Suspended Sediment Transport in a Rock-Bound, Macrotidal ... - BioOne

Journal of Coastal Research

29

2

358–371

Coconut Creek, Florida

March 2013

Suspended Sediment Transport in a Rock-Bound, Macrotidal Estuary: Han Estuary, Eastern Yellow Sea Hee J. Lee, Jun Y. Park, Sang H. Lee, Jeong M. Lee, and Tae K. Kim Marine Environments and Conservation Research Division Korea Institute of Ocean Science and Technology 787 Haeanro Ansan 426–744, Korea [email protected]

ABSTRACT Lee, H.J.; Park, J.Y.; Lee, S.H.; Lee, J.M., and Kim, T.K., 2013. Suspended sediment transport in a rock-bound, macrotidal estuary: Han estuary, eastern Yellow Sea. Journal of Coastal Research, 29(2), 358–371. Coconut Creek (Florida), ISSN 0749-0208. The Yellow Sea is fringed with deltas and estuaries on the Chinese and Korean coasts, respectively. The Korean estuaries, rock bound on the ria coast, are represented by the Han estuary, the largest in the eastern Yellow Sea. To reveal the relationship between extensive tidal flats and major channels in the Han estuary, suspended sediment transport was observed from the water column down to the bottom boundary layer. Transecting channels and deploying benthic tripods were accomplished at a number of critical sites during different spring tides, mostly under the fairweather conditions of dry seasons in 2006–08. The hydrodynamic measurements unraveled the role of macrotidal currents in estuarine suspended transport. According to lateral momentum analyses, cross-channel flows were governed by two forces: rotation and surface slope. The estimations of suspended flux and lateral momentum balance elucidated that, of the two candidate channels, the Yeomha Channel dominantly supplied riverine muds to the adjacent vast Ganghwa tidal flats. The other larger channel, Sukmo, was found to displace muds up the channel to the Han River mouth. Another offshore major channel, Jangbong, revealed small suspended flux, reflecting no substantial mud input of offshore origin into the Han estuary. This study may be timely in that most Korean estuaries have artificially developed and are facing various environmental problems.

ADDITIONAL INDEX WORDS: Rock-bound estuary, macrotidal, suspended sediment transport, lateral momentum analysis, Han River, Yellow Sea.

INTRODUCTION The estuary is a geologically peculiar setting in which wave, tidal currents, and freshwater all contribute to sedimentary processes (Dyer, 1989; Pritchard, 1967; Schubel and Kennedy, 1984). For tide-dominated environments, funnel-shaped estuaries have been sufficiently investigated to yield well-established estuarine models (Allen, 1991; Dalrymple and Choi, 2007; Dalrymple, Zaitlin, and Boyd, 1992). However, rockbound estuaries have also been frequently observed (Brothers et al., 2008; Fenster and FitzGerald, 1996; FitzGerald et al., 2000; Perillo, 1995; Woodroffe, 2002). These are usually dominated by former river valleys that now act as tidal channels. Many hydrographical investigations on the role of the channels have revealed that the momentum analyses of cross-channel flows are instrumental in evaluating sediment transport laterally onto the shoals or tidal flats (Chen and Sanford, 2009; Fugate, Friedrichs, and Sanford, 2007; Geyer, Signell, and Kineke, 1998; Woodruff et al., 2001). The hydrodynamics of cross-channel flows involves various driving DOI: 10.2112/JCOASTRES-D-12-00066.1 received 27 March 2012; accepted in revision 30 June 2012. Published Pre-print online 24 September 2012. Ó Coastal Education & Research Foundation 2013

forces, such as the Coriolis force, channel curvature, density gradient, local wind, and river discharge (Huijts et al., 2009; Huzzey and Brubaker, 1988; Lerczak and Geyer, 2004; Stacey, Burau, and Monismith, 2001; Valle-Levinson, Reyes, and Sanay, 2003). These analyses, though highly complicated, might be crucial to predicting potential changes in a coupled sedimentary system of tidal flats and adjacent channel due to artificial interventions in a tidal estuary. Numerous deltas and estuaries occur on either side of the Yellow Sea formed in Holocene transgression (Chough, Lee, and Yoon, 2000). Because of abundant sediment discharges from the large rivers, a series of deltas have developed on the Chinese coast. Among others, the Yangtze River delta is famous worldwide and well studied with respect to both modern and ancient sedimentological processes (Chen et al., 1999, 2000; Hori et al., 2001; Li et al., 2001; Liu et al., 2001; Wang et al., 2004; Yang, Eisma, and Ding, 2000; Zhang, 1999). In comparison, the rock-bound estuaries are scattered along the Korean ria coast, governed by relatively sediment-deficient small rivers. Although the Korean estuaries are closely associated with the sediments deposited in an eastern third of the Yellow Sea, these estuaries rarely have been investigated (Choi and Park, 2000; Kim, Choi, and Lee, 2006; Lee and Chu, 2001). For instance, the Han estuary, the largest in the eastern

