2799-12055-1-Pblongshore Transport of Suspended Sediment

7/28/2019 2799-12055-1-Pblongshore Transport of Suspended Sediment

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CHAPTER 56

LONGSHORETRANSPORTOFSUSPENDEDSEDIMENTby

JohnC .Fairchild*

ABSTRACTInexcessof800suspendedsedimentsampleswerecollectedfromstations

alongth eCityPier,Ventnor,NewJerseyan dJennettesPier,NagsHead,NorthCarolinausingatractor-mountedpumpsampler.ostsampleswerecollectedwithinthesurfzoneattheVentnorsite.tth eNagsHeadsite,samplecollectionsincludedthesurfzone,butgenerallyextendedoverawiderrangeofth enearshorezone.veragesamplingtimewas3minutes.Nozzleelevation variedfrom3inchesaboveth ebottom uptoamaximumaboutmid-depth,generallynotgreaterthan2.5feetabovebottom.aximumconcentrationsatVentnorrangedupto2.6pptby weightan datNagsHeadwereabout4.0ppt.ediansizeatVentnorrangedfrom0.12to0.15 mmandaveragedabout0.20mm indepthsof4feetan dlessatNagsHead.Resultsaresummarizedinaseriesofscatterplotswhichrelatesuspendedsedimentconcentrationtonozzleheight,waveheight,waterdepthan dsamplingdistancefromanobserved wave-breaker-line.esultsar ecomparedtoCERClaboratorydata,totwoexcerptedconcentrationsfrom unidirectionalflowtestsandtotheCERCTR-4designcurveoflongshore wav eenergyversuslongshoretransport.Introduction

Designcriteriaforproblemsinvolvingsedimenttransportby waveactioncouldbegreatlyimprovedbydevelopmentofempiricalrelationshipsfromfieldmeasurementscoveringa widerangeofwaveconditions. Inth eabsenceofadequatedataorpredictivetechniques,th ecoastalengineerhasreliedonhistoricalcompilationsofdredgingandsurveyrecordstoobtainlongshoretransportestimates. Bettertechniquesar eneededan dtherefore,fieldandlaboratorystudiesofsedimenttransportar eneededtogainaddedknowledgeofnaturalbeachprocesses,leadingtobetterpredictionsoflongshoretransportrates. Thispaperreportsonsomedataonsuspendedloadsgatheredinbothoceanandlaboratorywaves.

Longshoretransportofsuspendedsedimentisoneofth etwobasicmodesof waveinducedsedimenttransport,theother modebeingbedloadtransportwherebyth ebedsedimentispushedorrolledalongth enearshorebottombywaveaction,andtheresultingshearstressinth ebottomboundarylayer.Th etotalamountofsuspendedmaterialmovingalongagivenshoreoveragiventimedependsonth ecomplexinteractionsofseveralfactors,includingwaveconditions,waterdepth,beachslope,characteristicsofth ebeachmaterialsandtotalrangeofthetide. Importantamongth eseveralinter-

*ResearchHydraulicEngineer,CoastalEngineeringResearchCenter,Washington,D .C .

1069


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1070 COASTALENGINEERING actionsisthegeneration ofth elongshorecurrentduringthe wavebreakingprocess. Thiscurrenttransportslargeamountsofbottommaterialsforcedintosuspension bywave-generatedcurrentsan dturbulence.

Anearlyestimateofth elongshoretransportofsuspendedsedimentwa smadeby Watts( 1 ) ,usingpumpsamplingsofsuspendedsedimentfromapieratPacificBeach,California.pdatedestimatesofth elongshoretransportofsuspendedsedimentmay bemadefrom fieldan dlaboratorydata,includingsuspendedsedimentconcentrationsandrelated waveparameters,now on handatCERC( 2 ) . ThisdatawasobtainedattwoAtlanticcoastsites,andinth eCERClaboratorywavefacilitiesusing pumpsamplingtechniques.

