Full text of DOI:10.1007/BF02910308

Geosciences Journal
Vol. 5, No. 3, p. 251 262, September 2001
A seasonal model of surface sedimentation on the Baeksu open-coast inter-
tidal flat, southwestern coast of Korea
Faculty of Earth Systems and Environmental Sciences, Basic Science Institute, Chonnam National
University, Kwangju 500-757, Korea (e-mail: sschun@chonnam.ac.kr)
Byong Cheon Yang }
Seung Soo Chun
ABSTRACT: Detailed sedimentological studies such as surface
sediment distribution, facies changes and sedimentation rate have
been carried out over two years (1997−1998) in the Baeksu open-
coast intertidal flat, SW Korea. In the winter and spring seasons,
sand facies is relatively dominant on the flat resulting from the
strong and frequent influence of wind storms. Average sedimen-
tation rates are ca. 4 and 3 mm/month in winter and spring,
respectively. High temperature and weak wave energy would facil-
itate mud deposition over the sand-facies deposits during the sum-
mer with an average sedimentation rate of ca. 10 mm/month. In
fall, mud sediments deposited in summertime, are subjected to
erosion by strong waves. Sedimentation rate shows net erosion of
ca. −10 mm/month in average. Such a seasonal cyclic pattern in
deposition/erosion of surface sediments is most likely to reflect the
importance of monsoonal controls in open-coast intertidal flats in
temperate regions. The annual sedimentation rate of <0.5 mm/yr
suggests that the intertidal flat would be in steady or quasi-equi-
librium state.
son, 1983; Ren et al., 1985; Wells et al., 1990; Lee et al.,
1999). Mukherjee et al. (1987) firstly reported the impor-
tance of storm wave in sedimentary processes on open-
coast intertidal flats, especially with an emphasis on sea-
sonal changes in weather conditions in tropical tidal flats.
Further detailed investigation is necessary in various cli-
mate settings for full understandings in sedimentary pro-
cesses on open-coast-type intertidal flats.
The Baeksu coast, locates in the southwestern coast of
Korea, can be a representative of typical open, non-barred
macro-tidal flat in temperate region (Fig. 1). It is strongly
influenced by monsoon wind waves in the winter seasons.
This paper aims to reveal the seasonal variation of sedi-
mentation and surface facies, controlled by tide and wave
effects in the open-coast macro-intertidal flat setting.
2. ENVIRONMENTAL SETTING
2.1. Geomorphology
Key words: Baeksu intertidal flat, open-coast intertidal setting, surface
sediment facies, monsoonal control of sedimentation, sedimentation rate,
Yellow Sea
1. INTRODUCTION
The Baeksu intertidal flat is 4 6 km in width and 8 10
−
−
km in length (ca. 60 km2 in area), directly facing on Yellow
Sea to the northwest. Its northern coastal area directly con-
tacts with a steep coast (sea cliff), but its southern coast
boards farms and saltpans with nearly straight coastline
which have resulted from continuous reclamations over the
past few decades (Fig. 1). Input of terrestrial sediments is
mostly restricted because the gate in reclaimed dyke is
closed throughout the year except for the intense summer
flooding period (Yang and Chun, 2000; Chun et al., 2000).
Intricate tidal creeks are developed only in the front area
(uppermost tidal flat) of gate (Fig. 2). Proper salt marshes
are not developed here.
Intertidal flats are generally revealed as gently dipping
sea coasts with intricate tidal-creek system and less strong
wave action caused by geomorphological protection such as
barrier island (Reineck and Singh, 1980; Klein, 1985). The
intertidal flats are commonly characterized as three distinc-
tive zones according to the composition of sediments and
sedimentary structures: sand flat, mixed flat and mud flat
(Reineck and Singh, 1980). Sediment-transport processes
on the tidal flats show a contour-parallel fashion from high-
to low-water levels depending upon tidal current magnitude
(Klein, 1985, 1998). Within the classic view of depositional
processes in the tidal flats, current magnitude and tidal
range act dominantly as an enhancer of sediment transpor-
tation and deposition (Ginsburg, 1975; Klein, 1998).
