Each barrier island is different and unique in its own way and although no two are exactly alike, they may have many similarities. A typical mature barrier island consists of several distinct features or habitats including the ocean, beach, berm, primary dunes, secondary dunes, interdune meadows, shrub zone, maritime climax forest, salt marsh, tidal or mud flats and sounds.
Many barrier islands also have freshwater sloughs that form in low areas and between old dune ridges. Each habitat, although existing in close proximity to the others, has its own unique characteristics. They shelter and protect the mainland from the powerful forces of wind, waves, tides, currents and the ravages of storms and hurricanes.
They shelter the estuaries that form behind the barriers. They allow marshes to build up in the quiet waters of the sound. The marshes filter the pollutants that come in from mainland rivers and when the marsh grasses die they create detritus, a valuable food source for small marine organisms. Barrier islands provide valuable habitats for mammals, shellfish and fish including many endangered species, and offer a greater variety of bird species than any other ecosystem in the continental United States.
They also serve aesthetic purposes and for years, have inspired artists, poets, writers, bird watchers, boaters, picnickers, sunbathers and swimmers. Barrier islands are truly a most valuable resource. It is remarkable that although they constantly change and may become badly eroded some even disappear entirely , they can rebuild if left alone. There are several theories as to how barrier islands form.
During glacial advances, huge amounts of water were locked in glaciers causing the sea level to fall. Then, during interglacial periods when it was warmer, the ice melted, causing sea level to rise.
Georgia was never covered by glaciers but it was affected by the rising and falling of the sea level about 40 million years ago.
During the Pleistocene, in warm periods, sea level may have been as much as 50 to feet higher than today with the ancient coastline lying along a sand ridge known as Trail Ridge. During times of extreme cold, the shoreline lay as much as 80 miles seaward of its current position. The older islands formed thousands of years ago when sea level rose flooding the coastline. Dates for their formation are unclear and could have been any where from 25, to , years ago. Ridges of sand dunes were left above sea level and gradually built into barrier islands.
Over the next few thousand years sediments from rivers added material to fill in the area between these new islands and the mainland producing mud in which the vast expanses of salt marsh formed. As sea level stabilized these small remnants of old sand dunes gradually built into our older barrier islands.
Although barrier-island development is a risky business, these islands continue to be popular vacation and recreational attractions. Because barrier islands serve important functions protecting the coasts from storm damage, nurturing ecosystems and protecting wildlife , a balance is needed between conservation and further development.
Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe.
Environmental Science. Conservation Issues. How Barrier Islands Work. A sandbar barrier island protects the mangrove forest from ocean waves. Barrier-island Zones " ". Beach - consists of sand deposited by the actions of waves Dunes - formed from sand carried and deposited by winds.
Dunes are stabilized naturally by plants sea oats, bitter pancum and artificially by fences. The primary dune faces the ocean and may be followed by secondary and tertiary dunes inland. Barrier flat - also called backdune, overwash or mud flat formed by sediments that get pushed through the dune system by storms, such as hurricanes.
Grasses grow and stabilize these areas. Salt marsh - a low-lying area on the sound-side of a barrier island. Salt marshes are generally divided into high and low marsh areas. High marsh areas get flooded twice each month with the spring tides, while low marsh areas get flooded twice daily with the high tides.
Cord grasses stabilize the salt marsh area, which are one of the most ecologically productive areas amount of vegetation per acre on Earth.
In fact, the salt marsh ecosystems of the islands and the coast help to purify runoffs from mainland streams and rivers. Barrier-island Habitats " ". Beach Dune Barrier flat Salt marsh. Brown pelicans often feed on both the ocean and sound sides of barrier islands.
Sawgrass community on a barrier flat top and salt marsh community on the sound side bottom. Did You Know? Nature's Effects on Barrier Islands " ". Changes in Louisiana's Isle Dernieres barrier island before top images and after bottom images Hurricane Andrew in The arrows indicate identical, corresponding points on the top and bottom images.
Waves - Waves continually deposit and remove sediments from the ocean side of the island. Currents - Longshore currents that are caused by waves hitting the island at an angle can move the sand from one end of the island to another. For example, the offshore currents along the east coast of the United States tend to remove sand from the northern ends of barrier islands and deposit it at the southern ends.
Tides - The tides move sediments into the salt marshes and eventually fill them in. Thus, the sound sides of barrier islands tend to build up as the ocean sides erode. Winds - Winds blow sediments from the beaches to help form dunes and into the marshes, which contributes to their build-up.
Sea level changes - Rising sea levels tend to push barrier islands toward the mainland. Storms - Hurricanes and other storms have the most dramatic effects on barrier islands by creating overwash areas and eroding beaches as well as other portions of barrier islands.
Impact 1 - Wave erosion is confined to beach area. The eroded sands will be replenished in a few weeks to months and no significant change occurs in the system. Impact 2 - Waves erode the dune and cause the dune to retreat. This is a semi-permanent or permanent change to the system. This change in the system pushes the barrier island landward. Impact 4 - The storm surge completely covers the barrier island, destroys the dune system and pushes sediments landward approximately 0.
This is a permanent change to the barrier island or portions of it. The storm impact hazard scale top left: impact 1, top right: impact 2, bottom-left: impact 3, bottom-right: impact 4.
Fighting Erosion on Barrier Islands " ". In , the Cape Hatteras Lighthouse was carefully and slowly moved about one-half mile. Beach nourishment in Upham Beach, FL top: before; middle: soon after; bottom: one year later. Coastal Attractions. Development's Effects on Barrier Islands " ". Springer, Berlin, pp Google Scholar. Marine Geol — Ross, Stroudsburg, pp 5—43 Google Scholar.
Wiley-Blackwell Publisher. Bertrand, Paris, pp — Springer, Berlin. Ehlers J The morphodynamics of the Wadden Sea. Balkema, Rotterdam, pp Google Scholar. Geogr Rundschau — Google Scholar. University of Rhode Island, pp — Google Scholar.
Fisher JJ Barrier island formation: discussion. First presented May 1, John, N. Springer, Berlin, pp — Google Scholar. Gilbert GK The topographic features of lake shores. Inst Br Geogr —68 Google Scholar. Hayes MO The Georgia bight barrier system. Springer, Berlin, pp —, pp Google Scholar. Sedimentology — Google Scholar. Hoyt JH Barrier island formation. Ibrahim NA Sedimentological and morphological evolution of a coarse-grained regressive barrier beach, Horseneck Beach.
MS thesis, Boston University. Johnson DW Shore processes and shoreline development. Wiley, pp Google Scholar. The Water Institute of the Gulf. Baton Rouge, LA. McGee WJ Encroachments of the sea. Reprinted in: Barrier Islands, Benchmark papers in geology.
0コメント