Tombolo nearKarystos,Euboea, GreeceTombolo contrasted with other coastal landforms
Atombolo is a sandy or shingleisthmus. It is adepositionlandform by which an island becomes attached to the mainland by a narrow piece of land such as aspit orbar.[1] Once attached, the island is then known as atied island. The wordtombolo is from theItaliantombolo, meaning 'pillow' or 'cushion', and sometimes translated incorrectly asayre (an ayre is ashingle beach of any kind).
Several islands tied together by bars which rise above the water level are called atombolo cluster.[2] Two or more tombolos may form an enclosure (called alagoon) that can eventually fill with sediment.
The shoreline moves toward the island (or detachedbreakwater) owing to the accretion of sand in the lee of the island, where wave energy andlongshore drift are reduced and therefore deposition of sand occurs.
True tombolos are formed bywave refraction anddiffraction. 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.[3]
In the case of longshore drift due to an oblique wave direction, like atChesil Beach orSpurn Head, the flow of material is along the coast in a movement which is not determined by wave diffraction around the nowtied island, such as theIsle of Portland, which it has reached. In this and similar cases likeCádiz, 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 aspit, because the sand or shingle ridge is parallel rather than at right angles to the coast.
Tombolos demonstrate the sensitivity ofshorelines. 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.
^Glossary of Geology and Related Sciences. The American Geological Institute, 1957
^Easterbrook, Don T. (1999).Surface Processes and Landforms (Second ed.). Prentice Hall.ISBN0-13-860958-6.
^Neal, William; Orrin H. Pilkey; Joseph T. Kelley (2007).Atlantic Coast Beaches: A Guide to Ripples, Dunes, and Other Natural Features of the Seashore. Missoula, MT: Mountain Press Publishing Company. p. 272.ISBN978-0-87842-534-1.
