جزایر سدی-Barrier islands
Barrier islands
Elongate accumulations of sediment
formed in the shallow coastal zone and separated from the mainland by some
combination of coastal bays and their associated marshes and tidal flats.
Barrier islands are typically several times longer than their width and are
interrupted by tidal inlets. There are at least three possible explanations for
their origin: (1) longshore spit development and subsequent cutting of inlets;
(2) upward shoaling by wave action of subtidal longshore sand bars; and (3)
drowning of coastal ridges. All three modes of origin may have occurred. The
first two have been observed in recent times, the third is yet to be
conclusively demonstrated.
Formation
Modern barrier islands extend along
approximately 15% of the Earth's coasts. They are geologically quite young,
having been formed about 7000 years ago or less, as sea level slowed or came to
its present position during the Holocene transgression. Some are still in the
process of forming and gaining definition. The primary requisites for their
formation are a significant amount of available sediment, a place for it to
accumulate, and significant wave-energy influence. Tectonic setting is an
important control in the development of barrier islands, with most forming along
the American-type trailing edge coasts of mature continental margins such as the
east coasts of North and South America. The low-relief coastal plain and
adjacent shelf are served by well-developed drainage systems with abundant
sediment. Barriers are also well developed along marginal sea coasts such as the
Barrier
environments
It is not possible to discuss
barrier islands without considering the adjacent and closely related
environments within the coastal system. Beginning offshore and proceeding toward
land, the sequence of environments comprises shoreface, including the offshore
and nearshore; beach; dunes; washover fans, which comprise the back-island
flats; marsh; tidal flat; and coastal bay, which may be a combination of
estuarine and lagoonal environments leading to the mainland (Fig. 1). The
barrier island proper includes the beach, dunes, and washover fans; however, the
adjacent shoreface is well integrated with the barrier in terms of morphology,
processes, and sediments.
Fig. 1 Generalized profile across a barrier
island complex showing various environments.
Shoreface
The shoreface extends from the zone
where storm waves influence bottom sediment transport, shoreward to the low-tide
line. Typically it is subdivided into the outer shoreface, that area affected
only by storm waves, and the inner shoreface or nearshore zone, where normal
waves influence the bottom. The latter contains shallow longshore sand bars and
troughs, varying in number from one coastal location to another. The depths of
water marking the boundaries of the shoreface vary with location, depending upon
wave climate.
Beach
The barrier island itself begins at
the low-tide or outer limit of the beach. In many places the lower part of the
beach is characterized by an ephemeral bar and trough generally called a ridge
and runnel (Fig. 1). This small bar moves onto the upper beach under low-energy
wave conditions, either repairing earlier storm erosion or adding to the beach,
thereby causing the island to prograde. The upper intertidal or foreshore beach
extends up to the berm, where the foreshore and backshore beach environments
meet. The upper foreshore slopes toward the water and is the site of the uprush
and backwash of waves—the swash zone. The backshore part of the beach is a dry,
eolian environment except during storms. It tends to be horizontal or slightly
landward-sloping. See also: Eolian
landforms
Dunes
Coastal dunes occupy a wide range of
portions of the barrier island. The most common site of dune development is
immediately landward of the beach in the form of shore-parallel ridges. These
are called foredunes and may rise several meters above the beach. Barrier
islands that receive abundant sediment and tend to prograde seaward may have
numerous foredune ridges. See also:
Dune
Low dunes or low areas between dunes
may act as pathways for water and sediment during storm surges, producing
washover fans on the landward, flat side of the barrier. This part of the
barrier is generally only a meter or two above sea level and is covered with a
floral community that extends from grasses in the high areas down to marsh
grasses as the high intertidal zone is reached. Sediment that supports this
vegetation is delivered in the form of washover fans which coalesce to form a
washover apron—the landward part of the barrier. An individual fan may be about
a half meter in thickness and extend over several acres. Large storms may
generate sufficient surge to produce washover fans that traverse the entire
barrier and extend into the adjacent back-barrier bay.
