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 sphe ...
, a shoal is a natural submerged
ridge,
bank, 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 ...
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,
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
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 ...
, 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 conto ...
promotes
deposition
Deposition may refer to:
* Deposition (law), taking testimony outside of court
* Deposition (politics), the removal of a person of authority from political power
* Deposition (university), a widespread initiation ritual for new students practiced f ...
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 in the
sea, where they are classified as a type of
ocean bank, 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 mouths and
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 islands.
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 ...
, 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, 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 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
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 rest ...
, 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 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. 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
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 waters, anoxic) processes prevailing, especially in t ...
s 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
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.:
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
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 d ...
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,
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
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
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 isla ...
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
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