BBARRIER ISLANDSDefinition and occurrenceBarrier islands are elongate, shore-parallel accumulations ofunconsolidated sediment (primarily sand), some parts of which aresupratidal, that are separated from the mainland by bays, lagoons, orwetland complexes. They are most abundant along the coastlines of thetrailing edges of continental plates and of epicontinental seas and lakes(e.g., Caspian and Black Seas). They do not occur on coasts with tidalranges greater than around 4 m, because their primary mechanism waveaction is not focused long enough at a single level during the tidal cycleto form the island and the strong tidal currents associated with suchlarge tides transport the available sand to offshore regions. Barrierislands do occur, primarily as spit forms, on leading edge and glaciatedcoasts, but they are a minority coastline type in those areas.Barrier islands are the dominant coastline type along the Atlanticand Gulf coasts of the United States, most of them having been formedwithin the last 4,000–5,000 years during a near stillstand of sea level.Barrier islands on depositional coastsDepositional, coastal plain shorelines typically have barrier islandslocated between major river deltas and estuaries. Two types of barrierislands may be present, those that consistently migrate landward (retro-grading) and those that build seaward (prograding). The island typedepends upon the ratio of relative sea-level change to sediment supply.Diminished or low sediment supply and/or rapid sea-level rise promotesthe development of retrograding barrier islands and vice versa forprograding barrier islands.Retrograding barrier islands are composed of coalescing washoverfans and terraces that are overtopped at high tides, usually several timesa year (Figure B1). Stratigraphically, a relatively thin wedge of sand andshell of the washover terrace overlies backbarrier sediments, which aretypically composed of muddy sediments deposited in the lagoons orwetlands behind the islands. These islands are impractical sites forhuman development because of their constant landward migration.Prograding barrier islands are composed of multiple beach ridges.Many have a drumstick configuration because of selective sand accu-mulation on the updrift end of the island (the direction sand comesfrom). The most notable changes on prograding islands occur when theadjacent tidal inlets migrate or when the inlets expand dramaticallyduring hurricane storm surges.Stratigraphically, prograding barrier islands are composed of sand8–10 m thick (thickness depends upon wave size), which has progradedover offshore muds (Figure B1). When human development occurs onthese islands, buildings are usually secure from all but the most extremehurricanes, if they have been set back an adequate distance from thefront-line dunes and tidal inlets. But that security will vanish if a majorrise in sea level occurs (Hayes, 1996).The morphology of prograding barrier islands is controlled by acombination of wave and tidal forces (Hayes, 1979; Davis, 1994a).Under wave-dominated conditions, which most commonly occur inmicrotidal areas (tidal range ?2 m; Davies, 1964), the barriers are long,typically tens of kilometers, with widely spaced inlets that have largeflood-tidal deltas and small ebb-tidal deltas. Washover fans are com-mon and the islands are flanked on the landward side by bays and/orlagoons (Figure B2). Barrier islands along mixed-energy coasts (Hayes,1979), which typically occur in mesotidal areas (tidal range ? 2–4 m),are stunted and short (usually ?10 km) with abundant tidal inlets thatcontain large ebb-tidal deltas and small to nonexistent flood-tidaldeltas. These islands are flanked on the landward side by complex tidalchannels, tidal flats, and wetlands (Figure B3). Barrier islands do notoccur on tide-dominated coasts.Origin of barrier islandsBarrier islands are thought to most commonly originate in one of thethree possible ways: (1) by spit elongation (Fisher, 1967); (2) retreatingtransgressive barrier islands (Swift,1975);and (3)a process termedtrans-gressive–regressive interfluve hypothesis by Hayes (1994). In many partsof the world, it is clear that the source of sand for the existing barrierislands originated from an updrift headland, and as a spit extended awayfrom the headland it was cut into segments during storms, creating tidalinlets that eventually attained permanence. Swift (1975) stated that bar-rier islands originated at a lower stand of sea level and migrated over thedrowning coastal plain as sea level rose during the early Holocene. The“primary barrier”of Pierce and Colquhoun (1970), the nucleus for manyprograding barrier islands, no doubt originated in this way. According tothe transgressive–regressive interfluve hypothesis, as sea level rose, thetransgressive barrier eventually perched on the topographic high of aninterfluve located between major alluvial valleys that were carved duringthe last Pleistocene lowstand (Figure B4). Once sea level stabilizedaround 4500 years BP , prograding barrier islands developed in areas withadequate sediment supply. As the island grew, beach ridges progradedaway from the interfluve, with major tidal inlets forming at both ends ofthe island over the antecedent lowstand valleys (Moslow, 1980). Thismechanism explains the origin of many of the major prograding barrierislands along the coast of the southeastern United States.Barrier islands on leading edge and glaciated coastsThe somewhat rare barrier islands on the leading edge, west coast of theUnited States are, for the most part, relatively short spits that have built
