sand transport offshore the saemangeum dike, midwest coast of korea

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Sand Transport offshore the Saemangeum Dike, Midwest Coast of KoreaAuthor(s): Hee J. Lee and Seok LeeSource: Journal of Coastal Research, 27(1):153-165. 2011.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/JCOASTRES-D-10-00086.1URL: http://www.bioone.org/doi/full/10.2112/JCOASTRES-D-10-00086.1

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Sand Transport offshore the Saemangeum Dike,Midwest Coast of Korea

Hee J. Lee and Seok Lee

Marine Environment Research DepartmentKorea Ocean Research and Development InstituteAnsan, P.O. Box 29, Seoul 425-600, [email protected]

ABSTRACT

LEE, H.J. and LEE, S., 2011. Sand transport offshore the Saemangeum Dike, midwest coast of Korea. Journal of CoastalResearch, 27(1), 153–165. West Palm Beach (Florida), ISSN 0749-0208.

Geological investigations conducted over a vast seabed offshore the giant Saemangeum Dike on the west coast of Koreaindicate that the surface area of fine to very fine sands has rapidly expanded southward in recent years (2002–2008). Tounravel the processes responsible for this phenomenon, two field campaigns of hydrodynamic measurements werecarried out at the same station with an instrumented tripod during May 2007 and January 2008. The measurementsshow the definite occurrence of wind-generated residual currents that were more distinctive and consistent duringwinter compared with the relatively weak, inconsistent analogues of the late-spring season. The simple algebraiccalculations for bedload transport based on a cubic relationship between the bedload transport rate and steady currentssuggest that the southward movements of surface sands result from interplay of wintertime residual currents andmacrotidal currents. In addition, numerical model experiments illuminate that the dike construction has increased theN-S component of tidal currents over the study area. The sediment transport evaluations further suggest that thisartificial increase in tidal currents in the N-S direction could considerably enhance southward sand transport.

ADDITIONAL INDEX WORDS: Sediment transport, wind-generated currents, tidal currents, tidal models,Saemangeum Dike, southeastern Yellow Sea.

INTRODUCTION

The southeastern Yellow Sea is dominated by Holocene

transgressive sands on the offshore seabed, whereas the

coastline of Korea is characterized by extensive tidal flats of

mud-sand mixtures (Chough, Lee, and Yoon, 2000; Kim et

al., 1999; Lee, Chu, and Park, 1999; Lee et al., 1994). The

shallow water depths and strong macrotidal currents

inherited from an epicontinental setting with few estuarine

point sources have generally hindered mud accretion onto

the transgressive inner-shelf sandy surface. However, at the

same time, these sands appear to be hardly transported in

any direction for a considerable distance by tidal currents

because tides offshore are slightly flood dominant or near

symmetrical (Lie, Lee, and Cho, 2002). Furthermore,

representative sand grains of 0.08–0.17 mm for the well-

sorted, offshore transgressive sands (Lee et al., 1987) suggest

that the onset of their bedload transport most likely requires

additional forcing to the tidal current; a roughly estimated

skin shear velocity (u*cs , 0.015 m/s) by tidal currents with

nominal mean current velocity and water depth of 0.5 m/s

and 15 m, respectively, barely exceeds threshold shear

velocity (u*cr , 0.012 m/s) of the sand grains according to

the method of Soulsby (1997).

The Asian monsoon system prevailing over the Yellow Sea is

characterized by the pronounced seasonality of summer

precipitation and winter wind (Xiao and An, 1999; Zhang et

al., 2008). Accordingly, the Yellow Sea experiences energetic

wind-generated currents and waves during winter, which are

generally far less frequent during summer, resulting in a

strong seasonal contrast with respect to the hydrodynamic

conditions of sediment transport. It is therefore the combined

flow of tidal currents and wind-generated currents and waves

that make the southernmost part of the transgressive sands

offshore the Saemangeum Dike begin to move actively during

the winter season (Lee and Ryu, 2008). The winter monsoonal

forcing, a strong northwesterly or northerly wind, also plays a

major role in along-coast mud movements such that the muds,

temporarily accumulating on tidal flats near estuaries or on

relatively straight coasts in small embayments facing the

Yellow Sea in the NW direction, are winnowed seaward and

then transported south by littoral drift (Lee and Chough, 1989).

The littoral plume visits ria-type bays and is scattered on its

way; part of the suspended particles settle out in sheltered

areas, forming water-saturated mud flats (KORDI, 2005; Lee

and Chu, 2001; Lee et al., 2004). Therefore, monsoon, in

addition to macrotidal currents, should principally control the

dynamics of modern sediments in the southeastern Yellow Sea.

