sea-level rise, and analysis and predictive methods of
shoreline changes.
Classification of shorelines
Depositional and erosional coastlines
A sedimentary environment and its sub-environments in
a coastal environment can be either erosional or depositional. Erosion, progradation, and reworking of sediments
are important shore and nearshore processes that modify
coastlines. The details are discussed below.
Submergent and emergent coasts
In some coastal areas where uplift occurs, the sea level
falls fast relative to the land. Sea level will rise when the
coastal region subsides, resulting in a net change in sea
level of as much as 30 cm per century.
Wave-dominated coasts
Wave-dominated coasts, comprising accumulations of
detrital sands, undergo high levels of physical reworking
interspersed with periods of burial before finally being
deposited as present-day coastal deposits. Waves and
wave-induced currents are the dominant mechanisms for
moving and depositing sands on shorefaces and beaches
of the open coast, although winds, river discharge, tidal
currents, and Ekman flows act as transporting agents landward of the beach in estuaries and seaward of the
shoreface (Roy et al., 1994). In relation to the shoreface
and beach, open coastal types are determined by four
factors: (1) substrate gradient, (2) wave versus tidal range,
(3) sediment supply versus accommodation (Swift and
Thorne 1991), and (4) rates of sea-level change. At one
end are steep, high-energy, sediment-deficient coasts,
and at the other end are low-gradient, low-energy coasts
(Roy et al., 1994). The behavior of wave-dominated coasts
can be explained under two concepts: (1) geological inheritance or imprint of various land-forming processes that
have operated for a long period of geological time and
(2) wave-formed coastal deposits operating over shorter
periods of time (Roy et al., 1994).
Clastic and carbonate coasts
Clastic coasts are characterized by the relative abundance
of river materials and reworked by waves and tides. Carbonate coasts are those where calcareous sediment is produced, transported, deposited, lithified, and eroded. They
significantly differ from clastic coasts.
Paleoshorelines
A general history of relative changes in sea level and
shoreline migration has been documented on the continental shelves of the world oceans. Relic barrier shorelines of
Pleistocene age have been widely reported landward of
their modern analogues, while submerged Holocene barrier remnants are common features on modern continental
shelves. Barrier shorelines are characteristic features of
depositional environments. Barriers can originate from
a topographical ridge along the landward side of a beach
which may subsequently be partly submerged. The other
important characteristic feature of paleoshorelines is
cheniers, originally described by Howe et al. (1935) as
long, narrow, sandy ridges rising above the surrounding
marshes and forming the most conspicuous topographic
features along the southwestern Louisiana coast. Cheniers
are characterized by gently dipping littoral, sublittoral and
washover deposits, and some overlying dunes. Cheniers
and barrier shorelines can be identified by the shape and
extent of their respective deposits. A fundamental difference in the depositional process is the proximity of
a sizeable sediment source which can overload the distributive fores, viz., waves and tidal currents, along the adjacent shoreline.
Shoreline changes due to physical forcing
Shoreline processes operate on a narrow zone, and therefore the shoreline migrates landward or seaward
depending on changing sea level or uplift or subsidence
of coastal regions. These cyclic and noncyclic processes
change the position of the shoreline over various time
scales, from the daily and seasonal interaction of winds
and waves to over thousands of years due to secular
sea-level changes. During sea-level transgression, for
example, the shoreline migrates landward and vice versa
during sea-level regression. Furthermore, shoreline
changes are not constant through time and frequently
reverse in sign, i.e., accretion to erosion, or vice versa.
Most shorelines undergo patterns of erosion and accretion
on a daily and seasonal basis and may be unidirectional or
cyclic on a long-term basis. In the process, beach and nearshore sediments are deposited over vast regions. However,
the physical processes such as tides, waves, and nearshore
currents are most important for modifying shorelines.
Tides and tidal currents
In response to the gravitational attraction of the moon and
sun, some shorelines experience two almost equal high
tides and two low tides each day, called a semidiurnal tide.
Some locations experience only one high and one low tide
each day (called a diurnal tide), whereas some other locations experience two uneven tides a day or sometimes
one high and one low each day (called a mixed tide).
These regular fluctuations in the estuarine or ocean surface
vary from a few centimeters to >15 m. A complete tidal
cycle includes a flood tide that progressively covers
more and more of a nearshore area until high tide is
reached, followed by ebb tide, during which the nearshore
area is once again exposed. These regular fluctuations in
sea level constitute one largely untapped source of energy
as do waves, currents, and temperature differences in
seawater.
Tidal ranges are also affected by shoreline configuration. In offshore areas, where the direction of flow is not
restricted by any barriers, the tidal current is rotary; that
is, it flows continuously, with the direction changing
SHORELINE CHANGES
591
shoreline changes.