Most works in the tidal-flat sedimentology have focused
on the estuarine- and embayment-type environments during
the past few decades. On the contrary, open-coast tidal flats
have been less studied despite its common occurrence in
modern environments. In temperate regions, wave condi-
tions may be of much greater significance than tidal-current
magnitude, particularly in monsoon climate regions (Ander-
The line BA in the northern part of the Baeksu intertidal
flat is 3.2 km long with slope gradient of ca. 0.09o in aver-
age, whereas the line YS in the southern part is 2.8 km long
with slope gradient of 0.04o in average. These slope gradi-
ents are similar to those in the Gomso-bay (ca. 0.06o) and
Hampyong-bay (ca. 0.05o) intertidal flats, which locate in
the northern and southern coasts of study area, respectively.
The elevation of the upper tidal flat in the line BA is rel-
atively higher than that of line YS, but reverse is true for
lower-flat elevation, suggesting that line YS is monoto-

252
Byong Cheon Yang and Seung Soo Chun
Fig. 1. Location map of study area in
the Baeksu open-coast intertidal flat,
which faces directly to Yellow Sea
without seaward barriers. The two
transect lines have been set up for
gathering surface-sample, elevation-
change and can-core data. Dots indi-
cate sampling positions of surface sed-
iments for spatial distribution. Depth
contours are based on the MHLWL
=
(mean high low water level 0 m).
SRP is plate for measuring sedimenta-
tion rate.
nously flat-lying with broad mixed flat. Both slope profiles
gradually decrease toward the subtidal zone except for mor-
phological highs and intervening lows (Fig. 3). Those fea-
tures could be caused by the formation and movement of
intertidal swash bars on wave-dominated intertidal flats
(Davis et al., 1972). A prominent swash bar lies parallel to
the shoreline at 400 m of line YS from the shore. Over the
1997 1998 working period, this migrated 100 150 m land-
−
−
ward in winter while it was very stable during the summer.
Some small swash bars is growing on the both parts (Fig.
3), but others were actively welded to the southern coast.
This suggests that winter-storm wave forms many small-
sized swash bars on both areas, but active migration occurs
only on the southern part due to broad travel distance of
waves.
2.2. Weather Condition
Tides are semi-diurnal (diurnal inequality of about 1 m)
with a mean tidal range approximately of 3.9 m (mean neap
tide: 2.4 m, mean spring tide: 5.4 m). Tidal current shows
NNE direction in flood current and SSW in ebb current
(Chun et al., 2000).
Table 1 shows an averaged annual wind condition for two
working years from 1997 to 1998. It indicates that storms
(defined as wind speed over 13.9 m/s) are relatively fre-
quent in fall and winter as well as in early spring season.
Monsoon in the study area reveals dominantly northwest-
erly wind in winter and southerly wind in summer (Korea
Meteorological Administration, 1997, 1998). Spring wind
shows N-NW direction with a mean speed of about 2 m/s
(at 15 m above the MSL) and a significant wave height of
Fig. 2. Photographs of inner-flat area, in which surface sediment
consists predominantly of mud. Erosional patches of mud blanket
(arrows) and some ephemeral tidal creeks are formed in winter
season when strong wind wave frequently reworks the inner-flat
surface (A). Relatively stable tidal creeks characteristically occur
only in the innermost-flat area (B).
about 1.5 2.0 m (Korea Meteorological Administration,
−
1997, 1998). Storms occur at a frequency of 4 6 days/month
−

A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea
253
Fig. 3. Cross sections along two
transect lines, BA and YS. The slopes
in lower right corner of each section
indicate the averaged slopes.
Table 1. Weather condition from January, 1997 to December, 1998 near the Baeksu tidal flat (Wido weather station). Note that the aver-
aged wind direction is changed from northwesterly into southerly in the summer season (May to August) with a decrease in wind speed
(Korea Meteorological Administration, 1997 and 1998).
Feb. Mar. Apr. May Jun. Jul. Aug. Sep.
Oct.
Nov.
Dec.
Jan.
