Journal of Coastal Research23

4

1015–1027

West Palm Beach, Florida

July 2007

Hydrodynamics and Associated Morphological Variations on an
Estuarine Intertidal Sand Flat
Yusuke Uchiyama*
Littoral Drift Division
Port and Airport Research InstituteYokosuka 239-0826, Japan

ABSTRACTUCHIYAMA, Y., 2007. Hydrodynamics and associated morphological variations on an estuarine intertidal sand flat.
Journal of Coastal Research, 23(4), 1015–1027. West Palm Beach (Florida), ISSN 0749-0208.
Hydrodynamics, sediment suspension, and morphological response on an estuarine intertidal sand flat in Tokyo Bay,
Japan, are examined through a field experiment performed for 16 days in winter 2000 and using a bathymetry data
set based on a 6-year series of surveys. Topography of the sandy flat was found to fluctuate by approximately 8 cm
during the deployment, while the long-term accumulation rate is estimated with the surveyed bathymetries to be only
3.8 cm/y. Cross-spectral analysis of the measured data indicates that on the sandy tidal flat, semidiurnal or shorterperiod fluctuations in current velocity are mostly attributed to semidiurnal tides and waves, whereas wind above thesea generally drives diurnal or longer-period fluctuations. The field data also confirm that suspended sediment concentrations were highly correlated with bed shear stress, which is generated by combination of tidal current, windinduced current, and wind waves. Episodic erosion is observed on the sandy flat with high turbidities on ebbing phases.
Erosion evidently occurs at phases between the high slack and the mean tidal levels during ebb flows when combined
tidal, wind-driven, and wave-driven currents are maximized.
ADDITIONAL INDEX WORDS: Wetting and drying, topography change, bed shear stress, tidal oscillation, wind waves,wind-induced current.

INTRODUCTION
For preservation and rehabilitation of ecosystems on tidal
flats, wetlands and salt marshes, predicting morphologicalbehavior is a significant issue (P ATERSON, 1989). The intertidal sediments play a key role in providing habitat for resident
infauna, which act as a food resource for large communities
of shorebirds (FERNS, 1983; BAKKER et al., 1993). Even whenslight erosion occurs on tidal flats, it may have significant
impact on the resident benthos (KAKINO, 2000). Understanding of the hydrodynamics and associated morphological process on intertidal flats and wetlands is a prerequisite for thedevelopment of conceptual and quantitative models that are
essential to allow effective management of the intertidal
area.
Recently, efforts have been made to understand hydrodynamics, sediment transport, and resultant morphological process on mudflats located in the Dollard Estuary, the Netherlands (DYER et al., 2000; VAN DER LEE, 2000), Humber Estuary, UK (CHRISTIE et al., 1999), Severn Estuary, UK(O’BRIEN et al., 2000), Baie de Marennes-Oleron, France (BASSOULLET et al., 2000), the Wadden Sea, the Netherlands andDenmark (HOUWING, 2000; ANDERSON, 2001; ANDERSON andPEJRUP, 2001), and San Francisco Bay, USA (TALKE and STACEY, 2003). Sedimentological processes reported in these
DOI:10.2112/04-0336.1 received 26 August 2004; accepted in revision
18 May 2005.
* Present address: Institute of Geophysics and Planetary Physics,
University of California, Los Angeles, 3845 Slichter Hall, Los Angeles, CA 90095-1567, U.S.A. uchiyama@atmos.ucla.edu.

