© Bill Glude Alaska Avalanche Specialists 20111202
All photos and graphics © Bill Glude unless otherwise noted.
AVALANCHE TERRAIN
The Key Questions
1.) Could the terrain produce an avalanche?
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a.) Where you are?
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b.) Above you?
2.) What would the consequences be?
Path Characteristics
• Terms for parts of paths, see captioned
photo at right.
• Unconfined and confined paths.
• Single or multiple starting zones.
• Path size - it’s the small ones that catch
most people, not the big obvious ones.
Path History
• Human knowledge, history, observation.
• Debris.
• Vegetation clues:
• “Unauthorized logging”
• Tree damage
• Disaster species
• Dating slides by tree growth.
• Large forests slow but do not always stop big slides.
Slope Angles
• Slope angle is the most important terrain factor.
• This graph shows prime time in the 30° to 45° range, but the data is skewed toward lower angles because
most of it is from Intermountain and Continental snow climates. In Alaska, the peak is shifted to the right;
prime time angles are 35° to 50°+.
Slab Avalanche Frequency - Slope Angle
48
44
40
36
32
28
% 24
20
16
12
8
4
0
Series 1
20-25° 25-30° 30-35° 35-40° 40-45° 45-50° 50-55°
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If you are on flat or gentle terrain, are you in a runout zone? What is above you?
At 12° - 14°, slush flows can release in water-saturated snow.
At 25º, slabs only release during periods of unusual instability.
At 30º, slabs can release under a wider range of conditions.
At 35°-50°+, the slope angles are in the "prime time" range for large slabs in Alaska and other coastal and
transitional climates.
• At 60º+, large slabs still release, but spindrift avalanches and sluffs start to dominate.
• Cliffs are unlikely to produce large slabs because they are too steep to hold much snow, but you must still
watch the snow on lower-angle ledges and watch for hanging glacier ice.
Runout, or Alpha Angles
• A handy way to estimate potential runout distance in the field, the runout angle is measured from the toe of
the debris to the crown face, or the reverse.
• 25º - typical
• 20º - low, efficient avalanche
• 19° - lowest measured in SE Alaska
• ~12º - lowest measured
Factors Influencing Runout Distance
• Long runouts:
• Smooth transition.
• Steep slope right to bottom.
• Channeled and concentrated flow.
• Short runouts:
• Flat landing, “bellyflop” onto the flats.
• Rough terrain.
• Sharp turns or obstacles.
• Unconfined flow that spreads out and dissipates.
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Elevation
Elevation is a key factor influencing:
• Snow type and quantity.
• Wind intensity.
• Precipitation type, snow level.
• Avalanche activity.
• Special Southeast Alaska notes: Though
snow to sea level will help a slide maintain
speed and volume, large avalanches don’t
really care if there is no snow in the lower
track. They travel easily on bare ground.
• Avalanches can reach low elevations whenever conditions in their starting zones up
high are suitable. This is hard to remember
when all you see is rain and fog. The key
question is “What’s going on up high where
avalanches start?”.
Ground Conditions
Smooth slopes, more prone to glide avalanches
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Grass.
Tundra.
Low bushes.
Herbaceous plants.
Rough slopes reduce glide, may help break up and support slabs until covered.
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Rocks.
Tall bushes.
Open trees.
Beware, weaker faceted grains tend to form around rocks, bushes, and trees. They are often more functional
as stress concentration or weak points than as so-called “anchors”.
Forest
• Closed canopy helps stability because layers are discontinuous, broken up by tree wells, drips, tree plops,
logs, and stumps. But closed canopy in dry climates with small-trunk, short branch trees can still produce
large slabs.
• Open forest may just concentrate stress and makes far worse consequences if a slide does release. Trees
open enough for you to travel easily are open enough for avalanches to travel easily, too. Being swept
through trees is the major cause of avalanche trauma in North America.
Glaciers
• Flow stresses are slow, probably insignificant.
• Crevasses and bergschrunds break the continuity of snow layers. Effects vary, but are usually secondary.
• The old snow surface or glacier ice can function as a bed surface, especially in the summer or late spring.
Slope Configuration
• Convex profile areas are generally the most likely sections of a slope to release.
• Concave or straight profile sections may be less likely but the consequences are worse if you bring a slide
down on you from the middle or bottom of the slope. Some unstable snow conditions, notably graupel and
surface hoar, tend to be more pronounced in the hollows of the slope.
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Aspect
Aspect with respect to sun
• Shaded slopes tend to develop more surface hoar and faceted grains, weak snow types.
• Sunny slopes benefit in midwinter from moderate and more-uniform temperatures that tend to produce
rounded grains, rapid bonding, and a stronger snowpack, but too much sun leads to sun crusts or wet slides
as temperatures increase.
Aspect with respect to wind
Wind erosion reduces the load on potential weak layers
in the snowpack. Windward, eroded areas generally
are preferred ascent routes because they have less
load, though those thin snowpack areas often develop
weak faceted grains.
• Wind deposition loads slopes very rapidly and builds
sensitive windslab. Sheltered lee slopes are easy to
trigger when freshly loaded.
Wind effects - cornices
• Cornices form from drifting snow building out over a
slope change like a ridgetop or dropoff. They build
straight out, then creep makes them sag down like a
breaking wave.
