The Design of a Slotted Vertical Screen Breakwater.pdf

8/14/2019 The Design of a Slotted Vertical Screen Breakwater.pdf

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CHAPTER38THEDESIGNF SLOTTEDVERT ICALSCREENREAKWATER

J Gardner*, I Hownend*and Fleming**INTRODUCTIONThispaperdescribesChewavestudies,modeltestingandstructuraldesignofa250metrelongslottedverticalscreenbreakwaterforamarinaatPlymouth,England.Themarinai sbeingdevelopedbyPlymouthCityCounciltoprovideapurposebuiltfacilityforhostingthemajorlongdistanceracesthatstartorfinishatPlymouth,inadditiontotheusualmarinafacilitiesforprivateboatowners.AfterexaminingthreealternativelocationsPlymouthCouncilselectedasiteinthenorth-eastcornerofPlymouthSoundasshowninFigure1 .Thesiteisconfinedontwosidesbyexistingshippingchannelsandbyarockyshoreonthelandside.Exposuretowaveattacki slimitedtothesouthwestsector.BecausetheexistingseabedatthemarinesitewastheresponsibilityoftheDuchyofCornwall,anActofParliamentwasrequiredbeforePlymouthCouncilcouldcommenceconstruction.TheActcontainsclausesregulatingtheuseofthemarinaandthepermissablechangestothewaveconditionsintheadjacentshippingchannels.DESIGNCRITERIAThebasicmaritimecriteriagoverningthedesignare:-(i) WaveheightsinthemarinashouldnotexceedHs=0.3metresfor

morethan200hoursperyear.(ii) Withtheexceptionofa10metrecarriagewayontheSWfaceofthebreakwaterwaveheightsintheadjacentshippingchannels

shouldnotbeincreased,asaconsequenceofthebreakwater,bymorethan50 abovetheconditionsexistingbeforeconstruct ion.

(iii)Thebreakwatershouldbedesignedtowithstandthe100yearreturnperiodstormoccurringatanytidelevel.

* SeniorEngineer,SirWilliamHalcrow PartnersLtd,BurderopPark,UK

**Director,SirWilliamHalcrow PartnersLtd,BurderopPark,UK

1 8 8 1


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1882 COASTALENGINEERING-1986

oa. o


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SLOTTEDVERTIC LSCREENBRE KW TER8 8 3Thepreparationofconstructiondrawingsandtenderdocumenswasnotstarteduntilparliamentaryapprovalhadbeengiven.ThiswasobtainedinApril1985andPlymouthCouncildecidedthatthemarinashouldbereadyforhostingtheTwoHandedTransAtlanticRaceinJune1986.Toachievethisprogrammedetaileddesign,testingandtenderdocumentshadtobepreparedintheperiodMaytomid-Augustsothattenderscouldbereceivedbymid-SeptembertoallowconstructiontocommenceinNovember1985.EXISTINGWAVECONDITIONSDeepwaterswellfromtheEnglishChannelca nonlyreachthemarinaareafromthesouth-west.Themostseverewaveconditionswillthereforecomprisecombinedswellwavesandlocallygeneratedwindwavesarrivingfromthisdirection.DatafordeepwaterwaveconditionswereavailablefromtheInstituteofOceanographicSciences(IOS)intheform ofareportondatacollectedbyaWaveriderbuoylocatedneartheEddystoneLighthouse.Thedeepwaterwaveclimatewastransferredtothemarinasiteintwostages.(i) waverefractionbacktrackingandspectrumtransferfromdeep

watertothePlymouthSoundbreakwater.(ii)diffractionoftheresultingwaveenergyspectrumfromthe

PlymouthSoundbreakwatertothemarinasite.WaveheightsandperiodsforlocallygeneratedwaveswerecalculatedusingtheCarterEquationsderivedfromtheJonswapenergyspectrumformulation.WindspeedscalculatedfromdatarecordedatFortStanford,Plymouthandtheequationsusedforafetchlimitedgrowingseawere:

Hs=0.0163X-5UTz=0.439X- U-4whereXistheeffectivefetchinkilometresandUisthewindspeedinmetrespersecond.Theresultingwaveheightsatthemarinasitewereasfollows:ReturnPeriod Hs m) Hs m) Hs m)deepwater) local) combined)

1.5 0.8 1.71.4 0.75 1.61.2 0.7 1.41.1 0.6 1.3

1005010

ExaminationoftheAdmiraltyChartsforPlymouthshowsthattheapproachdirectionfortheswellwavesi slimitedbyPenlessPointtothewestandthePlymouthSoundandMountBattenBreakwaterstotheeast.Approachdirectionsforthecriticalwaveconditionarethereforelimitedtothesectorbetween210and220.


