Screening Level Risk Assessment for Low Hazard Dams

Screening Level Risk
Assessment for
Low Hazard Dams
2014 ASFPM Annual Conference
Laurie Carrette Zook, PE, CFM
© 2014 HDR, Inc., all rights reserved.
Screening Level Risk Assessment (SLRA)

Why SLRA for Low Hazard Dams?
Identify and Better Understand Risk and Benefits
o Document
o Make Decisions
o Provide Basis for Prioritization
o Communicate
o
Initial Screening of
Low Hazard Dams
10 sites at GTE, YELL, ROMO,
CHIC (3 sites), CHAT, and LAVO
 Deliverables:

Checklist Examination Report
o Screening Level Risk Assessment
(SLRA) Report
o

Work Completed in 2012
Methodology
Specific to National Park Service
(NPS) mission and values
 Based on best practices for high
and significant hazard dams
 Semi-Quantitative
 Scalable

To preserve unimpaired the natural and
cultural resources and values of the
national park system for the enjoyment,
education, and inspiration of this and future
generations
Risk Matrix
Low Hazard vs. High Hazard (typ.)
Very High
10-3
High
10-3
Moderate
10-4
Low
10-5
Remote
10-4
Probability Category
Likelihood
10-2
Very
High
High
Seismic right
abutment sliding
Moderate
10-5
Low
0
1
2
Consequences Level
Multi-attribute
3
4
Level 1
Level 2
Level 3
Consequence Category
Life Loss
Level 4
Steps and Process
Desktop Review & Potential Failure Mode Analysis (PFMA)-“lite”
 Site Inspection
 Checklist Examination Report
 Simplified Engineering Evaluations
 Risk Workshop
 Risk Screening Report

Desktop Review
Review of available information
 Generation of available mapping
 Preliminary identification of failure consequences
 Interviews with Park staff

PFMA – Lite
Failure Mode
Category
Static
Hydrologic
Seismic
PFM Description
Embankment piping/internal erosion
Foundation piping/internal erosion
Abutment piping/internal erosion
Piping along outlets or service spillway
Piping of material into open rock joints
Slope instability
Wave erosion of upstream slope
Large landslide creating seiche waves in reservoir
Channel erosion/back cutting due to underseepage
Increased threat of seepage-related failure during flood
Spillway damage due to toe erosion and/or headcutting
Dam breach due to overtopping
Earthquake-induced embankment cracking/damage leading to seepagerelated failure
Seismic slope instability with excessive deformations and failure by
overtopping
Embankment/foundation liquefaction leading to slope instability,
excessive deformations and failure by overtopping
Credible
PFM
X
X
X
X
Highlighted PFMs
PFM 1
PFM 2
Considered as part of PFM 2
PFM 3
X
X
X
PFM 4
X
X
X
PFM 5
Site Inspection
Verify PFMs and identify any new modes of failure for consideration
 Confirm failure consequences
 Collect field data to support simplified analyses

Site Inspection – Manzanita Dam
Site Inspection – Manzanita Dam
Site Inspection – Manzanita Dam
Checklist Examination Report
Document credible PFMs
 Summarize current conditions
 Summary failure consequences

=> Serves as Reference Document for the Risk Workshop
Simplified Engineering
Evaluations
Static PFMs
 Hydrologic PFMs
 Seismic PFMs

Estimated Seismic Hazard, Manzanita Lake Dam
Earthquake Return
Period in years
100
500
1,000
2,500
5,000
10,000
Source: USGS, 2012
Peak Ground
Acceleration
0.07g
0.17g
0.24g
0.35g
0.45g
0.57g
Estimated Flood Hazard, Manzanita Lake Dam
Flood Return
Period (years)
2
5
10
25
50
100
500
1,000
5,000
10,000
Peak Flow
(cfs)
95
155
215
320
435
605
1,290
1,785
3,775
5,215
Source: USGS
StreamStats, 2012
Source: Estimated
using linear
extrapolation from
the values above
Risk Matrix
Very High
10-2
Likelihood
High
10-3
Moderate
10-4
Low
10-5
Remote
Likelihood
Est. Annual Probability of Failure:
VERY HIGH – greater than 1E‐02
HIGH – 1E‐03 to 1E‐02
MODERATE – 1E‐04 to 1E‐03
LOW – 1E‐05 to 1E‐04
REMOTE – less than 1E‐05
0
1
2
3
Consequences Level
Consequences Level
Level 0 – minor or no damage
Level 1 – minor damage confined to NPS property
Level 2 – moderate damage/ less than 1 year recovery
Level 3 – significant damage/2‐5 year recovery
Level 4 – extensive, some permanent, damage/ long recovery 4
Confidence Estimates
Good – Rating is high; it is unlikely that additional information would change the rating.
Poor – Rating is low; additional information could very well result in a change to the rating.
Medium – In between Good and Poor
Consequences Categories
LEVEL
Level 0
Level 1
Level 2
Level 3
Level 4
DESCRIPTION
 Dam breach discharge would result in no resource, property, or environmental damage. Re-construction costs are expected to be minor.
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Minor property or environmental damage would be limited to NPS property
No permanently occupied structures, roads, or bridges are damaged.
Repair costs < $50K
No or minor impact on visitation or park resources
Damage would be limited to a few permanently occupied structures, recreation areas, roadways, and non-critical bridges in low lying areas on NPS
property only
Moderate property damage (~$100K).
Moderate environmental damage or loss of park resources
Moderate impact to recreation/visitation
Costs associated with re-constructing the project features and negative publicity associated with a dam failure would also be incurred.
Less than 1 year recovery.
Significant environmental damage or impacts to historical and cultural park resources. Damages possible or expected to extend off NPS property
Dam breach discharge would results in significant damage to permanently occupied structures, recreational areas, roadways, and bridges within the
inundation zone ($500K -$1M)
Moderate impact to recreation
Visible negative impact to scenic beauty
Multi-year recovery 2-5 years.
Long-term damage to threatened and endangered species and species habitat
Significant negative publicity expected
Extensive damage to park infrastructure (campgrounds, structures, utilities, roads, trails, picnic areas, bridges etc) in excess of $5Million including
possible impacts off of NPS property.
Damage to critical infrastructure (water supply, hospitals, schools etc)
Major impacts to historical and/or cultural park resources.
Loss of heavily visited/iconic park resource.
Life loss or injury due to dam failure is unlikely but plausible.
Recovery over very long time (5-10 years)
Risk Workshop
Final Screening Report
Documents Risk Workshop
 Summarizes PFMs and respective risks, consequences, and confidence
 Provides recommendations for safety of dam and operation and
maintenance actions

Lessons Learned
Great Communication Tool
 Metrics Need to be Consistent
with the Mission
 Small Experienced Team –
Value of Process

Questions?
[email protected]
© 2014 HDR, Inc., all rights reserved.
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