Barrier islands
Barrier islands
Can humans and barrier islands coexist?
A barrier island is a long, thin, sandy stretch of land oriented parallel to the mainland coast, which protects the coast from the full force of powerful storm waves. Between the barrier island and the mainland is a lagoon or bay. Barrier islands are dynamic systems that migrate under the influence of changing sea levels, storms, waves, tides, and longshore currents. Approximately 2,500 barrier islands are known to exist in open oceans, while thousands of barrier islands are found in low-energy settings such as bays. In the United States, barrier islands occur along gently sloping sandy coastlines such as those along the Gulf Coast and the Atlantic Coast as far north as Long Island, New York. They are, in contrast, absent along the generally steep and rocky Pacific Coast. Some of the better known barrier islands along the coast of the United States are Padre Island, Texas, the world’s longest barrier island; Sanibel and Captiva Islands, Florida; Cape Hatteras, North Carolina; and Assateague Island, Maryland.
Residential and recreational development on barrier islands has become a contentious issue in recent years. Although the islands serve as buffers against the sea by constantly shifting and changing their locations, the owners of homes, stores, and hotels on barrier islands often try to stabilize the shifting sand to protect their property. This is accomplished by beach hardening (the construction of engineered barriers) or beach nourishment (the continual replacement of sand that has been washed away during storms). In either case, the result is an interruption of the natural processes that formed the islands. Construction that prevents the naturally occurring erosion of sand from the seaward side of a barrier island, for example, may result in erosion problems when the landward side is consequently starved of sand. Beach nourishment projects typically require ever increasing amounts of sand to maintain a static beachfront, and are therefore economically viable for only short periods of time. Although coastal management activities have long been directed towards beach hardening and
nourishment, current scientific thinking suggests that the islands are more appropriately viewed as geologically transient features rather than permanent shorelines suitable for development.
Barrier island origins
Currently existing barrier islands are geologically young features that formed during the Holocene epoch (approximately the last 10,000 years). During that time, the rapid rise in sea level associated with melting glaciers slowed significantly. Although the exact mechanisms of barrier island formation are not fully understood, the decreasing rate of sea level rise allowed the islands to form.
Several conditions must be met in order for barrier islands to form. First, there must be a source of sand to build the island. The sand may come from coastal deposits or offshore deposits called shoals. In either case, the sand originated from the weathering and erosion of rock and was transported to the coast by rivers. In the United States, much of the sand composing barrier islands along Florida and the East Coast came from the Appalachian Mountains. Second, the topography of the coastline must have a broad, gentle slope. This condition occurs from the coastal plains of the mainland to the edge of the continental shelf along the Atlantic and Gulf Coasts. Finally, the waves, tides and currents in the area of a future barrier island must be strong enough to move the sand. Waves must be the dominant of these three water movement mechanisms.
Several explanations for barrier island development have been proposed. According to one theory, coastal sand is transported shoreward as sea level rises and, once sea level stabilizes, waves and tides work the sand into a barrier island. Another possibility is that sand is transported from shoals. Some barrier islands may form when low-lying areas of spits, extensions of beaches that protrude into a bay, are breached by the sea. Finally, barrier islands may form from sandy coastal ridges that become isolated and form islands as sea level rises.
Once formed, barrier islands do not persist as static landforms. They are dynamic features that are continuously reworked by wind and waves. Changes in sea level also affect barrier islands. Most scientists agree that sea level has been gradually rising over the last
thousand years, and this rise may be accelerating today due to global warming. Rising sea levels cause existing islands to migrate shoreward, and barrier islands off the Carolina coast are thought to have migrated 40-50 mi (64-80 km) during the Holocene epoch.
Barrier island zonation
Individual barrier islands do not stand alone; instead, systems of islands develop along favorable coastlines. The formation of an island allows other landforms to develop, each characterized by its dominant sediment type and by the water that shapes them. For example, each barrier island has a shoreline that faces the sea and receives the full force of waves, tides, and currents. This shoreline is often called the beach. The beach zone extends from slightly offshore to the high water line. Coarse sand and gravel are deposited along the beach, with finer sand and silts carried farther offshore.
