Silver Creek Dam Potential Failure Modes Analysis (PFMA)

8/6/2019 Silver Creek Dam Potential Failure Modes Analysis (PFMA)

1/88

FinalReport

Silver Creek Dam Potential Failure Modes

Analysis (PFMA)

11APRIL2011


2/88

ES1

EXECUTIVESUMMARYE.1 IntroductionSilverCreekDamislocatedonSilverCreek,approximately2milessoutheastofSilverton,Oregon. The

drainageareaforthedamisapproximately46squaremilesandextendssoutheastoftheprojectand

includesSilverFallsStatePark. ThedamwasdesignedbyCH2MhillandconstructedbetweenMay1973

andNovember1974fortheCityofSilverton,Oregon. Duetothehistoryofseepageandproximityof

thedamtoresidentsofSilvertondownstreamofthedam,theCityhasdecidedtoconstructan

automateddatacollectionandearlywarningsysteminordertotrytoimproveitsabilitytomonitorthe

conditionofthedamandtoprovidewarningtoresidentsifunsafeconditionsdevelopatthedam.

TheCityofSilvertonandthePortlandDistrictUSArmyCorpsofEngineers(USACE)haveagreedto

partneronthedesignandinstallationofthedatacollectionandwarningsystemaspartofthe

ContinuingAuthoritiesProgram(CAP). Aspartofthisproject,USACErecommendedthatapotential

failuremodesanalysis(PFMA)beconductedforthedam. ThepurposeofthePFMAwastoidentify

potentialmodesoffailureforthedaminordertoevaluatewhatmeasuresmaybetakentoreduce

eithera)thelikelihoodthatfailureofthedamwilloccurand/orb)theconsequencesifthedamshould

fail. TheresultsofthePFMAwillbeusedtofocusthescopeofworkforthedesignofthemonitoring

andearlywarningsystem.

E.2 ResultsofPFMAThePFMAteam,consistingofengineersfromUSACEandCityOperationsPersonnelidentified36

potentialfailuremodes. Ofthese,16werepreliminarilyidentifiedasbeingcredibleandsignificant.

Afterevaluatingthese16failuremodesfurther,someofthefailuremodeswerefoundtobesimilarand

werecombinedandsomewereconsideredcrediblebutnotsignificant. Afterfurtherevaluation,4

failuremodeswereconsideredcredibleandsignificant. Thecredibleandsignificantfailuremodeswere

identifiedas: SpillwaySlabJacking,DownstreamSillandStillingBasinScour,UpstreamSillScourand

Overtopping. Seepagewasconsidered,howeveritwasjudgedtobelessofaconcernthanthefour

significantfailuremodesidentified.

E.3 MajorFindingsandUnderstandingsDuringthePFMAsessionthefollowingMajorFindingsandUnderstandingswereidentified:1) TheDamappearstobewellmaintainedandprojectoperationsstaffismaintainingtheprojectto

thebestoftheirabilities.

2) AvarietyofdocumentswereavailableforreviewforthePFMA. AlistofthedocumentsisincludedasAppendixA. Ingeneral,therewasalackofconstructionphotosassociatedwithoriginal

construction(oneaerialphotowasavailablefromoriginalconstruction).

3) Generalaccesstotherightsideoftheprojectisbycar. Accesstotherightabutmentoftheprojectsite(includingtherightspillwaytrainingwallandfishladder)canbegainedonfoot.Accesstothe

embankment,lowleveloutletandleftabutmentofthedamrequiresaboat.


3/88

ES2

4) Undernormaloperations,poolrisestospillwaycrest(El.424feet)foraboutninemonthsofeveryyear. PoolofrecordisaboutEl.430feet(6feetoverthecrest).

5) DownstreamfloodingoccurseveryyearatSilverGardensRetirementHome,requiringsandbaggingeveryyear. Thiswillcontinueunlessstructuralmeasuresaretakentopreventflooding.

6) Thereisnostreamgaugingupstreamofthedamorforecastdataavailableattheproject. Asaresult,theCityisunabletopredictfloodinginadvancefromrainfall.

7) Therearenoweirstomeasureseepagepresentatthisproject. Thecitycurrentlymanuallymeasuresseepagewithabucketandstopwatchperiodically. Thismethodresultsininconsistent

dataandnoplottingofseepagedatahasoccurredtodate.

8) Thereisnoseepagecollection/measuringpresentontheleftabutment. Becauseofthehistoricalseepageconcerns,seepagewasextensivelydiscussed. ForthisPFMA,generalseepagewasbroken

downintoseepageleadingtoaspecificfailuremechanism(i.e.increasedseepagecausesan

increaseinupliftpressurebeneaththespillwayslabs,leadingtofailureofspillwayslabs,which

allowserosionoftheunderlyingfoundationmaterials,leadingtobreachofthedam).

9) Outletcapacityandreliabilityarekeystosafeprojectoperations. Theprojectcannotdewaterquicklyifneededforanemergency(exceptinsummer). Inaddition,therearereliabilityconcerns

basedonoperationhistory.

10) Inspectors/CityStaffcannotvieworinspecttheleftsideoftheDam(embankment,spillwayandabutment)withoutaboat. Thisresultsininfrequentandinadequateinspection.

11) Citystaffcannotintervenetostopdamfailurewithouttheuseofahelicopterorboat/barge.Placingmaterialatthedam,suchasrockorothermaterialstopreventorstopdamfailureis

difficult.

12) ThecurrentEmergencyActionPlan(EAP)doesnotallowforfullcontactwiththepopulation. Thecalllistforthecurrentemergencypublicnotificationssystem(EverbridgeCallSystem)isnot

complete. AccordingtoCityOperationsStaff,thesuppliersoftheoldnotificationsystem(Code

Red)willnotreleasethefulllisttotheCity. TheEverbridgesystemwillautomaticallycallall

landlinesinSilverton. However,Cityresidentswontbecontactedoncellularphonesunlessthey

havesignedupfortheEverbridgecallsystem(eveniftheysigneduporiginallyfortheoldRedCode

system).

13) TheProjectOperationsplanneedstriggerthresholdsforinspection(whentogoto24hoursurveillance,etc.)andwarning. Criticalelevationsneedtobeestablishedforthisprojectinorder

toensureadequateinspectionandearlywarning.

14) Knownfoundationfeatureswillcontinuetoseep(basaltinterflowzone@398,landslidetalusonrightabutment). However,theseareasarenotpronetopipinganddamfailure.

15) Visiblesignsofdistressarepresentatthedam(thespillwaytrainingwallsaretilted,stillingbasinscourisapparentatthesite,newand/orincreasedseepagehasbeenidentifiedinrecentyears).

16) TheCityreportsthatseepagehasincreasedinrecentyears(leftandrightsideofoutletworks,newpipeinstalledinFeb2010),butthereisnowayfortheCitytoquantifyormeasuretoverifyif

seepageisincreasingduetoalackofaweirorothermeasurementsystemthatcouldbeusedto

quantifythisseepage.

17) TheCityinspectsthedamtwiceperyearandtheStateinspectsthedamapproximatelyonceperyear. Inspectionrequiresboataccesssofrequentinspectionisdifficult.


4/88

ES3

18) Stillingbasinscourisevidentandhasbeenpreviouslyreportedinpostconstructioninspections.ThereisnorecordofscourinspectionormonitoringbytheCity. Originalrecommendedoperations

andmaintenancedocumentindicatesthatthestillingbasinispronetoscourandshouldbe

frequentlyinspected. Theinspectionswillrequiredivertingwaterfromthelowleveloutletandfish

ladderandpumpingfromthestillingbasinduringperiodsofnoflowthroughthespillway;oruseof

anROV.

19) PowerattheprojectisCitysuppliedandisonlycurrentlyavailableontherightsideoftheDam. Nobackuppower,suchasanemergencygenerator,isavailableatthesite. Somemethodof

alternativepower,suchasasolarpanel,maybeneededforanyproposedmonitoringsystem.

20) Someofthepiezometersarecurrentlynonfunctionalandsomeofthepiezometerstandpipesaresmallindiameter. Smalldiameterpiezometersmayprovedifficulttoautomate. Ifnew

piezometersarerequired,accesstotheembankmentwillhavetobegainedbyboatorhelicopter.

21) Operationspersonnelhavenomeansofverifyingthelowleveloutletgateposition(42in.or18inoutlet)operationscannotseetheoutletfromthecontrolstandlocatedontheleftbank.

22) Outletoperationduringhighflows(openorclosed)isnotdefinedintheoperationsmanual. Theoutletmaynotbestructurallysoundathighpoolswhenthegateisopenduetothepotentialfor

cavitation. Inaddition,theairintakefortheoutletmaybeinundatedinhighfloweventswhichwill

resultinsurgeflow.

23) Lowspotsarepresentattherightabutment. Thismeansovertoppingmayoccuratlowerelevations.

24) Nocurrenthydraulicmodeling/routinghasbeenperformedinordertodefinethresholds(100yrflood,PFM,etc.),poolsandtailwaters. Ifmodelingdatawasavailable,theabilitytosetmeaningful

thresholdsforanearlywarningsystemwouldbeimproved.

25) Whilethetopoftheembankmentisdesignedatelevation440,theclaycorewithintheembankmentonlyextendsuptoElev.435;thefilterextendsuptoElev.430. Increasedseepage

abovetheclaycoreshouldbeanticipatedwithpoollevelsgreaterthanEl.435. Inaddition,thelack

ofaclaycoreextendingtothetopofthedammayresultinovertoppingoccurringatelevations

belowelevation440(i.e.overtoppingmaybeginbeforethepoolreachesthetopofthedamdueto

wave/winderosionoftheembankment). Thepotentialforovertoppingtobeginatalower

elevationshouldbeconsideredwhendefiningtriggerpointsforanearlywarningsystem.

26) TheCitywasnotabletoproducecurrentsurveydatafortrueheightofembankmentcrest,thetopsofwallsorabutments. Settlementoftheembankmentorotherlowspotsmayresultin

overtoppingatlowerelevationsthananticipated. Knowingthelocationoflowspotsisimportant

fordetermininglocationsofstockingemergencymaterialsandtofocusfloodfightingefforts.

27) BasedontheresultsofanalysisperformedtodatebyCornforthConsultants,theembankmentandabutmentsareexpectedtobeadequatelystableunderseismicconditionsandliquefaction

susceptibilityislow.

28) Noseismicanalysisoftheconcretespillwayorspillwaywallhasbeenperformed. Itisanticipatedthatlossofthewallmayleadtofailureoftheembankment.(Wallsdesignedfor0.05g,currentMCE

fromUSGS=0.4g).

29) Thereappearstobeagapinthecoretrenchontherightabutmentbelowthetrainingwall. ThepresenceofagapatthislocationmayresultinseepageatpoolsaboveElev.430.


5/88

ES4

30) Interventionmaterials(sandbags,riprap)arenotcurrentlystockpiledonthesite.31) Thereareresidencelocationswithinveryshorttraveltimesdownstreamofthedam.Warningtime

wouldnotbepossibleforsomefailuremodes.

32) LossofpoolinsummermonthswilleliminatethedownstreamwatersupplyandforcetheCitytogotowaterconservation.

33) Alogboomwasinstalledattheendofconstructionattheupperendofthepoolbutisnolongerpresent. Buildupofreservoirrimdebriscouldclogthespillwayduringhighflowevents.

34) Largevegetationdebrisisexpectedtocontinuetocollectdownstreamofthespillwaybelowthestillingbasin,reducingchannelcapacity. Ifthismaterialcontinuestocollectovermanyyears,it

maycreateadownstreamhazardduringasubsequentlargefloodeventasoccurredinrecent

floodingeventsontheSandyRiverresultingindownstreamdamagetobridgesandproperty.

