Modelling answers tsunami questions �

issue 83 Sept 2006

Modellinganswers tsunami questionsNew research will help emergency plannersOle Nielsen, Jane Sexton, Duncan Gray and Nick Bartzis

TheIndianOceantsunamion26December2004demonstratedthepotentiallycatastrophicconsequencesofnaturalhazards.Inadditiontohumanitarianassistance,theAustralianGovernment’sresponseincludedtheestablishmentoftheAustralianTsunamiWarningSystem(ATWS)andgreaterpriorityforresearchintohazardandriskmodellingoftsunamiimpacts.

Determining tsunami riskGeoscienceAustraliaaimstodefinetheeconomicandsocialthreatposedtourbancommunitiesbynaturalhazardssuchastsunamis.Predictionsofthelikelyimpactsoftsunamiscanbemadethroughtheintegrationofearthquakeandtsunamihazardresearch,communityexposureandsocioeconomicvulnerabilities.Bymodellingthelikelyimpactsonurbancommunitiesasaccuratelyaspossibleandbuildingtheseestimatesintolanduseplanningandemergencymanagement,wecanbetterpreparecommunitiestorespondtotsunamiswhentheyoccur.

OnecriticalcomponentinunderstandingtsunamiriskisbeingexaminedbytheRiskAssessmentMethodsProject(RAMP)atGeoscienceAustraliawhichhasbeendevelopingahydrodynamicinundationmodellingtooldevelopedspecificallytoestimatetheconsequencesofpossibletsunamiimpactsonAustraliancommunities.

Modelling methodologyTsunamihazardmodelshavebeenavailableforsometime.Theygenerallyworkbyvirtuallyconvertingtheenergyreleasedbyasubductionearthquakeintoaverticaldisplacementoftheoceansurface.Theresultingwaveisthenpropagatedacrossasometimesvaststretchofoceanusingarelativelycoarselinearmodelbasedonbathymetrieswithatypicalresolutionoftwoarcminutes.

Themaximalwaveheightatafixedcontourlinenearthecoastline(say,50metres)isthenreportedasthehazardtocommunitiesashore.ModelssuchasMethodofSplittingTsunamis(MOST)(Titov&Gonzalez1997)andtheURSCorporation’sProbabilisticTsunamiHazardAnalysis(Somervilleetal2005)followthisparadigm.

Theseverityofahydrologicaldisasteriscriticallydependenton

complexbathymetricandtopographiceffectsneartheareaofinterest.Forexample,duringthe1993OkushiriIslandtsunami,averylargerun‑upwasobservedatonespecificlocation,whereassurroundingareasreceivedmuchlessinundation(Matsuyamaetal1999).Estimatingtheimpactofatsunamionaparticularcommunitythereforerequiresmodellingofthenonlinearprocessbywhichwavesarereflectedandotherwiseshapedbythelocalbathymetriesandtopographies.Thesecomplexeffectsgenerallyrequireelevationdataofmuchhigherresolutionthanisusedbythelinearmodels,whichtypicallyusedataresolutionsintheorderofhundredsofmetres(sufficienttomodellong‑wavelengthtsunamisinopenwater).Thedataresolutionusedbynonlinearinundationmodels,bycontrast,istypicallyinthetensofmetres.

TheANUGAmodel(Nielsenetal2005)—theresultofcollaborationbetweentheAustralianNationalUniversityandGeoscienceAustralia—issuitableforthistypeofmodelling.However,runninganonlinearmodelcapableofresolvinglocalbathymetric

Figure 1. DatarequirementsforanANUGAsimulationincludetopographyofthestudyarea,atriangularmesh,definitionofinitialandboundaryconditions,andanyforcingterms,suchaswindstress.Boundaryconditionscouldcaptureincomingwavesfromarangeofsources,suchasoutputfromothermodels,run‑offortidalvariations.

initial conditionmeshtopography

boundary condition model forcing terms

output

Modelling answers tsunami questions �

issue 83 Sept 2006

effectsandrun‑upusingdetailedelevationdatarequiresmorecomputationalresourcesthanthetypicalhazardmodel,makingitinapplicableforcompleteend‑to‑endmodellingofatsunamievent.

Wehaveadoptedahybridapproach,inwhichtheoutputfromahazardmodelsuchasMOSTisusedasinputtoANUGAattheseawardboundaryofitsstudyarea.Theoutputofthehazardmodelthusservesasaboundaryconditionfortheinundationmodel.Inthisway,werestrictthecomputationallyintensiveparttoregionswheredetailedunderstandingoftheinundationprocessisrequired.

