Coastal erosion investigation and management options...

Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire

Disclaimer:

While this document has been prepared with care it contains general information and does not profess to offer legal, professional or commercial advice. The Queensland Government accepts no liability for any external decisions or actions taken on the basis of this document. Persons external to the Environmental Protection Agency should satisfy themselves independently and by consulting their own professional advisors before embarking on any proposed course of action.

© Environmental Protection Agency RE508 August05Visit us online at www.epa.qld.gov.au

�Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.

Contents�.0 Introduction ........................................................................................................................ 22.0 Study area and site description ........................................................................................... 33.0 Coastal erosion issue .......................................................................................................... 54.0 Data collection and analysis ................................................................................................ 7 4.1 Investigationoflocalcoastalgeomorphologyandimplicationsforpresentday coastalprocesses...................................................................................................... 7 4.2 Comparisonofhistoricalshorelinepositions............................................................10 4.3 Waterlevels.............................................................................................................13 4.4 Waveclimate............................................................................................................13 4.5 Sedimenttransport..................................................................................................13 4.6 Beachprofileanalysisandchanges..........................................................................15 4.6.1 Comparisonofsurveyprofiles..................................................................................15 4.6.2 Potentialshorelinerecessionfromfuturestormerosion............................................ 17 4.7 Effectsofstormwaterdischargeonbeacherosion....................................................185.0 Key findings on erosion ......................................................................................................�96.0 Erosion management options for South Mission Beach .......................................................�9 6.1 Managedretreat.......................................................................................................19 6.2 Beachnourishment................................................................................................. 20 6.3 Localsandrelocation.............................................................................................. 23 6.4 Revetments............................................................................................................. 24 6.5 Groynesandartificialreefs...................................................................................... 257.0 Conclusions ...................................................................................................................... 268.0 Recommendations ............................................................................................................ 269.0 Additional information ...................................................................................................... 27 9.1 Statutoryobligations............................................................................................... 27 9.2 Stateassistancewithfunding.................................................................................. 27�0.0 References ........................................................................................................................28

List of figuresFigure 1. Locality plan of South Mission Beach including marine park and other protected areas. ........................................................................................................... 4Figure 2. Morphostratigraphic units of the dune areas from the Hull River to WongalingBeach–1951photography.(source:EPA) .................................................... 8Figure3. SoilprofilesoftheTamO’ShanterPointdunefield.(source:EPA) ..................................9Figure4.HistoricalaerialphotosofSouthMissionBeach1951to2002.(source:EPA) ................. 11Figure5. Changesinshorelineposition1951to2002. ................................................................12Figure 6. Location and number of survey lines. ...........................................................................14Figure7. Beachprofileateachsurveylinerecordedon22September2002 ................................15Figure8. Typicalbeachprofiles..................................................................................................16Figure9. Distanceofthehighestastronomicaltideleveltothe-1.0mcontour atSeptember2002. ....................................................................................................16Figure10.Estimatedshort-termbeachprofileresponsetoa1in100ARIstormevent basedonVellinga1995). .............................................................................................18Figure11.Indicativedesignnourishmentprofilesbasedonsandnourishmentvolumes providedintable3tofillerosionalloss. ..................................................................... 22

List of photosPhoto1. KennedyEsplanadeshowingtherelativelynarrowwidthofundevelopedesplanade seawardoftheroad.(source:EPA) ............................................................................... 3Photo2. PoorlyconstructedrockwallatthesouthernendofSouthMissionBeach. (source:EPA) ............................................................................................................... 5Photo3. BeachconditioninJune2001followingseaerosion.(source:EPA) ................................ 6Photo4. BeachconditioninNovember2003followingseaerosion.(source:EPA) ....................... 6Photo5. BeachconditioninMay2004followingseaerosioninMarch2004.(source:EPA) ......... 6Photo6. NorthernendofSouthMissionBeachfollowingerosioninMarch2004. Notethebroadintertidalzoneandhigh-tidebar.Itwouldbeexpectedthatthisbar willmoveuptheprofileincalmweatherandrebuildthedunes.(source:EPA) ............. 10Photo7. Scourchannelthroughtheforedunecausedbystormwateroverflow,May2004. (source:EPA) ..............................................................................................................18Photo8. Rockandsoilfillmaterialontheesplanadeexposedbyduneerosion.(source:EPA)....20

Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.2

�.0 Introduction Since1971beacherosionhasbeenreportedattheverysouthernendofSouthMissionBeachinthevicinityoftheboatramp.Theerosionhasprogressivelyextendednorthandnowvisiblyaffectsthecoastlineforsome800mtothenorth.Theerosionisaffectingthenarrowstripofparklandandthescarpfaceisclosetothesealedroadway,whichistheonlylegalaccesstomanyresidentialproperties.

SouthMissionBeachhasbeenidentifiedintheCardwellHinchinbrookRegionalCoastalManagementPlan(EnvironmentalProtectionAgency,2003)asapriorityareaforerosionmanagement(seePolicy2.2.2Erosionproneareas).Thisinvolvesinvestigatingthecauseoftheerosionanddeterminingaschemeofworksthatcanbeimplementedwithinenvironmental,economicandsocialconstraints.

TheEnvironmentalProtectionAgencyEPA(andpreviouslytheBeachProtectionAuthority)andCardwellShireCouncilhavediscussedtheerosionissueonanumberofoccasionsovertheyears.In1999,theEPApreparedareportentitledManagementoftheDune/BeachRidgesystematSouthMissionBeachforCouncil.Thisreportrecommendedoptionsforthemanagementofthedunalareas,butbeacherosionwasonlydiscussedgenerally,anddidnotquantifytheproblemorgivespecificsolutions.

AsaresultofdiscussionswithofficersoftheCardwellShireCouncil,theEPAagreedtoundertakeastudyofthebeacherosionatSouthMissionBeachtoassistCouncilinmanagingtheerosionproblemalongthissectionofcoast.

Theaimsofthestudyareto:

• examineanddescribethecoastalprocesses occurringinthevicinityofSouthMissionBeach;

• identifythecauseandmagnitudeoftheerosion problem;

• discussthevariousstrategiesformanagingthe erosionproblem;and

• providerecommendationsonaspecificprogramfor addressingtheerosionproblemforconsiderationby CardwellShireCouncil.

3Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.

2.0 Study area and site description

SouthMissionBeachisthesouthern-mostsectionofacoastalplainextendingfromTamO’ShanterPointinthesouthtoClumpPointinthenorth.ThisbeachcompartmentalsoincludesWongalingBeachandMissionBeachand,forthisstudy,iscalledtheMissionBeachcompartment.SouthMissionBeachisalowlying2kmsectionofcoastlocatedbetweenarockyoutcropatitssouthernendandWheatley’sCreektothenorth.InthevicinityofWheatleysCreeksandhasaccumulatedinabroad,moderatelyconvexsalientcausedbytheshelteringeffectsofDunkIslandtothesoutheast.Thisactsasacontrollingfeatureonthecoast,andSouthMissionBeachhasformedasaslightlyindentedembaymentbetweenthispointandtherockyoutcropatthesouthernend.Alocalityplanofthesiteispresentedasfigure1(nextpage).

Thebeachprofileconsistsofabroadgentlyslopingintertidalareaofabout100minwidthbackedbya

narrowhightidebeach10mto20minwidth.BehindthisisarelativelylowandnarrowsandybeachridgeplainformingabarriertotheHullRiverestuary.SouthMissionbeachhasbeenextensivelydevelopedforresidentialpurposes.A30mwideesplanadehasbeendesignatedoveritsentirelength.TheoriginalforeduneandswalenowcoveredbytheEsplanadehas,formostofitslength,beengraded,filledanddevelopedforroadandpark,andistypicallyonly1mabovehighestastronomicaltidelevel(HAT).Theerosionscarpiswithin6mto10moftheconstructedroadinsomeplaces.Behindthisareaisanarrowbeachridgeplain,whichishigherandbroaderthantheforedune.Housingandotherresidentialinfrastructurehasbeenestablishedalongthesebeachridges.EstuarineandfreshwaterwetlandsassociatedwiththeHullRiverbackthiswesternsideofthebeachridgeplain.

Photo1.KennedyEsplanadeshowingtherelativelynarrowwidthofundevelopedesplanadeseawardoftheroad.(source:EPA)


Figure 1. Locality plan of South Mission Beach including marine park and other protected areas.


3.0 Coastal erosion issueThesouthernendofthebeachhasexperiencedaslowbutpersistenttrendoferosionforatleast30years.Followingconstructionofaconcreteboatrampin1968,localisederosionimmediatelynorthoftherampemergedasaproblemin1971.TheCouncilrespondedbyconstructingarockwallonthescarpfacetoprotecttheimmediatelyadjacentroadandtoiletfacility.Therockwallhasbeenprogressivelyextendednorthwardfollowingthemigratingerosionscarp.Therockwallappearstohavehadtheundesirableeffectoftransferringtheerosionproblemnorthwards.Therockwallpresentlyextendsapproximately600malongthebeachandapersistenterosionscarpisevidentforapproximately200mfurthernorthoftherockwall.

Theproblemwiththecurrenterosionmanagementstrategyisthreefoldasdescribedbelow.

