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DamBreakRiskAssessmentinBakerValley(ChileanPatagonia)

by

ElisabettaNatale

B.S.CivilandEnvironmentalEngineeringUniversitdiPavia,2007

SUBMITTEDTOTHEDEPARTMENTOFCIVILANDENVIRONMENTAL

ENGINEERINGINPARTIALFULFILLMENTOFTHEREQUIREMENTSFORTHEDEGREEOF

MASTEROFENGINEERINGINCIVILANDENVIRONMENTALENGINEERING

ATTHEMASSACHUSETTSINSTITUTEOFTECHNOLOGY

JUNE2009

2009ElisabettaNatale.Allrightsreserved.

TheauthorherebygrantstoMITpermissiontoreproduceandtodistributepubliclypaperandelectroniccopiesofthisthesisdocumentinwholeorinpartinanymediumnowknownorhereaftercreated.SignatureofAuthor:__________________________________________________________________________

DepartmentofCivilandEnvironmentalEngineeringMay8,2009

Certifiedby:__________________________________________________________________________

DaraEntekhabiProfessorofCivilandEnvironmentalEngineering

ThesisSupervisorAcceptedby:__________________________________________________________________________

DanieleVenezianoChairman,DepartmentalCommitteeforGraduateStudents

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DamBreakRiskAssessmentinBakerValley(ChileanPatagonia)

by

ElisabettaNatale

B.S.CivilandEnvironmentalEngineeringUniversitdiPavia,2007

SubmittedtotheDepartmentofCivilandEnvironmentalEngineering

onMay8,2009inPartialFulfilmentoftheRequirementsfortheDegreeofMasterofEngineeringinCivilandEnvironmentalEngineering

ABSTRACTAnhydroelectricprojectwasproposedbyHidroAysnCompanyintheAysnRegionofChileanPatagonia.Itconsistedoftheinstallationoffivehydroelectricpowerstations,twoonRioBakerandthreeonRioPascua,withanaverageannualenergyproductionof18,430GWH.TheEnvironmentalImpactAssessment(EIA)presentedbyHidroAysnlackedofadamfailureimpactassessment,crucialtopreventorminimizetheimpactofunexpectedfloodingcausedbyadamfailure.AdambreakriskassessmentwasperformedforthesocalledBaker2,aproposedconcretegravitationaldamlocatedontheRioBaker,upstreamthesmallcommunityofCaletaTortel,anareaofconcernforpotentialflooding.UsingORSACode,softwaredevelopedbyaresearchcollaborationbetweentheUniversityofPaviaandtheUniversityofRomeLaSapienza(Italy),acomputationalmodelwasperformedtosimulatethefloodwavepropagationassociatedwithadambreakfailurescenario.Theareassubjecttofloodingweremappedonthedigitalelevationmodel(DEM)ofthesurfacetopography.ThesisSupervisor:DaraEntekhabiTitle:ProfessorofCivilandEnvironmentalEngineering

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1. Project Overview __________________________________________________________- 5 - Background __________________________________________________________________- 5 -

1.1 Location ___________________________________________________________________- 5 - 1.2 Water Rights _______________________________________________________________- 6 - 1.3 Energy Demand_____________________________________________________________- 7 - 1.4 Proposed Hydroelectric Project________________________________________________- 7 -

2. Glacial-Lake Outburst Floods (GLOFs) _______________________________________- 8 - 2.1 GLOF Frequency in the Aysn Region of Chilean Patagonia ______________________- 10 -

3. Dam Failure Impact Assessment ____________________________________________- 12 - Guidelines __________________________________________________________________- 12 -

3.1 Dam Site Inspection ________________________________________________________- 13 - 3.2 Data Collection ____________________________________________________________- 13 -

3.2.1 General Information______________________________________________________________ - 13 - 3.2.2 Dam and storage information_______________________________________________________ - 13 - 3.2.3 Topographic information __________________________________________________________ - 14 - 3.2.4 Hydrographic data _______________________________________________________________ - 15 - 3.2.5 Hydrologic data _________________________________________________________________ - 15 - 3.2.6 Downstream community information ________________________________________________ - 15 - 3.2.7 Determination of Flood Prone Areas _________________________________________________ - 16 - 3.2.8 Population at risk ________________________________________________________________ - 16 -

3.3 Accuracy of population at risk calculations _____________________________________- 17 - 3.4 Analytical Techniques ______________________________________________________- 18 -

3.4.1 Two-dimensional flow analysis _____________________________________________________ - 18 - 3.4.2 Simplified assessment ____________________________________________________________ - 18 - 3.4.3 Comprehensive assessment ________________________________________________________ - 18 - 3.4.4 Two or more dams on the same watercourse ___________________________________________ - 19 - 3.4.5 Other failure events ______________________________________________________________ - 19 -

3.5 Periodic Re-Assessment of Failure Impact Rating _______________________________- 20 - 4. Case study ______________________________________________________________- 21 - The Proposed Hydroelectric Dam Baker 2_________________________________________- 21 -

4.1 Dam Site__________________________________________________________________- 25 - 4.1.1 Topographic Information __________________________________________________________ - 25 - 4.1.2 Topographic Editing using ArcGIS 9.3 _______________________________________________ - 26 -

Dam-Break Modeling _________________________________________________________- 26 - 4.2 Calibration________________________________________________________________- 26 -

4.2.1 Hydrologic Data Collection ________________________________________________________ - 26 - 4.3 Dam-Breach Analysis _______________________________________________________- 26 -

4.3.1 NWS Simplified Dam-break model __________________________________________________ - 27 - 4.3.2 Generalized Ritter Dam-Break Solution ______________________________________________ - 28 - 4.3.3 Estimated Peak Discharge at Dam Site _______________________________________________ - 29 -

5. Dam-Break Simulation ____________________________________________________- 30 - ORSA Code _________________________________________________________________- 30 -

5.1 Numerical Solvers __________________________________________________________- 30 - 5.2 Surface Topography Acquisition and Analysis __________________________________- 30 -

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5.2.1 Breakline ______________________________________________________________________ - 30 - 5.2.2 Sections _______________________________________________________________________ - 30 - 5.2.3 Visualization ___________________________________________________________________ - 31 - 5.2.4 Assignation of Mannings Coefficient ________________________________________________ - 31 -

GLOF Simulation ____________________________________________________________- 32 - 5.3 Initial Conditions __________________________________________________________- 32 - 5.4 Boundary Conditions _______________________________________________________- 32 -

5.4.1 Upstream B.C. __________________________________________________________________ - 33 - 5.4.2 Downstream B.C.________________________________________________________________ - 34 -

5.5 Simulation Characterization _________________________________________________- 34 - 5.6 Results ___________________________________________________________________- 34 -

Dam-Break Simulation ________________________________________________________- 35 - 5.7 Initial Conditions __________________________________________________________- 35 - 5.8 Boundary Conditions _______________________________________________________- 35 -

5.8.1 Upstream B.C. __________________________________________________________________ - 35 - 5.8.2 Downstream B.C.________________________________________________________________ - 37 -

5.9 Simulation Characterization _________________________________________________- 37 - 5.10 Results ___________________________________________________________________- 37 -

5.10.1 Dam-Break Wave Outflow ______________________________________________________ - 37 - 5.10.2 Water Level__________________________________________________________________ - 40 - 5.10.3 Flooded Width________________________________________________________________ - 40 - 5.10.4 ORSA Code Output____________________________________________________________ - 41 -

6. Literature Review: Dam-Break Modeling _____________________________________- 43 - Hydrodynamic Model: Shallow Water Equations ___________________________________- 43 -

6.1 Numerical Solvers __________________________________________________________- 45 - 6.1.1 Lax-Friedrichs Scheme ___________________________________________________________ - 45 - 6.1.2 MacCormacks Scheme ___________________________________________________________ - 46 - 6.1.3 TVD Methods __________________________________________________________________ - 47 - 6.1.4 Roes Method___________________________________________________________________ - 48 - 6.1.5 Lax-Friedrich Type Scheme applied on a staggered grid _________________________________ - 49 -

6.2 ONE-Dimensional Dam-Break Simulation Codes ________________________________- 49 - 6.2.1 SMPDBK Simplified Dam Break Model______________________________________________ - 49 - 6.2.2 DAMBRK _____________________________________________________________________ - 50 - 6.2.3 CASTOR ______________________________________________________________________ - 50 - 6.2.4 HEC-RAS _____________________________________________________________________ - 50 -

6.3 Comparisons of Dam-Break Simulation Codes __________________________________- 52 - 6.3.1 SMPDBK vs. DAMBRK __________________________________________________________ - 52 - 6.3.2 HEC-RAS vs. ORSA _____________________________________________________________ - 53 -

6.4 Case Studies of Dam Failure: comparison of breach predictions____________________- 54 - 6.4.1 Simulation of the Big Bay Dam failure _______________________________________________ - 54 - 6.4.2 Simulation of the St. Francis Dam failure _____________________________________________ - 55 -

References __________________________________________________________________- 56 -

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1. ProjectOverview

Background1

1.1 LocationChileisaSouthAmericancountryflankedbyArgentinaandBoliviatotheEast,PerutotheNorth,andthePacificOceantotheWest.Withatotalareaof757,000sqkm,ChileisapproximatelytwicethesizeofMontana.Becauseofitsuniquegeographyspanningfromapproximately20to56latitude,Chilesclimatevariesgreatly.Generallytemperate,theclimatecanbecharacterizedasdesertinthenorth,Mediterraneaninthecentralregion,andcoolanddampinthesouth(CIA,2008).Chilesterrainisasvariedasitsclimatewithacombinationoflowcoastalmountains,afertilecentralvalley,andtheruggedAndesintheeast.(Figure1)

Figure1MapofChile(source:www.southwindadventures.com)

1FromAnalysisofproposedhydroelectricdamsinChileanPatagonia,KristenBurrall,GiannaLeandro,LauraMar,ElisabettaNatale,FlaviaTauro,MIT,USA.

