dune convergence/divergence controlled by residual current

Article, Author´s Postprint

Kubicki, Adam; Kösters, Frank; Bartholomä, AlexanderDune convergence/divergence controlled by residualcurrent vortices in the Jade tidal channel, south-easternNorth SeaGeomarine-Letters

Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/105160

Vorgeschlagene Zitierweise/Suggested citation:Kubicki, Adam; Kösters, Frank; Bartholomä, Alexander (2016): Duneconvergence/divergence controlled by residual current vortices in the Jade tidal channel,south-eastern North Sea. In: Geomarine-Letters 37 (1). S. 47-58.

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AutorenfassungdesArtikels:Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyre-sidualcurrentvorticesintheJadetidalchannel,south-easternNorthSea(2016)

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ErstveröffentlichunginGeo-MarineLetters;Vol.37;Heft1;Seite47-58 FüreinekorrekteZitierbarkeitistdieSeitennummerierung derOriginalveröffentlichungfürjedeSeitekenntlichgemacht.S.47

Duneconvergence/divergencecontrolledbyresidualcurrentvorticesintheJadetidalchannel,south-easternNorthSeaAdamKubicki1 FrankKösters2AlexanderBartholomä1Referencedoi:10.1007/s00367-016-0470-6ElectronicsupplementarymaterialTheonlineversionofthisarticlecontainssupplementarymaterial,whichisavailabletoauthorizedusers.PublishedOnline:05.October2016Abstract

A field of large to very large subaqueous duneswas investigated in the Jade tidal channel, south-easternNorthSea,betweenJanuary2006andOctober2011.Aground-truthed sidescansonarsedi-mentmapshowsthatthedunes,whicharelocatedontopofaconsolidatedclaysurface,arecomposedofmediumtocoarsesand.Aseriesof35consecutivehighresolutionbathymetricsurfacescollectedbymultibeamechosounderrevealedacomplexmigrationpatterninducedbythereversingtidalcurrents.Variouspartsofthedunefieldareundertheinfluenceofeitherebb-orflood-dominatedcurrents,asindicatedbyduneasymmetries.Althoughsomedunesmigrateatapaceexceeding100m/year, themajorityaredisplacedby30m/year inthedirectionofthe locallydominantcurrent.Inthedeepestpartofthechannel,however,duneswereobservedtoconvergehead-on,resultinginpracticallyzeronet transportwithminoroscillationsofsymmetricaldunesat theapex.Applying thenumericalUn-TRIMmodel for the simulationof the fair-weatherhydrology, a simplifiedmapof residual currentvectorsoverthedunefieldwasgenerated.Theresidualflowvectorsarefoundtoperfectlymatchthederived dunemigration vectors, suggesting that dune convergence is controlled by two counter-rotatingresidualcurrentvorticescausedbythelocalshapeofthetidalchannel.Asnosedimentbuild-upisobserved,amechanismofsedimentbypassingwithpotentialrecirculationmustexist,buthasnotyetbeenidentified.

IntroductionSubaqueousdunes,calledalsomarinesandwaves,areoneof themost fascinatingsedimentaryfeaturesoncontinental shelves.Theyaredefinedas flow-transversebedformswithwavelengthsexceeding0.6mandheightsgreaterthan0.075m(Ashley1990).Itiswelldocumentedthatdunemorphologies tend to adjust to changing hydrological conditions until a dynamic equilibrium isreached.Their shapes anddimensions reflect the interactionbetween critical currentvelocities,waterdepth,grainsizeandsedimentavailability,therelationshipsbetweentheseparametersap-


