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CHAPTER242

MeasurementsofWaveGeneratedBedformsChristopherD.Jette1andDanielM.Hanes2

AbstractA Hzultipleransducerrrayassedoeasureaveeneratedbedformsea rDuck,NC.Theransducerrrayonsistsof37ransducerswithcentertocenterspacingof .2centimeters.Seabedmeasurementsweremadewithapproximately1 millimeterverticaland2centimeterhorizontalresolution.Measuredripple imensionsreomparedohreeopularippleredictionodels.tsshownthaterrorsofover00percentwerefoundbetweenmeasuredandpredictedrippleheightsandlengthsfo rallripplemodelscompared.Errorsinpredicting ripplesteepnessrangedfrom37to5 5 percent.

DescriptionOfExperimentFieldmeasurementsofbedformsweremadeuringnexperimentattheArmyCorpsofEngineersFieldResearchFacilityinDuck,NCduringAugust23-25,995 .Thenstrumentsereeployedsing ensornsertionystemSIS)nheresearch ier.TheSISwasositionedtmultipleocationslongheross-shoreprofileduringtheexperiment.Bedformmeasurementseremadesing multipleransducerrrayMTA)

developedtheniversityflorida.heTAonsistsf7ltra-sonictransducersoperatingatafrequencyof5MHz.Thecenter-to-centerspacingofthetransducerss2mm.uchconfigurationllowsfo r0(1mm)erticalesolution and0(2cm )horizontalresolution.TheMTAcanscanaprofileinapproximately4 seconds.Thedistancetotheseabediscalculatedfromtheelapsedtimebetweenthepulsingofheransducerndheim ethichheeturnxceeds oftwareselectablethreshold. 1)esearchAssistant,Departmentof CoastalandOceanographicEngineering,

UniversityofFlorida,POBox116590 ,Gainesville,FL32611 ,USA.2)ssociateProfessor,DepartmentofCoastalandOceanographicEngineering,UniversityofFlorida,POBox116590 ,Gainesville,FL32611 ,USA.3129

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3130 OASTALENGINEERING996

/multiplexer,driver& cross-sect ionof Iecejvercircuitry,and mult i -e lement \dataoggert ransducerI- ---'._ -_^,'t ransducers.t ransmitted5MHzultrasonicsignalrecordedpoints

bottom profilelon9bottomP rofile: s a r i d . s e a : b f e t i ; .

Figure1 .SchematicoftheMTA InadditiontotheMTA,nunder-watervideocamerawaseployedinordertodocumentedformrientation.lectro-magneticurrentetersndressuretransducersereeployedoeasureheavendurrentonditions. Measurementsfheerticalistributionfuspendededimentycousticalbackscattermethodswerealsomade.Theatawasollectedn3minutebursts.Eachurstcontained31rofilestakenwiththeMTA.Current,pressure,andconcentration profiledatawerecollectedat 4Hz.

ConditionsDuringExperimentExploitinghemobilityoftheSIS,measurementsweremadeunderavarietyofwaveandsedimentconditions.Duringtheexperiment,anoff-shorebarwaspresent

atapproximately21 0metersfromthehighwaterline.easurementsweremadeat8separatecross-shorelocationsintheregionsoffshoreofthebar,onthebar,inthetrough,ndnheearshoreeach-face.Depthswheremeasurementsweremaderangedrom.6o .8meters.Theerticalinesnigure ndicateross-shorelocationswheremeasurementswereobtained.

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WAVE GENERATED BEDFORMS 3131

E i

< 5 5

10

> LU -1 0 100

=SouthsideofpierNorthsideofpier

60000000000Distancea longpier-m Figure2.Cross-shoreprofilesfrom NorthandSouthsidesofpierduringexperiment. 700Theedimentharacteristicsariedreatlyverheross-shorerofile.50valuesofthemostlyquartzsedimentrangedfrom 0.19 to1.67mm.TherangeofD50valuesersusunumberareplottednFigure3)longwithHmoa),eakwaveperiod(b),andwaterdepth(c).

