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Observations of Wave-Sediment Interactions on a Muddy Shelf

Permanent Link: http://ufdc.ufl.edu/UFE0022682/00001

Material Information

Title: Observations of Wave-Sediment Interactions on a Muddy Shelf
Physical Description: 1 online resource (122 p.)
Language: english
Creator: Jaramillo, Sergio
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: gravity, mud, nonlinear, waves
Civil and Coastal Engineering -- Dissertations, Academic -- UF
Genre: Coastal and Oceanographic Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In an effort to better understand the interaction between wave propagation and sediment transport processes in a muddy environment, this work discusses the data collection and analysis of high resolution coherent observations of waves, currents and sediment dynamics in the Atchafalaya shelf, Louisiana, USA. The data shows that strong swells associated with cold front passages induce bottom liquefaction, the formation of fluid mud layers that move seaward, and subsequently settle as the wave energy decreases. These rheological changes in the state of the bed occur in the time span of a storm questioning the practical applicability of sediment and wave evolution models based on single-phase mud rheology. This scenario also contradicts previous hypotheses that point to post-frontal upwelling to explain the accretion patterns in muddy coasts. It is found that bottom-induced wave energy dissipation increases gradually after the arrival of pre-frontal swells reaching a maximum of about 60% over 4 km. Contrary to what is suggested in previous studies, the results of this work show that fluid muds do not seem to particularly influence dissipation, instead, most of the dissipation takes place after the fluid mud is deposited and the seabed is, presumably, in an underconsolidated soft-mud state. This dissipation is not constrained to the swell band, as it is observed in the infra-gravity wave, and sea bands during high swell events. Further analysis indicates that nonlinear three wave interactions during these periods are significant and may contribute to the dissipation of energy in frequency bands that would otherwise not be affected by bottom effects. Similar nonlinear processes have been explored in previous numerical and theoretical studies, but have not yet been observed with the temporal and spatial resolution achieved in this experiment. The present observations will help to verify the relevance of such models. Future improvements to the measurement techniques applied in this work, should include vertical profiles of rheological properties of mud.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sergio Jaramillo.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Sheremet, Alexandru.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022682:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022682/00001

Material Information

Title: Observations of Wave-Sediment Interactions on a Muddy Shelf
Physical Description: 1 online resource (122 p.)
Language: english
Creator: Jaramillo, Sergio
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: gravity, mud, nonlinear, waves
Civil and Coastal Engineering -- Dissertations, Academic -- UF
Genre: Coastal and Oceanographic Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In an effort to better understand the interaction between wave propagation and sediment transport processes in a muddy environment, this work discusses the data collection and analysis of high resolution coherent observations of waves, currents and sediment dynamics in the Atchafalaya shelf, Louisiana, USA. The data shows that strong swells associated with cold front passages induce bottom liquefaction, the formation of fluid mud layers that move seaward, and subsequently settle as the wave energy decreases. These rheological changes in the state of the bed occur in the time span of a storm questioning the practical applicability of sediment and wave evolution models based on single-phase mud rheology. This scenario also contradicts previous hypotheses that point to post-frontal upwelling to explain the accretion patterns in muddy coasts. It is found that bottom-induced wave energy dissipation increases gradually after the arrival of pre-frontal swells reaching a maximum of about 60% over 4 km. Contrary to what is suggested in previous studies, the results of this work show that fluid muds do not seem to particularly influence dissipation, instead, most of the dissipation takes place after the fluid mud is deposited and the seabed is, presumably, in an underconsolidated soft-mud state. This dissipation is not constrained to the swell band, as it is observed in the infra-gravity wave, and sea bands during high swell events. Further analysis indicates that nonlinear three wave interactions during these periods are significant and may contribute to the dissipation of energy in frequency bands that would otherwise not be affected by bottom effects. Similar nonlinear processes have been explored in previous numerical and theoretical studies, but have not yet been observed with the temporal and spatial resolution achieved in this experiment. The present observations will help to verify the relevance of such models. Future improvements to the measurement techniques applied in this work, should include vertical profiles of rheological properties of mud.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sergio Jaramillo.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Sheremet, Alexandru.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022682:00001


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IwanttothankmyadvisorDr.AlexandruSheremet,forhisconstantsupportduringthiseventfulridefromLouisianatoFlorida.Ithinkitpaidohandsomelyforme.IalsowanttothankhimfortheoccasionalkickthatgotmetomoveandworkbetterwhenIfeltstucked.IalsowanttothankDr.MeadAllisonandDanDuncan,originallyatTulaneandnowattheUniversityofTexas.Theyweremyseamates,advisors,andfriendsduringlongdaysofworkintheAtchafalayashelf.TheyenduredKatrina,andRita,andstillfoundtimetodogoodscience.Ialsowanttothankmycommitteemembers,Dr.JaneMcKee-Smith,Dr.JohnJaeger,Dr.ArnoldoValle-Levinson,andDr.TianJianHsu,foragreeingtoexaminemydissertationinsuchshortnotice.Alloftheircommentshelpedmetohaveamorecompleteandconcisedocumentaswellasputtingthingsintheperspectiveofabroaderscienticcontext.EspeciallyhelpfulweremydiscussionswithDr.Hsuaboutboundarylayerprocessesandgravityows,aswellasDr.Valle-Levinson'shelpinmyratherclumsyapproachtotidalanalysis.IhavetothankCaptainSteveDartez,thenworkingatLouisianaStateUniversityeldsupportgroup,ashecollectedthedataIpresentinthiswork.Ifnotforhisprofessionalandcarefulwork,Iwouldhavebeenforcedtochooseaneweldofresearch.IhopeotherstudentsndthedatawhichIhelpedtocollectasusefulasIfoundhis.Mygratitudeisextensivetoallthestudentsinthecoastalgroup,speciallyAmy,Chloe,Ilgar,andUriah.Allofthemhelpedmeinsomewayalongmydoctorate,eitherwithtechnicaladvice,deployinginstruments,orbysharingapintofbeer.Thankstomymother,mysisterandmyfather,whohavebeenthinkingaboutmeintheseyearsdespitetheirownpersonalproblems,andinthedarkesthours.Theirloveandsupporthashelpedmetofocusandlearnwhatistrulyimportant.Lastly,IwanttothankmywifePaola;shehasbeenmycompanion,mysolace,andmyreason,fromMedellintoVancouver,BatonRouge,Gainesville,andsoontoHonolulu.Idonotknowwhereourjourneywillleadnext,butIknowthataslongasIamwithheritwillbeawonderfulplace. 4

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page ACKNOWLEDGMENTS ................................. 4 LISTOFFIGURES .................................... 6 ABSTRACT ........................................ 8 CHAPTER 1INTRODUCTION .................................. 10 1.1TheAtchafalayaShelfEnvironment ...................... 11 1.2Wave-MudInteractionModels ......................... 12 1.3ObservationalStudies .............................. 20 1.4BoundaryLayerUnderOscillatoryForcing .................. 22 1.5NonlinearTriadInteractions .......................... 26 1.6ResearchObjectives: .............................. 31 2EXPERIMENTALSETTINGS ........................... 33 2.1StudySites ................................... 33 2.2InstrumentationandSiteSampling ...................... 35 3OBSERVATIONS ................................... 42 3.1ExperimentA ................................. 42 3.2ExperimentB .................................. 61 4WAVEPROPAGATION ............................... 71 4.1SwellDissipation: ................................ 73 4.2ModelingOtherSourcesofDissipation .................... 75 4.3Near-bottomPhaseChanges ......................... 81 4.4FrequencyDependentDissipation ....................... 84 4.5NonlinearTriadCoupling ........................... 88 5SUMMARYANDCONCLUSIONS ......................... 101 APPENDIX ADATAANALYSISTECHNIQUES ......................... 107 A.1PowerSpectralDensity ............................. 107 A.2Higher-orderMoments(BispectralAnalysis) ................. 110 REFERENCES ....................................... 115 BIOGRAPHICALSKETCH ................................ 121 5

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Figure page 1-1SamplingstationsontheAtchafalayaShelf. .................... 13 1-2LaminarBoundarylayerunderoscillatoryforcing. ................. 24 2-1SurcialsedimentgrainsizedistributionontheAtchafalayaShelf. ........ 35 2-2CongurationofplatformT1andT2. ........................ 36 2-3CalibrationcurveforoneoftheOBS'deployedatT2. ............... 40 3-1Aqua-1MODISimageoftheGulfofMexicoandCOAMPSsurfacewindeldonMarch92006. ................................... 44 3-2WindandADCPwaveobservationsatT1duringExperimentA. ........ 45 3-3Wind,wavesandADCPcurrentobservationsatT1duringExperimentA. ... 46 3-4HarmonicanalysisofmeanwaterelevationatT1duringExperimentAusingt tide. ......................................... 47 3-5TidalanalysisofmeanwaterelevationatT1duringExperimentAusingaFouriertransformlter. .................................... 49 3-6CharacteristicsoftidalcurrentsandmeansurfaceelevationatT1duringExperimentA. ........................................... 50 3-7Non-tidalcurrentsandmeansurfaceelevationatT1duringExperimentA. ... 51 3-8ObservationsofwavesfromADCP(a)andPC-ADP(e),near-bottomcurrents,andsignalintensity(PC-ADP),duringtheMarch10thstorminExperimentA. 53 3-9PC-ADPdataqualityvericationduringthetwouidmudevents. ....... 54 3-10Estimationofgravityowspeedfortherstuidmudevent. .......... 61 3-11Estimationofgravityowspeedfortherststageoftheseconduidmudevent. 62 3-12Estimationofgravityowspeedforthesecondstageoftheseconduidmudevent. ......................................... 63 3-13Observationsofwaves,near-bottomcurrents,andsignalintensity(PC-ADP),duringExperimentBatplatformT2. ........................ 65 3-14Non-tidal,tidalcurrentsandmeansurfaceelevationatT2duringExperimentB. 66 3-15SuspendedsedimentconcentrationrecordedbytheOBSatT2duringExperimentB. ........................................... 67 6

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........................................... 70 4-1WaveobservationsatT1andT2duringtheMarch7th-14thstorm. ....... 72 4-2SwellenergyuxtransmissionbetweenT1andT2. ................ 75 4-3GeographicdomainandbathymetryusedinSWAN. ................ 79 4-4SignicantwaveheightinSWAN. .......................... 80 4-5Swellenergyuxtransmissionobtainedfromanon-stationarySWANrun. ... 81 4-6Cross-shorevelocityphasedierencesinthelowerwatercolumnaveragedovertheswellband(0.05-0.2Hz). ............................. 92 4-7Pre-stormverticalprolesofcross-shorevelocityphasedierences,andcoherence 93 4-8Prolesofcross-shorevelocityphasedierencesandcoherenceduringtherstuidmudevent,andtheinitialstageoftheseconduidmudevent. ....... 94 4-9Prolesofcross-shorevelocityphasedierencesandcoherenceduringthenalstageoftheseconduidmudevent,andafterthestormhaspassed. ....... 95 4-10Frequencydependentwaveattenuationcoecientki. ............... 96 4-11Frequencyspectracomparisonandcross-sectionsofwaveattenuationcoecientki. ........................................... 97 4-12Prolesofrmswavehorizontalvelocityduringtwouidmudevents. ...... 98 4-13ObservationsofnonlinearcouplingatT1duringthestormMarch8th-14th. .. 99 4-14Doubleintegralbii 100 7

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1958 ]showedabout80%waveenergylossoveronly2.6wavelengths,forlayerthicknessofabout1.3timesthewaveboundarylayerthickness.Similarlyremarkabledissipationrateshavebeenobservedintheeld,associatedwithunder-consolidatedmudseabeds[ 1981 ; 1995 ; 2000 ].Alongmuddycoastlines(typicallyassociatedwithlargeriverinedischarges,e.g. 1972 ])strongwavedissipationmaydiminishstormwaveheightsandresultingseverityofcoastalinfrastructuredamage.Mudinduceddissipationseemstonotbeconstrainedtoaspecicfrequencyband,namelylongwaves(0:050:2Hz)seemtobeinuencedbythepresenceofamuddybottom,raisingthequestionofhownonlinearwaveinteractionisaectedbytheobservedfrequencydependentdissipation,andviceversa.Inrelationtosedimenttransportprocesses,waveinducedliquefactionofmuddybottoms,andwaveinducedturbulenceinthebottomboundarylayer,havebeenpointedassomeofthemainmechanismsresponsibleforcohesivesedimenttransportintheformofgravityorturbiditycurrentsonthecontinentalshelf(e.g. 2000 2001 ).Thefeedbackloopofwaveinducedresuspensionofcohesivesedimentsandsedimentinducedwavedampingseemtohavedirectconsequencesintheaccretionpatternsofcoastslocatednearsignicantsourcesofmuddysediments(e.g. 1992 2005 ; 2006 ). 10

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1958 ; 1978 ; 1980 ; 1987 ; 1995 ; 2005 )ortheeectofwavesonmudbottoms( 1986 ; 1993 ; 2000 ; 2005 ).Themainpurposeofthisstudyistousedetailedeldobservationsofthesetwoprocessestogainaninsightintohowtheyaecteachother.Followingthisidea,inordertostudywavepropagationwithlimitedshoalingeects,theinterplaybetweenbottominducedfrequencydependentdissipationandnonlinearinteractions,thedesiredsiteforthisstudyisamuddyshelf,withshallowwaters,largescalesandmildslopes.Italsorequiresanenvironmentwerewaveinducedliquefactionandturbidityowscouldbeobserved.SuchconditionscanbefoundintheAtchafalayashelfinLouisiana.Thegeneralcharacteristicsofthisenvironmentandthedierentapproachestomodelwave-mudinteractionsareexplainednext. 1-1 )thatreceivesnearly30%ofthetotalMississippidischarge[ 1996 ]withanaveragesuspendedsedimentloadatSimmersport(1951-2000)of84milliontons/yroutofwhichonly17%issand[ 2000 ].Suchsedimentdischargehasleadtotheformationofcoarse-grainedsub-aerialdeltasatthemouthoftheAtchafalayaandWaxlakeoutletsintotheAtchafalayaBay( 1975 ; 1988 ; 1980 1989 )andasub-aqueouspro-deltamuddepositontheadjacentinnershelf( 2000 ; 2005 )whichisgrowingatratesupto3.8cm/yr[ 2005 ].Additionally,andcontrarytowhatoccursinmostoftheLouisianacoastandMississippidelta,whichisexperiencingsubsidenceandrapidlandloss,themudatsinthecheniercoastwestoftheAtchafalayaBayexperienceprogradationratesrangingfrom29m/yr[ 2005 ]to60-80m/yr[ 1989 ]. 11

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2000 )andthepassageofhighlyenergeticatmosphericcoldfronts( 1996 ; 1998 ; 2000 ; 2006 ).Themechanismproposedforthisaccretionisasfollows:highwavesassociatedwithpre-frontconditionsbreakdowntheplumederivedstraticationofthewatercolumnandresuspendthebottomsediments[ 2000 ],duringpostfrontconditionsthestraticationisrestoredandhigh-concentrationsuspensionsaretransportedonshorebycoastalupwelling[ 2006 ]andwestwardbyacoastalcurrentknownasthe\Atchafalayamudstream"[ 1981 ].Nearshore,duetomudinducedwaveattenuationtheconditionsarelessturbulentandthecoastisshelteredfromwaveenergy,howevertheconditionsarestillturbulentenoughtopreventdepositionandmaintainthishigh-concentrationmudlayer[ 1986 ],whichleadstotheobservedhighaccretionratesonthecheniercoast[ 2006 ].TidesintheAtchafalayaBayandadjacentcontinentalshelfarediurnalwithameanamplitudeof0.6m[ 1980 ],withtide-inducedcurrentsaveraginglessthan10cm/sindepthsshallowerthan10m[ 1986 ].Duringtheperiodoftheexperiment(February-March)coldfrontspassovertheareawithaperiodicityofabouttwoweeksaccompaniedbysealevelchangesandwind-inducedset-upandset-downaswinddirectionchangesfromsoutherlytonortherly.Associatedcurrentscanexceed60cm/s[ 2000 ; 1998 ].FrontalwindsalsorestricttheoshoreextentoftheAtchafalayariverplume;inwaterofdepthlessthan10mwavesbreakdownthestraticationinducedbythebuoyantplume[ 2000 ; 2006 ].Shallowerthan5meters,suspendedsedimentloadinthewatercolumncanreach200g/m3withbriefperiodsofnear-bottomuidmudwith10g/Lduringthepostfrontalstages[ 2000 ]. 12