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NEWBREAKWATER L A Y O U T

r

Figure2 Outline la nfornewmarina


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SLOTTEDVERTIC LSCREEN RE KW TER8 8 5UndercriticalwaveconditionsthewaveclimatetothewestandsouthofthemarinawillbeinfluencedbyshelteringanddiffractioneffectscausedbythecoastlineadjacenttoFisher'sNose(seeFigure2) .

Thetreatmentofwavediffractionwa sbasedontheSommerfeldsolutionaspresentedbyWiegel(1964).AHalcrowcomputermodelwa susedtoevaluatewaveheightfactorsatthenodesofasquaregridacrosstheareaofstudy.Waveheightfactorsbeingdefinedas:waveheightfactor=aveheighta tnode

heightofincidentwaveThemodelcouldbeusedtoevaluatewaveconditionsinthevicinityofsystemsofoneortwoimpermeablebreakwaters.Thebreakwaterscouldalsoassumeanydegreeofreflectionofincidentwaveenergy.TheshelteringeffectofFisher'snosewa srepresentedbyabreakwateralignednorth/south.TheresultingwaveheightfactorsaregiveninFigure3wherethefactorsareexpressedrelativetotheincidentwaveheightoffFisher'sNose.WAVECONDITIONSAFTERCONSTRUCTIONOFTHEMARINABREAKWATERThewavediffractionmodelwasrunforeachsegmentofthebreakwaterandtheresultsweresuperimposedtoprovideacompositediffractionsolution.Thusarelativelysimpleprogramprovidedaquickandrealisticanswer.Th eresultsforafullyreflectingbreakwaterareshowninFigure4 .ComparisonofFigure3withFigure4givesthepercentagechangeinwaveheightfactorsresultingfromconstructionofthebreakwater.Theseinitialresultsshowedthatwaveheightswouldincreasetoanunacceptablelevelinthetwoadjacentshippingchannels.Thewavediffractionmodelwa sre-runusingbreakwaterreflectioncoefficientslessthanunityandi twa sfoundthatacceptablewaveconditionscouldbeachievedwiththefollowingreflectioncoefficients:

SectionA-KR


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FishersNose

Figure: Wave eightfactorsorexistingsite////


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SLOTTEDVERTIC LSCREENBRE KW TER8 8 7Howeverthissitehasthreemajorconstraints:theseabedconsistsofsoftsiltupto30metresthickoverlyingbedrock;theareaavailableforthemarinaissmall;thespringtidalrangeis5metres. Averticalscreensupportedbypilesdriventobedrockwasconsideredtobethemostpracticalandeconomicsolutionwithintheseconstraints.Followingtheinitialnumericaldiffractionandreflectionanalysisvariousconfigurationsofperforatedscreenswereexamined.Examinationofexistingliterature(refs1 ,3 ,4and5 )indicatedthatadoublescreenwouldberequiredforSectionBwhereasasinglescreenwouldbesufficientforSectionA.Theextentofthedoublescreenwasabalancebetweenwavereflection,wavetransmissionintothemarinaandconstructioncost.Althoughexistingcasehistoriesweresufficeinttodemonstratethatasolutioncouldbefoundtosuitthebasicdesigncriteriatherewa sinsufficientinformationtopermitdetaileddesigntobecarriedout.Thefinalconfigurationofthescreenswasthereforebasedontheresultsofasetofsimpleflumetests.MODELTESTINGTomeetthetightdesignprogramme,modeltestingcommencedatthestartofthedetaileddesignphase.Thisthenenabledthefinalstructuraldesigntotakeaccountofthetestresults.ModeltestingwascarriedoutatHydraulicsResearch,Wallingfordina45metrelongwindwaveflume.Thepurposeofthetestswastomeasurethereflectionandtransmissioncoefficients(KgandKf )forsingleanddoublescreenswhilstchangingthefollowingvariablesidentifiedduringtheconceptualdesignstage:(i)aveperiodsbetween3and1 2seconds(ii)creenporosityvaryingbetween8 and35(iii)pacebetweenscreensvaryingbetween5metresand15metres(iv)lotshorizontalorvertical(v)hegapbetweenthebottomscreenandtheseabed(vi)heeffectofchangingtheplankthicknessfrom300mmto

