Estuarine, Coastal and Shelf Science 86 (2010) 352–362
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Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier.com/locate/ecss
Sediment dispersion pattern off the present Huanghe (Yellow River) subdelta
and its dynamic mechanism during normal river discharge period
Naishuang Bi a, b, Zuosheng Yang a, b, *, Houjie Wang a, b, Bangqi Hu a, b, Youjun Ji a, b
a
b
College of Marine Geosciences, Ocean University of China, 238 Songling Road, Qingdao 266100, PR China
Key Lab of Submarine Sciences & Prospecting Techniques, MOE, Ocean University of China, 238 Songling Road, Qingdao 266100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 February 2009
Accepted 4 June 2009
Available online 16 June 2009
Hydrological observations were conducted synchronically along three transects in the southeast, middle
and northeast off the present Huanghe (Yellow River) subdelta during normal-discharge (approximately
200 m3 s1) period on August 8–13, 2003. Suspended sediment fluxes and dispersion patterns off the
present Huanghe subdelta were studied based on the hydrographic data collected in these surveys. Along
each survey transect, tidal shear fronts were identified that in combination with the tidal currents were
the dominant factors controlling the pattern of sediment dispersal. Most of the river-laden suspended
sediment from the river mouth was transported via hypopycnal flow and was limited within the 5 m
isobath off the mouth due to the barrier effect of the tidal shear front and the weak river flow. In
northern and southern areas off the subdelta, the sediment fluxes at stations farther from the coast were
much higher than those at the nearshore ones, indicating that the river-laden sediments were transported to north and south offshore via deeper water areas at both sides of the river mouth. The tidal
shear fronts revealed in the northern and southern nearshore areas of the subdelta, jointly with tidal
currents barred the sediment transport from offshore to nearshore. This resulted in offshore sediment
deposition on the northern and southern parts of the subaqueous subdelta, rather than in the nearshore
area, thus forming nearshore erosion and offshore accumulation areas, respectively.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
China
present Huanghe subdelta
sediments
dispersion
tidal currents
geomorphology
1. Introduction
Worldwide, approximately 10–20 billion metric tons of fluvial
sediment are transported into the ocean through rivers every year
(Milliman and Syvitski, 1992). Most of these river-delivered sediment deposits in riverdelta systems, which are vital for deltacoast construction and for environmental preservation (Chen et al.,
2007a,b). With a drainage basin area of 794 712 km2 and a total
length of 5464 km in northern China (Fig. 1a), the Huanghe (Yellow
River) has historically had a low runoff (<60 109 m3 yr1), but one
of the largest sediment loads (10.8 109 t yr1) of any river in the
world (Milliman and Meade, 1983). The Huanghe has been discharging into the Bohai Sea since 1855, forming the modern
Huanghe delta with an accretion of more than 20 km2 yr1 (Pang
and Si, 1980). In total, eleven major shifts of the lower river course
occurred between 1855 and 1976 due to rapid channel siltation,
resulting in the formation of 8 subdeltas (Pang and Si, 1979; Fan
* Corresponding author.
E-mail address: [email protected] (Z. Yang).
0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ecss.2009.06.005
et al., 2006). The latest major shift occurred in 1976 and formed the
present Huanghe subdelta (Fig. 1b).
Suspended sediment dispersion in subaqueous delta in the form
of hypopycnal and hyperpycnal flows has been studied in detail
since the 1980s (Wiseman et al., 1986; Wright et al., 1986, 1988,
1990; Li et al., 1998; Wang et al., 2007b). Approximately one third of
the suspended sediment delivered from the Huanghe is deposited
around the subaerial delta, while the other two-thirds is transported to coastal areas and the Bohai Sea (Pang and Si, 1980; Wu
et al., 1994). Approximately 70% of sediment transported to coastal
areas is deposited in the subaqueous delta region no more than
15 km away from the mouth of the river (Qin and Li, 1983).
However, only 1% of the Huanghe sediment discharge is transported to the Yellow Sea through the southern part of the Bohai
Strait (Martin et al., 1993). The suspended sediment delivered from
the mouth of the Huanghe is transported westward along the coast
of Laizhou Bay (Jiang et al., 2000, 2004).
Shear fronts, interfaces between two bodies of water with
opposing flow directions or significantly different velocities, have
been observed in many estuaries (Nunes and Simpson, 1985;
Huzzey and Brubaker, 1988; Zhu, 1995; Li et al., 2001; Wang et al.,
2006b). The tidal shear front off the mouth of the Huanghe was first
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
Beijing
Maowusu
Desert
LOESS
PLATEAU
Middle reaches
CHINA
a
Upp
er r
eac
hes
Beijing
Lijin
(b)
es
h
ac
er
re
w
Lo
0 100 200km
38.4°
N
b
Bohai Bay
Bohai Sea
38.2°
A1
38°
A3
Transect A
A4
196
4-19
7
6
A2
37.8°
nghe
Hua
37.6°
Huanghe delta
37.4°
37.2°
Transect C
C5
sent
e
r
p
C4
1996
C3
19 C 1 C 2
76
-19
96
B 5 Transect B
B4
B3
La
B2
izh
B1
ou
Ba
y
Survey station
isobath
-5
Diaokou course
Qingshuigou course
0
10 20 km
37°
118.6°
118.8°
119°
119.2°
119.4°
119.6°
E 119.8°
Fig. 1. The Huanghe drainage basin, the location of the present Huanghe subdelta
(land area in shadow rectangle) (a) and the three survey transects, as well as the
survey stations (b).