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2000 ; 2003 ; 1998 ).ThewidthoftheshallowinnerAtchafalayashelf(the10-misobathisinsomeplacesabout50kmoshore),itssedimentaryfabric,andthevariabilityofthewaveclimatemakeitaprimelocationforthestudyofwave-bottominteractionundervariablemud-phaseconditions. 1981 ; 1986 ; 2003 )aswellasinseveralplacesaroundtheworld (e.g.Surinam, 1981 ;India, 1995 ;Guyana, 2007 ) .Toexplainthisprocessseveraltheoreticalformulationshavebeenintroducedwherethedissipationmechanismdependsonthestateofthebed,however,mainlyverheologicalmodelsdescribethedeformationpropertiesofthebottom.Thesewillbrieybediscussednext,althoughthisdoesnotrepresentacomprehensivereviewofallthestudiesmadetodateinthesematters.Themainintentionistoillustratethegeneralcharacteristicsandndingsforeachtypeofrheologyasitpertainstotheproblemofawaterwavepropagatingoveradeformablebed. 13

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SamplingstationsontheAtchafalayaShelf.a)QualitativefaciesmapofsurcialsedimentsontheAtchafalayaShelf(basedon 2005 ,unpublisheddata).RelictmudsrepresentolderHoloceneMississippideltaiclobedepositsthataremantledephemerallyanddiscontinuouslybymodernmudlayerlessthan30-cmthick.Themodernmuddepositrepresentstheextentofthesubaqueousmudclinoformdelta[ 2005 ]thatmergeswestof-92.5olongitudeintoaconcave,progradingshorefacedeposit[ 2005 ].Instrumentsweredeployedintwodistinctcongurationsontheoutertopsetandforesetareaoftheclinoform.Thelocations(T1,T2)forExperimentA,March1-14,2006,aremarkedbycircles.Thedistancebetweenthetwoplatformswasabout4km.ThelocationscorrespondingtoExperimentB(T1,T2),March15to25th,2006,aremarkedby\x"s.Thedistancebetweenthestationswasabout4km.b)Smoothedestimateofbathymetryprole(continuousline)alongatransectthroughthetwoExperimentAsites(circles,panela).ThelightgraydotsarefathometerreadingsfromtheR/VPelicancollectedinMarch2008alongtheprolebetweenT1andT2.Becauseoftheshipsdraft(4.5m),fathometerreadingswerenotobtainableforwaterdepthslessthanabout5.5m.Todenetheshallowersectionoftheseaoor,fathometerdatacollectedbyNRLinMay2007(darkgraydots)approximately1kmwestoftheplatformlineisalsoshown.SurveydatawascheckedagainstmeanwaterlevelrecordsatT1andT2.Themaximumslopeis0.0008,reachedatx=5km,slightlyoshoreoftheoshoreplatformT1.Notethepresenceofseaward-dippingsubsurfaceacousticreectorsintheR/VPelicandatathatindicateprogradationofthedeltafront. 14

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1957 ]asatwo-layermodelofaninvisciduidoveradenserviscousNewtonianuid,waveattenuationiscausedbyviscousdampingwithinthemudlayer.Themathematicalmodelwasbasedontheshallowwater(longwave)equationsanditssolutionsshowanexponentialdecaywithdistanceofpropagation.Themaximumdecayratewasreachedwhenthenon-dimensionallowerlayerdepth,~d=h2p 1978 ]modiedtheshallowwaterassumptioninthismodelandusedthelinearizedNavier-Stokesequationstomodelansmallamplitudewavetrainpropagatingovertwouidswithdierentdensitiesandviscosities,wheretheowinbothlayerswastreatedaslaminar.Theeectsofthedeformablebottomwhereanalyzedintermsofwaveheightattenuationandphaseshiftbetweenthewaveformatthetwolayers.TheirresultsaresimilartoGade'sinthesensethatthemaximumattenuationrateisfoundwhenthelowerlayerthicknessisontheorderoftheboundarylayerthicknessoftheloweruid(~d1:6).However,thismodelproducedverylargevelocitiesinthemudlayerunlessanunrealisticallylargemudviscosityisused[ 1988 ]. 2000 ]modeledtheprobleminasimilarway,withthelowerlayerassumedtohaveathicknessonthesameorderastheboundarylayer(maximumdampingat~d=1:5),andismuchthinnerthantheoverlyingwaterlayer.Anexplicitexpressionforthewaveattenuationrateunderthisconditionsisderived,andmudtransportinducedbywavemotionsisexamined,ndingthatthemudvelocityincreaseswithmudlayerthicknessbutdecreaseswhenthedierenceindensitybetweenthetwolayersincreases.Despiteitsshortcomings,andasthismodelisrelativelyeasytoimplementcomputationally,ithasbeenthemostusedbyresearchers(e.g. 2007 ;

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2008 ],inpreparation)toaccountfordampingbymudinwavemodelssuchasSWAN( 1999 ; 1999 ; 2005 ). wheretheYoung'smodulus,E,thePoissonratio,#,andtheshearmodulus,G,arerelatedthrough: 1977 ]presentedananalyticsolutionforawavepropagatingoveranon-rigidbottomassumingstressequilibriumwithinthesoil,calculatingsoilstresses,displacementsandthecorrespondentwavekinematics.Theyfoundthatthewavenumberincreasedastheshearmodulusofmuddecreased.Theeectsofsoilinertiawherelaterincludedintotheproblemby 1978 ],buttheattenuationofwaveamplitudewasnotaccountedforinthesestudies. 1989 ]showedthatforawavepropagatingoveraverticallystratiedelasticbed,thecarriergravitywavewouldloseenergytosidebandoscillations.Theseoscillationscouldbetaintedwithveryshort,smallamplitudeelasticwavesthatcaninteractwiththeviscousboundarylayeratthewater-mudinterface,increasingviscousattenuationofthepropagatingwave. 16

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@z=8>><>>:0;forjj<00sgn(@u=@z);forjj>0(1{4) 1987 ],studiedthecaseoflonggravitywavespropagatingoverathinviscoplasticmudlayerndingthataswavespropagatedthemudmotionwouldchangefromcontinuoustointermittent.Adrawbacktothismodelispresentedby 1988 ],pointingoutthatmostmudsexhibitapseudoplasticbehavioratlowratesofstrain,andthereforetheBinghamplasticbehaviorisonlyapproximate. Kp~q+ n@~q @t(1{5)whererpisthegradientoftheporepressure,~qisthedischargevelocityvector,Kpistheintrinsicpermeabilitycoecientthatrepresentsthecharacteristicsoftheporousmedia,isthedensityoftheuid,andnisthevolumetricporosityofthemedium. 1985 ]examinedwaveattenuationinaporo-elasticbed.Intheirmodelthemechanismsforwavedampingexaminedarebottomturbulence,percolation,andCoulombfrictionbetweensoilgrains.Thislatermechanismisfoundtobethemostdominant.SinceCoulombfrictionishighlynon-linearduetothedynamicsofteningoftheseabed,theyndthatlargewavesattenuatemuchquickerthansmallwaves. 1987 ]studieddissipativewavesinsideaporo-elasticbed,showingthatfreewavesmaycauselocalizedliquefaction,andthatwaveenergydampingcanoccurduetouid-solidrelativeowinsidetheporousbed. 17

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1988 ]. G_=_(1{7)wherethedotrepresentsderivativewithrespecttotime. 1980 ]examinedtheproblemofasmallamplitudewavepropagatinginaninviscidupperwaterlayeroverlyingalinearviscoelasticseabedthatbehavedaccordingtotheVoigtmodel.ThemathematicaldescriptionofthemodelisbasedontheNavier-Stokesequations,withthedierencethattheviscosityissubstitutedwiththecomplexparameter: (1{8)wheretherealpartrepresentstheviscousresponseandtheimaginaryparttheelasticeects.Thesolutionstothismodelprovidedthedispersionrelationandthedampingrate,showingthatforaxedvalueofelasticity,thereisalocalpeakinattenuationratewiththeattenuationraterstrisingwithviscosityandthendecreasingastheviscositygrowslarger.Forthecasewhenviscositydominates,therearenoelasticforcesthatcanbringthesoiltoitsinitialstateandextremelyhighattenuationratesarereached. 1980 ]followedasimilarformulationfortheproblemandfoundreasonableagreementwhencomparedtomeasurementsinEastBay,Louisiana[

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1993 ],whoshowsthatthehighwavedampingratecanbeexplainedbythepossibleresonanceeect.Laboratoryexperimentsby 1988 ]suggestthattheVoigtmodelperformsbetterthantheMaxwellmodelforsoftmuds. 1990 ]extendedthemodelof 1978 ]toamodelwherethebottombehavesasamultiplelayerVoigtmodel.Thesimulatedshearstressatthemud-waterinterfacewasfoundtobelargerthanthatobtainedbyassumingarigidmud,duetothedierencesinphasebetweentheoscillationsinwaterandmud. 2007 ]foundthatheaviermudhasaweakereectonwaveattenuation.Thisattenuationratedoesnotalwaysdecreaseastheshearelasticmodulusincreasesastherecouldbearesonancewhenthewavefrequencyapproachesthenaturalfrequencyoftheviscoelasticmudlayer.Whentheelasticshearmodulusvaluesaresmall,thewaveattenuationisampliedsignicantly.Theirresultscomparedwellwiththeexperimentaldataof 1990 ],buttheyregretthattherearenotavailablemeasurementstoverifythephasefunctionofthevelocitycomponentsandthewater-mudinterfacedisplacement.Theirresultsalsoshowthatthethinmudlayerassumptionisadequate. 1993 ]pointoutthatmostoftheabovementionedmodelsarelinearinteractionmodelswherethebottomsedimentpropertiesareprescribedapriori,independentoftheimposedoscillatorystrains.However,laboratorymeasurements[ 1993 ]showthatcohesivesedimentshavenonlinearbehavior,sincetheirfunctionsarepropertiesofthestateofstrain.Theirmeasurementsalsoshowthatmudsbehaveaselasticsolidsatlowratesofstrainandasviscousuidsathighratesofstrain,whileforintermediatevaluesoftherateofstrainthemudbehaveslikeaviscoplasticmaterial.Followingthisidea, 1993 ]developedawave-seabedinteractionmodel,whichassumesthattheviscoelasticpropertiesofmudareafunctionofthewave-inducedstrains,anditsolvesforthedepthofuidizationdependingontheimposedwaveheight.Theynoticedthatforthesamewaveheight,ashorterwaveperiodresultsinlargerelastic 19

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2006 ],whichndthattherateofwaveattenuationincreaseswhenthewavedirectionsisopposedtothecurrentdirection,andreducedwhentheyareinthesamedirection.Theyalsonotethattheeectsofcurrentsonwaveattenuationarehigherbyoneorderofmagnitudeformuddyseabedsthanforrigidbeds.Itisnotedthen,thattheapplicabilityofanyoftheabovemodelstoeldconditionsisconstrainedbycorrectlymatchingofthehypothesizeddissipationmechanismtotheactualsedimenttypeandphase.Thisiscomplicatedbythefactthateventhoughitisknownfromlaboratory,eld,andtheoreticalstudies(e.g. 1995 ; 2006 ; 2005 ; 1993 )thatthestateofmuddyseabedschangesunderdierenthydrodynamicconditions,withtheexceptionof 1993 ],noneoftheabovemodelsincludewaveorcurrentinducedmodicationstothesedimentstate.Mostofthemodelsdescribedaboveassumeasingle-phase,stablemudrheology,andpredictwidelydierentlevelsofeciencyofmud-inducedwavedissipation. 1993 ; 1993 ; 1995 ],ortheformationofnear-bedhigh-densitysuspensions(hyperpycnalows,uid-mudlayers),detailedeldobservationsoftheprocessarescarce.Liquefactionwasobservedtypicallybasedoncorescollectedafterstorms[ 1975 ], 1983 ]alsoreportsmassfailuresalongsubaqueousslopesinducedbystormwaveswhichdecreasefrom20-23mindeepwaterto3-4minwaterdepthsof12-21mintheMississippiDeltaregion.ObservationsinEastBay,Louisiana,by 20

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1985 ]measuredsignicantwaveattenuationandbottomverticalmotionaswavespropagatedfromdeeptoshallowwaterinEastBay.Theydiscussedthatprocessessuchaslinearrefractionandshoalingcanexplainforwaveheightreductionwhenthewaveheightsaresmall,howevertheypointoutthatasthedissipationincreasewithwaveheight,thedissipationmechanismshouldbeanonlinearfunction,stronglydependentonwaveamplitudeandweaklydependentonfrequency.Theirmeasurementsshowalagof180degreesbetweenthebottommotionandthesurfaceelevation(bottommovesdownasthepressureincreases).Sincethemeasuredmotionsarenotlargeenoughtoaccountfortheobservedwavedissipation,theysuggestthatmostofthedissipationmustoccurbetweentheirmeasurementstationswhenthebottomsedimentsmustbemuchsofter. 1981 ],reportedmeasurementsofsurfaceelevation,suspendedsedimentconcentrationandmuddensity,othecoastofSurinam,showingthatmuddysedimentswereresuspendedatbothtidalandwavefrequencies.Thesestudiesindicatethattheinteractionbetweenwavesandcohesivesedimentscouldgovernthemechanismsoferosionanddepositiononmuddycoasts,andcouldleadtoveryhighratesofsedimenttransport,evenunderrelativelyweakcurrents[ 1981 ].Unfortunately,sincemostofthesestudiesonlyobserveverticalmotionsofthebottomsedimentsthroughindirectmeans(e.g.,oatsplacednearthebottom,sedimentcoring),wehavenoinfromationondetailedsedimentresponse(e.g.,bottomliquefaction,formationofmudlayers),thatcouldhelptoelucidatethemechanismthatwasresponsibleforwaveattenuation.Morerecently,near-beduid-mudlayerformationonamuddyshelfwasobservedintheeld(Atchafalayashelf, 2000 ; 2005 ; 2006 ),butexperimentshavetypicallyfocusedeitheronsediment[ 2000 ]orhydrodynamics( 2005 ; 2008 ),thuslacking 21

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2007 ]usedthetwo-layerviscousuidmudapproachof 1958 ]andincorporateditintoSWAN(SimulatingWAvesNearshore, 1999 )totrytomodelwavedissipationintheGuyanacoast.However,thedatausedforthepredictionswerescarceandverywidespreadbothintimeandspacetoallowgoodvalidationoftheresults.Forinstance,theirobservationsoftheextentandthicknessoftheuidmudlayerwerederivedfromdual-frequencyechosoundingstransectsdoneinthe1960's[ 1962 ]ontheGuyanacoastandtheirmuddensitymeasurementswereobtainedinthe1970's[ 1972 ]alsoontheGuyanacoast,buttheirwaveparametersareextractedform 1981 ]measurementsintheSurinamcoast. 2008 ]alsousedSWANincombinationwiththeviscousmudformulationsin 2007 ]and 2000 ]toinvestigatetheimpactofanon-rigidooronthewaveclimateatCassinoBeach,Brazil.Theirmeasurementsofrheologicalandwaveparametersaremorecomprehensivethanin 2007 ],however,theyunderpredictwaveheights(overpredictdissipation)possiblyduetotheassumptionthattherheologicalstateofthebottomsedimentsremainsconstantduringthesimulation(i.e.,alwaysviscousuidmud),andtheassumedhomogeneityofthethicknessoftheuidmudlayerinthemodeledtransect.Onlythelatterassumptionwasaddressedintheirpaper,byperforminganinversionthatallowedthethicknessoftheuidmudlayertovaryinbetweenthewaveobservationsiteswhichresultedinabetterpredictionofwavedissipation.Existingobservations,nonetheless,consistentlyquestionthedirectapplicabilityofsingle-phaserheologymodelstoelddata.Thereisthereforetheneedtoobtainmeasurementsthatprovideanindicationofthestateofthebottomsedimentsatthesametimethatwaveandhydrodynamicobservationsarebeingcollected.Duetothe 22

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1992 ], @t=@p @x+@ @z(1{9)whereisthewaterdensity,uisthevelocityinthex-direction,tisthetime,pisthepressure,isthestress,andzistheverticalcoordinate.Outsidetheboundarylayer,theshearstressesvanish,soEq. 1{9 becomes: @x(1{10)whereu0isthefreestreamvelocity.ThelongitudinalpressuregradientremainsconstantintheboundarylayersoEq. 1{9 canbewrittenas, @z(1{11) 23