Marshes and
tidal flats
These are the landward continuation
of the back-barrier environment. Their extent tends to be related to the
combination of tidal range and morphology of the island; generally the higher
the tidal range, the wider the back-barrier marsh and tidal-flat complex.
Coastal
bay
The coastal bay that separates the
barrier island from the mainland may take on various characteristics. In many
areas it is an estuary, a bay that is diluted by fresh-water runoff from streams
and that has good tidal circulation with the open ocean. Examples are found
behind the Outer Banks barrier system of
Inlets
The continuity of barriers is
interrupted by inlets, which carry great quantities of water and sediment
between the open marine environment and the coastal bays. Inlet spacing, size,
and stability vary, depending upon the dominant processes controlling barrier
island morphology. Inlets may be closely spaced such as along the
Processes
A variety of physical processes
occur along the coast. These processes act to shape and maintain the barrier
island system and also to enable the barrier to slowly migrate landward as sea
level rises. The most important process in forming and maintaining barriers is
the waves, which also produce longshore currents and rip currents. Waves and
longshore currents dominate the outer portion of the barrier system, whereas
tidal currents are dominant landward of the barrier, although small waves may
also be present. Tidal currents also dominate inlet processes, although waves
also may influence this environment. On the supratidal portion of the barrier
island, the wind is the most dominant physical process.
Waves
Normal waves that cross the inner
shelf and break in the surf zone do not cause significant sediment motion until
they steepen and break in shallow water, typically over a longshore bar. Such
breaking causes much sediment to be temporarily suspended and generates local
currents that may carry sediment toward or away from the shore. Storm waves
cause sediment motion in relatively deep water. They also tend to remove
sediment from the beach and transport it seaward. The reverse situation exists
during low-wave-energy conditions. Examples of this condition occur seasonally
along much of the west coast of the United States. The winter storms produce
large waves which remove sediment from the beaches and transport it offshore,
where it is stored until the lower-wave-energy conditions of the spring. The
sediment then returns and the beach is restored. This cycle of erosional and
accretional beach is common globally and is related to stormy versus quiet wave
conditions.
Waves rarely approach the coast with
the crests parallel to the shore. Instead, they approach at some acute angle and
are refracted, thereby generating longshore currents that flow away from the
acute angle made by the wave and the coast. These currents are the primary
mechanism for distribution of sediment along the barrier beach and surf zone.
Longshore or littoral drift may cause a net transport of hundreds of thousands
of cubic meters of sediment to pass a given point in a year's time. Waves and
longshore currents, in concert with nearshore topography, cause formation of rip
currents that may transport sediment seaward. These currents are produced by the
temporary piling up of water between the inner longshore bar and the beach. This
unstable condition causes water to flow seaward, and the water does so through
low areas of the bar. These narrow currents are quite local but may be dangerous
to swimmers. See also: Ocean waves
Tides
Tidal currents carry great
quantities of water and sediment through the inlets and to a lesser extent in
the estuaries landward of the barrier islands. Much sediment is deposited just
landward of the inlet as a flood-tidal delta. The equivalent seaward
accumulation, the ebb-tidal delta, varies greatly in size and shape, depending
on wave and tidal interactions. Along many barriers, waves produce longshore
currents that cause inlets to migrate or even close; these are wave-dominated
inlets. By contrast, other inlets have strong tidal currents that cut deep and
stable inlets which show little wave influence; these are tide-dominated inlets.
Tides are also important landward of
the barriers in the estuaries, marshes, and tidal flats. Currents generated by
tides are a primary mechanism for moving sediment both in suspension and as
bedload. Flooding of low areas by tides provides a means for depositing sediment
on the marsh or tidal flat. Typically the rate of tidal sediment flux is
directly related to the tidal range.
See also: Ocean circulation
Wind
The wind is also an important agent
for sediment transport and therefore for molding barrier island morphology.
