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Avalanches - a warning. http://www.youtube.com/watch?v=6qVwIuznFW0. Avalanche prerequisites. snow accumulations and steep topography. Mean snow depth, February (cm). Avalanche fatalities (1998-9). Kangiqusualujjaq. Avalanche facts and figures (Canada).

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Avalanches a warning l.jpg
Avalanches - a warning

http://www.youtube.com/watch?v=6qVwIuznFW0


Avalanche prerequisites l.jpg
Avalancheprerequisites

  • snow accumulationsand

  • steep topography

Mean snow depth, February (cm)



Avalanche facts and figures canada l.jpg
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 deaths, N. America (2002-3)

Activity Fatalities

Skiers 25

Snowmobilers 23Climbers 5

Snowboarders 4

Hikers 1Total 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 one

of 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)

Mountains

5 - 10 m of snow?

City receives

~2.5m of snow per year


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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)

Place Deaths

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


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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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Avalanche

types 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, powdery layer of unbroken crystals (the weak layer). If thewind 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.


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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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Snow stability testing

Surface test Bench test

failure plane at depth

Images: Landry et al. (2001) Cold Reg. Sci. Tech. 33, 103-121.


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Effects of slope angle

rare

infrequent

60

most large slabs

frequent sluffs

45

frequent

rare

30

infrequent

rare

25

Point release

Slabs


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Avalanche hazard and aspect

leeward?windward?

north-facing? south-facing?

shaded sunnylittle T° fluc. large T° fluc.

Photo: R. Armstrong


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start

zone

track

run-out

zone


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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 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

  • 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: 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(Centre for Snow Studies, Grenoble, France)

3-phase model

SAFRAN

Predicts average weather for 23 zones in Alps;

Predicts snowpack changes; (errors tend to accumulate)

Predicts snow stability

CROCUS

MEPRA


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Protecting settlements 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

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 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

(snow nets)

~5 m high


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Andermatt, 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 timberSwitzerland.Village protected by fences to hold snowpack, and forest (cutting forbidden by C13th by-law)


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Protecting 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

transportation corridors:

e.g

Coquihalla Hwy.


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Protecting highway links 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

  • 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


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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?

above

below

Laternser and Schneebeli (2003) Int. J. Climatology 23, 733-750.


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Will global warming reduce the avalanche hazard in temperate alpine areas?

Above normal

Below normal

Scott and Kaiser(2003?) Amer. Met.Soc Conference; pdf 71795.


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Ice avalanches* alpine areas?

  • 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)


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Subsidence and local ground failure alpine areas?

before

= vertical displacement of the ground surface

D, v

after

Velocity

fast

slow

slight

expansive

soils

Vertical

displacement

surface

loading

sinkholes

large


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Sinkholes: alpine areas?

  • 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

Subsidence and local ground failure

Expansive soils

  • associated with smectite clays and frost-heaving

  • annual cost >$1000M in North America


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Sinkholes alpine areas?

  • 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)


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Sinkholes alpine areas?

Occur in soluble

carbonates or evaporites

limestone dolomitegypsum halite

Relative solubility

1 1150 7500


Slide45 l.jpg

Stage 1 - alpine areas?

Cavern

formation

Stage 2 -

Sinkhole

formation


Slide46 l.jpg

House for scale alpine areas?

Large sinkhole, central Florida


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Sinkhole formation in halite, Dead Sea alpine areas?

sinkholes collapse

above halite caverns

*

*

fresh

water

Dead Sea

halite


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1912 survey of one land section in Indiana, alpine areas?

showing numerous sinkholes


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Subsidence and local ground failure alpine areas?

  • 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


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Sinkhole detection alpine areas?(ground-penetrating radar imagery)

soil

sinkhole

limestone


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Sinkhole detection: alpine areas?electro-resistivity techniques



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Vertisol profile alpine areas?

Note blocky structure and uniform black upper horizons





Smectite clay minerals expansive soils l.jpg
Smectite clay minerals = expansive soils alpine areas?

H2O

Graphic: www.smianalytical.com


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Damage to buildings on expansive soils alpine areas?

Farm buildings, Idaho House, Texas


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How significant is the problem? alpine areas?

  • 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!)


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Future problems: alpine areas?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