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The10millionhectareAysnRegionishometotheRioBakerandisChilesleastpopulousadministrativeregionwithapproximately0.9person/sqkm(personalcommunication).Withsuchalowpopulationdensity,theregionisaprimeexampleofpristinenaturalconditions.ThewatershedthatcontainstheRioBakerencompassesawidevarietyofhabitatsfrommountainglacierstoriverbraidedfloodplains,andinthesevariedphysicalhabitatsalargearrayofplantandanimalspeciesthrive(Strittholt,2008).TheRioBakerisa170kmriverwhichoriginatesinBertrandLakeandemptiesintothefjordsofthePacificOceannearthesmallcommunityofTortel.DuetotheremotelocationoftheRioBakerwatershedfewstudieshavebeenconductedastothepotentialimpactsofdaminstallationontheregionsbiologyandecology(Strittholt,2008).Accordingly,thispaperelucidatesthekeyimpactsofthedamsinanapproachthatcombinesvariousresearchobjectivesthroughasystematicevaluation.

1.2 WaterRightsCurrently,waterrightsinChileareprivatizedunderasystemofwaterlawsimilartothatoftheWesternUnitedStates(personalcommunication).EndesaSpain,oneofthelargestelectricalcompaniesintheworld,controlsmorethan80%ofthewaterrightsontheRioBakerand,withthisjurisdictionalfreedom,Endesaendeavorstoinstalltwohydroelectricdamsontheriver.Thereareahostofproponentsforandopponentsagainstthedams.ProponentsarguethatthedamswillhelpsecureChilesenergyindependence,createjobs,andprovidepowerfortheminingindustrythatsupportstheChileaneconomy,whereasopponentsarguethatthedamswillirreparablydestroynaturalhabitatswhilesimultaneouslyincreasingenergycostsforChileans(personalcommunication).

1.3 EnergyDemandElectricitydemandinChilecentersonSantiagoandthenationsminingindustries(personalcommunication).IncreasedGDPandtransportinChilehaveheightenedenergydemandthroughoutthecountryandespeciallyinthenorthernregions.Installingthedamswilladdressthisincreaseddemand.Thecombinedcapacityoftheprojectwilltotal2,750megawatts,approximately20%ofSantiagoscurrentenergydemand.ThiselectricitywillbetransportednorthwardfromtheAysnRegiontoSantiagovia2,240kmoftransmissionlines,whichrequirea92meterwideclearcutswaththrough14nationalparks(InternationalRivers,2008).InadditiontosupplyingSantiagoandindustrywithelectricity,thedamsareasteptowardincreasingChilesdomesticenergysecurity.

1.4 ProposedHydroelectricProjectHidroAysnisajointventurebetweenEndesaChile,aSpanishenergycompany,andColbunS.A.,aChileanelectricitygenerationcompany(HidroAysn,2007).The

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HidroAysnprojectconsistsoffivehydroelectricdamsontworivers:theRioBakerandtheRioPascua.Thecombinedcapacityofthedamsystemwouldgenerate,onaverage18,430GWhofpowerannually,theequivalentof30%ofthepowercurrentlyinstalledintheCentralInterconnectedSystem(SIC)grid.Powergeneratedbythedamsconnectthroughlocaltransmissionlines,longdistancehighvoltagetransmissionlineswillconnectthedams,whichareintheAysngridtotheSIC(HidroAysn,2008).Ifconstructed,thefivedamswillinundate5,910hectares;further,damconstructionandoperationwilldisturbanadditional11,000hectares.Alocalelectricitygridwillcombinetheoutputfromthefivedamsandwillconvertalternatingcurrent(AC)todirectcurrent(DC)forlongdistancetransmission(HidroAysn,2008).The2,240kmtransmissionlinewillrequirefivethousand50metertowersspaced400metersapartthrougha70metereasement(PatagoniawithoutDamsCampaign).Sincegeneration,transmissionanddistributionunderChilesGeneralLawofElectricalServices,TranselecwilldesignthehighvoltagetransmissionlineseparatelyfromtheHidroAysndams.CONAMA,ChilesequivalentoftheUnitedStatesEnvironmentalProtectionAgency,willevaluatetheenvironmentalimpactofthedamsandthetransmissionlinesseparately.Thetransmissionlinesareagreatsourceofcontroversy.OnequestionsurroundsthetotalcapacityoftheDCline:iftheircapacityisgreaterthanthe2750MWthattheHidroAysendamprojectwouldprovide,willotherhydroelectricdevelopmentsfollow?

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2. GlacialLakeOutburstFloods(GLOFs)

Therecentglobalclimaticchangehasbroughtatremendousimpactonthehighmountainousglacialenvironment.Manyofthebigglaciersmeltedrapidlyandgavebirthtotheoriginofalargenumberofglacierlakes.GLOFsarenotanewphenomenonbutwiththeworldwiderecedingofglaciersandrisingtemperature,theprobabilityoftheiroccurrenceshasdramaticallyrisen.

Asglaciersretreat,glaciallakesformbehindmoraineoricedams.Thesedamsarecomparativelyweakandcanbreachsuddenly,leadingtoadischargeofhugevolumesofwateranddebris.Suchoutburstshavethepotentialofreleasingmillionsofcubicmetersofwaterinafewhourscausingcatastrophicfloodingdownstream.

Alakeoutburstcanbetriggeredbyseveralfactors:iceorrockavalanches,thecollapseofthemorainedamduetothemeltingoficeburiedwithin,thewashingoutoffinematerialbyspringsflowingthroughthedam(piping),earthquakesorsuddeninputsofwaterintothelakee.g.throughheavyrainsordrainagefromlakesfurtherupglacier.

Manyresearchershavedemonstratedthatinfrequent,largemagnitudefloodsinmountainenvironmentsaresignificantgeomorphicagentsbecauseonlythesefloodsarecapableofgeneratingthehydraulicforcesnecessarytoentrainandtransportthelargematerialthatconstitutesthechannelandvalleybottomintheseregions.

ErosionalanddepositionalfeaturesproducedbyGLOFvarylongitudinallyalongthefloodroutesascomplexinteractionsbetweenfloodhydraulics,boundaryconditions,sedimentsupply,andthetemporalsequenceandmagnitudeofpastfloods.Figure2

Thelargescaleerosionalanddepositionalfeaturescreatedbyanextremefloodinmountainousenvironmentsaretypicallypersistentfeaturesthatremainunalteredbecausenormalfloodsarenotabletoreworkthecoarsesediments(Cenderellietal.2003).

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Figure2GeomorphicmapsshowingthedistributionoferosionalanddepositionalfeaturesproducedbyaGLOF.Forthecrosssections,theelevationsarerelativeandinunitsofmetres(afterCenderelliandWohl,2001.ReprintedwithpermissionfromElsevierScience).

AGLOFmightbemodelledasashallowwatersurfacewavegeneratedbylandslides.

Insucheventstheprimaryriskisnotassociatedwiththeslidingmassbutinsteadtothewavesgeneratedbytheenergytransferredtothewaterbythecollision.Asecondaryriskisthepossibilityoflargescaledepositionofmaterialontheriverbedforminganartificialdamabletoaccumulatealargeamountofwatertemporarily,givingrisetofurthercatastrophicdamfailure.(Perezetal.2006).

AnappropriatenumericalmethodcouldbebasedonRoesapproximateRiemannsolver(see6.1.4),asORSAcodeone.MoreoverarecentstudydemonstratesthatonedimensionalmodellingofGLOFpropagationprovidesresultsbroadlycomparabletodataderivedfrommorecomplexsimulations.(Petterietal.2007).

