Environment Problems in the Coastal Zone

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Environment Problems in the Coastal
Chairs: Hideo Sekiguchi and Sanit Aksornkoae
3.1 Coastal Characteristics and Changes in Coastal
Yoshiki Saito
Geological Survey of Japan, AIST,
Central 7, Higashi 1-1-1, Tsukuba, Ibaraki,
305 8567 Japan

Understanding coastal dynamics and natural history is important in developing
a better understanding of natural systems and human impacts in coastal
zones. This chapter outlines the characteristics of sedimentary environments
in coastal zones which must be understood in order to manage and preserve
coastal environments.
3.1.1 Coastal Classification, Shoreline Migration,
and Controlling Factors

The world’s coastal environments and topography are classified into two types
on the basis of the changes which occurred during the Holocene when they
were particularly influenced by millennial-scale sea-level changes. Transgres-
sive coastal environments, where shorelines migrate landward, are characterized
by barriers, estuaries, and drowned valleys (Boyd et al., 1992). Regressive
coastal environments, where shorelines migrated seaward, consist of deltas,
strand plains, and chenier plains (Fig. 3.1.1).
Thus regressive shorelines at river mouths are called deltas, while trans-
gressive shorelines at river mouths are called estuaries. The latter consist of
drowned, incised valleys. In regressive environments, coastal lagoons sepa-
rated from the open ocean by barriers are well developed alongshore, whereas
estuaries cross the general coastline. A strand plain is a coastal system that
develops along a wave-dominated coast; it is characterized by beach ridges, a
N. Mimura (ed.), Asia-Pacific Coasts and Their Management: States of Environment.
© Springer 2008

H. Sekiguchi and S. Aksornkoae
FIG. 3.1.1. Coastal depositional systems. (After Boyd et al., 1992.)
foreshore, and a shoreface. A chenier plain is composed of muddy tidal flats
with isolated sand or shelly ridges that form episodically.
The global distribution of these coastal systems is controlled mostly by relative
sea-level changes, particularly eustatic sea-level changes and glacio- and
hydro-isostasy. After the last glacial maximum (LGM), about 20,000 years
ago, eustatic sea level (global sea-level changes) rose until 4,000 years ago,
and since then the sea level has been comparatively stable. However, a relative
(observed) sea-level change is locally determined by the combination of these
eustatic sea-level changes, isostatic effects of glaciers (glacio-isostasy)
and meltwater (hydro-isostasy), and local factors (e.g., tectonics, human-
caused subsidence etc.). Glacio-isostasy and hydro-isostasy have strongly
impacted Holocene sea-level changes on a global scale. In glacio-isostasy,
areas surrounding regions glaciated during the LGM that bulged because
of glacial loading, have since subsided; therefore, in such areas, the relative
sea level has risen on a millennial timescale. Thus the mid to southern parts
of North America, mid to southern Europe, and the Mediterranean region
have experienced a rising sea level through the Holocene as a result of glacio-
isostasy. The relative sea level has risen in these regions at a rate of ca.1 m/ky
for the last 7,000 years; therefore, transgressive systems are found in these
areas. On the other hand, most of Asia, Oceania, central to southern Africa,
and South America were far from glaciers during the LGM. Hence, although

3. Environment Problems in the Coastal Zone
direct influence from glaciers is less significant, the isostatic effects of melt-
water loads (the increased loading of seawater on the mantle) have also led to
Holocene sea-level changes in these regions. As a result of movement of the
mantle from beneath the ocean floor to under continental areas, land areas
have uplifted on a millennial timescale, resulting in a relative sea-level fall of
2–3 m during the last 6,000–7,000 years. Therefore, regressive coastal systems
are well developed in these areas. Most lagoons and estuaries that formed in
these regions during the early Holocene have been filled or abandoned during
the subsequent sea-level fall.
Sediment supply is also a key factor controlling shoreline migration. Although
the general distribution of coastal systems is controlled by relative sea-level
changes, the amount of sediment supply also influences shoreline migration.
Even when the relative sea level is rising, the shoreline may migrate seaward
if the sediment supply is high. The Mississippi and Nile deltas, both located
where sea level rose through the Holocene, are good examples of regressive
coastal systems developed during a sea-level rise. Conversely, along Australian
coasts, estuaries are well developed even though the relative sea level has fallen
over the last 6,000 years. The estuaries and lagoons that formed during the
early Holocene have persisted, remaining unfilled because of the very low sedi-
ment supply from that dry and ancient continent (Saito 2001, 2005b).
Figure 3.1.2 summarizes the relationship between sediment supply and
relative sea-level change with regard to shoreline migration. Barriers and
estuaries are typical coastal features when the shoreline is migrating land-
ward. A rise in sea level causes marine inundation of the incised valleys that
formed during the sea-level lowstand, resulting in the formation of drowned
valleys and estuaries. The sand composing the barriers is supplied mostly
from coastal erosion at headlands and by recycling marine sand, because river
mouths are in retreat during transgressive periods. As a result the distribution
of riverine sand is generally limited to within the estuarine head. The sea-level
rise leads to an increase in wave energy along coasts because of the increase
FIG. 3.1.2. Factors controlling shoreline migration. (Modified after Curray,1964.)