Theparagraphs whichfollow brieflydescribethefieldsampling,comparethefieldresultstoCERC-laboratorydataandtotw oexcerptsofdatafromunidirectionalflowtests,an dfinally,comparecomputedlongshoretransportratestotheCoastalEngineeringResearchCenterTR-4designcurveoflong-shoretransportversuslongshorewaveenergyflux.FieldSampling

Inexcessof800suspendedsedimentsampleswerecollectedfromstationsalongtheCityPier,Ventnor,NewJerseyandJennettesPier,NagsHead,NorthCarolina,usingatractor-mountedpumpsampler( 3 ).igure1isaschematicillustration ofthesampler onafishing pier.ostsampleswereobtained

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LONGSHORETRANSPORT 1071withinth esurfzoneatVentnor,N.J .under varyingintensities(typesan dproximities)ofwavebreaking.agsHeadsamples werecollectedinthesurfzone,butgenerallyextendedoverawideranddeeperrangeofth enearshorezone. Figure2isaschematicillustrationoftheareasofinterestinth esamplecollectionswhichshowsasurfzone,aswashzoneandanoffshorezone.Averagesamplingsof3 minutestimeweremadefromaheightof3inchesabovethebottomuptoamaximumheightataboutthe mid-depthlevel.achsample,asinitiallypumpedthrougha1/2-inchintakenozzle,wasa40gallonvolumeofwater-sedimentmixture whichwasdecantedinthefieldtoasample-sizequantityofwetsand.aboratoryanalysisan ddatareductionatCERCyieldedasuspendedsedimentconcentrationandasizedistributionforeachsample. Inthe Ventnordata,maximum concentrations wereabout2.6pptby weight(equivalentto3 80grams(.84lb)ofsandin40gallonsofseawater)an dminimum concentrationswereoftheorderof.025ppt(lessthan4gramsin40gallons).ntheNagsHeaddataconcentrationsabove1.0pptby weightoccurred morefrequentlywithtw omeasuredvalueswhichwereabove4ppt.However,there wasalsoagreaterpercentoccurrenceofconcentrationsbelow.0 1pptintheNagsHeaddata.ediandiameterofthesuspendedsedimentrangedfrom.1 2to.1 5mmforthe Ventnordataan daveraged0.20mm in depthsof4feetorlessatNagsHead. Indepthsof8to12feetatNagsHead,mediandiameterofsuspendedsedimentsamplesaveragedabout0.17mm.

OFFSHOREZONE

"Outer ar (Inner Bar,ow Tide

FIGURE:CHEMATICROFILEF URFO N E


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1072OASTAL ENGINEERING DiscussionofResultsfromSampleCollections

Table1liststh eprincipalvariablesconsideredinthisstudy. Theconcentration,C isconsideredtobeadependentvariabledetermined byan unknownfunctionofelevation,depth,waveheight,anddistancefromth ewave-breaker-line

Cw=f( EB,d ,H,SB) .ThevariablesQand9 ,areusedinthefinalsectionofthispaper.

Table1 .EFINITIONOFVARIABLESCsuspendedsedimentconcentration(pptbywgt)ER-elevationofsuction nozzle(ftabove bottom)dwaterdepthatsamplingstation(ft)Hsignificant waveheight(ft)S , ,-distancefromthewave-breaker-line(ft,+islandward)i > 3Qlongshoretransportrate(100,000yd/yr9 ,-anglebetweena wavebreakingcrestandtheshoreline(designated

of einFig.13 ,CERCR-4)TheresultsfromthesamplecollectionsatVentnor,N.J .an dNagsHead,

N .C .aresummarizedinaseriesofscatterplotswhichrelatesuspendedsedimentconcentrations,C in partsperthousand(ppt)by weighttothew4independentvariables. Sedimentconcentrationispresentedintheseveralfigureswhichfollowbysamplingstations,andbyallstationsplottedtogetherongeneralscatterplots. Th esamplingstationscatterplotsshowdatacollectedataspecificstationonth epierusuallywithinaperiodof3 hoursorless. Th egeneralscatterplotsar erepresentativeofallthedatacollectedandsomay have moresignificancethandatacollectedataspecificstationan doverperiodsof-say2to3 hours.

