Barrier Island Evolution
Beach Diagram
Introduction
• Three main theories of barrier island
formation
• Other theories-plate tectonics
• Barrier island migration and long shore
current
• Overwash and inlets
• Man-made attempts to stabilize beaches
Multiple Theories of Barrier
Island Formation
• Three original theories were developed between
1845-1890.
• Since that time, the theories have been reexamined and it has been determined that all three
theories explain the formation of different types of
barrier islands
• No one theory is correct and new studies have
shown that other factors such as plate tectonics,
sea level, and tidal ranges also have an effect on
the formation of barrier islands
Three main hypotheses for the
formation of barrier islands
• 1845-deBeaumont-Johnson concept of the
emergence of submarine bars
• 1885-Gilbert-Fisher idea of spit growth and
later breaching by inlets
• 1890-McGee-Hoyt hypothesis of beach
ridge submergence
Emergence of submarine bars
• 1845-deBeaumont suggested that islands such as
those found in the North Sea and the Gulf of
Mexico were formed by what is known as bar
emergence.
– Submarine bars are located offshore. When waves
come into the shore, they sweep across the shallow
bottom and pick up loose sand and deposit it on these
bars. The bars grow in size and rise above sea level to
become barrier islands.
Emergence Theory
• Waves break over a
submerged bar
• Bar emerges above sea
level due to deposited
sediment
• Bar develops into
barrier island and
lagoon
Support of deBeaumont
• 1919-Douglas Johnson reviewed deBeaumont’s
study and supported his hypothesis
• Johnson tested emergence and spit formation by
drawing profiles across a series of barrier islands.
He determined that islands were formed from
submerged bars because there was an area of
depression in front of the islands. This area of
depression provided the sediment needed to build
up the bars.
Support of deBeaumont, con’t
• He claims that islands formed by submarine
bars are common on shorelines with a low
slope. A low slope causes the waves to
break away from the shoreline and permits
the buildup of submerged bars off the coast
Tests of bar emergence and spit formation
• Island formed by a
spit, profile
intersection at coast
• Island formed from
emerged bar, profile
intersection is on the
mainland
Against deBeaumont
• If barrier islands were formed from the buildup
of submerged bars, there should be examples of
islands in various stages. This is not the case.
• Studies by John Hoyt in 1967 of the sediments
of barrier islands in Texas show no beach or
open marine sediments on the back side of the
island. Barrier islands that were formed from
bars should show evidence of marine fossils.
Spit Growth
• 1885-Gilbert proposed that barrier islands
were formed from spits by currents flowing
parallel to the coast-the longshore current.
Individual islands were created when the
spits were breached by waves during
storms, creating inlets.
Spit Formation
• Spit starts to grow
from point of land
• Spit is extended along
the coast by longshore
currents
• Spit is breached
during a storm,
forming a tidal inlet
and barrier island
Support of Spit Formation
• 1968-John Fisher agreed with Hoyt that the
lack of fossils argues against bar
emergence. He studied the lagoons behind
barrier islands and concluded if the island
was formed from a submerged ridge, there
should be evidence of soils or forests that
grew behind the ridge. He found no
sediments and determined islands must be
formed from breached spits.
Refute of spit formation
• Hoyt agrees that spit formation is on a limited
scale, in small segments of the coast where there is
enough sediment supply. He does not believe that
this theory supports the growth of large barrier
islands.
• If an island was formed from a spit, the original
marine beach should be preserved along the
mainland coast. Hoyt studied the mainland side of
lagoons and found no evidence of beach
sediment.
Dune-Ridge Submergence
• 1890-W.D. McGee suggested that barrier
islands were produced by drowning of the
coastal area during a rise in sea level
Beach Ridge Formation
• Dune ridge along the
coast
• Rise in sea level starts
to submerge dune
• Barrier island and
lagoon formed from
ridge
Support of Submergence
• 1967-Hoyt determined from his studies of
sediments that islands must be formed by
submergence of coastal dune fields or beach
ridges. Once the island was formed, waves
and the longshore current determined their
present-day shape
Other Theories
• 1971-Maurice Schwartz published a compilation
to draw the theories together to show that there are
multiple origins for barrier islands. He stated that
there are basically two types of islands:
– Primary-islands that were formed on land that is later
flooded by the sea
• Submergence
– Secondary-islands that developed seaward of the
primary coast on the continental shelf
• Spits and emergence
Role of Tectonics in Formation
• Three types of tectonic settings throughout
the world:
– Trailing Margins
– Collision Margins
– Marginal Seas
Collision Coast
Trailing Edge Coast
Blue-Continental
Red-Island Arc
Yellow-Neo-trailing
Pink-Afro-trailing
Green-Amero-trailing
Ligt Blue-Marginal Sea Coast
Percentages of Islands along
Coastlines
• Trailing Margins: 49%
– Atlantic Seaboard
• Collision Margins: 24%
– Western Central America
• Marginal Sea Coasts: 27%
– Baja California
Additional Factors
• Three additional factors contributing to formation
of barrier islands:
– Continental shelf width
– Sediment supply
– Tectonic setting
• Majority of islands are found on trailing margins
because all three factors are favorable
• However, islands are found on collision and
marginal sea coasts because they have a wide
coastal plain even though they do not have a wide,
flat continental shelf or a history of sediment
supply
Coastal Zone
Barrier Island Migration
• The beach system includes the area from the dunes
to 30 feet deep offshore. The sand in this system
is constantly moving.