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plying equally to unidirectional currents (rivers, geostrophic flows) and reversing tidal currents(e.g. Flemming 1988, 2000; Mazumder 2003;Bartholomäetal. 2008; Carle and Hill 2009;VanLandeghemetal.2012).Ingeneraltheduneasymmetry indi-catesthedirectionoftheprevailingcurrent,butantiasymmetrymigrationhasbeenalsoreported(Hanes2012).Inthecaseofperiodi-callyreversingflows,thesymmetry/asymmetryofthedunesis,inaddition,controlledbytheratiobetweenthe localfloodandebbflowvelocitieswhich, insimplifiedform, isalsoexpressed intheresidualtidalflowpattern.Moreover, localseabed topographycangreatly influencetheflowpat-tern,especiallyinmeanderingchannels(Hughes2012).The combination of complex topography,water level changes and reversing currents producesdunefieldswhicharecommonlyseparated intoflood-andebb-dominatedregions(e.g.Beldersonetal.1978;Harris1988;Harrisetal.1995;GonzalesandEberli1997;FeniesandFaugères1998;Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.48 Geo-MarineLetters;Vol.37;Heft1;Seite47-58BatesandOakley2004;CarleandHill2009;KubickiandBartholomä2011;VanLandeghemetal.2012). The line separating the ebb- and flood-dominated parts is called the zone of zero nettransport,alsoknownas abedloadconvergencezone.This line ismostlysharpandonly in rarecasesaccompaniedbysymmetricaldunebodies(e.g.LeBotandTrentesaux2004;Barnardetal.2006;Gómez et al. 2010; Fraccascia et al. 2016), let alone head-on dune convergence (e.g.VanLandeghemetal.2012).The flowconditions triggering thehead-onconvergencehas todatenotbeenfullyexplained,potentialcontrolby localresidualcurrentvectorshavingbeenhypothesizedbutnotverifiedinthefield(Idieretal.2011).Inthecourseofthepresentstudy,physicallyconvergingdunesdiscoveredintheJadetidalchannel(KubickiandBartholomä2011),south-easternNorthSea,presentedanidealopportunitytofurtherinvestigate the underlyingmechanisms of bedload convergence/divergence.The dune fieldwasmonitored for 6 years (January 2006–October 2011) by repetitivemultibeam echosounding ac-companiedbysidescansonarmappingcalibratedbygrabsamplesintheclosing12months.Inor-dertounderstandthecharacteristicdistributionofresidualcurrentvectors,anumericalmodelwasapplied tosimulate the localhydrology—amethodalreadyutilised in the1970s (e.g.Tee1976).Numericalmodellingoflocalhydrologyallowsaphysicallyconsistentbutspatiallylarger,synopticviewofresidualcurrentvectorsthancanbeachievedby insitumeasurements(Lietal.2008).Inthepast,suchmodellingwasnotonly limitedbecauseof insufficientcomputingpowerbutoftenalsotendedtofailinreproducingtruecurrentmagnitudesduetoinsufficientdataonseabedtopog-raphy(Hanes2009).Thissituationchangeddramaticallyascomputingpowerincreasedandbath-ymetric data acquisition exploded by the development ofmultibeam echosounders for seabedmappingaswellasLiDARtechniquesformappingthetopographyofintertidalareas.Nonetheless,in spite of the improvement in computing power and numericalmodels (e.g. CastroDíaz et al.2008),thereexistsnouniversalmodellingapproachtoreproduceaccuratelytheinteractionoffluidflowandsedimenttransportindynamicshallow-waterenvironments(VanRijnetal.2013).There-fore,numericalhydrologicalmodellingoftenstillactsasanaid inmorphodynamicstudies,ratherthanbeingthebaseofit(e.g.Fraccasciaetal.2016).


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Investigationsofseabeddynamicsthereforestilldependonrepetitiveinsitumeasurementsoflocaltopography.Themonitoringperiodofsubaqueousdunescommonlyspansperiodsrangingfromafewmonths(e.g.WhitmeyerandFitzGerald2008)toatleast15years(Zorndtetal.2011).Inviewofthis,theperiodcoveredinthepresentstudycanbeconsideredasmedium-term.Thenumericalmodellingofthelocalhydrologywasappliedfora2-weekperiodtocoveracompletespring–neaptidalcycle.Thespecificaimsof thepresentstudywere (1) todocument, for the first time,dunehead-onconvergence in a large tidalchannel athigh temporal andspatial resolution, and (2) toprovideaplausibleexplanationofthemechanismcontrollingthisphenomenon.PhysicalsettingThe Jade is a50-km long,slightlymeandering tidalchannel located in thesouth-easternGermanBight.ItisanaturalwaterwaytotheharboursofWilhelmshaven,whichincludethelargestGermannavalbase and theeasternmostdeep-watercontainer terminal in theNorthSea, the JadeWeserPort.Duetoitseconomicandmilitaryimportance,theJadefairwayismonitoredatmonthlyinter-valsbytheGermanauthoritiestoensuretherequiredwaterdepthof17.6mbelowSKNLAT(marinechartdatumcorrespondingtothelowestastronomicaltide;GötschenbergandKahlfeld2008).Thetimespanbetween thesurveys isnever longer than 1month.Tomaintain thedesigndepth, thefairway is regularly dredged, the dredge spoil being subsequently deposed at designated dumpsitesnearthemouthoftheJadeinproximitytothestudysite.Thesemi-diurnaltidesrangingfrom2.8minthenorthto3.8minthesouth(GötschenbergandKahlfeld2008)triggermaximumtidalcurrentvelocitiesexceeding1.5m/s(Grabemannetal.2004).Thetidalwaveisuniformlydistrib-utedacross thechannelwidth,andnoobviousstreamcoreexistsduringeither floodorebb tide(KahlfeldandSchüttrumpf2006).Theareaofinterestcoversca.14km2andissituatedinthenorthernpartoftheJadebetweentheislandsofMinsenerOogandOldooginthewestandthelighthouseMellumplateintheeast(Fig.1).Itcoversachannelwidthofca.3.2kmandincludesbothchannelflanksuptoawaterdepthof5mbelow SKNLAT .The eastern flank in thevicinityof the lighthouse isused as adump site for thedredgespoil from thenavigationchannel.Over11millionm3ofsedimentweredisposedofherebetween1994 and2001 (BfG/WSA2003).Noofficial reportsondumpingvolumesareavailableafterthisperiod,althoughthesitecontinuedtobeutilised.KubickiandBartholomä(2011)calcu-latedthattherehasbeenasurplusofca.0.13m/m2ofsedimentinthedeepestinvestigatedsectionofthechanneloftheJadefairwaybetween2005and2010.Ifthevalueholdstruefortheentireareaofinterest,afurther1.8millionm3couldhavebeendumpedinthisperiod.MaterialsandmethodsDuring themeasuringcampaigns inOctober2010,April2011andOctober2011,aResonSeaBat8125multibeamechosounderwasdeployedsimultaneouslywithaBenthosSIS-1624sidescanso-naronboardRVSenckenbergKubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.49 Geo-MarineLetters;Vol.37;Heft1;Seite47-58


