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Estuarine, Coastal and Shelf Science 86 (2010) 352–362

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Estuarine, Coastal and Shelf Science

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Sediment dispersion pattern off the present Huanghe (Yellow River) subdeltaand 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 College of Marine Geosciences, Ocean University of China, 238 Songling Road, Qingdao 266100, PR Chinab 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

Article history:Received 25 February 2009Accepted 4 June 2009Available online 16 June 2009

Keywords:Chinapresent Huanghe subdeltasedimentsdispersiontidal currentsgeomorphology

* Corresponding author.E-mail address: zshyang@ouc.edu.cn (Z. Yang).

0272-7714/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.ecss.2009.06.005

a b s t r a c t

Hydrological observations were conducted synchronically along three transects in the southeast, middleand northeast off the present Huanghe (Yellow River) subdelta during normal-discharge (approximately200 m3 s�1) period on August 8–13, 2003. Suspended sediment fluxes and dispersion patterns off thepresent Huanghe subdelta were studied based on the hydrographic data collected in these surveys. Alongeach survey transect, tidal shear fronts were identified that in combination with the tidal currents werethe dominant factors controlling the pattern of sediment dispersal. Most of the river-laden suspendedsediment from the river mouth was transported via hypopycnal flow and was limited within the 5 misobath off the mouth due to the barrier effect of the tidal shear front and the weak river flow. Innorthern and southern areas off the subdelta, the sediment fluxes at stations farther from the coast weremuch higher than those at the nearshore ones, indicating that the river-laden sediments were trans-ported to north and south offshore via deeper water areas at both sides of the river mouth. The tidalshear fronts revealed in the northern and southern nearshore areas of the subdelta, jointly with tidalcurrents barred the sediment transport from offshore to nearshore. This resulted in offshore sedimentdeposition on the northern and southern parts of the subaqueous subdelta, rather than in the nearshorearea, thus forming nearshore erosion and offshore accumulation areas, respectively.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Worldwide, approximately 10–20 billion metric tons of fluvialsediment are transported into the ocean through rivers every year(Milliman and Syvitski, 1992). Most of these river-delivered sedi-ment deposits in river�delta systems, which are vital for delta-coast construction and for environmental preservation (Chen et al.,2007a,b). With a drainage basin area of 794 712 km2 and a totallength of 5464 km in northern China (Fig. 1a), the Huanghe (YellowRiver) has historically had a low runoff (<60� 109 m3 yr�1), but oneof the largest sediment loads (10.8 � 109 t yr�1) of any river in theworld (Milliman and Meade, 1983). The Huanghe has been dis-charging into the Bohai Sea since 1855, forming the modernHuanghe delta with an accretion of more than 20 km2 yr�1 (Pangand Si, 1980). In total, eleven major shifts of the lower river courseoccurred between 1855 and 1976 due to rapid channel siltation,resulting in the formation of 8 subdeltas (Pang and Si, 1979; Fan

All rights reserved.

et al., 2006). The latest major shift occurred in 1976 and formed thepresent Huanghe subdelta (Fig. 1b).

Suspended sediment dispersion in subaqueous delta in the formof hypopycnal and hyperpycnal flows has been studied in detailsince the 1980s (Wiseman et al., 1986; Wright et al., 1986, 1988,1990; Li et al., 1998; Wang et al., 2007b). Approximately one third ofthe suspended sediment delivered from the Huanghe is depositedaround the subaerial delta, while the other two-thirds is trans-ported to coastal areas and the Bohai Sea (Pang and Si, 1980; Wuet al., 1994). Approximately 70% of sediment transported to coastalareas is deposited in the subaqueous delta region no more than15 km away from the mouth of the river (Qin and Li, 1983).However, only 1% of the Huanghe sediment discharge is trans-ported to the Yellow Sea through the southern part of the BohaiStrait (Martin et al., 1993). The suspended sediment delivered fromthe mouth of the Huanghe is transported westward along the coastof Laizhou Bay (Jiang et al., 2000, 2004).

Shear fronts, interfaces between two bodies of water withopposing flow directions or significantly different velocities, havebeen 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

a

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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 thesurvey stations (b).

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362 353

reported by Li et al. (1994) based on in-situ measurements in 1991during a period of high water discharge (approximately2600 m3 s�1 at Lijin Station). These measurements indicated thatsuspended sediment delivered from the mouth of the Huanghe wasaggregated and deposited rapidly along the shear front zone due tothe low velocity in this region. Wang et al. (2007b) presented thefeatures and results of the barrier effect on the suspended sedimentdispersion of tidal shear fronts off the mouth of the Huanghe using25 h in-situ observations at five time-series stations across thesubaqueous slope in 1995. Their results suggested that the hyper-pycnal flows generated near the mouth of the river were termi-nated within shallow waters due to the barrier effect of the shearfront. Nevertheless, their study area was local and limited to thearea near the Qingshuigou river mouth. An overall picture of riversediment dispersion over the whole present subdelta or the newriver mouth area (since 1996) has not yet been achieved, and it iscritical for understanding the pathway of sediment transport andflux off the Huanghe delta.