35) Projectstaffwasunawareofthepresenceof4footfilterdrainthatrunsfromthefilterzonetotheleftoftheoutletpipeandendsdownstreamimmediatelytotheleftoftheoutletstructure(this

mayormaynotexistsinceasbuiltdrawingsdonotshowitbutitisdiscussedinCH2MHill

correspondence). Ifitexists,thisdrainmaycontributetoseepageexitingnearthelowleveloutlet

structureatthetoeofthedam.

36) Therearenorecordsofconduitinspection/ROVinsidetheoutletpipe.37) Zone1materialsontheupstreamshelloftheembankmentconsistofwellgradedsilty,sandy

gravelandshouldactasaflowlimiter/pipingrestriction(filldevelopedvoids)shoulditinitiate.

38) Thelowlevelconduitpassesthroughthedamatthebaseoftheembankment. Itisfoundedonbedrockwhereitpassesbeneaththeimperviouscore. Theconduitissquareandhasseepage

collarson10footcenters. Imperviousbackfillaroundtheconduitwasplacedin4inchliftsand

compactedwithhandtampers.

E.4 Conclusions1) Failuremodesrelatedtoseepagethroughtherightabutmentlandslidematerialareconsidered

crediblebutnotassignificantasthe4credibleandsignificantfailuremodesidentifiedinthisreport.

Itwasjudgedthatevenunderhighpoolconditions,suchasElev.440(PMFconditions),therelative

increaseinhydraulicgradientcomparedtowhattheprojecthasexperiencedisnotenoughto

initiatepipinginthegenerallypervious,angular,basaltfragmentsthatcomprisethelandslide

deposit. However,itisimportanttocontinuemonitoringseepagetodetectanyincreasesthatmay

indicateapotentialdevelopingissueofconcern.

2)With

respect

to

the

significant

and

credible

failure

modes,

an

automated

early

warning

system

wouldbemosteffectiveatreducingriskassociatedwiththeovertoppingfailuremode.

3) Forovertoppingfailuremodes,itisanticipatedthat,ifproperlydesignedandmaintained,anautomatedearlywarningsystemmaybeabletoprovideearlywarningtodecisionmakersintimeto

reducetheriskassociatedwithovertoppingfailuremodes.

4) Initiationandprogressionofthefailuremodesassociatedwitherosionofthespillwayfoundationwouldbedifficulttomonitorwithanautomatedmonitoringsystemandthereforedifficultto


6/88

ES5

provideadvancedwarningoffailurebyanearlywarningsystem.Monitoringhorizontaldraindata

anddatafromanewpiezometerinstalledtotheleftoftheleftspillwaywallanddownstreamofthe

embankmentfilterzonecouldbeusedtomonitordevelopmentofthefailuremodeassociatedwith

upstreamsillscourandincreasedseepagethroughtherightabutment. However,visualmonitoring

forspillwayslaboffsetsandstillingbasinandupstreamsillerosionaremoredirectwaystoidentify

whetherthesefailuremodesareinitiating.

5) Forfailuremodesassociatedwithspillwayfoundationerosion,itisanticipatedthatthetimefrominitiationtouncontrolledreleaseofthepoolcouldbemonthsifthefailureoccurredundernormal

poolconditionsbutdays(possiblyhours)underhighflood(suchasa100yearevent). Duringhighflows,unlessinitiationisobserveddirectlybyCitystaff,minimalwarningwillbeprovidedtothedownstreampopulation forspillwayfailureinitiatedfailuremodesregardlessofwhetherawarningsystemisinstalled.

6) Ifproperlydesignedandmaintained,theautomateddatacollectionfunctionoftheproposedearlywarningsystemwillprovideabenefitofincreasedconsistencyandqualityofdata(including

piezometeranddraindata)collectedbythesystem. Alldatashouldbestoredforfuturereference.

Thisdatahelpstoassesstheconditionofthedamandimprovesunderstandingforhowthedam

respondsundervariouspoolandseasonalconditions.

7) TheautomateddatacollectionfunctioncouldalsobeusedtoprovideconfirmationofflowsthroughthelowleveloutletandfrequentupdatesontheelevationofSilverCreekDamdownstreamofthe

damtoimprovecoordinationoffloodresponseactivitiesassociatedwithhydrologiceventsthatare

notrelatedtodamfailure.

E.5 RecommendationsRegardingtheEarlyWarningSystem1) Theautomatedearlywarningsystemshouldincludeareservoirlevelmeasurementandraingage.

Multiplelevelthresholdsforreservoirlevelandraindatashouldbedefinedaspartofthedesignof

theearlywarningsystem. Ataminimum,thethresholdsshouldconsidertheelevationsofthetop

ofthecore(El.435)andthelowspottotherightoftherightabutmentwall(approx.El.438). In

addition,duetothegapinthecoretrenchontherightabutmentbelowthetrainingwall,theCity

shouldanticipatethepossibilityofincreasedseepagewithpoolsaboveElev.430andmonitorthis

location. Thesystemshouldprovidenotificationoftheexceedanceofthethresholdstoappropriate

decisionmakingpersonnelattheCity. Inaddition,itshouldcollectandstoredataforfuture

referenceandanalysis.

2)The

City

should

design

and

install

weir

collection

boxes

to

measure

the

flow

rate

of

known

seeps

anddrainflows,especiallyflowsassociatedwiththetoedrains. Changesintheseflowratesmay

indicatechangingconditionsatthedam.

3) Intheeventthatdrainflowsshowsustainedincreasesinflow,considerinstallationofanewpiezometertotheleftoftheleftspillwaywallforthepurposesofmonitoringpiezometricpressure

withinthelandslidedepositthatprovidesthefoundationforthespillway. Thewaterlevelinthis


7/88

ES6

piezometercouldbemonitoredwithanautomateddatacollectionsystemwiththresholdsassigned

toprovidenotification.

4) Thereliabilityoftheexistingpiezometersshouldbeevaluatedaspartofthedesignofthewarningsystem.

5) Forthepurposesofhistoricaldatacollection,theautomatedwarningsystemshouldbedesignedtocollectdatafromthepiezometersandweirstoimprovetheconsistencyofdatacollectionand

review.

6) Theautomatedsystemshouldsupportstorageofhistoricaldataforthepurposesofreviewinghistoricaltrendsandprovidingreportswhenneeded. Timehistoryplotsofdatashouldbereviewed

atleastmonthlybytheCityandmorefrequentlyifvisualobservationsindicatepotentialdeveloping

issuesofconcern.

7) ForthepurposesofimprovingfloodresponsebytheCity,theautomatedwarningsystemshouldbedesignedtocollectdataforwaterlevelatdownstreamlocationsasdeterminedbytheCity.

Multiplelevelthresholdscanbedevelopedtoprovideanassessmentoflevelofurgency.

8) TheCitysEmergencyActionPlanshouldbeupdatedtoincorporatetheearlywarningsystemandtoaddressnotificationofallcitizensimpactedbyflooding.

E.6 AdditionalRecommendationstoImproveDamSafetyandReliability1) Performasurveyofthedamalongthecrestofthedamfromleftabutmenttorightabutmentin

ordertodeterminethepresenceoflowpointsonthedam. Iftherearelowspots,theyshouldbe

raisedtothedesignelevation.

2) Performsurveyofspillwayandabutmentwallsforverticalandhorizontaldeflectioninordertoestablishabaselineforcomparisonwithfuturesurveys. Additionalsurveysshouldbecompletedat

leastevery5years. Ifsurveysindicatemovement,theresultsofthesurveysshouldbereviewedby

astructuralengineerforevaluation.

3) Inspectandsurveythestillingbasinforscourduringperiodwhenspillingisnotoccurring(summerlowpoolinspection)inordertoestablishabaselineforcomparisonwithfuturesurveys. Inspectthe

stillingbasinforincreasedscourafterlargeflowevents. Performtheinspectionsandsurveysat

leastevery5years.

4) Inspectthespillwayforverticaloffsetsbetweenslabsduringperiodwhenspillingisnotoccurring(summerlowpoolinspection). Themostcriticaloffsetisthedownstreamslabupliftedrelativeto

theupstreamslab;thiscreateslocalizedhighpressuresthatmaypenetratedownalongtheslab

jointandcreatehighupliftpressure. Ifdownstreamverticaloffsetsarepresent,grindthe

downstreamslabdowntoremoveoffsets. Recommendvisualinspectionsarecompletedatleast

onceperyear.

5) Visuallyinspectupstreamimperviousblanketforholesforitsfulllengthupstream(notjusttheupstreamsill)whenpossible. Visuallyinspectsillforerosionatleastonceperyearorafterlargespill

events.

6) InspecttheintegrityofthelowleveloutletpipeusingaRemotelyOperatedVehicle(ROV).


8/88

ES7

7) Evaluateandimplementoptionsforincreasingthereliabilityofthelowleveloutlethydraulicsystemandforconfirmationofgateopeningsize. Tothisend,thewarningsystemshouldincludecollection

ofdataontheflowthroughthelowleveloutlettoimprovetheabilitytoconfirmgateopeningsize.

8) TheCitysnewpublicnotificationsystem(Everbridge)willcallallCityofSilvertontelephonelandlinesbutdoesnotincludetheentirecontactlistfromtheprovideroftheoldpublicnotification

system(CodeRed). RecommendevaluatingwaystoupdatetheEverbridgesystemcontactlistforall

cellularphoneuserstotheextentpossible.

9) Performhydraulicmodelingtodeterminethefrequencyandmagnitudeandroutingofstormevents(determinerelationshipsbetweenfloodfrequency,peakflowanddownstreamwaterelevations).

TheresultsofthemodelingwouldhelptheCitywithdecisionsregardingbalancingdownstream

floodingimpactswithdamsafetyrisksandtoimprovetheCitysabilitytowarnthepopulation.

Oncetheresultsofthemodelingarecomplete,itmaybeappropriatetomodifytheoperationsplan

fortheproject.

10)TheCityshouldconsidermakingstructuralimprovementstopreventfloodingofresidentsatlowelevationlocations(suchasSilverGardensRetirementHome).

11)Theincreasedwoodydebrisbuildingupdownstreamofthestillingbasincreatesapotentialfordownstreamdamageduringfuturestormevents. Duringlargeevents,thismaterialmaybecome

dislodgedandcausedamagetobridgesandproperty. TheCityshouldconsiderremovingthis

materialevery5years.