reported by Li et al. (1994) based on in-situ measurements in 1991
during a period of high water discharge (approximately
2600 m3 s1 at Lijin Station). These measurements indicated that
suspended sediment delivered from the mouth of the Huanghe was
aggregated and deposited rapidly along the shear front zone due to
the low velocity in this region. Wang et al. (2007b) presented the
features and results of the barrier effect on the suspended sediment
dispersion of tidal shear fronts off the mouth of the Huanghe using
25 h in-situ observations at five time-series stations across the
subaqueous slope in 1995. Their results suggested that the hyperpycnal flows generated near the mouth of the river were terminated within shallow waters due to the barrier effect of the shear
front. Nevertheless, their study area was local and limited to the
area near the Qingshuigou river mouth. An overall picture of river
sediment dispersion over the whole present subdelta or the new
river mouth area (since 1996) has not yet been achieved, and it is
critical for understanding the pathway of sediment transport and
flux off the Huanghe delta.
To date, most publications that have discussed the Huanghe
sediment dispersion in the coastal region near the delta and the
Bohai Sea are largely based on datasets prior to 1996. Since that
time, several significant changes have occurred that directly affect
the sediment dispersal pattern off the delta: 1) The deltaic course
shifted significantly in 1996, and the river mouth moved
353
approximately 20 km northeast of the old river mouth (Fig. 1b); 2)
The annual water and sediment discharges from the Huanghe into
the sea were recorded at the Lijin Gauge (some 100 km upstream
from the river mouth, Fig. 1a) and have been drastically reduced
from 25.1 km3 to 634 Mt observed between 1976 and 1996–
7.49 km3 (29.8% of the previous value) and 150 Mt (23.7%) during
the period from 1997 to 2003 due to extensive human activities
(Wang et al., 2006b, 2007b); and 3) The Huanghe water and sediment discharges into the sea have been controlled since 2000 by
the operation of the Xiaolangdi Reservoir, the largest reservoir in
the mainstream, through the Project of ‘‘Artificial Regulation of the
Huanghe Water and Sediment’’ (Wang et al., 2005; Yang et al.,
2008). This project determined that high water and sediment
discharges into the sea are regulated, and only occur once or twice
a year for periods of approximately 15–30 days to scour the
riverbed and transport a relatively large amount of sediment into
the sea. Water flow into the sea is kept at low levels (<500 m3 s1)
for most of the year (e.g., 360 days in 2001). As a result, low water
flow with low sediment discharge into the sea is now the dominant
and normal hydrographic regime for sediment transport off the
present subdelta and the low water discharge in previous publications corresponds to the normal water discharge in this paper.
These recent changes have altered the boundary conditions and the
seasonal allocation of water and sediment in a year, which have
significant impacts on the dispersion of sediment off the present
subdelta.
Most previous studies have focused on the flood season of the
Huanghe when sediment discharge is high and a unique sediment
hyperpycnal flow from the river mouth to the sea is observed
(Wright et al., 1986, 1988, 1990; Li et al., 1998; Wang et al., 2007b).
Less attention has been paid to the sediment dispersion pattern
during normal water flow or to the dispersion pattern off northern
and southern parts of the present subdelta. No studies have yet
demonstrated the sediment dispersion process or have quantitatively assessed the general pattern of sediment dispersion in the
whole area off the present Huanghe subdelta.
This paper demonstrates suspended sediment transport
processes, fluxes, and the mechanism and dispersion pattern of
sediment off the present Huanghe subdelta during normal
discharge period through the new river course. Observations are
based on hydrographic data collected during synchronic multistation hydrographic time-series surveys along three transects in
the southeast, middle and northeast off the present Huanghe
subdelta in August, 2003. The geomorphological response of the
subaqueous delta to the suspended sediment dispersion is discussed based on multi-year observations of bathymetry over the
whole delta region.
2. Study area
The Bohai Sea, a receiving basin of the Huanghe sediment, is
a semi-enclosed shallow shelf sea with an average depth of
approximately 18 m (Wang, 1996). The Huanghe delta composed of
8 subdeltas is located to the west of the Bohai Sea. The tidal regime
is dominated by an irregular semi-diurnal tide with an average tidal
range of 0.6–0.8 m at the river mouth area that increases both
southwards and northwards, reaching 1.5–2.0 m in the south at
Laizhou Bay and Bohai Bay. The tidal currents have an average
speed of 0.5–1.0 m s1, and are in paralleling to the coast and flow
southward during the flood tide, move northwards during the ebb
tide. The tidal currents have a clockwise current rotation during the
transitional period of the tidal phase. The flood tidal duration is 60–
90 min longer than that of the ebb tide during one tidal cycle
(Cheng and Cheng, 2000). The waves off the Huanghe delta vary
strongly by season and are generated by local winds in the Bohai
354
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
Sea. The prevailing southerly waves in the winter are stronger than
the dominant northerly waves in the summer. The surface residual
currents, driven by the winds, move southward in the summer and
northward in the winter (Zhang et al., 1990).
The surface sediment of the seabed in the study area is generally
fine. The main compositions of the sediment are silt and clayey silt
with a grain size of less than 0.01 mm (Qin et al., 1985). Conversely,
sediments in the river mouth on the platform and delta front areas
are coarser and include some sand (Bornhold et al., 1986). The
suspended sediment delivered from the Huanghe is mainly transported northwest by ebb currents (Wiseman et al., 1986).