Suspended Transport in a Rock-Bound, Macrotidal Estuary

Yellow Sea, has received little attention, particularly from the sediment dynamical viewpoint. The Han estuary is structurally controlled (Figure 1). Accordingly, it hardly exhibits the sedimentological pattern modeled for conventional funnel-type estuaries. Tidal ranges in this region are ample (4–8 m), and monsoonal winds mostly from the NW yield wave-dominated environments over the estuary during winter. Apart from flooding seasons (June– August), those complex environments pose great challenges on investigating recent sedimentation of the Han River–derived terrigenous materials. Nonetheless, any investigation in this region might shed light on the evolution of extensive tidal flats scattered in the Han estuary. The results might also give a clue to exploring the likelihood of sedimentological relationships between the Han estuarine muds and the shelf muds in the central Yellow Sea (Lee and Chough, 1989). Gyeonggi Bay, including the Han estuary, is one of the most extensively developed bays along the Korean coast. Therefore, a variety of environmental problems have continually arisen, most of which have been associated with sedimentary processes, such as erosion and siltation of main channels and tidal flats. However, little information has been available to understanding their cause and effect. In this context, preliminary sediment-dynamical observations were accomplished in the Han estuary for the first time during 2006–08 (KORDI, 2008). A series of field surveys were carried out during this period to observe short-term (1–5 d) sediment transport in the water column and near the bottom boundary layer, mostly during the fair-weather conditions of dry seasons. Based on

Figure 1. Map showing study area in the Han estuary, Gyeonggi Bay. Heavy lines and filled triangles denote the location of channel transecting and the deployments of benthic tripods, respectively. SC ¼ Sukmo Channel, YC ¼ Yeomha Channel, JC ¼ Jangbong Channel.

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these measurements, the purpose of this study is to provide a basic framework of suspended sediment transport by tidal currents in the Han estuary.

GEOLOGIC SETTING The Han River (34,674 km2 drainage area) annually empties relatively large amounts of water (2.53 1010 m3) and sediments (7.53104–1.33106) into Gyeonggi Bay (Figure 1; Oh and Bang, 2003). Many islands and channels are present offshore the Han River mouth. Among the islands, Ganghwa Island, the largest island in the estuary, is bound by two large channels, Yeomha in the east and Sukmo in the west. These channels, together with another major channel, Jangbong in the south, run NE– SW or N–S. Ebb currents were reported to be generally stronger than flood currents in the channels (Woo and Je, 2002). Although rather complex, with the channels and tidal flats, the water depth in Gyeonggi Bay gradually deepens to SW and S (Figure 1). The suspended load of the Han River mostly comprises silt (mean ¼ ~6u), which dominantly covers the river and seabeds around the river mouth. However, its mean concentrations prominently decreased due largely to the construction of underwater dams in the midreaches of the river (Chang and Oh, 1991). Drill cores more than 30 m long retrieved from supratidal flats in Yongjong Island show that the cored sediments consist dominantly of illite (53%), with the subordinate remainder consisting of chlorite (27%), kaolinite (16%), and smectite (4%; Moon et al., 1997). Although the assemblage of clay minerals around the river mouth and adjacent nearshore is similar to that of Yongjong Island, the content of illite tends to increase offshore at the expense of kaolinite and chlorite (Park and Oh, 1991). Seismic profiling demonstrates that tidal sand ridges 16 m high and 8 to 12 km long occurred on the nearshore seabed off Incheon, with the crestline normal to the major tidal axis (Bang et al., 1994). In addition, some former sand ridges were found buried underneath that were of a similar scale to the surficial one. The internal structures of the sand ridges indicate that sands were transported south, away from the river mouth. A few local maps of surface sediment distribution (Lee, Yoo, and Park, 1992; Oh, 1990; Woo and Je, 2002) were compiled to yield a distributional mosaic of sediment facies for Gyeonggi Bay (Figure 2). In the coastal to nearshore area, sandy mud facies is dominant, with increasing muds on tidal flats. Far offshore and in the inner shelf sand facies occur, part of the extensive sand sheet in the eastern Yellow Sea. Lee et al. (1988) suggested that the sand deposit is a palimpsest, resulting from Holocene transgression. Between these two contrasting facies lies a transitional facies of muddy sand (Figure 2). Such a distribution of muds is most likely attributed to the Han River. The tidal flats on the southern coast of Ganghwa Island (called Ganghwa tidal flats) also show a transition of surface sediment facies from mud in the east to sand in the west. Woo and Je (2002) demonstrated that on the eastern muddy tidal flats, abnormally watery and soft muds gradually deposited with a maximum rate of 5 cm/y during 1997–2000. By comparison, the western sand–mud mixture and sand exhibited fluctuating