AlthoughtheenormousnumbersoflimitationsofaGLOFstudy,theresultsofahydrodynamicmodellingcanbeacosteffective.

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Thevulnerabilitymapsgeneratedthroughsuchtoolprovideascientificbasisforidentifyingwhoisvulnerableandwhichinfrastructureandagriculturallandismostlikelytobeaffected.Thisinformationcanbeofgreatvalueindevelopingbetteroptionsformanagementplansandtheirimplementation.(Bajracharyaetal.2007).

2.1 GLOFFrequencyintheAysnRegionofChileanPatagonia

UntilApril2008,noGLOFeventshadbeenrecordedintheNorthernPatagonianIcefield(NPI).Sincethatyear,thisphenomenonhasoccurredfourtimessofar.LagoCachet2,a5squarekilometerwideand100meterdeepglaciallakenearthesnoutoftheColoniaGlacier,hascreatedaGLOFbyemptyingitsvolumethroughasystemof8kmlongtunnels.ThewaterhasbeenreleasedintoLagoColoniawhichfeedsRioColonia.LagoColoniasincreaseinwaterlevelwasnotsignificant:itdischargedtheexcesswaterintotheRioBaker.

TheeffectofaGLOFintotheRioBakerresultsintoanincreaseofthewaterlevelandanassociatedwatertemperaturedrop(Table1).

Apr072008 Diff.

Oct082008 Diff.

Dec222008 Diff.

Mar052009 Diff.

max 3575 303% 3007 561% 3052 291% 3812 324%Q(m3/s)

average 1181 536 1048 1176

max 6.71 241% 5.85 522% 5.92 235% 7.06 254%WaterLevel(m) average 2.79 1.12 2.52 2.78

minimum 4.6 53% 4.5 62% 8.1 74% 3.8 36%Temperature(C) average 8.6 7.3 10.9 10.5

Table1SummaryofrecentGLOFs(Q,waterlevel,waterT)withacomparisonbetweenpeakvaluesandbasevalues

Apossibleexplanationforthesuddenoccurrenceoftheseeventscanbefoundintheclimatechangethatthisareahasexperiencedinthelastfourdecades.MeteorologicalstationsintheAysnregionhaverecordedahalfdegreeincreaseintemperaturesincetheearly1970s.Atthesametime,precipitationisdecreasing,causingdiminishediceformation.Glacialsystemsarebasedonanequilibriumthatisverysensitivetoclimatefluctuations.TheNPIisatemperateglacierthatcontainsicebelowthemeltingtemperaturethroughoutitsmass.Iceformationoccursonlyin

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winteranditisaresultofsnowcompaction.Ifprecipitationdecreasesandtemperatureincreases,therateatwhichtheglaciermeltswillbefasterthantherateatwhichiceisformed.Thisproducesamassshrinkage.TheColoniaGlacierhasbeenshrinkingsincethelastdecadebutthisfacthasnotproducedvisibleresultsuntiltheoutburstoccurred.

WithrespecttoLagoChachet2GLOFs,theincreaseintemperaturehasledtomultipleeffects:

Icemelting.TheresponsibleistheincreasedtemperatureoftheairthathasallowedthemeltingoftheglacierandconsequentlytheformationandfillingofLagoCachet2.

Bankthinning.Duetothedoubleactionoftheairandlaketemperature(thatreflectstheclimatechangeaswell),thebanksofLagoCachet2havereducedaswellandtheiceoverburdenissignificantlydecreased.

Tunnelformation.Thefracturesalreadypresentintheglacierhaveprobablyformedlargerandinterconnectedmoulinsthat,undertheeffectofthewarmerlaketemperatureduringtheoutbursts,havecontinuedtogrowandtoletthedrainageofthelakeitself.

Theroleoftheairtemperatureisparticularlyrelevantforthefrequencyoftheoutburst:therateatwhichLagoCachet2isfilledisaconsequenceofthewarmingtrend.

WhilethepeakrunoffofeachrecentGLOFeventwasalmostconstant,thetrendisalarming.Infact,thetimebetweeneventshasdecreasedalongageometricprogression.

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3. DamFailureImpactAssessment

Thedambreakinundationzoneisdefinedastheareadownstreamofadamthatwouldbeinundatedorotherwisedirectlyaffectedbythefailureofadam.

ThesimulationofdambreacheventsandtheassociatedfloodedareaarecrucialtoreducethreatsfromunexpectedfloodingcausedbyadamfailureandareoftenrequiredbyStateslegislationandtechnicalregulation.Thedamfailureimpactassessmentwoulddeterminewhetherthedamisareferabledam,ifthereisadangerofthedamfailingandwhichactionisnecessarytopreventorminimisetheimpactofthefailure.

AlthoughitseemsthattheChileanlawdoesntincludeanydamssafetyregulation,amassivedamprojectshouldbesupportedbyacompletefloodriskassessment.

Dambreakstudiesarenotonlyachievedtopursuesafetylegislation,oftenlacking,buttheyarefrequentlycommissionedbythedamownersinordertoavoidthecostlyupgradestotheirdamtomeetincreaseddamsafetystandardsthatareappliedasaresultofthedownstreamdevelopment.

ThedamfailureassessmentproceduresherepresentedfollowstheguidelinepublishedbytheDepartmentofNaturalResourcesandMinesoftheQueenslandGovernment,Australia(2002).

GuidelinesThemethodologyofdamfailureimpactassessmentshouldrequirethefollowingactivities:

Inspectionofdamsite Accuratecollectionofdata Identificationoffloodproneareas Identificationofpopulationatrisk Certificationofafinalwrittendamfailureimpactassessmentbyaregistered

professionalengineerandsubmissiontothechiefexecutive.

3.1 DamSiteInspectionSiteinspectionsaremandatoryandensurethattheinformationuponwhichthedamfailureimpactassessmentisbasediscorrectanduptodate,andalsoenablean

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appreciationofthecharacteristicsofthesite.Siteinspectionsmustincludeareasthatcouldbeaffectedbydamfailurebothupstreamanddownstreamofthedam.

Siteinspectionsareneededto:

Verifytheaccuracyofallmapping/aerialphotogrammetrythatisusedintheassessment

Verifytheexistenceofbuildingsandotherplacesofoccupationtojustifythefailureimpactratingidentifiedintheassessment

Identifyotherstoragesonthesamewaterway Identifybuildingsandotherplacesofoccupationalongwaterways,which

mayhousepopulationatrisk

Identifycatchmentmodificationworks(e.g.diversiondrainsandleveebanks).

3.2 DataCollectionTheappropriatenessandaccuracyoftheinformationincludedintheassessmentmustbejudgedbytheengineerthatcertifiesthefailureimpactassessment.Awidearrayofinformationneedstobecollectedtodeterminetheeffectsofadamfailure.Theseinclude:

3.2.1 GeneralInformationFloodsrelatedtodamfailurearegenerallysignificantlylargerthannaturalfloods.Informationthatshouldbeincludedintheassessmentis:

Availablehistoricfloodlevels Hydrographicdata Rainfall/runoffmodelresults Dambreakfloodmodelresultsundersunnydayandincremental

conditions.

3.2.2 DamandstorageinformationTheinformationusedtodeterminepotentialbreachcharacteristicsandincrementalfloodingeffectsshouldinclude:

Typeofdamandlocation(includinglatitudeandlongitude)

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Spillwaytypeandadequacy(includingfloodcontrolfacilitiessuchasgatesandsecondaryspillways)

Dimensionssuchasheightandlengthofembankmentsandthewidthofthecrest

Storagecapacitytofullsupplylevelandtothecrestofthedam(stagecapacitycurve)

Useofdamincludingcontentsofthestoragearea Possiblecausesandmodesoffailure Commentsondesign,foundationsandanyunusualconditions Designstudiesorreports.

Additionalpiecesofinformationshouldregardthestabilityofslopesandearthquakeeffects.

3.2.3 TopographicinformationSufficienttopographicinformationmustbeobtainedtoaccuratelydetermine:

Theshapeandslopeofthevalleydownstreamofallpotentialfailurelocations

Controlsonthedownstreamflow(i.e.culverts,vegetation,weirs,bridges,embankments,surfaceroughnessandtemporarystorageonthefloodplains)

Locationofmajordownstreamtributaries.Ifregionalmapsdonotprovidesufficientdetailforafailureimpactassessment,furtherinformationmayneedtobeobtainedfromsourcessuchas:

Roadmaps Orthographic,topographic,militaryandcadastralplans Surveyedcrosssections Aerialphotographs Satelliteimagery Localresidents.

Itisimportanttonotethatmappingoraerialphotogrammetrymaynotcontainrecentdevelopmentse.g.housesorotherplacesofoccupation.Informationcontainedinphotogrammetrythatplaysanintegralroleintheassessmentmustbeverifiedbysiteinspections.

