Shoal
In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body of water close to the surface or above it, which poses a danger to navigation. Shoals are also known as sandbanks, sandbars, or gravelbars. Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as a shoal complex.[1][2]
The term shoal is also used in a number of ways that can be either similar to, or quite different from, how it is used in geologic, geomorphic, and oceanographic literature. Sometimes, the term refers to either any relatively shallow place in a
Description
Shoals are characteristically long and narrow (linear) ridges. They can develop where a
Shoals can appear as a
A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as:
- Marine lagoons
- Brackish water estuaries
- Freshwater seasonal stream and river mouths and deltas.
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 kilometers along a coastline, often called barrier islands.
Composition
They are typically composed of
Formation
Shoaling can also refract waves, so the waves change direction. For example, if waves pass over a sloping bank which is shallower at one end than the other, then the shoaling effect will result in the waves slowing more at the shallow end. Thus, the wave fronts will refract, changing direction like light passing through a prism. Refraction also occurs as waves move towards a beach if the waves come in at an angle to the beach, or if the beach slopes more gradually at one end than the other.
Types
Sandbars and longshore bars
Sandbars, also known as a trough bars, form where the waves are breaking, because the breaking waves set up a shoreward current with a compensating counter-current along the bottom. Sometimes this occurs seaward of a trough (marine landform).
Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break.[3] Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.
Peresyp
In Russian tradition of geomorphology, a peresyp is a sandbar that rises above the water level (like a spit) and separates a liman or a lagoon from the sea. Unlike tombolo bars, a peresyp seldom forms a contiguous strip and usually has one or several channels that connect the liman and the sea.[4]
Harbor and river bars
A harbor or river bar is a sedimentary deposit formed at a harbor entrance or river mouth by the deposition of freshwater sediment or by the action of waves on the sea floor or on up-current beaches.
Where beaches are suitably mobile, or the river's
The formation of harbor bars that prevent access for boats and shipping can be the result of:
- construction up-coast or at the harbor — e.g.: breakwaters, dune habitat destruction.
- upriver development — e.g.: dams and reservoirs, riparian zone destruction, river bank alterations, river adjacent agricultural land practices, water diversions.
- gradingfor development.
- artificially created/deepened harbors that require periodic dredging maintenance.
In a
Geological units
In addition to longshore bars discussed above that are relatively small features of a
In places of reentrance 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.[5]
Human habitation
Since
An area in Northwest Alabama is commonly referred to as “The Shoals” by local inhabitants, and one of the cities, Muscle Shoals, is named for such landform and its abundance of Mussels.
See also
- Ayre (landform) – Shingle beaches in Orkney and Shetland
- Barrier Island– Coastal dune landform that forms by wave and tidal action parallel to the mainland coast
- Bank (geography) – Land alongside a body of water
- Coastal Barrier Resources Act — 1982 U.S. law
- Reef – A shoal of rock, coral or other sufficiently coherent material, lying beneath the surface of water
- Tombolo – Deposition landform in which an island is connected to the mainland by a sandy isthmus
- The Point of Sangomar – Sand spit located on the Atlantic Ocean at the mouth of the Saloum Delta
- Adam's Bridge – Chain of shoals between India and Sri Lanka
- List of shoals and sandbanks in the southern North Sea
References
- DOI). p. 116.
- ^ ISBN 0-922152-76-4
- ^ W. Bascom, 1980. Waves and Beaches. Anchor Press/Doubleday, Garden City, New York. 366 p
- ^ Федченко Г.П, 'О самосадочной соли и соляных озерах Каспийского и Азовского бассейнов 1870, p. 54
- ISBN 1-58603-616-5
- ^ C.Michael Hogan (2008) Morro Creek, ed. by Andy Burnham
- ^ Dick Morris (2008) Fleeced
- Jefferson Beale Browne(1912) Key West: The Old and the New, published by The Record company