Landslide Warning: Development of the Hong Kong System

1. Landslide Warning: Development of the Hong Kong System Major Landslide at Maracas, Trinidad - Late December 2002.

2. Why Do Landslides Occur? • Slope Profile • Geology • Material Properties • Water • Ground Water • Surface Water

3. How 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 > Factor of Safety if < 1 slope is likely to fail if > 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. (>10)

12. Categorisation of Landslide Events by Lumb (1975) Severe Minor Disaster Data points coloured RED and GREEN occurred after Lumb’s Paper

13. Rainfall Profile and Onset of Landslides Cumulative Rainfall 4 hours 20 hours Landslip Time (LT) (The time when first landslip is reported to FSD). Landslip Prediction Criteria (LPC) Warning Time (WT) (Rainfall predicted to reach LPC in 4 hours) Criteria Time (CT) The time when LPC are actually reached. Actual Cumulative Rainfall Predicted Cumulative Rainfall

14. 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 Lumb’s 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

16. Distribution of Automatic Rain Gauges in Hong Kong (mid 1984) Semi-Automatic Gauges (Royal Observatory) Automatic Gauges (GCO) Automatic Gauges (Royal Observatory) Built up areas N 0 5 10 km

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 09 04 00 20 16 12 400 300 200 100 0 Landslide Warning: 1/82 Issued at 09:00 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 09 04 00 20 16 12 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 09:00 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 09 04 00 20 16 12 Landslide Warning: 1/82 Issued at 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: 107 Situation with running 24 hr criterion 400 300 200 100 0 • Criterion was reached at approx 03:00 • BUT • 1st Landslide was reported at 02:00 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 09:00 on 29/05/82 Landslides reported: Total: 223 Squatters: 107 20 16 12 08 04 00 16 12 08 04 20 16 12 LW 5/82 05:50 – 16/08/82 Total: 98 Sq: 32 LW 2/82 06:15 – 31/05/82 Total: 91/ Sq: 40 LW 7/82 23:52 – 16/09/82 Total: 3 Sq: 3 00 LW 4/82 11:00 – 03/08/82 Total: 9 Sq: 5 LW 6/82 06:35 – 18/08/82* Total: 8 Sq: 2 16 06 LW 3/82 11:00 – 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. Red Landslides with No Warning! Green Landslide Warnings with Several Hours Warning Blue Landslide Warnings with 1 Hour Warning Performance of All LandSlip Warnings 1982 - 1983

24. All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82 Disastrous > 50 reported Landslides: Severe 10 - 50 Landslides Minor < 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