Journal of Marine Research, 67, 619 – 635, 2009

Asymmetries in tidal flow over a Seto Inland Sea scour pit
by Arnoldo Valle-Levinson1 and Xinyu Guo2
ABSTRACT
Underway profiles of current velocity were combined with stationary profiles of temperature andsalinity around a vertically mixed scour pit of the Seto Inland Sea throughout a semidiurnal tidal
cycle. This was done with the purpose of determining (a) whether flood flow is asymmetric relative toebb over a pit with weakly stratified conditions, and (b) whether there is a dynamic transition fromfrictionally dominated tidal flow to advectively dominated tidal flow over the pit. These questions
arose from previous studies elsewhere under stratified water columns, in contrast to the unstratified
conditions at the study site. Observations showed an acceleration of the flood tidal flow over the pit
and a deceleration during ebb. The flow acceleration over the pit during flood and deceleration during
ebb was attributed to asymmetric patterns of flow convergence/divergence. In turn, these divergence
patterns were influenced by the direction and strength of the baroclinic pressure gradient force, which
was 10 to 30% of the advective term, despite the relatively weak horizontal gradients of order
10⫺5 kg/m4. The non-negligible influence of the baroclinic pressure gradient was possible from therelatively large depths that exceeded 100 m at the deepest part of the pit, compared to the surrounding
depths of 30 m. From depth-averaged dynamical terms derived for flood and ebb phases of the tidal
cycle, it was found that the advective terms became more important than frictional terms over the
deep parts of the pit. Advection became more prominent than friction where the bottom slope
exceeded the value of the bottom drag coefficient (⬃0.003). Otherwise, frictional effects dominatedoutside the pit.

1. Introduction
Scour pits or bathymetric hollows are depressions in the ocean floor or elongated holesthat tend to be ubiquitous in estuarine systems (e.g., Davies and Brown, 2007). The
location of scour pits is often related to erosive processes caused by flow accelerations
around capes or headlands. Scour pits tend to be molded by the interaction of oscillatory
tidal flows with coastal morphology and sometimes under the influence of water column
density stratification. Oscillatory homogeneous flows moving over scour pits are expected
to decelerate upon their passage over the cross-section expansion represented by increasing
depths. This is analogous to flows moving past a sill (e.g., Armi, 1986). The residual flows
resulting from these oscillatory flows tend to depict re-circulating cells over the pit (Park

1. Civil and Coastal Engineering Department, University of Florida, Gainesville, Florida, 32611, U.S.A.
email: arnoldo@ufl.edu
2. Center for Marine Environmental Studies, Ehime University, Matsuyama, Japan.
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Journal of Marine Research[67, 5and Wang, 1991). Under stratified water columns, the flow reaction to the pit might bedifferent than that under unstratified conditions.
An oscillatory stratified flow interacting with a scour pit produces asymmetric distributions in hydrography (salinity and temperature) and flow (e.g., Davies and Brown, 2007;
Salas-Monreal and Valle-Levinson, 2009). This is also similar to a stratified flowinteracting with a sill or at a contraction (e.g., Farmer and Armi, 1986; Stenstrom, 2003).
The asymmetric distributions may consist of flow accelerating over the pit during flood anddecelerating over it during ebb (Salas-Monreal and Valle-Levinson, 2009). Alternatively,
the flow may decelerate during both phases of the tide (Davies and Brown, 2007) as it
moves over the pit. The deceleration still is asymmetric, showing more deceleration during
ebb phases than during flood phases. The reason for the flow acceleration over the pit
during flood was hypothesized to be a convergence of mass toward the scour pit(Salas-Monreal and Valle-Levinson, 2009). However, this mechanism of mass convergence does not explain the flow asymmetry between flood and ebb tides because there isalso mass convergence toward the pit during ebb.
The reason for the asymmetry was explained, on the basis of a conceptual model andnumerical solutions, as being caused by the presence of a horizontal density gradient(Cheng and Valle-Levinson, 2009). The conceptual model explained that a horizontal
density gradient, with a sign typical of that observed in estuaries (density decreasing
landward), weakens ebb flows and enhances flood flows over the scour pit. An important
aspect of the dynamics suggested by the observations in the Chesapeake Bay (SalasMonreal and Valle-Levinson, 2009) and by the idealized numerical solutions (Cheng andValle-Levinson, 2009), is that the flow dynamics are frictionally dominated outside the pit
and transition to advectively dominated inside the pit. A scaling analysis of the dynamicalterms involved in balancing the pressure gradient, i.e. advection and friction, indicates thatthe transition between dynamical regimes occurs over the area where the bathymetric slope
equals the bottom drag coefficient (⬃0.003).
This work investigates, with observations, two main issues that extend the findings ofthe research described above. First, it determines whether the flood/ebb asymmetries
caused by a horizontal density gradient over a scour pit are also observable in a system
where the water column is nearly vertically homogeneous but with weak horizontal density
gradients. Second, this work explores whether the transition in the dynamics fromfrictionally dominated flow outside the pit to advectively dominated flow over the pit isdetected in a weakly stratified system. A semidiurnal tidal cycle experiment, under calm
winds, was carried out over a scour pit in the highly energetic region of the Seto Inland Sea,
in Japan. Horizontal density gradients in this region were of order 10⫺5 kg/m4 or smaller.
Because of the weak density gradients observed during the period of measurements, nearly
symmetric distributions during ebb and flood, which are expected from homogeneous
water columns, were anticipated to appear over the scour pit sampled. However, notableebb/flood asymmetries, caused by the baroclinic pressure gradient, were observed in the2009]

