exercise 6 rick city landslides 2008

International Institute for Geo-Information Science and Earth Observation (ITC)

ISL 2004

RiskCityRiskCityExercise 5:Exercise 5:

Landslide risk assessment. Landslide risk assessment.

Cees van WestenUnited Nations University – ITC School for Disaster Geo-Information

ManagementInternational Institute for Geo-Information Science and Earth Observation (ITC)

Enschede, The NetherlandsE-mail: [email protected]

Associated Institute of the


ISL 2004

Landslide risk exerciseLandslide risk exercise

Three approaches will be done:• Qualitative landslide risk assessment• Semi quantitative • Quantitative

Input data


ISL 2004

Simple vulnerability assessmentSimple vulnerability assessment

Vuln_buildings:=iff(nr_buildings<5,”Low_v”,iff(nr_buildings<25, “Moderate_v”, ”High_v”))

This is of course a large simplification. If more time was available the vulnerability could be better evaluated using Spatial Multi Criteria Evaluation with many more criteria.

Mapping units

Vulnerability


ISL 2004

Qualitative risk assessmentQualitative risk assessmentQualitative_risk = Qualitative_risk [Susceptibility,Vulnerability]

Vulnerability

Susceptibility

Two dimensional table


ISL 2004

Semi-quantitative risk assessmentSemi-quantitative risk assessmentIt combines the susceptibility zones with the mapping units, and calculates the number of houses and people in High, Moderate and Low susceptibility zones

Number of buildings

Risk formula:

Risk =H*V*A

H= Hazard V= VulnerabilityA= Amount of Elements at risk

Risk formula:

Risk =H*V*A

H= unknown V= unknownA= Amount of Elements at risk


ISL 2004

Calculate Nr of buildingsCalculate Nr of buildingsCalculates the number of houses in High, Moderate and Low susceptibility zones

Total area of each mapping unit

Fraction of each unit with high, moderate and low susceptibility

Table mapping units

Table joinCross

Mapping units Susceptibility

LS_Risk_high = LSS_high * nr_buildings


ISL 2004

Calculate Nr of buildingsCalculate Nr of buildings

2699 buildings

7193buildings

16739buildings

The results obtained so far can be a bit misleading because the losses are shown per mapping unit, and not for individual buildings located in high hazard areas.

The number of buildings per mapping units were estimated earlier when a building footprint map was still not available


ISL 2004

Better estimation using Building mapBetter estimation using Building mapCalculates the number of houses in High, Moderate and Low susceptibility zones using Building map

Total area of each mapping unit

Fraction of each unit with high, moderate and low susceptibility

Results using mapping units

Table joinCross

Building map Susceptibility

2699 buildings

4426 buildings

7193buildings

9645buildings

16739buildings

22019buildings

Results using Building map

Cross

High Moderate Low


ISL 2004

Quantitative risk assessmentQuantitative risk assessment

4426 buildings

9645buildings

22019buildings

Risk = Hazard * Vulnerability * Amount How much percentage of the high, moderate and low

hazard classes may be affected by landsides? In which period will these landslides occur? What is the vulnerability to landslides?

Results using mapping unitsHigh Moderate Low

Known nowStill to doOnly susceptibility

Hazard = Spatial probability * Temporal probability

The temporal probability that landslides may occur due to a triggering event. Here we will link the return period of the triggering event with the landslides that are caused by it. We have differentiated return periods of: 50, 100, 200, 300 and 400 years.

The spatial probability that a particular area would be affected by landslides of the given temporal probability. This is calculated as the landslide density within the landslide susceptibility class.


ISL 2004

From susceptibility to hazardFrom susceptibility to hazardMillion dollar information!!!

Landslide related to different return periods

Landslide_ID map If the indication of the high, moderate and low areas susceptibility is correct, different landslide events with different return periods will give different distributions of landslides in these classes.

The probability can be estimated by multiplying the temporal probability (1/return/period for annual probability) with the spatial probability (= what is the chance that 1 pixel is affected)

Susceptibility

Cross

Density in high

Density in moderate

Density in low


ISL 2004

Calculating hazardCalculating hazardAssumption is that events with a larger return

period will also trigger those landslides that would be triggered by events from smaller return periods

Susc

eptib

ility

cla

sses

Return periods


ISL 2004

Calculating VulnerabilityCalculating VulnerabilitySimple assumption:

The more buildings there are with 3 floors or higher, the lower will be the landslide vulnerability, as it becomes less likely that large buildings will be

destroyed by landslides.

Vuln:=iff(PerVacant=1,0,1-(Perc3floor+Percover3floor))

Estimating landslide vulnerability is very complex.

It requites knowledge on the building types and on the expected landslide volumes and velocities.

These are difficult to estimate. In many study landslide vulnerability of

buildings is simply taken as 1, assuming complete destruction of the elements at risk.

This would, however, in our case give too exaggerated values of risk.


ISL 2004

Calculate lossesCalculate losses

Loss_50_high:=0.0181*vuln*nr_b_highLoss_50_moderate:= 1.31199E-06*vuln*nr_b_moderateLoss_50_low:= 5.96345E-07*vuln*nr_b_lowetc

Losses = Spatial Probability * Consequences Losses = Spatial P * V * A


ISL 2004

Calculate lossesCalculate losses

What can you conclude when you compare the spatial probabilities and consequences for the high, moderate and low susceptibility classes ?

Losses for a return period = sum of losses in high, moderate and low susceptibility areas


ISL 2004

Calculate riskCalculate riskPeriod


ISL 2004

Triangles and Triangles and rectangles rectangles

methodmethod

• The area under the curve is divided into trangles, which connect the straight lines between two points in the curve and have X-axis difference as difference between the losses of the two scenarios. Y-axis of the triangles is the difference in probability between two scenarios. The remaining part under the curve is then filled up with rectangles, as illustrated in the graph and table below.

This is the annual risk, taking the sum of the triangles and squares in the graph


ISL 2004

Cost / benefit analysisCost / benefit analysis• Defining risk reduction scenarios• Calculate investment costs• Calculate operation and

maintenance• Define project lifetime• Calculate Net Present Value• Calculate Internal Rate of Return

exercise 6 rick city landslides 2008

Documents