The Huanghe water and sediment discharges into the sea have
a significant seasonal variability due to the effects of the summer
monsoon. More than 60% of water and sediments are discharged
into the sea during the flood season (Wang et al., 2007b). The water
is highly turbid in the river mouth with an average suspended
sediment concentration of approximately 25 g l1 (Wright and
Nittrouer, 1995) and a peak value of approximately 200 g l1 (Ren
and Shi, 1986), forming prominent hyperpycnal flows in the bottom
layer of the water column during the flood season. However, since
the 1970s, the water and sediment discharges decreased dramatically due to a reduction of precipitation and construction of dams
within the drainage basin (Wang et al., 2006a, 2007a), resulting in
infrequent hyperpycnal flows, even during the flood season.
salinity and suspended sediment concentrations (SSCs) were
measured in the laboratory. The current velocities and temperatures were measured at the corresponding sampling layers using an
LS25-1A propeller current meter and a high-resolution Aanderaa
thermometer, respectively. A single frequency (208 2 kHz) echo
sounder bathometer was employed to record the water depth.
The salinity of the water samples was measured in the laboratory by a SYA2-2 salinometer using the Practical Salinity Scale. The
water samples were filtered through pre-weighed paired micropore filters of 47 mm diameter with a pore diameter of 0.45 mm by
pumping. The filters with sediments were washed three times with
distilled water to remove the remaining salt, and dried at 60 C
before being weighed again using a high-resolution electronic
balance. The suspended sediment concentrations (in g l1) were
calculated from the final sediment weights and volumes of filtered
water.
Bathymetric records of 36 offshore transects covering the area
from the coast to the outer edge of the Huanghe subaqueous delta
(at approximately the 16 m isobath) were collected in 1976 and
2003 to illustrate the deposition pattern of the present Huanghe
subdelta. These bathymetric data were collected every two years by
the Yellow River Conservancy Committee.
3. Data and methods
4.1. Hydrodynamics and tidal shear front
Synchronic time-series hydrographic surveys of multiple
stations along three transects in the southeast, middle and northeast off the present Huanghe subdelta (Fig. 1b) were conducted
from August 8–13, 2003 during the low-discharge period when the
water and sediment discharges at Lijin Station were approximately
200 m3 s1 and 200 kg s1, respectively. Observations were conducted under calm weather conditions with a maximum wind
speed of approximately 3 m s1. Five stations were surveyed in the
southeast transect (B) and the middle transect (C), and four stations
were surveyed in the northeast transect (A). Hydrographic data
including current velocity, temperature and water depth were
recorded, and water samples were collected for 25 h at each station
in 1-h time intervals. Water samples of 500 ml were collected at
three water layers, the surface and depths of 0.6 and 0.9, and the
4.1.1. Tidal currents off the present Huanghe subdelta
Based on the in-situ measurements, the mean surface tidal
current velocities in the ebb and flood tidal phases were calculated
using the method of Reiche (1938). The tidal currents off the
present Huanghe subdelta are reciprocating flows. In the northeast
and middle transects off the present Huanghe subdelta, the
currents flowed southward during the flood tide and northward
during the ebb tide (Fig. 2). The lowest tidal current velocities were
observed at the two stations closest to the shore, A1 and B1
(Table 1). The mean current velocities in the ebb and flood tides at
these stations were 21.9 and 24.5 cm s1, respectively. The tidal
currents at most stations in transect B, except for station B5, were
flowing from west to east and differed from the north–south
direction of the tidal currents at all stations along the northeast
4. Results
38.5°
N
a
Bohai Bay
b
Bohai Bay
Bohai Sea
Bohai Sea
A2 A3 A4
38°
A1 A2
A1
Huanghe delta
C1
e
ngh
Hua
C4
C3
C5
Huanghe delta
Hua
B5
B4
37.5°
B1
0
37°
118.5°
B2
e
B4
Laizhou Bay
B3
B3
B1
Laizhou Bay
50 cm s-1
0
119.5°
B5
5
B2
50 cm s-1
10 20 km
119°
C5
C4
C1C2 C
C3
ngh
C2
A3
A4
E
120°
118.5°
10 20 km
119°
119.5°
E
120°
Fig. 2. Mean vectors of the tidal current velocities at the surface layer in each station during flood tides (a) and ebb tides (b). The arrows marked in grey indicate the predicted
current directions based on the mean vectors of the tidal current.
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
Table 1
Mean tidal current velocities during flood and ebb tides at each station.
Station Mean velocity
Flood tidal phase
A1
A2
A3
A4
C1
C2
C3
C4
C5
B1
B2
B3
B4
B5
Ebb tidal phase
Magnitude
(cm s1)
Azimuth
(degree)
Consistency Magnitude
ratioa
(cm s1)
Azimuth
(degree)
Consistency
ratioa
24.5
41.6
44.9
44.1
42.4
40.4
41.8
58.5
42.1
21.9
33.3
41.7
28.9
41.4
147.3
162.6
134.3
146.1
73.8
79.6
143.2
142.0
152.6
223.9
302.2
282.4
261.4
192.0
0.90
0.95
0.93
0.85
0.99
0.99
0.99
0.96
0.89
0.90
0.97
0.92
0.87
0.96
321.9
348.6
337.8
342.4
73.8
79.6
323.9
329.2
320.2
103.9
132.8
102.7
87.3
49.8
0.92
0.90
0.87
0.86
0.99
0.99
0.95
0.90
0.90
0.93
0.91
0.96
0.95
0.99
24.5
38.9
40.5
39.0
61.4
67.4
35.6
46.6
40.3
22.2
30.8
52.3
41.8
50.5
a
Consistency ratio varying from 0.0 to 1.0 indicates the dispersion of the tidal
current direction. If consistency ratio is zero, the current directions follow a random
distribution. If consistency ratio is 1.0, all the current vectors are in the same
direction.
transect (A) and middle transect (C). During the flood tide, the
currents flowed southward and turned southwest when passing
the protrusion of the abandoned river mouth, Qingshuigou (station
B5), where the mean current speed was 41.4 cm s1 and the azimuth was 192.0 . The currents changed direction further west at
station B4 (28.9 cm s1/261.4 ) and began flowing northwest at
stations B3 (41.7 cm s1/282.4 ) and B2 (33.3 cm s1/302.2 ).
