-
A R E N C I T e c h n i c a l R e p o r t
w w w . r e n c i . o r g
Adapting Scienti�c Work�ows onNetworked Clouds
Using Proactive Introspection
Storm Surge Computations for the North Carolina SeaLevel Rise
Risk Management Study
TR-12-04
Brian Blanton, PhD Renaissance Computing Institute
(RENCI)University of North Carolina at Chapel
[email protected]
Prepared for:North Carolina Division of Emergency Management,
Office of Geospatial and Technology Management
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NCSeaLevelRiseRiskManagementStudy Page1
Preparedfor:
NORTHCAROLINADIVISIONOFEMERGENCYMANAGEMENTOFFICEOFGEOSPATIALANDTECHNOLOGYMANAGEMENT
StormSurgeComputationsfortheNorthCarolinaSeaLevelRiseRiskManagementStudy
Preparedby
BrianBlanton,Ph.D.
RenaissanceComputingInstitute
UniversityofNorthCarolinaatChapelHill
November12,2012
Version1.0
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CONTENTS
Contents...................................................................................................................................................2
1.
OverviewofADCIRCApplication.............................................................................................31.1.
TheUseofADCIRCtoEvaluatetheCoastalFloodingHazardfortheNorthCarolinaSeaLevelRiseRiskManagementStudy...............................................................................................................................31.2.
ReviewofMethodologyfortheNorthCarolinaFloodInsuranceStudy.......................................3
2.
ADCIRC-relatedTASKS................................................................................................................42.1.
Task1:SupportJPMSensitivityTest..................................................................................................42.2.
Task2:WindFieldGeneration...........................................................................................................62.3.
Task3:TidalDatumCode..................................................................................................................62.4.
Task4:PerformBaselineSimulations................................................................................................6
2.4.1
Equilibriumtidalsolution.............................................................................................................62.4.2
TidalDatums................................................................................................................................62.4.3
SimulationofHistoricalHurricanes..............................................................................................82.4.4
Productionstormsurgesimulations..........................................................................................92.4.5
Statisticalsynthesis....................................................................................................................11
2.5.
Task5:PerformSeaLevelRiseScenarioSimulations......................................................................122.5.1
20cmScenario:..........................................................................................................................132.5.2
40cmScenario:..........................................................................................................................142.5.3
60cmScenario:..........................................................................................................................152.5.4
80cmScenario:..........................................................................................................................162.5.5
100cmScenario:........................................................................................................................17
2.6.
Task6:Storminessstatisticalanalyses.............................................................................................182.6.1:20cmSLRandMid21stCentury:..................................................................................................192.6.2:40cmSLRandMid21stCentury:..................................................................................................202.6.3:40cmSLRandEnd21stCentury:..................................................................................................212.6.4:60cmSLRandMid21stCentury:..................................................................................................222.6.5:60cmSLRandEnd21stCentury:..................................................................................................232.6.6:80cmSLRandMid21stCentury:..................................................................................................242.6.7:80cmSLRandEnd21stCentury:..................................................................................................252.6.8:100cmSLRandMid21stCentury:................................................................................................262.6.9:100cmSLRandEnd21stCentury:................................................................................................27
2.7.
Overviewofstatisticalresults..........................................................................................................28
3.
CapeFearwaterlevels.............................................................................................................303.1.
IdealizedModelExperiments..........................................................................................................313.2.
IdealizedModelResults...................................................................................................................343.3.
ConnectionofIdealizedResultstoNC-SLRIS...................................................................................363.4.
IdealizedResultsConclusion............................................................................................................40
4.
References....................................................................................................................................41
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1. OVERVIEWOFADCIRCAPPLICATION
1.1.
TheUseofADCIRCtoEvaluatetheCoastalFloodingHazardfortheNorthCarolinaSeaLevelRiseRiskManagementStudy
Toaddress theevaluationof the coastal hazard component for
theNorthCarolina Sea LevelRise Impacts Study (NC-SLRIS), the
Renaissance Computing Institute (RENCI) proposed
anapproachthatfollowsconceptuallytheapplicationofthetidalandstormsurgemodelADCIRCforcomputingfloodhazardlevelsfortherecentFloodInsuranceStudy(FIS)forNorthCarolina’scoastalcounties.TheFISapproachusesahigh-resolutionnumericalmodelgridforstormsurgeandwavesbasedonrecenttopographicsurveysandbest-availablebathymetricdata,aswellasadvanced
statistical techniques for modeling North Carolina’s tropical storm
climate. Theapproach addresses conduction and management of needed
simulations on RENCI’s high-performancecomputers.
1.2.
ReviewofMethodologyfortheNorthCarolinaFloodInsuranceStudy
RENCIrecentlycomputedfloodhazarddatafortheNorthCarolinaFloodplainMappingProgram(NCFMP)FloodInsuranceStudy(FIS)forcoastalcounties.Thecomputationalsystem(Blanton,2008)developedfortheFISusesstate-of-the-artnumericalmodels,
includingthestormsurgeandtidalmodelADCIRC(Westerinketal,2008),andusescomputerresourcesatRENCIfortheactual
computations. This systemhasbeen testedseveral times in thepast
threeyears,andRENCIconsidersthissystemacceptablyrobustforthepurposesofNC-SLRIS.
The NC FIS project developed a comprehensive digital elevation
model (DEM) using
recentcoastalLIDARdataaswellasbest-availablebathymetricdata.
