In oceanography, geomorphology, and Earth science, 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.[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. ]
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.
Description
Shoals are characteristically long and narrow (linear) ridges. They can develop where a
stream,
river, or
ocean current promotes 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.
Shoals can appear as a coast in the sea, where they are classified as a type of ocean bank, or as Fluvial in rivers, streams, and .
A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as:
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 .
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,
Shingle beach, or even
). 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.
Formation
Wave shoaling is the process when surface waves move towards shallow water, such as a beach, they slow down, their
wave height increases and the
wavelength 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, 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 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.[W. Bascom, 1980. Waves and Beaches. Anchor Press/Doubleday, Garden City, New York. 366 p] 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.
[ Федченко Г.П, 'О самосадочной соли и соляных озерах Каспийского и Азовского бассейнов'' 1870, p. 54]
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 suspended load or 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 and in the lower course of the river. This situation will persist until the bar is coastal erosion by the sea, or the dammed river develops sufficient hydraulic 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.: and , riparian zone destruction, river bank alterations, river adjacent agricultural land practices, water diversions.
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Drainage basin erosion from habitat alterations — e.g.: deforestation, , 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 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
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
. 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 , , rias, and bays), sediments carried by a Longshore drift 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 .
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.[Mirko Ballarini, Optical Dating of Quartz from Young Deposits, IOS Press, 2006 146 pages, ]
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.
[ C.Michael Hogan (2008) Morro Creek, ed. by Andy Burnham] In modern times, these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage.
[Dick Morris (2008) Fleeced][Jefferson Beale Browne (1912) Key West: The Old and the New, published by The Record company]
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 Mussel.
See also
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— 1982 U.S. law
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The
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List of shoals and sandbanks in the southern North Sea
