Hervey Bay - insights from numerical modelling into the hydrodynamics of an Australian subtropical bay [Elektronische Ressource] / von Ulf Gräwe

Description

Hervey Bay - Insights from NumericalModelling into the Hydrodynamics of anAustralian Subtropical BayVon der Fakult¨at f¨ur Mathematik und Naturwissenschaften der Carlvon Ossietzky Universit¨at Oldenburg zur Erlangung des Grades undTitels einesDoktors der Naturwissenschaften (Dr. rer. nat.)angenommene Dissertationvon Herrn Ulf Gr¨awegeboren am 12. Juli 1974 in StralsundErstgutachter: Prof. Dr. J¨org-Olaf WolffZweitgutachter: Prof. Dr. Emil StanevTag der Disputation: 11. September 2009...Twenty years from now, you will be more disappointedby the things you didn’t do than by the ones you did do. Sothrow off the bowlines. Sail away from the save harbour.Catch the trade winds in your sails. Explore. Dream.Discover ... - Mark TwainAbstractHervey Bay, a large coastal embayment situated off the central eastern coast of Australia, isa shallow tidal area (average depth = 15 m), close to the continental shelf. It shows featuresof an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation(less than 1 m/year) and on average almost no freshwater input from rivers that drain intothe bay.The hydro- and thermodynamical structure of Hervey Bay and their variability are presentedhere for the first time, using a combination of three-dimensional numerical modelling andobservations from field studies. The numerical studies are performed with the COupled Hy-drodynamical Ecological model for REgioNal Shelf seas (COHERENS).

Sujets

• Naturwissenschaften

Informations

• Publié par: carl_von_ossietzky_universitat_oldenburg
• Publié le: 01 janvier 2009
• Nombre de lectures: 38
• Poids de l'ouvrage: 4 Mo

Extrait

HerveyBay-InsightsfromNumerical
ModellingintotheHydrodynamicsofan
AustralianSubtropicalBay

VonderFakulta¨tfu¨rMathematikundNaturwissenschaftenderCarl
vonOssietzkyUniversita¨tOldenburgzurErlangungdesGradesund
Titelseines

DoktorsderNaturwissenschaften(Dr.rer.nat.)

angenommeneDissertation

vonHerrnUlfGra¨we
geborenam12.Juli1974inStralsund

Erstgutachter:

.forP

.rD

Jo¨rg-Olaf

loW

Zweitgutachter:Prof.Dr.EmilStanev

TagderDisputation:

.11

September2009

...Twentyyearsfromnow,youwillbemoredisappointed

bythethingsyoudidn’tdothanbytheonesyoudiddo.So

throwoffthebowlines.Sailawayfromthesaveharbour.

taChc

eht

tradewindsin

Discover

...

ruoy

sails.Explore.

niawTkraM-

.mareD

Duetotheinversefeaturesandthusadensitydierencebetweentheshoreandopenocean

(withhigherdensitiesclosetothecoast),gravitycurrents

duringlateautumnandhave

associated

8

htiw

eseht

,swo

is

anaveragedurationof30days.

comparable

htiw

eht

atotl

lov

arereleased.Theyoccurmostly

ehT

emu

integrated

fo

Hervey

volume

.yaB

transport,

imSalzgehaltvonHerveyBay.DieUntersuchungendieserextremenFrischwassereintra¨geer-
gaben,dassdieFrischwasseraustauschzeitenengandiewindgetriebenenResiduenstro¨mungen
gekoppeltsind.
DieinversenEigenschaftenvonHerveyBayunddiedamitverbundeneDichtegradienten(mit
einerho¨herenDichteinKu¨stenna¨healsimoenenOzean)ko¨nneninstabileSchichtungenerzeu-
gen.Diesea¨ussernsichinDichtestro¨mungenunddamiteinemAusussvondichtemWasser
entlangdesGrundesvonHerveyBay.DieAusussereignissehabeneinemittlereDauervon
30TagenundsindmeistensaufdenSpa¨therbstbeschra¨nkt.Derdabeiauftretendeintegrierte
VolumentransportistvergleichbarmitdemVolumenvonHerveyBay.

