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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.
[edit] Sandbars and longshore bars
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.
[edit] Harbour and river bars
A harbour or river bar is a sedimentary deposit formed at a harbour entrance or river mouth by the deposition of sediment or the action of waves on the sea floor or adjacent beaches. A bar can form a dangerous obstacle to shipping, preventing access to the river or harbour in unfavourable weather conditions or at some states of the tide. Where beaches are suitably mobile, or the river’s suspended and/or bed loads are large enough, wave action can build up a bar to completely block a river mouth, damming the river, preventing access for boats or shipping, and causing flooding in the lower reaches of the river. This situation will persist until the bar is eroded by the sea, or the dammed river develops sufficient head to break through the bar.
[edit] Shoals as geological units
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]
[edit] Federal Laws
[edit] Examples
- Abidjan, Côte d'Ivoire
- Amelia Island, Florida, United States
- Anna Maria Island, Florida, United States
- Ap Lei Chau - Ap Lei Pai, Hong Kong, China
- Atlantic City, New Jersey, United States
- Barnegat Bay, New Jersey, United States
- Beecher Island, Colorado, United States
- Brigantine, New Jersey, United States
- Cape Canaveral, Florida, United States
- Cape St. Paul, Ghana
- Cheung Chau, Hong Kong, China
- Cumberland Island, Georgia, United States
- Dauphin Island, Alabama, United States
- Ediz Hook, Port Angeles, Washington, United States
- Fire Island, New York, United States
- Fort George Island' Florida, United States
- Galveston Island, Texas, United States
- Grand Isle, Louisiana, United States
- Great Yarmouth, England
- Hel Peninsula, Poland
- Hilton Head Island, South Carolina, United States
- Hutchinson Island, Florida, United States
- Lung Kwu Chau, Hong Kong, China
- Loe Bar, Cornwall, England
- Long Point, Ontario, Canada
- Longboat Key, Florida, United States
- Macau Isthmus, Macau, China
- Miami Beach, Florida, United States
- Minnesota Point, Duluth, Minnesota, United States
- Mission Beach, San Diego, California, United States
- Ninety Mile Beach, Victoria, Australia
- Ocean City, Maryland, United States
- Outer Banks, North Carolina, United States
- Padre Island, Texas, United States
- Port Said, Egypt
- Ponte Vedra Beach,Florida, United States
- Pui O, Hong Kong, China
- Qijin Island, Kaohsiung, Taiwan
- Rockaway, Queens, New York City, New York, United States
- Sable Island, Nova Scotia, Canada
- Santa Rosa Island, FL, Florida, United States
- Sandbar Resort, Boquete Island, Puerto Galera, Mindoro, Philippines
- Sha Chau, Hong Kong, China
- Shek O Headland - Tai Tau Chau, Hong Kong, China
- Ship Island, Mississippi, United States
- Sriharikota, Andhra Pradesh, India
- Stinson Beach, California, United States
- Talbot Island, Big, Florida, United States
- Talbot Island, Little, Florida, United States
- The Coorong, South Australia, Australia
- Twin Lakes, California, United States
- Vistula Spit, Poland
- Wadden islands, also known as West Frisian Islands, along the Dutch, German, and Danish coasts.
- Winona, Minnesota, United States
- Yim Tin Tsai - Ma Shi Chau, Hong Kong, China
- Zingst Peninsula, Germany
[edit] See also
[edit] Line notes
- , Mirko Ballarini, Optical Dating of Quartz from Young Deposits, IOS Press, 2006 146 pages, ISBN 158603616
[edit] References
- Bascom, W. 1980. Waves and Beaches. Anchor Press/Doubleday, Garden City, New York. 366 p.
(Source: Wikipedia) |
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