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Managing Landslide Hazard Risk in Sub Tropical Countries

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 7TJ. Acknowledgements: British Council, University of East Anglia,

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Managing Landslide Hazard Risk in Sub Tropical Countries

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  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 7TJ • Acknowledgements: • British Council, • University of East Anglia, • University of West Indies (Trinidad) • Hong Kong Government Major Landslide at Maracas, Trinidad - Late December 2002.

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

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

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

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

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

  7. Landslides:Engineering Modelling Methods • 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. But one must not be complacent

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

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

  10. Landslides:GIS Modelling Methods • 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 Accuracy is dependant on existence of a good unbiassed database of landslides and slopes

  11. Landslides:Categorisation of Slopes Cut Slope Fill Slope Retaining Wall “Natural” Slope e.g. North Coast Road, Trinidad

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

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

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

  15. Landslides:GIS Modelling Methods: Requirements for the future Landslides triggered by anthropogenic activity • 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. Deep seated landslide unaffected by anthropogenic activity

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

  17. Landslides:Statistical MethodsLandslide 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

  18. Rain Gauge Network in Hong Kong Built Up Areas

  19. Landslides: Management Hong Kong Approach • Historically: Reactive Approach to Landslides • Similar to present situation in Trinidad and Tobago • From 1977 onwards: • approach became progressively more pro-active • 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. Landslides: Management Click once on Slope to display data for 11SW-A/CR175 Slope Catalogue: Slope 11SW-A/CR175: Po Shan Road

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

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

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

  24. Landslides: Management Failure of slope in June 1972 Aerial Photograph of Slopes 11SW-A/CR175and 11SW-A/FR30

  25. Landslides: Landslide Warning System 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

  26. Landslides: Landslide Warning System Background to Warning System • 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)

  27. Landslides:Statistical MethodsLandslide Warning System (continued) • 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)

  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 criteria Cummulative Rainfall Cumulative Rainfall over previous 15 days 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Day

  30. Rainfall Profile and Onset of Landslides Cumulative Rainfall 4 hours 20 hours Landslip Time (LT) (The time when first landslip is reported to FSD). Landslip Prediction Criteria (LPC) 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 09 04 00 20 16 12 400 300 200 100 0 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: 107 Rainfall on Landslip Day (mm) 0 100 200 300 400 500 600 700 800 Antecedent Rainfall in previous 15 days (mm)

  34. Landslide Event 28 - 29th May 1982 09 04 00 20 16 12 Even with 24hr day plotting, the plot for 29th May should have been as follows 400 300 200 100 0 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: 107 Rainfall on Landslip Day (mm) 0 100 200 300 400 500 600 700 800 Antecedent Rainfall in previous 15 days (mm)

  35. Landslide Event 28 - 29th May 1982 09 04 00 20 16 12 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: 107 Situation with running 24 hr criterion 400 300 200 100 0 • Criterion was reached at approx 03:00 • BUT • 1st Landslide was reported at 02:00 when rainfall was about 220mm Rainfall on Landslip Day (mm) Even if Warning procedure has been operated correctly, warning would have been 1 hour too late! 0 100 200 300 400 500 600 700 800 Antecedent Rainfall in previous 15 days (mm)

  36. All Landslide Warning Incidents in 1982 400 300 200 100 0 09 04 00 20 16 12 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: 107 20 16 12 08 04 00 16 12 08 04 20 16 12 LW 5/82 05:50 – 16/08/82 Total: 98 Sq: 32 LW 2/82 06:15 – 31/05/82 Total: 91/ Sq: 40 LW 7/82 23:52 – 16/09/82 Total: 3 Sq: 3 00 LW 4/82 11:00 – 03/08/82 Total: 9 Sq: 5 LW 6/82 06:35 – 18/08/82* Total: 8 Sq: 2 16 06 LW 3/82 11:00 – 02/06/82* Total: 28/Sq: 12 16 0 100 200 300 400 500 600 700 800 Antecedent Rainfall in previous 15 days (mm)

  37. Red Landslides with No Warning! Green Landslide Warnings with Several Hours Warning Blue Landslide Warnings with 1 Hour Warning Performance of All LandSlip Warnings 1982 - 1983

  38. All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82 Disastrous > 50 reported Landslides: Severe 10 - 50 Landslides Minor < 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 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. Severe and Disastrous Landslide Events: with 1984 Scheme Existing Criteria Line - in use mid 1982 - mid 1984 Warning and Landslide Lines in use from mid 1984

  41. Landslides: Landslide Warning System 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.

  42. Landslides: Landslide Warning System The 1984 Warning Scheme • Simple to understand • On average ~ 0 - 7 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

  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. 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 1972. Caribbean Countries should learn from their experience. • Important to begin and resource fully the research needed to achieve these aims.

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