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Figure 1. (a) Seto Inland Sea with bathymetry and showing area of study. (b) Zoom of the study area
showing the location of the scour pit. (c) Further zoom showing sampling trajectory and location ofhydrographic profiles (CTD casts marked as black circles) over the bathymetry off Kajitori no-ha.

flow. In addition, a transition was identified from frictionally dominated dynamics outside
the pit to advectively dominated dynamics over the pit.
2. Study area
This study was carried out at a scour pit on Kurushima Strait, located off the northernend of Japan’s Shikoku Island in the Seto Inland Sea (Fig. 1). The strait is oriented roughly
in the NE-SW direction and is delimited to the NW by Osaki-Shimo Island and to the SE by
Kajitori-no-Ha peninsula. Offshore of the peninsula, ⬃750 m to the NNW, a well-definedscour pit with a length of 5 km (along-pit direction), width of 1.5 km (cross-pit direction),and maximum depth of 120 km (Fig. 1), cuts through the surrounding depths of ⬃30 m. At
the southwestern flank of the scour pit the depth increases from 30 m to 120 m over a
distance of ⬃1.5 km, representing a slope of 0.06. Over the northeastern flank of the pit thebottom slope is gentler, with typical values of 0.03. The rim of the scour pit is characterized
by the presence of megaripples with wavelengths of ⬃200 m and heights of ⬃6 – 8 m. As
will be described here, those megaripples affect tidal flows.
Tides in the entire Seto Inland Sea are mixed with semidiurnal dominance. Tidal

