Snow Mechanics / Avalanche Dynamics


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  • NOTE TO PRESENTER Copyright ©2000-2002 , Howard M. Laney. All rights reserved. Use of this material constitutes agreement with the following terms and conditions. This material is licensed for use by National Ski Patrol, Inc. (NSP) certified instructors, for NSP registered Avalanche courses only. The intent of the copyright is to maintain quality control over the product. Periodic upgrades to this presentation will be made available to qualified users, through NSP distribution channels. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - We have listed avalanche factors separately, but be aware that they are all acting simultaneously and interactively. Paradoxically, each factor may contribute either to stability or to instability, depending on what other factors are at play in any given situation. Sound interesting?
  • What is missing from the diagram? Should people be included as factors that produce avalanches?
  • To understand how this works, you need to know something about the characteristics of snow.
  • Gravity and temperature are constantly manipulating the snow pack. Stress occurs in three basic forms. All three stresses are active simultaneously in the snow pack. All three must cause brittle failure in the snow pack to produce an avalanche. How does gravity cause stress? How does temperature cause stress?
  • Difficult to clearly state which factors contribute to stability or instability. It can go either way, depending on its interaction with other factors.
  • Note the thick snow deposits at a lower elevation than the obvious snow line on the right half of the picture.
  • Knobs produce a triple threat for overloading.
  • What do 1-3 indicate? solar radiation striking the snow surface Penetration of light into the snow pack (small amount) Most light reflected back into the atmosphere What does 4-6 indicate? 4. Heat is radiated from the snow into the atmosphere. 5. Some penetrates the atmospheric layers 6. Most is reflected back toward the surface What does 7-9 indicate? 7. Heat energy may strike a cloud 8. Some heat is reflected back to the surface 9. Most works its way into the upper atmosphere (becoming 4) What can this tell us about changes in the snow pack on a clear or cloudy day? What can this tell us about changes in the snow pack on a clear or cloudy night?
  • Remember that snow is visco-elastic. It can deform and move in response to force (stress). The whole snow pack is slowly flowing down hill (Glide). Newer, less consolidated snow can glide faster than older, consolidated, compressed snow, so the upper layers move at a faster rate than the lower layers (creep) Explain how this relieves some stresses, but increases others.
  • Dig around an isolated rock or tree. Is the snow firmly bonded to it? Is the snow of the same structure and consistency as the rest of the adjacent snow pack? If not, why not? There is stress in contours, but it is most pronounced where the contour changes Why so? Rock interfaces consist of large outcroppings, cliff bands What happens there? Sun lines are where rock outcroppings, forest margins, or ridge lines create significant shading of a portion of a slope while the rest gets a dose of sun. How would this affect stress in the snow pack? How do pillows affect stress? Is the stress evenly or unevenly distributed? Slab margins are where the relatively thin areas of the snow pack. How does this contribute to stress? One might think that brush would serve as an excellent anchor point. Who has walked through the snow covering brush? Describe the experience. (constant falling through, large voids, etc.– they’ll get the point)
  • Remember my earlier question regarding the shape of a slope? What is a possible effect of a weak layer here? What effect does the concavity have on compression? What effect does the convexity have on tension? What’s a snow cushion? What is its effect here? What effect does a rock or cliff band have on tension of snow beneath it?
  • What are these? (glide cracks) Where else are they found? (downhill side of cliff bands, rock outcroppings, etc.) What do they indicate about slope stability? Indicate deformation of snow layers through movement downhill May indicate stress relief in the slab through deformation May indicate increase of tension stress in convex portions of the slab &/or loss of anchoring in tension zone May indicate increase in shear stress on underlying layers. May indicate location of isolated stress points within the slope.
  • Anchors are large objects fixed to the ground that may block or inhibit the down hill motion of snow. Remember the avalanche you were asked to classify earlier? What did the trees seem to do?.
  • Remember the previous slide on anchors? They are temporary at best.
  • So, what causes a snow pack to avalanche? What causes Loose Snow releases?
  • These avalanches are the result of a cascading effect, where one moving particle dislodges another particle, which then dislodge others, and so on.
  • Snow sliding off steep upper slopes can overload and strain the stability of lower slopes. They may also provide move snow loads to areas with less slope, which releases loading stress and actually has a stabilizing effect. They might also trigger slab avalanches on already unstable lower slopes. . . .And that brings us to Slab Avalanches
  • Guess what kind of snow has been known to avalanche on slopes of less than 20 degrees? Why? Demo Tip: Bring in a pan of wet slush. Scoop up a handful of slush and press it into a snowball, pressing out the water. It may hold together. Return the snowball to the pan & let it soak up some of the loose water. If it does not collapse on its own, lightly press on it with a thumb, it should collapse.
