Field observations of rip currents

236 Pages·2003·10 MB·English

by  MacMahan, James H

FIELDOBSERVATIONSOFRIPCURRENTS
By
JAMESH.MACMAHAN
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2003
Thisdissertationisdedicatedtomyfamilyandfriends,whosupportedmethroughoutthis
process.
ACKNOWLEDGMENTS
Asmyacademiccareercomestoaconclusion,Ihavemanypeopletothankwho
haveassistedmeeitherscientifically,socially,orboth,allofwhichhascontributedto
makingthisawonderfulandrewardingexperience.First,Iwouldliketoexpressmy
appreciationtoDr.RobertDean,whobelievedinmeinthebeginningandgavemethe
necessarylatitudetoallowmetodevelopasapersonandascientist.ThelessonsIhave
learnedfromthiswillaidinmyfutureandIamgratefulforDr.Deansunselfishnessand
wisdom.ThoughIprobablyhavefmstratedhimattimes,heneverindicateditandhe
alwayssupportedmeinmytimesofneed.
BeforeIstartedattheUniversityofFlorida,Dr.Deanplantedanideaof
investigatingripcurrentsinmyhead.SincethedayIsteppedfootinthedepartment,I
wantedtostudyripcurrents.ThoughIdidendupstudyingripcurrents,thepathwayto
thisgoalwasnotverydirect.Afterspendingthefirstspringsemesterwritingaproposal
totheOfficeofNavalResearchwithDr.Dean,IworkedattheUnitedStatesGeological
SurveywithDr.GuyGelfenbaumandDr.DougWilsoninvestigatingripplesoffthecoast
ofSt.Petersburg,FL.WhileworkingattheUSGS,GuyGelfenbaumlearnedofmy
interestinbuildingahydrographicsurveyingsystemusingapersonalwatercraftfor
investigatingripcurrents.Afteranunsuccessfulfirstroundinobtainingfundingtostudy
ripcurrents,GuysponsoredmetobuildthePWCsystemfortheSouthwestWashington
CoastalErosionStudy.ThisiswhenImetthe“oneandonly”PeterRuggerio,whoisone
cooldude.IhadablastthatsummercollectingbathymetricprofilesofftheWashington
andOregonshorelinesandstayinginsomeoftheshadiesthotelsinthePacificNorthwest.
IwouldalsoliketothankMarkHansenforsuggestingthatIgotoUFandworkwithDr.
Dean.MarkalsoassistedmewithmysurveyingquestionsduringthedesignoftheUSGS
PWC.DaveWegenerhelpedmeoutalotwiththeconstructionoftheUSGSPWC.
Atthispoint,wefinallyreceivedfundingtostudyripcurrentsinFlorida.Ibegan
collaboratingwithDr.RobertThieke,whoisoneofthemostarticulateandcaptivating
professorsthatIknow.Ihaveenjoyedmanyofourlongconversationsthatoriginated
overrathersimplequestions.Thoughourdreamdatasethasyettocometofimitionforan
intensecollaboration,Ihaveenjoyedourdiscussionsandlookforwardtofutureones.
IextendmyappreciationtoLyndaCharlesandTomWattersforfundingthe
coastaldepartmentfordevelopingaUFPWC,whichhasallowedmetocollaboratein
manyadditionalprojects,whicheventuallycontributedtothisbodyofwork.James
JoynermadetheUFPWCtopnotchandhasalwayssupportedandassistedmeinmy
efforts.
IthasbeenapleasuretoworkwithSidneySchofieldandVikAdamsinthefield
andoutatthelaboratory.VikisoneofthemostrelaxedfieldguysandIwillneverforget
thetimehewasinvertedina5footdeepjettedsandpitontheshore-breakofDaytona
BeachduringtherecoveryoftheAquadopp.Sidneywasalwaysavailabletoanswermy
questionsandIenjoyedourdiscussionsofelectronics,biking,andhisextracurricular
activities.Mr.VernonSparkmanalwaysgreetedmewithafriendly“hello”,butalways
IV
followeditbysomejokingremark.