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Whereextremeprecisionisrequired,extensive,detailedsurveysofthedownstreamvalleymaybenecessary.Insuchcircumstances,surveysmayalsoberequiredtolocateanddeterminenaturalsurfacelevelsatallbuildingsorotherplacesofoccupationthatarethoughttobeatrisk.

3.2.4 HydrographicdataTheresultsofadambreakanalysiswilldependonanumberofparameterssuchas:

Thesizeoftheavailablefloodstorage Theheightofthedam Thesizeandcapacityofitsspillway Theshapeofthevalleydownstreamofthedam.

Foranaccuratedambreakanalysis,thevaluesoftheManningroughnesscoefficient2shouldbevariedineachcrosssectionaccordingtothelocalsurvey.Forloweraccuracyanalyses,onlyoneroughnesscoefficientmightbesufficientinrepresentingthewholefloodplain.

Wherepreviousfloodrecordsexistintheriverorstreamreachunderconsideration,thehydraulicmodelshouldbecalibratedtomatchtheavailablefloodinundationdatasothatthenumericaldambreakmodelcanbedemonstratedtoapproximateactualflowconditions.Iftheserecordsarenotavailable,orareavailableforalimitedrangeofflows,someassessmentmustbemadeofthepotentialimpactontheaccuracyofthemodelledresults.Allmodellingmustbesubjectedtosensitivityanalysestotestsensitivitytomodelassumptions.

3.2.5 HydrologicdataDownstreamtributaryinflowsmayimpactonthedambreakflood,particularlyifpopulationcentresaresomedistancedownstreamofthedam.Simpleranalysesonsmallerdamswouldnotnormallyconsiderinflowsfromtributariesdownstreamofthedams.

2TheManningcoefficientisaroughnessparameterusedtomodelenergylossesinstreams.Unlessreasonabledischargeandwaterlevelcalibrationisavailable,referenceshouldbemadetostandardhydraulicengineeringtextsforappropriatevaluesoftheparameter.

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3.2.6 DownstreamcommunityinformationDownstreamcommunityinformationmustincludethelocation,numberandnatureofbuildings,otherplacesofoccupationandapprovedcampingandrecreationalareasinthefailureimpactzone.

Thisinformationmaybeobtainedfrommaps,personswithlocalknowledgeandemergencyactionplansforthedam.Recentaerialphotogrammetryalsoprovidesusefulinformationonthelocationofdownstreamstructures.Asstatedabove,siteinspectionsmustbeundertakentoverifydownstreamcommunityinformation(forexample,toensuretheinformationisuptodateandidentifiesbuildingsandotherplacesofoccupationobscuredbytrees).

3.2.7 DeterminationofFloodProneAreasThefloodpronearea,definedasthefailureimpactzone,istheareaaffectedbyfloodingasaresultofamajorfloodevent,e.g.thefailureofthedam.Failureimpactzonesmustbedeterminedforallfailureeventsspecifiedwithintheanalyticaltechniqueusedforthefailureimpactassessmentandforallotherfailureeventsrelevanttothedam.

Itshouldbenotedthatwhilethedamfailureimpactzoneisgenerallylocateddownstream,areasupstreamcanalsobeaffectedandshouldbeincludedwhererelevant(e.g.anupstreamareamaybeaffectedbytheabnormaloperationofdischargecontroldevicessuchasgatesorinflatablebags).

Amapshowingtheextentofthefloodproneareasmustbeincludedinthewrittenassessment.

3.2.8 PopulationatriskPeopleareconsideredpartofthepopulationatriskifbuildingsoranypartofthegroundwherethesebuildingsarelocatedliewithinthefailureimpactzone.

Whenthefailureimpactzoneisbeingdetermined,thenumber,locationandnatureofbuildingsandotherplacesofoccupationmustbeidentified.

Thepopulationatriskisthedifferencebetweenthepopulationatriskforaspecificdamfailureandthepopulationatriskfromthenaturalfloodingevenifdamfailuredoesnotoccur.

Damfailureduringaflood:somepeoplecouldbeatriskfromthenaturalfloodingevenifdamfailuredoesnotoccur.(Figure3)

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Figure3Damfailureduringaflood.(GuidelinesforFailureImpactAssessmentofWaterDams,StateofQueenland,2002)

Asunnydaydamfailure (whenfloodingisduetodamfailureonly):nobodyisatriskifthedamdoesnotfail.(Figure4)

Figure4Sunnydaydamfailure

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3.3 AccuracyofpopulationatriskcalculationsAvarietyoffactorsmayaffecttheaccuracyofpopulationatriskcalculations.Thesemustbeconsideredtoensurethereliabilityofpopulationatriskcalculations.Factorsinclude:

Theaccuracyofcrosssectionsusedintheanalysis Thelocationsofcrosssectionsusedintheanalysis Theaccuracyofthehydraulicmodelling Availabilityandaccuracy/reliabilityofcalibrationdataandthedegreeof

extrapolationrequiredtomodeldambreakflows

Assumedhydraulicroughnessparameters Assumedbreachdevelopmenttimes Locations,numbersandelevationsofbuildingsandotherplacesof

occupation.

Thedegreeofconservativenessshouldreflecttheamountofcalibrationdataavailabletodeterminestreamchannelroughnessforthewatercoursereachesinquestion.

Thewrittendamfailureimpactassessmentshouldincludeastatementontherangeoftheestimateofpopulationatriskforthecriticalcase.Suchanassessmentshouldindicatevaluesfortheupperlimitofpopulationatriskthatcouldreasonablybeexpectedasaresultoftheanalysisandasimilarlyderivedlowerlimitofpopulationatrisk.

3.4 AnalyticalTechniquesThreeanalyticaltechniquesmaybeusedinpreparingdamfailureimpactassessments.Thesearetwodimensionalflowanalysis,simplifiedassessmenttechniquesandcomprehensiveassessmenttechniques.Thesetechniquesmaybeusedaloneorincombination.Certifyingregisteredprofessionalengineersneedtobesatisfiedthatthetechniquesselectedandtheaccuracyofthemodelsdevelopedarereasonableforthesituationsunderconsideration.

3.4.1 TwodimensionalflowanalysisThisanalysiswilltypicallyneedtobeusedifthepopulationissituatedinarisklocationclosetoapossibledambreachandthereisariskthatthepopulationwillbeflooded.

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Modelsusedinsuchanalysesneedtobeabletosimulatethedynamicbehaviourofoverlandflowovercomplexgeometries,determininginundatedareasfortwodimensionalflows.

3.4.2 SimplifiedassessmentAsimplifieddamfailureimpactassessmenttechniquemaybejustifiedwherethereislittledoubtastothepopulationatriskandthecostofacomprehensiveassessmentisanticipatedtobehighrelativetothepotentialbenefits.Itinvolvestheconservativeuseoftopographicandhydrographicdataandanempiricallydeterminedbreachdischarge.

Thisisanapproximatetechnique,whichusesthenormaldepthatasectiontoestimatemaximumfloodlevelsatapointforagivendischarge.Assuchthistechniquedoesnottakeanybackwatereffectsintoaccount.Itmustnotbeusedwherebackwatereffectsareexpectedtobesignificantintermsoftheaffectedpopulationatrisk.Asidefromthebackwatereffects,theprincipalareasofuncertaintyaretheaccuracyofthestreamslopes,thecrosssections,andthelocationsandlevelsoftheimpactedbuildings.

Themaximumbreachdischargefromadamduringabreachingeventistypicallydeterminedusingempiricalrelationshipbasedonstatisticalanalysisofpreviousdamfailures.

3.4.3 ComprehensiveassessmentIfasimplifiedassessmentisnotaccurateenoughtoadequatelycalculatethepopulationatrisk,thenacomprehensivedambreakanalysismayberequired.Acomprehensiveassessmentisadetailedassessmentofthefailureimpactzoneandthepopulationatriskifthedamfails.Dambreakanalysesmustbeundertakenforarangeofdamfailurescenariosandusecurrenthydraulicmodelingpracticeandsuitablydocumentedandvalidatednumericalmodels.

Someestimateoftheaccuracyofeachnumericalmodelusedforthedambreakanalysismustbemadeandthisaccuracymustbetakenintoaccountinassessingpotentialpopulationatriskasindicatedintheprevioussections.Theimpactonpopulationatriskwillbegreatestinareaswithhigherpopulations(e.g.towns),anditmaybejustifiedtoselectivelyimproveaccuracyintheseareas.