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=@u @z(1{12)whereisthekinematicviscosity.SubstitutinginEq. 1{11 : @z2(1{13)Assumingafeestreamvelocityoftheform: 1{13 is: sin!tz (1{15)where !(1{16)isthethicknessofthewaveboundarylayer.Asanexample,takeawavewithvelocityamplitudeU=1m=s,aperiodof8s,and=106m2=s:Figure 1-2 (a)isshowntheverticalproleofuinsidetheboundarylayer(0
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LaminarBoundarylayerunderoscillatoryforcing.a)Verticalproleofnormalizedvelocityintheboundarylayeratthephaseofmaximumvalue;b)Phasedierenceinsidetheboundarylayerwithrespecttothefreestreamvelocity,positivedierencesrepresentaphaseleadoveru0. theboundarylayerthickness,assumingnoshearatthetopoftheboundarylayer: @t(u0u)dz=Z+z0z0@ @zdz=b(1{17)wherebisthebottomstressandz0isthezerovelocitylevel.Theboundarylayermomentumequationmaybesolvedusingthemixinglengthhypothesis[ 1926 ]: @z=u 1{18 hasthesolution: z0(1{19) 25

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1932 ],withkN=bedroughness.Eq. 1{19 satisestheboundaryconditionsforEq. 1{17 .Following 1992 ],theevolutionandverticalstructureoftheturbulentwaveboundarylayercanbeobtainedbyintroducingthedimensionlessparameter: 1{20 and 1{21 intoEq. 1{17 ,andaftersomealgebra( 1992 )thefollowingequationfortheevolutionofZisobtained: d(!t)=sin(!t)Z[exp(Z)Z1]1 kN!Eq. 1{22 wassolvednumericallyusingMatlab'sordinarydierentialequationsolver.ForawavewithanamplitudeU=1m=s,aperiodof8s,andbottomsedimentwithkN=1:5d,whered=1mmisthesanddiameter,thisexerciseproducesaphaseleadofthefrictionvelocityuoverthefreestreamvelocityu0of11.5degrees.Thisshowsanoticeablereductionofphasedierencesinducedbyturbulenteectscomparedtolaminarconditions. 1.2 )thereareseveralmodelsthatdescribetheinteractionbetweenwavesandmudinitsdierentstates.Allthesemodelsarefarmorecomplicatedthanthesimpleturbulentandlaminarcasesoverasolidbottomdescribedpreviously,howeverwecanextractsomecommonresultsregardingthephasedierencespredictedforthecaseofawavepropagatingoveramudbed.Mostofthestudiespoint 26

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1978 ]showvariationsbetween0to90degreesfordierentthicknessofanon-dimensionalizeduidmudlayer,(=22)1=2h2,where2istheuidmudviscosityandh2istheuidmudlayerthickness.Theresultoftheirmodelshowshighervaluesforsmallerdepthsintheuidmudlayer,whileatgreaterdepthstheinterfacialwavebecomesmoreinphasewiththefreesurfacewave.Theyalsopointoutthatforauidmudlayerofnon-dimensionalthicknessof1.119,themaximumvelocityintheupperlayer(waterwithnosediments)precedesthecrestarrivalbyasmallamount,whileinthelowerlayer(uidmud)thevelocitylagssignicantly,byalmost45o.Similarly, 1990 ]intheirviscoelasticVoigtmodelreportphaseshiftsof90-120degreesinthemudlayersrelativetothewaterlayer.Itbecomesapparentthat,asarstapproximation,intheabsenceofinsiturheologicalmeasurementsthecharacteristicsofthephasedierencedistributioninthewatercolumnmayhelptoinfertheevolutionofthedierentstatesofthebottomsediments. 27

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1{23 and 1{24 followthedispersionrelationshipforfreewaves: 1960 ].Indeep(kh1)andintermediatewaters(kh=O(1))triadsarenon-resonant(f6=0,ork6=0).Theso-calledboundinteractionofdispersivewaves[ 1963 ]occurwhentwofreewaves(f1;~k1)and(f2;~k2)forceasecondarywavecomponent(f3=f1f2;~k3=~k1~k2)thatdoesnotsatisfythedispersionrelationship 1{25 .Theamplitudeoftheseboundharmonicsremainsmallcomparedtothatoftheprimarywaves.Inshallownearshorewaters(kh1),wherefrequencydispersionisweakandthedirectionalspreadingofthewavesarrivingfromdeepwaterisreducedduetorefraction,near-resonantinteractionscanoccur,inwhichthesum(ordierence)componentofthetriadisclosetosatisfyingthedispersionrelationforfreewaves(thatisjf=fj1,jk=kj1),thenphaserelationsbetweeninteractingwavesvaryslightlyoverawavelength,whichallowsforsignicantenergytransferoverseveralwavelengths.Manystudies(i.e., 1962 ; 1984 ; 1993 ,andothers)haveshownthatthesetypeofinteractionscansignicantlyaectwavespectraevolutionoverfewwavelengths.Thesignicanceoftriadinteractionsandtheenergytransferredduetothismechanismdependsmainlyontheorderofmagnitudeofthenonlinearityasdeterminedfromtherelativewaveamplitude,a=h,andtherelativewater 28

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1963 ].Somedetailsaboutthestatisticsofpowerspectra,andhigher-ordermomentsisprovidedinAppendix A .ThepowerspectruminEq. A{1 inAppendix A ,forthediscretelysampledtimeseriesofwaterelevation()isdenedhereas, A{23 intheAppendix A ,becomes: 1994 ]: [S(f1)S(f2)S(f1+f2)]1=2(1{28)whereb2(f1;f2)isknownasthebicoherence,whichisameasureofthephasecoherencebetweenspectralcomponentsf1,f2. 29

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1982 ]and 1989 ]haveshownthatthebicoherencefollowsanoncentral2probabilitydistributionwhichisafunctionofthenumberofdegreesoffreedomoftheFouriertransformandthevalueofthebicoherence.Themagnitudeofthecondencelimitsdecreaseasboththedegreesoffreedomandbicoherencevalueincrease.Forthisreasonthelongerthetimeseries(moredegreesoffreedom)thebetterthebicoherenceestimates.Whilethebicoherenceindicatesifthereisphasecoherencebetweenspectralcomponents,thebiphase,(f1;f2),providesdetailsaboutthestructureofthenonlinearrelationbetweenphases: Re[B(f1;f2)](1{29)TheStokeswaveform(peakedcrests,attroughs)isassociatedwith(f1;f2)=0[ 1981 ],whileas!=2thewavesbecomeincreasinglypitchedforward,thesawtoothwaveforinstancewouldhave==2.Itisnotedthatatlowvaluesofthebicoherence,thebiphasebecomesunstable.Manyobservationalstudiesrelyonthedeterminationofthebispectratoestimatethemagnitudeofnonlineartriadinteractions. 1963 ]analyzedthebispectraofoceanwavesmeasuredat11mdepth,ndingcleartriadcouplingswiththespectralpeak.TheobservedbispectramatchedwellagainsttheoreticalestimatesobtainedusingaperturbationschemebasedontheNavier-Stokesequations.Itwasconcludednon-resonantorboundtriadinteractionswereresponsiblefortheobservedcoupling. 1985 ]usedthebispectratostudythenonlinearshoalingofwavesastheypropagatedfrom9to1mdepth.Theyshowhowthebicoherencevaluesincreaseasthedepthdecreasesindicatingstrongernonlinearinteractions.Theirestimatesofbiphasesat9msuggestsimilarnonlinearprocessesasthoseobservedby 1963 ],alsoasdepthdecreasedthebiphasesindicatedpitchingforwardofthewaves.Theyalso 30

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1990 ],foundthatthenear-resonanttriadinteractionsshouldexplaintheincreaseinenergyobservedat2fp,asthetheoryforlinearshoalingcouldnotaccountforit.However, 1992 ],usedaphasedaveragedapproachtomodelthedataof 1990 ],sincethemodelagreedwellwiththedata,itwasconcludedthatthenonlinearcouplingobservedwasduetoresonantco-linearinteractionsinstead. 1994 ]usedbispectralanalysistoestimatetherelativeamountsoffreeandforcedinfragravity(0.005-0.05Hz)waveenergyusingpressuredataat13mdepthinDuck,NorthCarolina.Thecontributionofforcedwavestothetotalinfragravityenergywasfoundtovarybetween0.1-30%,withthemaximumcontributionobservedwheninfragravitylevelsarelarger. 1995 ]analyzedthebispectraandtrispectra 31

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2003 ],whohypothesizethatshortwind-waves,throughnonlineartriadinteractions,maytransferenergytolongwaveswhointurndissipatesomeofthisenergybydirectlyinteractingwiththebottom. 1. Howdowavesandcurrentsaectsedimentdynamics(i.e.,uidization,formationofmudlayers,transport)inthemuddyAtchafalayashelf? 2. WhatarethecharacteristicsofwaveenergydissipationintheAtchafalayashelf? 3. Whatistheinuenceofbottomsedimentstateonwavepropagation? 4. ArewavenonlinearitiessignicantintheAtchafalayashelf?Toinvestigatethesequestions,twoeldexperimentswereconductedduringMarchof2006intheAtchafalayaShelf(Louisiana,USA,Figure 1-1 ),inacross-shore(ExperimentA,circlesinFigure 1-1 )andinanalong-isobathconguration(ExperimentA,crossesinFigure 1-1 ).Chapter 2 describesindetailtheexperimentalsettings,sedimentologicalcharacteristicsofthearea,thestudysites,instrumentationused,andsamplingschemes.Chapter 3 presentswatercolumnobservationsofwave,current,andtidesatthesamplingstations.Increasedspatialresolutioninthebottomboundarylayerprovidedinsightintonear-bedandsurcialsedimentresponsetowaveandcurrentforcinganditssignicanceforsedimenttransportprocessesintheAtchafalayaShelf.Next,throughtheuseofspectralanalysisandnumericalmodeling,theeectsofbottominduceddissipationonwavepropagationareinvestigated.Cross-spectralanalysisofbottomboundarylayervelocitydataoeredvaluableinformationaboutthestateofbottomsediments,theirinteractionwiththeoverlyingow,andrelatedeectsonwavedissipation.BispectralanalysisofsurfaceelevationprovidedestimatesofthesignicanceofnonlinearwavecouplingontheshallowandmuddyAtchafalayaShelf.InthenalChapter,asummaryoftheresultsobtainedinthisworkispresentedregardingbothwave-inducedsedimenttransportandassociatedmud-inducedwave 32

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A 33

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1-1 )theplatformswerearrangedinacross-shoreconguration,withtheinnerplatform(4-misobath)designedtocoincidewiththeoutertopsetareaofthesubaqueousmuddeltaicclinoformandtheouterwiththesteeperforesetzoneataboutthe5misobath.Thesesiteswerepickedutilizingsub-bottomdatafromanEdgetechX-Star1{12kHzCHIRPproler(Figure10,lineCin 2005 ]).DuetotheverymildslopeofthispartoftheAtchafalayashelf,thissitepresentsverydesirableconditionsforstudyingwavepropagationwithsmallshoalingeectsandlimitedrefraction.ExperimentBhadanalongisobathconguration,withbothplatformslocatednearthe5-misobath,alongshorebetweenthenavigationchannelandMarshIsland(crossesinFigure 1-1 ).Thesesitescoincidewith 2005 ]CHIRPLineAforesetarea.Toavoiddamagebyshrimptrawleractivity,theinstrumentsweredeployedtetheredtooil/gasplatforms,andthustheirexactlocationwassubjecttooil/gasplatformsavailability.Near-bottomhigh-densitysedimentsuspensions(uid-muds)hadpreviouslybeenobservedinthisarea,likelydueeithertosettlingofsuspendedsedimentinthewakeofenergeticevents(coldfronts, 2000 ; 2006 ;cyclonicstorms, 2005 ),ortogravitationalowsofhighdensitysuspensionsgeneratedontheinnershelf( 2005 ]noteaccumulationratesof2to3.8cm/yearintheforesetzone,5-8mdepth).Overall,thesedimentologyofsurcialinthisareaiscomplex,withlargeadjacentpatchesofdierentsedimentcharacteristics,andalarge-scalealongshelfningofAtchafalayaRiver-derivedsediments:grainsizegradesfromsandyandclayeysiltsintheEastneartheAtchafalayadredgedshipchanneltosiltyclayswithlessthan5% 34

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2005 ; 2005 ].Thisisalsoreectedinsilt/claycontentsinthemudfraction:greaterthan3%nearthedredgechannelanddecreasingprogressivelywestwardoppositeMarshIslandtoabout0.2to0.5%.Thistrendseemstoberelatedtopreferentialsorting,ascoarsematerials(sandandcoarsesilt)aredepositedclosertotheriverinesourceduringresuspensioneventswhilenersedimentsaretransportedfurtherwestwardbycoastalcurrents,withonlybriefinterruptionsduringthecoldfrontpassageswhenpost-frontalnortherlywindsadvectsurfaceplumesedimentsoshore[ 2000 ].GivenpreviousstudieshaveshowninterannualvariationsinsedimenttypeatspeciclocationsontheAtchafalayainnershelf[ 2000 ],thedistributionofsedimenttypeontheshelfwassampledbytheNavalResearchLaboratory,StennisSpaceCenter(NRL)on3-5May2006usingasurfacegrabsampler.Figure 2-1 showstheresultsofsedimenttypeanalysisbasedongrabsamplescollectedalongthreetransects:onetotheWestofTrinityshoal(notdiscussedhere)andtwoacrosstheAtchafalayaShelf;onesouthofMarshIsland,crossingtheshelfWestofTrinityShoaloverthesiteofExperimentBandtheotherpassingoverthesitesofExperimentAandShipShoal(Figures 1-1 and 2-1 ).ThesamplesagreefairlywellwiththequalitativepictureinFigure 1-1 .WhiletheShipShoalareaandclosetoTrinityShoaltypicallyoshoreofthe6-misobatharedominantlysand(upto95%),overallthesurcialsedimentonAtchafalayaShelfintheexperimentalareaduringtheperiodofourstudyappearstobemixtureofalmostequalpartsofsiltandclay,withtracesofsand(lessthan5%)andshellgravel(lessthan1%).Althoughthesedimentsampleswerecollectedaboutonemonthaftertheendoftheexperiments,April-May2006wasaperiodofrelativecalmintheNorthernGulfofMexico,thereforeitisreasonabletoassumethateventhoughsomeconsolidationisexpectedtohaveoccurredduringthisinterval,thesedimentgrainanalysisshouldnotbemuchdierentthanatthetimeofourexperiments. 35

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SurcialsedimentgrainsizedistributionontheAtchafalayaShelf(sampledbyNRLon3-5May2006).a)Mapoftheshelfwithsiteswheregrabsamplingwasdonemarkedbycircles(arrowsmarkthelocationofT1andT2platforms,alsoFigure 1-1 ).Thedistributionofsand,silt,andmudconstituentsversusdistancefromtheshoremostsamplingsite,measuredalongtheb)easterntransect;c)westerntransect. 1-1 ),theapproximatepositionsare:[T1:]Latitude:29o15.6'N,Longitude:91o34.26'W[T2:]Latitude:29o13.4'N,Longitude:91o34.77'WInthesecondphase(ExperimentB,March15to25)theplatformsweremovedtotheWesttoanalongshoreconguration(crossesinFigure 1-1 ).Thepositionsare:[T1:]Latitude:29o19.38'N,Longitude:91o51.16'W[T2:]Latitude:29o21.43'N,Longitude:91o47.15'W 36

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CongurationofplatformT1(a)andT2(b),showinginstrumentsdeployedandtheirinitialpositionwithrespecttotheseabed.Arrowsrepresentthedirectionoftheacousticbeams. Consistentwiththetypicaltimewindowforpeakwaveactivity,oneenergeticfrontalpassagewasrecordedduringExperimentAandanumberofweakshort-waveeventsduringExperimentB.Eventhoughshorterandlessenergetic,thesecondphasehasprovidedvaluableobservationsofsedimentdynamicsinlow-energy,sediment-richenvironments. Theinstrumentsweredeployedforabout14daysandservicedbetweentheindividualdeploymentsbybringingthemonboardship,downloadingdata,replacingbatteries,andcleaningbiofoulingfromsensors.Platformswerere-deployedapproximately24-48hafterservicing.Figure 2-2 showsaschematicoftheinstrumentsandtheircongurationatthetwoplatformsdeployed.Theinstrumentationusedfocusedonobservationsofhydrodynamicsandsuspendedsedimentconcentration(SSC)inarangeofapproximately1meterabovethebottom(mab).TheupperwatercolumnwasmonitoredonlyatatplatformT1usinganupward-lookingAcousticDopplerCurrentProler(ADCP,1-MHzNortekAWAC)withtransducersatapproximately1.1mab(Figure 2-2 a).TheADCPprovided2-minutevelocityaveragesin50-cmhighbins.Withablankingdistanceofapproximately0.45,this 37

