Shoal

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Sandbar between St. Agnes, Isles of Scilly and Gugh in Cornwall
Sandbar between St. Agnes, Isles of Scilly and Gugh in Cornwall
A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji
A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji

A shoal is a somewhat linear landform within or extending into a body of water, typically composed of sand, silt or small pebbles. Alternatively termed sandbar or sandbank, a bar is characteristically long and narrow (linear) and develops where a stream or ocean current promote deposition of granular material, resulting in localized shallowing (shoaling) of the water. Bars can appear in the sea, in a lake, or in a river. Alternatively a bar may separate a lake from the sea, as in the case of an ayre. They are typically composed of sand, although could be of any granular matter that the moving water has access to and is capable of shifting around (for example, soil, silt, gravel, cobble, shingle, or even boulders). The grain size of the material comprising a bar is related to the size of the waves or the strength of the currents moving the material, but the availability of material to be worked by waves and currents is also important.

The term bar can apply to landform features spanning a considerable range in size, from a length of a few meters in a small stream to marine depositions stretching for hundreds of kilometres along a coastline, often called barrier islands.

In a nautical sense, a bar is a shoal, similar to a reef: a shallow formation of (usually) sand that is a navigation or grounding hazard. It therefore applies to a silt accumulation that shallows the entrance to or the course of a river or creek.

Contents

A sandbar off of Suffolk County, Long Island, New York, August 2006.
A sandbar off of Suffolk County, Long Island, New York, August 2006.
Shoals in the Mississippi River at Arkansas and Mississippi.
Shoals in the Mississippi River at Arkansas and Mississippi.

Bars that occur at or off the shoreline of a sea or a lake are related to beaches and might be considered offshore features of a beach (Bascom, 1980). At times when larger waves attack the beach berm, some of the beach material is redistributed offshore to become a longshore bar or sandbar, possibly visible at low tide. This bar forms (sometimes seaward of a trough) where the waves are breaking, because the breaking waves set up a shoreward current with a compensating counter-current along the bottom. Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break (Bascom, 1980). Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.

In addition to longshore bars discussed above that are relatively small features of a beach, the term shoal can be applied to larger geological units that form off a coastline as part of the process of coastal erosion. These include spits and baymouth bars that form across the front of embayments and rias. A tombolo is a bar that forms an isthmus between an island or offshore rock and a mainland shore.

The largest of the geological units of this kind is a barrier island, such as occur along the East Coast of the United States, along the Gulf coast, along the southern coast of Belize and many other locations worldwide.

In places of re-entrance along a coastline (such as inlets, coves, rias, and bays), sediments carried by a longshore current will fall out where the current dissipates, forming a spit. An area of water isolated behind a large bar is called a lagoon. Over time, lagoons may silt up, becoming salt marshes.

In some cases shoals may be precursors to beach expansion and dunes formation, providing a source of windblown sediment to augment such beach or dunes landforms.[1]

 This article or section may contain original research or unverified claims.
Please improve the article by adding references. See the talk page for details. (October 2007)

  1. ^ Mirko Ballarini, Optical Dating of Quartz from Young Deposits, IOS Press, 2006 146 pages, ISBN 158603616

  • Bascom, W. 1980. Waves and Beaches. Anchor Press/Doubleday, Garden City, New York. 366 p.
Retrieved from "http://en.wikipedia.org/wiki/Shoal"
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