3.4.4 TwoormoredamsonthesamewatercourseSometimes,twoormoredamsoccuronthesamewatercourse.Insuchcircumstances,itmustbeassumedthatthefailureofanupstreamdammaytrigger

20

thefailureofdownstreamdams.Ifthedownstreamdamcannotstorethecontentsoftheupstreamdamwithoutfailure,thecombinedeffectofmultipledamfailuresmustbeconsideredwhendeterminingtheincrementalpopulationatriskfortheupperdamforfailureevents.Similarly,iffailureofadownstreamdamcouldcontributetothefailureofanupstreamdam(suchasthrougharapiddrawdownfailureifheadwatersofthedownstreamdambackupagainsttheupstreamdam),thepotentialfailureoftheupperdammustbeconsideredwhendeterminingtheincrementalpopulationatriskofthelowerdamforfailureevents.

Thedamfailurecaseproducingthehighestincrementalpopulationatriskmustbeusedtodeterminethefailureimpactratingforthedam.

3.4.5 OtherfailureeventsIftheregisteredprofessionalengineerconsidersthatotherfailureeventscouldresultinahigherincrementalpopulationatrisk,thesefailureconditionsmustbeconsideredanddescribedinthewrittendamfailureimpactassessment.Thesefailuresmayinclude:

Storageriminstability. Factorssuchasdeterioration,oldage,designorconstructionfaultsandpoor

maintenance.

Damageduetofire,windandescapeofwaterintominingtunnels/shaftsbeneathreservoirs.

Vandalism.

3.5 PeriodicReAssessmentofFailureImpactRatingThedamownersshouldprovideaperiodicreassessmentofthedamfailureincludingwhetherornottherehasbeensubstantialchangesin:

thestreamchannelcrosssectionsandroughness theembankmentandspillwaygeometry themagnitudeofthedesignfloods thedownstreampopulation

Thepopulationatriskmustberecalculatedaspartofeachreassessmentofthefailureimpactrating.

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4. CasestudyThedambreakanalysishereafterdescribedissupplying,althoughinasmallportion,thetotalabsenceofadamfailuresafetyassessmentintotheEIApresentedbyEndesa.

ThestartingcasestudyoftheanalysisconsistedofthesimulationofarecentGlacialLakeOutburstFlood(GLOF)event.Thisfirststepallowedtocalibratethemodelcomparingthemathematicalresultsofthesoftwarewiththerealdatacollectedduringtheevent.Furthermore,theanalysisgivesanadditionalinsightontheGLOFphenomenon.

Afterthecalibrationandtheevidenceofconsistencyofthemodel,theactualdambreakwavefloodinghasbeenimplementedandsimulated.

TheProposedHydroelectricDamBaker2ThesocalledBaker2isaproposedconcretegravitationaldam,whosedesignlifeis40years,thatwouldbelocated2kmupstreamtheconfluenceoftheRioElSaltnintotheRioBaker.Theprojectcapacityofthestationis360MWandtheaverageannualpowerproducedwouldbe2530GWh.

Thedesigndischargeis1275m3/s,whiletheaverageannualdischargeoftheRioBakeris948m3/s.ThemaindesignaspectsofthedamaredescribedinTable2.

DamBaker2:Concretegravitytype

Elevation(m) 40

Height(m) 93

Lenght(m) 320

Width(m) 8

Reservoirsurface(ha) 3600

Reservoirvolume(hm3) 380

Table2Baker2:designcharacterization

ThedamBaker2,accordingtotheEIApublishedbyHydroaysn,ismeantnottohaveanysignificanteffectontheregulationofflowregime,namelyitwouldcauseflowvariationnotgreaterthanthe5%ofthenaturaldailyandmonthlyvariation.Theaverageretentiontime,forthedesigndischarge,is8hours.

22

Figure5Baker2reservoir(fromHydroAysenEIA)

23

Figure6RioBakerdownstreamdamBaker2(fromHydroAysenEIA)

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4.1 DamSiteOnJanuary2009anonsitefieldtripintheAysnRegionhasbeenorganized.

DuringthatperiodtheRioBakerwatershedhasbeenanalyzedcarefullywithacomplementaryraftexperiencefromhisoriginBertrandLake,fedbygeneralCarreraLake,untilhisdeltaintothePacificOceannearthetownofCaletaTortel.

TheonsiteevaluationhasallowedappreciatingthecharacteristicofthevegetationsurroundingthewaterwayandtheriverbedgranulometryinordertodefinetheappropriatevaluesoftheManningcoefficientdownstreamthelocationoftheproposeddamBaker2.

ThesiteinspectionofCaletaTortelgavethoroughinformationaboutthecontextofthemajortownthreatenedbytheimpactofapotentialdambreakfailure.

4.1.1 TopographicInformationWithintheprocessofdatacollection,thegatheringoftopographicmaterialhasbeenthemostchallenging.TheprincipalofficeoftheDireccinGeneraldeAguas(DGA),Chilesstatebodyresponsibleforpromotingthemanagementandadministrationofwaterresources,hasntbeenabletoprovideanyorthographic,topographic,militaryorcadastralplanofthevalley.

TheonlyavailabletopographicdatahavebeenprovidedfreelybytheU.S.GeologicalSurveywebsite.

TheDigitalElevationModel(DEM)providedbytheUSGSisaproductoftheShuttleRadarTopographyMission(SRTM)isajointprojectbetweenNASAandNGA(NationalGeospatialIntelligenceAgency)tomaptheworldinthreedimensions.SRTMutilizeddualSpaceborneImagingRadar(SIRC)anddualXbandSyntheticApertureRadar(XSAR)configuredasabaselineinterferometer,acquiringtwoimagesatthesametime.Theseimages,whencombined,canproduceasingle3Dimage.FlownaboardtheNASASpaceShuttleEndeavourFebruary1122,2000,SRTMsuccessfullycollecteddataover80%oftheEarthslandsurface,forallareabetween60degreesNand56degreesSlatitude.

SRTMdatawereprocessedfromrawradarechoesintodigitalelevationmodelsattheJetPropulsionLaboratory(JPL)inPasadena,CA.Theseoriginaldatafileshadsamplesspaced(posted)atintervalsof1arcsecondoflatitudeandlongitude(about30metersattheequator).ThesedataweretheneditedbytheNationalGeospatialIntelligenceAgency(NGA,formerlytheNationalImageryandMappingAgency),anddistributedaspartoftheirDTEDproductset.

Theediting,alsoreferredtoasfinishing,consistedofdelineatingandflatteningwaterbodies,betterdefiningcoastlines,removingspikesandwells,andfilling

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smallvoids.Thissetispubliclyavailableattwopostings:1arcsecondfortheUnitedStatesanditsterritoriesandpossessions,and3arcsecondsforregionsbetween60degreesNand56degreesSlatitude.(Source:http://seamless.usgs.gov).

4.1.2 TopographicEditingusingArcGIS9.3ThedatadownloadedfromtheUSGSwebsitewasa90meterDEMaffectedbysomeissuese.g.occasionalbaddataontheriverbedandholesinthedatasetincertainplaces.

TheDEMhasbeencleanedandelaboratedusingArcMapfromArcGIS9.3.

Thedatasethasbeenimported,reducedtotheareaofinterest,thewatershedhasbeendefinedcarefully,andtheoccasionalholeshasbeenlocatedandeliminatedperformingamovingaverage.Thedatasetprovidedbythemovingaveragehasbeenmergedretainingforthefinalmapthebestdataavailable(thefirst)fromeachpassageoftheaverage.

DamBreakModeling

4.2 CalibrationAfundamentalprocessinthedamfailureimpactassessmentisthecalibrationofthemodel.Theexistenceofpreviousfloodrecordsallowscalibratingthemodelmatchingtheavailablefloodinundationdatatothenumericaldambreakmodelresults.Thisanalysisteststheaccuracyofthemodeladopted.

TheeventsimulatedtocalibratethemodelhasbeentheGlacialLakeOutburstFloodrecordedonMarch5th2009.

ThischoicewasdrivenbythefactthattheGLOFunderconsiderationisthelatestandtheonewiththebiggestpeakdischargerecordedsofar.TherearestillmanyconcernsabouttheimpactofGLOFsonthesafetyofthedownstreamBakervalley,withorwithoutdam,hencethisfloodsimulationisanadditionaltoolforabetterunderstandingofthephenomenonandhispotentialimpact.

4.2.1 HydrologicDataCollectionThehydrologicdatadownloadedfromtheDGAwebsiteenabledthedefinitionoftheflowhydrographassociatedtotheGLOFeventanalyzed.

AdetaileddescriptionoftheGLOFimpactsimulationwillbetreatedinChapter5.Nevertheless,inordertogiveconsistencytothenextparagraphs,itisanticipatedthattheresultsofthecalibrationprocessguaranteedthemodelaccuracydesired.

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4.3 DamBreachAnalysisThemostimportantcomponentofadambreakanalysisisthedefinitionofreasonablebreachparameters,whicharehighlydifficulttobeaccuratelypredicted.