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Figure 2. Distribution of surface sediments in the Han estuary and adjacent areas. Compiled from a few local distributions by Lee, Yoo, and Park (1992); Oh (1990); and Woo and Je (2002).

accumulation rates, frequently with dominant erosion (2 to4 cm/y), during 2002–03 (Kim, 2006). Gyeonggi Bay is under a monsoonal climate, with strong winds from the N, NNW, or NNE in winter and relatively mild winds from the SSW during summer. Meteorological data for 30 years (1971–2000) indicate that annual mean temperature is 12.28C and annual precipitation reaches 1344.2 mm, mostly concentrated between June and August (KMA, 2001). Near Incheon Harbor, the tidal prism in spring is 2.3 times greater than during neap, with mean tidal ranges of 7.99 and 3.46 m in spring and neap, respectively (Oh, 1995; Oh and Bang, 2003). The duration of flood phase and tidal range decreases up the Han River.

MATERIALS AND METHODS All measurements except for the one transecting across the Jangbong Channel (JL) were accomplished during the dry

Figure 3. Monthly precipitation at Seoul in the watershed of the Han River during 2006–08 (data from KMA, 2006, 2007, 2008).

seasons (Figure 3). Along the selected channel transects, current velocities were measured with a 1-MHz vesselmounted acoustic Doppler profiler (ADP, SonTek). The transects were repeatedly run across major channel entrances (Sukmo, Yeomha, and Jangbong) for one spring tidal cycle in different field surveys (Figure 1). The current velocities were recorded at 10-second intervals with a vertical bin size of 1 to 2 m. Onboard the returning ship, conductivity–temperature– depth (CTD) casts were taken at two to three stations with a SeaBird Electronics SBE-19þ recorder. Salinity and temperature were continuously measured at 5-second intervals. Each cast took less than 1 minute. In addition, water samples were collected at the same stations at three water levels (surface, middle, and lower) with a vacuum pump. Each transect took less than 20 minutes to complete and was repeated 8 to 16 times for one tidal cycle. In the laboratory, the water samples were filtered through 0.45-lm Millipore membranes and dried overnight at 608C. The suspended sediment concentrations (SSCs) were calculated from the dry weight of the filtered particles. Deployments of a benthic tripod called TISDOS were performed near the channel transects and on the Ganghwa tidal flats for 3 to 5 days (Figure 1 and Table 1). All measurements indicated fair-weather conditions, with waves mostly less than 0.1 m high. The benthic tripod was equipped with a Doppler current sensor (DCS3620, Aanderaa Instruments), one to two optical backscatter sensors (OBSs, Seapoint), and a Digiquartz depth sensor. Some tripods contained a SeaBird Electronics SBE-37 recorder to measure 1-minute averages of salinity and temperature at 10-minute intervals. The current sensor averaged 2-Hz, 30-second to 1-minute burst records at 10-minute intervals, whereas OBSs continuously measured the SSCs at 4 Hz. The electrical signals of the OBSs were calibrated in a recirculating tank using in situ bottom sediments following Downing and Beach (1989). The calibration of the acoustic signals from the ADP into the SSCs was attempted in vain, with poorly fitted regression curves from the preexisting calibration equations (Gordon, 1996). A Digiquartz recorded pressure variations due to water depth and waves in a sequence of 2-Hz, 8.5-minute bursts every 10 minutes. The details of surface sediment texture at the tripod sites are shown in Table 1. More details of the tripod configuration can be seen elsewhere (Lee, Jo, and Chu, 2006; Lee et al., 2004). A cross-channel momentum equation was considered to examine lateral circulations (Fugate, Friedrichs, and Sanford, 2007): dv ] ]v u2 ]g g ]q Az þ fu þ ðzÞ ¼0 ð1Þ þg dt ]z ]z ]y q ]y R where t is time; z is water depth; u and v are along-channel and cross-channel velocities, respectively; f is the Coriolis parameter; R is the radius of the channel curvature; g is the acceleration due to gravity; g is sea surface elevation; q is density; and Az is vertical eddy viscosity. Positive x and u are landward, and positive y and v are to the right looking landward; negative z is below sea surface level. Equation (1) shows relationships among the Coriolis forces, rotation, surface slope, density gradient, friction, and local and advective acceleration. To gain insight into the sign and relative



