UNDERSTANDING A “PERFECT” WAVE; A STUDY OF GIANT SURF BENEATH THE CLIFFS OF MOHER ALEXANDER HART, SIDDHI JOSHI, GARRET DUFFY, DAMIEN GUIHEN, MARTIN WHITE Department of Earth and Ocean Sciences, School of Natural Sciences, Na onal University of Ireland, Galway
Introduc on 0 “Aileens” is the name given by surfers to the interna onally renowned wave which N ‐8 breaks below the 200m high Cliffs of Moher, on the northwest coast of Co.Clare, 4km to ‐20 the southwest of Doolin village. As a giant plunging breaker (right), it has a steep face which allows for high board speed, and a large vortex or tube which allows surfers to ‐28 ride “in the barrel”. ‐36 Objec ves MAP ‐44 The aim of this project was to iden fy the factors producing such a phenomenal wave. s ‐56 Syndeposi onal faul ng occurs along the Cliffs of Moher (Wignall & Best 2004) and it Depth was hypothesised that in this case it had caused a seabed topographic feature, (Meters) comprising a shoal of shallow water protruding into deeper water. This shallow water 1 km would cause wave refrac on and wave focusing, with a subsequent increase in Aran Is. amplitude. This is an integral part of many world class surfing loca ons (Scarfe et al. Doolin 2003). Bathymetry surveys were carried out and data entered into a flexible mesh Cliffs of Moher domain (far right) which allowed computer modelling of surf condi ons.
Coastline N Fault
200m Tullig Cyclothem
Gull Island Fm.
Shallow Deep Water Cronogort Sandstone Member Water
Clare Shale Geology Photographs of the cliff (above) taken from the sea show a large Bathymetry (A) Georeferenced image showing a 3D composite of both bathymetry datasets, viewed from the southwest. The larger image syndeposi onal normal fault, responsible for the forma on of the Two bathymetry surveys were undertaken to determine the is a close up, clearly showing the feature upon which Aileens breaks. reef, as the sandstone is more resistant to erosion than shale. morphology of the seabed upon which the wave breaks. A over In order to accentuate features the image is shown with ten mes Stra graphy determined a er Hodson (1953) and Sevastopulo Kongsberg Simrad EM 3002 mul beam depth profiler aboard the ver cal exaggera on. (B) is produced from the singlebeam survey. (2009). This fault can also be seen in aerial photographs and in the RV Cel c Voyager was used to map the area offshore of the reef. The wave breaks at the pinnacle just above (C). (D) is from the mul beam bathymetry image (below). A small cra with a singlebeam echosounder, mounted on a mul beam survey, hence it shows far more detail. The large spikes in stable outrigger, was used to study the shallowest areas following 20 this image are probably boulders, which have rolled to the bo om of a procedure pioneered by Mead & Black (2001). the reef, and appear elongated due to the ver cal exaggera on. The N fault runs through here, but cannot be seen from this angle. Parallel Depth (Meters) lines (E) throughout the image are a sinusoidal heave error introduced as the survey vessel moved up and down. (F) is probably 25 Above: Apparatus used for shallow water bathymetry survey. a bedding plane. A line of raised ground (G) running parallel to the Above right: Composite image of singlebeam and mul beam fault is probably related to it, and is possibly another fault axis. bathymetry datasets. Fault
30
Above 285° 52.99 9 52.985 100m 8 Maximum 275° 7 Wave Heights 265° 6 (Meters) In 35 52.980 Scenario B 255° 52.98 5 4 245° 3 2 235° 52.975 1 Below 230° 52.97 0 Discussion Syndeposi onal faul ng has led to a shallow reef being formed of ‐9.44 ‐9.43 ‐9.43 ‐9.425 ‐9.45 ‐9.44 ‐9.43 ‐9.42 turbidite sandstone beds, surrounded by deeper water where so er 52.985 Mean Wave Scenario Swell Swell Significant Maximum shale has been preferen ally eroded. Due to wave refrac on this Direc on showing Height Period Wave Height Wave Height shallow area causes concave focusing of wave energy, leading to an wave refrac on (Meters) (Seconds) (Meters) (Meters) increase in wave height. The sudden transi on from deep to shallow around the reef in A 4.4 13 5.1‐5.3 8.8‐9.1 water causes the waves to break steeply (Scarfe et al. 2003). Wave 52.980 Scenarios B (top modeling using Mike 21 shows maximum wave heights of nine B 4 16 5.1‐5.3 8.8‐9.1 le ) and C meters, when the model is forced with a 4.4m swell, traveling from (bo om le ). C 4 10 4.1‐4.3 7.3‐7.6 the west with a period of 13s. This agrees strongly with observa ons 52.975 D 2.5 10 2.9‐3.1 5.2‐5.5 made by surfers. This study, an undergraduate thesis project, has E 2.5 16 3.8‐4.0 6.7‐7.0 highlighted how the interplay of geological and oceanic processes produces what is one of Europe's top surfing waves and one of Numerical modeling Ireland’s most iconic coastal features. Bathymetry data were input into a smoothed flexible mesh domain in the Mike 21 Spectral Wave model. Grid spacing was smallest in the area over the reef, as calcula ons are performed for each individual node of the mesh. As the waves at Aillenasharragh have been observed to break best at low de, a dal height of one meter was simulated. The model was forced with different swell scenarios based on recordings from the Marine Ins tutes M1 Buoy (bo om right). As swell reaches shallower water, wave crests become aligned with the contours of the reef, and focus wave energy towards it (le , top and bo om). Concentra ng wave energy in a smaller area leads to an increase in amplitude, resul ng in the giant breakers observed (top right). Greater refrac on was seen to occur in larger swell and in swell with longer wavelengths.
References HODSON, F. 2001. The beds above the Carboniferous Limestone in North‐West County Clare, Eire. Quarterly Journal of the Geological Society. 109, 259‐283 MEAD, S.T. & BLACK, K.P. 2001. Field studies leading to the bathymetric classifica on of world‐class surfing breaks. In: BLACK, K.P. (ed,), Natural and Ar ficial Reefs for Surfing and Coastal Protec on, Journal of Coastal Research, Special Issue No.29, pp. 5‐20 SCARFE, B. E., ELWANY, M. H. S., MEAD, S. T., & BLACK, K. P. 2003. The Science of Surfing Waves and Surfing Breaks; A Review. Scripps Ins tu on of Oceanography Technical Report, Scripps Ins tu on of Oceanography, UC San Diego SEVASTOPULO G.D. 2009. Carboniferous (Silesian). In: The Geology of Ireland. Dunedin Academic Press Ltd, Edinburgh. WIGNALL, P.B. & BEST J.L. 2004. Sedimentology and kinema cs of a large, retrogressive growth‐fault system in Upper Carboniferous deltaic sediments, western Ireland. Sedimentology 51, 1343–1358