Thebreachwidthandbreachtimehaveagreatinfluenceontheforecastoftheoutflowandthefloodedareadownstreamthedam.Thedevelopingtimeforabreachisdefinedasthepointwheredamfailureisimminentandendwhenthebreachhasreacheditsmaximumsize.Forrelativelysmallreservoirs,thedambreakpeakoutflowusuallyoccursbeforethebreachiscompletelydeveloped,resultingfromaconsistentdropinreservoirlevelsduringtheformationoftherupture,whileinlargereservoirsthepeakoutflowoccursassoonasthebreachhasextendedtoitsmaximumsize.

Breachparametersandrelateddambreakpeakoutflowhavebeendefined,usingthelimitedavailabledata,adoptingtwomethods:

TheNWSSMPDBKmodeldevelopedbytheNationalWeatherService(NWS)

TheGeneralizedRitterDamBreakSolution(1892)suggestedbytheUSACE

4.3.1 NWSSimplifiedDambreakmodelTheempiricalpredictorequationsusedbytheNWSSMPDBKmodelarebasedonregressionanalysesofanumberofhistoricalbreaks,andtheyhavebeenultimatelydescribedbyFroehlichin1995.

NWSSMPDBKPeakdischarge:

3

1.3

+=

d

p

HCT

CbQ where:b

aSC4.23= and

Qp=Peakflow(cfs)

Sa=SurfaceArea(Ac)

Hd=BreachHead(ft)definedasDepthofWaterattimeofBreaching

b=Breachwidth(ft)

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T=Time(Minutes)

NWSSMPDBKconcretegravitydamBreach:

b=5Hd

where:

b=Breachwidth(ft)

Hd=BreachHead(ft)definedasDepthofWaterattimeofBreaching

Theestimatedbreachwidthresultingfromthemodel,giventheavailabledata,isb=265m.

ThecorrespondingpeakoutflowisQp=61098m3/s.

4.3.2 GeneralizedRitterDamBreakSolutionForcompletenessandtoprovideanalternativefloodforecast,thepeakdischargehasbeencalculatedalsoaccordingtotheUSACEGuidelinesforCalculatingandroutingaDam

BreakFlood.

Peakdischarge:

23

278

dp HgbQ =

where:

Qp=Peakflow(m3/s)

Hd=BreachHead(m)definedasDepthofWaterattimeofBreaching

b=Breachwidth(m)

g=gravitycoefficient(m/s2)

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Thestandardguidelinesrecommendtoconsiderastandarddambreachcorrespondingtothe30%ofthetotaldamlength:

b=0.3L

where:

b=Breachwidth(m)

L=Damlength(m)

Theestimatedbreachwidthresultingfromthemodel,giventheavailabledata,isb=96m.

ThecorrespondingpeakoutflowisQp=34375m3/s.

4.3.3 EstimatedPeakDischargeatDamSiteInordertoguaranteeacautiousanalysis,thefinaldecisiondisposedtochoseforthemostconservativeestimationofthepeakflow,thatistheQpestimatedthroughtheNWSSMPDBKmodel.

ThebreachtimehasbeenthereforeevaluatedusingtherelationsadoptedbytheNWSSMPBDBKmodelforaconcretegravitydam:

40d

bH

t =

where:

tb=BreachTime(minutes)

Hd=BreachHead(ft)definedasDepthofWaterattimeofBreaching

Theestimatedbreachwidthresultingfromthemodelis7.6minutes.

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5. DamBreakSimulation

ORSACodeORSACodeisa1Dimensionalcomputationalmodelwhichsimulatesfloodwavepropagationandidentifiesfloodproneareasastheareaswherethegroundislowerthanthecomputedwaterelevation.ThesoftwarehasbeendevelopedbyaresearchcollaborationbetweentheUniversityofPavia3andtheUniversityofRome,LaSapienza4.

Thehydraulicsimulationreproducesdifferentscenariosmappingtheresultantfloodedareasonthedigitalelevationmodel(DEM)ofthesurfacetopography.

5.1 NumericalSolversORSACodeadoptstwodifferentfinitevolumessolverstointegratetheShallowWaterEquations,thesimplified1Dgoverningequationsofunsteadyopenchannelflow:theRoeschemeandamodifiedLaxFriedrichsscheme.

AswillbediscussedinChapter6,thosemathematicalmodelssolvetheSWEwritteninconservativeform,whichallowsthesoftwaretoreproducethediscontinuitiesofaflowregimevariationswithoutincurininstabilitiesofthesolution.

Theshockcapturingcapabilityofthesoftwarewhendealingwithtranscriticalflowregime,whichismainlyduetosuddenchangesinthechannelgeometry,haspracticalinterest.

5.2 SurfaceTopographyAcquisitionandAnalysisTheRioBakerDigitalElevationModel(DEM),alreadyprocessedinapreviouspassage(see4.1),hasbeenuploadedandverified.ORSACodeinterfaceallowstoautomaticallyoutliningthecontourlinesforagivendistance.Thecontourlineshavebeendefinedevery20meters.

5.2.1 BreaklineThebreaklineisapolilinethathasbeendrawnintotheriverbedinordertodefinetheaxislineofthechannel.ThebreaklinestartsatthelocationoftheproposeddamBaker2andhasbeentraceduntiltheRioBakerdelta;itstotallengthis65km.

3L.Natale,G.Petaccia,M.Zanotti4F.Savi

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5.2.2 SectionsThedefinitionofthecrosssectionsisanaccuratepassageofthetopographyediting.Theoutputvaluesofthecalculationarealwaysreferredtoaparticularsection.Hence,forathoroughunderstandingofthefloodedareainacertainlocation,forinstanceCaletaTortel,anattentivechoiceisveryimportant.Thecrosssectionsarealwaysperpendiculartothebreakline.66sectionshavebeenedited.

5.2.3 VisualizationAlthoughthecorrelationbetweenORSACodetopographicinterfaceandanothertopographictoolisoptional,itresultedofgreatimportanceinunderstandingtheplanimetryandinconfirmingthecoherencyoftheinformation.

SomeinterestingcomparisonbetweenORSACodeinterfaceandGoogleEarthimagesareshowninfiguresherebelow.Figure7

Figure7RioBakerdownstreamBaker2dam.ViewfromGoogleEarthandORSACodeinterface

5.2.4 AssignationofManningsCoefficientManningsroughnesscoefficientvaluesareusedintheManningsformulaforflowcalculationinopenflowchannels.Roughnesscoefficientsrepresenttheresistancetofloodflowsinchannelsandfloodplains.

Roughnesscoefficientshouldbevariedtoaccountforseasonalvariability.Floodsoftenoccurduringthewinterwhenthereislessvegetation,whileinspringvegetationgrowthisdenser.Nevertheless,thecalibrationcasestudy,assaid,has

31

beenasimulationofaGLOF,phenomenonalwayshappeningduringwarmseason(Spring/Summer)whenicemeltingoccurs.ForthisreasontheManningscoefficientchoicereferstothespringvegetationgrowth.

Foramoreaccuratedambreakanalysis,thevaluesoftheManningroughnesscoefficientshouldbevariedineachcrosssectionaccordingtothelocalsurvey.Whenlocalanalysesarenotaccurateenough,onlyoneroughnesscoefficientmightbesufficientinrepresentingthewholefloodplain.

Onaccountoftheseconsiderations,theManningsroughnesscoefficientassignedhasbeen0.03625(Table3).

SurfaceMaterial ManningsRoughnessCoefficientn

Earthchannelgravelly

0.025

Earthchannelstony,cobbles 0.035

Floodplainspasture,farmland

0.035

Floodplainslightbrush 0.05

Averagevalue 0.03625

Table3ManningsRoughnessCoefficientvalues

GLOFSimulationThesimulationoftheGlacialLakeOutburstFloodofMarch5th2009,andtherelatedfloodedareahavebeenafundamentalprocessforthemodelcalibration.(see4.2)

Thenextparagraphscontaintheassumptionandthecharacterizationswhichresultedmoresuitableforafairmatchingoftheavailablefloodinundationdatatothenumericalmodeloutputs.

5.3 InitialConditionsORSACodeinterfaceisabletosimulatethewavepropagationduringaflood,specifyingacertainsectionanditswaterlevel,orasituationofsunnyday,assumingdrybed.

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Theappropriatechoicehasbeentheassumptionoftheconditiondrybed(orsunnyday).

5.4 BoundaryConditionsThedefinitionoftheBoundaryConditions,thatistheassignationinacertainsectionofaknownvaluefromwhichproceedinthecalculusoftheunknowns,isacriticalpassageoftheanalysisanditisalwaysaffectedbyacertaindegreeofincertitude.ThemanipulationoftheB.C.,evaluatingtheirdifferentimpactsontheevolutionoffinalresults,isanimportantgoalofthecalibrationprocess.

HereinafterfollowsadescriptionoftheB.C.finallyassignedtotheGLOFcasestudy.

