Barrier islands are coastal landforms and a type of dune system that
are exceptionally flat or lumpy areas of sand that form by wave and
tidal action parallel to the mainland coast. They usually occur in
chains, consisting of anything from a few islands to more than a
dozen. They are subject to change during storms and other action, but
absorb energy and protect the coastlines and create areas of protected
waters where wetlands may flourish. A barrier chain may extend
uninterrupted for over a hundred kilometers, excepting the tidal
inlets that separate the islands, the longest and widest being Padre
Island of Texas. The length and width of barriers and overall
morphology of barrier coasts are related to parameters including tidal
range, wave energy, sediment supply, sea-level trends, and basement
controls. The amount of vegetation on the barrier has a large
impact on the height and evolution of the island.
Chains of barrier islands can be found along approximately 13-15% of
the world's coastlines. They display different settings, suggesting
that they can form and be maintained in a variety of environmental
settings. Numerous theories have been given to explain their
1 Constituent parts
2.1 United States
2.4 New Zealand
4 Formation theories
4.1 Offshore bar theory
4.2 Spit accretion theory
4.3 Submergence theory
5 Ecological importance
6 See also
9 External links
The shoreface is the part of the barrier where the ocean meets the
shore of the island. The barrier island body itself separates the
shoreface from the backshore and lagoon/tidal flat area.
Characteristics common to the lower shoreface are fine sands with mud
and possibly silt. Further out into the ocean the sediment becomes
finer. The effect from the waves at this point is weak because of the
Bioturbation is common and many fossils can be found here.
The middle shore face is located in the upper shoreface. The middle
shoreface is strongly influenced by wave action because of its depth.
Closer to shore the grain size will be medium size sands with shell
pieces common. Since wave action is heavier, bioturbation is not
The upper shore face is constantly affected by wave action. This
results in development of herringbone sedimentary structures because
of the constant differing flow of waves.
Grain size is larger sands.
The foreshore is the area on land between high and low tide. Like the
upper shoreface, it is constantly affected by wave action. Cross
bedding and lamination are present and coarser sands are present
because of the high energy present by the crashing of the waves. The
sand is also very well sorted.
The backshore is always above the highest water level point. The berm
is also found here which marks the boundary between the foreshore and
backshore. Wind is the important factor here, not water. During strong
storms high waves and wind can deliver and erode sediment from the
The dunes are typical of a barrier island, located at the top of the
backshore. Dunes are made by the wind. See Coastal Dunes for more
information. The dunes will display characteristics of typical aeolian
wind blown dunes. The difference here is that dunes on a barrier
island typically contain coastal vegetation roots and marine
Lagoon and tidal flats
The lagoon and tidal flat area is located behind the dune and
backshore area. Here the water is still and this allows for fine
silts, sands, and mud to settle out. Lagoons can become host to an
anaerobic environment. This will allow high amounts of organic rich
mud to form. Vegetation is also common.
They are seen most prominently on the United States' East
Gulf Coast, where every state, stretching from Maine to Florida and
Florida to Texas on each coast has at least part of a barrier island,
stretching to more than twenty-five for Florida. However, this chain
is international. It starts in Quebec's Magdalen
Islands and ends in
Mexico. No barrier islands are found on the Pacific coast of the
United States due to the rocky shore and short continental shelf,
however barrier peninsulas can be found. Barrier islands can also be
seen on Alaska's Arctic coast.
Islands can also be found in Maritime Canada, and other places
along the coast. A good example is found at Miramichi Bay, New
Portage Island as well as Fox
Island and Hay Island
protect the inner bay from storms in the Gulf of Saint Lawrence.
Coast has numerous barrier islands and barrier
Barrier islands are more prevalent in the north of both of New
Zealand's main islands. Notable barrier islands in
New Zealand include
Matakana Island, which guards the entrance to Tauranga Harbour, and
Rabbit Island, at the southern end of Tasman Bay. See also Nelson
Boulder Bank, below.
Barrier islands can be observed in the Baltic Sea and are a distinct
feature of the Wadden Islands, which stretch from the Netherlands to
Lido di Venezia
Lido di Venezia is a notable barrier island which has for
centuries protected the city of
Venice in Italy.
Islands can be observed on every continent on Earth, except
Migration and overwash
Water levels may be higher than the island during storm events. This
situation can lead to overwash, which brings sand from the front of
the island to the top and/or landward side of the island. This process
leads to the evolution and migration of the barrier island.
Outer barrier in Long Island.
The Mississippi-Alabama barrier islands guarding
Mobile Bay and
Scientists have proposed numerous explanations for the formation of
barrier islands for more than 150 years. There are three major
theories: offshore bar, spit accretion, and submergence. No single
theory can explain the development of all barriers, which are
distributed extensively along the world's coastlines. Scientists
accept the idea that barrier islands, including other barrier types,
can form by a number of different mechanisms.
There appears to be some general requirements for formation. Barrier
island systems develop most easily on wave-dominated coasts with a
small to moderate tidal range. Coasts are classified into three groups
based on tidal range: microtidal, 0–2 meter tidal range; mesotidal,
2–4 meter tidal range; and macrotidal, >4 meter tidal range.
Barrier islands tend to form primarily along microtidal coasts, where
they tend to be well developed and nearly continuous. They are less
frequently formed in mesotidal coasts, where they are typically short
with tidal inlets common. Barrier islands are very rare along
macrotidal coasts. Along with a small tidal range and a
wave-dominated coast, there must be a relatively low gradient shelf.
Otherwise, sand accumulation into a sandbar would not occur and
instead would be dispersed throughout the shore. An ample sediment
supply is also a requirement for barrier island formation. The last
major requirement for barrier island formation is a stable sea level.