  • To understand how this works, you need to know something about the characteristics of snow.
  • The weight of the skier caused the weak layer to collapse, all but eliminating shear strength on the bed surface. Tensile strength is overloaded and fails. Shear strength along the flanks is all that’s left to hold the slab in place – not enough – it, too, quickly fails. The slab is free to move. This is now considered the most common process.
  • What are some characteristic properties of Snow? Viscous Elastic Resilient Dynamic (metamorphism) Stress What are the forms of stress? Which form is most important? What must stress do to initiate an avalanche Strength What factors tend to increase snow pack strength? What factors tend to decrease snow pack strength? Stress Relief What dynamics relieve stress in a snow pack? Compare viscous properties with elastic properties as they relate to avalanche potential.
  • Snow Mechanics / Avalanche Dynamics

    1. 1. Snow Mechanics/Avalanche Dynamics
    2. 2. Objectives – can you: <ul><li>describe how gravity, temperature and slope angle affect the snow pack over time? </li></ul><ul><li>identify potential stress points in the snow pack? </li></ul><ul><li>describe differences between point and fracture initiation? </li></ul>
    3. 3. Triangle factors interact dynamically to produce avalanches Snow pack Weather Terrain
    4. 4. Avalanches occur when the strength of the snow pack is overcome by stress to produce failure. Stress Strength
    5. 5. Snow Pack Stress
    6. 6. Factors that add/reduce stress <ul><li>Add stress or weaken strength* </li></ul><ul><ul><li>Loading </li></ul></ul><ul><ul><li>Slope angle </li></ul></ul><ul><ul><li>Contour </li></ul></ul><ul><ul><li>Diurnal temp. change </li></ul></ul><ul><ul><li>Free water flow </li></ul></ul><ul><ul><li>Metamorphism (faceting) </li></ul></ul><ul><li>Reduce stress or add strength* </li></ul><ul><ul><li>Surface Friction </li></ul></ul><ul><ul><li>Settlement </li></ul></ul><ul><ul><li>Anchors </li></ul></ul><ul><ul><li>Metamorphism (rounding) </li></ul></ul><ul><ul><li>Internal motion (deformation) </li></ul></ul>
    7. 7. Gradual loading increases settlement. Rapid loading leads to brittle failure
    8. 8. Rapid Loading Awakens the Dragon
    9. 9. Microclimate Loading Can Be Extreme
    10. 10. Energy exchange (as heat)
    11. 11. Internal motion
    12. 12. Stress Points <ul><li>Isolated anchors </li></ul><ul><li>Contour changes </li></ul><ul><li>Rock interfaces </li></ul><ul><li>Sun lines </li></ul><ul><li>Pillows </li></ul><ul><li>Slab Margins </li></ul><ul><li>Brush </li></ul>
    13. 13. Contour stress
    14. 14. Rock Interfaces
    15. 15. Anchors
    16. 16. Anchors get covered
    17. 17. Avalanche Dynamics <ul><li>Loose Snow Releases </li></ul><ul><li>Wet Snow Releases </li></ul><ul><li>Slab Releases </li></ul>
    18. 18. Loose Avalanche Release
    19. 19. Consequential factors
    20. 20. Slush / Wet Release
    21. 21. Slab Release <ul><li>Slab? </li></ul><ul><li>Consolidated snow </li></ul><ul><li>Cohesive new snow </li></ul><ul><li>Weak Layer? </li></ul><ul><li>Graupel </li></ul><ul><li>Surface Hoar </li></ul><ul><li>Rime </li></ul><ul><li>Facets </li></ul><ul><li>Crusts </li></ul><ul><li>Trigger? </li></ul><ul><li>Rapid loading </li></ul><ul><li>Rapid Temperature Change </li></ul>
    22. 22. The trigger is the event that tips the balance between strength and stress. Stress Strength Stress Relief
    23. 24. Avalanche Characteristics by Climate Zone Factors Maritime Intermountain Continental Altitude 4,000-7,000 7,000-11,000 10,000-14,000 Cumulative Snowfall 600” + 400” 250” Storm Snowfall Many 12”+ Some 12”+ Few 12”+ Temperature Seldom<0 o F <0 o & >32 o F <32 o F Wind Moderate Strong Very strong Usual Type of Avalanche Slab (damp-wet) Slab (damp-dry) Slab (dry) Release Direct Direct Often Delayed Example Squaw Valley, CA Alta, UT Berthoud Pass, CO
    24. 25. Recap: Strength vs. Stress <ul><li>Properties of snow </li></ul><ul><li>Stress </li></ul><ul><li>Strength factors </li></ul><ul><li>Stress relief </li></ul>