Barrier islands are detached linear
islands of sand and/or gravel(rare) that run parallel to the shore and are back
by a bay, lagoon, marsh, or tidal flat. Barrier islands are wave and wind
built landforms and form in wave dominated or mixed energy environments, typically
microtidal or mesotidal environments.
Location of barrier islands
- Most barrier islands are along
trailing edge and marginal sea coasts where continental shelves are wide and
gently sloping. Barriers originating on steep shelves generally would not
survive rising sea level.
- Most barrier island in North
America are along the Atlantic and Gulf Coast.
Characteristics and morphology
of barrier islands
- Barrier islands are elongate
islands of sand which parallel the shore and are separated from the mainland
- Bay (microtidal/lmix energy
- Marsh and tidal creek
system (mesotidal/mix energy tide dominated)
- Barrier islands typically
occur in chains; each island is separated from its neighboring islands
The number of inlets per length along an island chain is reflective of the
combined influence of waves and tides. High wave energy tends to close
keeps them open. Barriers on glaciated coasts, such as the New England
coast, may be locally anchored to an isolated
- Barrier islands range in length
from 3 to 100 km in length and <1 to 3 km
- As alluded to above, barrier
island length reflects hydrographic regime, which controls the
size and number of inlets.
Storms produce inlets, tides keep them open, and waves transport sediment
that fills them in. The balance of these processes determines the
of the barrier islands and number of inlets along a coast. Long barrier
islands typically form in regions of low tidal range and moderate
- Barrier island width primarily
reflects sediment supply and relative sea level fluctuations. Rising
sea level is causing erosion and landward retreat (transgression)
of most barrier islands. However, barrier islands receiving large
amounts of sediment can build
seaward (prograde) despite the rise in sea level. Width can also vary
with prevailing wind pattern.
Winds blowing across the island promote the formation of large transverse
dunes and wide barriers. Narrow barriers are associated with prevailing
winds that blow along the barrier axis.
- The morphology of a typical
East coast barrier is shown in figures 1 and 2.
1. Morphology of a typical east coast barrier. Redrawn from Godfrey,
|Figure 2. Plum Island,
MA. Photo taken and low tide. Takes some time to locate the following
features then place the cursor over the image to check your interpretation:
migrating bar, backshore
the berm and foredune ridge), dunes, vegetated overwash,
forest, high marsh and low marsh.
- Barrier Islands may consist
of one or more beach ridges, depending on whether the system is prograding
or transgressive. (Discussed below.)
- Beach ridges, the highest
portion composed of dune sediments, represent successive shoreline positions
formed during progradation.
- Transgressive barrier
islands commonly lack multiple beach ridges unless they had an earlier
history of progradation.
- Beach ridges, which are
transverse dunes, run parallel to the shore at the time of their
and may be 1 to
- Adjacent beach ridges may
be separated by low areas called swales.
- Back barrier region
- The lagoon, bay, marsh,
or tidal flats backing the barrier islands range form 2 to 20 km in width.
- Transfer of sediment into
the back barrier region is important to the survival of a transgressive
barrier system. Sediment is transported into the back barrier region by:
- Overwash during storms.
Dominant process in microtidal systems
- Flood tidal
inlet migration. Dominant process in mesotidal systems
- Wind. Blows sand from
the beach face to the back shore. Once dunes are formed they tend
to migrate landward.
- spit formation
- As will be discussed later,
human intervention typically interrupts the natural flow of sediment,
thereby preventing a barrier island system from migrating and keeping
up with rising sea level.
Barrier islands are normally
composed of sand size sediment but in some instances a substantial
of gravel may be present. (e.g., some New England Barriers).
Barriers with substantial
gravel most likely formed by spit growth from a till or rocky
Attached barrier beaches
Attached barriers are sand and/or
gravel ridges that are attached to the shore and are backed by a bay, lagoon,
marsh, or tidal flat.
- A baymouth barrier is
barrier beach that is attached at both ends to headlands and backed
by a bay
- A spit is a protrusion
or linear beach extending from a headland. Spits are attached at one
However, continued growth may extend a spit to a downdrift headland
where it can become attached thereby evolving into a barrier beach.
- A sand or gravel beach
ridge is a relatively small beach ridge backed by a marsh
or tidal flat.
- Attached barrier beaches
are commonly smaller in size and more variable in composition than barrier
- These beaches are not separated
from the mainland or other barrier beaches by inlets
- Any theory of barrier island
formation must account for the following characteristics:
- Morphology: elongate parallel
to shore and narrow in width
- Composed predominantly
of sand size material
- Islands form chains
- Involves the building of
- Waves approaching the coast
would stir-up bottom sediments
- Where the waves break they
would lose their energy and the sediment would be deposited
- This sediment would accumulate
and an offshore bar would be formed
- Once the bar became subaerial
eolian processes would build it up vertically
- Problems with De Beaumont
- Studies conducted by
McKee and Steward (1961) in wave tanks showed that offshore bars could
be built no higher than sea level
- When ½ the bar reaches
a certain height the wave action would carry sediment form the top of
the bar landward.
- If barriers formed in
this manner why don't be see them in the various stages of formation
along the coast today?
- Absence nearshore deposits
landward of the barriers.
- If a barrier formed
from an offshore bar the open ocean conditions should have prevailed
prior to barrier island formation
- This means that landward
of the barrier nearshore fauna and sediment should be found.