It is especially important for sea level to remain relatively
unchanged during barrier island formation and growth. If sea level
changes are too drastic, time will be insufficient for wave action to
accumulate sand into a dune, which will eventually become a barrier
island through aggradation. The formation of barrier islands requires
a constant sea level so that waves can concentrate the sand into one
Offshore bar theory
In 1845 the Frenchman
Elie de Beaumont
Elie de Beaumont published an account of barrier
formation. He believed that waves moving into shallow water churned up
sand, which was deposited in the form of a submarine bar when the
waves broke and lost much of their energy. As the bars developed
vertically, they gradually rose above sea level, forming barrier
Spit accretion theory
Grove Karl Gilbert
Grove Karl Gilbert first argued in 1885 that the
barrier sediments came from longshore sources. He proposed that
sediment moving in the breaker zone through agitation by waves in
longshore drift would construct spits extending from headlands
parallel to the coast. The subsequent breaching of spits by storm
waves would form barrier islands.
Isles Dernieres in 1853 and 1978. Wave action detaches Isles Dernieres
from the mainland.
William John McGee
William John McGee reasoned in 1890 that the East and Gulf coasts of
United States were undergoing submergence, as evidenced by the
many drowned river valleys that occur along these coasts, including
Raritan, Delaware and Chesapeake bays. He believed that during
submergence, coastal ridges were separated from the mainland, and
lagoons formed behind the ridges. He used the Mississippi-Alabama
barrier islands (consists of Cat, Ship, Horn, Petit Bois and Dauphin
Islands) as an example where coastal submergence formed barrier
islands. His interpretation was later shown to be incorrect when the
ages of the coastal stratigraphy and sediment were more accurately
Along the coast of Louisiana, former lobes of the Mississippi River
delta have been reworked by wave action, forming beach ridge
complexes. Prolonged sinking of the marshes behind the barriers has
converted these former vegetated wetlands to open-water areas. In a
period of 125 years, from 1853 to 1978, two small semi-protected bays
behind the barrier developed as the large water body of Lake Pelto,
leading to Isles Dernieres's detachment from the mainland.
An unusual natural structure in
New Zealand may give clues to the
formation processes of barrier islands. The
Boulder Bank, at the
entrance to Nelson Haven at the northern end of the South Island, is a
unique 13 km-long stretch of rocky substrate a few metres in
width. It is not strictly a barrier island, as it is linked to the
mainland at one end. The
Boulder Bank is composed of granodiorite from
Mackay Bluff, which lies close to the point where the bank joins the
mainland. It is still debated what process or processes have resulted
in this odd structure, though longshore drift is the most accepted
hypothesis. Studies have been conducted since 1892 to determine the
speed of boulder movement. Rates of the top-course gravel movement
have been estimated at 7.5 metres a year.
Barrier islands are critically important in mitigating ocean swells
and other storm events for the water systems on the mainland side of
the barrier island, as well as protecting the coastline. This
effectively creates a unique environment of relatively low energy,
brackish water. Multiple wetland systems such as lagoons, estuaries,
and/or marshes can result from such conditions depending on the
surroundings. They are typically rich habitats for a variety of flora
and fauna. Without barrier islands, these wetlands could not exist;
they would be destroyed by daily ocean waves and tides as well as
ocean storm events. One of the most prominent examples is the
Louisiana barrier islands.
North Frisian Barrier Island(s)
Virginia Barrier Islands
New York Barrier Islands
Texas barrier islands
Bald Head Island
^ Garrison, J.R., Jr., Williams, J., Potter Miller, S., Weber, E.T.,
II, McMechan, G., and Zeng, X., 2010, "Ground-penetrating radar study
of North Padre Island; Implications for barrier island interval
architecture, model for growth of progradational microtidal barrier
islands, and Gulf of Mexico sea-level cyclicity:" Journal of
Sedimentary Research, v. 80, p. 303–319.
^ a b Davis Jr., p. 144.
^ Durán Vinent, Orencio; Moore, Laura J. "
Barrier island bistability
induced by biophysical interactions". Nature Climate Change. 5 (2):
^ a b Smith, Q.H.T., Heap, A.D., and Nichol, S.L., 2010, "Origin and
formation of an estuarine barrier island, Tapora Island, New Zealand:"
Journal of Coastal Research, v. 26, p. 292–300.
^ Lorenzo-Trueba, J.; Ashton, A. (2014). "Rollover, drowning, and
discontinuous retreat: Distinct modes of barrier response to sea-level
rise arising from a simple morphodynamic model". Journal of
Geophysical Research: Earth Surface. 119 (4).
^ a b Davis Jr., p. 147
^ Boggs, S., Jr., 2012, Principles of Sedimentology and Stratigraphy:
New Jersey, Pearson Education, Inc., 585 p.
^ Coastal Services Center, NOAA’s Coastal Services Center Barrier
Islands: Formation and Evolution Archived August 8, 2010, at the
Wayback Machine.. Accessed April 10, 2011.
^ Davis Jr., pp. 144–145
^ Davis Jr., p. 145
^ Morton, p. 2
^ M. R. Johnson (2001). "Nelson
New Zealand Journal of
Geology and Geophysics. 44: 79–88.
^ Stone, G.W., and McBride, R.A., 1998, "
Louisiana barrier islands and
their importance in wetland protection: forecasting shoreline change
and subsequent response of wave climate:" Journal of Coastal Research,
v. 14, p. 900–915.
Davis Jr., Richard A.; FitzGerald, Duncan M. (2004), Beaches and
Coasts, United Kingdom: Blackwell Publishing,
Morton, Robert A. (2007), Historical Changes in the
Islands and the Roles of Extreme Storms,
Sea level, and human activities (PDF), U. S. Geological Survey
Headlands and bays
Large-scale coastal behaviour
Integrated coastal zone management
Region of freshwater influence