Barrier Island Formation from Beach Erosion and Sand Deposition Created Jekyll Island and Other Barrier Islands
Jekyll Island is a barrier island formation, created by the forces of wind and current and tide. Barrier islands are found all over the world. In the United States, barrier islands occur on both coasts.
On the east coast they run from Maine to Florida. They form on the Gulf coast, also. On the Pacific side, they're mostly elongated spits. Even the Southeast and Northern coasts of Alaska support barrier islands.
No one knows exactly how barrier island formation occurs, but there are several theories.
- The "Waves Concentrate on Sand Bars in Shallow Water Adjacent to the Mainland" Theory Some experts believe waves take sand from higher areas and move it towards the land, where another bar forms. Over time, this new bar builds up. As waves eventually break over the top, more sand is deposited. The process continues until you have a fully developed barrier island, with large dunes and vegetation communities
- The "Connected Sand Spits" Theory This one says that maybe barrier island formation occurred when long spits of sand that were connected to the mainland became separated when a big storm like a hurricane breached the narrow, low-lying connection, leaving the spit stranded and cut off from the shore.
- The "Low-Lying Coastal Dune" Theory Maybe dunes along a low coast provided the core material for a barrier island formation? During a period of rising sea levels, the sea might have flooded these areas. Over time, sediment deposited by wave action made these submerged dunes higher until they formed islands.
Barrier islands are relatively young. Most are from 3000 to 7000 years old.
Whatever theory you subscribe to, barrier islands are the result of the complex interaction of waves, available sediment, and sea level change. Like building a sand castle, first you need the sand. Add a more or less stable sea level and wave action over time, and you can grow a barrier island.
Barrier Island Zones -
Several environmental zones can be present on barrier islands. The number of zones depend on the size of the island or its stage of growth.
- The Near Shore Environment -This zone is in the water, immediately seaward of the shore line, and includes the area of breaking waves, and any longshore sand bars and trough depressions that may be present. Formation of longshore bars and troughs will depend on the slope of the sea bed. Off the coast of Jekyll Island, the Continental Shelf is relatively flat, so troughs and longshore bars are apt to appear. The gentler the slope, the more bars and troughs. Storms and wave actions are constantly changing the shapes of the longshore sandbars within the Near Shore environment.
- The Beach Environment -We all know the beach environment. It's the gently sloping area beginning at the shore line and extending landward, which helps to protect the upland environments. The beach zone encompasses the flat sand area and the dunes, and many man-made structures like sea walls.The Beach zone includes two sub-zones, the Foreshore and the Backshore. The Foreshore extends from the low tide to high tide. The Backshore is mostly dry except during severe storms and storm tides. It's looser sand is more subject to wind erosion - sand tends to be blown landward. This is what forms the larger dunes. Vegetation begins to take root in this zone, slowing the wind and allowing further sand depositing, gradually increasing back dune size. There tends to be a balance of forces at work in barrier island formation. Generally, in the winter, storms scour the beach, causing erosion. Then, in the calmer summer months, current created by low wave action return sand from the longbars offshore to the beach. Long term erosion, however, is another story. Many factors may contribute to long term erosion, including violent storms, high rates of sea level rise, or natural or man-made interruptions in the longshore system that transports sand along a beach. Inappropriate shoreline development can aggravate the problem.
- The Dune Environment -The Dune environment continues to grow with opportunistic vegetation taking root and trapping more sand, eventually creating high dunes that offer excellent protection to inland areas. Birds and some wildlife start colonizing the dunes, also.
- The Washover Environment -Sometimes, large storm surges will wash completely over an island, or channel through lower spaces between the dunes, carrying sediment to the other side of the island. This is called a washover event. The sediments deposited form a fan shape in the bay or estuary, essentially extending the island towards the mainland. The barrier island actually migrates inland. Washover events help a barrier island conserve its mass, as sand removed from the shoreline is deposited on the island's backside.
- The Wetland Environment -This fringe environment is highly vegetated. In the lower latitudes, mangroves dominate here. The middle and higher latitudes support salt marsh, which provides great erosion stabilization. Salt marsh also serve up a nutrient smorgasbord to a great many marine invertebrates, mammals, reptiles, and birds.
- The Tidal Flat Environment -The tidal flat is an un-vegetated intertidal zone on the landward side of a barrier island. Bottom dwelling invertebrates live here, like oysters and crabs. Being in the intertidal zone, it's highly influenced by the tides.
Wave and tide continue to impact barrier island formation, producing currents traveling parallel to the shore and causing erosion. Where two barrier islands are separated by an inlet, tidal currents create sand bodies at the inlet's mouth. These bodies may interrupt normal beach forming processes. Loss of beach sand may occur.
Sometimes, humans attempt to influence beach formation by dredging for sand in outlying areas and depositing it along the beach front. This is called beach nourishment, but it's only a stop gap measure. If beach erosion has been occurring, it will again, and the expensive beach nourishment will be for naught.
There are two general types of barrier island formation.
- Wave Dominated Islands -Waves interacting with longshore currents create long, narrow barrier islands.
- Mixed Energy Barrier Island Formation -Waves and tide combine to create islands of unique size and shape. Islands are wide at one end, narrow at the other, resembling a chicken leg. They're called drumstick islands. Jekyll Island is a drumstick island. Drumsticks tend to be more stable than wave dominated islands.
Barrier islands function as protectors of the mainland, providing a buffer against severe storms. They're also wonderful wildlife habitat, harboring crabs, shore birds, gulls, terns, deer, raccoon, squirrels and gators, to name a few.
Plants that colonize the islands can include sea oats, sand live oak, myrtle oak, southern wax myrtle, magnolia, and slash pine, among others. The barrier flats include clams, mussels, cordgrass and sawgrass.
Despite their seeming permanence, barrier islands are ephemeral, on a geologic scale very young, and here today, gone tomorrow, prey to the dynamic forces that created them. While they fulfill their protective functions well, severe hurricane force winds and tidal surges can alter them drastically - along with the human habitation settlements that are built on them.
Humans would do well to remember this. Our best building efforts can come down around our heads in the span of an instant.
Because nothing lasts forever - including barrier islands.
Return From Barrier Island Formation to the Wild Side
