development of all barriers distributed worldwide. More
and more studies (e.g., Schwartz, 1971; Hayes, 1979;
Leatherman, 1979; Leatherman, 1985) have shown that
the formation and development of barrier islands are
a result of multiple processes.
Boundary constraints
Although the environmental conditions required for formation of barrier islands are relatively flexible, there still
exist some boundary constraints. According to the statistics from Stutz and Pilkey (2011), barrier islands are most
abundant (~63 % of the total) on tectonically stable,
trailing edge continental margins as such environments
provide favorable boundary conditions (e.g., abundant
sediment supply, small ratio of tidal range to mean wave
height) for the formation of barrier islands. Of the
remaining barrier islands, ~21 % are located on marginal
seas, and only ~16 % are found on collision margins. Most
(~58 %) of the barrier islands existing on collision margins
are developed on delta lobes favored by a low-gradient
shoreface produced by abundant riverine sediment input;
the rest are located on wide coastal plains. Barrier islands
rarely form on narrow continental shelves with an upper
shoreface slope larger than 0.8
, in which sediment tends
to move offshore rather than accumulating onshore.
Another significant boundary factor influencing the formation and development of barrier islands is sea-level
change. A stable sea level is a prerequisite for the formation
of barrier islands. Most barrier islands are quite young,
being formed during the last ~6,000 years when the global
sea level became relatively stable with only minor fluctuations. A stable sea level with small rates of change (within
millimeters per year) in the mid- to late Holocene restricts
tides and wave actions to a small-range coastal area (i.e.,
hydrodynamically active zone). Sediment transport within
this area became increasingly important to shape the modern coastline. Driven by wave and aeolian processes, an
excess of sediment supply to a local accommodation zone
would eventually build up new land above the water surface. Holocene barrier spits and islands present such examples. Holocene barrier islands are low-lying structures made
of unconsolidated sediment, with the highest part at the
dune crests, which is normally only meters above the water
level. Thus they are quite vulnerable to high water-stand
impacts induced by storms or floods. Without sufficient
sediment supply to compensate the increased accommodation zone, continuous sea-level rise would cause a barrier
island to shrink and migrate landward.
Besides a sufficient sediment supply to feed the formation of a barrier island, the “quality” of the sediment supply is also critical for the fate of the island. Sediment
supply with a larger proportion of sand and coarse material
is able to sustain stronger hydrodynamic impacts than fine
sediment such as mud and clay. Thus, sandy substrate and
shoreface are more durable than a muddy one to maintain
a barrier island. The wave-tide regime is also an important
factor influencing the morphogenesis of barrier islands.
Beaches and barrier islands are products of wave action.
They develop most easily on wave-dominated coasts with
small to moderate tidal range. Only ~12 % of barrier
islands develop in tide-dominated regions (with the ratio
of mean tidal amplitude to mean annual wave height generally larger than three according to Davis and Hayes,
1984), and they are rarely found in areas with a tidal range
larger than 4 m.
Barrier island morphodynamics
Among different types of coastal landforms, barrier
islands have the most variable morphology. They are constantly shaped by winds, tides, and waves and, on a longer
time scale, can shift landward or seaward due to oscillations of sea level and variations in the sediment supply
(Masetti et al., 2008).
Depending on the relative importance of waves to tides
in determining the coastal morphology, three types of
coastal environments can be classified: wave dominated,
tide dominated, and mixed energy. In wave-dominated
coasts, barrier islands are elongate and narrow due to the
impact of longshore drift. Inlets produced by tides or storm
breaching migrate fast for the same reason. Washover features are prominent, and flood deltas are well developed
but ebb deltas are small or nonexistent (Hayes, 1979).
Along with an increase of tidal effect, inlets play a more significant role in shaping the barrier island morphology. Substantial ebb deltas can develop, and barrier islands become
shorter and wider as a result. As tidal range increases, these
features become more prominent. When the tidal range is
high enough and overwhelms the wave effects, barrier
islands cannot develop and inlet deltas are confined to elongated stringers oriented with the dominant tidal currents.
In addition to tide and wave actions, development of
barrier islands is also affected by other processes (e.g., stochastic extreme events, sea-level change, tectonic movements, and fluvial input). Barrier islands evolve and
migrate parallel or normal to the mainland in response to
these processes. The shore-normal evolution of barrier
islands corresponds to two types of behavior: namely,
regression and transgression, respectively. Barrier transgression refers to an onshore migration of the landform
and an overlapping of deeper water sediment over
shallower lagoon deposits. Leatherman (1979) summarized three main processes controlling barrier island transgression, which, in the order of importance, are inlet
dynamics, overwash, and aeolian transport. In some areas
with a thick and compressible substrate (e.g., the Virginia
barrier coast), auto-compaction also contributes to the barrier island transgression (Leatherman, 1985). In response
to the increased impacts of these processes induced by an
eustatic sea-level rise, three modes have been proposed
to describe a subsequent evolution of a barrier island:
(1) a continuous landward migration across the underlying
substrate to higher elevations; (2) a disintegration of the
island due to insufficient sediment supply and backshore
relief to sustain inundation during stochastic extreme
BARRIER ISLAND
49
and more studies (e.g., Schwartz, 1971; Hayes, 1979;
Leatherman, 1979; Leatherman, 1985) have shown that
the formation and development of barrier islands are
a result of multiple processes.