9/88

i

TableofContents1.0 INTRODUCTION...................................................................................................................................... 1

2.0 BACKGROUND........................................................................................................................................ 1

3.0 PFMAPROCESS...................................................................................................................................... 2

4.0MAJORFINDINGSANDUNDERSTANDINGS............................................................................................ 3

5.0CREDIBLEANDSIGNIFICANTFAILUREMODES........................................................................................ 6

5.1SpillwaySlabJacking(PFMCinAppendixB)...................................................................................... 6

5.1.1Description................................................................................................................................... 6

5.1.2FactorsthatIncreasetheLikelihoodofthisPFM......................................................................... 7

5.1.3RiskReductionMeasures............................................................................................................. 7

5.1.4WarningTime............................................................................................................................... 7

5.2 DownstreamSillandStillingBasinScour(PFMDandPFMEinAppendixB).................................... 7

5.2.1Description................................................................................................................................... 7



5.2.4WarningTime............................................................................................................................... 8

5.3Scourofupstreamspillwaysill(PFMFinAppendixB)....................................................................... 8

5.3.1Description................................................................................................................................... 8



5.3.4WarningTime............................................................................................................................... 9

5.4 Overtopping(PFMJ,KandNinAppendixB)..................................................................................... 9

5.4.1Description................................................................................................................................... 9



5.4.4WarningTime............................................................................................................................. 10

5.5 RightAbutmentSeepage................................................................................................................. 10

6.0APPLICABILITYOFANEARLYWARNINGSYSTEMTOSIGNIFICANTFAILUREMODES........................... 10

7.0CONCLUSIONSANDRECOMMENDATIONS........................................................................................... 12

7.1Conclusions....................................................................................................................................... 12

7.2Recommendations............................................................................................................................ 13


10/88

ii

7.2.1 AutomatedDataCollectionandEarlyWarningSystemRecommendations............................ 13

7.2.2 AdditionalRecommendationstoImproveDamSafetyandReliability.................................... 14

Figures

Figure1SitePlan

Figure2SketchofSpillwaySlabJackingFailureMode

Figure3SketchofScourofDownstreamSpillwaySillandStillingBasinFailureMode

Figure4SketchofScourofUpstreamSpillwaySillFailureMode

AppendicesAppendixA References

AppendixBPFMANotes

AppendixCSiteVisitNotesandPhotographs

AppendixDProjectDrawings


11/88

1

1.0 INTRODUCTIONSilverCreekDamislocatedonSilverCreek,approximately2milessoutheastofSilverton,Oregon. Silver

CreekistributarytoPuddingRiverapproximately6milesbelowtheproject. Thedamwasdesignedby

CH2MHillandwasconstructedbetweenMay1973andNovember1974. Thedamwasconstructedto

storewaterformunicipalwatersupplyandrecreation. Duetothehistoryofseepageandproximityof

thedamtoresidentsofSilvertondownstreamofthedam,theCityofSilvertonhasdecidedtoconstruct

anautomateddatacollectionandearlywarningsysteminordertotrytoimproveitsabilitytomonitor

theconditionofthedamandtoprovidewarningtoresidentsifunsafeconditionsdevelopatthedam.

TheCityofSilvertonandthePortlandDistrictUSArmyCorpsofEngineers(USACE)haveagreedto

partneronthedesignandinstallationofthedatacollectionandwarningsystemaspartofthe

ContinuingAuthoritiesProgram(CAP). Aspartofthisproject,USACErecommendedthatapotential

failure

modes

analysis

(PFMA)

be

conducted

for

the

dam.

The

purpose

of

the

PFMA

was

to

identify

potentialmodesoffailureforthedaminordertoevaluatewhatmeasuresmaybetakentoreduce

eithera)thelikelihoodthatfailureofthedamwilloccurand/orb)theconsequences,ifthedamshould

fail. FailureisdefinedinUSACEEngineeringRegulation(ER)111021156SafetyofDamsPoliciesand

ProceduresandbythePFMAteamassudden,rapidanduncontrolledreleaseofimpoundedwater. A

failuremodeisadetaileddescriptionofapossiblechainofeventsfrominitiationthroughfailure. Riskis

definedastheprobabilityofaload(suchasapoollevelorearthquake)multipliedbytheprobabilityof

failure(giventheload),multipliedbytheconsequencesofthefailure. Asaresult,thePFMAprovidesa

meansforidentifyingriskattheproject. TheresultsofthePFMAprovideasenseofwhatpossible

modesoffailureanautomatedearlywarningsystemmightbeeffectiveatreducingriskbutalsofor

thosethatthesystemmaynotbeabletoeffectivelyreducerisk. ThePFMAalsoprovidesvaluable

informationthatcanbeusedtodeterminewhatdatashouldbecollectedfromtheautomateddata

collectionandearlywarningsystem. InformationlearnedfromthePFMAcanalsoinformtheCityof

criticalaspectsorareasontheprojectthatrequirespecialattention.

2.0 BACKGROUNDThedamwasconstructedbetweenMay1973andNovember1974. Uponfirstfilling,seepagewas

observedontherightsideoftheearthembankment. Figure1isasiteplanforthedam. Thereservoir

wasloweredshortlythereafterandaseriesofhorizontaldrainswereinstalledintofoundationmaterials

below

the

right

side

of

the

earth

embankment.

In

addition,

a

buttress

filter

layer

berm

was

placed

on

muchoftherightsideoftheembankmenttointerceptseepagenotcapturedbythedrains.

InJune1981,USACEandtheOregonWaterResourcesDepartmentcompletedaPhase1Inspection

Report. ItidentifiedtheSilverCreekDamasahighhazarddambecauseofthepotentiallossofliferisk

andthelevelofpotentialpropertydamageimmediatelydownstreamofthedam. Theinspection

evaluatedabutmentandfoundationconditions,embankmentstability,hydraulicandhydrologic


12/88

Embankment

Spillway

Fish Ladder

Left Abutment

Right Abutment

Figure 1. Site Plan


13/88

2

conditions,andstructural/mechanicalfeatures. Theinspectionfoundthedamtobeinsatisfactory

conditionforcontinuedoperation.

InMarch1993,amagnitude5.6earthquake(ScottsMills)occurredwithanepicenterapproximately10

mileseastoftheCityofSilverton. Themagnitudeoftheearthquakeanditslocationnearthedam

causedincreasedfocusonseismicsafetyintheregion. InJuly1999,theCitycontractedtoCornforthConsultantstoperformanassessmentofthestabilityofthedamundersteadystate,rapiddrawdown

andseismicconditions. Theanalysisindicatedthatthedamdemonstratedadequatefactorsofsafety

foreachoftheconditionsconsidered.

AttherequestoftheCityofSilverton,adambreakanalysiswasperformedbyPhilipWilliamsand

AssociatesinJanuary2000. Theanalysiswasperformedtoprovideinformationtobeincorporatedinto

anEmergencyActionPlan(EAP)fortheproject. TheanalysisconcludedthatfullbreachofSilverCreek

DamwouldbecatastrophictotheCityandthatelapsedtimefromdamfailuretothepassageofthe

floodwaveislessthanonehour. ItwasrecommendedinthisreportthattheCityinstallsanearly

warningsystematthedam.

In2002,SquierAssociatesperformedapreliminarydesignreportforanearlywarningsystem. The

designreportincludedrecommendationsforanearlywarningsystembasedon3failuremechanisms:

seepagefailureundernormaloperatingconditions,seismicdeformationsandinstabilityand

overtoppingfailuresresultingfromflooding.

USACEregulations(ER111021156)nowrequirethataPFMAprocessbeperformedforallmajordam

modificationsorevaluationofinterimriskreductionmeasures. In2010,USACEdecidedthataPFMA

wasnecessarytoevaluatetheapplicabilityofanearlywarningsystemtoreduceriskatSilverCreek

Dam.

3.0 PFMAPROCESSThefirststepintheprocesswastoidentifytheteammembersthatwouldparticipateinthePFMA. In

general,PFMAteamsincludeafacilitator,cofacilitatorandtechnicalexperts. Themakeupofthe

technicalexpertsonaPFMAteamdependsuponthenatureoftheproject. Projectsthathaveahigh

degreeofcomplexitymayrequireteammembersfromeachofStructural,Geotechnical,Mechanicaland

ElectricalEngineeringandHydraulicsandHydrologydisciplinesinadditiontomembersfamiliarwith

operationalaspectsofthedam. ForSilverCreekDam,itwasdecidedthatthecomplexityoftheproject

withregardtoelectricalandmechanicalequipmentwassuchthatteammembersrepresentingthe

otherdisciplinesandtheCitysoperationalteammemberhavesufficientknowledgeinthisareato

addressissuesinthesedisciplines.

Oncetheteamwasidentified,eachoftheteammembersreviewedbackgroundinformationonthe

project. ProjectinformationwasgatheredbytheCityofSilvertonandwastransmittedtoUSACEprior

tothestartofthePFMA,whichwasperformedasafocusedworkshop,between10and14January,

2011. EachoftheteammembersreviewedthebackgroundinformationpriortothePFMAworkshop,so

thateachmemberhadfamiliaritywiththeprojectbythetimeoftheworkshop. Onday1,theteam


14/88

3

continuedtoreviewbackgroundinformationasagroupandsummarizedinitialmajorfindingsand

understandings(seeSection4.0). Onthesecondday,theteamconductedasitevisittotheprojectso

thattheteamcouldbecomemorefamiliarwithsiteconditions. Theremainderoftheweekwasspent

identifying,describingandevaluatingpotentialfailuremodesfortheproject. Theteamidentifiedthe

majorfeaturesoftheprojectandbasedontheteamsexperienceandtrainingforperformingPFMAs

identifiedpossiblefailuremodes.Withthislistofpotentialfailuremodes,theteamdevelopeda

preliminaryratingastohowsignificanteachpotentialmodewas. Byconvention,USACEassignseach

modeoneofthefollowingcategories:

a) Credible: aphysicallyplausiblefailuremode.b) Significant(orCredibleandSignificant): thesefailuremodesareasubsetofcrediblefailure

modes. Theprobabilityoffailureandassociatedconsequencesapproachcloselyorexceeda

tolerablerisklimitguideline.

c) Noncredible: notaphysicallyplausiblefailuremode.AppendixBincludesthedetailednotesthatweretakenduringtheworkshop.

4.0MAJORFINDINGSANDUNDERSTANDINGSThissectionincludesalistofthemajorfindingsandunderstandingsabouttheprojectthatwere

identifiedfromreviewofthebackgroundinformation,thesitevisitandduringthePFMAworkshop:

1) TheDamappearstobewellmaintainedandprojectoperationsstaffismaintainingtheprojecttothebestoftheirabilities.

2) AvarietyofdocumentswereavailableforreviewforthePFMA. AlistofthedocumentsisincludedasAppendixA. Ingeneral,therewasalackofconstructionphotosassociatedwithoriginalconstruction(oneaerialphotowasavailablefromoriginalconstruction).

3) Generalaccesstotherightsideoftheprojectisbycar. Accesstotherightabutmentoftheprojectsite(includingtherightspillwaytrainingwallandfishladder)canbegainedonfoot.Accesstothe

embankment,lowleveloutletandleftabutmentofthedamrequiresaboat.

4) Undernormaloperations,poolrisestospillwaycrest(El.424feet)foraboutninemonthsofeveryyear. PoolofrecordisaboutEl.430feet(6feetoverthecrest).

5) DownstreamfloodingoccurseveryyearatSilverGardensRetirementHome,requiringsandbaggingeveryyear. Thiswillcontinueunlessstructuralmeasuresaretakentopreventflooding.

6) Thereisnostreamgaugingupstreamofthedamorforecastdataavailableattheproject. Asaresult,theCityisunabletopredictfloodinginadvancefromrainfall.

7) Therearenoweirstomeasureseepagepresentatthisproject. Thecitycurrentlymanuallymeasuresseepagewithabucketandstopwatchperiodically. Thismethodresultsininconsistent

dataandnoplottingofseepagedatahasoccurredtodate.

8) Thereisnoseepagecollection/measuringpresentontheleftabutment. Becauseofthehistoricalseepageconcerns,seepagewasextensivelydiscussed. ForthisPFMA,generalseepagewasbroken


15/88

4

downintoseepageleadingtoaspecificfailuremechanism(i.e.increasedseepagecausesan

increaseinupliftpressurebeneaththespillwayslabs,leadingtofailureofspillwayslabs,which

allowserosionoftheunderlyingfoundationmaterials,leadingtobreachofthedam).

9) Outletcapacityandreliabilityarekeystosafeprojectoperations. Theprojectcannotdewaterquicklyifneededforanemergency(exceptinsummer). Inaddition,therearereliabilityconcerns

basedonoperationhistory.

10) Inspectors/CityStaffcannotvieworinspecttheleftsideoftheDam(embankment,spillwayandabutment)withoutaboat. Thisresultsininfrequentandinadequateinspection.