ThisDEMwasusedtodevelopthe ADCIRC grid that is being used for the
flood hazard simulations. Additionally, the JointProbabilities
Method (JPM) approach to model the current tropical storm
population (P.Vickery, Applied Research Associates) represented an
advanced application of JPM
tosubstantiallyreducethenumericalmodelcomputationalresourcerequirements.
IntheNCFISsystem,theparametricboundarylayermodelHBL(Vickeryetal,2009)modelsthetropical
stormwind and pressure fields. Stormparameters forHBL are derived
through
theabove-mentionedJPMapproach.TheextratropicalcomponentismodeledwithanalyzedwindandpressurefieldsfromOceanWeather,Inc.
IntheNCSLRRMScontext,theprimarycomponentsoftheNCFISwillbeused.Thisincludesthe
model ADCIRC, a comprehensive ADCIRC grid for the region, and a
modification of thestorm tracks developed for the FIS, considered
the baseline storm population for SLR
RMS.Thesestorms,andmodificationsthatrepresentfuturestormclimates,aredescribedelsewhereintheTask2approach(ARA,PeterVickery).TheexistingextratropicalstormsetfortheFISwillbeusedforanyrequiredextratropicalsimulations.
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2. ADCIRC-RELATEDTASKS
TheRENCI/UNCscopeconsistsof6 tasks,numbered the sameas in
theRFDO. Task3
(TidalDatumCode)wasnotpartofthefinalRENCIproject;however,weretainthesamenumberingas
in theRDFO for clarity. The specific tasks are as follows,
eachofwhich is detailed in
thesubsequentsectionsofthereport.Task1:SupportJPMSensitivityTestswithARATask2:WindFieldGenerationTask3:TidalDatumCodeTask4:PerformBaselineSimulations–present-dayADCIRCgridandstormclimateTask5:PerformFutureSLR/GeomorphologicalSimulations–ADCIRCgridsand/orincreasedmeansealevelthatcorrespondto20cm,40cm,60cm,80cm,and100cmscenarios.Task6:StorminessIncorporation
Tasks 1, 2, and 6 are to compute specific analyses and/or inputs
to themain computationaltasks (4and5).
Tasks4and5requiredthemajorityof
theeffortandconsumedsubstantialcomputationalresources.
2.1. Task1:SupportJPMSensitivityTest
Task 1 of RENCI’s component of the NC-SLRRIS project provides
computational support toAppliedResearchAssociates (ARA)
todeterminea reduced setof storm tracks foruse in
theBaselinecomputationofcoastalhazards.Tothisend,RENCIhasconductedADCIRCsimulationsonacoarseADCIRCgridusingstormtrackdataprovidedbyARA.
RENCIprovidedthestormsurgeresultsbacktoARAforfurtheranalysis.
RENCIreceivedtwosetsofcandidatestormtracks from ARA:
NC-Reduced-set-1-B-Caseand NC-Reduced-set-2-BandC-Case. Thesesets
contain 450 and 294 tracks,respectively. Each set was converted
fromtheARA“hur”formattotheADCIRCfort.22format, and the surge
response wascomputed on a coarse grid that covers theNorth Carolina
region and surroundingwaters(Figure1). Thisgridhas4636nodesand 8703
elements, does not include
land(i.e.,doesnotsupportlandinundation),andexecutesveryfast
inaserialcomputationalmode. Maximum surge levels were Figure 1:
Coarse ADCIRC grid used for storm
trackselectionscreening.
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recordedateachof117coastalnodesinNorthCarolina(indicatedbythereddotsinFigure1).Inadditiontothetworeducedtracksets,RENCIalsocomputedthecoarsegridsurgeresponsetothefullNCFIStrackset,whichcontains675tropicalstorms.
Each simulation computes amaximum storm surge, an example
ofwhich is shown inError!Reference source not found. for a
relatively intense landfalling storm (dp4r3b2c3h1l1).
Foreach“scenario”orsetoftracks,themaximumsurgeresultsaregatheredandthereturnlevelsarecomputedusingtheJointProbabilitiesMethodasdescribedbyARA.Figure3showsthe1%water
level at each coastal node in the coarse grid,with the numbering as
indicated
above.Changesincoastlineorientationandcontinentalshelfwidthareevident.Comparedtothe1%surgelevelscomputedwiththefullFIStrackset(NCFMP_Final_EQSpacing),thermsdifferenceforbothreducedsetsis11cm.ThisissummarizedintheError!Referencesourcenotfound..
Table 1: Basic statistics for comparison of surge results from
the three track sets.
Thecomparisonismadeforthe1%returnlevelatthecoastalnodes.
Rms min maxSet-1-B-Case .11 -.29 .27Set-2-BandC-Case .11 -.35
.25
Figure 2: Example of coarse grid surgeresponse to a landfalling
storm. The stormtrack is shown with the black line, and
thewaterlevelisshownincolorinmeters.
Figure 3: 1% Surge return level at
coastalnodelocationsforthestormsurgecomputedfromthethreetracksets.Theabscissaisthenumbering
of the coastal nodes (1-117) asindicated in Error! Reference source
notfound..
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2.2. Task2:WindFieldGenerationEachtropicalstormdefinedas
inputtothecomputationalsystemrequireswindandpressurefieldstoforcethemodels.Inthistask,RENCIcomputedthewindandpressurefieldsassociatedwitheachofthe294tropicalstormsdefinedbyARA,usingARA’sHBLwindmodel.Thisstormsuitewasused
for themaincomputational scenarios
(baseline,20cm,40cm,60cm,80cm,and100cmADCIRCgrids).