To date, most publications that have discussed the Huanghesediment dispersion in the coastal region near the delta and theBohai Sea are largely based on datasets prior to 1996. Since thattime, several significant changes have occurred that directly affectthe sediment dispersal pattern off the delta: 1) The deltaic courseshifted significantly in 1996, and the river mouth moved

approximately 20 km northeast of the old river mouth (Fig. 1b); 2)The annual water and sediment discharges from the Huanghe intothe sea were recorded at the Lijin Gauge (some 100 km upstreamfrom the river mouth, Fig. 1a) and have been drastically reducedfrom 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%) duringthe period from 1997 to 2003 due to extensive human activities(Wang et al., 2006b, 2007b); and 3) The Huanghe water and sedi-ment discharges into the sea have been controlled since 2000 bythe operation of the Xiaolangdi Reservoir, the largest reservoir inthe mainstream, through the Project of ‘‘Artificial Regulation of theHuanghe Water and Sediment’’ (Wang et al., 2005; Yang et al.,2008). This project determined that high water and sedimentdischarges into the sea are regulated, and only occur once or twicea year for periods of approximately 15–30 days to scour theriverbed and transport a relatively large amount of sediment intothe sea. Water flow into the sea is kept at low levels (<500 m3 s�1)for most of the year (e.g., 360 days in 2001). As a result, low waterflow with low sediment discharge into the sea is now the dominantand normal hydrographic regime for sediment transport off thepresent subdelta and the low water discharge in previous publi-cations corresponds to the normal water discharge in this paper.These recent changes have altered the boundary conditions and theseasonal allocation of water and sediment in a year, which havesignificant impacts on the dispersion of sediment off the presentsubdelta.

Most previous studies have focused on the flood season of theHuanghe when sediment discharge is high and a unique sedimenthyperpycnal 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 patternduring normal water flow or to the dispersion pattern off northernand southern parts of the present subdelta. No studies have yetdemonstrated the sediment dispersion process or have quantita-tively assessed the general pattern of sediment dispersion in thewhole area off the present Huanghe subdelta.

This paper demonstrates suspended sediment transportprocesses, fluxes, and the mechanism and dispersion pattern ofsediment off the present Huanghe subdelta during normaldischarge period through the new river course. Observations arebased on hydrographic data collected during synchronic multi-station hydrographic time-series surveys along three transects inthe southeast, middle and northeast off the present Huanghesubdelta in August, 2003. The geomorphological response of thesubaqueous delta to the suspended sediment dispersion is dis-cussed based on multi-year observations of bathymetry over thewhole delta region.

2. Study area

The Bohai Sea, a receiving basin of the Huanghe sediment, isa semi-enclosed shallow shelf sea with an average depth ofapproximately 18 m (Wang, 1996). The Huanghe delta composed of8 subdeltas is located to the west of the Bohai Sea. The tidal regimeis dominated by an irregular semi-diurnal tide with an average tidalrange of 0.6–0.8 m at the river mouth area that increases bothsouthwards and northwards, reaching 1.5–2.0 m in the south atLaizhou Bay and Bohai Bay. The tidal currents have an averagespeed of 0.5–1.0 m s�1, and are in paralleling to the coast and flowsouthward during the flood tide, move northwards during the ebbtide. The tidal currents have a clockwise current rotation during thetransitional 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 varystrongly by season and are generated by local winds in the Bohai

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362354

Sea. The prevailing southerly waves in the winter are stronger thanthe dominant northerly waves in the summer. The surface residualcurrents, driven by the winds, move southward in the summer andnorthward in the winter (Zhang et al., 1990).

The surface sediment of the seabed in the study area is generallyfine. The main compositions of the sediment are silt and clayey siltwith 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 areasare coarser and include some sand (Bornhold et al., 1986). Thesuspended sediment delivered from the Huanghe is mainly trans-ported northwest by ebb currents (Wiseman et al., 1986).

