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					     Managing Landslide Hazard Risk in
          Sub Tropical Countries

                     Keith Tovey
             Reader 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
 Major Landslide at Maracas, Trinidad - Late December 2002.   1
  Managing Landslide Hazard Risk in Sub
           Tropical Countries

• Introduction
• Modelling Methods
  – Engineering Models
  – GIS Methods
  – Statistical Methods
• Management Issues
• Conclusions


                                          2
                  Landslides: Introduction

   Consequences of Landslides
       •Injury
       •Death
       •Economic Loss
       •Disruption to Transport Links

Landslide            Stability Assessment
Preventive
Measures
 Design      Temporarily     Landslide         Landslide
  Cost          Safe         Warning
 Build                                        Consequence

Safe at the moment    Remedial Measures     Remove Consequence
                                                             3
            Landslides: Removing the Consequence

 Manchester




   Main
Manchester –
 Sheffield
Road (A625)


Alternative route – only suitable for light
      vehicles – gradient of 1 in 4
                                              1 km   4
 Landslides: Removing the Consequence




Landslides in Kowloon East 28th - 31st May 1982
                                                  5
       Landslides: Engineering Modelling Methods
             Man’s Influence (Agriculture /Development)



                                Geology

                   Material Properties    Slope Angle
 Hydrology          (Shear Strength)
                                                               Loading


Landslide                Stability Assessment
Preventive
Measures
 Design        Temporarily         Landslide              Landslide
  Cost            Safe             Warning
 Build         But only for specific slopes               Consequence

Safe at the moment        Remedial Measures        Remove Consequence
                                                                         6
Landslides: Engineering Modelling Methods

• 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.
         But one must not be complacent
                                                 7
berms




        8
            Landslides: GIS Modelling Methods


                           Soil Type     General Slope
Hydrology    Geology                                      Land Use
                                          (and aspect)



Cataloguing slopes
 and landslides

    Database of existing               Classification into potential
        Landslides                            Areas of Risk


  Identification of areas for          General Planning Guidelines
 detailed Engineering Study                of Landslide Risk
                                                                       9
    Landslides: GIS Modelling Methods

• 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

  Accuracy is dependant on existence of a good
   unbiassed database of landslides and slopes
                                                 10
Landslides: Categorisation of Slopes
e.g. North Coast Road, Trinidad


                Cut Slope

                                     ―Natural‖ Slope

   Fill Slope




                            Retaining Wall

                                                       11
                         Landslide at Maracas
                           December 2002




December 2004 – note
the slide is much more
       extensive

                                                12
                               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
                                                   13
                           Slope before failure at
                                  Couva




   Slope after Landslide




Slide by Derek Gay, UWI                              14
    Landslides: GIS Modelling Methods:
        Requirements for the future

            Landslides
•          triggered by
    Cut Slopes
•         anthropogenic
    Fill Slopes
              activity
•   Retaining Walls
•   Hybrids: Cut/Retaining Wall / Fill/Retaining Wall
                                             Deep seated
•   ―Natural‖ Slopes - is there a better word?
                                        landslide unaffected
     slopes where there has been no anthropogenic activity, or
                                          by anthropogenic
     where there is such activity it causes small changes to the
                                               activity
     geometry of the slope so that the Factor of Safety is
     largely unaffected.


                                                                   15
                Landslides: Statistical Methods
Historical Database of                              Rainfall Data
Landslide Occurrence

                          Research to correlate
                         Rainfall with Landslide
                               Incidence
   Antecedent                                      Current/ Predicted
    Rainfall                                           Rainfall
                   Prediction of exactly when
                  landslides are likely to occur

    Issue warnings to                        Mobilise Emergency
     affected people                               Teams

          Aim: to minimise injury and loss of life                  16
         Landslides: Statistical Methods
          Landslide Warning System
• 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.
 Method aims to alert people to impending danger so
they can seek safety during critical periods – it will not
                   prevent landslides                        17
Rain Gauge Network in Hong Kong




                  Built Up Areas   18
                      Landslides: Management
Hong Kong Approach
• Historically: Reactive Approach to Landslides
      Similar to present situation in Trinidad and Tobago
• From 1977 onwards:
          approach became progressively more pro-active
•       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.
                                                                     19
               Landslides: Management




                          Click once on Slope to display data for 11SW-A/CR175




Slope Catalogue: Slope 11SW-A/CR175: Po Shan Road                                20
              Landslides: Management




Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road   21
                              Background Information And Related Document
Feature Reference Number           11SW-A/CR 175
GIU Cell Ref.                      11SW7D8 - NKT’s Classification still in use!
Location
Sift Class
                                   18M NORTH OF 21 PO SHAN ROAD
                                   C1
                                                                                                                   Feature Registration
Map Sheet Reference (1:1000) 11SW- 7D
Coordinates                         Easting : 832718 Northing : 815676
                                                                                                                   Form for Po Shan
Aerial Photos                       Photo No.                                  Year                                Road Slope
                                    1824-5                                     1972


Nearest Raingauge Station
(Station Number)
                                    Knowles Building, Hong Kong University (H04)
                                                                                                                     11SW-A/CR175
Data Collected On                   11/14/97
Date of Construction, Subsequent
Modification & Demolition
                                    N/A                                                                             Major Disaster in
Related Reports/Files or Documents File/Report                                 Ref. No
                                                                                                                      June 1972
                                    Previous Instability                       16/6/72 63/11SW-A, 64/11SW-A
                                                                               17/6/72 66/11SW-A, 18/6/72
                                    Previous Instability
                                                                               69/11SW-A
                                    Development                                GCI 2/B7/113
                                                                               IIB Po Shan Rd Area 3/76, Mid
                                    BP Reports

                                    GEO
                                                                               Level Rpt 3/86
                                                                               Mid Levels Study Rpt
                                                                                                                       Landslide
                                    LPM                                        Considered in 91 Selected              Preventative
                                    Pre-SIRST                                  Field Sheet                              Measure
                                                           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 - READ
                                    Disclaimer                                                                                            22
              Landslides: Management




Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road   23
Landslides: Management




             Failure of slope in June 1972
            Aerial Photograph of Slopes
            11SW-A/CR175 and
            11SW-A/FR30
                                             24
         Landslides: Landslide Warning System
 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                                   25
      Landslides: Landslide Warning System


   Background to Warning System
Two Approaches
• Detailed Warning - e.g. 1. Conduit Road

          automatic piezometer gives warning when
          ground water level gets above a critical level
          as determined by Slope Stability Analysis

• Warning based solely on Rainfall

          Aim to give warning when a significant
                                                    (>10)
          number of landslides are likely to occur.
                                                            26
         Landslides: Statistical Methods
      Landslide Warning System (continued)
• 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)


                                                                     27
            ANTECEDENT CONDITIONS.

•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.
                                                                28
Basis of Lumb’s Predictor



                                                          24 – hour
                                                          criteria




                                  Cumulative Rainfall
                                  over previous 15 days




            2   3 4   5 6   7   8 9 10 11 12 13 14 15
                                Day

                                                                      29
             Rainfall Profile and Onset of Landslides
        Cumulative
         Rainfall


                                        4 hours
                           20 hours


 Landslip                                          Landslip Time (LT) (The
Prediction                                         time when first landslip is
 Criteria                                          reported to FSD).
  (LPC)


               Warning Time (WT)
               (Rainfall predicted to     Criteria Time (CT)
               reach LPC in 4 hours)      The time when LPC are
                                          actually reached.

     Actual Cumulative Rainfall         Predicted Cumulative Rainfall
                                                                             30
  First Landslide Warning System (1977 - 1979)
                             450
                             400
      Rainfall on Day (mm)
                             350
                             300
                             250
                             200
                             150
                             100
                              50
                               0
                                   0      100      200     300     400    500       600    700       800
                                                         Antecedent Rainfall (mm)

                                   Minor Incident                        Severe Incident
                                   Disastrous Incident                   RED Warning (1977 - 1979)
                                   Amber Warning (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.
                                                                                                           31
Landslide Warning System 2: (1980 - mid 1983)
 450
 400
 350
 300
 250
 200
 150
 100
  50
   0
       0   100     200       300       400     500      600    700   800

                 Minor Incident              Severe Incident
                 Disastrous Incident         RED Warning
                 AMBER Warning


Advantage: Much easier to identify whether WARNING
should be called - even when chart is not to hand.
                                                                           32
                                          Landslide Event 28 - 29th May 1982

                                400
Rainfall on Landslip Day (mm)




                                300




                                                  09          Landslide Warning: 1/82
                                200               00          Issued at 09:00 on 29/05/82
                                                  20          Landslides reported:
                                                              Total:       223
                                                  04          Squatters: 107

                                100               16


                                                  12
                                0
                                      0     100        200    300        400        500     600   700   800
                                                  Antecedent Rainfall in previous 15 days (mm)