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Journal of Marine Research[67, 5amplitudes can reach 2 m at Kurushima Strait and Bisan Strait (Yanagi et al., 1982), which
are the narrowest sections in the middle part of the Inland Sea. Bisan Strait, as aside
information, is at the other northward protrusion of Shikoku Island, to the east of
Kurushima Strait. Tidal currents at the narrowest part of Kurushima Strait, a few
hectometers farther into the Inland Sea from the scour pit, can reach more than 2 m/s while
over the scour pit they can exceed 1 m/s. For reference, tidal currents can reach up to 5 m/s
at Naruto Strait (Yanagi and Okaichi, 2007). The density gradients tend to be weak over theentire Seto Inland Sea because of the elevated tidal dissipation at most of the straits, where
the currents accelerate and maintain a well-mixed water column. Despite the weak verticaland horizontal gradients, as will be shown here, the horizontal baroclinic pressure gradients
may be influential in the dynamics of the system because its depth typically exceeds 30 m.
The geometry of the scour pit, its relatively short length and width, plus its remarkablebottom slopes, made it an ideal natural setting to sample the three-dimensional influence ofa scour pit on flow distributions. Furthermore, the strength of the tidal flows (⬃1 m/s) inthis area ensured the detection of flow alterations caused by the pit as shown next.
3. Data collection
Underway profiles of current velocity were combined with conductivity-temperaturedepth (CTD) profiles at fixed stations over a scour pit in the Kurushima Strait (Fig. 1) withthe purpose of describing the influence of the scour pit on tidal flows. Underway data werecollected over a sampling track that cut the scour hole once in the longitudinal direction
and twice in the transverse direction (Fig. 1) during one semidiurnal tidal cycle on May 31,
2007. Current velocities were collected with a 307.7 kHz RD Instruments acoustic Doppler
current profiler (ADCP) mounted on a pipe attached to the starboard side of the R/V Tobiuo(Flying Fish) of Ehime University. Acoustic pings with bin depths of 2 m were recorded
every second and averaged every 10 seconds while cruising at ⬃2 m/s. This sampling
scheme yielded a spatial resolution of ⬃20 m in the horizontal. The first ADCP bin wascentered at a depth of 2.76 m, resulting from a blanking interval of 0.6 m. The last usablebin was located at 90% of each profile depth because of side lobe effects from the angled
ADCP transducers. A total of 9 transect repetitions of the along-pit transect and 7
repetitions of the two cross-pit transects were carried out during the semidiurnal sampling
period.
Hydrographic profiles were obtained at 5 stations on the along-pit transect. Two stations
were located at each end of the transect, outside the scour pit. One CTD station was
occupied over the deepest part and two more stations were located over the slopes of thescour pit. The profiles were recorded with an Alec Instruments CTD, (ACL208-PDK) witha vertical resolution of 0.1 m and an accuracy of 0.3% for depth, ⫾0.02°C for temperature,and ⫾0.02 mS/cm for conductivity. The sampling scheme allowed a vertical resolution of
0.1 m in the CTD profiles. The 5 stations were occupied only during the first transect
repetition. After identifying vertically mixed conditions at every one of the 5 stations(⬍0.02 top-to-bottom difference in salinity and temperature), only the stations at the end of

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the transect were sampled subsequently. The salinity, temperature and density difference
between these two CTD stations at the end of the transect remained relatively constant(⬍0.04 kg/m3 between end stations) throughout the sampling period.
In addition to identifying instantaneous velocity fields as recorded underway, the ADCP
data were time-averaged for flood periods and for ebb periods. This allowed the descriptionof the typical distributions of flood and ebb flows over the scour pit. These average floodand ebb fields were also used to calculate depth-averaged advective accelerations andbottom frictional effects to investigate whether they changed along the transect. The CTD
data were used to calculate the influence of the baroclinic pressure gradient to thedepth-averaged dynamics, comparatively to the advective acceleration.

4. Data description
Sampling on both phases of the tide depicted acceleration of the tidal currents over thepit during flood and deceleration during ebb. This was attributed to asymmetric but
consistent divergence patterns between flood and ebb. Such asymmetry in divergence
patterns was ultimately related to the influence of the horizontal density gradient as
depicted next.
a. Flood phase
Sampling over the scour pit began during the flood phase of the tidal cycle (Fig. 2a). It isclear from the along-pit section that the instantaneous flow accelerated over the pit during
flood. Outside the pit, on the upstream side relative to the tidal flow, the tidal current was⬍0.4 m/s whereas in the deepest part of the pit, only two kilometers away, it was ⬃1 m/s.
The acceleration (gradient in shades) was evident at both the surface and bottom, but was
greater at the bottom and mid depths, allowing the flow to reach faster velocities(⬎1.2 m/s) than at the surface. As explained later, this acceleration was favored by thebaroclinic pressure gradient. The cross-pit transect on the upstream side of the pit, i.e. tothe southwest of the deepest point of the pit, showed lateral convergence and a spiraling
effect in the vertical plane (Fig. 2b). The lateral flow from the headland located to the southof the pit added mass to the hollow. The figure shows how the lateral flow cascaded down
to the deepest part of the pit while decelerating from ⬃0.5 m/s to ⬃0.1 m/s. The lateralcascading developed on both flanks of the pit and favored the spiraling recirculation
mentioned above. Analogously, the cross-pit transect on the downstream side of the pit,i.e., to the northeast of deepest point of the pit, displayed lateral divergence (Fig. 2c). Thiswas related to the flood flow expanding laterally to the sides of the pit as it moved past the
deepest point.
Consistent with the quasi-instantaneous snapshots captured during sampling, the average flood flows showed along-pit flow acceleration over the pit (Fig. 3a) synchronized withlateral flow convergence on the upstream side (Fig. 3b) and divergence on the downstreamside of the pit (Fig. 3c). The longitudinal transect showed greatest flood flows in the scour