• Therockwall,formostofitslength,hasbeenpoorlyconstructedintermsoffootingdepth,armorsizeandwallporosity,andformuchofitslengthconsistssimplyofdumpedrock.Itisconsideredthatitwouldneitherhaltthepresenttrendofcoastlinerecessionnorprovideprotectionagainsttheeffectsofaseverestormevent.Insomeplacesthewallhasslumped,smallerrockhasbeendispersedacrossthebeachandtheerosionscarphasretreatedbehindthewallalignment.

• Constructionofthewallontheerosionscarpmeansthatthereislittle,ifany,sanddunebufferleftbetweentherockwallandthebeach.Thisleavesthewallcontinuouslyexposed,whichreducesthevisualamenityofthisprimerecreationalasset.

• Theongoingconstructionandmaintenance,andtherequiredupgradingoftherockwallwillbeanincreasingfinancialliabilitytotheCouncil.Atthesametimetherecreationalvalueofthebeachwilldeclineasthebeachleveldecreasesandtheuseablebeachnarrows.

Photo2.PoorlyconstructedrockwallatthesouthernendofSouthMissionBeach.(source:EPA)


Photo3.BeachconditioninJune2001followingseaerosion.(source:EPA)

Photo4.BeachconditioninNovember2003followingseaerosion.(source:EPA)

Photo5.BeachconditioninMay2004followingseaerosioninMarch2004.(source:EPA)


4.0 Data collection and analysis

4.� Investigation of local coastal geomorphology and implications for present day coastal processes

TounderstandtheerosionproblematSouthMissionBeach,anoverviewofthegeomorphologyandthelarge-scalesedimenttransportpathwaysandcoastalprocessesintheregionwereinvestigated.

Holmes(1993)assessedthegeomorphologyanderosionvulnerabilityoftheMissionBeachcoastaspartofalargerregionalstudy.ThiscoastconsistsofanarrowsandplainextendingfromtheboatrampatSouthMissionBeachtoClumpPoint.ThiscoastalplainconsistsofbothPleistocenesandsdepositedduringaprevioussealevelhigh>100,000yearsbeforepresent(BP),andHolocenebeachridges,depositedupto6000yearsBP.Theselandformsaredetailedinfigure2.TheHolocenebeachridgesdirectlyonlapthePleistoceneridges.ThecoastalplainisnarrowestatSouthMissionBeachandwidestatWongalingBeach.SouthofTamO’ShanterPointisawideHolocenebeachridgebarrier.TheHolocenebarrierisbetween350mand460minwidthandextendsfromTamO’ShanterPointtotheHullRivermouthasacontinuousseriesofridges.Threemajorridgesoccuralongthelengthofthebarrier.TheHolocenebarrieronlapsPleistocenebeachridgeswhichextendsouthfromTamO’ShanterPoint.

Holmes(1993)indicatedthattheMissionBeachembaymentdidnothaveanexternalsourceofsandandcouldessentiallybeconsideredapocketbeachwithafinitesandresource.ThiswasbasedonthefactthattheTullyRiveristheonlypossibleexternalsourceofsedimentandthatnotransportpathwaycouldbefoundaroundTamO’ShanterPoint.ItwasalsodeterminedthattheMissionBeachsandhadalowerPotassium-feldsparcontentthanTullyRiversandindicatingthattheMissionBeachsandhasundergonesomeformofweatheringandisnotrecentlyderivedfromtheTullyRiver.HeconcludedthatmostlikelysourceofsandalongMissionBeachwasfromthecontinentalshelf.Thissandwasprobablytransportedonshoreduringthemarinetransgression(10,000to6000yearsBP)andearlystillstand(post6000yearsBP).

FurtherinvestigationofthestratigraphyandgeomorphologyoftheareawasundertakeninDecember2002todeterminethehistoryandageofgeomorphicunitsofthesite.Thisinvestigationincludedananalysis

ofaerialphotographsandasoilprofileexaminationbybackhoetrenching.ThepurposeoftheworkwastoconfirmwhetherasedimentsupplyfromsouthofTamO’ShanterPointtoSouthMissionBeachonceexistedinrelativelyrecentgeologicaltimesandifso,atwhattimeovertheHolocenehadtheformationoftheTamO’Shantertombolocutoffthesandsupply,resultingintheobservedtrendoferosion.ThiswouldassistindeterminingwhethererosionatSouthMissionBeachiscausedbyageologicallyrecentdisruptioninsandsupplyfromtheTullyRiver/HullRiverscompartmenttoSouthMissionBeachcausingalong-termrealignmentofthecoast,whetheritiscausedbyanepisodicredistributionofsandwithintheMissionBeachcompartment,orbysomeothercause.

Fourtrenchesupto5mdeepwereexcavatedonthetombolowestandsouthofTamO’ShanterPoint,andarecentseptictankexcavationandsoiltestborelogonaseawardresidentiallotatSouthMissionBeachwereexamined.Thelocationoftheseisindicatedonfigure2(nextpage).Unfortunately,noshellmaterialthatcouldbeusedfordatingtheduneemplacementswasrecoveredfromanyofthetrenches.Thiswasprobablyduetotheageandextensiveweatheringofthesesoilsinatropicalclimate.Adescriptionofthesoilprofilesisgiveninfigure3(followingpage).

Theageofthedunesandscouldonlybeinferredfromthecharacteristicsofthesoilprofileandbarriermorphology.Thedunesoilsexaminedhadwelldevelopedprofilesindicativeofextensiveweathering,exceptfortheSouthMissionBeachprofile.ThesoilprofilecharacteristicsofanincreasingwidthoftheorganicallyenrichedA-horizonanddevelopmentofanorangebrownB-horizondeepeningwithdepthareconsistentwithincreasingageofthebarriersequencelandward.Pye(1981)hasindicatedthatinhumidtropicalconditionstheobservedreddeningofdunesandsmayoccurwithin10,000to100,000years,butwithadiversenatureofcontrolsonreddening,timealoneisnotthedeterminingfactor.

Soilprofilecharacteristicsasshowninfigure3indicatetheHolocenebarrierformedrapidlyaftertheendofthepostglacialmarinetransgression(estimatedat5600-6100yearsBP).ThesoilprofileofthetomboloimmediatelywestofTamO’ShanterPoint(locationno.2)indicatedthatitisprobablyPleistocene,butmaybeHoloceneinpart.Inanyevent,itisinferredthatthetomboloclosedthesandtransportpathwaybehindthePointtoSouthMissionBeachfromatleastthemidHolocene.


Figure2.MorphostratigraphicunitsoftheduneareasfromtheHullRivertoWongalingBeach–1951photography.(source:EPA)


AninterestingfeatureofthedunemorphologyinthevicinityofTamO’ShanterPointwastheincreasingelevationofthedunecrestsandswaletroughslandward.ThisisconsistentwithfindingsfromotherresearchonmorphostratigraphyofbarriersystemsinnorthQueensland(MasselinkandLessa,1995)whichindicatesealevelattheendofthepostglacialmarinetransgressionwas1.5to2mhigherthanatpresent,probablyduetoisostaticrebound,wherethelandlevelisraisedandthereisarelativedropinsealevel.

TheundifferentiatedsoilprofilefromSouthMissionBeachindicatesarelativelyshortperiodofweatheringduetoitsyoungage.However,ifthisbarrierwasformedinthemidHolocenebytheonshoremigrationofshelfsands,theapparentyoungagecanonlybeinterpretedasreworkingoftheexistingsandbyseaerosionanddunereconstruction.

Thefindingsfromthiscomponentoftheinvestigationwere:i) ThegapbetweenthemainlandandTamO’ShanterPointhasbeenclosedbyatombolosinceatleastthemidHolocene,andthereislittlelikelihoodofanysignificantlongshoretransportofsandaroundTamO’ShanterPoint.ThereforethereisnosignificantmoderndaysandsupplytotheMissionBeachcompartmentfromtheHullorTullyRivers.

ii) TheMissionBeachcompartmentisalargepocketbeachwithafinitesandresourceheldbetweenTamO’ShanterPointandClumpPoint.SandinthecompartmentwasprobablyderivedfromoffshoremarinedepositsandemplacedduringtheearlyHolocene.

iii)TheSouthMissionBeachHolocenebeachridgesarerelativelyuniformandintactovermostofthecompartmentwithnoevidenceoflong-termreworkingorerosion,suchasbarriertruncation.However,evidencesuggestsarecent(50to500years)reworkingofthemostseawardbeachridge.Thiswasthoughttoresultfromcycloneactivity,wheresandfromthebeachridgeiserodedandstoredinanoff-shorebar.Duringsubsequentcalmweatherthesandistransportedonshoreandabeachridgerebuilt(TaylorandStone,1996).• Thelong-termnetlittoraldriftforthiscoastal sectorisassessedtobeclosetozero.

• Thelargemostseawardbeachridgethatoccurs alongthelengthoftheEsplanadeofSouth MissionBeachappearstoberelativelyyoungand wasprobablyformedbyamajoreventreworking existingdepositsintherecentpast(<500years BP).

Figure3.SoilprofilesoftheTamO’ShanterPointdunefield.(source:EPA)

Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.�0

4.2 Comparison of historical shoreline positionsHistoricalaerialphotographyoftheregioncapturedin1951(figure2)wassourcedfromtheDepartmentofNaturalResourcesandMinestodeterminethestateofthecoastlineatSouthMissionBeachbeforesignificantresidentialdevelopmentoccurred,asshownonthemostrecentaerialphotographytakenin2002(figure4).

