Module 8 Flashcards
Snow Avalanches(2 types)
Definition: masses of snow that separate from snow pack and slide or flow downslope
slide – movement as a coherent mass of snow
flow – coherent mass of snow that rapidly disintegrates into small particles moving independently of one another
volumes range from several m3 to more than
1 million m3
Avalanche Size(4)
1.Sluff
-small snow slide that normally would not bury anyone
-relatively harmless
=length <50m
- Small
- avalanche stops on the slope
- can bury, injure, or kill a person
- length <100m
3.Medium
-avalnche runs to bottom of the slope
-can bury and destroy a care, destroy small buildings
lenghth <1000m
- Large
- avalanche runs out over areas significants less steep then 30 degrees; mmay reach valley bottomand run up the lower part of the opposing slope
- can bury and destroy large trucks, trains, and buildings
- length >1000m
Snow climatology:Snowfall and accumulation effected by..5
latitude – seasonal/annual net radiation/energy balance
altitude – atmospheric temperature decreases with altitude (i.e., lapse rate = -10oC/1000 m)
proximity to moisture source – moist air masses generate more precipitation
slope angle – snow accumulates on slopes less than 45o; sloughs off on steeper slopes
wind redistributes snow to form cornices and slabs
snow accumulated on slopes less then __ degrees angle
45
snow cornices
Image result for snow cornices
A snow cornice or simply cornice (from the Italian cornice meaning “ledge”) is an overhanging edge of snow on a ridge or the crest of a mountain and along the sides of gullies.
snow slab
A cohesive layer of snow formed when wind deposits snow onto leeward terrain. Wind slabs are often smooth and rounded and sometimes sound hollow
point-release avalanches
failure of small volumes of loose snow (i.e., flows)
often occur after heavy snowfall events
loose snow is unstable because:
1) increased mass on slope;
2) snow crystals have had little time to bond to one
another
failing snow initiates failure in adjacent snow pack; produces a distinct V-shaped, downslope-widening trough
-small runout and small volume
What does a point-release avalanche look like?
failing snow initiates failure in adjacent snow pack; produces a distinct V-shaped, downslope-widening trough
slab avalanches
failure of coherent mass of snow (i.e., slide)
initiated by fracturing of snow pack along a weak layer at depth
failure propagates along weak layer
slab slips downslope with top of slab moving more rapidly than bottom of slab
slab is bounded by crown and flank fractures
Which type of avalanche should we be worried about most?
Slab Avalanche
Slab avvalanche diagram
-5 main parts on pg.190
.slab .flank(sides) .crown fracture(top end of avalnche where it broke away) .layered snowpack .weak layer where failure occurs
-
Stauchwall
german word for runout
Strength of the Snowpack is influenced by:4
strength of snow pack influenced by:
grain size and grain type
degree of bonding between ice crystals; compaction (affected by snowfall amounts)
presence of anchors (e.g. rocks, vegetation)
temperature: heating by solar radiation (affected by slope aspect); snow pack thermal gradient
slope aspect
In physical geology, aspect is the compass direction that a slope faces. For example, a slope on the eastern edge of the Rockies toward the Great Plains is described as having an easterly aspect.
define avalanche weak layer and how it develops(3)
Weak layers – avalanches require a buried weak layer and an overlying stronger layer
weak layers develop in several ways:
changes in air temperature during snowfall events
hoar frost formation within the snow pack
hoar frost formation at surface of snow pack
hoar frost
a grayish-white crystalline deposit of frozen water vapor formed in clear still weather on vegetation, fences, etc.
“Right Side Up”
vs.
“Upside Down”
“Right Side Up” storm – air temperatures are warm when the snow starts falling, and then become colder
snow is light and fluffy on top and becomes more dense with depth; results in a strong layer at depth
“Upside Down” storm - increasing air temperatures during a snowfall event
heavy, denser snow lies on top of lighter snow; results in a slab of more dense snow lying over a weak layer of less dense snow, providing the necessary ingredients for slab avalanches
If you see ____ you know there might be trouble
hoar
Surface Hoar
Vs.