3.5
average wind speed (m/s)
3
2.3
2.1
2.6
16.2
SE
2
1.7
15.5
SW
2
1.8
16.4
SW
3
1.8
2.4
2.9
20
NW
6
2.6
16.7
NW
7
2.8
18.8
NW
9
maximum wind speed (m/s) 18.2
and direction
average storms (days/month) 10
17.6
17.4
16.1
16.5
18.9
NW WNW NW NNW
SSW NNW
6
6
4
2
6
during spring period. In summer, wind direction is changed
Precipitation concentrates from June to August and tem-
in-situ
into SE SW with much weaker wind speed (<2 m/s), perature is highest in summer. Seawater temperature
−
resulting in much subdued wave height, ranging from 0.5 to measured nearby Chilsando during winter (National Fisheries
1.0 m. In the fall season, wind speed and storm frequency Research and Development Institute, 1997, 1998) never falls
increase suddenly and wind direction changes again to below freezing point with ca. 5oC in average. Field obser-
northwesterly wind. Mean wind speed and significant wave vation has not detected ice floe effect during winter in the
height are very similar to those of spring season. In this study area, compared with cold regions (Dionne, 1974, 1976;
period, however, storms are more frequent, ranging from 5 Dalrymple et al., 1991).
to 7 days/month. In winter, wind is the strongest over the
mean speed of 5 6 m/s, resulting in the significant wave 3. FIELD MEASUREMENTS AND ANALYSES
−
height of 1.5 2.0 m (Korea Meteorological Administration,
−
1997, 1998; National Fisheries Research and Development Insti-
For the measurement of the slope variation on the flat,
tute, 1997, 1998). Storm frequency is up to 9 to 10 days/month. two-transect lines were accurately leveled, using both elec-

254
Byong Cheon Yang and Seung Soo Chun
Fig. 4. Plate for measuring sedimentation rate (Sedimentation Rate
Plate, SRP).
tronic optical distance measuring instrument (Wild T2 Uni-
versal Theodolite and Geodimeter 220) and leveling ins-
trument (Pentax Pal 2S Level). In order to allow subse-
quent sampling at identical positions, thin iron stakes were
driven into the flat at every 100 m. Surface sediments were
sequentially (about two- or three-month intervals) taken
from the topmost several millimeters (less than 1 cm deep
from surface) at each station using spatula. For the spatial
distribution pattern of surface sediments the additional
samples were obtained during the summer and winter of
1998 at 62 stations, using Van Veen grab sampler (Buller
and McManus, 1979). Sampling locations were spaced in
a regular grid intervals of 0.5’ in latitude and 0.5’/0.75’ in
longitude.
Grain-size analyses of surface sediments were performed
by sieving and pipetting after removal of organic matters
and calcium carbonates with 10% H2O2 and 0.1 N HCl
(Carver, 1971). Dried sand fractions were analyzed using
Fig. 5. Triangle diagrams for the classification of surface sedi-
ments. In the summer season, fine-grained sediments are dominant
(A), whereas most sediments consist dominantly of sands in the
winter season (B). This seasonal pattern of sediment composition
Ro Tab Sieve Shaker with 0.5 phi sieve interval over 15 would be due to typically different energy regimes of both seasons.
−
minutes; mud fractions were analysed by pipette method.
Sediment types are identified according to Folk’s (1968)
scheme (Fig. 5) and statistical parameters of grain-size dis- 4. CHARACTERISTICS AND DISTRIBUTION OF
tribution are calculated by moment method.
SURFACE SEDIMENTS
Bottom-elevation change was measured by use of bur-
ied stainless plate (Fig. 4; Anderson et al., 1981; Carling, 4.1. General Spatial Distribution
1982; Ryu, 1998, Lee et al., 1999) along the transect lines
at 200 to 300 m intervals that were driven into subbottom
The surface sediments can be classified into three types
about 30 cm deep (the plate is referred hereafter as SRP; according to the sand/mud ratio: sand facies (sand, more
Sedimentation Rate Plate; see Fig. 1 for location). The than 70%), mixed facies (sand, 40 70%) and mud facies
−
SRP-buried area was healed for 2 3 months before the (sand, less than 40%). Clay component in the surface sed-
−
first base-line measurement. The incremental depth iments is characteristically very low throughout the year.