(a)

o0.5

20-o oa s D L 0

X1 X X X X

1 1 1 1 X X X * * X

1

X X X X X X1 X X X X X

1 X * X X X * 9

10 15 (b )0 25 30i X 1 1 ~ X X X X X X X X X ifx x X X X1. X X X X X1 X X X X1 X X X -1 1 1010

i5 Q15 (c )0 25 30

0

E ioL O D

xxxxxxxx** X X X****xxx*xxx x

10 15 (d )0 25 305K5K3tS 5K5 5lf5K5) 5

10 15 RunNumbe r20 25 30

Figure3.Hmo(a),peakwaveperiod(b),waterdepth(c),andD5 0(d )vs.Runnumberfo rSISexperiment

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3132 COASTALENGINEERING996

Mostofthewavedatatakenoverthethreedayperiodcontainedatleasttwo,if notthreefrequencypeaksnthesurfaceelevationpectrum.Formanyoftherunstherewasalowfrequencycomponent(12-15ec.period)generatedfromhurricaneFelix,whichwaseveralhundredmilesoff-shore,aswellsamoderatefrequencycomponent(6-8sec.period),andgenerallyintheafternoon,ahigherfrequency(4-5 sec.period)ocallygeneratedcomponent.Hmowaveheightsvariedfrom0 .2to0 .9 meters.uringmostfhexperimentheredominateaveirectionthe experimentsitewasdirectlyon-shore.

Forostfheunseasurementsereadeffshorefhereakpoint,howeverduringruns1hrough21omewaveswerenotedtobreakinthevicinity ofthenstruments.tslsootedthat,ftertransformingthe ressurepectrumsintourfacelevationpectrums,nheajorityfasesheigherrequencycomponentecameheeakrequency.onsequently,heaveeriodsednmodelpredictionwasthelowestperiodcomponentinthewaveprofile.

0.040.035

0.025

0.015

0.1.2.3.4.1.2.3.FrequencyinH z requencyinH z Figure4.Representative correctedsurfaceelevationspectrumsfromSISexperimentforruns1(a)and13(b)withHmowaveheightsof0.72and 0.28 metersrespectively

MeasurementsBedformsereresenthroughouthexperiment,oweverheireometriesvariedgreatlyoverthebeachprofile.Measuredbedform heightsrangedfrom7to5 0 mm,andbedform lengthsrangedfrom8 0to000mm.Activerippleconditionswereobservedduringalltimesofdatacollection.Formostofthefilesunder-watervideowasvailableoocumentear-bededimentotionndipplerientation.

Significantripplemigrationwasnotobservedduringanyofthe13minuteruns.

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Bedformrofileswereollectedt ateoften rofileserminute.Forhe measurements resentedherein, singlerepresentativeprofilewasoundoreachminuteofdata(1 0scans).Anexampleofthethirteenrepresentativeprofilesfo ran entirerunisshowninfigure5(a-c).Inthesefigures,eachprofilehasbeenoffset by-3mmfromthepreviousprofilefo rcomparativedisplay.Thetopprofilerepresentsthefirstminuteofdataandcontainscirclesateachofthe37measureddatapoints.

( a ) 10-0-

-10--20--30--40--50-

5 0 1 005 0005 0005 0005 0 150005 0HorizontalDistance -mm Figure5 .Representative profilesfo reachminuteofa13minuterunfromruns1(a),13(b),and26(c).Datafrom firstminuteofrunisplottedwithcircles. 450

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Inordertocomparethesedatatomodels,thebedformsneedtobecharacterizedinermsf wavelengthndeight.nreviousorkhisasypicallyeen accomplishedyisualnspection.Wehereforehose hresholdethododeterminerippleheightandlengthdimensionsforeachrepresentativerippleprofile.Thismethodfirstfoundthepeaksandtroughsalongaprofile.Then,ifthedistancebetweendjacenteaksndroughsmet ertainhresholdriterion,heywererecorded.Fromheseeaksndroughsverageippleeightsndengthswerefound.hethresholdwaschosenconservatively,thatis,tomaximizerippleheightsandlengths.Figure(6 )snexampleofarepresentativeprofilewiththethresholdripplecrestsandtroughsmarked.Table1 .Measuredhydrodynamicparametersandrippledimensionsfo rfieldexperiment.