High-resolutionaerialphotographyoftheSouthMissionBeachareatakenin1974,1991and2002wererectifiedusingpermanentfeaturessuchashousesandroadscommontoeachtimeseries,andcomparedtoquantifythehorizontalchangesinshorelinepositionoverthis28-yearperiod.Aerialphotographyfrom1951wasalsoassessedbutaccuratecomparisonsofshorelinepositioncouldnotbemadeduetothelownumberofpermanentfeaturesfromwhichtorectifytheimages.ThesehistoricalviewsofSouthMissionBeachareshowninfigure4.Thespatiallyreferencedshorelinepositionsobtainedfromeachdatewereoverlaidonthe2002rectifiedimageandmeasurementsofshorelinechangearepresentedinfigure5.Somelimitedcomparisonshavebeenincludedfromthe1951aerialphotographywherepermanentfeatureswereidentified,butaccuracyisreduced.

Shorelinecomparisonsover28yearsfrom1974to2002indicatedthatthebeachbehaviourhasbeencomplex,asdescribedbelow:• inthesouthernsectionshorelineretreathasbeen

minorandintheorderof3m.Progressiveconstructionofarockwallonthescarpfacehaslimitedfurtherretreat;

• inthesoutherntocentralsectionshorelineretreathasbeenmorerapid,particularlyinthepast10years,withtypicalretreatof4mto7mandthegreatestretreatof13matanareaofstormwaterdischargenearDouglasStreet;and

• inthecentraltonorthernsection,minoraccretionofapproximately7mhasoccurred,mainlyasalowlightlyvegetatedduneterracethatprobablycomesandgoesovertime.

AtthecentralpartofSouthMissionBeachtherateofshorelineretreatincreasedbetween1991and2002upto0.88mperyear.Erosionhasprobablybeenacceleratedbyconstructionoftherockwalltothesouth,whichhasconcentratederosioninthisarea.

Photo6.NorthernendofSouthMissionBeachfollowingerosioninMarch2004.Notethebroadintertidalzoneandhigh-tidebar. Itwouldbeexpectedthatthisbarwillmoveuptheprofileincalmweatherandrebuildthedunes.(source:EPA)

��Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.

Figure4.HistoricalaerialphotosofSouthMissionBeach1951to2002.(source:EPA)


Figure5.Changesinshorelineposition1951to2002.

�3Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.

Shorelinechangesbetween1951and1974couldnotaccuratelybedeterminedduetothepoorqualityoftheearliestphotographyavailableandlackofpermanentfeaturestoreferencethephotosatvariousdates.Theonlyobvioustrendwassignificantaccretiononthenorthernpartofthebeach.ThisisbasedontheassumptionthatthecentrelineofKennedyEsplanadewasconstantbetween1951and1974.

Whilehorizontalrecessionalongthiscoastisonlyminor,therecessionrelativetotheparkwidthbacktotheedgeofthesealedroadhasbeensignificant,with25to50percentofthevegetatedparkseawardoftheroadhavingbeenlostatsomelocations.

4.3 Water levelsTable1providesthetidalplanesfortheregion,basedonpublisheddataforClumpPoint.Table1alsoprovidesestimatedstormtidelevels(RustPPK,1995).

Level relative to AHD

1:500yearAverageRecurranceInterval(ARI)stormtide*

3.20m

1:100yearARIstormtide* 2.64mHighestAstronomicalTide(HAT) 1.91mMeanHighWaterSprings(MHWS) 0.93mMeanLowWaterSprings(MLWS) -1.00mLowestAstronomicalTide(LAT) -1.68m

4.4 Wave climateOnlongsandybeachessuchasSouthMissionBeach,theoverallwaveclimatehasastronginfluenceonthealignmentofthecoast.Ingeneral,thebeachalignmentrespondstothe“average”wavedirectionandsignificantlocalchangesmaybecausedbyvariationsinthewaveclimate.

Thesiteisexposedtorelativelylowtomediumwaveconditions.Thesearelocallygeneratedwindwavesdrivenmainlybypredominantsoutheasterlytradewindsandtheseabreeze.Theoffshoreislands(particularlyDunkIsland)andTamO’ShanterPointprovideadegreeofshelteringthatreducestheenergyoftheinshorewaveclimate.Theoccasionalimpactoftropicalcyclonescanresultinthegenerationofextremewaves.Theseareintheformofswellwaves,propagatingfromnearthecentreofthecyclone,andlocallygeneratedwindwaves.AdirectassessmentofthewaveclimateaffectingSouthMissionBeachhasnotbeenmadeaspartofthepresentstudy.

TheJamesCookUniversityMarineModelingUnit(JCU,2003)hasundertakenaprogramofnumericalmodelingandassociatedstatisticalanalysisoftropicalcyclone

Table1StormTideStatistics–CardwellRegion*Note:EstimatesforTullyHeads-doesnotincludeanallowanceforwavesetup.

wavesforthemajorityoftheGreatBarrierReefarea.ThisworkwasfundedthroughtheCooperativeResearchCentrefortheGreatBarrierReefWorldHeritageArea.Foralocationapproximately35kmeastofDunkIsland,thesignificantwaveheightcorrespondingtoa50yearaveragerecurrenceintervalisestimatedas4.5m.TheresultsdonotallowanaccurateestimationoflikelyextremewaveconditionsintheleeofDunkIsland,howeveranindicativeestimateisintheorderof3.5m.

4.5 Sediment transportSedimenttransportonthecoastatSouthMissionBeachoccursbytwoprincipalmechanisms.

Longshore sediment transport:Thisisaresponseofabeachtoincomingwaveenergyreachingtheshallowwaterneartheshoreatanangletothebeach.Thewaveswillmovesandalongthebeachastheystirupsedimentonthebottom.Overagivenperiodanddependingonwaveenergyanddirectionthetotal(orgross)longshoresedimenttransportmaybesignificantassandismovedonbothdirectionsalongthecoast.However,thenetlongshoresedimenttransportmaybesmall.OverrelativelylongtimescalesthisislikelytobethecaseatSouthMissionBeach,basedontheassessmentofgeologicalcontrols.

Cross-shore sediment transport:Themostimportantformofcross-shoresedimenttransportistheresponseofabeachtoelevatedwaterlevels(stormtides)andstormwaves.Duringstormconditionsthebeachprofileflattenscausingarecessionoftheupperbeach.Generallythesanderodedfromtheupperbeachisdepositedinthenear-shoreareaandisslowlyreturnedduringcalmerwaveconditions.

AdirectassessmentofthelongshoresedimenttransportratesaffectingSouthMissionBeachhasnotbeenmadeaspartofthepresentstudy.Contributionstolongshoretransportaremadeby:

• southeasterlytradewinds;

• thedailyseabreeze.Thishastheeffectofrotatingthewind-wavedirectiontowardstheperpendiculartothecoastline;

• seasonalnorth-easterlywinds;and

• theeffectofseaandswellwavesgeneratedbytheoccasionaltropicalcyclone.

TheanalysisofthelongshoresedimenttransportatSouthMissionBeachiscomplicatedbythelackofrepresentativewindrecordsfortheregionthatarerequiredtoassessthelocallygeneratedwind-waveclimate.Applicablerecordedwinddata,fromapreliminaryreview,appeartobeinfluencedbytopographicfactors.


Smallchangestothelongshoresedimenttransportmaybecausedbyrelativelysubtleclimatevariations(fore.g.atypicalyear-to-yearvariabilityofthestrengthandnetaveragedirectionofthetradewindsmaycausechangestotheequilibriumalignmentofthebeach).Overalongsectionofbeachthiscancausemarkedeffectsonthecoastalalignmentneareachend.InthecaseofSouthMissionBeachthisislikelytobenoticeableatthesouthernendofthebeach.

Figure 6. Location and number of survey lines.

Duringyearswhenthesouth-easterlywindsarefrequent,thenetannuallongshoretransportwouldbetothenorth.Thisisprobablybalancedbyyearswhenstrongnorth-easterlywindsoccuroratropicalcycloneoccursnorthoftheregion,whennetannuallongshoretransportwouldbetothesouth.


-10 0 10 20 30 40 50 60 70 80 90 100Distance (metres)

-2

-1

0

1

2

3

4

Hei

gh

tab

ove

AH

D(m

etre

s)

Survey LinesLine 1Line 2Line 3Line 4Line 5Line 6Line 7Line 8Line 9Line 10Line 11Line 12Line 13Line 14

HAT level (1.19m)

MHWS level (0.93m)

MLWS level (-1.0m)

4.6 Beach profile analysis and changesAspartofthepresentstudy,CardwellShireCouncilcompletedaninitialsetof16beachprofilesurveysforthelengthofSouthMissionBeachinSeptember2002.Thesurveysextendfromthebackshoreareaouttoapproximatelythelowestastronomicaltidelevel(figure6).