The Huanghe water and sediment discharges into the sea havea significant seasonal variability due to the effects of the summermonsoon. More than 60% of water and sediments are dischargedinto the sea during the flood season (Wang et al., 2007b). The wateris highly turbid in the river mouth with an average suspendedsediment concentration of approximately 25 g l�1 (Wright andNittrouer, 1995) and a peak value of approximately 200 g l�1 (Renand Shi, 1986), forming prominent hyperpycnal flows in the bottomlayer of the water column during the flood season. However, sincethe 1970s, the water and sediment discharges decreased dramati-cally due to a reduction of precipitation and construction of damswithin the drainage basin (Wang et al., 2006a, 2007a), resulting ininfrequent hyperpycnal flows, even during the flood season.

3. Data and methods

Synchronic time-series hydrographic surveys of multiplestations along three transects in the southeast, middle and north-east off the present Huanghe subdelta (Fig. 1b) were conductedfrom August 8–13, 2003 during the low-discharge period when thewater and sediment discharges at Lijin Station were approximately200 m3 s�1 and 200 kg s�1, respectively. Observations were con-ducted under calm weather conditions with a maximum windspeed of approximately 3 m s�1. Five stations were surveyed in thesoutheast transect (B) and the middle transect (C), and four stationswere surveyed in the northeast transect (A). Hydrographic dataincluding current velocity, temperature and water depth wererecorded, and water samples were collected for 25 h at each stationin 1-h time intervals. Water samples of 500 ml were collected atthree water layers, the surface and depths of 0.6 and 0.9, and the

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Fig. 2. Mean vectors of the tidal current velocities at the surface layer in each station duricurrent directions based on the mean vectors of the tidal current.

salinity and suspended sediment concentrations (SSCs) weremeasured in the laboratory. The current velocities and tempera-tures were measured at the corresponding sampling layers using anLS25-1A propeller current meter and a high-resolution Aanderaathermometer, respectively. A single frequency (208 � 2 kHz) echosounder bathometer was employed to record the water depth.

The salinity of the water samples was measured in the labora-tory by a SYA2-2 salinometer using the Practical Salinity Scale. Thewater samples were filtered through pre-weighed paired micro-pore filters of 47 mm diameter with a pore diameter of 0.45 mm bypumping. The filters with sediments were washed three times withdistilled water to remove the remaining salt, and dried at 60 �Cbefore being weighed again using a high-resolution electronicbalance. The suspended sediment concentrations (in g l�1) werecalculated from the final sediment weights and volumes of filteredwater.

Bathymetric records of 36 offshore transects covering the areafrom the coast to the outer edge of the Huanghe subaqueous delta(at approximately the 16 m isobath) were collected in 1976 and2003 to illustrate the deposition pattern of the present Huanghesubdelta. These bathymetric data were collected every two years bythe Yellow River Conservancy Committee.

4. Results

4.1. Hydrodynamics and tidal shear front

4.1.1. Tidal currents off the present Huanghe subdeltaBased on the in-situ measurements, the mean surface tidal

current velocities in the ebb and flood tidal phases were calculatedusing the method of Reiche (1938). The tidal currents off thepresent Huanghe subdelta are reciprocating flows. In the northeastand middle transects off the present Huanghe subdelta, thecurrents flowed southward during the flood tide and northwardduring the ebb tide (Fig. 2). The lowest tidal current velocities wereobserved at the two stations closest to the shore, A1 and B1(Table 1). The mean current velocities in the ebb and flood tides atthese stations were 21.9 and 24.5 cm s�1, respectively. The tidalcurrents at most stations in transect B, except for station B5, wereflowing from west to east and differed from the north–southdirection of the tidal currents at all stations along the northeast

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ng flood tides (a) and ebb tides (b). The arrows marked in grey indicate the predicted

Table 1Mean tidal current velocities during flood and ebb tides at each station.

Station Mean velocity

Flood tidal phase Ebb tidal phase

Magnitude(cm s�1)

Azimuth(degree)

Consistencyratioa

Magnitude(cm s�1)

Azimuth(degree)

Consistencyratioa

A1 24.5 147.3 0.90 24.5 321.9 0.92A2 41.6 162.6 0.95 38.9 348.6 0.90A3 44.9 134.3 0.93 40.5 337.8 0.87A4 44.1 146.1 0.85 39.0 342.4 0.86C1 42.4 73.8 0.99 61.4 73.8 0.99C2 40.4 79.6 0.99 67.4 79.6 0.99C3 41.8 143.2 0.99 35.6 323.9 0.95C4 58.5 142.0 0.96 46.6 329.2 0.90C5 42.1 152.6 0.89 40.3 320.2 0.90B1 21.9 223.9 0.90 22.2 103.9 0.93B2 33.3 302.2 0.97 30.8 132.8 0.91B3 41.7 282.4 0.92 52.3 102.7 0.96B4 28.9 261.4 0.87 41.8 87.3 0.95B5 41.4 192.0 0.96 50.5 49.8 0.99

a Consistency ratio varying from 0.0 to 1.0 indicates the dispersion of the tidalcurrent direction. If consistency ratio is zero, the current directions follow a randomdistribution. If consistency ratio is 1.0, all the current vectors are in the samedirection.

