What is a barrier island?

A typical meso-tidal barrier island, Pawleys Island S.C.  breached by Hurricane Hugo (from Davis, 1994).

A barrier island is an elongated body of sand, which runs parallel to the mainland shore and is separated from the main land by a lagoon. A barrier island is usually comprised of several parts which are shown it the figure below.

A schematic of a barrier island system with components of a barrier island system labeled (from Reinson, 1992)

Common Components of a Barrier Island System.

1.Beach - Seaward deposits of well-sorted sands with a steeply sloping bank into the ocean.The beach is made up of the subtidal zone or shore face (the area of the beach, which is below the mean low tide mark, the intertidal zone or beach (which is the area between the subtidal zone and the dune formations).
2.Dunes - Located just landward of the beach is dune complex or backshore setting.This area reworked by wind and occasional large storms.These processes form landward dipping beds.
3.Washover - Washovers are formed during large storm events when storm surges overrides the island and transports deposits of dune and beach sand all the way to the back barrier lagoon and marsh. These washovers provide a mechanism that increases the width of the island by providing a pathway that relocates aeolian sands from the beach and shore face. A barrier island is an elongated body of sand, which runs parallel to the mainland shore and is separated from the main land by a lagoon. A barrier island is usually comprised of several parts which are shown it the figure below.

 

Several washouts over a barrier island (from Prothero 1997)

4. Ebb and Flood Tidal Deltas - These are deposits of sediment that are carried by tidal currents. The Ebb Tidal Deltas form on the seaward side of the island and are sediments carried by the outgoing (or ebb) tide.Flood Tidal Deltas are formed on the landward side of barrier islands and are sediments carried by the incoming (or flood) tide.

 

Aerial image of a flood tidal delta (from Prothero, 1977)

5. Main Tidal Channel - These are channels which connect the back-barrier lagoons with the ocean environment.
6. Lagoon -The area on the landward side of the barrier island.It is a quiescent setting with very little wave action, which leads to finer grained sediment deposits.It is within these areas which organic rich mud accumulates.
7. Tidal Flat - Areas of fine grain sediment accumulation in the barrier lagoon, that exists between the low and high tide marks.
8. Marsh - Located directly landward of the tidal flats, marshes are marked by the presence of grasses and other wetland vegetation.