Beachprofilesrecordedateachsurveylinearepresentedinfigure7.Profiles1to3arewithinthebeginningofthesalientfeature,andprofiles4to5displayasmallnearshorebar.Profiles6to9indicateanactivelyerodingsectionofcoastandprofiles.Profiles10to14areonthesectionofthecoastwherearockwallhasbeenconstructed.Profiles15and16arewithinanareamodifiedbyconstructionofaboatramp.Typicalprofilesforeachoftheseareasareshowninfigure8.

4.6.1 Comparison of survey profilesNohistoricalbeachprofileorhydrographicsurveydataareavailablefortheSouthMissionBeacharea.Intheabsenceofhistoricalsurveydata,someinformationonsandvolumelostovertimefromthebeachbyerosionmaybederivedbyacomparisonofcurrentsurveylines.Asanapproximation,itcanbeassumedthattheprofilesatthenorthernendofthebeachrepresenttheun-eroded,“natural”stateofthebeach.Therelativelossofvolumefromthebeachprofilesatthesouthernendofthebeachcanbedeterminedbycomparingtheprofiledata,takingprofile4asthebase.

Figure7showsallthesurveydataoverlaidtoacommonunderwaterpoint(-1mAHD).Itcanbenotedthat,apart

fromprofiles1and3,thereislittlechangebetweenprofileshapesparticularlyinthelowerbeachsections.Differencesarenoticeableataroundthe+1mlevel,whichisapproximatelymeanhighwaterspringstidallevel.

Toinvestigatethisfurthertherelativepositionofthe+2mcontour(approximatelyHATorthelevelofthetoeofthefrontaldune)relativetothe–1mcontour(MLWS)hasbeenderived(figure9)andprovidesasimplemeasureoftheoverallerosionandretreatoftheupperbeach.Themostlandwardpositionofthe+2mcontouroccursatprofile9,600mnorthoftheboatramp.Thisistheprofileimmediatelynorthoftheendoftherockwall.

Thisfindingillustratesthemodeoferosionoccurringatthesouthernendofthebeach.Theupperbeach(representedbythe+2mcontour)showsrecessionofupto7mcomparedtoprofilesfurthernorth,butthelowersectionofthebeachisrelativelyunaffected.Thebeachinfrontoftherockwallhasconstrainedlandwardretreatandtheforeshoreimmediatelyseawardexhibitsaloweredlevel.Therefore,erosionoccursprimarilyontheupperbeachaboveMHWS,butwhererecessionispreventedbytheconstructionofarockwall,erosioncontinuesseawardofthewallresultinginaloweringofthebeachlevel.

Thevolumeofsanderodedovertimecanbeestimatedbycomparingprofile4,representingatypicalequilibriumbeachprofile,andallotherprofiles.Sandvolumedifferencesbetweenthe-1mcontourandapoint80mlandwardofthe–1mcontourisshownintable2.

Figure7.Beachprofileateachsurveylinerecordedon22September2002


2100

2000

1900

1800

1700

1600

1500

1400

1300

1200

1100

1000 900

800

700

600

500

400

300

200

100

Distance along beach north of boat ramp (metres)

80

75

70

65

60

Dis

tan

cefr

om

HA

Tto

-1.0

met

reco

nto

ur

(m)

1

3

4

56

7 8

9

10

1112

1314

Figure8.Typicalbeachprofile

Figure9.Distanceofthehighestastronomicaltideleveltothe-1.0mcontouratSeptember2002.

Thetotalvolumedeterminedthroughsummationofsandlostovertheentirebeachisestimatedat14,639m3,withthebulkofthiscomingfromthesoutherntocentralsectionofthisbeach.Themosterodedsectionofthisbeachhasexperiencedasandlossof14m3/mofcoast.

Itshouldbenotedthattheaboveestimationofsandlossassumesthatthebeachprofileatthe–1mcontourisstableandhasnotchangedovertime.Asnoprevioussurveydatatothislevelisavailable,thisassumptioncannotbetested.Therefore,itwouldbeprudenttoincludeasafetyfactorof40percentinanycalculationof

sandlosstominimizetheriskofanunderestimationofsandloss.Sandlossisthereforerecalculatedat20,494m3andforconvenienceisroundedto20,000m3.

Assumingthatthiserosionhasoccurredovera28yeartimeperiod,theaverageannuallossfromerosioniscalculatedatabout700m3peryear.ThisisarelativelysmallamountcomparedtoothersitesalongtheQueenslandcoast(e.g.WooriminCabooltureShireisasimilarmoderatewaveenergycoastandislosingabout30,000m3ofsandperyear).

-10 0 10 20 30 40 50 60 70 80 90 100Distance (metres)

-2

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

etre

s)

Survey LinesLine 4 - Uneroded profileLine 9 - eroded profileLine 11 - eroded profile with rock wall

HAT = 1.91m

MLWS = -1.00m

MHWS = 0.93m


Survey line (profile) number

Chainage north from boat ramp

Sand volume difference between each profile and

profile 4

Section length Sand volume change per 100m of beach

(m) (m3/m) (m) (m3) 1 2100 2.3 200 230

2000 230 2 1900 -1.95 200 -195*

1800 -195* 3 1700 -6.21 200 -621#

1600 -621# 4 1500 0 200 0

1400 0 5 1300 -7.65 200 -765

1200 -765 6 1100 -9.22 200 -922

1000 -922 7 900 -14.07 200 -1407

800 -1407 8 700 -11.16 100 -1116 9 600 -15.35 100 -1535 10 500 -15.35 100 -1535 11 400 -7.65 100 -765 12 300 -6.08 100 -608 13 200 -6.37 100 -637 14 100 -6.23 100 -623

Total of losses (-) -13,628

Table2Sandlossfrombeachsectionscomparedtoanunerodedbeachsectionrepresentedbysurveyline4.*Profiledataofpoorqualityhencecalculatedvolumesmaybeinaccurate#ProfileaffectedbycreekmouthhencecalculatedvolumesmaybeinaccurateNote1.Profiles15and16werenoincludedduetodistortionsinthebeachprofileintroducedbyconstructionoftheboatramp.

4.6.2 Potential shoreline recession from future storm erosion Anestimateofshort-termbeacherosionfromastormeventmaybedeterminedbythemethodofVellinga(1983).Thismethodassumesanequilibriumbeachprofilethatdevelopsinresponsetoelevatedwaterlevelsandwaveaction.Thecomputedprofileisafunctionofwaveheightandbeachsedimentproperties.TheVellingamethodislimited,asitdoesn’ttakeintoaccounttimevaryingconditionsduringastormanddescribestheresponseoftheupperbeachonly.Despiteitsshortcomings,itisanacceptedempiricalmethodwidelyusedforestimationoferosionpotentialonothersectionsoftheQueenslandcoast.

AVellingaanalysishasbeenundertakenonprofile13,whichrepresentsthebeachatthesouthernendofthestudyarea.Keyparametersusedintheanalysiswere:

• averagerecurrenceintervalforstormevent= 100years(extremecyclonicevent);

• peakstormwaterlevel=2.99mAHD;

• peaksignificantwaveheight=3.5m;and

• mediansedimentdiameterD50=0.21mm.

Theanalysisresultsshowarecessionatthetoeofthefrontalduneintheorderof45misrequiredtoachieveanequilibriumbeachprofileattheelevatedwaterlevel(figure10).Theactualamountofrecessionwilldependonthedurationoftheelevatedwaterlevelandthetidalinfluenceatthetime.Sucharecessioneventmaynotoccurasasingleeventandcouldresultfromthecumulativeeffectofaseriesofeventsoverashorttimeperiod.Thiswouldprovidetheopportunityforshort-termremedialactions,suchassandpushingfromtheintertidalzonetotheupperbeach.


Figure10.Estimatedshort-termbeachprofileresponsetoa1in100ARIstormeventbasedonVellinga1995).

4.7 Effects of storm water discharge on beach erosionStormwaterfromtheroadwayalongSouthMissionBeachisdischargedtosumpsorsoakagepitsontheforeduneseawardoftheroadwayandallowedtopercolateintothesand.Beacherosionalongthiscoastappearstobeworsenedgenerallyseawardofstormwaterdisposalsumps.

Thesumptypedisposalsystemdoesnotappeartohavethecapacitytohandleintenserainfalleventswhichcausesystemoverflows.Thisisobservedtoresultinpondingofwaterbehindthedunecrest,andlocallyconcentratedsurfaceflow.Thissystemmayalsohavealimitationinthatthesumpwilllocallyelevategroundwaterlevelsandwherethisisneartheerosionscarpface,thesandbecomesfluidisedandismoresusceptibletoerosion.Aspondedwaterbreachesthedunecrestsignificantscourholesareformed,whichcanfurtherconcentratewaveerosion(seephoto7).

Photo7.Scourchannelthroughtheforedunecausedbystormwateroverflow,May2004.(source:EPA)

0 20 40 60 80 100 120 140 160Distance (metres)

-2

-1

0

1

2

3

4

Hei

ghtA

bove

AH

D(m

etre

s)

Existing profile at survey line 13Predicted post storm profile (Vellinga)

HAT 1.91m

MHWS 0.93

MLWS -1.0

Potentialerosion loss


Thedirectdischargeofstormwatertotheoceanacrossthebeachappearstobetheonlyviablealternativeatthislocationtominimisethisimpact.However,unlessmanagedcorrectly,disposalofstormwatertotheupperbeachhasthepotentialtocauseextensivelocalscour.Suitablydesignedoutletstructureswouldberequiredtominimiseproblems.

