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Estuarine, Coastal and Shelf Science 86 (2010) 322–330

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Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

Dynamics and spatial variability of near-bottom sediment exchangein the Yangtze Estuary, China

Hong Liu a, Qing He a,*, Zhengbing Wang b,c, Gert Jan Weltje c, Jing Zhang a

a East China Normal University, State Key Laboratory of Estuarine and Coastal Research, Shanghai 200062, People’s Republic of Chinab Deltares, WL j Delft Hydraulics, P.O. Box 177, NL-2600MH Delft, The Netherlandsc Delft University of Technology, Faculty of Civil Engineering and Geosciences, P.O. Box 5048, NL-2600GA Delft, The Netherlands

a r t i c l e i n f o

Article history:Received 8 February 2009Accepted 16 April 2009Available online 22 April 2009

Keywords:Yangtze (Changjiang) Estuarygrain-size distributionsnon-linear sediment mixingselective depositionsediment exchange rate

* Corresponding author.E-mail address: qinghe@sklec.ecnu.edu.cn (Q. He)

0272-7714/$ – see front matter Crown Copyright � 2doi:10.1016/j.ecss.2009.04.020

a b s t r a c t

This study was conducted to examine the spatial variations in the exchange between near-bottomsuspended and sea-bottom sediments in the Yangtze Estuary and adjacent region, as well as to explorethe fate of suspended sediments in the study area. The relationship between the sand, silt, and claycontents of the sediments was analyzed by log-ratio analysis, which revealed a non-linear function forselective deposition and a wide range of grain-size distributions in the estuary. This finding does notconform to the nearly constant clay/silt ratios reported for other tidal basins around the world, due tonon-linear sediment mixing under complex hydrodynamic conditions. The sediment exchange processesin the Yangtze Estuary were quantified based on the principle of mass balance. The average grain-sizedistribution of near-bottom suspended sediments from the estuary showed that approximately 49% ofthe riverine sediments accumulated in the mouth bar area, while the rest, which is primarily composedof fine-grained sediments, is transferred to the outer estuary and deposited in the form of flocs. Thespatial distribution of the sediment exchange ratios demonstrated that small amounts of suspendedsediment were deposited onto the seabed of the upper estuary (exchange ratio< 0.1), because the fine-grained suspended sediments in this region were transported to the mouth bar area by the ebb-domi-nated tidal flow. The sediment exchange ratios in the outer estuary also showed very low values (<0.1)due to the oceanic currents offshore that prevented the diffusion of riverine sediments further seaward.Intensive sediment exchange occurred in the inner estuary due to the sand-mud mixing which wascontrolled by bidirectional tidal flows. In addition, a high sediment exchange ratio occurred in the muddyarea (>0.8) seaward of the river mouth, which implies that this is the present-day depocenter of Yangtzemud. The sediment exchange rates obtained by combining the dimensionless exchange ratios and bulksediment accumulation rates, were found to be 2–3 cm/yr in the muddy depocenter, which extends tothe south of the river mouth (from 122� E to 123� E longitude, at 31� N latitude).

Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Sediment grain-size distributions (GSDs) have been extensivelystudied because they provide valuable information regarding theorigins of sediments. In addition, analysis of GSDs is an effectivemethod of evaluating sediment transport, deposition, and size-selective erosion. Early studies of grain size primarily focused onestablishing links between sedimentary environments andsummary statistics of GSDs, such as the mean, standard deviation,and skewness (Shepard, 1954; Folk and Ward, 1957; Friedman,1979). In later studies, the search for environmental fingerprints

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was abandoned, and a more dynamic concept in which spatialpatterns of summary statistics were assumed to reflect the netsediment transport pathways was adopted (McLaren, 1981; Gaoet al., 1994; Le Roux and Rojas, 2007). Parametric curve-fitting(Visher, 1969; Leys et al., 2005) has also been a popular method ofcharacterizing GSDs. This technique is based on the assumption thatobserved (polymodal) GSDs can be regarded as mixtures of specificanalytical functions, such as the normal, lognormal, or log-hyperbolicdistributions (see Weltje and Prins, 2007, for a critical review).

Alternative approaches to the analysis of GSDs have beendeveloped based on the principles of compositional data analysis(Aitchison, 1986). In this approach, GSDs are regarded as spectrarather than distributions falling into a particular parametric class.These spectra are represented by a series of mass fractions thatcorrespond to discrete size ranges. Traditionally, classification of

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H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330 323

gravel-free marine sediments has been based on the proportions ofsand, silt, and clay, which lend themselves to visualization internary diagrams (Pejrup, 1988; Flemming, 2000). Such composi-tional representations assume that GSDs are mixtures of a limitednumber of fixed sediment types, and that as such, they are verysimilar to parametric curve-fitting exercises. Generalization of themixture representation of GSDs has been achieved through the useof so-called end-member models that have been employed tostatistically ‘‘unmix’’ compositional data (Weltje, 1997). End-member modeling has been applied to GSDs of deep-sea cores andaeolian deposits in order to quantify the interplay of provenanceand selective deposition (Weltje and Prins, 2003; Prins et al., 2007).

