Inoceanography,geomorphology, andgeoscience, ashoal is a natural submergedridge,bank, or bar that consists of, or is covered by,sand or other unconsolidated material, and rises from the bed of abody of water close to the surface or above it, which poses a danger to navigation. Shoals are also known assandbanks,sandbars,gravelbars, orbars. Two or more shoals that are either separated by sharedtroughs or interconnected by past or present sedimentary and hydrographic processes are referred to as ashoal complex.[1][2]
The termshoal 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 astream,lake,sea, or other body of water; a rocky area on theseafloor 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.[2]
Shoals are characteristically long and narrow (linear) ridges. They can develop where astream,river, orocean current promotesdeposition ofsediment andgranular 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 episodicsea level rise or by the erosion and submergence of inactivedelta lobes.
Shoals can appear as acoastal landform in thesea, where they are classified as a type ofocean bank, or asfluvial landforms in rivers, streams, andlakes.
A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as:
The termbar can apply tolandform 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 calledbarrier islands.
They are typically composed ofsand, 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 evenboulders). Thegrain 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.
Wave shoaling is the process whensurface waves move towards shallow water, such as a beach, they slow down, theirwave height increases and thedistance between waves decreases. This behavior is calledshoaling, and the waves are said to shoal. The waves may or may not build to the point where theybreak, 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 alsorefract waves, so the waves change direction. For example, if waves pass over a slopingbank 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.
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 atrough (marine landform).
Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break.[3] Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.
In Russian tradition ofgeomorphology, aperesyp is a sandbar that rises above the water level (like aspit) and separates aliman or alagoon from the sea. Unliketombolo bars, aperesyp seldom forms a contiguous strip and usually has one or several channels that connect the liman and the sea.[4]
A harbor or river bar is a sedimentary deposit formed at aharbor 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'ssuspended orbed 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 ofaquatic ecology, causing the formation ofestuaries andwetlands in the lower course of the river. This situation will persist until the bar iseroded by the sea, or the dammed river develops sufficienthead to break through the bar.
The formation of harbor bars that prevent access for boats and shipping can be the result of:
In anautical sense, abar is a shoal, similar to areef: a shallow formation of (usually) sand that is anavigation orgrounding hazard, with a depth of water of 6 fathoms (11 meters) 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 thetide.
In addition to longshore bars discussed above that are relatively small features of abeach, the termshoal can be applied to larger geological units that form off a coastline as part of the process of coastal erosion, such asspits andbaymouth bars that form across the front ofembayments andrias. Atombolo is a bar that forms anisthmus between anisland or offshore rock and amainland shore.
In places of reentrance along a coastline (such asinlets,coves, rias, and bays), sediments carried by alongshore 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, becomingsalt 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.[5]
Sinceprehistoric times, humans have chosen some shoals as a site of habitation. In some early cases, the locations provided easy access to exploit marine resources.[6] In modern times, these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage.[7][8]
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 ofmussels.
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