Furthermore,toavoidunnecessarycomputations,ANUGAworkswithanunstructuredtriangularmeshratherthantherectangulargridstypicallyusedbyhazardmodels.Theadvantageofanunstructuredmeshisthatdifferentregionscanhavedifferentresolutions,allowingcomputationalresourcestobedirectedwheretheyaremostneeded.Forexample,onemightuseveryhighresolutionnearacommunityorinanestuary,whereasacoarserresolutionmightbeenoughfordeeperwater,wherethebathymetriceffectsarelesspronounced.

Toimplementascenario,themodellerrequiressuitableinitialconditions(suchasatidalheight),boundaryconditions(suchasmodeldatafromasubductionzoneearthquake),forcingterms(suchaswind)and,importantly,bathymetricandtopographicdataforthestudyarea(figure1).Thecalculatedrun‑upheightandresultinginundationashoreisdeterminedbytheseinputs,aswellasthecellresolution.

Thedatashouldideallycaptureallcomplexfeaturesoftheunderlyingbathymetryandtopography,andcellresolutionshouldbecommensuratewiththeunderlyingdata.Anylimitationsintheresolutionandaccuracyofthedata,includingthecellresolution,willintroduceerrorstotheinundationmapsaswellastotherangeofmodelapproximations.

Figure 2. Exampleofinundationmapprovidedtoemergencymanagers.Here,themapisembeddedinaGISproduct,enablingemergencymanagerstousetheoutputasadecisionsupporttool.

Modelling answers tsunami questions �

issue 83 Sept 2006

Tsunami impact on the North West shelfHistoricalevidenceoflargetsunamigenicearthquakesoffSumatrawithimpactsontheWesternAustraliancoastlinesuggeststhatcommunitiesandinfrastructurealongthatcoastlineareatriskoftsunamiinundation(Cummins&Burbidge2004).

Tobetterunderstandtherisk,particularlyforthesignificantpetroleumproductioninfrastructureofftheNorthWestShelfandneartheSundaArctrench,theFireandEmergencyServicesAuthority(FESA)inWesternAustraliastruckacollaborativeresearchagreementwithGeoscienceAustralia.InitialpriorityareasareOnslow,PortHedland,Karratha,Dampier,Broome,BusseltonandPerth.ThestudyhasbroughttogetheranumberofgroupswithinGeoscienceAustraliatosupporttheFESAproject.ThestudyareasforthefirstprojectmilestoneareOnslowandPortHedland.

TheboundaryconditionhasbeendefinedbytheEarthquakeandTsunamiHazardProjectmodelofanMw9earthquakegeneratedeastofJavabytheSundaArctrench(Mwisalogarithmicmeasureofearthquakesize,similartotheRichterscalebutbettersuitedtoverylargeevents).Thiseventisplausible,buttherecurrencerateisnotyetknown.TheearthquakeandsubsequenttsunamiwaveindeepwateraresimulatedbyMOST,whichoutputswaterheightandvelocityinspaceandtime.ANUGAthenusesthisinformationandpropagatesthewavethroughtheshallowwaterandonshore.

Thecollationofdatahasprovedtobeachallengingtask.GeoscienceAustralia’sPetroleumandMarineDivisionhassourcedavailablehydrographiccharts(‘fairsheets’)forregionsidentified

ontheNorthWestShelf.Digitisationofsomeofthesechartsisneeded,andmatchingtheentiredatasetrequiressuitablemetadatatobeavailable(whichitseldomis,especiallyforolderdatasets).ThankstotheNationalMappingandInformationGroupwithinGeoscienceAustralia’sGeospatialandEarthMonitoringDivision,offshoreandonshoredatasetsforOnslowandPortHedlandhavebeendeliveredtoRAMPforinundationmodelling.

Oncetheinundationmodellinghasbeencompleted,structuraldamageandcontentslossestimatescanbemade.RAMPengineeringmodelsandthenationalbuildingexposuredatabase(NBED)arebroughttogethertodevelopadamageestimateforeachsimulation.TheNBEDcontainsinformationaboutresidentialbuildings,people,infrastructure,structurevalueandbuildingcontent,andhasbeencreatedsothatconsistentriskassessmentsforarangeofnaturalhazardscanbeconducted.ThedamageestimatesusetheNBEDinformationandpredictprobabilityofcollapseasafunctionofthebuildingtype,locationandinundationdepthatthebuildingandfloorlevels.

