oceanography Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamic ...

geomorphology Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...

, and geoscience, a shoal is a natural submerged

ridge A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The line ...

, 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 to near the surface. It often refers to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute 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 or quite different from how it is used in geologic, geomorphic, and oceanographic literature. Sometimes, this term refers to either any relatively shallow place in a

stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, 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

, 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 delta lobes. Shoals can appear as a

coastal landform The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...

in the sea, where they are classified as a type of

ocean bank An ocean bank, sometimes referred to as a fishing bank or simply bank, is a part of the seabed that is shallow compared to its surrounding area, such as a shoal or the top of an seamount, underwater hill. Somewhat like continental margin, conti ...

, or as fluvial landforms in rivers, streams, and lakes. 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 mouth A river mouth is where a river flows into a larger body of water, such as another river, a lake/reservoir, a bay/gulf, a sea, or an ocean. At the river mouth, sediments are often deposited due to the slowing of the current reducing the carrying ...

s and deltas. The term ''bar'' can apply to

landform A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, ...

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 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 Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

, cobble,

shingle Shingle may refer to: Construction *Roof shingles or wall shingles, including: **Wood shingle ***Shake (shingle), a wooden shingle that is split from a bolt, with a more rustic appearance than a sawed shingle ***Quercus imbricaria, or shingle oak ...

, or even

boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...

s). The

grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refer ...

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 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 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 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 Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...

(marine landform). Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break. Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.

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 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 A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may ...

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 A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...

habitat destruction. * upriver development — e.g.: dams and reservoirs, riparian zone destruction, river bank alterations, river adjacent agricultural land practices, water diversions. *

watershed Watershed is a hydrological term, which has been adopted in other fields in a more or less figurative sense. It may refer to: Hydrology * Drainage divide, the line that separates neighbouring drainage basins * Drainage basin, called a "watershe ...

erosion from habitat alterations — e.g.: deforestation, wildfires, grading for development. * artificially created/deepened harbors that require periodic

dredging Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing da ...

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

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

inlet An inlet is a (usually long and narrow) indentation of a shoreline, such as a small arm, bay, sound, fjord, lagoon or marsh, that leads to an enclosed larger body of water such as a lake, estuary, gulf or marginal sea. Overview In marine geogra ...

s, coves, rias, and bays), sediments carried by a

longshore current Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction ...

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

Since

prehistoric Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use of ...

times, humans have chosen some shoals as a site of habitation. In some early cases, the locations provided easy access to exploit marine resources. In modern times, these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage. Jefferson Beale Browne (1912) ''Key West: The Old and the New'', published by The Record company

References

{{Authority control Coastal and oceanic landforms Fluvial landforms Hydrology Coastal geography Physical oceanography . Oceanographical terminology Islands by type hu:Turzás pl:Mielizna