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1 Managing Landslide Hazard Risk in Sub Tropical Countries Major Landslide at Maracas, Trinidad - Late December 2002. Keith Tovey Reader in Environmental.

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Presentation on theme: "1 Managing Landslide Hazard Risk in Sub Tropical Countries Major Landslide at Maracas, Trinidad - Late December 2002. Keith Tovey Reader in Environmental."— Presentation transcript:

1 1 Managing Landslide Hazard Risk in Sub Tropical Countries Major Landslide at Maracas, Trinidad - Late December Keith Tovey Reader in Environmental Sciences, HSBC Director of Low Carbon Innovation. University of East Anglia, Norwich, UK, NR4 7TJ Acknowledgements: British Council, University of East Anglia, University of West Indies (Trinidad) Hong Kong Government

2 2 Introduction Modelling Methods – Engineering Models – GIS Methods – Statistical Methods Management Issues Conclusions Managing Landslide Hazard Risk in Sub Tropical Countries

3 3 Landslide Remedial Measures Safe at the moment Landslide Preventive Measures Landslides: Introduction Design Cost Build Consequence Remove Consequence Stability Assessment Temporarily Safe Landslide Warning Injury Death Economic Loss Disruption to Transport Links Consequences of Landslides

4 4 Main Manchester – Sheffield Road (A625) Alternative route – only suitable for light vehicles – gradient of 1 in 4 Landslides: Removing the Consequence Manchester 1 km

5 5 Landslides in Kowloon East 28th - 31st May 1982 Landslides: Removing the Consequence

6 6 Landslide Remedial Measures Landslide Preventive Measures Mans Influence (Agriculture /Development) Landslides: Engineering Modelling Methods Geology Design Cost Build Consequence Remove Consequence Stability Assessment Temporarily Safe Landslide Warning Hydrology Material Properties (Shear Strength) Slope Angle Loading But only for specific slopes Safe at the moment

7 7 Applicable to very specific locations only Can have moderate to good accuracy for spatial predictions where information exists Moderate accuracy for temporal predictions (good if accurate ground water temporal variations are available) Poor for overall spatial coverage Is costly to implement. Landslides: Engineering Modelling Methods But one must not be complacent

8 8 berms Landslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002

9 9 General Planning Guidelines of Landslide Risk Classification into potential Areas of Risk Soil Type Landslides: GIS Modelling Methods Geology Hydrology General Slope (and aspect) Land Use Database of existing Landslides Identification of areas for detailed Engineering Study Cataloguing slopes and landslides

10 10 Good spatial (geographic) coverage of likelihood of landslides Poor to moderate prediction of precise locations of landslides Effective use of resources Poor accuracy for temporal predictions –i.e. precisely when landslides occur Landslides: GIS Modelling Methods Accuracy is dependant on existence of a good unbiassed database of landslides and slopes

11 11 e.g. North Coast Road, Trinidad Fill Slope Retaining WallNatural Slope Cut Slope Landslides: Categorisation of Slopes

12 12 Landslide at Maracas December 2002 December 2004 – note the slide is much more extensive

13 13 December 9 th Landslide 3 km beyond Las Cuevas as seen on TV half of road blocked Landslide 11 th December 2004 at approximately 13:00 1 km before Las Cuevas half of road blocked

14 14 Slope before failure at Couva Slope after Landslide Slide by Derek Gay, UWI

15 15 Landslides: GIS Modelling Methods: Requirements for the future Cut Slopes Fill Slopes Retaining Walls Hybrids: 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. Landslides triggered by anthropogenic activity Deep seated landslide unaffected by anthropogenic activity

16 16 Landslides: Statistical Methods Prediction of exactly when landslides are likely to occur Issue warnings to affected people Mobilise Emergency Teams Historical Database of Landslide Occurrence Rainfall Data Research to correlate Rainfall with Landslide Incidence Antecedent Rainfall Current/ Predicted Rainfall Aim: to minimise injury and loss of life

17 17 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. Landslides: Statistical Methods Landslide Warning System Method aims to alert people to impending danger so they can seek safety during critical periods – it will not prevent landslides

18 18 Rain Gauge Network in Hong Kong Built Up Areas

19 19 Historically: Reactive Approach to Landslides Similar to present situation in Trinidad and Tobago From 1977 onwards: approach became progressively more pro-active Hong Kong Approach Landslides: Management Proactive Control of all New Developments > Engineering / Geotechnical Control Categorisation of Slopes and Landslides > Develop a Robust Database > Identify critical issues and areas affected: GIS > Planning Policies > Identification of Critical regions for Preventative Measures Development of Landslide Warning System.

