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Barrier islands are coastal landforms and a type of
dune system A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...
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, United States. Sometimes an important inlet may close permanently, transforming an island into a peninsula, thus creating a barrier peninsula, often including a
beach 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 ...
, barrier beach. The length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range,
wave energy Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC). Waves are generated by win ...
, sediment supply, sea-level trends, and basement controls.Davis Jr., p. 144. The amount of vegetation on the barrier has a large impact on the height and evolution of the island. Chains of barrier islands can be found along approximately 13-15% of the world's coastlines.Smith, Q.H.T., Heap, A.D., and Nichol, S.L., 2010, "Origin and formation of an estuarine barrier island, Tapora Island, New Zealand:" ''Journal of Coastal Research,'' v. 26, p. 292–300. They display different settings, suggesting that they can form and be maintained in a variety of environments. Numerous theories have been given to explain their formation. A human-made offshore structure constructed parallel to the shore is called a
breakwater Breakwater may refer to: * Breakwater (structure), a structure for protecting a beach or harbour Places * Breakwater, Victoria, a suburb of Geelong, Victoria, Australia * Breakwater Island, Antarctica * Breakwater Islands, Nunavut, Canada * Br ...
. In terms of
coastal morphodynamics Coastal morphodynamics (i.e. the dynamics of beach morphology) 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 ...
, it acts similarly to a naturally occurring barrier island by dissipating and reducing the energy of the waves and currents striking the coast. Hence, it is an important aspect of
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. The hydrodynamic impact of especially waves, tides, storm surges ...
.


Constituent parts

;Upper shoreface The shoreface is the part of the barrier where the ocean meets the shore of the island. The barrier island body itself separates the shoreface from the backshore and
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'' ...
/
tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...
area. Characteristics common to the upper shoreface are fine sands with mud and possibly silt. Further out into the ocean the sediment becomes finer. The effect of waves at this point is weak because of the depth.
Bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
is common and many fossils can be found in upper shoreface deposits in the
geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, san ...
. ;Middle shoreface The middle shoreface is located in the upper shoreface. The middle shoreface is strongly influenced by wave action because of its depth. Closer to shore the sand is medium-grained, with shell pieces common. Since wave action is heavier, bioturbation is not likely. ;Lower shoreface The lower shoreface is constantly affected by wave action. This results in development of herringbone
sedimentary structure Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
s because of the constant differing flow of waves. The sand is coarser. ;Foreshore The
foreshore The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species of ...
is the area on land between high and low tide. Like the upper shoreface, it is constantly affected by wave action.
Cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...
and
lamination Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materials ...
are present and coarser sands are present because of the high energy present by the crashing of the waves. The sand is also very well sorted. ;Backshore The
backshore The backshore area of a beach extends from the limit of high water foam lines to dunes or extreme inland limit of the beach. It is only affected by waves during exceptional high tides or severe storms.Whittow, John (1984). ''Dictionary of Physical ...
is always above the highest water level point. The
berm A berm is a level space, shelf, or raised barrier (usually made of compacted soil) separating areas in a vertical way, especially partway up a long slope. It can serve as a terrace road, track, path, a fortification line, a border/ separation ...
is also found here which marks the boundary between the foreshore and backshore. Wind is the important factor here, not water. During strong storms high waves and wind can deliver and erode sediment from the backshore. ;Dunes
Coastal dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
, created by wind, are typical of a barrier island. They are located at the top of the backshore. The dunes will display characteristics of typical aeolian wind-blown dunes. The difference is that dunes on a barrier island typically contain coastal vegetation roots and marine bioturbation. ;Lagoon and tidal flats The
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'' ...
and
tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...
area is located behind the dune and backshore area. Here the water is still, which allows fine silts, sands, and mud to settle out. Lagoons can become host to an
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
environment. This will allow high amounts of organic-rich mud to form. Vegetation is also common.


Location

Barrier Islands can be observed on every continent on Earth, except Antarctica.