Figures3and4ar esinglestationscatterplotswhichshowth everticaldistributionofsuspendedsedimentconcentrationfortw opierstationsatVentnor,andtw opierstationsatNagsHead,respectively.otethatth eordinateineachfigurerepresentsconcentrationsofsuspendedsediment,Cinpptby weightandthattheabscissarepresentssamplingnozzleheights,E ,infeetabovetheoceanbottom.wosetsofpointsar eplottedintheright-sidegraphinFigure3 ,withth eopencirclesrepresentingsamplingsbefore1200hoursan dthefilledcirclesrepresentingsamplingsafter1300hours. It wasobservedduringthesamplingsthattherewasanabruptincreaseinthewaveactivityatabout1300hoursandthisgraphshowshowquicklyth econcentrationlevelrespondstoth eincreased wav eheight,an dhowmuchflatterth etrendisforth ehigherwaves( H1.82ft).


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LONGSHORE TRANSPORT0 7 5Also,noteth econsiderabledifferenceinth esteepnessofthescattertrendsinFigure4forth eNagsHeaddata.asedonastudyofnumerousplots,itwasfoundthatahigher waveormoreintense waveactivity wasusuallyassociated withth eflatterdistributiontrends.

Figure5relatesconcentrationofsuspendedsedimenttothedistanceofthesamplingstationfrom anobserved wave-breaker-lineforth eVentnordata.Th etopgraphrelatessuspendedsedimentconcentrationtodistancefrom amoving wave-breaker-lineforasinglestationduringaneighthourperiod.Thebottom graphshowsthesametypeofrelationshipbu tforshorterperiodsoftimeonthreeseparatedates. Basedonth eauthor'sexaminationofth eeffectofotherfactorssuchaswaveheight,waterdepth,nozzleheight,itishisjudgementthatthetypeofwavebreaking(surging,plungingorspilling)iswhatcausedth eparticulardistributionofth ethreedatagroupsshowninthisscatterplot.

Thenextthreefigures,6 ,7and8are morerepresentativeofal lthedatacollectedthanth elastthreefigureswhichgaveresultsfordatacollectedatasinglestationandgenerallyovertimesoftw oorthreehours.Th efirstofthese,Figure6 ,relatessuspendedsedimentconcentration,Cinpptbyweight,todistancefromthe wave-breaker-line,S fortwonozzleheightrangesaboveth eocean bottomatVentnor,N .J . Thenextscatterplot,Figure7 ,showstherelationshipofsuspendedsedimentconcentration,Cotheparameter,H/d,waveheightover waterdepth.otethatth eleftgraphinFigure7showstheresultsforthe Ventnordataandth eright-sidegraphshowsresultsforth eNagsHeaddata.igure8concludesthisphaseofth eresultswith anadditionalgraphrelatingsuspendedsedimentconcentration,C toth eparameter,H/d,waveheightoverwaterdepth,forsamplescollectedabove0.5feetan dlessthan1.0feetaboveth eocean bottom. Th elegendoftheleftsidegraphforthe Ventnordatadividesth edataintotw oclasses,oneclassforsamplescollectedseawardofthe wave-breaker-line,anotherclassforsamplescollectedshorewardofthe wave-breaker-line.otethatth eright-sidegraphforth eNagsHeaddatainFigure8appearstohaveagapinthedatascatterappearingjustaboutwherethepeakoccursinth edatascatterfortheVentnordata,leftgraphinthefigure.

TheresultspresentedinFigures5 -8maybeexplainedasfollows:stheshallowwaterwavesapproach breaking,theconcentrationofsuspendedsedimentincreasessharplyfrombarelynon-breaking wavestofullybreakingwaveswithshorewardadvancean dwithincreaseinH/d(waveheightoverwaterdepth). Scatterconfigurations(NoteFigures7and8 )indicatethatth esuspendedsedimentconcentration peaksnearth ewavebreakerindex(H/d=0.78)(4)an dthatconcentrationsdecreasegradually withtheshorewardadvanceofspillingtranslatory wavesandwithfurtherincreaseinH/d.

Thenexttw ofigurespresental ldatacollectedinthesefieldstudies..Thefirstofthese,Figure9 ,relatessuspendedsedimentconcentrationto(waveheightsquared).hesedatashowvery muchscatter. In notingth escatterinthisfigure,itmustberecognizedthattheplottedconcentrations werecollectedatarangeofelevationsaboveth eoceanbottom-Erom0.25to2.50feet. Scatterplotspresentedearlierhaveindicatedthatthiselevationvariablesignificantlyaffectsthetotalscatterinthegraphs.hescattertrendinth eto pgraphinFigure9appearstochangenoticeablynearan2Hf1.5andtrendtowardtheupperright.


