Shoal

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Sandbar between St Agnes and Gugh on the Isles of Scilly, off the coast of Cornwall, England, United Kingdom GughSandbar.JPG
Sandbar between St Agnes and Gugh on the Isles of Scilly, off the coast of Cornwall, England, United Kingdom
A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji WayaWayasewa.jpg
A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji
Sandbar between Nosy Iranja Be and Nosy Iranja Kely
(Nosy Iranja, Madagascar) Nosy Iranja 097.jpg
Sandbar between Nosy Iranja Be and Nosy Iranja Kely
(Nosy Iranja, Madagascar)

In oceanography, geomorphology, and geoscience, 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 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. [1] [2]

Contents

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 the 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. [2]

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 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:

The term bar can apply to landform features spanning a considerable range in size, from a length of a few metres 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, 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, 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

Amanohashidate in Miyazu, Kyoto Prefecture, Japan 1 Amanohashidate panorama 2014.jpg
Amanohashidate in Miyazu, Kyoto Prefecture, Japan
White Island in Camiguin, Philippines Hibokwhiteisl.jpg
White Island in Camiguin, Philippines
A sandbar off Suffolk County, Long Island, New York, USA, August 2006. SandbarLongIsland1.jpg
A sandbar off Suffolk County, Long Island, New York, USA, August 2006.

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. [3] Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.

Harbour and river bars

The Doom Bar sand bank extends across the River Camel estuary in Cornwall, England, UK DoomBarRiverCamelCornwall2jpg.jpg
The Doom Bar sand bank extends across the River Camel estuary in Cornwall, England, UK

A harbor or river bar is a sedimentary deposit formed at a harbor entrance or river mouth by: the deposition of freshwater sediment, or the action of waves on the sea floor or 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 damming 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 to break through the bar.

The formation of harbor bars can prevent access for boats and shipping, can be the result of:

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 6 fathoms (11 metres) 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 harbour in unfavourable weather conditions or at some states of the tide.

Shoals as geological units

Shoals in the Mississippi River at Arkansas and Mississippi, USA. Mississippi River-sand bars.jpg
Shoals in the Mississippi River at Arkansas and Mississippi, USA.

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. These include 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 re-entrance 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. [4]

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. [5] In modern times these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage. [6] [7]

See also

Related Research Articles

Beach Area of loose particles at the edge of the sea or other body of water

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 shells or coralline algae. Sediments settle in different densities and structures, depending on the local wave action and weather, creating different textures, colors and gradients or layers of material.

Estuary Partially enclosed coastal body of brackish water

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 environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.

Tombolo Deposition landform in which an island is connected to the mainland by a sandy isthmus

A tombolo is a sandy isthmus. A tombolo, from the Italian tombolo, meaning 'pillow' or 'cushion', and sometimes translated as ayre, is a deposition landform by which an island becomes attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island.

Fluvial processes Processes associated with rivers and streams

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 fluvioglacial is used.

Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.

Spit (landform) Coastal bar or beach landform deposited by longshore drift

A spit or sandspit is a deposition bar or beach landform off coasts or lake shores. It develops in places where re-entrance occurs, such as at a cove's headlands, by the process of longshore drift by longshore currents. The drift occurs due to waves meeting the beach at an oblique angle, moving sediment down the beach in a zigzag pattern. This is complemented by longshore currents, which further transport sediment through the water alongside the beach. These currents are caused by the same waves that cause the drift.

Longshore drift Sediment moved by the longshore current

Longshore drift from longshore current is a geological process that consists of the transportation of sediments along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction. Oblique incoming wind squeezes water along the coast, and so generates a water current which moves parallel to the coast. Longshore drift is simply the sediment moved by the longshore current. This current and sediment movement occur within the surf zone. The process is also known as littoral drift.

Barrier island Coastal dune landform that forms by wave and tidal action parallel to the mainland coast

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 a few islands to more than a dozen. They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish. A barrier chain may extend uninterrupted for over a hundred kilometers, excepting the tidal inlets that separate the islands, the longest and widest being Padre Island of Texas. Sometimes an important inlet may close permanently, transforming an island into a peninsula, thus creating a barrier peninsula. The length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls. The amount of vegetation on the barrier has a large impact on the height and evolution of the island.

Groyne Structure extending into a body of water

A groyne is a rigid hydraulic structure built perpendicularly from an ocean shore or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concrete, or stone. In the ocean, groynes create beaches, prevent beach erosion caused by longshore drift where this is the dominant process and facilitate beach nourishment. There is also often cross-shore movement which if longer than the groyne will limit its effectiveness. In a river, groynes slow down the process of erosion and prevent ice-jamming, which in turn aids navigation.

Coastal morphodynamics refers to the study of the interaction and adjustment of the seafloor topography and fluid hydrodynamic processes, seafloor morphologies and sequences of change dynamics involving the motion of sediment. Hydrodynamic processes include those of waves, tides and wind-induced currents.

