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  • PooleBayNearshoreBeachReplenishmentTrialPremonitoringreport

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    PooleBayNearshoreBeachReplenishmentTrial

    Premonitoringreport

    DrTravisMason&JonEvans,ChannelCoastalObservatory

    21May2015

    ListofFigures

    Figure1:Laserscanner..........................................................................................................................4Figure2:5yeardifferencemodel..........................................................................................................5Figure3:Orthophotographyofbeachtrialarea,2001to2013...........................................................6Figure4:Profile5f00509.........................................................................................................................8Figure5:Profileenvelope5f00509.........................................................................................................8Figure6:BeacherosionatSandbanksfollowingwinter2013/14storms.............................................9Figure7:Swathbathymetry,HI1366(CrownCopyright).....................................................................10Figure8:Bathymetryprofile5f00509..................................................................................................11Figure9:Bathymetryprofileenvelope5f00509..................................................................................11Figure10:Longertermchangeinbathymetrycrosssectionalarea(trialarea).................................12Figure11:Longertermchangeinbathymetrycrosssectionalarea(PooleBayfrontage).................13Figure12:Seabedsedimenttypes(westernPooleBay)HI1366(CrownCopyright)...........................15Figure13:Seabedsedimenttypes(trialarea)HI1366(CrownCopyright)..........................................16Figure14:ParticleSizeAnalysisforsampleBP09................................................................................17Figure15:DatawellDirectionalWaveriderbuoyoffBoscombe..........................................................18Figure16:LocationofBoscombeDirectionalWaveriderbuoy...........................................................18Figure17:WaveroseforBoscombeDWR...........................................................................................19Figure18:StormcalendarforBoscombeDWR...................................................................................20Figure19:HsexceedanceatBoscombeDWR.....................................................................................21Figure20:WaveRadarREXonSwanagePier.......................................................................................21

  • PooleBayNearshoreBeachReplenishmentTrialPremonitoringreport

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    1. INTRODUCTION

    Anextensivemonitoringprogramme is inplace toassess theevolutionofa trial seabed rechargeproject. Theprimarypurposeofthemonitoring istoestablishwhethermaterial fromtheseabedrechargemovesonshoretothebeach,butakeysecondaryaimistodeterminethepotentialimpactsofthedepositionuponsensitive/protectedmarine features,particularlygiventheproximityofthePooleRocksMCZ. The trialmonitoringprogrammecomprises the followingactivities to track thedispersalofthedepositedmaterial:

    beach(topographic)surveys swathbathymetry(multibeam)surveys seabedtracerstudy wave,currentsandturbiditymonitoring tidalmonitoring siltmonitoring

    However,an important facetofthenearshore trialmonitoringprogrammewillbe toexamine theresultsincontextwithlongertermnaturalchanges,bothintermsofthetransportratesandpathwaysoftherecharge,andinthehydrodynamicconditionsexperienced.AnextensivedatabaseofmorphoandhydrodynamicconditionsinPooleBayisavailable,datingbackto2002.Theaimofthisreportistoanalyseandquantify longerterm (~15years)environmentalconditions,basedondataexistingpriortothestartofthetrial.Thereportwillfocuson:

    sedimentmobility sedimenttype hydrodynamicswaveclimateandtidalregime

    2. EXISTINGDATA

    AsummaryofthepretrialbaselinedataisgiveninTable1.Mostofthepreexistingdatahasbeencollectedaspartofthenationalnetworkofregionalcoastalmonitoringprogrammes,and is inthepublicdomainasOpenGovernmentLicencedataviatheChannelCoastalObservatoryswebsite.Thelevel of detail of all surveys is consistent and all are carried out to a rigorous specification andsubsequentlyqualitycontrolled.

    Topographicdata includesbeachsurveysconductedatatemporal intervalofapproximatelyeverythreemonths,andincludespoststormsurveys.Singlebeambathymetrysurveyshavebeencarriedout annually. An extensive swath bathymetry (multibeam) survey was carried to InternationalHydrographicOrganisationOrder1astandardbytheMaritimeandCoastguardAgency(MCA)aspartoftheUKCivilHydrographyProgrammein2012;thesedataaremadefreelyavailablebytheMCA.

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    Area Surveytype Standard,coverageandresolution Comments

    Intertidalbeach

    Beachsurveys

    RTKGPSsurveysorlaserscans(2profilesurveysplusonebaselinesurveyfromwhichaDTMcanbederived).Crossshoreprofilesareat100mintervals.SurveysextendfromtheseawalltotheMeanLowWaterSpringscontour.Includespoststormprofilesurveys

    3surveysperyear,2003topresent

    Lidar 1mresolution,shorewardofMeanLowWatercontour3yearlysince2003,latestsurvey2013/14

    Aerialphotography

    10cmresolution,shorewardofMLWScontour

    2001,2002,2005,2008and2013

    Seabed

    Swath(multibeam)bathymetry

    IHOOrder1a(navigationsafetystandard),100%seafloorcoverage,1mresolution

    ConductedforMaritime&CoastguardAgency,2012

    Depositiontrialarea 2013Singlebeambathymetry

    Crossshoreprofilelinesat50mintervals Annually,2002to2014

    Substrate

    Surficialseabedsedimenttype,derivedfromswathbathymetrysurveyplusbackscatterandsedimentsampling