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Fig.1LocationofthestudyareainameanderoftheJadetidalchannel(Table1).Thesurveyareawasexpandedcompared toearlierstudies in thisregion(KubickiandBartholomä2011) inorder toalso investigate theeastern flankof the Jade tidalchanneland thevicinityoftheMellum lighthouseuptoawaterdepthof5m,whichisthevessel’ssafeunder-keelclearance.TheReson SeaBat8125multibeam echosounderoperating at455kHzwas interfacedwithaMagellanAquarius5002LongRangeKinematicGlobalNavigationSatelliteSystem.Thiscon-figurationachievesverticalandlateralaccuraciesofbetterthan0.05m(Ernstsenetal.2006).Theechosounderhasafixedrecordingswathwidthofaboutthreetimesthewaterdepth,whichiscon-siderablysmallerthanthatrecordedbysidescansonar.TheBenthosSIS-1624sonar,adual-frequencysystemscanningat123and382kHzwithalong-andacross-trackresolutionofbetterthan0.1m,wassettorecorda200-mwideswath.Fullspatialcov-erage of the areawas therefore achieved only by the sidescan sonar,whereas the bathymetricswathswere separatedby regular gaps.Thiswasvisible especially in the shallowwater,wheremanymoremultibeam trackswereneeded toachieve fulldatacoverage,butwerenotmeasured


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duetothelimitedtimeframeofthecampaigns.Aftereachmeasuringcampaign,thebathymetryandbackscatterdatasetswerespatiallymatchedinArcGISinordertomanuallydelineatethecrestsofduneshigherthan1m,whichareherereferred toas largedunessensuAshley(1990).ThisduneheightlimitwasarbitrarilychosentoensureidentificationofindividualdunecrestsevenTable 1 Summary of in situ data used in this study in various measuring campaigns

(MBESmultibeamechosounder,SSSsidescansonar,SESsedimentechosounder)

Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.50 Geo-MarineLetters;Vol.37;Heft1;Seite47-58aftertime intervalsof6months.Smallerdunesmigratefasterand individualcrestscanthereforeno longerbetracedatthechosentime interval.Thecrestsof largeduneswerealsoclearlyrecog-nisableonsonographsduetomorphology-inducedcontrasts inbackscatterrecords.Thisalloweddelineatingcrestsevenwhereswathbathymetrylackedcoverage.Themonthlybathymetric surveys for fairwaymonitoringperformedby theWSAWilhelmshavenwere carriedoutwith anAtlasFansweep20-200multibeam echosounder.This systemoperateswithafrequencyof200kHzandachievesaverticalaccuracyof0.05m±0.2%ofthewaterdepth.Thesesurveyswere limitedtotheactualfairwaywidthofca.300m.Inordertoalsomonitortheareaadjacenttothefairway,thelateralsurveywidthwasincreasedtwiceayear.Afteridentifyingsitesrequiringdredging,thebathymetricdatawereconvertedtoaregularlyspacedgridof2mandarchived.For thepurposeof thepresent study,32 availabledatasets collectedbetween January2006 andOctober2010wereused (seeFig.2a, locationA).Basedon thesedata,digital terrainmodels (DTMs)weregenerated for time intervalsspanningperiodsof 2weeks to 7monthsbe-tween individual surveys.After assigning the same colour scale to thebathymetryvalues, a 32-framemoviewasgeneratedwhichhighlightedanychanges in the lateralpositionsof the large toverylargedunesinthesurveyarea(seeElectronicsupplementarymaterialavailableonlineforthisarticle).Inaddition, a sedimentdistributionmapwascompiledaftercalibrating thesidescanbackscatterintensities recorded inOctober2010with118 Shipek grab samples collectedbetweenFebruary2010andApril2011(Table1).Suchacalibrationwasadequateduetoasurprisingstabilityofthesedimentdistributionmapobservedinconsecutivesonographs(Fig.3).Thesediment


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Fig.2SedimentologicalandmorphologicaldatafromOctober2010overlainbysimplifiedbathyme-

try(cf.Fig.1).aHighresolutionbathymetryrecordedbymultibeamechosounder.BSedi-mentmapderivedfromsidescansonarrecords.CBedformswithcrestsofdifferentheights


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markedbydifferentcolours.dCrestpositionsofthe largestdunes inOctober2010,April2011andSeptember2011,andvectorsmarkingthechange

Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.51 Geo-MarineLetters;Vol.37;Heft1;Seite47-58sampleswere analysed in the laboratory after standarddesaltingbydialysis andseparating thevolume intogravel,sandandmudfractions.Gravelfractions(grainsize>2mm)weresievedme-chanically,whereasthesandfractions(grainsizesbetween2mmand63μm)wereanalysedbyasettling tube(MacroGranometer;Brezina1979)and themud fractions(grainsizes<63μm)byaSedigraphIII(Micromeritics,Inc.).Theresultswerecombined intograin-sizedistributionplotsatquarter-phi intervals.Sedimentpatches identifiedonsonographswereclassifiedaccording totheprimarycompositionofthemajorityofsedimentsamplescollectedwithingivenpatchareasfollow-ingtheUdden-Wentworthclassificationscheme(Wentworth1922).WaterlevelsandflowvelocitiesoftheJade-WeserestuaryweresimulatedwiththeUnTRIMmodel-lingsystem.UnTRIM(CasulliandZanolli2005)usesafinitevolume–finitedifferencemethodwhichsolves themomentumand transportequationsonahorizontallyunstructuredgrid. Itwassetupthreedimensionallywithaspatialresolutionofabout200mby300mover thestudyareaandaverticallyconstantresolutionof1m.Thus,individualdunesarenotresolvedbythemodel,whichinsteadsimulatestheoverallflowregimeoftheJade.Roughnessistakentobemainlyduetospa-tiallyuniformsmall-scaleripples.Theoverallmodeltopographyrepresentstheyear2002basedonechosoundingandLiDARdata(dataprovidedbytheFederalWaterwaysandShippingAdministra-tion,BAW,andtheFederalMaritimeandHydrographicAgency,BSH)butwasupdatedinthestudyareawithmorerecentmeasurements fromOctober2010.Themodel issetupas aprocessstudywithapplicationofrealisticforcing.WindstressatthesurfacewasobtainedfromtheoperationalGermanNationalMeteorologicalService(DWD)weatherforecastmodel(COSMO/LM,e.g.Domsetal.2002).Atthe lateralopenseaboundaries,water levelsfromaBAWmeasurementcampaign inJune2002wereprescribed.


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Fig.3SidescansonarmosaicscollectedinaOctober2010,bApril2011andcOctober2011.dSed-

imentdistributionchartsofgrabsamplesusedforcalibrationofthesonographsKubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.52 Geo-MarineLetters;Vol.37;Heft1;Seite47-58


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Themodelwasrunfor4weeksoffair-weatherconditions(only2dayswithwindsreaching7Bfatthe AlteWeser offshore gauge), and the last 2weekswere used for analysis of a characteristicspring–neapcycle.Salttransportwastakenintoaccountby themodelbutbaroclinic flow isof less importance inthe Jadedue tovery low inflowoffreshwater(ReineckandFlemming1990).Inthemodel,riverdischargewasonlyconsideredfortheWe-serriver.Validationofthemodelwassuccessfullyperformedonthebasisoftheoriginal2002set-up(BAW2009)andalsoincludedsedimenttransport(Köstersetal.2014)which,however,wasnotanalysed for thisstudy.Foreachcomputationalnode in themodel, the floodandebb tidesweredeterminedandanaveragefloodandebbtidalcurrentvelocitywascalculated.Inaddition,thecur-rentvelocityforeachnodewassummedoverafulltidalcycleandsubsequentlyaveragedforthemodelled springneap cycle. The resulting vector field describes the Eulerian residual flow fieldphysicallyconsistentoverthemodeldomain.Results

BathymetryandsedimentsTheJadechanneltopographywasinvestigatedatwaterdepthslargerthan5m(SKNLAT).Thecross-sectionofthechannel inthestudyareashowsthe troughofachannelorientatedNNW–SSEwiththedeepestpointlocated35mbelowSKNLAT(Fig.2a).ThetroughislocatedclosertothewesternchannelflankoffMinsenerOogandOldoog,resultinginasteeperwesternflank(ca.1°)incompari-sontotheeasternflank(ca.0.5°),whichhasmorethecharacterofaterrace.Bothflanksofthechannelarecoveredbysandydepositswithabroadgrain-sizespectrum(Fig.2b).Theshallowestpartsofboth flanksarecomposedof finesand(2–3phi), ineachcasecoarseningtowardsthechannelthalweguptograinsizesrangingbetweenmediumsandandgravel.Bedformswerepresent ineachof thesediment types(Fig.2c).Largestdunesreaching9.4m inheightand300minlengthhadgrainsizesrangingfromfinesandtomediumgravel.Theprimarylargetoverylargeduneswerecoveredbysecondarydunesofsmallerdimensions,whichappeared tobe lessstableovertime.Someofthedunebodieswereuptoseveralkminlateralextent,connectingbothchannelflankswithcrestsalignednormaltotheshore.Sonographsrecordedduringthethreemeasuringcampaigns inOctober2010,April2011andOc-tober2011showlittlechangesintheoverallsedimentpattern(Fig.3)butthemigrationofindivid-ual dunes reached several tens ofmetres (Fig. 2d). Sediment classification uncertainties weremostlyassociatedwithlargevolumesofshellhashpassingthroughthearea.Shellstrappedindunetroughsarerecordedascoarse