The Saemangeum area is dominated by the Saemangeum

Dike that encloses the vast estuarine areas in the midwest

coast of Korea, governed by two rivers, Mangyung and Dongjin

(Figure 1). Dike construction commenced in 1991 for the initial

purposes of creating paddy fields from reclaimed tidal flats. It

DOI: 10.2112/JCOASTRES-D-10-00086.1 received 6 June 2010;accepted 17 August 2010.Published Pre-print online 12 November 2010.’ Coastal Education & Research Foundation 2011

Journal of Coastal Research 27 1 153–165 West Palm Beach, Florida January 2011

was completed without any open gaps in 2006, followed by

extensive civic rather than agricultural developments inside

the dike due to changing economic interests. The emplacement

of a giant dike in the estuarine environment resulted in large-

scale, unpredicted changes in the bathymetry and sediment

facies on the seabed both outside and inside the dike (Lee and

Ryu, 2008). Along the dike, erosion occurred on both sides, and

hence muddy sediments underneath were exposed on the

Figure 1. Map showing the Saemangeum area dominated by Saemangeum Dike (thick solid line) consisting of four sectors (I to IV). Dots stand for sampling

sites of surface sediments in front of sector IV within the modeling area for tidal currents (dashed-line box). The solid triangle represents station S4-1 for

hydrodynamic measurements with a benthic tripod, and a solid square and a solid circle the meteorological stations at Buan and the Gogunsan Archipelago,

respectively. The wind data for 20-year averages and the measurement period were derived from the two meteorological stations, respectively. MGR 5 mouth

of the Geum River; MMR 5 mouth of the Mangyung River; MDR 5 mouth of the Dongjin River.

154 Lee and Lee

Journal of Coastal Research, Vol. 27, No. 1, 2011

seabed (KORDI, 2003). The open gaps of the dike also led to a

deepening of the associated tidal channels inside the dike. In

particular, sands offshore the dike were found to move

southward at a seemingly much faster rate in recent years

(KORDI, 2006). This phenomenon appears to be man-made to a

large extent, owing to the dike with additional help from the

winter monsoonal forcing. To find out what physical conditions

the dike changed and how these changes accelerated the

transport of sands, we conducted field observations of the

benthic boundary layer (BBL) with an instrumented tripod on

the sandy seabed off the dike. A tidal model was used to

simulate tidal regimes with and without the dike for the

offshore of the Saemangeum area. Our objectives were to

document the existence of residual or wind-driven currents

during winter; to reveal the dike-induced changes in the tidal

currents; and to prove, through simple calculations with a

generalized bedload transport equation, the nonlinear increase

in the southward sand movement by the combination of the two

aforementioned effects under the macrotidal regime.

STUDY AREA

The Saemangeum Dike is a gigantic offshore structure (33 km

long) confining both riverine water and sediments within an

estuarine area on the midwest coast of Korea (Figure 1). It took

about 15 years (1991 to 2006) to complete the dike. Since three

gaps (two of them at the sluices in sectors I and II and the other

in the middle of sector IV of the dike) remained open during

construction, the combined system of the dike and the trapped

estuary appeared to act as a huge natural barrier-lagoon

environment. However, the gaps allowed little sediment

exchange between the inside of the dike and offshore because

of the existence of thresholds formed of piles of net-tied rock

fragments across the gaps. Instead, large moats were carved on

the seabed on either side of the gaps due to flow separation and

associated erosion (Chu et al., 2001). The gap on sector IV was

completely closed by June 2003, and the other two on sectors I

and II were closed with the sluices by April 2006. Inside the

dike, coarse-grained sediments have rapidly accumulated on the

subtidal areas resulting in creation of additional tidal flats and

incipient land soils evolving from the former supratidal flats

(KORDI, 2004; Lee and Ryu, 2008). Offshore the dike, deep

erosional troughs formed along the dike as moats during dike

construction. This erosion (of up to 12 m of sediments vertically)

exposed the underlying, hardened muddy sediments (KORDI,

2003). Farther offshore sector IV, there are no apparent signs

yet of variations in sediment types and topography triggered by

dike construction, except for the surficial sandy area, part of the

transgressive sand sheet, which was prominently expanded in

recent years (KORDI, 2006).