Spatial patterns of sea-bottom GSDs are important to sedi-mentologists and civil engineers because studies of estuarinegeomorphology and validation of numerical models of sedimenttransport are based on such patterns. Moreover, the grain-sizecharacteristics of sea-bottom sediments are of increasing concernto biogeochemical researchers because the nutrients and pollutantscarried by fine-grained sediments play a significant role inbiogeochemical processes (McKee et al., 2004).

Many studies of the relation between sedimentation processesand GSDs of sea-bottom sediments in the Yangtze Estuary havebeen conducted (Chen et al., 2007). These studies include an eval-uation of sea-bottom sediment types in the estuary (Dong andDing, 1988), estuarine sedimentary characteristics (Liu et al., 1995),a particle compositions of sea-bottom sediment (Li et al., 1995),sedimentation on the subaqueous Yangtze Delta (Chen et al., 2000),and dynamic sedimentation environment and sediment transportpatterns in the North Branch of the Yangtze Estuary (Jia et al., 2001;Yang and Liu, 2002). Sea-bottom sediments of the outer estuary arerelict, and represent reworked, nearshore deposits of the late-Pleistocene sea-level lowstand (Yan and Xu, 1987; Qin et al., 1987;Chen et al., 2000). Previous studies in the Yangtze Estuary havefocused on the sediment grain-size distributions and sedimenta-tion processes rather than sediment dynamics. In the present study,the characteristics of sea-bottom sediment grain size were evalu-ated by log-ratio analysis to gain a better understanding of thesediment dynamics of input, exchange and output areas in theYangtze Estuary.

Ongoing discussions regarding the relation between sedimentdynamics and GSDs (Hartmann and Flemming, 2007) demonstratethat many problems associated with grain-size analysis and inter-pretation have not been resolved, despite more than 70 years ofsedimentological research. One of the outstanding issues is deter-mination of the exact nature of the exchange between suspendedand sea-bottom sediments, which is particularly important forunderstanding sediment transport and the fate of suspendedsediments in estuarine and coastal areas (Hossaina and Eyreb,2002; Jiang and Wang, 2005; Ren and Packman, 2007). In thispaper, we present an integrated data set of the GSDs of suspendedand sea-bottom sediments to analyze sediment dynamics in theYangtze Estuary, and apply the principle of mass balance to analyzesediment exchange processes. The results allow us to trace theYangtze River sediments along the estuary and into the adjacentshelf. In addition, the combination of our results with long-termaccumulation rates based on 210Pb dating allows us to identify thecurrent depocenter of the Yangtze Estuary mud belt.

2. Study area

The Yangtze River is the fourth largest river in the world in termsof water discharge and sediment load (Chen et al., 2001; Yang et al.,2007). The mean annual total runoff at Datong HydrologicalGauging Station is 903.4�109 m3/yr (1950–2005) and the meanannual suspended sediment load from the Yangtze River

approaches 414�109 kg/yr (1951–2005). The mean concentrationof suspended sediments in the estuary is 0.461 kg/m3 (1951–2005),and the mean grain size of the suspended sediments is 9 mm (1987–2005) (CMWR, 2006). The tidal limit of the Yangtze River is locatedat Datong, and the tidal flow limit is located at Jiangyin. The modernYangtze Delta dates back to the mid-Holocene epoch. At around7000 yr BP, the rate of post-glacial sea-level rise had decreasedsufficiently to initiate progradation of deltaic deposits (Chen andStanley, 1993; Chen et al., 2000; Wang et al., 2005).

Hydrographic conditions in the Yangtze Estuary and the adja-cent region are governed by a southward current of relatively coldand brackish water known as the Jiangsu Coastal Current in thenorth and the Zhejiang-Fujian Coastal Current in the south. Thesecurrents, which are most active during winter, carry water andsediments from the Yangtze River southward along the inner shelf.In addition, there is a northward flow of warm and saline water onthe outer shelf that is known as the Taiwan Warm Current. Thiscurrent intensifies during summer in response to the prevailingsoutheast monsoon (Qin et al., 1987; Liu et al., 2006). Largeamounts of fine-grained sediments from the Yangtze River Basinare deposited in the estuary and on the subaqueous delta. Indeed,about half of the sediments entering the estuary are deposited inthe river-mouth area (Chen et al., 1985), while the rest are believedto be carried southward along the coast by littoral currents.

