Week 9, Mass Movements and Avalanches

Question 1

Gravity is always toward the ____ of the earth, not ____ to surface.

Answer 1

Gravity is always toward the centre of the earth, not perpendicular to surface.

Question 2

For an object to move parallel to a slope we need 2 criteria:

Answer 2

Must overcome

Friction (shake or kick boulder) AND inertia (rain)

Question 3

How water increases instability

Answer 3

• adding weight to soil or porous rock
• expanding and weakening clay minerals
• decrease rock cohesion, also called cement dissolution
• subsurface erosion
• increasing pore pressure due to burial weight, destabilizes soil
• raising water table

Question 4

How can we trigger a mass movement event?

Answer 4

• heavy rain
• earthquakes
• thawing of frozen ground
• surface disruption by humans

Question 5

1 kg boulder experiences __N of force.

Answer 5

1 kg boulder experiences 9.8N of force.

Question 6

Mass Movement Classification

Answer 6

Downward: Falls and subsidence

Down and outward: Slides and flows

Question 7

Mass Movement Classification: Falls

Answer 7

Individual blocks detach along fractures

Question 8

Mass Movement Classification: Subsidence

Answer 8

Collapse into void, dominantly vertical downward movement, moves as separate blocks.

Question 9

Mass Movement Classification: Slides

Answer 9

Slide of blocks on surface

Question 10

Mass Movement Classification: Flows

Answer 10

No sliding surface, more as fluids. (Includes creep)

Question 11

2 types of slides

Answer 11

rotational and translational

Question 12

Rotational Slide

Answer 12

Decrease in the driving mass, increase in the resisting mass. Does not typically slide for long distances.

Question 13

Translational Slide

Answer 13

Mass slides on surfaces of weakness, can slide for long distances.

Block Slides: Material remains intact
Debris Slides: Material deforms as it slides
Lateral Slides: Underlying material breaks and flows

Question 14

Creep

Answer 14

Slowest but most common failure.

Caused by successive expansion/contraction due to freezing/thawing, wetting/drying of clay minerals heating/cooling.

Expansion due to heating it is perpendicular to the surface while contraction is due to gravity.

Question 15

Complex Events

Answer 15

Mass movements that combine falls, slides, and flows along their path.

Question 16

Subsidence can happen fast or slow, what do these speeds look like?

Answer 16

Fast: Sudden collapse into subsurface void
Slow: Slow sag due to compact of water saturated sediments (Like Italy) or long term removal of ground’s water.

Question 17

Inertia

Answer 17

The tendency of an object to stay in place

Question 18

Mass Movement Mitigation

Answer 18

• Reinforce Hazard: unload head, reinforce body, support toe
• Contain Hazard: wire mesh, steer flows in direction of least harm
• Support Hazard: buttress overhanging blocks or weak layers
• Protect against hazard: tunnels snowsheds
• Sea to sky highway

Question 19

Avalanche Path

Answer 19

Starting Zone: Steepest slope: 30-40 degrees
Track: Guided by topography: 20-30 degrees
Run-out zone: < 20 degrees

Question 20

Avalanche size scale

Answer 20

Logarithmic:

Size 1: 10 tonnes, 10 metres, 1 kilopascal of force
Size 2: Can kill people
Size 3: Danger to highways
Size 4: Rare
Size 5: Massive, very rare

Question 21

Avalanche Types

Answer 21

• loose snow/sluff

- slab avalanche

Question 22

What is needed for a slab avalanche?

Answer 22

• cohesive slab on top
• weak layer to fail and slide on
• most common on slope > 30 degrees
• most commonly triggered by extra weight of skier

Question 23

Snow becomes denser and more cohesive with

Answer 23

age.

Question 24

Explain how hoar frost creates weakness in the snow pack?

Answer 24

Hoar develops on the surface, when buried it can be deadly because it is so weak. Wind can cause loading.

Question 25

Indicators of wind loading:

Answer 25

• cornices (out of wind)

- rime growing on object (into wind)