Understanding How Barrier Islands Function

Barrier islands, also known as barrier spits, are found along coastlines worldwide. They are particularly prominent along the eastern seaboard of North America, stretching from New England down the Atlantic Coast, and further south along the Gulf Coast to Mexico.

Barrier islands are a sought-after destination for tourists, with popular vacation spots ranging from Atlantic City, New Jersey, to Miami Beach, Florida. Many people own vacation homes or condos here, and the development of barrier islands for tourism is on the rise. Despite their popularity, these islands are delicate and ever-changing ecosystems that play a crucial role in coastal geology and ecology. Increased development has not only threatened these fragile environments but also heightened the risk of property damage each year from hurricanes and Nor'easters.

This article delves into the delicate ecosystems of barrier islands. We will examine their formation, the various habitats and life they support, and how they evolve over time. Additionally, we will explore the impact of development and the recreational activities that draw tourists to these islands each year.

Formation of Barrier Islands

Barrier islands are elongated, narrow deposits of sand or other sediments found offshore, running parallel to the coastline. Some of these islands can stretch for up to 100 miles (160 km) or more. They are separated from the mainland by a shallow sound, bay, or lagoon. Chains of barrier islands are common along coastlines, and they are often divided by narrow tidal inlets, like those found in the Outer Banks of North Carolina.

The process by which barrier islands form remains complex and is not entirely understood. The prevailing theory suggests that these islands began to take shape around 18,000 years ago at the end of the Ice Age. As the glaciers melted and receded, rising sea levels inundated areas behind the beach ridges of that time.

As the waters rose, they transported sediments from the beach ridges, depositing them in the shallow waters just beyond the newly formed coastal ridges. The ongoing action of waves and currents continued to carry these sediments, gradually forming barrier islands. Additionally, rivers brought sediments from the mainland, which settled behind these islands and contributed to their development.

Barrier islands serve two essential roles. Firstly, they shield the coastline from intense storm damage. Secondly, they offer sanctuary for various wildlife by providing critical habitats. Now, let's explore the different components of a typical barrier island.

Barrier-island Zones

A typical barrier island is structured with several distinct zones, ranging from the ocean side to the sound side:

Each of these zones supports unique plant and animal life, which we will explore in the following section.

Barrier-island Habitats

Despite their narrowness, barrier islands are home to multiple distinct habitats. As we've already established, these habitats include the beach, dune, barrier flat, and salt marsh. Each habitat has unique conditions and wildlife, which we will explore in more detail.

Beach Habitat

The barrier island's beach habitat lies on the ocean side. Much like a desert, this area lacks fresh water, yet a significant portion of the beach is covered by salt water twice a day (during storms, the water can reach all the way to the base of the dunes). The plants and animals in this intertidal zone (the area between tides) must withstand prolonged exposure to both salt water and dry air.

On the beach, plant life is scarce, with only algae that have been washed ashore. Bacteria inhabit the spaces between the grains of sand, where the water from the surf seeps through. The animals on the beach include burrowing creatures such as mole crabs and clams, which filter-feed during high tides, burrowing worms that feed on bacteria in the sand, ghost crabs that scavenge, and various shorebirds like sandpipers, seagulls, and pelicans, which feed on crabs, burrowing animals, and fish from offshore.

Dune Habitat

The dunes are nourished by rainwater and surf and are occasionally flooded during intense storms. Despite this, the dunes remain a rather challenging environment with high salt content, sandy soil, and limited fresh water.

Plants like sea oats and bitter pancum help stabilize the dunes. Their root systems anchor the sand in place, and their shoots reduce wind speed, which allows sand to settle. Along the dunes, you'll find various crabs, especially ghost crabs. You'll also spot birds such as gulls and terns that prey on the animals living in the dunes.

Beyond the dunes, some islands are home to maritime forests containing shrubs and trees like Sand Live Oak, Myrtle Oak, Slash Pine, and Magnolia. The wildlife in these forests includes various snakes, opossums, skunks, raccoons, and foxes.

Barrier-flat Habitat

On certain islands, barrier flats follow the dunes. The primary plants here include cordgrass and sawgrass. These areas are regularly flooded during high tides. The mud and sediments are rich in anaerobic bacteria due to the lack of oxygen in the sediments. These bacteria break down the abundant organic material in the sediments, including dead plants and animals.

In the muddy wetlands, animals filter-feed on bacteria and plankton from the tidewaters or consume bacteria from the mud. These creatures include clams, mussels, snails, and worms. Various fish enter and leave with the tides. Fiddler crabs feed on the bacteria in the mud. Ghost crabs and blue crabs eat bacteria, small invertebrates, and small fish. Numerous birds, including seagulls, egrets, and pelicans, prey on the fish, crabs, and invertebrates.

