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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