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Summary articles Hazards and Risk Assessment (GEO4-4425)
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Topic 1Landslide hazard and risk zonation – why is it still so difficult (2005)
Landslide risk (Varnes 1984) = the expected number of lives lost, persons injured,
damage to property and disruption of economic activity due to a particular
damage phenomenon for a given area and reference period. Risk can be
quantified as the product of vulnerability, cost or amount of the elements at risk
and the probability of occurrence.
Total risk = hazard x vulnerability x amount
It is a simple formula but is very difficult to quantify. Difficult to locate the exact
element at risk and the possible locations of the landslides. Hazard is the most
difficult component to establish.
Risk curve = relation between all events with different probabilities, and the
corresponding losses.
There are 4 basic inputs required:
1. environmental factors,
2. triggering factors
3. historic landslide occurrences
4. elements at risk.
Quantitative risk assessment is feasible for geotechnical engineering on a site
investigation scale and the evaluation of linear features (e.g., pipelines and
roads). However, the generation of quantitative risk zonation maps is still
difficult.
- Information on date, type and volume of a
landslide: mapping is difficult because
every landslide has different characteristics
- Determination of spatial and temporal
probability of landslide: once it has
occurred, the slope conditions have
changes so there are different
circumstances so it is not likely that the
same event will occur (frequency-
magnitude)
- Modelling of runout
- Assessment of landslide vulnerability:
limited damage data available
Landsldie risk approaches
1. Landslide inventory-based probabilistic approach
2. Heuristic approach: direct-geomorphological mapping or indirect-
combination of qualitative maps
3. Statistical approach: bivariate or multivariate statistics
4. Deterministic approach
New advances and challenges for the future
- Use of very detailed topographic data
- Generation of event-based landslide inventory maps
, - Use of these maps in spatial-temporal probabilistic modelling: landslide
initiation modelling
- Land use and climate change scenarios in deterministic modelling
Earthquake-induced chains of geologic hazards: patterns, mechanisms
and impacts
Large earthquakes initiate chains of surface processes that last much longer than
the brief moments of strong shaking. Most trigger landslides, of which the
deposits can remobilize during heavy rainfall and evolve into debris flows.
Earthquakes abruptly raise rates of erosion, sediment transport and deposition.
Earthquake magnitude and depth are controls on the degree of landscape
disturbance, modulated by the incoming seismic waves, topography, rock-mass
properties, groundwater conditions and other factors.
Spatial frequency distribution of earthquake-triggered landlides are influenced by,
source material, movement mechanisms, ground-motion characteristics and
sizes. The shaking can also weaken the slope. Landslide dams can fail years later
as well as floods.
Coseimic landslides (EQTL) = mass movements triggered within seconds to
minutes of strong ground shaking: soil failures, rock falls, slides and avalanches.
Mapping can be done by
1. visual image interpretation (+ expertise, collaboration, - subjective, time-
consuming and accuracy)
2. (semi) automated classification based on spectral characteristics (+ rapid,
- cloud-free)
3. (semi) automated classificated based on elevation differences (+ small
differences, - pre-earthquake DEM are low quality)
4. field investigation (+ essential evidence, - small areas)
Power law for frequency-area distribution can be used for the event-magnitude
scale. Landslide volume is difficult (pre and post-landslide is needed).
Uncertainties: 1) mapping accuracy, 2) coalescing landslides, 3) overlapping
sources and 4) geometric distortions. Coseimic landslides can be triggered by
seismicity and predisposing factors like soil/rock mass properties, slope geometry
and structural features. The magnitude of earthquakes, type of faulting and
aspertises are important controlling factors. Future research: calibration,
accessibility layers, ground-motion characterization and solve scale effect.
, Post-seismic geological hazard and sediment cascade
- formation and failure of EGTL dams: attenuate or accentuate water and
sediment fluxes, channel geometry and stability, and valley evolution.
Controls: 1) seismic intensity, 2) steep hillslope and uneven topography 3)
weak and weather lithologies and 4) high soil moisture or pore-water
pressures. The volume topography and hydrologic factors are also
important. Landslide dams can fail through overtopping (water spilling
over), piping (internal erosion by seepage) and slope failure (pressure >
resistance).
- Landslide remobilization, debris flows and post-seismic landslides: rainfall
can cause new landslides, which feed debris flows.
- Sediment cascade: sediment prone to reworking. Aggradation fills, reduced
accommodation space and changes in flooding affect post-seismic
resilience and recovery. Landslide connectivity = fraction and rate of
coeseismic landslide material being delivered to channels as well as the
underlying processes: 1) loose material mobile during rain, 2) low-order
channels can transport, 3) debris flow entrain material 4) high rare flood.
Suspectilibity: classify terrain into zones with different vulnerability and
probabilities.
Risk = probability of harmful consequences and expected losses (death, injury,
property, livelihoods, economic activity or environment damaged).
Risk = temporal probability x spatial probability x elements at risk x vulnerability
Risk management: life cycle of hazards, duration, extent and elements at risk.
1. What can go wrong
2. What is the likelihood that it will go wrong?
3. What are the consequences if it goes wrong?
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