Barrier islands are a coastal landform, a type of dune system and sand island, where an area of sand off the coast has been formed by wave and tidal action parallel to the mainland coast. Barrier islands are commonly formed in long, narrow systems parallel to shorelines and are shaped by waves, tides, sediment movement, and sea-level change, which cause them to shift, grow, or erode over time. They protect coastlines by absorbing energy, and create areas of protected waters where wetlands may flourish. A barrier chain may extend for hundreds of kilometers, with islands periodically separated by tidal inlets. The longest barrier island in the world is Padre Island of Texas, United States, at long.Garrison, J.R., Jr., Williams, J., Potter Miller, S., Weber, E.T., II, McMechan, G., and Zeng, X., 2010, "Ground-penetrating radar study of North Padre Island; Implications for barrier island interval architecture, model for growth of progradational microtidal barrier islands, and Gulf of Mexico sea-level cyclicity:" Journal of Sedimentary Research,' v. 80, p. 303–319. Sometimes an important inlet may close permanently, transforming an island into a barrier peninsula, often including a barrier beach. Barrier islands are related to other coastal landforms such as barrier beaches and spits, which share similar physical features but differ primarily in their degree of attachment to the mainland and the presence of water on their landward side.Stripling, S.; Bradbury, A. P.; Cope, S. N.; Brampton, A. H. (2008). Understanding barrier beaches: R&D Technical Report FD 1924/TR (PDF) (Report). United Kingdom Department for Environment, Food and Rural Affairs. Though many are long and narrow, 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.Davis & FitzGerald (2004), p. 144. The amount of vegetation on the barrier has a large impact on the height and evolution of the island.
Globally, there have been approximately 1,500 barrier islands identified, totaling over 15,000 kilometers in combined length. Barrier islands are most abundant in the northern hemisphere with North America accounting for over one-third of the barrier islands and Asia nearing one-quarter of the total.Stutz, Matthew L.; Pilkey, Orrin H. (2001). "A Review of Global Barrier Island Distribution". Journal of Coastal Research: 15–22. ISSN 0749-0208.
There are chains of barrier islands along approximately 13 to 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 proposed to explain their formation.
A human-made offshore coastal engineering structure constructed parallel to the shore is called a breakwater. Its coastal morphodynamic effect is to dissipate and reduce the energy of the waves and currents striking the coast in the same way as a naturally occurring barrier island.
Constituent parts Barrier islands function as part of an interconnected coastal system where environments exchange both sediment and energy, where changes to one component can influence the entire barrier system. Upper shoreface The shoreface is the part of the barrier where the ocean reaches 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 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 is common and many fossils can be found in upper shoreface deposits in the geologic record.
Middle shoreface The middle shoreface is located in the upper shoreface. The middle shoreface is strongly influenced by wave movement 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 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 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 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 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 wind-blown dunes. The difference is that dunes on a barrier island typically contain coastal vegetation roots and marine bioturbation. They also help Barrier Islands grow.
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 anaerobic environment. This will allow high amounts of organic-rich mud to form. Vegetation is also common.
Processes 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. During these washover events, the sand that is being transported may end up on the landward side, beyond the dune, forming a washover fan. The overwash can continuously alter the surface of the barrier, requiring the ecosystem to adapt to periodic disturbances. This process leads to the evolution and migration of the barrier island.
Types of migration Barrier island systems are dynamic and they may shift positions over time through three main types of migration. Lateral migration happens when sand is pushed along the coast, causing erosion at one end and accumulations at the other. This results in sideways movement along the shoreline. Prograding/regressive migration is when sand builds up on the ocean-facing side of the barrier island, leading to growth on the seaward side. Transgression migration occurs when the barrier island is being built up toward the mainland. This is commonly caused by the sea levels rising or the land sinking."Island Migration - SEPM Strata". www.sepmstrata.org. Retrieved 2026-02-03.
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.
Critical barrier width can be 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.
Location
Barrier islands can be observed on every continent on Earth, except Antarctica. They occur primarily in areas that are tectonically stable, such as "trailing edge coasts" facing (moving away from) ocean ridges formed by divergent boundaries of tectonic plates, and around smaller marine basins such as the Mediterranean Sea and the Gulf of Mexico.Davis (2016), p. 8 Areas with relatively small tides and ample sand supply favor barrier island formation.
Australia 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.
K'gari (formerly known as 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, had 29 (in 1997) in just along the west (Gulf) coast of the Florida peninsula, plus about 20 others on the east coast and several barrier islands and spits along the panhandle coast. Padre Island, in Texas, is the world's longest barrier island; other well-known islands on the Gulf Coast include Galveston Island in Texas and Sanibel and Captiva Islands in Florida. Those on the East Coast include Miami Beach and Palm Beach in Florida; Hatteras Island in North Carolina; Assateague Island in Virginia and Maryland; Absecon Island in New Jersey, where 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.
Canada 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 and Hay Island protect the inner bay from storms in the Gulf of Saint Lawrence.
Mexico Mexico's Gulf of Mexico 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, at the southern end of Tasman Bay. See also Nelson Harbour's Boulder Bank, below.
India The Vypin Island in the Southwest coast of India in Kerala is 27 km long. It is also one of the most densely populated islands in the world.
Indonesia