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
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.
Jekyll Island is a Barrier Island with Distinctive Zones
Barrier islands are relatively young. Most are from 3000 to 7000 years old.
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
Barrier Island Zones -
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
- 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
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
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.
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