11) Citystaffcannotintervenetostopdamfailurewithouttheuseofahelicopterorboat/barge.Placingmaterialatthedam,suchasrockorothermaterialstopreventorstopdamfailureis

difficult.

12) ThecurrentEmergencyActionPlan(EAP)doesnotallowforfullcontactwiththepopulation. Thecalllistforthecurrentemergencypublicnotificationssystem(EverbridgeCallSystem)isnot

complete. AccordingtoCityOperationsStaff,thesuppliersoftheoldnotificationsystem(Code

Red)willnotreleasethefulllisttotheCity. TheEverbridgesystemwillautomaticallycallall

landlinesinSilverton. However,Cityresidentswontbecontactedoncellularphonesunlessthey

havesignedupfortheEverbridgecallsystem(eveniftheysigneduporiginallyfortheoldRedCode

system).

13) TheProjectOperationsplanneedstriggerthresholdsforinspection(whentogoto24hoursurveillance,etc.)andwarning. Criticalelevationsneedtobeestablishedforthisprojectinorder

toensureadequateinspectionandearlywarning.

14) Knownfoundationfeatureswillcontinuetoseep(basaltinterflowzone@398,landslidetalusonrightabutment). However,theseareasarenotpronetopipinganddamfailure.

15) Visiblesignsofdistressarepresentatthedam(thespillwaytrainingwallsaretilted,stillingbasinscourisapparentatthesite,newand/orincreasedseepagehasbeenidentifiedinrecentyears).

16) TheCityreportsthatseepagehasincreasedinrecentyears(leftandrightsideofoutletworks,newpipeinstalledinFeb2010),butthereisnowayfortheCitytoquantifyormeasuretoverifyif

seepageisincreasingduetoalackofaweirorothermeasurementsystemthatcouldbeusedto

quantifythisseepage.

17) TheCityinspectsthedamtwiceperyearandtheStateinspectsthedamapproximatelyonceperyear. Inspectionrequiresboataccesssofrequentinspectionisdifficult.

18) Stillingbasinscourisevidentandhasbeenpreviouslyreportedinpostconstructioninspections.ThereisnorecordofscourinspectionormonitoringbytheCity. Originalrecommendedoperations

andmaintenancedocumentindicatesthatthestillingbasinispronetoscourandshouldbe

frequentlyinspected. Theinspectionswillrequiredivertingwaterfromthelowleveloutletandfish

ladderandpumpingfromthestillingbasinduringperiodsofnoflowthroughthespillway;oruseof

anROV.

19) PowerattheprojectisCitysuppliedandisonlycurrentlyavailableontherightsideoftheDam. Nobackuppower,suchasanemergencygenerator,isavailableatthesite. Somemethodof

alternativepower,suchasasolarpanel,maybeneededforanyproposedmonitoringsystem.


16/88

5

20) Someofthepiezometersarecurrentlynonfunctionalandsomeofthepiezometerstandpipesaresmallindiameter. Smalldiameterpiezometersmayprovedifficulttoautomate. Ifnew

piezometersarerequired,accesstotheembankmentwillhavetobegainedbyboatorhelicopter.

21) Operationspersonnelhavenomeansofverifyingthelowleveloutletgateposition(42in.or18inoutlet)operationscannotseetheoutletfromthecontrolstandlocatedontheleftbank.

22) Outletoperationduringhighflows(openorclosed)isnotdefinedintheoperationsmanual. Theoutletmaynotbestructurallysoundathighpoolswhenthegateisopenduetothepotentialfor

cavitation. Inaddition,theairintakefortheoutletmaybeinundatedinhighfloweventswhichwill

resultinsurgeflow.

23) Lowspotsarepresentattherightabutment. Thismeansovertoppingmayoccuratlowerelevations.

24) Nocurrenthydraulicmodeling/routinghasbeenperformedinordertodefinethresholds(100yrflood,PFM,etc.),poolsandtailwaters. Ifmodelingdatawasavailable,theabilitytosetmeaningful

thresholdsforanearlywarningsystemwouldbeimproved.

25) Whilethetopoftheembankmentisdesignedatelevation440,theclaycorewithintheembankmentonlyextendsuptoElev.435;thefilterextendsuptoElev.430. Increasedseepage

abovetheclaycoreshouldbeanticipatedwithpoollevelsgreaterthanEl.435. Inaddition,thelack

ofaclaycoreextendingtothetopofthedammayresultinovertoppingoccurringatelevations

belowelevation440(i.e.overtoppingmaybeginbeforethepoolreachesthetopofthedamdueto

wave/winderosionoftheembankment). Thepotentialforovertoppingtobeginatalower

elevationshouldbeconsideredwhendefiningtriggerpointsforanearlywarningsystem.

26) TheCitywasnotabletoproducecurrentsurveydatafortrueheightofembankmentcrest,thetopsofwallsorabutments. Settlementoftheembankmentorotherlowspotsmayresultin

overtoppingatlowerelevationsthananticipated. Knowingthelocationoflowspotsisimportant

fordetermininglocationsofstockingemergencymaterialsandtofocusfloodfightingefforts.

27) BasedontheresultsofanalysisperformedtodatebyCornforthConsultants,theembankmentandabutmentsareexpectedtobeadequatelystableunderseismicconditionsandliquefaction

susceptibilityislow.

28) Noseismicanalysisoftheconcretespillwayorspillwaywallhasbeenperformed. Itisanticipatedthatlossofthewallmayleadtofailureoftheembankment.(Wallsdesignedfor0.05g,currentMCE

fromUSGS=0.4g).

29) Thereappearstobeagapinthecoretrenchontherightabutmentbelowthetrainingwall. ThepresenceofagapatthislocationmayresultinseepageatpoolsaboveElev.430.

30) Interventionmaterials(sandbags,riprap)arenotcurrentlystockpiledonthesite.31) Thereareresidencelocationswithinveryshorttraveltimesdownstreamofthedam.Warningtime

wouldnotbepossibleforsomefailuremodes.

32) LossofpoolinsummermonthswilleliminatethedownstreamwatersupplyandforcetheCitytogotowaterconservation.

33) Alogboomwasinstalledattheendofconstructionattheupperendofthepoolbutisnolongerpresent. Buildupofreservoirrimdebriscouldclogthespillwayduringhighflowevents.

34) Largevegetationdebrisisexpectedtocontinuetocollectdownstreamofthespillwaybelowthestillingbasin,reducingchannelcapacity. Ifthismaterialcontinuestocollectovermanyyears,it


17/88

6

maycreateadownstreamhazardduringasubsequentlargefloodeventasoccurredinrecent

floodingeventsontheSandyRiverresultingindownstreamdamagetobridgesandproperty.

35) Projectstaffwasunawareofthepresenceof4footfilterdrainthatrunsfromthefilterzonetotheleftoftheoutletpipeandendsdownstreamimmediatelytotheleftoftheoutletstructure(this

mayormaynotexistsinceasbuiltdrawingsdonotshowitbutitisdiscussedinCH2MHill

correspondence). Ifitexists,thisdrainmaycontributetoseepageexitingnearthelowleveloutlet

structureatthetoeofthedam.

36) Therearenorecordsofconduitinspection/ROVinsidetheoutletpipe.37) Zone1materialsontheupstreamshelloftheembankmentconsistofwellgradedsilty,sandy

gravelandshouldactasaflowlimiter/pipingrestriction(filldevelopedvoids)shoulditinitiate.

38) Thelowlevelconduitpassesthroughthedamatthebaseoftheembankment. Itisfoundedonbedrockwhereitpassesbeneaththeimperviouscore. Theconduitissquareandhasseepage

collarson10footcenters. Imperviousbackfillaroundtheconduitwasplacedin4inchliftsand

compactedwithhandtampers.

5.0CREDIBLEANDSIGNIFICANTFAILUREMODESDuringthePFMA,theteamidentified36potentialfailuremodes(PFMs). Ofthese,16werepreliminarily

identifiedaspossiblybeingcredibleandsignificant. Afterevaluatingthese16failuremodesfurther,

someoftheidentifiedfailuremodeswerefoundtobesimilarandcombinedandsomewereconsidered

crediblebutnotsignificant. Intheend,4failuremodeswereconsideredcredibleandsignificant. Inthis

section,eachofthesesignificantfailuremodesaredescribed,alongwithpossibleactionsormeasures

thatcanbetakentoreduceriskassociatedwiththefailuremode. ThePFMAnotes(AppendixB)

includesmoredetailedinformationregardingtheevaluationofeachfailuremode.

5.1SpillwaySlabJacking(PFMCinAppendixB)5.1.1DescriptionSpillwayflowimpactsonverticaloffsetinspillwayslabswhichresultfromupliftpressureorslab

settlement(eitherunderlongtermstaticconditionsorseismicallyinduced). Themostcriticaloffsetis

thedownstreamslabupliftedrelativetotheupstreamslab,whichcreateslocalizedhighpressuresthat

maypenetratedownalongtheslabjointandcreatehighupliftpressure. Landslidefoundationmaterials

areexposedinitiatingerosion. Exposedtalus/landslideishighlyerodibleandscourwillprogress.

Continuedlossoffoundationmaterialresultsinadditionalslabfailuresupstreamandadditional

foundationexposureanderosion. Erosionprogressestotheupmostslaberodingdowntobasalt

resultinginuncontrolledrelease. Figure2isasketchofthisfailuremode.


18/88

Figure2.

SpillwaySlab

JackingFailureMode


19/88

7

5.1.2FactorsthatIncreasetheLikelihoodofthisPFM Jointsarekeyshapedwithpremoldedjointfillerbetweenslabswithaplasticwaterstop

embeddedapproximately3inchesbelowtopofspillwayslab(nodowelingorstructuraltie

betweenslabs).

Constructionfinalreportdocumentsdifficultywithrockanchorinstallationduetopoorrockquality

and

saturated

foundation

(water

in

anchor

holes).

Visualobservationduringthesitevisitshowedturbulence/flowdisturbanceontheleftsideofthespillwayatthetoe.

Spillwayflowsoccurapproximately9monthsoutoftheyear. Poolwasverycalmimmediatelydownstreamofendofspillwaythenturbulentdownstreamof

calmarea(possibledeeppool).

Nodata/inspectionexceptvisualflowdisturbancesdocumentedduringsitevisit. Drainsarenotinspectedorcleaned.

5.1.3RiskReductionMeasures Attimesofnonspilling,inspectforoffsetsbetweenspillwayblocks. Grindslabsflushatjoints. Anchortheslabs. Fillvoids/scourholeswithconcrete(holesaroundandunder).

5.1.4WarningTime Warningopportunitytime(timefromabilitytodetectfailuremechanismtoinitiationofthe

mechanism)is0ornegative. Itcanbeseenafteraslabhasfailed(maybe).

Breachformationtime(timefrominitiationofbreachtolossofdam): fornormalpoolsmonths,andforhighflowsgreaterthan100yrsdays,maybehours.

5.2 DownstreamSillandStillingBasinScour(PFMDandPFMEinAppendixB)

5.2.1DescriptionSpillwayflowinitiateserosionofthebasaltandspillwaystillingbasinconcrete. Erosioncontinues,

underminingthespillwayconcreteandfoundation. Failure/underminingofthespillwayfoundationat

thetoeresultsinlossofspillwayslabsupport. Lossofslabsupportpermitsslabfailure,exposingthe

talus/landslidefoundation. Exposedtalusishighlyerodibleandscourwillprogress. Continuedlossof

foundationmaterialresultsinfailureofadditionalslabsupstreamandadditionalfoundationexposure

anderosion. Erosionprogressestotheupmostslab,resultinginallspillwayslabshavingbeendisplaced

anderodingdowntobasaltresultinginuncontrolledrelease. Figure3isasketchofthisfailuremode.

