Barrier island processes

[From the U.S. Government Printing Office, www.gpo.gov]

CRC 161A

COASTAL ZONE
INFORMATION CENTER

BARRIER ISLAND PROCESSES

STEPHEN B..BENTON

HEAD, TECHNICAL SERVICES,

NORTH CAROLINA,DEPARTHENT OF
=f NATURAL RESOURCES
CC) AND
tic COMMUNITY DEVELOP14ENT
7Z:

OFFICE,OF COASTAL MANAGEMENT

September 30, 1981.

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1981

BARRIER ISLAND PROCESSES

Barrier Islands

Barrier islands such as North C�Lrolinals Outer Banks.and other.beaches
are a product of wind, wave, and current en 'ergy.along the ocean, land inter-
face. They are found world wide along continental margins characterized by
low tectonic or mountain building activity, and broad, gently sloping.Coastal
Plains. Physically, they consist of a pile of loose unconsolidated sand and
shell resting on the sediment layers whichmake up the Coastal Plain and
continental shelf. Except where large dunes have developed,such as Jockey's
Ridge, North Carolina's barrier islands are typically 40 feet or less thick.
They range from approximatelyl/4 to 4@miles in width and are generally
separated from the mainland by sounds, lagoons,. rivers, or marsh systems.
Barrier island systems are .broken into individuai'island segments by inlets.
The inlets allow a tidal exchange of water between the ocean and sounds or
lagoons and provide an outlet for river discharge.

There is considerable.variation in the size a:nd shape of barrier islands.
This is a function of differences in the amount of sediment available for
their formation, island orientation with respect to prevailing winds, wave
and storm climate, the presence or absence of topographic relief.on the
coastal plain sediment surface (relict barrier islands) and a host'of other
conditions and variables.

Barrier islands in North Carolina can, however, be grouped into three
basic types; simple, complex and compound (Figure 1). A simple barrier
island consists of the following minimum elements or zones which, from ocean
to estuary include; the nearshore or littoral zone, berm, frontal dune system,
back dune flat and marsh or tidal flat zone. Complex barrier islands generally
have the same elements with the addition of a back barrier dune complex. This
generally reflectsa surplus in the sediment supply-necessary for basic
barrier island maintenance.

Compo und barrier islands consist of the elements of a simple or complex
barrier island developed against and on a remnant of a relict barrier which
.was produced several thousands of years in the past. These are best known
from the Georgia Sea Islands though Kitty Hawk Woods, Collington Island and,,,
the Ocracoke Village portion of.Ocracoke Island have been proposed to be
remnants of a relict barrier island.

Sea Level Fluctuations

For the past two million years, the coastal areas of the world have
been subject to rel 'atively rapid (in a geological sense) changes in sea level,..
These changes have been brought about by the alternate lock up and release,
of water from the ocean basins by the continental ice sheets of the "ice
ages" or Pleistocene. During this time, at least four'major glacial events
have occurred which resulted in a lowering of sea level by as much as 400
feet below its present level. When these ice sheets melted, sea1evel rose
to as*high as 200 feet above its present level.

The last of the major glacial events ended 18,000 years ago and sea
level has risen to its present levelsince that time, a proces.s that is
continuing today., Mo t t t dies *ndicate that the current rate of
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Shoreline Change and Sea Level Rise

As sea level has risen and fallen,. the location of the shoreline has
migrated back and forth across the Coastal Plain and Continental Shelf. Since
sea level is presently rising, the shoreline is migrating westward resulting
in shoreline recession or erosion. Though,the@current sea level rise of one
foot per century doesn't sound very significant, the horizontal translation
on the low slope of the Coastal Plain ranges from 100.to 1000 times the vertical
change (Figure 2). In northeastern North Carolina, one must travel inland as
far as 75 miles.to reach an elevation of 25 feet above'sea level. A large
portion of mainland Dare and, Hyde County is.less than five feet above sea level.

Long Term Shoreline Change Rates.In North Carolina

Long term shoreline change or beach erosion varies considerably along the
North Carolina coast'. Generally, th e north-south trending coastal segments
erode faster than east-west trending segments. Both erode faster in the.,
vic inity of inlets. Some portions of the shoreline are accreting or building
seaward. Of these, some appear to be relatively short term or temporary pheno-
mena while others (the areas forming the western arm of the three capes) have
a net accretion which has been going on for up to 4,000 years.

Erosion rates range. generally4from Z,to 6 feet per year over,the long term.
Near inlets, the erosion may range up to.25 feet per year, This is not to say
that a given stretch of.beach with an' erosion rate of 6 feet per year will erode
6.feet every year. Erosion generally occurs in jumps of perhaps 10 to 50 feet
as a result of large storms which only,occur once in 2 to 5.years or more. Near
inlets, however, the erosion may be more regular and even occur when there
are no storms.