                                                                                                              33
                                          Landslide Event 28 - 29th May 1982

                                400        Even with 24hr day plotting, the plot for 29th
                                                 May should have been as follows
Rainfall on Landslip Day (mm)




                                300


                                                                               Landslide Warning: 1/82
                                                                               Issued at 09:00 on 29/05/82
                                                                    09         Landslides reported:
                                200               00                           Total:       223
                                                  20                           Squatters: 107
                                                                    04

                                100               16


                                                  12
                                0
                                      0     100        200    300        400     500        600       700    800
                                                  Antecedent Rainfall in previous 15 days (mm)

                                                                                                                   34
                                          Landslide Event 28 - 29th May 1982
                                                             Situation with running 24 hr criterion
                                400               09
                                                             Landslide Warning: 1/82
                                                             Issued at 09:00 on 29/05/82
                                                             Landslides reported:
                                                  04         Total:       223
Rainfall on Landslip Day (mm)




                                300                          Squatters: 107


                                                              Criterion was reached at approx 03:00
                                                                              BUT
                                200               00              1st Landslide was reported at 02:00
                                                  20              when rainfall was about 220mm

                                100
                                                  16     Even if Warning procedure has been
                                                         operated correctly, warning would have
                                0                 12     been 1 hour too late!
                                      0     100        200       300        400        500   600   700   800
                                                  Antecedent Rainfall in previous 15 days (mm)

                                                                                                               35
                        All Landslide Warning Incidents in 1982
400     09      Landslide Warning: 1/82
                Issued at 09:00 on 29/05/82
                                                   20
                Landslides reported:
                Total:       223
                Squatters: 107

        04                                         16
300
                                                   12




200     00                                                          16
        20                           20
                                                                    12


                                                   08
        16
                                                        LW 5/82     08
100                                  16                 05:50 –          LW 2/82
             LW 7/82                                    16/08/82         06:15 – 31/05/82
             23:52 –      00         12 LW 4/82         Total: 98        Total: 91/ Sq: 40
             16/09/82                   11:00 –         Sq: 32                                         LW 6/82
                                                                                                  16   06:35 –
        12   Total: 3                   03/08/82
             Sq: 3                      Total: 9                       LW 3/82                         18/08/82*
                                                   04               04 11:00 – 02/06/82*          06
                                        Sq: 5                                                          Total: 8
                                                   00                  Total: 28/Sq: 12      16
0                                                                                                      Sq: 2



    0          100          200     300      400      500     600    700                                       800
                          Antecedent Rainfall in previous 15 days (mm)

                                                                                                                     36
Performance of All LandSlip Warnings 1982 - 1983
Warning                 Criteria    Warning     Forecast    First
                         Time                    Gain      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

  Red        Landslides with No Warning!
  Green      Landslide Warnings with Several Hours Warning
 Blue        Landslide Warnings with 1 Hour Warning
                                                                      37
                    All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall

                                     450
24-hour Rainfall during Event (mm)

                                                                                      Disastrous
                                     400                                              Severe
                                                                                      Minor
                                     350                                              Null Event

                                     300                                              New 1983 Criteria


                                     250                                     Criteria for low antecedent
                                                                             rainfall reduced to conform
                                     200                                     to actual 1st landslide in
                                                                             Event 1/82
                                     150

                                     100

                                      50

                                       0
                                           0   100   200   300   400   500      600           700    800
                                 Antecedent 15-day Rainfall (mm)
         Disastrous > 50 reported Landslides: Severe 10 - 50 Landslides
         Minor      < 10 Landslides         : Null Event: No reported Landslides                           38
               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??
                                                                          39
                                     Severe and Disastrous Landslide Events: with 1984 Scheme
                                     450
                                                                            Disastrous: > 50 Landslides
24-hour Rainfall during Event (mm)
                                                                            Severe: 10 - 50 Landslides
                                     400                                    Existing Criteria
                                                                            Warning Line
                                     350                                    Landslide Line

                                     300

                                     250

                                     200

                                     150

                                     100

                                      50

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

Existing Criteria Line - in use mid 1982 - mid 1984
Warning and Landslide Lines in use from mid 1984                                                            40
           Landslides: Landslide Warning System
  Landslide Warnings: The Final (1984) 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.

                                                                         41
           Landslides: Landslide Warning System


                     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

                                                       42
               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
                                                             43
                 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.


                                                          44

				
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