Assessing Seismic Assessing Seismic (Cascadia Subduction (Cascadia Subduction Zone) Risk at Oregon Zone) Risk at Oregon

DamsDams

Keith Mills, P.E., G.E.Keith Mills, P.E., G.E.

Dam Safety EngineerDam Safety Engineer

Oregon Water Resources Oregon Water Resources DepartmentDepartment

Oregon Statutory DamsOregon Statutory Dams• 10 feet or higher10 feet or higher• Over 9.2 acre feet of Over 9.2 acre feet of

waterwater• Design review/approvalDesign review/approval• Inspection frequency Inspection frequency

by hazard classificationby hazard classification• High – annual inspectionHigh – annual inspection

• Plus Emergency Action Plus Emergency Action Plans (EAPs)Plans (EAPs)

• Significant – Once every Significant – Once every 2-3 years2-3 years

• Low – Once every 5-6 Low – Once every 5-6 yearsyears

• Federal Agencies Federal Agencies responsible for their responsible for their damsdams

• Dam information in Dam information in database on the webdatabase on the web

High Hazard 61/130

Significant Hazard 152/175

Low Hazard 733/939 1244

OWRD Seismic Hazards OWRD Seismic Hazards AssessmentAssessment

36 high hazard dams closest to 36 high hazard dams closest to Cascadia S.Z.Cascadia S.Z.

• Dams west of longitude 123.0 Dams west of longitude 123.0 degreesdegrees

• 3 State regulated dams - current 3 State regulated dams - current and satisfactory seismic analysisand satisfactory seismic analysis

• 1 1 State regulated dam with State regulated dam with analysis showing failure at 0.15 g analysis showing failure at 0.15 g

• 3 State regulated dams with 3 State regulated dams with indicators of liquefaction indicators of liquefaction potentialpotential

• 1 USBR dam - unsatisfactory 1 USBR dam - unsatisfactory seismic analysis (deformation)seismic analysis (deformation)

• Remaining dams unknown (to Remaining dams unknown (to OWRD) but with no major OWRD) but with no major apparent indicators of liquefiable apparent indicators of liquefiable or highly compressible or highly compressible foundations or with hydraulically foundations or with hydraulically placed embankment materials.placed embankment materials.

Priorities for Seismic AnalysisPriorities for Seismic AnalysisNeed for seismic stability Need for seismic stability

modificationsmodifications Big Creek Upper Big Creek Upper

and Lower dams and Lower dams (Newport) (Newport) In progress In progress

(HDR/Cornforth)(HDR/Cornforth) Bear Creek dam Bear Creek dam

(Astoria)(Astoria) Re-investigation (City Re-investigation (City

- OWRD scoping now)- OWRD scoping now) Lower Pony Creek Lower Pony Creek

dam dam (Coos Bay – North (Coos Bay – North Bend)Bend) Proposed in dam Proposed in dam

owners next budgetowners next budgetNorthwest State Regulated High Hazard dams

Big Creek DamsBig Creek Dams

65 feet of sandy silt below lower dam65 feet of sandy silt below lower dam Water treatment plant at base of damsWater treatment plant at base of dams Half of SPT’s for lower dam: N = 0Half of SPT’s for lower dam: N = 0

N = 0N = 0 not that uncommon around coastal not that uncommon around coastal estuariesestuaries

HDR lead consultantHDR lead consultant Subsurface investigation completeSubsurface investigation complete Soil testing and analysis in progressSoil testing and analysis in progress Cornforth – Cornforth – seismic site characterizationseismic site characterization (just (just

completed)completed)

Big Creek Lower

ML (sandy) with half

blow counts N=0

Bear Creek DamBear Creek Dam Concrete dam on fractured rock Concrete dam on fractured rock Dam above Knappa-Svensen – east of AstoriaDam above Knappa-Svensen – east of Astoria Harza analysis (soil mechanics) – failure at Harza analysis (soil mechanics) – failure at

0.15 g0.15 g Emergency Action Plan (EAP) has Level 3 Emergency Action Plan (EAP) has Level 3

Special emergency conditionSpecial emergency condition Self evacuate like those in Tsunami zoneSelf evacuate like those in Tsunami zone

OWRD D.S. in discussions with City of AstoriaOWRD D.S. in discussions with City of Astoria Field investigation with instrumentation – uplift Field investigation with instrumentation – uplift

pressure, rock strength, concrete strength pressure, rock strength, concrete strength (cracks)(cracks)