ru n I loc. epth Hmo Tp j D5 0 tm(1) tm(4) tm(8) l tm(13);htm(1) htm(4) htm(8)jhtm(13) (m ) (m ) (m ) j (s) mm) mm) mm ) mm) mm) (mm) mm) mm) mm)

i l . M 7 . . J > . . . . ? ^ t . ? . . . J . . . . 9 : n . . j . . . . ? . 7 . ? . . j . . 9 . - . ? . ? . . . i . . . . l l . 9 . ? . . . . i ? J L . 1 9 3 . J . . . . 1 9 7 . . . J .? ;I.! H...jZ!KIi"ZIZX!MLX513?!1X~9:?0^ Z"!?ZX!!M7ZX!ML[!"9Z?0^Z3ZX. ' . 'M7Z1Z9:M.XZ>Î!"!ZXIM7ZOiiLl"5"7ZI]i^^ XXZX 4TZiilZIZ1 . . .'ZX!H^'X3!:?jDZ9:?5!l!i^^^ 1 ? " Z ZZ?ZX!!M7ZQ:7]0Z9"?O3^Z3ZXIM7ZO:?OZ9X?03^Zl l9ZT"Mt lX"9XCjZMOZ4î"jffi'TT$4T"o3r'i'j&'To^ o""To"1"iZXQZiii-OZi:!$X"^ZZiCX!M7ZIZi!$JLiZ9^ZZ:LX"?7ZIZi:SUZiQ .Z ZJ ! IZX"??C1Z1 : ? 0 "?L^ ^ ^ZX?ZXI?ClZi:7jQllOZi^.............^...^.^^ . . . . . ^ . . . . ^ ...^........ZXCXZMC!t3!?OZ9l?OZ?^ ZjpZXIMOZClClZÔ"!^ J'2?""Il2p'2;25 T"a 'J"7iTj"o 2""]""l98""['4"l9'"}" " 6 ' . 3 7 " " l "7 * ii ] Q- -J"- -Y' fT' f'T^5'T'""4 0 "T 4o""j0 " " 'Z"?i03i?~nIO!!MiIZ. ' .?4Z.X3SOZ?:?OZ9:.^5""T'"T8T'T"3"86T'"a63''] '"8]bo"T'"i^^ 3""r"-^Q""r"22'""T0I"T^se'Ii'asb^Y'aeb^. . . . . . . . . . . | . . . . ^ . . . . ^ . . . . _ . . _ . . j . . . . . 8"X ' " i65"T '2 "7^ ..._....,...^p...,...._p._

Q " " T - " Y 5 5 " -]""2o"7"'o;2T'T"'6"4b"1''"ll47'T"'liy'T'"7^^^^87 6 T7Formodelomparisons,achunofatawasummarizedntoour rofiles.Theseprofileswerefromminutes,4,8 ,and3ofeach3minuterun.Thereasonsforthisweretominimizetheamountofclutterinmodelcomparisons,whileatthe

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WAVEGENERATEDBEDFORMS 3135

sametimeretaintherangeofripplegeometriespresentduringeachun .Measuredrippleengthltm)ndeighthtm)longwithummariesofydrodynamicndsedimentconditionsarepresentedintable fo reachrun.

20-10-

o-og-10-> -20-

-30

Thresh.RippleHt.= Thresh:Ripplel_t.= 10 84

i i0 5 0 1 00 350 40050005000HorizontalDistancein mm Figure6.Representativerippleprofilewiththresholdmethodresultsfo rfirstminuteofdatafromrunnumber13.

450

ModelComparisonsMeasurededformimensionsereomparedohreeredictiveodels;Nielsen( 1 981 ) ,WibergandHarris(1994) ,ndGrantandMadsen1981 ) .Foreach

runmeasuredvaluesofwaterdepth,Hmowaveheight,eakperiod,ndsedimentpropertieswerenteredntohepredictivemodels.Measuredbedform imensionswere lottedwithhemodelurves,fossible,nrderoetterllustratehe relationshipsbetweenmeasuredvaluesandpredictedvalues.