Depth Hoar
Surface Hoar:
develops during clear and calm conditions in the evening; promotes radiative cooling of snow surface
humid air lies over cold snow surface; promotes frost formation
forms a thin, fragile and persistent weak layer in the snowpack
Depth Hoar:
heat moves from warm to cold, and moisture follows the same gradient
moisture in the form of water molecules is constantly moving upward from the relatively warm ground surface below the snow through the porous snow pack
moisture condenses on lower sides of crystals, causing them to grow (and have razor-sharp edges), and sublimating from the tops of the grains, making the tops rounded
Avalanche Motion: Slide
sliding motion occurs in avalanches moving up to 40 km/hr
after the snow fails and has overcome initial friction, it can accelerate rapidly downslope
slabs break into smaller fragments and snow glides along the surface with little mixing and turbulence
Avalanche Motion: Dry Snow
avalanches become turbulent (i.e., flows) when velocities exceed about 40 km/hr
dry flowing avalanche contains a dense core of snow particles (10-30 cm in diameter)
finer particles mix with air at the front and along the upper surface of moving snow forming a powder cloud
velocity tends to be greater in the center of the flow
avalanche generally moves along the surface of the terrain, uninfluenced by small irregularities
avalanches become turbulent (i.e., flows) when velocities exceed about ___ km/hr
40
Avalanche Motion: Wet Snow
wet snow avalanches develop in the same manner as dry snow avalanches but have no powder cloud
moving snow is dense and composed of rounded particles with a diameter of 10 cm to rounded lumps up to several metres; contains intergranular water
wet snow avalanches tend to flow in channels and are easily deflected by irregularities in the terrain
Differences between Dry and Wet Snow on velocities of avalanches:
Small
Medium
Large
Small (<1000 m3)= up to 10 up to 10
Medium (<10,000 m3)= 10-20 10-35
Large (>10,000 m3)= 20-35 35-60
Wet Left, Dry right
3 Components of Avalanche Paths:
Start zone – location on hillslope where snowpack fails
Track – path along which avalanche travels and accelerates to achieve its highest velocity
Run-out zone – location where avalanche decelerates and snow is deposited
Avalanche Triggers: 5
1.Increase in mass: of materials on slope
addition of snow during and soon after snow storms
weight of people traversing slopes
2.Slope angle:
most important terrain factor for avalanche initiation
slope angles >60o – frequent sluffs
slope angles 45o-60o – frequent point-release avalanches
slope angles 30o-45o – frequent large slab avalanches
slope angles <30o – wet snow avalanches only
3.Wind Loading: wind erodes snow from the windward (upwind) side of obstacles, such as a ridge, and deposits the same snow on the leeward (downwind) terrain
wind loading is a common denominator in most avalanche accidents; wind can deposit snow 10 times more rapidly than snow falling from the sky
wind-drifted snow is often much denser than non-wind loaded snow; adds significant weight on top of buried weak layers
4.Slope Aspect:
– orientation of slope with respect to wind and incident sunlight
leeward slopes may accumulate large amounts of snow in cornices or wind slabs
wind-deposited snow commonly consists of stronger and weaker layers; increased risk of slab avalanches
“warm” slopes – both south- and west-facing – are prone to point release avalanches in sunny and warm weather
“cold” slopes – both north- and east-facing – are prone to point release avalanches in cold weather
- Slope Shape
- convexity of slopes vs concavity
Slope angle:
most important terrain factor for avalanche initiation
slope angles >60o
slope angles 45o-60o
slope angles 30o-45o
slope angles <30o
– frequent sluffs(rare)
– frequent point-release avalanches(Infrequent)
– frequent large slab avalanches(Infrequent)
– wet snow avalanches only(Rare)
Leeward
on or toward the side sheltered from the wind or toward which the wind is blowing; downwind.
“Warm Slopes”
Vs.
“Cold Slopes”
“warm” slopes – both south- and west-facing – are prone to point release avalanches in sunny and warm weather
“cold” slopes – both north- and east-facing – are prone to point release avalanches in cold weather
North facing slopes receive little heat from the sun in mid- winter
South facing slopes receive much more heat from the sun in mid- winter
east facing slopes catch sun only in the morning when temperatures are colder while west facing slopes catch the sun in the warm afternoon
consequently, east facing slopes are colder than west facing slopes.
cold snowpack tends to develop more persistent weak-layers, such as surface hoar or depth hoar, than a warm snowpack
majority of avalanche accidents occur on north and east facing slopes
where strong surface melting creates wet snow conditions, it is just the opposite of a dry snow pack
south and west facing slopes will usually produce more wet avalanches than more shaded slopes
majority of avalanche accidents occur on ____ and _____ facing slopes
north and east
-cold snowpack tends to develop more persistent weak-layers, such as surface hoar or depth hoar, than a warm snowpack
_____ and _____
facing slopes will usually produce more wet avalanches than more shaded slopes
south and west
-where strong surface melting creates wet snow conditions, it is just the opposite of a dry snow pack
Slope shape:
convex
vs. concave
Convex:
.No compressive support asgravity pulls snowaway from the low angle, anchored snow above
Concave:
Compressive support zone where gravity pulls snowinto the ground or lower layers helping to anchor it
In Canada, avalanche risk is most likely in regions with more then __ cm snow depths like…4
Canadian Cordillera – Alberta, B.C., Yukon, NWT
- eastern Canadian Arctic – Baffin, Devon, Ellesmere Islands
- Torngat Mountains, Newfoundland & Labrador
- north shore of St. Lawrence River, Gaspé Peninsula, Québec
Minimizing Avalanche Risk
- Create Hazard Maps
- Structures located in start zone-avalanche support structures to support the snow pack or reduce snow accumulation above start zone
- Structures located in track or run-out zone-berms, splitting wedges, snow sheds
berms, splitting wedges, snow sheds are examples of
Structures located in track or run-out zone- to mitigate avalanches
snow fences are an example of
Structures located in start zone-avalanche support structures to support the snow pack or reduce snow accumulation above start zone
Avalanche Stability and Strength Tests(2)
compression test:
application of vertical force on a column of undisturbed snow
rutschblock test: skier steps, then jumps, on a block of snow, 2 m across and 1.5 m long
assign a score based on the force required to release a block from upper part of snow column
Avalanche Danger scale(5)
Low(Green): unlikely and generally safe
Modertae(Yellow):unlikely but use caution
Considerable(Orange):possible and be cautious
High(Red): likely and travel not reccomended
Extreme(Red with black Lines): certain and travel should be avoided