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362 355

transect (A) and middle transect (C). During the flood tide, thecurrents flowed southward and turned southwest when passingthe protrusion of the abandoned river mouth, Qingshuigou (stationB5), where the mean current speed was 41.4 cm s�1 and the azi-muth was 192.0�. The currents changed direction further west atstation B4 (28.9 cm s�1/261.4�) and began flowing northwest atstations B3 (41.7 cm s�1/282.4�) and B2 (33.3 cm s�1/302.2�).However, the dominant direction of flow during the flood tide wassouthwest at station B1, where the mean velocity was 21.9 cm s�1

and the azimuth was 223.8�. Thus, a swirling water mass wasformed off the southern part of the delta due to the protrusion ofthe abandoned river mouth at Qingshuigou. During the ebb tide,the currents flowed southeastward off the southern part of thedelta and turned northeastward at station B5 after passing Qing-shuigou. The current then flowed northwest after passing thecurrent river mouth, continuing northwest all the way to thenorthern part of the delta (Fig. 2).

4.1.2. Tidal shear front off the present Huanghe subdeltaA tidal shear front was recorded off the abandoned Qingshuigou

river mouth in 1994 and 1995 (Li et al., 1994; Wang et al., 2007b). Afront was also observed along the three offshore transects in 2003.The tidal shear front was detected in our surveys from the surfaceto the bottom of the water column, and was most obvious in thesurface layer. Accordingly, we used the records of the tidal currentsin the surface layer to show the evolution of the tidal shear frontsalong the three transects (Fig. 3). The shear front along transect Coff the present Huanghe river mouth occurred in the first hours ofboth the flood and ebb tidal phases. Two types of tidal shear frontwere clearly identified, inner-ebb-outer-flood type (IEOF) andinner-flood-outer-ebb type (IFOE). The IEOF type of shear frontoccurred at 19:00–21:00 on August 9 between stations C3 and C4when the tidal currents at station C3 were ebbing (flowing north-west) while those at station C4 were flooding (flowing southeast;Fig. 3a). The front then moved seaward and was found betweenstations C4 and C5 at 21:00–22:00 on August 9. The IFOE shearfront was recorded at 1:00–3:00 on August 10 between stations C3and C4 when the tidal currents at station C3 were flooding, whilethose at station C4 were ebbing. The front then moved seaward anddisappeared at about 4:00 on August 10. Two types of tidal shearfronts occurred alternately within one tidal cycle. The movement of

the tidal shear front indicated by our records agreed with theconclusion presented by Wang et al. (2007b). In addition, the totalduration of the two types of tidal shear fronts off the Huanghemouth was approximately 4–5 h, which was in agreement withprevious studies (Wang et al., 2007b). The tidal shear front alongtransect A was also observed based on the current velocity records(Fig. 3b). It lasted 3–5 h during one tidal cycle and alternatedbetween the two types of tidal shear fronts, moving from a shallowto a deep area, the same as along transect C. The front disappearedaround station A3 at a water depth of approximately 13 m. Qiaoet al. (2008) suggested that the tidal shear front off the river mouthwas caused by a tidal phase gradient along the delta slope, and thetopography, a steep slope, was the dominant factor causing theformation of the tidal shear front off the Huanghe mouth. Addi-tionally, they concluded that the tidal shear fronts could begenerated in both the region of the present river mouth and in theregion of the abandoned Diaokou river mouth area due to thestrong slopes in both of these areas (locations are shown in Fig. 1b).The tidal wave propagates southward from the area by the northernHuanghe delta (Shi and Zhao, 1985) and forms a tidal shear front bythe abandoned river mouth area due to the tidal phase gradientalong the delta slope. The tidal phase gradient still exists when thetidal wave propagates southward, indicating that the tidal shearfront could occur along the east coast of the Huanghe delta butwould not be limited to the area near the river mouth. Thus, therecords of the tidal shear front in the northern part of the study area(along transect A) confirmed the numerical simulation resultsreported by Qiao et al. (2008). The tidal shear front along transect Boccurred between stations B1 and B2, but was quite different fromthe fronts observed along transects C and A in two ways. First, thefront in transect B only occurred during the flood tide and lastedthrough 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 floodtides as it occurred in transects A and C. Second, it did not occur atneighboring station B3 or at other stations further seaward, whichmeans that it did not move from the nearshore area to the offshorearea (Fig. 3c). We suggest that these differences arose as follows:the currents turned during the flood tide, changing from southwardto northwestward towards the southern coast after passing theprotruding abandoned Qingshougou river mouth, and flowedthrough stations B4, B3 and B2 into the southern sea area next tothe subdelta. This resulted in increased water levels in the areabetween the southern coastline and transect B. When the floodtidal currents oriented towards the coast and those flowingnorthwest reached the northwestern coast of Laizhou Bay, theywere forced to turn southeast, flowing seaward from the nearshorearea at station B1, following the local topography and the risingwater level (Fig. 2). Thus, the current directions at station B1 werethe opposite of those at station B2, resulting in the formation ofa shear front between stations B1 and B2.

