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QUANTITATIVE LANDSLIDE HAZARD ASSESSMENT

Jordi Corominas

Department of Geotechnical Engineering andGeosciences

Civil Engineering School of BarcelonaTechnical University of Catalonia - UPC

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Van Westen et al, 2005

LANDSLIDE RISK ASSESSMENT

GIS environment

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Quantitative Assessment and Zoning of Landslide Hazard

Some questions:

May landslide all susceptibility maps be considered spatialprobability maps?

How can landslide hazard maps be derived from the landslidesusceptibility maps?

Are landslide susceptibility maps necessary to prepare hazardmaps?

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Hazard assessment based on frequency of the trigger

Logistic regressionChen & Wang, 2007Nat. Hazards, 42: 75-89

Small scale maps

Multiply the spatialprobability and the probability of the landslidetrigger (i.e. rainfall)

Some critical issues:

Which are the potentialtriggers?

Which probability? That ofthe critical rainfall?

What will be the probabilityin case of a rainfall muchhigher than the criticalrainfall?Usually neither landslide size nor runout are

taken into account

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PREPARING LANDSLIDE HAZARD MAPS FOR MORLEs

Small scale maps

Multiple Occurrence of Regional LandslideEvents (MORLE)

Homogeneous area. The exact location ofThe failure is not known. Neither landslidesize nor runout are taken into account

Landslide susceptibility mapmight not be necessary

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From Reid & Page, 2002

Relationship between stormMagnitude and areal landslidedensity for forest and pastureLand. Curved lines indicatethe 95% confidence intervalfor the unconstrained regressions. MaximumMagnitudes for which landslides werenot generated are indicated by arrows in the x-axis

PREPARING LANDSLIDE HAZARD MAPS FOR MORLEs

The relation betweenthe magnitude of thetrigger and that of thelandsliding event has to be established

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From Reid & Page, 2002

Expected frequency ofstorms as function ofthe storm magnitude

PREPARING LANDSLIDE HAZARD MAPS FOR MORLEs

and the f-M relation ofthe landslide trigger

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0.00 0.10 0.20 0.30 0.40 0.50

Normalised Storm Rainfall

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PREPARING LANDSLIDE HAZARD MAPS FOR MORLEs: some restrictions

density of landslides (events/km2) vs the Normalised Storm Rainfall (NSR) (Govi & Sorzana, 1980).

The relation betweenthe magnitude of thetrigger and that of thelandsliding event is notalways evident

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MEDIUM TO LARGE SCALE MAPS

What are the landslide suceptible areas?

Areas where slope failure may occur

Areas potentially affected by the arrival of the landslidemass (runout distance)

There is a general consensus that susceptible areas shoulddisplay both the source and runout

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MEDIUM TO LARGE SCALE MAPS

Probability atthe landslidesource

Area potentiallyaffected

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Corominas & Moya, 2008

Medium to large scale maps: where is landslide frequencydetermined?

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Preparing magnitude-frequency curves

The goal is to determine the frequency (probability of occurrence) foreach magnitude class

Guthrie & Evans, 2004, NHESS, 4: 475

Landslide inventories should include data on the landslide magnitude

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Jakob 2005

Hazard analysis for different magnitudes (scenarios withdifferent probability of occurrence): spatial extent

Landslide susceptibilitymaps may not be necessaryalthough runout analysis it isoften required to define thepotentially affected areas(scenarios)

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Hazard levels

Lateltin et al. 2005

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Hazard matrix: intensity – frequency relationship

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HAZARD MATRIX: INTENSITY – FREQUENCY RELATIONSHIP

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Landslide intensity

Intensity: a set of spatially distributed parameters describingthe destructiveness of the landslide (Hungr, 1997)

i.e. velocity, kinetic energy, total displacement, flow discharge, etc

The landslide mechanism has a strong influence on theconsequences and damages. Large landslides are notnecessarily more damaging than small ones.

The expected damage depends on the location of theexposed element in relation to the landslide

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Landslide intensity descriptors

Cascini, 2003

Rock avalanches, large rock slides

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LANDSLIDE INTENSITY DESCRIPTORS

Lateltin et al. 2005

Rock avalanches and large rock slides will be alwaysranked as very high intensity events (catastrophic events)

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from Jakob 2005

Landslide intensity descriptors

In case of rapid and very rapid landslides, intensity is closelyrelated to the landslide size (magnitude)

In case of small size landslides the spatial distribution of theintensity is a key issue

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Small size landslides: intensity is more appropriate thanthe magnitude (it is a spatially distributed parameter)

Design of remedial and protectivemeasures are based on landslideintensity calculations

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Hazard analysis for different magnitudes (scenarios withdifferent probability of occurrence): intensity computation

Scenario IIb: V = 10000 m3 ; µ = 0.15; C = 9 m1/2/s

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yes no

Potentiallyaffected area

Landslidesource

Landslide inventory

Frequency/ magnitude

Quantitativelandslide hazard

Spatially distributed quantitative landslide hazard map

Intensityassessment

Runoutanalysis

Repetitivefailures

Dormantlandslides

Sequence ofreactivation events

Magnitude/frequencyLocal vs. global

Sequence offailures

# local & global landslide reactivations/yr

Magnitude/ frequency

Quantitativehazard

Individual landslides

Multiplelandslides

yes no

Landslide inventory

Sequences of eventsLandslide frequency# landslides/Km2/year

Semi-quantitative hazard# landslides/Km2/year

Quantitative hazard# landslides/Km2/year per each magnitude class

Quantitative hazard# landslides/Km2/yearper each magnitude class

Runoutanalysis

Site specific, linear feature, etc

Short displacement landsides

Large displacementlandslides

zoning

Magnitude/ frequency

Different hazard assessment approaches based on frequency of the landslides (Corominas & Moya, 2008)

Hazard assessment based on the frequency of the landslides

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Corominas, J. & Moya, J. 2008. A review of assessing landslide frequencyfor hazard zoning purposes. Engineering Geology (in press)

Hungr. O. 1997. Some methods of landslide hazard intensity mapping. In D. Cruden & R. Fell (editors). Landslide risk assessment. A.A. Balkema, Rotterdam. pp. 215-226

Jakob, M. 2005. A size classification for debris flows. Engineering Geology, 79: 151-161

Lateltin, O. ; Bonnard, Ch.; Haemig, Ch. & Raetzo, H. 2005. Landslide riskmanagement in Switzerland. Landslides, 2: 313-320

Reid, L.M. & Page, M.J. 2002. Magnitude and frequency of landsliding in a large New Zealand catchment. Geomorphology 49: 71-88

References