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Journal of Marine Research[67, 5Figure 2. Instantaneous flow during flood phase of the tidal cycle. (a) Along-pit section (SW is to theleft) showing flow acceleration in the deepest part of the pit. The along-pit flow is shown in shades
and in arrows, moving from left to right during flood. (b) Cross-pit section on the southwestern(upstream relative to the flood flow) side of the pit showing flow convergence (looking toward theSW). Arrows represent lateral flow and shaded contours represent along-pit flow moving toward
the observer. (c) Cross-pit section on the northeastern side of the pit showing flow divergence(looking toward the SW). Arrows represent lateral flow and shaded contours represent along-pit
flow moving toward the observer.

pit, but concentrating near the bottom (Fig. 3a). This transect also displayed increased
speeds as the flood flow moved to shallower depths over the downstream end of the pit. The
lateral transects showed that on the upstream side of the pit, the greatest flood flow
appeared on the deepest part of the cross-section (Fig. 3b) but was displaced to the sideslopes on the downstream cross-section (Fig. 3c). The scenario of convergence of flow

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Figure 3. Same as Figure 2 but showing averages for the entire flood phase of the tidal cycle. Flowmoves from left to right on (a) and toward the observer on (b) and (c).

upstream of the pit and downstream divergence was also apparent during the ebb phase but
it was less robust than in flood. This is depicted next.
b. Ebb phase
The instantaneous flow pattern during the ebb phase of the tidal cycle was asymmetricwith respect to that during flood. During ebb, the instantaneous flow speed decreased fromaround 1 m/s outside the pit to ⬍0.5 m/s in the deepest part (Fig. 4a). Ebb flows thenaccelerated again as they moved on to shallow water. Upstream of the deepest part of thepit relative to ebb flow, to the northeast of the pit, the cross-pit section showed flow
convergence patterns (Fig. 4c). The lateral flows during ebb were weaker than during floodand the recirculation on the vertical plane was not as well defined as that observed in flood(Fig. 2b). Nonetheless, this pattern indicated addition of mass to the pit just upstream of the626

Journal of Marine Research[67, 5Figure 4. Instantaneous flow during ebb phase of the tidal cycle. (a) Along-pit section (SW is to theleft) showing flow deceleration in the deepest part of the pit. The along-pit flow is shown in shaded
contours and in arrows, moving from right to left during ebb. (b) Cross-pit section on thesouthwestern side of the pit showing flow divergence (looking toward the SW). (c) Cross-pit
section on the northeastern (upstream relative to the ebb flow) side of the pit showing flow
convergence (looking toward the SW). Arrows in (b) and (c) represent lateral flow and shaded
contours represent along-pit flow moving away from the observer.

deepest part. Downstream of the deepest part of the pit, relative to the ebb flow, thecross-pit section showed weak lateral flows with a vague pattern (Fig. 4b). In general, thedivergence patterns were consistent with those suggested, but not exactly observed, by
Salas-Monreal and Valle-Levinson (2009) in the sense that convergence developed
upstream of the pit and divergence appeared downstream. The divergence pattern emergedin both phases of the tidal cycle. In these observations, it is evident that the divergence2009]

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patterns were stronger and more robust in the flood phases than in the ebb phases of thetide. This asymmetry develops because the convergence pattern acts with the baroclinicpressure gradient to accelerate the flow toward the pit during flood but acts against thepressure gradient acceleration during ebb (Cheng and Valle-Levinson, 2009).
c. Baroclinic pressure gradient influence on tidal flow
The influence of the baroclinic pressure gradient on the flow speed V moving over asinusoidal scour pit of length L and depth h 2 , relative to the ambient depth outside the pit
h 1 , may be explained conceptually with a simplified, Bernoulli-type, frictionless momentum balance that yields (Cheng and Valle-Levinson, 2009):
V 2 ⫽ V o2 ⫺ 2g␩ ⫺ 2

