Barrier Islands

Barrier islands are an important part of the natural, economic, and recreational landscape of the North American coast. These elongated and detached bodies of unconsolidated sand or gravel lie above the high-tide level and are separated from the mainland by a lagoon, a marsh, or a sound. The islands are most prominent on wide and gently sloped trailing edge coasts, although they can form on collision coasts where there is an adequate supply of sediment. The islands typically occur in chains separated from neighboring islands by inlets, which are spaced depending on the relative strength of wave and tide forces. Long (wave-dominated) barriers tend to form where tidal range is small, and waves are able to close breaches that form during storms (e.g., from Southern Texas to the Northwest Florida coast). In contrast, mixed-energy barriers form through a combination of wave and tide forcing, leading to short barriers that are wider on one end (a drumstick) and separated by stable inlets with large sand shoals (e.g., the Georgia coast).

Theories of barrier island formation include (a) the subaerial growth of near-shore bars, (b) the long-shore progradation and eventual breaching of spits, and (c) the submergence of coastal dune ridges during the last glacial low-stand. Most barrier islands are less than 7,000 years in age, suggesting that they formed during a period when sea level was gradually rising, most probably as a combination of all the above mechanisms. Once formed, the morphology, composition, and stability of a barrier island depends on the relative importance of waves and tide, the availability of sediment, and relative sea-level rise. If there is a sufficient supply of sediment from offshore or alongshore, the island progrades seaward, leading to a regressive barrier, able to maintain itself against rising sea level. However, most modern barrier islands are retreating landward in response to both global (eustatic) sea-level rise and local subsidence at a rate that depends on the slope of the continental shelf. Despite an almost constant rise in sea level, barrier island transgression (rollover) tends to only occur during extreme storms capable of breaching the foredune and distributing sediment to the back of the island in the form of washover fans (see second photo).

The ability of coastal dunes to control the rate and timing of transgression depends on the ability of dunes to redevelop in height and extent from sediment delivered from the beach or wash-over fans by the wind following storms. Barrier islands affected by frequent storm events tend to be low in profile and are susceptible to island overstepping (island inundation). A lower frequency of storms or a large supply of sediment leads to larger dunes that can recover quickly following a storm, thereby limiting washover and promoting island rollover. A change in the rate of sea-level rise or the frequency of hurricanes and [Page 184][Page 185]tropical storms and a reduction in sediment supply through the construction of groins and jetties may lead to more rapid transgression and shoreline erosion.

Santa Rosa Island, a wave-dominated barrier island in northwest Florida, photographed in 2004

Source: Author.

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