Avalanches - a warning

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

Avalancheprerequisites

• snow accumulations

and

• steep topography

Mean snow depth, February (cm)

Avalanche fatalities (1998-9)

Kangiqusualujjaq

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

Source: New Scientist

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.

Juneau, Alaska(a city at risk)

City receives~2.5m of snow per year

Mountains5 - 10 m of snow?

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)

Avalanchetypes and

triggers

from ‘The Province’Jan. 04, 1998

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 Failures are a result of layered snowpacks

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.

Surface hoarice crystals commonly ~10 mm

long

Photo: K.Williams

Strengthening of surface hoar layer over time

Avalanches

Graph: Chalmers and Jamieson (2003) Cold Reg. Sci. Tech. 37, 373-381.

Snow stability: Rutschblock test

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.

startzone

track

run-outzone

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.

Avalanche protection structures

(snow nets) ~5 m high

Andermatt,Switzerland.

Village protected by

fences to hold snowpack, and forest (cutting forbidden by

C13th by-law)

Protecting transportation

corridors: e.g

Coquihalla Hwy.

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

Large sinkhole, central Florida

House for scale

Sinkhole formation in halite, Dead Sea

Dead Seahalite

freshwater

sinkholes collapseabove halite caverns

* *

1912 survey of one land section in Indiana,showing numerous sinkholes

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

Sinkhole detection(ground-penetrating radar imagery)

soil

limestone

sinkhole

Sinkhole detection: electro-resistivity techniques

Global distribution of vertisols

Vertisol profile

Note blocky structure and uniform black upper horizons

Vertisols - ‘Gilgai’

Vertisols-dry season shrinkage and cracking

Vertisols - ‘Slickensides’

Smectite clay minerals = expansive soils

Graphic: www.smianalytical.com

H2O

Damage to buildings on expansive soils

Farm buildings, Idaho House, Texas

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