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Table 1. Summary values (mean and range) of hydrodynamic parameters with estimated residual current velocities and suspended sediment (SS) flux from benthic-tripod sites. Site

Date

Channel S3 04–06/03/08

V (m/s)

SSCs (g/L)

D (m)

T (8C)

S (PSUs)

d50 (mm)

0.3

0.43 (~0–0.82) 0.35 (~0–0.74) 0.37 (~0–0.94)

0.38 (~0–1.18) 0.25 (~0–1.19) 0.23 (~0–0.48)

16.5 (13.4–20.0) 18.0 (14.2–21.7) 10.9 (6.8–14.8)

2.4 (2.1–2.9) 20.1 (19.7–20.6) 20.2 (19.1–21.4)

29.3 (27.4–30.0) 26.3 (24.2–28.0) 17.5 (8.8–26.5)

0.032

0.09 (98)

5500 (3.48)

0.041

0.06 (478)

2800 (398)

0.040

0.16 (2718)

2800 (2708)

0.37 (~0–0.71) 0.38 (~0–1.50) 0.15 (~0–0.22) 0.35 (~0–0.74)

0.72 (~0–2.02) 2.37 (~0–10.31) 0.12 (~0–0.39) 0.12 (~0–0.45)

—

—

0.110

0.04 (2918)

1400 (2228)

—

—

0.087

0.07 (3328)

4200 (2858)

—

—

0.016

0.05 (2968)

431 (1238)

13.8 (12.9–15.1)

26.2 (18.1–29.6)

0.018

0.17 (688)

J1

24–26/07/06

0.7

Y2

09–11/10/07

0.3

Tidal flat TF1 21–26/09/06

0.7

TF2

21–26/09/06

0.2

TF6

01–04/05/07

0.4

TF7

01–04/05/07

0.4

4.6 (0–7.8) 3.2 (0–5.3) 3.0 (0–4.3) 3.7 (0–5.9)

Vr (m/s)

SS Flux (kg/m2/d)

h (m)

3600 (698)

h ¼ measurement height above seabed, V ¼ current velocity, SSCs ¼ suspended sediment concentrations, D ¼ water depth, T ¼ temperature, S ¼ salinity, PSUs ¼ practical salinity units, d50 ¼ median size of seabed grains, Vr ¼ residual current velocity.

magnitude of the forces with the measurement constraint and errors given in this study, the forces were grouped simply into four components: rotation, density gradient, surface slope, and acceleration and friction following Fugate, Friedrichs, and Sanford (2007). These components were estimated from the in situ channel observations as follows: Rotation¼ fu u2/R, where f¼0.953 104 s1 and R¼5, 18, and 18 km for the Sukmo, Jangbong, and Yeomha Channels, respectively Density gradient ¼ (z)(g/q)dq/dy from the measurements of temperature and salinity along individual transects Surface slope ¼ g]g/]y ¼ (1) 3 depth average of (Rotation þ Density gradient) Acceleration and friction ¼ dm/dt þ friction ¼ (1) 3 (Rotation þ Density gradient þ Surface slope) In addition, based on the gradients of density and current speeds, the Richardson number Ri was calculated as Ri ¼

gdq du 2 = qdz dz

ð2Þ

In the following figures, the normalized Richardson number log10(Ri/0.25) is presented. Residual (tidally averaged) current speeds in both the alongchannel and the cross-channel directions were calculated from the channel measurements of currents. The current data from individual cross-sections were interpolated onto 10 3 16, 10 3 37, and 10 3 9 grids for the Sukmo, Jangbong, and Yeomha Channels, respectively. The grids were designed to have proportional vertical spacing and uniform transverse spacing. This grid design was chosen for this study, although Perillo and Piccolo (1998) suggested a more complicated design in which the spacing is proportional in both directions. The chosen grid design was considered accurate enough to display the contrasting flow characteristics between the trough and the shoal of the channels in the macrotidal Han estuary. In a grid design with proportional transverse spacing, both water and seabed could be somewhat difficult to locate from a horizontal axis

scaled merely from 0 to 1. According to the relief of the seabed and variable tide, the vertical resolutions varied spatially and temporally in the range of 0.5 to 3.5 m. In comparison, the horizontal resolution was constantly 100 m. The resulting averaged sections indicated the distribution of current velocities through the relative water depth (0 to 1) at each point along the transect.