Behind the beach are sand dunes. Wind and plants such as sea oats form dunes, but occasionally dunes are inundated by high water and may be reworked by storm surges and waves. On wide barrier islands, the landscape behind the foredunes gently rolls as dunes alternate with low-lying swales. If the dunes and swales are well developed, distinct parallel lines of dune ridges and swales can be seen from above. These differences in topography allow some soil to develop and nutrients to accumulate despite the porous sandy base. Consequently, many medium to large barrier islands are host to trees (which are often stunted), bushes, and herbaceous plants. Smaller or younger barrier islands may be little more than loose sand with few plants.
The back-barrier lies on the shoreward side of the island. Unlike the beach, this zone does not bear the full force of waves. Instead, the back-barrier region consists of a protected shoreline and lagoon, which is influenced by tides more than waves. Water may occasionally rush over the island during storms, carrying beach and dune sand and depositing it in the lagoon. This process, called rolling over, is vital to the existence of barrier islands and is the method by which a barrier island migrates landward. Sand washover fans in the lagoons are evidence of rolling over. Salt marsh, sea grass, and mudflat communities develop along the sheltered back-barrier. These communities teem with plant and animal life and their sediments are rich in organic matter.
Finally, barrier islands are associated with tidal inlets and tidal deltas. Tidal inlets allow water to move into and out of bays and lagoons with rising and falling tides. Tidal inlets also provide a path for high water during storms and hurricanes. As water moves through an inlet, sand is deposited at both ends of the inlet, forming tidal deltas. Longshore currents may also deposit sand at the delta. Eventually, the growing deltas close the inlet and a new inlet appears elsewhere on the island, usually at a low-lying spot. The size and shape of the inlet are determined by various factors, including the size of the associated lagoon and the tidal range (the vertical height between high and low tide for the area). A large tidal range promotes the formation of many inlets, thereby creating shorter and wider barrier islands referred to as drumsticks. In addition, the larger the lagoon and the greater the tidal range, the deeper and wider the inlet due to the large quantity of water moving from ocean to lagoon and back. Deep, wide inlets occur where the main
KEY TERMS
Inlet— A narrow watery channel next to a barrier island that leads from the sea to a bay or lagoon.
Lagoon— A shallow, semi-enclosed body of water connected to the sea by an inlet. Shoal—A sandy area submerged in shallow water.
Storm surge— A significant, sudden, and temporary rise in sea level, associated with high winds and very low pressure, accompanying hurricanes.
source of energy shaping the coastal area is tides, or tides in conjunction with waves. In contrast, wave-dominated areas form long barrier islands with narrow bays and narrow, shallow inlets.
Can humans and barrier islands coexist?
Barrier islands bear the full force of coastal storms and hurricanes, buffering the mainland coast. This often occurs at the expense of the island. Although the processes creating and maintaining barrier islands have been occurring for thousands of years, they have only become of concern in the last few decades. Billions of dollars worth of real estate development on barrier islands is now threatened by migrating beaches as sand continues to be reworked and transported by natural forces. Cities such as Miami Beach and Atlantic City are on barrier islands. Engineering efforts to stop erosion through beach nourishment projects, seawalls, and other means are merely temporary fixes against the powerful forces of nature. Some engineered structures can actually accelerate the rate of erosion.
Laws in some coastal states prohibit building between the sea and the dunes closest to the sea. Some laws prohibit the rebuilding of structures lost or damaged due to storms and erosion. Preservation may be the best long-term solution to ensure the future of these islands, but for many people the desire for beach resorts is a more immediate concern.
See also Coast and beach.
Resources
BOOKS
Boggs Jr., Sam. Coastal Principles of Sedimentology and Stratigraphy (4th Edition). New Jersey: Prentice Hall, 2005.
Frankenberg, Dirk. The Nature of the Outer Banks, Environmental Processes, Field Sites, and Developmental Issues, Corolla to Ocracoke. Chappel Hill, N.C.: The University of North Carolina Press, 2001.
Kaufman, W., and O. Pilkey. The Beaches Are Moving. Durham, N.C.: Duke University Press, 1983.
Pilkey, Orrin H. and Mary Edna Fraser. A Celebration of the World’s Barrier Islands. New York: Columbia University Press, 2003.
