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In order to understand current
processes acting on barrier islands, we must investigate the forces that
created them. These islands are normally in dynamic equilibrium with the
natural forces of sea level, tides,
and waves. The movement
of sand from one reservoir (barrier island, beach,
dune, continental
shelf, etc.) to another is a key mechanism to maintaining this equilibrium.
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| Change in sea
level is caused from a combination of processes. The total volume of
water in our oceans can be effected by such events as glaciation,
global warming,
the displacement of sea water by increased rates of seafloor
spreading and underwater volcanism, and the amount of sand deposited
or eroded at any given time. Sea level has been rising at a rate of 10 -
15 cm over the past 100 years (www-class.unl.edu,
2001). This rate seems too slow to notice, but over thousands of years,
the change is significant. |
Locally, sea level can be affected by regional uplift
or subsidence (raising or lowering) of the coastline. Such is the case with
Morris Island, just north of Folly. The Morris Island Lighthouse was located
on the island as recently as the 1940's and is now located located 400 meters
offshore (Neal, et al, 1984). This lighthouse is isolated due to two natural
processes. First, the land is subsiding (or dropping) and secondly, sea
level is rising. These processes alone are not enough to cause it to be
inundated by the ocean. The third factor is human-induced. The construction
of the Charleston Harbor Jetties
has cut off the sand supply to Morris Island.
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At the
end of the last major glaciation, 18,000 years ago, sea level was approximately
150 feet below its current position (www-class.unl.edu, 2001).
At that time, sea level began to rise from the glacial melting caused
by global temperature rise. Although there is much debate regarding the
formation of barrier islands, many scientists agree that this melting
of glaciers gave rise to them.
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Barrier islands are located parallel
to the mainland, separated by estuaries,
bays and lagoons, on
a gently sloping continental shelf.
These islands provide protection to the mainland from the brunt of ocean
waves and are considered dynamic environments, as they are constantly
being shaped and reshaped, eroded
and accreted. Their
formation depends on sand supply, wave energy, and tidal fluctuations.
These islands typically occur in chains which are separated from each
other by tidal inlets.
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Over time as sea level rises, transgressive
barrier islands are thought to migrate toward the mainland (OCRM,
2000). The reason why these islands appear to migrate is because the front
side of the island is constantly eroded by wave
action. These islands lack healthy dune
systems and vegetation that act as sand anchors and barriers on the beach.
This deficiency makes the island susceptible to erosion,
allowing waves to carry sediment from the beaches and dunes to the backside
of the island. Due to the flooding of ocean waves over the island during
storms, sand is deposited as washover
fans behind the dunes. Therefore, the backside of the island grows
by the buildup of washover- transported sand.
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Image provided by http://euphrates.wpunj.edu/faculty/pardir/Courses/Oceanography
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Click
here for a close up of the "Structure of a Typical Barrier Island"
diagram. Notice the lagoonal peat in the diagram. If the beach sand is
constantly washed over to the backside of the island, old marsh peat
becomes exposed at the beachfront. This provides direct evidence that
islands migrate landward in a process known as "island rollover."
In fact, today's beach is where the marsh used to be (see Figure below).
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Picture provided
by Leslie R. Sautter
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