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2-2 a)alsohousedadownward-lookingSontek1.5MHzPulseCoherentAcousticDopplerProler(PC-ADP),thatsampledthevelocityproleandpressureat2Hzcontinuouslyfortheentiredurationoftheexperiment,in173-cmbinscoveringtherangefromthebottomto51cmab(centimetersabovethebottom),withablankingdistanceofabout0.6mfromtheinstrumenthead(distancefromheadtobottomwasabout1.1m).Theinstrumentalsorecordedpressureat2Hzanddatafromitsinternaltiltsensorandcompass.DatafromanadditionalParoscienticpressuretransducers(mountedat60cmab)samplingat2Hzinburstsof2hoursand50minutesevery3hours,wereloggedtogetherwithconductivity-temperature(CT,MicroCat,at85cmab)thatwassampledonceevery20minutes,andinformationfromtwoturbidity probes(Analite195,McVanInstruments), mountedatapproximately45and115cmab.Turbiditydatawererecordedattheendofa3-hrmeasurementburstasaone-minuteaverage.PlatformT2(Figure 2-2 b)hadanidenticalPC-ADPthatinadditionloggeddatafrom2OBS's(D&AOpticalBackscatteranceSensor)locatedat50and75cmab,andaSeabirdMicroCAT(conductivityandtemperature)locatedat115cmab.ThesamplingschemeusedatT2wasslightlydierent:10-minlong,2-Hzmeasurementburstsevery30minutes,samplingin60binseach2-cmhigh,(10cmblankingdistance).InthesecondhalfoftheexperimentaSequoiaScienticLaserInSituScatteringandTransmissometrydevice(LISST-100X)wasusedtoprovideanindependentopticalmeasurementofturbidityandtomonitorparticleandocsize(datanotreportedhere)at115cmab.Featuresassociatedwithnear-bottomsuspendedsediment(e.g.,lutoclines)werealsomonitoredonT2withasingle-frequency(600kHz)AcousticBackscatterSensor(ABS)madebymarine 38

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39

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2{1 .Normally,thecountnumberwillgrowtoamaximumvaluewithincreasingconcentration,butafterthispeakisreached,thecountnumberdecreasewithincreasingconcentration.Asanexample,thettedcurveisshowninFigure 2-3 fortheOBSat75cmablocatedinT2/M2withthecoecients(with95%condencebounds):a=1.056e+05(9.654e+04,1.146e+05),b=-0.01515(-0.01631,-0.014),c=-1.063e+05(-1.152e+05,-9.738e+04),d=-0.07799(-0.08447,-0.07151),andr2=0:9786.Inthiscase,thecountnumbergrowsuptoapeakatabout30-gm/l,forvalueshigherthanthis,thecountsnumberdecreases,meaningthatforaparticularcountnumbertherecouldbetwodierentconcentrationvaluesassociated.Theimplicationisthatinordertondabestmatchforaparticularcountnumber,thehistoryoftheOBSresponsetosuspendedsedimentconcentration(SSC)hastobetakenintoconsideration,andcross-checkedagainstthevaluesofechointensityfromthePC-ADP,andABS.Theruledfollowedhereisthattherstpartofthecurve(0-30g/l)wasusedforttingthedataunlessthecountnumbersgreaterthan60000werereachedconsistentlyduring1-minute,togetherwithhighechointensityvalues,inwhichcasethesecondpartofthecurve(30-250g/l)wasused.ThesameprocedurewasusedtotthedatafromtherestoftheOBS,althoughduetothesamplingschemeusedatT1/M1(onemeasurementevery3-hr),thettothatdataisconsiderablymoreuncertain.Usingthe95%condenceintervalcalculated(dashedlinesin 2-3 )givesameanrangeofuncertaintyof5:37g/lforconcentrationslowerthan30g/l,and13:3g/lforconcentrattionsgreaterthan30g/l.Theroot-mean-squarederrorforthetinFigure 2-3 iscalculatedas,rmse=r 2{1 .Forthecalibrationrange0-30g/lrmse=3:53g/l,andfortherange30-250g/l,rmse=9:8g/l.Itisnotedthatduringthis 40

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CalibrationcurveforoneoftheOBS'deployedatT2.Thedashedlinerepresentsthe95%condenceinterval. workSSCvaluesobservedwerealwaysbelow30g/lduetotherelativelyhighlocationoftheOBSsensors.Calibrationexperimentsattemptedwiththeacousticsensors(PC-ADPandABS)werenotsuccessfulandarestillbeingstudied.ThesamplingschemeusedfortheprogrammingofthePC-ADPinuencethevelocityrangesthatcanbesampledwithcondence,whentheserangesareexceeded,theobservationsarenotdeemedreliable.InthecaseofthePC-ADPatT1,theschemeusedallowedamaximumhorizontalvelocitymeasurementof1.09m/s,andamaximumverticalvelocityof0.28m/s.ThesamplingschemeatT2allowedthePC-ADPtosamplemaximumhorisontalvelcitiesof1.14m/s,andmaximumverticalvelocitiesof0.3m/s.Duringoperation,thePC-ADPtransmitsaseriesofsoundpulses(pings).thequality,anduncertaintyofthemeasurementscollectedcanalsobeassessedthroughthecorrelationlevels,andsignal-to-noiseratios.Theinstrumentcalculatesthecorrelationbewteentwo 41

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42

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3-1 ).Incontrast,ExperimentBwascharacterizedbylowenergyatmosphericandseaconditions,withonlyoccasionalweakpulsesofwaveactivity.Detailsofthehydrodynamicconditionsandsurfacesedimentresponseduringbothexperimentsarefurtherdiscussedinthenextsections. 1997 ]. 3-1 and 3-2 a),withashortperiodofreductiontoabout5m/sonMarch10likelycorrespondingthefrontpassingovertheobservationsite.Justpriortothearrivaltothefront(eveningofMarch9)signicantwaveheightincreasedto2m(thewaterdepthattheexperimentalsiteswasabout5m).Relativelyhighwinds(about10m/s)continuedforseveraldaysafterthat(Figure 3-2 a);followinga1-dayhighwavepulse(March11),waveenergydecayedsteadily.Figure 3-1 bshowsthemapofnumerically 43

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3-2 ashowstheevolutionofsignicantheightsoftheshortwaveband(seas,f>0:2Hz)andlongwaves(swell,f0:2Hz),asobservedbytheT1ADCP.Thespectralestimatesareobtainedusingthemanufacturer'ssoftware(Nortek,QuickWave).Theheightsareestimatedbasedontherstspectralmoment4q Rf2f1S(f)df,wherefisthefrequencyandf1andf2thespectralbandlimits.Thedistinctionbetweenseaandswellisarbitrarybutjustiedbyobservations(e.g., 2003 )andbythedierentdynamiccharacteristicsoftheseaandswell:seasarecharacterizedbyafastresponsetolocalwindforcingandweakinteractionwiththeseabed.Forexample,duringtheMarch7th-14thstorm,highseas(upto1-mheight)withrelativelyvariabledirectionwererecordedassoonasthewindintensied,changingdirection(Figures 3-2 aandc).Swellsarrivedattheobservationsitelater,withasteadydirectionofpropagation(towardNW)associatedwiththedominantfetchalignment,approximatelySEtoNW.Atthepeakofthestormswellheightreachesupto1.5m,withapeakperiodofabout9s(Figure 3-2 a,b).Itisnotedthatinthedirectionalspectra( 3-2 c)thedirectionscorrespondingtofrequencycomponentswithenergylessthan5%ofthetotalenergyatanyparticulartimehavebeenexcluded.Sincebottomsedimentstateisexpectedtoshowsignicantcorrelationwithswellenergy,inthediscussionbelowwewillassumethatbottomsedimentprocessesaredrivenmainlybytheswellenergyandwillneglecttheeectsoftheseaband.Amoredetailedanalysisofwavepropagationprocesseswillbepresentedinsection 4 .SurfacecurrentsanddirectionsfromtheADCPareshowninFigure 3-3 b-c.Strong(80cm/sgoingtotheNW)surfacecurrentsareobservedduringExperimentA(Figure 3-3 b-c),generallycoincidingwithpeaksofwindandwaveactivity((Figure 3-3 a),howeversomeofthestrongestcurrentsdonotseemtoberelateddirectlytoanincreaseinwind 44

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a)Aqua-1MODISimageoftheGulfofMexicoonMarch92006,19:11hours,withshorelinemarkedinred.b)SurfacewindeldonMarch92006at21:00hours(numericalsimulationrunningCOAMPS,NRLStennisSpaceCenter,MS).Arrowsareproportionaltowindvelocity.Maximumvelocityrepresentedhereis15.3m/s.OnMarch10,thefrontstartedmovingeastward,causingatemporarydecreaseinwindintensityasitpassedovertheobservationsites. andwaveenergy.Thisraisedthequestionofhowsignicanttidalcurrentsareintheareaofourmeasurements.AclassicharmonicanalysiswasperformedusingtheMatlabpackaget tide[ 2002 ]basedonmeanwaterelevationdataobtainedfromthepressuresensorsatT1.DuringtheLATEXproject[ 1998 ]themaintidalconstituentsoftheTexas-LouisianashelfwereidentiedtobeO1(period25.82hr ), K1(period23.93hr),andM2(12.4hr)withmeantidalrangesofabout60cm.Usingthisconstituentsfortheharmonicanalysis,itisfoundthattheyaccountfor73%ofthemeanwaterlevelvariance(Figure 3-4 a),but,therearestilldiurnalandsemidiurnaloscillationspresentintheresidualelevation(Figure 3-4 b).Thisisanindicationthattherearespectralpeaksinthewaterelevationspectrumthatarenotproperlyresolvedusingharmonicanalysis.Ameasureofthefrequencyresolutionneeded 45

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WindandADCPwaveobservationsatT1(circles,Figure 1-1 )duringExperimentAMarch7th-14th,2006.a)WindintensityanddirectionatT1(numericalsimulations,COAMPS);signicantwaveheightofsea(red)andswell(blue);b)evolutionofnormalizedfrequencypowerspectrumofwaves(frequencyspectraateachtimepointarenormalizedforunitvariance.Theblacklineshowsthe(arbitrary)boundarybetweenthesea(highfrequencies)andswellband.c)distributionofwavepropagationdirection.Forbothwindandwavesweusethe\ow"convention(e.g.Northmeans\propagatingnorthward"). toresolvespectralpeaksinneighboringfrequencybandsisgivenbywhatisknownasthe\Rayleighcriterion"[ 1957 ],fT=1,whereTrepresentsthelengthoftherecord(i.e.,spectralpeakswithf<1=Tcannotbeproperlyresolved).Thelengthofourrecords(13.6days)isjustbarelylongenoughtoresolvetheK1andO1modes,asthespectralbandwidthofouranalysisisf=1/13.6day1,whileastrictapplicationoftheRayleighcriterionwouldrequiref1/13.7day1toresolvethesetwocomponents.ForthisreasontheRayleighcriterionint tidewasrelaxedto0.98.Interestinglyenough, 46

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Wind,wavesandADCPcurrentobservationsatT1(circles,Figure 1-1 )duringExperimentAMarch7th-14th,2006.a)WindintensityanddirectionatT1(numericalsimulations,COAMPS);signicantwaveheightofsea(red)andswell(blue);b-c)CurrentspeedanddirectionsfromADCPmeasurements.Thebluelinesinpanelsb)andc)representthemeanfreesurfaceelevation.Directionsarerepresentedhereastheowdirection(Nmeansnorthward). theinertialoscillationmodehasaperiodofabout24.5hoursatthe29.3Nlatitudeoftheexperimentalsites,meaningthatatthisresolutionitisinthesamefrequencybandwithconstituentsO1andK1,andlikelyatresonance[ 1978 ; 1996 ]withthem.Ourtimeseriesarenotsucientlylongtoresolvesuchoscillationsfromastronomicalcomponentsinneighboringfrequencybands,forexample,toseparatetheinertialoscillationfromK1wouldrequireatimeseriesof42.85days.Adetailedtidalanalysisisbeyondthescopeofthiswork,andsincethelimitationsimposedbythelengthofourrecord,thedecisionwasmadetoinsteadlterthedata 47

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HarmonicanalysisofmeanwaterelevationatT1duringExperimentAusingt tide [ 2002 ] .a)Meanwaterelevationrawdata(solidline),andtidalsignalusingconstituentsO1,K1,andM2(dashedline);b)meanwaterelevationresidual.Notethatdiurnalandsemidiurnaloscillationsarestillpresentintheresidual. toeliminatealloscillationswithperiodsbetween40-4hours,whichincludeinertial,astronomical,andotherdiurnalandsemi-diurnalforcings(e.g.,seabreezeeects).Thistreatmentallowsustoobservetheeectsofthecoldfrontpassageonthehydrodynamicmeasurementsinamoredirectmanner.Theprocedureforlteringademeanedtimeseries,x(t),isasfollows:theFouriertransform,X(t),ofthetimeseriesiscalculated,then,fortwoarbitraryfrequenciesf1=1/40hr1andf2=1/4hr1: 48

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3-5 )accountsfor85%ofthemeanwaterlevelvariance.Inasimilarmanner,themainspectralpeakscanbeidentied,bylimitingtheintervalf1ff2toonlyincludeaspecicspectralpeak.Figure 3-6 ashowsthespectraldensity(Eq. A{6 )calculatedfromthemeanwaterleveldataatT1.AsthespectralpeakassociatedwithconstituentsO1,andK1iswelldenedandnarrowenough(2-fwidth,Figure 3-6 a),forsimplicity,wewillidentifythetwofrequencybandswithK1andO1andignoreinertialoscillations.ThepeakcorrespondingtomodeM2isalsoprominentbutwider(widthabout4f),sointhiscaseEq. 3{1 andEq. 3{2 become: Zf2+2ff12fjX(f)j2df;f12fff2+2f(3{3) 3-6 b,canddshowsthephaseportraitsofthemostenergetictidalconstituents(varianceFigure 3-6 d),O1(period25.82hr),K1(period23.93hr),andM2 (12.4hr),usingS-NandW-Evelocities.Figure 3-7 showsnon-tidal(residual)componentsofsurfaceelevation(deviationfromthemeanwaterlevel,measuredbythepressuresensorontheADCP)andcurrents,observedatT1(ExperimentA,March1-142006).Bothresidualsurfaceelevationandcurrentsarecorrelatedwithmeteorologicalforcing.Maximummeteorologicalsetup/setdownobserved(waterdepthisapproximately5meters)isabout20cm.Residualcurrentstendtobeconcentratedatthesurface,withtheexceptionofasetdown-setup 49

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TidalanalysisofmeanwaterelevationatT1duringExperimentAusingaFouriertransformlter.a)Meanwaterelevationrawdata(solidline),andtidalsignal(dashedline);b)meanwaterelevationresidual. sequenceofeventscorrespondingtotheMarch7th-14thstorm.Thelargestsetdown(March7)isobservedbeforethefrontreachestheexperimentsite,associatedwithmildNWwindsthatpeakat5m/s(Figure 3-7 a);theeventshowsrelativelyintense(about50cm/s)surfacecurrentsintherst1-msurfacelayeranddecayingtoabout30cm/sbelow.ThestrongestsetupisobservedonMarch11,coincidingwithsteady,10-m/sSEwinds,andweaker(upto25cm/s)butrelativelydepth-uniformcurrentswhichlastedforabout2days. 3-8 comparesobservednear-bottomvelocitydistributionduringtheMarch7th-14thstorm,tosonarbackscatterintensityvaluesofthePC-ADPatbothplatforms. 50

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CharacteristicsoftidalcurrentsandmeansurfaceelevationrecordedbytheADCPatT1duringExperimentA(March1-14,2006,circlesinFigure 1-1 ).d)Spectraldensityofwaterlevelvariance.b,c,d)Phase-portraitsoftidalcomponentsO1,K1andM2,respectively; AtstationT2thereisnoADCPtoobtainwavedata,then,thewavevariancedensityspectraiscalculatedusingthePC-ADPpressuresensormeasurements.Each10-minburst(2-Hzsamplingrate)ofpressuremeasurementswasscaledtoheight(m)usingthewaterdensity,,calculatedfromthetemperatureandsalinitymeasurementsfromtheMicroCat-CTandgravity(g).Followingtheproceduredescribedinsection A.1 ofAppendix A ,thedatawasdetrended,dividedin64-slongsegmentswith50%overlap,andtapperedwithaHanningwindow,thepowerspectrawereestimatedwithabout16degreesoffreedom,andafrequencyresolutionof0.0078Hz.Thechi-squared90%condenceintervalwith16degreesoffreedomis[0:9895S(f);1:0543S(f)],oramaximumnormalizederrorof1%ofthecalculatedspectra.Thespectraarethencorrectedto 51

