JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, , doi: /jgrc.20410, 2013

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1 JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, , doi: /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 Guan 3 Received 23 March 2013; revised 23 July 2013; accepted 5 August 2013; published 22 October [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, , doi: /jgrc 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: /jgrc 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 , China. (songdh@ouc.edu.cn) American Geophysical Union. All Rights Reserved /13/ /jgrc 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 ( ), 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 m 3 [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 5555

2 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 ( ) 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 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 m 3 s 1 [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 (<63 m). It provides a good example of a turbid estuary, with a near-bed suspended sediment concentration (SSC) of over 4 kgm 3 [e.g., 5556

3 He et al., 2001; Shi et al., 2006; Liu et al., 2011]. The YRE is a partially mixed mesotidal estuary with mean tidal range of 2.66 m and maximum tidal range of 4.62 m at the Zhongjun tide station in the South Passage (Figure 1), which decrease upstream to 2.21 m and 4.48 m at Wusong tide station in the South Channel [Wu et al., 2012]. Under suitable conditions of current and turbulence, fluid muds can form in the YRE; the extent of this varies with flood and dry seasons, ebb and flood tides, gravitational circulation, and storms [Li et al., 2001]. They also found that the South Passage offers the best conditions for fluid-mud formation in all four outlets, followed by the North Passage. [7] Our field-work site A0 ( E, N) was located in the middle of the DNC, North Passage (Figure 1), where the mean depth is about 9 m and the bottom sediments consist of soft mud with slightly more silt than clay. A bottom quadrapod was deployed at the station over a 7 day period. It was equipped with two ADCPs and one ADV to measure flow velocity, and two Optical Backscatter Sensors (OBS 5þ) to measure the turbidity. Unfortunately, the OBS at 0.34 m above bed (mab) did not work in the highly turbid water and returned abnormal data. A pressure sensor (RBR 420) with a CTD data logger was also used to record water surface level variation, temperature, salinity, and density. The quadrapod instrumentation and the sampling schemes are summarized in Table 1 and some of the results are plotted in Figure 2. The collected data, starting at 18:00 on 29 March and ending at 06:00 on 5 April 2009 (þ0800, Beijing Time Zone), covering a spring-to-neap transition from Julian day to 94.25, are used to analyze the sediment transport and validate the numerical model in Part II of this paper. During this period, sea surface wind records with 1 h intervals were also obtained from Sheshan station (Figure 1) and are shown in Figure 2c. [8] In addition, vertical profiles of temperature, salinity, and turbidity were measured at 1 h intervals during a spring tidal day (Julian day ) and a neap tidal day (Julian day ), respectively (Figure 3), by a CTD (Seabird 19) with an OBS 3þ probe attached. When the current is strong, the probes cannot reach the nearbottom layer. Therefore, in the following study, the RBR salinity data are used to correct the bottom layer at 1.72 mab, and interpolation to fill the voids in the CTD readings. This method is also used to process the SSC data. Thus, Figure 3 shows the vertical profile from 1.5 m under the sea surface to 1.72 mab for salinity, but to 0.63 mab for SSC. [9] To calibrate the turbidity (unit: NTU) measured by OBS 3þ probe, water sampling was concurrently taken with CTD deployment. The relationship between NTU and SSC was found using a least-square regression with 115 samples (R 2 ¼ , see Figure 4a): SSC kg m 3 ¼ 0:00273 NTU 0:0578: [10] Laboratory test was also conducted to calibrate the OBS 5þ in the bottom quadrapod. A tank filled with water obtained at our study site was settled until all suspended sediment removed from the water column. The slurry (an amalgam of fine sediment from the field bed) was introduced to the waterbody with vigorously stirring to ensure a homogeneous mixture. Turbidity was measured and recorded by OBS 5þ, and meanwhile a water sample was extracted for SSC measurement. The level of suspended solids was gradually increased to obtain sufficient data to derive the NTU-SSC relationship (Figure 4b). For this study, within a coefficient of determination R 2 ¼ , the relationship is given by 12 water samples as: SSC kg m 3 ¼ 0:0134 NTU 0:5771: [11] The ADV in the bottom quadrapod was set to record at a sampling rate of 8 Hz, which had 1025 samples within a 128 s burst for every 30 min. After the 128 s temporal average, the turbulent flow components u 0, v 0, and w 0 indicates quasinormal probability distributions; variances u 0 u 0, v 0 v 0, and w 0 w 0 (the overbar denotes a time average) are positively skewed with high kurtosis; and covariances u 0 w 0, v 0 w 0, and u 0 v 0 are usually negatively skewed with high kurtosis [Cao et al., 2012]. These characteristics remain consistent throughout the deployment, which are typical behaviors of tidal boundary layers as described by Gross and Nowell [1985] and Kim et al. [2000]. In addition, the velocities measured with the ADV at 0.77 mab were compared with velocity measurements at bin 13 from the downward-looking ADCP; the correlation coefficient between the two data sets is 0.96, indicating good agreement between two measurements [Cao et al., 2012]. [12] In this study, the turbulent kinetic energy (TKE) at 0.77 mab can also be estimated by using the ADV measured turbulent flows as: TKE ¼ 1 2 u0 u 0 þ v 0 v 0 þ w 0 w 0 ; which is shown in Figure 2d. [13] Using a well-established PUV method (Matlab Toolkit supplied by Nortek, available at kusa.com/usa/knowledge-center/table-of-contents/waves), Table 1. The Quadrapod Instrument Depth and Sampling Schemes Instrument Measurement Elevation (Meter Above Bed) Data Intervals (min) Range Cell (m) Nortek ADV (1025-sample average) RDI ADCP Downward (120-ping average) 0.02 Upward (120-ping average) 0.40 D&A OBS5þ (2-sample average) (2-sample average) RBR (60-sample average) 5557