studies have usually been described in comparison with laboratory data (TEISSON et al., 1993), and there is a growing
literature on the influence of mixtures of various sizes of sediments (e.g., MITCHENER et al., 1996; TORFS et al., 1996) andbiological factors (e.g., RHOADS and BOYER, 1982; PATERSON,
1989; WIDDOWS et al., 1998; AUSTEN et al., 1999; ANDERSON,
2001). Furthermore, AMOS et al. (1988) have documented thatthe subaerial exposure often has considerable influence on
bed strengthening. Some studies have examined hydrodynamics that potentially control sediment transport and morphological process on mudflats by measuring tidal currents,
wind-induced currents, wind waves, oceanic swells and infragravity waves, density-driven circulation, and drainage processes (e.g., LE HIR et al., 2000; TALKE and STACEY, 2003).
Sediment transport on tidal flats occurs over a variety oftime scales, ranging from seconds to seasons. DALRYMPLE etal. (1991), in a paper on cohesive sediment deposition on a
mudflat in the Bay of Fundy, indicate that historical deposition may be divided into two phases, namely, a short-term
rapid erosion and subsequent aggradation, and longer-term
gradual but quasiequilibrium topographic fluctuations. They
also reported that deposited sediments are intensely disturbed by benthic organisms. The combination of morphological processes in different time scales may control overall development of intertidal areas. Intertidal topographic variations have been reported to be generally minor as hydrodynamic forcing has basically been observed to be small in
previous studies (e.g., CHRISTIE et al., 1999). For example, KIRBY et al. (1993) demonstrated that bed elevation varies only
3–5 cm a year in Strangford Lough, Northern Ireland. How-

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Figure 2. Grain size distributions of bottom sediments sampled at stations 1 and 2. Median grain sizes are estimated to be 170–190 ␮m.

Figure 1. The location of Banzu intertidal sand flat, Tokyo Bay, Japan.
Bed profiles were surveyed along transect A–A ⬘ in the vicinity of the
Obitsu River mouth.

ever, intermittent episodic processes are considered to dominate the morphodynamics on intertidal areas. Storms can
produce significant sediment transport and cause topographic changes that are likely to occur for several years without
storms. Episodic fluvial outflows can also induce significant
accretion. Recent studies have reported that considerable
short-term morphological variations of 10–30 cm on a mudflat in the Severn Estuary (W EST and WEST, 1991; WHITEHOSE
and MITCHENER, 1998; O’BRIEN et al., 2000). Therefore, morphological processes on intertidal flats, not only in the longterm but also in the short term, provide an ‘‘open’’ question.
Intertidal flats are characterized by tidal submergenceemergence cycles with the additional effect of benthic disturbance and exposure to the air. The fluctuations in water surface elevation induce hydrodynamic conditions that generate
sediment processes such as formation of ripples, dunes, channel networks, and so on. These external forces are responsible
for sediment transport through producing bed shear stresses
affected by nonlinear wave-current interaction and are attributed to erosion and accretion of the bed.
The purpose of the present study is to demonstrate shortand long-term morphological behavior of a sandy tidal flat

rather than mudflats reported in the previous studies. The
prime objective is to investigate episodic (short-term) andgradual (long-term) morphological processes on the sandy
flat, to investigate mechanisms of erosion related to estimating bed shear stresses caused by combined effects of waves
and currents, and to examine detailed hydrodynamics, sediment suspension, and associated morphodynamic processes
in contrast to long-term topography variations. Biological effects on sediment suspension and subaerial exposure are neglected in the present study but should be considered in future works.

FIELD MEASUREMENT
Study Site
Field measurements were carried out for 16 days, February
7–22, 2000, on an intertidal sandy flat at Banzu situated on
the eastern shore of Tokyo Bay, Japan (Figure 1). This embayment is characterized by a well-mixed semidiurnal mesotidal condition. During rainy seasons (June and September), massive freshwater influx is supplied by three majorrivers. Whereas Tokyo Bay was previously fringed by a hugearea of intertidal flats until the late 1950s, some 20,800 haof the flats have been reclaimed over past decades (M INISTRY
OF TRANSPORT, 2000). The study site has been left from coastal developments to become the largest intertidal flat in Tokyo
Bay.
Banzu sandy flat comprises an intertidal area of 7.6 km 2
(760 ha) above the MLLW level. The maximum spring tidal
range is about 2.0 m. Bed sediments at the study site arecharacterized by well-sorted fine sand with a median grain
size of 170–190 ␮m (Figure 2). A preliminary study showedthat tidal exchange is the major source of suspended and dis-

