Lecture 4: Landslides (mass wasting)

Question 1

Where can mass wasting occur?

Answer 1

A mass wasting event can occur any time slope becomes unstable.

Sometimes, as in creep, slope is unstable at all times and process is continuous.

But other times, triggers can occur that cause a sudden instability to occur.

Question 2

Preconditions (“causes”)

Answer 2

Weathering- Increase in pore water pressure (rainfall, snowmelt, pipes)

Increase in slope angle

Head loading

Changes to water table

Removal of vegetation

Question 3

Triggers of landslides (mass wasting)

Answer 3

Increase in pore water pressure (rainfall, snowmelt, pipes)

Earthquake shaking

Human induced shaking (blasts, trucks)

Volcanic eruption

Unknown triggers

Question 4

Example of major triggers

Answer 4

-Earthquakes (major shock)

– Heavy rainfall, Sudden Snowmelt
Changes in hydrologic conditions- water reduces gain to grain contact- which reduces the angel of repose- can saturate the rock- increasing its weight

• Volcanic eruptions
  Shock like explosions- can trigger earthquakes- melting of snow or crater lake empties- causes large amounts of water to mix with regolith-reducing grain to grain contact- causing debris/mud flows and land slides

Question 5

Other example of landslide trigger

Answer 5

Slope modification

By humans or by natural causes.
• Changes slope angle so no longer at angle of repose.
• A mass wasting event can restore slope to its angle of repose.

Undercutting

FIRE - can cause removal of vegetation, resulting in the regolith becoming less fixed overtime.

(iv) ADDED MASS can make a region more prone to mass wasting.
Examples
• waste material • mining tailings • structures
• water leakage

MINOR  SHOCKS can  also  trigger  mass  movements.
Examples
• heavy  trucks  on road
• trees  blowing  in  wind
• human  made  explosions

Question 6

Mapping triggered landslide events

Answer 6

Reconnaissance of area affected:
helicopter or field survey.
• Locations and sizes of individual landslides: remote sensing (aerial photography, satellite images).
• When remote sensing is not possible: archives (newspapers, social media, diaries, photographs).
• Novel techniques: LiDAR, seismometers, UAVs, crowdmapping.

Question 7

Mapping triggered landslide events - stereoscope

Answer 7

Stereo-pair of images from Landsat/SRTM imagery.

To view stereo pair, cross eyes slightly until a third dot appears between the two at the top. The new centre image is in 3D.

Two perspectives of the same image – one for each eye.
Gives 3D image with exaggeration of vertical.

Question 8

Stereoscope problems

Answer 8

• Completeness
• Bias towards larger landslides
• Bias towards landslides with impacts
• Temporal persistence
• Time between event and photographs taken (which
landslides were caused by a specific event?)
• Lack of metadata/communication