H. Sekiguchi and S. Aksornkoae
in water depth, resulting in increases in both coastal erosion and the sediment
supply to barriers.
Shoreline migration is controlled mostly by sea-level changes and sediment
supply. However, even if sea level is stable or falling, a shoreline with little sediment
supply is likely to migrate landward. On a wave- or storm-dominated coast,
the nearshore zone is typically erosional because of wave action. Transgres-
sion may thus occur along such coasts even during periods of falling sea level.
Sea cliffs developed along some coasts during the Holocene illustrate this
In addition to sea-level changes and sediment supply, waves and tide are
important controlling factors in coastal environments. This is because they
move sediment particles, resulting in deposition or erosion. The main difference
between waves and tide in terms of general sediment movement normal to
the shoreline is the direction of sediment movement. During storms, except
for washover sediments deposited on the land, most sediments are moved
seaward by offshore bottom currents. These, in combination with gravitational
sediment movement in the nearshore zone (shoreface or delta front slope),
result in an erosional environment. Energetic conditions affecting the bottom
sediments increase landward. The foreshore (intertidal zone) experiences the
highest wave energy (wave swash and backwash). The wave influence decreases
offshore, resulting in offshore fining of the sediments.
On the other hand, tidal currents cause asymmetric sediment movements
and tend to move sediments landward. Flood tidal currents result in more
landward movement of sediment than do ebb tidal currents, a phenomenon
known as tidal pumping. The amount of energy available for sediment movement
depends on the tidal current, particularly in terms of water depth. Energetic
conditions increase offshore, resulting in onshore fining of the sediments.
Therefore, on a tide-dominated coast, sediment is accreted onto coasts, and
finer sediments are found landward and coarser sediments offshore.
3.1.2 Wave- or Storm-Dominated Coast
On a wave- or storm-dominated coast, the coastal zone from onshore to
offshore consists of dunes, backshore, foreshore, upper shoreface, lower
shoreface and shelf. In general, the shoreface zones have the steepest gradi-
ent on the shelf, forming a step between the onshore plain and the shelfal
Coastal Sediments and Their Succession
On accumulating or progradational beaches, the succession of coastal sedi-
ments consists, in ascending order, of lower shoreface, upper shoreface,
foreshore, backshore, and dunes (Saito 1989, 2005a, Fig. 3.1.3). This is the
typical succession on a wave- or storm-dominated sandy coast. The shore-
face, located in the nearshore zone, has a concave topography created by
wave action. The upper shoreface, also called the inshore, is characterized

3. Environment Problems in the Coastal Zone
FIG. 3.1.3. Coastal features on a wave- and storm-dominated coast. (Modified after
Saito, 1989.)
by bar and trough topography as a result of being constantly influenced by
waves and wave-induced currents. Rip currents and the landward or sea-
ward migration of bars result in the tabular and trough cross-stratification
that characterize upper shoreface sediments. Two- and three-dimensional
wave ripple structures are also commonly found. These sedimentary facies
reflect mostly fair-weather wave conditions. The upper shoreface sediments
overlie the lower shoreface sediments, which are characterized by swaley
cross-stratification (SCS) or hummocky cross-stratification (HCS). HCS
displays low-angle (less than 15°) erosional lower set boundaries with sub-
parallel and undulatory laminae that systematically thicken laterally, and
scattered lamina dip directions (Harms et al., 1975). SCS is amalgamated
HCS with abundant swaley erosional features. These sedimentary structures
are thought to be formed by the oscillatory currents of storm waves inter-
acting with offshore-directed currents.
During storms beaches are eroded and longshore bars migrate seaward.
Strong (long-period) oscillatory currents caused by storm waves agitate
sea-bottom sediments at the shoreface. Some of the sediments are transported
offshore by bottom currents caused by coastal set up and gravity currents.
Oscillatory currents related to calming storm waves produce HCS/SCS in the
shoreface to inner shelf region overlain by wave ripple lamination. HCS and
SCS are found only in sediments composed of coarse silt to fine sand. Similar wave
conditions form large dunes in coarse-grained sediments. As lower shoreface
sediments are deposited mainly during storms, there is a sharp boundary
between upper and lower shoreface sediments. This is formed by bar migration,