change was taken from flat surface to SRP with an inter-
Inner flat, composed mostly of mud facies (silt sedi-
val of 1 2 months, using fine metal ruler. Sedimentation ments), is restricted within the narrow region from coastline
−
rate is simply calculated from the values of depth changes to ca. 200 m away from the shore. Its surface is covered
(Table 2).
with silty mud regardless of the seasonal change of dynamic

A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea
255
Table 2. Sedimentation rate along the two transect lines on the Baeksu intertidal flat. Note that annual net deposition is recorded as 0.5
mm/year over two years. It suggests that dynamic condition in this open-coast intertidal flat would be considered as quasi-equilibrium
state even under severe change of seasonal sedimentation rate.
Seasonal Sedimentation Rate: Line-BA (mm)
Summer
Fall
Winter
Spring
Summer
Fall
Winter
Spring Cumula- two year rate of sedi-
6/97-8/97 9/97-11/97 12/97-2/98 3/98-5/98 6/98-8/98 9/98-11/98 12/98-2/99 3/99-5/98 tion(mm) mentation(mm/2 years)
−
−
−
−
−
−
−
BA-1
BA-2
BA-4
BA-7
BA-10
18.6
-
10.6
12.4
4.2
2.4
15.8
16.8
3.5
−
0.2
5.5
52
12.5
2.6
28.8
65.4
0.1
1.2
2.7
0
−
−
−
26
−
−
−
1
4.3
10.5
8.8
16.5
−
−
−
−
−
−
27
12.4
9
1.5
5.2
0.7
9.3
21
8.9
26
13.7
−
1.4
−
−
−
−
−
−
−
−
−
11.5
34.7
5.5
1.8
0.5
15.3
−
−
26
-
25.1
0.6
0.2
11.5
0.6
−
−
−
2.7
−
−
−
BA-13
BA-15
BA-17
20.9
16.1
22.2
32.5
26.9
13.6
15.5
18.5
38.9
33.8
1.5
10
20.2
0.3
5.1
34.1
1.4
−
7.4
-
15.8
20.2
36.4
21
69.8
2.9
−
−
−
−
22.1
24.4
8.5
−
20.5
8.2
9.2
48.9
4.9
5.9
20.8
39.8
35.4
0.9
−
16.1
−
−
BA-19 16.6
BA-23
BA-27 16.2
13
17.5
1.0
−
46.3
3.3
0.4
4.3
6.2
-
-
−
81.9
7
2.6
102.5
148.7
-
-
-
-
−
2.4
BA-31
31.7
43.8
75.6
-
-
-
-
−
13.6
−
6.3
−
3.3
Average
11.7
15.5
9.8
0.6
9.1
18.1
0.8
Seasonal Sedimentation Rate: Line-YS (mm)
Summer Fall Winter
6/97-8/97 9/97-11/97 12/97-2/98 3/98-5/98 6/98-8/98 9/98-11/98 12/98-2/99
Spring
Summer
Fall
Winter Cumulation Two year rate of sedi-
(mm)
mentation (mm/2 years)
−
48.3
−
49.8
−
55.3
−
47.5
−
45.5
−
72.9
−
YS-2
YS-4
YS-8
YS-12
YS-15
YS-19
YS-24
YS-28
303.6
40.4
-82.1
33.7
23.5
-117.4
106.4
5.6
−
3.4
−
−
−
38.1
29.7
10.2
91.3
17.2
4.3
4.8
−
−
−
66
36.4
71
29.9
17
3.5
18.8
43.6
21.9
9.1
14.2
60.7
43.8
0.5
8.5
0.9
0.2
0.5
−
31.4
50
−
57.5
9.5
−
−
−
−
19.9
9.3
25.8
27.3
87.4
100.8
4.6
5.3
−
44.8
−
26.0
−
1.4
Average
122.1
-53.2
1.3
13.5
19.8
-9.5
-66.3
4.2
0.2
condition (Fig. 2). This suggests that this slightly embayed the upper middle-flat along the inner margin of mud facies
inner-flat zone has been sheltered against waves. Intricate with showing narrow belt. Sand facies occurs only on some
tidal creeks are formed only in this region.