4.2. Suspended sediment dispersion along three transects off thepresent Huanghe subdelta

4.2.1. Suspended sediment dispersion off the river mouth alongtransect C

The salinity increased seaward along transect C. The lowestsalinity values were recorded at stations C1 and C2, located in theriver channel (shown in Fig. 1b), with a maximum value ofapproximately 1.0 during the whole tidal cycle, indicating that theriver water dominated this part of the channel and that almost nosalt water intrusion was occurring from the sea (Fig. 4a, b). Thewater was much more turbid in the river channel than in theHuanghe river mouth. The highest SSC value of approximately1.3 g l�1 was recorded at station C2 during slack water after the ebb

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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 linesindicating 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 tidalshear 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) 352–362356

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Fig. 4. Vertical and temporal variations of salinity (in black contour) and SSCs (in color, in g l�1) at stations C1(a), C2(b), C3(c), C4(d) and C5(e).

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362 357

tide. The SSCs ranged from 0.1 to 0.7 g l�1 at station C1, much lowerthan those at station C2. The structures of the water masses at twostations in the river channel were fairly uniform throughout thesurvey. The salinity increased rapidly at station C3 and variedperiodically from 13.0 to 30.0 with the tidal phase. Observationswith lower salinity values and evident stratification were recordedat the end of the ebb tides and at the beginning of the flood tidesafter 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 l�1.The high SSC values were observed at roughly the same time as thelow salinity values, indicating that the river effluent could reachstation C3 at the end of ebb tides and during the early flood tides.The temporal variation of the salinity at station C4 was almost thesame 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 alsodetected in the first hours of the flood tides. The SSCs at station C4decreased further, reaching values of 0.01–0.1 g l�1 varied as thoseat station C3. The salinity at station C5 (30.0–32.0) was the highestamong all stations during the whole tidal cycle, with the smallestfluctuations observed at the surface and bottom layers.The watercolumn 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 l�1, values thatwere much lower than those at other stations, implying that theHuanghe effluent has little impact on the variation of SSCs atstation C5.

4.2.2. Suspended sediment dispersion in the northeast of thepresent Huanghe subdelta along transect A

The salinity along transect A increased seaward from station A1to A4. The salinity was lower at station A1 than at the other stations

along transect A throughout the entire tidal cycle (Fig. 5). Salinityvalues varied between 31.0 and 32.0 without any evident periodicvariation with the tidal phase, while the SSCs in flood tides wereslightly higher than those in the ebb tides. For example, the SSCsduring the flood tide between 12:00–18:00 on August 8 wereslightly higher (0.03–0.34 g l�1) than those in the ebb tide between18:00–1:00 on August 9 (0.01–0.14 g l�1), indicating that the rivereffluent carried by the ebb currents had little impact on the water atstation A1 (Fig. 5a). The salinity at station A2 seemed to varyslightly with the tidal phase, decreasing during ebb phases witha minimum value of approximately 31.5 and increasing during floodtides with a maximum value of approximately 32.8. The SSCs atstation A2 ranged from 0.002 to 0.02 g l�1, and were much lowerthan those at station A1 (Fig. 5b). Variations in the salinity at stationA3 were similar to those at station A2 (Fig. 5c). However, thepattern of SSC variation at station A3 seemed to be quite differentfrom that at station A1, as indicated by the high turbidity in boththe 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) comparedto those observed during flood tides (Fig. 5a and c). The bottom SSCeven exceeded 0.45 g l�1 at 6:00 on August 9 (ebb tide) at stationA3. At station A4, the salinity fluctuated slightly and was higherthan the salinity values observed at the other stations throughoutthe tidal cycle, whereas the SSCs were much lower than those atstations A1 and A3 (Fig. 5d).

