Barrier Islands

Before it is possible to define how barrier islands work, it’s important to have an understanding of what barrier islands are. Barrier islands are those narrow little strips of sand and beach grass that line the eastern North American seacoast, where people build vacation homes and sea birds flock to hatch their eggs. Though they are often barely more than specks of land with names, a few barrier islands approach 100 miles in length.

Barrier islands are rarely the same from year to year. Their coastlines are ever in flux; the sand and loose soil are at the mercy of the wind, of tides, and of storms. If you’ve ever read the stories about Chincoteague Island and her “outrider” sister, Assateague Island, then you probably have a good idea what they’re about. Barrier islands do exist in other areas of the world, and even in other areas surrounding North America, but eastern coastal conditions are ideal for their formation.

Nature intended barrier islands as the first line of defense for the mainland against heavy weather. You can see evidence of this function by watching the few islands that exist off Louisiana’s coast, which are slowly eroding away, eventually to disappear forever.

Researchers are already growing concerned about the fate of the Gulf coast wetlands these islands protect, particularly in light of the disappearance of one island already in 2004. As the loss of these islands progresses over time, some scientists predict that the brackish water lake, Lake Ponchartrain, will eventually become a bay in the toe of Louisiana’s boot, due to storm erosion.

So how much science can there be behind a narrow strip of sand? Quite a lot. Due to their unique and surprisingly consistent construction, barrier islands absorb a great deal of the erosive effects that hurricanes would have. How do they perform this function? The simplicity of science will surprise you.


Barrier islands share many features, despite any differences in their sizes or in the areas in which they are formed. They all consist of sand or another sediment, dropped along the coastline. Barrier islands often run in chains along a coast, and always parallel to it, separated from the coast by a shallow body of water.

Theoretically, the first barrier islands were created when glaciers receded and melted, creating the ideal conditions for their formation. Over time, silt, sand, and sediments carried by the rivers and oceans interacted with the troughs and ridges formed by the action of the glaciers, producing narrow strips of land. Barrier islands eventually emerged from these activities, built on the foundations provided by the continuous introduction of new raw materials. All of these islands had, and still have, the following features:

  • Beach
  • Dunes
  • Barrier Flat
  • Salt Marsh

With this deceptively simple construction, barrier islands not only protect the coastline, but also provide breeding and nesting grounds for all kinds of seabirds. Let’s take a look at how these elements perform their necessary functions.


The beach is formed by the action of waves, tides, and rivers, all of which deposit the sand or other sediment that gathers on the ocean side of the island. The beach does not support any plant growth itself. However, some animals do live on the beach, such as various species of crabs and shorebirds, worms, and clams. A large portion of the beach gets flooded twice daily due to tides, although it is not entirely submerged except in the case of high tides or waves caused by storms.


Dunes are created out of blown sand, which builds up with the assistance of a stabilizing agent. Natural stabilizers include plants and rocks. Dunes can also develop through the assistance of man-made stabilizers such as fences and buildings. There may be several lines of dunes, ranging from the largest “primary” dunes to the smallest “tertiary” dunes.

Dunes form on the ocean side of the island; the primary dune is that which is closest to the ocean. Dunes play an important role in the health of a barrier island. One of the reasons that Louisiana’s barrier islands are disappearing more quickly than islands along the East Coast is because they lack most of the sandy deposits that form dunes.

The dune environment is friendlier to animal and plant life than is the beach. As on the beach, crabs and shorebirds populate the dunes; however, its friendlier environment permits some varieties of plants to take root. Although the majority of these plants are grasses, some of the larger, more stable islands also support some trees and shrubs.

On such islands, there may be small animal wildlife, such as skunks, snakes, and raccoons. All of these plants allow larger and more stable dune landscapes to be formed, as their roots stabilize the sand, permitting the dunes to hold and collect more sand.

Barrier Flat

Islands that have no trees and shrubs have barrier flats that follow the dunes. Dirt and other sediments that have blown over or through the range of dunes to the interior of the island form these flats, which consist of wet mud, cordgrass, and saw grass. The barrier flat is also called the “overwash,” since water that breaches the dunes is trapped and filtered into the island’s ecology. Like the beach, barrier flats are flooded during the high tide. These flats are home to a wide variety of animal life, such as crabs, clams, fish, and birds.

Salt Marsh

Marshes form on the side of the island facing the mainland coast. They are divided into the high marsh and low marsh areas. Low marsh areas are flooded with each of the high tides that affect the island, just like the beach and barrier flats. The high marsh is flooded less frequently, approximately twice in a month. The plant and animal life is similar to that of the barrier flats, and of salt marshes on the mainland.

Despite the relative stability of all of these systems, barrier islands are continual works in progress. Any movement of water or wind affects the surface of these islands; their shifting costs are dramatically affected by storms. Although a barrier island’s beach or dunes are always being built up or partially torn down, the loss of either on a permanent basis can damage the island’s ecology.

If the dune system is compromised, then storm winds and waters can damage the barrier flats, changing the form of the island forever. At the very least, the sediments deposited by the storm can shift the flats and marsh toward the mainland; at the very worst, this loss can destroy the island. By taking the brunt of the damage caused by high seas and winds produced by severe storms and hurricanes, barrier islands protect delicate shoreline ecological systems that exist on the mainland coast.

Despite the valuable contributions that barrier islands make to the coastline, barrier islands are constantly under very human threat. Human beings build roads, bridges, and jetties that change the flow of water, and thus, the way sand is deposited along the island beaches. In addition, because of the beauty of the island landscapes, people find barrier islands welcome places to build vacation homes.

Permanent changes are made to the barrier flats and salt marshes to accommodate the building of homes, businesses, and infrastructure. The changes that human settlements create affect the islands’ ability to function properly. These changes can increase the speed of erosion, or even cause various portions of the islands to break away during storms. Several states are sponsoring environmental studies, to determine the viability of developing barrier islands.

As destructive as human settlement can be to barrier island ecology, humans can also prevent the islands from disappearing. The process by which this prevention is done is called “renourishment,” also called “erosion mitigation.” Rather than creating storm jetties to slow erosion, an action that actually speeds erosion in some cases, sediment is strategically placed in areas of need, allowing the sediment to be absorbed into the island systems.

Barrier islands provide homes for shorebirds and nesting places for migratory birds. They perform a valuable service, preserving delicate mainland ecology. Despite their relative delicacy, barrier islands are a valuable first line of defense against severe storms. It is up to human beings to reverse some of the damage being done by the development of these islands, keeping this line of defense from being lost forever.

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