A tombolo is a sandy or shingle isthmus. It is a deposition landform by which an island becomes attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island. The word tombolo is from the Italian tombolo, meaning 'pillow' or 'cushion', and sometimes translated incorrectly as ayre (an ayre is a shingle beach of any kind). Several islands tied together by bars which rise above the water level are called a tombolo cluster. Two or more tombolos may form an enclosure (called a lagoon) that can eventually fill with sediment.
Formation
The shoreline moves toward the island (or detached breakwater) owing to the accretion of sand in the lee of the island, where wave energy and longshore drift are reduced and therefore deposition of sand occurs.
Wave diffraction and refraction True tombolos are formed by wave refraction and diffraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then bend around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drift on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there, conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up, the beach shoreline, known as a spit, will connect with an island and form a tombolo.
Unidirectional longshore drift In the case of longshore drift due to an oblique wave direction, like at Chesil Beach or Spurn Head, the flow of material is along the coast in a movement which is not determined by wave diffraction around the now tied island, such as the Isle of Portland, which it has reached. In this and similar cases like Cádiz, while the strip of beach material connected to the island may be technically called a tombolo because it links the island to the land, it is better thought of in terms of its formation as a spit, because the sand or shingle ridge is parallel rather than at right angles to the coast.
Morphology and sediment distribution Tombolos demonstrate the sensitivity of shorelines. A small piece of land, such as an island, or a beached shipwreck can change the way that waves move, leading to different deposition of sediments. Sea level rise may also contribute to accretion, as material is pushed up with rising sea levels. Tombolos are more prone to natural fluctuations of profile and area as a result of tidal and weather events than a normal beach is. Because of this susceptibility to weathering, tombolos are sometimes made more sturdy through the construction of roads or parking lots. The sediments that make up a tombolo are coarser towards the bottom and finer towards the surface. It is easy to see this pattern when the waves are destructive and wash away finer grained material at the top, revealing coarser sands and cobbles as the base.
Examples
Some of these may be simple isthmuses, and not have the deposition creation that defines a true tombolo.
Image gallery
See also
Ayre (landform) – Shingle beaches in Orkney and Shetland Bar – Natural submerged sandbank that rises from a body of water to near the surfacePages displaying short descriptions of redirect targets Causeway – Route raised up on an embankment Cuspate foreland – Geographical features found on coastlines and lakeshores Isthmus – Strip of land connecting two larger areas Peninsula – Land feature Tied island – Island that is connected to land only by a tombolo Shoal – Natural submerged sandbank that rises from a body of water to near the surface
References
External links
Geology.About.com's page on tombolos (useful for its descriptive photograph) Tombolo in Sainte-Marie, Martinique (useful for its photos and description) further reading on Detached breakwaters from Vlaams Instituut voor de Zee in Belgium further reading on coastal structures from Prof. Leo van Rijn in Holland
