Presentation on theme: "Managing Landslide Hazard Risk in Sub Tropical Countries Keith Tovey"— Presentation transcript:
1 Managing Landslide Hazard Risk in Sub Tropical Countries Keith Tovey Reader in Environmental Sciences,HSBC Director of Low Carbon Innovation.University of East Anglia,Norwich, UK, NR4 7TJAcknowledgements:British Council,University of East Anglia,University of West Indies (Trinidad)Hong Kong GovernmentMajor Landslide at Maracas, Trinidad - Late December 2002.
2 Managing Landslide Hazard Risk in Sub Tropical Countries IntroductionModelling MethodsEngineering ModelsGIS MethodsStatistical MethodsManagement IssuesConclusions
3 Landslides: Introduction Consequences of LandslidesInjuryDeathEconomic LossDisruption to Transport LinksStability AssessmentLandslidePreventive MeasuresTemporarilySafeLandslide WarningLandslideDesignCostBuildConsequenceRemedial MeasuresRemove ConsequenceSafe at the moment
4 Landslides: Removing the Consequence ManchesterMain Manchester – Sheffield Road (A625)Alternative route – only suitable for light vehicles – gradient of 1 in 41 km
5 Landslides: Removing the Consequence Landslides in Kowloon East 28th - 31st May 1982
6 Landslide Landslides: Engineering Modelling Methods Geology Man’s Influence (Agriculture /Development)GeologyHydrologyMaterial Properties (Shear Strength)Slope AngleLoadingStability AssessmentLandslidePreventive MeasuresTemporarilySafeLandslide WarningLandslideDesignCostBuildConsequenceBut only for specific slopesRemedial MeasuresRemove ConsequenceSafe at the moment
7 Landslides: Engineering Modelling Methods Applicable to very specific locations onlyCan have moderate to good accuracy for spatial predictions where information existsModerate accuracy for temporal predictions (good if accurate ground water temporal variations are available)Poor for overall spatial coverageIs costly to implement.But one must not be complacent
8 bermsLandslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002
9 Landslides: GIS Modelling Methods HydrologyGeologySoil TypeGeneral Slope (and aspect)Land UseCataloguing slopes and landslidesDatabase of existing LandslidesClassification into potential Areas of RiskIdentification of areas for detailed Engineering StudyGeneral Planning Guidelines of Landslide Risk
10 Landslides: GIS Modelling Methods Good spatial (geographic) coverage of likelihood of landslidesPoor to moderate prediction of precise locations of landslidesEffective use of resourcesPoor accuracy for temporal predictionsi.e. precisely when landslides occurAccuracy is dependant on existence of a good unbiassed database of landslides and slopes
11 Landslides: Categorisation of Slopes e.g. North Coast Road, Trinidad “Natural” SlopeCut SlopeFill SlopeRetaining Wall
12 December 2004 – note the slide is much more extensive Landslide at MaracasDecember 2002December 2004 – note the slide is much more extensive
13 3 km beyond Las Cuevas as seen on TV half of road blocked December 9thLandslide3 km beyond Las Cuevas as seen on TVhalf of road blockedLandslide 11th December at approximately 13:001 km before Las Cuevashalf of road blocked
14 Slope before failure at Couva Slope after LandslideSlide by Derek Gay, UWI
15 Landslides: GIS Modelling Methods: Requirements for the future Landslides triggered by anthropogenic activityCut SlopesFill SlopesRetaining WallsHybrids: Cut/Retaining Wall / Fill/Retaining Wall“Natural” Slopes - is there a better word?slopes where there has been no anthropogenic activity, or where there is such activity it causes small changes to the geometry of the slope so that the Factor of Safety is largely unaffected.Deep seated landslide unaffected by anthropogenic activity
16 Landslides: Statistical Methods Historical Database of Landslide OccurrenceRainfall DataResearch to correlate Rainfall with Landslide IncidenceAntecedent RainfallCurrent/ Predicted RainfallPrediction of exactly when landslides are likely to occurIssue warnings to affected peopleMobilise Emergency TeamsAim: to minimise injury and loss of life
17 Landslides: Statistical Methods Landslide Warning System Poor prediction for spatial location of Landslides.Potentially effective use of resources to minimise death and injury.Moderate ability to predict when landslides are likely to occur.Requires automatic recording of rainfall over short periods of time (e.g. 5 – 15 minute intervals).Requires a robust historic database of landslides and associated rainfall.Method aims to alert people to impending danger so they can seek safety during critical periods – it will not prevent landslides
19 Landslides: Management Hong Kong ApproachHistorically: Reactive Approach to LandslidesSimilar to present situation in Trinidad and TobagoFrom 1977 onwards:approach became progressively more pro-activeProactive Control of all New Developments> Engineering / Geotechnical ControlCategorisation of Slopes and Landslides> Develop a Robust Database> Identify critical issues and areas affected: GIS> Planning Policies> Identification of Critical regions for Preventative MeasuresDevelopment of Landslide Warning System.
