of coastal landforms were influenced by various factors,
viz., the coastal processes, sea-level changes, and tectonics.
These landforms are modified by a variety of dynamic processes and the driving forces include framework geology,
oceanographic processes, river-mouth processes, sediment
supply, and human activity (FitGerald et al., 2008). Coastal
landforms are extremely variable and coastal habitats
change over a range of spatial and temporal scales, and recognition of these variations is necessary for effective planning and management (Woodroffe, 2007). Barrier islands,
wetlands, and other parts of coastal systems might have
a threshold, and, when the limits of threshold are exceeded,
the landforms become unstable and prone to irreversible
changes in form and position (Williams and Gutierrez,
2009).
Coastal systems exhibit two distinct types of coastal
landforms: depositional and erosional. Erosional coastal
landforms typically exhibit high relief and rugged topography, which include sea cliffs, wave-cut platforms, and
stacks. The depositional coastal landforms include barrier
islands, beach ridges, cheniers, tidal flats, mudflats, etc. In
this chapter, we focus on the depositional coastal landforms and their characteristics.
Barrier Islands
Coastal barriers and spits are often regarded as similar
coastal forms in terms of beach deposition projecting
across coastal bays. While barriers tend to bridge the bay
by joining the mainland at each end, spits are only
attached at one end. However, many barriers show crossbarrier breaks or breaches through which the sea may enter
on a permanent or intermittent basis, thus forming barrier
islands (Figure 1). Coastal barriers are complex constructional morphological features involving deposition by
waves, wave-generated currents, tidal currents, and wind
activity (Hayes, 1979). A barrier exhibits two
morphodynamic units – a seaward beach face and
a landward facing back-barrier slope – and these two units
develop when the barrier is gravel dominated (Orford
et al., 1996). As sand becomes the dominant component,
a third environment comprising aeolian dunes can appear
at the top of the beach face (barrier crest) and spread onto
the backslope. Current flows may have been responsible
for the initial submarine platform under the barrier, but
with wave action forcing, onshore migration of the barrier
takes place in combination with fine sedimentation characteristics of the low-energy back-barrier bay. Tidal currents also become dominant once barrier islands appear.
Sediment supply and the type of sediment are major
controls on barrier development with which a behavioral
distinction can be drawn between sand-dominated barriers
and gravel-dominated barriers. This distinction has
a spatial basis with gravel-dominated barriers being more
prevalent in mid-upper latitudes compared to sanddominated barriers, which reflects the greater potential
of coarse material in high latitudes as a residue of late
Quaternary glacigenic processes.
Coastal bars
Coastal bars can be broadly defined as aggradational
ridges of sediments whose formation, morphology, and
behavior are determined by interactions between waves,
currents, tides, local slope, and grain size. Bars occur
along beach, river delta, estuary, and continental shelf
environments with a wide range of sizes, types, and orientation (Figures 1 and 2). Beach morphology undergoes
cycling change, promoting offshore sediment transport
and bar formation during winter; while during summer
when the oceanographic conditions are calmer, the landward migration of the bar and eventual welding to the
beach face take place. However, the existence of such
“winter” and “summer” profiles is not universal, as both
barred and non-barred profiles occur at all times in
some areas, while in others only one type may persist
throughout the year. Furthermore, cyclic beach response
at timescales much shorter than seasons can result
in barred profiles (Short, 1979). Types of bars are often
distinguished based on their alongshore planform shape
and orientation relative to the shoreline as linear, shoreparallel, sinuous, or crescentic with a trough separating
them from the shoreline. Some of the coastal areas consist
of alternating transverse bars, welded to the shoreline and
are separated by channels occupied by rip currents
(Figure 2). Bar type is strongly related to wave energy
level with linear bars developing under high-energy
conditions, crescentic bars during intermediate energy,
and transverse bars during lower wave energy levels.
Under very low-energy conditions, a bar may become
fully welded to the beach and appear as a flat terrace at
low tide. These types of bar configurations are common
on microtidal beaches and may grade into each other as
energy levels vary. A number of classifications
exist describing both bar types and the continuum of bar
evolution (e.g., Greenwood and Davidson-Arnott, 1979;
Short and Aagaard, 1993; Wijnberg and Kroon, 2002).
Coastal lagoons
The term lagoon describes a stretch of salt water
separated from the sea by a low sandbank or coral reef
(Figures 1 and 3). Coastal lagoons (Figure 3) are mostly
estuarine, usually shallow, and have generally been partly
or wholly sealed off from the sea by the deposition of spits
or barriers, by localized tectonic subsidence, or by the
growth of coral reefs. They are best formed on transgressive coasts, particularly where the continental margin has
a low gradient, and sea-level rise is slow. The lagoons
are ephemeral features and their depths and areal extent
gradually decrease due to sedimentation from inflowing
rivers, as well as accumulation of sediment washed in
from the sea, wind-blown material, and chemical and
organic deposits. Lagoons range in size from less than
a kilometer to more than a 1000 km, and they occur on
about 12 % of the length of the world’s coastline (Bird,
2000). They can be classified on the basis of infilling or
increasing in size (Nichols, 1989). The infill of some
144
COASTAL LANDFORMS
viz., the coastal processes, sea-level changes, and tectonics.