5.2.2FactorsthatIncreasetheLikelihoodofthisPFM Spillwaywasextendedduetopoorfoundationconditions(talus)


20/88

Figure

3.

Sp

illway

Downstream

Sill

an

d

St

illi

ng

Bas

in

Scour

Fa

ilure

Mo

de


21/88

8

Constructionfinalreportdocumentsdifficultywithrockanchorinstallationduetopoorrockqualityandsaturatedfoundation(waterinanchorholes).

Oct.1975inspectionmentionsscouratthesill. Visualobservationduringthesitevisitshowedturbulence/flowdisturbanceontheleftsideofthe

spillwayatthetoe.

Foundationbedrock

was

described

as

poor

to

fair

quality.

Originalinspectionmanualindicatedconcernabouterosionofthestillingbasinandrecommends

frequentinspectionsandmonitoringforerosion. (MaximumhistoricalpoolofElev.430correspondstoflowofapproximately5,000cfs).

Spillwayflowsoccurapproximately9monthsoutoftheyear. Poolwasverycalmimmediatelydownstreamofendofspillwaythenturbulentdownstreamof

calmarea(possibledeeppool). Thismaybeindicativeofapossibledeeppoolcreatedbyscour.

5.2.3RiskReductionMeasures Visuallymonitorforscourandsurveythedownstreamsillandstillingbasin. Armorstillingbasin. Anchorthespillwayslabs. Fillvoids/scourholeswithconcrete(holesaroundandunder).

5.2.4WarningTime Warningopportunitytimeis0ornegative. Itcanbeseenafteraslabhasfailed(maybe). Breachformationtimefornormalpoolsmonths;andforhighflowsgreaterthan100yrs

days,maybehours.

5.3Scourofupstreamspillwaysill(PFMFinAppendixB).5.3.1DescriptionScourpresentontheupstreamsillofthespillwaypermitsadditionalflowunderthespillway. Theresultingflowovertaxesthespillwaydrainsystemcreatingfullupliftonthespillwayandslabs. Increasedupliftandcyclesofloadingdisplacespillwayslabs(byoffset,settlement,anchoragefailure,etc.)resultinginexposedhighlyerodibletalus/landslidefoundation. Erosionprogressesthroughcontinuedlossofslabsupportandscour. Scourprogressesupstreamtofullpoolresultinginuncontrolledreleasedowntobedrock(approximatelyEl.375). Figure4isasketchofthisfailuremode.5.3.2FactorsthatIncreasetheLikelihoodofthisPFM

Constructionfinalreportdocumentsdifficultywithrockanchorinstallationduetopoorrockqualityandsaturatedfoundation(waterinanchorholes).

Spillwayflowsoccurapproximately9monthsoutoftheyear. Upstreamimperviousblankethashistoricallyerodedandbeenrepairedwithpitrunrockwhich

doesnotprovidethesameseepagecutoffthattheoriginalblanketwouldprovide.


22/88

Figure

4.

Upstre

am

Sp

illway

Sill

Scour

Fa

ilure

Mod

e.


23/88

9

Eachrepaircycleindicatesremovalofimperviousblanket,resultinginreducedscourbutincreasedflowbeneaththespillway.

5.3.3RiskReductionMeasures Ensurespillwaydrainsareeffective. Ensureupstreamclayblanketremainsimpervious. Anchorslabsifneeded. Addpiezometeradjacenttothespillwayasanindicatorofhydraulicpressureunderthespillway

slabs. Calculateacceptableupliftpressurethresholdatwhichslabsbecomedislodgedand

monitorforthesepressures.

5.3.4WarningTime Warningopportunitytimeis0ornegative. Itcanbeseenafteraslabhasfailed(maybe). Breachformationtimefornormalpoolsmonths,andforhighflowsgreaterthan100yrs

days,maybehours.

5.4 Overtopping(PFMJ,KandNinAppendixB).5.4.1DescriptionPoolrisesabovetheEl.435(topoftheclaycore),waveactioninitiateserosionofsandy/gravelinthe

embankmentupstreamshellandcrest. Materialsloughingupstreamanddownstreamdecreasesthe

crestwidth/seepagepathleadingtoovertoppingandcontinuedovertoppingleadstodowncuttingof

theembankment. Breachoccurs,downcuttingtothevalleyfloorleadingtouncontrolledreleaseand

downstreamconsequences.

5.4.2FactorsthatIncreasetheLikelihoodofthisPFM CoreonlydesignedtoEl.435. Historicaldebrisblockagecouldraisethepool. Insufficientfreeboard ProjectcannotpassthePMF(accordingtoPMFestimatedin1981USACEReport) Flashypool(i.e.poolrisesquicklywithraineventsduetoasmalldrainagebasin). Actualcrestelevationvariesandisunknownduetoalackofsurveydata. Truecrestofdamcould

belowerresultinginmorefrequentovertoppingatalowerthanpredictedlevel.

LowspotoccursatrightabutmentatEl.438wouldpermitovertoppingatlowerelevationsthanPMForrestofproject.

Insufficientdrawdowncapacity Landslideproneareasinthebasin(surge)

5.4.3RiskReductionMeasures Raiselowspots Armortheupstreamface


24/88

10

Armorthedownstreamface

5.4.4WarningTime Warningopportunitytimeishoursbutcouldbeincreasedwithanearly/floodwarningsystem.. Breachformationtimeisapproximately2hours.

5.5 RightAbutmentSeepageSeepagethroughtherightabutmentlandslidedepositisconsideredanimportantissuebutfailure

modesassociatedwiththisconditionwerenotconsideredascriticalasthesignificantfailuremodes

identifiedabove. Thereservoirhasgenerallyfilleduptothespillwaycrest(El424ft)eachyear.When

observed,seepagehasbeenclearwithnoevidenceofpipinginitiating. Also,infrequentobservationsat

higherpoolsabovespillway(poolofrecordis430feet),doesnotprovideanyevidenceof

turbidity/cloudyseepagetosuggestthatpipingmayhaveinitiatedoroccurred. Thereisnoreasonto

believethatgoingfrompoolofrecordofEl430toa100yearpoolofEl431(oneadditionalfoothigher

or2percentincreaseindepthofwaterdowntothebaseoftheembankment)wouldchangethe

hydraulicgradientsufficientlytoinitiatepiping. APMFeventwouldraisepool10feetabovepoolof

record(20percenthigher). A20percentincreaseseepagegradientwasthoughtnottobesufficientto

initiatepipingoftheperviouslandslidedebris. However,itisimportanttocontinuemonitoringseepage

todetectanyincreasesthatmayindicateapotentialdevelopingissueofconcern.

6.0APPLICABILITYOFANEARLYWARNINGSYSTEMTOSIGNIFICANTFAILUREMODESThetermearlywarningsystemcanbeinterpretedtoencompassavarietyofsystemsthatcould

potentiallybebeneficialforreducingriskatthisproject. Inaverysimpleform,anearlywarningsystem

couldsimplybeafloodwarningsystemthatutilizesrainfalland/orreservoirwaterleveldatato

providenotificationofelevatingpoolconditions. Inthissimpleform,thesystemwouldbeprettybasic

andhaveonlythatfunction. Inmoreadvancedform,thesystemcouldmonitordatafromavarietyof

instruments,comparethatdatatopredeterminedalgorithmsorthresholdsbasedonseveraldifferent

failuremodesandprovidenotificationwhencertainconditionsaremet. Thistypeofsystemmightbe

referredtoasanearlydamfailurewarningsystem. Forthepurposesofthisreport,itisassumedthat

theCitydesirestoinstallasystemconsistentwiththelatterdescription.

For

failure

modes

that

have

parameters

that

are

directly

related

to

the

initiation

and

further

developmentofthosefailuremodesthatcanbemonitoredthroughautomation,itispossiblethata

systemcouldbedesignedtoprovideearlydetectionandnotificationofthesemodesoffailure. The

effectivenessofthissystemwouldberatedbasedonthedegreetowhichtheinitiationand

developmentofthemostsignificantfailuremodes(i.e.thosethathavethehighestlikelihoodofcausing

significantdownstreamconsequences)canbedetectedbytheautomatedsysteminsufficienttimeto

bothnotifydecisionmakersandtoallowtimeforinterventionand/orevacuation. Clearly,then,the


25/88

11

effectivenessofthesystematagivenprojectwilldependuponthenatureofthemostsignificantfailure

modesatthatproject.

ThesignificantfailuremodesforSilverCreekDam,basedonthePFMAworkshop,weresummarizedin

Section5above. Ofthesignificantpotentialfailuremodesidentified,anearlydamfailurewarning

systemwouldbemosteffectiveatprovidingwarningfortheovertoppingfailuremodedescribedin

Section5.4. Thesystemcouldutilizecriteriasuchasreservoirlevelortherateofriseofthepoollevel

(orcombinationofthetwo)toprovideearlydetectionofovertoppingconcerns. Thesystemcould

providenotificationatvariousthresholdstoindicatehowsignificanttheconditionsare. Animportant

elevationforovertoppingconcernsisElev.435,thetopoftheimperviouscoresectionofthedam. The

coresectionisthelowpermeabilityzonewithinthedamthatprovidestheabilityofthedamto

impoundwater. Increasedseepagethroughtheembankmentfillabovetheimperviouscorezonewould

beexpectedtooccurforpoollevelsaboveEl.435sincethepoolisabovethetopofthecore. In

addition,waveactionatthesehighpoollevelsmayerodethecrestofthedamresultinginpotential

overtoppingatelevationslessthanthecurrentcrestofthedam. Section7includesadditional

considerationsandrecommendationsregardingovertopping.

Theothercredibleandsignificantfailuremodesarerelatedtorapiderosionofthefoundationmaterial

beneaththespillwayduetothelossofspillwayslabs(seeSections5.1through5.3). Themosteffective

meansofdetectingadevelopingconditionforthesefailuremodesistoregularlyinspectforoffsets

betweenadjacentslabjointsandforscourinthestillingbasinandontheupstreamsill. Themostcritical

offsetisthedownstreamslabupliftedrelativetheupstreamslabwhichcreateslocalizedhighpressures

thatmaypenetratedownalongtheslabjointandcreatehighupliftpressurebeneaththeslab. Forthe

upstreamsillscourfailuremode,describedinSection5.3,itisanticipatedthatprogressionofthisfailure

modewouldresultinincreasedseepageinthehorizontaldrainsinstalledwithinthelandslidedeposit. A

sustained,

gradual

increase

in

seepage

during

spilling

that

is

not

related

to

rainfall

would

be

expected

to

occur. Theearlywarningsystemshouldincludeautomatedcollectionofdraindatafromthedrains

(eitherindividuallyorsomedrainscanbecombined). Thiswillrequireconstructionofweirstocollect

theflow. TheCitycouldalsoconsiderinstallinganewshallowpiezometertomonitordevelopmentof

thisfailuremode. Thepiezometercouldbeinstalledtotheleftoftheleftspillwaywalldownstreamof

thefilterzonetoadepthofapproximately10feetbelowthebaseofthespillwaywithapressuresensor

forautomatedmonitoring. Increasedupliftpressurebeneaththespillwayshouldresultinincreased

piezometriclevelinthispiezometerduetohydraulicconnectivityoflandslidematerialbeneaththe

spillwayandthatscreenedbythepiezometer.Whiletheweirsandpiezometermightprovideindication

ofprogressionofthisfailuremode,visualinspectionoftheupstreamsillandclayblanketisamore

directandeffectivemeansfordetectingiferosionisoccurring. Also,itisimportanttonotethatthe

automatedsystemwouldprobablynotbeabletoprovideearlydetectionandwarningfortheslab

jacking(Section5.1)andstillingbasinanddownstreamsillscourfailuremodes(Section5.2).