However, the dominant direction of flow during the flood tide was
southwest at station B1, where the mean velocity was 21.9 cm s1
and the azimuth was 223.8 . Thus, a swirling water mass was
formed off the southern part of the delta due to the protrusion of
the abandoned river mouth at Qingshuigou. During the ebb tide,
the currents flowed southeastward off the southern part of the
delta and turned northeastward at station B5 after passing Qingshuigou. The current then flowed northwest after passing the
current river mouth, continuing northwest all the way to the
northern part of the delta (Fig. 2).
4.1.2. Tidal shear front off the present Huanghe subdelta
A tidal shear front was recorded off the abandoned Qingshuigou
river mouth in 1994 and 1995 (Li et al., 1994; Wang et al., 2007b). A
front was also observed along the three offshore transects in 2003.
The tidal shear front was detected in our surveys from the surface
to the bottom of the water column, and was most obvious in the
surface layer. Accordingly, we used the records of the tidal currents
in the surface layer to show the evolution of the tidal shear fronts
along the three transects (Fig. 3). The shear front along transect C
off the present Huanghe river mouth occurred in the first hours of
both the flood and ebb tidal phases. Two types of tidal shear front
were clearly identified, inner-ebb-outer-flood type (IEOF) and
inner-flood-outer-ebb type (IFOE). The IEOF type of shear front
occurred at 19:00–21:00 on August 9 between stations C3 and C4
when the tidal currents at station C3 were ebbing (flowing northwest) while those at station C4 were flooding (flowing southeast;
Fig. 3a). The front then moved seaward and was found between
stations C4 and C5 at 21:00–22:00 on August 9. The IFOE shear
front was recorded at 1:00–3:00 on August 10 between stations C3
and C4 when the tidal currents at station C3 were flooding, while
those at station C4 were ebbing. The front then moved seaward and
disappeared at about 4:00 on August 10. Two types of tidal shear
fronts occurred alternately within one tidal cycle. The movement of
355
the tidal shear front indicated by our records agreed with the
conclusion presented by Wang et al. (2007b). In addition, the total
duration of the two types of tidal shear fronts off the Huanghe
mouth was approximately 4–5 h, which was in agreement with
previous studies (Wang et al., 2007b). The tidal shear front along
transect A was also observed based on the current velocity records
(Fig. 3b). It lasted 3–5 h during one tidal cycle and alternated
between the two types of tidal shear fronts, moving from a shallow
to a deep area, the same as along transect C. The front disappeared
around station A3 at a water depth of approximately 13 m. Qiao
et al. (2008) suggested that the tidal shear front off the river mouth
was caused by a tidal phase gradient along the delta slope, and the
topography, a steep slope, was the dominant factor causing the
formation of the tidal shear front off the Huanghe mouth. Additionally, they concluded that the tidal shear fronts could be
generated in both the region of the present river mouth and in the
region of the abandoned Diaokou river mouth area due to the
strong slopes in both of these areas (locations are shown in Fig. 1b).
The tidal wave propagates southward from the area by the northern
Huanghe delta (Shi and Zhao, 1985) and forms a tidal shear front by
the abandoned river mouth area due to the tidal phase gradient
along the delta slope. The tidal phase gradient still exists when the
tidal wave propagates southward, indicating that the tidal shear
front could occur along the east coast of the Huanghe delta but
would not be limited to the area near the river mouth. Thus, the
records of the tidal shear front in the northern part of the study area
(along transect A) confirmed the numerical simulation results
reported by Qiao et al. (2008). The tidal shear front along transect B
occurred between stations B1 and B2, but was quite different from
the fronts observed along transects C and A in two ways. First, the
front in transect B only occurred during the flood tide and lasted
through the whole flood tide (approximately 6 h in one tidal cycle),
but did not occur during the first 2–3 h of either the ebb and flood
tides as it occurred in transects A and C. Second, it did not occur at
neighboring station B3 or at other stations further seaward, which
means that it did not move from the nearshore area to the offshore
area (Fig. 3c). We suggest that these differences arose as follows:
the currents turned during the flood tide, changing from southward
to northwestward towards the southern coast after passing the
protruding abandoned Qingshougou river mouth, and flowed
through stations B4, B3 and B2 into the southern sea area next to
the subdelta. This resulted in increased water levels in the area
between the southern coastline and transect B. When the flood
tidal currents oriented towards the coast and those flowing
northwest reached the northwestern coast of Laizhou Bay, they
were forced to turn southeast, flowing seaward from the nearshore
area at station B1, following the local topography and the rising
water level (Fig. 2). Thus, the current directions at station B1 were
the opposite of those at station B2, resulting in the formation of
a shear front between stations B1 and B2.