The Yangtze Estuary is a mesotidal estuary with a mean tidalrange of 2.66 m. The estuary consists of complex shoal-channelsystems (He et al., 2001). The morphology of the Yangtze Estuary ischaracterized by three bifurcations and four outlets (Fig. 1a).Analysis of sediment accumulation rates based on 210Pb dating ofsediment cores indicates low sedimentation rates in the outerestuary, and high sedimentation rates in the muddy areas of theYangtze Estuary (Wei et al., 2007) (Fig. 1b).

To evaluate the vertical sediment exchange in the YangtzeEstuary and the adjacent region, the average GSDs of near-bottomsuspended sediments in the areas of input, exchange, and outputwere calculated. For this study, the input area was defined as theupper reaches of the estuary from Jiangyin to Changxing Island(Fig. 1a). The exchange area was defined as the estuarine mouthfrom Changxing Island to the 20 m isobath. The output area wasdefined as the area seaward of the Yangtze Estuary.

3. Materials and methods

Sea-bottom sediment samples (n¼ 570) were collected from theYangtze Estuary and the adjacent region between 2003 and 2006(Fig. 1a) using a grab (clamshell) sampler. The 2 cm thick surfacelayer of the bottom samples was used for grain size analysis. Sus-pended sediment samples (n¼ 423) were collected from the innerestuary in February 2003 (over a period including the spring andneap tides; sampling sites CJ01–CJ15; Fig. 1b), and from the outerestuary in July 2006 (sampling sites LJ01–LJ08; Fig. 1b). The estu-arine samples were collected using a horizontal iron water samplerat relative heights of 0.0H, 0.2H, 0.4H, 0.6H, 0.8H and 1.0H, where His the water depth above the bed. The outer estuary samples werecollected at relative heights of 0.0H, 0.5H, and 1.0H. For the presentstudy only the suspended sediment samples taken at relativedepths of 1.0H were used. Preparation was identical for the sus-pended and sea-bottom sediment samples. Prior to grain-sizeanalysis, organic matter was removed by adding H2O2 (10%).Aggregates were then dispersed by the addition of (NaPO3)6 andsubsequent ultrasonic treatment. Grain-size analyses were carriedout on a Coulter LS 100Q grain-size analyzer, which subdivides eachsample into 85 size fractions between 0.3 mm and 1000 mm.

Because GSDs represent compositional data which are governedby constant-sum and non-negativity constraints, the raw data

Fig. 1. Study area and sampling sites of sea-bottom and suspended sediments. (a) Bathymetry and sampling sites of sea-bottom sediments (subdivided into five types). (b) Samplingsites of suspended sediments and sedimentation rate [cm yr�1] based on 210Pb dating of sediment cores. Samples of sites of CJ01–CJ15 were collected in February 2003, while thosefrom sites of LJ01–LJ08 were collected in July 2006. Sedimentation rate data are from Duan et al. (2005) and Liu et al. (2006).

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330324

collected in this study are difficult to interpret. However, compo-sitional data may be transformed to log-ratios (Aitchison, 1986) togive a representation that is unconstrained. Such transformed datamay be subjected to standard methods of statistical analysisbecause log-ratios can take on any value between C�N;þND. In thisstudy, we utilized the log-ratio method to analyze the relationshipsbetween the sediment components of CY, CT, and CS, which repre-sent the clay (Y), silt (T), and sand (S) contents of the sea-bottomsediments. Two log-ratio values were defined: lnfCY=CTg, which isthe log-ratio of clay to silt, and lnfCS=ðCY þ CTÞg, which is the log-ratio representation of the sand content.

The observed sea-bottom sediment grain size distribution is thesum of the flux of the suspended sediment mass to the seabed inthe form of single grains and flocs. The grain diameter at which theflux of mass to the seabed is equal to the single grain and flocdepositions is the floc limit (df). Thus, sediment grains larger than df

are primarily deposited as single grains, while those smaller than df

are primarily deposited as flocs (Curran et al., 2004). An integratedanalysis of current velocities, salinity, and in-situ floc-size distri-butions shows that the critical diameter for the flocculation of finecohesive sediments in the Yangtze Estuary is 32.5 mm (Tang, 2007).Therefore, in this study, we adopted 32.5 mm as the floc limit (the

critical diameter for the flocculation). This allowed us to estimatethe mass fractions of flocculated and cohesionless sediments for allmeasured (disaggregated) GSDs, which were denoted as mf and mc,respectively.