The long-term (1980–2004) wind data from a meteorological

station at Buan (Figure 1) show that the northwesterly winds

(from the NW, WNW, and NNW) have been dominant over the

Saemangeum area, blowing 31% of the year on average

(Figure 2; KORDI, 2005). The occurrence of strong wind speeds

is highest for the northwesterly winds: 19.7% and 4.8% for the

ranges of 0.3–3.3 m/s and 3.4–7.9 m/s, respectively, as

compared with 12.0% and 3.1% for the subordinate southerly

winds (Figure 2). Based on these wind data, the mean peak

significant wave heights by the northwesterly winds are

estimated to be in the range of 1.0–5.8 m. While the study

area is still characterized by the macrotidal regime with a tidal

range over 5.0 m (NORI, 2005, 2006), the tidal regime

experienced an abrupt decrease by up to 50% in current

intensities offshore the dike after closure of the two gaps into

sluices in sectors I and II in 2006. On the other hand, the

closure of the gap in sector IV exerted much less influence on

the tidal range and current speed (Lee et al., 2008). The

amplitude of M2, the most dominant constituent, was estimat-

ed at about 2.1 m before the gap in sector IV was closed in 2003

(KORDI, 2007). A weak horizontal water circulation is inferred

to occur due to a slight tidal asymmetry offshore sector IV (Kim

et al., 2008). Before dike construction, the major tidal axis

offshore sector IV was generally in the E-W direction with peak

current speeds on the order of 0.5 m/s. By comparison, tidal

current speeds inside the dike exceeded 1 m/s along main tidal

channels near the river mouths (Lee et al., 2008).

MATERIALS AND METHODS

A benthic tripod, called the Tidal Sedimentary Dynamics

Observational System, was deployed to investigate hydrody-

namic characteristics in the BBL at station S4-1 offshore sector

IV (Figure 1). Measurements of water depths, currents,

temperature, salinity, and suspended sediment concentrations

(SSCs) were performed twice at the same station during the

periods of 15–20 May 2007 and 22–27 January 2008. Wind data

for the measurement periods were obtained from an insular

meteorological station in the Gogunsan Archipelago near

station S4-1 (Figure 1).

Water depth was continuously measured 1.2 m above the bed

(AB) using a Digiquartz pressure transducer at 2 Hz through-

out the measurements, and significant wave heights (Hs) were

Figure 2. Scatter diagram of percent wind speeds for the Saemangeum

area based on the 20-year (1981–2000) wind statistics from the Buan

meteorological station (see Figure 1), after KORDI (2006). Note that winds

from the NW, WNW, and NNW show the most frequent occurrence of the

highest speed class of 3.4–7.9 m/s.

Sand Transport offshore a Giant Dike 155


estimated from time series of water depth by up-zero-crossing

methods. Currents were recorded 1.0 m AB as 30-second (or 1-

minute) averages every 10 minutes with an acoustic Doppler

current sensor (DCS3620, Aanderaa Data Instruments, Ber-

gen, Norway). For temperature and salinity, a thermistor or a

conductivity and temperature recorder (SBE 37-SM MicroCAT,

Sea-Bird Electronics, Inc., Bellevue, Washington) was used to

record 10-minute averages every 30 minutes. The SSCs were

measured continuously at 4 Hz at the same level as that of the

currents (plus another level of 0.65 m AB for the late-spring

measurements) using the Optical Backscatter Sensor (OBS) of

Seapoint Sensors, Inc. (Exeter, New Hampshire). The OBS was

calibrated in a recirculating tank with in situ sediments

obtained from the station following Downing and Beach

(1989). Unfortunately, the OBS malfunctioned during the

wintertime measurements when a fragmental fishing net

wrapped around it. The textural data from the station were

obtained using a standard method of dry sieving at 0.5 W

intervals for the sand fractions, and with a Sedigraph 5100 for

the mud fractions after organic matter and calcium carbonates

were removed with 10% H2O2 and 0.1 N HCl, respectively. The

mean sizes for the winter and late-spring seasons were 0.13 mm

(94% sand) and 0.09 mm (78% sand), respectively.

The numerical model adopted was a modified version of the

Princeton Ocean Model using primitive equations (Blumberg

and Mellor, 1987). The wetting-drying scheme of Flather and

Heaps (1975) was incorporated to represent a huge extent of

tidal flats in the Saemangeum area. The model domain covers

77 km and 72 km from north to south and from east to west,

respectively, including all the sectors of the Saemangeum Dike

(Figure 1). The model grid system consists of 10 vertical sigma

intervals with a 300-m horizontal resolution. With the tidal

elevation forced on the open boundary, the harmonic constants

of eight constituents (M2, S2, N2, K1, O1, M4, MS4, and MSf) were

evaluated in the numerical modeling to simulate changes of

tidal current regime offshore sector IV during the course of dike

construction. We compared three phases of dike construction:

before dike construction (Case 1), during dike construction with

an opening gap (Case 2), and after closure of the gap (Case 3).