2.3. Task3:TidalDatumCodePrior to initiation of the RENCI tasks,
the explicit task to develop a tidal datum code
wasremovedfromthescope.However,thisoriginaltaskismaintainedinthescopingandreportingdocumentstomaintainconsistentnumberingbetweenthetaskorder,proposals,andcontracts.Thetidaldatumcodewasdevelopedindependentof,andexternalto,theNC-SLRISproject.
2.4.
Task4:PerformBaselineSimulationsAbaselinescenariowascomputedonthefirstprojectADCIRCgrid(Version4.3.2).Thisscenarioreflectsthepresent-daystormclimateandthegridisareducedversionofthelargerFISgrid.All
simulations in thisandsubsequentSLR scenariosused
thecoupledADCIRC+SWANmodel,version 49.60. For this present-day
scenario, as well as the subsequent SLR scenarios,
thefollowingsetofactivitieswasperformed.
Resultsforthebaselineactivitiesareshowninthissection.ResultsfromtheSLRsimulationsareshowninSection2.5below.
2.4.1
EquilibriumtidalsolutionAnequilibriumtidalsolutionwascomputedonthescenariogrid,withthesametidalelevationboundary
conditions thatwereused in theNC FIS tidal validation study (Egbert
et al, 1994).The main model outputs from this simulation include
the global elevation and velocityharmonic analysis files and a
station velocity file. The stations for velocity output
werespecifiedacrosseachtidalinletandrivermouth.Thisoutputwasusedbysubsequentprojectanalyses.Themainsimulationparametersareasfollows:
2.4.2
TidalDatumsTidaldatumswerethencomputedateachADCIRCgridnodefromtheglobalharmonicanalysisfileoftheequilibriumtidalsolution.ThisincludestheusualtidaldatumsofMeanHigherHighWater
(MHHW),Mean HighWater (MHW),Mean LowWater (MLW), andMean Lower
LowWater(MLLW),aswellascumulativedistributionsoftidalheightsneededforthesubsequentJPMandESTstatisticalanalyses.GIS-compatiblefilesforeachdatumwerealsoproduced.
TimeStep 0.5secRunLength
77.625days(150M2tidalcycles)RampupLength
10.35days(20M2tidalcycles)TidalConstituents
M2,S2,N2,K2,K1,O1,P1,Q1HarmonicAnalysisPeriod
Days10.35through77.625
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Eachsurfaceisinitiallydefinedonlyoverwater,sincetheharmonicanalysisresultsaresensitivetothepercentoftimethatanode
iswettedduringtheharmonicanalysisperiod. Inordertouse the tidal
datums over land for the surge statistical analyses, the datum
surfaces areextended inlandtocoverthearealextendofthesurgeresults.
Thedatumsarecomputed inMSL and converted toNAVD88 as the last step
in the analysis. Figure 4
showsMHHWandMLLWwaterlevelsaboveNAVD88forthebaselinescenario.
Figure4:Tidaldatumsfrombaselinescenarioequilibriumtidalsimulation.Left)MeanHigherHighWater(MHHW).
Right)MeanLowerLowWater(MLLW). UnitsaremetersrelativetoNAVD88.The
computed datum values were compared to the NOAA-published values
(Table 2).
TheNOAAvaluesarepublishedrelativetothestationdatum,sotheMSLlevelhasbeensubtractedfromtheNOAAvaluesforcomparisontothecomputeddatums,whicharerelativetoADCIRC’smeansealevel.Overall,thecomputedtidaldatumsarewithin2-6cmoftheobservedvalues.
Table2:TidalDatumComparisonforBaselineEquilibriumTidalSolution.UnitsaremetersrelativetoMSL.
NOAA ADCIRC NOAA ADCIRC MHHW-MSL MHHW MLLW-MSL MLLW
Duck,NC 0.58 0.59 -0.54 -0.51OregonInletMarina,NC 0.18 0.21
-0.18 -0.18
CapeHatterasFishingPier,NC 0.56 0.55 -0.49 -0.46Beaufort,NC 0.56
0.59 -0.52 -0.51
Wilmington,NC 0.68 0.72 -0.74 -0.69WrightsvilleBeach,NC 0.69
0.71 -0.62 -0.62
Southport,NC 0.74 0.74 -0.71 -0.66SunsetBeach,NC 0.87 0.83 -0.81
-0.76
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2.4.3 SimulationofHistoricalHurricanesTwo historical events
(Fran 1996; Isabel 2003) were simulated. These simulations
includedtides,witha45-daytidalspinupperiodpriortotheonsetofthewindandpressureforcing,andstartingequilibriumadjustment
factors for thespecific startdate. Themaximumwater
levelandwaveheightswereretainedforvisualizationaswellasotherprojectanalyses.
Table3:Startandendtimesforthevalidationsimulationcomponents,andrunlengthsindays.
Storm TidalStartDate MetStartDate SimulationEndDate
Metlength,TotalRunLength[days]
Fran(1996) 1996-07-1600Z 1996-08-3000Z 1996-09-0700Z
7.8,52.8
Isabel(2003) 2003-07-3100Z 2003-09-1400Z 2003-09-1912Z
5.5,50.5
The maximumwater levels from the two historical tropical storm
simulations are shown inFigure5. HurricaneFranmaximumwater
levelsare largestalongthecoast
fromWrightsvilleBeachsouthwardtoFryingPanShoals,withthehighestlevelsreaching3.25meters.CapeFearRiverlevelsareabout1.0to1.25meters.HurricaneIsabelcausedlargerinlandeffects,mainlyinthePamlicoSoundarea,withthemaximumlater
levelsgenerally
inthelowerNeuseRiver.ThesesolutionswerecomparedtotheobservedhighwatermarksthatwereanalyzedfortheNCFIS.TheerrordistributionsareshowninFigure6.Bothsolutionsarerelativelyskillful,withrmserrorsof0.38and0.30meters,andareconsistentwiththevalidationresultsfromtheNCFIS.