Councilwillneedtoseekfurtherdesignadviceregardingthismatter.Itisnotedthatanumberofnewproductshavebeendevelopedutilizinggeotextilematerialsthatmaybeofuseincludingsandfilledcontainers,andscouraprons.

5.0 Key findings on erosioni) ThecoastlineatthesouthernendofSouthMission

Beachisexperiencingarelativelysmallbutpersistenttrendoferosion.Theerosiontrendappearstohavestartedatthesouthernendandhassteadilyprogressednorthward.Arockwallhasstoppedthecoastlinerecessionatthesouthernendofthebeachbuthaseffectivelytransferredtheerosiontothenorth.Activebeacherosionisnowapparentnearthecenteroftheembaymentadjacenttotheendofthewall.ThenortherncoastofSouthMissionBeachhasexperiencedsandaccretionandcoastlineprogradation.

ii) Themosterodedsectionofcoastaroundprofile9(figure6)hasexperiencedshorelinerecessionintheorderof4mto7mwhichisaannualiserateof0.24m.Themaximumpointofrecessionattheendoftherockwallwas13.4mwhichisanannualisedrateof0.9m.

iii) Basedonacomparisonofbeachprofiles,atotallossof20,000m3ofsandisestimatedalongthesouthernandcentralsectionofthisbeachoverthelast28years.

iv) AreviewofthegeomorphologicalfactorsaffectingsedimentsupplytoSouthMissionBeachhasdeterminedthattheTamO’ShantertoClumpPointcompartmentisaclosedsystemwithnolongshoresupplyofsandfromtheHullorTullyRivers.Theonlysignificantsandsupplywasoffshoremarinedepositsinthemid-Holoceneassociatedwiththeendingofthepostglacialmarinetransgression.Thepresentbeachhasformedinresponsetozeronetsedimentsupplyandthereforethelong-termaveragelongshoresedimenttransportrateatanypointalongthebeachisnegligible.

v) Themostprobablecauseofthepresentbeacherosionproblemsisthereforeanepisodicresponsetoachangeintheprevailingwaveclimate,whichhasresultedfromasubtleshiftinthebalancebetweenthevariousclimaticfactorsthatdrivelongshoresedimenttransport.Itislikelythatthe

erosionatthesouthernendofthebeachhasresultedinanassociatedaccretionofthesalientfeatureatthenorthernendofthebeachnearWheatley’sCreek.

vi) Thefactorsthatcancausechangestotheprevailingwaveclimateincludeinter-annualvariabilityofthesouth-easterlytradewindsandtheeffectsofoccasionaltropicalcycloneevents.

vii) Theesplanade,constructedontheforeduneandclosetotheerosionscarp,isvulnerabletostormerosion.Astormevent,producingwaterlevelscorrespondingtoa100yearaveragerecurrenceintervalmaypotentiallycauseupto45mofrecession.Thiswouldpotentiallydestroymostoftheroadandmaythreatenadjacenthouses.

viii) Thepresentrockwallisgenerallypoorlyconstructedandhasbeendamagedbysmalltomoderateerosionevents.Forthemostpart,itwouldofferlittleprotectionagainstaseverestormevent.

ix) Erosionhasbeenlocallyaggravatedbythedischargeofstormwatertosoakagepitsonthefrontalduneandtheoverlandflowofpondedstormwaterovertoppingthedunecrest.

6.0 Erosion management options for South Mission BeachTherearearangeofoptionsavailabletotheCounciltomanagetheerosionatSouthMissionBeach.Thesearedetailedbelowwiththeprosandconsofeachprovided.Specificrecommendationsonthefavorederosionmanagementoptionsareprovidedinsection8.0.

6.1 Managed retreat Managedretreatallowscoastalerosiontocontinueunhindered,byrelocatingassetsfurtherlandwardaserosionprogresses,toavoidloss.Itiscommonlyreferredtoasthebufferzoneconcept,wheresanderodedfromthedunesduringstormsismovedoffshoreandreturnstobuildthedunesduringcalmweather.Thisisalow-coststrategybutrequiresthedunalbufferareastobeofadequatewidth,andismostapplicablewhenerosioniscyclical.

AtSouthMissionBeach,theslowerosionalongsectionsofcoastisexpectedtocontinueintheshorttomediumterm,andatthecurrentrate,theexistingdunalareacanonlyaccommodateerosionforthenext5to10yearsbeforetheroadisdirectlyaffected.Thereisalsoinsufficientlandbetweentheroadandtheforeshoretoaccommodateshort-termcyclicalerosionwithoutdirectlythreateningtotheroadandotherpublicinfrastructure,andthereisinsufficientspaceforthesetoberelocated.Furthermore,thesuspectedpresenceoffillmaterialintheduneswaleunderminesthevalueofthisareaasasourceofsandfornaturalbeachreplenishment.


ThelossofparklandamenityisalsoofconcerntotheCouncilassomeresidentshaveidentifiedthisparklandamenityasanimportantfeatureofSouthMissionBeach.Managedretreatthereforeisnotconsideredaviableoptionformostoftheactivelyerodingsectionsofthestudyarea.

6.2 Beach nourishmentBeachnourishmentisthereplenishmentofbeacheswithimportedsandandisoneofthebestavailablemeansofrestoringbeacheswhereerosionhasbecomeaproblem.Itwidensthebeachanddunestherebypreservingexistingbeachamenityandincreasesthebufferingcapacityofthedunesagainststormattacktoprotectdevelopment.Sandnourishmentincreasesthewidthoflandavailabletoaccommodateerosion,whileatthesametimeallowingthebeachtocontinuetobehavenaturally.

Sandplacedinonlyafewselectedareasofagenerallyerodingcoastlineorplacedtoformonlytheupper(visible)partofthebeachwillusuallybedispersedquicklythroughthebeachsystem.Thesandisnotlostbutremainsintheactivebeachsystem,resultinginsomegainoverallbutaless-than-desiredgainforindividualbeaches.Toachievesuccessfulbeachrestoration,expectationsoftheresultmustbeclearly

articulatedandsandrequirementsmustbecarefullycalculatedtoachievethese.

Theonedisadvantageofbeachnourishmentisthatthesandwillcontinuetobeerodedandongoingtop-upnourishmentmayberequiredtomaintainthedesiredbeachprofile.Thismaybeseenbysomeasatemporarysolutionandawasteofmoney.

Animportantaspectofbeachnourishmentisthatanysandaddedtothebeachcomesfromasourceoutsideoftheactivebeachsystem.Theactivebeachsystemincludestheareasofpresentdaysandmovementsincludingthedunes,beachandoffshoresandbars.Thesandintroducedtherebyrepresentsagaininthequantityofsandcontainedwithinthebeachsystemratherthanjustatransferfromonepartofthesystemtoanother.

Itisalsoimportantthatthegrainsizeofnourishmentsandissimilartothatoftheexistingsand.Thisensuresthattheslopeofthebeachprofileremainscompatiblewiththepre-nourishmentprofile,andthatsandmovementsasaresultofnaturalprofileadjustmentsareminimised.

Alow-costalternativetopurchasingsandfromanoutsidesourceistousecleansandsurplustorequirementsfromCouncilprojectssuchasroadworksorpipelineinstallations,orfromprivatedevelopment

Photo8.Rockandsoilfillmaterialontheesplanadeexposedbyduneerosion.(source:EPA)

2�Coastal erosion investigation and management options for South Mission Beach, Cardwell Shire.

sitesormaintenancedredging.Councilcanalsoimposearequirementoncertaindevelopmentapprovalsthatallsurpluscleansandfromsuchsitesisplacedonbeachesanddunes.Withinthedesignatederosionpronearea,buildingworksinvolvingtheexcavationofcleansandcanberequiredtoplacethismaterialelsewherewithintheerosionpronearea.

Beachnourishmentworksmayattracta25percentStatesubsidythroughtheLocalGoverningBodies’CapitalWorksLoanSubsidySchemeadministeredthroughtheDepartmentofLocalGovernmentandPlanning

Application to South Mission BeachBeachnourishmentcanbeusedatSouthMissionBeachto:

i) replaceerosionallossesandrestorethebeachtothepreexistingprofile.SouthMissionBeachisexperiencingmoderateerosionovermorethan1000mofcoastlinewithacalculatedannualsandlossof700m3,andalong-termlosssince1974of20,000m3(table2).Thesequantitiesareconsideredrelativelysmallcomparedtootherareasofthecoastandarethereforeamenabletoremediationbybeachnourishment.Thesandvolumesrequiredtooffsetthehistoricalerosionallossesispresentedintable3,andtherequiredpostnourishmentbeachprofilesarepresentedinfigure11.

ii) increasethewidthofthedunestocreateawiderbufferzoneseawardoftheroad.Figure10demonstratesthatanextremecyclonicevent(1in100ARI)maycauseerosionofthedunesandalandwardrecessionofthecoastbyupto45m.Ananalysisoftheexistingdunewidthindicatesthattheduneswouldhavetobeincreasedinwidthseawardoftheroadbyupto35msothatthiserosionallosscouldbeaccommodatedwithinthedunalzone.Thesandvolumetoconstructthisbufferzoneisestimatedat180,000m3forthelengthofSouthMissionBeach.Alessersandvolumecouldbeconsideredtoincreasethelevelofprotectionagainstlowerintensitystormevents,andwouldalsoassistinprotectingtheremainingtreesontheEsplanade.Anincreaseindunewidthto20mseawardoftheroadedgeisconsideredtobeareasonablyfunctionalwidthincrease.Thiswouldrequireanadditional20,500m3ofsandplacedonthesouthernandcentralsectionofthebeach.Adetaileddescriptionofsandquantitiesrequiredandplacementlocationforanincreaseindunewidthisgivenintable3.

