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OCE 2001C

Barrier Islands

Introduction:

Barrier islands are long, narrow low-lying islands that parallel the coast and are separated from the mainland by a bay or lagoon. They are associated with passive margin coastlines. Along the East coast of the United States, barrier islands parallel the coast from New York south to Florida and they parallel the Gulf Coast from Marco Island north around the Gulf of Mexico and south to Mexico. The Outer Banks of North Carolina are barrier islands. Anastasia Island, FL is an example of a barrier island off Florida’s east coast. Along the Gulf Coast, Clearwater Beach is on a barrier island. We will visit Honeymoon Island, which is a barrier island.

Barrier Islands may form in more than one way. Along the east coast, the long narrow islands like Assateague Island, MD-VA or the Outer Banks of North Carolina probably represent ancient beaches/beach ridge deposits that formed when sea level was lower during the last ice age and became islands as sea level rose. The barrier islands off Florida’s Gulf Coast are thought to have formed as sand was trapped in high and irregular areas of the bedrock as sea level rose. Some islands like Georgia’s Sea Islands are parts of the mainland that have become stranded by rising sea levels.

Barrier islands are in constant motion and tend to move in more than one direction at the same time. Look up barrier islands in your textbook and look up methods of erosion control especially groins and jetties in your textbook. Be sure you understand how waves more sand along a beach, wave refraction, and the longshore current because all of these forces move barrier islands. These topics are covered in your text. Wave and wind erosion moves sand along the island beaches. The longshore current will cause the island to move by removing or eroding sand from one area and depositing it in another area. The tidal currents will move sediment through the inlets between islands. The islands may move very rapidly during storms including hurricanes and may overwash or submerge during storms. This is part of the natural system and helps replenish sand on the island. The islands are also moving with rising sea level. The ocean side of the island tends to erode. The lagoon or back barrier is usually marshy and the marsh plants trap and hold sediment causing that side of the island to grow or accrete. This means that the islands are moving toward the mainland.

Go to the following link and read about barrier islands:

http://science.howstuffworks.com/environmental/conservation/issues/barrier-island.htm

Watch this video on beaches. This beach does not have dunes unlike our beaches along the Gulf Coast.

http://www.youtube.com/watch?v=A9c5UEXTNkE

Watch this video on dunes.

http://www.youtube.com/watch?v=OiAs1-VCsXs&feature=relmfu

Information on Assateague Island

http://www.ohranger.com/assateague-island/geology

Answer the following questions using your text, the links above and the map on the last page of this exercise:

Describe how waves move sand along a beach.
How does the longshore current form? How does the longshore current move sand along a coastline?
What is overwash and how does it affect a barrier island?
Describe how barrier islands move or migrate with the tides, longshore current, and storms.
What do you think will happen to barrier islands as sea level continues to rise?
Look at the figure of Assateague Island on the following page. How far has the north end of Assateague moved from 1849 to 1980? Give your answer in total kilometers as well as kilometers/year. Assuming this rate of migration does not change, how long will it take the island to migrate to the mainland?
Along the south end of Fenwick Island and north end of Assateague Island, the longshore current flows from north to south. Do you think the jetties that were built to stabilize Ocean City Inlet have caused some of the changes to the north end of Assateague? If so, describe how you thing the presence of the jetties changed Assateague?

Southern Fenwick Island and Northern Assateague Island, Md-VA