5.4.1 UpstreamB.C.Consideringasupercriticalflow,twoconditionshavebeenassignedintheinitialupstreamsection:

Fr=1,wherethecriticalvalueoftheFroudenumber5indicatesthattheinitialsectionisacontrolsection

GLOFhydrograph(representedinFigure8),basedonthehydrologicdatadownloadedfromtheDGAwebsite.

5TheFroudenumber,Fr,isadimensionlessvaluethatdescribesdifferentflowregimesofopenchannelflow.TheFroudenumberisaratioofinertialandgravitationalforces.

hgvFr = where:

vistheflowvelocitygisthegravitationalaccelerationhisthehydraulicdepth

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GLOF_March 5th 2009

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3 /s)

Figure8HydrographoftheGLOFeventthathappenedonMarch5th2009

Thetotaldurationofthefloodwavehavebeenchosentobe20hours,aftersomeattemptsandanevaluationoftheaverageflowvelocity,inordertoguaranteethereachingoftheclosuresection(thesimulationstopsassoonasthewaveiscompletelyreleasedfromtheinitialsection).

5.4.2 DownstreamB.C. Fr=1

Assigningastateofcriticalflowhastwodifferentimplicationsdependingontheflowwavestatearrivingatthesection.Ifcasethewaveissupercritical,thisB.C.isredundant.Incasethewaveissubcritical,thisB.Crepresentstheclosureofthenumericalsimulation.Thelastcaseisthemorelikely,becauseitrepresentsawavejumpingintotheocean,wheretheactualwaterdepthisbelowcriticaldepth.

5.5 SimulationCharacterization CFL=0.16,thesolutionstabilityhasbeenguaranteed Dt=0.1s,whereDtistheminimalintegrationtimestep

6TheCourantFriedrichsLewycondition(CFL)isaconditionforconvergencewhilesolvingthehyperbolicpartialdifferentialequationswithtimeexplicitschemes.Thetimestepmustbesmallerthanacertaintimeintervalinordertohavestabilityintheresults.CFLvariesbetween0.1and1.

34

5.6 ResultsThemaximumfloodexperiencedintheBakerValleyduetoaGLOFcanbedepictedbyagraphonwhichmaximumfloodedwidthscalculatedbythesoftwareORSACodeareplottedagainstdrainagearea.

Thiswayofrepresentingthesimulationresultsprovidesthemostconcisedescriptionofthecatchmentsizemagnitude.

Theresultsareconsistentwiththeavailablefloodinundationdata(DGAreport,March6,2009).Themodelisthereforecalibrated.

Envelope Curve (GLOF)

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Distance (m)

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ded

Wid

th (m

)

Figure9GLOFfloodedwidthenvelopecurve

DamBreakSimulationThesimulationoftheGlacialLakeOutburstFloodofMarch5th2009,andtherelatedfloodedareahavebeenafundamentalprocessforthemodelcalibration(see4.2).

Thenextparagraphscontaintheassumptionandthecharacterizationswhichresultedmoresuitableforafairmatchingoftheavailablefloodinundationdatatothenumericalmodeloutputs.

5.7 InitialConditionsORSACodemodelingtools,asillustratedinpreviousparagraphs,cansimulatedifferentscenariosandtheresultingdownstreaminundationmap.Duetothespare

35

availabilityofhydrologicdata,theanalysisconsidersasunnydayscenario,whichaccountsfornonrainfallcausesoffailure.

Thescenarioassumesthatthebreachwilloccurundernormaloperatingconditions,i.e.thereisnoaddedinflowtothereservoirtoincreasethepoolnormalfullstorage.

5.8 BoundaryConditions

5.8.1 UpstreamB.C.Consideringasupercriticalflow,twoconditionshavebeenassignedintheinitialupstreamsection:

Fr=1,wherethecriticalvalueoftheFroudenumberindicatesthattheinitialsectionisacontrolsection(DamBreachhappens)

Hydrographfordambreakwave(representedinFigure10)

Thedesignhydrographhasbeenestimatedusingatwoparametergammadistribution,commonlyusedtoestimatequantilesofhydrologicalrandomquantities.

Thegammadistributionisveryflexibleinfittingdata.Itsparametershavebeengaugedinordertofulfillthefollowingrequirement:

peakoutflowQp=61098m3/s(definedthroughtheNWSSMPDBKmodel(see4.3.1)

breachtimetb=7.6min(definedthroughtheNWSSMPDBKmodel(see4.3.1)

areaunderthecurve3600ha(Baker2designcharacterization)

36

Dam-Break wave

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40000

50000

60000

70000

0 20 40 60 80 100120140160180200220240260280300320340360380400420440460

time(min)

Q(m

3 /s)

Figure10Dambreakwavehydrograph

5.8.2 DownstreamB.C. Fr=1

Assigningastateofcriticalflowhastwodifferentimplicationsdependingontheflowwavestatearrivingatthesection.Ifcasethewaveissupercritical,thisB.C.isredundant.Incasethewaveissubcritical,thisB.Crepresentstheclosureofthenumericalsimulation.Thelastcaseisthemorelikely,becauseitrepresentsawavejumpingintotheocean,wheretheactualwaterdepthisbelowcriticaldepth.

5.9 SimulationCharacterization CFL=0.01,thesolutionstabilityhasbeenguarantee Dt=0.1,whereDtistheminimalintegrationtimestep

37

5.10 Results

5.10.1 DamBreakWaveOutflowThefollowingchartsrepresentthedambreakwavedischargedownstreamBaker2damalongthewaterway.Eachgraphcorrespondstotheindicatedtime.

Forabetterunderstandingoftheresults,thegraphshavebeencomparedindifferentcharts.

Figure11showshowthedischargedecreasesrapidlyintime.

After2hoursfromthebeginningofthedambreakwavethedischargesaretoosmalltobecomparedwiththestartingoutflow,forthisreasontheyarepresentedindifferentcharts.Figure12,Figure13andFigure14showthefinaldevelopmentoftheoutflows.

0

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90000

100000

0 10000 20000 30000 40000 50000 60000 70000 80000

distance (m)

Q (m

3/s) t=0.5 hrs

t=1 hrt=1.5 hrs

Figure11Dambreakdischargeafter0.5hours,1hourand1.5hours

38

0

1000

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7000

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9000

0 10000 20000 30000 40000 50000 60000 70000 80000

distance (m)

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3/s)

t=2 hrst=2.5 hrs

Figure12Dambreakdischargeafter2and2.5hours

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distance (m)

Q (m

3/s) t=2.5 hrs

t=3 hrst= 3.5 hrs

Figure13Dambreakdischargeafter2.5,3and3.5hours

39

0

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600

800

1000

1200

0 10000 20000 30000 40000 50000 60000 70000 80000

distance (m)

Q (m

3/s) t=3 hrs

t= 3.5 hrst=4 hrs

Figure14Dambreakdischargeafter3,3.5and4hours

5.10.2 WaterLevelThefollowingchartshowshowthewaterheightsdevelopalongdifferentsectionsatdifferenttimes.Figure15

Thehigherwaterlevelpresentedatthefinalsectionsoftheprogressiveareconsistentwiththephysicalrealityofthecase:thefinalsectionscorrespondtotheriverdelta,wherethevalleywidens:theriverflowvelocitydecreasesandconsequentlythewaterlevelincreases.

After3hourstheimpactofthedambreakwaveonthelastsectionsoftheBakerValleyisnomoresignificant.

40

water height

0

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35

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45

0 10000 20000 30000 40000 50000 60000 70000 80000

distance (m)

heig

ht (m

.a.s

.l.)

riverbedt=0.5 hrst= 1 hrt=3 hrs

Figure15Waterlevelsat0.5,1and1.5hours

5.10.3 FloodedWidthThegraphpresentedhereinafter(Figure16)showsthefloodpropagationandthefloodrouting.Theenvelopecurveisobtainedbyplottingthelargestfloodpeaksversusthedrainageareaandrepresentsthemaximumfloodedwidthineachsectionevaluatedinallthetimeintervals.

Thechangeinshapeofthefloodedareacorrespondstothechangeinshapeofthevalley,whichnarrowsandenlargessuccessivelymanytimesfromtheBaker2damtotheriverdelta.

41

Envelope Curve

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)

Figure16Floodedwidthenvelopecurve

5.10.4 ORSACodeOutputORSACodepresentsafloodsimulationtoolwhichallowstheusertovisualizethefloodpropagationinrealtime.AnimageoftheresultingfloodedareaisgiveninFigure17.

ThefirstredlinecorrespondstotheBaker2damproposedlocation,whiletheyellowlinescorrespondtoeachdesignsection.

Figure18isfocusedontheriverdelta,whichcoincideswiththelocationofCaletaTortel,themainvillageofthearea.With320inhabitants(Census,2002),CaletaTortelconsistsmainlyofstilthousesbuiltalongthecoastforseveralkilometres.Insteadofconventionalstreetstherearewoodenwalkways.