Theproblemwillbeinfindingasupplyofsuitablesandandatareasonablecost.Foursandsourceshavebeenidentifiedandarebrieflydescribedbelow.

i) Offshoresandeitherfromtheriverdeltastothesouthorshelfsandseveralkilometresoffshore.ThemostprospectivesourceofthissandisconsideredtobethenearshoreareasbetweentheHullRivermouthandTamO’ShanterPoint,duetoitsaccessibility,modernaccumulativehistory,minimaladverseimpactonlocalcoastalprocessesandexpectedlowfinescontent.Thissourceofsandmaynotbeeconomicallyviableforobtainingsmallquantities(<20,000m3)duetothehighcostsofresourceidentification,approvalprocessing,dredgecommissioningandenvironmentalmanagementintheGreatBarrierReefMarinePark.Theeconomicsofusingnearshoresandmayimproveiflargerquantitiesofsandwereneededforbotherosionmitigationandwideningofthedunalbuffer.PreliminarydiscussionswillberequiredwiththeGreatBarrierReefMarineParkAuthorityandtheQueenslandParksandWildlifeServicetodetermineifaccesstothissandispossible.Followingonfromthis,ageotechnicalinvestigationofpotentialsandsourceswouldberequiredtoconfirmthesuitablilityofthesandintheborrowareas,anddiscussionsheldwithdredgeoperatorstoevaluatepracticalityandcost.

ii) OnshoresandfrombeachridgeplainssuchasthatbehindTamO’ShanterPoint.Muchofthissandisonfreeholdlandandextractionthereforerequirestheconsentoftheowner.ArecenttrialbeachnourishmentprojectatSouthMissionBeachusingsuchsanddemonstratedtheviabilityofthisoptionintermsofcostandpracticality.

iii)RiversanddredgedfromtheTullyRiver.WhilethisisaviableoptionastheCouncilcurrentlyholdsdredginglicensesfortheTullyRiver,orcouldobtainsandfromcommercialoperators,thecostoftransporttoSouthMissionBeachcouldproveprohibitive.

iv) Surplussandfromprivateorlocalgovernmentprojects,includinginstallationofseweragepipes,roadupgrading,excavationforprivatedevelopmentsuchasbuildingsorprawnfarmponds.

Anysandsourceneedstobeofasimilarormoderatelylargersizegradingandtohaveafinescontentoflessthan5percenttobesuitable.

Adisadvantageofbeachnourishmentistheperceptionofitshighcostanditstemporarynature,thatis,itisnotseenasapermanentsolution.However,inthepresent


case,arelativelysmallvolumeofsandimportedtotheaffectedsectionofthebeachwouldprovidesignificantbenefits.AnynourishmentsanderodedfromtheplacementareaatthesouthernendofSouthMissionBeachwouldbetransportednorthandthereforeofbenefittothebeachestothenorthoftheexistingrockwall.

Artificiallyplacedsandisgenerallynotasstableasanaturalbeachduetoitstendencytoberedistributedacrosstheintertidalzoneandalongshore.Anourishedbeachhasanabnormalalignment(thehighwatermarkisformedseawardofthepre-existingcoastline)andanover-steepprofile.Duetothesefactors,thereistypicallyanadvantagetoslightlyoverfillanourishedbeachandtoplacesomeormostofthesandintheintertidalzone.

Beachnourishmentworkscouldbecompletedinstages.Thismayhaveadvantagesintermsofresourcesand

funding,andthemonitoringoftheinitialstageswouldprovidevaluableinformationtoassistinimprovingthedesignoffurtherstages.

Thenourishmentsandwillbevulnerabletowinderosionandmaybeblownlandwardsontotheparkorroad.Thesandsurfaceshouldbevegetativelystabilisedwithnativedunegrassesandrunnerssoonafterbeingplaced.A5mwide“starterstrip”ofplantsincludingsandspinifexgrassandgoatsfootshouldbeestablishedonthelandwardedgeofthenewsand.Plantsshouldbeestablishedat1mto1.5mcentres.Thiscanbecombinedwithfertilizingtheexistingnativedunevegetationwithanitrogenousfertiliseratarateof50kgofnitrogenperhaperapplication,repeatedeverytwotothreemonthsasrequired.

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 14

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 13

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 12

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 11

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

LEGENDProfileatSeptember2002Designnourishedprofile

Profile 5

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 6

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 7

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 8

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)

Profile 9

-10 0 10 20 30 40 50 60Distance (m)

-1

0

1

2

3

4

Hei

ghta

bove

AH

D(m

)Profile 10

Figure11.Indicativedesignnourishmentprofilesbasedonsandnourishmentvolumesprovidedintable3tofillerosionalloss.


Survey line/profile

number

Chainage north from boat ramp

Current distance

from road edge to HAT

Nourishment sand

required to fill erosion

loss

Estimated HAT

extension Seaward

New distance

from road edge

Additional sand

required to extend

distance from road

edge to HAT to 20m

Additional sand

required to extend

distance from road

edge to HAT to 45m

(m) (m) (m3) (m) (m) (m3) (m)

1 2100

2000 >45

2 1900 >45

1800 >45

3 1700 43 43 1000

1600 31 31 7000

4 1500 37 37 4000

1400 35 35 5000

5 1300 33 500 0 33 6000

1200 29 500 <2 29 8000

6 1100 25 500 <2 25 10,000

1000 25 1000 4 29 8000

7 900 15 1500 5 20 12,500

800 15 2000 5 20 12,500

8 700 12 2500 5 17 1500 14,000

9 600 11 3000 7 18 1000 13,500

10 500 10 3000 5 15 2500 15,000

11 400 14 2000 0 14 3000 15,500

12 300 11 2000 0 11 4500 17,000

13 200 14 1000 0 14 3000 15,500

14 100 10 500 0 10 5000 15,500

Total sand volume

20,000 20,500 180,000

Table3Effectsofsandnourishmentvolumesonincreasingthewidthofthedunesystem(HATextensionseaward)

6.3 Local sand relocationWhereerosionisconsideredaslocalised,wherethesandismovedonlyarelativelyshortdistanceoffshoreoralongthecoast,somerelieffromerosioncanbegainedfrommechanicallyrelocatingthesandbacktowhereitwaserodedfrom.Inthesimplestexample,sandcanbescrapedfromtheintertidalareaoradjacentsandbarsimmediatelyoffshoreandmovedtotheupperbeach.Thisusuallyhasonlyshorttomedium-termbenefitsastheplacedsandwillberedistributedbywaveandtidalactiontoreformtheoriginalequilibriumprofile.Thatis,itwillwanttogetbacktoitsoriginalshape.Thefurtheroffshorethesandistakenfromandtheshallowerthematerialisskimmed,theslowertheredistributionprocesswillbe.

Sandcanalsobemovedfromaccumulationareasupcoastordowncoastfromtheerodedarea,usuallyatspitsorcreekmouths.Thismayinvolvelongerandmorecomplextransportarrangements.

Localsandrelocationisusuallymoreusefulformanagingshort-termerosioneventsassociatedwithstormsorextremetides,andpocketsoferosionassociatedwithstormwaterwashouts.


Application to South Mission BeachAtSouthMissionBeachsandissometimespushedfromtheupperbeachtoerosionscarpsoraroundthebaseoftreestoprotectagainstkingtidesorothershort-termevents.Thistechniqueprovidesonlyshort-termbenefit.

ThisstudyhasindicatedthatsanderodedfromthesouthernendofSouthMissionBeachtendstoaccumulatetothenorthinthevicinityofWheatleysCreek.Thereforethisareacouldpotentiallybeusedasasourceofsandtoreplenishthesouthernbeach.Thisisconsideredaslocalsandrelocationratherthanbeachnourishment,asthesandborrowareaisstillpartoftheactivebeachsystem.Twooptionsforremovingsandareidentifiedbelow:

i) WheatleysCreekmouthisoccasionallyopenedtothesouthtoovercomechannelmigrationtothenorth,whichaffectsfreeholdproperties.Theremaybeanopportunityforsandremovedtoberelocatedsouthatthesametime.

ii) Thebeachareafor500mimmediatelysouthofWheatleysCreekcouldbeconsideredasasourceofnourishmentsand,asthisareaisslowlybuildingup.However,itissuggestedthattheborrowquantitybelimitedto5000m3andberemovedfromtheupperintertidalzonetolimitbeachrecessionadjacenttotheborrowarea.Beachrecessionshouldbelessthan1mandasthissiteiswellnorthofresidentialdevelopment,theerosionshouldnotbeofconcern.Observationofthebeachresponsewouldberequiredbeforefurthersandremovalisconsidered.Thissourcewillthereforeonlyprovideaportionofthenourishmentsandrequirement.

