Engineering Geology and Geohazards Lecture 9: Landslides and Karsts Flashcards
How common are landslides
Landslides are the most widespread and common geological hazard on Earth.
– Earliest on record: Yi and Lo Rivers, Central China, 1767 BC
June and August prominent months for landslide deaths (due to rainfall).
Where do landslides occur?
Everywhere, especially:
– Young mountains
– Coastlines
– Volcanoes
– Remobilized ancient landslides
Describe the effects of landslides
- Loss of life
- Destruction of property
- Defence measures (e.g., barriers,
drainage) - Repairs
- Environmental effects:
– Fertile soils stripped away
– Ecosystem damage
Describe the secondary effects of landslides
● Tsunami/ waves
● Earth dams:
○ Collapse
○ Flooding
● Increased sedimentation into rivers
& deltas:
○ Fish stocks
○ Water quality
○ Flood risk
What are the landslide triggers
- Rainfall
- Earthquakes
- Volcanic deformation/eruption
- Erosion & Alteration
Describe an example of rainfall triggering a landslide
Hurricane Mitch, October 1998
* Hurricane stalls for 6 days over Central America as much as 2000 mm/m2 rainfall.
Floods;
~10 to the power of 6
Las Casitas volcano debris flow, 16 km long, destroyed at least four villages and killed 2,000+.
~10,000 deaths;
~1.5 million homeless
* Damage ~US$ 5 billion
* Most destructive Atlantic hurricane for 200 years
Describe an example of a landslide triggered by an earthquake
Huascarán, Peru, 1970
M 7.7, 150 km
15:23 local time, 31 May
50-100 million cu. m.
16 km at ~280 km/hr
Destroyed Yungay
and smaller communities.
Killed 18,000
Describe different landslide speeds
Sluggish m/hr - m/century
Intermediate m/hr to m/s
Catastrophic >m/s
Describe slides (in the context of landslides)
Slides
* Emplaced in minutes-hours
* Common: embankments, coasts
(Dorset)
* Volumes ~m3
to ~100,000’s m3
* Failure depths ~m to ~10’s m, hence
mostly soil and weathered rock
(regolith)
* Weak rock, easily mobilised by rain
Describe mudflows
Material flows downslope as mixture of rock fragments and wet mud/clay.
- Soils, clay-rich rocks, volcanoes.
- Volumes most ~10,000 cu.m or less; some 10 cu.km
- Speed: 0.1-10 km/hr
- Highly destructive
Describe the Storegga landslide
The Storegga Slide, occurred around ~8.1 ka(8.1 thousand years ago)
* Instantaneous detachment of 2400 to 3200 km3 of sediment, covering an area of 95000 km2.
- Increasingly detailed bathymetric mapping of the ocean floors has revealed many well-preserved landslide deposits of comparable volumes.
Typical hazards:
– destroy communications lines on sea bed
– trigger tsunami
– threaten shipping
– threaten sea-bed habitats
Describe Giant, catastrophic landsides / sturzstroms
Rare, unique type of landslide
consisting of soil and rock
- Great horizontal movement when compared to its initial vertical drop - as much as 20 or 30 times the vertical distance.
- Flow across land fairly easily, and their
mobility increases when volume
increases.
Define a Karst
Karst are landscapes created by the dissolution of soluble rocks
● Soluble rocks: rocks susceptible to dissolution
● Most frequent karst:
○ Carbonates: Limestones/Dolomite
○ Evaporites: Halite & Gypsum
○ Quartzite
Karts can cause collapse features: sinkholes (cenote, torca or dolines), uvalas and poljes
Define dissolution
The process in which solids (like minerals) are disassociated and the ionic components are dispersed in a liquid (usually water).
Dissolution speed depends on the rock type and environmental conditions
(acidity, temperature)
● Creates multitude of geomorphological features:
○ Underground - Typically cave systems
○ Overground- characteristic landscapes and depressions
● Soluble rocks pose a frequent and serious hazard to human live and activities
What variables affect the development and the rate of development of karsts
Speed of development and evolution depends on:
- Lithology (Gypsum, Halite, Limestones, Dolomites, Quartzite)
- Temperature and climate (Latitude)
- Ph
- Natural (organic matter/ vegetation)
-Anthropogenic (Pollution)
- Atmospheric CO2