Inadditiontothebenefitsdescribedabove,anautomatedwarningsystemthatincludesdatacollection

fromtheexistingpiezometersandnewdrainagecollectionweirsnearthetoeofthedamwouldprovide

benefitintermsofincreasingthefrequencyofmeasurementsfromtheseinstruments. Althoughthe

existingpiezometerswerenottiedtosignificantfailuremodes,theyaretiedtocrediblefailuremodes


26/88

12

thattheteamidentified. Traditionally,suchmeasurementshavebeentakenmanuallyonceperyearattheproject,althoughtherehavebeengapsoftimewhereseveralyearshavegonebywithouttakingmeasurements. Piezometricwaterlevelsanddrainflowratesareimportantindicatorsofdamperformance. Theseinstrumentsshouldbemonitoredconsistentlyatleastoncepermonth(withautomation,higherfrequencyiseasilyobtained)tounderstandseasonalfluctuationsandlongtermtrends. Withpropermaintenance,theautomatedsystemwillconsistentlycollectthedata,improvingtheunderstandingofthebehaviorofthedamandreducingefforttoreviewthehistoricaldata.Adustmentstothelowleveloutletgatesaremadeusingmanualcontrolslocatedatthecrestofthedamadjacenttotherightspillwaytrainingwall. Currently,operationscannotverifythelowleveloutletgatepositionwhenadjustmentsaremade. Theoutletisnotvisiblefromthislocationinordertoconfirmthattheactualgateadjustmentsweremade. Thewarningsystemshouldcollectdataforflowoutofthelowleveloutletsothatconfirmationofgateadjustmentscanbeobtained.AnautomatedwarningsystemcouldalsobeusedtonotifyCitystaffwhenimportantthresholdwaterlevelsdownstreamofthedamarereached. Althoughthisnotificationmaynotbetiedtofailureofthedam,itmaybeusefulfortheCitytoknowwhendownstreamwaterlevelsreachcertainelevationstohelpcoordinatefloodresponse. Thesystemcouldmonitordownstreamwaterlevelsatvariouslocationsandcouldbeprogrammedtoprovidenotificationatmultipleelevationstoindicatehowseveredownstreamfloodingconditionsarebecoming.

7.0CONCLUSIONSANDRECOMMENDATIONS7.1ConclusionsAlistofmajorfindingsandunderstandingsfromthePFMAworkshopisprovidedinSection4. Thissectionprovidesconclusionsspecificallyrelatedtotheproposedearlywarningsystem.1) ThePFMAworkshopresultedintheidentificationof4significantandcrediblepotentialfailure

modes. Threeofthe4modeswererelatedtoerosionofthespillwayfoundationduetoeitherslabjacking,erosionofthestillingbasinorerosionoftheupstreamsillofthespillway. Thefourthmodeisovertoppingofthedam.

2) Failuremodesrelatedtoseepagethroughtherightabutmentlandslidematerialareconsideredcrediblebutnotassignificantasthe4modesidentifiedabove. Itwasjudgedthatevenunderhighpoolconditions,suchasElev.440(PMFconditions),therelativeincreaseinhydraulicgradientcomparedtowhattheprojecthasexperiencedisnotenoughtoinitiatepipinginthegenerallypervious,angular,basaltfragmentsthatcomprisethelandslidedeposit. However,itisimportanttocontinuemonitoringseepagetodetectanyincreasesthatmayindicateapotentialdevelopingissueofconcern.

3) Withrespecttothesignificantandcrediblefailuremodes,anautomatedearlywarningsystemwouldbemosteffectiveatreducingriskassociatedwiththeovertoppingfailuremode.


27/88

13

4) Forovertoppingfailuremodes,itisanticipatedthat,ifproperlydesignedandmaintained,anautomatedearlywarningsystemmaybeabletoprovideearlywarningtodecisionmakersintimeto

reducetheriskassociatedwithovertoppingfailuremodes.

5) Initiationandprogressionofthefailuremodesassociatedwitherosionofthespillwayfoundationwouldbedifficulttomonitorwithanautomatedmonitoringsystemandthereforedifficultto

provideadvancedwarningoffailurebyanearlywarningsystem.Monitoringhorizontaldraindata

anddatafromanewpiezometerinstalledtotheleftoftheleftspillwaywallanddownstreamofthe

embankmentfilterzonecouldbeusedtomonitordevelopmentofthefailuremodeassociatedwith

upstreamsillscourandincreasedseepagethroughtherightabutment. However,visualmonitoring

forspillwayslaboffsetsandstillingbasinandupstreamsillerosionaremoredirectwaystoidentify

whetherthesefailuremodesareinitiating.

6) Forfailuremodesassociatedwithspillwayfoundationerosion,itisanticipatedthatthetimefrominitiationtouncontrolledreleaseofthepoolcouldbemonthsifthefailureoccurredundernormal

poolconditionsbutdays(possiblyhours)underhighflood(suchasa100yearevent). Duringhighflows,unlessinitiationisobserveddirectlybyCitystaff,minimalwarningwillbeprovidedtothedownstreampopulation forspillwayfailureinitiatedfailuremodesregardlessofwhetherawarningsystemisinstalled.

7) Ifproperlydesignedandmaintained,theautomateddatacollectionfunctionoftheproposedearlywarningsystemwillprovideabenefitofincreasedconsistencyandqualityofdata(including

piezometeranddraindata)collectedbythesystem. Thisdatahelpstoassesstheconditionofthe

damandimprovesunderstandingforhowthedamrespondsundervariouspoolandseasonal

conditions.

8) TheautomateddatacollectionfunctioncouldalsobeusedtoprovideconfirmationofflowsthroughthelowleveloutletandfrequentupdatesontheelevationofSilverCreekDamdownstreamofthe

damtoimprovecoordinationoffloodresponseactivitiesassociatedwithhydrologiceventsthatare

notrelatedtodamfailure.

7.2Recommendations7.2.1AutomatedDataCollectionandEarlyWarningSystemRecommendations1) Theautomatedearlywarningsystemshouldincludeareservoirlevelmeasurementandraingage.

Multiplelevelthresholdsforreservoirlevelandraindatashouldbedefinedaspartofthedesignof

theearlywarningsystem. Ataminimum,thethresholdsshouldconsidertheelevationsofthetop

ofthecore(El.435)andthelowspottotherightoftherightabutmentwall(approx.El.438). In

addition,duetothegapinthecoretrenchontherightabutmentbelowthetrainingwall,theCity

shouldanticipatethepossibilityofincreasedseepagewithpoolsaboveElev.430andmonitorthis

location. Thesystemshouldprovidenotificationoftheexceedanceofthethresholdstoappropriate

decisionmakingpersonnelattheCity. Inaddition,itshouldcollectandstoredataforfuture

referenceandanalysis.


28/88

14

2) TheCityshoulddesignandinstallweircollectionboxestomeasuretheflowrateofknownseepsanddrainflows,especiallyflowsassociatedwiththetoedrains. Changesintheseflowratesmay

indicatechangingconditionsatthedam.

3) Intheeventthatdrainflowsshowsustainedincreasesinflow,considerinstallationofanewpiezometertotheleftoftheleftspillwaywallforthepurposesofmonitoringpiezometricpressure

withinthelandslidedepositthatprovidesthefoundationforthespillway. Thewaterlevelinthis

piezometercouldbemonitoredwithanautomateddatacollectionsystemwiththresholdsassigned

toprovidenotification.

4) Thereliabilityoftheexistingpiezometersshouldbeevaluatedaspartofthedesignofthewarningsystem.

5) Forthepurposesofhistoricaldatacollection,theautomatedwarningsystemshouldbedesignedtocollectdatafromthepiezometersandweirstoimprovetheconsistencyofdatacollectionand

review.

6) Theautomatedsystemshouldsupportstorageofhistoricaldataforthepurposesofreviewinghistoricaltrendsandprovidingreportswhenneeded. Timehistoryplotsofdatashouldbereviewed

atleastmonthlybytheCityandmorefrequentlyifvisualobservationsindicatepotentialdeveloping

issuesofconcern.

7) ForthepurposesofimprovingfloodresponsebytheCity,theautomatedwarningsystemshouldbedesignedtocollectdataforwaterlevelatdownstreamlocationsasdeterminedbytheCity.

Multiplelevelthresholdscanbedevelopedtoprovideanassessmentoflevelofurgency.

8) TheCitysEmergencyActionPlanshouldbeupdatedtoincorporatetheearlywarningsystemandtoaddressnotificationofallcitizensimpactedbyflooding.

7.2.2Additional

Recommendations

to

Improve

Dam

Safety

and

Reliability

1) Performasurveyofthedamalongthecrestofthedamfromleftabutmenttorightabutmentin

ordertodeterminethepresenceoflowpointsonthedam. Iftherearelowspots,theyshouldbe

raisedtothedesignelevation.

2) Performsurveyofspillwayandabutmentwallsforverticalandhorizontaldeflectioninordertoestablishabaselineforcomparisonwithfuturesurveys. Additionalsurveysshouldbecompletedat

leastevery5years. Ifsurveysindicatemovement,theresultsofthesurveysshouldbereviewedby

astructuralengineerforevaluation.

3) Inspectandsurveythestillingbasinforscourduringperiodwhenspillingisnotoccurring(summerlow

pool

inspection)

in

order

to

establish

a

baseline

for

comparison

with

future

surveys.

Inspect

the

stillingbasinforincreasedscourafterlargeflowevents. Performtheinspectionsandsurveysat

leastevery5years.

4) Inspectthespillwayforverticaloffsetsbetweenslabsduringperiodwhenspillingisnotoccurring(summerlowpoolinspection). Themostcriticaloffsetisthedownstreamslabupliftedrelativeto

theupstreamslab;thiscreateslocalizedhighpressuresthatmaypenetratedownalongtheslab

jointandcreatehighupliftpressure. Ifdownstreamverticaloffsetsarepresent,grindthe


29/88

15

downstreamslabdowntoremoveoffsets. Recommendvisualinspectionsarecompletedatleast

onceperyear.

5) Visuallyinspectupstreamimperviousblanketforholesforitsfulllengthupstream(notjusttheupstreamsill)whenpossible. Visuallyinspectsillforerosionatleastonceperyearorafterlargespill

events.

6) InspecttheintegrityofthelowleveloutletpipeusingaRemotelyOperatedVehicle(ROV).7) Evaluateandimplementoptionsforincreasingthereliabilityofthelowleveloutlethydraulicsystem

andforconfirmationofgateopeningsize. Tothisend,thewarningsystemshouldincludecollection

ofdataontheflowthroughthelowleveloutlettoimprovetheabilitytoconfirmgateopeningsize.

8) TheCitysnewpublicnotificationsystem(Everbridge)willcallallCityofSilvertontelephonelandlinesbutdoesnotincludetheentirecontactlistfromtheprovideroftheoldpublicnotification

system(CodeRed). RecommendevaluatingwaystoupdatetheEverbridgesystemcontactlistforall

cellularphoneuserstotheextentpossible.

9) Performhydraulicmodelingtodeterminethefrequencyandmagnitudeandroutingofstormevents(determinerelationshipsbetweenfloodfrequency,peakflowanddownstreamwaterelevations).

TheresultsofthemodelingwouldhelptheCitywithdecisionsregardingbalancingdownstream

floodingimpactswithdamsafetyrisksandtoimprovetheCitysabilitytowarnthepopulation.

Oncetheresultsofthemodelingarecomplete,itmaybeappropriatetomodifytheoperationsplan

fortheproject.

