Landslide Warning:

1
Landslide Warning Development of the Hong Kong
System
15342 days ago Aberfan 13168 days ago
Major Landslide at Maracas, Trinidad - Late
December 2002.
2
Why Do Landslides Occur?

• Slope Profile
• Geology
• Material Properties
• Water
• Ground Water
• Surface Water

3
Can we predict whether a Landslide will occur?

• Survey the Profile
• Test Material Properties
• Estimate / measure where water table is.
• Analyse Stability using
• Infinite Slope
• Method of Slices

gt Factor of Safety if lt 1 slope is likely to
fail if gt 1 slope should be stable
4
Some Questions

• 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

5
Consequences of Landslides

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

Hong Kong Approach

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

• Landslide Warnings.
• Categorise slopes
• Landslide Preventative Measures (LPM)

6
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
7
Significant Landslide Events in 1982 in Hong Kong.
Disaster
Disaster
N. K. Tovey arrived in Hong Kong on 26th March
1982
8
Landslides in Kowloon East 28th - 31st May 1982
9
Frequency of Landslide Incidents
10
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

11
Background to Warning System
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.
(gt10)
12
Categorisation of Landslide Events by Lumb (1975)
Severe
Minor
Disaster
Data points coloured RED and GREEN occurred after
Lumbs Paper
13
Rainfall Profile and Onset of Landslides
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ANTECEDENT CONDITIONS.

• 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.

15
Basis of Lumbs Predictor
24 hour criteria
Cummulative Rainfall
2 3 4 5 6 7 8 9
10 11 12 13 14 15 Day
16
Rain Gauge Network in Hong Kong
Built Up Areas
17
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.
18
Landslide Warning System 2 (1980 - mid 1983)
Advantage Much easier to identify whether
WARNING should be called - even when chart is not
to hand.
19
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)
20
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)
21
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)
22
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)
23
Performance of All LandSlip Warnings 1982 - 1983
24
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

25
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??
26
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.
27
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
28
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
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15342 days ago Aberfan 13168 days ago