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JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 5555–5567, doi:10.1002/jgrc.20410, 2013

Suspended sediment transport in the Deepwater Navigation Channel, Yangtze River Estuary, China, in the dry season 2009: 1. Observations over spring and neap tidal cycles Dehai Song,1,2 Xiao Hua Wang,1,2 Zhenyi Cao,3 and Weibing Guan3 Received 23 March 2013; revised 23 July 2013; accepted 5 August 2013; published 22 October 2013.

[1] The in situ data in the Deepwater Navigation Channel (DNC), Yangtze River Estuary (YRE), China, in the dry season 2009, shows spring tides associated with greater maximum velocities, more mixing, less stratification, and diffused fluid mud; whereas neap tides are associated with smaller maximum velocities, greater stratification, inhibited mixing, and stratified fluid muds. The balance of salt flux indicates the seaward salt transport is dominated by fluvial flows, and the landward salt transport is generated by compensation flows during spring tides, but shear effects during neap tidal cycles. The balance of suspended sediment flux illustrates the offshore sediment transport is dominated by fluvial flows as well, but the onshore transport is induced by tidal-pumping effects on spring tides, and shear effects on neaps. The suspended sediment transport is strongly affected by the salinity distribution and salinity-gradient-induced stratification in the DNC. The spring-neap asymmetry is generated by the estuarine gravitational circulation during low-flow conditions; while the flood-ebb asymmetric stratification within a tidal cycle is due to the semidiurnal tidally movement of the salt front. Citation: Song, D., X. H. Wang, Z. Cao, and W. Guan (2013), Suspended sediment transport in the Deepwater Navigation Channel, Yangtze River Estuary, China, in the dry season 2009: 1. Observations over spring and neap tidal cycles, J. Geophys. Res. Oceans, 118, 5555–5567, doi:10.1002/jgrc.20410.

1.

Introduction

[2] Suspended sediment transport in the Yangtze River Estuary (YRE) has been studied by many researchers since the 1980s [e.g., Yang et al., 1982; Milliman et al., 1984, 1985; Beardsley et al., 1985; Su and Wang, 1986; Shen et al., 1993; Li and Zhang, 1998; Hamilton et al., 1998; Shi and Kirby, 2003; Shi et al., 2003]. It has been found that individual differences constrained by regional conditions may be vital in the YRE [Shen et al., 1993]. Although extensive field studies have been carried out to study systematically the environmental background, temporal and spatial variations, flocculated settling, characteristics and distribution of fluid mud, and geobiochemical process in the YRE, the physical mechanisms responsible for the Companion paper to Song and Wang [2013] doi:10.1002/jgrc.20411. 1 Key Laboratory of Physical Oceanography, Ministry of Education, Qingdao, China. 2 School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Canberra, ACT, Australia. 3 State Key Laboratory of Satellite Ocean Environment Dynamics, Hangzhou, China. Corresponding author: D. Song, Key Laboratory of Physical Oceanography, Ministry of Education, Qingdao 266003, China. ([email protected])

©2013. American Geophysical Union. All Rights Reserved. 2169-9275/13/10.1002/jgrc.20410

suspended sediment transport in the YRE are complex and our knowledge is still far from complete. [3] In this paper, we focus on the North Passage of the YRE and an engineering project called the Deepwater Navigation Channel (DNC), which started in 1998 and was completed in 2011 (Figure 1). The project created a 92 km long channel with a water depth of 12.5 m below the mean lowest low water (MLLW) along the North Passage and South Channel. In addition, two dikes of length 48.1 km to the south of the channel and 49.2 km to the north, and 19 groynes, 30 km in total length, were constructed to increase current speed and decrease sediment deposition in the North Passage. However, since the completion of the first phase (1998–2000), a silting problem began to attract attention, as the annual amount of deposit to be dredged to maintain the DNC was far greater than the original estimate of 30 million m3 [Liu et al., 2011]. The fluvial bed-load sediment has been reduced dramatically due to extensive hydroengineering projects in the river basin, such as Three Gorges Dam, which act as sediment traps [Chen and Zong, 1998; Yang et al., 2006]. Therefore, the mass of the observed deposits in the DNC is more as a result of the redistribution of sediment due to local erosion and deposition, than of the direct input of sediment from the river. [4] As a consequence of the DNC project, the morphology of the North Passage has changed significantly, which inevitably affects the dynamic processes in the North Passage, and even in the YRE. The flow pattern along the main channel of