Figure5:Historical simulationmaximumwater level for thebaseline
(present-day)scenarioforHurricanesFran(1996)andIsabel(2003).Thestormtracksareshownwiththeblackline.WaterlevelunitsaremetersNAVD88.
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2.4.4
ProductionstormsurgesimulationsTheproductionstormsurgesimulationsare
themaincomputational step in
thebaselineandSLRscenario.ThisstepcarriedouttheproductiontropicalandextratropicalstormsimulationsrequiredfortheESTandJPMreturnperiodanalyses.Generally,theextratropicalstormswererunfirstinorderfortheESTstatisticalprocessingtooccurconcurrentlywiththetropicalstormsimulations.Thecombinednumberofstormsimulations(294tropical+21extratropical=315)tookabout4weeks
to compute. Anoverviewof theproduction simulation results is
shownnext. Therankedstormsurgevaluesareshown inFigure7 for
threecoastal locations.
Bothextratropicalandtropicalstormsareincluded.Generally,theextratropicalstormsurgesareoflarger
magnitude north of Cape Hatteras (Duck). The 1993 extratropical
storm
causedsignificantsurgesintheCapeFearRiver,seenasthegreencircleataboutstormnumber275.Themaximumofall
stormmaxima is shown inFigure8. The
largestvaluesexceed5metersalongthecoast in
theWrightsvilleBeacharea,with3+meters in the
lowerCapeFear,upperNeuse,NewandWhiteRivers. Since theseare
themaximumwater levels
reachedacrossallstorms,theyrepresentannualoccurrencelevelsnear0.1%(1in1000years).
Figure 6: Error distribution for historical baseline
simulations. Left) Hurricane Fran
(1996).Thermserroris0.38m.Right)HurricaneIsabel(2003).Thermserroris0.30meters.
00.050.1
0.150.2
0.25
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
Relativ
eFreq
uency
Error[m]
FranBaselineError
00.050.1
0.150.2
0.25
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
Relativ
eFreq
uency
Error[m]
IsabelBaselineError
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Figure 7: Ranked storm surges for three coastal locations for
the baseline
productionsimulations.Circlesmarktheextratropicalsimulationresults.Thereare315totalstormsinthestormpopulation.
Figure8:MaximumateachADCIRCnodeacrossallbaselinesimulations.Unitsaremeters
NAVD88.
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2.4.5
StatisticalsynthesisThefinalstepisthestatisticalsynthesisofthetropicalandextratropicalresults,wheretheJPMprobabilities
are used with the tropical production simulation results to compute
the waterlevels associated with the 10%, 4%, 2%, 1%, .2%, and .1%
annual chances of
occurrence.Likewise,theEmpiricalSimulationTechnique(EST)
isusedtoderivereturn levelsatthesamefrequencies. All EST analyses
were carried out at Dewberry. The final return levels are
astatisticalcombinationofthe(independent)JPMandESTresults.
AllstatisticalmethodsusedareconsistentwiththeNCFIS,andeach(JPM,EST,combinations,andincorporationoftides)arefullydescribedinpriorprojectdocuments,theNCFISIDS#3document,aswellastheESTUser’s
Guide (Scheffner et al, 1999). The final data set from this step is
the
combined(JPM+EST)returnlevelsforthe10%,4%,2%,1%,.2%,and.1%annualchancesofoccurrence.SummaryresultsareshowninFigure9.Theleftpanelshowsthe1%waterlevelfortheNorthCarolinacoastalwaters.Thecolorscalemaximumissetto5metersforeasiercomparisontowater
levels for theSLRscenariosdescribedbelow. Water
levelsarehighestalong theopencoast in theWrightsville Beach area
and lowest in the sounds. The Neuse River levels
arehigherthanthesoundlevels,consistentwiththeFISresults.TherightpanelshowstheannualchancelevelsatDuck,WrightsvilleBeach,andWilmingtonnodes.
Figure9:Left)1%waterlevel(mNAVD88)forthebaselinescenario.Right)Waterlevelsatstandardoccurrencefrequenciesatthreelocations.
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2.5. Task5:PerformSeaLevelRiseScenarioSimulationsThe sea level
rise scenarios were computed in the same manner as the baseline
(Task 4)scenario. In addition to the equilibrium tides, historical
storms, production simulations, andstatistical synthesis, one
additional step was carried out. In order to
determinegeomorphologicalchangesforascenario’sADCIRCgrid,RENCIconductedanequilibriumtidalsimulationonthepreviousscenario’sgridbutwiththecurrentscenario’sSLRamount;inotherwords,
the effect of the SLR amount on the tides on the previous
geomorphologicalconfiguration/grid. Tidal datums and tidal
velocities across the region’s tidal inlets
werecomputedandprovidedtoDewberryforfurtherapplicationtothescenario’sADCIRCgrid.Each
project sea level rise amount (20, 40, 60, 80, 100 cm) represents a
“loop” through thebaselineprocedure. Themaximumstormsurgesurfaces
for thehistoricalstormsareshownfirst, and then 1% surface and the
return levels at the three selected coastal locations
areshowninFigure10throughFigure14.The historical storm results show
the generally expected behavior of increased water
levelsalongtheopencoastintheareaswherethestormshadthelargestimpacts.ThesurgeimpactsinthelandareabetweenPamlicoandAlbemarleSoundsbecomesincreasinglyfloodedasMSLincreases,withwaterlevelsreaching1.5metersforFran(1996).Forexample,thisareafloodsinthe100cmcaseandhaswater
levelsataboutthe1%level (comparetheFranwater
leveland1%waterlevelinFigure14).Thisareadoesnotfloodinthebaselinethrough40cmcases.Open-coastwater
levels tend to increaseatamountsconsistentwith the increasedsea
level.Physically,thisisbecausetheaveragewaterdepthandcontinentalshelfwidthdonotchangedappreciably
as themean sea level increases. Since themeandepth and shelfwidth
are
theprimaryfactorsthatcontrolregionalsurgeresponse,andthereislittlechangeisthedepthandshelf
width, surge increases essentially linearly with the mean sea level
increase. In thesheltered waters, however, the simple linear
response seen along the open coast is
notexpected,generallyduetononlineareffects.Particularly,thearealextentoflandbelowmeansea
level increasesrapidlyasmeansea level increases,andthusthe
lower-lying, flatterareas(e.g., land west of Pamlico Sound) exhibit
substantially increased inundation in the
end-memberscenario(100cm).