Left abutment –fractured rock and leakage

Foundation – fractured rock, unknown uplift pressure

Concrete Gravity dam built 1912 and raised in 1953

Pony Creek DamPony Creek Dam Embankment damEmbankment dam Designed in 1987Designed in 1987 No seismic analysis No seismic analysis

apparentapparent Above coastal Above coastal

sloughslough Foundation Foundation

materials uncertainmaterials uncertain Seismic analysis Seismic analysis

recommended and recommended and under considerationunder consideration

Water treatment Water treatment plant at base of damplant at base of dam

Dams to current seismic Dams to current seismic standardstandard2 rock-fill dams

Rock Foundation and abutments

Very little seepage

Designed to current seismic standards

1 earth-fill dam on rock - Upper Pony Creek Dam – Cascadia and crustal design1 earth-fill dam on rock - Upper Pony Creek Dam – Cascadia and crustal design

Hillsboro Hillsboro (Barney (Barney reservoir) reservoir) DamDam

McGuire Dam

McMinnville W&L

Preparing for CascadiaPreparing for Cascadia• Tsunami Tsunami (rapid, no direct (rapid, no direct

effects on high hazard effects on high hazard dams)dams)

• Large (extremely) Large (extremely) landslides with landslide landslides with landslide dams (LiDAR)dams (LiDAR)

• Extreme liquefaction Extreme liquefaction coastal valleyscoastal valleys

• Bridge failures Bridge failures (Bruce - (Bruce - ODOT)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 Likely very limited coastal access by road coastal access by road

• Inspections by helicopter (they Inspections by helicopter (they will be in high demand)will be in high demand)

• Engineered DamsEngineered Dams unlikely unlikely to be highest priority to be highest priority emergencyemergency Mapping tool with earthquakes (1

orange dot)

Emergency Action PlansEmergency Action Plans• Oregon Template – will Oregon Template – will

present at ASDSO annual present at ASDSO annual meeting)meeting)

• Notification short and up frontNotification short and up front• Emergency detection and Emergency detection and

evaluationevaluation• Level 3 Special – Severe shaking Level 3 Special – Severe shaking

evacuation – no notificationevacuation – no notification• Probable modes of failureProbable modes of failure

• Liquefaction, crest Liquefaction, crest displacement, slope displacement, slope instability, concrete failure, instability, concrete failure, rock abutment displacementrock abutment displacement

• Inundation (dambreak flood) Inundation (dambreak flood) mapsmaps

• Sunny day for CSZ quake Sunny day for CSZ quake • Responsibilities - emergency Responsibilities - emergency

manager – know maps and manager – know maps and what to do – will they have what to do – will they have resources?resources?

Low hazard dam failed in 2011, in eastern Oregon near Baker City

Rapid Dam BreachRapid Dam Breach

Coordination with Coordination with emergency managersemergency managers

• Evacuation responsibilities and resourcesEvacuation responsibilities and resources• Inspection of infrastructureInspection of infrastructure

• May take a long timeMay take a long time

• EAPs for dams close to CSZEAPs for dams close to CSZ• Emergency Level 3 SpecialEmergency Level 3 Special

• Oregon Emergency Response CouncilOregon Emergency Response Council• Dam Safety Engineer is currently OWRD Dam Safety Engineer is currently OWRD

representativerepresentative• OWRD engineers input into Oregon Cascadia PlanOWRD engineers input into Oregon Cascadia Plan• Also have roles in flood and droughtAlso have roles in flood and drought

Seismic Analysis IssuesSeismic Analysis Issues

Possible conservatism in FLAC and Possible conservatism in FLAC and Newmark analyses Newmark analyses (Scoggins Consultant Review (Scoggins Consultant Review Board)Board)

Actual behavior of dams in Subduction Actual behavior of dams in Subduction EventEvent Waiting to learn from dam performance during Waiting to learn from dam performance during

Tohoku great earthquakeTohoku great earthquake ML behavior in subduction zone earthquakeML behavior in subduction zone earthquake Seismic analysis of fractured rock Seismic analysis of fractured rock

abutmentsabutments

SummarySummary Aware (too much) of CSZ and what it can doAware (too much) of CSZ and what it can do Using available resources on high priority damsUsing available resources on high priority dams

EAP grantsEAP grants HEC-RAS/HMS inundation analysis for hazard HEC-RAS/HMS inundation analysis for hazard

classificationclassification Seismic analysis coastal damsSeismic analysis coastal dams

Very conservative seismic analyses are Very conservative seismic analyses are unhelpful for evaluating risk at existing damsunhelpful for evaluating risk at existing dams

Ground access to dams closest to Cascadia will Ground access to dams closest to Cascadia will at best be difficult after a major S.Z. earthquakeat best be difficult after a major S.Z. earthquake Repair of damaged dams will be prolongedRepair of damaged dams will be prolonged