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SONG ET AL.: SS TRANSPORT IN DNC YRE: OBS

Figure 1. Bathymetry map of the Yangtze River Estuary with detailed structure of the Deepwater Navigation Channel project, including channel cell names and groyne numbers. In the top figure, the red star gives the quadrapod position A0 and the blue dots show (B) Beicao, (Z) Zhongjun, (W) Wusong, and (S) Sheshan stations. the North Passage changed from a rotational current into almost rectilinear flow due to the construction of dikes and groynes, and geometrically controlled eddies may be produced in the groyne areas [Hu and Ding, 2009; Jiang et al., 2012]. Furthermore, due to the construction of two dikes, the horizontal sediment transport between the North Channel and South Passage [Su and Wang, 1986; Chen et al., 1999] has been blocked off. Hence, the suspended sediment transport in the North Passage needs to be reanalyzed. [5] The in situ data in our study were collected in the dry season in 2009, when the last phase (2006–2011) of the DNC was under construction. At that time, the dikes and groynes were completed, and the DNC was already dredged to 10.5 m below the MLLW. The data indicate a highly turbid zone in the down-channel section of the DNC, which might be related to the estuarine turbidity maximum (ETM). Wu et al. [2012] shows a flood-dry season variation of sediment resuspension and trapping in the turbidity maximum zone. In this paper, the spring-neap and flood-ebb variation of the suspended sediment transport in this dry season will be illustrated. Physical mechanism controlling the suspended sediment transport in the DNC will be investigated and discussed. The study site and the field measurements are presented in section 2. The data analysis

and results are offered in section 3, followed by discussion in section 4, and conclusions in section 5.

2.

Study Site and Data Acquisition

[6] The Yangtze River is a multichannel estuary with three-level bifurcations and four outlets (North Branch, North Channel, South Channel, North Passage, and South Passage, see Figure 1 for details) separated by islands and shoals. It is the third longest river in the world and has a multiyear averaged discharge of 29,300 m3 s1 [Shen et al., 1993]. The river discharge has a strong seasonal variation, in which approximately by 70% of the runoff occurs in the flood season from May to October, and only 30% in the rest of the year, the dry season. Historically, the Yangtze River is the fourth largest in terms of sediment discharge [Milliman and Syvitski, 1992]. The annual mean suspended sediment load from the Yangtze River approaches 480 million tons. It has been estimated that 40% of the sediment load is deposited in the estuary [Milliman et al., 1985]. More than 95% of the suspended sediment load in the YRE consists of fine sediments ( 0 indicates a fast-rising tide and 0() < 0 shows a fast-ebbing tide. However, the sign of 0(u) depends on the direction of the flood current. To be consistent with the duration asymmetry, in this study we reverse the original sign of 0(u) calculated by equation (1) and still let 0(u) > 0 represent for flood-dominant velocity and 0(u) < 0 for ebb dominance. Tidal asymmetry quantified by equation (1) is identical to the approach proposed by Song et al. [2011] ; using just sample skewness misses the chance to discuss the roles of different combinations of tidal constituents in tidal asymmetry. [17] As shown in Figure 5a, the water level had a positive 0() during spring tides, but varied from positive to negative during neap tides as the flood duration became longer. Giving the observed tidal elevation as the summation of N individual constituents (n):

ðt Þ ¼

N X

n ¼

N X

n¼1

an cos ð!n t ’n Þ;

ð2aÞ

n¼1

where an is the amplitude, !n ¼ 2/Tn is the frequency, Tn is the period, and ’n is the phase of constituent n, the contribution of different combinations of tidal constituents to duration asymmetry can be estimated via [Song et al., 2011], based on the long-term water level records at Beicao tide station about 9.5 km upstream from the site A0 (Figure 1): 3 2 2 a1 !1 a2 !2 sin ð2’1 !3=2 N