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Fig.4aInnomarSES-2000parametricsub-bottomprofilerrecordofthreeduneslocatedonacon-

solidated clay surface (dots)collected inAugust2011 (seeFig.2a, locationC). SedimentsamplesarefromFebruary2010andtheplasticcontainerare10cmhigh.Verticalexagger-ationof the cross-section ~15×. b 3Dview of these dunes recorded bymultibeam echo-sounder inOctober 2010 (yellow), April 2011 (orange) and October 2011(red). Noteincompletecoverageofdifferentdatasets,andnorthwardmigrationofdunes inallcases.Bluelinemarksthepositionofcross-sectionina

Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.53 Geo-MarineLetters;Vol.37;Heft1;Seite47-58materialonacousticbackscatter images.Inaddition,grabsamplingshowedthatshellsseemedtobeeasilytransportedupthestosssideofthedunes,therebyBcorruptingthegroundtruthingofthesonographsat largespatialscales(Fig.4a).Anothercharacteristic featureof thearea is thepres-


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enceofaconsolidatedbedbeneaththedunes,exposedpartshavingbeenidentifiedindunetroughsatseverallocations(shownasdottedlineinFig.4a).Successfullyrecoveredsamplesconsistofir-regularly shaped cracked fragmentsof consolidated claywithbarnacle shells attached (Fig.4a).Thistypeofchannelbedoftenoutcropsbetweendunebodiesand,duetoitsroughtexture,canalsoeasilybemistakenforagravellyseabed.Itwasthereforenotalwayseasytounequivocallyidentifythematerialconstitutingdifferentgenerationsofdunes.Inordertoresolvethisissue,oneofthelargestduneswasinvestigatedacrossitsentirelengthwithaseriesof31sedimentsamplesrecoveredinApril2011(Fig.5).Ingeneral,ahighcontentofmedi-umtocoarsesandwasrecorded.Sedimentcoarseningtowardsthecrestoftheduneanditsleesideisvisibleinthecontentsofcoarsesand,verycoarsesandandgravel.Therewerealsoseveralsitesalongthestossslopeatwhichthemediumsandcontentwasover10%higherthanatneighbouringsites,therebyreplacingthecontentofcoarsesand.Thesuccessivesamplesmayhavebeenrecov-ered from either crestsor troughsofsecondarybedforms superimposedon the largedune.Theincreaseinmediumsandcontentmaythusbeassociatedwithsmallerdunes6–7mlongandca.0.5mhigh(Fig.5c).

SeabeddynamicsIn order to identify probable paths of dunemigration, dune asymmetrieswere analysed eventhoughastudybyHanes(2012)showsthatresultsofsuchanapproacharenotalwaysstraightfor-ward.Basedonthelee-sideexposition,flood-andebb-dominatedbedformshadbeenidentifiedonearlieroccasions(KubickiandBartholomä2011).InOctober2010thesurveyareawasexpanded,whichalsoallowedalargercoverageofderivedmigrationvectors.Thesameareawasscannedcon-sistently inApril2011 to traceanychanges indunepositionsand toverify theassumption thatasymmetriesareinducedbymodernhydrologicalconditions.Oncethemigrationpathswereveri-fied, itwasessential tocheckwhether themigrationwasperhaps temporarily inducedbystormeventsinthewinterof2010/2011,orwhethertheobservedvectorsweretheeffectoftheconstant-lyactingtidalforces.InthismannertheOctober2011surveyconfirmedthatthedunesweremi-grating independentlyofhigh-energyeventsandthatthemigrationratecouldbecalculatedquitereliably(Fig.4b).Theanalysisofcrestalignmentsinthethreetimestepsshowedavarietyofdirectionsofsedimenttransportaswellasdifferentmigrationratesthroughoutthearea(Fig.2d).The longestdunes lo-cated in themiddleof thesurveyareaweremigratingnorthwardswithaslightbendtowards theNNWfollowingthegeneralchannelarchitecture.Someoftheduneswere


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Fig.5aCross-sectionsshowingsecondarybedformsrecordedoverthevolumetricallylargestdune

(seeFig.2a,locationB)duringthefloodphase(red)andtheebbphase(green).Notea1-mverticalshiftoftheplotstoavoidoverlapping.bLocationsofcross-sectionson3Dmodel.


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Reddots31grabsamplingsitesused toplotpanelc. cGrain-sizecompositionbasedongrabsamplesacrossthedune.Bathymetriccrosssectionisdelineatedinwhite