Finally,theGISteamwithinRAMPdevelopsthedecisionsupporttool(figure2),whichincludesANUGAoutputs,inundationmaps,timeseriesfordefinedpointlocations,anddamageestimates.Theseoutputsareincludedaslayersinthe

Figure 3. Snapshotofvisualisationoftsunamiinundation,NorthWestShelf(photographcourtesyofDepartmentofLandInformation,WesternAustralia).

Modelling answers tsunami questions �

issue 83 Sept 2006

decisionsupporttool,withaerialphotographyandoverlaysofcriticalinfrastructure,suchasroads.VisualisationsdevelopedfromtheANUGAoutput(figure3)areusefultomodellersandplannersalikeforunderstandingthebehaviourofthetsunami.

Theoutcomeisatacticaldecisionsupporttoolforusebyoperationalemergencymanagersastheymakedecisionsonhowtomitigaterisktocoastalcommunities.Inparticular,thetoolwillprovide:

abetterunderstandingofnationaltsunamiriskandresourcingrequirementsforparticularcommunitiesandregions

scenariosforawiderangeoftsunamieventsforwhichcasualtyandinfrastructureconsequencesarepredicted,andagainstwhichemergencymanagementcapabilitycanbeassessed

real‑timeconsequencepredictiontoolsfortacticalusebyemergencymanagerstoobtainassessmentsoftsunamiimpactandexpectedconsequencestoguideinitialresourcedeployment.

Further studiesThepreliminaryhazardmodellinghasidentifiedcommunitiesmostatriskfromtsunamisgeneratedbysubductionzoneearthquakes.Moredetailedmodelling,whichwillbeavailablebytheendof2006,willprovideinformationonreturnperiodssothattsunamiriskcanbedetermined.ThisisconsistentwithRAMP’sobjectiveofdefiningthenationalriskfromarangeofrapid‑onsetnaturalhazardsinastandardisedandconsistentway.

Therelativetsunamiriskcanbemeasuredusingthemodellingtechniqueswehavedescribed,providingastrategicaidtoemergencyplanning.Inaddition,theprecomputedsimulationsandriskmapswillformalibraryofscenariosfortheATWS,aidingmitigation,

•

•

•

warning,responseandcommunityrecoveryintheeventofatsunamidisaster.

TherecentmeetingoftheAustralianTsunamiWorkingGroupacknowledgedtheutilityofdetailedimpactmodellingformitigatingtheeffectsoftsunamis.However,thebiggestbarriertosuchmodellingistheunavailabilityofreliable,high‑resolutionbathymetryandelevationdata.GeoscienceAustraliahasdevelopedasetofguidelinesforstateagencies,outliningtherequirementsforthecollectionofsuchdata.Theseguidelineswillassisttheexchangeofdatabetweenagenciesandguidethirdpartiesinthecollectionofdata.

OtherstateagencieshaveexpressedinterestinconductingstudiessimilartothosebeingdoneforFESA.GeoscienceAustraliaiscommittedtoworkingwithstateemergencymanagerstounderstandtsunamirisk,andwillcontinuetoconductdetailedstudiesinareasofnationalinterest.

Modelling answers tsunami questions �

issue 83 Sept 2006

More information

phone OleNielsenon+61262499048email ole.nielsen@ga.gov.au

References

CumminsP&BurbidgeD.2004.Smallthreat,butwarningsoundedfortsunamiresearch.AusGeoNews75,4–7.MatsuyamaM,WalshJ&YehH.1999.TheeffectofbathymetryontsunamicharacteristicsatSisanoLagoon,PapuaNewGuinea.GeophysicalResearchLetters26(23):3513–3516.NielsenO,RobertsS,GrayD,McPhersonA&HitchmanA.2005.Hydrodynamicmodellingofcoastalinundation.MODSIM2005InternationalCongressonModellingandSimulation,ModellingandSimulationSocietyofAustralianandNewZealand,518–523.www.mssanz.org.au/modsim05/papers/nielsen.pdfSomervilleP,ThioH‑K&IchinoseG.2005.Probabilistictsunamihazardanalysis.ReporttoGeoscienceAustralia.TitovV&GonzalezF.1997.ImplementationandtestingoftheMethodofSplittingTsunami(MOST)model.NOAATechnicalMemorandumERLPMEL‑112.

Related sites

AusGeo News 81 GeoscienceAustralia’simpactmodellingprotectingAustralialink www.ga.gov.au

RiskAssessmentMethodsProjectwww.ga.gov.au/urban/projects/ramp/inundation.jsplink