DISTRIBUTION OF SURFACE SANDS

The surface sands off sector IV underwent alternating

expansion and contraction on a large spatial scale during the

period 2002–2008 (Figure 3). The 2002 sand sheet was not

present at all in a distributional map of surface sediments in

1998 (Chu et al., 2001). By 2005, the study area was covered

predominantly with surface sands. However, surface sands

were found to be remarkably restricted to much smaller areas

in 2007 because of extensive dredging of sands. Because the

dredging operations were hampered or even ceased owing to

diminishing sand deposits and an environmental concern, the

surface sands again expanded in just 1 year (Figure 3). Even

though the texture of the sands was rather uniform with mean

size in a relatively narrow range of variations (0.09–0.25 mm),

there was still a tendency toward slight decrease of mean size to

the south (KORDI, 2003). Together with the finding that a

train of sand waves were marching south or southwest (Lee and

Ryu, 2008), this textural trend attests to the prominent

phenomenon of the rapid expansion of surface sands offshore

the dike through southward sand movements. The sands,

however, lay on the surface in a veneer form less than 1 cm

thick in newly expanded areas, reflecting that the sand

transport was marginal enough to cover the relatively vast

areas (KORDI, 2008).

HYDRODYNAMIC OBSERVATIONS

Winter

Figure 4 shows time series of data obtained at a spring tide

during 22–27 January 2008. The water depth ranged between

7.6 m and 14.4 m, indicating tidal ranges of 5–6 m with a

diurnal inequality of up to 1 m. During the measurements,

winds blew mostly from the NW or NE with maximum speeds of

16 m/s, and current speeds were strongest, exceeding 0.4 m/s,

on 23–25 January when the waves were highest (Hs 5 2.0–

2.5 m). For the rest of the measurement periods, wind speed

was less than 10 m/s with Hs varying from 0.5 m to 1.5 m, and

peak current speeds were usually in the range of 0.2–0.3 m/s.

The water depth and current speed data in this period

demonstrate that flood tidal currents were generally dominant.

This coherency between the maximum speeds of currents and

wind reflect that there were residual currents caused by winds.

The residual currents were estimated from the current

measurements that were de-tided by a harmonic analysis with

three constituents (M2, K1, and M4) and low-pass filtered by a

25-hour moving averaging technique (Figure 5). It is clear that

the speeds of residual currents or wind-driven currents vary in

rather good accordance with wave heights or wind speeds

(Figures 4 and 5). The residual currents were all directed south

or SW except for a few hours on 26 January 2008, with the

Figure 3. Comparison of distributional areas of surface sand (shaded

area) off sector IV during the period 2002–2008. Note that the sandy area

experienced remarkable contraction in 2007 due to sand dredging. The

unshaded area is the seabed of mud-sand mixtures over which the surface

sand moves. For sampling sites of the surface sediments, see Figure 1.

156 Lee and Lee


maximum residual speeds estimated at 0.1 m/s during the

highest waves on 23–24 January 2008. The asymmetry in the

measured currents was variable over time such that tidal

currents seemed to be markedly ebb dominant during the

highest waves; then as waves waned, the ebb dominance

diminished, eventually overturned by flood dominance (Fig-

ure 4). Such a seemingly variable tidal asymmetry should be

caused by southward-flowing residual currents superimposed

on inherently flood-dominated tidal currents. The major tidal

axis is in the direction of NE-SW, with flood currents flowing

NE. The temperature generally showed a gradual decrease

over time from about 6.0uC to lower than 4.5uC (Figure 4).

Late Spring

The BBL measurements were also carried out at a spring tide

during 15–20 May 2007, when water depth varied between

7.8 m and 14.8 m. This and other tidal characteristics were

virtually similar to those of the winter measurements (Fig-

ure 6). Tidal currents were consistently flood dominant (up to

0.1 m/s greater than the ebb flow), with peak flood current

speeds around 0.4 m/s (Figure 6). Wind direction during the

measurements was in the range of S-W with speeds up to 14

m/s. Although strongest southerly winds took place early in

Figure 4. Time series of hydrodynamic data, including water depth, currents, significant wave height (Hs), temperature, and wind speed and direction from

station S4-1 during 22–27 January 2008. The bottom boundary layer of 1 m of the seabed was measured with an instrumented tripod. Wind data were

recorded at a meteorological station in the Gogunsan Archipelago. Lines and dots in the second and fourth panels represent current speed/direction and wind

speed/direction, respectively. Currents and wind are oriented, respectively, toward and from the azimuth measured clockwise relative to the north. For

locations of hydrodynamic and meteorological stations, see Figure 1.