H. Sekiguchi and S. Aksornkoae
The lower shoreface topography depends on the inner-shelf topography.
Because typical shoreface topography can form only on a gently sloping to
flat basal surface, no clear shoreface topography can form in the steep shelf
regions of active plate margins. Thus, sometimes only the upper shoreface is
referred to as the shoreface. In middle latitudes, typical storms are summer
typhoons and winter storms. However, tropical regions closer to the equator do
not experience such storms. Therefore, wave conditions and sediment distribution
in tropical regions are different from those of middle latitudes. Development
of bars and troughs is weak, and they are located at much shallower depth
than storm-dominated coasts in middle latitudes.
The coastal succession and sedimentary facies reflect the current velocities
under fair-weather and storm conditions as well as seaward-decreasing energy
conditions. Under fair-weather conditions the bedforms (sedimentary struc-
tures) found from the foreshore to the upper and lower shoreface are upper plane
beds (parallel lamination), 3D and 2D subaqueous dunes (trough and tabular
cross-bedding, respectively), and 3D and 2D ripples (ripple lamination). On the
other hand, under storm conditions, beaches are eroded and the lower shoreface
resembles an upper flow regime characterized by long-period oscillatory waves,
resulting in the formation of HCS and SCS. Ripples are formed in shelf regions.
The preservation potential of storm deposits is higher than that of sediments
deposited during fair weather, particularly in the lower shoreface and offshore
areas. However, in tropical regions, sediments deposited under fair-weather con-
ditions are relatively well preserved because storms are infrequent.
Key Boundaries and the Mud Line
There are three important boundaries on storm- or wave-dominated coasts: one
between the upper and lower shoreface, one between the lower shoreface and
the inner shelf, and one at approximately 50–60 m water depth on the shelf.
The upper shoreface is characterized by longshore currents and along-
shore sediment movement. On the upper shoreface, longshore bars
migrate frequently. They often move landward during fair weather, carried
by breakers. The positions of the outermost longshore bars are relatively
stable. These bars are thought to be formed during storm waves. Sediments
in the upper shoreface are relatively coarse grained, forming dunes and 2D
and 3D ripples. Thus active morphological change and sediment movement
are typical on the upper shoreface. However, they are not typical on the
lower shoreface under fair-weather conditions. Small ripples are often
found, but alongshore sediment movement is not active in the lower shore-
face. Most sediments are storm generated (HCS/SCS). These differences
between the upper and lower shoreface result in a clear erosional bound-
ary and time gap. The water depth of this boundary ranges from 4 to 8 m,
depending on wave conditions.
Under fair-weather conditions sediment movement and its budget form
a closed system in the upper shoreface. The closure depth is located at the
boundary between the upper and lower shoreface. However, during storms,