4 Figure 2. (a) Suspended sediment concentration (blue) measured at 0.62 mab and salinity (red) measured at 1.72 mab at site A0; (b) ADCP measurement of current velocity at different layers (red at 6.16 mab, blue at 2.96 mab, and green at 0.62 mab; positive indicates ebb) and tidal elevation (black) at site A0; (c) observed wind at Sheshan station; (d) calculated turbulent kinetic energy at 0.77 mab (red line shows the daily averaged result); and (e) significant wave height based on the ADV measurement at 0.77 mab at site A0. the significant wave height and period, direction and spreading at the spectral peak can be calculated from arrays of sequential velocity and pressure time series measured by the ADV in the bottom quadrapod, which is shown in Figure 2e. 3. Data Analysis and Discussion [14] Measurements indicate strong spring-neap and flood-ebb variations in salt transport, suspended sediment transport, and vertical mixing in our study site. In this section, those variations will be illustrated, and the mechanisms will be discussed. As YRE is a tidal-dominated estuary, tidal-induced asymmetries would be important to the suspended sediment transport. Here, a skewness-based approach will be used to show the asymmetric pattern in water levels, current speeds, and suspended sediment transport between spring and neap tidal cycles. Then, a flux decomposition method proposed by Dyer [1974] will be used to analyze the governing mechanisms, by which the 5558

5 Figure 3. (left) (a) vertical distribution of salinity (unit: psu), (b) suspended sediment concentration (unit: kgm 3 ), (c) gradient Richardson number (shown as log 10 (Ri g /0.25); the thick black line indicates log 10 (Ri g /0.25) ¼ 0, i.e., Ri g ¼ 0.25), (d) along-channel current velocity (unit: m s 1, positive seaward), (e) cross-channel current velocity (unit: m s 1, positive pointing to north-dike-side) in the observed spring tides. The right-hand plot gives as the left-hand plot but in the subsequent neap tidal cycles. The CTD measurement is corrected by the bottom-deployed measurement, and linear interpolation used to fill the gaps. The voids between the upward-looking ADCP (2.96 mab) and downward-looking ADCP (0.86 m) are linearly interpolated. spring-neap variation in salt and suspended sediment transport is generated. Finally, different performance of vertical mixing and stratification between flood and ebb tidal phases and between spring and neap tidal cycles will be explored Tidal Asymmetry [15] Nidzieko and Ralston [2012] proposed that tidal asymmetry (i.e., duration asymmetry and velocity skew) can be quantified by the sample third moment about zero, divided by the sample second moment about zero to the 3/2 power: 0 3 3=2 2 ; ð1aþ [16] The sample mth moment about zero is defined as m ¼ 1 X N n m i N 1 ; i¼1 ð1bþ where N is the total number of samples n i. Tidal elevation time derivative where is tidal elevation) and tidal current velocity (u) can be applied in equation (1) to quantify the duration asymmetry 0 () and velocity skew 5559