Figure 5. Time series of residual currents derived from the current

data shown in Figures 4 and 6 by a tidal harmonic analysis. Note

that strong currents of up to 0.1 m/s occurred during the wintertime

measurements.



the measurements (17 May 2007), waves in this period

were less than 0.5 m. In contrast, the highest waves, with

Hs 5 1.3 m, were recorded in the subsequent less strong winds

from the west (8–10 m/s on 18 May 2007; see Figure 6). This is

because the Gogunsan Archipelago should have blocked

southerly winds against the measurement site to the north.

The evaluated residual currents were relatively weak (maxi-

mum 0.04 m/s) and unanimously flowing N or NE (Figure 5).

The SSCs were controlled mainly by waves, with maximum

values of 0.4 g/L occurring 0.65 m above the bed at the highest

waves (Figure 6). In general, the SSCs exceeded 0.1 g/L, far

above the background value of 0.03 g/L, when Hs was larger

than 0.5 m. Temperature gradually increased from 14.0uC to

over 15.0uC , whereas salinity showed a slight decrease from

31.7 to 30.7 psu with time (Figure 6).

RESULTS OF TIDAL MODELING

Numerical model experiments show remarkable changes

in tidal currents during dike construction (Cases 2 and 3) as

compared with the initial conditions without the dike (Case

1) in the offshore area of sector IV (Figure 7). In Case 2,

during construction with an opening gap, water exchange

was concentrated at the gap. Except for the area around the

gap, however, tidal currents tended to flow parallel to the

dike as a result of counterclockwise rotation of the major axis

(Figure 7b). The spatial pattern of tidal currents was further

modified after closure of the gap (Case 3). With the gap

closed, tidal currents were rotated all along the dike, enough

to flow parallel to the dike over the offshore area up to 3–

4 km from the dike (Figure 7c). It is remarkable that the N-S

Figure 6. Time series of hydrodynamic data, including water depth, currents, significant wave height (Hs), temperature, wind speed and direction, and

salinity from station S4-1 during 15–20 May 2007. The bottom boundary layer of 1 m of the seabed was measured with an instrumented tripod. Wind data

were recorded at a meteorological station in the Gogunsan Archipelago. Lines and dots in the second and fourth panels represent current speed/direction and

wind speed/direction, respectively. Currents and wind are oriented, respectively, toward and from the azimuth measured clockwise relative to the north. For

locations of hydrodynamic and meteorological stations, see Figure 1.

158 Lee and Lee


component of tidal currents (hereafter, NST) was increased

by dike construction (Figure 8). The comparison between

Case 1 and Case 2 shows a definite increase (up to 0.2 m/s) in

the NST in the model area except for a small patch around

the gap (Figure 8a). Case 3 suggests an even greater

increase in the NST (maximum 0.24 m/s) in most of the

model area than in Case 2 (Figure 8b). The comparison of

Case 3 with Case 1 hence indicates a consistent increase over

the entire model area in the NST after closure of the gap

(Figure 8c). Therefore, dike construction induced the rota-

tion of tidal currents counterclockwise for the major tidal

axis to be parallel with the dike, resulting in the increase of

the NST up to over 0.2 m/s—a prevailing hydrodynamic

consequence of this artificial structure.

Figure 7. Calculated tidal current ellipses of M2: (a) Case 1, before dike construction, (b) Case 2, during construction with a gap, and (c) Case 3, after closure

of the gap. The ellipses are plotted at four-grid intervals.



NONLINEAR TRANSPORT EFFECTS OFCOMBINED RESIDUAL AND TIDAL CURRENTS

A number of formulas for the dimensionless bedload

transport rate (W) have been suggested in diverse theoretical

and experimental results (Gadd, Lavelle, and Swift, 1978;

Hardisty, 1983; Ribberink, 1998; Soulsby, 1997; Soulsby and

Damgaard, 2005; Vincent, Young, and Swift, 1981). Among

them, we adopted a simple, workable one:

W~ c1h0:5 h{hcrð Þ ð1Þ

Here, h 5 t0 / gr(s 2 1)d is the Shields parameter; c1 is a

constant; and hcr is the threshold Shields parameter, where

Figure 8. Differences in the N-S component amplitude of M2 tidal currents: (a) Case 2–Case 1, (b) Case 3–Case 2, and (c) Case 3–Case 1. The shadowed area

denotes positive differences. Contour interval is 0.02 m/s. For explanations of the cases, see Figure 7.

