Definition Common Landforms Barrier Islands Barrier Spits Welded Barriers Tombolos and Salients

Definition

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).


Continent
Percent of Continent’s Coastline
Percent of World’s Barrier Coastlines
North America
17.6
33.6
Europe
5.3
8.4
South America
12.2
10.3
Africa
17.9
8.6
Australia
11.4
5.6
Asia
13.8
5.6

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)
Back to the top


Common landforms
Barrier Islands

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 island system
with mutiple spit complexes, including the distinct cuspate spit at the lower right. (USAS EROS Data Center, 2001)
Back to the top
Barrier Spits

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)
Back to the top
Welded Barriers

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)
Back to the top

Continue to the next page
Copyright 2004, Jennifer Bruce