Definition

A “coastal barrier” is a barrier that lies between a sea/lake/lagoon and some landform or feature that is non-coastal or at least more landward than the immediate modern or Holocene coastal landform or group of landforms. It may be that the next landward feature is another coastal barrier or ancient rocks. In this use of the term, it is merely a barrier between the water and other land or landforms, and it is likely consistent with the original use of the term (Johnson, 1919).

Description

In the American literature, it has been common to refer to a barrier as a barrier island, due to the predominance of these types of barriers on the East Coast of the USA. However, a barrier island is only one type of a suite of types of coastal barriers. Dillenburg and Hesp (2009) state that “a coastal barrier is a shore parallel structure, formed by an accumulation of sand, gravel, shells, and small amounts of organic material due to the action of waves, tides and winds” (p. 1). In some cases, a barrier may not be shore parallel, particularly where spits are forming (Zenkovich, 1967).

Hesp and Short (1999) define a coastal barrier as “a shore-parallel, sub-aerial and sub-aqueous accumulation of detrital sediment formed by waves, tides and aeolian processes. It constitutes a definable coastal landform or sequence of landforms which is clearly separate in age, lithology, and/or form from adjacent, underlying or landward landforms. The barrier may block off or impound drainage from the hinterland, but this is not a prerequisite for definition as a barrier” (p. 308). Coastal barriers may be progradational (building seawards), retrogradational (eroding landwards), or stable (Morton, 1994), and they may be transgressive where sea level is rising and regressive where sea level is falling (Hesp and Short, 1999).

There are a variety of coastal barrier types and many varying terms for these types. On an evolutionary continuum, coastal barriers range from barrier islands (i.e., a barrier separated from the mainland by a lagoon or sea with no connections to the mainland at either end, thus a true island) to attached barriers (i.e., barriers that are attached to the mainland and may merely be a beach or have dune fields which transgress or climb the mainland terrain) (Figure 1). Where the barrier is predominantly a beach and attached to the mainland, it has also been termed a bayhead beach (Johnson, 1919) and mainland beach (Roy et al., 1994). In between these types are various types such as barrier spits (barrier connected at one end to the mainland) and bay barriers (barrier connected at both ends and extending across a bay) (Shepard, 1960; Dillenburg and Hesp, 2009).

Coastal Barriers, Figure 1
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Stages in the development and retrogression of a barrier. (After Davis (1912) from Johnson (1919)). The models illustrate some of the barrier types from attached barrier (5) to barrier islands (1).

Full size image

The subaerial morphology of coastal barriers may range from a beach and backshore, overwash terrace, overwash fans and nebkha (discrete small dunes), beach and foredune, beach and beach ridge, multiple foredunes (relict foredune plains), beach ridge plains, foredune and blowouts, parabolic dunefields and transgressive dunefields, or combinations of these. All of the dune types may be found on progradational, retrogradational, or stable barriers.

Cross-references