IextendmyappreciationtoDonaMossandLucyHammfortreatingmeas
individualandnotasastudent,therebyallowingmetounderstandthefundingand
proposalprocess.Ilearnedalotofvaluablelessonsfromthem,whichwillassistmein
thefuture.Thankstotherestoftheofficestaff:Thelma,Donna,Carol,andDebra.
Duringthesummerof2000,Dr.EdThorntoncametovisitthedepartment,ashe
wasontheexternalvisitingcommittee.Thestudentshadtheopportunitytomeetwiththe
committeeandIwasanxiouslyinterestedinchattingwithEd.IspokewithEdforabout
anhourandhementionedthepotentialopportunityformetocometoMontereyinthe
futuretoassistincollectingbathymetricmeasurements.Severalmonthslaterwhilesitting
atmydesk,IreceivedacallfromDistinguishedProfessorThorntonaskingmeifIwas
interestedinheadingtoMontereyinthespringof2001.1wasexcitedaboutthe
opportunity,butIneverrealizedhowsignificantthisopportunitywouldbe.Iamprobably
theonlystudenttoturndownaFulbrightFellowshiptostudyartificialsurfingreefsin
NewZealandtoworkintheUnitedStates.
DuringtheMontereyexperiment,knownasRIPEX,Igottheopportunityto
participateinalargefieldexperiment.Thiswasadreamcometrue,asImissedallofthe
Duckexperiments.Theexperimentwasablast.Duringthisexperiment,ImetEdand
TimStanton’screw:EdithGallagher,RobWyland,MarkOrzech,andJimStockel.Greg
MillerandIstayedatthebeachfrontNavalPostgraduateSchoolbedandbreakfast,where
theshowersdidnotworkinitially.Afriendlyguy,AdReniers,Edspost-doctoralfellow,
broughtGregandmeintohishouseforahotshowerandgoodmeal.Ididnotrealizeitat
V
thetime,butAdwouldbeeomeagreatfriend,amentor,andpersonforwhomIinspireto
be.Adunselfishlyallowedmetoutilizetheripcurrentdataarray,whichhesuggestedfor
hisownscientificpursuits,thathasnowbecomeasignificantportionofmydissertation.I
mustthankStellaandFemkeforbeingmyadoptedMontereyfamily.Iwilltreasurerthe
timesatChe’Renters.MycollaborationswithEdandAdhavejustbegunandIlook
forwardtothefutureinteractions.
IwouldliketothankDanHanes,anoriginalcommitteemember,forassistingme
onproposals,providingusefulscientificdiscussion,andlettingmeteachinhisclasses.I
wouldalsoliketoextendmyappreciationtotheremainingcommitteemembers.Dr.Ochi
andDr.Mossa
BesidesthescientificfolksthatIhavemetalongtheway,mycomradesin
Gainesvillehavealwayssupportedme.Susan“Powell”DevineandKerryAnnDonahue
introducedmetoGainesvilleallowingmetocontinuebeingmyselfwithoutskippinga
beat.ThanksgoestoA1Browder,HugoRodquiez,andEricThostesonforteachingme
thewaysofthedepartmentandsharingtheirenthusiasmforcoastalengineering.Ihave
hadrewardingexperienceswithassistingJasonEngleandGregoryMillerintheir
Masters.IextendmyappreciationtoJonMiller,Oleg,Justin,LoraineChiale,andJack
Puleo.Thanksforatwogreatyearswithmyoriginalclass,thoughnotoneofthemare
stillhere;CrisWeber,DaveAltman,LisaHeckman,BethCranston,HeatherSummerell,
andSeanMulcahy.IextendmyappreciationtotheStatsgroupforprovidingsome
diversityandthefootballblock(STA);JeffThompson,WolfgangYank,BrianCaffo,and
PatchesJohnson.Thenewgroup(RobertWeaver,KristenSplinter,andJodiEshleman)
VI
ofcoastalstudentshavebeenalotoffun.
