UNDERSTANDING A “PERFECT” WAVE; A STUDY OF GIANT SURF

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Introduc)on“Aileens”isthenamegivenbysurferstotheinterna6onallyrenownedwavewhichbreaksbelowthe200mhighCliffsofMoher,onthenorthwestcoastofCo.Clare,4kmtothesouthwestofDoolinvillage.Asagiantplungingbreaker(right),ithasasteepfacewhichallowsforhighboardspeed,andalargevortexortubewhichallowssurferstoride“inthebarrel”.

Objec)vesTheaimofthisprojectwastoiden6fythefactorsproducingsuchaphenomenalwave.Syndeposi6onalfaul6ngoccursalongtheCliffsofMoher(Wignall&Best2004)anditwashypothesisedthatinthiscaseithadcausedaseabedtopographicfeature,comprisingashoalofshallowwaterprotrudingintodeeperwater.Thisshallowwaterwouldcausewaverefrac6onandwavefocusing,withasubsequentincreaseinamplitude.Thisisanintegralpartofmanyworldclasssurfingloca6ons(Scarfeetal.2003).Bathymetrysurveyswerecarriedoutanddataenteredintoaflexiblemeshdomain(farright)whichallowedcomputermodellingofsurfcondi6ons.

DiscussionSyndeposi6onalfaul6nghasledtoashallowreefbeingformedofturbiditesandstonebeds,surroundedbydeeperwaterwheresoRershalehasbeenpreferen6allyeroded.Duetowaverefrac6onthisshallowareacausesconcavefocusingofwaveenergy,leadingtoanincreaseinwaveheight.Thesuddentransi6onfromdeeptoshallowwatercausesthewavestobreaksteeply(Scarfeetal.2003).WavemodelingusingMike21showsmaximumwaveheightsofninemeters,whenthemodelisforcedwitha4.4mswell,travelingfromthewestwithaperiodof13s.Thisagreesstronglywithobserva6onsmadebysurfers.Thisstudy,anundergraduatethesisproject,hashighlightedhowtheinterplayofgeologicalandoceanicprocessesproduceswhatisoneofEurope'stopsurfingwavesandoneofIreland’smosticoniccoastalfeatures.

UNDERSTANDINGA“PERFECT”WAVE;ASTUDYOFGIANTSURFBENEATHTHECLIFFSOFMOHER

ALEXANDERHART,SIDDHIJOSHI,GARRETDUFFY,DAMIENGUIHEN,MARTINWHITEDepartmentofEarthandOceanSciences,SchoolofNaturalSciences,Na6onalUniversityofIreland,Galway

ReferencesHODSON,F.2001.ThebedsabovetheCarboniferousLimestoneinNorth‐WestCountyClare,Eire.QuarterlyJournaloftheGeologicalSociety.109,259‐283MEAD,S.T.&BLACK,K.P.2001.Fieldstudiesleadingtothebathymetricclassifica6onofworld‐classsurfingbreaks.In:BLACK,K.P.(ed,),NaturalandAr:ficialReefsforSurfingandCoastalProtec:on,JournalofCoastalResearch,SpecialIssueNo.29,pp.5‐20SCARFE,B.E.,ELWANY,M.H.S.,MEAD,S.T.,&BLACK,K.P.2003.TheScienceofSurfingWavesandSurfingBreaks;AReview.ScrippsIns:tu:onofOceanographyTechnicalReport,ScrippsIns:tu:onofOceanography,UCSanDiegoSEVASTOPULOG.D.2009.Carboniferous(Silesian).In:TheGeologyofIreland.DunedinAcademicPressLtd,Edinburgh.WIGNALL,P.B.&BESTJ.L.2004.Sedimentologyandkinema6csofalarge,retrogressivegrowth‐faultsysteminUpperCarboniferousdeltaicsediments,westernIreland.Sedimentology51,1343–1358

BathymetryTwobathymetrysurveyswereundertakentodeterminethemorphologyoftheseabeduponwhichthewavebreaks.AKongsbergSimradEM3002mul6beamdepthprofileraboardtheRVCel:cVoyagerwasusedtomaptheareaoffshoreofthereef.AsmallcraRwithasinglebeamechosounder,mountedonastableoutrigger,wasusedtostudytheshallowestareasfollowingaprocedurepioneeredbyMead&Black(2001).