RESULTS Hydrodynamic Characteristics Sukmo Channel The peak tidal currents observed from the Sukmo transect (SL) on 11 May 2006 show spatially contrasting tidal dominance: flood dominance on the shoal and ebb dominance on the trough (Figure 4a). In general, the currents over a deep trough beside Ganghwa Island remain much stronger than those over the shallow shoal near Sukmo Island throughout the measurements. Cross-channel flows are observed with maximum speeds of 0.2 to 0.3 m/s (Figure 4b). Crosssectional–wide lateral circulation tends to occur only during flood, which is largely clockwise (Figure 4b). Salinity and temperature over the trough exhibit that the water column is considerably mixed during the midtidal phases (Figure 5a). Such a relatively low degree of stratification can also be shown by the distributions of water density and the Richardson number (Figures 4c and d). The SSCs over the trough indicate settling out of suspended sediments during ebb into low water and then resuspension processes during flood in the range of 100 to 300 mg/L (Figure 6a). Near the seabed of the trough, tripod measurements at station S3 show that the SSCs generally vary with tidal currents, which are rather flood dominant (Figure 7). Herein, the maximum values of SSCs amount to 0.2 to 0.4 g/L (Figure 7).

Jangbong Channel The flood-season measurements along JL on 25 July 2006 display no pronounced tidal asymmetry in current speeds.


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Figure 4. Midflood and midebb sections representing one tidal-cycle hydrographic measurements along the transect in the Sukmo Channel (SL) on 11 May 2006. (a) Along-channel velocity. (b) Cross-channel velocity. (c) rt. (d) Log10normalizedRi, which has negative values in shaded areas. Ri ¼ Richardson number. Letters A, B, and C in panel (c) denote the location of the CTD cast and water sampling. The seabed profile was somewhat variable during measurements due to a shift in course to avoid fishing net. For the location of SL, see Figure 1.

Along-channel current speeds are constantly stronger on a trough than on the shoal (Figure 8a). However, the near-bed currents from station J1 at the trough clearly demonstrate flood dominance. The cross-channel flows reach 0.1 m/s, with lateral circulations largely restricted to the trough (Figure 8b). These circulations are reversed during a tidal cycle, clockwise during midflood but anticlockwise during midebb. Considerable stratifications develop throughout a tidal cycle,

expressed in the distributions of water densities and the Richardson number, as well as salinity and temperature (Figures 5b, 8c, and 8d). The SSCs over the trough are generally higher during ebb than during flood, ranging from 10 to 50 mg/L (Figure 6b). In contrast, the SSCs near the seabed at station J1 show a reversed pattern of tidal variations, with higher flood values reaching the maximum of 0.6 g/L.



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Figure 5. One tidal cycle (12.5 h) of salinity (PSUs) and temperature (8C) over the deepest portions of (a) the Sukmo transect (SL), (b) the Jangbong transect (JL), and (c) the Yeomha transect (YL). For the location of the transects, see Figure 1.

Yeomha Channel The Yeomha Channel is a long, relatively narrow, shallow tidal channel in contrast to the Sukmo and Jangbong Channels (Figure 1). Current speeds measured from the Yeomha transect (YL) on 10 October 2007 and at tripod station Y3 during 9–11 October 2007 all indicate remarkable ebb dominance, with a maximum of about 1.4 m/s at the surface during midebb (Figure 9a). Throughout the tidal cycle, along-channel current speeds often reach 1.0 m/s, even 1 to 2 m from the seabed, whereas cross-channel current speeds are mostly less than 0.1 m/s (Figures 9a and b). The clockwise lateral circulation becomes distinctive during flood (Figure 9b). Salinity and temperature over the trough show almost vertically mixed profiles throughout the tidal cycle (Figure 5c). Water densities also suggest well-mixed waters with a distinctive lateral gradient (Figure 9c). However, the Richardson number remains higher than 0.5 during midflood because of small differences in current speeds between the surface and the near-bed (Figure 9d). The surficial SSCs exceed 300 mg/L during ebb compared to less than 100 mg/L during flood (Figure 6c). However, the SSCs in the lower water level reach more than 500 mg/L at both flood and ebb, with the maximum value of 1.2 g/L during flood. The near-bed SSCs from station Y2 also show peak flood values similar to the ebb counterpart, despite the large asymmetry of current speeds.

