The Role of Wave Diffraction in the Formation of St. Ninian's Ayre (Tombolo) in Shetland, Scotland

Abstract

St. Ninian's Ayre (sand tambala), on the south-west coast of Shetland, occurs in an environment of deeply plunging hard-rock cliffs which enclose the area containing the tombolo so that the maximum fetch of local 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 tambala is so deep that wave refraction in the vicinity is restricted. Thus, the various explanations usually offered for the origin of tombolos are precluded or incomplete. Satellite image and airphotos show that wave patterns in the vicinity of the tombolo result from the prevailing ocean swell from the west being diffracted round the island, on either side. As the sea shallows, the diffracted swell is increasingly modified by refraction, before breaking on either side of the tombolo. The shorelines on either side of the tombolo are nearly parallel to the crests of the diffracted and refracted swell-derived waves that these break along the length of the tambala at the same instant. Local wind driven waves have little effect. The tombolo owes its position and shape directly to the diffraction and refraction of the ocean-swell waves transporting sand from the adjacent sea floor into the area where the waves meet behind the island. Longshore drift acts only within the confines of the tombolo and matches its shape to that the crests of the waves break over it.