Classification of shorelines
Depositional and erosional coastlines
A sedimentary environment and its sub-environments in
a coastal environment can be either erosional or depositional. Erosion, progradation, and reworking of sediments
are important shore and nearshore processes that modify
coastlines. The details are discussed below.
Submergent and emergent coasts
In some coastal areas where uplift occurs, the sea level
falls fast relative to the land. Sea level will rise when the
coastal region subsides, resulting in a net change in sea
level of as much as 30 cm per century.
Wave-dominated coasts
Wave-dominated coasts, comprising accumulations of
detrital sands, undergo high levels of physical reworking
interspersed with periods of burial before finally being
deposited as present-day coastal deposits. Waves and
wave-induced currents are the dominant mechanisms for
moving and depositing sands on shorefaces and beaches
of the open coast, although winds, river discharge, tidal
currents, and Ekman flows act as transporting agents landward of the beach in estuaries and seaward of the
shoreface (Roy et al., 1994). In relation to the shoreface
and beach, open coastal types are determined by four
factors: (1) substrate gradient, (2) wave versus tidal range,
(3) sediment supply versus accommodation (Swift and
Thorne 1991), and (4) rates of sea-level change. At one
end are steep, high-energy, sediment-deficient coasts,
and at the other end are low-gradient, low-energy coasts
(Roy et al., 1994). The behavior of wave-dominated coasts
can be explained under two concepts: (1) geological inheritance or imprint of various land-forming processes that
have operated for a long period of geological time and
(2) wave-formed coastal deposits operating over shorter
periods of time (Roy et al., 1994).
Clastic and carbonate coasts
Clastic coasts are characterized by the relative abundance
of river materials and reworked by waves and tides. Carbonate coasts are those where calcareous sediment is produced, transported, deposited, lithified, and eroded. They
significantly differ from clastic coasts.
Paleoshorelines
A general history of relative changes in sea level and
shoreline migration has been documented on the continental shelves of the world oceans. Relic barrier shorelines of
Pleistocene age have been widely reported landward of
their modern analogues, while submerged Holocene barrier remnants are common features on modern continental
shelves. Barrier shorelines are characteristic features of
depositional environments. Barriers can originate from
a topographical ridge along the landward side of a beach
which may subsequently be partly submerged. The other
important characteristic feature of paleoshorelines is
cheniers, originally described by Howe et al. (1935) as
long, narrow, sandy ridges rising above the surrounding
marshes and forming the most conspicuous topographic
features along the southwestern Louisiana coast. Cheniers
are characterized by gently dipping littoral, sublittoral and
washover deposits, and some overlying dunes. Cheniers
and barrier shorelines can be identified by the shape and
extent of their respective deposits. A fundamental difference in the depositional process is the proximity of
a sizeable sediment source which can overload the distributive fores, viz., waves and tidal currents, along the adjacent shoreline.
Shoreline changes due to physical forcing
Shoreline processes operate on a narrow zone, and therefore the shoreline migrates landward or seaward
depending on changing sea level or uplift or subsidence
of coastal regions. These cyclic and noncyclic processes
change the position of the shoreline over various time
scales, from the daily and seasonal interaction of winds
and waves to over thousands of years due to secular
sea-level changes. During sea-level transgression, for
example, the shoreline migrates landward and vice versa
during sea-level regression. Furthermore, shoreline
changes are not constant through time and frequently
reverse in sign, i.e., accretion to erosion, or vice versa.
Most shorelines undergo patterns of erosion and accretion
on a daily and seasonal basis and may be unidirectional or
cyclic on a long-term basis. In the process, beach and nearshore sediments are deposited over vast regions. However,
the physical processes such as tides, waves, and nearshore
currents are most important for modifying shorelines.
Tides and tidal currents
In response to the gravitational attraction of the moon and
sun, some shorelines experience two almost equal high
tides and two low tides each day, called a semidiurnal tide.
Some locations experience only one high and one low tide
each day (called a diurnal tide), whereas some other locations experience two uneven tides a day or sometimes
one high and one low each day (called a mixed tide).
These regular fluctuations in the estuarine or ocean surface
vary from a few centimeters to >15 m. A complete tidal
cycle includes a flood tide that progressively covers
more and more of a nearshore area until high tide is
reached, followed by ebb tide, during which the nearshore
area is once again exposed. These regular fluctuations in
sea level constitute one largely untapped source of energy
as do waves, currents, and temperature differences in
seawater.
Tidal ranges are also affected by shoreline configuration. In offshore areas, where the direction of flow is not
restricted by any barriers, the tidal current is rotary; that
is, it flows continuously, with the direction changing
SHORELINE CHANGES
591