In this study, sediment exchange refers to the verticalexchange between the near-bottom suspended and the sea-bottom sediments and reflects the results of the suspendedsediment deposition characteristics and sediment dynamics. Aquantitative analysis of the vertical sediment exchange processwas conducted by evaluating the constraints imposed by thesediment budget, according to which the sum of the masses ofthe exchange and output sediments should equal the mass of theinput sediments:

GI ¼ pGE þ ð1� pÞGO (1)

where GI, GE, and GO are the GSDs of the input, exchange, andoutput sediments, respectively. Each GSD is a vector of k elements(mass fractions of grain-size classes) which sum to unity, or 100%.The unknown, p, is a proportion (0 � p � 1) termed the exchangeratio. The least-squares solution to this constrained linear mixingproblem is given by:

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330 325

p ¼Pk

j¼1

�GEj� GOj

��GIj� GOj

�

Pkj¼1

�GEj� GOj

�2 (2)

To convert these proportions to rates of sediment exchangebetween the water column and the bed (VE), we need independentinformation regarding the bulk sediment accumulation rates (VB):

VE ¼ pVB (3)

where VE is the rate of sediment exchange, p is the exchange ratio,and VB is the sediment accumulation rate. The sediment accumu-lation rate data were obtained from Duan et al. (2005) and Liu et al.(2006), and are shown in Fig. 1b.

4. Results

4.1. Sea-bottom sediment types and mean grain size

Sea-bottom sediments were classified using the methoddescribed by Shepard (1954) and plotted in a ternary diagram ofclay (0.5–4 mm), silt (4–62.5 mm), and sand (62.5–2000 mm)contents. Five primary sediment types occur in the Yangtze Estuaryand the adjacent region: clayey silt, sand, silty sand, sandy silt, andsilt (Fig. 2). Clayey silt is the dominant sediment type (46% of all 570sea-bottom sediment samples) and is primarily deposited in thelower reaches of the Yangtze Estuary (Turbidity Maximum Zone), inthe muddy area, and on the outer estuary. Sand (24%) is present inthe upper reaches of the estuary, on the shoals, and in the primarychannels of the Turbidity Maximum Zone. The relict deposits arealso composed of sand. Silty sand (15%), sandy silt (11%), and silt(4%) are present in the estuarine mouth bar area and in the upperNorth Branch.

The spatial distribution of the mean diameter of the sea-bottomsediments (Fig. 3a) shows a clear coarse–fine-coarse trend from theupper estuary to the outer estuary. On average, the diameter ofsediments on the sea-bottom in the South Branch vary from 150 mmto 220 mm and displays a general fining towards the mouth of theestuary. The mean grain size in the North Branch is finer than thatof the South Branch, with values in the range of 50–120 mm in theupper reaches, and 15–60 mm in the lower reaches.

Fig. 2. Ternary diagram of sand/silt/clay proportions of 570 samples taken from thestudy area (sampling sites referring to Fig.1a). (Y, clay; T, silt; S, sand; TY, silty clay; SY,sandy clay; YT, clayey silt; ST, sandy silt; YS, clayey sand; TS, silty sand; S–T–Y, sand–silt–clay).

The mean diameter of sea-bottom sediment in the mouth bararea of the estuary is more variable and reflects the feedbackbetween morphology and sediment exchange. The coarsest sea-bottom sediments in the three main outlets are present in theNorth Channel, while the finest are present in the South Passage(Fig. 3a).

In the southeastern portion of the estuary, sea-bottom sedi-ments are fine, with a mean diameter of less than 20 mm. The meandiameter of the outer continental shelf deposits ranges from150 mm to 300 mm (Fig. 3a).

The clay component is transported out of the estuary anddeposited in the outer estuary (Fig. 3b). The silt component istransported eastward and southward through the estuary anddeposited in the South Passage and further southward along thecoast (Fig. 3c). The sand component is primarily deposited in theupper reaches of the estuary (Fig. 3d).

4.2. Log-ratio analysis of sea-bottom sediment grain size

To examine the relations between the three sediment compo-nents, the data were plotted using two different methods (Fig. 4).Fig. 4a and b provides a straightforward visualization of the rela-tions between the three sediment components, which appear tovary with sand content. Three domains can be distinguished:

(1) Segment 1: CS< 1%; CY and CT are constant (approximately 30%and 70%, respectively).

(2) Segment 2: 1%< CS< 20%; CT is constant (approximately 70%);CY is negatively correlated with CS and decreases to about 20%.

(3) Segment 3: CS> 20%; CY and CT are negatively correlated with CS

and decrease to 0%.

The log-ratio plot, which is shown in Fig. 4c, provides an alter-native to Fig. 4a and b, as well as to the conventional ternaryrepresentation shown in Fig. 2. Of particular interest is the rela-tionship between sand content and the clay/silt ratio in segments 2and 3, which changes from a negative correlation to a positivecorrelation. The sediments in segment 1 are present in theexchange and output areas, which are characterized by relativelyfine sediments. The sediments in segment 2 are dominant in theexchange and output areas, which are characterized by segregationof fine and coarse sediments. The sediments in segment 3 arepresent in the input and exchange areas, which are characterized byrelatively coarse sediments. The variation among the differentsegments suggests that non-linear sediment mixing is reflected inthe spatial changes of sea-bottom sediments from the upperestuary (input area) via the mouth area (exchange area), to theouter estuary (output area).

