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Barrier Islands, a coastal landform and a type of barrier system, are relatively narrow strips of sand that are parallel to the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen. Excepting the tidal inlets that separate the islands, a barrier chain may extend uninterrupted for over a hundred kilometers, the longest and widest being Padre Island. 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.
Chains of barrier islands can be found along approximately thirteen percent of the world's coastlines, some displaying different settings, suggesting that they can form and be maintained in a variety of environmental settings. Numerous theories have been given to explain their formations.
Explanations for the development of barrier islands have been proposed by numerous scientists over more than 150 years. They can be grouped into three major theories: offshore bar theory, spit accretion theory and submergence theory. No single theory can explain the development of all barriers distributed extensively along the world's coastlines. Scientists accept the idea that barriers, including other barrier types, can form by a number of different mechanisms.
However, there are 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 most dominantly 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 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, there will not be enough time for wave action to accumulate sand into a dune, which will eventually become a barrier island through aggradation. Barrier islands need the sea level to remain constant so that waves can concentrate the sand into one location.
One of the earliest ideas to explain barrier island formation was published in 1845 by the Frenchman Elie de Beaumont. He believed that waves moving into shallow water churned up sand, which was deposited in the form of a submarine bar when the waves broke and lost much of their energy. As the bars accreted vertically, they gradually built above sea level, forming barrier islands.
American geologist Grove Karl Gilbert first argued in 1885 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 parallel to the coast. The subsequent breaching of spits by storm waves would form barrier islands.
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 Bays. He believed that during submergence coastal ridges were separated from the mainland, forming lagoons behind the ridges. He used the Mississippi-Alabama barrier islands (consists of Cat, Ship, Horn, Petit Bois and Dauphin Islands) as an example where coastal submergence formed barrier islands, but his interpretation was later shown to be incorrect as the coastal stratigraphy and sediment ages 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 had been transforming to the large water body of Lake Pelto, leading to Isles Dernieres's detachment from the mainland.
An unusual natural structure in New Zealand may give clues to the formation processes involved in barrier islands. The Boulder Bank, at the entrance to Nelson Haven at the northern end of the South Island, is a unique 13 kilometer-long stretch of rocky substrate a few metres in width. It is not strictly a barrier island itself, 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 speed of boulder movement. Rates of the top course gravel movement have been estimated at 7.5 metres a year.
Barrier islands play an enormous role in mitigating ocean swells and other storm events for the water systems behind on the mainland side of the barrier island. 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. Without barrier islands, these wetlands could not exist and would be destroyed by daily ocean waves and tides as well as ocean storm events. One of the most prominent examples is that of the Louisiana barrier islands.