426
14 Tides and Waves on Vegetated Coasts
export at ebb tide, from creek to the mangrove are 360 and 260 gC /m of creek
length, respectively. The bulk of the organic carbon in the mangrove area is
in dissolved form with maximum concentration of about 2.5 mg/I. Dissolved
organic carbon accounts for 85% of the total organic carbon. These values are
much higher than those in nearby coastal waters. A study suggests that organic
carbon may be partitioned between creek and mangrove forest, with the major
change in the molecular weight of the DOe in the mangroves occurring between
flood and ebb tides.
Removal of mangroves may increase water turbidity and hence decrease primary productivity in tidal creaks and adjacent coastal waters. Unfortunately,
mangroves have declined worldwide, particularly in the South-East Asia, where
losses in the last three decades have reached 70-80% (Sasekumar et al., 1994).
On the other hand, in some areas efforts to plant mangroves on an extensive
scale are observed. Mazda et al. (1997a) demonstrated the usefulness of mangrove reforestation for coastal protection in the Tong King delta, Vietnam.
14.3 Tidal Flats
14.3.1 Tidal and Wave Motion on Tidal Flats
Tidal flats are usually defined as sandy to muddy or marshy flats emerging
during low tide and submerging during high tide. The frequency of tidal flat
inundation, examined in a perspective of months or years, indicates that approximately the lower 25% of the extreme tidal range is intermittently exposed,
while the higher 25% is intermittently inundated, leaving only the central 50%,
corresponding to the above definition (Amos, 1995).
Tidal fiats can be found in all climate regions, in arid and wet tropical or
subtropical regions as low-latitude tidal flats, in temperate regions as midlatitude tidal fiats, and in areas influenced by ice as high-latitude tidal flats.
All of them exhibit a common relationship in grain size, benthic flora and
fauna diversity and abundance, surface morphology and slope. However, two
major groups of tidal flats are apparent: sandy tidal fiats and muddy tidal
fiats. The first group corresponds to the mean inorganic suspended sediment
concentration (SSe) of the inundating water of less than 1 g/l, while for muddy
tidal fiats, the sse is generally greater than 1 g/l (Amos, 1995).
The tides on tidal flats are characterized by a fast start of the flood current,
prolonged period of high water, then slack water followed by a short, fast ebb
current. Such a current pattern develops primarily in the small tidal creeks
adjacent to the fiats. The current velocities on tidal fiats are much smaller,
mostly below 0.5 m/s and they change with water depth, according to the
formula C = /9h, where h is the water depth. Tidal currents on the fiats stop
almost immediately at high water, but in the creeks they often continue to
flow for up to half an hour after high water. Creeks are not filled (or emptied)
simultaneously by the tidal water, and filling time depends on the speed of
propagation of tidal wave in the creeks. As a result, a residual current pattern
14 Tides and Waves on Vegetated Coasts
export at ebb tide, from creek to the mangrove are 360 and 260 gC /m of creek
length, respectively. The bulk of the organic carbon in the mangrove area is
in dissolved form with maximum concentration of about 2.5 mg/I. Dissolved
organic carbon accounts for 85% of the total organic carbon. These values are
much higher than those in nearby coastal waters. A study suggests that organic
carbon may be partitioned between creek and mangrove forest, with the major
change in the molecular weight of the DOe in the mangroves occurring between
flood and ebb tides.
Removal of mangroves may increase water turbidity and hence decrease primary productivity in tidal creaks and adjacent coastal waters. Unfortunately,
mangroves have declined worldwide, particularly in the South-East Asia, where
losses in the last three decades have reached 70-80% (Sasekumar et al., 1994).
On the other hand, in some areas efforts to plant mangroves on an extensive
scale are observed. Mazda et al. (1997a) demonstrated the usefulness of mangrove reforestation for coastal protection in the Tong King delta, Vietnam.
14.3 Tidal Flats
14.3.1 Tidal and Wave Motion on Tidal Flats
Tidal flats are usually defined as sandy to muddy or marshy flats emerging
during low tide and submerging during high tide. The frequency of tidal flat
inundation, examined in a perspective of months or years, indicates that approximately the lower 25% of the extreme tidal range is intermittently exposed,
while the higher 25% is intermittently inundated, leaving only the central 50%,
corresponding to the above definition (Amos, 1995).
Tidal fiats can be found in all climate regions, in arid and wet tropical or
subtropical regions as low-latitude tidal flats, in temperate regions as midlatitude tidal fiats, and in areas influenced by ice as high-latitude tidal flats.
All of them exhibit a common relationship in grain size, benthic flora and
fauna diversity and abundance, surface morphology and slope. However, two
major groups of tidal flats are apparent: sandy tidal fiats and muddy tidal
fiats. The first group corresponds to the mean inorganic suspended sediment
concentration (SSe) of the inundating water of less than 1 g/l, while for muddy
tidal fiats, the sse is generally greater than 1 g/l (Amos, 1995).
The tides on tidal flats are characterized by a fast start of the flood current,
prolonged period of high water, then slack water followed by a short, fast ebb
current. Such a current pattern develops primarily in the small tidal creeks
adjacent to the fiats. The current velocities on tidal fiats are much smaller,
mostly below 0.5 m/s and they change with water depth, according to the
formula C = /9h, where h is the water depth. Tidal currents on the fiats stop
almost immediately at high water, but in the creeks they often continue to
flow for up to half an hour after high water. Creeks are not filled (or emptied)
simultaneously by the tidal water, and filling time depends on the speed of
propagation of tidal wave in the creeks. As a result, a residual current pattern