Landslide Risk Management

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• Landslide Risk Management
• Westlynn and Pemberton Heights Escarpments
• Landslide Risk Assessment
• Presentation to District Council
• December 3, 2008
• Michael Porter, Sam Fougere
• BGC Engineering Inc.

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Outline Westlynn and Pemberton Heights
Escarpments

• Landslide Risk Management Program
• Preliminary Landslide Hazard Assessment (Nov 07)
• Risk Scenarios, Recommendations
• Landslide Risk Assessment (Nov 08)
• Risk Estimation Individual and Societal Risk
• Risk Evaluation
• Remediation Options
• Recommendations

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Study Areas
Westlynn
Pemberton Heights
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Landslide Risk Management Program
Nov 2007 Report
Nov 2008 Report
Nov 2008 Report
Nov 2008 Report
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Preliminary Landslide Hazard Assessment

• Completed in November, 2007
• Four main landslide risk scenarios identified
• - based on presence or absence of evidence of
  slope instability and the presence or absence of
  homes at the base of the escarpment
• INCREASING RISK from A to
  D

RISK SCENARIO RESIDENTIAL DEVELOPMENT AT THE BASE OF THE SLOPE HISTORIC LANDSLIDE ACTIVITY OR POTENTIALLY HAZARDOUS CONDITION IDENTIFIED
A NO NO
B YES NO
C NO YES
D YES YES
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Preliminary Landslide Hazard Assessment

• Recommendations (Nov 07)
• Scenario A no further work
• Scenario B periodic geotech review
• Scenario C remotely possible these sites may
  pose unacceptable individual or societal risk
• Scenario D may pose unacceptable individual
  or societal risk but further investigation
  required to conclude this

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Follow-up Risk Assessment - Site Selection

• DNV retained BGC to conduct follow-up risk
  assessment for the Scenario D points of
  interest
• Sites/properties investigated about 30 m either
  side of Scenario D points of interest
• 11 Points of Interest 24 Properties

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Landslide Risk Estimation

• Risk PSLIDE x N (fatalities per year)
• (Risk Hazard x Consequence)
• Consequence expected number of fatalities
• N PSH x PTS x V x E

Based on position of homes and runout behaviour
of previous slides on the Berkley Escarpment
based on consequences of previous slides at the
Berkley Escarpment
12 to 16 hrs per day
4 people per home
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Landslide Runout

• Consequence a function of the angle to the crest
  of the escarpment

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Risk Evaluation

• Interim landslide risk tolerance criteria for
  existing developments
• Tolerable Risk 10-4 per year (110,000 chance)
• ALARP principal demands lower risk, where
  practical
• In early 2008, DNVs Natural Hazards Task Force
  recommended continued use of the interim criteria

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Individual Risk Estimates

• Risk faced by a single individual exposed to one
  or more landslide hazards
• Interim Tolerance Criteria
• Existing slopes 10-4 per year (110,000)
• New development 10-5 per year (1100,000)

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Individual Risk Estimates

• Identified as shaded properties on the report
  maps
• One Unacceptable Red Zone Property (gt10-4)
• but drainage improvements implemented prior to
  completion of this study
• 14 ALARP Yellow Zone Properties (10-4 10-5)
• 12 properties at the base of the escarpment
• 2 properties at the crest of the escarpment

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Societal Risk Estimates

• Risk imposed upon society by a single landslide
• Interim Tolerance Criteria
• F-N Curves

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Societal Risk Estimates

• No Unacceptable Zones Properties
• 18 ALARP Zone
• 6 Broadly Acceptable
• Shown as Yellow or Blue stars on the report maps

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Discussion

• Landslide risk assessment addresses risk of loss
  of life from landslides triggered by heavy
  rainfall
• Landslide risks in the vicinity of the Scenario
  D points of interest are currently tolerable
• Risks are lower than found along the Berkley
  Escarpment (generally less fill less development
  at the base of the escarpments)
• Keep risks under review and reduce further, if
  practical (ALARP)

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Remedial Options

• Drainage improvements, followed by fill removal,
  found to be most cost effective

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Recommendations

• Encourage homeowners along crest of escarpments
  to connect their downpipes to the storm drain
  system
• Continue to provide educational material to the
  public about how to reduce landslide risk when
  living near steep slopes (pamphlet in circulation
  at present)
• Prioritize Scenario C sites for follow-up risk
  assessment once results of other preliminary
  landslide studies are available

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Five Landslide Preliminary Study Areas

• Preliminary landslide study areas work in
  progress at
• Capilano River East
• Mosquito Creek
• Mount Fromme East
• Deep Cove
• Riverside West
• Reports due February 2009

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• Thank you
• Report available at
• www.dnv.org

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