    2012,2013,2014

    Diverecords InnerPoolePatchRocks UniversityofSouthamptonSiltdeposition

    Hydrodynamics

    Waves DatawellDirectionalWaveriderMkIII July2003topresent

    Tides

    NationalNetworkAclassgauge,bubbler,15minuteelevationsplusresiduals

    1996topresent,managedandqualitycontrolledtoEuropeanSeaLevelServicestandardsbyNTSLF

    GLOSSstandardWaveRadarREXonSwanagePier,10minuteelevationsplusresiduals

    2007topresent,managedandqualitycontrolledtoEuropeanSeaLevelServicestandardsbyCCO

    Table1:Summaryofpretrialbaselineinformation

  • PooleBayNearshoreBeachReplenishmentTrialPremonitoringreport

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    3. SEDIMENTMOBILITY

    3.1 BeachtopographyTopographicsurveyshavebeencarriedouteitherusingafixedbase laserscanner(Figure1),orwithRTKGPS,based on the Environment Agency's Specification fortopographicsurveys. Thescannercapturesmillionsofdatapointswithinaradiusofabout150m.Thescannerismovedalongthebeachtoensurethattherearenoblind spots in data capture. The beach structures,backingcliffsordunesarealsocapturedingreatdetail.Thelasercannotreliablycollectdataoverawetsurface,so the beach area close toMLWS is captured by asurveyortakingcontinuousRTKGPSmeasurements,ataminimum intervalof5m,and includingallbreaksofslope.TheaccuracyofboththelaserscanandRTKGPSsystemsis:

    Plan 15mm Vertical 15mm

    Theresultingpointcloudofthousandsofdatapoints isdownsampledtoproduceaDigitalTerrainModel(DTM)ataresolutionof1m.ArecentDTMcanbesubtractedfromanearlierDTMtoproduceadifferencemodel,quantifyingtheelevationchangesbetweenthetwosurveys.Anexampleof5yearchangesalongthis frontage isshownatFigure2,wherethedifference inelevation iscolourcoded,redrepresentingerosionandblueaccretion.Notethatalthoughverticaldifferencesof15mmcanbeidentified,inpractiseonlyelevationdifferencesof25cmcanrealisticallybeconsideredasanythingotherthannoise,giventheinherentdailychangesinthebeachcausedbypeoplewalkingonthebeachandotherhumanactivity.

    Figure1:Laserscanner

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    Figure2:5yeardifferencemodel

    Furthermore,thereareextensivebeachmanagementoperationsalongtheentirePooleBayfrontage,witharesultthatitisverydifficulttoassesspurelynaturalchange.Forexample,approximately1.1millionm3of sanddredged fromPooleHarbour channels and approacheswasused to replenishbeachesatSwanage,PooleandBournemouthduringthewinterof2005/2006.ThePoolefrontagewas recharged with 450,000 m3 of sand in 2006 with topup recharge of 139,000 m3 inNovember/December2014.ThemanagementactivitiesonthebeachcanbeseenclearlyinFigure3,whichshowstheaerialimageryofthebeachtrialareasince2001.ThenetvolumetricchangesalongthebeachtrialareasaregiveninTable2,forsurveysafterthemajorrechargein2006.Sedimenttransportvolumesandratesarecalculatedoveranareacommontoallsurveys.ThenetvolumetricchangesgiveninTable2representbroadlyannualvariations(sincethefullbaselinesurveystakeplaceonceperyearusuallyinthespring)andthusmaydisguisevariationsonaseasonalorshortertimescale.Theannualbaselinesurveysaresupplementedbybeachprofilestwiceayear,typically inthesummerandautumn (the locationoftheprofiles issuperimposedonFigure 10 below). Using the profile survey results, the changes in crosssectional area can becalculatedrelativetoaMasterProfilewhichboundsthemeasuredprofiletypicallyrepresentedbytheseawallatthebackofthebeachandtheelevationofMeanLowWaterSpringsbeingtheseawardboundary.Figure4andFigure5showatimeseriesofsurveysatProfile5f00509(theclosesttothedepositionarea),Thelengthyperiodofstormsduringthewinterof2013/2014leftthebeachdenudedofsand(Figure6).

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    Figure3:Orthophotographyofbeachtrialarea,2001to2013

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    Surveydates No.days

    Netvolumetricchange(m3)

    Netvolumetricchangeperday

    (m3day1)

    Sedimenttransportrate(m3m2day) Comments

    28/06/2006 20/06/2007 358 33,776 94 0.0006 RTKGPSsurveys

    20/06/2007 12/06/2009 724 43,261 60 0.0004 RTKGPSsurveys

    12/06/2009 17/06/2010 370 12,026 33 0.0002 RTKGPSsurveys

    17/06/2010 0