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
island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atoll An atoll (), sometimes known as a coral atoll, i ...

s 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
wetland A wetland is a distinct ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles ...

s may flourish. A barrier chain may extend uninterrupted for over a hundred kilometers, excepting the tidal
inlet An inlet is an indentation of a shoreline, usually long and narrow, such as a small bay or arm, that often leads to an enclosed body of water, body of salt water, such as a Sound (geography), sound, bay, lagoon, or marsh. Overview In sea coasts, ...

s that separate the islands, the longest and widest being
Padre Island Padre Island is the largest of the Texas barrier islands and the world's longest barrier island. The island is located along Texas's southern coast of the Gulf of Mexico and is noted for its white sandy beaches. Meaning ''father'' in Spanish langua ...
of Texas. Sometimes an important inlet may close permanently, transforming an island into a
peninsula A peninsula ( la, paeninsula from 'almost' and 'island') is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el ...

, thus creating a barrier peninsula. The length and width of barriers and overall morphology of barrier coasts are related to parameters including
tidal range Tidal range is the height difference between high tide High Tide may refer to: * High tide, the state of tide when the water rises to its highest level Film and television * High Tide (1947 film), ''High Tide'' (1947 film), an American film noi ...
wave energy Wave power is the capture of energy of wind waves to do useful mechanical work, work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power (physics), power is a wave energy converte ...
, sediment supply,
sea-level trends
sea-level trends
, and
basement controls
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 man-made offshore structure constructed parallel to the shore is called a breakwater. In terms of
coastal morphodynamics Coastal morphodynamics (i.e. the dynamics of beach A summer tourism at the Yyteri Beach in Pori, Finland">Pori.html" ;"title="Yyteri Beach in Pori">Yyteri Beach in Pori, Finland. A beach is a landform alongside a body of water which con ...
, 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.

Constituent parts

;Lower 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/tidal flat area. Characteristics common to the lower 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 is common and many fossils can be found in lower shoreface deposits in the geological record. ;Middle shoreface The middle shore face 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. ;Upper shoreface The upper shoreface is constantly affected by wave action. This results in development of herringbone sedimentary structures because of the constant differing flow of waves. The sand is coarser. ;Foreshore The foreshore is the area on land between high and low tide. Like the upper shoreface, it is constantly affected by wave action. Cross bedding and Lamination (geology), lamination 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 Sorting (sediment), sorted. ;Backshore The backshore is always above the highest water level point. The berm 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 Dune#Coastal dunes, Coastal dunes, 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 processes, 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 and tidal flat 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 Hypoxia (environmental), anaerobic environment. This will allow high amounts of organic-rich mud to form. Vegetation is also common.


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


Moreton Bay, on the east coast of Australia and directly east of Brisbane, is sheltered from the Pacific Ocean by a chain of very large barrier islands. Running north to south they are Bribie Island, Moreton Island, North Stradbroke Island and South Stradbroke Island (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 seen most prominently on the United States' East Coast and Gulf Coast, where every state, stretching from Maine to Florida and Florida to Texas on each coast has at least part of a barrier island, stretching to more than twenty-five for Florida. This chain is international. It starts in Quebec's Magdalen Islands and ends in Mexico.
Padre Island Padre Island is the largest of the Texas barrier islands and the world's longest barrier island. The island is located along Texas's southern coast of the Gulf of Mexico and is noted for its white sandy beaches. Meaning ''father'' in Spanish langua ...
, Texas, is the world’s longest barrier island; other well-known islands on the Gulf Coast include Galveston Island in Texas and Sanibel, Florida, Sanibel Island in Florida. Those on the East Coast include Miami Beach, Florida, Miami Beach, Palm Beach, Florida, Palm Beach, and Cape Canaveral in Florida; Cape Hatteras in North Carolina; Assateague Island in Virginia and Maryland; Absecon Island in New Jersey, where Atlantic City, New Jersey, Atlantic City is located; and Jones Beach Island and Fire Island, 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.


Barrier Islands can also be found in Maritime Canada, and other places along the coast. A good example is found at Miramichi Bay, New Brunswick, where Portage Island as well as Fox Island, New Brunswick, Fox Island and Hay Island protect the inner bay from storms in the Gulf of Saint Lawrence.


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, which guards the entrance to Tauranga Harbour, and Rabbit Island, New Zealand, Rabbit Island, at the southern end of Tasman Bay / Te Tai-o-Aorere. See also Nelson Harbour's Boulder Bank, below.


Barrier islands can be observed in the Baltic Sea and are a distinct feature of the Wadden Islands, which stretch from the Netherlands to Denmark. Lido di Venezia and Pellestrina are notable barrier islands of the Lagoon of Venice which have for centuries protected the city of Venice in Italy. Chesil Beach on the south coast of England developed as a barrier beach.


;Migration and overwash Water levels may be higher than the island during storm events. This situation can lead to overwash, 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 Tidal range is the height difference between high tide High Tide may refer to: * High tide, the state of tide when the water rises to its highest level Film and television * High Tide (1947 film), ''High Tide'' (1947 film), an American film noi ...
: 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 rise, 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. 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 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 (landform), 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 Spit (landform), spits extending from headlands 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 Coast of the United States, East and Gulf Coast of the United States, Gulf coasts of the United States were undergoing submergence, as evidenced by the many drowned river valleys that occur along these coasts, including Raritan Bay, Raritan, Delaware Bay, Delaware and Chesapeake Bay, Chesapeake 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 (consists of Cat Island (Mississippi), Cat, Ship Island (Mississippi), Ship, Horn Island (Mississippi), Horn, Petit Bois Island (Mississippi), Petit Bois and Dauphin Island, 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, at the entrance to Nelson, New Zealand, Nelson Haven at the northern end of the South Island, 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 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. 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 :Barrier islands of Louisiana, 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 *Virginia Barrier Islands *New York Barrier Islands *Texas barrier islands *Sea Islands *Long Beach Island *Bald Head Island, North Carolina, Bald Head Island



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External links

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