Screening Level Risk Assessment for Low Hazard Dams

Transcription

1 Screening Level Risk Assessment for Low Hazard Dams 2014 ASFPM Annual Conference Laurie Carrette Zook, PE, CFM 2014 HDR, Inc., all rights reserved.

2 Screening Level Risk Assessment (SLRA) Why SLRA for Low Hazard Dams? o Identify and Better Understand Risk and Benefits o Document o Make Decisions o Provide Basis for Prioritization o Communicate

3 Initial Screening of Low Hazard Dams 10 sites at GTE, YELL, ROMO, CHIC (3 sites), CHAT, and LAVO Deliverables: o Checklist Examination Report o Screening Level Risk Assessment (SLRA) Report Work Completed in 2012

4 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

5 Risk Matrix Low Hazard vs. High Hazard (typ.) Very High Likelihood Very High High Moderate Low Remote Probability Category High Moderate Seismic right abutment sliding Consequences Level Multi-attribute Low Level 1 Level 2 Level 3 Level 4 Consequence Category Life Loss

6 Steps and Process Desktop Review & Potential Failure Mode Analysis (PFMA)- lite Site Inspection Checklist Examination Report Simplified Engineering Evaluations Risk Workshop Risk Screening Report

7 Desktop Review Review of available information Generation of available mapping Preliminary identification of failure consequences Interviews with Park staff

8 PFMA Lite Failure Mode Category Static Hydrologic Seismic PFM Description Credible PFM Highlighted PFMs Embankment piping/internal erosion X PFM 1 Foundation piping/internal erosion X PFM 2 Abutment piping/internal erosion X Considered as part of PFM 2 Piping along outlets or service spillway X PFM 3 Piping of material into open rock joints Slope instability X 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 X Dam breach due to overtopping X PFM 4 Earthquake-induced embankment cracking/damage leading to seepagerelated failure X Seismic slope instability with excessive deformations and failure by overtopping X PFM 5 Embankment/foundation liquefaction leading to slope instability, excessive deformations and failure by overtopping X

9 Site Inspection Verify PFMs and identify any new modes of failure for consideration Confirm failure consequences Collect field data to support simplified analyses

10 Site Inspection Manzanita Dam

11 Site Inspection Manzanita Dam

12 Site Inspection Manzanita Dam

13 Checklist Examination Report Document credible PFMs Summarize current conditions Summary failure consequences => Serves as Reference Document for the Risk Workshop

14 Simplified Engineering Evaluations Static PFMs Hydrologic PFMs Seismic PFMs Estimated Flood Hazard, Manzanita Lake Dam Estimated Seismic Hazard, Manzanita Lake Dam Earthquake Return Period in years Peak Ground Acceleration g g 1, g 2, g 5, g 10, g Source: USGS, 2012 Flood Return Period (years) Peak Flow (cfs) ,290 1,000 1,785 5,000 3,775 10,000 5,215 Source: USGS StreamStats, 2012 Source: Estimated using linear extrapolation from the values above

15 Risk Matrix Likelihood Very High High Moderate Low 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 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 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

16 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. 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)

17 Risk Workshop

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21 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

22 Lessons Learned Great Communication Tool Metrics Need to be Consistent with the Mission Small Experienced Team Value of Process

23 Questions? 2014 HDR, Inc., all rights reserved.

24 2014 HDR Architecture, 2014 HDR, Inc., all rights reserved.