powerpoint presentation - snow avalanches€¦ · jan. 03, 1998 • 6 heli-skiers die in kokanee...
TRANSCRIPT
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Avalanches - a warning
http://www.youtube.com/watch?v=6qVwIuznFW0
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Avalancheprerequisites
• snow accumulations
and
• steep topography
Mean snow depth, February (cm)
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Avalanche fatalities (1998-9)
Kangiqusualujjaq
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Avalanche facts and figures(Canada)
• range in size from few 100 m3 to 100 x 106
m3.• most occur in remote mountain areas.• >1 million events per yr in Canada• 100 avalanche ‘accidents’ (casualties,
property damage) reported per yr.• Estimated that 1 avalanche in 3000 is
potentially destructive.
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Avalanche fatalities per year: North America
0
5
10
15
20
25
30
35
40
85-6 86-7 87-8 88-9 89-0 90-1 91-2 92-3 93-4 94-5 95-6 96-7 97-8 98-9 99-0 00-1 01-2 02-3 03-4
Canada
USA
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Source: New Scientist
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Avalanche deaths, N. America (2002-3)
Activity Fatalities
Skiers 25
Snowmobilers 23
Climbers 5
Snowboarders 4
Hikers 1
Total 58
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“Avalanches kill
eight in B.C.”
Headline in “The Province”
(Jan. 04, 1998)
“we have a real disaster on our hands ….this is oneof the worst weekends on record”
Alan Dennis, Canadian Avalanche Centre
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Kootenay avalanches, Jan. 03, 1998
• 6 heli-skiers die in Kokanee Glacier Park
• 2 skiers die on Mt. Alvin, near New Denver
• 1 snowmobiler dies (4 buried) near Elliot Lake
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Avalanches in inhabited areas (e.g. the Alps)
• On 9th February 1999 in the afternoon a large avalanche destroyed 17 buildings on the edge of Montroc and killed twelve: vertical drop 2500m to 1300m, horizontal length 2.25Km, deposit depth 6m. The map shows known avalanche paths in the area, with the 1999 avalanche circled.
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Juneau, Alaska(a city at risk)
City receives~2.5m of snow per year
Mountains5 - 10 m of snow?
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Snowfall and avalanche hazards
More than 70 people died in the Alps in the winter of
1998-9 as a result of avalanches resulting from the heaviest snowfalls in
50 yrs. There was extensive damage to
property (e.g. Morgex, Italy), and many tourists
were stranded.
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Deaths in villages (1998-9)
Kangiquasualujjaq, Qué 9 in school gym
Darband, Afghanistan 70 in village
Gorka, Nepal 6 in village
Le Tour, France 12 in ski resort/village
Galtuer, Austria 20 in ski resort/village
Place Deaths
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Bruce Tremper Staying Alive in Avalanche Terrain, (Mountaineer’s Books):
“most avalanches happen during storms but most avalanche accidents occur on the sunny days following storms. Sunny weather makes us feel great, but the snow-pack does not always share our opinion”.
And elsewhere: People who are most likely to die are those whose skills at their sport (e.g. snowboarding) exceed their skill at forecasting avalanches.So, some basics…..
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Avalanche triggers
• Snowstorms dump thick snowpacks over surface hoar (increased weight)
• Vehicles or skiers increase weight on pack
• Surface heating (sunshine, warm airmass) weakens snowpack
• Gravitational creep
• Shaking (seismic, explosives), but rarely low noise (shouts, aircraft overhead)
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Avalanchetypes and
triggers
from ‘The Province’Jan. 04, 1998
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Avalanche types I:Point-release
• start at a point in loose, cohesionless snow;
• downslope movement entrains snow from sidewalls
• in dry snow they are relatively small
• in wet snow they can be large and destructive
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Avalanche types II:Slabs
•layers of cohesive snow may fail as a slab
•can be triggered from below•fracture must occur around the perimeter (crown, flanks and toe
[or stauchwall])•depth controlled by depth to
failure plane
crown
flank
toe
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Slab avalanches Failures are a result of layered snowpacks
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Slab avalanches: dry and wet
Dry avalanches moveat 50-200 km/h;
develop powder clouds
Wet avalanches moveat 20-100 km/h;(denser & slower)
most dangerous!
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Formation of weak layers in snowpacks
• In calm conditions snow settles as a fluffy, powderylayer of unbroken crystals (the weak layer). If thewind speed increases, a layer of dense brokencrystals settles on top (the slab).
• Cold air over a thin snowpack can create ‘depthhoar’ near the base of the snowpack. Water vapoursublimates from pores in snow onto ice crystals(produces a weak layer).
• Surface hoar forms on cold, clear nights. Icecrystals are large and have weak cohesion.
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Surface hoarice crystals commonly ~10 mm long
Photo: K.Williams
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Strengthening of surface hoar layer over time
Avalanches
Graph: Chalmers and Jamieson (2003) Cold Reg. Sci. Tech. 37, 373-381.
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Snow stability: Rutschblock test
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Surface test Bench test
failure plane at depth
Snow stability testing
Images: Landry et al. (2001) Cold Reg. Sci. Tech. 33, 103-121.
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Effects of slope angle
Point release Slabs
60
45
30
25
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Avalanche hazard and aspect
Photo: R. Armstrong
leeward? windward?
north-facing? south-facing?shaded sunnylittle T° fluc. large T° fluc.
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startzone
track
run-outzone
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Effects of clearcutting in mountainous terrain.
A wet slab avalanche was generated from a
clearcut block on a 37°slope at Nagle Creek,
BC (1996). It split into six separate avalanche paths, which destroyed
$400K of timber
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Avalanche forecasting
• Wind speed:hazard increases if wind >25 km/h.
• Snowfall forecast:<0.3 m snow depth - no hazard.>1.0 m - major risk.
• Temperature change:hazard increases if T >0°C.
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Avalanche forecasting:(Centre for Snow Studies, Grenoble, France)
SAFRAN
CROCUS
MEPRA
Predicts average weather for 23 zones in Alps;
Predicts snowpack changes; (errors tend to accumulate)
Predicts snow stability
3-phase model
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Protecting settlements
In Switzerland and some parts of US ‘red zones’ have avalanche return intervals <30 yrs or large avalanches (impacts >30 kPa) <300 yrs. Building is prohibited in these areas.
In ‘blue zones’ the upslope walls of a building must be reinforced or include a deflecting wedge.
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Avalanche protection structures
(snow nets)~5 m high
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Andermatt,Switzerland.Village protected by fences to hold
snowpack, and forest (cutting forbidden by C13th by-law)
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Protecting transportation
corridors: e.g
Coquihalla Hwy.
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Protecting highway links
Boston Bar (Coquihalla Highway)•71 avalanche paths producing ~100 events / yr.•RI varies from < monthly to ~25 yrs.•Forecasts from 5 weather stations (4 in alpine)•Defences:- snowsheds (#5 shed cost $12M)- raised highway; deflection dams; check dams- use of artillery and ropeways to initiate controlled events