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2.5.1 20cmScenario:
Figure10:20cmscenariohistoricalstormmaximumsurge,1%combinedwaterlevel,andreturnlevelsforthethreeselectedstationlocations.
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2.5.2 40cmScenario:
Figure11:40cmscenariohistoricalstormmaximumsurge,1%combinedwaterlevel,andreturnlevelsforthethreeselectedstationlocations.
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2.5.3 60cmScenario:
Figure12:60cmscenariohistoricalstormmaximumsurge,1%combinedwater
level,andreturn levelsforthethreeselectedstationlocations.
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2.5.4 80cmScenario:
Figure13:80cmscenariohistoricalstormmaximumsurge,1%combinedwaterlevel,andreturnlevelsforthethreeselectedstationlocations.
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2.5.5 100cmScenario:
Figure14:100cmscenariohistoricalstormmaximumsurge,1%combinedwaterlevel,andreturnlevelsforthethreeselectedstationlocations.
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2.6.
Task6:StorminessstatisticalanalysesTheeffectsofpossiblechangesintropicalcycloneareincorporatedintothescenariostatisticalsynthesesbyreweightingtheJPMstormweights.Noadditionalordifferentstormsimulations(eitherwind/pressureorADCIRCsimulations)wererequired.
Thereweightingprocedureandresultsisdescribedelsewhereintheprojectdocumentation.ARAprovidedtwosetsofrevisedstormweights;
SetA for themid 21st century, and Set B for the end 21st century.
For
eachcombinationofSLRscenarioandstorminesssetinTable4,numbered1-9,theJPMreturnlevelswererecomputedusingtherevisedstormweightsandthepreviouslycomputedsurgeresultsfromtheSLRscenario.
Results forthe1%surfaceandthereturn levelsatthethreeselectedcoastal
locationsareshownbelow,foreachofthestorminessscenarios,
inFigure15throughFigure23.
Table4:Storminessandscenariocombinations.
StorminessApplication
Value,m TimeFrame a b0 2010-2100 0.1 2050-2100 0.2 2050-2100 1
0.3 2075-2100 0.4 2100 2 30.6 2100 4 50.8 2100 6 71 2100 8 9
StormSeta: Mid21stcenturyStormSetb: Endof21stcentury
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2.6.1:20cmSLRandMid21stCentury:
Figure15:1%water leveland return levels for storminess
combination1of20 cmSLRandmid-centurystormclimate.
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2.6.2:40cmSLRandMid21stCentury:
Figure16:1%water leveland return levels for storminess
combination2of40 cmSLRandmid-centurystormclimate.
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2.6.3:40cmSLRandEnd21stCentury:
Figure17:1%waterlevelandreturnlevelsforstorminesscombination3of40cmSLRand
end-centurystormclimate.
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2.6.4:60cmSLRandMid21stCentury:
Figure18:1%waterlevelandreturnlevelsforstorminesscombination4of60cmSLRand
mid-centurystormclimate.
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2.6.5:60cmSLRandEnd21stCentury:
Figure19:1%water leveland return levels for storminess
combination5of60 cmSLRandend-centurystormclimate.
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2.6.6:80cmSLRandMid21stCentury:
Figure20:1%water leveland return levels for storminess
combination6of80 cmSLRandmid-centurystormclimate.
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2.6.7:80cmSLRandEnd21stCentury:
Figure21:1%water leveland return levels for storminess
combination7of80 cmSLRandend-centurystormclimate.
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2.6.8:100cmSLRandMid21stCentury:
Figure22:1%waterlevelandreturnlevelsforstorminesscombination8of100cmSLRandmid-centurystormclimate.
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2.6.9:100cmSLRandEnd21stCentury:
Figure23:1%waterlevelandreturnlevelsforstorminesscombination9of100cmSLRandend-centurystormclimate.
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2.7. OverviewofstatisticalresultsThe following figures show the
return levels for the different scenarios for each
stationseparately. TheDuck andWrightsville locations are
representativeof theopen coasts northandsouthofCapeHatteras,
respectively. For theSLRscenarios (Figure24), return
levelsaregenerallylower,foraspecificrecurrencefrequency,alongtheOuterBanks,andtheincreaseinwaterlevelatlowerfrequenciesisless.Thisisdueprimarilytotherelativelysmallerimpactofthetropicalstormsurgesresultsinthecombinedwaterlevels.Essentially,theoppositeoccursinthelowerpartoftheNCcoastalwaters;waterlevelsatspecificfrequenciesarelargerthanthose
in theOuter Banks, and levels increasemore as the
frequencydecreases.