Pilkey, Orrin H., Tracy Monegan Rice, and William J. Neal Pilkey. How to Read a North Carolina Beach: Bubble Holes, Barking Sands, and Rippled Runnels. Chappel Hill, NC: The University of North Carolina Press, 2004.
OTHER
Earth & Sky. “Transcript—Barrier Islands.” July 18, 2002 <http://www.earthsky.org/radioshows/barrier-islands> (accessed October 9, 2006).
Earth & Sky. “Transcript—Holding the Line on Barrier Islands.” October 6, 2006 <http://www.earthsky.org/radioshows/holding-the-line-on-barrier-islands> (accessed October 9, 2006).
United States Geological Survey. “USGS Fact Sheet: Louisiana Barrier Islands.” 1995. <http://www.marine.usgs.gov/fact-sheets/Barrier/barrier.html> (accessed October 9, 2006).
United States Geological Survey. “Coasts in Crisis.” 1997 <http://pubs.usgs.gov/circular/c1075/(accessed October 9, 2006).
Elaine L. Martin
Barrier Islands
Barrier islands
A barrier island is a long, thin, sandy stretch of land oriented parallel to the mainland coast, which protects the coast from the full force of powerful storm waves. Between the barrier island and the mainland is a lagoon or bay. Barrier islands are dynamic systems that migrate under the influence of changing sea levels, storms, waves, tides , and longshore currents . Approximately 2,100 barrier islands are known to exist around the world. In the United States, barrier islands occur along gently sloping sandy coastlines such as those along the Gulf Coast and the Atlantic Coast as far north as Long Island, New York. They are, in contrast, absent along the generally steep and rocky Pacific Coast. Some of the better known barrier islands along the coast of the United States are Padre Island, Texas, the world's longest barrier island; Sanibel and Captiva Islands, Florida; Cape Hatteras, North Carolina; and Assateague Island, Maryland.
Residential and recreational development on barrier islands has become a contentious issue in recent years. Although the islands serve as buffers against the sea by constantly shifting and changing their locations, the owners of homes, stores, and hotels on barrier islands often try to stabilize the shifting sand to protect their property. This is accomplished by beach hardening (the construction of engineered barriers) or beach nourishment (the continual replacement of sand that has been washed away during storms). In either case, the result is an interruption of the natural processes that formed the islands. Construction that prevents the naturally occurring erosion of sand from the seaward side of a barrier island, for example, may result in erosion problems when the landward side is consequently starved of sand. Beach nourishment projects typically require ever increasing amounts of sand to maintain a static beachfront, and are therefore economically viable for only short periods of time. Although coastal management activities have long been directed towards beach hardening and nourishment, current scientific thinking suggests that the islands are more appropriately viewed as geologically transient features rather than permanent shorelines suitable for development.
Barrier island origins
Currently existing barrier islands are geologically young features that formed during the Holocene epoch (approximately the last 10,000 years). During that time, the rapid rise in sea level associated with melting glaciers slowed significantly. Although the exact mechanisms of barrier island formation are not fully understood, the decreasing rate of sea level rise allowed the islands to form.
Several conditions must be met in order for barrier islands to form. First, there must be a source of sand to build the island. The sand may come from coastal deposits or offshore deposits called shoals. In either case, the sand originated from the weathering and erosion of rock and was transported to the coast by rivers . In the United States, much of the sand composing barrier islands along Florida and the East Coast came from the Appalachian Mountains. Second, the topography of the coastline must have a broad, gentle slope. This condition occurs from the coastal plains of the mainland to the edge of the continental shelf along the Atlantic and Gulf Coasts. Finally, the waves, tides and currents in the area of a future barrier island must be strong enough to move the sand. Waves must be the dominant of these three water movement mechanisms.
Several explanations for barrier island development have been proposed. According to one theory, coastal sand is transported shoreward as sea level rises and, once sea level stabilizes, waves and tides work the sand into a barrier island. Another possibility is that sand is transported from shoals. Some barrier islands may form when
low-lying areas of spits, extensions of beaches that protrude into a bay, are breached by the sea. Finally, barrier islands may form from sandy coastal ridges that become isolated and form islands as sea level rises.
