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Managing Landslide Hazard Risk in Sub Tropical Countries
Major Landslide at Maracas, Trinidad - Late December 2002.
Keith ToveyReader 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
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• Introduction
• Modelling Methods– Engineering Models– GIS Methods– Statistical Methods
• Management Issues
• Conclusions
Managing Landslide Hazard Risk in Sub Tropical Countries
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Landslide
Remedial MeasuresSafe at the moment
LandslidePreventive Measures
Landslides: Introduction
DesignCostBuild Consequence
Remove Consequence
Stability Assessment
TemporarilySafe
Landslide Warning
•Injury•Death•Economic Loss•Disruption to Transport Links
Consequences of Landslides
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Main Manchester –
Sheffield Road (A625)
Alternative route – only suitable for light vehicles – gradient of 1 in 4
Landslides: Removing the Consequence
Manchester
1 km
5Landslides in Kowloon East 28th - 31st May 1982
Landslides: Removing the Consequence
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Landslide
Remedial Measures
LandslidePreventive Measures
Man’s Influence (Agriculture /Development)
Landslides: Engineering Modelling Methods
Geology
DesignCostBuild Consequence
Remove Consequence
Stability Assessment
TemporarilySafe
Landslide Warning
HydrologyMaterial Properties
(Shear Strength)
Slope AngleLoading
But only for specific slopes
Safe at the moment
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• 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
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berms
Landslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002
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General Planning Guidelines of Landslide Risk
Classification into potential Areas of Risk
Soil Type
Landslides: GIS Modelling Methods
GeologyHydrologyGeneral Slope (and aspect) Land Use
Database of existing Landslides
Identification of areas for detailed Engineering Study
Cataloguing slopes and landslides
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• 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
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e.g. North Coast Road, Trinidad
Fill Slope
Retaining Wall
“Natural” Slope
Cut Slope
Landslides: Categorisation of Slopes
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Landslide at Maracas
December 2002
December 2004 – note the slide is much more
extensive
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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
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Slope before failure at Couva
Slope after Landslide
Slide by Derek Gay, UWI
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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
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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
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• 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 MethodsLandslide Warning System
Method aims to alert people to impending danger so they can seek safety during critical periods – it will not
prevent landslides
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Rain Gauge Network in Hong Kong
Built Up Areas
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• Historically: Reactive Approach to LandslidesSimilar 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.
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Click once on Slope to display data for 11SW-A/CR175
Slope Catalogue: Slope 11SW-A/CR175: Po Shan Road
Landslides: Management
21Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road
Landslides: Management
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Background Information And Related DocumentFeature Reference Number 11SW-A/CR 175
GIU Cell Ref. 11SW7D8 - NKT’s Classification still in use!
Location 18M NORTH OF 21 PO SHAN ROAD
Sift Class C1
Map Sheet Reference (1:1000) 11SW- 7D
Coordinates Easting : 832718 Northing : 815676
Aerial Photos Photo No. Year
1824-5 1972
Nearest Raingauge Station(Station Number)
Knowles Building, Hong Kong University (H04)
Data Collected On 11/14/97Date of Construction, SubsequentModification & Demolition
N/A
Related Reports/Files or Documents File/Report Ref. No
Previous Instability 16/6/72 63/11SW-A, 64/11SW-A
Previous Instability17/6/72 66/11SW-A, 18/6/7269/11SW-A
Development GCI 2/B7/113
BP ReportsIIB Po Shan Rd Area 3/76, MidLevel Rpt 3/86
GEO Mid Levels Study Rpt
LPM Considered in 91 Selected
Pre-SIRST Field Sheet
DH-Order (To Be Confirmed with Buildings Department)
NONE
Advisory Letter (To Be Confirmed with Buildings Department)
NONE
Remarks N/A
Follow Up Actions N/A
Maintenance Responsibility Government
Responsible Government Department - HyD
(Based on SIMAR results from Lands Department) IMPORTANT - READDisclaimer
Feature Registration Form for Po Shan Road Slope
11SW-A/CR175
Major Disaster in June 1972
Landslide Preventative
Measure
23Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road
Landslides: Management
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Failure of slope in June 1972
Aerial Photograph of Slopes
11SW-A/CR175 and
11SW-A/FR30
Landslides: Management
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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
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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
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• 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 MethodsLandslide Warning System (continued)
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•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.