6 Figure 4. Calibration of the turbidity (in NTU) measured by (a) the OBS 3þ probe and (b) the OBS 5þ probe with water sampling. Linear fits (black lines) between turbidity and suspended sediment concentration (SSC unit: kgm 3 ) are achieved with 95% confidence level shown in blue lines. 0 (u), respectively. Here, we use a running discrete lunarday window (24.84 h 149 samples) to compute 0. Note that 0 () > 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 ): ðþ¼ t XN n ¼ XN a n cos ð! n t n Þ; ð2aþ n¼1 n¼1 where a n is the amplitude,! n ¼ 2/T n is the frequency, T n 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): ¼ a2 1!2 1 a 2! 2 sin ð2 1 2 Þ! 3=2 ð2bþ 1 2 X N a 2 i!2 i i¼1 for pairs of tidal constituents with frequency relationship (2! 1 ¼! 2 ) and ¼ a 1! 1 a 2! 2 a 3! 3 sin ð 1 þ 2 3 Þ! 3=2 ð2cþ 1 2 X N a 2 i!2 i 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 M 2 and M 4 contributes most ( 2 ¼ 0.205) and followed by the triplet of M 2,S 2, and MS 4 ( 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 m 3 s 1 to 19,300 m 3 s 1 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 5560

7 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) 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 Term Salt Flux (psum 2 s 1 ) Sediment Flux (kgm 1 s 1 ) Spring Neap Spring Neap T T T T T T T Total Landward Seaward

8 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 h t u z T1 T2 þ h hu z;t c z;t i þ h t hu z;t c z;t i T6 T7 þ hui h t c z þ h u z c z þ h t u z c z T3 T4 T5 where u ¼ uz; ð tþ and c ¼ cz; ð 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 ¼huiþu z and c ¼hciþc z, where u z and c z are the deviations of the observed values from the depth-averaged values at a particular time. Because of tidal fluctuations, hui ¼hui þu z;t and hci ¼hci þc z;t, where u z,t and c z,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 h t, where h t 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 ð3þ (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 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 (Ri g ) were calculated from the buoyancy frequency (N 2 ) over the vertical shear (S 2 h ) to describe the relative stability of the stratified shear flow: Ri g ¼ N 2 Sh @z 2 2 ; 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 Ri g, normalized by the critical value, Ri c 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 Ri g < 0.25 or log 10 (Ri g /0.25) < 0, the flow is unstable and the turbulence mixing is enhanced, whereas when Ri g > 0.25 or log 10 (Ri g /0.25) > 0, the flow is stable and the mixing is inhibited. [24] During spring tides, the distribution of Ri g shows a better mixed water-column on ebb tides than flood tides. The higher Ri g 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 Ri g water is gradually lifted from the bottom to the surface. In this stably stratified water, the ð4þ 5562

9 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 Ri g. 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 s 1 ) 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

10 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 h with the depth-mean density ¼ 1 D ð Þgzdz; ð5aþ Z h dz; ð5bþ 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 m 3 ) 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 Ri g, 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 m 3 ) 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 s 1, 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

11 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; ð6þ s 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 Z Z tot ¼ 1 ð D w w Þgzdz þ 1 C C gzdz h D h 1 Z s D w C w C gzdz; ð7þ h where w ¼ 1 D R h wdz, C ¼ 1 D R h Cdz, and wc ¼ 1 D R 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 kgm 3, 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 in Figure 7c) or higher turbidity (Julian day 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 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 kgm 3 in the fluid mud layer [e.g., Li and Zhang, 1998; He et al., 2001] from 5 kgm 3 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 s 1 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. 5. 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 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 5565

12 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 Ri g 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 (LP ). 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