Once formed, barrier islands do not persist as static landforms. They are dynamic features that are continuously reworked by wind and waves. Changes in sea level also affect barrier islands. Most scientists agree that sea level has been gradually rising over the last thousand years, and this rise may be accelerating today due to global warming . Rising sea levels cause existing islands to migrate shoreward, and barrier islands off the Carolina coast are thought to have migrated 40-50 mi (64-80 km) during the Holocene epoch.
Barrier island zonation
Individual barrier islands do not stand alone; instead, systems of islands develop along favorable coastlines. The formation of an island allows other landforms to develop, each characterized by its dominant sediment type and by the water that shapes them. For example, each barrier island has a shoreline that faces the sea and receives the full force of waves, tides, and currents. This shoreline is often called the beach. The beach zone extends from slightly offshore to the high water line. Coarse sand and gravel are deposited along the beach, with finer sand and silts carried farther offshore.
Behind the beach are sand dunes. Wind and plants such as sea oats form dunes, but occasionally dunes are inundated by high water and may be reworked by storm surges and waves. On wide barrier islands, the landscape behind the foredunes gently rolls as dunes alternate with low-lying swales. If the dunes and swales are well developed, distinct parallel lines of dune ridges and swales can be seen from above. These differences in topography allow some soil to develop and nutrients to accumulate despite the porous sandy base. Consequently, many medium to large barrier islands are host to trees (which are often stunted), bushes, and herbaceous plants. Smaller or younger barrier islands may be little more than loose sand with few plants.
The back-barrier lies on the shoreward side of the island. Unlike the beach, this zone does not bear the full force of waves. Instead, the back-barrier region consists of a protected shoreline and lagoon, which is influenced by tides more than waves. Water may occasionally rush over the island during storms, carrying beach and dune sand and depositing it in the lagoon. This process, called rolling over, is vital to the existence of barrier islands and is the method by which a barrier island migrates landward. Sand washover fans in the lagoons are evidence of rolling over. Salt marsh, sea grass, and mudflat communities develop along the sheltered back-barrier. These communities teem with plant and animal life and their sediments are rich in organic matter .
Finally, barrier islands are associated with tidal inlets and tidal deltas. Tidal inlets allow water to move into and out of bays and lagoons with rising and falling tides. Tidal inlets also provide a path for high water during storms and hurricanes. As water moves through an inlet, sand is deposited at both ends of the inlet, forming tidal deltas. Longshore currents may also deposit sand at the delta . Eventually, the growing deltas close the inlet and a new inlet appears elsewhere on the island, usually at a low-lying spot. The size and shape of the inlet are determined by various factors, including the size of the associated lagoon and the tidal range (the vertical height between high and low tide for the area). A large tidal range promotes the formation of many inlets, thereby creating shorter and wider barrier islands referred to as drumsticks. In addition, the larger the lagoon and the greater the tidal range, the deeper and wider the inlet due to the large quantity of water moving from ocean to lagoon and back. Deep, wide inlets occur where the main source of energy shaping the coastal area is tides, or tides in conjunction with waves. In contrast, wave-dominated areas form long barrier islands with narrow bays and narrow, shallow inlets.
Can humans and barrier islands coexist?
Barrier islands bear the full force of coastal storms and hurricanes, buffering the mainland coast. This often occurs at the expense of the island. Although the processes creating and maintaining barrier islands have been occurring for thousands of years, they have only become of concern in the last few decades. Billions of dollars worth of real estate development on barrier islands is now threatened by migrating beaches as sand continues to be reworked and transported by natural forces. Cities such as Miami Beach and Atlantic City are on barrier islands. Engineering efforts to stop erosion through beach nourishment projects, seawalls, and other means are merely temporary fixes against the powerful forces of nature. Some engineered structures can actually accelerate the rate of erosion.
Laws in some coastal states prohibit building between the sea and the dunes closest to the sea. Some laws prohibit the rebuilding of structures lost or damaged due to storms and erosion. Preservation may be the best long-term solution to ensure the future of these islands, but for many people the desire for beach resorts is a more immediate concern.
See also Coast and beach; Dune.
Resources
books
Bascom, W. Waves and Beaches. Garden City, New York: Anchor Press/Doubleday, 1980.
Carter, R. W. G. Coastal Environments. San Diego: Academic Press, 1991.
Davis, R. A. Depositional Systems. New Jersey: Prentice-Hall, 1983.
