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Assessing Seismic (Cascadia Subduction Zone) Risk at Oregon Dams

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Presentation on theme: "Assessing Seismic (Cascadia Subduction Zone) Risk at Oregon Dams"— Presentation transcript:

1 Assessing Seismic (Cascadia Subduction Zone) Risk at Oregon Dams
Keith Mills, P.E., G.E. Dam Safety Engineer Oregon Water Resources Department

2 Oregon Statutory Dams 10 feet or higher Over 9.2 acre feet of water
Design review/approval Inspection frequency by hazard classification High – annual inspection Plus Emergency Action Plans (EAPs) Significant – Once every 2-3 years Low – Once every 5-6 years Federal Agencies responsible for their dams Dam information in database on the web High Hazard 61/130 Significant Hazard 152/175 Low Hazard 733/

3 OWRD Seismic Hazards Assessment 36 high hazard dams closest to Cascadia S.Z.
Dams west of longitude degrees 3 State regulated dams - current and satisfactory seismic analysis 1 State regulated dam with analysis showing failure at 0.15 g 3 State regulated dams with indicators of liquefaction potential 1 USBR dam - unsatisfactory seismic analysis (deformation) Remaining dams unknown (to OWRD) but with no major apparent indicators of liquefiable or highly compressible foundations or with hydraulically placed embankment materials.

4 Priorities for Seismic Analysis Need for seismic stability modifications
Big Creek Upper and Lower dams (Newport) In progress (HDR/Cornforth) Bear Creek dam (Astoria) Re-investigation (City - OWRD scoping now) Lower Pony Creek dam (Coos Bay – North Bend) Proposed in dam owners next budget Northwest State Regulated High Hazard dams

5 Big Creek Dams 65 feet of sandy silt below lower dam
Water treatment plant at base of dams Half of SPT’s for lower dam: N = 0 N = 0 not that uncommon around coastal estuaries HDR lead consultant Subsurface investigation complete Soil testing and analysis in progress Cornforth – seismic site characterization (just completed)

6 ML (sandy) with half blow counts N=0
Big Creek Lower ML (sandy) with half blow counts N=0

7 Bear Creek Dam Concrete dam on fractured rock
Dam above Knappa-Svensen – east of Astoria Harza analysis (soil mechanics) – failure at 0.15 g Emergency Action Plan (EAP) has Level 3 Special emergency condition Self evacuate like those in Tsunami zone OWRD D.S. in discussions with City of Astoria Field investigation with instrumentation – uplift pressure, rock strength, concrete strength (cracks)

8 Concrete Gravity dam built 1912 and raised in 1953
Left abutment –fractured rock and leakage Foundation – fractured rock, unknown uplift pressure

9 Pony Creek Dam Embankment dam Designed in 1987
No seismic analysis apparent Above coastal slough Foundation materials uncertain Seismic analysis recommended and under consideration Water treatment plant at base of dam

10 Dams to current seismic standard
2 rock-fill dams Rock Foundation and abutments Very little seepage Designed to current seismic standards Hillsboro (Barney reservoir) Dam McGuire Dam McMinnville W&L 1 earth-fill dam on rock - Upper Pony Creek Dam – Cascadia and crustal design

11 Preparing for Cascadia
Tsunami (rapid, no direct effects on high hazard dams) Large (extremely) landslides with landslide dams (LiDAR) Extreme liquefaction coastal valleys Bridge failures (Bruce - ODOT) Oregon Seismic Lifelines Identification Technical Documentation (CH2MHill for ODOT) - SECTION 3 Evaluation of Roadway Damage Risk Due To Dam Seismic Damage Likely very limited coastal access by road Inspections by helicopter (they will be in high demand) Engineered Dams unlikely to be highest priority emergency Mapping tool with earthquakes (1 orange dot)

12 Emergency Action Plans
Oregon Template – will present at ASDSO annual meeting) Notification short and up front Emergency detection and evaluation Level 3 Special – Severe shaking evacuation – no notification Probable modes of failure Liquefaction, crest displacement, slope instability, concrete failure, rock abutment displacement Inundation (dambreak flood) maps Sunny day for CSZ quake Responsibilities - emergency manager – know maps and what to do – will they have resources?

13 Rapid Dam Breach Low hazard dam failed in 2011, in eastern Oregon near Baker City

14 Coordination with emergency managers
Evacuation responsibilities and resources Inspection of infrastructure May take a long time EAPs for dams close to CSZ Emergency Level 3 Special Oregon Emergency Response Council Dam Safety Engineer is currently OWRD representative OWRD engineers input into Oregon Cascadia Plan Also have roles in flood and drought

15 Seismic Analysis Issues
Possible conservatism in FLAC and Newmark analyses (Scoggins Consultant Review Board) Actual behavior of dams in Subduction Event Waiting to learn from dam performance during Tohoku great earthquake ML behavior in subduction zone earthquake Seismic analysis of fractured rock abutments

16 Summary Aware (too much) of CSZ and what it can do
Using available resources on high priority dams EAP grants HEC-RAS/HMS inundation analysis for hazard classification Seismic analysis coastal dams Very conservative seismic analyses are unhelpful for evaluating risk at existing dams Ground access to dams closest to Cascadia will at best be difficult after a major S.Z. earthquake Repair of damaged dams will be prolonged

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