2 ¼ 4

1 2

X i¼1

a2i !2i

’2 Þ

ð2bÞ

for pairs of tidal constituents with frequency relationship (2!1 ¼ !2) and 3

3 ¼ 2

a1 !1 a2 !2 a3 !3 sin ð’1 þ ’2 ’3 Þ !3=2 N X 2 2 1 ai !i 2

ð2cÞ

i¼1

for triplets of tidal constituents with frequency relationship (!1 þ !2 ¼ !3). It shows the fast-rising tide in the North Passage is mainly due to nonlinear effects, in which the pair of M2 and M4 contributes most (2 ¼ 0.205) and followed by the triplet of M2, S2, and MS4 (3 ¼ 0.165). [18] However, a negative 0(u) shows a freshwaterdischarge-induced ebb-dominant current from surface to bottom during spring tides, when the water is well mixed. Due to the high-stratification environment, velocity skew differs at different layers during neap tides. Ebb dominance is greater near the surface (6.16 mab), but less near the bottom (0.62 mab). The daily-averaged river discharge at the Datong hydrologic station, about 600 km upstream from the YRE, decreased gradually from 20,500 m3 s1 to 19,300 m3 s1 during the period of quadrapod deployment. The discharge difference has only an insignificant effect on the variable velocity skew of the surface. Thus, the enhancement of ebb-dominant velocity near the surface is mainly due to the spring-neap modulation, i.e., the weakened fast-rising water level increases the ebb-dominant surface current on neap tides. On the other hand, net upstream bottom flow is weak or absent during periods of weak stratification, but it is important when the system is highly stratified [Jay and Smith, 1990]. This can be seen in Figures 5a or 5b, where the ebb-dominant current in the lower layer is dramatically reduced during neap tides by an enhanced upstream flow. The difference between the inter-tidal asymmetries at the surface and the bottom indicates shear

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Figure 5. (a) The skewness-based along-channel surface-velocity skew (blue), bottom-velocity skew (green), duration asymmetry (black), and sediment flux skew (red) based on the bottom quadrapod measurements; positive values indicate fast-rising duration asymmetry and flood-dominant velocity skew or flux skew. (b) The vertical distribution of along-channel (AC) and cross-channel (CC) current velocity skew; (c) vertical distribution of along-channel and cross-channel salt flux skew; and (d) vertical distribution of along-channel and cross-channel suspended-sediment flux skew during the spring and neap tide, respectively; in along-channel direction, negative indicates a downstream net transport; while in cross-channel direction, negative indicates a slope-to-channel net transport (i.e., from the south-dike-side to the north-dike-side).

effects increase from spring to neap tides, which may enhance the salt intrusion (see Table 2 Term T6, discussed below). 3.2. Salt Transport [19] The bottom temperature measurement reveals a small-magnitude semidiurnal tidal oscillation between 10 C and 12 C (not shown). The tidal salinity variation indicates a salt front crossed the bottom quadrapod (Figure 2a). Observations discussed here (Figures 3a and 3f) illustrate a situation where the system is relatively well mixed on the spring tides, but highly stratified on the subsequent neap tides. As noted by Jay and Smith [1990], the decrease in tidal range during low-flow conditions favors such a transition to a two-layer flow system and a longer salinity intrusion length. Figure 2a also shows an increased minimum salinity on the bottom during neap tides, which

Table 2. The Balance of Salt Flux and Suspended Sediment Flux Through a Unit-Width Cross Section Based on the CTD Measurements Salt Flux (psum2 s1)

Sediment Flux (kgm1 s1)

Term

Spring

Neap

Spring

Neap

T1 T2 T3 T4 T5 T6 T7 Total Landward Seaward

21.314 8.327 1.110 0.613 1.001 0.257 0.066 14.294 9.197 23.491

10.437 1.760 0.260 0.191 0.027 2.243 0.070 6.600 4.194 10.794

3.174 1.291 0.098 1.661 0.115 0.389 0.101 0.083 3.456 3.373

0.564 0.109 0.001 0.071 0.014 0.271 0.018 0.116 0.466 0.582

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indicates a landward shift of the bottom salt front as the weakened tidal excursion cannot push the salt wedge out of the DNC. This is also consistent with a decrease in river discharge during these neap tides, but it probably plays a minor role in the increase in bottom minimum salinity, as mentioned above. Furthermore, the salt-wedge intrusion during neap tides can be understood in terms of the nontidal, baroclinic response to a change in the intensity of vertical mixing [Linden and Simpson, 1988]. The extremely high salinity with larger tidal variation on neap tides shows a stronger salinity-gradient structure in the frontal zone, compared with a more diffuse frontal structure observed on spring tides. [20] The dispersion of salt can be analyzed by the method proposed by Dyer [1974], in which the mean salt flux through a unit-width cross section can be calculated as: F ¼ hhuci ¼ h hui hci þ hci ht uz þ hui ht cz þ h uz cz þ ht uz cz T2