limited areas, forming linear body on the upper flat. The
(Halophytes) field and some cheniers occur in this flat. The distribution of surface sediments shows a seaward fining
cheniers are very small in size with 4 5 m long and 30 40 trend, which is different from that of other intertidal flats
Sueda Japonica
−
−
m wide, compared with well known worldwide cheniers where surface sediments show mostly a seaward coarsening
(Woodroffe et al., 1983; Chappel and Grindrod, 1984). They pattern (Reineck and Singh, 1980).
are most probably of storm origin like those of other west-
ern coast of Korea, which predominantly migrate during 4.1.2. Winter distribution
storm periods (Lee et al., 1994; Yang, 2000). Except only
Most intertidal sediments in winter are composed pre-
for the restricted inner flat area, the flat shows a distinctive dominantly of sand facies with some isolated mud-facies
seasonal variation of surface-sediment distribution.
patches (Fig. 6B). Like the summer distributional pattern, it
also shows crudely a seaward fining trend in that sand con-
tent slightly decreases seaward with an increase in mud
4.1.1. Summer distribution
Mud facies is predominant in summer, which covers up content, except on the inner flat. Mud facies is distributed
to about 70% of the flat (Fig. 6A). Mixed facies occupies only on the southern and northern coastal areas. Some mud-

256
Byong Cheon Yang and Seung Soo Chun
Fig. 6. Spatial distributional pattern in the summer (A) and winter (B) in the Baeksu tidal flat. Samples were taken with a team of KORDI
(Korea Ocean Research & Development Institute).
facies zones in southern and northern part could be attrib-
Surface sediments in the early summer dominantly exhibit
uted to mud trapping in small tidal channels in the upper mud facies in the lower flat and mixed facies in the middle
tidal flat (Fig. 6B). The summer front of inner-flat mud- flat, respectively. Sand facies is present only in the upper
facies zone are slightly retreated by the winter expanding of flat (Figs. 6A and 7). With time, most intertidal flat is get-
sand facies. Mixed facies occurs in the restricted northern ting draped with thick mud deposit (5 30 cm thick) (Fig.
−
coastal area.
11). However, sand contents in the upper middle and lower
upper flats (between 0.4 to 0.9 km) are high, similar to those
of spring (Fig. 7B). Inner-flat region also slightly extends
offshore during this period. Similar to the trend in other sea-
4.2. Temporal Variation of Surface Sediments
Surface sediments taken from the two-transect lines are sons, sand content decreases offshore with an increase of
mostly made up of two distinct particle populations: the mud content (Figs. 7B and 8). These textural characteristics
fine sand and the silt fractions (Figs. 7 and 8). Clay fraction would reflect relatively weak energy condition, resulting
is minor. Gravels are rare to absent. In the upper flat on Line from subdued wave effect, and relatively rapid settling of
BA, sand grains are slightly coarser than those in the lower the fine-grained particles by temperature enhancement
flat (Fig. 8). Some small-scale tidal beaches are developed (Kröel and Flemming, 1998).
in the narrow steep shore zones, especially near rocky cliffs.
The mud sediments, deposited in summer, have experi-
Coarse sand grains (ca. 2 ) and some gravels are found on enced severe erosion in the early fall, probably caused by
φ
the uppermost tidal flat around the line YS, where several increased wind intensity. Erosion takes place firstly in the
cheniers are superposed. These features suggest that the upper flat and progresses into the middle flat, resulting in a
migration of some low-relief bars or shoals would play an textural change into sand facies (Figs. 7C, 8 and 9). The
important role for the supply of coarse particles in the upper sand deposits typically show high amounts of mud fraction.
intertidal flat probably during storm, and also the adjacent Shallow tidal creeks and mud patches are formed on the
tidal beach deposits would be attributed partly to the impor- sandy tidal-flat surface. Whereas mud deposits in the lower
tant source of coarse materials.