Salt-marsh Habitat

The sound-side of a barrier island is typically dominated by the salt marsh. These salt marshes resemble those found on the coastal mainland. Similar to the barrier flats, salt marshes are frequently flooded with seawater during high tides, and the plants and animals that inhabit these areas are quite similar to those found in the barrier flats.

Nature's Effects on Barrier Islands

Barrier island formations are constantly evolving, influenced by several natural forces including the following:

How Storms Affect Barrier Island Chains

The effects of storms on barrier islands vary based on the characteristics of the storm (such as storm surge and waves) as well as the elevation of the barrier island when the storm strikes. To gauge the severity of the storm's impact, the U.S. Geological Survey (USGS) has developed the following "hazard scale":

For more details, see USGS: Mapping Coastal Change Hazards.

Fighting Erosion on Barrier Islands

Many barrier islands are becoming sought-after vacation destinations. Resort developments are taking place, including high-rise hotels, condos, and golf courses. To make room for these buildings, sand dunes are often flattened, and salt marshes and barrier flats are filled in. In some areas, humans have attempted to combat the natural erosion processes of barrier islands, though with varying success. Let's examine two notable examples.

Cape Hatteras Lighthouse

Barrier islands like Hatteras Island, NC, home to the iconic Cape Hatteras Lighthouse, are in a constant state of transformation. Offshore currents continuously carry sand from one part of the island and deposit it at the opposite end, reshaping the island over time.

The Cape Hatteras Lighthouse has been a beacon for sailors since the 1800s. Due to the erosion of the surrounding beach, the lighthouse was at risk of falling into the Atlantic Ocean. In 1989, authorities made the decision to move the lighthouse from its original location, where it had stood for over a century, to a new inland site about 2,900 feet (roughly 0.5 miles/0.8 km) away. Extensive reports, studies, and public hearings delayed the move, but it was finally completed in 1999.

Upham Beach, Florida

Longshore currents and wave action can cause significant erosion, transforming a beach in a short period. To maintain the beach, humans often replenish it with sand dredged from other areas, a process known as beach nourishment. This procedure is costly, often running into the millions of dollars.

As demonstrated below, beach nourishment at Upham Beach in west-central Florida proved to be short-lived. Within a year, the offshore currents had eroded the newly added sand. At best, beach nourishment serves as a temporary, expensive measure to counteract the relentless movement of barrier island sands. In the upcoming section, we will explore the significant impacts that development has had on barrier islands.

Development's Effects on Barrier Islands

Development has significant impacts on barrier island ecosystems, which are inherently dynamic. Environmental experts, including renowned geologist Orrin Pilkey from Duke University, have raised concerns about the risks of constructing on these islands. Let's explore two examples of barrier islands that have undergone drastic changes due to development.

Ocean City, Maryland

Ocean City, situated at the southern tip of Fenwick Island on Maryland's eastern shore, has long been a favorite beach destination. Large hotels began to appear in the 1920s, and by the 1950s, development surged and continued at a rapid pace for nearly 30 years. In the 1970s, concerns about the island's environmental health emerged, leading to laws that prohibited the dredging of channels and filling of wetlands.

A hurricane created the Ocean City Inlet in 1933, separating Fenwick Island from Assateague Island to the south. To ensure the inlet remained navigable, the U.S. Army Corps of Engineers installed two rock jetties. While the jetties helped stabilize the inlet, they disrupted the natural flow of sand transported north to south by longshore currents.

As a result, sand accumulated behind the northern jetty, while the sand south of the jetty was rapidly washed away. This increased erosion has pushed Assateague Island nearly half a mile (.8 km) inland. In just a short period, human interventions have permanently altered the island's natural structure.

Topsail Island, located in North Carolina, is a well-known beach destination along the state's coast.

Topsail Island, a popular vacation spot on North Carolina’s coast, saw extensive development with the rise of condominiums and beach houses. In September 1996, Hurricane Fran made landfall near Wilmington, NC, with its counterclockwise winds pushing heavy waves and storm surges over the island.

The hurricane caused severe erosion across the island, leading to overwashes. Multiple areas of the island were eroded, and the only highway was severely damaged. Residential properties were destroyed, with damages estimated at several hundred million dollars.

Barrier island development is a precarious endeavor, yet these islands remain popular for vacations and recreational use. Given their critical role in shielding coasts from storm damage, supporting ecosystems, and protecting wildlife, a balance must be struck between development and conservation.