Coastal geography Study of the region between the ocean and the land

Coastal geography is the study of the constantly changing region between the ocean and the land, incorporating both the physical geography and the human geography of the coast. It includes understanding coastal weathering processes, particularly wave action, sediment movement and weather, and the ways in which humans interact with the coast

Cuspate foreland Geographical features found on coastlines and lakeshores that are created primarily by longshore drift

Cuspate forelands, also known as cuspate barriers or nesses in Britain, are geographical features found on coastlines and lakeshores that are created primarily by longshore drift. Formed by accretion and progradation of sand and shingle, they extend outwards from the shoreline in a triangular shape.

Depositional environment Combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment

In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases, the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.

Beach evolution occurs at the shoreline where sea, lake or river water is eroding the land. Beaches exist where sand accumulated from centuries-old, recurrent processes that erode rocky and sedimentary material into sand deposits. River deltas deposit silt from upriver, accreting at the river's outlet to extend lake or ocean shorelines. Catastrophic events such as tsunamis, hurricanes, and storm surges accelerate beach erosion.

Sedimentary budget

Sedimentary budgets are a coastal management tool used to analyze and describe the different sediment inputs (sources) and outputs (sinks) on the coasts, which is used to predict morphological change in any particular coastline over time. Within a coastal environment the rate of change of sediment is dependent on the amount of sediment brought into the system versus the amount of sediment that leaves the system. These inputs and outputs of sediment then equate to the total balance of the system and more than often reflect the amounts of erosion or accretion affecting the morphology of the coast.

Coastal engineering

Coastal engineering is a branch of civil engineering concerned with the specific demands posed by constructing at or near the coast, as well as the development of the coast itself.

Sedimentary structures Geologic structures formed during sediment deposition

Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.

Canterbury Bight

The Canterbury Bight is a large bight on the eastern side of New Zealand's South Island. The bight runs for approximately 135 kilometres (84 mi) from the southern end of Banks Peninsula to the settlement of Timaru and faces southeast, exposing it to high-energy storm waves originating in the Pacific Ocean. The bight is known for rough conditions as a result, with wave heights of over 2 metres (6.6 ft) common. Much of the bight's geography is shaped by this high-energy environment interacting with multiple large rivers which enter the Pacific in the bight, such as the Rakaia, Ashburton / Hakatere, and Rangitata Rivers. Sediment from these rivers, predominantly Greywacke, is deposited along the coast and extends up to 50 kilometres (31 mi) out to sea from the current shoreline. Multiple hapua, or river-mouth lagoons, can be found along the length of the bight where waves have deposited sufficient sediment to form a barrier across a river mouth, including most notably Lake Ellesmere / Te Waihora and Washdyke Lagoon

Flat coast Shoreline where the land descends gradually into the sea

At a flat coast or flat shoreline, the land descends gradually into the sea. Flat coasts can be formed either as a result of the sea advancing into gently-sloping terrain or through the abrasion of loose rock. They may be basically divided into two parallel strips: the shoreface and the beach.

Hapua

Hapua is the Māori term for river-mouth lagoons on mixed sand and gravel (MSG) beaches which form at the river-coast interface where a typically braided, although sometimes meandering, river interacts with a coastal environment that is significantly affected by longshore drift. The lagoons which form on the MSG coastlines are common on the east coast of the South Island of New Zealand and have long been referred to as hapua by the Māori. This classification differentiates hapua from similar lagoons located on the New Zealand coast termed waituna. Hapua are often located on paraglacial coastal areas where there is a low level of coastal development and minimal population density. Hapua form as the river carves out an elongated coast-parallel area, blocked from the sea by a MSG barrier which constantly alters its shape and volume due to longshore drift. Longshore drift continually extends the barrier behind which the hapua forms by transporting sediment along the coast. Hapua are defined as a narrow shore-parallel extensions of the coastal riverbed. They discharge the majority of stored water to the ocean via an ephemeral and highly mobile drainage channel or outlet. The remainder percolates through the MSG barrier due to its high levels of permeability. Hapua systems are driven by a wide range of dynamic processes that are generally classified as fluvial or marine; changes in the balance between these processes as well as the antecedent barrier conditions can cause shifts in the morphology of the hapua, in particular the barrier. New Zealand examples include the Rakaia, Ashburton and Hurunui river-mouths.

References

  1. Rutecki D, Nestler E, Dellapenna T, Pembroke A (2014). "Understanding the Habitat Value and Function of Shoal/Ridge/Trough Complexes to Fish and Fisheries on the Atlantic and Gulf of Mexico Outer Continental Shelf". Draft Literature Synthesis for the U.S. Dept. of the Interior, Bureau of Ocean Energy Management. Contract # M12PS00031. Bureau of Ocean Energy Management (DOI). p. 116.
  2. 1 2 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. ISBN   0-922152-76-4
  3. W. Bascom, 1980. Waves and Beaches. Anchor Press/Doubleday, Garden City, New York. 366 p
  4. Mirko Ballarini, Optical Dating of Quartz from Young Deposits, IOS Press, 2006 146 pages, ISBN   1-58603-616-5
  5. C.Michael Hogan (2008) Morro Creek, ed. by Andy Burnham
  6. Dick Morris (2008) Fleeced
  7. Jefferson Beale Browne (1912) Key West: The Old and the New, published by The Record company