ThewaterlevelbehavioratWilmingtoniscomplexanddiscussedfurtherinSection3.
Figure24:SLRScenarioreturnlevelsatthethreemodelnodesforDuck,WrightsvilleBeach,andWilmington.
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Figure 25 shows the return levels for the storminess scenarios.
Generally, the qualitativeresults are similar to the SLR scenarios
in terms of the behavior north and south of
CapeHatteras,aswellasintheupperCapeFear.TheimpactsofthestorminessJPMmodificationsare
most notable at the Wrightsville Beach location, where the return
levels at
lowerfrequenciesarehigherthanthoseforthepresent-daystormclimate(asshowninFigure25).
Figure25:StorminessScenarioreturnlevelsatthethreemodelnodesforDuck,WrightsvilleBeach,andWilmington.
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3. CAPEFEARWATERLEVELS
During analyses of theNC-SLRISmodel-generated products for the
statistical flood
elevationsurfaces,itisnoticedthat,intheWilmingtonarea,SWELvaluesdonotincreaseinproportionto
the increasedMSL increment imposed on the scenario. This area is of
particular
concerngiventheplacementofaNOAAtidegaugeneartheturningbasinandthedataanalysisofthewaterlevelrecordinthecontextofchanneldredgingactivitiesandtidalamplitudes.At
face value, smaller increases in SWEL than expected seem
problematic and concern
wasplacedonRENCI’snumericalmodelresults.ThisunexpectedbehaviorisshowninFigure26fortheJPMresults,alongapproximatelyequidistantpointsfromtherivermouth(0km)toabout30
km upstream of Wilmington, taking the westward channel branch to
the west of EagleIsland.Theupperplot shows the1%SWELvaluesalong
theCapeFearRiver including tidesand
theerrortermfromtheNCFISproject.ThelowerplotshowstheJPMlevelswithoutthetidalanderror
contributions. Considering first the JPM+Tides+Error results,
the1%values increasebyaboutthesea level increaseattherivermouth.
Asthescenario“increases”,thewater levelsincrease in the lower river
region,butdonot increaseasmuchasexpectedupstreamof
theWilmingtonarea(atabout45kmfromtherivermouth).Additionally,Itappearsthatthe40cmwater
levels are actually larger than the 3 later scenarios, at least in
a limited region nearWilmington.
Figure26:Along-channel1%JPMwaterlevelsfortheNCSLRISscenarios.Top)1%JPMlevelswithtidesanderrorterm.Bottom)1%JPMlevelswithsurge-only.
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Thelowerplot(JPM,notides,noerror)indicatesthat,evenwithoutthetides,thefloodwaterlevelsdonotbehaveinastrict“bathtub”sense.Inresponsetothisconcern,RENCIhasscrutinizedthemodeloutputanddeterminedthatthemodelresponsesarewellwithinexpectations,giventheextentofgeomorphologicalchangestothe
underlying scenario topography and bathymetry, which determines the
ADCIRC gridelevations. Additionally, the unexpected behavior of
SWEL levels can be explained
byconsideringthetidalresponsetoariverchannelthat1)stronglyconverges
intermsofcross-channelwidthanddepthconvergingand2)thathasasubstantial
intertidalstorageareathatalso increases as sea level increases.
This storage volume (defined as the volume of
watercontainedbetweenthechanneltopo/bathyandtheMHWandMLWwatersurfaces)generallyincreases
as sea level increases. However, the storage volume may change due
tomorphological considerations that are themselves a complicated
response of increased
sealevels,modifiedtidalregime,etc.Ofcourse,inADCIRC,thetopo/bathyisstatic,meaningthatforagiven
scenarioand tidal regime, theeffectsof the sea level increasecanbe
reasonablyunderstood.Apurely
linear,“bathtub”perspectivemotivatestheexpectationthatSWEL,tidalamplitudes,and
tidal datums increase relatively monotonically with the imposed
static sea levelincrement. This would be largely correct if
geometric considerations of the river could beignored and (more
importantly) if therewere no imposed geomorphological changes on
theriverbasin. In the limitofachannelwithverticalwalls (i.e.,no
floodplain),
thenthebathtubexpectationisessentiallycorrect.Foradetailedanalysisofidealizedchannelresponsetotidalforcingundersealevelincreases,seeFriedrichs,Aubrey,andSpeer(1990).
In rivers with complex geometries, the tidal water level response a
sequence of sea levelincreases depends critically on how the
intertidal storage volume increases. As this volumeincreases, tidal
amplitudes may decrease as the incoming tidal wave is reflected
less.
Theconsequenceofdecreasingreflectionisthattheprogressivecharacterofthewaveincreases,inresponse
to increased conveyance upstream to fill the increased volume. If
the
volumeincreaseissmall,thentheincreasedmassfluxneededtofillthatvolumeisalsorelativelysmalland
tidal amplitudeswill decrease a small amount. However, if the
volume change is
large,thenthetidepropagationcharacteristicscanchangesubstantially.
3.1. IdealizedModelExperimentsTo illustrate this dependency, and
in addition to the engineering and coastal
oceanographicliterature,wehaveconductedasequenceoftidalsimulationswith
idealizedrivers.
ThebasicCapeFeargeometriccharacteristicsareusedtodefine
thealong-channeldependenceonthecross-channelwidthanddepthprofiles.Inparticular,cross-channeldepthprofilesweretakenupstreamintheNCSLRISBaselineand100cmgridsandusedtodefinethetopo/bathy
intheidealized grids. Themouth of the Cape Fear River is about 1
kmwide, but the river
widthexpandssubstantiallyimmediatelyupstream.Thisisindicatedinthegridfiguresbelowasthewideningoftheriverbetween0and3km.
