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]]
thumb|A tidal sandbar connecting the islands of [[Waya Island|Waya and Wayasewa of the Yasawa Islands, Fiji]]
thumb|Sandbar between Nosy Iranja Be and Nosy Iranja Kely<br />([[Nosy Iranja, Madagascar)]]
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 in oceanography, geomorphology, and geoscience, as sandbanks, sandbars, gravelbars, or bars. 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.
Other uses of the term shoal either similar to or quite different from its application in geologic, geomorphic, and oceanographic literature, include 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. 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.
Harbor and river bars
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 or bed loads 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 estuaries and wetlands in the lower course 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.
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.
- watershed erosion from habitat alterations — e.g.: deforestation, wildfires, 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 grounding hazard, with a depth of water of 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
thumb|Shoals in the [[Mississippi River at Arkansas and Mississippi, USA.]]
However, 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 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.
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.
Human habitation
Humans have inhabited shoals since prehistoric times. In some cases the locations provide easy exploitation of marine resources. In modern times, these sites are sometimes chosen for their water access or view, but many such locations are prone to storm damage.
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
- — 1982 U.S. law
- The
- List of shoals and sandbanks in the southern North Sea
References
hu:Turzás
pl:Mielizna