, February,
March, and January. Avalanches caused by thaw occur
most often in April.
What is the typical setting or circumstances
in which avalanches occur?
- About 90% of all avalanches start
on slopes of 30-45 degrees; about 98% of all avalanches
occur on slopes of 25-50 degrees. Avalanches release
most often on slopes above timberline that face away
from prevailing winds (leeward slopes collect snow
blowing from the windward sides of ridges.) Avalanches
can run, however, on small slopes well below timberline,
such as gullies, road cuts, and small openings in
the trees. Very dense trees can anchor the snow to
steep slopes and prevent avalanches from starting;
however, avalanches can release and travel through
a moderately dense forest.
What are the dangers associated with
the hazard?
- Loss of life and injury
- Property damage
- Blocked highways, roads, and trails.
Return
to Avalanche Table of Contents
Avalanche
Safety:
What can be done in case of
an emergency?
- Avoidance.
- Surviving avalanches can depend
on luck; therefore, it is always better to avoid them
in the first place. Remember that only 1 of 3 victims
buried without a beacon survives. If you are caught,
first try to escape to the side, or grab a tree or
rock. If you are knocked down, get rid of your poles,
skis and or snowshoes, and a heavy pack. Swim with
the avalanche to try to stay on top and avoid trees.
When the avalanche slows down, reach the surface or
make an airpocket.
- Avalanche victim survival depends
on rapid location and extrication by companions. Victims
generally do not have enough time for successful rescue
by outsiders. Groups must posses self-sufficient rescue
skills and equipment (beacon, probe, & shovel).
- Call park incident command (rangers,
etc.) in a park unit.
- Contact local emergency personnel
(911).
What can be done in terms of prevention?
- Avoidance is the best policy.
- Education.
- Planning prior to activity in the
area.
- Education and understanding. Interpreters
work in partnership with the scientific community
to ensure that complex information can be conveyed
accurately, and in a form that is comprehensive and
relevant to non-specialists.
Return
to Avalanche Table of Contents
Economic and
Physical Losses Associated with Avalanches:
What damages are associated with avalanches?
- Loss of capital assets (homes, buildings,
structures and transportation networks), injuries
and deaths.
What economic costs are associated
with avalanches?
- Avalanches can cause loss of life,
damage property, and disrupt transportation corridors.
During a severe winter, avalanches can close highways.
Recreation and ski areas can also close due to avalanches
or the threat of avalanches. Figures are not available
to document monetary losses, but such expenses are
a regular winter occurrence for state transportation
departments and ski areas. Losses can also be realized
by the timber industry, homeowners, power companies
and recreation resorts.
Return
to Avalanche Table of Contents
Avalanche
Photos:
 |
 |
| Battleship Avalanche |
Wet Slab Avalanche |
Avalanche
Links:
State Avalanche Links:
Other Avalanche Links:
Return
to Avalanche Table of Contents
Parks at Risk:
Any park with topographic relief and
significant snowfall.
Return
to Avalanche Table of Contents
Quick Facts
and Trivia About Avalanche Hazards:
- From 1950-2000 there have been 447
fatalities in the US from Avalanches.
- Investigation into a residence
destroyed by avalanches provides an excellent example
of poor avalanche hazard land-use planning. A residence
near Sundance Ski Resort, Utah was a mountain dream
home constructed in an avalanche runout zone. Structural
damage and economic loss could have been prevented
if adequate land-use planning and engineering guidelines
were followed. On February 13, 1986, the residence
was struck by an avalanche which substantially damaged
the structure. A law suit, Transamerica Insurance
Company v. Sundance Development Corporation and others
was filed and eventually dismissed without a conclusive
ruling. On February 25, 1993, the residence was struck
by a second avalanche destroying the remaining structure
and damaging adjacent homes. Mapping of vegetation
patterns and avalanche terrain could have provided
design information for avalanche magnitude, frequency
and runout behavior. Mitigation of the avalanche hazard
is critical to avoid additional damage to adjacent
homes and possible loss of life. Hazard mitigation
could include both operational and structural methods.
Return
to Avalanche Table of Contents
Stories from
Parks Threatened by Avalanche Hazard:
From Ranger Reports:
- WEST GLACIER, MONT. – Recent
snow accumulation on the Going-to-the-Sun Road melted
sufficiently last weekend to warrant a decision by
Glacier National Park officials Tuesday afternoon
to announce plans to hold a West Side spring snow
plowing "Show Me" Day of Going-to-the-Sun
Road on Thursday, May 16, weather and snow conditions
permitting. Each spring, weeks of intense work are
required by National Park Service personnel to clear
the steep and precipitous Going-to-the-Sun Road of
snow, rocks, and avalanche debris before vehicles
may travel over Logan Pass. The Bird Woman Falls Overlook,
near "Weeping Wall" west of Logan Pass,
serves as the West Side viewpoint from which participants
can watch and photograph the crew working in the Haystack
Creek area. Park Rangers will be on hand to answer
questions and explain plowing operations. The event
is so popular that hundreds of visitors join park
rangers each year. Spotting scopes will also be available
at the viewing area.
- Crater Lake Ranger Presented with
Exemplary Act Award
On December 7, 2002, National Park Service Ranger
Randy Benham was presented with the Department of
Interior's Exemplary Act Award in a ceremony at park
headquarters. Chief Park Ranger Dave Brennan presented
the award on behalf of Superintendent Chuck Lundy,
"In recognition of Randy’s courageous actions
that resulted in the rescue of a two avalanche victims
at Crater Lake National Park."
Return
to Avalanche Table of Contents
|