4.2. Suspended sediment dispersion along three transects off the
present Huanghe subdelta
4.2.1. Suspended sediment dispersion off the river mouth along
transect C
The salinity increased seaward along transect C. The lowest
salinity values were recorded at stations C1 and C2, located in the
river channel (shown in Fig. 1b), with a maximum value of
approximately 1.0 during the whole tidal cycle, indicating that the
river water dominated this part of the channel and that almost no
salt water intrusion was occurring from the sea (Fig. 4a, b). The
water was much more turbid in the river channel than in the
Huanghe river mouth. The highest SSC value of approximately
1.3 g l1 was recorded at station C2 during slack water after the ebb
356
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
a
1
IEOF
IFOE
IEOF
IFOE
station C3
Velocity (m s-1)
0
-1
1
station C4
0
1
-1
station C5
0
-1
17:00
22:00
03:00
August 9
b
1
08:00
13:00
IFOE
IEOF
18:00
August 10
Time (hour)
IFOE
IEOF
station A1
Velocity (m s-1)
0
-1
station A2
1
0
1
station A3
-1
station A4
1
0
-1
0
-1
08:00
13:00
18:00
August 8
c
23:00
Time (hour)
04:00
09:00
August 9
1
station B1
0
1
Velocity (m s-1)
-1
station B2
0
1
-1
station B3
0
1
-1
station B4
0
1
station B5
-1
0
-1
11:00
16:00
August 11
21:00
02:00
Time (hour)
07:00
12:00
August 12
Fig. 3. A comparison of current vectors of tidal currents in the surface layer along transects C (a), A (b) and B (c). The slanting lines represent the vectors, with the length of the lines
indicating the current magnitude and the angle (in degrees) indicating the direction of the current (N ¼ 0 ). The marked shadow areas indicate the periods for the formation of tidal
shear fronts, and the widths of the shadow areas correspond to the durations of tidal shear fronts.
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
357
g l-1
g l-1
3
Ebb
Flood
Flood
Station C1
Depth (m)
Station C2
2
0
1
0.6
0.3
0.53
1
0.2
0.1
Ebb
5
Ebb
Flood
Flood
Station C3
0
0.35
0.25
0.20
2
0.15
1
0.10
0.05
Ebb
Flood
Ebb
Station C5
15
10
5
0
17:00
22:00
August 9
03:00
08:00
13:00
18:00
0.2
0
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
3
Flood
0.4
0
0.30
4
1.2
0.8
0.4
.53
0
Depth (m)
3
Flood
1.0
0.5
2
0
e
b
Ebb
Flood
0.6
0
c
0.7
Ebb
0.54
Depth (m)
a
Ebb
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0
d
Ebb
Ebb
Flood
12
Flood
Station C4
10
8
6
4
2
0
17:00
22:00
August 9
03:00
08:00
Time (hour)
13:00
18:00
August 10
Time (hour) August 10
Fig. 4. Vertical and temporal variations of salinity (in black contour) and SSCs (in color, in g l1) at stations C1(a), C2(b), C3(c), C4(d) and C5(e).
tide. The SSCs ranged from 0.1 to 0.7 g l1 at station C1, much lower
than those at station C2. The structures of the water masses at two
stations in the river channel were fairly uniform throughout the
survey. The salinity increased rapidly at station C3 and varied
periodically from 13.0 to 30.0 with the tidal phase. Observations
with lower salinity values and evident stratification were recorded
at the end of the ebb tides and at the beginning of the flood tides
after slack water, e.g., at 1:00–4:00 and 12:00–15:00 on August 9
(Fig. 4c). The SSCs at station C3 decreased rapidly to 0.01–0.4 g l1.
The high SSC values were observed at roughly the same time as the
low salinity values, indicating that the river effluent could reach
station C3 at the end of ebb tides and during the early flood tides.
The temporal variation of the salinity at station C4 was almost the
same as that at station C3, although higher salinities (25.0–32.0)
were observed at this station in comparison to station C3 (Fig. 4d).
A highly stratified water column with a low salinity value was also
detected in the first hours of the flood tides. The SSCs at station C4
decreased further, reaching values of 0.01–0.1 g l1 varied as those
at station C3. The salinity at station C5 (30.0–32.0) was the highest
among all stations during the whole tidal cycle, with the smallest
fluctuations observed at the surface and bottom layers.The water
column at this station was not as stratified as at station C4 (Fig. 4e).
The SSCs at station C5 ranged from 0.01 to 0.06 g l1, values that
were much lower than those at other stations, implying that the
Huanghe effluent has little impact on the variation of SSCs at
station C5.
4.2.2. Suspended sediment dispersion in the northeast of the
present Huanghe subdelta along transect A
The salinity along transect A increased seaward from station A1
to A4. The salinity was lower at station A1 than at the other stations
along transect A throughout the entire tidal cycle (Fig. 5). Salinity
values varied between 31.0 and 32.0 without any evident periodic
variation with the tidal phase, while the SSCs in flood tides were
slightly higher than those in the ebb tides. For example, the SSCs
during the flood tide between 12:00–18:00 on August 8 were
slightly higher (0.03–0.34 g l1) than those in the ebb tide between
18:00–1:00 on August 9 (0.01–0.14 g l1), indicating that the river
effluent carried by the ebb currents had little impact on the water at
station A1 (Fig. 5a). The salinity at station A2 seemed to vary
slightly with the tidal phase, decreasing during ebb phases with
a minimum value of approximately 31.5 and increasing during flood
tides with a maximum value of approximately 32.8. The SSCs at
station A2 ranged from 0.002 to 0.02 g l1, and were much lower
than those at station A1 (Fig. 5b). Variations in the salinity at station
A3 were similar to those at station A2 (Fig. 5c). However, the
pattern of SSC variation at station A3 seemed to be quite different
from that at station A1, as indicated by the high turbidity in both
the surface and bottom layers at station A3 during ebb tides (e.g..