migratingatarateexceeding100m/year,butmoretypicalvalueswerearound30m/year.Dunesonthewesternchannelflank,bycontrast,migratedsouthwards(atarateofca.80m/year)untiltheyreachedthechannelthalwegwhereadunefieldwaspropagatingintheoppositedirection(ca.60m/year).Anothergroupofdunes,locatedintheSEoftheeasternchannelflank,wasmorestableataSSWmigrationofonlyabout10m/year.Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.54 Geo-MarineLetters;Vol.37;Heft1;Seite47-58Amongallthebedformmigrationpaths,thosefacing inoppositedirectionattractedspecialatten-tion.Theborderbetweenflood-andebb-dominatedduneswasdelineatedbyconnectingsectionsofduneswithsymmetricstossandleeangles.Suchsectionswerefrequentlyidentifiedatthecentreofdunebodies.As laterallyadjacentsectionsweremigrating inoppositedirections,theopposingmovementresulted inbendingofcrest linesandoccasionalcalvingofdunes.Thezoneofconver-genceofoppositemigrationpathswas locatedexactlybelow the fairway.Bathymetricdata fromthefairwaymonitoringsurveysfurthercorroboratedtheseobservations.Thus,aseriesof32con-secutiveDTMsbasedondatacollectedbetween January2006andOctober2010confirmedcon-stantmovementofdunes inoppositedirections,as illustrated inFig.6.Datasets fromAprilandOctober2011were excluded from this figuredue to insufficient coverage.With the aidofdunecrestpositionsderivedfromsidescansonardata,however,itwasconfirmedthatthedunescontin-uedtomigratealongtheirconsecutivepathsforanotheryear.Theconvergencezoneischaracterisedbylittlechangeoverthe5yearscoveredbythedata.Neithersedimentaccumulationnorbedformheighteningwereobserved. Inonecase a sectionof aduneshowedsuchsmallspatialoscillationsthatitspositionanddimensionscouldbetakenasbeingsta-bleovertheentire5-yearperiod(Fig.6).Withlateraldistancefromthecentreofconvergence,themigrationratesofbedformsincreased.Withinthefairwaysectionthedunemigrationratewasuptoca.50m/year.Eventhoughthelatestdatasetsfrom2010and2011coveralargerarea,theydonotprovideanycluesastothefateofthesedimentintheconvergencearea.Duetothefactthatthewesternchannelflankwassosteep,safenavigationwasdifficultandthepre-designedsurveygridwasthusrarelyfullyaccomplishedinthispartoftheJadechannel.Sedimenttransportonthewest-ernchannelflankcouldthereforenotbeassessedbasedonbathymetrydifferences.Forthisreasonthehydrodynamicsofthissectionofthechannelwassimulatedbynumericalmodellinginordertounderstandhow tidalcurrents interactover thisarea,whichcouldgive indirectclueson forcingdirections.Themodellingresultsshowedthatthetidalcurrentexceeds1m/sinthemiddleofthechanneldur-ingbothfloodandebbphases(Fig.7b,c),andthatitprogressivelyslowsdowntowardsbothchan-nelflanks.Inaddition, itwasrevealedthat,atanyonetime,thetidalwavewasofuniformheightacrosstheentirewidthofthechannel,andthattheturnofthetidefromebbtofloodwasabruptwithonlyashortslack-waterperiod.ThemapwiththeEulerianresidualflowvectorsshowsthat,


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duetothebendingchanneloftheJade,floodandebbvelocitiesarenotperfectlysymmetrical.Thus,averaging tidalvelocitiesovera full tidalcyclereveals that theresultingresidualcurrentvectorsshowacomplicatedpatternofsecondarycirculation(Fig.7d),whichcorrespondsverywelltothespatialpatternofdunemigration(Fig.2d).Themainvectorofebbdominancerunsalong theS–Naxisandcoincideswith thegreatestduneheightsinthearea.Flankingthisvector,threetide-inducedresidualcurrentvorticesareidentified,ofwhichaclockwisevortexislocatedontheeasternflankintheSEofthesurveyarea.Thewesternflank,bycontrast,isaffectedbyaclockwisevortexinthenorthandacounter-clockwiseoneinthesouthwhich,incombination,produceahorizontalcirculationpatternintheshapeofan„8“.Sincethetwovorticesrotateinoppositedirections,thecurrentvectorsbothfacewestwardsatthecentreofthe“8“.Thissectionpreciselycorrespondstothezoneofduneconvergenceidentifiedinthesed-imentologicaldata.Thewestward-facingvectorsthusexplainthe lackofanyduneheighteningorsedimentaccretionintheconvergencezone,althoughtheprecisemechanismofsedimentbypass-ingandpotentialrecirculationhasnotyetbeenidentified.

Fig.6DynamicsofthedunefieldbelowtheJadefairwayhighlightedbyfiveexamplesofbathymet-

ricrecordsoutof32analysedinthisstudy.Ontopofthegrey-scaledbathymetryofOcto-ber 2010, a planimetric view of five selected large dune crests and the change of theirpositionbetweenJanuary2006andOctober2010ispresentedincoloursdarkeningfromyellowtobrown(seeFig.2a,locationA).ThedatasetsaregeographicallyAligned


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Kubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.55 Geo-MarineLetters;Vol.37;Heft1;Seite47-58

Fig.7Resultsofthenumericalmodellingofthehydrologyinthestudyarea.aSimplifiedbathyme-

tryandmodelgridshapencomparedtotheextentoftheOctober2010survey.bDepthand


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time-averagedfloodcurrentvectors.cDepth-andtimeaveragedebbcurrentvectors.dRe-sidualcurrentvectorswithschematicinterpretation(whitearrows).Noteexaggerationofvectormagnitudesinpaneldrelativetopanelsbandc