Australia

Moreton Bay, on the east coast of Australia and directly east of
Brisbane Brisbane ( ) is the capital and most populous city of the Australian state of Queensland, and the third-most populous city in Australia and Oceania, with a population of approximately 2.6 million. Brisbane lies at the centre of the Sou ...
, is sheltered from the Pacific Ocean by a chain of very large barrier islands. Running north to south they are
Bribie Island Bribie Island is the smallest and most northerly of three major sand islands forming the coastline sheltering the northern part of Moreton Bay, Queensland, Australia. The others are Moreton Island and North Stradbroke Island. Bribie Island i ...
,
Moreton Island Moreton Island (Mulgumpin) is an island on the eastern side of Moreton Bay on the coast of South East Queensland, Australia. The Coral Sea lies on the east coast of the island. Moreton Island lies northeast of the Queensland capital, Brisbane. ...
,
North Stradbroke Island North Stradbroke Island ( Jandai: ''Minjerribah''), colloquially ''Straddie'' or ''North Straddie'', is an island that lies within Moreton Bay in the Australian state of Queensland, southeast of the centre of Brisbane. Originally there was on ...
and
South Stradbroke Island The South Stradbroke Island ( Indigenous: ''Minjerribah''), colloquially ''South Straddie'', is an island that lies within Moreton Bay in the Australian state of Queensland, south of Brisbane and forms the northern end of Gold Coast. The islan ...
(the last two used to be a single island until a storm created a channel between them in 1896). North Stradbroke Island is the second largest sand island in the world and Moreton Island is the third largest. Fraser Island, another barrier island lying 200 km north of Moreton Bay on the same coastline, is the largest sand island in the world.


United States

Barrier islands are found most prominently on the United States' East and Gulf Coasts, where every state—from Maine to Florida (East Coast) and from Florida to Texas (Gulf Coast)—features at least part of a barrier island. Many have large numbers of barrier islands; Florida for instance has twenty-five. Padre Island, Texas, is the world’s longest barrier island; other well-known islands on the Gulf Coast include Galveston Island in Texas and
Sanibel Sanibel is an island and city in Lee County, Florida, United States. The population was 6,382 at the 2020 census. It is part of the Cape Coral-Fort Myers, Florida Metropolitan Statistical Area. The island, also known as Sanibel Island, constitut ...
and Captiva Islands in Florida. Those on the East Coast include Miami Beach and Palm Beach in Florida;
Hatteras Island Hatteras Island (historically Croatoan Island) is a barrier island located off the North Carolina coast. Dividing the Atlantic Ocean and the Pamlico Sound, it runs parallel to the coast, forming a bend at Cape Hatteras. It is part of North Carol ...
in North Carolina;
Assateague Island Assateague Island is a long barrier island located off the eastern coast of the Delmarva Peninsula facing the Atlantic Ocean. The northern two-thirds of the island is in Maryland while the southern third is in Virginia. The Maryland sectio ...
in Virginia and Maryland;
Absecon Island Absecon Island is a barrier island located on the Jersey Shore of the Atlantic Ocean in Atlantic County, New Jersey. On the island from north to south are the resort communities of Atlantic City, Ventnor, Margate, and Longport. The island ends a ...
in New Jersey, where
Atlantic City Atlantic City, often known by its initials A.C., is a coastal resort city in Atlantic County, New Jersey, United States. The city is known for its casinos, boardwalk, and beaches. In 2020, the city had a population of 38,497.
is located; and
Jones Beach Island Jones Beach Island is one of the outer barrier islands off the southern coast of Long Island in the U.S. state of New York. Etymology It is named for Major Thomas Jones, who first came to Long Island in 1692, where he proceeded to build the ...
and
Fire Island Fire Island is the large center island of the outer barrier islands parallel to the South Shore of Long Island, in the U.S. state of New York. Occasionally, the name is used to refer collectively to not only the central island, but also Lon ...
, both off Long Island in New York. No barrier islands are found on the Pacific Coast of the United States due to the rocky shore and short continental shelf, but barrier peninsulas can be found. Barrier islands can also be seen on Alaska's Arctic coast.