To evaluate the mode of deposition of the sea-bottom sedi-ments, the relationship between the log-ratio of the flocculatedfraction to the cohesionless fraction, lnfmf=mcg, and the meandiameter, Md, were analyzed. A clear linear trend was observedbetween lnfmf=mcg and Md (Fig. 5). Relatively fine-grained sedi-ments, which are described by lnfmf=mc > 0g , display a nearlyperfect correlation between Md and lnfmf=mcg. These findingsindicate simple linear mixing of two sediment fractions with fixedcharacteristics. Relatively coarse-grained sediments, which aredefined by lnfmf=mc < 0g , display an increased scatter, whichimplies size sorting within the cohesionless sediment population.Points that fall near the line in Fig. 5 represent mixtures of theflocculated sediment population and relatively coarse cohesionlesssediments. The sea-bottom sediments in the input area areprimarily deposited in the form of single grains, which are mainlydeposited as flocs in the output area (Fig. 5). The sediments in the

Fig. 3. Spatial distribution of the mean diameter (a), contents of clay (b), silt (c) and sand (d) in sea-bottom sediments.

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330326

exchange area are deposited in the form of single grains as well asflocs.

The spatial distribution of lnfmf=mcg in the study area (Fig. 6)demonstrates that the sediments in the upper reaches of theestuary and the mouth bar of the North Channel are primarilydeposited in the form of single grains, as implied by the negativevalue of lnfmf=mcg. The sea-bottom sediments in the South andNorth Passages, as well as the southeast portion of the estuary areprimarily deposited as flocs, as demonstrated by the positive valuesof lnfmf=mcg.

4.3. Sediment exchange characteristics

Sediments supplied to the estuary play an important role in theevolution of estuarine morphology and nearshore biogeochemicalprocesses. The vertical exchange between the near-bottom sus-pended and sea-bottom sediments in the estuary involves repeatedcycles of flocculation, deposition, and resuspension and governs thespatial segregation of sediments according to grain size. The sedi-ments that remain in the estuary contribute to the evolution ofestuarine morphological structures, such as channel-bank systems,shoals, mouth bars, and nearshore subaqueous deltas. The sedi-ments that escape from the estuary are transported to the outerestuary and along the coast.

The GSDs of near-bottom suspended and sea-bottom sedimentswere analyzed to examine the vertical sediment exchangeprocesses in one estuarine outlet from the South Branch to theNorth Channel. Great differences were observed in the GSDs of thesuspended and sea-bottom sediments in the input and outputareas, which indicate that few suspended sediments were depos-ited onto the seabed (Fig. 7). However, the GSDs of the sea-bottomsediments in the exchange area are similar to those of the sus-pended sediments due to the intensive vertical sediment exchange.As a result, quasi-bimodal patterns are present in some GSDs of sea-bottom sediments in this area, and the finer peak is the result ofsediment exchange (Fig. 7).

The average GSDs of near-bottom suspended sediments clearlyillustrate the exchange process in the exchange area (Fig. 8). Thedistribution of the suspended sediments reveals a fine–coarse-finetrend in the Yangtze Estuary that is converse to the trend of the sea-bottom sediments. On average, the suspended sediments in theexchange area are coarser than those of the input or output areas.Figs. 7 and 8 describe the variation in the suspended sediment GSDsfrom the sediment transport processes in the estuary. The results ofthis study indicate that the vertical sediment exchange in the mouthbar area (the exchange area) occurs more frequently than in theupper estuary (the input area) or the outer estuary (the output area).

Substitution of the GSDs of sea-bottom sediments, as GE, into Eq.(1) gives the local exchange ratio of sea-bottom and suspendedsediments. The average suspended sediment GSDs of the input andoutput areas, represented by GI and GO, are shown in Fig. 8. Thiscalculation was conducted for all 570 GSDs (Fig. 9a). High values ofp indicate that the suspended sediments are deposited on the localseabed and contribute to the estuarine morphology, whereas lowvalues indicate that the suspended sediments are not deposited onthe local seabed, but are transported further seaward. The highestsediment exchange ratio, which was observed in the muddy area ofthe Yangtze Estuary, was approximately 0.8. This finding indicatesthat the exchange between suspended and sea-bottom sedimentsoccurs frequently in this area. The exchange ratio of the upperestuary and the outer estuary was found to be lower than 0.1, whichindicates that only small amounts of suspended sediments aredeposited onto the seabed (Fig. 9a).