160 Lee and Lee


t0 5 bed shear stress, g 5 acceleration due to gravity, r 5 water

density, s 5 the ratio of densities of grain and water, and d 5

the diameter of grains. This study focuses on the active

transport events during peak tidal currents in spring or during

strong winds when the shear stress imposed by energetic water

movements far exceeds the threshold shear stress of the seabed

sediment grains. For simplicity, we can obtain an approximat-

ed formula for the bedload transport rate as a result of ignoring

the term hcr in Equation (1): W 5 c1h1.5. With t0 , U2, where

U 5 mean current speed, Equation (1) can be expressed finally

as a function of only the mean current speed:

W~ c2�UU3 ð2Þ

In order to delineate the role of residual currents superim-

posed on tidal currents in bedload transport during winter, the

N-S components of tidal currents are considered because the

residual currents in winter mostly flow southward owing to the

monsoonal winds from N or NW. A schematic diagram of the

relationship among southerly residual or wind-driven currents

(u), mean south component of ebb and north component of flood

currents (UeS and UfN, respectively), and the resultant currents

in the ebb and flood directions (UeS + u and UfN 2 u,

respectively) is shown in Figure 9.

Given that the shear velocities by all types of currents in the

schematic diagram exceed the threshold shear velocity, u*cr, of

the seabed sands, the quantities of the bedload transport during

one tidal cycle by tidal currents (Qt), wind-driven currents (Qw),

and combined wind-driven and tidal currents (Qtw) are:

Qt ~ 0:5c2T �UU3eS{ �UU3

fN

� �ð3Þ

Qw ~ c2Tu3 ð4Þ

Qtw ~ 0:5c2T �UUeSzu� �3

{ �UUfN{u� �3

h ið5Þ

where T 5 period of 1 tidal cycle < 12.5 hours. The difference

between Qtw and Qt + Qw is:

Qtw{ QtzQwð Þ~ 1:5c2Tu �UU2eSz �UU2

fNz �UUeSu{ �UUfNu� �

ð6Þ

Because the values of UeS and UfN are generally much greater

than the value of u, the sum of the items in parentheses in

Equation (6) should always be positive. The positive sign of the

difference, Qtw 2 (Qt + Qw), means that the bedload transport

by combined tidal and residual currents exceeds a simple

addition of the transports by the tidal current and residual

current. The rationale of Equations (2)–(6) can also be applied to

a large extent to the suspended load that is expressed on the

basis of theory and empirical formulas (Van Rijn, 1984;

Williams and Rose, 2001) as a function of mean current speeds

raised by the highest power of 3.4 (which can be rounded off to 3

for practical use herein) by Soulsby (1997).

The winter season measurements (Figures 4 and 5) clearly

exemplify the nonlinear effects of combined tidal and residual

currents on sand transport. They show that UeS is approxi-

mately 0.32 m/s, UfN is 0.35 m/s, and u is 0.1 m/s. With

T 5 45,000 seconds (5 12.5 hours) and c2 5 20 s3/m3 according

to Soulsby (1997), Equations (3)–(5) give northward tidal

current transport Qt 5 4548 seconds, wind-driven current

transport Qw 5 900 seconds to the south, and southward

transport by combined tidal and wind-driven currents

Qtw 5 26,308 seconds, respectively. The simple addition of Qt

and Qw will be a northward transport of 3648 seconds.

Therefore the combined tidal and wind-driven currents cause

a seven-fold increase in transport and also reverse the

transport direction from the simple addition of transports by

tidal current and wind-driven current.

DISCUSSION

Sediment transport observations on the west coast of Korea

have frequently documented that bottom sediments are easily

resuspended in the case of combined waves and currents with

the SSC over one order of magnitude greater than by tidal

currents alone, even at spring tide (KORDI, 2005; Lee and Ryu,

2007; Lee et al., 2004). It has been well established that orbital

currents of waves nonlinearly increase shear stress on the

seabed in combination with steady currents (Christoffersen and

Jonsson, 1985; Soulsby, 1997; Wiberg and Smith, 1983; You,

1995). To see how this nonlinear coupling of combined flows

influences sediments and water, we attempted to evaluate basic

parameters regarding sediment transport at station S4-1. Since

the tripod configuration was somewhat primitive for the

benthic boundary layer measurements to be applied to the

complicated performance of sediment transport calculations, a

pragmatic sediment transport model by Li and Amos (2001)

was instead adopted to determine semiquantitatively a few

major factors governing sand movements over the study area.

