landslide hazards in central america and the … filelandslide hazards in central america and the...
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Landslide hazards in Central
America and the Caribbean:
climate change, early-
warning systems and land-
use planning
José Cepeda DIC MSc PhD
NGI / ICG / UiO
NorLARnet conference
13 November 2009
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Basic definitions
Landslide: the movement of
a mass of rock, debris, or
earth (soil) down a slope
Debris flow: saturated
debris running along an
established channel at
extremely rapid velocities (≥
18 km/h)
Introduction Framework Thresholds Runout Recommendations
Santa Ana volcano, El Salvador
October 2005
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Basic definitions
Landslide susceptibility:
A quantitative or qualitative ranking
of slopes from stable to unstable.
A choice of quantitative measure:
Factor of Safety (FS).
FS > 1: no landslides
FS < 1: landslides occurSan Salvador volcano,
El Salvador, October 2008
Introduction Framework Thresholds Runout Recommendations
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Basic definitions
Rainfall threshold for
landslide triggering
Boundary curve or surface
separating triggering and non-
triggering rainfall events.
Duration (h)
Intensity
(mm/h)+
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+ ++
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+
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-
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--
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-- -- -
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Why are landslides important?
Exposed area: 3.7 million km2
Exposed population: 300 million (5% of world population)
High risk zones:
• 820 000 km2
• 66 million people
Source: global landslide hotspots (Nadim et al., 2006)
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Impact of landslides in Latin
America
Nadim et al. (2006)
Rank Region Deaths per
million habitants
per year
1 Central America 10.47
2 South America 7.78
3 Central Asia 1.17
Worldwide impact (1980-2000)
Petley et al. (2005)
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Importance of rainfall-induced
landslides
• Significant in total number
of landslides:
~ 80%
• Numerous transform in
debris flows:
~ 90%
• Debris flows:
• Long runout distances:
several kilometres
• High velocities: > 18 km/h,
but often > 100 km/h
ww
w.u
nic
ae
n.fr
ww
w.u
sg
s.g
ov
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Main objective
Development of tools for risk management of rainfall-
induced landslides, focusing on implementation in tropical
regions and in developing countries.
Why tropical regions and developing countries?
• Worldwide landslide hotspot regions: 6 of 9 in developing
countries (5 with tropical climates)
• Rainfall data from tropical regions for assessing
thresholds: 4.7%
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Time-trend analysis
1. Intensity-Duration (ID) for series of
n·ΔT years
2. Time binning of ID into n periods
of ΔT years each: T1, T2, ..., Tt, ...,
Tn
3. For t = 1 to n
a. Create ID plot for Tt period
b. Create N-i+1 bins with a set of
thresholds
c. Assess and plot parameter for k-th bin
4. Perform a time-trend analysis of
parameter in k-th bin
Introduction Framework Thresholds Runout Recommendations
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Time-trend analyses of
precipitation data
Hourly rainfall data from Pico del Este (Puerto Rico)
during 1973-2006
Probabilities of triggering
landslides: 0.25 to 0.40
Introduction Framework Thresholds Runout Recommendations
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Correlation with ENSO:
Nicaragua (1958-2005)
-3 -2 -1 0 1 2 310
20
30
40
50
60
Southern Oscillation Index
Numberof days
per yearthat
exceedGEA2008threshold
-3 -2 -1 0 1 2 31.2
1.4
1.6
1.8
2
2.2
Intensityof stormsper yearthat exceed GEA2008 threshold(mm/h)
R2 = 0.35
R2 = 0.001
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Rain
fall
para
mete
r 1
Rainfall parameter 2 Time
Th0- , Thn
Th0+
Thi
Pre-event Event Post - event
t0+
1
2
3 4
5
t0- ti tn
Δ0 ΔnΔi
Δtst
Rainfall parameter 1: intensity, n-day antecedent rainfall, etc.
Rainfall parameter 2: duration, 24-hour rainfall, etc.
Th: threshold curves
Δ0: reduction in threshold during weakening stage
Δi: increase in threshold at time tiΔn: increase in threshold during healing stage
tn – t0+ : duration of healing stage
Time-variable thresholds
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Threshold reduction
due to eruptive activity
Threshold reduction
due to eruptive activity
Threshold increases as erosion
and lahars occur
Application: Montserrat
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Model for local rainfall
thresholds
Generalised function DAI n ][ 2
1
α
lk DAIandDAIl
pl
k
nk
,
,1
,
,122
Using a combination of n-day and p-day
antecedent precipitation
i.e., threshold is exceeded when an event
occurs at or above both surfaces
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Model for local rainfall thresholds
Metropolitan Area of San Salvador (AMSS), El Salvador
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Local rainfall thresholds for
AMSS
• Total number of tested models: 210
• Each model was tested using 50 000
combinations of parameters
• Total number of models and parameters
combinations: > 10 million
• Implementation of optimisation of models
and parameters in Matlab
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Local rainfall thresholds for
AMSS
7-day criterion 150-day criterion
43.01
72322 DAI d
43.01
15028534 DAI dAND
Reduction in false alarms: 51 to 5
Currently in use by
salvadoran authorities
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Backanalysis of debris flow runout
at San Salvado volcano, El Salvador
• Debris flows in 1934,
1982, 2008
• 1982 event:
• 4 km runout
• ~300 000 m3
• ~400 killed
• Current exposure:
~ 21 500 persons
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Forward predictions for return period > 92 years
Introduction Framework Thresholds Runout Recommendations
Currently in use for updating of land-use plans
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To local and national authorities
and decision makers
• Tools have to be updated as more data
becomes available, particularly in regards to
rainfall thresholds.
• Improve the availability and quality of historic
and future landslide data.
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Further research
• Investigate conceptual framework in the lower
and upper ranges of rainfall durations.
• Test optimisation of rainfall threshold
parameters analysing other types of threshold
models.
• Test the new models and applications with other
datasets.
Introduction Framework Thresholds Runout Recommendations
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References
Cepeda, J., Høeg, K., and Nadim, F. (in press). "Landslide-triggering
rainfall thresholds: A conceptual framework." Quarterly Journal of
Engineering Geology and Hydrogeology.
Cepeda, J., Díaz, M. R., Nadim, F., Høeg, K., and Elverhøi, A. (in
review). "An empirical threshold model for rainfall-induced
landslides: application to the Metropolitan Area of San Salvador, El
Salvador." Submitted to Landslides.
Cepeda, J., Chávez, A., and Cruz, C. (in review). "Procedure for the
selection of runout model parameters from landslide back analyses:
application to the Metropolitan Area of San Salvador, El Salvador."
Submitted to Landslides.
Cepeda, J., and Devoli, G. (2008). "Rainfall thresholds for landslide
triggering following volcanic ash eruptions and earthquakes."
European Geosciences Union General Assembly 2008, Vienna,
Austria. www.cosis.net/abstracts/EGU2008/03879/EGU2008-A-
03879.pdf