Canada

Barrier Islands can also be found in Maritime Canada, and other places along the coast. A good example is found at
Miramichi Bay Miramichi Bay is an estuary located on the west coast of the Gulf of St. Lawrence in New Brunswick, at the mouth of the Miramichi River. Miramichi Bay is separated into the "inner bay" and the "outer bay", with the division being a line of unin ...
, New Brunswick, where Portage Island as well as Fox Island and Hay Island protect the inner bay from storms in the Gulf of Saint Lawrence.


Mexico

Mexico's Gulf Coast has numerous barrier islands and barrier Peninsulas.


New Zealand

Barrier islands are more prevalent in the north of both of New Zealand's main islands. Notable barrier islands in New Zealand include
Matakana Island Matakana Island is located in the western Bay of Plenty in New Zealand's North Island. A long, flat barrier island, it is in length but rarely more than wide. The island has been continuously populated for centuries by Māori tribes that are ...
, which guards the entrance to
Tauranga Harbour Tauranga Harbour is the natural tidal harbour that surrounds Tauranga CBD and the Mount Maunganui area of Tauranga, New Zealand, and which flows into the Pacific Ocean at Mount Maunganui. The harbour is effectively two flooded river systems se ...
, and Rabbit Island, at the southern end of Tasman Bay / Te Tai-o-Aorere. See also Nelson Harbour's
Boulder Bank The Boulder Bank ( Māori: ''Te Taero a Keropa'' or ''Te Tāhuna o Tama-i-ea'') is a very unusual naturally formed landform in Nelson, New Zealand. It is a 13 kilometre long stretch of rocky substrate which begins at the Mackay Bluff and ends a ...
, below.


Europe

Barrier islands can be observed in the Baltic Sea from Poland to Lithuania as well as distinctly in the Wadden Islands, which stretch from the Netherlands to Denmark.
Lido di Venezia The Lido, or Venice Lido ( it, Lido di Venezia), is an barrier island in the Venetian Lagoon, northern Italy; it is home to about 20,400 residents. The Venice Film Festival takes place at the Lido late August/early September. Geography ...
and Pellestrina are notable barrier islands of the Lagoon of Venice which have for centuries protected the city of Venice in Italy.
Chesil Beach Chesil Beach (also known as Chesil Bank) in Dorset, England is one of three major shingle beach structures in Britain.A. P. Carr and M. W. L. Blackley, "Investigations Bearing on the Age and Development of Chesil Beach, Dorset, and the Associat ...
on the south coast of England developed as a barrier beach.


Processes

;Migration and overwash Water levels may be higher than the island during storm events. This situation can lead to
overwash Overwash is the flow of water and sediment over a coastal dune or beach crest during storm events (or other situations with high water). 'Overwash' may refer only to the landward flux of water due to overtopping of a dune system while 'washover' may ...
, which brings sand from the front of the island to the top and/or landward side of the island. This process leads to the evolution and migration of the barrier island. ;Critical width concept Barrier islands are often formed to have a certain width. The term “critical width concept” has been discussed with reference to barrier islands, overwash, and washover deposits since the 1970s. The concept basically states that overwash processes were effective in migration of the barrier only where the barrier width is less than a critical value. The island did not narrow below these values because overwash was effective at transporting sediment over the barrier island, thereby keeping pace with the rate of ocean shoreline recession. Sections of the island with greater widths experienced washover deposits that did not reach the bayshore, and the island narrowed by ocean shoreline recession until it reached the critical width. The only process that widened the barrier beyond the critical width was breaching, formation of a partially subaerial flood shoal, and subsequent inlet closure. For the present discussion, critical barrier width is defined as the smallest cross-shore dimension that minimizes net loss of sediment from the barrier island over the defined project lifetime. The magnitude of critical width is related to sources and sinks of sand in the system, such as the volume stored in the dunes and the net long-shore and cross-shore sand transport, as well as the island elevation. The concept of critical width is important for large-scale barrier island restoration, in which islands are reconstructed to optimum height, width, and length for providing protection for estuaries, bays, marshes and mainland beaches.


