Tombolo: Wikis

  
  

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Tombolo at Stockton Island, Apostle Islands, Wisconsin.

A tombolo (Italian, from Latin tumulus – mound) or sometimes ayre (Old Norse Eyrr – gravel beach) is a deposition landform in which an island is 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 is called a tombolo cluster.[1] Two tombolos form an enclosure called a lagoon that might eventually fill with sediment.

Contents

Formation

The act of a spit reaching out to an island is far from random. By looking at the geomorphology of tombolos it is possible to understand how they are made.

Wave Refraction

"True" Tombolos are formed by wave refraction. As waves near an island they are slowed down by the shallow water surrounding it. These waves then refract or “bend” around the island to the opposite side as they approached. The wave pattern created by this water movement causes a convergence of longshore drifting 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 [2].

Lateral Longshore Drift

In the case of Chesil Beach or Spurn Head, the flow of material is along the coast in a movement which is not determined by the now tied island, such as Portland, which it has reached. In this and similar cases, whilst 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 or otherwise.

Morphology and man

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 the easy weathering, tombolos are sometimes man-made more sturdy as roads and maybe 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 at finer grained material at the top revealing coarser sands and cobbles as the base. Eustatic sea level rise may also contribute to accretion as material is pushed up with rising sea levels. This is the case with Chesil Beach (which connects the Isle of Portland to Dorset in England), notable because the shingle ridge is parallel rather than perpendicular to the coast.

Tombolos help to understand the sensitivity of shorelines. A small piece of land, such as an island, can change the way that the waves are moving which then leads to different deposition of sediments.

List of notable tombolos

See also

References

  1. ^ Glossary of Geology and Related Sciences. The American Geological Institute, 1957
  2. ^ Easterbrook, Don T. Surface Processes and Landforms, Second Edition. 1999 Prentice Hall Inc.

External links








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