Coastal Processes and Shoreline Change

Though sea level rise is the underlying cause of shoreline recession, the
work is performed by wind, waves and currents, particularly during storms.
Several 'interacting coastal processes are involved which affect the ocean beach
system. The beach, which consists of the storm berm, berm,swash zone and off-
shore bar system (Figure:3) is.an.effective energy sponge. During normal day
to day conditions, much of tbe energy of the waves is dissipated by the per-
meable slope of the swa,sh.zone as the wave wash. moves up oT% and filters into.,
the berm., A portion,of, the energy is..,[email protected] on moving satid.both up and
back down the face of the swash-zone.

Since waves generally approach the coast at.an angle, a current is pro-
duced which flows parallel with the shoreline. $and is moved.along the shore-
line by the current as littoral drift. In North Carolina, this drift is pre-
dominantly from nprth.to south,and has been estimated to be as much aq 370,000
cubic yards per y Iear net near Oregon Inlet by the Arm@ corps of. Engineers-.....

Energy levels increase during storms affecting dramatic changes on the beach
system. 'High winds associated with storm conditions increase the wave height and
decrease the wIave length or distance between wave crests,(Figure 4). This results
in an increase in the'.w'ave energy which must be dissipated by the beach system.
The larger and more closely spaced waves also increase the long shore

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current flow and its transport of littoral drift. In addition, low-atmospheric
pressures associated with storms cause a doming of water on the ocean surface
called a storm surge. The increased elevation of the water surface moves the
swash zone higher up the beach to the storm berm. During very large storms,
the swash zone may move all the way up to.the frontaldune (Figure 3).'

During storms, sand held in storage on the beach is transported offshore
and formed into one or more bars which help dissipate the energy of storm waves.
This, in combination with the onshore shift of the swash zone produces a much
broader and more effective energy dissipation zone (Figure 5). Generally, most
of the sand moved offshore moves back onto the beach when normal energy con-
ditions return.

Frontal dunes occur at a point landward of the beach,where wave energy
affects the barrier infrequently enough for pioneering vegetation such as American
beach grass (Ammophila breviligulata) or sea oats (Uniola Paniculata) to become
established. These,plants have developed-special adaptations to salt spray and
the wind blown (eolian) sedimentation processes dominating this portion of the
environment. Once established, the grasses buffer wind energy causing sand
blowing from the beach or inland to.be dropped and accumulate in mounds around
the plants. Burial stimulates growth of these frontal dune plant species so
an upward growth of the mound of sand occurs. Dune grasses spread laterally
relatively rapidly by sending out a. system of rhizomes. As upward growth of
the dune continues, new rhizomes 'are sent laterally just below the sediment
surface. This produces a structural framework within the dune allowing it to
maintain slopes considerably greater than the angle of repose for dry sand.
This framework of rhizomes is commonly visible throughout the entire thickness
of a dune truncated by large storm activity.

Generally, the. -frontal dunes continue to grow upwards as a series of hum-
mocky mounds along the back side of the beach until they reach equilibrium with
predominant wind energy conditions and sediment supply. During very large
storms, some of the sand held in storage within frontal dunes is carried offshore
and incorporated in the offshore bar system. Another portion of the sand is
carried landward as ov.erwash (adding elevation t.o the barrier). When the storm
is over, the process of frontal dune development begins again, normally at a
point somewhat landward of its original'location.-

Sand carried offshore from the beach and frontal dune system during a
large storm can result in a temporary shorelinel,retreat of as much as 500 feet.
Thirty to 40 feet of temporary retreat is common during even relatively small
storms. As this sand moves back onto the beach, generally within a year, most
of this shoreline retreat is reversed. A large-storm may only result in a
net permanent loss of 10 feet.

Shoreline Recession and Long Term Barrier Island Processes

Over the past 15 years researchers have found that barrier islands can
and do respond to ri ,sing.sea level by migrating landward. This is accomplished
primarily through inlet development, overwash and eolian processes. The ro-le
each plays varies somewhat from island to island and through time.

Volumetrically,. inlets are the primary mechanism for moving sand from the
ocean side to the back or estuarine side of barrier islands. Sediment budget
studies indicate nearly 75% of the landward transport of sediment on North
Carolina barrier islands is through inlets. Some of the sediment moving along
the ocean beach-(l .ongshore transport) moves into the inlets through the, inlet
channels and is deposited on Lrge shoals (flood tide deltas) which form on the

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estuarine side of b Iarrier islands. As these inlet channels migrate (up and
down the barrier islands within inlet migration zones in the southern half of
the state or to the south in the northern half af the state), the actively
building flood tide deltas also migrate producing,broad platforms.on the
estuarine,side along the inlet migration zone.' This platform adds width to
the island and provides.a base on which the other island migration processes
build.

Overwdsh and eolian processes add elevation tothe backbone of barrier
islands and provide a gradual landward shift in the island center of mass.
These processes also provide a mechanism for the landward shift of all of the
barrier island.elements or zones which insures the continuation of energy
equilibirum requirements. For example, the periodic landward shift of a
frontal dune allows it to maintain,the temporal stability period necessary for
successful.vegetation development as the beach narrows through shoreline erosion.

