View source for Shoal

{{Short description|Natural submerged sandbank that rises from a body of water to near the surface}}
{{About|underwater landforms|fish behavior|Shoaling and schooling|effects of shallow water on waves|Wave shoaling|other uses|}}
{{redirect-multi|2|Shoals|Sandbank}}
{{Use mdy dates|date=November 2021}}
{{EngvarA|date=November 2021}}
[[File:GughSandbar.JPG|right|thumb|Sandbar between [[St. Agnes, Isles of Scilly|St Agnes]] and [[Gugh]] on the [[Isles of Scilly]], off the coast of [[Cornwall]], [[England]], [[United Kingdom]]]]
[[File:WayaWayasewa.jpg|thumb|A tidal sandbar connecting the islands of [[Waya Island|Waya]] and Wayasewa of the [[Yasawa Islands]], [[Fiji]]]]
[[File:Nosy Iranja 097.jpg|thumb|Sandbar between Nosy&nbsp;Iranja&nbsp;Be and Nosy&nbsp;Iranja&nbsp;Kely<br />([[Nosy Iranja]], [[Madagascar]])]]

In [[oceanography]], [[geomorphology]], and [[Earth science|geoscience]], a '''shoal''' is a natural submerged [[ridge]], [[bank (geography)|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''', '''gravelbars''', or '''bars'''. Two or more shoals that are either separated by shared [[crest and trough|troughs]] or interconnected by past or present sedimentary and hydrographic processes are referred to as a '''shoal complex'''.<ref name="RuteckiOthers2014a">{{cite web |vauthors=Rutecki D, Nestler E, Dellapenna T, Pembroke A |year=2014 |url=http://www.boem.gov/Final-Draft-Report/ |title=Understanding the Habitat Value and Function of Shoal/Ridge/Trough Complexes to Fish and Fisheries on the Atlantic and Gulf of Mexico Outer Continental Shelf |work=Draft Literature Synthesis for the U.S. Dept. of the Interior, Bureau of Ocean Energy Management. Contract # M12PS00031 |publisher=[[Bureau of Ocean Energy Management]] ([[U.S. Department of the Interior|DOI]]) |page=116}}</ref><ref name="NeuendorfOthers2005a">Neuendorf, K.K.E., J.P. Mehl Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. {{ISBN|0-922152-76-4}}</ref>

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 [[stream]], [[lake]], [[sea]], or other body of water; a rocky area on the [[seafloor]] within an area mapped for navigation purposes; or a growth of vegetation on the bottom of a deep lake, that occurs at any depth, or is used as a verb for the process of proceeding from a greater to a lesser depth of water.<ref name="NeuendorfOthers2005a"/>

== Description ==
Shoals are characteristically long and narrow (linear) ridges. They can develop where a [[stream]], [[river]], or [[ocean current]] promotes [[deposition (sediment)|deposition]] of [[sediment]] and [[granular material]], resulting in localized shallowing (shoaling) of the water. Marine shoals also develop either by the in-place drowning of barrier islands as the result of episodic [[sea level rise]] or by the erosion and submergence of inactive [[Deltaic lobe|delta lobes]].

Shoals can appear as a [[coast|coastal landform]] in the [[sea]], where they are classified as a type of [[ocean bank]], or as [[Fluvial|fluvial landforms]] in rivers, streams, and [[lake]]s.

A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as:
* Marine [[lagoon]]s
* [[Brackish water]] estuaries
* Freshwater seasonal stream and [[river mouth]]s and [[river delta|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 island]]s.

=== Composition ===
They are typically composed of [[sand]], although they 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 (geology)|cobble]], [[Shingle beach|shingle]], or even [[boulder]]s). The [[Particle size (grain size)|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.

== Formation ==
{{main|Wave shoaling}}

[[Wave shoaling]] is the process when [[ocean surface waves|surface waves]] move towards shallow water, such as a beach, they slow down, their [[wave height]] increases and the [[wavelength|distance]] between waves decreases. This behavior is called ''shoaling'', and the waves are said to shoal. The waves may or may not build to the point where they [[Breaking wave|break]], depending on how large they were to begin with, and how steep the slope of the beach is. In particular, waves shoal as they pass over submerged sandbanks or reefs. This can be treacherous for boats and ships.

Shoaling can also [[Refraction|refract]] waves, so the waves change direction. For example, if waves pass over a sloping [[Bank (geography)|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===

[[File:DonuzlavLakeMap.png|thumb|left| [[Donuzlav]], Crimea, separated by a ''peresyp'' from the [[Black Sea]]]]
====Sandbars and longshore bars====
[[File:1 Amanohashidate panorama 2014.jpg|thumb|[[Amanohashidate]] in [[Miyazu, Kyoto|Miyazu]], [[Kyoto Prefecture]], [[Japan]]]]
[[File:Hibokwhiteisl.jpg|thumb|[[White Island (Philippines)|White Island]] in [[Camiguin]], Philippines]]
[[File:SandbarLongIsland1.jpg|thumb|A sandbar off [[Suffolk County, New York|Suffolk County]], [[Long Island]], [[New York (state)|New York]], US, August 2006.]]
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 (geology)|trough]] (marine landform).

Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break.<ref>W. Bascom, 1980. ''Waves and Beaches''. Anchor Press/Doubleday, Garden City, New York. 366 p</ref> Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.

====Peresyp====
{{main|Peresyp}}
In Russian tradition of [[geomorphology]], a ''peresyp'' is a sandbar that rises above the water level (like a [[spit (landform)|spit]]) and separates a [[liman (landform)|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.<ref> Федченко Г.П, 'О самосадочной соли и соляных озерах Каспийского и Азовского бассейнов''  1870, [https://books.google.com/books?id=0U0XAAAAYAAJ&dq=%D0%9F%D0%B5%D1%80%D0%B5%D1%81%D1%8B%D0%BF%D1%8C&pg=PA54 p. 54]</ref>

====Harbor and river bars====
{{Further|River bar|Mouth bar}}

[[File:DoomBarRiverCamelCornwall2.jpg|thumb|The [[Doom Bar]] sand bank extends across the [[River Camel]] estuary in [[Cornwall]], England, UK]]
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 [[suspended load|suspended]] or [[bed load]]s are large enough, deposition can build up a sandbar that completely blocks a river mouth and dams the river. It can be a seasonally natural process of [[aquatic ecology]], causing the formation of [[estuary|estuaries]] and [[wetland]]s in the lower course of the river.  This situation will persist until the bar is [[coastal erosion|eroded]] by the sea, or the dammed river develops sufficient [[hydraulic head|head]] to break through the bar.

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.: [[breakwater (structure)|breakwaters]], [[dune]] [[habitat destruction]].
* upriver development — e.g.: [[dam]]s and [[reservoir]]s, [[riparian zone]] destruction, river bank alterations, river adjacent agricultural land practices, [[Interbasin transfer|water diversion]]s.
* [[Drainage basin|watershed]] [[erosion]] from [[habitat]] alterations  — e.g.: [[deforestation]], [[wildfire]]s, [[Grading (engineering)|grading]] for development.
* artificially created/deepened harbors that require periodic [[dredging]] maintenance.

=====Nautical navigation=====
In a [[nautical]] sense, a ''bar'' is a shoal, similar to a [[reef]]: a shallow formation of (usually) sand that is a [[navigation]] or [[Ship grounding|grounding]] hazard, with a depth of water of {{convert|6|fathom|m|0|abbr=off|sp=us}} or less. It therefore applies to a silt accumulation that shallows the entrance to or course of a river, or creek.  A bar can form a dangerous obstacle to shipping, preventing access to the river or harbor in poor weather conditions or at some states of the [[tide]].

==Geological units==
[[File:Mississippi River-sand bars.jpg|thumb|Shoals in the [[Mississippi River]] at [[Arkansas]] and [[Mississippi]], USA.]]
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, such as [[spit (landform)|spit]]s and [[baymouth bars]] that form across the front of [[embayments]] and [[ria]]s. A [[tombolo]] is a bar that forms an [[isthmus]] between an [[island]] or offshore rock and a [[mainland]] shore.

In places of reentrance along a coastline (such as [[inlet]]s, [[cove]]s, rias, and bays), sediments carried by a [[Longshore drift|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 marsh]]es.

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.<ref>Mirko Ballarini, ''Optical Dating of Quartz from Young Deposits'', [[IOS Press]], 2006 146 pages, {{ISBN|1-58603-616-5}}</ref>

==Human habitation==
Since [[prehistoric]] times, humans have chosen some shoals as a site of habitation. In some early cases, the locations provided easy access to exploit marine resources.<ref>[http://www.megalithic.co.uk/article.php?sid=18502 C.Michael Hogan (2008) ''Morro Creek'', ed. by Andy Burnham]</ref> In modern times, these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage.<ref>Dick Morris (2008) ''Fleeced''</ref><ref>[[Jefferson Beale Browne]] (1912) ''Key West: The Old and the New'', published by The Record company</ref>

An area in Northwest Alabama is commonly referred to as "[[Florence–Muscle Shoals metropolitan area|The Shoals]]" by local inhabitants, and one of the cities, [[Muscle Shoals, Alabama|Muscle Shoals]], is named for such landform and its abundance of [[mussel]]s.

==See also==
{{Commons category}}
* {{annotated link|Ayre (landform)}}
* {{annotated link|Barrier Island}}
* {{annotated link|Bank (geography)}}
* {{annotated link|Coastal Barrier Resources Act}} — 1982 U.S. law
* {{annotated link|Reef}}
* {{annotated link|Tombolo}}
* The {{annotated link|Point of Sangomar}}
* {{annotated link|Adam's Bridge}}
* [[List of shoals and sandbanks in the southern North Sea]]

==References==
{{Reflist}}

{{coastal geography}}
{{Rivers, streams and springs}}
{{Authority control}}

[[Category:Shoals| ]]
[[Category:Coastal and oceanic landforms]]
[[Category:Fluvial landforms]]
[[Category:Hydrology]]
[[Category:Coastal geography]]
[[Category:Physical oceanography]]
[[Category:Barrier islands|.]]
[[Category:Oceanographical terminology]]
[[Category:Islands by type]]

[[hu:Turzás]]
[[pl:Mielizna]]