3. Environment Problems in the Coastal Zone
when some foreshore and upper shoreface sediments are transported offshore,
the closure depth is deepened. Therefore, the sediment budget of the upper
shoreface in a shore-normal section is fixed during fair weather, and negative
during storms because of sediment loss due to offshore transport. If the sedi-
ment supply to the upper shoreface is not enough to compensate for the
sediment loss by alongshore sediment movement, coastal erosion will occur
along such a coast. As alongshore sediment transport for sands and gravels
occurs only in the upper shoreface zone, it is important that coastal structures
such as groins and jetties do not cross the whole of the upper shoreface zone
and cut off alongshore sediment movement. If the depth and length of such
structures are such that the upper shoreface is blocked, sediments will not be
transported downcurrent beyond the structures, resulting in coastal erosion
in downcurrent areas. Most human-caused coastal erosion is the result of ces-
sation of alongshore sediment transport.
The second important boundary is between the lower shoreface and the
shelf. Wave ripples are often found in a lower shoreface. These are composed
of fine to very fine sand. Muddy sediments are rare in the lower shoreface. The
mud line is usually defined as the most landward boundary of muddy areas. If
it is between the shoreface and shelf it is called the nearshore mud line. This
depth is very important because it is regarded as the fair-weather wave base
for sediment movement. This boundary is at about 15 m water depth on a
storm-dominated coast in middle latitudes (coasts facing the Pacific Ocean or
the Japan Sea) and at less than 10 m water depth on a wave-dominated coast
at low tropical latitudes.
The last boundary is the storm wave base. There are two kinds of storm
wave base. One is for sediment movement by storm waves, and the other is for
bottom erosion. The erosional wave base is deeper than that for simple sediment
movement. The erosional wave base is regarded as the maximum depth
of bottom sediment movement of 0.5-mm sand grains caused by storm waves.
It is thought to be at 50–60 m water depth in areas facing the open ocean. The
storm wave base also coincides with the boundary between neritic sand
and offshore mud when the inner shelf is steep and shoreface topography is
not clear. This mud line is known as an offshore mud line.
All of the above characteristics apply to sandy coasts. However, on
coasts that receive abundant mud, sediment distributions are different. In
general, sediments from foreshore to shoreface are finer than on sandy
coasts. A common characteristic of both sandy and muddy coasts is that
the coarsest sediments are found around the boundary between the upper
and lower shoreface.
Sediment Sources
Understanding sediment sources is an important prerequisite to the devel-
opment of countermeasures against coastal erosion. There are three major
sediment sources for coastal sediments: rivers, sediment supplied by coastal
erosion from coastal cliffs or headlands, and recycled marine sediment. Most

H. Sekiguchi and S. Aksornkoae
sands and gravels supplied by rivers are deposited in the river-mouth area,
except for hyperpycnal flows. Sands deposited in the upper shoreface or delta
front platform are removed by waves and transported alongshore by long-
shore currents, forming bars and foreshore deposits, or offshore by storm
waves and offshore-directed bottom currents. Sands supplied from sea cliffs
and headlands are also transported alongshore. These point-source sands are
transported alongshore and accreted onto the foreshore (beaches), resulting in shore-
line migration seaward. However, intense storms pick up these sediments and
transport them offshore. Thus, these coastal sediments are regarded as a line
source of offshore sediments. Sediment recycling is very important on both
wave-dominated and low-energy coasts. Barriers in the northeastern Gulf of
Mexico and in the Wadden Sea of the North Sea are composed of recycled
sands. Some barriers are maintained during transgression by the recycling of
both overwash sediments and sediments of retreating barriers. However, at
present, some beaches on barriers in the Wadden Sea are maintained by beach
nourishment. Mud sediment sources are also from rivers and coastal erosion.
Most mud is transported in suspension via various pathways to the offshore.
Coastal erosion occurs as a result of an imbalance between sediment supply and
removal. The construction of jetties, groins, and harbors interrupts along-
shore sediment transport, resulting in a decrease in the sediment supply.
Seasonal wind changes (e.g., in a monsoon climate) cause the direction and
strength of alongshore sediment transport to change. A decrease in the sedi-
ment discharge of rivers due to dam construction, irrigation, or sand mining
in channels and river banks is also a cause of coastal erosion. An increase in
water depth in nearshore zones induces an increase in wave energy, resulting
in increased sediment transport offshore. A relative sea-level rise due to a
eustatic sea-level rise or ground subsidence also accelerates coastal erosion.
The specific causes of coastal erosion must thus be understood before coun-
termeasures can be developed.
3.1.3 Tide-dominated
Tide-dominated coasts differ from storm- or wave-dominated coasts in terms
of sediment transport and coastal morphology. Very wide, flat morphology
that is well developed in the intertidal to subtidal zones is called a tidal flat.
Bars and trough topography are also found in these zones. Two directional
currents, the flood current and the ebb current, give the sediment transport
a characteristic pattern. The capacity for sediment transport of flood and
ebb currents is controlled by current velocity and duration. In general, sedi-
ment transport by flood currents exceeds that of ebb currents, resulting in a
prevailing landward sediment transport. This phenomenon is called the tidal
pump. Moreover, current velocity increases with water depth, resulting in
more energetic conditions offshore. Therefore, sediment deposits on a tide-
dominated coast show a landward fining (seaward coarsening) distribution
(Fig. 3.1.4). Typically, sandy sediments in subtidal zones change to muddier