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LONGSHORETRANSPORT08 1Inthenextall-datascatterplot,Figure10 ,Csplottedagainstw

theparameter,H / E _ ,waveheightoversamplingnozzleelevationaboveth eocean bottom.hetotalscatterintheseplots,especiallythatfortheVentnordataontheleft,issignificantlylessthaninth elastFigureofC

2versusH ;thereducedscatterinFigure10resultslargelyfromth einclusionofE_inthedenominatorofth eabscissa,whichaccountsforth eeffectofuvariablenozzleelevationaboveth ebottom. By overlayingthetwosectionsofthisfigure,itwasobservedthattheVentnordatahasasomewhatsteeperslopethantheNagsHeaddata. Thismay beexplained byth efactthatth emediandiameterofth eVentnorsand(0.12-0.15mm)issignificantlylessthanthatofth eNagsHeadsand(about0.20mm),sothatth eVentnorsandismoreuniformlymixed,andbeingsignificantlysmaller maybesustainedinsuspensionatrelatively higherelevationsbyagivenenergyorturbulenceinput.

Particlesizeeffectsinrelationto wave-inducedsuspendedsedimentar esummarizedinFigure11 . Thetw oscatterplotsinthisfigurerelateth emedianparticlesizean dthecoarser5 thpercentileofth esamplestowaterdepthan dnozzleheightabovebottomfortheNagsHeaddata. Thesescattertrendsshowaverygradualdecreaseinmedianparticlesizewithincreaseinwaterdepth,an dalsowithincreaseinE ,nozzleheightabovebottom. ScattertrendsfortheVentnordata wereverysimilarbutanactualpointsplotfortheVentnordatawasnotincludedduetospacelimitations. Instead,asthelegendin Figure11shows,avisualbestfitcurvefromth eactualVentnordataisdisplayednearthebottomofeachgraph.

Figure12isacompositeplotofsuspendedsedimentconcentration versuswaveheightsquared,asindicatedforsamplingelevation,E


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1084 COASTALENGINEERING 1 1 1 1III 1 1 1 1 III 1 1 1 1 1 III 1 1 1 1II

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O SPTB, = 0.32~0.48ft.,T=.31e c. O 72ft.tonk, = 0.25-0.50ft,T=l.8sec.

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LONGSHORETRANSPORT08 5 conditionsareverydifferentfromthoseinth esurfzone.owever,muchdata,andempiricalandtheoreticaldevelopmentareavailablefrom unidirec-tionalflowstudieswhichwillprovideusefulconcepts,relationshipsandideastotestoutinsurfzonestudies.LongshoreTransportRateFromSuspendedSedimentData

Figure12 ,discussedinth etwoparagraphsaboveindicatedtherelation-shipbetweenrepresentativeconcentrationsofsuspendedsedimentan dtheirrespective waveheightsquared( a waveenergyparameter).furtheran dinterestinguseoftheCERCfieldan dlaboratorydataonsuspendedsedimentmaybe madebyapproximatinglongshoretransportratesforcomparison withthetransportratesindicatedbytheTR-4designcurveoflongshore waveenergyfluxversuslongshoretransportrate.(6)omputationsmay be madebyaformuladeveloped byGalvinan dincludedinhisabstractatth e12thConferenceonCoastalEngineering.(7)hisformulaisasfollows:2Q=DgcT($H, )in 9 , ,wheregisgravity,cissuspendedsedimentconcentrationinpptby volume,6isawavebreakerindexd,/H, ,takenas1.3,H ,iswavebreaker height,9 ,isth eanglebetweenawavebreakingcrestan dth eshoreline3an dDisacoefficientequalto11.68whichchangesft/secintounitsof100,000yd3/yr.