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Non-tidalcurrentsandmeansurfaceelevationrecordedbytheADCPatT1duringExperimentA(March1-14,2006,circlesinFigure 1-1 ),versustime.a)De-tidedsurfaceelevation,windintensityanddirection(numericalsimulations,COAMPS);b-c)verticaldistributionofde-tidedcurrentspeed(20minaverages)anddirection.Thebluelinesinpanelsb)andc)representthemeanfreesurfaceelevation.Directionsarerepresentedhereastheowdirection(Nmeansnorthward). accountforattenuationofthepressuresignalwithdepth.Usinglinearwavetheory[ 1991 ]: coshkh(3{5)wherepdynisthedynamicpressure(totalpressure-meanpressure),andiswaterelevation.Sincethespectra,S(f),areproportionalto2,thespectraatthesurfacecanbeobtainedfrom: 52

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coshkh(3{7)wherezistheheightoftheinstrumentabovethebottom,histhetotaldepthofthewatercolumn,andkisthewavenumberobtainedfromthelineardispersionrelationship2=gktanh(kh),correspondingtoeachfrequency(=2f)ofthewavespectrum,wheregisgravity.Sincethedepthoftheinstruments(about3matT2)makesitdiculttoestimatevariancedensityvaluesinfrequencybandswithf>0:5Hz(wherefisthefrequency),aspectraltailproportionaltof4isused.Estimatedsignicantwaveheightsfortheseaandswellband(Figure 3-8 a,e)showtheevolutionofwaveactivity.Asdiscussedabove,theresponseofthebedsedimentwiththeswellisexpectedtobestronglycorrelated.Atthepeakofthestorm,swellsshowsignicantdissipationoverthe4-kmdistancebetweentheplatforms,withroughly70%ofswellenergyrecordedatT1reachingT2atthepeakofthestorm(March10th,30%dissipation).Theresponseofbedsedimenttowave-currentactivitycanbeinferredbycomparingtwoseparatepiecesofinformation.ThewhitelineinFigure 3-8 d,hcorrespondstothelocationofthemaximumbackscatterintensity,andishereassumedtobeanestimateofthestrongestreection(acousticimpedance)nearthebed.Intheabsenceofanotherreectivesurface,thelocationoftheacousticbottomshouldcoincidewiththebed,becausethestrongestreectionshouldbeassociatedwiththestrongestverticalgradientofdensityandseismicwavevelocity.AnindependentestimateofthebedpositionfromthePC-ADPdataisgivenbythezero-velocityelevation,referredtohereasthehydrodynamicbottom.TheredlineonFigure 3-8 b,fmarksthepositionofthehydrodynamicbottombasedontheobservedverticalstructureofnear-bedvelocity.AtplatformT2,thetwobed-positionestimatesagreewithina3-cm(onePC-ADPbinheight)errorduringtheentireperiodoftheexperiment.AtplatformT1(sitedontheforesetareaoftheclinoform),theestimatesagreeduringpre-andpost-frontalstages 53

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ObservationsofwavesfromADCP(a)andPC-ADP(e)pressuresensor,near-bottomcurrents,andsignalintensity(PC-ADP),duringtheMarch10thstorminExperimentA.Left:platformT1.Right:platformT2.a,e)Swell(frequency<0:2Hz)andsea(frequency>0:2Hz)calculatedfromthePC-ADP;b,f)verticalstructureofnear-bottomcurrentspeed;c,g)near-bottomcurrentdirection;d,h)echointensity(normalizedbetween0and1).Backscatterintensityisnotcorrectedtoaccountforsignalattenuationduetosuspendedsedimentconcentration.Whiteline:approximatepositionofmaximumreectionsurface;redline:approximatepositionofzerovelocity(hydrodynamicbottom).

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(b) Figure3-9. PC-ADPdataqualityvericationduringthetwouidmudevents.a)March10,00:00hrs;b)March11,01:00hrs.Leftpanel:1-minuteaveragedverticalprolesofsignal-to-noiseratio(circlesandsolidline),withtheminimumacceptablelevellocatedat10-dB(solidverticalline),andping-to-pingcorrelationvalues(xanddashedline),withtheminimumacceptablecorrelationmarkedat25%(dashedverticalline).Rightpanel:1-minutelongrawtimeseriesofcross-shorevelocityat7measurementcells. 55

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3-9 presents1-minuteaveragesofSNRandping-to-pingcorrelationvalues,and1-minuterawtimeseriesofvelocitydataduringthetwouidmudeventsmentionedabove.ThisplotshowsthatforthemeasurementsinFigure 3-9 a(March10,00:00hrs),thedataareacceptabledownto1.05-1.08mfromtheinstrumenthead,whilefortheintervalduringtheseconduidmudevent( 3-9 b,March11,01:00hrs)thedataareacceptabledownto0.98-1.02mfromtheinstrumentheadwhereSNRandcorrelationvaluesfallbelowtherecommendedlimits.Althoughnoturbiditysensorsweredeployedcloseenoughtothebedtoallowarmattributionofuidmud(concentration>10kg/m3),thepresenceofamobileandrelativelydense(stronglyreectinglutocline)bottomlayerofsuspendedsedimentsisconsistentwithauidmudoccurrence.Thetwohigh-turbidityeventsappeartohavedistinctcharacteristics.Thestartofthersteventcoincideswiththemarkedincreaseoftheswellheight(from1to1.5m)asthefrontapproachestheobservationsite,andwindstrengthensandbecomessoutherly.Theowwithintheuidmudlayerisconsistentlyoshore(towardSouth)withthecurrentsintheentirewatercolumn.Theprocessisconsistentwithabedliquefaction/resuspensionprocess.NotethatfromMarch2nd-9th,whentheuidizationprocessstarts,thepressurerecordindicatesameandepthof5.46-m,whilefromMarch11th-4ththepressurerecordindicatesameandepthof5.62m,suggestingthattheinstrumentplatformsankinthe 56

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3-8 d).Thesecondeventisobservedinthewakeoffrontalpassage,underlessintensewaveaction(swellheightofabout0.8m);itappearstobelessmobile(speed5cm/s);andtheoshorehyperpycnalowisintheoppositedirectiontotheoverlyingwatercolumn.Thereisnosinkingoftheinstrumentplatformassociatedwiththissecondhyperpycnalevent.Thesecharacteristicsaremoreconsistentwithasedimentsettling/advectionmechanism,perhapssimilartoshelfgravityows(e.g.EelRivershelf; 2000 ). Neitheroftheseeventsisconsistentwiththeoverallnear-bottomshorewardowobservedby 2006 ]atacheniercoaststationoppositetheaccretionarymudatsnearFreshWaterBayou.Itmaybethatadierentdynamicmechanismdominatessedimenttransportalongtheconcave-shorefacemudatsofthecheniercoastthantheAtchafalayamudclinoform.However, 2006 ]estimatesarebasedoncurrentvelocitiessampled(byow-orientedwatercolumnproler)nocloserthanabout50cmtothebed,andourdatashowsthatdenserowsaremovingoshoreonlyinthelowerapproximately30cmab.Figures 3-8 e-hshowPC-ADPrecordsfromtheuppertopsetsiteofplatformT2(comparewithT1,Figures 3-8 a-d).Bothplatformsrecordanabruptmodicationofbottompositionatthebeginningofthestorm,likelyassociatedwiththeinitialliquefactionofthebedbywaveactivity.AtT2thebottompositionasdenedbythestrongest-reectionsurfacemovesupbyabout5cm.Sinkingdepthislikelydependentonbothplatformdesign(weightandsurfaceareaofthefeet)andgeotechnicalbedproperties.Platformsinkingisveriedbymeasurable,butslight,changesinPC-ADPmagneticcompassheading(15degreesheadingdirenece)andtilt(1degreeinpitchand1.5degreesinroll)atthesetimes.Sinkingofthisplatformisalsosupportedbypressurerecords,whichindicateadierenceinthemeandepthfromMarch2ndtoMarch14thofabout 57

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2001 ],toinvestigatetheeectsofcurrentsandwavesonthesetypeofows.Thetheoreticalmodelof 2001 ]assumesthatthereisabalancebetweenthedown-slopepressuregradientinducedbythenegativebuoyancyofhighloadsofsuspendedsedimentandfrictionaldragforces.Thisbalancecanbeexpressedas[ 2002 ]: 2001 ],indicatesthatongentleslopes,feedbackbetweenturbulentowsandsediment-induceddensitystraticationwithinagravityowmaintainsthegradient 58

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(@u=@z)2=gs(@c=@z) (@u=@z)2=B u2max(3{11)WhenRi<1=4,intenseturbulencesuspendsadditionalsediment,increasingBandRi,whileforRi>1=4,decreasedturbulencecausessedimenttosettle,decreasingBandRi.Duetothisfeedbackmechanism,inthepresenceofasignicantsourceofsediments,RitendstoremainclosetothecriticalvaluesofRicr=1=4[ 1994 ].Initially,inthecasestreatedinthisstudyitisassumedthatduringtheuidmudeventsthereisenoughsuspendedsedimentandshearwithinthewaveboundarylayer,tomaintaintheRichardsonnumberaround1/4.Underthisassumption,Equations 3{8 and 3{11 canbesolvedforthegravityowspeedtoget: 3-10 ),whiletheseconduidmudevent(March11th)hasbeendividedintoarststage(Figure 3-11 )withstrongerobserveddownslopecurrent(Figure 3-11 c),andasecondstage(Figure 3-12 ),withweakerdownslopecurrents(Figure 3-12 c).ForthesakeofclarityIwillrefertotheseeventsasasFM-1,FM-2,andFM-3,respectively.Frompanelsa-cinFigures 3-10 3-11 ,and 3-12 ,itisclearthatinallthreecasesthewavevelocitygreatlyexceedscurrentvelocitiesbelowthelutocline(dashedlinesinplots).Thissuggeststhattheseowsarewave-supported,sincetheshearinducedby 59

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3{14 ,Figures 3-10 d, 3-11 d,and 3-12 d,compareestimationsofwaveandcurrentsupportedgravityowspeed,ug,withmeasuredo-shorecurrentsuc,insidethewaveboundarylayeratstationT1.Inallcases,thegravityowspeedhasbeencalculatedusingthebottomslopeatT1,=0:0006,adragfrictioncoecientofCd=0:003[ 2001 ],andtimeseriesofalong-shorecurrent(20-minutevelocityaverage),vc,andtherootmeansquareofwavevelocity,uwbothmeasuredatthetopoftheboundarylayer.Thewavevelocityhasbeencalculatedasuw=uuc,whereuisthecross-shorevelocitysampledat2Hz,anducisthe20-minutemeancross-shorevelocity.Forexample,inthecaseofFM-1inFigure 3-10 ,theuw(Figure 3-10 a)andvc(Figure 3-10 b),correspondtothemeasurementcelllocatedat0.95mfromtheinstrument,andthegravityowmeasured(circlesinFigure 3-10 d)correspondtoanarithmeticmeanofdown-slopevelocity,uc,insidethewaveboundarylayer(0.95-1.08mfromtheinstrument).DuringFM-1(Figure 3-10 )thecalculatedgravityowvelocityisconsistentlylowerthantheobservedvelocity(ug
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3-11 c)indicatesthatthehighestsedimentconcentrationmuststillbeconnedtothewaveboundarylayer,sinceahigherconcentrationofsedimentswouldhavetheeectofslowingdownthevelocityinthislayer.Thissediment-inducedreductionofvelocityinsidethewaveboundarylayerisalsonoticeableduringFM-3(Figure 3-11 c),wheretheloweringofthelutoclinealsosuggestsincreasedsettlingrates.Despitethedisagreementsbetweenmeasuredandcalculatedgravityowspeed,inallthreecasesthevelocityofthegravityowcalculatedisofthesameorderofmagnitudeasthemeasuredvelocity,suggestingthatatleastasarstorderapproximationthebalanceinEquation 3{8 canexplainthebasicphysicsofthisow.Then,ifweusetheobservedcross-shorecurrent(uc)asthetruegravityowvelocity(i.eug=uc),andadragcoecientCd=0:003,wecaneasilycalculatethebuoyancyanomalyBfromEquation 3{8 ,andsubsequentlyRifromEquation 3{11 ,obtainingforFM-1,FM-2,andFM-3meanvaluesofB=0:04;0:024;0:0073m2=s2,andRi=0:31;0:3;0:18,respectively.ThehigherthancriticalRichardsonnumberinthersttwocasesseemstoindicatethatanabundantsupplyofeasilysuspendedsedimentavailableinthewaveboundarylayerisreducingturbulenteects[ 2001 ].Resultsusingthissimplemodelareverysensitivetothechoiceofdragcoecient.Toestimatethevalidityofthedragcoecientused,adierentapproachcanbetouseEquation 3{11 toobtainthebuoyancyanomalyB;assumingthattheRichardsonnumberremains1/4,andthenuseEquation 3{8 tocalculatethedragcoecient.ThiscalculationproducesmeanvaluesofCd=0:0026;0:0026;and0:0043,andmeanvaluesofB=0:032;0:02;0:001m2=s2forFM-1,FM-2,andFM-3respectively.Theseresultsindicatethatthedragcoecientwasindeedtohighforthersttwocases.Allthevaluesreportedhereforthedragcoecient,RichardsonnumberandbuoyancyanomalyfallwithinreportedvaluesintheEelshelf,andotherlocationsinLouisiana(e.g., 2001 ; 2000 ).Toobtainmorereliableestimatesofthesevariables, 61

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EstimationofgravityowusingtheanalyticalmodelofWrightetal.(2001)fortherstuidmudevent.a)uw;b)vc;c)uc,prolesaveragedoverthedurationoftherstuidmudevent.d)Timeseriesofmeasureddown-slopevelocityuc(circles)averagedbetweenlevels0.95-1.08mfromtheinstrument,andcalculatedgravityowspeedug(solidline).ThesolidlinemarkstheapproximatelocationofthelutoclineasobtainedfromthePC-ADPmaximumechointensityaveragedduringtheintervalshowninpanel(d)(approximately8hours).Thedashedlinemarkstheapproximatelocationofthewaveboundarylayer. aswellasthesedimenttransportuxintheregion,amoresophisticatedapproach,suchasnumericalmodelingwouldbenecessary.Also,itisappropriatetopointoutthatthescalingtoobtainaconstantRi1=4inEquation 3{11 wasobtainedby 1994 ]foratidallydominatedow,anditisunclearifthisscalingwouldreallyholdforwavedominatedconditionssuchastheonesdescribedinthisstudy. 62

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EstimationofgravityowusingtheanalyticalmodelofWrightetal.(2001)fortherststageoftheseconduidmudevent..a)uw;b)vc;c)uc,prolesaveragedoverthedurationoftherststageoftheseconduidmudevent.d)Timeseriesofmeasureddown-slopevelocityuc(circles)averagedbetweenlevels0.89-1.02mfromtheinstrument,andcalculatedgravityowspeedug(solidline).ThesolidlinemarkstheapproximatelocationofthelutoclineasobtainedfromthePC-ADPmaximumechointensityaveragedduringtheintervalshowninpanel(d)(approximately5hours).Thedashedlinemarkstheapproximatelocationofthewaveboundarylayer. 3-13 showsasummaryoftheconditionsduringtherst10daysofthe14-dayperiodatthewesternplatformsiteT2ofExperimentB.Theperiodwascharacterizedbysignicantshortwaveactivity,withtypicalshortandsteepseas,sometimesexceeding1-mheight;however,wewouldexpectwavestoaectsignicantlythebottomsedimentonlyduringswellevents.InExperimentBasinglesucheventwasrecorded,onMarch20-21(Figure 3-13 a)whenswellsreachedabout90cmsignicantheight.Near-bottom 63

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EstimationofgravityowusingtheanalyticalmodelofWrightetal.(2001)forthesecondstageoftheseconduidmudevent..a)uw;b)vc;c)uc,prolesaveragedoverthedurationofthenalstageoftheseconduidmudevent.d)Timeseriesofmeasureddown-slopevelocityuc(circles)averagedbetweenlevels0.89-1.02mfromtheinstrument,andcalculatedgravityowspeedug(solidline).ThesolidlinemarkstheapproximatelocationofthelutoclineasobtainedfromthePC-ADPmaximumechointensityaveragedduringtheintervalshowninpanel(d)(approximately6hours).Thedashedlinemarkstheapproximatelocationofthewaveboundarylayer. velocities(Figure 3-13 b-c)reached40cm/sduringmaximumebb/oodtide(tidalanalysisofcurrentsandmeanwaterelevationisconsistentwithobservationsduringExperimentA).ObservationsofPC-ADPandABSbackscatterintensityatT2conrmthepresenceofalong-lasting(entireperiod)andslow-moving(velocities5cm/s),20-to30-cmthickhyperpycnallayerwithwelldevelopedlutocline,movinginthesamedirectionastheoverlyingwatercolumn(strongercurrentsareinthealong-shelfdirectiongoingtotheEast,Figure 3-13 c).ThiscontrastswithExperimentA,wheretheuidmudlayersseem 64