19:00–23:00 on August 8 and 05:00–10:00 on August 9) compared
to those observed during flood tides (Fig. 5a and c). The bottom SSC
even exceeded 0.45 g l1 at 6:00 on August 9 (ebb tide) at station
A3. At station A4, the salinity fluctuated slightly and was higher
than the salinity values observed at the other stations throughout
the tidal cycle, whereas the SSCs were much lower than those at
stations A1 and A3 (Fig. 5d).
4.2.3. Suspended sediment dispersion in the southeast of the
present Huanghe subdelta along transect B
For the salinity of five stations along transect B, the salinity at
station B1 was lower than that at the other stations, and was found
to decrease during ebb tides and increase during flood tides
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
a
Flood
Flood
Ebb
Station A1
6
Depth (m)
Ebb
4
2
0
Depth (m)
c
Flood
14
12
10
8
6
4
2
0
9:00
14:00
August 8
Ebb
19:00
Flood
0:00
Time (hour)
Ebb
Station A3
5:00
10:00
g l-1
0.36
0.32
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0
0.44
0.4
0.36
0.32
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0
b
Flood
Flood
Ebb
Ebb
g l-1
0.020
0.018
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0
Station A2
10
8
6
4
2
0
d
Flood
14
12
10
8
6
4
2
0
9:00
August 9
Flood
Ebb
Ebb
Station A4
32.8
358
14:00
August 8
19:00
0:00
Time (hour)
5:00
10:00
0.026
0.024
0.022
0.020
0.018
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0
August 9
Fig. 5. Vertical and temporal variations of salinity (in black contour) and SSCs (in color, in g l1) at stations A1(a), A2(b), A3(c) and A4(d).
(Fig. 6a). However, the salinity at the other four stations had an
opposite behavior, decreasing during flood tides and increasing
during ebb phases (Fig. 6b–e). Station B2 had the highest salinity
along transect B in the surface, middle and bottom layers during the
whole tidal cycle, with a small fluctuation of 32.5–33.1. The SSCs at
stations B1 and B2 were much lower than those at the other
stations along transect B, ranging between 0.005 and 0.05 g l1
(Fig. 6a, b). The salinity decreased from stations B2 to B5 with the
enhanced fluctuation, while the SSCs increased significantly at
station B3 (e.g., 19:00–23:00 on August 11 and 3:00–6:00 on
August 12) and ranged from 0.02 to 0.2 g l1 (Fig. 6c). SSC values
then decreased slightly at station B4, but were much higher than
gl-1
gl-1
a
Ebb
6
Flood
Ebb
5
Depth (m)
Flood
Station B1
4
3
2
1
0
c
Ebb
Flood
Ebb
Flood
Station B3
Depth (m)
6
4
2
0
e
Ebb
Flood
Ebb
Depth (m)
Flood
Station B5
6
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0
b
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
d
0.40
Flood
Ebb
Flood
Station B2
0.07
0.06
0.05
6
0.04
4
0.03
2
0.02
0.01
0
0
Ebb
5
Flood
Ebb
Flood
Station B4
0.10
0.08
4
0.06
3
2
0.04
1
0.02
0
11:00
0
16:00
August 11
21:00
02:00
07:00
12:00
August 12
0.35
5
0.30
4
0.25
3
0.20
2
0.15
1
0.10
0
11:00
Ebb
8
0.05
16:00
August 11
21:00
02:00
07:00
12:00
0
August 12
Fig. 6. Vertical and temporal variations of salinity (in black contour) and SSCs (in color, in g l1) at stations B1 (a), B2 (b), B3 (c), B4 (d) and B5 (e).
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
those at stations B1 and B2 (Fig. 6d). The water at station B5 had
a salinity range of 30.0–32.5 and was more turbid, with a peak SSC
value of approximately 0.4 g l1 in the surface and bottom layers,
higher than the peak SSC values at the other stations (Fig. 6e). The
water column structure along transect B was quite uniform during
the 25 h survey.
4.3. Sediment fluxes along three transects off the present
Huanghe subdelta
The sediment flux at each station was calculated based on the
SSC value and the corresponding current velocity. The sediment
fluxes along transect C showed that the river-laden sediment was
transported northeastward parallel to the river channel and into
the sea at stations C1 and C2. Flux values were approximately
6.6 kg s1 at station C1 and increased to approximately
14.0 kg s1 m1 at station C2 (Fig. 7). At station C3, flux values
decreased dramatically to approximately 0.7 kg s1 m1, approximately 0.5% that at station C2, indicating that the alongshore
transport of river-delivered sediments within the 5 m isobath was
dominant. The suspended sediments at both stations C3 at 5 m and
C4 at 11 m dispersed southeastward with sediment fluxes of 0.7–
0.9 kg s1 m1, indicating that the suspended sediment was
primarily carried from the northern area by flood currents. The
sediment flux at station C5 decreased dramatically to approximately 0.1 kg m1 s1 in comparison to 0.9 kg m1 s1 at station C4.
The landward transport of sediment flux at station 5 indicated that
the transport of river-laden sediments was mostly confined within
the nearshore region shallower than 15 m.
Sediment fluxes along transect A showed that sediment was
transported to the southwest at the nearshore station (A1) with
a net sediment flux of approximately 0.13 kg s1 m1 (Fig. 7),
implying that the suspended sediment at station A1 was largely
derived from resuspension from the abandoned Diaokou river
mouth, since the SSCs at station A1 were higher during flood tides
than those observed during ebb tides (Fig. 5). In contrast, the suspended sediment at stations A2, A3 and A4 was transported to the
northwest with net sediment fluxes of approximately
38.5°
0.01 kg m-1 s-1
0.1 kg m-1 s-1
1.0 kg m-1 s-1
10.0 kg m-1 s-1
N
Bohai Bay
1.3
0.1
A1
38.0°
A2
Bohai Sea
0.2
A3
A4
0.1
Huanghe delta
0.1
C3
C1 C2
C5
C4
6.6 14.0
e
gh
uan
0.6
H
0.1 B3
37.5°
B2
B4
B5
0.9
1.1
0.5
Laizhou Bay
0.4
B1
0.05
0
37.0°
118.5°
10 20 km
119.0°
119.5°
E 120.0°
Fig. 7. Sediment fluxes at all stations based on 25 h measurements. The flux magnitudes are indicated by the underlined numbers, and the arrows indicate the direction.