T1

T3

T4

T5

þ h huz;t cz;t i þ ht huz;t cz;t i T6

T7

ð3Þ

where u ¼ uðz; tÞ and c ¼ cðz; tÞ is current velocity and scales at depth z and time t, respectively. The over bar denotes a tidally averaged value and the angled brackets denote a depth average. At any depth, u ¼ hui þ uz and c ¼ hci þ cz , where uz and cz are the deviations of the observed values from the depth-averaged values at a particular time. Because of tidal fluctuations, hui ¼ hui þ uz;t and hci ¼ hci þ cz;t , where uz,t and cz,t are the deviations from the depth-averaged values over a tidal cycle. The tidal height fluctuations can be written as h ¼ h þ ht , where ht is the deviation of the tidal height from the mean depth. According to Dyer [1974], the first two terms on the righthand side of equation (3) are associated with ‘‘nontidal drift,’’ T1 being the result of river flow and T2 being a compensation flow (Stokes drift) for landward transport on the partially progressive tidal wave; T3 is due to the correlation of the tidal-period variations of tidal height and salinity; T4 is the correlation of the tidal-period variations of salinity and current; T5 is the third-order correlation of the tidal-period variations of salinity, velocity, and tidal height; T6 is the mean shear effect; and T7 is the covariance of the shear effect and tidal height. The values of each term are listed in Table 2 for one spring tidal day and one neap tidal day, respectively. As shown in the table, the two advective terms (T1 and T2) are reduced in magnitude from spring to neap due to the decreased tidal excursion. T3–T5 are tidal dispersion terms, which are also reduced from spring to neap due to the reduced tidal range. Interestingly, the term T6, which indicates a landward salt flux, increases in magnitude from the spring to neap due to a strengthened vertical-salinity-gradient-induced shear effect. During neap tides, the increased velocity shear (Figure 5b) in the water column transports the bottom salt landward. Note that the salt imbalance between landward and seaward flux might be due to a lack of lateral resolution or the bottom-unreachable salinity measurements. [21] We apply equation (1) to the CTD measurement to find the vertical distribution of the salt-flux skewness

(Figure 5c). It shows an ebb-dominant salt transport with a less variation in vertical pattern due to the well-mixed water during these spring tides. However, the downstream salt transport is significantly reduced near the bottom as salt intrusion is enhanced during neap tides. The change of salt-flux skew in opposite directions between the surface and bottom indicates an intensification of the stratification on neap tides. 3.3. Suspended Sediment Transport [22] Figure 2a shows highly-turbid waters near the bottom in the DNC, which varies in the spring and neap tidal cycles. During spring tides (Figure 3b), a high SSC is usually formed at high slack water and a low SSC at low slack water. The measured SSC has a same variation with the salinity, which indicates turbid water intrudes into the DNC on flood tides. This might be related to the ETM movement, as several field studies have already found the turbidity maximum occurs in the down-channel section of the North Passage [e.g., Shi and Kirby, 2003; Wu et al., 2012]. And the site A0 is on the landward side of the ETM. [23] In addition, the effect of stable salinity-gradientinduced stratification on the suspended sediment transport needs to be evaluated, as the tidal mixing asymmetry has been proposed to generate an upstream net transport of suspended sediment [Geyer, 1993]. In estuaries, strain-induced periodic stratification (SIPS) [Simpson et al., 1990] is a dominant mechanism creating tidal mixing asymmetry in the presence of a longitudinal density gradient. Tidal currents stratify the water column through the straining of the density field during ebb tides, but destratify it during flood tides, which leads to a residual flow seaward near the surface and landward near the bottom. The influence of SIPS on processes of stratification and destratification will be discussed in section 4. Here, gradient Richardson numbers (Rig) were calculated from the buoyancy frequency (N2) over the vertical shear (Sh2) to describe the relative stability of the stratified shear flow: g @ @z N2 Rig ¼ 2 ¼ 2 2 ; @u Sh þ @v @z