Seven valuesofthelongshoretransportrate havebeencomputed,usingGalvin'sformulaalong withtheircorrespondinglongshorecomponentsofwaveenergyatbreakingusingtheformula,Ea/wave=1/8P g H , ( 1-M~)sin9 ,co s9 ,IC '

LbInFigure13thesesevenratesan dtheircorrespondinglongshore waveenergiesar eshownasplottedpointsforcomparisonwithth eTR-4designcurveoflongshore waveenergyfluxversuslongshoretransportrate.

Wave breakeranglesusedinthecomputationswereobtainedfromanomographofd/Loversus0ndertheassumptionthatthedeepwateranglebetween wavecrestandshorelinecouldbeapproximated byanaveragevalueof30.avebreakerangles( 9 , )wereobservedinconjunctionwithth ecollectionofsuspendedsedimentsamplesatVentnor,N .J .aluesobservedrangedfrom3to10degreesforlo wheightswell waves(1-2ft)withwave periodof7secondsormore.BreakerangleswerenotobservedinthecollectionofsuspendedsedimentatNagsHead,N .C .arge wavetank valuesof9 .ar enecessarilyhypothetical,sincethisisatestfacility where wave breakerangle( 6 , )isactuallyzero.

Th eplottedpointsofcomputedlongshoretransportratesinFigure13comparereasonablywell withth esuggesteddesigncurve. ThedottedlineinthefigureparallelingtheTR-4designcurveextendsthecomparisontoincluderecentindications byInman( 8 )an dDas( 9 )thatthecurveshouldbechangedtoreflectrm svaluesofwaveheight.heTR-4( 6 )designcurve(solidline)isbasedonth esignificantwaveheight.heauthorrecognizesthatsignificantthough notunreasonableassumptionshavebeen madetoobtainthisplot.


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1086 COASTAL ENGINEERING = !MIII! 1llllll 1 I'M: m i n i Ililill 1 \k FieldDotaV e n t n o r ,N.J.,ag Head ,N . C .an d * _

- Miss ion o y,Calif. ~ ~ CERCargeWavean kData / - /// A -1 SPT0af o //////_ 'P/ _ ~ A '/ ~ 'A/- //// _ _ _ //V E E qu iv a l en tCurveus i ng // ^ u g g e s t e do rDesignse = rm s a lues \ '/ CERC,TR-4 _ _ V / \// ~ ~ = E 0-S-Numbero fWavese rDay) ~ = // (Sinbo sb KR -- // where E 0=DeepWaterWaveE nergy er - // Wav e, K R=RefractionCoefficient //= 'V * an d ngleetween reak ingWaveCres t = : an dheeach ~ ~ ~ // ~ / '///*~ s

I iIIIIII inn 1 Mlll 1 1 lllllll 1 ii o -3o -2tr1oO 1o 2E 0-Longshoren e r g ynMillions f t.Lbs.Pe rDayPe rf.f eachF I G U RE3.O N G S H O R ERANSPORTRO MUSPENDEDSEDIMENTATACOMPAREDOCERCTR-4

L O N G S H O R ERANSPORTCURVE SummaryndConclusionsThispaperhaspresentedasummaryofsuspendedsedimentdatafromtwoAtlanticCoastsiteswithimplicationsforitsuseincompilinglongshoretransportdesigncriteriawithinthecontextofthree main pointsasfollows:1 .iscussionofth eresultsfromthefieldcollectionsofsuspended

sedimentatVentnor,N .J .an dNagsHead,N .C . Theseresultsshowlargevariationinthesuspendedsedimentconcentration,bu tthatth econcentrationsdiddependonelevationabovebottom,waveheight,an dpositioninthesurfzone.

2 .omparisonofthesefieldresultswithearlierCERCfielddata( 1 )fromPacificBeach,California,and withCERClaboratory measurements( 2 )showedatrendwhensuspendedsedimentconcentration wasplottedagainstwaveheightsquared.hiscomparisonalsoincludedtw osetsofsuspendedsedimentcon-centration measurementsfrom unidirectionalflow( 5 )-oneinth eMissouriRiverandoneinalaboratoryflume,underth eassumptionthattoafirstapproximationth eflowdepthisanalogoustoth e wavebreakerheight.