20 Landslides: Management Click once on Slope to display data for 11SW-A/CR175Slope Catalogue: Slope 11SW-A/CR175: Po Shan Road
21 Landslides: Management Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road
22 Landslide Preventative Measure Feature Registration Form for Po Shan Road Slope11SW-A/CR175Major Disaster in June 1972Landslide Preventative Measure
23 Landslides: Management Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road
24 Landslides: Management Failure of slope in June 1972Aerial Photograph of Slopes11SW-A/CR175 and11SW-A/FR30
25 Landslides: Landslide Warning System Requirements:It should:1) provide sufficient warning of an eventto alert general publicto mobilise Emergency Servicesto open temporary Shelters2) predict IN ADVANCE all serious EVENTS3) minimise number of false alarmsThree criteria can be in conflict:How long should warning be?Longer the time, the less accurate will be prediction– more false alarms
26 Landslides: Landslide Warning System Background to Warning SystemTwo ApproachesDetailed Warning - e.g. 1. Conduit RoadWarning based solely on Rainfallautomatic piezometer gives warning when ground water level gets above a critical level as determined by Slope Stability AnalysisAim to give warning when a significant number of landslides are likely to occur.(>10)
27 Landslides: Statistical Methods Landslide Warning System (continued) Research needed to correlate incidence of landslides with rainfallantecedentcurrentpredictedHong Kong scheme ~ mid 1980sResearch needed to adapt ideas to local conditions in Trinidad and Tobago.Emergency Services need clear guidelines on how to react.Reporting system needed to notify public (via radio/ television)
28 ANTECEDENT CONDITIONS. Are Slopes more susceptible to failure if there has been prolonged rainfall on preceding days?How should Antecedent rainfall Conditions be incorporated.Lumb (1975) - 15-day antecedent conditions.charts for Warning Purposes based both on Rainfall on Day AND Antecedent conditions.Most simple model uses simple cumulative 15-day antecedent rainfall.Could use a weighted system with days more distant weighted less.Lumb favoured simple approach.
29 Basis of Lumb’s Predictor 24 – hour criteriaCummulative RainfallCumulative Rainfall over previous 15 daysDay
30 Rainfall Profile and Onset of Landslides Cumulative Rainfall4 hours20 hoursLandslipPrediction Criteria(LPC)Landslip Time (LT) (The time when first landslip is reported to FSD).Warning Time (WT) (Rainfall predicted to reach LPC in 4 hours)Criteria Time (CT)The time when LPC are actually reached.Actual Cumulative Rainfall Predicted Cumulative Rainfall
31 First Landslide Warning System (1977 - 1979) AMBER and RED Warnings issued when predicted 24 hour rainfall would plot above relevant line.A Problem: Difficult to use without direct access to Chart.
32 Landslide Warning System 2: (1980 - mid 1983) Advantage: Much easier to identify whether WARNING should be called - even when chart is not to hand.