Above:Apparatususedforshallowwaterbathymetrysurvey.Aboveright:Compositeimageofsinglebeamandmul6beambathymetrydatasets.

GeologyPhotographsofthecliff(above)takenfromtheseashowalargesyndeposi6onalnormalfault,responsiblefortheforma6onofthereef,asthesandstoneismoreresistanttoerosionthanshale.Stra6graphydeterminedaRerHodson(1953)andSevastopulo(2009).Thisfaultcanalsobeseeninaerialphotographsandinthemul6beambathymetryimage(below).

NumericalmodelingBathymetrydatawereinputintoasmoothedflexiblemeshdomainintheMike21SpectralWavemodel.Gridspacingwassmallestintheareaoverthereef,ascalcula6onsareperformedforeachindividualnodeofthemesh.AsthewavesatAillenasharraghhavebeenobservedtobreakbestatlow6de,a6dalheightofonemeterwassimulated.ThemodelwasforcedwithdifferentswellscenariosbasedonrecordingsfromtheMarineIns6tutesM1Buoy(bo0omright).Asswellreachesshallowerwater,wavecrestsbecomealignedwiththecontoursofthereef,andfocuswaveenergytowardsit(le2,topandbo0om).Concentra6ngwaveenergyinasmallerarealeadstoanincreaseinamplitude,resul6nginthegiantbreakersobserved(topright).Greaterrefrac6onwasseentooccurinlargerswellandinswellwithlongerwavelengths.

(A)Georeferencedimageshowinga3Dcompositeofbothbathymetrydatasets,viewedfromthesouthwest.Thelargerimageisacloseup,clearlyshowingthefeatureuponwhichAileensbreaks.Inordertoaccentuatefeaturestheimageisshownwithten:mesver:calexaggera:on.(B)isproducedfromthesinglebeamsurvey.Thewavebreaksatthepinnaclejustabove(C).(D)isfromthemul:beamsurvey,henceitshowsfarmoredetail.Thelargespikesinthisimageareprobablyboulders,whichhaverolledtotheboRomofthereef,andappearelongatedduetothever:calexaggera:on.Thefaultrunsthroughhere,butcannotbeseenfromthisangle.Parallellines(E)throughouttheimageareasinusoidalheaveerrorintroducedasthesurveyvesselmovedupanddown.(F)isprobablyabeddingplane.Alineofraisedground(G)runningparalleltothefaultisprobablyrelatedtoit,andispossiblyanotherfaultaxis.

N

Shallow Water

Tullig Cyclothem

Deep Water

Gull Island Fm.

Clare Shale

Fault

Cronogort Sandstone Member

52.985

52.980

52.975

Scenario SwellHeight(Meters)

SwellPeriod

(Seconds)

SignificantWaveHeight(Meters)

MaximumWaveHeight(Meters)

A 4.4 13 5.1‐5.3 8.8‐9.1

B 4 16 5.1‐5.3 8.8‐9.1

C 4 10 4.1‐4.3 7.3‐7.6

D 2.5 10 2.9‐3.1 5.2‐5.5

E 2.5 16 3.8‐4.0 6.7‐7.0

MeanWaveDirec6onshowingwaverefrac6onaroundthereefinScenariosB(topleR)andC(booomleR).

52.985

52.980

52.975

‐9.44‐9.43‐9.43 ‐9.425

52.99

52.98

52.97

‐9.45‐9.44‐9.43 ‐9.42

Above285°

275°

255°

265°

245°

235°

Below230°

98

543210

67

MaximumWaveHeights(Meters)InScenarioB

20

100m

25

30

35

Fault

N

Depth(M

eters)

MAP

N0

‐8

‐20

‐28

‐36

‐44

‐56

Depth(Meters)

1kmAranIs.

CliffsofMoher

Doolin

Coastline

200m