1 Managing Landslide Hazard Risk in Sub Tropical Countries Major Landslide at Maracas, Trinidad -...

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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


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


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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


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Failure of slope in June 1972

Aerial Photograph of Slopes

11SW-A/CR175 and

11SW-A/FR30


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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


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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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0 100 200 300 400 500 600 700 800



400

300

200

100

0

0020

16

12

09

04


Even with 24hr day plotting, the plot for 29th May should have been as follows

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0 100 200 300 400 500 600 700 800



400

300

200

100

0

09

04

0020

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Situation with running 24 hr criterion


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


20

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.


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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


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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.

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