- Extensive coring
along the GA and TEX coast has shown this not to be true
Gilbert (1885): Spit Progradation
- Gilbert believed that material
which built the barriers came not from offshore sources but rather from along
the shore by the process of longshore transport.
- Spits will form which will
then be breach to form barrier islands.
Leontyev and Nikiforov
(1966): Higher Still Stand Theory
- Barriers formed from offshore
bars which were built during a higher sea level stand
- They also suggested uplift
as a secondary mechanism of raising offshore bars above sea level.
- Theory depends on higher
sea level still stands. (This may not be a problem in glaciated high-latitude
regions if the bars can be shown to have formed during deglaciation)
addition the absence of open marine sediments landward
of the barrier poses the same objection to the offshore bar theory.
Hoyt (1967): Beach Ridge
- Where the ocean meet the
land in many instances beaches will be formed.
- The wind may create high
dune ridges immediately landward of the beach.
- Crane beach: 12-15 m
- Outer banks, NC; 15-35
- Barriers along SC; 1-12
- Coalescing of these dunes produced
- If during the formation of
the beach ridges or most likely afterward there is submergence (rise in sea
level) the area immediately landward of the barrier dune ridge will be flooded
to form a lagoon, bay, etc.
Stage 1: Approximate
15, 000, when sea level was 85 meters below present beach ridges developed
along the late Pleistocene shoreline which was much farther out along
the continental shelf.
Stage 2. Rising sea
level at the end of the Pleistocene results in breaching of the beach
ridge and flooding of the region behind it . The beach ridge becomes
a barrier island backed by a bay or lagoon.
Stage 3. The barrier
system migrates landward across the shelf and sea level continues to
Figure 2. Stages of barrier island formation according to Hoyt. Modified
from Pilkey and others 1978.
Note that on the gentle slope of the continental shelf a meter rise
in sea level results in almost a kilometer of landward migration.
barriers are called transgressive (figure 2) and prograding barriers that are
building out are termed regressive. Most modern Holocene barriers, especially
along the East coast, are transgressive in response to rising sea level. However
some barriers that receive a large supply of sediment are regressive despite
the present rise in sea level.
- The resultant form, composition,
and stability of a barrier island ultimately depends on the relative influence
of each of the following:
- Sediment supply
- Sea level fluctuations
- Slope of the continental
shelf (figure 2)
Cross-section through a typical transgressive barrier island. Redrawn
from Godfrey 1976. Note the shoreface deposits are rolling back over
grassland and back barrier marsh deposits.
- Retreat results when the
rate sea level rise overcomes the rate of sediment supply
- Process of shoreward migration
is accomplished by the transfer of sediment to the back barrier environment
- When the barrier island
is attacked by waves sediment is transported both offshore by waves
the barrier by overwash or inlet formation (Effects
of Hurricane Isabel Sept 03)
- Overwash: The discontinuous
flow or pulse of sediment charged water which occurs in response to
the storm wave runup and storm surge over-topping
- overwash occurs where
there is a low or breach in the foredune ridge
- Storm wave carry sand
which is deposited in a tongue- or fan-shaped deposit called an overwash
- If islands are backed
by marsh deposits the overwash sediment may bury the marsh
- The overwash process, which
erodes sediment from the front and carries it to the back, is a cannibalistic
process which preserved the existence of the barrier.
- In mesotidal regions, sand
that is transported offshore eventually migrates along shore and is
to the back barrier region by waves and flood currents through inlets.
Formation if tidal deltas and inlet migration help to fill in the back
and allow the barrier to migrate shoreward.
- Problems with transgressive
systems along the east coast (e.g. North Carolina):
- Due to the rise in sea
level the islands are migrating shoreward
- The rate of
shoreward migration is determined by:
- Slope of the shelf
area over which it is migrating
- Rate of sea level
- Sediment supply
- intervention by man
- The steeper the
slope the slower the migration
- The faster the
rise in sea level the faster the rate of migration
- sea level is
presently rising a little over 1 foot/century
- North Carolina:
Island-migration rate is 100-1000 times the rate of sea level
rise, depending on the slope. (For every foot of sea level rise
the island migrates 100 to 1000 feet inland)
- Regressive (prograding) barrier
islands: seaward-building barrier island systems
- Examples: Galveston Island
and Padre Island, Texas; East Frisian Islands, West Germany
- Sand is supplied to
the barrier faster than sea level rise
- results in a progradation
of the shoreline
Barrier Islands:A Vanishing Resource-USGS fact sheet
Coast Louisiana Coastal Restoration Web Site (USGS)
Coastal erosion homepage (USGS)
Impacts on the Coastal Environment
Florida coastal Studies Project
Guide: History of the Outer Banks
- Geomorphology from Space
- ** East
Beach, RI virtual field trip
NC Natural Guide to Barrier Island Dynamics
effect, sea level rise, and barrier islands Case Study of Long Beach
Island, New Jersey, By James G. Titus Environmental Protection Agency
Mapping of Myrtle Island of the Virginia Coast Reserve, Charles R. Carlson
and Raymond D. Dueser, Utah State: Nice air photos
erosion rates along the Texas Coast: Maps with brief discussion
- ** West
Central Florida virtual field trip
Stratigraphy and Depositional History of the Central South Carolina Coast
and Inner Shelf: Implications to Coastal Change , Jack L. Kindinger
and others, USGS Center for Coastal
of Geological Sciences, Salem State
College, Salem, MA