These landforms are modified by a variety of dynamic processes and the driving forces include framework geology,
oceanographic processes, river-mouth processes, sediment
supply, and human activity (FitGerald et al., 2008). Coastal
landforms are extremely variable and coastal habitats
change over a range of spatial and temporal scales, and recognition of these variations is necessary for effective planning and management (Woodroffe, 2007). Barrier islands,
wetlands, and other parts of coastal systems might have
a threshold, and, when the limits of threshold are exceeded,
the landforms become unstable and prone to irreversible
changes in form and position (Williams and Gutierrez,
2009).
Coastal systems exhibit two distinct types of coastal
landforms: depositional and erosional. Erosional coastal
landforms typically exhibit high relief and rugged topography, which include sea cliffs, wave-cut platforms, and
stacks. The depositional coastal landforms include barrier
islands, beach ridges, cheniers, tidal flats, mudflats, etc. In
this chapter, we focus on the depositional coastal landforms and their characteristics.
Barrier Islands
Coastal barriers and spits are often regarded as similar
coastal forms in terms of beach deposition projecting
across coastal bays. While barriers tend to bridge the bay
by joining the mainland at each end, spits are only
attached at one end. However, many barriers show crossbarrier breaks or breaches through which the sea may enter
on a permanent or intermittent basis, thus forming barrier
islands (Figure 1). Coastal barriers are complex constructional morphological features involving deposition by
waves, wave-generated currents, tidal currents, and wind
activity (Hayes, 1979). A barrier exhibits two
morphodynamic units – a seaward beach face and
a landward facing back-barrier slope – and these two units
develop when the barrier is gravel dominated (Orford
et al., 1996). As sand becomes the dominant component,
a third environment comprising aeolian dunes can appear
at the top of the beach face (barrier crest) and spread onto
the backslope. Current flows may have been responsible
for the initial submarine platform under the barrier, but
with wave action forcing, onshore migration of the barrier
takes place in combination with fine sedimentation characteristics of the low-energy back-barrier bay. Tidal currents also become dominant once barrier islands appear.
Sediment supply and the type of sediment are major
controls on barrier development with which a behavioral
distinction can be drawn between sand-dominated barriers
and gravel-dominated barriers. This distinction has
a spatial basis with gravel-dominated barriers being more
prevalent in mid-upper latitudes compared to sanddominated barriers, which reflects the greater potential
of coarse material in high latitudes as a residue of late
Quaternary glacigenic processes.
Coastal bars
Coastal bars can be broadly defined as aggradational
ridges of sediments whose formation, morphology, and
behavior are determined by interactions between waves,
currents, tides, local slope, and grain size. Bars occur
along beach, river delta, estuary, and continental shelf
environments with a wide range of sizes, types, and orientation (Figures 1 and 2). Beach morphology undergoes
cycling change, promoting offshore sediment transport
and bar formation during winter; while during summer
when the oceanographic conditions are calmer, the landward migration of the bar and eventual welding to the
beach face take place. However, the existence of such
“winter” and “summer” profiles is not universal, as both
barred and non-barred profiles occur at all times in
some areas, while in others only one type may persist
throughout the year. Furthermore, cyclic beach response
at timescales much shorter than seasons can result
in barred profiles (Short, 1979). Types of bars are often
distinguished based on their alongshore planform shape
and orientation relative to the shoreline as linear, shoreparallel, sinuous, or crescentic with a trough separating
them from the shoreline. Some of the coastal areas consist
of alternating transverse bars, welded to the shoreline and
are separated by channels occupied by rip currents
(Figure 2). Bar type is strongly related to wave energy
level with linear bars developing under high-energy
conditions, crescentic bars during intermediate energy,
and transverse bars during lower wave energy levels.
Under very low-energy conditions, a bar may become
fully welded to the beach and appear as a flat terrace at
low tide. These types of bar configurations are common
on microtidal beaches and may grade into each other as
energy levels vary. A number of classifications
exist describing both bar types and the continuum of bar
evolution (e.g., Greenwood and Davidson-Arnott, 1979;
Short and Aagaard, 1993; Wijnberg and Kroon, 2002).
Coastal lagoons
The term lagoon describes a stretch of salt water
separated from the sea by a low sandbank or coral reef
(Figures 1 and 3). Coastal lagoons (Figure 3) are mostly
estuarine, usually shallow, and have generally been partly
or wholly sealed off from the sea by the deposition of spits
or barriers, by localized tectonic subsidence, or by the
growth of coral reefs. They are best formed on transgressive coasts, particularly where the continental margin has
a low gradient, and sea-level rise is slow. The lagoons
are ephemeral features and their depths and areal extent
gradually decrease due to sedimentation from inflowing
rivers, as well as accumulation of sediment washed in
from the sea, wind-blown material, and chemical and
organic deposits. Lagoons range in size from less than
a kilometer to more than a 1000 km, and they occur on
about 12 % of the length of the world’s coastline (Bird,
2000). They can be classified on the basis of infilling or
increasing in size (Nichols, 1989). The infill of some
144
COASTAL LANDFORMS