冋

冉 冊册

␲y
g ⳵␳
g ⳵␳ h 2L
h y⫺2
1 ⫺ cos,␳ o ⳵y 1␳ m ⳵y ␲L(1)where V o is the flow speed as it enters the pit, y is the along-pit direction, ␩ is the sea
surface elevation, g is the acceleration caused by gravity, ␳ o is a reference water densityand ⳵␳/⳵ y is the along-pit horizontal density gradient. Note that the flow entering the pit V owill be reduced if the other three terms are positive. In particular, note that if the density
gradient is positive, i.e. density increasing in the direction of the flow, as in ebb periods,
then V will be reduced in the pit. Conversely, if the density gradient is negative as in floodphases of the tidal cycle, then the flow speed will increase over the pit. This conceptualargument was supported by numerical solutions (Cheng and Valle-Levinson, 2009).
Returning to the observations in Kurushima Strait, the average ebb flows also depicted adeceleration over the scour pit as seen in the along-pit section (Fig. 5a). The cross-pit
section also showed the largest along-pit current amplitude over the deepest part of the
upstream section (Fig. 5c) but moving to the slopes on the downstream cross-section (Fig.
5b). The lateral flow patterns were rather vague on both cross-sections, unlike the average
patterns observed during flood (Fig. 3b, c).
d. Vertical component of the flow
It is noteworthy, as an aside, that most fields recorded with the towed ADCP exhibitedcoherent patterns that followed the bathymetry. For instance, the vertical flow showed an
overall descending pattern as the ebb flow entered the pit and an ascending pattern on
leaving the pit (Fig. 6a). There were also coherent undulations of order 200 m wavelength
likely triggered by the small mount (or megaripple) appearing at a distance of 3.7 km. The
undulations were reasonably well resolved as the spatial precision of the averaged
measurements was ⬃20 m. Although the overall descending/ascending flow pattern is
expected for a fluid moving past a hole in the bottom, its observation with a towed ADCP,
which tends to move up and down while at tow, was unanticipated. A similar vertical flow
structure could be gleaned from the flood flow (Fig. 6b) in the deepest part of the scour pit,
where the flow followed the bathymetry. Furthermore, the vertical component of the flowclosely followed the megaripples that appeared on the edges of the channel. The

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Journal of Marine Research[67, 5Figure 5. Temporal averages of the observations collected during ebb phases of the tidal cycle (sameas Fig. 3, but for ebb). Flow moves from right to left on (a) and away from the observer on (b)and (c).

megaripples, like the flow undulations, are roughly 200 m in wavelength and 5–10 m tall,and therefore tend to modify the flow. The ripples actually seem to affect the entire watercolumn, as seen in Figure 6. The interaction of the tidal flow with such ripples generates
barotropic waves with wavelengths of the order of the bathymetry and heights that are oneorder of magnitude smaller than the ripple height. These wave properties are visual
estimates derived from the surface expression of the waves.
e. Mean flow
The tidally averaged flow resulting from the interaction with the scour pit is shown in
Figure 7. Both the tidally averaged surface flow and the tidally averaged depth-mean flow
are included to show that the mean flow had practically no vertical structure. These flow

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Figure 6. Contours of vertical component of velocity (in shades) for (a) ebb and (b) flood stages of
the tidal cycle. Horizontal arrows represent the direction of flood or ebb.

fields were derived from the tidal average obtained along the sampling trajectory and theninterpolated onto a uniform grid through Delaunay triangulations (e.g., Fang and Piegl,
1992, 1993). The mean fields depicted mainly northeastward flow with convergence on thesouthwestern flank of the pit and divergence on the northeastern flank. The convergence/
divergence pattern seems to be part of a pair of recirculating gyres: a counterclockwise
gyre over the northwestern part of the pit and flanks and a clockwise gyre over thesoutheastern part. Obviously, a geographically more extensive sampling trajectory isneeded to resolve these gyres. Along the longitudinal axis of the pit, the net flow is in theflood direction, which likely results from the asymmetric enhancement of tidal flood flows
over the pit. This agrees with the direction of the baroclinic pressure gradient force. The
dynamical implications of such flow patterns are explored next in the Discussion section.
5. Discussion
This section concentrates on exploring the dynamical implications associated with theasymmetric flow patterns over the scour pit from flood to ebb tides. In particular, it isessential to determine the role of the horizontal density gradient, in terms of the baroclinicpressure gradient, in causing such asymmetry, as suggested by Cheng and Valle-Levinson(2009) and Eq. (1). Also, it is essential to determine whether frictional or inertial630

Journal of Marine Research[67, 5Figure 7. Residual flow after one tidal cycle (a) at the surface and (b) depth-averaged. The fields arevery similar, which illustrates practically no vertical structure in the mean flows. Flows are drawn
over the bathymetry (scale shown on right) of the scour pit.