The data describing the sedimentation rate are shown Fig. 1b.These data were derived from 210Pb dating of sediment cores andthen converted to exchange rates using Eq. (3). The resulting plot isshown in Fig. 9b. The low VE values indicate that the depositionalrate of the suspended sediments is relatively low and that mostsuspended sediments are transported out of the estuary. The highVE values indicate that most suspended sediments are depositedonto the seabed of the muddy area (the distal part of the SouthPassage).

0

10

20

30

40

50

60

0.001 0.01 0.1 10 100

Input

Exchange

Output

1 3

0

20

40

60

80

100

0.001 0.01 0.1 1 10 100

1 3

-3

-2

-1

0

-12 -10 -8 -6 -4 -2 0 4

ln{CS/(CY+CT)}

ln (

CY

/CT

)

1 3

1

2

CY

(

)C

T (

)

2

CS ( )

CS ( )

2

a

b

c 2

Fig. 4. Relationship between the contents of sand (CS), clay (CY), and silt (CT) in sea-bottom sediments. (a) Conventional display of CY to CS; (b) conventional display of CT

to CS; (c) log-ratio display (Input, exchange and output legend see Fig.1; 1, 2, 3 in thefigure are bottom grain-size segments, discussed in text).

0

1

2

3

4

5

6

7

8

-5 -4 -3 -2 -1 0 1 2 3 4

ln {mf/mc}

Md

(Φ)

Input

Exchange

Output

Fig. 5. Relationship between the mean diameter [F] and lnfmf=mcg of sea-bottomsediments (Input, exchange and output legend see Fig. 1).

Fig. 6. Spatial distribution of lnfmf=mcg of sea-bottom sediments.

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330 327

5. Discussion

5.1. Interpretation of sea-bottom grain size characteristics

The South Branch is currently the most active channel of theYangtze Estuary (Fig. 1). The majority of the sea-bottom sedimentsin the South Branch are relatively coarse, and represent bed loadderived from the Yangtze River Basin. Due to selective deposition,the fine particle sediments are transported into the mouth bar areaand the outer estuary, while the coarse sediments are deposited inthe upper reaches of the estuary (Fig. 2). Consequently, the meandiameter of the sea-bottom sediments is larger than that of otherchannels (Fig. 3).

Prior to the 18th Century, the North Branch was the majorchannel of the Yangtze Estuary. However, flow has graduallydecreased over the past 200 years. By the early 20th Century, thewater discharge from the North Branch had been reduced toapproximately 25% of the total discharge of the estuary, and by theend of the 1950s less than 2% of the total discharge passed throughthe North Branch (Chen et al., 1988). There is also a net influx of saltwater into the North Branch, and the sea-bottom sedimentsprimarily originate from the deposition of fine-grained suspendedsediments. Moreover, the distribution of sea-bottom GSDs in theupper reaches of the North Branch is affected by the tidal bore(Chen et al., 2003). Therefore, the mean diameter of the sea-bottomsediments in the upper reaches is larger than that of the lowerreaches (Fig. 3).

The GSD pattern in the mouth bar area is more complex. This isbecause the vertical sediment exchange between the suspendedsediments and the sea-bottom sediments occurs more frequentlythan in other regions (Fig. 3). Due to the Coriolis effects, themajority of fine-grained sediments are transported to the outerestuary through the South Passage (Chen et al., 1988), where thegrain size of sea-bottom sediments is relatively low. The dischargeratio of the North Channel is much larger than that of the South orNorth Passages (Chen et al., 1988). Furthermore, the currentvelocities of the North Channel are much larger than those of otherchannels (Liu et al., 2007). Consequently, the grain size of the sea-bottom sediments is largest in the North Channel.

The southern portion of the estuary is one of the primarydepocenters of fine-grained suspended sediments escaping fromthe Yangtze Estuary, and the sea-bottom sediments in that area arerelatively fine (Fig. 3).

5.2. From grain size to sediment dynamics

A nearly constant ratio between clay and silt has been reportedfor various tidal systems worldwide. Ternary diagrams presentedby Flemming (2000) show a constant clay/silt ratio in the following

0

2

4

6

8

10

12

14

Nearly bottomsuspended sediment

Sea-bottom sediment

CJ01

0

2

4

6

8

10

12

14CJ10

0

1

2

3

4

5CJ11

0

1

2

3

4

5CJ13

0

2

4

6

8LJ01

0

2

4

6

8

10

12

14

0.1 1 10 100 1000 0.1 1 10 100 1000

0.1 1 10 100 1000 0.1 1 10 100 1000

0.1 1 10 100 1000

Grain diameter (µm)0.1 1 10 100 1000

Grain diameter (µm)

LJ02

Fre

quen

cy/

dF

requ

ency

/ d

Fre

quen

cy/

d

Fre

quen

cy/

dF

requ

ency

/ d

Fre

quen

cy/

d

Fig. 7. Comparison of the grain size distribution of near-bottom suspended and sea-bottom sediments in the Yangtze Estuary. Sampling names are shown in the top left corner. SeeFig. 1b for the sampling sites. Samples from CJ01 and CJ10 were collected from the in the input area, which those from CJ11, CJ13 and LJ01 were collected from the exchange area, andLJ02 was collected from the output area.

