Managing Landslide Hazard Risk in Sub Tropical Countries

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
Landslide
Design Cost Build
Safe at the moment
4
Landslides Removing the Consequence
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
Mans 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
General Slope (and aspect)
Land Use
Soil Type
Geology
Hydrology
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
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
1300 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
• gt Engineering / Geotechnical Control
• Categorisation of Slopes and Landslides
• gt Develop a Robust Database
• gt Identify critical issues and areas affected
  GIS
• gt Planning Policies
• gt 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
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.
(gt10)
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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 Lumbs 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
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 0
Landslide Warning 1/82 Issued at 0900 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
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 0900 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
Situation with running 24 hr criterion
400 300 200 100 0
Criterion was reached at approx 0300 BUT 1st
Landslide was reported at 0200 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 0900 on
29/05/82 Landslides reported Total
223 Squatters 107
20 16 12 08 04 00
20 16 12
16 12 08 04
LW 5/82 0550 16/08/82 Total 98 Sq 32
LW 2/82 0615 31/05/82 Total 91/ Sq 40
LW 7/82 2352 16/09/82 Total 3 Sq 3
00
LW 4/82 1100 03/08/82 Total 9 Sq 5
LW 6/82 0635 18/08/82 Total 8 Sq 2
16 06
LW 3/82 1100 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
Performance of All LandSlip Warnings 1982 - 1983
38
All Rainstorm Events Daily Rainfall vs
Antecedent Rainfall
Disastrous gt 50 reported Landslides Severe 10
- 50 Landslides Minor lt 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.
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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
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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.