ð4Þ

@z

where g is the gravitational acceleration, is the seawater density, z is the vertical coordinate, and u and v are the eastward and northward current velocities, respectively. The variability of Rig, normalized by the critical value, Ric ¼ 0.25, is shown in Figures 3c and 3h. To identify flood/ebb phases of the tide, the along-channel current velocity is also given in Figures 3d and 3i. Both theory and observations indicated that in the stratified shear flow with Rig < 0.25 or log10 (Rig/0.25) < 0, the flow is unstable and the turbulence mixing is enhanced, whereas when Rig > 0.25 or log10 (Rig/0.25) > 0, the flow is stable and the mixing is inhibited. [24] During spring tides, the distribution of Rig shows a better mixed water-column on ebb tides than flood tides. The higher Rig on flood tides is caused by the salt front intrusion through site A0 with strong vertical-salinitygradient. As shown in Figure 3c, since the beginning of flood, higher Rig water is gradually lifted from the bottom to the surface. In this stably stratified water, the suspension

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of sediment is confined near the bottom, which can be regarded as the landward edge of the ETM. Until high slack water, the SSC is keeping increased as the ETM is moving landward. However, at high slack water, the suspended sediment begins to settle, but might have a settling lag, and forms a highly concentrated suspension layer, which can cause damping of turbulence generated in the bottom boundary [Wolanski et al., 1992]. On ebb tides, as the salt front retrieves back to the seaward of the site A0, the lesssaline water becomes well mixed with smaller Rig. Due to the seaward movement of the ETM, SSC is largely reduced on ebb tides, but the fine sediment can be suspended to the surface again by strong turbulent mixing (Figure 3c). [25] There is a distinct difference between the spring and neap tides ; the overall stratification is much stronger during neap tides, when turbulent mixing is highly suppressed (Figure 3h). The structure of salt front is changed to be sharper with stronger salinity gradient (i.e., a salt wedge, greater salinity oscillation in Figure 2a). The ETM should also be highly stratified in the vertical with stronger SSC gradient in the horizontal (Figures 2a and 3g). It seems a small amount of fine sediment can be suspended to the surface only on ebb tides when the turbulent mixing is enhanced due to the salt wedge moving to the seaward of the site A0 (Figure 3g). In general, the SSC is lower during neap tides than spring tides, due to the reduced bed erosion. Evidence can also be found in Figure 6, which indirectly presents the lutocline and fluid mud using ADCP data, as the phase of ADCP signals may be corrupted by the presence of a nepheloid layer or a lutocline [Cao et al., 2012]. It seems that during spring tides the strong bottom shearstress erodes the fluid mud as well as the bed on ebb tidal phases, and the resuspended sediment is transported downstream by the river flow (Figure 6). Then at low slack water, the deposited sediment begins to accumulate until the following flood tides, which erodes the bed again but less than ebb tides. The cycle continues with fluid-mud thickness increasing as tidal range reducing from spring tides to neap tides. During neap tides, in such a low-energy condition, the fluid-mud layer that forms is typically not completely reentrained, as it is during spring tides, which

leads to an increase in the fluid-mud thickness and a rise of the lutocline in the water column. The fluid mud is cyclically deposited and entrained, which may dynamically change the conditions of turbidity and stratification. During neap tides, the reentrainment of the fluid mud by tidal shear flow is more important than bed erosion. However, the vertical transport of sediment from the bed to the water column is reduced due to the strengthened stratification. We note that SSC drops quickly after the lower high tide (i.e., at midnight of the Julian day 93 and 94 in Figure 2a), when water with a smaller velocity but a longer slack period (see the gradient of velocity in Figure 2b) favors more rapid deposition of sediments. [26] Equation (3) is also applied to SSC data to study the suspended sediment balance (see Table 2). The residualflow-induced flux (T1 and T2) makes the largest two contributions to the suspended sediment transport. The sum of terms T3–T5 can be regarded as the transport due to a tidal-pumping effect, which is significant on spring tides and dominant (48.1% of the total) in the landward transport. All the terms are significantly reduced in magnitude on neap tides except the term T6, which dominates the onshore suspended sediment transport (58.2% of the total landward). This indicates that the shear effect could also move the suspended sediment landward on neap tides due to the intrusion of salt water. We apply equation (1) to the sediment flux at 0.62 mab, and find that the near-bed suspended sediment is transported offshore on spring tides but onshore on neap tides (Figure 5a). The former is due to strong resuspension coupled with an ebb-dominant velocity skew, while the latter due to saline water intrusion. The vertical distribution of the suspended sediment flux skewness (Figure 5d) is also calculated using the CTD data. It too shows opposite patterns during spring and neap tides. During spring tides, this indicates an upstream net suspended sediment transport in the upper layer, but a downstream net sediment transport in the lower layer, as the sediment is mostly suspended to the upper layer on flood tides when the water column is well mixed. This is consistent with the remarkable tidal-pumping effect on spring tides. During neap tides, it favors an ebb-dominant