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LONGSHORETRANSPORT08 73 .easonablecorrelationwasfoundbetweenlongshoretransportrates,

computedfromrepresentative valuesofsuspendedsedimentconcentrationan dwaveconditions,andtheTR-4designcurveofwaveenergyfluxvslongshoretransportrates.

Th edata an dresultspresentedanddiscussedappeartosupportthefollowingconclusions:

1 .ielddatasuggestthat3-minuteaveragesofsurfzoneconcentrationin wavesupto4feethigh,showsignificantvariationinconcentration withelevationaboveth ebottom. Th ehigherthelevelof waveactivitythelessconcentrationvarieswithelevationabovebottom,andhenceth eflatterthetrendinthedistributioncurve.

2 .catterconfigurationsalsosuggestthatfor wavesapproachingtheirbreakingdepth,theconcentrationofsuspendedsedimentrisessharplyjustbeforethe wavebreaks,peakingata waveheight-to-waterdepthratioofabout0.78(theoreticalbreakerindex). Concentrationthendropsoff moreslowlyshorewardofth einitialwavebreaking;

3 .oncentrationsmeasuredinthefieldcomparerealisticallywiththoseobtainedinCERClargean dsmallwavetanktests;4 .ediansizeofthesuspendedsedimentsamplesdecreasesgraduallywithincreaseinwaterdepthan dwithincreaseinsamplingelevationaboveth eocean

bottom. Thereissomeimplicationthataflatterdistributionofsandsize withnozzleheight,asfoundinthesmallersizeVentnorsand,maybeindicativeofhigherconcentrationsforsmallermoreuniformly mixedsands;

5 .nempiricalcheckonlongshoretransportratescomputedfromfieldandlaboratorydataonsuspendedsedimentcomparereasonably well withth eTR-4suggesteddesigncurve.

ACKNOWLEDGEMENTThispaper waswritten whiletheauthor wasamemberofth eCoastalProcessesBranch,Research Division. C .J .Galvin,Jr.,Chief,CoastalProcessesBranchreviewedthispaper.Datapresentedinthispaper,LongshoreTransportofSuspendedSediment,

unlessotherwisenoted,wereobtainedfromresearchconductedbyth eUnitedStatesArmyCoastalEngineeringResearchCenterunderth eCivilWorksresearchanddevelopmentprogramofth e UnitedStatesArmyCorpsofEngineers.ermissionofth eChiefofEngineerstopublishthisinformationisappreciated.hefindingsofthispaperarenottobeconstruedasofficialDepartmentofth e Armypositionunlesssodesignatedbyotherauthorizeddocuments.


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1088OASTALENGINEERING REFERENCES

1 .atts,G .M. ,"DevelopmentandFieldofaSamplerforSuspendedSedimentinWave Action,"BeachErosionBoardTechnical MemorandumNo.34,March1 95 3.

2 .airchild,J .C,"SuspendedSedimentSamplinginLaboratoryWaveAction,"Beach ErosionBoardTechnical MemorandumNo.115,June1 95 9.3 .airchild,J .C.,"A Tractor-MountedSuspendedSandSampler,"Coastal

EngineeringResearchCenterR.4-66,ReprintedfromShoreandBeach,June1966.

4 .airchild,J .C,"SuspendedSedimentConcentrationintheSurfZone,"Transactions Abstract(EOS),AmericanGeophysicalUnion,April1971.

5 .SedimentTransportationMechanics: SuspensionofSediment"ProgressReport,Task CommitteeonPreparationofSedimentation Manual,HY5 ,ASCE,Sept1 96 3.

6 .ShoreProtection,Planningand Design,"CoastalEngineeringResearchCenter,TechnicalReportNo.4 ,ThirdEdition,1 96 6.

7 .alvin,C .J .Jr.,"AGrossLittoralDriftRateFormula," Abstracts,12thConferenceonCoastalEngineering,1970.

8 .nman,D .L .an dJ .D.Frautschy,"LittoralProcessesan dth eDevelopmentofShorelines,"CoastalEngineering,1 9 6 6 ,ASCE,p .511-536.

9 .as ,M.M. ,"LongshoreSedimentTransportRates,"CoastalEngineeringResearchCenter MiscellaneousPaperNo.1-71,Sept1971.

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2799-12055-1-Pblongshore Transport of Suspended Sediment