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The idealized river is 70 km longwith a constriction in channel
and bankwidth at about 40km.
Thisplaces“Wilmington”atabout45kmupstream. Thisbaselinegeometry is
shown inFigure27.
Figure27:Idealizedbaselinegridgeometry.Left)Planviewofgrid,with2,0,and-10metergrid
elevation contours. The thick black line is the grid’s exterior
boundary. Right)
Cross-channeldepthsat10kmintervalsalong-channel.Theprofileat50km(blacksquares)isthenarrowestpartofthechannel.
To represent the end-member SLR scenario of a substantially
altered storage volume in
theupper(northofWilmington)CapeFearRiver,thebaselinegriddepthprofilesaremodifiedtobroadenANDdeepenthestorageupstreamofabout50km.ThisgridisshowninFigure28.Inadditiontoa1-meterincreaseinmeansealevel,theupperbasinisdeepenedtoincreasethevolume.Eachconfigurationincludesadeepcentralchannelwithvariable-widthalong-channelfloodplainbanks.
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Figure28:
Idealizedgridgeometrywithincreasedstoragecapacityontheupperbasin.Left)Planviewofgrid,with2,0,and-10metergridelevationcontours.Thethickblacklineisthegrid’sexteriorboundary.Right)Cross-channeldepthsat10kmintervalsalong-channel.Theprofileat50km(blacksquares)isthenarrowestpartofthechannel.
Threesimulationswereconductedtoshedlightonthetidaldynamicsinchannelswithvaryingtidalstoragevolumesinanupperbasin:
1. Baselinegeometry(Figure27)withnosealevelincrease.2.
Baselinegeometrywitha1-metersealevelincrease-ThisisthegridinFigure27with
adatumoffsetof+1meter.3.
Deeperupperbasingeometrywitha1-metersealevelincrease(Figure28).
Simulationparametersinclude:1. 10dayslongwitha2dayrampup.2.
Harmonicanalysisoverdays6-10.3.
ElevationboundaryconditionofanM2tidewith1-meteramplitudeand0phase.4.
Norotation.5. ConstantManning’sN.
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3.2.
IdealizedModelResultsThenatureofthetidesintheseidealizedscenariosisinvestigatedbylookingattheprimarytideelevation
response in the along-channel direction. Figure 29 shows the
along-channel
M2elevationamplitude(black)andphase(red)forthethreescenarios.Forallscenarios,thetidalamplitudedropsslightlyduetotheexpansionoftheriverwidthimmediatelyupstreamoftheopenboundary.
Case1and2 (bothon thebaselinegridgeometry)amplify slightly
fromtheminimum amplitude at 10 km. However, the case1 amplitude
increases relative to theboundary condition by about 10%. The
distance at which the amplitudes begin to
dropsubstantiallyismostupstreamincaseone,atabout55km.Case3(deeperupperbasinAND1mSLR)exhibits
this characteratabout45km,which is in thevicinityof “Wilmington” in
theidealizedgeometry.
Figure29:Along-channelM2Elevationamplitudeandphaseforthe3idealizedcases.
In the case 1 scenario, the tide amplifies due to the convergent
natureof the channel.
ThisamplificationdecreasesastheaveragewaterdepthincreasesANDtheriverwidthswiden(lessconvergent).
The M2 elevation phase increases upstream (later arrivals) and
undergoessubstantial lags upstream of about 50 km. The phases are
all about 60 deg in the idealizedWilmington area, but as the water
deepens and storage volume increases, the tide
arrivesearlierintheupperbasinarea.
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Next,weconsiderthealong-channeltidalvelocitycomponent.ThisisshowninFigure30.Thevelocity
amplitudes are all relatively smaller and relatively the same in
the lower
riverarea.However,justdownstreamofthechannelconstriction,thevelocityincreasesduetotheconvergingchannel.Forthebaselinegrid(case1and2),theupstreamvelocitypeaksatabout1.35m/s,withthemaximumoccurringmoreupstreamincase1.Theabruptchangesinspeedare
due to the transition of the channel depths. In case 3 (deeper
upper basinwith 1m
SLincrease),thepeakvelocityexceedsthatattheopenboundaryandoccursmoredownstreamrelativetothebaselinegridcases.Themorerapiddropinspeedisduetotherapidlyincreasingstoragearea.
Figure30:Along-channelM2velocityamplitudeandphaseforthe3idealizedcases.
While the elevation and velocity amplitude and phase are
illustrative of the impacts ofincreased storage volumes in the
upper river basin, they themselves do not reflect thetransition of
the incoming tidewave frommore standing (case 1) tomore progressive
(case3).Recallthatforastandingwave,thewaterlevelandvelocityareoutofphaseby90degrees,andforapurelyprogressivewavetheelevationandvelocityareexactly
inphase. Soit
isthephasedifferencebetweenelevationandvelocitythatismostrelevanttothenatureofthetidalwave.Figure31showsthisdifferenceforthe3idealizedcases.Inthebaselinecase1,thetidebecomes
more standing in character (relative to the open boundary phase
difference) andshifts toward a mixture of standing and progressive
upstream of the narrowest part of
thechannel.Thesmallestphasedifferenceisabout30degrees.Thecase2(baseline+1m)phase
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difference is relatively similar
tocase1,withan“earlier”minimumnear50kmand
lessofadifferenceinthelowerchannelarea.Theminimumisstillabout30degrees.However,
in case 3, the character is substantially different from the
baseline cases.