Figure 6. The along-channel current velocity (unit: m s1) measured by the downward-looking ADCP (positive seaward). The contaminated data are left blank [see Cao et al., 2012, for details]. Tidal elevation (unit: m) during the ADCP measurement is also given on top to identify the flood/ebb tidal phases and the spring/neap tidal cycles. 5563


suspended sediment transport in the upper layer and a flood-dominant transport in the lower layer due to the enhanced salt intrusion near the bottom.

4.

Discussion

[27] To study the competing influences of stratification and mixing, potential energy anomaly has been found to be an excellent measure for the stability of the water column [e.g., Simpson and Bowers, 1981; Simpson et al., 1990; Simpson and Souza, 1995; Souza and James, 1996], which can be easily quantified from field observations and defined by Simpson [1981] as: ’¼

1 D

Z

ð Þgzdz;

ð5aÞ

h

with the depth-mean density ¼

1 D

Z

dz;

ð5bÞ

h

the mean water depth H, the sea surface elevation , the total water depth D ¼ þ H, the gravitational acceleration g, and the vertical coordinate z. For a given density profile, ’ (J m3) represents the amount of mechanical energy required to instantaneously homogenize per volume unit of water column. The local change of ’ has been attributed to several physical mechanisms [Burchard and Hofmeister, 2008; de Boer et al., 2008], among which the classic SIPS defined by Simpson et al. [1990] and the advection of a vertical density structure by a depth-mean current without deformation can explain most of the variability [de Boer et al., 2008]. Due to the limitation of the observation in this study, we simply calculate the instantaneous ’ via equation (5) using the CTD measurements (Figures 7a and 7b). Basically, ’ tells the same stories with Rig, but in a more quantitative manner. More work is required to completely mix the water column during neap tides (Figure 7b) than spring

tides (Figure 7a). A local maximum of ’ is usually visible around high slack water, indicating salt front intrusion. The second peak occurring at the beginning of ebb during both spring and neap tides is probably induced by the advection, as the tidal straining typically results in the greatest stratification at the end of ebb [e.g., Simpson et al., 1990; Burchard and Hofmeister, 2008]. Further observation or numerical simulation is needed to investigate this phenomenon. On the following ebb, the salt front moving downstream generates an almost completely mixed water column during spring tides and a largely reduced stratification during neap tides. We speculate the advection-induced period stratification plays a more significant role than SIPS at site A0. [28] In addition, the asymmetric turbulent mixing during spring-neap tidal cycles and flood-ebb tidal phases can also be illustrated by turbulent kinetic energy (TKE). The 128 s averaged TKE shows a significant difference between neap and spring tides, but a weak asymmetry between ebb and flood phases (Figure 2d). Nunes Vaz and Simpson [1994] pointed out that stratification is produced primarily from the elastic straining at diurnal-tidal or semidiurnal-tidal frequencies, but that the estuarine gravitational circulation tends to dominate at a fortnightly spring-neap frequency. In this case, the semidiurnal tidally movement of the salt front contributes to the asymmetric stratification within a tidal cycle. The salt-front water generates a relatively stable stratification during flood tides; however, the moving away of salt front leaves a well-mixed water on site A0 during ebb tides. Figure 2d also reveals very weak turbulent mixing at the midnight of the Julian day 93 and 94, which may lead to a rapid fall in the SSC. This is because sedimentladen waters need a significant fraction of the TKE to maintain the sediment in suspension [Wolanski et al., 1992]. [29] To examine the lateral process on the suspended sediment transport, the vertical distribution of the crosschannel velocity skew and flux skewness (Figures 5b–5d) are also calculated using the ADCP and CTD data, respectively. The cross-channel flow is one order of magnitude