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330328

five intertidal environments: the macrotidal flats in Jiangsu Prov-ince, China; the Wadden Sea in Denmark; the Dyfi Estuary inWales; the Minas Basin in the Bay of Fundy, Canada; and the MuguLagoon in the USA. The phenomenon also occurs in two open-shelfenvironments: the Bering Shelf and the Central Gulf of the AlaskaShelf. An analysis conducted by Van Ledden (2003) also shows thatclay/silt ratios in various tidal basins of the North Sea (the WaddenSea, the Ems-Dollard Estuary, and the Western Scheldt) vary withina narrow range, between 0.16 and 0.25.

To determine if this principle also applies to the Yangtze Estuarysystem, clay/silt ratios were analyzed in the estuary and the adja-cent region. The results revealed that the ratio between silt and clayin the Yangtze Estuary is not constant, but varies systematicallyacross the estuary (Fig. 4c). The clay/silt ratio in the Yangtze Estuary

0

1

2

3

4

0.1 1 10 100 1000

Grain Diameter (µm)

Input

Exchange

Output

Fre

quen

cy/

d

Fig. 8. Average grain-size distributions of near-bottom suspended sediments inYangtze Estuary (Input, exchange and output legend see Fig. 1).

Fig. 9. Spatial distribution of the vertical sediment exchange ratio p (a) and sedimentexchange rate [cm yr�1] (b).

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330 329

and the adjacent region varies from 0.11 to 0.86, which is a muchlarger range than has been reported from other areas (Flemming,2000; Van Ledden, 2003). This increased range likely occurs due tothe non-linear sand–mud mixing under complex hydrodynamicconditions. Apparently, the interplay of erosion, deposition, andmixing in the bed itself produces non-linear effects that are morepronounced in the Yangtze Estuary system than in other areas, inwhich constant silt/clay ratios seem to be a reasonable assumption.A possible explanation for our results is provided by a study con-ducted by Mitchener and Torfs (1996), which revealed that addingsand to mud or vice versa increased the erosion resistance of theresulting sediment mixture and led to reduced erosion rates whenthe critical shear stress for erosion was exceeded. The results of thatstudy also demonstrated that the most significant effect on erosionresistance occurred when small percentages of mud were added tosand.

Although non-linear sediment mixing appears to be common inthe Yangtze Estuary fine-grained sediments ðlnfmf=mc > 0Þgdisplay a simple linear mixing of the flocculated and cohesionlessfractions, which are primarily deposited as flocs (Fig. 5). The coarse-grained sediments ðlnfmf=mc > 0Þg display a selective deposition ofthe cohesionless sediment fraction, which is primarily deposited assingle grains. High rates of floc deposition are recorded in the outerestuary between 122� E and 123� E along the coast (Fig. 6). The fine-grained sediment belt along the south coast indicates the deposi-tion of southward-transported suspended sediments from theYangtze Estuary in the form of flocs.

5.3. Integration of exchange ratios and accumulation rates

The GSDs of the suspended and sea-bottom sediments visuallyillustrate the vertical sediment exchange processes in the differentsections of the Yangtze Estuary (Fig. 7). The bulk suspended sedi-ment exchange ratio in the Yangtze Estuary can be calculated if theaverage GSD of suspended sediments at the exchange area (Fig. 8) issubstituted into Eq. (1) as GE. The results of this calculation gives anexchange ratio of p¼ 49.4%. This means that 49.4% of the sus-pended sediments supplied from the river basin are deposited inthe Turbidity Maximum Zone and contribute to the morphologicalevolution of the estuary. These results are similar to those of studiesin Yangtze Estuary conducted by Milliman et al. (1985), Shen(2001), and Liu et al. (2006), which provided estimates of 40%, 42%,and 47%, respectively. Hence, more than 50% of the suspendedsediments escape from the estuary and are transported southwardto be deposited in the outer estuary.

More detailed information regarding the sediment exchange inthe study area may be obtained by analysis of the spatial variationof sediment exchange ratios. Small amounts of suspended sedi-ments were deposited onto the seabed of the upper estuary(sediment exchange ratio p< 0.1) because the fine-grained sus-pended sediments in this region were transported to the mouthbar area by the ebb-dominated tide flow (Fig. 9a). The sedimentexchange ratios in the outer estuary also show very low values(p< 0.1) because the diffusion of suspended sediments in theouter estuary was blocked by the oceanic currents offshore (thesouthward Jiangsu Coastal Current and the northward TaiwanWarm Current). However, intensive sediment exchange occurredin the inner estuary due to sand–mud mixing, which wascontrolled by the bidirectional tidal flow (Fig. 9a). The mostfrequent sediment exchange in the muddy area, which was evi-denced by an exchange ratio of about 0.8, was observed at thedepocenter of the suspended sediment. The results also show thatthe southward transport pathway of suspended sediments in theYangtze Estuary is located between 122� E and 123� E longitudealong the coast (Fig. 9a). This coast-parallel belt of high sediment

exchange ratios is consistent with the pattern of longshorecurrents.