4.2.3. Suspended sediment dispersion in the southeast of thepresent Huanghe subdelta along transect B

For the salinity of five stations along transect B, the salinity atstation B1 was lower than that at the other stations, and was foundto decrease during ebb tides and increase during flood tides

00.040.080.120.160.200.240.280.320.360.40.44

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9:00 14:00 19:00 0:00 5:00 10:00Time (hour)

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August 8 August 9 August 8 August 9

Fig. 5. Vertical and temporal variations of salinity (in black contour) and SSCs (in color, in g l�1) at stations A1(a), A2(b), A3(c) and A4(d).

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362358

(Fig. 6a). However, the salinity at the other four stations had anopposite behavior, decreasing during flood tides and increasingduring ebb phases (Fig. 6b–e). Station B2 had the highest salinityalong transect B in the surface, middle and bottom layers during thewhole tidal cycle, with a small fluctuation of 32.5–33.1. The SSCs atstations B1 and B2 were much lower than those at the other

00.0050.0100.0150.0200.0250.0300.0350.0400.045

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August 11 August 12

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Fig. 6. Vertical and temporal variations of salinity (in black contour) and SS

stations along transect B, ranging between 0.005 and 0.05 g l�1

(Fig. 6a, b). The salinity decreased from stations B2 to B5 with theenhanced fluctuation, while the SSCs increased significantly atstation B3 (e.g., 19:00–23:00 on August 11 and 3:00–6:00 onAugust 12) and ranged from 0.02 to 0.2 g l�1 (Fig. 6c). SSC valuesthen decreased slightly at station B4, but were much higher than

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August 11 August 12

Cs (in color, in g l�1) at stations B1 (a), B2 (b), B3 (c), B4 (d) and B5 (e).

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362 359

those at stations B1 and B2 (Fig. 6d). The water at station B5 hada salinity range of 30.0–32.5 and was more turbid, with a peak SSCvalue of approximately 0.4 g l�1 in the surface and bottom layers,higher than the peak SSC values at the other stations (Fig. 6e). Thewater column structure along transect B was quite uniform duringthe 25 h survey.

4.3. Sediment fluxes along three transects off the presentHuanghe subdelta

The sediment flux at each station was calculated based on theSSC value and the corresponding current velocity. The sedimentfluxes along transect C showed that the river-laden sediment wastransported northeastward parallel to the river channel and intothe sea at stations C1 and C2. Flux values were approximately6.6 kg s�1 at station C1 and increased to approximately14.0 kg s�1 m�1 at station C2 (Fig. 7). At station C3, flux valuesdecreased dramatically to approximately 0.7 kg s�1 m�1, approxi-mately 0.5% that at station C2, indicating that the alongshoretransport of river-delivered sediments within the 5 m isobath wasdominant. The suspended sediments at both stations C3 at 5 m andC4 at 11 m dispersed southeastward with sediment fluxes of 0.7–0.9 kg s�1 m�1, indicating that the suspended sediment wasprimarily carried from the northern area by flood currents. Thesediment flux at station C5 decreased dramatically to approxi-mately 0.1 kg m�1 s�1 in comparison to 0.9 kg m�1 s�1 at station C4.The landward transport of sediment flux at station 5 indicated thatthe transport of river-laden sediments was mostly confined withinthe nearshore region shallower than 15 m.

Sediment fluxes along transect A showed that sediment wastransported to the southwest at the nearshore station (A1) witha net sediment flux of approximately 0.13 kg s�1 m�1 (Fig. 7),implying that the suspended sediment at station A1 was largelyderived from resuspension from the abandoned Diaokou rivermouth, since the SSCs at station A1 were higher during flood tidesthan those observed during ebb tides (Fig. 5). In contrast, the sus-pended sediment at stations A2, A3 and A4 was transported to thenorthwest with net sediment fluxes of approximately

118.5° 119.0° 119.5° 120.0°37.0°

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0.01 kg m-1 s-1

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Fig. 7. Sediment fluxes at all stations based on 25 h measurements. The flux magni-tudes are indicated by the underlined numbers, and the arrows indicate the direction.

0.1 kg s�1 m�1, 1.34 kg s�1 m�1 and 0.19 kg s�1 m�1, respectively.The sediment flux at station A3 was much higher than that at all theother stations, comprising approximately 76.5% of the total sedi-ment flux along transect A. This suggests that the transport of river-laden sediment from the river mouth was primarily northwardthrough station A3.

The sediment fluxes along transect B showed that the sus-pended sediment was transported eastward or southeastwardaround the head of the river mouth that had been active between1976 and 1996 (stations B3, B4 and B5 in Fig. 7). However, thesuspended sediment at station B2 was transported northwestwardin the opposite direction from sediment at stations B3, B4 and B5,with much lower flux values (0.10 kg s�1 m�1) in comparison tothose at stations B3 (0.4 kg s�1 m�1), B4 (0.5 kg s�1 m�1) and B5(1.1 kg s�1 m�1). The sediment at station B1 was transportedsouthward along the coast, the opposite direction of the sedimentat station B2 with a low flux of 0.05 kg s�1 m�1, indicating thata very small amount of suspended sediment was delivered to thenearshore area.

