Geology Reference
In-Depth Information
tidal cycle under the storm climate, violently disturb-
ing the intertidal morphology over a wider area (Mao
1987
; Shi and Chen
1996
; Fan et al.
2006
). The wave-
break zone tends to stall respectively at low and high
water line for longer time, hypothetically accounting
for the development of inner and outer swash ridges as
discussed in the former section.
Episodic high-energy events occur infrequently, but
are the fiercest force to produce marked erosion and
deposition cycles on the tidal flats. A large wave or
storm event usually lasts several hours to days, so the
resulted tidal-flat erosion and deposition pattern should
be revealed only by a finer time scale than the life cycle
of the events. Considering the complex interactions of
waves and tides and a philosophy that a finer time scale
tends to have a smaller spatial attribute, the episodi-
cally induced erosion/deposition phenomena should
therefore be examined over a finer spatial scale.
Following this, an experiment was carried out along a
cross-profile on the Nanhui Mudbank (Changjiang
Delta) in 1999, through using graduated-stake eleva-
tion-monitoring technology (Fan
2001
). Sixty-two
stakes were fixed on the intertidal ground with a dis-
tance of 30-50 m between two neighboring stakes, and
they were regularly monitored every single or 2 days.
The result shows that net erosion switches on when
waves exceed 1.5 m high during non-typhoon condi-
tions (Fig.
9.17
). Serious erosion occurs during peak
storm periods with a maximum of >15 cm deflations
over two to four tidal cycles at some locations (Fan
2001
). There are generally existent two erosion zones
separated by the accretional zone. It was hypothesized
that the erosion and the deposition zones were respec-
tively produced by series of wave breaking and reform-
ing processes over the gentle and broad mudflats
(3.4 km wide across entire intertidal zone with a mean
tidal range of 2.6 m) before the wave was dying out
(Fig.
9.18
, Fan et al.
2006
). The sites of erosion or
deposition change alternatively into deposition or ero-
sion over a next few tidal cycles (Fig.
9.17
), denoting
the same mechanism that waves tend to break over the
previous deposition zones because of shoaling and
produce new erosion zones, and vice versa for the pre-
vious erosion zones which turn into wave-reforming
area to promote deposition.
the neap-spring cycle, exerting great impacts on the
tidal-flat development. Tide modulation of storm
waves was clearly exhibited by the hydrodynamic data
from a field experiment on the Baeksu tidal flats (south-
west Korea) in February 1999, where the semidiurnal
tidal range varies from 2.3 m at neap to 5.5 m at spring
with a mean of ~3.9 m (Kim
2003
). The instrumenta-
tion system to monitor wave and current was deployed
on the lower intertidal flats for 2 weeks, catching two
invading winter storms with main wind direction
toward the shore. The first storm was stronger than the
second in terms of maximum wind speed (~18 m/s vs.
~14 m/s). However, the storm-induced waves were
larger for the second (weaker) storm than the first
storm in terms of the maximum significant wave height
(3 m vs. 2 m). The discrepancy of weaker storm gene-
rating larger waves is actually ascribed to the wave-depth
relationship, in that the occurrence of weaker storm at
spring tide tends to allow larger waves penetrate into
the intertidal flats because of higher water depth rising
by spring tide, whereas neap tide is potential to damp
storm energy more seaward (Kim
2003
).
The fact that the intensity of storm-related processes
is greatly modulated by the neap-spring cycle was also
addressed by Fan et al. (
2006
) in terms of the erosion
magnitude and the distribution of erosion zones across
the intertidal flats at the Nanhui Mudbank. They dis-
cussed that a weaker storm at spring tides was poten-
tial to induce more intense erosion than a stronger
storm at neap tides (Fig.
9.18
). It is not only due to the
higher wave energy and the related mightier vertical
scouring capability at spring tides than at neap tides
which are determined by water depth as discussed
above, but also linked with the higher spring current
speed and the related mightier horizontal advection
capability than neap tides. Consequently, more sedi-
ment is suspended and carried out of the erosion zone
by the combined action of storm waves and currents at
spring tides than at neap tides, producing higher mag-
nitude of the erosion.
Seasonal alternations of accretion and erosion are
the most significant and widely addressed features
of the tidal flats (Ren
1985
; Shi and Chen
1996
;
O'Brien et al.
2000
; Yang et al.
2005
; Fan et al.
2006
;
Yang et al.
2008b
). The processes leading to this peri-
odic development are principally related to seasonal
wind climate change, and less to other factors like
changes in tidal level, floral and faunal distributions,
solar intensity, and estuarine turbidity maximum loca-
tion. The Baeksu tidal flat in southwest Korea has more
9.4.1.2 Intermediate Cycles (Neap-Spring
Tidal Cycle to Season)
The modulation effect of large-wave processes by tides
is significantly different over a single tidal cycle and