(advective) effects are responsible for balancing the influence of the pressure gradient. It
has been proposed that frictional effects balance the pressure gradient outside the pit andinertial effects become more dominant in the pit (Salas-Monreal and Valle-Levinson,
2009; Cheng and Valle-Levinson, 2009). These observations offer the opportunity to
explore those concepts further. In order to investigate the ideas presented above and the
dynamical implications, the spatial structure of four variables is described. The four
variables are the vertically averaged flow, the volume transport per unit channel width, theadvective or inertial accelerations in the momentum balance and the frictional terms. Thesefour variables are calculated for flood periods and compared to those calculated for ebb
periods.
During the flood phase of the tidal cycle, the vertically averaged along-pit flow,
calculated from the temporal mean of all flood measurements (Fig. 3a), showed an increaseover the pit (Fig. 8a, shaded region) but also as the tidal flow came out of the pit toward theshallow water column. However, the transport per unit width (m2/s) was a mirror image ofthe bathymetry, increasing only over the pit (Fig. 8b, shaded region). This was anindication of mass convergence toward the pit during this phase of the tidal cycle, as2009]

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Figure 8. (a) Depth-averaged flow, (b) transport per unit width, (c) dynamical terms, and (d) bottom
profile for flood phases of the tide. Overbars denote vertically averaged quantities.

illustrated in Figures 2b and 3b. The decrease in transport outside the pit, despite theacceleration of the tidal flow, was consistent with the mass divergence illustrated in Figures
2c and 3c.
With respect to the dynamic terms, frictional effects F (in m/s2) were represented
through parameterization of the bottom stress ␶b:
F⫽C bv关V兴
␶b
⫽
␳ oH
H

(2)

where ␳ o , as before, is a reference water’s density, H is the water column depth, which
varies with distance along the pit, C b is a bottom drag coefficient taken as 0.003, v is thealong-pit flow, V is the flow speed and the overbar indicates depth-averaged quantities.
These frictional effects are compared to the advective effects, calculated with the along-pit
flow as v⳵v/dy , where ⳵ y is the spatial resolution of the measurements (20 m). Figure 8cshows that the inertial or advective term is larger than the frictional term over the deepest
part of the pit (distance 2– 4 km). Over the shallower parts (distance 4 – 6 km) of the pit thedominance becomes less evident and advection even becomes comparable to the frictionalterm in places, as it is before and on entering the pit (before the shaded area).
The competition between advection and bottom friction can be characterized by scaling
these terms. Friction may be scaled as C b V s2 /H and advection as V s2 /L, where V s is a

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Journal of Marine Research[67, 5Figure 9. Same as Figure 8 but for the ebb phase.

velocity scale and L is a horizontal length scale. The velocity scale is really irrelevant
because it vanishes upon comparing both dynamic terms. Such comparison yields that bothterms are the same when C b equals H/L or the bottom slope. From this scaling it can besaid that inertia will dominate over friction where the bottom slope is greater than C b⬃0.003. On the other hand, friction will dominate where H/L is smaller than C b , (e.g.,
Salas-Monreal and Valle-Levinson, 2009). This can help explain why the advective term isgreater than friction in the pit and suggests a change in the dynamics governing the motion
over the pit relative to the shoals surrounding it. This difference is also observed during theebb phase of the tidal cycle.
During ebb, the vertically averaged along-pit flow, calculated from the temporal averageof the ebb flow observations (Fig. 5), shows a clear decrease in the pit (Fig. 9a, shaded
region), after an initial increase (between ⬃4 and 5 km). Such a decrease is expected from
Bernoulli-type dynamics in which the flow speed decreases as it goes through an expansionof the cross-section (e.g., Eq. (1) with no density gradient). This was in contrast to theacceleration observed during flood (Fig. 8a) and also consistent with the effects of thebaroclinic pressure gradient opposing the ebb tidal flows more strongly at greater depths.
On the other hand, the transport per unit width shows an increase over the pit (Fig. 9b,
shaded region). This increase is caused, as discussed in the description of the flow fields
during ebb, by convergence of mass from the sides of the pit. In terms of the dynamical
implications, the inertial or advective accelerations again become larger than frictional
effects over the pit (Fig. 9c, shaded region). The transition where advection begins to2009]