Ganghwa Tidal Flats Two sets of tripods were occupied on the mid- to lower Ganghwa tidal flats at different times: stations TF1 and TF2 represent the sand flats, and stations TF6 and TF7 represent the mud flats (Figure 1 and Table 1). On the sand flats, peak

Figure 6. One tidal cycle (12.5 h) of suspended sediment concentrations (SSCs, mg/L) over the deepest portions of (a) the Sukmo transect (SL), (b) the Jangbong transect (JL), and (c) the Yeomha transect (YL). For the location of the transects, see Figure 1.

tidal currents are much stronger at station TF2 than at station TF1. The progressive plot of tidal currents shows a spiral offshore track at both stations. The peak SSCs range between 1.0 and 2.5 g/L. In general, the SSCs vary well with current speeds. The behavior of tidal currents on the mud flats is quite different between the two stations TF6 and TF7. Tidal currents are strongest at both the beginning of flood and the end of ebb at station TF7 (Figure 7), whereas they vary little over the tidal cycle at station TF6. The residual currents, however, are directed westward at both stations. The SSCs depend exclusively on the current speeds at station TF7 as compared with station TF6, where the SSCs gradually change, increasing during flood and decreasing during ebb. On the whole, the SSCs remain less than 0.3 g/L at both stations.

Residual Currents along and across Channel The Sukmo Channel shows landward residual currents on the shoal but seaward residuals on the western flank of the trough (Figure 10a). For the water mass over the trough,


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Figure 7. Examples of time-series measurements from the tripod stations S3 and TF7. Waves were insignificant during most tripod measurements. For the location and measurement details of the stations, see Figure 1 and Table 1, respectively.

average current data are not available due to some incompletely measured sections. The magnitude of residual alongshore currents ranges from 0.05 to 0.10 m/s. Lateral residual flows, largely less than 0.05 m/s, direct NW, except for the lower water column over the western flank of the trough where currents flow SE (Figure 10a). In the Jangbong Channel, the current data from the trough are also missing—and likewise for the Sukmo Channel (Figure 10b). The residual alongchannel currents flow seaward over the shoal except for the northwesternmost 500 m, in which vertically uniform flows characteristically occur in both the along- and the crosssections (Figure 10b). This appears to be related to the existence of an opening gab between Jangbong Island and adjoining small isles (Figure 1). The residual along-channel current magnitude mostly exceeds 0.1 m/s. By comparison, the lateral residual flow is less than 0.05 m/s, rotating anticlockwise over the shoal (Figure 10b). The residual along-channel currents in the Yeomha Channel, in the range of 0.15 to 0.30 m/ s, are the greatest among the three channels (Figure 10c). They are unanimously directed seaward with a net water flux of 1.86 3103 m3/s. The lateral residual flow (,0.08 m/s) seems to rotate clockwise over the trough.

Cross-Channel Momentum and Estuarine Suspended Flux The analyses of cross-channel momentum show that the rotation and surface slope forces govern the momentum balance in all three channels (Figure 11). These two opposite forces are maximized during midflood in the Sukmo and Yeomha Channels but during midebb in the Jangbong Channel (Figure 11). This is because of the different directions of channel curvature between the former two and the latter. The largest values of the two forces are estimated at 3.5 to 5.0 3 104 m s2 from the Sukmo Channel. By comparison, the Yeomha Channel yields the largest density gradient forces (0.65 3 104 m s2). Although the acceleration and friction forces are comparatively less than 13104 m s2, their vertical distribution appears to be in good agreement with the rotational direction of lateral flows in the three channels (Figures 4b, 8b, 9b, and 11). The net suspended flux through the channel was calculated by summing the products of the SSCs and corresponding velocity through the water column along the transects of the three channels. The input values of SSCs were based on the vertical distribution of the SSCs from water samples (e.g., Figure 6). For the zones of the trough deeper than 25 m in the Sukmo and Jangbong Channels, the velocities were linearly



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Figure 8. Midflood and midebb sections representing one tidal-cycle hydrographic measurements along the transect in the Jangbong Channel (JL) on 25 July 2006. (a) Along-channel velocity. (b) Cross-channel velocity. (c) rt. (d) Log10normalizedRi. Ri ¼ Richardson number. Letters A, B, and C in panel (c) denote the location of the CTD cast and water sampling. The seabed profile was somewhat variable during measurements due to a shift in course to avoid fishing net. For the location of JL, see Figure 1.

interpolated from the values at 25 to 0 m at the seabed. The results indicate seaward transport of suspended sediments in the Yeomha Channel, in contrast to landward suspended flux in the Sukmo Channel (Figures 12a and c). For the Jangbong Channel, the suspended fluxes are mutually evasive between the trough and the shoal (Figure 12b). Although the Jangbong suspended flux was measured during the flooding season, it is insignificant as compared with those of the Sukmo and Yeomha Channels, which are similar to each other. In addition, the

suspended flux from each tripod station was calculated by integrating over a tidal cycle the vector products of the instantaneous SSCs and current velocity. As a result, the near-bed suspended flux is landward on the troughs of the Sukmo and Jangbong Channels (Figure 12d). However, the Yeomha tripod station shows a lateral transport of suspended sediments toward the tidal flat. On the Ganghwa tidal flats, suspended sediments appear to move roughly westward at all stations (Figure 12d).


