©2000-2002 Mike Laney

Snow Mechanics/Avalanche Dynamics

©2000-2002 Mike Laney

Objectives – can you:

• describe how gravity, temperature and slope angle affect the snow pack over time?

• identify potential stress points in the snow pack?

• describe differences between point and fracture initiation?

©2000-2002 Mike Laney

Triangle factors interact dynamically to produce avalanches

Snow

Snow

pack

pack W

eather

Weather

TerrainTerrain

©2000-2002 Mike Laney

Avalanches occur when the strength of the snow pack is overcome by stress to

produce failure.

Stress Strength

©2000-2002 Mike Laney

Snow Pack Stress

©2000-2002 Mike Laney

Factors that add/reduce stress• Add stress or

weaken strength*– Loading– Slope angle– Contour– Diurnal temp.

change– Free water flow– Metamorphism

(faceting)

• Reduce stress or add strength*– Surface Friction– Settlement– Anchors– Metamorphism

(rounding)– Internal motion

(deformation)

©2000-2002 Mike Laney

Gradual loading increases settlement. Rapid loading leads to brittle failure

©2000-2002 Mike Laney

Rapid Loading Awakens the Dragon

©2000-2002 Mike Laney

Microclimate Loading Can Be Extreme

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Energy exchange (as heat)

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Internal motion

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Stress Points

• Isolated anchors• Contour changes• Rock interfaces• Sun lines

• Pillows• Slab Margins• Brush

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Contour stress

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Rock Rock InterfacesInterfaces

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Anchors

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Anchors get covered

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Avalanche Dynamics

• Loose Snow Releases• Wet Snow Releases• Slab Releases

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Loose Avalanche Release

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Consequential factors

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Slush /Wet

Release

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Slab Release

Slab?• Consolidated

snow• Cohesive new

snow

Weak Layer?• Graupel• Surface Hoar• Rime• Facets• Crusts

Trigger?•Rapid loading

•Rapid Temperature Change

©2000-2002 Mike Laney

The trigger is the event that tips the balance between strength and stress.

Stress Strength

Stress Relief

©2000-2002 Mike Laney

Avalanche Characteristics by Climate Zone

Factors Maritime Intermountain ContinentalAltitude 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<0oF <0o & >32oF <32oF

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

©2000-2002 Mike Laney

Recap: Strength vs. Stress

• Properties of snow• Stress• Strength factors• Stress relief