359
0.1 kg s1 m1, 1.34 kg s1 m1 and 0.19 kg s1 m1, respectively.
The sediment flux at station A3 was much higher than that at all the
other stations, comprising approximately 76.5% of the total sediment flux along transect A. This suggests that the transport of riverladen sediment from the river mouth was primarily northward
through station A3.
The sediment fluxes along transect B showed that the suspended sediment was transported eastward or southeastward
around the head of the river mouth that had been active between
1976 and 1996 (stations B3, B4 and B5 in Fig. 7). However, the
suspended sediment at station B2 was transported northwestward
in the opposite direction from sediment at stations B3, B4 and B5,
with much lower flux values (0.10 kg s1 m1) in comparison to
those at stations B3 (0.4 kg s1 m1), B4 (0.5 kg s1 m1) and B5
(1.1 kg s1 m1). The sediment at station B1 was transported
southward along the coast, the opposite direction of the sediment
at station B2 with a low flux of 0.05 kg s1 m1, indicating that
a very small amount of suspended sediment was delivered to the
nearshore area.
5. Discussion
5.1. The process and mechanism of suspended sediment dispersion
The tidal currents off the present Huanghe subdelta are reciprocating flows with lower velocity in nearshore areas, and form
a swirling of water body in the southern part off the present
Huanghe subdelta due to the protrusion of the abandoned Qingshuigou mouth (Fig. 2). The tidal currents in combination with the
tidal shear fronts which were identified along each transect off the
present Huanghe subdelta (Fig. 3) control the suspended sediment
dispersal in study area.
The barrier effect of the tidal shear front on the river-laden
sediment dispersion seemed to be effective in trapping suspended
sediment as the low water and sediment discharges weakened the
extension of the river plume off the river mouth. The diluted water
was restricted within the 5 m isobath due to the impact of the shear
front, and was transported northward by tidal currents during ebb
tides. However, during the transition period from the ebb tide to
flood tide, the river water characterized by a low salinity and a high
SSC was transported to the deeper sea due to the clockwiserotating currents (Pang and Jiang, 2003) and the disappearance of
a shear front. Therefore, the turbid water was transported southward by flood currents passing by stations C3 and C4, resulting in
a decrease in salinity and an increase in SSCs during flood tides
(Fig. 4), along with the southward net sediment fluxes during the
two tidal cycles at these two stations (Fig. 7). Additionally, the
maximum SSC in the bottom water layer was approximately
0.2 g l1, with a corresponding salinity value of approximately 25 in
the river mouth area during the field survey (Fig. 4), and was much
lower than 30 g l1, the critical SSC for the formation of hyperpycnal
flow (Pang and Yang, 2001). Therefore, the suspended sediment
dispersion off the river mouth was predominantly in the form of
hypopycnal flow during the low-discharge period, in contrast with
the hyperpycnal flow observed by Wang et al. (2007b).
This relatively turbid water mass could not be carried close to
the shore due to the barrier effect of the shear front formed in the
shallower area and transported northward again by the ebb
currents in the next tidal cycle through a deeper sea area, resulting
in the SSC increasing during ebb tide (Fig. 5) and the northward net
sediment flux of one tidal cycle at station A3 (Fig. 7). This water
mass did not pass through the shallower area barred by the shear
front between stations A1 and A2. This result generally agreed with
the suspended sediment dispersion shown in a LANDSAT image
acquired on May 5, 1998 (Fig. 8a). This image also illustrated that
360
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
a
b
r fr
ea
sh
t
on
Huanghe delta
Huanghe delta
r fr
ea
sh
t
on
0
10
20km
May 25, 1998
0
10
Aug. 28, 1999
20km
Fig. 8. Sketch maps of the suspended sediment dispersal of the present Huanghe subdelta during the ebb tidal phase (a) and the flood tidal phase (b). The LANDSAT images were
taken on May 25, 1998 (a) and August 28, 1999 (b).
accumulation occurred in the deeper offshore areas in the two
accumulation–erosion transient zones. This indicates that the
alongshore deposition of the river-laden sediment took place in the
offshore areas in the northern and southern areas of the
subaqueous delta, coinciding with the net sediment fluxes along
4260
a
Bohai
Sea
Huanghe
delta
4240
Hu
an
gh
e
b
Northing (km)
4220
A1 A2
4200
A3 A4
Erosion centers
C5
C4
C1
C2 C3
Accumulation
centers
4180
B5
1
4160
5.2. Geomorphological response of the subaqueous delta to the
suspended sediment dispersion
B4
B3
B1
Contour intervel:1m
B2
-1
the turbid nearshore water along transect A seemed to be consistent with the resuspended sediment from the northern abandoned
river mouth area. The seaward transport in this area was obstructed
by the shear front between stations A1 and A2.
