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Barrier Islands: Always changing
By JEFF SELINGO, Wilmington Morning Star
Published: 08/25/96
Barrier islands are systems. Each barrier island is molded by its unique set of
forces. The interaction of conditions that creates a barrier island is sometimes hard to
predict and in some cases little understood. One thing is known for sure: Barrier islands
are in constant motion. Here are some of the island-molding forces that affect the coast
of Southeastern North Carolina.
Inlet Dynamics
An inlet is the channel that allows exchange of water between the sound and
ocean sides of adjacent barrier islands. Larger sounds have deeper inlets because of the
larger amount of water that must flow the the tides.
Natural inlet closure: Tidal deltas in the mouth of an inlet hold sand, pushing
the channel flow to another area or closing the inlet altogether. Naturally closed inlets
can be the most stable areas of an island because of their wide buffer of sand.
Artificially filled inlets are less stable because the fill sand is often taken from the
tidal delta.
Inlet migration: Tides bring sand through the channels between barrier islands.
As sand builds up on one end of an island and in the tidal deltas, the angle of the
channel can change quickly. This can in turn change the flow of sand deposits. Currents
racing out of the inlet can protect portions of the beach from sand-scouring waves,
allowing new sand buildup. Combinations of these and other factors make inlet migration
volatile and unpredictable.
Inlet formation: Some inlets are formed when storms push unusually large amounts
of water into sounds. When the storm surge passes, the excess water is trapped in the
sound and cannot flow out of existing inlets fast enough to keep up with the post-surge
drop in the ocean's level. This trapped water may overwash a low-lying area of the
island, `popping it like a cork,' and quickly cutting a new channel.
 Waves
Waves bring sand to beaches, and they also remove it:
Fair-weather waves supply sand to the beach by moving it in from the continental
shelf.
High-intensity storm waves cut into the beach, moving sand offshore.
Longshore Currents
As row upon row of waves break on the shore, some of their shoreward energy is
deflected into longshore currents. When you're swimming in the ocean, the longshore
current is the one that carries you down the beach from your beach towel.
The longshore current is a major supplier of sand to beaches. Sand is churned up
by waves, suspended in the water and carried along by the longshore currents, which
deposit it grain by grain farther down the beach.
Overwash
In low-lying areas, storms push sand across the island and into the lagoon area
beyond. This extends the island landward; marsh grass soon grows on the new sound-side
sand. This gain of sand landward is often accompanied by loss of sand on the ocean side of
the island. The sound side of the island gradually becomes the oceanfront as the island
`rolls over on itself.'
Sources: William Cleary, Geology Dept., UNCW; 'The Corps and the Shore' by
Orrin Pilkey Jr. and Katherine Dixon; 'From Currituck to Calabash: Living With North
Carolina's Barrier Islands' by Orrin Pilkey Jr., William Neal, Orrin Pilkey Sr. and
Stanley Riggs
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