Week 9, Mass Movements and Avalanches Flashcards

1
Q

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

A

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

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2
Q

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

A

Must overcome

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

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3
Q

How water increases instability

A
  • 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
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4
Q

How can we trigger a mass movement event?

A
  • heavy rain
  • earthquakes
  • thawing of frozen ground
  • surface disruption by humans
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5
Q

1 kg boulder experiences __N of force.

A

1 kg boulder experiences 9.8N of force.

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6
Q

Mass Movement Classification

A

Downward: Falls and subsidence

Down and outward: Slides and flows

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7
Q

Mass Movement Classification: Falls

A

Individual blocks detach along fractures

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8
Q

Mass Movement Classification: Subsidence

A

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

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9
Q

Mass Movement Classification: Slides

A

Slide of blocks on surface

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10
Q

Mass Movement Classification: Flows

A

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

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11
Q

2 types of slides

A

rotational and translational

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12
Q

Rotational Slide

A

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

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13
Q

Translational Slide

A

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

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14
Q

Creep

A

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.

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15
Q

Complex Events

A

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

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16
Q

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

A

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.

17
Q

Inertia

A

The tendency of an object to stay in place

18
Q

Mass Movement Mitigation

A
  • 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
19
Q

Avalanche Path

A

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

20
Q

Avalanche size scale

A

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
21
Q

Avalanche Types

A
  • loose snow/sluff

- slab avalanche

22
Q

What is needed for a slab avalanche?

A
  • 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
23
Q

Snow becomes denser and more cohesive with

A

age.

24
Q

Explain how hoar frost creates weakness in the snow pack?

A

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

25
Q

Indicators of wind loading:

A
  • cornices (out of wind)

- rime growing on object (into wind)