Landslide Warning: Development of the Hong Kong System Major Landslide at Maracas, Trinidad - Late...

Landslide Warning:

Development of the Hong Kong System

Major Landslide at Maracas, Trinidad - Late December 2002.

Why Do Landslides Occur?

•Slope Profile

•Geology

•Material Properties

•Water

•Ground Water

•Surface Water

•Survey the Profile

•Test Material Properties

•Estimate / measure where water table is.

•Analyse Stability using

•Infinite Slope

•Method of Slices

How can we predict whether a Landslide will occur?

> Factor of Safety

if < 1 slope is likely to fail

if > 1 slope should be stable

•Is it realistic to do a full analysis on all Landslides to this detail?

•In some critical cases …….YES

•Impossible to analyse all slopes under prevailing climatic conditions to give adequate warning

Some Questions

•Injury•Death•Economic Loss•Disruption to Transport Links

Consequences of Landslides

•Landslide Warnings.

•Categorise slopes

•Landslide Preventative Measures (LPM)

•Historically: Reactive Approach to Landslides•From 1977 became pro-active

Hong Kong Approach

Date 24 hrRainfall

StormType

Land-slides

Casualties Evacuation Category

25/08/76 416.2 STSEllen

314 57 2400 D

12/06/66 401.2 Trough 100 35 8500 D29/05/82 394.3 Trough 498 48 8000 D17/10/78 379.8 STS

Nina15 1 No data S

16/08/82 362.4 STS Dot 62 9 1500 D17/06/83 346.7 Trough 114 2 600 D27/09/65 338.2 TS

Agnes9 4 200 S

17/08/71 327.8 T Rose 10 7 No data S13/10/64 303.8 T Dot 8 39 8000 S13/06/68 287.2 Trough 10 27 200 S16/06/72 280.1 Trough >15 21 D18/06/72 275.1 Trough dozens 229

7800D

11/05/72 271.1 Trough dozens 0 6000 S30/04/75 150.0 Trough 21 3 No data S

Serious/Disaster Landslide Events 1963 - 1983

Number of Landslides as reported in South China Morning Post

Significant Landslide Events while NKT was in Hong Kong

Number ofReported Landslips

Rainfall (mm)DAY

Total AffectingSquatters

Antecedent15 days

24 hrrainfall

LandslipRED

Warning Issued

198223/4 6 5 71 59.928/4 6 5 180.8 36.87/5 9 5 321.6 55.9

29/5 223 107 188.8 258.4 *31/5 91 40 456.9 205.5 *2/6 28 12 663.7 22.1 *3/7 3 1 125.2 54.31/8 4 4 89.9 114.93/8 9 5 207.6 204.2 *

16/8 98 32 375.6 334.2 *18/8 8 2 669.9 47.6 *16/9 3 3 199.1 73.8 *Total 488 221 7 issued

Significant Landslide Events in 1982 in Hong Kong.

N. K. Tovey arrived in Hong Kong on 26th March 1982

Disaster

Disaster

Landslides in Kowloon East 28th - 31st May 1982

0

2

4

6

8

10

121930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

Fre

qu

en

cy

Minor

Severe

Disaster

War Years - No Data

Missing Data 1974 - 1977

GCO Established

NKT arrives in Hong Kong

Frequency of Landslide Incidents

Requirements of A Landslide Warning System

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

Two Approaches

1. Detailed Warning - e.g. 1. Conduit Road

2. 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

0

100

200

300

400

500

0 100 200 300 400 500 600 700 800Antecedent 15 day Rainfall (mm)

Rai

nfa

ll o

n D

ay o

f In

cid

ent (

mm

)

Minor Incident

Severe Incident

DisastrousIncident

Disaster

Minor Severe

Categorisation of Landslide Events by Lumb (1975)

Data points coloured RED and GREEN occurred after Lumb’s Paper

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

•Are Slopes more susceptible to failure if there has been prolonged rainfall?

•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.

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

0 5 10

km

N

Semi-Automatic Gauges (Royal Observatory) Automatic Gauges (GCO)

Automatic Gauges (Royal Observatory) Built up areas

Distribution of Automatic Rain Gauges in Hong Kong (mid 1984)

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.

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.

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


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


0 100 200 300 400 500 600 700 800



400

300

200

100

0

09

04

0020

16

12

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!

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

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

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

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??

Landslide Warnings: The Final 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.

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

The 1984 Warning Scheme

•Is simple to understand

•On average there would be 0 - 7 Warnings in a Year

•up to one third would be false alarms

•two-thirds would identify all serious/disastrous events correctly although half of the events would have less than 10 Landslides and thus be classified as minor.

•Has the option of Selected Warnings.

i.e. Warning Issued locally if 10 or more Automatic Gauges in Urban Area meet criteria

or a single gauge in New Territories.

BUT - first use of New Scheme……………..

Improvements to Selected Warning Schemes were introduced in 1999

Landslide Warning: Development of the Hong Kong System Major Landslide at Maracas, Trinidad - Late...

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