barrier island

n.
A long, relatively narrow island running parallel to the mainland, built up by the action of waves and currents and serving to protect the coast from erosion by surf and tidal surges.

Elongate, narrow accumulations of sediment which have formed in the shallow coastal zone and are separated from the mainland by some combination of coastal bays and marshes. They are typically several times longer than their width and are interrupted by tidal inlets. Although their origin has been widely discussed, at least three possibilities exist: longshore spit development and subsequent cutting of inlets; drowning of old coastal ridges; and upward shoaling of subtidal sediment accumulations. All three may have occurred; however, the last seems most likely and most prevalent.

Barrier islands must be considered in terms of the adjacent and closely related environments within the coastal system. Beginning offshore and proceeding landward, the sequence of environments crossed is shoreface, beach, dunes, back-island flats or marsh, coastal bay, marsh, and mainland. The barrier island proper consists of the beach, dunes, and back-island flats or marsh; however, of the remaining environments, at least the shoreface is closely integrated with the barrier island in terms of morphology, processes, and sediments. See also Dune.

A variety of physical processes exists along the coast. These processes act to shape and maintain the barrier-island system and also to enable the barrier to migrate landward as sea level continues to rise. The most important process in the barrier-island system is the waves, which also give rise to longshore currents. Waves and longshore currents dominate the outer portion of the barrier system, whereas tidal currents are dominant landward of the barrier, although small waves may also be present. Tidal currents are most prominent in and adjacent to the inlets. On the supratidal portion of the barrier island, the wind is the most dominant physical process.

Barrier-island sands represent one of the best sources of oil and gas, with the tight organic-rich source rocks being in the form of the bay and shelf muds and the barrier itself being the reservoir rock. These elongate sand bodies have been sought by exploration geologists for decades. The Tertiary sequences of the Texas Gulf coasts are an example of such barrier systems which have been very productive. See also Coastal landforms.

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A long, narrow, offshore island, usually having beaches and dunes on the seaward side, often with lagoons on the landward side, such as the islands enclosing Pamlico Sound, off the coast of North Carolina. The formation of barrier islands has been variously explained as:

▪the drowning of beach ridges;
▪the aggradation of submarine bars;
▪the progradation of spits

.

Any low sandy island that is parallel to the mainland and protects it from storms. (See Cape Hatteras.)

Barrier islands normally change location every ten to thirty years, a fact that makes coastline management a major problem in environmental policy.