The maximum wave orbital velocity, Ub, is estimated at 0.5–

0.9 m/s during the highest waves in the wintertime measure-

ments, exceeding tidal currents of 0.4–0.5 m/s (Figure 10). The

outstanding effects on bed shear stress of combined wave and

currents are obvious by comparing the skin shear velocities of

wave-current (u*cws) and current only (u*cs) that are directly

related to the bedload transport. It is clear that waves

sufficiently escalate shear velocities above the threshold shear

velocity (u*cr) of 0.01 m/s to move sand grains for most of the

wintertime measurement duration; otherwise, sand mobility

would take place only by the strongest tidal currents during

spring tide (Figure 10). In contrast, the late-spring measure-

Figure 9. Schematic of resultant currents from the addition of tidal and

residual currents in the N-S direction in the winter season.



ments show relatively lower wave orbital velocity due to the

blocking of wind from south of the Gogunsan Archipelago. Even

the highest waves during the late-spring measurements could

marginally initiate sand movements (Figure 11).

Storms are usually accompanied by strong winds, high

waves, and wind-driven currents. Such energetic storm events

have been studied in many continental shelves with respect to

sediment transport (Drake and Cacchione, 1992; Green et al.,

1995; Gross, Isley, and Sherwood, 1992; Li, Amos, and Heffler,

1997; Madsen et al., 1993; Puig, Palanques, and Guillen, 2001;

Williams and Rose, 2001). However, these measurements,

located preferentially in the mid- to outer shelves deeper than

30 m, placed a focus on the boosting effects of large waves on

sediment transport. Among them, Madsen et al. (1993) have

illuminated from the time series current data that definitely

wind-driven currents occurred in the range of 0.1–0.4 m/s by an

extreme storm (Hs up to 4.5 m in water of 8 m) on the inner shelf

of the southern Middle Atlantic Bight. However, they concen-

trated on the effects of huge waves on some current-related

transport parameters rather than any relative contribution of

tidal and wind-driven currents to sediment transport. Thus the

core issues of the present study—the characteristics of wind-

driven currents concerning differences between windy and

calm conditions or seasonality, and hence their contributions to

sediment transport and redistribution in association with tidal

currents—have been rarely addressed.

Even though waves play a significant role in setting bed

sediments in action and increasing turbidity of the water

column, net sediment transport basically depends on the

intensities and persistence of unidirectional currents, whether

tidal, wind/wave-derived, or both. The monsoon winds are

proved to induce residual currents flowing in the downwind

direction offshore the dike. Although the evaluated residual

currents may contain a portion related to the pressure gradient

due to differing mean sea levels, the wind forcing should be

mostly responsible for the residual-current regime in that the

magnitude and direction of the residual currents exclusively

correlate with the wind climate (Figure 5). The pronounced

residual currents are shown to allow the intrinsically flood-

dominant tidal regime to appear reversely ebb dominant

(Figure 10). The generation of residual currents by winds is

also well reported by surface observations with high-frequency

radars in the study area (Kim et al., 2008; Son et al., 2007). Kim

et al. (2008) estimated surface currents of up to 0.1 m/s flowing

downwind by northerly winds of over 8 m/s in October 2004.

According to the long-term wind data (Figure 2), it appears

that the wintertime winds from NW or N, the most frequent

and strongest through the year, are often creating the most

prominent residual currents directed SE or S. It is also well

known that the basin-wise mean circulation in the Yellow Sea

is dependent on seasonal wind conditions. Based on a

numerical experiment with wind forcing, Jacobs, Hur, and

Riedlinger (2000) have suggested that the strong northerly

wintertime wind generates the southward flows along both

coastal areas, in contrast to the northward flows over the

central trough of the Yellow Sea. Moon, Hirose, and Yoon

Figure 10. Time series of current speed, wave orbital velocity (Ub), and various bed shear velocities from the wintertime measurements. Evaluation of

orbital and bed shear velocities is according to Soulsby (1997). Note the ebb (E) dominance overturned from the flood (F) dominance by winds. The dotted line

in the top panel indicates estimated tidal current speeds derived by subtracting residual-current speeds from the measured total currents. For the residual

current time series, see Figure 5. u*cw 5 combined current-wave total shear velocity, u

*cws 5 combined current-wave skin shear velocity, u*cs 5 current skin

shear velocity, and u*cr 5 critical shear velocity.