01

Contents

1Introduction

3

2TheRegionandData8
2.1TheRegion......................................8
2.2Data..........................................10

3Modeldescription12
3.1GeneralfeaturesofCOHERENS..........................12
3.2Boundaryconditions.................................12
3.3Modeldesign.....................................15

4Barotropiccirculations16
4.1Tidalforcing.....................................16
4.1.1Modelvalidation...............................16
4.1.2Tidalmixing.................................17
4.2Residualcirculations.................................18
4.3Waterexchange....................................21
4.3.1Setup.....................................22
4.3.2Flushingtime.................................24
4.3.3Residencetime................................26
4.3.4Originofreplacementwater.........................27

5Baroclinicprocesses29
5.1ModelValidation...................................29
5.2StraticationwithinHerveyBay..........................33
5.3Inversestateandhypersalinity...........................35
5.4Evaporationinducedcirculations..........................39

6Impactofclimatevariability43
6.1Thedryingtrend...................................43
6.1.1Trendsinfreshwatersupply.........................43
6.1.2Hypersalinityandinversestate.......................44
6.1.3Residualcirculations.............................45

1

Cnoettns6.1.4Salinityux..................................46
6.1.5ImpactoftheEastAustralianCurrent(EAC)...............46
6.2Shorttermvariability................................47
6.2.1Catchmentarea................................48
6.2.2Riverdischargestatistics..........................49
6.2.3Floodingevents................................49
6.2.4Theoodof1999...............................51
6.2.5Floodresponse................................52

7Gravitycurrents54
7.1Releaseofgravitycurrents..............................54
7.1.1Formation...................................54
7.1.2Pre-conditioning...............................55
7.1.3Down-slopepropagation...........................56
7.1.4Fateoftheplume...............................57
7.2Impactoffreshwaterreduction...........................57

8Conclusion

59

AParticletrackingschemes61
A.1Introduction......................................61
A.2TheLagrangianmodel................................62
A.2.1Numericalapproximation..........................63
A.2.2Boundaryconditions.............................65
A.3Idealisedtestcases..................................66
A.3.11-Ddiusion.................................66
A.3.21-Dresidencetime..............................68
A.3.32-Dcorrelationtest..............................69
A.4Conclusion......................................71

2

1Introduction

Estuarieshavealwaysattractedhumansettlements.Shelteredharbours,goodshinggrounds,
accesstotransportalongrivershavebeenimportantreasonswhypeoplehavesetupcities
alongthecoastalshoresformillennia.Thevarioushumanusesofestuariesaectthewater
qualityandthehealthoftheestuarineecosystem.Asthehumanpopulationgrewsignicantly
duringthe19thand20thcenturyandisexpectedtogrowfurtherinthenextcenturies,human
settlementsalongestuarineshoresincreaseinsize.Withabout70%oftheglobalpopulation
livingwithinthecoastalzone,distancetotheshorelessthan100km(e.g.Cohenetal.
[1997]),theinuenceofhumanactivityuponcoastalmarineenvironmentsisimmense.More-
over,thechallengesandpotentialthreatsduetoclimatechange,astheexpectedriseinsea
level,thepossibleincreaseinfrequencyormagnitudeofweatherextremesputanenormous
pressureonthelifeincoastalregions.
Theincreaseinknowledgeandunderstandingofcoastalhydrodynamicsandwaterexchange
cannotpreventforinstancetheoccurrenceofoilspills,wastedumping,toxicalgaeblooms,
orclimatechange.However,theknowledgeofthecomplexprocessesincoastalwaterscan
leadtothedevelopmentofadoptionstrategies,constructionofprotectedhabitats,redirection
offairways,orconstructionofcoastalprotections.Thus,wecanreducetheimpactoffuture
threatsandchallengesorcanbettercopewiththem.
Theunderstandingoftheinteractionofthenearshoreregionwiththeopenocean,theimpact
ofclimatechangebutalsotheinuenceoffreshwaterdischargeinthecoastalzoneisinparticu-
larimportantforAustralia.TheIntergovernmentalPanelonClimateChange(IPCC)predicts
adecreaseinprecipitationovermanysubtropicalareassuchastheEastcoastofAustralia,
American-CaribbeanandtheMediterranean[IPCC,2007].Theobserveddecreaseinprecip-
itation[Shietal.,2008a]alongtheeastcoastofAustraliadistortedthebalancebetween
evaporation/precipitation.Moreover,duetoanincreaseinseasurfacetemperature,coral
bleachingisasevereissueintheGreatBarrierReef[BerkelmansandOliver,1999;Glynn,
2006;Hoegh-Guldberg,2009].Further,heavyprecipitationeventsareprojectedtobecome
morefrequentovermostregionsthroughoutthe21stcentury.Thiswouldaecttheriskof
ashoodingandurbanooding.Theseoodsareexpectedtoushhugeamountsofwater,
ofurban/ruralorigin,intothecoastalregions,withtheconsequenceofseverestressonthe
localora/fauna.Thus,thesubtropicalregionsofAustraliaoeranexcellentresearcharea,
toinvestigatetheinterplaybetweenevapotranspiration,regionaloceancirculations/response
andclimatechange.