5. Discussion

5.1. The process and mechanism of suspended sediment dispersion

The tidal currents off the present Huanghe subdelta are recip-rocating flows with lower velocity in nearshore areas, and forma swirling of water body in the southern part off the presentHuanghe subdelta due to the protrusion of the abandoned Qing-shuigou mouth (Fig. 2). The tidal currents in combination with thetidal shear fronts which were identified along each transect off thepresent Huanghe subdelta (Fig. 3) control the suspended sedimentdispersal in study area.

The barrier effect of the tidal shear front on the river-ladensediment dispersion seemed to be effective in trapping suspendedsediment as the low water and sediment discharges weakened theextension of the river plume off the river mouth. The diluted waterwas restricted within the 5 m isobath due to the impact of the shearfront, and was transported northward by tidal currents during ebbtides. However, during the transition period from the ebb tide toflood tide, the river water characterized by a low salinity and a highSSC was transported to the deeper sea due to the clockwise-rotating currents (Pang and Jiang, 2003) and the disappearance ofa shear front. Therefore, the turbid water was transported south-ward by flood currents passing by stations C3 and C4, resulting ina decrease in salinity and an increase in SSCs during flood tides(Fig. 4), along with the southward net sediment fluxes during thetwo tidal cycles at these two stations (Fig. 7). Additionally, themaximum SSC in the bottom water layer was approximately0.2 g l�1, with a corresponding salinity value of approximately 25 inthe river mouth area during the field survey (Fig. 4), and was muchlower than 30 g l�1, the critical SSC for the formation of hyperpycnalflow (Pang and Yang, 2001). Therefore, the suspended sedimentdispersion off the river mouth was predominantly in the form ofhypopycnal flow during the low-discharge period, in contrast withthe hyperpycnal flow observed by Wang et al. (2007b).

This relatively turbid water mass could not be carried close tothe shore due to the barrier effect of the shear front formed in theshallower area and transported northward again by the ebbcurrents in the next tidal cycle through a deeper sea area, resultingin the SSC increasing during ebb tide (Fig. 5) and the northward netsediment flux of one tidal cycle at station A3 (Fig. 7). This watermass did not pass through the shallower area barred by the shearfront between stations A1 and A2. This result generally agreed withthe suspended sediment dispersion shown in a LANDSAT imageacquired on May 5, 1998 (Fig. 8a). This image also illustrated that

0 10 20km

shear front

a

May 25, 1998

Huanghe delta

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Aug. 28, 1999

Huanghe delta

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 weretaken on May 25, 1998 (a) and August 28, 1999 (b).

A1 A2A3 A4

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N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362360

the turbid nearshore water along transect A seemed to be consis-tent with the resuspended sediment from the northern abandonedriver mouth area. The seaward transport in this area was obstructedby the shear front between stations A1 and A2.

The turbid water in the southern area off the delta was derivedfrom the river mouth, flowing southward during the flood tides andturning westward into Laizhou Bay after the protrusion of theabandoned Qingshuigou river mouth as observed on the LANDSATsatellite image acquired on May 2, 2000 (Fig. 8b), demonstratingthat the water did not arise from the resuspension of the surfacesediment as documented by previous publications (e.g., Jiang andWang, 2005). Jiang and Wang (2005) suggested that resuspensionmight be the major source of the fine suspended sediment inLaizhou Bay. However, there was no evident relationship betweenthe shear stress estimated on the near bottom velocity and theobserved SSC, suggesting that the fine sediment off the Huangheriver mouth that was transported southward would be the primarysediment source for the suspended sediment in Laizhou Bay.

The tidal shear front, which occurred throughout the flood tidephase between stations B1 and B2, prevented the turbid water withhigh salinity from reaching the eastern coast of the southern delta.During the ebb tides, the tidal currents flowed southeastward at allstations except for station B5 (Fig. 2), carrying the turbid wateraway from the southeastern coast of the delta. Therefore, thisturbid water mass could not reach the southeastern coast of thedelta during the entire tidal period (Fig. 6). Thus, a narrow andrelatively clear band of water with low salinity and low SSC wasformed between the turbid water mass and the southeastern coastof the delta parallel to the shoreline, as shown by the satellite imageacquired on August 28, 1999 (Fig. 8b).