Nielsen1 9 8 1 )erived ifferent redictivemodelsorregularlaboratory)ndirregularfield)onditions.Comparisonsncludedereineferoherregularrfieldripplemodels.TheNielsen( 1 9 8 1 )bedform modelcharacterizesflowconditionsusingsedimentmobilitynumberQ) ,grainroughnessShield'sparameter(62.5),andthewavefrictionfactor"f2.5 "givenbySwart(1974) .Theseparametersaredefinedasfollows

6.5=0.5/5y/ -A . 5 exP 5.21 3 2.5O, 5.977

equation1

equation2

(a( 0yO-)gD equation3where"D "isthemeangraindiameter,"a" isthenear-bedwavesemi-excursion," s" isheedimentpecificravity,g"sheccelerationof ravity,ndoshe

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angularfrequencyofthewaves.TheNielsen( 1 981 )rregularwavemodelfornon-dimensionalripplesteepness,height,andlengtharerespectively T|/k=0.342-0.34(e2. 5 )025

Ti/a=21*F185or4/> 1 0 r|/a=0.275-0.022^5or JMJ? o

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10Mobil i tyNumber(b )

10Mobil i tyNumber(c )

10

10'

10 0 0GrainRoughnessShieldParamete rFigure8 .ComparisonofmeasuredfieldrippledimensionsandNielsen( 1 981 )modelcurvesfo rheight(a),length(b),andsteepness(c).

WibergandHarris(1994)reexaminedexistingrippledatafromoscillatoryflowsinbothflumeandfieldstudiestoconstructamodeltopredictripplegeometryforalltypes ofscillatorylownvironments. Wibergndarris (1994) classified

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bedformsccordingoheatioofhewaveboundaryayerhicknessndippleheightOo/n).romhisatio,heippleswereclassifiedsrbital,norbital,rsuborbitalbythefollowingcriteria

Table2 .WibergandHarris(1994)rippleclassificationflowconditionsippleclassification do/Tlano

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WAVEGENERATEDBEDFORMS 3139

104

103

(a )

ano ,=o rb +^-''

1021

m 3 (b )+ +

10*

10 1 01

ln

do/Ht.Figure9.WibergandHarris(1994)ripplemodelswithmeasuredvalues.(a)

/QB / in'/JC / .ca> -^ap < Db// o> X~&B)>R|1 / TDoS/ + C D

101 Q- /////a ) o

10'0'MeasuredLength(cm) MeasuredHeight(cm) MeasuredSteepnessFigure10.WibergandHarris(1994)modelripplecomparisonwithmeasurementsTheGrantandMadsen(982)ripplemodelusestheratioofthemaximumvalueoftheskinfriction(shearstress)underthewave( T * sf)tohecriticalvalueofshearstressorhenitiation fsedimentmotionx*)cr.Also dimensionlessedimentparameter(S*)isused.ThisripplemodelwasbasedprimarilyontheCarstensetal.(1969)laboratoryoscillatoryripplestudy.

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TwoeparateegionsoftransporttagesweresednheGrantndMadsen( 1 982 )model.Thefirststage,whenheflowconditionsrelesshanhebreakoffpointefinedyquation1 8 ) ,snownshequilibriumtage.Withinheequilibriumange,ippleteepnesss maximumndheippleengthsaidto scalewithhenear-bottomexcursionmplitude.Whenflowonditionsexceedthebreakoffpoint,conditionsaresaidtobeinthebreakoffregion;theripplelengthissaidtonolongerbeinequilibriumandadecorrelationbetweenipplelengthandnear-bottomxcursionmplitudeccursGrantndMadsen1982) ) .Ashelow intensityincreaseswithinthebreakoffregion,ripplesteepnesscontinuestodecreasetoapointwhereripplesarenolongerpresent.TheGrantandMadsen( 1 982 )ripplepredictionmodelisasfollows:whenthetransportstageislessthanthebreakoff point