3

1Introduction

Inthesesubtropicalclimateswhereevaporationislikelytoexceedthesupplyoffreshwaterfrom
precipitationandriverrun-o,largecoastalbays,estuariesandnearshorecoastalenvironments
areoftencharacterisedbyinversecirculationsandhypersalinityzones[TomczakandGodfrey,
2003;Wolanski,1986].Aninversecirculation/estuary/bayischaracterisedbysub-surfaceow
ofsalinewaterawayfromazoneofhypersalinitytowardstheopenocean.Thisowtakes
placebeneathalayerofinowingoceanicwaterandleadstosaltinjectionsintotheocean
[BrinkandShearman,2001].Secondly,inversecirculationsarecharacterisedbyareversed
densitygradient.Theriverinefreshwaterinputandthereforelowdensitiescontrolthecoastal
zoneinregularestuariesorbays.Inverseestuariesorbaysontheotherhandarecharacterised
byhighsalinitiesinthecoastalzonewithinversegradientsforsalinityanddensitydirecting
oshorewithminimaldirectoceanicinuence.ExamplesforsuchregionsincludetheGulf
ofCalifornia[Lavinetal.,1998],estuariesinMediterranean-climateregions(TomalesBay,
California;Largieretal.[1997]),SpencerGulf[Lennonetal.,1987],theRiaofPontevedra
[deCastroetal.,2004]andtheGulfofKachchh[Vethamonyetal.,2007].
Highevaporationduringsummerleadstoanaccumulationofsaltintheheadwaterofthese
inversebaysorestuaries.Followingtheseasonintoautumnandwinter,thesewatermassesare
subsequentlycooledandcanbecomegravitationallyunstable.Undercertaincircumstances,
theycanevolveintogravitycurrentsorplumesthatowoutofthebayintothedeeperocean
adjacenttothecontinentalshelf.Duetostrongtidalandwindinducedmixing(eithervertically
orhorizontally)theseeventsshouldbeofshortduration.Ecientmixinghomogenisesthe
watercolumnandinsteadofatwo-layerstructureinthevertical,oneobservesamorehorizon-
tallydistributedfrontalsystem[LoderandGreenberg,1986].Thesegravityowsarenotonly
restrictedtothesubtropicalregions.Theycanalsooccurinmidlatitudes[Burchardetal.,
2005]oreveninhighlatitudes[FerandAdlandsvik,2008].Inthelattercases,thetriggering
iscausedbyinowofhighsalinewaterinclosedseasoraccumulationofsaltduetofreezing.
Despitedierentmechanismsthatleadtothecreationofunstablestraticationsandrelax-
ationintogravityows,theyareallcontrolledbytheinteractionofearthrotation,friction,
topographyandpressuregradient[Sh