The turbid water in the southern area off the delta was derived
from the river mouth, flowing southward during the flood tides and
turning westward into Laizhou Bay after the protrusion of the
abandoned Qingshuigou river mouth as observed on the LANDSAT
satellite image acquired on May 2, 2000 (Fig. 8b), demonstrating
that the water did not arise from the resuspension of the surface
sediment as documented by previous publications (e.g., Jiang and
Wang, 2005). Jiang and Wang (2005) suggested that resuspension
might be the major source of the fine suspended sediment in
Laizhou Bay. However, there was no evident relationship between
the shear stress estimated on the near bottom velocity and the
observed SSC, suggesting that the fine sediment off the Huanghe
river mouth that was transported southward would be the primary
sediment source for the suspended sediment in Laizhou Bay.
The tidal shear front, which occurred throughout the flood tide
phase between stations B1 and B2, prevented the turbid water with
high salinity from reaching the eastern coast of the southern delta.
During the ebb tides, the tidal currents flowed southeastward at all
stations except for station B5 (Fig. 2), carrying the turbid water
away from the southeastern coast of the delta. Therefore, this
turbid water mass could not reach the southeastern coast of the
delta during the entire tidal period (Fig. 6). Thus, a narrow and
relatively clear band of water with low salinity and low SSC was
formed between the turbid water mass and the southeastern coast
of the delta parallel to the shoreline, as shown by the satellite image
acquired on August 28, 1999 (Fig. 8b).
Coastline of 2003
The erosion–accumulation pattern of the subaqueous delta was
quantitatively estimated based on the bathymetric data recorded
from 36 offshore transects off the Huanghe delta in 1976 and 2003.
A drip-like accumulation area with an irregular edge was formed
around the present sub-delta, including two accumulation centers
around the present river mouth and the abandoned Qingshuigou
river mouth. The two erosion areas were separated from each other
by the accumulation area with two distinct accumulation–erosion
transient zones north and south of the delta, respectively (Fig. 9).
Erosion took place in the shallower nearshore areas and
4140
Coastline of 1976
20640
20660
20680
20700
Easting (km)
Fig. 9. The net erosion–accumulation morphology of the Huanghe delta based on 36
bathymetric survey lines along the delta coast in 1976 and 2003 (a). The offshore
boundary of the delta is identified as the 15 m water depth contour. The accumulation
area was marked by a shadow with solid contour lines, and the erosion areas were
indicated by dashed contour lines. The dashed lines in panel (b) indicate the positions
of the 36 bathymetric survey lines.
N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362
transects A and B (Fig. 7). The accumulation area extended northward via the deeper sea area, and the erosion area was located in
the shallower area along transect A. The gradient of the accumulation thickness between stations C3 and C4 was very high, and
decreased sharply around station C5 (Fig. 9), suggesting that most
of the river-laden sediments deposited on the landward side of
station C3, and station C5 (depth of approximately 15 m) with an
accumulation thickness of approximately 2 m was close to the
seaward boundary of river-laden sediment transport. A slight
erosion zone where the coastline retreated landward, located on
the southeastern coast of the delta, corresponded with the position
of a clear water belt that was clearly observed on satellite images.
Thus, fast seaward propagation of the central part of the subdelta
with the retreat of its northern and southern coastline back
towards the land was the direct consequence of the sediment
dispersion.
Although high water and sediment discharges have made
a significant contribution to the delta accretion, both in the past and
during the water-sediment regulation periods over the past 7 years,
the sediment dynamics and processes we discussed based on the
low-discharge observation still played an important role in the
river-laden sediment dispersal and thus controlled the geomorphology of the delta. Therefore, the sediment dynamics and delta
response would provide a good reference for the safety of the
coastal dike and development of the oil fields on the delta.
Note that strong wave action, especially in the winter seasons,
could significantly reshape the deltaic geomorphology, as the high
stress induced by waves in the shallow water could cause notable
resuspension of sediments (e.g., Wang et al., 2006c). Observations
near the Huanghe delta in 1987 indicated that storm-induced
strong wave action directly induced a prominent slope failure of the
subaqueous delta, and the down-slope transport of resuspended
sediments reshaped the slope (Prior et al., 1989).
6. Conclusions
Tidal shear fronts with different formation mechanisms were
found along three transects off the present Huanghe subdelta. The
combined shear fronts and alongshore tidal currents were the
major dynamic factors controlling the sediment dispersion near
the present subdelta. Most of the sediment that was delivered to
the sea in the form of hypopycnal flow was deposited within the
5 m isobath off the river mouth due to the barrier effect of the tidal
shear front. The river-laden sediment was transported northward
or southward through the deeper water at both sides of the river
mouth under the joint effect of the shear fronts and tidal currents,
but not through the shallower nearshore water along the coast.
These observations were generally in agreement with the suspended sediment dispersal pattern indicated by satellite images.
Two shear fronts and tidal currents in the northern and
southern shallower nearshore areas of the delta prevented the
sediment transport from the offshore areas towards nearshore
areas, resulting in offshore sediment deposition in the northern and
southern parts of the subaqueous delta, rather than in the nearshore areas. Thus, two inside erosion-outside accumulation transition zones were formed off the northern and southern parts of the
delta, respectively.
Human activity has caused sharp decreases in water and sediment discharge from the river to the sea, and the area of accumulation and volume of the active river mouth is expected to decrease
in the future due to insufficient sediment supply, while the erosion
areas are expected to extend. These future challenges must be
considered by the delta conservation community and in the future
development plans of the Shengli Oil field in this region.
361
Acknowledgements
This work was supported by NSFC projects (No. 40676036 and
40876019) and by the National Fundamental Research Program of
Ministry of Science & Technology, China (No. 2002CB41404). We
thank Professor Zhongyuan Chen from East China Normal University for his valuable comments. We are grateful to the anonymous
reviewers for their constructive comments and helpful suggestion
to improve the manuscript.
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