and lower-middle flats (>1.3 km) remain uneroded and
In the spring season, the Baeksu intertidal flat can be show a relative higher content of clay fraction than other
divided into two distinctive zones except for the relatively seasons (Figs. 7C and 8). The high contents of mud fraction
stable inner flat: sand-facies (0.2 1.3 km) and mixed-facies in surface sediments suggest that some part of eroded fine
−
(>1.3 km) zones (Figs. 7A and 8). Sand content generally sediments may be resupplied through the ephemeral tidal
decreases toward subtidal zone with increasing mud con- creeks (Fig. 9).
tent. Sand grains consist dominantly of very fine sand, and
In winter, the surface sediments are rather simple in com-
muds are composed mostly of coarse to medium silt. Sand position and texture. Mean grain size constantly occurs at
content ranges 70 85% from shore to 1.3 km, and 40 60% ca. 3.5 , except for that of the inner flat. Sand fraction in
−
−
φ
from 1.3 km to end of survey line (Fig. 8).
most flats is over 80% in content but decreases offshore

A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea
257
Fig. 7. Seasonal variation of relative grain-fraction ratio in surface sediment along the transect line BA during 1997 (see Fig. 1 for loca-
tion). The most characteristic feature is a seaward fining trend with an increase in mud content at the cost of sand content toward offshore,
unlikely those of other intertidal flats. Inner-flat (<200 m) sediment consists predominantly of muds with coarse-silt modal size (see Fig. 8).
Fig. 8. Variation of grain size mode in the surface sediments (see Fig. 3 for zonation). The highest sand content occurs in the spring and
winter, whereas the finest mode occurs in the summer and fall.

258
Byong Cheon Yang and Seung Soo Chun
Fig. 10. Incremental depth change over two working years in SRP
positions. Note that the thickness range increases offshore, indi-
cating that wave reworking is stronger in the lower flat than in the
upper flat. An exceptional case occurs in the summer, 1998 due to
heavy rainfall (arrows). In the upper flat, bottom elevation increases
in the fall of 1998 (filled triangle), probably because a great
amount of sediments is supplied by typhoon.
by expanding of sand flat, probably caused by strong bot-
tom reworking by waves.
Fig. 9. Surface features of intertidal flat in fall season. Ephemeral
tidal creeks are formed by erosion of mud layer due to increasing
wind wave (A). Exposed sand flat and mud patches (B). Mud peb-
bles would be formed by breaking of mud patches due to attacks
of strong wave (C). These pebbles are often preserved in subbot-
tom under the condition of rapid sedimentation (Yang et al., 2000).
Triangles indicate burrows, and large arrows indicate mud pebbles.
Scale in 4 cm long.
Regardless of seasonal change, the surface sediments on
the Baeksu intertidal flat reveal a seaward fining trend (Fig.
7), unlikely to those in other tidal flats with almost all sea-
ward coarsening trend (Reineck and Singh, 1980; Klein,
1985). It has not yet been understood how the seaward fin-
ing trend could be developed on the intertidal flat.
5. SEDIMENTATION RATE
with increasing mud content (Figs. 7D and 8). These grain-
Many SRP data, especially in the YS line, were missed in
size distributions suggest that strong wave action would summer because mud deposition was so rapid and severe
prevent suspended particles from settling down even on the that many plates were completely buried by mud. Sedimen-
upper flat. The extent of inner-flat mud sediments is reduced tation rates in the Baeksu intertidal flat vary seasonally: net

A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea
259
deposition in summer and net erosion in fall (Fig. 10; Table
2). Mud thickness, deposited in the summer period, ranges
from ca. 5 cm in the upper flat to ca. 30 cm in the lower flat
(Fig. 11). The average sedimentation rate demonstrates 11.7
and −6.3 mm/month in the summer periods of 1997 and
1998, respectively. Maximum rate is recorded as 122.1 and
19.8 mm/month in the line YS during the summertime of
1997 and 1998, respectively. Such a feature is further sup-
ported by the spatial distribution of surface sediment in the
summer season (Fig. 6A). The low rate of deposition in
1998 could be attributed to a typhoon, passed near the west-
ern coast of Korean Peninsula before field period (Korea
Meteorological Administration, 1998).