10)TheCityshouldconsidermakingstructuralimprovementstopreventfloodingofresidentsatlowelevationlocations(suchasSilverGardensRetirementHome).

11)Theincreasedwoodydebrisbuildingupdownstreamofthestillingbasincreatesapotentialfordownstreamdamageduringfuturestormevents. Duringlargeevents,thismaterialmaybecome

dislodgedandcausedamagetobridgesandproperty. TheCityshouldconsiderremovingthis

materialevery5years.


30/88

SILVERCREEKDAMPOTENTIALFAILUREMODESANALYSIS(PFMA)REPORT

APPENDIXAREFERENCES


31/88

A1

HistoricalDocuments DesignMemoranda/Analysis

DesignLettertoStateEngineer,CH2M,28March1973,(FILE241D4,towardsback) ProjectInformationlettertoSilvertonMayor,CH2MHill,10April1973,(FILE241D4,

towardsback)thisincludesinfoontheoutlets,suggestedO&M,fishfacilities.

DetailedProjectReportSection205SilverCreekDamEarlyWarningSystem,CorpsofEngineers,August2004.

SilverCreekDam,Section205PreliminaryAssessment,CorpsofEngineers,DraftMaterial,8July2003.

SilverCreekDamBreakAnalysis,FinalReport,PhilipWilliams&Associates,Ltd.,January18,2000.(Haveentireprojectfile).

SilverCreekDamEarlyWarningSystemPreliminaryDesignReport,SquierAssociates,April2002.

ConstructionControlData FinalConstructionReport,SilverCreekDam,CH2MHill,13January1975.(redfile)

TripReportsandFieldInspectionReports Inspectiontripreport,CH2MHill,26August1975. (FILE241D4) Setof16OregonWaterResourceDepartmentInspectionReportsbetween1981and2010.

AsBuiltDrawings Specifications

Projectplansandspecifications,March1973. ConstructionPhotographs

Aerialphotoduringconstruction(File#319). Geotechnical/Geological/GeophysicalInvestigations

AnEngineeringReportonSubsurfaceExplorationandPreliminaryDesignAnalysisfortheConstructionofaDamonSilverCreek,CH2M,May1966.(Thisinvestigationappearstobe

atalocationdownstreamoftheactualdamsite).

SubsurfaceInvestigationofaDamSiteonSilverCreek,CH2MHill,April1973.(Includesgeologicmapping,boringlogsandsoilandrockdescriptionsatthecurrentdamsite(B)and

ataproposed damsitedownstreamofthecurrentlocationlocationA).

BoringLogs(includedinabovereports)

LivingDocuments OperationandMaintenance(O&M)Manual

SuggestedInspection,OperationandMaintenanceofSilverCreekDam,nodate,signedskw.

EmergencyActionPlan(EAP)

WaterControlManual(WCM) SuggestedInspection,OperationandMaintenanceofSilverCreekDam,nodate,signed

skw.

ISO90007.11Damoperatingprocedures,includeswaterdemandsandinspectionchecklist


32/88

A2

PeriodicInspectionandContinuingEvaluation PeriodicInspections

SilverCreekDam,Phase1InspectionReport,NationalDamSafetyProgram, OregonWaterResourcesDepartment,June1981.

IntermediateInspectionso AnnualInspectionseriesofStateDamSafetyinspectionso Incidents

Damseepagememo,CityofSilverton,Feb2010. Previouslyundetectedseepagewasencounteredneardownstreamoutletstructure,aseepagemonitoringpointwas

constructed,photosareincluded.

InstrumentationData SilverCreekDamPiezometerInstallationContractorCH2MHill,11978,File#319.(File

containsspecsanddesigndetailsfornewpiezometersappearstobeforpiezosP3through

P10,includessomemanualplotsandtablesofpiezoanddraindataforfirstcoupleyears

followingconstruction).

Spreadsheetsandtablesofpiezoanddraindatafor1975through2007.

EvaluationandModificationReports SeismicSafetyEvaluation

SeismicStabilityAnalysis,SilverCreekDam,CornforthConsultants,21July1999. DamModificationReports

Descriptionandconstructionphotosofintakerepairperformedin1993poolwaslowered anddescriptionandphotosofadditionalrepairsin2002.

Filefolderwithcorrespondenceandsomesketchesofrepairworkperformedasaresultof1996flood.

PlansandSpecificationsforConcreteRepairsproject,July9,2009. OperationandMaintenance(O&M)Program

SuggestedInspection,OperationandMaintenanceofSilverCreekDam,nodate,signedskw.

OtherDocuments FloodInsuranceStudy,U.S.DepartmentofHousingandUrbanDevelopment,FederalInsurance

Administration,September1978(includesfloodprofilesdownstreamfor10year,50year,100

yearand500yearfloods).

Seriesofuntitledplansthatappeartobeinundationmaps. Currentratingcurve(No.9) forSilverCreekatdownstreamgagingstation. Ratingcurveandflowdataforconditionspriortodam. Setofplandrawingsassociatedwithgettingpermitsforthedam.


33/88

SILVERCREEKDAMPOTENTIALFAILUREMODESANALYSIS(PFMA)REPORT

APPENDIXBPFMANOTES


34/88

B1

PHYSICALDATA

TopofDam: 440(439.4) TopofSpillwayCrest: 423.67onnorthendcurbwith424toSouth. Outletinvert: 376.8 TopofCore: 435.0 FishLadderPipeInvert: 421.0 TopofFilter: 430 Crestwidth=20ft Basemax=340ft Elev.OfBaseofDamisapprox.370ft Maxheightapprox.70ft. Embankment490ftlong,spillway160ftlong Outletexitinvert: 371.0(from376.8) Filterzoneis4ftwide Coreminimum20feetwide Coretrenchis24ftwidetoElev370

GEOTECH

Core4inminussandy,clayeysilt(ML)lowplasticity Zone1upstreamanddownstreamsilty,sandygravel(alluviumfromriverbottomreservoir

borrow)

Zone2downstreamsiltyrockfragments Structuralbackfill3in.minusrock Drainageberm3in.minuscrushedrock Filterzonewellgraded,4ftwide,3in.minus,lessthan4%fines North(Right)AbutmentrepresentedbyB13

SiltDeposit(Landslide)ontop Top25ftextremelypermeable(fractured,loose,open,4in.angular,leakedonfirst

fillingatspillway)

Siltytalusontopoffracturedbasalt BedrockisatElev.375(80ftdown)

South(Left)AbutmentrepresentedbyB12andB16(250ftdownstream) Plasticsilt,hasclaytoelev.410 Sitsonfracturedbasalt B16showsbouldersandgravelsdownstream

Bedrock/FoundationRock Northlandslide,porous Spillwaysameasnorthabutment SouthinterflowzoneatElev.398(2basaltflows)

LiquefactionPotentiallow(maybesomesmallisolatedzones)


35/88

B2

Coretrenchmaterialwascompactedinthewet(hadtodewaterduringcompaction) OBE=0.0875g MCE=0.4g

HYDRAULICSANDHYDROLOGY

Pools

Normalpool424to424.5

WinterOctApr424to427

AugustandSeptember440,PMF>440

Whatistailwater?? ********Datagap********

Flows

NormalQinriver: 50cfs@El424(leakagethroughfishladder)

Maxpool:200cfs@El424(leakagethroughfishladderandspillway)

[email protected],tailwaterisaboutEl.231.

PMF22,682cfsinflow,22650release

Floodstage/bankfull: El.228.2,4milesdownstream

Gagestation@WebStreettakescontinuousreadingsandsendsanautoemailalertwhentailwateris

within4ftofEl.228.2

WhogetsemailAndygetsit,whenitsrainingPolicedoacheckofthis.

Currentlynoflowpredictionsatthisprojectlookforrain.


36/88

B3

Thegageisatthelowspot,thisareacouldbebuiltupwithawall.

NextlowspotisaroundEl.235

SandbaggingisdoneatSilverGardensFamilyCare(El.227)

Highschoolisatsameelevation.

[email protected]

Traveltimetooutofbankforpipingfailurefrombreachanalysisis52min.

Overtopping21minutestoleavebank@JamesStreet

Flowintownfor21hoursminimum(overtopping)

TreatmentPlantismuchfurtherdownstreambutatElev.204. Nolossofatthissite(propertydamage).

Pool429isapprox.equaltobankfullattailwaterof227.

Everyyeariftheybuilda3tallsandbagbermthereisnootherflooding.

*Conclusion232isfloodtailwaterofconcern.

OPERATIONS

42in.gateneverused,cycledonceperyear. Lasttimeoperated(Winter2009)hadtoreplacehydraulic

cylinderinthecontrolbox. Ittook1weektohavethisrepaired. Thisistheonlywaytodewater. Inthe

winterthiswillnotdrawdownthelake. IntheSummer,ittakesabout1daytodrawdownthelake.

18in.gateisaregulatingline,adjustedforQ.

INTERVENTION

******OPISSUE*****Accessonlyaccesstoembankmentsideisbyboat(orhelicopter). Therefore,

thereisnoabilitytoplaceemergencymaterials. Therefore,cantstopapipeorbreach.

Nointerventioncurrentlypossible.

Currentlyittakes20to30minutesforacrewpersontogettothesite;ittakes10minutestogetthere

fromtheCitycenter


37/88

B4

ResponseAndycallskeypersonwithcellphoneorradiotoactivateEOCCitymanager(1),Police

chief(2),Policecaptain(3)andFireDepartment(4). Andywouldprovidearecommendationonaction

totake.

Ifbadenough,acoderedisgiven: automatedcallgoesouttopeopleonaCodeRedlist. TheCitypaysa

companythatmaintainstheredcodelist;theCityhasanewsystem(EverbridgeCallSystem)butCode

RedcompanywontprovidetheCitywithitslist. TheEverbridgesystemwillautomaticallycallall

landlinesinSilverton. However,Cityresidentswontbecontactedoncellularphonesunlesstheysigned

upfortheEverbridgecallsystem(eveniftheysigneduporiginallyfortheoldCodeRedsystem).

EvacuationroutesareintheEAPbuttherehasbeennopopulationeducationonevacuation(nopublic

meetings).

PERFORMANCEHISTORY

Firstfillingin1974,seepagewasobservedonNov.7,1974,whenthepoolwasat401.5,nextdaywater

wasseenflowingoutthepipe(4intoedrain). Downstreamwetareasbroadenedattherightabutment

throughthelandslidedepositmaterials;citycrewsinstalled3horizontalsurfacedrains. Seepagewas

believedtobeoccurringupstreamoftheclayblanketbutaclaylayerforcedflowintothefilterand

embankment.

Nov.21,1974toNov.29,1974,11horizontaldrainswereinstalledtomonitorseepageratesanda

downstreambermwasinstalled. Abilitytosolvetheproblemwasadverselyimpactedbynotbeingable

togetaccessupstreamofthedam.

August,1975 SeepagewasobservedbetweenStations3+00and3+60justabovethefilterand

buttresslayer;itwasrecommendedtoaddadditionalfilterrock. Deteriorationofclayblanketwasalso

observedtotherightofthespillwayentrancechannel. Weeklyobservationofblanketwas

recommended. Fillerosionholeatupstreamendofspillway. Placegravelfiltermaterialon

downstreamfaceofdam. Filteranddrainageblanketwasrecommendedandapparentlyinstalled. It

alsoappearsthata12to15ftdeep,approx.3footwidecutofftrenchwasinstalledupstreamoftheright

endoftheembankment.

Nopiezometerdatabetween1975and1979.

Piezometer#2droppedofffrom1975to1982. Thispiezometerisjusttotheleftofthespillwaywhere

thecutofftrenchwasinstalled.

