Barrier systems can be found on 15% of the world's coasts, on all of the
continents except Antarctica and at every latitude, including nearly half
of the Atlantic and Gulf of Mexico coastlines (Davis, 1994).
Percent of Continent’s Coastline
Percent of World’s Barrier Coastlines
Distribution of barrier coastlines worldwide (Cromwell, 1971).
All barrier forms consist predominantly of sandy sediment separated from the continent by a
shallow body of water, are greater in length than they are wide, and typically have a straighter
seaward margin and a more lobate or cuspate landward margin (Shepard, 1960). They need an ample
supply of sediment (typically sand), a favorable geomorphic setting and the existence of the
appropriate processes that will shape and maintain the landforms (Davis, 1994). They can be
less than 100 meters wide and more than several kilometers in length, as shallow as 2 meters
to more than 20 meters deep (Davis, 2004).
There are three different parts to the barrier: the beach, the interior, and the
backbarrier (also called the landward margin). The beach is the dynamic part of the barrier,
where sediment is deposited and eroded by longshore drift, waves, tides and storms. The
barrier interior usually contains dunes and ridges, some of which may be vegetated. This
is the area that is typically developed by humans, which traps the sand and disrupts beach
nourishment. Between the barrier and the mainland is an area of calm water, such as a lagoon,
bay, tidal flat, marsh or swamp.
Plum Island, Massachusetts, with some barrier features labeled. (Photo by Lindley Hanson)
When most people think of barrier systems, they often think of barrier islands.
Barrier islands are usually elongate and orientated parallel to the shore, backed by
lagoons on the shoreward side of the island. They are commonly found in groups, and
they typically develop on coasts with low-gradient shelves, low tidal ranges and ample
sediment supply (Davis, 1994).
Cape Hatteras, South Carolina. This shoreline has an extensive barrier
with mutiple spit complexes, including the distinct cuspate spit
at the lower right. (USAS EROS Data Center, 2001)
Created by longshore sediment drift, spits build out from land into
open water in places where the shoreline curves back away from the direction
of longshore drift. They require lots of sediment input and need a stable platform
to build upon, and can range in length from a few tens of meters to a few kilometers
(Davis, 2004). Often transient, they are easily breached and reshaped by larger storms.
They often create a bay or marsh behind them, and they take many different forms
including cuspate spits (which form when longshore transport comes from two directions
and sand piles up in a triangle) and recurved spits (which form curved beach ridges as
longshore drift is redirected by wave action, and the spit becomes hook-shaped). The
curved ends are the result of wave action, or because the spit reaches the edge of its
coastal marine platform. Large recurved spits are common at the mouths of
large rivers, such as the Hudson and Delaware Rivers (Davis, 1994).
Cape Fear, North Carolina. Both sides of the inlet on the southern edge have recurved spits,
as evidenced by their sweeping shape and the presence of multiple ridges at the
distal end. (Photo by Allen Glazner)
Formerly called baymouth bars, welded barriers are attached to land at both ends,
often from headland to headland. They form on irregular coasts with lots of sediment
and moderate to high wave energy, and can contain a mixture of longshore beach sediment and
eroded headland sediment. They typically close off an embayment.
This welded barrier in southern California has entirely closed off the bay behind it. (Photo by Steven Kite)
Tombolos and Salients
These landforms also require lots of sediment from longshore transport. Wave
refraction around offshore island (typically not a barrier island) produces reverse
transport on its shoreward side island, interrupting the sediment movement on the coast,
and sediment builds from the coast (forming a salient) until it connects with island
(forming a tombolo). Salients and tombolos tend to occur when the distance between
the island and the shoreline is less than four times the size of the island.
Sediment has outbuilt from the shore to connect this island, forming a tombolo.
(Photo from Biology 3361 lecture, Mount Allison University, New Brunswick)
Copyright 2004, Jennifer Bruce