A landform, a long, narrow island just offshore from the coastline of a landmass, running parallel to the inner coast.   A barrier to strong ocean waves, the island defines a sheltered bay behind itself.  Barrier islands are composed mostly of sand, and all have wide sandy beaches;  many barrier islands have a large sand dune separating the beach from the rest of the island.

Barrier islands form mostly along flat coastlines such as the coastline adjacent to the Atlantic Coastal Plain of North America. Most stretches of coastline whee barrier islands appear are undergoing a rise in sea level. As it rises, the island moves in a direction towards the inner coast (which is also moving).

Once, the most popular theory of barrier island formation said that wave action pushed sand inland from deeper ocean, piling up an underwater dune parallel to the coast.  This dune would eventually break the surface and turn the job of transporting sand over to the wind.   The wind piles up sand dunes, allowing vegetation to grow on it.

Theory 1 -- building up and underwater dune (cross section of coastline)

                    ____________                   _         ____________
~~~~~~~~~~~~~~~~~~~/                    ~~~~~~~~~~/ \~~~~~~~/
______->_/\_______/                     ______->_/   \_____/

Another theory states that sand from coastal erosion is carried by longshore drift along the coastline until coastal geometry makes the current carry the sand out to sea.    The current loses carrying capacity as it heads offshore, depositing the sand it carries and building up a spit extending parallel to the coastline.  The spit extends itself farther and farther until it forms a coastline the longshore drift can follow.  At some point, a storm (such as a hurricane) will overwash the spit and cut it off from the mainland, yielding a barrier island.

Theory 2 -- spit elongation (top view).

-> current ->                          -> current ->
___--_---_____                         ___--_---_____________
              \                                       .----))) <- spit
            __/                                    __/
            --_-_  __-__-___-                      --_-_  __-__-___-
                 ||                                     ||

-> current ->      -> current ->
___--_---________  _____  __________
            --_-_  __-__-___---_-__-

However, many scientists believe that the rise in sea level has something to do with it. In this theory, sand dunes build up along a shoreline while sea level drops or is relatively stable. When the sea begins to rise, however, the area behind the dune is drowned, forming the back bay. Most of the world's barrier islands, then, would be formed from the rise in sea level that has accompanied the end of the last Ice Age.

A barrier island is an important part of a coastal ecology:

  • It protects the mainland from erosion.
  • The bay side of the island develops into wetlands
  • Conditions on the island result in the evolution of specially adapted species.
However, a barrier island's long, sandy beach makes it attractive to humans, just the perfect spot for recreation areas and getaway homes.  Some are ideal locations for resort cities, fishery bases, or seaports.  The largest barrier island city is Lagos, Nigeria, whose population now exceeds 10 million.   Important North American barrier island cities are Galveston, Texas and Miami Beach, Florida.

Development of a barrier island results, naturally, in changes to the island ecology, with similar destruction of habitat to development of other areas.   As bad as this may be, there is worse:  Development often results in the removal of the island's protective dune, leaving the island itself vulnerable to the next storm.

In places where sea level is rising, such as the East Coast of the United States, the island migrates inland, just as the inner coast does.  It's impractical to move a beachfront home or a billion-dollar hotel inland (such as was done with Cape Hatteras Lighthouse) every few years, especially when someone else owns the property behind you.   Thus, development tries to fix the island in place, resulting in a war between man and the hungry coastline.

For a good example, consider Fenwick Island and Assateague Island on Maryland's Atlantic coast.   Before the Ocean City Inlet was carved out by a 1934 hurricane, the two islands were connected1, resulting in an unbroken beach.  The creation of the inlet allowed Ocean City to develop on Fenwick Island to the north, and Assateague Island became a national park.

So, although the southern end of Fenwick Island has been fixed in place by jetties, seawalls, and beach replenishment  projects, the northern end of Assateague Island has been allowed to migrate inland naturally.  The islands are now almost completely offset from each other.

 _/\_ _/   | |
|   / \    | |
|  \__/    | |
|          / |
\         /  /
 \       /  /
  \     /  |  <-Fenwick Island
   \    |  |    (Ocean City)
    \   |  |
     |  |__|_
     | -----
     | ||
    /  ||
   /   /|  <- Assateague
  /   / |

This is one war people can't win:  Eventually a storm will cause the island to migrate catastrophically instead of gradually. Maryland spends millions every year for beach replenishment and dune reconstruction.

1more accurately, the inlet cut the southern end of Fenwick Island off, and another inlet to the south filled in, attatching the chunk of island to Assateague to the south.

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