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2 3 4 5 6 7 8 9 10 11 12 13 14 15
Day
24 – hour criteria
Cumulative Rainfall over previous 15 days
Cu
mm
ula
tiv e
Ra i
nfa
ll
Basis of Lumb’s Predictor
30
20 hours4 hours
Lan
dsl
ipP
red
icti
on
Cri
teri
a(L
PC
)
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
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0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Antecedent Rainfall (mm)
Rain
fall
on
Da
y (
mm
)
Minor Incident Severe Incident
Disastrous Incident RED Warning (1977 - 1979)
Amber Warning (1977 - 1979)
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.
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0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Minor Incident Severe Incident
Disastrous Incident RED Warning
AMBER Warning
Landslide Warning System 2: (1980 - mid 1983)
Advantage: Much easier to identify whether WARNING should be called - even when chart is not to hand.
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Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
0 100 200 300 400 500 600 700 800
Antecedent Rainfall in previous 15 days (mm)
Rainfall on Landslip Day (mm)
400
300
200
100
0
0020
16
12
09
04
Landslide Event 28 - 29th May 1982
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Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
0 100 200 300 400 500 600 700 800
Antecedent Rainfall in previous 15 days (mm)
Rainfall on Landslip Day (mm)
400
300
200
100
0
0020
16
12
09
04
Landslide Event 28 - 29th May 1982
Even with 24hr day plotting, the plot for 29th May should have been as follows
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Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
0 100 200 300 400 500 600 700 800
Antecedent Rainfall in previous 15 days (mm)
Rainfall on Landslip Day (mm)
400
300
200
100
0
09
04
0020
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Situation with running 24 hr criterion
Landslide Event 28 - 29th 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!
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09
04
0020
16
12
Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
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16
12
20
16
12
08
0400
00
16
12
08
04
16
0616
LW 2/8206:15 – 31/05/82Total: 91/ Sq: 40LW 4/82
11:00 – 03/08/82Total: 9Sq: 5
LW 6/8206:35 – 18/08/82*Total: 8Sq: 2
LW 3/8211:00 – 02/06/82*Total: 28/Sq: 12
LW 5/8205:50 – 16/08/82Total: 98Sq: 32
LW 7/8223:52 – 16/09/82Total: 3Sq: 3
0 100 200 300 400 500 600 700 800Antecedent Rainfall in previous 15 days (mm)
400
300
200
100
0
All Landslide Warning Incidents in 1982
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Warning Criteria
Time
Warning Forecast
Gain
First
Landslip
No. Date (CT) Time (WT) (N) Time
1/82 29.05.82 0300 - 0400 0900 (-)b 0123
2/82 31.05.82a 0600 - 0700 0615 0 1351
3/82 02.06.82a not reached 1140 NA NR
4/82 03.08.82 1300 — 1400 1100 2 NR
5/82 16.08.82 0500 - 0600 0550 0 1009
6/82 18.08.82a not reached 0635 NA NR
7/82 16.09.82 not reached 2352 NA NR
1/83 27.03.83 2300 — 2400 2355 0 0011
2/83 08.04.83 not reached 1102 NA NR
3/83 17. 06.83 0800 - 0900 0745 1 0840
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
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0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Antecedent 15-day Rainfall (mm)
24-h
ou
r R
ain
fall
du
rin
g E
vent
(m
m) Disastrous
Severe
Minor
Null Event
New 1983 Criteria
All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall
Disastrous > 50 reported Landslides: Severe 10 - 50 LandslidesMinor < 10 Landslides : Null Event: No reported Landslides
Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82
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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??
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0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Antecedent 15-day Rainfall (mm)
24-h
ou
r R
ain
fall
du
rin
g E
ven
t (m
m)
Disastrous: > 50 LandslidesSevere: 10 - 50 LandslidesExisting CriteriaWarning LineLandslide Line
Existing Criteria Line - in use mid 1982 - mid 1984
Warning and Landslide Lines in use from mid 1984
Severe and Disastrous Landslide Events: with 1984 Scheme
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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 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
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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
Landslides: Landslide Warning System
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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
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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.