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Morphodynamics on an Intertidal Sand Flat

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cember to February), when the field measurements were performed. Thus, the effects of the river outflow on hydrodynamics and associated sedimentological processes may be assumed negligible in the present analyses. In addition, because of the historical reclamations of the hinterland, there
exist almost no marshes or vegetation on Banzu tidal flat
except on the delta formed around the mouth of Obitsu River(Figure 3).
The wave climate around the study area is dominated bysmall wind waves. Tokyo Bay is characterized as a semienclosed estuary, and the bay entrance faces southwest to shelter the study area from the prevailing wind and associatedwind waves entering the bay from the Pacific Ocean. Waveheights and periods are limited because of small fetches fromthe shoreline that comprise two peninsulas and the sand spitat the bay mouth. A number of wave measurements havebeen performed within the bay. NAGAI et al. (2001) estimateda significant wave height H1/3 of 0.5 m or less in 88.6% of the
waves during 1991–2000. Wave height H1/3 of 1.0 m was recorded 0.5% of the time at the Daini-Kaiho Wave Observatory
situated in the channel close to the spit at the bay entrance
during this decade. They also indicated a significant wave
period T1/3 to be 5.0 s or more in 1.4% of the waves, suggestingthat swells and infragravity waves in the study site are minor.

Figure 3. Aerial photo of the study site (Banzu intertidal sand-flat, located off Kisarazu, Chiba Prefecture, Japan).

solved material, although the Obitsu River mouth is locatedadjacent to the study site (KUWAE et al., 1998). As detailed in
the Fluvial Effect subsection in the Results section, freshwater discharge from the adjacent river (Figure 1) directly
onto the sandy flat was generally minor during winter (De-Methods
A line referred to as transect A–A⬘ was established near
the river mouth, as shown in Figure 1, for the purpose of
conducting bathymetric surveys on February 7 and 22, 2000,
using a surveyor’s level and staff. Instrumentation layout at
two monitoring stations, both of which were positioned alongtransect A–A⬘, is illustrated in Figure 4. At station 1, water
surface elevation was measured using an ultrasonic wave
gauge (KENEK Co., Ltd.) mounted at a height of 4.5 m from

Figure 4. Instrumentation layout (left, station 1; right, station 2).

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Figure 5. Bottom profiles along transect A–A ⬘ surveyed on February 7
and 22, 2000, and their difference, ⌬h, along with the positions of stations
1 and 2.

the bed at a sampling frequency of 5 Hz, and a photoelectric
sand-surface meter (SPM-7, Sanyo Sokki Co., Ltd.) was buried in the bed to monitor bottom elevation at 10-min intervals. The accuracy of the bed elevation measurement is ⫾5mm according to the instrument specification. At station 2,
an optical backscattering sensor (MTB-16K, Alec Electronics
Co., Ltd.) and a three-dimensional electromagnetic current
meter (ACM-16M, Alec Electronics Co., Ltd.) were installed
to measure suspended sediment concentrations and current
velocity at 10 cm above the bed at a frequency of 2 Hz. Fluvial
discharge and water level of the Obitsu River were measured
at 1-h intervals by the Obitsu Gate Operation Office locatedat the point 5.6 km upstream from the river mouth. Wind
velocity was monitored hourly by the Japan Meteorological
Agency at north Kisarazu, about 3.0 km north of the study
site. The OBS was calibrated in a laboratory immediately
after the deployment with the suspended sediments sampled
at the study site. A bucket of about 10 L painted in black andan electrically driven propeller were used to mix the seawaterand the sediments. The obtained calibration curves are linearwith very small variances, and there is almost no differencein the regression relations calculated with the bed and suspended sediments. These results are considered quite different from those for mudflats, presumably because the sediments are mostly composed of fine sand at the study site.