162 Lee and Lee


(2009) have conducted numerical experiments with wind and

tide, showing that the significant southward coastal-current

system is formed by northerly wind bursts in winter, although

current speeds tend to be suppressed by tide. These numerical

results for wintertime circulation are supported by long-term

current mooring data from the Yellow Sea (Hsueh, 1988).

Equation (6) suggests that the nonlinear increase of

transport by the combined tidal and wind-driven currents

depends on the magnitude of wind-driven current speed (u) and

the mean ebb and flood tidal currents. Therefore, the increase

of bedload transport caused by combined tidal and residual

currents could be most pronounced in the macrotidal regime

compared to the micro- to mesotidal regimes. In this regard, the

macrotidal southeastern Yellow Sea can be an ideal site for

featuring the winter monsoonal influence on seabed sand

movements. Such a tidal-current nonlinear effect may well be

further enhanced offshore the dike, since the Saemangeum

Dike has led to a noticeable increase in the N-S component of

tidal currents. Therefore, winter monsoonal winds, the macro-

tidal regime, and the dike emplacement together should

contribute to the abnormally expanded surface sands observed

offshore Saemangeum Dike during recent years (Figure 3).

CONCLUSIONS

The interplay of winter monsoon, macrotidal currents, and

Saemangeum Dike has been considered as the most likely

cause for the rapid expansion of sandy areas off the dike in the

midwest coast of Korea. To address this proposition, a dual set

of time series of hydrodynamic data for the bottom boundary

layer was obtained with an instrumented tripod from a

measurement station over the sandy area, each representing

a 6-day span of spring tide in the winter and late-spring

seasons. In addition, tidal models were performed using

various conditions associated with dike construction to simu-

late changes in the tidal regime, particularly in the N-S

component of tidal currents. A simple formula, W 5 cU3, was

adopted to describe the relationship between the bedload

transport rate and the steady currents (residual, tidal, or both)

in order to corroborate the seemingly erratic phenomenon of

sand expansion in recent years.

The time series of current speeds show that a residual

current controlled by winds took place in both seasons. Winter

winds persistently from NW or NE caused the residual

currents to reach up to 0.1 m/s and consistently flow downwind

toward the S or SW. By comparison, the late-spring residual

currents were weak (,0.04 m/s) because the southerly winds

were blocked by the Gogunsan Archipelago. The presence of

remarkable residual currents during winter even led to a

reverse of current asymmetry from flood-dominant to ebb-

dominant on the windiest days. Waves were also constantly

higher during winter than during late spring under the

characteristic wind climate. Bed shear stress should have

conspicuously increased with high waves, facilitating mass

transport of fine to very fine sand grains over the study area,

especially during winter.

Figure 11. Time series of current speed, wave orbital velocity (Ub), and various bed shear velocities from the late-spring measurements. Evaluation of orbital

and bed shear velocities is according to Soulsby (1997). Note the flood (F) dominance little changed by winds (E denotes ebb). The dotted line in the top panel

indicates estimated tidal current speeds derived by subtracting residual-current speeds from the measured total currents. For the residual current time

series, see Figure 5. u*cw 5 combined current-wave total shear velocity, u

*cws 5 combined current-wave skin shear velocity, u*cs 5 current skin shear

velocity, and u*cr 5 critical shear velocity.



The algebraic calculations for bedload indicate that super-

imposing the residual current on tidal currents, both flowing in

the same direction, allows for a nonlinear increase in the

bedload transport of sands, which depends largely on the

amplifying function of the tidal current velocity. These findings

suggest that the wintertime residual currents flowing south

may well act as a more efficient driving agent in sand transport

than before dike construction because tidal modeling demon-

strates a definite increase in the N-S component of tidal current

velocities by Saemangeum Dike. Therefore, the dike modula-

tion of tidal regime together with the combination of winter-

time residual currents and macrotidal currents most probably

resulted in the rapid southward expansion of surface sands

offshore the dike.

ACKNOWLEDGMENTS

This research was part of the project ‘‘Saemangeum Coastal

Research for Marine Environmental Conservation’’ funded by

the Ministry of Land, Transport and Maritime Affairs, Korea

(grant no. PM54893), and was additionally supported by the

Korea Ocean Research and Development Institute (grant

no. PE98462). We thank M.J. Kim, T.K. Kim, and J.M. Lee for

their technical assistance. We also thank Dr. Michael Li, who

significantly improved the contents of the paper.

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sand transport offshore the saemangeum dike, midwest coast of korea

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