THE ROLE OF WAVE DIFFRACTION IN THE FORMATION OF ST-NINIAN AYRE (TOMBOLO) IN SHETLAND, SCOTLAND

St. Ninian's Ayre (sand tombolo), on the south-west coast of Shetland, occurs in an environment of deeply plunging hard-rock cliffs which en close the area containing the tombolo so that the maximum fetch of loc al wind-driven waves is no more than 2 km. As a result of the presence of the plunging cliffs, there are no adjacent beaches to supply sand to the tombolo by longshore drift. The sea around St. Ninian's Island, at the seaward end of the tombolo, is so deep that wave refraction in the vicinity is restricted. Thus, the various explanations usually of fered for the origin of tombolos are precluded or incomplete. Satellit e image and airphotos show that wave patterns in the vicinity of the t ombolo result from the prevailing ocean swell from the west being diff racted round the island, on either side. As the sea shallows, the diff racted swell is increasingly modified by refraction, before breaking o n either side of the tombolo. The shorelines on either side of the tom bolo are nearly parallel to the crests of the diffracted and refracted swell-derived waves that these break along the length of the tombolo at the same instant. Local wind driven waves have little effect. The t ombolo owes its position and shape directly to the diffraction and ref raction of the ocean-swell waves transporting sand from the adjacent s ea floor into the area where the waves meet behind the island. Longsho re drift acts only within the confines of the tombolo and matches its shape to that the crests of the waves break over it.