Avalanches

Question 1

What is an avalanche?

Answer 1

They are flows which move under the influence of gravity

They can be channelized or unconfined

In this sense, they are similar to pyroclastic flows, debris flows, etc.

Question 2

Effects of Avalanches

Answer 2

Direct effects:
Impact
Burial

Indirect effects:
Tsunamis generated if an avalanche enters a lake

Question 3

The Path of an Avalanche

Answer 3

3 parts:
Starting Zone
Track
Run out Zone

Avalanches follow the topography, usually form and are pushed through gulleys

Question 4

How avalanches occur

Answer 4

Shear Strength: Internal resistance to movement

Shear Stress: Force causing movement parallel to the slope, increases with slope angle

Safety Factor= (Shear Strength/Shear Stress)

If the safety factor is less than one, the slope is unstable and prone to failure

Gravity:
Gp is the gravity that right angles the slope
Gs is the gravity that parallels the slope
As slope angle increases, Gp decreases and Gs increases
Once Gs exceeds shear strength, the mass fails

Question 5

Two types of initial failure

Answer 5

Loose Snow Avalanche: Loose snow running over the slope, less dangerous
○ Snow is dilute
Slab Avalanche: More dangerous
○ Snow is denser, less mixing with atmosphere

Avalanches are more likely to form at a 35 degree slope

Question 6

Structure of an Avalanche

Answer 6

→ Head is twice the height of the rest of the avalanche
○ Its mixing with the air form the atmosphere (slows it down)
○ Material is flowing form the body to the head, making it denser
→ Different parts of the flow are moving at different velocities, the head is moving fastest
→ Also there is a vertical difference in snow density
○ The lower portion is usually denser and more hazardous
○ The upper portion is more dilute, but still turbulent

Question 7

Two Types of Avalanches

Answer 7

Flow avalanches:
Originate from slab failure
Slower than powder avalanches
Granular Flow
Extremely dense
Freezes upon settling because of particle movement which heats it up
Do not flow around obstacles

Powder Avalanches
Originate form loose snow failure
Velocities can exceed 360 km/hr
Composed of a mixture of air and snow which makes it less dense
Turbulent Flow
Flow around obstacles because upper more dilute portion acts as a fluid

Question 8

Causes of Avalanches

Answer 8

Most triggers for avalanches involve human activities (skiing, snowboarding, ice climbing)

The number of fatalities from avalanches has been increasing

Dry slabs and leeward slopes are most prone to avalanches, where an overhanging cornice indicates leeward exposure

If you aren’t found within 20-30 minutes, your chances of survival decrease significantly

Question 9

Mitigation

Answer 9

Avoid steep slopes, gullies

Close high-hazard areas to reduce risk and vulnerability

Set off explosive charges to artificially induce avalanches and remove the source material (unstable snow)

Use reforestation to stabilize slopes and snow

Use of defense structures