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3-13 e)clearlyrecordasecondaryacousticreectingsurfacebelowthelutocline,whichisinterpretedasthebed.Themoredetailed(0.9cmdepthbins)ABSrecordalsoshowsevidenceoflutoclinesettling(increasinghyperpycnallayerconcentrationand/orparticledeposition)duringlowcurrentshearstressintervals(\A"inFigure 3-13 e).Duringlutoclinere-formationinthewaningphaseoftheseevents,lutoclineelevationandbackscatterintensityvarieswidelyintheABSrecord,suggestingsettling-inducedchangesinlayerconcentrationmediatedbyturbulentmixing. Althoughenergetic,theepisodicshortwavesburstsofactivitydonotreachdeepenoughinthewatercolumntoaectthenear-bottomlayer.Severalpulsesofbottomcurrentsrapidlyerodethelutocline(\B"inFigure 3-13 e),sometimescompletelyexposingtheunderlyingbedintheABSrecord.ThesinglelargerswelleventonMarch20-21causesanexpansionofthehyperpycnallayerandabackscatterintensityreductionoftheacousticlutoclinesurface(\C"inFigure 3-13 e)consistentwithreductioninsuspendedsedimentconcentration.Inthisdataset,currentsappeartoexertthedominantinuenceonsuspendedsediments.Forexampleacombinationofsetup/setdowninducedresidualcurrents(Figure 3-14 b)seemtoberesponsibleformostofthelutoclineerosionobservedonMarch19,whiletidalcurrentsseemtobeplayingasmallerroleonthesameevent(Figure 3-14 e).TheoppositeisobservedonMarch21stwhenlutoclineerosionseemtobesolelyduetostrongtidalcurrents.SuspendedsedimentdatacollectedatplatformT2duringthisperiodisshowninFigure 3-15 .ThetwoOBSwereplacedtoohighinthewatercolumntoobserve 65

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Observationsofwavesandnear-bottomcurrentsandsonarsignalintensity(PC-ADP),duringExperimentBatplatformT2.a)Swell(frequency<0:2Hz)andsea(frequency>0:2Hz)fromPC-ADPpressure;b)verticaldistributionofcurrentspeed;c)currentdirection;d)PC-ADPbackscatterintensity(normalizedbetween0and1).Whitelineinpanelsb-dmarkstheapproximatepositionofmaximumreectionsurface;e)normalizedABSbackscatterintensity.Redlineonpanele)marksthelocationofthemaximumreectionsurface.Thebedisclearlyidentiedbyanadditionalpeakinthebackscatter.Thelettersmarkperiodscharacterizedbydierentuid-mudregimes:A)settling;B)erosion,followedbyre-formation;C)expansionduetoresuspensionbywaves. 66

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Non-tidal,tidalcurrentsandmeansurfaceelevationrecordedbythePC-ADPatT2duringExperimentB(March14-25,2006,crossesinFigure 1-1 ),versustime.a,d)De-tided/tidalsurfaceelevation(blueline),windintensityanddirection(numericalsimulations,COAMPS);b,e-c,f)verticaldistributionofde-tided/tidalcurrentspeed(20minaverages)anddirection.Whitelineinpanelsb,e-c,fmarkstheapproximatepositionofmaximumreectionsurface.Directionsarerepresentedhereastheowdirection(Nmeansnorthward). 67

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SuspendedsedimentconcentrationrecordedbytheOBSatT2duringExperimentB.a)ABSbackscatterintensity(alsoFigure 3-13 e);thedashedlinemarksthepositionofthelowestturbiditysensor(OBS).b)SuspendedsedimentconcentrationrecordedbythetwoOBSat50cmab(redline)and75cmab(blueline,Figure 2-2 b). directlythelutocline,whichwasabout20-25cmabmostofthetime.However,duringthemud-layerexpansionofMarch20associatedwiththeswellevent,thelutoclinerosetoabout40cmab,within10cmofthelowestOBS,whichregisteredaSSCofapproximately20kg/m3(Figure 3-15 b).Thissuggeststhatontheorderof100kg/m3SSCvalueswithinthelayerareprobablyappropriate.Event\A"inExperimentB,Figure 3-13 (eveningofMarch17thtomorningofMarch18th)isremarkablypersistentunderapparentlyverylowforcing(meancurrentlessthan3cm/swithinthemudlayer,Figure 3-13 b-c),andowinginthealongshoredirection(roughlyE-W).Thelayermaintainedanalmostconstantlutoclineheight(only 68

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3-15 ).Theweakforcingandthenegligibleslopesuggeststhatthesedimentwasmaintainedinsuspensionbyabalanceofwave-inducedturbulenceandsettling. 1998 ]foundthatundersuchabalancetheheightHofthelutoclinecanbeexpressedas 1998 ].Giventhelowadvectionvelocities,therelativestationarityofthelutoclineheightandupper-columnconcentrationvalues(Figure 3-16 b),andthemaximumvaluesrecordedbythelowestOBS,thethesuspended-sedimentconcentrationwasestimatedtobeconstantatabout50g/l;withadiameterofthefundamentalparticle(whosedensityiss)d0=3m,aoccharacteristicdiameterd=60m,andfractaldimensionnf=2:15(e.g., 1994 ),theresultingvaluesofthemud-ocdensity[ 2007 ]andmeanvolumetricconcentrationarem=1233kg/m3,andCv=0:04.WaveamplitudeabwasestimatedastheverticalmeanoftheRMSorbitalvelocity(Figure 3-16 c)atbetweenthebottomandtheheightofthelutocline(inferredfromtheABSbackscatter).Thesettlingvelocitywasextrapolatedfrom 2005 ]Fig.6aforahinderedsettlingregimeasws=2107m/s.TheresultsofthesimulationsareshowninFigure 3-16 a).Astheparametersthatcharacterizethemudsuspensionarekeptconstantthroughoutthesimulationperiod,thetrendsinthesimulationreectonlythevariationsofthecharacteristicamplitudeandperiodofthewaveforcing,whichareestimatedas10-minaverages,basedonPC-ADP 69

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3{15 )agreewiththeABSobservations,andalsocaptureitsslowlydecreasingtrendinresponsetothedecreaseofthewaveorbitalvelocity.Theresultssupportthehypothesisthatthelayerissupportedbywave-inducedturbulence. 70

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DetailverticalvelocitystructureandevolutionoflutoclineatT2duringExperimentB,March2006,fortheperiodbetweenMarch17th-18th(\A"inFigure 3-13 ).Verticalproleofa)RMSwaveorbitalvelocityandb)meancurrentspeed(10-minaverages).c)adetailofABSbackscatterintensity.Circlesinpanelc)representthelutoclineheightscalculatedusingequation 3{15 .Eachdotrepresentsa10-minaverage.Thearrowmarksthemeasurementburstforwhichtheverticalprolesina-b)areshown. 71

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3.1.2 .InthefollowingdiscussionwewillfocusonthestormassociatedwiththecoldfrontthatpassedovertheexperimentalsitefromMarch7th-14th.Figure 4-1 showsasummaryofwaveconditionsduringthisstormatbothstations.Figure 4-1 a,eshowstheevolutionofwinddirectionandintensityandwaveheights.Asdiscussedinsection 3.1.1 ,weobservethatseasrespondfasttolocalwindforcing,reachingabout1-mheightassoonasthewindpicksupandrotateswiththewind(Figure 4-1 a,e).Swelldevelopsandarriveslater,witharelativelysteadydirection(fromthesoutheast)(Figure 4-1 d).Atthepeakofthestormswellreachesheightsofupto1.5matT1,andabout1.3matT2,withpeakperiodsofabout9sforbothstations(Figure 4-1 c,g).Thedierencesinseaheightsatbothstationsafterthestormpeakshavepassed(afterMarch11th,seasatT2are50%higherthanatT1),areanartifactofthedierentmethodsusedtocorrectthewavespectra,atthedepthoftheinstrumenttosurfaceheights,andnotaphysicalmechanism.Thatis,whiletheADCPsoftwareusedforT1appliesaspectraltailroughlyatfrequencieshigherthan0.3-Hz,thelinearcorrectionthatisappliedatT2includesaspectraltailforfrequencieshigherthan0.5-Hz.TheresultisthatthesoftwareusedforanalyizingtheADCPwavedataunderestimatesthemagnitudeofhigherfrequencywavesatT1.Panels 4-1 b,fshowforreferencetheobservedPC-ADPreturnintensitysignal,illustratingtheresponseofbedsediment(section 3.1.2 foradetaileddiscussion).Thetwolinesareindependentestimatesofthebottomlocation,onebasedonthemaximumreectionsurface(white)andtheotheronthezero-velocitylevel(pink).Asarguedinsection 3.1.2 ,instanceswhereadisagreementisobservedbetweenthetwoestimatesareoccurrencesofhigh-densitysuspensions. 72

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WaveobservationsatT1(left)andT2(right)duringtheMarch7th-14thstorm,versustime:a,e)winddirectionandintensity,signicantheightsofsea(frequency>0.2Hz)andswell(frequency0:2Hz);b,f)PC-ADPechointensity.Twoestimatesofthepositionofthebottomareshown:whiteline{maximumreectionsurface;pinkline{zerovelocitylevel.Signalisnotcorrectedforattenuationduetonear-bottomsuspendedsediment;c,g)evolutionofnormalized(totalvariance=1m2)frequencyspectraobtainedfromADCP,andPC-ADPpressuremeasurementsrespectively;d)wavedirectionsfromADCPmeasurements.Allthedirectionsareindicatedasgoingto. 2003 ]reportanomalousdissipationofshortwavesontheAtchafalayashelf.Theyhypothesizethatshort-wavedissipationistheresultofacombinationofnonlinearinteractions,activeintherelativelyshallowwateroftheshelf,andstrong,mud-inducedswelldissipation.Nonlinearitiesallowenergytoowfromtheshort-wavespectralbandtotheswellband,whereitisdissipatedthroughdirectwave-bottominteraction.SinceintheAtchafalayaShelfthebottomismuddy,thisdissipationisexpectedtobehigh. 73

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4-1 b,f,andFigure 3-8 )suggestthatthestateofbottomsedimentschangesignicantlyduringthetimeofthewaveeventinducedbycoldfrontpassage.Asmentionedabove,twouidmudeventshavealreadybeenidentied,however,littleisknownaboutthesedimentstatebeforeandafterthepassageofthecoldfront.Severalofthestudiesmentionedintheintroductorypartofthiswork(e.g., 1958 ; 1978 ; 2000 )pointatuidmudlayersasresponsibleforincreasedwaveattenuation.Inthischapter,weusethewaveobservationscollectedtotesttheseideas. 74

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4-1 d)andarelongenough(7-12s)tointeractwiththebottom(depthatT1is5.5m,andatT2is4.5m).Thus,asimpleestimateofthenettransmissionrateofswellenergyuxcanbeobtainedbycomparingmeasurementsatT2andT1.Figure 4-2 bshowstheevolutionofswellenergytransmissionfromT1toT2,estimatedasthefractionofswellenergyuxatT1thatreachesT2: netswellenergyuxtransmission=FT2 4-2 a),thetransmissionrateshowssignicantscatter(dotsinFigure 4-2 b),butmeanvaluesareconsistentlynear1,indicatingnegligiblelevelsofenergydissipation.Thetransmissionratestartstodecrease(scatterisalsosignicantlyreduced)atthetimeoftherstarrivalofthestrongerswellsonMarch9thandsteadilydecreaseuntilreaching0.3(70%energydissipation)byMarch12thandremainsatthatapproximateleveluntiltheendofthedeploymentonMarch14th.Duringthishighdissipationperiod,modulationsinducedbythetideareclearlyobserved,sinceduringlowtidedissipationmustincrease,andviceversa.Tomarktheoccurrencesofuidmud,theechointensitylevelsfromthePC-ADParepresentedinFigure 4-2 c.Interestingly,dissipationdoesnotseemtobenoticeablyaectedbytheuidmudeventsdiscussedabove,whereasseveralstudies(e.g. 1958 ; 1978 ; 2000 ,etc)pointatuidmudlayersasresponsibleforwaveattenuation.Instead,maximumdissipationratesareobservedafterthestormhaspassedandtheuidmudsarenolongerpresent.Also,contrarytowhatcouldbeexpectedwhenswellenergylevelsarelower,afterthestorm,thetransmissionratedoesnotcomebackto 75

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a)WavespectraevolutionatT1;b)swellenergyuxtransmissionbetweenT1andT2(fractionofswellvariancerecordedatT1thatreachesT2);c)PC-ADPreturnsignalintensity,versustime.Onpanelb)bluepointsrepresentvaluescomputedusing1-hrestimatesofvariances;theredcurveisa2-daymovingaverage.Onpanelc)thewhitelinemarksthelocationofthemaximumreectingsurface,andthepinklinemarksthehydrodynamicbottom(zerovelocity). theoriginalpre-stormlevels,but,asmentionedabove,remainslowuntiltheendofthedeployment.Itisimportanttonotethattheenergydissipationratecalculatedinexpression( 4{3 ),includesthecombinedeectsofallvariance-alteringprocesses.Processessuchasmud-induceddissipation,whitecapping,windgeneration,nonlinearinteractions,wavebreaking,canallhappenbetweenstationsT1andT2.Sinceinthisworkthemaininterestisrelatedtobottominducedwavedissipation,thenextsectionpresentsnumerical 76

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1974 ].Usingtheactionbalanceequationwavepropagationmodelscanbedividedintorst,second,andthirdgenerationmodels.Intherstgenerationmodels,eachenergycomponentisevaluatedindependently,therefore,nonlinearenergytransfersarenotconsidered.Secondgenerationmodelshandlethenonlinearitiesbyparametricmethods,forexamplebyapplyingareferencespectrumtoreorganizetheenergyoverthefrequencies.Thirdgenerationmodelscalculatethenonlinearenergytransfersexplicitly,thatis,thewavespectraiscalculatedwithoutassumingapredeterminedshape.TheSWANmodel(SimulatingWavesNearshore, 1999 ; 2005 )isathird-generationmodelwavemodelthathasbeenextensivelyusedinthenearshore.Itallowsfortherepresentationofseveralwavegenerationanddissipationmechanisms,suchasgenerationbywind,dissipationbywhitecapping,bydepth-inducedwavebreaking,bybottomfrictionandnonlinearwave-waveinteractionsinbothdeepandshallowwater.InSWANthewaveactionbalanceequationiswrittenas, @t+@cxN @t+@cyN @t+@cN @t+@cN @t=@Stot 77

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2007 ]or 2008 ].Inordertoseparatetheeectsofallthemechanismsfrombottominduceddissipation,thewavemodelSWANwasusedtosimulatewavepropagationintheAtchafalayaShelf.Thelatestversionavailableforpublicuse,SWANCycleIIIversion40.41,wasusedforthisstudy.SWANwasconguredwitharectangularnumericalgriddesignedtohavestationT1atthecenterofthesouthernboundary(Figure 4-3 ).Thegridresolutionis400mintheeast-westdirectionand460minthenorth-southdirection.ThedirectionalwavespectrummeasuredatT1(90directionsby99frequenciesevery20minutes)isconvertedtoaformatreadablebySWANanditisusedasauniformconditionalongthesouthernboundary.Thecornersofthisboundarywerechosentobeatapproximately20kmeastandwestofT1inordertoreduceerrorsderivedfromthefactthatwaveconditionsintheotherthreeboundariesareunknown.ThewindforcingwasobtainedfromnumericalforecastsbasedontheCOAMPS(coupledocean/atmospheremesoscalepredictionsystem, 78

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)runattheNavalResearchLaboratory,StennisSpaceCenter,MS.Modeledwindsareavailableevery3hoursfortheentiredurationoftheexperiment.ThebathymetrywasdownloadedfromtheNationalGeophysicalDataCenter 4-4 )evenwithouttheinclusionofbottom-induceddissipationandtriadinteractions.However,adetailedcomparisonbetweenmeasurementsandmodelingresultsisnotnecessary,sincethegoalofthisexerciseistoquantifytherelativeimportanceofwhitecapping,breaking,four-waveinteractions,andwindinputbetweenT1andT2.Inthatsense,theonlyrequirementforthesimulationisthattheresultsagreequalitativelywiththemeasurements,andthattherunsarestable.Inthisregardthisgoalissatisfactorilyachieved.