Prevailing winds along the coast typically have some onshore component. The
backbeach zone, or at low tide the entire beach, serves as a sediment source for
onshore winds that accumulate this sediment behind the beach in the form of
dunes. Wind also causes sediment to blow from the dunes back on the washover
apron or marsh and also directly into the bay. Along parts of Padre Island in
southern Texas, wind has blown so much sediment into Laguna Madre that it is
essentially filled. See also:
Nearshore processes; Wind
Sediment
budget
The combined processes operating in
the various sedimentary environments of the barrier island complex are best
viewed through a sediment budget (Fig. 2). There are two distinct sources of
sediment for this system (excluding biogenic or skeletal debris): the bays and
the shoreface. Both of these environments receive feedback from other
environments also. Shoreface sediment is transferred primarily to the beach,
with small amounts going directly to the inlet. Beach sediment moves toward four
sites: dunes, alongshore to inlets, over the barrier by storm washover, or
seaward to the shoreface. Dunes also serve as a sediment source for washover
fans, and they contribute as well to the back-island environments and bays
through eolian transport of sediment. The marshes and other low-relief,
back-island areas receive sediment from several sources, but do not provide
sediment except in the case of severe storms, such as hurricanes, when even
these low-energy environments are eroded.
Fig. 2 Sediment budget for barrier island
complex. Solid lines are major interactions; broken lines are relatively minor
intersections.
The coastal bays show great
variability in the sediment budget. Those bays that have little or no runoff
from land or receive little tidal influence (lagoons) have little sediment input
except from washover and blowover. Many bays receive much sediment runoff from
the mainland and are subjected to pronounced tidal influence (estuaries). These
bays provide sediment to marshes and tidal flats and some to inlets. They
receive sediment from the same sources as lagoons, with the addition of inlets.
Morphodynamics
The response of the barrier island
system to the above processes results in various sizes and shapes of barriers
and their related inlets. These morphodynamics are dictated primarily by the
interaction of wave- and tide-generated processes. Barrier islands are
restricted to wave-dominated coasts and to coasts where there is a mixture of
wave and tide influence. Tide-dominated coasts do not permit barrier islands to
develop, due to the dominance of on- and offshore flux. Barrier islands on
wave-dominated coasts are long and narrow with abundant and widespread washover
fans (Fig. 3a). Inlets tend to be widely spaced, small, and unstable due to the
dominance of waves primarily through longshore currents.
Fig. 3 Maps showing major coastal environments
and directions, and directions of coastal processes of (a) wave-dominated
barrier islands and (b) mixed-energy barrier islands. Large arrows indicate wave
direction, and small arrows indicate direction of sediment transport. (After S.
P. Leatherman, ed., Barrier Islands from the Gulf of St. Lawrence to the Gulf of
Mexico, Academic Press, 1979)
Mixed-energy coasts produce barrier
island systems that have short islands that are wide at one end with large,
fairly stable inlets resulting from the combination of wave and tide influence
(Fig. 3b). Strong tidal currents in the inlets develop a substantial ebb-tidal
delta that has an outer lobe smoothed by wave action. As waves are refracted
around the ebb delta, there is a zone of longshore current reversal downdrift of
the regional littoral drift direction. This condition causes sediment to become
trapped and accumulate as prograding beach ridges. The remainder of the island
is wave-dominated and is served by longshore currents moving in the direction of
regional littoral drift. This part of the barrier island tends to be narrow and
characterized by washover fans. Mixed-energy barriers have been termed drumstick
barriers because of this shape.
Sediments and
sedimentary structures
Although the barrier island system
contains a broad spectrum of sediment types and sedimentary structures, each
sedimentary environment can be characterized by its own suite. Knowledge of how
these features formed and their spatial relationships with one another is
important in recognizing and interpreting ancient barrier island systems
preserved in the rock record.