Figure 7. The potential energy anomaly (unit: J m3) for the observed (a) spring and (b) neap tides. The black solid and dashed lines indicate ’sal and ’tot, respectively. The red lines give depth-integrated along-channel current velocity (unit: m s1, positive seaward) and blue lines give tidal elevation (unit : m); both are of five time exaggeration with gray lines indicating zero. The proportion of ’sed to ’tot is also given for the observed (c) spring and (d) neap tides. 5564


smaller than along-channel velocity during ebb tides, but the difference is reduced during flood tides (Figures 3e and 3j). This generates a greater velocity skew in cross-channel direction than that in along-channel direction (Figure 5b), although the former has smaller current speeds. Furthermore, it indicates a net salt and sediment transport from the slope to the deep channel (from the south-dike-side to the north-dike-side) except an opposite flow below 1 m above bed (Figures 5c and 5d). Based on this analysis, we can draw a picture that on flood tides a lateral flow occurs, which might be generated by a geometrically controlled eddy in the groyne area [Jiang et al., 2012]. It transports the suspended sediment from the slope to the channel on surface layers. The lateral flow becomes stronger on late flood, when the SSC on site A0 is slightly reduced (Figure 3b). However, on ebb tides the along-channel flow is so strong that the lateral flow is almost vanished. Generally, the cross-channel flow or flux has a more asymmetric vertical pattern on neap tide than that on spring tide, but the lateral suspended sediment transport shows much smaller spring-neap variation, compared to the along-channel transport. [30] Sediment-induced stratification might be important in some muddy estuaries, such as the Yellow River Estuary, China [Wang and Wang, 2010]. Here, the contribution to stratification by salinity and SSC on site A0 can be evaluated based on the field measurements. According to Adams and Weatherly [1981], the effect of SSC on the equation of state is introduced using a bulk-density relation: ¼ w þ 1 w C; s

5. ð6Þ

where w is the seawater density calculated via the equation of state and s is the sediment bulk density. Considering equation (6) into equation (5), we can obtain ’tot ¼

Z 1 ðw w Þgzdz þ C C gzdz D h h Z 1 C w C gzdz; s D h w 1 D

Z

1.72 mab; thus ’sed is much smaller than ’sal. However, the turbidity-induced stratification is still increased after the maximum flood and ebb currents (Figure 7d), when entrainment on fluid mud is enhanced and more sediment is suspended into upper layers. Furthermore, based on the historical field measurement, the SSC may rapidly reach over 30 kgm3 in the fluid mud layer [e.g., Li and Zhang, 1998; He et al., 2001] from 5 kgm3 at 0.63 mab in this study. It indicates that such a great turbidity gradient would play a much more significant role in the BBL stratification processes than the salinity gradient during neap tides. [32] The wave parameters calculated through the ADV measurement show a maximum significant wave height of 0.75 m (Figure 3e), with about 0.1 m s1 maximum bottom orbital velocity. Compared with the tidal current in the DNC, waves appear to play an insignificant direct role in sediment suspension. However, we cannot rule out wave effects on sediment suspension in shallow waters (Jiuduansha Shoalwater and Hengsha Shoalwater) or on the pore pressure buildup and resulting liquefaction of the bed [Lambrechts et al., 2010]. Other evidence is that the SSC shows nontidal fluctuations between Julian days 91 and 93 in Figure 2a, which coincides with the southerly wind anomaly in Figure 2c. The southeastward outlet is favored by a southeasterly or southerly wind, which generates larger waves in the DNC by giving a larger fetch. Therefore, the disagreement between SSC and salinity might be caused by the southerly wind-generated waves.

ð7Þ

R R R where w ¼ D1 h w dz, C ¼ D1 h Cdz, and w C ¼ D1 h w Cdz. [31] In this study, the first term (’sal) on the right-hand side of equation (7) indicates the salinity-induced stratification; while the last two terms (’sed) turbidity-induced stratification including the interaction between salinity and turbidity. Assuming s ¼ 2650 kgm3, the ’tot is plotted in Figure 7a for spring tides and Figure 7b for neap tides. The proportion of ’sed to ’tot is also shown in Figures 7c and 7d. During spring tides, the sediment can be suspended throughout the water column; thus, the turbidity-induced stratification takes 20–60% of the total stratification in the potential energy anomaly; nevertheless, when the water is of less salinity (Julian day 88.8–88.9 in Figure 7c) or higher turbidity (Julian day 88.5–88.6 in Figure 7c), ’sed is comparable with or even larger than ’sal. During neap tides, the upper water column is of less turbidity due to the enhanced salinity-induced stratification ; a lutocline is formed in the bottom boundary layer (BBL) over the fluid mud layer (Figure 6), but our calculation is limited to above

