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Refrozen Snow Layers and Avalanche Release

Background Material

Conrad Koziol David Marchant

Outline

•  IntroducAon to Avalanches •  Snowpack •  Wet Layers and Crusts

•  Snow Metamorphism

•  Slab Avalanche Mechanics

•  Field Methods

What is an Avalanche?

“An avalanche is an abrupt and rapid flow of snow, oKen mixed with air and water, down a mountainside.” ‐ Wikipedia

• Avalanches kill more than 150 people per year, and cost millions of dollars every year

• The 2008/2009 avalanche season has already caused 16 fataliAes in Western Canada

• Two main types of avalanches

Loose Snow Avalanches

•  Loose, cohesionless snow sliding down a mountainside

•  Occur on steep slopes when light, unconsolidated snow sits on top of a consolidated snow pack

Slab Avalanches

•  A cohesive block of snow that moves iniAally as a single unit

•  “Can reach speeds of of 30 km/hr in the first three seconds, and can accelerate to a speeds of 130 km/hr in the first 6 seconds” ‐ Avalanche.org

Snowpack

•  The total accumulaAon of packed snow on the ground

•  Consists of layers of snow with different properAes

Snowpack Layers

•  Not all snow is the same

•  Some possible variables include: •  Density •  Moisture Content •  Shape & Crystal Structure

•  ProperAes of snow layers change over Ame

Snowpack Layers

Snowpack FormaAon of Wet Layers and Crusts

•  Form when the top of the snow is melted by one or more sources

•  When wet layers freeze into crusts they provide an excellent sliding surface for slab avalanches

•  The causes of wet layers are important to consider when anAcipaAng the distribuAon of crusts.


Rain Crusts – Formed when the snow surface is we`ed by rain and then freezes

– Factors •  ElevaAon: Temperature must be above freezing •  Slope: Water penetrates deeper into gentle slopes

•  Aspect: Windward slopes receive more rain than leeward

– Primary Months: November, March, and April


Temperature Crusts –  Formed by melAng of the top of the snow by air temperature

–  Factors •  ElevaAon: The air is warmer at lower elevaAons

•  Wind: Higher wind increases the melAng rate

•  Aspect: Windward slopes receive more wind, hence higher melAng

–  Primary Months: March and April


Sun Crusts – Formed by melAng of the Snow by solar radiaAon

– Factors •  Aspect: Whether the slope is south facing

•  Slope: Steeper slopes receive more sunlight

– Primary Months: March and April

Snowpack Snow Metamorphism

•  The snow in snow pack are constantly changing •  Two types of snow Metamorphism

–  Equilibrium Metamorphism (Rounding) •  simplifies the original crystal, making it more round •  snow becomes more stable

–  KineAc Metamorphism (FaceAng) •  turns the snow grains into angular shapes with sharp corners and flat faces

•  snow becomes less stable


•  Snowpack ProperAes – Temperature

•  Temperature through the snowpack is not constant •  The snow close to the ground remains close to 0˚C

•  The snow close to the surface is close to the air temperature

•  Temperature Gradients in the Snowpack


•  Snowpack ProperAes – Moisture

•  The air filling the pores of the snow pack is saturated (100% relaAve humidity)

• Warm air can hold more water than cold air •  Vapor pressure increases with increasing temperature • Water vapor moves from areas of high vapor pressure to areas of low vapor pressure

•  The speed at which vapor moves is proporAonal to the magnitude of the pressure gradient, which is proporAonal to the temperature gradient


A Temperature Gradient

A Pressure Gradient

Moving Air


Warm, Saturated Air Parcel

Moves to a Cooler Area

Cool, Super‐Saturated Air Parcel

Needs to Get Rid of Some Water


 Where does the water get deposited?

• Bernoulli's principle • The vapor pressure as air moves over a convex shape is greater than the vapor pressure as air moves over a concave shape


Rounding


If the pressure gradient due to the concavity in the crystals is great enough to overcome the the verAcal pressure gradient in the snowpack the excess

water vapor is deposited in the concaviAes.


FaceAng


If the pressure gradient due to the concavity in the crystals is NOT great enough to overcome the the verAcal pressure gradient in the snowpack the

excess water vapor moves past concaviAes and is deposited directly on

the surface of the first grain it encounters


Most important factor – The Magnitude of the temperature gradient

 FaceAng or Rounding?