Discussion

SeabedmobilityandimplicationsfordredgingmanagementWithoutlongtimeseriesofinsitucurrentmeasurements,onecanonlyspeculateoncriticalveloci-tiesachievedatvarioustidalphasesandunderdifferentweatherconditionsoverparticularseabedareas. In thepresentstudy,however, thisshortcoming iscompensatedbyauniquedatasetof35highresolutionbathymetric surveys spanning 6years.Thesedata show,without anydoubt, thatbedformmigrationoccursindependentlyofstormevents.Thelargetoverylargedunesinthestudyareamigrateatratesof30–100m/year.Thesemigrationratesare faster than thoserecorded insimilartidalinletsalsocomprisingmediumtocoarsesands(32m/yearinBartholdyetal.2002;21m/yearinCuadradoandGómez2011).Inaddition,smallerbedformswereobservedonnumerousoccasionstocompletelychangedirec-tion inresponse to thereversingtidalcurrents.Thisphenomenonwasnotedwhenneighbouringechosoundingtransectscrossingthesamesecondaryduneswerescannedatdifferenttidalphases(Fig.5a).Thisobservationstrengthensthenotionthatthemediumsandandfinerfractions intheareaaremobilisedbytidalcurrentsduringanyweatherconditionsonadailybasis.TheexperimentbySvensonetal.(2009)onalargedunelocatedca.4kmsouthofthestudyareademonstratedthatthemeasured frictionvelocitieswereable tomobiliseeven finegravel. In thatcase, thedunere-spondedtothedailytidesbyanup-crestcoarseningtrendinsedimentsorting—afeaturealsoob-servedover the largestdunes in thepresentstudyarea (Fig.5c).Moreover,bedformsofvarioussizeswereidentifiedinallsedimenttypesranginginmeandiametersfromfinesandtogravel.Thissuggeststhattheentirechannelbedisregularlyexposedtotidalcurrentsexceedingcriticalveloci-tiesforsedimentmobilisation.Asmoothchannelbedcomposedofconsolidatedclay,asidentifiedinthestudyarea,wouldnormal-lybean indicationforaprolongedabsenceofanysedimentcover.Instead,thearea iscoveredbyfullydevelopedduneswhencomparedwithglobaldataondunedimensions(Fig.8;cf.Flemming1988).Thiscouldmean that theobserveddunesare a fairly recentphenomenon,beingpossiblyrelatedtothedumpingofdredgespoilneartheKubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.56 Geo-MarineLetters;Vol.37;Heft1;Seite47-58


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Fig.8Plotofduneheight(H)vs.dunelength(L)forthestudyarea.Meanandmaximumtrendlinesofsubaqueousbedformsaswellastheoverallscatteroftheglobaldatasetareplotted inthebackgroundforcomparison(extractedfromFlemming1988)

lighthousewhichhasprovidedanewsedimentsourcefromwherethesedimenthasbeenredistrib-utedby the tidalcurrents.Thishypothesis isstrengthenedbytheobservation thatdunes locatedsouthofthe lighthouse,up-streamwithrespecttoresidualcurrentvectors,are lowerandshorterthan theonessituatedcloser tothe lighthouse(Fig.2c).This findingmaybeuseful toauthoritiesstrivingforanoptimisationofdredginganddumpingstrategies.Whenevaluatingthebathymetricchangesoverthepast6years,itseemsthatthedunesaremerelypassingthroughthissectionofthechannelandthatthechannelwouldonceagainbecomevoidofsedimentifthesourceweretobecutoffby,forexample,closingthedumpingsite.Ifthesedimentsourcewereindeedthedumpsite(e.g.Wienbergetal.2004),thenoneshouldbeabletopreciselycalculatethetimeofnaturalredistributionoftheexogenicmaterial.Onecouldalsoconsidercon-trollingdunedimensionsbydumpingonlycertaingrainsizesandavoidingothers.Thus,theavaila-bilityofsedimentscoarserthanfinesandwouldbeconducivetoforminglargetoverylargedunesinthearea.Decreasingthegrainsizeofdumpedmaterialwouldresultintheformationofsmaller,more rapidlymigrating dunes and thereby avoid navigational obstacles in the form of elevatedcrestheights.


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Observedsedimenttransportpatterninrelationtomodelledhydrology