The sediment exchange ratio, p, is a relative measure thatdescribes the local balance between suspended and sea-bottomsediments, which can be transformed to the sediment exchangerate (VE) using sedimentation rate. The low value of VE in the upperestuary indicates that a small amount of suspended sediments aredeposited in this area (Fig. 9b). Conversely, the high sedimentexchange rate in the muddy area with a value of greater than 2 cm/yr implies that this area is the depocenter of the Yangtze mud.Beyond the depocenter, the sediments are transported southwardalong the coast.

These findings provide information regarding the transportpathway and the fate of suspended sediments in the YangtzeEstuary. On the decadal to centennial time scale associated with ourresults, a clearly defined depocenter of Yangtze mud is present inthe south of the estuary, between 122� E and 123� E longitude, andaround 31� N latitude (Fig. 9b).

6. Conclusion

The near-bottom sediment exchange in the Yangtze Estuary wasinvestigated through analysis of the GSDs of the sea-bottom andnear-sea-bottom suspended sediments. In addition to the conven-tional approach to grain-size analysis, which involves classificationand the calculation of summary statistics such as Md, we appliedconcepts from compositional data analysis to delineate specificpatterns in the data. The relations between sand, silt, and claycontent, as determined by log-ratio analysis, indicates that selectivedeposition is a non-linear function of the sediment mixturecomposition. In addition, the nearly constant clay/silt ratios thathave been reported for many other tidal basins do not apply to theYangtze Estuary due to the non-linear sand–mud mixing which isgoverned by the complex hydrodynamic conditions in the system.

The floc limit (df, estimated as 32.5 mm) was used to distinguishthe mass fractions of flocculated (mf) and cohesionless (mc) sedi-ments. The spatial distributions of lnfmf=mcg showed that thesediments in the upper reaches of the estuary are primarilydeposited in the form of single grains, while the sea-bottom sedi-ments in the South and North Passages and the outer of the estuaryare mainly deposited as flocs.

Subsequent analyses were conducted in an attempt to quantifythe sediment exchange processes in the estuary and the adjacentregion based on the principle of mass balance. Based on the averageGSDs of the suspended sediments, approximately 49% of the sedi-ments from the Yangtze River accumulate in the inner estuary,while the rest are transferred to the outer estuary.

The spatial distribution of the sediment exchange ratiosdemonstrated that small amounts of suspended sediment aredeposited onto the seabed of the upper estuary (exchangeratio< 0.1), because the fine-grained suspended sediments in thisregion are transported to the mouth bar area by the ebb-dominatedtidal flow. The sediment exchange ratios in the outer estuary alsoshow very low values (p< 0.1) due to the oceanic currents offshorethat prevent the diffusion of riverine sediments further seaward.However, intensive sediment exchange occurs in the inner estuarydue to the sand–mud mixing, which is controlled by the bidirec-tional tidal flows. In addition, the high sediment exchange ratio thatoccurs in the muddy area (>0.8) at the river mouth implies that thisarea is the depocenter of the Yangtze mud.

Conversion of the dimensionless exchange ratio, p, to theexchange rates of the fine-grained sediments was achieved byconsidering the bulk sediment accumulation rates. On the decadalto centennial time scale associated with our results, a clearlydefined depocenter of Yangtze mud with the exchange rates of

H. Liu et al. / Estuarine, Coastal and Shelf Science 86 (2010) 322–330330

greater than 2 cm/yr is present in the south of the estuary. Thisdepocenter, which extends to the south of the estuary, is locatedbetween 122� E and 123� E longitude, and around 31� N latitude.Our study of sediment exchange rates provides a clear picture ofsuspended sediment transport pathways in the Yangtze Estuaryand the adjacent region, and allows us to trace the fate of sus-pended sediment supplied by the Yangtze River.

Acknowledgements

We thank the field work group of the 973 Program for theircontribution. This study was funded by the funds for CreativeResearch Groups of China (Grant No. 40721004), the 973 Program(Grant No. 2008DFB90240), the Shanghai Science and TechnologyFoundation (Grant No. 07DJ14003-01), and the PhD ProgramScholarship Fund of ECNU 2008 (Grant No. 20080009). The authorsalso thank Prof. Zhongyuan Chen and Dr. Maotian Li for theirconstructive comments, which substantially improved the originalmanuscript.

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