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Figure 9. Midflood and midebb sections representing one tidal-cycle hydrographic measurements along the transect in the Yeomha Channel (YL) on 10 October 2007. (a) Along-channel velocity. (b) Cross-channel velocity. (c) rt. (d) Log10normalizedRi, which has negative values in shaded areas. Ri ¼ Richardson number. Letters A and B in panel (c) denote the location of the CTD cast and water sampling. The seabed profile was somewhat variable during measurements due to a shift in course to avoid fishing net. For the location of YL, see Figure 1.

DISCUSSION The estimations of suspended flux in the channels indicate that the Yeomha Channel is a major conduit of suspended sediments seaward from the Han River (Figure 12). Suspended sediments are transported landward rather than seaward through the Sukmo Channel. This results from the distinctive ebb dominance and higher SSCs of the Yeomha Channel compared with the Sukmo Channel, although the latter is

much larger than the former. The SSCs from the two channels, though measured in different seasons (May and October), still appear to be comparable to each other, because both measurements avoided flooding seasons (June–August; Figure 3). Part of the suspended mud from the Yeomha Channel is most likely to move toward the Ganghwa tidal flats to the west. This is supported by the benthic measurements on the channel and tidal flats (Figure 12) and by the distribution of surface



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Figure 10. Cross-sections of along- and cross-channel residual currents (m/s) for one tidal cycle from (a) the Sukmo transect (SL), (b) the Jangbong transect (JL), and (c) the Yeomha transect (YL). hz/h ¼ ratio of the water depth at z to the total water depth at a point along transect. For the location of the transects, see Figure 1.


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Figure 11. Representative vertical profiles of the forces incorporated in the lateral momentum analyses for the channels. (a) Sukmo Channel, midflood. (b) Jangbong Channel, midebb. (c) Yeomha Channel, midflood. Each channel shows the common dominant forces, rotation, and surface slope, which are greatest at the selected tidal phase. See the text for details of lateral momentum analyses.

Figure 12. Schematic diagram showing suspended flux estimated from the channel and benthic-tripod measurements. (a) Sukmo transect. (b) Jangbong transect. (c) Yeomha transect. (d) Benthic boundary layer. These estimations represent tidal-current–driven flux in dry-season, fair-weather hydrodynamic conditions.

sediments that suggests muds spread out from the Yeomha Channel (Figure 2). However, relatively far from the Han River mouth, the Jangbong Channel proved to contribute little to a suspended sediment budget for the Han estuary. The SSCs in the Jangbong Channel were lowest among the three channels despite the measurements conducted during a flooding season (July). This may mean that suspended sediments of an offshore origin hardly account for the mud accumulation in the Han estuary. The lateral flow analyses suggested an occurrence of clockwise (looking landward) circulation during flood in the Sukmo and Yeomha Channels (Figures 4b and 9b). Huijts et al. (2009) modeled lateral circulation that would be anticlockwise during ebb and clockwise during flood in the Northern Hemisphere due to Coriolis forces and mass conservation of along-channel tidal flows. In addition, the modeling of Lerczak and Geyer (2004) demonstrated that tidally averaged lateral circulation is asymmetric, with stronger near-bed flows to the left flank of the channel. They further suggested that this asymmetry may preferentially construct shoals on the left, resulting in asymmetric channel profiles. Likewise, the clockwise lateral circulation shown in both the flood and the residual sections of the Yeomha Channel may significantly contribute to the westward dispersal of turbid near-bed channel waters (Figures 9b and 10c). This near-bed suspended flux was observed at the Yeomha Channel seabed from tripod station Y3 (Figure 12d). Furthermore, the westward mud transport matches well with the source–sink relationship between the Yeomha Channel and the Ganghwa tidal flats suggested earlier. A stretch of tidal flats also develops on the shoal west of the Sukmo Channel (Figure 1). In the Jangbong Channel, the lower relatively turbid waters show no distinctive westward movements onto the shoal through a tidal cycle (Figures 8 and 10). The close relationships between the shoal and the channel can be found elsewhere (Chen and Sanford,