If ΔT10 < TGF ⇒ Rounding Occurs

If ΔT10 > TGF ⇒ Faceting Occurs

ΔT10 ⇒ Change in temperature over 10cm


• What is TGF?

•  Depends on the temperature, density and grain/pore size of the snow

• Typically TGF=10˚‐20˚ C m‐1


Rate of Metamorphism • FaceAng occurs much faster than rounding • The rate depends on the magnitude of the temperature gradient, the temperature and the density of the snow

• Snow can become faceted in a few hours when ∆T=100˚C m‐1

• Snow can become faceted in a few days when ∆T=10˚C m‐1

Snowpack Types of Snow

New Snow

• It falls from the sky

• All snow begins this way

• Can form either a loose layer or a slab depending on its properAes


Surface Hoar

•  Ice crystals formed when water vapor in the air condenses directly into the solid phase onto a very cold surface • CondiAons of FormaAon:

• Surface RadiaAon • Cool Air

• Creates weak layers in the snowpack


Faceted Snow

•  The product of kineAc metamorphism

• Consists of large, angular crystal that bond poorly with each other

• Snow layers weaken as they become faceted


Depth Hoar

• Faceted snow that forms at the base of the snowpack

• Common in early winter, where the ground can be warm, but the surface of the thin snowpack is extremely cold.

• Very weak layer within the snowpack


Rounded Snow

•  Formed as a result of Equilibrium Metamorphism (eg low ∆T)

• Consist of small rounded crystals

• Rounded snow crystals bond well together, and form strong cohesive layers within the snow pack

Slab Avalanches Forces Within the Snowpack

Driving Force

• The component of gravity acAng parallel to the slope

• Greater on steeper slopes

Slab Avalanches Forces Within the Snowpack

Opposing Force

• FricAon between the slab and the underlying layer

• Dependant on the ProperAes of the snowpack

Slab Avalanches Anatomy of a Slab Avalanche

Slab Avalanches Steps to Slab Failure

1.  Shear Failure of the Snowpack –  Component of gravity acAng parallel to the slope

+ any external forces > than the staAc fricAon between the slab and the underlying layer

–  Fracture propagates along bed surface

2.  Tensile Failure at the Crown –  Slab breaks away from the rest of the snowpack

along the upper boundary

Slab Avalanches Steps to Slab Failure

3. Shear Failure in the Flanks –  Slab breaks away from the rest of the snowpack

along the sides

4.  Compressive Failure in the Staunchwall

Slab Avalanches ContribuAng Factors

Field Methods How do we Determine the Risk Associated with a Snowpack?

• A Number of tests can be conducted in the field to determine the properAes of the snowpack • Test Site: • Want a representaAve site.

• Need feel and consistency for these methods

Field Methods Rutschblock Test

Results - Rutschblock Test: What it means Fails while isolating the block 1 Red light Fails while stepping onto the block 2 Red light

Fails with an easy weighting 3 Red light Fails with one easy jump 4 Yellow light Fails with one hard jump 5 Yellow light Fails with several hard jumps 6 Green light Doesn't fail 7 Green light

Field Methods Snow Pits

• Exposes the layers of the snowpack

• Run Compression, Shear, and Hardness tests

• Typically 1.5 meters deep

• harder for humans to trigger avalanches deeper than that

Field Methods Hardness Test

Results - Compression Test: What it means Breaks when articulating from the wrist Red light Breaks when articulating from the elbow Yellow light Breaks when articulating from the shoulder Green light

Isolate 5 sides of a cube, and hit it progressively harder.

Field Methods Compression Test

• Isolate 5 sides of a cube, insert shovel behind, and see how hard it is to shear the cube off.

• Repeat for different layers

Field Methods Shear Test

On Thursday…

•  ’FormaAon of Refrozen Snowpack Layers and Their Role in Slab Avalanche Release’ ‐ Jamieson

•  SecAons 3, 4.4‐4.6, 5, 6 •  The effect of wet layers and crusts in the faceAng process •  Effect of latent heat on snowpack •  Case Studies

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