RESULTS
Short- and Long-Term Morphological Variations
Figure 5 shows bottom profiles surveyed on February 7 and22, 2000, along transect A–A⬘ and their difference, ⌬h. The
profile of the sandy tidal flat is found to have a mean slopeof about 1 : 1850, and ⌬h is very small, ranging from ⫹1 cm
to ⫺4 cm. FURUKAWA et al. (2000) exhibited a series of bottom
topography data of Banzu flat measured along transect A–A ⬘from October 1994 at intervals of 1–6 months. From theirdata, long-term variation of the sediment volume along transect A–A⬘ per unit alongshore length was calculated (Figure6). The figure indicates that Banzu sandy flat is graduallyaccreting in the long term at about 45 m3/m/y. By dividing by
the length of transect A–A⬘, 1200 m long, this value can beconverted into a profile-averaged long-term accretion rate of

Figure 6. Long-term accumulation evaluated from the bottom survey
data along transect A–A⬘.

3.8 cm/y. In many tidal wetlands and flats of the world, theaccretion rate is only a few millimeters per year (e.g., FREY
and BASAN, 1985). Therefore, the accretion rate near the rivermouth of Banzu sandy flat is 10 to 30 times greater thanthose in other studied sites in Europe and America (STEVENSON et al., 1985; FRENCH and SPENCER, 1993; LEONARD et al.,
1995; ANDERSON and PEJRUP, 2001).
Figure 7 displays the time series of bed level, h, measured
at station 1 by SPM-7: (a) original time series (hourly average), (b) 24-h moving averaged bed level representing medium-term fluctuations, (c) difference between (a) and (b), representing short-term fluctuations, and (d) root-mean-square(R.M.S.) of (c) calculated for every 4 hours. As shown in Figures 7a and 7b, the seabed appeared to be eroded by about 8
cm during the first 4 days with small erosions and accumulations, and then gradually recovered to the initial elevation.
The magnitude of the short-term erosion occurring on February 8–10 is nearly equivalent to the accretion occurring on
February 15–16, as shown in Figures 7c and 7d.
Figure 8 shows time series of tidal elevation averaged forevery 30 minutes, the bottom elevation at station 1, and theturbidity (suspended sediment concentrations) averaged forevery 10 minutes at station 2. The elevations displayed here
are relative to T.P. in meters, where T.P. is the mean lower
low water (MLLW) level of Tokyo Bay. The spring-neapspring tidal cycle and diurnal inequalities are visible in the
tidal data. Figures 5 and 8 show that station 1 was mostly
submerged over the duration of the deployment, whereas station 2 was exposed during almost every low-water phase. The
turbidity (Figure 8) fluctuated in response to the R.M.S. ofthe bed elevation (Figure 7d), indicating that on February 8
and 9, for instance, high turbidity is observed whenever the

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Morphodynamics on an Intertidal Sand Flat

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Figure 7. Time series of bed level, h, measured at station 1: (a) 1-hour averaged original time series; (b) 24-hour moving averaged bed level; (c) difference
between (a) and (b); (d) root-mean-square of (c).

seabed is eroded. In addition, small amounts of erosion andaccretion occur with the semidiurnal tidal fluctuations, probably because the wave height, which generally induces thesediment suspensions and resultant bed level variations,varies in response to the tides and has its maximum at highwater.

Hydrodynamics
Figure 9 shows significant wave height (H1/3) and period(T1/3) at station 1 and principal wave directions as estimatedfrom covariance of the current velocity. Figure 10 represents
wind velocity vectors at Kisarazu and temporally averaged

velocity vectors of horizontal currents and water depth at station 2. Wave conditions are relatively severe at the earlystage of the deployment as the wave height reached 0.8–0.9
m with a period of about up to 4 s. Mild conditions prevailed
for the next 5 d, followed by medium conditions in the laterstage. The principal wave direction is basically from N or
NNW, which means that the waves propagated mostly in thealongshore direction, and thus, refraction can be ignored on
the sandy flat, although it is significant on sandy beaches.
The wave height fluctuates according to the water depth
variation, i.e., tides, as shown in Figures 8 and 9, because H1/3
seems to be high during high water but low during low water.