77/A=0.16(T,1//(T.)CT)

7l/a.=0.22(T./(T.)crpandathighertransportstages,abovethebreakoff range

Tj/A=0 .285 .06(T ,1 //(T.)Cf)",

!7/a( 1=0.48S.O J ,(Tv(T.)cr)"1 'Swhere

S,=(d/4v)[(S-l)gD]andthebreakoffpointisdefinedas

[T,s//(T.)cr]B=1.85.06

equation13

equation14

equation15

equation16

equation17

equation18 .wherev'shekinematic iscosityofwater,D'shegraindiameter,S'shespecificgravityofthesediment,and'g'istheaccelerationofgravity.

AcomparisonofGrantandMadsen1 9 8 2 )predictedrippleheightsndlengthsversusmeasuredaluesshownnigure1 .taneeenhatheGrantndMadsen( 1 982 )modeloverp-redicted ripplelengthandheightforalmosteveryrun.

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m2 (a)< * ' /< f c > //

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3142 OASTALENGINEERING996

ConclusionsThemultipletransducerarray(MTA)hasproventheabilitytomeasurebedforms

inthefieldenvironmentwith0(1mm)verticalnd0(2cm )horizontalesolution.ComparisonsofoscillatoryfieldrippledatacollectedwithheMTAndthemostpopularredictiveodelshowrrorsfver00ercentorllodelsnpredictingrippleheightsandlengths.Averageerrorsof37to5 5percentwerefoundinpredictingripplesteepness.Sucherrorscouldbeduetoerrorsinhepreviouslycollecteddatasetsusedtoconstructthesemodels,ortothecurrentunderstandingofthemechanicsofrippleormationndeometricquilibriumwithhelowield.Moreinvestigationisnecessaryinthisfieldinordertoimprovebedformprediction models.Acknowledgments

TheauthorswishtoackowledgethefinancialsupportoftheU.S.Officeof NavalResearch,CoastalSciencesProgram,thefieldassistanceprovidedby theFieldResearchFacility,U.S.ArmyCorpsofEngineers,andthehelpofChuckBroward,EricThosteson,MikeKrecic,andCarlMiller.ReferencesBagnold,R.A.,Motionofwavesinshallow water:Interactionbetweenwavesandsandbottoms,Proc.R.Soc.LondonSer.A,87,-15 ,946.Carstens,M.R.,R.M.Neilson,andH.D.Altinbilek,Bedformsgeneratedinthelaboratoryunderoscillatoryflow:Analyticalandexperimentalstudy,Tech.Memo.28 ,U.S.ArmyCoarpsofEng.,CoastalEng.Res.Center,June1969.Dingier,J.R.,andInmanD.L.,Wave-formedripplesinnear-shoresands,Proc.FifteenthConf.CoastalEngng.,Amer.Soc.CivilEng.,1976 .Grant,W.D .,andO.S.Madsen,Movablebed roughnessinunsteadyoscillatoryflow,J.Geophys.Res.,87,469-481 ,1982 .Nielsen,P.,Dynamicsandgeometryofwave-generated ripples,J.Geophys.Res.,8 6,6467-6472 ,1981 .Nielsen,P.,Fieldmeasurementsofthetime-averagedsuspendedsedimentconcentrationunderwaves.CoastalEngn.,8 , 51 -72 ,984 .Sleath,J.F.A.,Seabedmechanics,WileyInterscience,NewYork,984.Swart,D.H.,Offshoresedimenttransportandequilibrium beachprofiles.Delft.Hydr.Lab.Publ.No.31 ,974.Vincent,C.E.,andP.D.Osbourne,Bedform dimensionsandmigrationratesundershoalingandbreakingwaves,Unpublished manuscript(1992) .Wiberg,P.L.,andC.K.Harris,Ripplegeometryinwavedominatedenvironments,J.Geophys.Res.,99,775-789 ,1994.Wikramanayake,P.N.,Velocityprofilesandsuspendedsedimenttransportinwave-currentflows.Ph.D.Thesis.Dept.ofCivilEng.,MIT,Cambridge,Mass.,1993.

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