33 Landslide Event 28 - 29th May 1982 4003002001000904Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total:Squatters:Rainfall on Landslip Day (mm)00201612Antecedent Rainfall in previous 15 days (mm)
34 Landslide Event 28 - 29th May 1982 Even with 24hr day plotting, the plot for 29th May should have been as follows400300200100Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total:Squatters:0904Rainfall on Landslip Day (mm)00201612Antecedent Rainfall in previous 15 days (mm)
35 Landslide Event 28 - 29th May 1982 Situation with running 24 hr criterion400300200100090400201612Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total:Squatters:Criterion was reached at approx 03:00BUT1st Landslide was reported at 02:00 when rainfall was about 220mmRainfall on Landslip Day (mm)Even if Warning procedure has been operated correctly, warning would have been 1 hour too late!Antecedent Rainfall in previous 15 days (mm)
36 Antecedent Rainfall in previous 15 days (mm) All Landslide Warning Incidents in 1982400300200100090400201612Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total:Squatters:20161208040020161216120804LW 5/8205:50 – 16/08/82Total: 98Sq: 32LW 2/8206:15 – 31/05/82Total: 91/ Sq: 40LW 7/8223:52 – 16/09/82Total: 3Sq: 300LW 4/8211:00 – 03/08/82Total: 9Sq: 5LW 6/8206:35 – 18/08/82*Total: 8Sq: 21606LW 3/8211:00 – 02/06/82*Total: 28/Sq: 1216Antecedent Rainfall in previous 15 days (mm)
37 Performance of All LandSlip Warnings 1982 - 1983 RedLandslides with No Warning!GreenLandslide Warnings with Several Hours WarningBlue Landslide Warnings with 1 Hour Warning
38 All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82Disastrous > 50 reported Landslides: Severe LandslidesMinor < 10 Landslides : Null Event: No reported Landslides
39 Landslide Warnings: The Problems 1. Antecedent Condition leads to confusion - (Incident 1/82)2. Must use rolling 24 hour scheme3. Previous Analysis (e.g. Lumb) has been based on 24 hr day basis4. Total Rainfall in day will not generally be a good correlator as final cumulative 24 hr rainfall (whether day or rolling) will occur AFTER Landslides have occurred.5. Some Landslides Events will occur after very low Antecedent Rainfall6. Some Landslides Events occur after short periods of very intense rainfall.7. It is difficult to predict with accuracy future rainfall.Is it sensible to continue with Antecedent Rainfall Condition??
40 Severe and Disastrous Landslide Events: with 1984 Scheme Existing Criteria Line - in use mid mid 1984Warning and Landslide Lines in use from mid 1984
41 Landslides: Landslide Warning System Landslide Warnings: The Final (1984) Approach1. Abolish Antecedent Criteria - base solely on Rolling 24hr approach.2. When Rainfall exceeds 100 mm in a period of 24 hours and is expected to exceed 175 mm (total) within 4 hours: CONSIDER issuing a LANDSLIDE WARNING.If weather conditions suggest that Rainfall will cease shortly then issue could be delayed.3. If Rainfall exceeds 175 mm then Landslides are likely and Warning should now be issued regardless of whether rain is likely to cease shortly4. Landslide Warning should be issued regardless of above if rainfall in any one hour exceeds 70 mm in any one hour in Urban Area.
42 Landslides: Landslide Warning System The 1984 Warning SchemeSimple to understandOn average ~ Warnings in a Yearup to one third are false alarmsidentifies all serious/disastrous eventsabout one third of warnings classified as minor(i.e. less than 10 landslides).Further Improvements were introduced in 1999
43 Landslides: The Way Forward the Engineering Approach is justified in a few casesNew developments / highways etcGIS methods are powerful and cost effectiveBUTRequires development of a robust DatabaseCatalogue of Slope Types (whether failed on not)Catalogue of LandslidesTrinidad and Tobago (Carribean) can build on an improve on the scheme developed in Hong Kong.Research needed to enhance GIS prediction of landslidesIncorporate Geotechnical information
44 Landslides: Conclusions Interdisciplinary Research incorporating all three approaches is important for effective management of slopes and mitigation of adverse effects of landslides.Proactive Management of slope hazards will be more cost effective in the long term.Hong Kong woke up to the seriousness of the issues following disastrous landslides in Caribbean Countries should learn from their experience.Important to begin and resource fully the research needed to achieve these aims.