The outer shoreface is typically
composed of muddy sand or sandy mud with thin layers of sand. Bioturbation is
nearly ubiquitous, and the surface may have inactive ripples or an irregular
undulating surface from a previous storm. Discrete sand layers may show
small-scale ripple cross-stratification and are related to storm events. The
inner shoreface or nearshore zone displays great variety in sedimentary
structures and has generally moderately to well sorted sand which may be a
mixture of terrigenous and biogenic particles. Bedforms include symmetrical,
wave-formed, two-dimensional ripples in the outer part, with three-dimensional
megaripples on the seaward side of the longshore bars, where waves steepen
producing combined flow conditions, and plane beds near the bar crests, where
waves break. This sequence is repeated as each of the longshore bars is
traversed by waves. The result is a complex but predictable pattern of bedforms
and corresponding stratification types. Rip channels have seaward-oriented
bedforms and cross-stratification due to the seaward transport of sediment in
this localized environmnent.
The distinct zones within the beach
also are readily distinguishable by their sediments and structures. Foreshore
sediments are generally moderately to well sorted sand or gravel. Sorting values
increase from worst to best from the base of the foreshore to the berm crest.
Stratification is pronounced and slopes seaward at angles generally less that 7
or 8°. The great dynamics of this environment and the pronounced temporal
changes in energy levels cause a range in the size of sediment particles
deposited at any given time; some layers are sand, some are shell hash, and some
are a combination. Erosion on the beach is typically marked by lag accumulations
of dark, heavy minerals, resulting in thin layers of such minerals as zircon,
garnet, magnetite, and rutile.
The backbeach is more uniformly
finer-grained and better sorted that the adjacent foreshore. This is a response
to eolian-dominated conditions, which may produce a thin surface deflation
pavement of shell debris. Stratification is nearly horizontal and may be
disrupted by burrows of various arthropods.
Dunes on the barrier island are
characterized by fine, well-sorted sand which is dominantly quartz but may also
contain carbonate shell debris. Dunes show the highest sorting values in the
barrier system. There is a general trend of decreasing grain size and increasing
sorting from the lower foreshore to the dunes. The surface of dunes is commonly
covered with small wind-generated ripples. Internal stratification is dominated
by large-scale cross-stratification formed as the active portion of the dune
migrates. Dips range widely from about 10° to greater than 30°; generally a
particular barrier is characterized by a given range in cross-stratification
dips. Azimuths of the cross-stratification typically reflect prevailing and
predominant wind directions.
The low-lying, back-island area is
composed of moderately sorted sand dominated by horizontal, plane-bed
stratification developed under sheet flow conditions of washover deposition.
Roots and benthic invertebrates may destroy some of the stratification.
Algal-mat-covered wind tidal flats may be widespread in the supratidal zone of
some back-barrier areas. The blue-green algae stabilize the sediment under
normal conditions but are torn up during storms, thereby permitting erosion by
storm surge waters.
Marshes act as efficient sediment
traps and contain a combination of mud, plant debris, and sand. Suspended
sediment is transported onto the marsh as the tide rises, especially during
storm surge conditions. Particles settle, and are trapped by the grass,
resulting in slow accumulation of sediment. Washover fans may encroach upon the
marsh, providing sand layers within the peaty muds.
Tidal flats receive sediment as the
tides rise and fall, bringing both bedload sand and suspended mud. These
sediments commonly accumulate in thin alternations, forming tidal bedding. The
subtidal portion of the estuaries associated with barrier islands tends to be
dominated by mud with variable amounts of terrigenous sand and shell gravel.
Oyster shell debris is an important constituent of many low-energy estuaries,
such as along the Atlantic and Gulf coasts of the United States. Some estuaries
are influenced by strong tidal currents and have coarser sediment with bedforms,
such as the Bay of Fundy in Nova Scotia, Canada.
Lagoons accumulate little
terrigenous sediment because of the lack of mechanisms to transport it. Biogenic
material and the precipitation of carbonate and evaporite minerals in some
lagoons provide most of the sediment, with some contribution from both washover
and blowover.
Inlets contain a wide variety of
sediment texture and bedforms. Cross-stratification ranges from small-scale to
large-scale, depending on the size of the bedforms present. A generally bimodal
direction to this cross-stratification may be present, representing flooding and
ebbing tidal currents.