Conclusions

[33] To study the suspended sediment transport in the Yangtze River Estuary, especially in the Deepwater Navigation Channel, we analyzed the data measured in the DNC in late March and early April 2009. During the dry season, the tidal current is comparable with the river discharge in the middle-channel section of the DNC; thus, intertidally and intratidally asymmetric phenomena are evident in our measurements. In general, the observations in the frontal zone show spring tides associated with greater maximum velocities, more mixing, less stratification, and diffused fluid mud (when present). Neap tides are associated with smaller maximum velocities, greater stratification (due to salt), subdued mixing, and stratified fluid mud. The springneap asymmetry can be attributed to the estuarine gravitational circulation during low-flow condition, which supplies a high-energy environment (a well-mixed estuary) during spring tides but a low-energy environment (a highly-stratified estuary) during neap tides. [34] Seaward salt transport is dominated by fluvial flows, and landward salt transport is generated by compensation flows during spring tides but shear effects during neap tides. Seaward sediment transport is dominated by fluvial flows as well, but that the landward suspended sediment transport is determined by tidal-pumping effects during spring tides, and shear effects during neap tides. The difference between salt and sediment transport in this study is due to their conservative and nonconservative characteristics respectively. The suspended sediment transport is strongly affected by the salinity distribution and salinitygradient-induced stratification in the DNC. For instance, the height of sediment resuspension is usually constrained

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by strong stratification. Therefore, the ETM in the DNC may have the same structure variation with the salt front between spring and neap tides, i.e., relatively well mixed in vertical with diffuse gradients in horizontal on spring tides but highly stratified in vertical with sharp gradients in horizontal on neap tides. [35] On site A0, the semidiurnal tidally movement of the salt front contributes to the asymmetric stratification within a tidal cycle. The turbulent mixing is determined by salt front movement rather than the local tidal straining effect between flood and ebb tides. The distribution of Rig illustrates two types of water column: water within the salt front has a more stable stratification; meanwhile water landward of the salt front is well mixed. The potential energy anomaly also confirms that advection describes the displacement of salinity structure by the depth-averaged current without deformation overweighs the tidal-straining effect at our study site. The horizontal advection is dominated in the suspended sediment transport on this spot; however, it may also be important that the enhanced stratification at high slack water drops the suspension on the surface. The lateral process generates a net sediment flux from the slope to the deep channel, which reduces the SSC on late flood tides; however, it might still play a minor role in the suspended sediment transport in the DNC. In the upper layer, the turbidity-gradient-induced stratification may comparable with salinity-gradient-induced stratification; whereas we speculate the former is much more important in the BBL. [36] Two neap events were found in our observations, which have been seldom reported in previous studies on the YRE. One is the increased minimum salinity during neap tides, which might be related to the bottom salt intrusion. This can be understood as a baroclinic response to the reduced vertical mixing. The other event is the rapid drop in SSC during neap tides. Based on the data we obtained, this can be explained by the turbulent kinetic energy being too weak to support sediment in suspension. Nevertheless, we speculate that the retrieval of the ETM may also reduce the SSC on site A0, as the falling event begins at the initiation of ebb tides. [37] Due to the limited in situ data, numerical simulation is required to further understand the processes controlling sediment trapping in the DNC. Thus, a three-dimensional wave-current-sediment coupled model will be established in Part 2 of this paper to investigate mechanisms on suspended sediment transport in the North Passage, Yangtze River Estuary, China. [38] Acknowledgments. D.S. has been supported by the China Scholarship Council and the University of New South Wales (UNSW) Research Publication Fellowship for his PhD study in Australia. X.H.W. was supported by 2011 Australian Research Council/Linkage Projects (LP110100652). This work was supported by the National Basic Research Program of China (grant 2010CB428704), the National Nature Science Foundation of China (grant 41276083), and the scientific research fund of the Second Institute of Oceanography, SOA (grant JT1007). This paper benefited from reviews by Andrew Kiss and Peter McIntyre at UNSW Canberra, and two anonymous reviewers. This is a publication of the SinoAustralian Research Centre for Coastal Management, paper 15.

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