A shoal, sandbar (or just bar in context), or gravelbar is a somewhat linear landform within or extending into a body of water, typically composed of sand, silt or small pebbles. A spit or sandspit is a type of shoal. Shoals are characteristically long and narrow (linear) and develop where a stream or ocean current promotes deposition of granular material, resulting in localized shallowing (shoaling) of the water. Shoals can appear in the sea, in a lake, or in a river. Alternatively a bar may separate a lake from the sea, as in the case of an ayre. They are typically composed of sand, although 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.
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 kilometres along a coastline, often called barrier islands.
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 six fathoms or less. It therefore applies to a silt accumulation that shallows the entrance to the course of a river or creek.
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 behaviour 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 diffract waves, so the waves change direction. For example, if waves pass over a sloping sandbank 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.
The largest of the geological units of this kind is a barrier island, such as occur along the East Coast of the United States, along the Gulf coast, along the southern coast of Belize and many other locations worldwide.
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.
Category:Physical oceanography Category:Coastal geography Category:Hydrology Category:Fluvial landforms Category:Coastal and oceanic landforms
da:Sandbanke de:Sandbank et:Ajurand es:Barra (relieve) eo:Sablejo fr:Banc de sable fy:Sânplaat id:Gosong pasir he:שרטון hi:रेती hu:Turzás nl:Zandbank ja:砂州 no:Sandbanke nn:Sandbanke pl:Mielizna pt:Barra ru:Побочень sr:Спруд sv:Sandbank tl:Bahura uk:Бар (смуга суходолу) vls:Zandbank zh:沙洲This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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