Thepartitioningofthewavebetweenstandingandprogressivealmostimmediatelymovestowardsprogressive,withthewavebecomingalmostentirelyprogressiveinthe“Wilmington”area,anda“rapid”transitiontowardalmostfullystandinginthedeeperupperbasin.
Ofcourse,atthehead of the channel in all cases, there is
significant reflection and hence an almost
entirelystandingwavephasedifferencenear90deg.
Figure31:
Along-channelphasedifferencebetweenelevationandthealong-channelvelocityfortheM2tide
3.3.
ConnectionofIdealizedResultstoNC-SLRISIntheNC-SLRISproject,asequenceofSLRamounts
isaddedtotheBaselinegridoragridtowhich some level of
geomorphological adjustments have been imposed. The 100cm
end-memberscenariohasasubstantiallyincreasedstoragevolume,ascomparedtojustaddingsealevel
increases to the baseline geometry. This is illustrated in Figure
32, which shows
thechangeinstoragevolumewithincreasingSLforthe6projectscenarios.Theredlineisthetidalstoragevolume(incubickm)belowthescenario’sMSL,using
thegrid for thatscenario.
Thegreenlineisthevolumebelowmeanhighwater.Thevolumeincreasesarelarge,witha5-foldincreaseinthestoragevolumebelowMSL.Forcomparison,thebluelineisthevolumebelowMSLforeachscenariobutcomputedrelativetothebaselinegrid.
Noteaslightlyexponential
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increaseassea level increases,anexpectedfeatureofvolume
increasewith
increasingwaterlevelsinbasinswithgentlyslopingsidewalls.
Figure32:
TidalstoragevolumesintheCapeFearRiver,upstreamofClarksadEagleIslands,fortheprojectscenarios.Thebluelineindicatesstoragecomputedwiththesealevelincreasebutwiththebaselinegridtopography/bathymetry.
The impactof tidal storagechanges (increases)on theM2
tidepropagatingup
theCapeFearRiverisshowninFigure33.Thegeneralcharacteristicsareverysimilartotheidealizedresultsdiscussed
above. For the scenarios that are close to the Baseline (00 cm, 20
cm, 40 cm), intermsofupperbasinstoragevolume, there isa
relativelysmallchange in
thetidalelevation,althoughthereisaslightamplitudeincreaseduetoslightlydeeperwaterandlittlegeometricchangeupstreamofWilmington.
However, as the storage volume increases (red line, Figure32)
upstream ofWilmington, tidal amplitudes drop and the phase is
lagged. The
amplitudedifferencebetweenthescenarioend-membersisabout60cm.
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Figure33:Along-channelM2tidalelevationamplitudeandphaseintheCapeFearRiver.
The phase angle between the elevation and velocity, indicative
of the mixture
betweenstanding(toward90deg)andprogressive(toward0deg),isshowninFigure34fortheBaselineand100cmscenarios.Duetotherealisticandmorecomplexriver/channelcharacteristics,thevelocity
exhibits much more variability than in the idealized cases.
Nonetheless, the phaseangle for the 100 cm scenario is
substantially more toward 0 (progressive, in phase) in thelower
part of the river than in the Baseline scenario. The baseline
scenario is generally
anapproximatebalancebetweenstandingandprogressiveandoveralldoesnotchangeasmuchasinthe100cmscenario.
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Figure34:Along-channelphasedifferencebetweenM2tidalelevationandvelocity.OnlytheBaseline(blue)and100cm(red)scenariosareshown.
Thestoragevolumeincreasesalsoaffectstormsurgesinasimilarway,althoughthediagnosticquantities
like phase angle not meaningful in this context. To illustrate the
impact of
thestoragevolumeonthesimulatedwaterlevels,thealong-channelmaximumwaterlevelforthe1993
extratropical storm (Storm of the Century) is shown in Error!
Reference source
notfound.Figure35.ThewaterlevelresponseintheBaselinescenarioisthatthesurgeisamplifiedup
the channel to the Wilmington area, upstream of which there is a
reduction of
waterlevel.Thischaracteristicisgenerallythecaseforthefirst3scenarios.However,asthestoragevolumeincreases,twothingshappen.1)WatervolumeflowspasttheWilmingtonarea,fillinginthestorageincrease,andraisingwaterlevelsintheupperbasin.2)This
volume of water is no longer stored in the lower part of the river,
as indicated by
thegenerallyflatwaterlevelinthe80and100cmscenarios.Inotherwords,thewaterthatwouldotherwise
fill in the lower river floodplain is, in later scenarios, being
driven into the largerbasinsupstreamofWilmington.
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Figure35:Along-channelwaterlevelfortheextra-tropicalstorm1993.
3.4. IdealizedResultsConclusion
Theimplicationsofthisdecreasedtidalamplitudewithincreasingstoragevolumearethatthetidal
contributions to the statistical flood levelswill be less in
“later” scenarios, inwhich thelarger storage volumes occur. In
these idealized experiments the tidal amplitude
at“Wilmington”decreasesfromabout1.1mto0.6m, implyingthat if
thenon-tidalSWEL
levelsincreaseby1meterbetweentheNCSLRISend-members,thenitcanbeexpectedthatthetotalcombinedSWEL+tideswillnotincreaseatthesamelevelasthesealevel
increase.
Itseemsunlikelythateffectsofchanneldredgingcanhaveanappreciableimpactonthetidalandsurgedynamics
in the Cape Fear River system, although local (to the gauged
location) effects areprobable.
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