20 20 Click once on Slope to display data for 11SW-A/CR175 Slope Catalogue: Slope 11SW-A/CR175: Po Shan Road Landslides: Management

21 21 Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road Landslides: Management

22 22 Feature Registration Form for Po Shan Road Slope 11SW-A/CR175 Major Disaster in June 1972 Landslide Preventative Measure

23 23 Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road Landslides: Management

24 24 Failure of slope in June 1972 Aerial Photograph of Slopes 11SW-A/CR175 and 11SW-A/FR30 Landslides: Management

25 25 Requirements: It should: 1) provide sufficient warning of an event to alert general public to mobilise Emergency Services to open temporary Shelters 2) predict IN ADVANCE all serious EVENTS 3) minimise number of false alarms Three criteria can be in conflict: How long should warning be? Longer the time, the less accurate will be prediction – more false alarms Landslides: Landslide Warning System

26 26 Two Approaches Detailed Warning - e.g. 1. Conduit Road Warning based solely on Rainfall automatic piezometer gives warning when ground water level gets above a critical level as determined by Slope Stability Analysis Aim to give warning when a significant number of landslides are likely to occur. (>10) Background to Warning System Landslides: Landslide Warning System

27 27 Research needed to correlate incidence of landslides with rainfall antecedent current predicted Hong Kong scheme ~ mid 1980s Research 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) Landslides: Statistical Methods Landslide Warning System (continued)

28 28 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. ANTECEDENT CONDITIONS.

29 Day 24 – hour criteria Cumulative Rainfall over previous 15 days Cummulative Rainfall Basis of Lumbs Predictor

30 30 20 hours 4 hours Landslip Prediction Criteria (LPC) Warning Time (WT) (Rainfall predicted to reach LPC in 4 hours) Cumulative Rainfall Actual Cumulative Rainfall Predicted Cumulative Rainfall Landslip Time (LT) (The time when first landslip is reported to FSD). Criteria Time (CT) The time when LPC are actually reached. Rainfall Profile and Onset of Landslides

31 31 First Landslide Warning System ( ) 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 32 Landslide Warning System 2: ( mid 1983) Advantage: Much easier to identify whether WARNING should be called - even when chart is not to hand.

33 33 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: Antecedent Rainfall in previous 15 days (mm) Rainfall on Landslip Day (mm) Landslide Event th May 1982

34 34 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: Antecedent Rainfall in previous 15 days (mm) Rainfall on Landslip Day (mm) Landslide Event th May 1982 Even with 24hr day plotting, the plot for 29th May should have been as follows

35 35 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: Antecedent Rainfall in previous 15 days (mm) Rainfall on Landslip Day (mm) Situation with running 24 hr criterion Landslide Event th May 1982 Criterion was reached at approx 03:00 BUT 1st Landslide was reported at 02:00 when rainfall was about 220mm Even if Warning procedure has been operated correctly, warning would have been 1 hour too late!

36 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: LW 2/82 06:15 – 31/05/82 Total: 91/ Sq: 40 LW 4/82 11:00 – 03/08/82 Total: 9 Sq: 5 LW 6/82 06:35 – 18/08/82* Total: 8 Sq: 2 LW 3/82 11:00 – 02/06/82* Total: 28/Sq: 12 LW 5/82 05:50 – 16/08/82 Total: 98 Sq: 32 LW 7/82 23:52 – 16/09/82 Total: 3 Sq: Antecedent Rainfall in previous 15 days (mm) All Landslide Warning Incidents in 1982

37 37 Performance of All LandSlip Warnings Red Landslides with No Warning! Green Landslide Warnings with Several Hours Warning Blue Landslide Warnings with 1 Hour Warning

38 38 All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall Disastrous > 50 reported Landslides: Severe Landslides Minor < 10 Landslides : Null Event: No reported Landslides Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82

39 39 Landslide Warnings: The Problems 1. Antecedent Condition leads to confusion - (Incident 1/82) 2. Must use rolling 24 hour scheme 3. Previous Analysis (e.g. Lumb) has been based on 24 hr day basis 4. 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 Rainfall 6. 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 40 Existing Criteria Line - in use mid mid 1984 Warning and Landslide Lines in use from mid 1984 Severe and Disastrous Landslide Events: with 1984 Scheme

41 41 Landslide Warnings: The Final (1984) Approach 1. 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 shortly 4. Landslide Warning should be issued regardless of above if rainfall in any one hour exceeds 70 mm in any one hour in Urban Area. Landslides: Landslide Warning System

42 42 The 1984 Warning Scheme Simple to understand On average ~ Warnings in a Year up to one third are false alarms identifies all serious/disastrous events about one third of warnings classified as minor (i.e. less than 10 landslides). Further Improvements were introduced in 1999 Landslides: Landslide Warning System

43 43 Landslides: The Way Forward the Engineering Approach is justified in a few cases New developments / highways etc GIS methods are powerful and cost effective BUT Requires development of a robust Database Catalogue of Slope Types (whether failed on not) Catalogue of Landslides Trinidad and Tobago (Carribean) can build on an improve on the scheme developed in Hong Kong. Research needed to enhance GIS prediction of landslides Incorporate Geotechnical information

44 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.


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