3. Environment Problems in the Coastal Zone
Increasing terrestrial effects, e.g. evaporation
Decreasing effects of tidas
high tidal flats
Decreasing effects of waves
middle tidal fla
Salting cliff
Sandy loly tidal flat
Shell bank
Sand waves
Tidal creek
100 cm
10-50 cm
Mud cracks
10-50 cm
10 cm
Salt ripples
100 cm
10-50 cm
FIG. 3.1.4. Coastal features of a tide-dominated coast. (After Semeniuk, 2005.)
sediments in the intertidal zone and in the vegetated supratidal zone. From
the upper part of the intertidal zone to the supratidal zone salt marshes are
well developed, particularly in coastal lagoons and estuaries. In tropical to
subtropical regions, mangroves are found between the mean tide level and the
high tide level. Vegetation effectively traps fine-grained sediments transported
from offshore by the tidal pump. Fine-grained sediments supplied by rivers
are transported alongshore and are trapped in tidal estuaries and tidal flats
by tidal processes.
3.1.4 Impact of Sea-level Rise
Sea-level rise has affected coastal morphology and systems on a millennial
timescale, but a short-term sea-level rise also affects coastal environments.
The future sea-level rise due to global warming is expected to be 11–88 cm
by the year 2100 (IPCC 2001). On a wave- and storm-dominated coast, the
shoreface topography is thought to represent an equilibrium profile controlled
by waves and sediments (sea level 1 in Fig. 3.1.5). When sea level rises, a new
equilibrium profile is formed. As the sediment supply is generally not enough
to fill all the accommodation space to maintain the shoreline position, the
shoreline retreats and a new profile forms at the new shoreline position (sea

H. Sekiguchi and S. Aksornkoae
FIG. 3.1.5. Shoreface erosion due to a sea-level rise. (After Bruun 1962; Saito 1989.)
level 2 in Fig. 3.1.5). Some of the shoreface sediments are eroded in the process
of forming the new equilibrium profile. This phenomenon is called shoreface
erosion according to the Bruun principle or rule. It is important to note that
erosion occurs not only at the shoreline but also in the shoreface region, at
water depths of less than about 15 m. The sea-level rise leads to an increase
of wave power caused by the increase in water depth. Coastal erosion of cliffs
and strand plains will also be accelerated by a sea-level rise.
Mimura and Kawaguchi (1996) estimated that sand beach erosion will
occur at all Japanese beaches when sea level rises. Their results show a 60%
loss of beaches with a 30-cm sea-level rise, an 80% loss with a 65-cm rise, and
a 90% loss with a 100-cm rise. Although a sea-level rise would also increase the
sediment supply to beaches as a result of cliff erosion, that was not considered
in this estimate. Because most cliffs in the Japanese islands that used to supply
sediments to beaches are now protected from wave action by concrete blocks,
this additional supply is no longer overly large.
On the other hand, the impact of a sea-level rise on a tide-dominated coast
is not modeled according to the Bruun rule. The increase in water depth
causes more energetic conditions in such coastal zones. This changes the
distribution of sand and mud and causes some shoreline erosion by increased
wave action.
The northern coast of the Gulf of Thailand is a good example of the
impact of a relative sea-level rise on a muddy coast. Subsidence due to ground-
water pumping has occurred not only in Bangkok but also in the coastal zone
south of the city. The mouth of the Chao Phraya River is located south of
these areas. At the river mouth, and in the neighboring coastal zones, more
than 60 cm of subsidence occurred during the 1960s to 1980s, resulting in
severe coastal erosion (Vongvisessomjai 1992; Vongvisessomjai et al., 1996).
The shoreline retreat was 700 m in total up to the early 1990s. The main causes
of this erosion and retreat are submergence and an increase of wave energy as
a result of the nearshore zone being deepened by subsidence. As the nearshore
zone has a very gentle slope of 1/1000, the more than 60 cm of subsidence
(deepening) directly caused an increase in wave energy. Moreover, destruction
of mangroves in conjunction with shrimp pond farming has enhanced