• Evolution of barrier islands includes:
–
–
–
–
–
–
Sand bars moving up to the beach
Beaches flattening and sand moving offshore
Islands retreating toward the mainland
Islands migrating along the coast
Islands prograding toward the ocean
Inlets opening, closing, or migrating
Movement on the Beaches
• Sand is constantly moving due to wind, waves and
currents
• During fair weather, gentle waves cause a gradual
movement of sand bars toward the mainland.
– Net movement of sand is onshore
• During storms, powerful wind and waves erode the
shorelines
– Storm surges and waves may pass through breaks in dunes
and deposit sand in washover fans behind the dunes
– Storms can pull the sand offshore and flatten out the beach
– Net movement of sand is offshore
Longshore Current
• Waves typically strike a beach at an angle,
causing a current to run parallel to the coast
– In NC, the longshore current runs from North to
South
• Longshore current allows sediment to be
carried down the beach.
Longshore Current
Movement landward
• Transgression is the movement of islands
landward
– Rate of sea level rise overcomes the amount sediment
supply
• Transgression is caused by waves carrying
sediment from the beaches and dunes to the back
of the barrier island and depositing it in washover
fans.
• The natural movement of a barrier island is to
migrate landward as sea level rises.
– Front side of the island is eroded by wave action
– Back side must grows from the buildup of washover
Coastline Moving Landward
Evidence of Transgression
• Studies of sediment on barrier islands show peat,
tree stumps, oyster shells and layers of mud on the
beach side
– Once were part of the marsh, now found on the beach
• Marsh has a floor of thick, black mud from the
ocean side
• As the island rolls back, the beach moves back
over the old marsh. Today’s beach is where the
marsh used to be.
– Island rolls over on itself like a conveyor belt
Movement Seaward
• Progradation is the movement of islands seaward
• Process is different from transgression because the
island itself does not move
• Sediment is pushed on shore, building up the
ocean side of the island
• The addition of sediment causes the development
of multiple beach-dune systems
• Shoreline on the ocean side moves seaward while
the landward back side stays stationary
– Brought about by an excess of sediment that can not be
carried away by the longshore current
Coastline Moving Seaward
Barrier Island Changes
• Barrier islands can experience both
transgression and progradation at the same
time or change back and forth over time
• Barrier islands may change the shape of the
shoreline, but generally maintain their
position
Shoreline Changes
Coastline Changes
Hog Island Changes
Changes to Hog Island Over the
Past 350 Years
Movement Parallel to the Coast
• Barrier islands may experience erosion on
one end and a build-up of sediment on the
other end
• Migration of inlets can contribute to
changes in shape and size of barrier islands
Effects of Inlets
• Inlets are important because the are a site of
storage and transportation of sand
• Sand being carried by the longshore current
gets carried into tidal inlets
• Inlets move in three general ways
Shifting Inlets
• Breathing Inlets expand and contract
• Migrating inlets form in one place and
migrate with the longshore current
• Returning inlets migrate, but may return to
their original position
Mason’s Inlet
Shell Island
• October 1989
May 1990
Average Distance Change=180ft/0.6 yr
Shell Island
• November 1993
Average Distance Change=
670 ft/3.6 yrs
November1995
Average Distance Change=
440 ft/1.9 yrs
Shell Island 1994-1998
•
Inlet after Hurricane Hugo-SC
Tidal Deltas
• There are two types of deltas:
– Ebb tidal delta develops on the ocean side of
the island
– Flood tidal delta develops on the lagoon side of
the island; protected from effects of waves
• Built by the sediment deposition that
accumulates when a current suddenly slows
at the mouth of a channel, losing its
carrying capacity
Deltas, Inlets, Islands
On-line References
• http://books.nap.edu/books/030904806/html/70.ht
ml#pagetop
• http://coastal.er.usgs.gov/hurricanes/dennis/buxton
/
• http://members.nbci.com/igorz/MIWO.html
• http://www.beachbrowser.com/Archives/Environm
ent/A…/Barrier-Islands-Always-changing.ht
• http://www.csc.noaa.gov/products/nchaz/htm/lidto
po.htmhttp://www.lacoast.gov/Programs/CWPPR
A/Watermarks/Fall99/barrier.fa99.htm
References
• http://www.ncsu.edu/coast/shell/migration/migrati
on.html
• http://www.ncsu.edu/coast/shell/terms.html
• http://www.salem.mass.edu/~lhanson/gls214/gls21
4_tec_clas.htm
• http://www.seafriends.org.nz/oceano/beacheng.ht
m#sea%20walls
• http://www.seashell.com/kbphotos.htm
• http://www.vcrlter.virginia.edu/~bph/NSFTalk/NS
FTalk.html
References
• Davis, Richard, 1994, The Evolving Coast: W.H.
Freeman and Company, New York, pp 166-183.
• Fox, William, 1983. At the Sea’s Edge, PrenticeHall, Inc., pp. 151-175.
• Pilkey, Orrin, 1990. The Beaches are moving
(videorecording).
• Schwartz, Maurice, 1973, Benchmark Papers in
Geology Barrier Islands: Dowden, Hutchinson and
Ross, Inc., Stroudsburg, Pa.