B1

B2

B3

4140

4160

20640 20660 20680 20700Easting (km)

Coastline of 2003

Coastline of 1976

Contour intervel:1m

-1

Fig. 9. The net erosion–accumulation morphology of the Huanghe delta based on 36bathymetric survey lines along the delta coast in 1976 and 2003 (a). The offshoreboundary of the delta is identified as the 15 m water depth contour. The accumulationarea was marked by a shadow with solid contour lines, and the erosion areas wereindicated by dashed contour lines. The dashed lines in panel (b) indicate the positionsof the 36 bathymetric survey lines.

5.2. Geomorphological response of the subaqueous delta to thesuspended sediment dispersion

The erosion–accumulation pattern of the subaqueous delta wasquantitatively estimated based on the bathymetric data recordedfrom 36 offshore transects off the Huanghe delta in 1976 and 2003.A drip-like accumulation area with an irregular edge was formedaround the present sub-delta, including two accumulation centersaround the present river mouth and the abandoned Qingshuigouriver mouth. The two erosion areas were separated from each otherby the accumulation area with two distinct accumulation–erosiontransient zones north and south of the delta, respectively (Fig. 9).Erosion took place in the shallower nearshore areas and

accumulation occurred in the deeper offshore areas in the twoaccumulation–erosion transient zones. This indicates that thealongshore deposition of the river-laden sediment took place in theoffshore areas in the northern and southern areas of thesubaqueous delta, coinciding with the net sediment fluxes along

N. Bi et al. / Estuarine, Coastal and Shelf Science 86 (2010) 352–362 361

transects A and B (Fig. 7). The accumulation area extended north-ward via the deeper sea area, and the erosion area was located inthe shallower area along transect A. The gradient of the accumu-lation thickness between stations C3 and C4 was very high, anddecreased sharply around station C5 (Fig. 9), suggesting that mostof the river-laden sediments deposited on the landward side ofstation C3, and station C5 (depth of approximately 15 m) with anaccumulation thickness of approximately 2 m was close to theseaward boundary of river-laden sediment transport. A slighterosion zone where the coastline retreated landward, located onthe southeastern coast of the delta, corresponded with the positionof a clear water belt that was clearly observed on satellite images.Thus, fast seaward propagation of the central part of the subdeltawith the retreat of its northern and southern coastline backtowards the land was the direct consequence of the sedimentdispersion.

Although high water and sediment discharges have madea significant contribution to the delta accretion, both in the past andduring the water-sediment regulation periods over the past 7 years,the sediment dynamics and processes we discussed based on thelow-discharge observation still played an important role in theriver-laden sediment dispersal and thus controlled the geomor-phology of the delta. Therefore, the sediment dynamics and deltaresponse would provide a good reference for the safety of thecoastal 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 highstress induced by waves in the shallow water could cause notableresuspension of sediments (e.g., Wang et al., 2006c). Observationsnear the Huanghe delta in 1987 indicated that storm-inducedstrong wave action directly induced a prominent slope failure of thesubaqueous delta, and the down-slope transport of resuspendedsediments reshaped the slope (Prior et al., 1989).

6. Conclusions

Tidal shear fronts with different formation mechanisms werefound along three transects off the present Huanghe subdelta. Thecombined shear fronts and alongshore tidal currents were themajor dynamic factors controlling the sediment dispersion nearthe present subdelta. Most of the sediment that was delivered tothe sea in the form of hypopycnal flow was deposited within the5 m isobath off the river mouth due to the barrier effect of the tidalshear front. The river-laden sediment was transported northwardor southward through the deeper water at both sides of the rivermouth 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 sus-pended sediment dispersal pattern indicated by satellite images.

Two shear fronts and tidal currents in the northern andsouthern shallower nearshore areas of the delta prevented thesediment transport from the offshore areas towards nearshoreareas, resulting in offshore sediment deposition in the northern andsouthern parts of the subaqueous delta, rather than in the near-shore areas. Thus, two inside erosion-outside accumulation tran-sition zones were formed off the northern and southern parts of thedelta, respectively.

Human activity has caused sharp decreases in water and sedi-ment discharge from the river to the sea, and the area of accumu-lation and volume of the active river mouth is expected to decreasein the future due to insufficient sediment supply, while the erosionareas are expected to extend. These future challenges must beconsidered by the delta conservation community and in the futuredevelopment plans of the Shengli Oil field in this region.

Acknowledgements

This work was supported by NSFC projects (No. 40676036 and40876019) and by the National Fundamental Research Program ofMinistry of Science & Technology, China (No. 2002CB41404). Wethank Professor Zhongyuan Chen from East China Normal Univer-sity for his valuable comments. We are grateful to the anonymousreviewers for their constructive comments and helpful suggestionto improve the manuscript.

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