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Figure 8. Bottom elevation and tidal elevation at station 1 and turbidity at station 2.

The principal wave directions roughly coincide with the winddirections. Therefore, in order to determine contributions of
the wind action and the tidal forcing to the enhancement ofwave height, H1/3, cross-spectral analysis has been conducted.
Figure 11 shows the cross spectra (represented by spectrum,
coherence squared, and phase) between (a) significant wave
height (H1/3) and N–S component of wind velocity (Wn-s) and(b) H1/3 and ␩ (tide : water depth at station 2). The significant
wave height is shown to correlate appreciably with ␩ overbroad frequency bands with almost no phase lag. By contrast,
coherence between H1/3 and the wind velocity is generally
small, although it is slightly larger at the lower frequencies.
These results demonstrate that H1/3 is fluctuating basicallyin response to the tide, not to the local wind, yet its long-term(at frequency of about 0.01 cycle/h or less) variation is caused
mainly by the wind. This result is important because waves(short-period wind waves) are usually believed to be induced
by local wind forcing. However, the local water depth is alimiting factor for the development of waves on the extremely
shallow intertidal flat.
Next, causes of the variation in the horizontal current ve-

locity are investigated. As shown in Figure 10, the horizontal
current velocity appears to vary in the long term in responseto the fluctuations of the wind velocity and the wave height.
The current field on the sandy flat is also considerably influenced by the tide because the semidiurnal inversions in thecurrent directions occur with the tidal oscillations. As a consequence, the currents on Banzu intertidal flat are simultaneously affected by the wind, the tide, and the waves. Figure
12 shows the cross-spectra between (a) Wn-s and N–S component of horizontal current velocity (Un-s), (b) H1/3 and Un-s,and (c) ␩ (tide : depth at station 2) and Un-s. The power spectral density of Un–s has an intense peak in the vicinity of thesemidiurnal period (T ⫽ 12 h), although it is relatively low
over periods longer than 24 h. The distinct peaks at the semidiurnal period can also be found in the spectra of ␩ and H1/3,
although there is no visible peak in the wind velocity spectrum. The estimated coherences show that the current variation is obviously correlated with ␩ and H1/3 at a period of 12
h. Because the coherence between Un–s and ␩ is slightly larger
than that between Un–s and H1/3, the current velocity fluctuation is considered mainly due to the tide which influences

Figure 9. Significant wave height, H1/3, and period, T1/3, at station 1 and principal wave directions.

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Morphodynamics on an Intertidal Sand Flat

1021

Figure 10. Wind velocities at Kisarazu and current velocities and water depth at station 2.

on the development of the waves. On the other hand, thecoherence between Wn–s and Un–s is high over periods longer
than about 24 h, although the longer-period component ofUn–s does not have such a high correlation with the wave
height and the tide. Consequently, the wind velocity mainly
affects the current field on Banzu sandy flat in the diurnal
or longer-period fluctuations, whereas the tide and the waves
induce the semidiurnal or shorter-period fluctuations.

HUYNH-TANH and TEMPERVILLE, 1991; SOULSBY, 1997), and an
approach in which the bed shear stress is estimated using
Reynolds stresses calculated from the fluctuating componentsof the current velocity (DYER and SOULSBY, 1988; FRENCH and
CLIFFORD, 1992) is chosen. Assuming that the region in thevicinity of the seabed, where the velocity measurements were
performed, is situated in a ‘‘constant-shear layer,’’ the bed
shear stresses can be calculated using the following equations:

Fluvial Effects
Because the study site is situated near the river mouth,
influence of the fluvial input from the adjacent Obitsu River
must be considered in the analyses. Figure 13 gives a timeseries of fluvial discharge and water elevations of the ObitsuRiver monitored at 5.6 km upstream from the river mouth.
Although the river discharge during the duration of the measurement fluctuates slightly, it is minor compared with annual variation of the discharge displayed in Figure 14. The
tidal elevation is observed to affect the riverine water levels
during high tides, although they are generally lower than theriverine water elevations during low tides. Therefore, it isconcluded that the freshwater discharge from the adjacentriver was presumably small and can be ignored in the present
analysis.

