Presentation on theme: "Avalanches - a warning"— Presentation transcript:
Avalanches - a warning
Avalanche prerequisites 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 m 3 to 100 x 10 6 m 3. 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
Source: New Scientist
Avalanche deaths, N. America (2002-3) ActivityFatalities Skiers25 Snowmobilers23 Climbers 5 Snowboarders 4 Hikers 1 Total58
“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
Kootenay avalanches, Jan. 03, 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 Mountains m of snow?
Snowfall and avalanche hazards More than 70 people died in the Alps in the winter of 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 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 move at km/h; develop powder clouds Wet avalanches move at 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 hoar ice 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,
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,
Effects of slope angle Point release Slabs frequent sluffs frequent rare infrequent rare most large slabs rare
Avalanche hazard and aspect Photo: R. Armstrong leeward?windward? north-facing?south-facing? shaded sunny little T° fluc.large T° fluc.
start zone track run-out zone
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: 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 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 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
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, above below
Will global warming reduce the avalanche hazard in temperate alpine areas? Scott and Kaiser (2003?) Amer. Met.Soc Conference; pdf 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 m 3 of ice swept 20 km down-valley, killing ~100 people and burying a village. A similar event occurred in the same valley in Kolka Glacier avalanche debris *cf. Mt.Yungay, Peru (1970)
Subsidence and local ground failure = vertical displacement of the ground surface D, v Vertical displacement Velocity slight large slow fast sinkholes expansive soils surface loading 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 collapse e.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
Occur in soluble carbonates or evaporites Relative solubility limestone dolomite gypsum halite
Large sinkhole, central Florida House for scale
Sinkhole formation in halite, Dead Sea Dead Sea halite fresh water sinkholes collapse above 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
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: