Avalanches - a warning
http://www.youtube.com/watch?v=6qVwIuznFW0
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.
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
Avalanche deaths, N. America (2002-3)
Activity Fatalities
Skiers 25Snowmobilers 23Climbers 5Snowboarders 4Hikers 1Total 58
“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
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
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.
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.
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
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…..
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)
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
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
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!
Formation of weak layers in snowpacks
• In calm conditions snow settles as a fluffy, powdery layer of unbroken crystals (the weak layer). If the wind speed increases, a layer of dense broken crystals settles on top (the slab).• Cold air over a thin snowpack can create ‘depth hoar’ near the base of the snowpack. Water vapour sublimates from pores in snow onto ice crystals (produces a weak layer).• Surface hoar forms on cold, clear nights. Ice crystals are large and have weak cohesion.
Strengthening of surface hoar layer over time
Avalanches
Graph: Chalmers and Jamieson (2003) Cold Reg. Sci. Tech. 37, 373-381.
Surface test Bench test
failure plane at depth
Snow stability testing
Images: Landry et al. (2001) Cold Reg. Sci. Tech. 33, 103-121.
Effects of slope angle
Point release Slabs
60
45
30
25
frequ
ent s
luffs
frequent
rareinfrequent
infrequentra
re
most large
slabs
rare
Avalanche hazard and aspect
Photo: R. Armstrong
leeward? windward?
north-facing? south-facing?shaded sunnylittle T° fluc. large T° fluc.
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
Avalanche forecasting
• Wind speed:hazard increases if wind >25 km/h.
• Snowfall forecast:Snowfall forecast:<0.3 m snow depth - no hazard.>1.0 m - major risk.
• Temperature change: hazard increases if T >0°C.
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
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.
Andermatt,Switzerland.
Village protected by
fences to hold snowpack, and forest (cutting forbidden by
C13th by-law)
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
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Will global warming reduce
the avalanche hazard in
temperate alpine areas?
Data from Switzerland show
that snowpacks in the 1990’s were
significantly thinner than in any
decade since the 1930’s. Natural
variation or global warming?
Laternser and Schneebeli (2003) Int. J. Climatology 23, 733-750.
above
below
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Will global warming reduce the avalanche hazard in temperate alpine areas?
Scott and Kaiser (2003?) Amer. Met.Soc Conference; pdf 71795.
Below normal Above normal
Ice avalanches*• On September 21, 2002 the terminus of the
Kolka Glacier in the Caucasus Mountains collapsed, and some 4 M m3 of ice swept 20 km down-valley, killing ~100 people and burying a village. A similar event occurred in the same valley in 1902.
Kolka Glacier
avalanchedebris
*cf. Mt.Yungay, Peru (1970)
Subsidence and local ground failure
= vertical displacement of the ground surface D, v
Vert
ical
dis
pla
cem
en
t
Velocity
slight
large
slow fast
sinkholes
expansivesoils
surfaceloading
before
after
Subsidence and local ground failure
Expansive soils
Sinkholes:
•associated with soluble rocks - carbonates and evaporites plus mining activities•annual cost ~$10M in North America
Subsidence: •associated with tectonics, surface loading,agricultural drainage and fluid extraction•annual cost ~$100M in North America
•associated with smectite clays and frost-heaving•annual cost >$1000M in North America
• Characterized by rapid surface collapsee.g. New Mexico (1918) a sinkhole 25m wide by 20 m deep formed in a single night.
• Individual holes small, but may be locally numerous
• Collapse behaviour unpredictable; often triggered by heavy rain, which causes loading of soil and sinkhole collapse (e.g. in Pascoe Co., Florida., twice as many sinkholes are reported in wet season vs. dry season)
Sinkholes
Sinkholes
Occur in soluble carbonates or evaporites
Relative solubility
limestone dolomite gypsum halite
1 1 150 7500
ShaleSoftLst.HardLst. springcaverns
ShaleSoftLst.HardLst. springcavernsSinkholeCavern roof/conduit collapse
Stage 1 -Cavernformation
Stage 2 -Sinkholeformation
Sinkhole formation in halite, Dead Sea
Dead Seahalite
freshwater
sinkholes collapseabove halite caverns
* *
Subsidence and local ground failure
• Effects - damage to urban and suburban infrastructure
• Detection - e.g. GPR and ER (see next slide)
• Mitigation - non-intensive land uses on affected land to minimize hazard
How significant is the problem?
• Expansive soils are the #1 cause of structural damage to buildings and urban infrastructure (roads, sidewalks, pipelines) in the US.
• Annual losses ~ US-$2 - $7 G (probably x2 the amount associated with all other natural hazards!)
Future problems:
e.g. Dallas, TX• Expansive soils (= ‘low
urbanization potential’) are predominant on the interfluves of the plains of north Texas.
• Suburban construction is increasingly moving onto these soils in as low and medium risk soils reach their development capacity (>50% of new construction on these soils in some counties).
Source: Williams (2003) Environmental Geology 44: 933-938
Top Related