Lecture 4: Landslides (mass wasting) 3

Question 1

Hazard /Susceptibility Assessment of Mass Wasting-Short‐term prediction issues:

Answer 1

(i) For earthquake triggered events, the problems inherent in earthquake prediction are present.
(ii) Slopedestabilization&undercuttingtriggered
events require constant attention of those observing the slopes [many of whom are not educated in problems inherent in such processes].

Question 2

Hazard /Susceptibility Assessment of Mass Wasting- more

 Short‐term prediction issues:

Answer 2

(iii) Mass‐wasting hazards from volcanic
eruptions can be predicted with same degree of certainty that volcanic eruptions can be predicted [threat has to be realized and warnings heeded!].

(iv) Hydrologic conditions such as heavy precipitation can be forecast with some certainty, and warnings issued to areas that might be susceptible to mass‐wasting processes caused by such conditions.
(v) But…with heavy precipitation, even if forecast, which hill slope of millions that exist will be most vulnerable?

Question 3

Hazard /Susceptibility Assessment of Mass Wasting

 Some LANDSLIDE WARNING SIGNS

Answer 3

Springs, seeps, or saturated ground in areas that have not typically been wet before.
B. New cracks or unusual bulges in the ground, street pavements or sidewalks.
C. Leaning telephone poles, trees, retaining walls or fences.
D. Soil moving away from foundations.
E. Ancillary structures such as decks and patios tilting and/or moving relative to the main house.
F. Tilting or cracking of concrete floors & foundations.
G. Broken water lines and other underground utilities.

Question 4

Prevention & Mitigation of Mass Wasting

Answer 4

 Slope Assessment:
• All slopes are susceptible to mass‐wasting hazards if a triggering event occurs.
• Thus, all slopes should be assessed for potential mass‐wasting hazards.
 Mass‐wasting events can sometimes be avoided by employing engineering techniques to make slope more stable.
 Engineering mitigation technique examples:
(i) Steep slopes can be covered or sprayed with SHOTCRETE, CONCRETE, or METAL MESH.
(ii) RETAINING WALLS can be built to stabilize slope.

Question 5

Prevention & Mitigation of Mass Wasting

 Engineering mitigation technique examples:

Answer 5

(vi) Over‐steepened slopes could be graded to reduce the slope to the natural angle of repose.
 Other mitigation technique examples:
(i) In mountain valleys subject to mudflows, plans could be made to rapidly lower levels of water in human‐made reservoirs to catch and trap the mudflows.
(ii) Land use planning: humans should avoid high risk areas
- warning signs
-Education
-monitor high risk slopes

Question 6

Secondary hazards

Answer 6

Landslides can also trigger other hazards: 
 Tsunamis
 Volcanic eruptions
 Further landslides
 Floods

Question 7

Secondary Hazards- Landslide triggered tsunamis

Answer 7

Displacement of water by either:
• Landslides falling into the water
• Submarine landslides
E.g., Papua New Guinea, 1998:
• Mw 7.0 Earthquake
• Triggered underwater landslide
• Tsunami hit the coast
• 2,183 fatalities

Question 8

Secondary Hazards-Landslide triggered volcanic eruptions

Answer 8

Unloading of a volcano by landslides reduces confining pressures, changing lithospheric stress and strength conditions.
• Material input into lava has also been shown to trigger the nucleation of bubbles, triggering an eruption.
E.g., Mount St Helens, USA, 1980
• Pressure build up of magma created bulge on the
flank of the volcano
• Failed suddenly in a large debris avalanche
• Release of pressure resulted in eruption

Question 9

Secondary Hazards- Landslide triggered further landslides

Answer 9

Landslide deposition on the head of a slope adds weight, which can trigger a landslide
• Landslides can re-activate
• Release of sediment which can
then mobilise in future storms
E.g., East Cuchara, Panama, 1986
• Landslide landed on the head of an already active
landslide
• Added extra weight and momentum to the
landslide
• 10 ha in area

Question 10

Secondary Hazards- Landslide triggered floods

Answer 10

Landslide blocking river can create a dam, which restricts the flow of water.
• Landslides can add sediment to a river, changing the flow dynamics.
E.g., Attabad, northern Pakistan, 2010
• Unclear what triggered the landslide
• 45 million cubic meter landslide blocks
Hunza river
• 22 km long, 100 m deep lake forms over 5
years
• Risk of catastrophic failure of the landslide
dam