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

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

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 e.g. North Coast Road, Trinidad
“Natural” Slope
Cut Slope
Fill Slope
Retaining Wall

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

13. 3 km beyond Las Cuevas as seen on TV half of road blocked
December 9th
Landslide
3 km beyond Las Cuevas as seen on TV
half of road blocked
Landslide 11th December 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
Historical Database of Landslide Occurrence
Rainfall Data
Research to correlate Rainfall with Landslide Incidence
Antecedent Rainfall
Current/ Predicted Rainfall
Prediction of exactly when landslides are likely to occur
Issue warnings to affected people
Mobilise Emergency Teams
Aim: 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

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. Landslide Preventative Measure
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/CR175 and
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 Methods Landslide 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
Day

30. Rainfall Profile and Onset of Landslides
Cumulative Rainfall
4 hours
20 hours
Landslip
Prediction 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
400
300
200
100 09 04
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters:
Rainfall on Landslip Day (mm) 00 20 16 12
Antecedent 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 follows
400
300
200
100
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters: 09 04
Rainfall on Landslip Day (mm) 00 20 16 12
Antecedent Rainfall in previous 15 days (mm)

35. Landslide Event 28 - 29th May 1982
Situation with running 24 hr criterion
400
300
200
100 09 04 00 20 16 12
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters:
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!
Antecedent Rainfall in previous 15 days (mm)

36. Antecedent Rainfall in previous 15 days (mm)
All Landslide Warning Incidents in 1982
400
300
200
100 09 04 00 20 16 12
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters: 20 16 12 08 04 00 20 16 12 16 12 08 04
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
Antecedent Rainfall in previous 15 days (mm)

37. Performance of All LandSlip Warnings 1982 - 1983
Red
Landslides with No Warning!
Green
Landslide Warnings with Several Hours Warning
Blue 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 1/82
Disastrous > 50 reported Landslides: Severe 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 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 ~ 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 Caribbean Countries should learn from their experience.
Important to begin and resource fully the research needed to achieve these aims.