Mass Wasting

Question 1

What is Mass Wasting?

Answer 1

Downslope movement of Earth materials (soil, sediment, debris, rock…) in response to gravity

Question 2

what is an example of a mass wasting event?

Answer 2

Northern Taiwan 2010

300m of highway buried up to 10m deep

Question 3

When does mass wasting occur?

Answer 3

When: downslope (driving) forces > retaining (resisting) forces

Question 4

what is shear stress (τ)?

Answer 4

downslope force from gravity (g) acting on mass on a slope

Question 5

what is shear strength/resistance (S)?

Answer 5

retaining forces due to particle cohesion & friction, anchoring by plant roots, etc

Question 6

How can we estimate slope stability?

Answer 6

Factor of Safety (Fs) = resisting force (S) / driving force (τ)
Fs> 1 (or S > τ) = stable
Fs< 1 (or S < τ) = unstable

Question 7

What is the critical slope angle?

Answer 7

....it depends
higher angle (steeper slope) for more coherent material

Question 8

what factors control mass wasting?

Answer 8

Strength of slope materials
–weathering: ↓cohesion, ↓S
–additional mass: ↑τ
Water content
–low: adds cohesion, ↑S (e.g., moist sand more stable than dry sand)
–high: ↓friction, ↓S (e.g., saturated sand)
Slope oversteepening
–Undercutting: ↑slope, ↑τ

Question 9

what is slope oversteepening?

Answer 9

Road cut oversteepens the slope:↑slope angle, ↑shear stress -> landslide

can be natural e.g. wave undercutting

Question 10

what are some examples of triggering mechanisms?

Answer 10

Earthquakes
loud noises
heavy rainfall
deforestation
undercutting
blasting (construction)

cause sudden ↓S,↑τ

Question 11

what are some examples of earthquakes that triggered landslides?

Answer 11

Denali M7.9 earthquake, Alaska, 2002

Haida Gwaii 2012 M7.8

Question 12

what are the main types of mass wasting?

Answer 12

(1) Creep and solifluction (slow)
(2) Slumps and slides
(3) Falls (and avalanches)
(4) Flows

Question 13

what are the main types of mass wasting distinguished by?

Answer 13

rate of motion
material
moisture content
coherency

Question 14

what is soil creep?

Answer 14

(most common mass movement)
slow, gradual downslope movement of surface materials
(coherent- low H2O content)
Soil creep (~1 cm/yr)
curved tree trunks, deformed roads, tilted posts, strata bent in downslope direction…
slow

Question 15

what is solifluction?

Answer 15

Water can’t penetrate permafrost -> upper active layer becomes water logged and flows
slow

Question 16

what are slumps?

Answer 16

(rotational slides)
faster
Downslope sliding of soil or rock along a curved detachment surface
e.g. Quebec 2010, 4 fatalities

Question 17

what are slides?

Answer 17

(translational)
Moderate to rapid failure of material along a planar surface
can move fast or slow

Question 18

what are falls/avalanches?

Answer 18

Rapid movement of material either through the air or downslope
- rockfalls, snow avalanches, debris/rock avalanches
-10 – 100s km/hr
Material often travels laterally several times its vertical fall distance

Question 19

what was the Hope Slide rock avalanche (BC jan 1965)?

Answer 19

47 million m3 (2nd largest landslide in Canada)

4 fatalities

Question 20

what was the Lituya Bay Rockfall & tsunami (Alaska 1958) triggered by?

Answer 20

earthquake

Question 21

what was the Frank Slide, AB (Apr 1903) rock avalanche triggered by?

Answer 21

freeze-thaw weathering
30 million m3
70+ fatalities

Question 22

what was the Mt Meager rock avalanche (BC Aug 6 2010)?

Answer 22

(Largest landslide recorded in Canada: 48 million m3)

Question 23

what are flows?

Answer 23

Rapid movement of materials (as viscous fluid)… soil, mud, debris, water

Question 24

what are earthflows?

Answer 24

flow moves downslope as a viscous fluid (slow or rapid)

Question 25

what are mudflows?

Answer 25

flowing mixture of debris & water, usually down a channel (e.g., lahar)

Question 26

what are debris flows?

Answer 26

flow where coarse material is predominant- very common in the Canadian Cordillera where heavy rainfall mobilizes unstable debris

Question 27

what were the Vargas, Venezuela debris flows (1999) triggered by?

Answer 27

rainfall
Flows exceeded 50 km/hr
10,000–30,000 fatalities

Question 28

how many fatalities were there as a result of the Johnsons Landing (Kootenay Lake) debris flow (Jul 2012)?

Answer 28

4

Question 29

what negative impacts is human action having on mass wasting?

Answer 29

Creation of new landscape features
–reshaping the Earth’s surface e.g. channelized rivers, road cuts, open pit mines..
Alteration of processes -> affect driving/ resisting force balance
–deforestation
–alteration of natural drainage

Question 30

how can human action prevent/delay mass wasting?

Answer 30

Increase resisting forces (↑S), reduce driving forces (↓τ)

• Stitching (rock bolts) (↑S)
• Shotcrete
• Re-vegetate slopes
• Improve drainage

Question 31

what can humans do if mass wasting is inevitable?

Answer 31

Monitor -> early warning
Protect people (e.g. snow sheds)
Move people away from harm