Formation theories

Scientists have proposed numerous explanations for the formation of barrier islands for more than 150 years. There are three major theories: offshore bar, spit accretion, and submergence. No single theory can explain the development of all barriers, which are distributed extensively along the world's coastlines. Scientists accept the idea that barrier islands, including other barrier types, can form by a number of different mechanisms.Davis Jr., p. 147 There appears to be some general requirements for formation. Barrier island systems develop most easily on wave-dominated coasts with a small to moderate tidal range. Coasts are classified into three groups based on tidal range: microtidal, 0–2 meter tidal range; mesotidal, 2–4 meter tidal range; and macrotidal, >4 meter tidal range. Barrier islands tend to form primarily along microtidal coasts, where they tend to be well developed and nearly continuous. They are less frequently formed in mesotidal coasts, where they are typically short with tidal inlets common. Barrier islands are very rare along macrotidal coasts. Along with a small tidal range and a wave-dominated coast, there must be a relatively low gradient shelf. Otherwise, sand accumulation into a sandbar would not occur and instead would be dispersed throughout the shore. An ample sediment supply is also a requirement for barrier island formation. The last major requirement for barrier island formation is a stable sea level. It is especially important for sea level to remain relatively unchanged during barrier island formation and growth. If sea level changes are too drastic, time will be insufficient for wave action to accumulate sand into a dune, which will eventually become a barrier island through
aggradation Aggradation (or alluviation) is the term used in geology for the increase in land elevation, typically in a river system, due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount o ...
. The formation of barrier islands requires a constant sea level so that waves can concentrate the sand into one location.


Offshore bar theory

In 1845 the Frenchman
Elie de Beaumont Elie and Earlsferry is a coastal town and former royal burgh in Fife, and parish, Scotland, situated within the East Neuk beside Chapel Ness on the north coast of the Firth of Forth, eight miles east of Leven. The burgh comprised the linked ...
published an account of barrier formation. He believed that waves moving into shallow water churned up sand, which was deposited in the form of a submarine
bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar * Chocolate bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a layer of cloud * Bar ( ...
when the waves broke and lost much of their energy. As the bars developed vertically, they gradually rose above sea level, forming barrier islands.


Spit accretion theory

American geologist Grove Karl Gilbert first argued in 1885 that the barrier sediments came from longshore sources. He proposed that sediment moving in the breaker zone through agitation by waves in longshore drift would construct spits extending from
headlands A headland, also known as a head, is a coastal landform, a point of land usually high and often with a sheer drop, that extends into a body of water. It is a type of promontory. A headland of considerable size often is called a cape.Whittow, John ...
parallel to the coast. The subsequent breaching of spits by storm waves would form barrier islands.