DISCUSSION
Bed Shear Stress
The bed shear stresses are calculated to investigate thevarious mechanism of sediment suspension on Banzu sandy
flat. Different methodologies have been proposed to evaluate
the bed shear stress in terms of wave-current interaction (e.g.,
GRANT and MADSEN, 1979; SOULSBY and HUMPHERY, 1990;␶uw ⫽ ⫺␳u⬘w⬘ ,(1)

␶vw ⫽ ⫺␳v⬘w⬘ ,

(2)

2
2
␶b ⫽ 兹␶uw
⫹ ␶vw
,

(3)

where (u⬘, v⬘, w⬘) ⫽ (u ⫺ U, v ⫺ V, w ⫺ W) is a fluctuating
velocity vector, (u, v, w) is a measured current-velocity vector,(U, V, W) is a burst-averaged velocity vector over 15 s of burst
intervals, ␶uw and ␶vw are the vertical components of Reynolds
stress in the directions of u and v, ␳ is the density of sea water(1025 kg/m3), and overbars represent an ensemble averaging
operator. It might be better to decompose the current velocity
into mean flow, wave component, and turbulence. However,that is unfortunately not possible because the sampling frequency of the current meter is too low (2 Hz). One thus must
be careful that the bed shear stress ␶b estimated may include
wave components.
Figure 15 shows temporal variations of the current bed
shear stress ␶b and the turbidity Cs measured at station 2.
The turbidity Cs clearly fluctuates in response to ␶b. It is verified that the high turbidities, occurring intermittently on 8,
9, 15, and 21 February, for instance, are generated whenhigher bed shear stress are observed. Figure 16 supports this

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Figure 11. Cross-spectra (a) between significant wave height, H1/3, and N–S component of wind velocity, Wn-s, and (b) between H1/3 and ␩ (tide water
depth at station 2).

because the bed shear stress and the turbidity are highly correlated with the correlation coefficient of R ⫽ 0.856.
Previous studies have shown that modes of sediment transport are related to the Shields parameter, ⌿m, which is thenondimensional bed shear stress under oscillatory flow withconsideration of underlying sediment properties. The relationship between ⌿m and ␶b is given by⌿m ⫽ u* 2/(␳s/␳ ⫺ 1)dg ⫽ ␶b/(␳s ⫺ ␳)dg

(4)

where u* is bed friction velocity (m/s), ␳s and d are the density
and median grain size of the bed sediment (2690 kg/m3 and0.19 mm), ␳ is the seawater density, and g is the gravitational
acceleration (9.80 m/s2). By converting the previously proposed threshold Shields parameters (SHIBAYAMA and HORIKAWA, 1982) into ␶b using equation (4), one obtains ␶b 艐 0.2
N/m2 for the minimum value for initial generation of bed
load, and ␶b 艐 0.6 N/m2 for the minimum value for transition
to suspended load. As shown in Figure 15, whenever the bed
sediments are highly suspended, ␶b is much greater than 0.6.
Therefore, suspended load dominates on the intertidal sandy
flat when high turbidity is measured.
Wave action is well known to have the largest effect on