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(b) Figure4-3. a)Generallocationofthesimulationregion.b)BathymetryusedforSWANsimulations. Figure 4-5 b,showsthenetswellenergyuxtransmissioncalculated(asin 4{3 )usingSWAN'sestimatesatT1andT2.Figure 4-5 a,showsthePC-ADPechointensityatT1asareferencetoidentifyoccurrencesofuidmud.ThetimeevolutionofthedierentsourcetermsatT2isshowninFigure 4-5 c,thegreatestdissipationcomesfromwhitecapping,andbreakingbuttheseeectsaremostlybalancedbywindinputandnon-linear4-waveinteractioneects.Itisseenthatatthetimeswhereitsobservedthegreatestdissipationintheeldmeasurements(March9th-14th),SWANresultsinanoveralldissipationthatis 80

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(b) Figure4-4. a)SignicantwaveheightsimulatedbySWAN(solidline)versusmeasurementsatT2(circles).b)SignicantwaveheightsimulatedbySWANforMarch9th,19:00hrs. 81

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a)PC-ADPreturnsignalintensity,versustime,thewhitelinemarksthelocationofthemaximumreectingsurface,andthepinklinemarksthezerovelocitylevel;b)Swellenergyuxtransmissionrateobtainedfromanon-stationarySWANrun,bluepointsrepresentvaluescomputedusing20-minestimatesofvariances,theredcurveisa2-daymovingaverage;c)Evolutionofdierentinput/dissipationtermsfromSWANatT2. verysmall,suggestingthatmostoftheswellenergyuxdissipationobservedbetweenT1andT2mustcomefrombottom-inducedeects.Havingdeterminedthattheobservedenergydissipationismostlikelyduetobottominducedeects,thenextquestioniswhythemaximumobserveddissipationoccursafterthestormhaspassed,andwhythereisnotamoredistinctiveindicationofuidmudeectsinthedissipationcurve. 3-8 d,h)suggestthatthestateofsurcialsedimentschangessignicantlyduringthepassageoftheMarch7th-14thcoldfront.However,direct 82

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1.4 ,thestructureofthevelocityinthebottomboundarylayercanprovideaninsightintothestateofbottomsediments.Whenwavespropagateoverarigidbottom,thephaseofthevelocityinsidetheboundarylayerwillleadthephaseofthefreestreamvelocity.Typicalvaluesofthisphaseleadrangebetween45degreesforlaminarowstoabout12degreesforturbulentowconditions.Instead,whenwavespropagateoveradeformablebottom,thereisaphaselagbetweenthevelocityinsidetheboundarylayerwithrespecttothefreestreamvelocity,withvaluesthatvarydependingonthethicknessofthelowermudlayer,andonthestateofthelayer(viscous,viscoelastic,etc).Toanalyzethestructureofthevelocityinthenear-bottom,estimatesofcross-spectra(Equation A{8 )ofvelocityfromthePC-ADPmeasurementswereobtained,andthephasedierences,betweenthevelocityattheuppermostcell(0.63mfromtheinstrumenthead)andtherestofthelowercells,arecalculatedusingEquation A{11 .ThecoherenceofeachestimateiscalculatedusingEquation A{12 .Thecross-spectraiscalculatedfollowingtheproceduredescribedin A.1 .The2Hztimeseriesofcross-shorevelocityweredividedin3-hrsegment.Eachsegmentisdetrendedanddividedinblocksof256measurements(128s),with50%overlap,andtamperedwithaHanningwindow,whichresultsinapproximately168degreesoffreedomforeachestimate,andaf=0:0078Hz.Thechi-squared90%condenceintervalwith16degreesoffreedomis[0:9923Sxy(f);1:0118Sxy(f)],oramaximumnormalizederrorof0.7%ofthecalculatedcross-spectra.Figure 4-6 apresentsthephasedierences((f))averagedovertheswellfrequencyband(0.05-0.2Hz),bluecolorsrepresentphaselagsandredcolorsphaseleadsofthevelocityinlowercellswithrespecttotheuppermostvelocitycell.Ingeneral,low 83

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4-6 c)thatcanenhanceturbulenceinthewatercolumnreducingthecoherenceofthewavesignal.Phaseleadsaredominantintheperiodbeforethestartofthestorm(March1st-9th,Figure 4-6 a)suggestingthatthebottombehavesasarigidsolid,typicalvaluesleadsarebetween5to10degrees(Figure 4-7 panels1,and2).Thesevaluesarewithintheorderofmagnitudeexpectedfromthetheorydescribedinsection 1.4.2 ,forinstanceifwetakeatypicalpre-stormvalueofwaveperiodof5s,afreestreamvelocityof0.1m/sandaclaysizeparticlediameterof3m,theestimatedphaseleadofnearbedtofreestreamvelocity,inaturbulentregime,isabout7.5degrees.Whenthestormisatitspeak(March9th-11th)andtheoccurrencesofuidmudareobserved,duringtherstevent(March10th,point3inFigure 4-6 a)thecoherencevalues(Figure 4-6 b)arelow,probablyduetoenhancedturbulenceinducedbythestrongwavevelocitiesduringthistime( 4-6 d),howeverthephaseleadsobservedduringtheseperiod(5-10degrees,Figure 4-8 panel3)alsofallwithintherangeexpectedfortheturbulentboundarylayerofwavespropagatingoverarigidbed.Theseconduidmudevent(March11th)showshighcoherencevaluesandamixturebetweenphaseleadsduringtheinitialpartoftheevent(point4inFigure 4-6 a,andpanel4inFigure 4-8 ),andphaselagsinthelaterstages(point5inFigure 4-6 a),suggestingthatthisisprobablyaperiodoftransitionbetweenthedynamicsofawavepropagatingoverarigidbottomtomuddominateddynamics.Thehighcoherenceinthewavesignalobservedduringthisperiodcouldbeanindicationofturbulencedampingduetotheincreaseindensitywithintheuidmudlayerwithrespecttotherstevent.Althoughitisbeyondthescopeofthiswork,inordertovalidatetheprevioushypothesis,morecarefultestsareneededtoinvestigatethedampingofturbulenceduringtheseuidmudevents.Phaselagvaluesduringthislaterstageoscillatebetween5-20degrees(Figure 4-9 point5).Usingtheoreticalestimatesofphaselagfromstudiesofwavedissipationoversoft(Newtonian)muds,suchasthoseinFigures4and5in 1978 ]and

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2000 ],respectively,foraphaselagof20degrees(about0.5sfora10speriodwave),onecanobtainanon-dimensionalthicknessoftheuidmudlayer,~d;ofabout1.9.Then,wecangetanestimateofviscosityinthemudlayer(2)from2=2fd2=2~d.Forexample,withamudlayerthickness,d=16cm(maximumechointensitylineatpoint5inFigure 4-6 a),awavefrequency,f=0:11Hz,andanon-dimensionalmudthicknessof~d=1:8theresultantmudviscosityis0.0024m2/s.Thisvaluefallswithinrangesofuidmudviscosity,whicharecommonlyreportedbetween0.01-0.0001m2/s[ 2007 ].FromMarch11th-14thafterthestormhaspassedandthemaximumdissipationisobserved,phaselagsaredominant(e.g.,point6inFigure 4-6 a),withastrongcoherencethroughoutmostofthewatercolumn(Figure 4-6 b).Phaselagrangesof5-30degreesnearthebed(panel6,Figure)suggestthatthebottom,afterthestormhaspassed,behavesasasoftdeformablebed.Interestingly,usingFigures4and5in 1978 ],and 2000 ],fora30degreesphaselag(0.83sfora10speriodwave),thecorrespondentnondimensionalthicknessoftheuidmudlayerisabout1.6,whichisveryclosetothevalueforthisvariable(1:1~d1:5)wheretheyreportthemaximumdissipationmodeled.Thisresultseemstosuggestthatalthoughtheacousticinstrumentsdonotregisteradierencebetweenthemaximumechointensityandtheminimumvelocityvalue(thecriterionusedheretodetermineauidmudevent),thebottomisbehavingasaviscousuidmud.Phaselagvaluesobtainedassumingalowermudlayerthatbehavesasaviscoelasticsoftmud,rangebetween60and180degrees[ 1990 ],whichareconsiderablyhigherthanthoseobservedinthepresentstudy.However,itmustbenotedthatthesehighphaselagsareallreportedbelowtheinterfacebetweenwaterandmud.IfthestateofthebottommudinthisstageofExperimentAissignicantlydenserthanduringtheabovementioneduidmudevents,theacousticinstrumentscannotpenetrateintothemudlayer,andthusthephaselagsweobserveareonlyneartheinterfaceandcanbe 85

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2003 ]regardingthedissipationofwaveenergyintheshortwavebands,itisusefultoexpandtheanalysisofwaveenergydissipationtofrequencybandsotherthantheswellband.Swellvariancedissipationrateisusefulinthesensethatitgivesanestimationofthemagnitudeofbottom-induceddissipationinthearea,however,thisisaquantitythatisnotusedinmostmodelsdescribingwave-mudinteractions.Ingeneralthequantitythatismostusedtodescribewaveattenuationistheimaginarypartofthewavenumber.ConsideramonochromaticwavewitharandomamplitudeAandarandomphase:(x;t)=<[Aexpi(kx2ft+)],withcomplexwavenumbervectork=kr+iki,canbeexpressedas,(x;t)=<[Aexp(kix)exp(i)expi(krx2ft)]Wecancalculatethevarianceofas2()=hi,wheretheoperatorh:idenotestheaverage,as,hi=1 2A2exp(2ikix)SinceRS(f)df=2(),wecanwriteRS(f)df=1 2hA2iexp(2ikix).ConsideringnowanenergydissipationbetweenT1(x1)andT2(x2)as, 2A2exp(2kix2) 2A2exp(2kix1)=exp(2kix2)(4{7) 86

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2x2lnRf2f1S2(f)df 4-10 bshows3-hraveragesofkiduringExperimentA.Thedissipationcoecienthasbeenalsoaveragedover21frequencybandsbetween0.0156-0.34Hz,thatis,withaf=0:0156Hz.Highvaluesofkirepresenthighdissipationrates,whilenegativevaluesindicateenergyinputbetweenstationsT1andT2.InordertoeliminateshoalingeectsbetweenT1andT2,theattenuationcoecientiscalculatedhereusingthevarianceuxF=S(f)Cg,whereCgisthegroupvelocitycalculatedusingthelinearwavetheory(Equation 4{2 ).CoincidingwithwhatwasobservedinFigure 4-2 c,energydissipationstartstoincreaseashighwavesarrivetothesamplingstationsonMarch9th(Figure 4-2 a).Thereappeartobebasicallyfourperiodswithdierentdissipationcharacteristics:1)FromMarch9th-11th,energydissipationremainsmostlyconstrainttotheswellband(0.05-0.2Hz);2)duringtheseconduidmudevent( 4-10 b),thepeakindissipationshiftsfrom0.1toabout0.15Hz,andenergydissipationisobservedovermorefrequencies,withmostofthedissipationobservedinthelaterstagesoftheuidmudevent;3)themaximumvalueofdissipationisobservedaftertheuidmudeventsareover,withpeaksofki=0.15km1,atabout0.16HzearlieronMarch12th;and4)lateronMarch12ththedissipationpeakisobservedtomoveotheswellbandintotheseaband(about0.22Hz).Highdissipationvaluesoverfrequenciesbetween0.1-0.3Hzcontinuetobeobserveduntiltheendofthedeployment. 87

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4-11 a-d.Theseguresqualitativelyagreewith 1996 ]estimatesobtainedfrommodelingandlaboratorystudiesofviscoelasticmudinduceddissipation.Themajordierenceinourcasearethenegativevaluesofwaveattenuationobservedathigherfrequencies,whicharemostlikelyduetoenergyinputbythewindandnonlinearenergytransfers.Bothoftheseeectswouldalsocontributetomakethecurveofdissipationversusfrequencylesssteepthanwouldotherwisebeintheabsenceofsuchinputprocesses.However,ingeneralitisclearthatdissipationissmallatlowandhighfrequenciesandmaximuminbetween. 1996 ]provideaqualitativeexplanationbasedonthemeanrateofenergydissipation,D,derivedby 1991 ]: 4{9 ,thelargerthegradientofthehorizontalvelocity,thelargerthedissipation.Sincethevelocitygradientismostlylimitedtothewaveboundarylayer(WBL),whichisproportionalto(22=)1=2,foraverylowfrequencywave,theWBLtendstobemuchlargerthanthemudthickness,andthereforethevelocitygradientisrelativelygentle,resultinginsmallenergydissipation.Inthehighfrequencycase,viscouseectsarelimitedtoaverythinlayerclosetothebottomofthemudlayer,thisleavestherestofthemudlayerwithalesssteepvelocitygradientandthusinthiscasetheenergydissipationisalsosmall.ForaviscousuidmudthemaximumdissipationoccurswhenthethicknessofthemudlayerisclosetothethicknessoftheWBL.Inthiscasethevelocitygradientisthesteepestwheremaximuminternalfrictionoccursresultingingreaterenergydissipation.Inthisstudy,velocitymeasurementsinsidethemudlayercouldonlybeobtainedduringthetwouidmudeventsatT1.Figure 4-12 showsprolesofrootmeansquarewavevelocitycorrespondingtotherstuidmudevent(Figure 4-12 a),andlaterstage 88

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4-12 b).TheapproximatelocationofthelutoclineismarkedwithadashedlineanditwasobtainedfromthemaximumreectingsurfaceidentiedbythePC-ADP.Thelocationoftheupperlimittothewaveboundarylayerisdenedhereasthemeasurementcellwherethemaximumgradientinhorizontalvelocityisfoundanditsmarkedinthegurebythesolidline.Inbothcasesthethicknessofthemudlayerisabout1.6timesthethicknessofthewaveboundarylayer(i.e.~d=1:6),andisonthesameorderofmagnitudeasthevaluefoundthroughthephaselaganalysis(~d=1:9)intheprevioussection.Inthesecases,usingthepeakfrequenciesf=0:11Hzfortherstuidmudevent(Figure 4-11 a)andf=0:13Hzfortheseconduidmudevent(Figure 4-11 b),andawaveboundarylayerthickness,=0:1m,weobtainamudviscosityof2=0:0035m2=s,and2=0:0041m2=s,respectively.Thissmallincreaseinmudviscositymaycontributeinparttothehigherratesofdissipationobservedduringtheseconduidmudevent.Itcanbehypothesizedthatafterthesetwouidmudeventshaveended,andsmallerwaveheightsareobserved(Figure 4-10 a),theviscosityofthemudlayercontinuetoincreaseduetoconsolidationprocesses,inducingthestrongdissipationratesobserved(Figure 4-11 c,d). 1.5 ,thebispectrumhasbeenwidelyusedinnearshorestudiestoanalyzethesignicanceofthree-waveinteractions.Thebispectrumestimatespresentedherearecalculatedfor3-hrsegmentsofacousticsurfacetrack(AST,distancetowatersurface)collectedwithasamplingrateof4HzbytheADCPatT1.Eachsegmentis 89

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1{27 ,andinthedenominatorofEquation 1{28 .Thepowerspectraldensityandtherealpartofthenormalizedbispectra,b(f1;f2)areshowninFigure 4-13 ,forfourdierentperiodsduringtheMarch9th-14thcoldfrontpassage.Attheearlystagethestorm(March9th,Figure 4-13 a),thepowerspectrumshowsapredominantpeakatfp=0.14Hz.Thebispectrarevealsapositiveridgeforthisfrequency,highestatthe45olineandthentaperingo,withasecondaryridgeformedattherstharmonic2fp=0:28Hz(i.e.f1=f2=fp=0:14,f3=f1+f2=2fp=0:28Hz).Thisresultimpliesastronginteractionofthepeakwithitself,whichleadstothedoublefrequencypeakobservedinthepowerspectrum.Positivevaluesofb(f1;f2)indicateenergytransfertohigherfrequenciesthroughsum-frequencyinteractions.Thiscanbeinterpretedasthepeakingofthewavecrestswithresultantharmonicsthatareinphasewiththefundamentalone[ 1963 ].Negativevaluesofb(f1;f2)indicateenergytransfertolowerfrequenciesthroughdierence-frequencyinteractions.Forinstance,theobservedvalueb(0:14;0:06)=0:25Hz-1/2canbeinterpretedasduetotheinteractionofthemainpeakat0.14Hzandasidepeakat0.2Hz,whichproducesthedierencefrequency0.06Hz.Lesspronouncedpeaksalsoshowsum-frequencyinteractionsat(f1;f2)=(0:21;0:03)Hz,and(f1;f2)=(0:32;0:03)Hz.Atthepeakofthestorm(March9,20:00hrs,Figure 4-13 b),thebispectrashowsstrongcouplingat(f1;f2)=(0:12;0:12)Hz(thefp,fp,2fpinteraction),the(f1;f2)= 90