2009; Fugate, Friedrichs, and Sanford, 2007; Geyer, Signell, and Kineke, 1998; Woodruff et al., 2001). On the whole, the flows in the three channels were most likely to be partially mixed during the measurements, according to the methods of Dyer (1997). He proposed a simple indicator of the stratification state in estuaries, S/S0, where S is the difference of salinity between the surface and the bottom and S0 is the mean salinity. The estimated values of the indicator for the three channels range between 0.1 and 0.5, suggestive of the partially mixed state. Lateral circulations may be common in the channels of the Han estuary regardless of location and season. Without channel bends, the triangular bathymetry of channels could substantially generate lateral flows by bed friction and lateral variations of water depth. These basic physical forcings create both horizontal and vertical gradients of along-channel velocities and thus lateral flows through tidal rectification processes and Coriolis deflection (Huijts et al., 2009; Huzzey and Brubaker, 1988; Li and Valle-Levinson, 1999). Therefore, the direction of lateral flows may depend on the magnitude of friction relative to Coriolis acceleration, i.e., the vertical Ekman number (Valle-Levinson, Reyes, and Sanay, 2003). For instance, the direct effects of varying water depth on alongchannel flows can be seen in the tidally averaged cross-section of the Sukmo Channel, in which the along-channel residuals are mutually evasive between the shoal and the flank of the trough (Figure 10a). Such a water-depth–dependent residual current system has also been documented from the estuaries of the Delaware and Chesapeake Bays (Valle-Levinson and Lwiza, 1997; Wong, 1994). In addition, during the flooding season, when stratification is prominently intensified by the culminating freshwater discharge, a triangular channel could produce lateral flows simply by a combination of boundary mixing and Coriolis forcing (Chen and Sanford, 2009). Because the three channels of the Han estuary retain distinctively asymmetric bathymetry, the laterally varying water depth or sloping bottom may play a basic role in lateral momentum balance in any hydrographic environment of the Han estuary. The tidal variations of the SSCs from the tripod measurements suggest that most bottom sediments in channels, as well as tidal flats, could be resuspended by tidal currents alone (Figure 7). If waves would be superposed on tidal currents, the resuspension processes might be greatly enhanced. This is because the shear stress imposed to the seabed increases nonlinearly by the combined waves and currents (Soulsby, 1997). Offshore the midwest coast of Korea, the prevailing effects of waves on bed shear stress were quantitatively manifested from the wintertime benthic-boundary measurements (Lee and Lee, 2011). However, the directions of suspended flux in the channels of the Han estuary (Figure 12) may be more or less maintained even during winter. In the Han estuary, local wind fetch seems to be relatively small due to narrow channels and an abundance of islands and tidal flats. The small fetch seldom induces persistent wind-generated currents. On tidal flats, even though a spell of strong waves can induce abrupt changes in the distribution of surface sediments, tidal currents will most probably recover their normal distributions in a steady, prolonged way. During the summertime flooding seasons, the Yeomha Channel may still act as a

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principal supplier of suspended sediments to the sea. By comparison, it is difficult to predict whether suspended flux in the Sukmo Channel would be seaward or landward in this period, based on the dry-season results from this study alone. The lateral momentum of the two channels will be remarkably changed with the vigorous, turbid freshwater plumes. Therefore, it is still necessary to address flash-flood consequences to the tidally driven transport scheme of suspended sediments in the Han estuary.

CONCLUSIONS A network of tidal flats and channels, characteristic of a rockbound, macrotidal estuary, were sediment dynamically investigated from the Han estuary. The results illuminated that a complicated fine-grained sedimentary system could be basically explicable with estuarinewise suspended transport observations. In the Han estuary during the dry season, the channel transect measurements suggested that the Yeomha Channel played a predominant role in supplying the Han River–derived suspended sediments to the estuary. Another major channel, Sukmo, proved to contribute little to the estuarine mud repository, particularly the Ganghwa tidal flats. The direct linkage of the Yeomha Channel to the Ganghwa tidal flats was elaborated by lateral-flow analyses and benthic measurements, in addition to a simple distribution map of surface sediments. The momentum balance of rotation and surface slope forces caused lateral suspended flux to feed the tidal flats. The offshore suspended sediments were considered an insignificant source of the estuarine muds, based on the Jangbong Channel measurements. These lateral momentum analyses and suspended flux estimation might be further extended to offer a complete picture of sediment dynamics in the Han estuary channels with flash-flood observations. This study may underscore that a number of rock-bound estuaries on the Korean coast need sediment-dynamical explorations before their environments would be increasingly degraded artificially.

ACKNOWLEDGMENTS This study was supported by the Korea Ocean Research and Development Institute (Grant No. PE98735). The manuscript was improved by critical reviews from anonymous reviewers.

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