1. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 RiskCity Exercise 5: Landslide risk assessment. Cees van Westen United Nations University – ITC School for Disaster Geo-Information Management International Institute for Geo-Information Science and Earth Observation (ITC) Enschede, The Netherlands E-mail: westen@itc.nl Associated Institute of the

2. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Landslide risk exercise Three approaches will be done: Qualitative landslide risk assessment Semi quantitative Quantitative Input data

3. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Simple 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

4. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Qualitative risk assessment Qualitative_risk = Qualitative_risk [Susceptibility,Vulnerability] Vulnerability Susceptibility Two dimensional table

5. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Semi-quantitative risk assessment It 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= Vulnerability A= Amount of Elements at risk Risk formula: Risk =H*V*A H= unknown V= unknown A= Amount of Elements at risk

6. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculate Nr of buildings Calculates 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 join Cross Mapping unitsSusceptibility LS_Risk_high = LSS_high * nr_buildings

7. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculate Nr of buildings 2699 buildings 7193 buildings 16739 buildings 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

8. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Better estimation using Building map Calculates 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 join Cross Building mapSusceptibility 2699 buildings 4426 buildings 7193 buildings 9645 buildings 16739 buildings 22019 buildings Results using Building map Cross HighModerateLow

9. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Quantitative risk assessment 4426 buildings 9645 buildings 22019 buildings 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 units HighModerateLow 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.

10. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 From susceptibility to hazard Million 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

11. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculating hazard Assumption is that events with a larger return period will also trigger those landslides that would be triggered by events from smaller return periods Susceptibility classes Return periods

12. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculating Vulnerability Simple 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.

13. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculate losses Loss_50_high:=0.0181*vuln*nr_b_high Loss_50_moderate:= 1.31199E-06*vuln*nr_b_moderate Loss_50_low:= 5.96345E-07*vuln*nr_b_low etc Losses = Spatial Probability * Consequences Losses = Spatial P * V * A

14. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculate 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

15. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Calculate risk Period

16. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Triangles and rectangles method 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

17. International Institute for Geo-Information Science and Earth Observation (ITC) ISL 2004 Cost / 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