![Nosy Iranja 097](https://upload.wikimedia.org/wikipedia/commons/3/36/Nosy_Iranja_097.jpg)
In
oceanography,
geomorphology, and
geoscience
Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four sphere ...
, a shoal is a natural submerged
ridge,
bank
A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets.
Because ...
, or bar that consists of, or is covered by,
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...
or other unconsolidated material and rises from the bed of a
body of water
A body of water or waterbody (often spelled water body) is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such a ...
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,
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
,
sea
The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...
, or other body of water; a rocky area on the
seafloor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...
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
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...
, or
ocean current
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours ...
promotes
deposition of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
and
granular material
A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when grains collide). The constituents that compose gra ...
, 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
Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...
or by the erosion and submergence of inactive
delta lobes.
Shoals can appear as a
coastal landform in the
sea
The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...
, where they are classified as a type of
ocean bank, or as
fluvial landforms
In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvio ...
in rivers, streams, and
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s.
A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as:
* Marine
lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...
s
*
Brackish water
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
estuaries
* Freshwater seasonal stream and
river mouths and
deltas
A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarel ...
.
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
Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from ...
s.
Composition
They are typically composed of
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...
, although they could be of any granular matter that the moving water has access to and is capable of shifting around (for example,
soil
Soil, also commonly referred to as earth or dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debri ...
,
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
,
gravel,
cobble
Cobble may refer to:
* Cobble (geology), a designation of particle size for sediment or clastic rock
* Cobblestone, partially rounded rocks used for road paving
* Hammerstone, a prehistoric stone tool
* Tyringham Cobble, a nature reserve in Tyr ...
,
shingle, or even
boulders). 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
In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at th ...
move towards shallow water, such as a beach, they slow down, their
wave height
In fluid dynamics, the wave height of a surface wave is the difference between the elevations of a crest and a neighboring trough. ''Wave height'' is a term used by mariners, as well as in coastal, ocean and naval engineering.
At sea, the te ...
increases and the
distance
Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
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
Break or Breaks or The Break may refer to:
Time off from duties
* Recess (break), time in which a group of people is temporarily dismissed from its duties
* Break (work), time off during a shift/recess
** Coffee break, a short mid-morning res ...
, 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
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
waves, so the waves change direction. For example, if waves pass over a sloping
bank
A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets.
Because ...
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
![SandbarLongIsland1](https://upload.wikimedia.org/wikipedia/commons/4/43/SandbarLongIsland1.jpg)
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. 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
![DoomBarRiverCamelCornwall2jpg](https://upload.wikimedia.org/wikipedia/commons/5/56/DoomBarRiverCamelCornwall2jpg.jpg)
A harbor or river bar is a sedimentary deposit formed at a
harbor
A harbor (American English), harbour (British English; see spelling differences), or haven is a sheltered body of water where ships, boats, and barges can be docked. The term ''harbor'' is often used interchangeably with ''port'', which is a ...
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 load
The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load.
Bed load moves by rolling, sliding, and/or saltating (h ...
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
An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The tw ...
, causing the formation of
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...
and
wetland
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The p ...
s in the lower course of the river. This situation will persist until the bar is
eroded
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...
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
A breakwater is a permanent structure constructed at a coastal area to protect against tides, currents, waves, and storm surges. Part of a coastal management system, breakwaters are installed to minimize erosion, and to protect anchorages, h ...
,
dune habitat destruction
Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...
.
* upriver development — e.g.:
dam
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...
s and
reservoir
A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation.
Reservoirs can be created in a number of ways, including contro ...
s,
riparian zone
A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...
destruction, river bank alterations, river adjacent agricultural land practices,
water diversion
Interbasin transfer or transbasin diversion are (often hyphenated) terms used to describe man-made conveyance schemes which move water from one river basin where it is available, to another basin where water is less available or could be utilized ...
s.
*
watershed erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
from
habitat
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
alterations — e.g.:
deforestation
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated ...
,
wildfire
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
s,
grading for development.
* artificially created/deepened harbors that require periodic
dredging maintenance.
=Nautical navigation
=
In a
nautical
Seamanship is the art, knowledge and competence of operating a ship, boat or other craft on water. The'' Oxford Dictionary'' states that seamanship is "The skill, techniques, or practice of handling a ship or boat at sea."
It involves topics ...
sense, a ''bar'' is a shoal, similar to a
reef
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...
: a shallow formation of (usually) sand that is a
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land 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
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide tables ...
.
Geological units
![Mississippi River-sand bars](https://upload.wikimedia.org/wikipedia/commons/e/ea/Mississippi_River-sand_bars.jpg)
In addition to longshore bars discussed above that are relatively small features of a
beach
A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc sh ...
, 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
A baymouth bar is a depositional feature as a result of longshore drift. It is a sandbank that partially or completely closes access to a bay.
Background
These bars usually consist of accumulated gravel and sand carried by the current of lon ...
that form across the front of
embayments
A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a nar ...
and
ria
A ria (; gl, ría) is a coastal inlet formed by the partial submergence of an unglaciated river valley. It is a drowned river valley that remains open to the sea.
Definitions
Typically rias have a dendritic, treelike outline although they ca ...
s. A
tombolo
A tombolo is a sandy or shingle isthmus. A tombolo, from the Italian ', meaning 'pillow' or 'cushion', and sometimes translated incorrectly as ''ayre'' (an ayre is a shingle beach of any kind), is a deposition landform by which an island becom ...
is a bar that forms an
isthmus
An isthmus (; ; ) is a narrow piece of land connecting two larger areas across an expanse of water by which they are otherwise separated. A tombolo is an isthmus that consists of a spit or bar, and a strait is the sea counterpart of an isthmus ...
between an
island
An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island ...
or offshore rock and a
mainland
Mainland is defined as "relating to or forming the main part of a country or continent, not including the islands around it egardless of status under territorial jurisdiction by an entity" The term is often politically, economically and/or dem ...
shore.
In places of reentrance along a coastline (such as
inlets,
cove
A cove is a small type of bay or coastal inlet. Coves usually have narrow, restricted entrances, are often circular or oval, and are often situated within a larger bay. Small, narrow, sheltered bays, inlets, creeks, or recesses in a coast are o ...
s, 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 marsh
A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is domin ...
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.
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. 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]
See also
*
*
*
* — 1982 U.S. law
*
*
* The
References
{{Authority control
Coastal and oceanic landforms
Fluvial landforms
Hydrology
Coastal geography
Physical oceanography
.
Oceanographical terminology
Islands by type
hu:Turzás
pl:Mielizna