During the fall season when summer muddy sediments
experience severe erosion, sedimentation rate is −17.8 mm/
month in 1997 and −3.1 mm/month in 1998 on the line YS,
and −4.6 mm/month in 1997 and 0.2 mm/month in 1998 on
the line BA. Higher erosional rate on the line YS is con-
spicuous. Rather slight erosion in 1998 may be attributed to
the temporary redeposited mud after a typhoon.
In the winter and spring seasons, sedimentation rate
reveals slightly positive values (Fig. 10), probably because
of sand supplies from offshore into intertidal flat by the
combined effects of wave and tidal current (Fig. 11). Sed-
imentation rates are 5.2 mm/month in 1997 and −1.1 mm/
month in 1998 on the line BA, and 0.4 mm/month in 1997
on the line YS.
The two-year sedimentation rate, despite varying season-
ally, reveals small depositional rate (<0.5 mm/yr), indicat-
ing that sediment flux in this area could be considered as
dynamically steady or quasi-equilibrium state.
6. DISCUSSION
Fig. 11. Cancore peels in the summer (A and B) and winter (C and
D). Note that most sediments consist of mud in the summer,
whereas wave generated structures are conspicuous in winter. Peels
of C and D show stratified and slightly inclined stratified sands at
the base and ripple cross lamination above. These suggest that the
former is commonly interpreted in the tidal-flat deposits as a result
of waxing-stage storm, and the latter is formed under fair-weather
condition.
Rapid sedimentation of mud is observed during summer
season. The top mud layer thins landward from ca. 30 cm
on the lower flat to 5 cm on the upper flat (Fig. 11). Most
mud flats have been known to occur in the upper tidal flat
where settling and scour lags promote fine-grained deposi-
tion (Van Straaten and Kuenen, 1958; Postma, 1961). In the
Baeksu intertidal flat, the inner mud flat is restricted and
meager in areal extent. On the contrary, extensive mud flat
develops in summer in the outer flat region. Although the Baeksu intertidal flat include high content of coarse silt (5
dynamics for the mud deposition in summer has not been ). That gives us some difficulties to classify and differen-
φ
understood well yet, it may be a direct consequence of high tiate the surface sedimentary facies based on the content of
concentration of suspended sediment in encroaching sea those particles. For this reason, the two-tier system based on
water.
measurement in the subtidal area reveals that sedimentary structure and sand/mud ratio would be sug-
In-situ
suspended sediment concentration is up to 8,000 ppm in gested for the reasonable facies classification of surface
near bottom (Chun et al., 2000). Further detailed study for sediments in this flat. In general, sand is, however, domi-
mud-flat sedimentation would be needed to explain the fea- nant component in the Baeksu intertidal flat even in the
ture.
The dynamic behavior both of coarse silt and fine sand is could be regarded as sand-dominated intertidal flat.
very similar in water column (Rees, 1966; Southard and The surface sediment in the Baeksu intertidal flat char-
Harms, 1972; Stanley, 1974). Most surface sediments in the acteristically shows a seaward fining trend, compared with
muddy sediments. It implies that the Baeksu intertidal flat

260
Byong Cheon Yang and Seung Soo Chun
Fig. 12. A suggested seasonal model of surface-sediment distribution (sedimentary facies) in the open-coast Baeksu intertidal flat. In win-
ter and spring seaons, sand facies is dominant in the tidal flat due to heavy and strong wave action. In summer, mud flat gradually trans-
gresses landward (B). Mud patches and ephemeral tidal creeks are formed in fall when wind wave increases again (C).
other tidal flats displaying a seaward coarsening trend. This to be discussed further, especially in the open-coast inter-
unique facies distribution is rather comparable to that of tidal setting. Observation and description based on detailed
tidal beach. Indeed, significant wave height of 1.5 2.0 m sedimentological studies with seasonal changes should be
−
(Korea Meteorological Administration, 1997, 1998) in the warranted for much correct information to reconstruct the
study area through most seasons except for summer is ancient bodies as well as to interpret modern open-coast
almost identical to that of tidal beaches generally of 1 2 m tidal-flat settings.