sediment suspension on sandy beaches, particularly in surfzones, and therefore, the correlation between the turbidity
and wave energy evaluated from the linear wave theory using
the significant wave height is investigated as shown in Figure 17. Note that the waves and current data were separately
measured at stations 1 and 2. In spite of this discrepancy,
the correlation coefficient R is evaluated to be 0.689. This isconsidered sufficiently large, although somewhat less thanthat between ␶b and turbidity.
On the other hand, because hydrodynamics in shallow estuaries is strongly affected by local wind (e.g., WU and TSANIS,
1995), the shear stress induced by the wind action at thewater surface, ␶s, is evaluated using the following equation:␶s ⫽ Cf␳a(W2n-s ⫹ W2e-w)(5)where Cf is the friction coefficient at the sea surface ( ⫽ 2.5⫻ 10⫺3), ␳a is the atmospheric density (⫽ 1.2 kg/m3), and(Wn–s, We-w) is a horizontal wind-velocity vector. One can expect that the surface wind stress may be related to the bed
stress caused by wind, particularly when water-depth variation is negligible. However, as shown in Figure 18, there isalmost no correlation between ␶b and ␶s. This fact again sup-Journal of Coastal Research, Vol. 23, No. 4, 2007

Morphodynamics on an Intertidal Sand Flat

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Figure 12. Cross-spectra (a) between Wn-s and N–S component of horizontal current velocity, Un-s, (b) between H1/3 and Un-s, and (c) between ␩ and Un-s.

ports the results of the cross-spectral analyses (Figure 12).
Consequently, increment of ␶b and the resultant sedimentsuspension are not directly influenced by wind action, being
primarily attributed to the tides and the waves.

Phases of High Concentration of Suspended Sediment
Next, the phases when the high concentrations of suspended sediment are measured are investigated. Conditional sam-plings are carried out to estimate the ensemble averages (inother words, the phase-averaged pictures) of the wind andcurrent velocities, the tidal elevations, the significant wave
height, and the turbidity. The condition is defined here as theturbidity exceeding 40 ppm (i.e., bed shear stress to exceed
about 3.5 N/m2). The conditional samplings (e.g., YUAN and
MOKHTARZADEH-DEHGHAN, 1994) are executed to obtain theensemble averages for five cases of high concentrations mea-

Figure 13. Discharge and water level of the Obitsu River measured at 5.6 km upstream from the river mouth. Upper panel: River discharge. Lower
panel: Tidal elevation at station 2 and river water level.

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Figure 14. Annual variation of the Obitsu River discharge measured at 5.6 km upstream from the river mouth in 1999.

sured at 2100 on February 8, 0820 and 1940 on February 9,1240 on the 15th, and 1810 on the 21st. The estimated ensemble averages are shown in Figure 19, in which the phaseof high turbidity is adjusted to the elapsed time t ⫽ 0 h foreach case. The current velocity, the wind velocity, and the
bed shear stress apparently have highest values near thephase of the high turbidity. In contrast, the peak phases of
the wave height and the tidal elevation do not correspond to
the phase of high turbidity. The turbidity is found high during ebb tide, which begins at around t ⫽ ⫺1.5 h, and becomes

the highest at the phase between the high slack and themean water, 1.5 h after the high slack water.
Intertidal flats are reported quite sensitive to action ofwind-induced waves because they are extremely shallow,
which favors the generation of high bed shear stresses that
promote sediment resuspension (FREEMAN et al., 1994). In
other words, a net transport of suspended sediments may besignificantly dependent on wind speed and direction, as suggested by PEJRUP (1986), CHRISTIE et al. (1999), and BASSOULLET et al. (2000). The wave height has its peak at the highFigure 15. Bed shear stress (solid line) and turbidity (dotted line) at station 2.

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Morphodynamics on an Intertidal Sand Flat

Figure 16. Correlation between bed shear stress, ␶b, and turbidity.

1025

Figure 18. Correlation between bed shear stress and wind shear stressat surface.

slack phase because it fluctuates in response to the tide asshown in Figure 11b. If only the waves are predominant ingenerating the bed shear stress as in surf zones, the highest
turbidity may not appreciably lag behind the high-waterphase. In general, tidal current velocities become largest between high slack and low slack phases during flooding andebbing. If the semidiurnal tidal component is dominant as inTokyo Bay, the maximum tidal velocities, as well as resultant
bed shear stresses, appear at 3 h and 9 h after the low slack.
However, the estimated phase of high turbidity in Figure 19
is found at 1.5 h after the phase of the high water and the
maximum wave height, and at 1.5 h before the tidal current
velocity and the bed shear stress become maximal. This dis-Figure 17. Correlation between wave energy and turbidity.