• Cornices break
• More easily than you expect.
• Farther back than you expect.
• And faster than you expect.
• If you walk out to the edge of a snow-covered ridge, you are likely to be on the cornice even if there are rocks
on both sides. Ridges usually scallop back between the rocks. You must have solid rock supporting you if
you want to look over the edge.
• When approaching snow-covered ridges, assume there is a cornice there until you prove otherwise. Usually
the ridge bends somewhere and allows a view over the edge from a solid slope.
• Cornice consequences include the fall when it breaks, the chunks of hard, heavy snow bouncing downhill,
and their tendency to trigger the slopes they land on.
Wind effects - reading wind direction from snow forms
• Depositional forms like drifts have their steep face downwind, to the lee.
• Erosional forms like sastrugi, the corrugated, rough snow surface in areas with high wind intensity, have their
steep face upwind, to the windward.
• Mountain winds are strongly affected by topography, may eddy and change direction frequently. Check a
number of wind clues before you decide, and keep reevaluating as you travel.
• Travel in low risk terrain during a windstorm to study how wind forms develop.
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Consequences - Bad Consequence Spots and Terrain Traps
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Trees - the leading cause of traumatic avalanche injury and death in North America.
Cliffs and rocks - second, but more chance of fatal injury than trees.
Gullies and hollows - the third major terrain trap.
Slopes lacking escape routes.
Slopes lacking “islands of safety”.
Abrupt transitions, road cuts.
Creek beds.
Water.
Brush.
Narrows or choke points.
Crevasses.
Bergschrunds.
Ice.
Multiple consequences.
Consequences - Trees!
• Overall, roughly 10 - 15% of people completely buried
in avalanches are dead from trauma.Trauma accounts
for about 30% of avalanche deaths.
• In Europe only 5-6% are killed by trauma. Most skiing is
above treeline.
• In North America the trauma death rate is about 25%, 5
times the European rate. This is mostly due to trees.
• Studies in Canada show that 67% of avalanche trauma
deaths there are from impacts with trees.
• In a Utah study, cliffs and rock bands were a “distant
second” from trees as sources of traumatic injury, fewer
in number but more likely to be fatal.
• Bottom line: Trees are very bad-consequence terrain!
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Consequences - Avalanche Dynamics
• Slower-flowing avalanches tend to follow gullies, channels, and low spots.
• Faster powder and mixed motion avalanches tend to go
straighter. Momentum prevails.
• Side slopes and double fall lines can take slides where
you might not expect them to go.
• Evaluate micro-terrain carefully!
Consequences - What You See
in the Movies
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We work with some of the best athletes in the world when filming.
They take risk management seriously. No one drops in casually. None of these pro’s have a death wish!
We evaluate consequences and snow stability with care.
We spend hours safeguarding each shot.
We seek well-supported cornices to jump, avoid the big overhanging ones.
We plan for and manage our sluffs.
And despite all our precautions, it is still a dangerous business.
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Avalanche Terrain Classification
20080403 Draft, Adapted by Southeast Alaska Avalanche Center from Parks Canada Model
Terrain Complexity
Factors
1 Easy
+
2 Moderate
0
3 Difficult
-
Route Options or
Alternatives
Numerous, terrain allows
multiple choices.
A selection of choices of
varying exposure, options
to avoid avalanche paths.
Limited chances to reduce exposure, avoidance not possible.
Slope Angle
Angles generally < 30°.
Mostly low angle, isolated slopes > 35°.
Variable, with many >
35°.
Terrain Traps &
Consequences
Minimal, some creek slopes
or cutbanks.
Some depressions, gullies or hollows, and/or
avalanche terrain
above, trees, brush,
cliffs, rocks, narrows
creeks, or water with
some good escape
routes or “islands of
safety”.
Many depressions,
gullies or hollows,
cliffs, hidden slopes
above gullies, cornices, trees, brush,
cliffs, rocks, narrows,
creeks, or water with
few escape routes or
“islands of safety”.
Avalanche Frequency
30 year return, ≥ D2 size.
Yearly for < D2 size.
3 year return for ≥ D2
size.
Yearly for < D3 size.
Yearly for ≥ D3 size.
Interaction with
Paths
Runout zones only.
Single path or paths with
separation.
Numerous and overlapping paths.
Exposure Time
None, or limited exposure
crossing runouts only.
Isolated exposure to
start zones and tracks.
Frequent exposure to
start zones and tracks.
Glaciation
None or smooth.
Generally smooth with
isolated bands of crevasses.
Broken or steep sections of crevasses,
icefalls or serac exposure.
Start Zone Density
Limited starting zone terrain.
Some start zones; isolated avalanche paths
leading to valley bottom.
Large expanses of start
zones; multiple paths
leading to valley bottom.
Runout Zone
Characteristics
Solitary, well-defined areas,
smooth transitions, spread
deposits.
Abrupt transitions or depressions with deep deposits.
Multiple converging runout zones, confined
depositions area, steep
tracks overhead.
Slope Shape
Uniform.
Some convexities.
Convoluted.
Notes:
Terrain may fit into multiple classes. Consider all the variables and note that there are some default properties.
Properties with a bold italicized descriptor automatically default into that or a higher terrain class. Nonitalicized descriptors are considered as carrying less weight.
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