Kaufman, W., and O. Pilkey. The Beaches Are Moving. Garden City, New York: Anchor Press/Doubleday, 1979.
Leeder, M. Sedimentology and Sedimentary Basins. London: Blackwell Science, 1999.
periodicals
Bennett, D. "Paying for Sand." Audubon (September/October 1993): 132.
Hawes, E. "Castles in the Sand." The New York Times Magazine (July 1993): 24–32.
Overby, P. "Beachfront Bailout." Common Cause (Summer 1993): 12–17.
Stuller, J. "On the Beach." Sea Frontiers (December 1994): 28–34.
Tibbetts, J. "On Shifting Sands." E: The Environmental Magazine (July/August 1993): 19–21.
other
Earth & Sky. "Transcript—Barrier Islands." July 18, 2002 [cited October 19, 2002]. <www.earthsky.com/2002/es020718.html>.
List, J. "USGS Fact Sheet: Louisiana Barrier Islands." 1995 [cited October 19,2002]. <www.marine.usgs.gov/factsheets/Barrier/barrier.html>.
Pilkey, O. H. "Engineered Barrier Islands: Lifeless Piles of Sand." 2002 [cited October 19, 2002]. <www.gsa.confex.com/gsa/2002AM/finalprogram/abstract_40202.htm>.
Elaine L. Martin
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Inlet
—A narrow watery channel next to a barrier island that leads from the sea to a bay or lagoon.
- Lagoon
—A shallow, semi-enclosed body of water connected to the sea by an inlet.
- Shoal
—A sandy area submerged in shallow water.
- Storm surge
—A significant, sudden, and temporary rise in sea level, associated with high winds and very low pressure, accompanying hurricanes.
Barrier Islands
Barrier islands
A barrier island is a long, thin, sandy stretch of land, oriented parallel to the mainland coast that protects the coast from the full force of powerful storm waves. Between the barrier island and the mainland is a calm, protected water body such as a lagoon or bay. Barrier islands are dynamic systems, constantly on the move, migrating under the influence of changing sea levels, storms, waves, tides , and longshore currents. In the United States, barrier islands occur offshore where gently sloping sandy coastlines, as opposed to rocky coastlines, exist. Consequently, most barrier islands are found along the Gulf Coast and the Atlantic Coast as far north as Long Island, New York. Some of the better known barrier islands include Padre Island of Texas, the world's longest; Florida's Santa Rosa Island, composed of sugar-white sand ; Cape Hatteras of North Carolina, where the first airplane was flown; and Assateague Island near Maryland, home of wild ponies.
Barrier islands are young in geologic terms. They originated in the Holocene Epoch , about 4,000–6,000 years ago. During this time, the rapid rise in sea level, associated with melting glaciers from the last ice age, slowed significantly. Although the exact mechanisms of barrier island formation aren't fully understood, this slowdown of sea level rise allowed the islands to form.
In order for barrier islands to form, several conditions must be met. First, there must be a source of sand to build the island. This sand may come from coastal deposits or offshore deposits (called shoals); in either case, the sand originated from the weathering and erosion of rock and was transported to the coast by rivers . In the United States, much of the sand composing barrier islands along Florida and the East Coast came from the Appalachian Mountains. Next, the topography of the coastline must have a broad, gentle slope. From the coastal plains of the mainland to the edge of the continental shelf , this condition is met along the Atlantic and Gulf Coasts. Finally, the forces of waves, tides and currents must be strong enough to move the sand, and of these three water movement mechanisms, waves must be the dominant force.
Several explanations for barrier island development have been proposed. According to one theory, coastal sand was transported shoreward as sea level rose, and once sea level stabilized, wave and tidal actions worked the sand into a barrier island. Another possibility is that sand was transported to its present location from shoals. Barrier islands may have formed when low-lying areas of spits, extensions of beaches that protrude into a bay as a result of deposition of sediment carried by longshore currents, were breached by the sea. Finally, barrier islands may have formed from sandy coastal ridges that became isolated from low-lying land and formed islands as sea level rose.
Once formed, barrier islands are not static landforms ; they are dynamic, with winds and waves constantly reworking and moving the barrier island sand. Changes in sea level also affect these islands. Most scientists agree that sea level has been gradually rising over the last thousand years, and this rise could be accelerating today due to global warming . Rising sea level causes existing islands to migrate shoreward.