Boundary constraints
Although the environmental conditions required for formation of barrier islands are relatively flexible, there still
exist some boundary constraints. According to the statistics from Stutz and Pilkey (2011), barrier islands are most
abundant (~63 % of the total) on tectonically stable,
trailing edge continental margins as such environments
provide favorable boundary conditions (e.g., abundant
sediment supply, small ratio of tidal range to mean wave
height) for the formation of barrier islands. Of the
remaining barrier islands, ~21 % are located on marginal
seas, and only ~16 % are found on collision margins. Most
(~58 %) of the barrier islands existing on collision margins
are developed on delta lobes favored by a low-gradient
shoreface produced by abundant riverine sediment input;
the rest are located on wide coastal plains. Barrier islands
rarely form on narrow continental shelves with an upper
shoreface slope larger than 0.8
, in which sediment tends
to move offshore rather than accumulating onshore.
Another significant boundary factor influencing the formation and development of barrier islands is sea-level
change. A stable sea level is a prerequisite for the formation
of barrier islands. Most barrier islands are quite young,
being formed during the last ~6,000 years when the global
sea level became relatively stable with only minor fluctuations. A stable sea level with small rates of change (within
millimeters per year) in the mid- to late Holocene restricts
tides and wave actions to a small-range coastal area (i.e.,
hydrodynamically active zone). Sediment transport within
this area became increasingly important to shape the modern coastline. Driven by wave and aeolian processes, an
excess of sediment supply to a local accommodation zone
would eventually build up new land above the water surface. Holocene barrier spits and islands present such examples. Holocene barrier islands are low-lying structures made
of unconsolidated sediment, with the highest part at the
dune crests, which is normally only meters above the water
level. Thus they are quite vulnerable to high water-stand
impacts induced by storms or floods. Without sufficient
sediment supply to compensate the increased accommodation zone, continuous sea-level rise would cause a barrier
island to shrink and migrate landward.
Besides a sufficient sediment supply to feed the formation of a barrier island, the “quality” of the sediment supply is also critical for the fate of the island. Sediment
supply with a larger proportion of sand and coarse material
is able to sustain stronger hydrodynamic impacts than fine
sediment such as mud and clay. Thus, sandy substrate and
shoreface are more durable than a muddy one to maintain
a barrier island. The wave-tide regime is also an important
factor influencing the morphogenesis of barrier islands.
Beaches and barrier islands are products of wave action.
They develop most easily on wave-dominated coasts with
small to moderate tidal range. Only ~12 % of barrier
islands develop in tide-dominated regions (with the ratio
of mean tidal amplitude to mean annual wave height generally larger than three according to Davis and Hayes,
1984), and they are rarely found in areas with a tidal range
larger than 4 m.
Barrier island morphodynamics
Among different types of coastal landforms, barrier
islands have the most variable morphology. They are constantly shaped by winds, tides, and waves and, on a longer
time scale, can shift landward or seaward due to oscillations of sea level and variations in the sediment supply
(Masetti et al., 2008).
Depending on the relative importance of waves to tides
in determining the coastal morphology, three types of
coastal environments can be classified: wave dominated,
tide dominated, and mixed energy. In wave-dominated
coasts, barrier islands are elongate and narrow due to the
impact of longshore drift. Inlets produced by tides or storm
breaching migrate fast for the same reason. Washover features are prominent, and flood deltas are well developed
but ebb deltas are small or nonexistent (Hayes, 1979).
Along with an increase of tidal effect, inlets play a more significant role in shaping the barrier island morphology. Substantial ebb deltas can develop, and barrier islands become
shorter and wider as a result. As tidal range increases, these
features become more prominent. When the tidal range is
high enough and overwhelms the wave effects, barrier
islands cannot develop and inlet deltas are confined to elongated stringers oriented with the dominant tidal currents.
In addition to tide and wave actions, development of
barrier islands is also affected by other processes (e.g., stochastic extreme events, sea-level change, tectonic movements, and fluvial input). Barrier islands evolve and
migrate parallel or normal to the mainland in response to
these processes. The shore-normal evolution of barrier
islands corresponds to two types of behavior: namely,
regression and transgression, respectively. Barrier transgression refers to an onshore migration of the landform
and an overlapping of deeper water sediment over
shallower lagoon deposits. Leatherman (1979) summarized three main processes controlling barrier island transgression, which, in the order of importance, are inlet
dynamics, overwash, and aeolian transport. In some areas
with a thick and compressible substrate (e.g., the Virginia
barrier coast), auto-compaction also contributes to the barrier island transgression (Leatherman, 1985). In response
to the increased impacts of these processes induced by an
eustatic sea-level rise, three modes have been proposed
to describe a subsequent evolution of a barrier island:
(1) a continuous landward migration across the underlying
substrate to higher elevations; (2) a disintegration of the
island due to insufficient sediment supply and backshore
relief to sustain inundation during stochastic extreme
BARRIER ISLAND
49