150(vii)creenstiltedorverticalThemodelscalewas1:15andrandom waveswereused.Wavemeasurementsweremadeusingthreewaveprobesandincidentandreflectedspectracalculatedusingananalysisprogram.Themethoddeterminesthereflectionandtransmissioncoefficientsforeachfrequencyconsideredintheincidentwavespectrum.Asthemodeltestsprogressedsomechangesweremadetothetestingsequencetakingaccountofthepreviousresultsandtherequirementsofthestructuraldesign,whichwa sproceedingsimultaneously,therebyavoidingtestsunlikelytoyieldausefulsolution.Inall54testswerecarriedoutandtheresultsaregivenbyHydraulicsResearchLaboratory(1986).TheprincipalresultsaresummarisedinFigures5 ,6and7 .


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COASTALENGINEERING-1986

Optimum Solution

WaveTransmission

WaveReflection

30 35Porosity

Figure5- ReflectioncoefficientKR)SransmissioncoefficientKT)Porosityorsingleverticalcreen1-0

16 orosity

5 6 7 8 9 10 12 3 14 15 16 17Distancebetweenscreensm )

Figure6- Reflectioncoefficient Distanceetweencreens


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SLOTTEDVERTICALSCREENBREAKWATER 1889

987

6

0-i

OptimumSolution

43

1r1 T15 20 305

Porosity35

Figure Reflectioncoefficient Frontcreendetailsorscreenpacingofm

8007 i ll

K2S] HBSKHffBB}

MH W S5- 5

N H W N 5 6 l


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Forthesinglescreen:(i) aporosityof8 gaveacceptablevaluesofKgandKf(see

Figure5 )(ii) theperformanceofhorizontalslotswassimilartothatof

vertical.Horizontalslotswerethereforeadoptedbecausethisorientationsuitedtheproposedconstructionmethod

(iii)a metregapbeneaththescreengaveunacceptablewavetransmissionresults

(iv) thehydraulicperformanceof150mmplankswassimilartothatof300mmplanks.

Forthedoublescreen:(i) adoublescreenwithafrontscreenwith16 porosityspaced8

metresfromasolidbackscreengavesuitablereflectioncoefficients(seeFigures6and7 )

(ii) theperformanceofhorizontalslotswassimilartothatofvertical.Horizontalslotswerethereforeadoptedbecausethisorientationsuitedtheproposedconstructionmethod

(iii)a metregap beneaththescreensgaveunacceptablewavetransmissionresults

(iv) thehydraulicperformanceof150mmplankswassimilartothatof300mmplanks

(v) tiltingthescreensdidnotshowanyimprovementoververticalscreens.

FINALWAVESTUDIESUSINGMODELTESTRESULTSThenumericaldiffractionmodelpreviouslydiscussedwasre-runusingtheresultsfromtheflumetestasfollows:

avePeriod(sees) |BreakwaterSection |g K^ g Kf Kg K^ii r

|.6.3.6.4 .6.4(single,8 porosity)

.4.0.2.0 .4.0(double,16 porosity

mspace)I.9.0.9.0 .9.0(single,solid)


