Snow Avalanches

Question 1

Snow Avalanches

Answer 1

Masses of snow that separate from snow pack and slide or flow downslope.

Question 2

Slide

Answer 2

Movement as a coherent mass of snow

Question 3

Flow

Answer 3

Coherent mass of snow that rapidly disintegrates into small particles moving independently of one another

Question 4

Snowfall and accumulation influenced by:

Answer 4

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

Question 5

Avalanche initiation

point release avalanches

Answer 5

point-release avalanches

failure of small
volumes of loose snow (i.e., flows)

often occur after heavy snowfall events

failing snow initiates failure in adjacent snow pack; produces a distinct V-shaped, downslope-widening trough

Question 6

loose snow is unstable because:

Answer 6

1) increased mass on slope;
2) snow crystals have had little time to bond to one
another

Question 7

slab avalanches

Answer 7

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

Question 8

strength of snow pack influenced by:

Answer 8

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

Question 9

Weak layers

Answer 9

avalanches require a buried weak layer and an overlying stronger layer

Question 10

weak layers develop in several ways:

Answer 10

changes in air temperature during snowfall events

hoar frost formation within the snow pack

hoar frost formation at surface of snow pack

Question 11

“Right Side Up” storm

Answer 11

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

Question 12

“Upside Down” storm

Answer 12

ncreasing 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

Question 13

Surface Hoar

Answer 13

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

Question 14

Depth Hoar

Answer 14

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

Question 15

Avalanche Motion - Slide

Answer 15

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

Question 16

Avalanche Motion – Dry Snow

Answer 16

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

Question 17

Avalanche Motion – Wet Snow

Answer 17

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

Question 18

Start zone

Answer 18

location on hillslope where snowpack fails

Question 19

Track

Answer 19

path along which avalanche travels and accelerates to achieve its highest velocity

Question 20

Run-out zone

Answer 20

location where avalanche decelerates and snow is deposited

Question 21

Increase in mass of materials on slope

Answer 21

addition of snow during and soon after snow storms

weight of people traversing slopes

Question 22

Slope angle

Answer 22

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

Question 23

Avalanche Triggers

Answer 23

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-drifted snow is often much denser than non-wind loaded snow; adds significant weight on top of buried weak layers

Question 24

wind loading

Answer 24

is a common denominator in most avalanche accidents; wind can deposit snow 10 times more rapidly than snow falling from the sky

Question 25

slope aspect

Answer 25

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

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

Question 26

“warm” slopes

Answer 26

both south- and west-facing – are prone to point release avalanches in sunny and warm weather

Question 27

“cold” slopes

Answer 27

both north- and east-facing – are prone to point release avalanches in cold weather

Question 28

North facing slopes

Answer 28

receive little heat from the sun in mid- winter

Question 29

South facing slopes

Answer 29

receive much more heat from the sun in mid- winter

Question 30

east facing slopes

Answer 30

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.

Question 31

Avalanche risk greatest in regions characterized by steep slopes and annual mean snow depths (AMSD) > 50 cm

Answer 31

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

Question 32

Historical changes in pattern of avalanche mortality

Answer 32

development of transportation corridors

resource development in alpine environments

backcountry recreation

Question 33

Avalanche Fatalities in Canada

Answer 33

January, 1999 Fatalities: 9; injured: 25
Kangiqsualujjuaq, QC. Tonnes of snow cascaded down a cliff
and knocked out a school gymnasium wall.
Ten other buildings were evacuated.

February, 1965 Fatalities: 26; injured: 22
Granduc Mine, BC. Snow avalanche struck sleeping quarters of
mining camp.

March, 1910 Fatalities: 62
Rogers Pass, BC. Workers clearing snow from previous
avalanche on CP tracks buried by second avalanche.

Question 34

Minimizing Avalanche Risk

Location of infrastructure

Answer 34

assessment of recurrence interval of avalanche events

includes data on frequency and volume of avalanche events

construction of hazard maps: incorporates recurrence interval (i.e., 300 years) and potential impact forces (i.e., > 30 kPa) of avalanche events

Question 35

structures located in start zone – avalanche support structures designed to:

Answer 35

1) support snow pack, or

2) reduce snow accumulation above start zone

Question 36

Minimizing Avalanche Risk

Answer 36

explosives employed to release accumulation of snow on mountain slopes

prevent the release of large avalanches

Question 37

Stability and Strength Tests

compression test

Answer 37

application of vertical force on a column of undisturbed snow

Question 38

Stability and Strength Tests

rutschblock test

Answer 38

skier steps, then jumps, on a block of snow, 2 m across and 1.5 m long