Barrier island migration is'a relatively long term phenomena and not
readily recognizable during one person's lifetime. Unlike many other geological
processes, however, it has been significant withintistoric times. Durifig the
300 years European man has lived on North Carolina's barrier islands, shoreline
changes canbe measured in thousands of feet, more than 20 inlets have opened,
migrated and closed,@another 20 or so are still open and migrating, and much
of the.present barrier island land has had an inlet passing through at one time
or another.

Selected Bibliography

Baker, E. J. (ed.), 1980, Hurricanes and coastal storms, Papers presented
at a national conference: Florida Sea Grant College, Report No. 33,
219 p.

Baker, S., 1978, Storms,people:and'property,in coastal North Carolina: U.N.C.
Sea Grant Publication. UNC-SG-78-15, 82 p.

Bascom,.W., 1964,.Waves and beaches: The dynamics of the ocean surface:
Anchor Books, Doubleday and Company., Inc. Garden City,NY, 267 p.

Benton, S. B., 1979, Environmental assessment: Proposed ordinance'clearance
at the U.S. Army Corps of Engineers CERC Facility, Duck, North Carolina:
North Carolina Department of Natural Resources and Community Development,
Office of Coastal Management, .Raleigh, N.C., 12 p.

Clark,J..(ed.), 1976, Barrier islands and beaches: Technical Proceedings
of th.e.1976 Barrier Islands Workshop, Annapolis, Maryland: The Conser-
vation Foundation, Washington, D. C., 149 p.

Cleary, W. J., and Hosier, P. E.,,1977, New Hanover Banks: Then and now:
U.N.C. Sea Grant Publication,' number UNC-SG@77-14, Raleigh, N.C., 69 p.

Collier, C.'A.,11976,-Bulkhead and revetment effectiveness, cost and construction,
2hd.'edition: Florida Department of Natural Resources

Dolan, R., Hayden, B;. P.,.and Heywpod,@J. E., 1978, A new photogrammetric
method for determining shoreline erosion: Coastal Engineering, v. 2,
p. 21-39.

Dolan, R., Hayden, B.,P., May, P.,_-and May, S., 1.980, The reliability of
shoreline changemeasurements from aerial photographs: Shore and Beach,
v. 48, No. 4, p. 22- 29.

Fisher, J. J., 1967, Development pattern of relict beach ridges, Outer Banks
barrier chain, North Carolina: PhD. Dissertation, University of North
Carolina, Chapel Hill, N. C. 250 p.

Gares, P. A., Nordstrom, K. F. and Psuty, N. P., 1979, Coastal dunes: Their
function, del@neation, and management: Center for Coastal and Environ-
mental Studies -.Rutgers, New.Brunswick, N..@J., 112 p.

Godfrey.,-P..J., and Godfrey, M., 1976, Barrier island ecology of Cape
Look Lookout National Seashore and vicinity, North Carolina: National
Park Service Monograph Series, Number 9, 160 p.

Goldsmith, V. (ed.), 1977, Coastal processes and resulting forms of sediment
accumulations, Currituck Spit, Virginia - No rth Carolina: Virginia
Institute of Marine Science, Gloucester Point, VA., 4:6 p. plus contributions.

Leatherman S. P., (ed.), 1979,-Barrier islands: From the Gulf of St. Lawrence
to the.Gulf@ of.M.exico: Academic Press,' New York, N. Y., 325 p.

N.' C. Council of Civil Defense, 1955, State of North Carolina long-range
hurricane rehabilitation project: North Carolina Council of Civil
Defense, Raleigh, N.- C., 64 p.

North Carolina Marine Science Council, 1981, Coastal study: Final report
to the 1981 North Carolina General Assembly: North Carolina Department
of Administration, Raleigh, NC, 83 p.

Pilkey, 0. H., Jr., Pilkey, 0. H., Sr., and Turner, R., 1975, How to live
with an island: A handbook to Bogue Banks, North Carolinaz North
Carolina Department of Natural and Economic Resources, Raleigh, N.C., 119p.

Pilkey, 0. H., Jr., Neal, W. J., Pilkey, 0. H.,. Sr., Riggs, S. R., 1980, From
Currituck'to Calabash: Living.with North Carolina's barrier islands:
North Carolina Science and Technology-Research Center, Research Triangle
Park, N. C., 244 P.

Ranwell,,D. S., 1972, Ecology of salt marshes and sand dunes: John Wiley and
Sons, Inc., New York, N. Y., 258 p.

Riggs, S. R.,.andlenton, S. B.,.19,77, The notthern,Outer Banks of North
Carolina: AAayman's field'trip guide through a dynamic barrier island
system: Institute for Coastal-andMarine Resources, East Carolina
Carolina University, Greenville, N. C., 39 p.

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