Barrier islands do not stand alone in a geologic sense. A whole system of islands develops along favorable coastlines. The formation of these islands allows other landforms to develop, each characterized by their dominant sediment type and by the water that helps form them. For example, each barrier island has a shoreline that faces the sea and receives the full force of waves, tides, and currents. This shoreline is often called the beach. The beach zone extends from slightly offshore (subtidal, or underwater) to the high water line. Coarser sands and gravels are deposited here, with finer sands and silts carried farther offshore.
Behind the beach are sand dunes . Wind and plants (such as sea oats) help form dunes, but occasionally dunes are inundated by high water and may be reworked by storm surges and waves. On wide barrier islands, the landscape behind the fore-dunes gently rolls as dunes alternate with low-lying swales (marshy wet areas). If the dunes and swales are well developed, distinct parallel lines of dune ridges and swales can be seen from overhead. These differences in topography allow some soil to develop and nutrients to accumulate despite the porous, sandy base. Consequently, some barrier islands are host to trees (which are often stunted), bushes, and herbaceous plants. Other narrower or younger barrier islands may be little more than loose sand with few plants.
On the shoreward side of the main body of the island is the back-barrier. Unlike the beach, this zone does not bear the full force of ocean waves. Instead, the back-barrier region consists of a protected shoreline and lagoon, which is more influenced by tides than waves. Occasionally, during storms, water may rush over the island carrying beach and dune sand and deposit the sand in the lagoon. This process, called rolling over, is vital to the existence of barrier islands and is the method by which a barrier island migrates landward. Characteristic sand washover fans in the lagoons are evidence of rolling over. Because the back-barrier region is sheltered, salt marsh, sea grass, and mudflat communities develop. These communities teem with plant and animal life and their muddy or sandy sediments are rich with organic matter.
Finally, barrier islands are characterized by tidal inlets and tidal deltas. Tidal inlets allow water to move into and out of bays and lagoons with rising and falling tides. Tidal inlets also provide a path for high water during storms and hurricanes. As water moves through an inlet, sand is deposited at both ends of the inlet's mouth, forming tidal deltas. Longshore currents may also deposit sand at the delta . Eventually the deltas fill in with sand and the inlet closes, only to appear elsewhere on the barrier island, usually at a low-lying spot. The size and shape of the inlet are determined by various factors, including the size of the associated lagoon and the tidal range, or the vertical height between high and low tide for the area . A large tidal range promotes the formation of numerous inlets, thereby creating shorter and wider barrier islands referred to as drumsticks. In addition, the larger the lagoon and the greater the tidal range, the deeper and wider the inlet due to the large quantity of water moving from ocean to lagoon and back. Deep, wide inlets occur where the main source of energy shaping the coastal area is tides or tides in conjunction with waves. In contrast, wave-dominated areas form long barrier islands with narrow bays and narrow, shallow inlets.
See also Beach and shoreline dynamics; Gulf of Mexico; Offshore bars; Tropical cyclone; Wave motions
Barrier Island
Barrier island
An elongated island that lies parallel to, but mostly separate from, a coastline. Barrier islands are composed of sediments, mainly sand, deposited by longshore currents, wind, and wave action. Both marine and terrestrial plants and animals find habitat on barrier islands or along their sandy beach shorelines. These islands also protect coastal lagoons from ocean currents and waves, providing a warm, quiet environment for species that cannot tolerate more violent wind and wave conditions. In recent decades these linear, sandy islands, with easy access from the mainland, have proven a popular playground for vacationers. These visitors now pose a significant threat to breeding birds and other coastal species. Building houses, roads, and other disruptive human activities can destabilize dunes and expose barrier islands to disastrous storm damage. In some cases, whole islands are swept away, exposing protected lagoons and delicate wetlands to further damage. Major barrier island formations in North America include those along the eastern coasts of the mid-Atlantic states, Florida, and the Gulf of Mexico.
barrier island
barrier island
barrier island
a. the building up of submarine bars;
b. spit progradation parallel to the coast and segmentation by inlets;
and
c. submergence of subaerial coastal beach ridges by a rise in sea-level. Barrier islands are most common in areas of low tidal range.