Landslide causes and landslide hazard assessments Flashcards
What is an example showing stable slopes by McColl 2022?
3 slopes (Dipping strata in stable formation, vegetated regolith on moderate dip, fractured steep slope rock)
How do the slopes form McColl 2022 shift to being moderately unstable?
Dipping strata is undercut expsoing bedding planes/ weak layer
Regolith - deforestation and loss of cohesion/ interception
Rock slope - Freeze thaw causing fracture growth
WHat happens when a slope shifts from stable to moderately unstable?
Becomes more sensitive to environmental change
When a slope becomes actively unstable what might the triggering factor be like?
The event which causes failure might be much smaller or the same size as what the slope has coped with in the past but is now no longer strong enough to resist
What are the actively unstable slope types from McColl 2022?
DIpping strata= Rock slide
Regolith = Earth flow
Steep rock - Rock fall
What are preconditioning factors?
inherent characteristics to a slope independent of time
What are some examples of preconditioning factors?
Lithology
Presence of weak layers
Fractures and faults
Slope orientation
Contrast in geomechanical properties
What is the timescale of preparatory factors?
Long term can bring slope to be marginally unstable
What are some examples of preparatory factors?
Fluvial/marine/glacial erosion
Glacial retreat (debuttressing)
Weathering (e.g. frost)
Sheet jointing
Deforestation
Human activity at toe
Intrinsic strength degradation
What is the timescale of triggering factors?
Short term
Transient (vary with time)
Overlap with preparatory
What are some examples of triggering factors for landslides?
Earthquakes
Snowmelt *
Water pressures*
Intense rainfall *
Loading of slope
Slope toe undercut
(either natural or manmade)
(Intrinsic strength degradation)
* resulting in high pore pressure
What might slope stability be like overtime for a stable slope with a factor of safety of 1.5?
Slope is stable but partially reduced by rainfall but reocvers with drainage, Erosion at toe causes big reduction in stability with no recovery as material gone, prolonged wet period and earthquake continue reduction until a rainfall event which might be relatibely small causes failure
What is intrisic strength degradation?
Preparatory but sometimes triggering
Chemical decomposition (pore fluids, stress corrosion) or stress- strain processes (e.g. frost, root) cause fracture development or growth
What scale will intrinsic strength degradation work?
Microscopic: nucleation and growth of
small fractures,subcritical fracture propagation
Small fractures and imperfections, imbalanced stress concentration, further growth of cracks at the tips
What scale are earthquake triggered hazards?
Regional scale hazard – Earthquakes and storms can generate many simultaneous events (i.e., potentially tens of thousands) with spatial variation in landslide type, size, frequency
What are earthquake triggereing landslides influenced by?
EQ depth, slip, and mechanism; and local geomorphology, slope materials, geology, hydrology, land use
Landslides might not occur in high frequency next to EQ depends on characteristics
What was landslide distribution like for Wenchuan 2009?
Most landslides on hanging wall, within 10 km of fault rupture
Areas of high landslide density along the coseismic rupture, but also the steep sides of major river valleys
Concentrated in south – where the fault had a more gently dipping, thrust geometry
In the north, geometry was steeper, with a strike-slip component – note the much narrower distribution
Most of the landslides were in fractured rock masses
How much rain was experinced before the landslides of central italy sept 2022?
419mm in 9 hours
How many landlsides occured as a result of the rainfall in central italy sept 2022?
1687
What were landslides like from sept 2022 in central italy?
Shallow and fast moving
Channelised – high efficiency
Some hill slopes entirely denuded
Entrainment of debris along course
How does rainfall intensity/ duration affect stability?
Intensity over short duration less affect on stability then prolonged rainfall
What is hazard zonation?
Divison of land in homogeneous areas or domains and ranking of these areas according to their degrees of actual or potential hazard caused by mass movement
Why are landslide inventories needed?
fundamental step as they should give insight into the location of landslide phenomena, the types, failure mechanisms, causal factors, frequency of occurrence, volumes and the damage that has been caused
What is needed to gve a temporal chnage insight to landslides?
Based on multitemporal aerial photo interpretation
Based on a particular trigger event (e.g. mapping immediately after typhoon or earthquake)