Theresultsofthedambreakanalysisshowahighrisklevelforthepopulationlivingintheaforementionedarea.

Thissimplifiedassessmentisnotaccurateenoughtoadequatelycalculatethepopulationatrisk.Amorecomprehensiveassessmentofthefailureimpactzonerequiresdetailsaboutthelocaldistributionofthepopulation,thelocation,thenumber,theelevationofeachbuildingandthedefinitionofthemostcriticalzones(e.g.hospitals,school).

ORSACodeisabletodefineaccuratelythefloodedarea,nonethelessaplanimetricbackgroundisnecessarytointerprettheactualriskforthedownstreampopulationandaccomplishathoroughdambreakimpactassessment.

42

Figure17ORSACode,overviewoffloodpropagation

43

Figure18ORSACode,floodpropagationattheRioBakermouthandimageoftheareadownloadedfromGoogleEarth

44

6. LiteratureReview:DamBreakModeling

Indamsafetyprogramsandfloodforecast,dambreakmodellingisanindispensabletooltoevaluatedaminducedriskandtosupportemergencyplan,optimizingresponseeffortsanddirectingfirstresponseteamstodamagedareas.

However,abetterunderstandingoffloodpredictabilityandmodelefficiencyisneededbeforesuchsystemscanbeeffectivelyimplemented.

Thissectionpresentsandcomparessomeonedimensionalnumericalmodelsandtheirbasicmathematicalandnumericalexpressions.

HydrodynamicModel:ShallowWaterEquationsDespitethethreedimensionalstructureofthedetailedfreesurfaceflows,itispossibletosimplifythegoverningequationsofunsteadyopenchannelflowincasesinwhichthelongitudinalflowscaleisgreaterthanthetransversaldimensionsandtheproblemisapproximatedasvariableonlyinthespatialdirectionalongthemainflow(Prezetal.2006).(Figure19).

TheconservationofmassandmomentumalongthedirectionofthemainflowisgovernedbytheshallowwaterorSaintVenantequations,derivedfromNavierStokesequationsbymakingcertainassumptions.

Thisisareasonableapproachinmanypracticalapplicationsassumingthatwaterisincompressible,pressuresarehydrostatic,verticalaccelerationsarenegligible,andwavesarenondispersive.TheSaintVenantequationsareanonlinearhyperbolicsystemmathematicallyanalogoustothecompressibleEulerequations.

UndertheDeSaintVenanthypothesis,theconservationlawsare(Cungeetal.1980)

RxF

tU =

+

With

( ) ,, TQAU =

,, 12 T

gIA

QQF

+=

45

( )TfSSgAgIR )(,0 02 +=

Aisthewettedcrosssectionalarea,Q,thedischargeandg,theaccelerationduetogravity.I1representsahydrostaticpressureforcetermasdescribedinCungeetal.(1980).

I2accountsforthepressureforceinavolumeofconstantdepthhduetolongitudinalwidthvariations.

S0isthebedslope;Sfstandsfortheenergygradeline.

GarciaNavarroetal.(1999)defineitintermsoftheManningsroughnesscoefficientnas:

342

2||

RA

nQQS f = ,

ThehydraulicradiusisR=A/P,Pbeingthewettedperimeter.

TheJacobianmatrixofthesystemis

==

uucUFA

210

22 ,

Whereu=Q/Aand bgAc /= .AccordingtotheformulationofGarciaNavarroetal.(1999),theeigenvaluesandeigenvectorsofArespectivelyare:

cua =2,1

( )Tcue = ,12,1 .

46

Figure19.Onedimensionalflowalongxdirection.(Prezetal.2006).

6.1 NumericalSolversThefirstdifficultyinanycaseofintegrationofasystemofquasilinearpartialdifferentialequationsisthechoiceofthenumericalscheme.

Thenumericalmodellingofunsteadyflowinriversisacomplicatedtaskandthedifficultiesgrowasthepretensionstoobtainbetterqualityormoregeneralsolutionsdo(Cungeetal.,1980).

Currentlythereisawiderangeoftechniquessuitablefornumericalintegrationofshallowwaterflowequations(SWE).However,ithastraditionallybeendifficulttohaveasinglemethodabletoreproduceautomaticallyanygeneralsituation.

Hereafter4schemesaredescribed.Theyareallexplicitintime.

Scheme1.isusuallyadoptedforsubcriticalflowsbecauseitpresentsinstabilitiesduringflowregimetransitions(passagethroughcriticalphase)orsupercriticalflows;Scheme2.isapplicablealsoforflowswithafewflowregimetransitions;Schemes3.and4.aregeneralforsubcriticalandsupercriticalflow.

Schemes1.,2.,and3.arefinitedifference(FD)whileScheme4.and5.arefinitevolume(FV).

6.1.1 LaxFriedrichsSchemeTheLaxFriedrichs(LxF)methodisabasicmethodforthesolutionofhyperbolicpartialdifferentialequations(PDEs).Itsuseislimitedbecauseitsorderisonlyone,butitiseasytoimplement,applicabletogeneralPDEs,andhasgoodqualitativepropertiesbecauseitismonotoneandveryrobust.TheLxFmethodisoftenusedtoshowtheeffectsofdissipation,butitisnotactuallyadissipativemethod(Strikwerda1989).

47

Forthegeneralcaseofhomogeneousconservationsystem

0=+

xF

tU

theprocedureofnodalupdatingofaregulargridforeachtimesteptis:

( ) ( )nininininini FFxtUUUU 11111 221 +++ ++= 0

48

Correctorstep

( )pipipici FFxtUU 1= Thefinalsolutionisgivenby

( )cipini UUU +=+ 211 .

6.1.3 TVDMethodsWiththetotalvariationdiminishing(TVD)method,someclassicalnumericalschemesarereformulatedimprovingtheirperformancewithoutgreateffort.

Onlymonotonicitypreservingschemes,whichneverprovidenonphysicalsolutions,areTVD.GodunovstheoremprovesthatonlyfirstorderlinearschemesaremonotoneandarethereforeTVD.

GarciaNavarroetal.(1999),proposeareformulationoftheMacCormackmethodaddingaconservativeandnonlinearmodificationoftheschemetomakeitnonoscillatoryatlocalextremawithoutlosingitsaccuracyinotherregionsoftheflow.

Afirstorder(LaxFriedrichs),asecondorder(MacCormack)andahighresolution(TVDMacCormack)explicitfinitedifferenceschemesareappliedtothesimulationof1Dunsteadyflowinrivers(GarciaNavarroetal.1999).

Whileforsmoothersituationthethreemethodsperformwell,thepresenceofextremeslopesandstrongchangesintheirregulargeometryrepresentasourceofnumericalerrors.

Inthesekindsofproblem,theTVDversionoftheMacCormackschemehasaclearsuperiority,overallinmassconservationability.(Figure20).

49

Figure20ImprovementprovidedbytheTVDscheme.(GarciaNavarroetal.1999).

6.1.4 RoesMethodItsastrategyforobtainingnumericalsolutionstohyperbolicinitialvalueproblems.(Roe,1981).

TheRoesapproximateRiemannsolverappliedtoanupwindfinitevolumemethodhasprovedefficientforunsteadyshallowwaterproblemsoverafixedbedbothinsimplecasesandincasesinvolvingvariablebedslopeandchannelwidth(GarcaNavarroand

VzquezCendn1999),itsaccuracyandstabilityconstraintshavingbeenstudiedinBurgueteandGarciaNavarro(2001).

Themethodisbasedonthelocallinearizationachievedbymeansofanapproximated

Jacobianmatrix J~ ,suchthat

UJF = ~

50

with ii UUU = +1 andbuiltaccordingtoaseriesofnecessaryconditionsforconservationandconsistency(HubbardandGarcaNavarro2000).

ThefirstorderexplicitRoesmethodissuitableforthedescriptionofrigidlandslidemovementsandtheirinteractionwithshallowwaterbodies.(Perezetal2006).

6.1.5 LaxFriedrichTypeSchemeappliedonastaggeredgridThisisasemiimplicitIorderschemeappliedonastaggeredgrid;themomentumequationiswrittenaccordingtothecharacteristicsoftheflowinthecurrentnode.Thisscheme,developedbyNataleandSavi(1992)modifyingtheschemeproposedbySielecky(Vreugdenhil1989),isintuitivefromthephysicalpointofviewandeasytoimplement.Themomentumequationisfullyexplicitandreads:

( ) 2/1112/1

2/12/11

2/1 +++

++++ +

= iiii

ini

ni StgIgIx

tMtQQ

where

= ++++

+n

i

n

sisii A

QA

Qx

M2/1

2

2/1

2

2/12/1

1