Relocationofsandisusuallyachievedusingscrapersandortrucktransport.ArelativenewdredgingtechniqueknownastheSandShiftermayalsobeworthconsideringatthissite.Thisisarelativelylowcost,methodwheresandisextractedusingasubmergedsuctionheadandpumpedasasandslurrytothereplenishmentarea.Significantinvestigationwouldberequiredtoestablishtheapplicabilityofthistechniquetothissite,includingcoresamplingtoidentifyanadequateresourceofsuitablesandatdepthsofupto6m.

Theuseofsandfromthislocationwouldbesubjecttoanassessmentofenvironmentalimpacts.

6.4 RevetmentsRevetmentwallsaregenerallysuccessfulatcontrollingshorelinerecession.However,revetmentscantransfererosionoftheadjoiningbeachbylockingupsandreservesinthedunes,andpreventanewbeachfrombuildingup.Byconstructingarevetmentwallalongtheexistingerosionescarpment,sandlandwardofthewallisisolatedfromtheactivebeachsystemandthevolume

ofsandavailablefornormalbeachmovementsislimitedtothatseawardofthewall.Duringastormeventwhensandiswashedoffshore,limitedsandsupplycanbequicklyeroded,resultinginthereductionofbeachlevels.Whenwavesbreakagainstthewall,thelossofsandisfurtherhastenedaswaveenergyispartiallyreflectedfromthewallandtherateofsandtransportalongthebeachisincreasedasthewallsinduceorincreasenearshorecurrents.

Revetmentwallsdonotnecessarilypreventtheaccumulationofsandontothebeachduringperiodsofaccretion.However,revetmentsalmostalwaysleadtoincreasederosioneffectsatthedowndriftendofthewall.Accordingly,revetmentsshouldonlybeconsideredwhenprivatepropertyorpublicinfrastructurethatcannotberelocatedisunderdirectthreat,andshouldbelocatedasfarlandwardaspracticabletomaintainadunalbufferzoneandbeachamenity.

Application to South Mission BeachIndiscussingtheapplicabilityofrevetmentwallstoaddresstheerosionproblematSouthMissionBeachtwomattersneedtobeconsidered:

• Whetherrockwallsareanappropriatesolutiontostoperosion;and

• Anappropriatedesignandalignmentofanysuchrevetment.

Itisconsideredthatthecontinueduseofrockwallprotectiontomitigatethelong-termtrendoferosionisnotaneffectiveoption.Asalreadyevident,thissimplyresultsinthetransferoftheproblemfurthernorthward,thusleadingtoaneedtofurtherextendthewall.Thisapproachalsolowersthebeachprofileseawardofthewallcausingprogressivelossoftheusablebeach.Furthermore,awallconstructedonthescarpfaceislikelytobecomeapermanentvisualfeatureonthebeach,degradingtheamenityvalueofthearea.

ItisrecognisedthatthepresentconfigurationofthebeachfrontareaofSouthMissionBeachisvulnerabletodamagefromextremewavesandelevatedwaterlevelsassociatedwithaseveretropicalcyclone.TheroadandotherinfrastructuresuchastheCounciltoiletblocknearJackeyJackeyStreetareatriskofdamage.

TheexistingrevetmentwallsatSouthMissionBeachhavebeenconstructedbyenddumpingrockoverthescarpfaceinresponsetolocalisederosionevents.Thewallshavenotbeendesignedtowithstandaspecificevent,whichdeterminesarmorsize,anddonotincludetoescourprotection,orafilterlayerandsignificantcrestelevationtopreventcollapsefromthewashingofsedimentsfrombehindthewall.Theexistingrevetmentwall,initspresentcondition,wouldprobablyprovideonlylimitedprotection.


Anupgradedrevetmentwallwouldberequiredtoreducethepresenterosionrisk,andiftheCouncildecidedtoproceed,theEPAwouldrecommendthatthewallbedesignedtowithstandaminimumonein50yearaveragerecurrenceintervalstormevent(seeTable1).TheCouncilwouldneedfurtherstructuraldesignadvicebeforeproceedingtoactualconstructionofawall.Asacostindicatorofanadequatelydesignedrevetment,arecentlyconstructedrockrevetmentwallatHollowaysBeachatCairnscostapproximately$3000/m.Optimizingthisdesigntolocalconditionsasdetailedbelowmayreducethiscosttounder$2000/m.

Itmaybecostprohibitivetoconstructawallovertheentire1600mlengthofKennedyEsplanadeatanindicativecostof$3.2million.Duetothepresenttrendoferosion,itmaynotbefeasibletomaintainaduneandbeachalongthesouthernmostsectionofSouthMissionBeachatalltimes.Accordingly,thesectionofwall(e.g.southfromJackeyJackeyStreet)shouldberetainedandupgraded.

IfconstructionofarockrevetmentwallalongthelengthofKennedyEsplanadeiscontemplated,thewallshouldbedesignedsothatitremainsburiedasfarbehindtheactivebeachprofileaspossible.Thewallwouldthereforefunctionprincipallyasabackstoptolimitcoastalrecessiononlyduringseverestormevents.Ingeneralterms,thealignmentofarockrevetmentwallshouldbelocatedasfarlandwardaspossible.Theintentionistoencouragethemaintenanceofasandybeachseawardofthewallandthereforereducetheexposureofthewalltodirectwaveaction.Thisincreasestheeffectivenessofthewallduringstorms,minimisesadverseimpactstotheadjacentbeachandavoidsexcessiveongoingmaintenancecosts.

Inthisscenario,thedesignofanupgradedrevetmentwallcouldbeoptimisedbyacceptingalimitedamountofdamagetothewallduringasevereevent.Thiswouldreducetheinitialcostswhilestillprovidinganacceptablelevelofprotection,asfollows:

• Thewallisintendedtoprotecttheroadfromwaveattackonly,andnotadjacentprivatedevelopmentfromstormsurge.

• Thewallcrestlevelwouldbesetatabouttheroadsurfaceheight.Thewallmaybeovertoppedduringdesignconditions,andthereforesomeinundationanddamagetotheroadsurfacemaybeexpected.

• Thearmorsizecouldbereducedsuchthatthewallwouldpartially“fail”duringtheworstofthestormconditionsalthoughthebulkofthewallwillremaininplacetolimittheamountofsubsequenterosion.(Thelikelyfailuremodeofthistypeofseawallisthedislodgementofarmorunitsbywaveactionfollowedbyslumping).Atypicalarmorsizerangeis500-2000kgwithamediansizeof1500kgisindicated.In

thepresentcasethemedianarmorsizecouldbereducedtosay1000kg,althoughtheminimumsizeof500kgshouldberetained.Thiscouldalsoreducecostsofsourcingmaterialandsubsequenthandling.

• Thedepthofthetoeofthewallshouldbesettoalevelthatwouldwithstandunderminingduringthedesignstormconditions.Typicallythetoeofaseawallwouldbeconstructedtoatleastthelowestastronomicaltidelevel.However,assumingthevolumeofsandonthebeachseawardofthestructurehasbeenmaintainedviaanourishmentprogram,ahighertoelevel(saymeanlowwaterspringstidallevel)couldbeused.

ItshouldbenotedthattheaboveremarksarepreliminaryadviceonlyandtheCouncilshouldseekdetailedstructuraldesignadvice.

6.5 Groynes and artificial reefsGroynesconstructedperpendiculartothecoastlinecanbeusedtotraplongshoresedimenttransport,producinganaccumulationofsandontheupdriftsideofthegroyne.However,thedowndriftbeachiscorrespondinglystarvedofsand,resultinginanerosionproblembeingtransferredtoandconcentratedinthisarea.Unlessthisdowndrifterosionisacceptable,theconstructionofgroyneswithoutassociatedbeachnourishmentwillnotsolveerosionproblems.Accordingly,theconstructionofgroyneswithoutassociatedbeachnourishmentshouldnotbeconsidered,astheerosionproblemwillbetransferredfurtheralongthebeach.AtSouthMissionBeachwherethereisnonewsandenteringthebeachcompartmentfromthesouth,virtuallyallsandwouldhavetobeimportedtoreconstructthebeach.

Groynesareoftenbuiltinconjunctionwithbeachnourishmentprogramswherethegroyneactsasanendstructuretoretainthenourishmentsandandextendthelifeoftheworks.However,thereisnonaturalendpointforsuchastructureonthisbeach.Intheabovesituationsgroyneswithbeachnourishmentwillhaveasignificantlyhighercostthanbeachnourishmentalone.

GroynesarenotconsideredanacceptablesolutionfortheerosionatSouthMissionBeachforthereasonslistedabove,andforthesignificantchangestheywouldcausetothevisualamenityandusabilityofthebeach.

Artificialreefscanmodifythelocalwaveclimateandextendthelifeofbeachnourishment.Buttheymayhavesimilardisadvantagestogroynes,anditisexpectedthattheywouldhaveasignificantlyhigherinvestigationandestablishmentcost.