Management
Continued pressures of growing
populations have caused increased development of barrier islands, primarily for
residential, commercial, and tourism purposes. This development along with the
apparent increase in the rate of sea-level rise over the past several decades is
causing severe problems. Natural barrier island processes include landward
migration as washover occurs, and many tidal inlets tend to migrate along the
coast. Construction of buildings, roads, and other infrastructure tends to lend
some degree of permanence to barrier islands in that these structures are not
designed to move as the island environments migrate.
Stabilization caused by structures
commonly causes problems in various ways. Seawalls are designed to protect
buildings or other structures from wave attack and coastal erosion. They also
prohibit washover and landward migration, and commonly fail as wave action
scours underneath them. Because of their nature, they prevent beaches from
developing as barrier island retreat takes place. Structures that are
perpendicular to the coast, such as groins and jetties, interrupt the littoral
drift of sediment along the coast. These structures cause considerable sediment
to pile up on their updrift side and result in erosion on the downdrift side
because of lack of sediment supply. This situation is much like the sediment
that is impounded behind a dam in a reservoir. Jetties are similar in their
nature and are typically large because of their role in inlet stabilization.
These concrete and rip-rap structures do a good job, but in the process they are
also effective sediment traps on the updrift side, and the downdrift side of the
inlet suffers serious erosion problems due to lack of sediment. Several
locations around the United States now have sediment-bypassing systems to
counteract this problem, but they are costly and commonly inoperable due to
mechanical problems. New technology is needed to provide more efficient sand
transfer across structured inlets.
There are federal, state, and local
zoning restrictions on construction type and location, and other management
programs that are in place. Most states have zoning that restricts construction
on barriers through the implementation of some type of minimum setback from the
active beach and dune environment. Some also prohibit seawalls and have
limitations on rebuilding after loss due to storms. Extensive permitting
procedures are involved in nearly all barrier island construction. New strict
construction codes have resulted in tremendous reduction in structural damage
due to severe storms such as hurricanes. Conditions associated with Hurricane
Opal along the Florida panhandle coast in 1995 offer a good example. Of the
nearly 2000 homes and other buildings along the open coast that were in the path
of this storm, about 1000 incurred serious damage or were destroyed. Not one of
the destroyed homes was built under recent construction codes, whereas homes
built under these codes experienced no major damage.
Soft construction has become the
primary method for protection of upland barrier properties. The most utilized
approach is through beach nourishment, but various methods of constructing dunes
and extensive vegetation programs are also common. All are designed to provide
protection and stabilization to barrier environments while maintaining the
esthetic appearance of the area and permitting natural processes to act on the
islands.
Beach nourishment is expensive and
temporary but has been effective in most places. This involves taking beach sand
from a source location, generally offshore, and placing it on the beach in a
specific design configuration. This solution typically costs a few million
dollars per mile and lasts 5–10 years before additional nourishment is required.
The cost of such projects is typically shared among federal, state, and local
levels in decreasing proportions.
Although great strides have been
made in coastal management over the past few decades, there is still a long way
to go. Current regulations need to be enforced, and exceptions—which are too
common—minimized. Like so many current environmental problems, the coastal zone
is subjected to ever-increasing pressures of population and the related
development. Barrier island systems are among the most affected. See also: Coastal engineering
Richard A. Davis, Jr.
Bibliography
R. W. G. Carter and C. D. Woodroffe
(eds.), Coastal Evolution, 1994
R. A. Davis, Jr. (ed.), Geology of
M. L. Schwartz (ed.), The Encyclopedia
of Beaches and Coastal Environments, 1982
این وبلاگ تمامی موضوعات و مقالات و اطالاعات تخصصی زمین شناسی را که از سایتهای علمی جهان برگرفته شده در اختیار بازدیدکنندگان محترم قرار می دهد.گفتنی است که مطالب موجود در این وبلاگ در نوع خود بی نظیر بوده و از هیچ وبلاگ ایرانی ای کپی برداری نشده است و اگر هم شده منبع آن به طور کامل ذکر شده است.