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SLOTTED VERTICAL SCREENBRE KW TER8 9 1Theresultsshowedthatwaveheightsoutsidethebreakwaterwerewithintherequiredlimitsexceptforonesmallareatothesouthofthebreakwater.I tshouldbenotedthatthediffraction-reflection-transmissionanalysisdoesnottakeaccountofwave-waveinteraction,wavebreakingandthelateralinternaldiffractionofwaves.Asthesenon-lineareffectswouldallleadtoareductioninresultantwaveheightsi twasdecidedthatthissmallareadidnotjustifyanychangesintheproposedbreakwater.TheresultsalsoshowedthattransmissionintothemajorityofthemarinahadbeenretainedbelowthetargetvalueofHs=0.3metres.Asmallareainthesouth-westcornershowinghighercontourswouldbedeletedbyanadditionallengthofdoublewavewall.STRUCTURALDESIGNThetypicalstructuraldetailsforthedoublescreenareshowninFigure8.Thesinglescreenissimilarexceptthattherearscreenandhalfofthedeckisdeleted. TypicalisometricviewsofbothtypesofscreenareillustratedinFigure9 . Thewaveloadsonthescreenaretransmittedtothebedrockbythepairsofrakingpiles.Themaximumcompressionandtensionloadsareapproximately2500kNand1800kNrespectively.Thehighcompressionloadsaretakenbyendbearingontherockandforthisreasonthepiletoesarepluggedwithconcrete.Thetensionloadsaretakenbydeadanchorsdrilledbeyondthepiletoesandgroutedintobedrock.Theverticalpilei sacompositepilewiththeuppersectioncomprisinganexternalsleevegroutedontotheinnerpileaftertheinnerpilehasbeendriventobedrock.Theexternalpileisaprefabricatedsectionwithflangesforwavescreenplankfixing.Theuseofthecompositepileallowedtheinnerpiletobedesignedfortheforcesbelowthebedlevel,withthegroutedsleeveabovebedlevelprovidingtheadditionalstiffnesstocarrythehighmomentsinducedbythewavescreen.Thisarrangementalsohelpedtoeliminatealignmentproblemswiththeplankfixings.Thescreenplankingmaterialisgreenhearttimberandthedecki sconcrete.Handrailsandlightsareprovidedonthedecktoprovideapublicpromenadeandviewingarea.Thecostofthebreakwater,excludingpontoonsandothermarinaitems,i sapproximately1.8million,ofwhichapproximately65 i sthecostofthetubularsteelpiles.TheContractorfortheprojectwasDeanandDyball(Western)LtdfromExeter.Figure10showstheprojectnearingcompletion.


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Marina Doublescreenprovides-1aximumprotection Singlecreenconstruction

Figure Typicalviewsofsingleanddoublecreenbreakwater

Figure0: MarinaNearingCompletion


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SLOTTEDVERTIC LSCREENBRE KW TER8 9 3CONCLUSIONBasedonvisualinformationduringthe4monthperiodsincethebreakwaterwa scompletedwavereflectionintotheshippingchannelsappearstobewellwithinthepermissiblelevels.Withouttheuseofaslottedverticalscreenthemarinaprojectwouldnothavebeenallowedtoproceed,butusingtherelativelyinexpensivemethodsofanalysisandtestingdescribedinthispaperapracticalandaestheticallyacceptablesolutionhasbeenfound.ACKNOWLEDGEMENTTheauthorsareindeptedtotheCityEngineer,Plymouthforhispermissiontopublishthispaper.

REFERENCES1 .utchinsonPS ,RaudkiviAJ ,Casehistoryofaspacedpile

breakwateratHalfMoonBayMarina,Auckland,NewZealand,ProcCoastalEngineeringConf,ASCE,Houston,Sept1984.

2 .ydraulicsResearch,Wallingford,1985,Plymouthmarinaeventsbase-perforamnceofwavescreens,ReportNoEX1327.

3 .akunoS ,1983,Reflectionandtransmissionofwavesthroughverticalslittypestructures,ProcCoastalStructures83,ASCE,Arlington,March1983.

4.akunoS ,1984,Water-wavereflectionandtransmissionbyaninfitearrayofverticalflatplates. Proc32ndJapanNationalCongressforAppliedMechanics,UniversityofTokyoPress.

5 .ekckmannJ ,BighamGN ,DixonR0,1983,Reflectioncharacteristicsofawave-absorbingpier,ProcCoastalStructures83,ASCE,Arlington,March1983.

6.iegelRL ,1964-OceanographicalEngineering,rentice-HallInc.

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