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Avalanches - a warning

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

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  1. Avalanches - a warning http://www.youtube.com/watch?v=6qVwIuznFW0

  2. Avalancheprerequisites • snow accumulationsand • steep topography Mean snow depth, February (cm)

  3. Avalanche fatalities (1998-9) Kangiqusualujjaq

  4. 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.

  5. Avalanche fatalities per year: North America

  6. Source: New Scientist

  7. Avalanche deaths, N. America (2002-3) Activity Fatalities Skiers 25 Snowmobilers 23Climbers 5 Snowboarders 4 Hikers 1Total 58

  8. “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

  9. 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

  10. 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.

  11. Juneau, Alaska(a city at risk) Mountains 5 - 10 m of snow? City receives ~2.5m of snow per year

  12. 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.

  13. 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

  14. 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…..

  15. 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)

  16. Avalanche types and triggers from ‘The Province’ Jan. 04, 1998

  17. 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

  18. 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

  19. Slab avalanches Failures are a result of layered snowpacks

  20. 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!

  21. 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.

  22. Surface hoarice crystals commonly ~10 mm long Photo: K.Williams

  23. Strengthening of surface hoar layer over time Avalanches Graph: Chalmers and Jamieson (2003) Cold Reg. Sci. Tech. 37, 373-381.

  24. Snow stability: Rutschblock test

  25. Snow stability testing Surface test Bench test failure plane at depth Images: Landry et al. (2001) Cold Reg. Sci. Tech. 33, 103-121.

  26. Effects of slope angle rare infrequent 60 most large slabs frequent sluffs 45 frequent rare 30 infrequent rare 25 Point release Slabs

  27. Avalanche hazard and aspect leeward?windward? north-facing? south-facing? shaded sunnylittle T° fluc. large T° fluc. Photo: R. Armstrong

  28. start zone track run-out zone

  29. 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

  30. Avalanche forecasting • 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.

  31. 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

  32. 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.

  33. Avalanche protection structures (snow nets) ~5 m high

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

  35. Protecting transportation corridors: e.g Coquihalla Hwy.

  36. 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

  37. 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.

  38. 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.

  39. 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)

  40. 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

  41. 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 Subsidence and local ground failure Expansive soils • associated with smectite clays and frost-heaving • annual cost >$1000M in North America

  42. 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)

  43. Sinkholes Occur in soluble carbonates or evaporites limestone dolomitegypsum halite Relative solubility 1 1150 7500

  44. Stage 1 - Cavern formation Stage 2 - Sinkhole formation

  45. House for scale Large sinkhole, central Florida

  46. Sinkhole formation in halite, Dead Sea sinkholes collapse above halite caverns * * fresh water Dead Sea halite

  47. 1912 survey of one land section in Indiana, showing numerous sinkholes

  48. 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

  49. Sinkhole detection(ground-penetrating radar imagery) soil sinkhole limestone

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