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4-13 c),wherethebispectrashowsalsostronginteractionsbetweenthepeakfrequencyandtherstharmonic,at(f1;f2)=(0:1;0:1)Hz,and(f1;f2)=(0:2;0:1)Hz.Althoughwithlesservalues,inthewaneofthestorm(March12th,19:00hrs,Figure 4-13 d),themainbispectralsum-frequencyinteractionsobservedintheseafrequencyband,(f1;f2)=(0:28;0:16)Hzand(f1;f2)=(0:35;0:11)Hz.Dierence-frequencyinteractionisobservedintheinfragravitybandat(f1;f2)=(0:13;0:05)Hz.Thedierence-frequencycouplingobservedintheswellbandat(f1;f2)=(0:16;0:09)Hzcouldbeduetothedierenceinteractionbetweenfrequenciesnearthepeakat0.25Hzandthepeakat0.16Hz.Thebispectraduringthislaterstageseemstoclearlyindicatetransferofenergyfromhighertolowfrequenciesashypothesizedby 2003 ].Fromtheseresultsitbecomesclearthatthreewaveinteractionsareactiveduringthecoldfrontpassage,transferringenergybetweendierentfrequencybands.Asmentionedbefore,theeectofsuchexchangeofenergycanresultinthedissipationofenergyinfrequencybandsthatshouldotherwisenotbeaectedbythebottom.Comparedtotheestimationofthepowerspectra,thereissignicantstatisticaluncertaintyinthebispectralestimatesobtained.Toincreasethecondenceofthebispectralestimates,longertimeseriesthatarestatisticallystationaryareneeded.However,particularlyinthecaseofstormswhichcanevolvefairlyquickly,thestationarityassumptionishardertomeet.Thisinherentinstabilitycanbegreatlyreducedbyintegration. 1994 ]showsthatthecontributionofforcedwavestothetotalenergyonafrequencyrange[fmin;fmax],canbeobtainedbythedoubleintegrationof 91

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1994 ]alsoshowthatjbiij2isapproximatelythefractionofenergyinthefrequencyrange[fmin;fmax]thatisnonlinearlyforced.InFigureisshowntheresultofthecalculationofjbiij2,fortwodierentcases:therstcaseshowsthecontributionofforcedwavesintheseaandswellbands(0:05Hzf<1Hz)totheinfragravityband[fmin=0:005Hz;fmax=0:05Hz](bluelineinlowerpanelofFigure),andthesecondcaseshowsthecontributionofforcedwavesintheseaband(0:2Hzf<1Hz)totheswellband,thatis[fmin=0:05Hz;fmax=0:2Hz](redlineinlowerpanelofFigure).Thisplotshowsthatnonlinearlyforcedinfragravitywaveenergyrepresentsaround10-15%oftotalinfragravityenergyduringthestorm,andnonlinearforcedswellwaveenergyrepresentsamaximumof30%oftheswellenergyatthepeakofthestorm.Itmustbementionedthattheseestimatesofnonlinearlyforcedwaveenergyaremostreliablewhenthedirectionalspreadofthewavemeasurementsislow,thereforetheyaremorerobustduringthestormwhenthewavedirectionisalmostexclusivelyfromtheSEinthefrequencyrangeof0-0.5Hz.EventhoughthecontributionofseaenergyintotheswellbandisrelativelysmallatT1,thecumulativeeectofnonlineartransferofenergybytriadinteractionsbetweenT1andT2maybesignicantanditcouldnotablyalterthedissipationofenergybetweenthestations.Toinvestigatethisprobability,numericalmodelingofwavepropagationinthisareaincludingboththeeectsofaviscousuidmudlayerandnonlinearthree-waveinteractionsisneeded.Howeverthiseortisbeyondthescopeofthisstudy,anditisrecommendedasfuturework. 92

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a)Cross-shorevelocityphasedierencesinthelowerwatercolumnaveragedovertheswellband(0.05-0.2Hz),prolescorrespondingtothenumbers1,2,3,4,5,and6areshowningures 4-7 4-8 ,and 4-9 .b)Coherence;c)Cross-shorevelocityamplitudeaveragedover3hrs;d)Rootmeansquarevalueofcross-shorevelocity.Thewhitelinerepresentsthemaximumreectancesurface,measurementsbelowthehydrodynamicbottomhavebeenmaskedforclarity. 93

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Pre-stormverticalprolesofcross-shorevelocityphasedierences,andcoherence,correspondingtothenumbersinFigure 4-6 a:(1)March7,22:00hrs;(2)March9,04:00hrs. 94

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Prolesofcross-shorevelocityphasedierences,andcoherenceduringtherstuidmudevent,andtheinitialstageoftheseconduidmudevent.TheplotscorrespondtothenumbersinFigure 4-6 a:(3)March10,01:00hrs;(4)March11,01:30hrs. 95

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Prolesofcross-shorevelocityphasedierences,andcoherenceduringthenalstageoftheseconduidmudevent,andafterthestormhaspassed.TheplotscorrespondtothenumbersinFigure 4-6 a:5)March11,13:30hrs;6)March12,05:00hrs. 96

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a)SeaandswellwaveheightsatT1;b)PC-ADPreturnsignalintensity,versustime,thewhitelinemarksthelocationofthemaximumreectingsurface,andthepinklinemarksthezerovelocitylevel;c)Waveattenuationcoecientkiaveragedover3hrperiods.ThewavespectrausedforthisestimationiscalculatedatthedepthofthePC-ADP(i.e.nocorrectiontosurface). 97

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Frequencyspectracomparison(leftpanels)andcross-sectionsofwaveattenuationcoecientki(rightpanels)versusfrequency.a)March10th,rstuidmudevent;b)March11th09:30hrs,nalstageoftheseconduidmudevent;c)March12th03:30hrs,maximumdissipationobservedaftertheuidmudevents;d)March12th12:30hrs,peakindissipationshiftstowardhigherfrequencies.Themaximumerrorusingachi-squared90%condenceintervalisaround1%oftheestimatedspectrum. 98

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Prolesofrmsofwavehorizontalvelocityduringtwouidmudevents.a)March10th,rstuidmudevent;b)March11th09:30hrs,nalstageoftheseconduidmudevent.TheapproximatelocationofthelutoclineisobtainedfromthePC-ADPmaximumacousticechointensity.Thelocationofthewaveboundarylayer(WBL)istakenwherethemaximumgradientinwavevelocityisobserved. 99

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b) c) d) Figure4-13. ObservationsofnonlinearcouplingatT1duringthestormMarch8th-14th.Upperpanelsshowthespectraldensity,S(f),andlowerpanelsshowtherealpartofthebicoherence,b2(f1;f2).a)March9,00:00hrs;b)March9,20:00hrs(stormpeak);c)March10,19:00hrs;d)March12,19:00hrs.ThecontoursunitsareHz1=2.

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DoubleintegralbiiinEquation 4{11 ,calculatedfor1-hrsegmentsofsurfaceelevationmeasurementsduringexperimentAatT1. 101

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3-8 a-d)therstoccurrence(March10th)coincideswithnearbottomseawardcurrentvelocitiesofabout5cm/s;2)thedirectionofthecurrentintheupperwatercolumnreversestoshorewardimmediatelyaftertherst(andlargest)pulseofwaveactivity(eveningofMarch10th),whilenear-bottomvelocityremainsaround5cm/s;3)swellenergyduringthesecondevent(March11th)issignicantlylower,4)thenear-bottomturbiditylayermovesseaward,oppositetothewatercolumn,and,nally,5)verylittleofthistypeofactivityisseenattheinshoreplatformT2(Figure 3-8 e-h).Theseobservationssuggestascenariothatinvolveswave-inducedgravitycurrents(e.g., 2000 ; 2001 ; 2002 ;andmanyotherobservationsandmodels).Therstenergetic 102

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3-8 d).Inthisscenario,thesourceofsedimentistheentireshelf,butthestructureofresuspensionanduid-mudgenerationchangesinthecross-shore.Liquefaction/resuspensionismostintenseattheoutersite,consistentwithashorewarddecreaseinwaveenergyduetobottominduceddissipation.Incipient(diluted),shoreward-owinguidmudlayerslikelyformat(andonshoreof)theinnersite(T2),andincreaseinthicknessanddensityastheyowandaccumulateatthedeeperpartoftheshelf.Furthereldwork,withincreasedspatialresolution,isnecessaryinordertoverifythisscenario.However,neitheroftheseeventsagreeswiththeoverallnear-bottomupwellingobservedby 2006 ]atacheniercoaststationoppositetheaccretionarymudatsnearFreshWaterBayou.Itmaybethatadierentdynamicmechanismdominatessedimenttransportalongtheconcave-shorefacemudatsofthecheniercoastthantheAtchafalayamudclinoform.However, 2006 ]estimatesarebasedoncurrentvelocitiessampledaboveabout50cmfromthebed,andourdatashowsthatdenserowsaremovingoshoreonlyinthelowerapproximately30cmab.Assumingthatthedownslopeowsobservedareindeedgravityows,theanalyticalmodelof 2001 ]wasappliedwithsatisfactoryresultsproducinggravityowvelocitiesveryclosetotheobservedvelocitiesinsidethewaveboundarylayer.Themodelindicatesthateasilysuspendedsedimentwasavailableduringtheuidmudeventsdiscussed,andthattheincreaseofcohesivesedimentsinthewaveboundarylayereectivelydampedturbulenceincreasingtheRichardsonnumbertovaluesabovecritical(Ri>1=4).Themodelalsosuggeststhattheappropriatedragcoecientvaluesforthese 103

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2001 ; 2007 ; 2007 ).FluidmudsobservedduringExperimentBonthewesternedgeoftheclinoformagreewithpreviousobservationsofwestwardcoastalcurrents(\Atchafalayamudstream", 1981 ]).TheylastlongerthantheonesobservedduringxperimentA,andshowweakerows(orderof1-3cm/s),andappeartobecontrolledmainlybywaveturbulence,whilecurrenteectsseemtobelimitedtotheerosionofthelutoclineandtheadvectionofsedimentsabovethewaveboundarylayer.Thelackofinformationaboutthelocalbathymetryprecludesusfromdrawinganyconclusionabouttheoriginandowmechanism.Theuidmudlayercouldbeslowlyowingdownalocalslopeor/andpoolingattheexperimentsite,beingmaintainedbythewave-inducedturbulence.Thesetofobservationspresentedhere,especially,thebedreworkingduringtheMarch7th-14thstorm,showsclearlythatthestateofthebedsedimentschangessignicantlyduringthelifetimeofastorm,aswellaswithinrelativelysmallgeographicdistances(afewkminthecaseofExperimentA).Thesendingsquestiontheapplicabilityofsinglephaserheologicalmodelsforbothsedimenttransportandwavedissipation,aswellasthetreatmentofbottomconditionsasspatiallyhomogeneous.DissipationofwaveenergyuxbetweenstationsT1andT2ismeasuredduringExperimentA.TheseobservationsshowstrongdissipationstartingwiththearrivaloftherststormswellsinMarch9th,increasingsteadilyuntiltheendofthedeploymentonMarch14th.Maximumdissipationratesontheorderof60%areobservedafterthemainstormpeakshavepassed.Themeasurementsofwavedissipationobtainedarenetestimates,inthesensethatallphysicalprocessesthatcontributetoenergydissipation,andgenerationareincluded(bottom-induceddissipation,whitecapping,windenergyinput,four-waveinteractions,andwavebreaking).Toassesstheimportanceofother 104

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1978 ; 2000 ).Phaselagsobservedaftertheswellshavediminishedandthemaximumdissipationisobserved,suggesingthatnondimensionalthicknessoftheuidmudlayerisontheorderof1.6,coincidingwiththevaluesofnondimensionalthicknesswhere 1978 ]and 2000 ]obtainedthehighestdissipationrates.However,ifthestateofthebottommudattheendofExperimentAissignicantlydenserthanduringtheuidmudevents,theacousticinstrumentscannotpenetrateintothemudlayer,andthephaselagscanbemuchhigherinsidethemud.Thiswouldbeexpectedinthecaseofaviscoelasticmudsuchastheonemodeledin 1990 ]. 105

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4-10 a),theviscosityofthemudlayercontinuetoincreaseduetoconsolidationprocesses,inducingthestrongdissipationratesobserved(Figure 4-11 c,d).However,toidentifymorepreciselythewavedissipationmechanism(viscousuidmudvs.viscoelasticmud)wouldrequiredirectobservationsoftheevolutionofrheologicalparametersnearandinsidethemudbed.Instrumentationcapableofcollectingthistypeofdataispresentlynotavailableforelddeployments.Theimportanceofnonlineartriadinteractionsisinvestigatedthroughtheuseofbispectralestimates.Thenormalizedbispectrumshowsstrongharmonicfrequencycouplingduringthepeakofthestorm,notunlikethatreportedovershoalingonsandybeaches(e.g., 1985 ]; 1993 1995 ]; 1990 ],andmayothers).Inthewakeofthestormmoremoderatecouplingisobserved,butitdoesnotinvolveaclearcouplingbetweenharmonics.Nonlinearinteractionsduringthisstageinvolvesmainlybispectralpeaksinthehigher(sea)frequencybandindicatingtransferofenergytolowerfrequencybands.Thistypeofinteractionmayleadtothedissipationofenergybybottomeectsfromfrequenciesthatwouldnotbedissipatedifnonlineartriadcouplingisnotpresent.Nonlinearlyforcedinfragravitywaveenergyrepresentsaround10-15%oftotalinfragravityenergyduringthestorm,andswellwaveenergynonlinearlyforcedbyspectralcomponentsintheseabandrepresentsamaximumof30%oftheswell 106

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3-8 a-b.DuringthersthighswelleventaroundMarch10th,theliquefactionofthebottomandsubsequentformationoftheuidmudlayerrequiresmuchmoreenergy(higherswells)thantheresuspensionandformationoftheseconduidmudlayerofMarch11th(lessenergeticswells),thereasonislikelytoberelatedtothefactthataftertherstswellpeakthereisnotmuchtimeallowedforconsolidationofthebottom,sothatthesecondswellpeakndsthemudinthebottomeasiertoresuspend.Inturn,thefactthatthemaximumwaveattenuationisobservedafterthehigherswellshavealreadypassedseemstoberelatedtothefactthatthebottomrheologyhasbeenmodiedbytheoccurrenceoftheseprevioushighswelleventsandisnowdierent(bottommudismoreconsolidatedandviscous)thanduringthepreviousuidmudevents.Forexample,ifonewouldliketoderiveconclusionsabouttheforcingnecessarytocreateauidmudlayersimilartothatobservedduringMarch11th,onewouldneedtoconsiderthecombinedeectonthebottomofthetwohighswelleventsandnotonlythelaterone. 107

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1971 ]. 108

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A{4 ): 109

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1. Noiseispresentintherecords. 2. Thesystemrelatingx(t)andy(t)isnotlinear. 3. ~Gxx(f)=2 1971 ]: 1. Thedatasetwasdividedinequalsegmentsandeachsegmentwasdemeanedanddetrended. 110

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ThedatainasegmentwastruncatedorpaddedwithzerossothatN=2p.Theeectofthiszeropaddingistoreducethefrequencyspacingfandincreasethenumberofestimatesforagivensegmentlength. 3. Theresultingsequenceismultipliedbyawindowfunction.IngeneralthefunctionusedistheHanningwindow:w(n)=0:51cos2n N1wherenisanintegerwithvaluesbetween0nN1. 4. Thesegmentsareoverlappedbygenerally50%inordertolimittheenergy\leakage"attheedgesofthewindow. 5. 6. ~Gxx(f)wascalculatedaccordingto( A{14 )andaveragedoverallthesegments.AnanalogousprocedurewasfollowedforthecalculationsoftheXPSD.Amoredetailaccountoftheseprocedurescanbefoundin 1971 ]. A{2 ),andthirdcumulantsofazero-meanstationaryprocessaredenedby: 111

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112

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113

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1963 ],thephysicalinterpretationofthespectraandthebispectracanbeobtainedusingthecomponentsdZ(f)oftheFourier-Stieltjesrepresentationofthestationaryprocessx(t):x(t)=Z1dZ(f)e2ftThen

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115

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SergioJaramillowasborninMedellin,Colombia.Heobtainedhisundergraduatedegreeinnavalsciencesandlaterinphysicaloceanography,fromtheColombianNavalAcademy.In2004,heobtainedhisMastersdegreeinphysicaloceanographyfromtheUniversityofBritishColumbiainVancouver,Canada,workingwithDr.SusanAllentryingtounderstandthedynamicsofowaroundsubmarinecanyons.Since2004,hehasbeenworkingwithDr.AlexandruSheremetdoingeldstudiesoftheinteractionsbetweensurfacewavesandmuddyseaoorsintheAtchafalayaShelf,Louisiana,USA.HecompletedhisdoctoratedegreeincoastalengineeringattheUniversityofFloridain2008. 122