Tombolo

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A tombolo is a sandy or shingle isthmus. A tombolo, from the Italian , meaning 'pillow' or 'cushion', and sometimes translated incorrectly as ayre (an ayre is a shingle beach of any kind), 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.

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) due to 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

Adam's Bridge (until 1480), between India and Sri Lanka
Aupōuri Peninsula, New Zealand
Barrenjoey Headland, Pittwater, New South Wales, Australia
Beavertail Point, Conanicut Island, Rhode Island, United States
Bennett Island, De Long Group, Russia
Biddeford Pool, Maine, United States
Bijia Mountain, China
Bruny Island, Tasmania, Australia
Burgh Island, Devon, England
Cádiz, Andalucía, Spain
Chausey, Manche département, France (two features connecting the main island and two smaller outcrops)
Chappaquiddick Island, Martha's Vineyard, Massachusetts, United States
Charles Island, Connecticut, United States
Chesil Beach, Portland, Dorset, England
Cheung Chau, Hong Kong
Crimea, Ukraine
Eaglehawk Neck, Tasmania, Australia
S'Espalmador, Formentera, Spain
Fingal Bay, New South Wales, Australia
The Rock of Gibraltar
Grand Island National Recreation Area, Michigan, United States
Gugh, St Agnes, Isles of Scilly, England
Gwadar, Pakistan
Hakodate, Hokkaido, Japan
Howth Head, Dublin, Ireland
Inishkeel Island, Narin, Ireland
Kapıdağ Peninsula, Balıkesir, Turkey
Knappelskär, Nynäshamn, Sweden
Konet Island, Kuala Sungai Baru, Malacca, Malaysia
Kurnell, Sydney, New South Wales, Australia
Lake Pomorie, Bulgaria
Langness, Derbyhaven, Isle of Man
Las Palmas de Gran Canaria, Canary Islands, Spain
Little Heart's Ease, Newfoundland and Labrador beach, Newfoundland Canada
Llandudno, North Wales
Louds Island at Muscongus Bay, Maine, United States
Maharees, Dingle Peninsula, Ireland
Mare Island, Vallejo, California, United States
Maria Island, Tasmania
Maury Island, Washington, United States
McMicken Island State Park, Washington, United States
Miquelon, Saint-Pierre and Miquelon, France
Monemvasia, Laconia, Peloponnese, Greece
Monte Argentario, Tuscany, Italy
Mont-Saint-Michel, Normandy, France
Moses' Pass (Whale Tail), Ballena National Marine Park, Uvita, Costa Rica
Mount Maunganui, New Zealand
Mount Taipingot, Rota, Northern Marianas
Nahant, Massachusetts, United States (a natural tombolo, but connected to the mainland by a causeway)
Nissi beach, Ayia Napa, Cyprus
Ormara, Pakistan
Palisadoes, Kingston, Jamaica
Peniche, Portugal
Peniscola, Castellon, Spain
Porchat Island, Itararé Beach, São Vicente, Brazil
Presqu'ile Provincial Park, Ontario, Canada
Presque Isle, Michigan, United States
Presqu'ile de Giens, Hyères, France
Quiberon, France
Ras Hafun, Somalia
Sainte-Marie, Martinique, France
Scotts Head, Dominica
Shaman's Island, Douglas, Alaska, United States
Sharp Island, Sai Kung District, Hong Kong
Silver Strand (San Diego), Coronado, California, United States
St Michael's Mount, Cornwall, England
St Ninian's Isle, Shetland Islands, Scotland
Sveti Stefan, near Budva, Montenegro
Tam Hai, Quang Nam province, Vietnam
University Beach, Ward Island, Corpus Christi, Texas, United States
Uummannaq in North Star Bay, Greenland
Vatersay, Scotland – the island comprises two sections connected by a broad tombolo
Zhifu Island, Yantai, China

Some of these may be simple isthmuses, and not have the deposition creation that defines a true tombolo.