Avalanches a warning

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Avalanches - a warning
http//www.youtube.com/watch?v6qVwIuznFW0
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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.
• gt1 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
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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
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,
(Mountaineers 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 IPoint-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 IISlabs

• 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 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.

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(No Transcript)
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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
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most large slabs
frequent sluffs
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frequent
rare
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infrequent
rare
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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

• Wind speedhazard increases if wind gt25 km/h.
• Snowfall forecastlt0.3 m snow depth - no
  hazard.gt1.0 m - major risk.
• Temperature change hazard increases if T gt0C.

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Avalanche forecasting(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
In Switzerland and some parts of US red zones
have avalanche return intervals lt30 yrs or large
avalanches (impacts gt30 kPa) lt300 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 lt 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
1990s were significantly thinner than in any
decade since the 1930s. 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

• 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
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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Subsidence and local ground failure
Expansive soils

• associated with smectite clays and frost-heaving
• annual cost gt1000M in North America

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Sinkholes

• 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
Occur in soluble carbonates or evaporites
limestone dolomite gypsum halite
Relative solubility
1 1 150 7500
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Stage 1 - Cavern formation Stage 2
- Sinkhole formation
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House for scale
Large sinkhole, central Florida
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Sinkhole formation in halite, Dead Sea
sinkholes collapse above halite caverns


fresh water
Dead Sea
halite
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1912 survey of one land section in
Indiana, showing numerous sinkholes
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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

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Sinkhole detection(ground-penetrating radar
imagery)
soil
sinkhole
limestone
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Sinkhole detection electro-resistivity
techniques
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Global distribution of vertisols
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Vertisol profile
Note blocky structure and uniform black upper
horizons
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Vertisols - Gilgai
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Vertisols-dry season shrinkage and cracking
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Vertisols - Slickensides
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Smectite clay minerals expansive soils
H2O
Graphic www.smianalytical.com
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Damage to buildings on expansive soils
Farm buildings, Idaho
House, Texas
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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!)

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Future problemse.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 (gt50 of new
  construction on these soils in some counties).
• Source Williams (2003) Environmental Geology
  44 933-938