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Snow Avalanches

Avalanches are natural disasters characterized by falling masses of snow and ice, posing significant threats to life, property, and transportation routes. They result from complex interactions involving terrain, weather, and snowpack dynamics. Knowing their formation factors is crucial for recreationists and communities alike. This overview discusses avalanche types, accident statistics, forecasting methods, and the latest research in avalanche management. Recognizing the risks and understanding how to mitigate them can enhance safety in both backcountry and ski area environments.

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Snow Avalanches

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  1. Snow Avalanches

  2. what is an avalanche? • a falling mass of snow and/or ice • a mass-wasting process • analogous to debris flows or mudslides • a natural hazard - threat to life and property • an expression of earth system complexity

  3. why do we care? • recreation • ski areas • backcountry • transportation • highways • railroads • communities/structures • avalanches are cool Westwide Avalanche Network

  4. outline • overview of: • avalanche interests • accident statistics • avalanche formation factors • current research

  5. ski area avalanche management • snow compaction • skier traffic • boot packing • intentional avalanche release • explosives • ski cutting • cornice management Westwide Avalanche Network

  6. backcountry recreation • no active control • who gets caught – trends in avalanche statistics • avalanche accidents • forecasting • education • be your own snow expert Westwide Avalanche Network

  7. us avalanche fatalities by winter

  8. us avy fatalities by state

  9. us avy fatalities by age group

  10. fatalities by activity

  11. recent trends • recent explosion in backcountry use • technological advances • ski gear • snowmobile power and design • “Extreme” hype • availability of avalanche education Gallatin National Forest Avalanche Center

  12. fatalities by state

  13. fatalities by age group

  14. fatalities by activity

  15. avalanches and transportation • highways and railways • control and reduction • high cost of link closure • potential for disaster

  16. transportation • active control WSDOT www.gaz-ex.com

  17. passive control www.eng.ucalgary.ca WSDOT www.eng.ucalgary.ca

  18. avalanche hazard to communities • primarily a European issue • higher alpine population density • resort development in US

  19. types of avalanches • loose snow (point release) • slab • soft slab • hard slab • distinction based on snow cohesiveness • can be wet or dry snow

  20. Loose Snow Avalanche • Tear drop shape • Unconsolidated • Wet or Dry • Often only surface snow • Point release • Sluff • Easier to predict

  21. slab avalanche terminology Starting zone Track Runout zone

  22. slab avalancheterminology • crown face • bed surface • flanks • stauchwall Class demonstration, stupid!

  23. avalanche formation factors • terrain • weather • snowpack • humans • good news: the snowpack is stable the majority of the time

  24. terrain • Is the terrain capable of producing an avalanche?

  25. terrain • factors to consider: • slope angle • slope size and consequences • slope shape • vegetation and trees • runout • aspect with respect to wind • elevation

  26. Is this avalanche terrain? terrain

  27. slope angle 60° • NOTE: referring to the steepest part of the slope 45° 30° 25° 0°

  28. slope shape • convexities and concavities zone of compression zone of tension Gravity’s influence snow slab

  29. Trigger Points ROCK TREE CLIFF CONVEX CONCAVE PILLOW

  30. vegetation and trees • indicators of avalanche activity • tree “flagging” • secondary growth • trees can anchor snow

  31. vegetation and trees • ground cover affects: • effective snow depth • heat transfer ~ snow metamorphism in basal layers

  32. vegetation change • implications of • climate change • timber cutting • creation of starting zones • forest fires • removes ground cover, thins trees • large avalanche events

  33. terrain trap

  34. weather • Is the weather affecting the snow stability? • Precipitation (snow or rain) • Wind • Temperature

  35. precipitation • addition of mass to the snowpack • rate of addition is important • stress vs. strength strength stress θ compression gravity θ

  36. wind • snow redistribution • change in snow density • cornices

  37. Aspect In Relation To Wind WIND WIND Lee Loaded Cross Loaded

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