Submergence theory

William John McGee reasoned in 1890 that the East and Gulf coasts of the United States were undergoing submergence, as evidenced by the many drowned river valleys that occur along these coasts, including Raritan, Delaware and
Chesapeake Chesapeake often refers to: *Chesapeake people, a Native American tribe also known as the Chesepian * The Chesapeake, a.k.a. Chesapeake Bay *Delmarva Peninsula, also known as the Chesapeake Peninsula Chesapeake may also refer to: Populated plac ...
bays. He believed that during submergence, coastal ridges were separated from the mainland, and lagoons formed behind the ridges. He used the
Mississippi–Alabama barrier islands The Mississippi–Alabama barrier islands are a chain of barrier islands in the Gulf of Mexico along the coasts of Mississippi and Alabama enclosing the Mississippi Sound. The major islands are Cat Island, Ship Island, Horn Island, Petit Bois I ...
(consists of
Cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
, Ship,
Horn Horn most often refers to: *Horn (acoustic), a conical or bell shaped aperture used to guide sound ** Horn (instrument), collective name for tube-shaped wind musical instruments *Horn (anatomy), a pointed, bony projection on the head of various a ...
,
Petit Bois Petit Bois is a communal section in the Arcahaie commune in the Arcahaie Arrondissement, in the Ouest department of Haiti. See also *Arcahaie Arcahaie ( ht, Lakayè) is a commune in the Arcahaie Arrondissement, in the Ouest department of H ...
and Dauphin Islands) as an example where coastal submergence formed barrier islands. His interpretation was later shown to be incorrect when the ages of the coastal stratigraphy and sediment were more accurately determined. Along the coast of Louisiana, former lobes of the Mississippi River delta have been reworked by wave action, forming beach ridge complexes. Prolonged sinking of the marshes behind the barriers has converted these former vegetated wetlands to open-water areas. In a period of 125 years, from 1853 to 1978, two small semi-protected bays behind the barrier developed as the large water body of Lake Pelto, leading to Isles Dernieres's detachment from the mainland.


Boulder Bank

An unusual natural structure in New Zealand may give clues to the formation processes of barrier islands. The
Boulder Bank The Boulder Bank ( Māori: ''Te Taero a Keropa'' or ''Te Tāhuna o Tama-i-ea'') is a very unusual naturally formed landform in Nelson, New Zealand. It is a 13 kilometre long stretch of rocky substrate which begins at the Mackay Bluff and ends a ...
, at the entrance to Nelson Haven at the northern end of the
South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman S ...
, is a unique 13 km-long stretch of rocky substrate a few metres in width. It is not strictly a barrier island, as it is linked to the mainland at one end. The Boulder Bank is composed of
granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gr ...
from Mackay Bluff, which lies close to the point where the bank joins the mainland. It is still debated what process or processes have resulted in this odd structure, though longshore drift is the most accepted hypothesis. Studies have been conducted since 1892 to determine the speed of boulder movement. Rates of the top-course gravel movement have been estimated at 7.5 metres a year.


Ecological importance

Barrier islands are critically important in mitigating ocean swells and other storm events for the water systems on the mainland side of the barrier island, as well as protecting the coastline. This effectively creates a unique environment of relatively low energy,
brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
. Multiple wetland systems such as lagoons, estuaries, and/or marshes can result from such conditions depending on the surroundings. They are typically rich habitats for a variety of flora and fauna. Without barrier islands, these wetlands could not exist; they would be destroyed by daily ocean waves and tides as well as ocean storm events. One of the most prominent examples is the Louisiana barrier islands.Stone, G.W., and McBride, R.A., 1998, "Louisiana barrier islands and their importance in wetland protection: forecasting shoreline change and subsequent response of wave climate:" ''Journal of Coastal Research,'' v. 14, p. 900–915.


See also

* North Frisian Barrier Island *
Outer Banks The Outer Banks (frequently abbreviated OBX) are a string of barrier islands and spits off the coast of North Carolina and southeastern Virginia, on the east coast of the United States. They line most of the North Carolina coastline, separatin ...
* Virginia Barrier Islands *
New York Barrier Islands The Outer Barrier, also known as the Long Island and New York City barrier islands, refers to the string of barrier islands that divide the lagoons south of Long Island, New York from the Atlantic Ocean. These islands include Long Beach Barrier I ...
*
Texas barrier islands The Texas barrier islands are a chain of barrier islands in the Gulf of Mexico along the Texas Gulf Coast. The islands enclose a series of estuaries along the Texas coast and attract tourists for activities such as recreational fishing and dolphin ...
* Sea Islands * Long Beach Island * Bald Head Island


Notes


References

* *


External links

{{coastal geography Physical oceanography Coastal geography Hydrology Coastal and oceanic landforms Oceanographical terminology Islands by type