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Encyclopedia > Barrier island

In geography, a bar is a linear shoaling landform feature within a body of water. Bars tend to be long and narrow (linear) and develop where a current (or waves) promote deposition of particles, resulting in localized shallowing (shoaling) of the water. Bars can appear in the sea, in a lake, or in a river. They are typically composed of sand, although could be of any particulate 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 size of the particles 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 over a considerable range in size, from just a few meters 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 grounding hazard.

Contents

Sandbars and longshore bars

Bars that occur at or off the shoreline of a sea or a lake are related to beaches and might be considered offshore features of a beach (Bascom, 1980). At times when larger waves attack the beach berm, some of the beach material is redistributed offshore to become a longshore bar or sandbar, possibly visible at low tide. This bar forms (sometimes seaward of a trough) where the waves are breaking, because the breaking waves set up a shoreward current with a compensating counter-current along the bottom. Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break (Bascom, 1980). Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.


Bars as geological units

In addition to longshore bars discussed above that are relatively small features of a beach, the term bar 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.


The largest of the geological units of this kind are the barrier islands, such as occur along the East Coast of the United States, as well as along the Gulf coast.


In places of reentrants 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.


See also: geomorphology, earth science.


Examples

References

  • Bascom, W. 1980. Waves and Beaches. Anchor Press/Doubleday, Garden City, New York. 366 p.



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