Draindataisnottakenfrequentlyenoughtounderstandwhattheflowsarerespondingto.

Seepageobservedatrighthandside100ftdownstreamofstillingpool.


38/88

B5

June1981Inspection

Poollevelwas424.4;normalpool.

Nouncontrolledseepageobserved;seepageonlyobservedinthehorizontaldrains.

Phreaticsurfacehasbeenestablished;primarilycontrolledbyseepagethroughrightabutmenttalus.

Conclusionsafedam,blanketandcoretrenchworking.

Recommendationtoreaddrains.

1993ScottsMillsEarthquake

EarthquakewaslargerthanOBE(operatingbaseearthquake)

Sourcewas8milesfromproject.

Mw=5.6

Nodamage.

1993InletStructureRepair

2002InletGateRepair

Mostlymaintenancework,newhydrauliclineslocatedinaconcretetrench.

Between1981andFeb.2010noevidenceofseepagerelateddistress. Knownseepagequantitieshave

beenmeasuredapproximatelyannually;seepagehasoccurredwhereyouwouldanticipateit. No

evidenceofturbidseepage. Nosignsofslopemovement.

UpstreamandDownstreamFlowIssues

1. Lowspotsforflooding(SilverGardens)*****OPSISSUE*****closeproximity,awareness2. Flashyreservoir3. Landslidesupstreamofpoolindrainagebasincausesupstreamdamandsurge(littleorno

warning)contributestoovertoppingfailuremodes.


39/88

B6

BRAINSTORMINGPOTENTIALFAILUREMODES

Step1Identifyfeatures,locationsandissuesoftheprojectforPFMsbasedon background

informationandsitehistoryreviewandthesitevisit.

Step2Ofthefeaturesidentifiedinstep1,identifywhichofthesehavePFMsofconcernassociated

withthem.

Step3DoublecheckthatallstandardGeotechinitiationmechanismshavebeenaccountedfor.

Step4RankPFMsthatwearemostconcernedwithprovidingwarningsfor. Trytoaddressmost

significant.

DefinitionDocumentationreferstothearealeftofthespillwayastherightabutment. Weconsider

thisareaasembankment. Theabutment(onrock/slide)istotherightofthespillway.

Identificationofpotentialfailuremodesbasedonbrainstorming.

Spillway

1. Spillwayslabjacking(drainconcerns)normalflowconditions,erodiblematerial2. Spillwaysilldownstreamscouronleftsidenormalflowconditions,erodiblefoundationand

concreteirregularityatthelocation

3. Spillwaybasinscournormalflowconditions,nosurveydata,previousscour(isitonrock/seam?)

4. Spillwayupstreamsillscournormalflowconditions,happenedbefore,clayblanketmaynotbefullypresent,canleadtoupliftofspillwayslab

5. Spillwayrightupstreamwalljoint6. Spillwayrightdownstreamtrainingwalloffset/stabilitynormalflow,wallfalls,erosionand

headcutting

7. Spillwayleftdownstreamtrainingwalloffset/stability8. Spillwaytailwater/channelcapacityPMFconditions9. Spillwayfoundationseepage/stability/settlement(potentialfordifferentialsettlementdueto

differentfoundationupstreamanddownstream)combinewithPFMs1,2,5and6

10.DebrisblockagePMFconditions,havebumperbutnologboom;considerplacingboomupstreamofclayblanket.

RightAbutment

1. Surfacerunofferosionnormalflowconditions,noexitforwateronrightabutment,2. Abutmentseepage,Integrityofupstreamclayblanket,lowhydraulicgradient


40/88

B7

3. Inadequatetrainingofwateronrightabutment(upstreamofspillway)PMFflowconditions,abutmentscour.

4. Lowspotadjacenttorightabutmentwall(groundappearstobearound438(438.5). Scourathighpools.Shallowembedmentofwall,losewall,thiscouldleadtoreleaseatpoollevelatEl.

430*****OPSISSUE*******

5. Landslidepresenceandcreepmovement.6. Seismicstability/liquefactionCornforthreport7. Seepageatrightwall,nocore,goingthroughlandslidedepositwithrockfragments,lowhead,

lowgradient.

FishLadder

1. Spillwaydrainontherighthandsideoftheladder. Drainisapreferentialseepagepathwayalongsidethedrainthroughthegranularbackfillaroundthepipe;settlementhasbeen

observed. Normalflowconditions,slowheadcut,lotsofwarningtime.

2. Wallstabilitydifferentialmovementbetweenadjacentblocksobserved. Flowinladderislow.3. Ballmillingwithinthefishladderchambers(cobblesandbouldersobserved).LowQandvelocity

offlow.******OPISSUE******

4. Operabilityoffishladdergates.Ladderonly,nolifesafetyconsequences.*****OPISSUE*****

Outlets/Conduit/Controls

1. Reliabilityofhydrauliclinesandcontrols,includingsubmergedhydrauliccylinderonthegate.Happened

before

where

the

gate

breaks,

Normal

flow,

1

in

20

chance

of

failure,

no

drawdown

capabilities,stuckopen,drainpool,losedrinkingwatercapability,*******OPISSUE********

2. Seepageonbothsidesoftheoutlet,aroundseepagecollars.Normalflowconditions*********must address********

3. Conduitintegrityearthquakeloadingatjoints,internalerosion;foundedonbedrock,reinforced,castinplace,seepagecollars******OPISSUE*****

4. Hydrauliccapacityoftheoutlets,200cfsismaxdischarge(operationatPMFcouldcreateproblemsifoperatingduringPMF.ContributingfactortootherPFMs(notaPFMonits

own)*****OPISSUE*****

5. Inabilitytoknowwhatthegatepositionis.ContributingfactortootherPFMs(notaPFMonitsown)********OP

ISSUE********

6. MisoperationofoutletvalvePMFconditionsdoesnotaddconsequencestoovertoppinginPMF*****OPISSUE*****(notaseparatePFM)

7. Seismicinducedfailureoftheintaketowerfailureoftheintakedoesnotresultinovertoppingorreleasegreaterthandownstreamchannelcapacity.


41/88

B8

Embankment

1. StabilitystaticandEQ,1981reportand1999Cornforthreport,highfreeboard(notlikelytoloseduetodeformation)

2. Foundationseepageandpipingseemat398,expectseepagebutnotpipingduetobedrockfoundation,noevidenceofinstability

3. Corestability/overtopping heightofcoreabout5feetbelowcrest,analyzedinstability,testedfor40years.

4. Filterzonesize,capacity,stabilityinEQ;designedfilter,historicallyperformed5. Overtopping(corelessthancrest)Lackofconsistentsurveyonverticalsettlementpotential

lowspots.

6. Embankmentseepageandpiping(11drainsthatflow,3drains/berm/75trench/blanket/2workingpiezometers) burrowinganimals,treesandvegetation******OPSISSUE*****

LeftAbutment

1. Seepageandpiping,stabilityofinterflowzoneatElev.398;stable,shouldntfailduetoseepageandstability,hasplasticfines(samematerialascore).

2. Siltfoundationseepageandpiping,pipingatElev.398seamcausesinternalerosionofsilt,createsapipeinsiltanduncontrolledrelease

ReviewGeotechnicalofInitiationMechanisms

Ofthe28IMsforseepageandpiping,thefollowingIMsweredeemedapplicabletothisstructure(the

othersdonotapply).

IM1crossvalleydifferentialsettlementcredible,notsignificantbasedongoodperformance(no

evidence)andmostlikelytobecontributingtoseepageandpipingfailuremechanismwhichiscovered

elsewhere,uniformabutmentslopeoutlet#2.

IM14corefoundation/abutmentcontactatrightabutmentpoorcompactionresultsinpossible

seepageandpipingalongconduitOutlet#2PFM(coveredelsewhere).

IM17seepageadjacenttoconduitcoveredunderoutlet#2PFM

IM18featuresallowingseepageintoanonpressurizedconduitnotsignificantbecausemorelikelyto

seepalongconduits

IM19seepageadjacenttospillwayorabutmentwallallreadycoveredunderspillwayPFMs5,6,7

IM20seepageduetocrackorgapadjacenttospillwayorabutmentwallallreadycoveredinspillway

PFMs5,6,7


42/88

B9

IM22backwarderosionandpipingthroughacohesionlessfoundationnotcredible,havecoretrench

thatcutsitoff

IM27scouralongthecontactinbedrock

Note=: ThePFMsweregivenapreliminaryratingregardingcredibilityandsignificance. Thefollowing

abbreviationswereused: C&S=credibleandsignificant,C=credible,notsignificantandNC=not

credible. TheresultsoftheevaluationsarelistedbelowfortheidentifiedPFMs.

Spillway

1. (C&S)Spillwayslabjacking(drainconcerns)normalflowconditions,erodiblematerial2. (C&S)Spillwaysilldownstreamscouronleftsidenormalflowconditions,erodible

foundationandconcreteirregularityatthelocation

3. (C&S)Spillwaybasinscournormalflowconditions,nosurveydata,previousscour(isitonrock/seam?)

4. (C&S)Spillwayupstreamsillscournormalflowconditions,happenedbefore,clayblanketmaynotbefullypresent,canleadtoupliftofspillwayslab

5. (C&S)Spillwayrightupstreamwalljoint6. (C&S)Spillwayrightdownstreamtrainingwalloffset/stabilitynormalflow,wallfalls,

erosionandheadcutting

7. (C&S)Spillwayleftdownstreamtrainingwalloffset/stability8. (C&S)Spillwaytailwater/channelcapacityPMFconditions9. Spillwayfoundationseepage/stability/settlement(potentialfordifferentialsettlementdue

to

different

foundation

upstream

and

downstream)

combine

with

PFMs

1,2,5

and

6

10.(C&S)DebrisblockagePMFconditions,havebumperbutnologboom;considerplacingboomupstreamofclayblanket.

RightAbutment

1. (NC)Surfacerunofferosionnormalflowconditions,noexitforwateronrightabutment,2. (NC)Abutmentseepage,Integrityofupstreamclayblanket,lowhydraulicgradient3. (NC)Inadequatetrainingofwateronrightabutment(upstreamofspillway)PMFflow

conditions,abutmentscour.

4. (C&S)Lowspotadjacenttorightabutmentwall(groundappearstobearound438(438.5).Scourathighpools.Shallowembedmentofwall,losewall,thiscouldleadtoreleaseatpool

levelatEl.430*****OPSISSUE*******

5. (NC)Landslidepresenceandcreepmovement.6. (NC)Seismicstability/liquefactionCornforthreport7. (C)Seepageatrightwall,nocore,goingthroughlandslidedepositwithrockfragments,low

head,lowgradient.


43/88

B10

FishLadder

1. (C)Spillwaydrainontherighthandsideoftheladder. Drainisapreferentialseepagepathwayalongsidethedrainthroughthegranularbackfillaroundthepipe;settlementhasbeen

observed. Normalflowconditions,slowheadcut,lotsofwarningtime.

2. (C)Wallstabilitydifferentialmovementbetweenadjacentblocksobserved. Flowinladderislow.

3. (C)Ballmillingwithinthefishladderchambers(cobblesandbouldersobserved).LowQandvelocityofflow.******OPISSUE******

4. (C)Operabilityoffishladdergates.Ladderonly,nolifesafetyconsequences.*****OPISSUE*****

Outlets/Conduit/Controls

1. (C)Reliabilityofhydrauliclinesandcontrols,includingsubmergedhydrauliccylinderonthegate.Happenedbefore

Silver Creek Dam Potential Failure Modes Analysis (PFMA)

Documents

Transcript of Silver Creek Dam Potential Failure Modes Analysis (PFMA)