7.0 ConclusionsItisconcludedthat:

i) ThesouthernendofSouthMissionBeachisexperiencingslowbutchroniclong-termerosion,wheresandisbeingtransportedfromthesouthernandcentralendsofthebeachanddepositedonthenorthernend.Thisisconsideredtobeanaturalcoastalprocessredistributingsedimentwithinthisbeachcompartment,withtransportcurrentlytothenorth.

ii) KennedyEsplanadefrontingSouthMissionBeach,isrelativelynarrowandhasbeendevelopedforroadandparkuse.Thisroadandassociatedinfrastructurearevulnerabletolossordamagefromseaerosionandshorelinerecessionthatmayoccurduringaseveretropicalcycloneduetothelimiteddunalbufferzoneseawardoftheroadandparkfacilities.

iii) Therevetmentwallconstructedalongthesouthernsectionofthiscoastisconsideredinadequatetopreventerosionduringaseveretropicalcyclone,andinsomesectionsthewallhasbeencompletelyineffectiveagainsterosion,withthescarpfaceretreatingbehindthewall,andmaterialfromthecollapsedwallbeingspreadoverthebeach.

iv) TherevetmentwallhasledtoanaccelerationoferosionandshorelineretreatinthesouthernpartofSouthMissionBeach,andcontinuationofthewallnorthwillacceleratetheprogressionoftheerosionproblemfurthernorth.Furthermore,constructionofthewallhasisolatedsandbehinditfromactivecoastalprocesses.Astheerosionprocesscontinuesandsedimentcontinuestoberemoved,theupperbeachinfrontofthewallwillnarrowandthebeachlevelwillgenerallydrop,affectingbeachamenityandusability.Itisconsideredthatthecontinueduseofarevetmentwallalonetoprovideprotectionandmitigatethelong-termtrendoferosionisnotaneffectivestrategyifbeachamenityistoberetained.

v) Therelativelyslowrateoferosionofthissectionofcoastmakesbeachnourishmentanattractiveerosionmanagementstrategyifasourceofsandatreasonablecostcanbeidentified.Beachnourishmentcanbothincreasethewidthofthedunalbufferzone,increasingthelevelofprotectiontoinfrastructureontheEsplanade,andmaintainorimprovebeachamenity.

vi) Infrastructurebehindthebeachisvulnerabletolossbyerosionduringextremecyclonicevents.Beachnourishment,inadditiontothatrequiredforthereplacementoferosionallosses,couldbeusedtowidenthebeachanddunesystemtoincreasethebufferingcapacityofthedunesystemagainstseaerosionduringstormevents.

vii) RockrevetmentprotectionworksfortheentirelengthofKennedyEsplanade(constructedtoanadequatestandard)wouldbeexpensive.Basedonanindicativecostof$2,000-$3,000/m,totalcostscouldexceed$3M.Thecostcouldprobablyonlybejustifiedifthepresentlevelofriskofstormerosiondamagetotheroadwasconsideredunacceptable.ThesectionofwallfromtheboatramptoJackeyJackeyStreetisofasignificantsizeandclosetotheroad,andshouldbeupgradedtoanappropriatestandard.Rubblerevetmentwallsfurthernorthshouldberemovedpriortotheplacementofnourishmentsand.ItshouldbenotedthatarockrevetmentwalleastoftheEsplanadeshouldonlybeconsideredfortheprotectionofCouncilassets.Worksfortheprotectionofprivatepropertyshouldbeconstructedwithinoradjacenttothefreeholdpropertyboundary.

viii) Thecurrentmethodofdisposingofstormwaterintosoakagepitsintheforedunehashadtheeffectofraisinggroundwaterlevels,fluidizingthesandandlocallyacceleratingtheerosionoftheupperbeach.Analternativestormwaterdisposalmethod,whichdischargesstormwaterdirectlytothebeach,isrequiredtoovercomethisproblem.

8.0 RecommendationsProvidedbelowarerecommendationsonpreferredactionsformanagingseaerosionatSouthMissionBeachforconsiderationbyCardwellShireCouncil:

i) Beachnourishmentshouldbeundertakenonthecentralandsouthernsectionsofthebeachtoreplacehistoricalseaerosionlosses,therebyre-establishingadunalzoneandraisingthebeachlevel.Thiswilloffsetthepresentsandlossfromthisareaandincreasetheprotectionfromfuturestormeventsprovidedbytheforedune.Aminimum20,000m3ofimportedsandisrequiredtorestorethebeachtothestatethatexistedsome30yearsago,andwouldprovideforthewideningoftheupperbeachbyupto7minthemosterodedsectionofthiscoast.

ii) Averageannualbeachrenourishmentatanannualisedrateof700m3shouldbeundertakentomatchongoinglosses.Thiscanbeachievedbyinclusionintheinitialnourishmentprogramofa10-yearlossprovision,orbyperiodictopups.

iii) Wideningofthedunalzonebybeachnourishmentinadditiontoi)andii)aboveshouldbeconsideredasameansofminimizingtheimpactoffutureerosioneventsoninfrastructureontheesplanade.Aminimumof20,000m3ofsandwouldberequired,dependingonthelevelofprotectionrequired.


iv) ThesouthernsectionofrockrevetmentwallfromtheboatramptoJackeyJackeyStshouldbeupgradedtoastandardappropriatetowithstanddamagefromaseverestormeventwithanARIof50yearsminimum.Rubblewallstothenorthshouldberemovedandthereconstructeddunerelieduponastheprimaryprotectionagainstwaveattackandseaerosion.

v) Theuseofsoakagepitsontheforeduneforstormwaterdissipationshouldbediscontinued,withstormwaterbeingdischargeddirectlyacrossthebeach.Thenumberofstormwateroutletsshouldbeminimisedasispracticalandstormwaterflowimpactsmanagedbytheuseofenergydissipaters,andwaterqualitymanagedthroughtheuseoflittertraps.DueregardshouldbegiventotherequirementoftheEnvironmentalProtection(Water)Policy1997forurbanstormwaterqualitymanagement.

vi) TheplacementofpermanentdevelopmentontheEsplanadeshouldbeavoided,butwherethisisnotpossible,locatedasfarlandwardasispractical.

vii) Allpublicinfrastructure,includingpower,waterandsewerageshouldbelocatedonthelandwardsideoftheroadtominimiseriskoflossduringfutureerosionevents.

viii) Parkfacilitiesshouldbelocatedasclosetotheroadedgeasispractical,ordesignedtoberelocatableintheeventofanerosionthreat.

ix) Monitoringofbeachprofilesshouldbeundertakenannuallytomonitormovementofthenourishmentsandandimproveestimatesofrenourishmentvolumes.

x) Anysandnourishmentprogramshouldincluderevegetationworkstoreinstatethesandtrappingdunevegetationtostabilisethedunesandpreventwinderosion.Thiswouldalsoneedtoincludecontrolofpedestrianaccesstothebeachandreplacementoftreespreviouslylosttoerosion.

9.0 Additional information9.1 Statutory obligationsAnyworksundertakenatSouthMissionBeachmayrequireanapprovalundertheCoastalProtectionandManagementAct1995,IntegratedPlanningAct1997andapermitundertheMarineParksAct1982administeredbytheEnvironmentalProtectionAgency.TheuseofoffshoresourcesofsandmaybeconstrainedbybeinglocatedintheGreatBarrierReefMarineParkworldheritageareaandaStateMarinePark,asdescribedintherelevantMarineParkZoningPlan.

9.2 State assistance with fundingCertainworksforbeachprotectionpurposes,includingbeachnourishmentandrevegetation,maybeeligibleforaStateGovernmentsubsidyof25percentofthecostoftheworksundertheLocalGoverningBodies’CapitalWorksLoanSubsidySchemeadministeredbytheDepartmentofLocalGovernmentandPlanning.Othertypesofworksmaybeeligibleforalowerlevelofsubsidy.


�0.0 ReferencesBeachProtectionAuthority(1997).StormTideStatistics–CardwellRegion,ConservationStormTideDataReportNo1.DepartmentofEnvironment,Queensland.

CoastalManagementBranch(1997).Management of the Dune/BeachRidgeSystematSouthMissionBeach.Internalreport.DepartmentofEnvironment,Queensland.

EnvironmentalProtectionAgency(2003).Cardwell-HinchinbrookRegionalCoastalManagementPlan.QueenslandGovernment.

Holmes,K.H.(1993).TullyRiverCoastalSector.MarineandCoastalGeologyUnitProjectReportMA50/4,DepartmentofMineralsandEnergy,Queensland.

JamesCookUniversity(2003)http://tsunami.jcu.edu.au/atlas/wave_atlas.shtml

Masslink,G.andG.Lessa.(1995).Barrier Stratigraphy on theMacrotidalCentralQueenslandCosatline,Australia.JournalofCoastalResearch,Vol11No2,pp454-477.

Pye,K.(1981).RateofDuneReddeninginaHumidTropicalClimate.MacmillianJournalsLtd.

Rust,PPK(1995)StormTideStatistics–Cardwellregion.

Taylor,M.andG.W.Stone,(1996).BeachRidges: AReview.JournalofCoastalResearch,Vol.12No3,pp612-621.

Vellinga,P.(1983).PredictiveComputationalModelforBeachandDuneErosionDuringStormSurges.Proceedings,AmericanSocietyofCivilEngineersSpecialtyConferenceonCoastalStructures,1983,pp806-819.

Coastal erosion investigation and management options...

Documents

Transcript of Coastal erosion investigation and management options...