Figure 19. Ensemble averaged wind velocities, current velocities, tidal
elevation, significant wave height, bed shear stresses, and suspended sediment concentrations calculated with conditional sampling when the turbidity is in excess of 40 ppm.

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tuarine intertidal sandy flat in the semienclosed Tokyo Bay,
Japan, has a morphological process consisting of long-term
gradual accretion, episodic erosions, and subsequent accumulations. The long-term accretion rate is estimated to be
only about 3.8 cm/y on the basis of the surveyed data, whereas bed elevation varies by approximately 8 cm during theshort-term measurement conducted in 2000, when fluvial discharge can be ignored and relatively severe wave condition
appears as significant wave height exceeds 0.8 m during the
deployment. Episodic erosion and subsequent gradual accretion are significant for the short-term morphological processes. Modes of bottom sediment movement during the measurement are examined using the Shields parameter. Erosion
occurs in response to generation of high concentrations of suspended sediments at phases between the high slack waterand the mean water during ebb, when the combined wave
and current interactions are predominant. On Banzu sandy
flat, a key mechanism to produce high turbidity is considered
to be the combination of high waves, strong ebb-tidal current,
and acceleration as a result of wind blowing in the directionof ebb-tidal currents. By contrast, the bed elevation tends toaccrete immediately after the erosion episodes as the wave
height decreases. The waves are observed to develop and attenuate in response to the tidal elevations, suggesting thatthe short-term cyclic morphological processes also are attributed to the tides.

Figure 20. Conceptual illustration of the current generation on Banzu
intertidal flat: (a) tidal elevation; (b) velocities of wind-induced current,
tidal current, and wave-driven oscillatory flow; and (c) resultant total
current velocity. Tide is assumed to consist of only the semidiurnal component. High water and low water are set here to be at 3 h and 9 h,
respectively.

crepancy suggests that combined wave and current interaction is essential to generation of the bed shear stress as conceptually illustrated in Figure 20. If the contributions ofwaves and currents to produce the bed shear stress are equivalent (i.e., the amplitude of tidal currents and wave orbital
velocities are assumed to be the same), and the direction ofwind coincides with that of ebb tidal currents, the total velocity has the maximum value during ebbing at the phase of
1.5 h after the high water, as shown in Figure 20c. When theamplitudes of tidal currents and wave orbital velocities are
not the same, the phase of high turbidity is evidently different from that shown in Figure 19. When wind blows in the
direction of flood tidal currents, the maximum concentration
appears at a phase 1.5 h before the high water. This result
supports the importance of a combination of waves, tidal currents, and wind-induced currents to hydrodynamics, associated sediment transport, and topography variations on the
sandy tidal flat.

CONCLUSIONS
The results of a 16-day field measurement on hydrodynamics, sediment suspension and topography variations and a
6-year series of bed-profile surveys demonstrate that an es-

ACKNOWLEDGMENTS
The author is grateful to Dr. Keita Furukawa of National
Institute for Land and Infrastructure Management, Japan,
for his cooperation in making available the long-term topography data surveyed on Banzu tidal flat. Acknowledgement
is made to Dr. Tomohiro Kuwae of Port and Airport ResearchInstitute (PARI), Japan, Dr. Munemasa Nomura of Tohoku
University, Japan, Mr. Tsuyoshi Nakashima of Ministry ofLand, Infrastructure and Transport, Japan, and Mr. Tomoaki
Fujino of Toyo Construction Co., Ltd., Japan, for their earnest
help with the field measurements. The author would like toexpress sincere appreciation to Dr. Yoshiaki Kuriyama and
Dr. Kazumasa Katoh of PARI, Professor Terry Healy of the
University of Waikato, New Zealand, and an anonymous reviewer for their thorough and constructive comments on the
manuscript. This study was partially supported by Sumitomo
Foundation.

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