IwouldliketothankPaulDevine,LeeGordon,andPaulGayesfortheirsupport
invariousavenues.Iwillcontinuetointeractwiththesefolks.
Iwouldliketothankmymomandmysisterforsupportingmeandunderstanding
mygoalsanddreams.
Finally,IwouldliketothankHeatherforemotionallysupportingmeduringmy
highs,lows,andmyinjuries.Shetrulyunderstandsmyhardshipsanddreamsandhas
helpedmeembraeethem.
Vll
1 TABLEOFCONTENTS
2 Page
ACKNOWLEDGMENTS
3
ABSTRACT
CHAPTER
INTRODUCTION
4 PresentWork .
RIPCURRENTS-BACKGROUND
RipCurrentMeasurementsandObservations 10
FieldSetting 10
LaboratoryExperiments 13
RipCurrentModeling 14
MassTransport 17
RipCurrentMorphodynamics 19
HYDROGRAPHICSURVEYINGFROMAPERSONALWATERCRAFT
Introduction 23
NearshoreBathymetricSurveyingSystem 26
Instruments 26
SamplingScheme&Error 34
FieldTests 38
Results 39
Conclusions 43
FEASIBILITYOFMEASURINGCURRENTSINTHENEARSHOREFROMA
PERSONALWATERCRAFT
Introduction
Vlll
RipCurrentApplication 48
Instrumentation 48
FieldExperiment:RIPEX 50
SamplingErrors 55
Discussion 56
Conclusion 57
5 RIPCURRENTSTABILITY
DatasetandMethodology .58
ConcurrentWaveandLongshoreCurrentMeasurements .64
ChronologicalStabilityofRipCurrentChannels .66
RRiippSSttaabbiilliittyyPPeerriioodd21::((SMeaprtcehm2b4e,r11909,819-9A8ug-uDsetc2e1m,b1e9r9288),1998) ..6678
RipStabilityPeriod3;(January19,1999-March3,1999) .68
RipStabilityPeriod4:(April10,1999-August27,1999) .68
RipStabilityPeriod5:(September25,1999-January12,0000) . .69
RipStabilityPeriod6:(March25,2000-July26,2000) .70
Discussion .70
Conclusion .77
6 RIPEXANDMORPHODYNAMICS
Introduction ,83
RIPEX-SBEExperiment ,86
Instrumentation .86
BathymetricandBeachfaceSurveys .89
WaveClimate .92
ComplexBeachMorphology .96
SpatialSedimentDistribution .97
BeachCusps .99
BathymetricMeasurements 101
VideoImages 103
Erosional-AccretionsTrends 105
RipCurrentFlowKinematics 107
SeaBreezeInfluence 119
Summary 119
7 RIPCURRENTPULSATIONS 121
Introduction 121
TheRIPEX-SBEExperiment 126
IX
ObservationofInfragravityEnergy
Cross-shoreandAlongshoreSpectralStatistics 132
EdgeWavesandLeakyWaves
Cross-shoreStructureofWaveGroupsandLongWaves 143
RipCurrentPulsations
AlongshoreStructureofCurrentVelocities 151
Cross-correlationofInffagravityWaves 153
Cross-correlationsofInffagravityMotionswithRipCurrentPulsations156
Discussion 1CO
Conclusion
8 VERYLOWFREQUENCYMOTIONS 163
Introduction ^63
FieldSite ^69
VeryLowFrequencyMotions 1^1
RipCurrentConceptualModeling 191
Discussion ^95
Conclusion
9 SUMMARY,CONCLUSIONS,ANDRECOMMENDATIONSFORFUTURE
WORK
REFERENCELIST 213
BIOGRAPHICALSKETCH 222
X