−
(Short, 1991). The Baeksu intertidal flat, however, lacks
well-defined ridge and runnel system and is very gentle in 7. CONCLUSIONS
slope gradient, ranging from 0.09o to 0.04o. Tidal beaches
are relatively steep sloped commonly exceeding 1o (Short,
To understand the sedimentation in open-coast intertidal
1991; Masselink and Hegge, 1995). The deposits also show setting, the detailed study on the surface sediment and sed-
some tidal bedding and facies zonation typical in tidal-flat imentation rate was carried out in the Baeksu intertidal flat,
setting (Chun et al., 2000; Yang, 2000; Yang and Chun, 2000). southwestern coast of Korea, for two years (1997 1998).
−
These indicate that the study area should be regarded as a As it is significantly affected by monsoon, the textural char-
tidal flat rather than a tidal beach.
acteristics of surface sediment and the change of sedimen-
Even though the depositional environments of coasts are tation rate also reflect well the seasonal weather condition.
successfully classified by Boyd et al. (1992), the evident The results are summarized as a simple seasonal model in
differentiation between tidal sandflat and tidal beach needs Figure 12, based on two-year observation. Most mud patches

A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea
261
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the model should be considerably simplified and concep-
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SPRING: Surface sediments consist of two facies zones:
Chun, S.S., Yang, B.C., Lee, I.T. and Lee, H.J., 2000, Non-barred,
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mixed facies (40 70% in sand, >1.3 km) and sand facies
−
(>70% sand, 1.3 0.2 km). Sedimentation rate shows slight
−
net deposition with ca. 3 mm/month in average. Poor devel-
opment of mud-facies deposits can be accounted by fre-
quent storms and sand supply from offshore by combination
of waves and tidal currents.
Dalrymple, R.W., Makino, Y. and Zaitlin, B.A., 1991, Temporal and
spatial patterns of rhythmite deposition on mud flats in the mac-
rotidal Cobequid Bay-Salmon River estuary, Bay of Fundy, Can-
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SUMMER: Surface sediments are dominated by mud
facies that covers ca. 70% of the flat. Mixed and sand facies
are minor in areal extent. Sedimentation rate is 10 mm/month
in average. Mud deposit reaches up to ca. 30 cm in thick-
ness. It is suggested that high insolation and subdued wave
encourage mud deposition, which aid to stabilize intertidal
fine-grained sediments.
Davis, R.A.Jr., Fox, W.T., Hayes, M.O. and Boothroyd, J.C., 1972,
Comparison of ridge and runnel systems in tidal and non-tidal
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Lawrence tidal flats, Quebec. Canadian Journal of Earth Science,
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FALL: With increasing wind storms in frequency and
magnitude, significant mud erosion occurs across the inter-
tidal flat. Ephemeral tidal creeks, mud patches and mud
pebbles are formed on the tidal flat, resulting from mud ero-
sion. With time, most sediments change from mud into sand
facies. Sedimentation rate shows the net erosion with ca.
−10 mm/month in average.
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WINTER: Intertidal sediments are dominated by sand
facies. Sedimentation rate is ca. 4 mm/month in average.
Sand would be supplied from offshore by wind wave and
storms. Formation and landward movement of swash bars
also appeals to storm influence.
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ACKNOWLEDGEMENTS: The research was supported through
the grant to S.S. Chun by the Korea Science and Engineering Foun-
dation (KOSEF 98-0703-04-01-3). We thank Drs. H.J. Lee, S.B. Kim,
R.W. Dalrymple and I.T. Lee for discussions and our colleagues, T.S.
Chang, C.S. Sohn, Y.S. Baek and Y.W. Kim, for supports in the field.
Lee, H.J., Chun, S.S., Chang, J.H. and Han, S.J., 1994, Landward
migration of isolated shelly sand ridge (chenier) on the macrotidal
flat of Gomso Bay, west coast of Korea: controls of storms and
typhoon. Journal of Sedimentary Research, 64, 886−893.
Lee, H.J., Chu, Y.S. and Park, Y.A., 1999, Sedimentary processes of
fine-grained material and the effect of seawall construction in the
Daeho macrotidal flat-nearshore area, northern west coast of
Korea. Marine Geology, 157, 171−184.
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