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Figure 10. Distribution of (a) water density along the pit (white symbols represent location of
measurements), (b) baroclinic pressure gradient along the bottom of the along-pit section, and (c)bathymetry of the along-pit section.

dominate over bottom friction in the dynamical balance emerges again where the bottom
slope approximately equals the bottom drag (0.003).
It appears then that there is convergence of mass toward the pit at both flood and ebb
phases and that the competition between advective and frictional accelerations is similar atboth tidal phases. The question that arises then is what causes the convergence to bestronger during flood and an acceleration of the tidal flow during flood and deceleration
during ebb? As suggested in the last sentence of the Ebb phase subsection of the Data
Description section (section 4) and by the conceptual analysis of Eq. (1), the answer belies
in the influence of the baroclinic pressure gradient on the tidal flow (Cheng andValle-Levinson, 2009). Even though hydrographic profiles exhibit rather weak verticalstratification and weak horizontal density gradients (0.04 kg/m3 in 6 km of the along-pit
transect) (Fig. 10a), the baroclinic pressure gradient along the hollow is not negligible (Fig.
10b). The values calculated from the along-pit transect of Figure 10a are of order
10⫺5 m/s2 or 10 to 30% of the advective accelerations. This is because the baroclinicpressure gradient is proportional to the depth of the water column, which exceeds 100 m inthe pit. These observations support the model results of Cheng and Valle-Levinson (2009)that indicate that the essential ingredient for the flood/ebb asymmetries in the speed of thetidal flows over the pit is the presence of a baroclinic pressure gradient influence. It is thisgradient that causes asymmetric convergence patterns from flood to ebb. During flood, thebaroclinic pressure gradient acts in conjunction with the barotropic pressure gradient thatdrives the tidal flow. The superposition of forces increases with depth and causes the634

Journal of Marine Research[67, 5near-bottom acceleration in the pit during flood. During ebb, the tidal flood is hindered at
depth by the baroclinic pressure gradient, most effectively in the great depths of the pit.
It can be reasonably argued that the flow patterns observed in this part of the Seto Inland
Sea may also be produced by the headland, Kajitori-no-ha, protruding toward the scour pit(Fig. 1). The headland is expected to favor convergence of mass toward the hollow during
flood phases of the tide and divergence during ebb phases of the tide. The convergence ofmass produced by Kajitori-no-ha is likely to produce flood flow accelerations over the pit.
However, the observed accelerations are better distinguished near the bottom, instead of at
the surface or throughout the water column as should be expected from the effects of theheadland. Moreover, the numerical results that indicated flow accelerations over a scour pit
during flood phases of the tide, the same results that motivated this study (Cheng andValle-Levinson, 2009), were obtained for a channel of constant width. The numericalmodel results indicated that the horizontal density gradient causes the asymmetries
observed in this study. Those two details (a) the fact that the observed flood flow
accelerations are most prominent near the bottom than at the surface, and (b) the agreementof the numerical results obtained in a straight channel with our observations, strongly
suggest that the change in channel width plays a secondary role in the flood flow
accelerations over the scour pit. Further numerical simulations that examine the presenceof a headland near the scour pit will better elucidate the relative role played by morphology
and by density gradients on the flow patterns observed.
6. Conclusion
The main finding of this study was that asymmetries in the tidal flow from flood tide to
ebb tide over a scour pit can develop even under seemingly homogeneous hydrography.
However, careful attention must be paid to the size of the baroclinic pressure gradientbecause ultimately it is the main agent that causes the asymmetry. This study also confirms
previous assertions of the transition in dynamics from friction-influenced flow outside thepit to advection-dominated flow inside the pit. The transition may be found where thebottom slope at the pit equals the bottom drag coefficient (⬃0.003).
Acknowledgments. The authors express their sincere gratitude to Mr. H. Ohnishi, the captain of
R. V. Tobiuo and to Drs. P.-H. Chang and D. Takahashi for their valuable help with the field work.
X. Guo was supported by a Grant-in-Aid for the Global COE Program from the Ministry of
Education, Culture, Sports, Science and Technology, Japan (MEXT), and the Japan Society for the
Promotion of Science (JSPS). AVL acknowledges support from the U.S. National Science Foundation projects 0551923 and 0726697. The manuscript was completed while AVL was on leave at theCentre for Water Research, University of Western Australia (2294).
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Received: 11 June, 2009; revised: 27 October, 2009.