Due to the local channel topography, a complicated pattern of residual currents has developed,whichwas identifiedevenon the relativelycoarsegridof thenumericalmodelcompared to thedunelengths.Ithastobekeptinmindherethatthesimplifiedbathymetryusedinthemodeldoesnotresolveindividualdunes.However,therequiredassumptionsofthenumericalmodel—e.g.av-eragingthevariabletopographywithinindividualgridcells—wereshowntobe insignificantwithrespecttotheadvantageofabroaderspatialviewandtheopportunitytoanalysetheflowfieldinaphysicallyconsistentwayusingcharacteristic tidalparameters.Amajorresultof thestudy isthefactthatthetransportvectorsderivedfromtracingthecrestlinesareinidealagreementwiththeresidual current vectorsmodelled for fair-weather conditions. Thus, the locations of the largestdunescoincidewiththehighestresidualcurrents,whichare locallyebb-dominant.Smallerdunesalsoclearlymigrateinthedirectionoftheresidualcurrents.Becausethemodelledcurrentveloci-ties lackdetailedvalidation forthe2010bathymetry, theyshould in the interimbetreatedasanapproximation.Thesedimentologicaldata,ontheotherhand,providemultipleindicatorsthatcriti-calvelocitiesformobilisingcoarsetoverycoarsesandareexceededduringeverydaytidalaction.Nevertheless, themodel resolution ishigh enough to reproduce the residual currentvortices atexactlythesamelocationsasindicatedbytheobserveddunepathways.Suchresidualcurrentvor-ticesareanaturalphenomenoninenvironmentscharacterizedbyreversingtidalcurrents,butitisarareexperiencetobeabletoobservetheireffectindunemigrationpathsontheseabed insuchdetail.In the study area, theneighbouring counter-rotatingvorticeshave resulted indunes convergingalongoneaxis.Interestingly,nosedimentaccretionisobservedatthepointofconvergenceinthecourse of the 6 years ofmonitoring (Fig. 6).Moreover, even a buffer zone of ca. 200 m radiusaroundtheconvergencepointremainedrelativelystable,beingcharacterisedbysymmetricaldunebodies andonlyminor shifts in thepositionsofdune crests. In combinationwith thenumericalmodelresults,thewestward-directedflowvectorsatthecentrebetweenthetwocounter-rotatingvorticessuggestthatthesedimentbypassesinthatdirection,althoughthiscouldnotbeverifiedonthegroundbydunemigrationpatterns.Unfortunately, itwasnotpossible toestablish theexactnatureofthebypassingmechanismbecausedetailedmeasurementsonthewesternbankwerenotavailabletothisstudy.Whilenoevidenceofanaccretingwesternbankinthevicinityoftheconver-gencezoneisavailable,acleardepocentrewasobservedinthenorth-westerncornerofthestudyarea (see animation in theElectronic supplementarymaterial).This accumulation is spatially inperfectagreementwiththeresidualcurrentvortexwhichprobablytransportsthesedimentawayfromtheconvergencezonetowardsthenorthalongtheshoreoftheMinsenerOog island,beforeeventuallybendingeastwardspreciselyatthelocationofthedepocentre(seeFig.7d).Itcan thusbehypothesized that thesediment trappedby theopposing residualcurrentvorticesover thewesternchannelbank isconstantlybeingrecirculated.As this featurecouldnotbecon-firmedbytheavailablesedimentologicaldata,thehypothesisshouldbetestedbyacoupledhydro-dynamic–sedimentKubicki,Kösters,Bartholomä:Duneconvergence/divergencecontrolledbyresidualcurrentvorti-cesintheJadetidalchannel,south-easternNorthSea. S.57 Geo-MarineLetters;Vol.37;Heft1;Seite47-58


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transportsimulationusingahigher-resolutiongridsupportedbymoredetailedground-truthingintheareaofinterestConclusionsThemeanderingsectionof theouter Jade tidalchannel,which isapparentlybeingsuppliedwithmaterialderived fromanearbydredge-spoildumpingsite,wasmonitoredmonthlyovera6-yearperiodbymeansofhigh-resolutionbathymetricmappingtotracedunemigrationratesanddirec-tions.Thefieldinvestigationswerecomplementedbynumericalmodellingofthefair-weatherhy-drology.Themainresultsandimplicationscanbesummarizedasfollows:

– Themodel-derivedresidualcurrentvectorsareinperfectspatialagreementwithdatacol-lectedinsitu.

– Themaindirectionandhighestvelocityofresidualebbcurrentflow isassociatedwiththelargestdunes,whicharecomposedofmediumtocoarsesand.

– Residualcurrentvorticesformonbothsidesofthechannel.Theseare largeenoughtobetracedontheseabedbyduneasymmetries.

– Twocounter-rotatingvorticesover thewesternchannelbank result induneconvergencealongoneaxis.

– Theconvergencezoneremainedstableovertheentire6-yearmonitoringperiod,suggest-ingequilibriumconditionsbetweenhydrologyandmorphology.

– Itishypothesizedthatsedimentbypassestheconvergencezonetobeconstantlyrecirculat-ed.

Acknowledgements

ThisresearchwasfundedbytheFederalMinistryofEducationandResearch(BMBF/KFKI)aspartof the „Model-based analysis of long-term morphodynamic processes in the German Bight(AufMod)“project(03KIS083and03KIS088).TheauthorswishtoacknowledgeSandraBüllesandAxelGötschenbergfromtheWSAWilhelmshavenforenrichingthestudywitharchivedbathymetricdata.OursincerethanksgotothecaptainKarlBaumannandthecrewoftheRVSenckenbergfortheirnavigationalskillsduringthemeasuringcampaigns.ArnulfMöllerisacknowledgedforacquir-ingandpost-processingthemultibeambathymetricdata.AstridRaschkeisthankedforherhelpinsedimentanalysis.Lastbutnot leastBurghardW.Flemming,DanielM.Hanesandananonymousrevieweraredeeplyacknowledged forconstructivecriticsand remarks thatenriched themanu-script.CompliancewithethicalstandardsConflictofinterestTheauthorsdeclarethatthereisnoconflictofinterestwiththirdparties.


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