Sediment trapping by haloclines of a river plume in ...

Continental Shelf Research 82 (2014) 1–8

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Sediment trapping by haloclines of a river plume in the Pearl River Estuary Jie Ren, Jiaxue Wu n Center for Coastal Ocean Science and Technology (CCOST), School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China

art ic l e i nf o

a b s t r a c t

Article history: Received 7 August 2013 Received in revised form 10 March 2014 Accepted 19 March 2014 Available online 21 April 2014

Sediment trapping by the halocline of a river plume was investigated over a spring-neap tidal cycle in the 2010 dry season in the Pearl River Estuary. Benthic tripod observations and concurrent shipboard measurements were conducted to examine mean and turbulent flows, and sediment distributions. The field observations showed that suspended particles are apparently concentrated on the halocline of the river plume, forming a patchy high-concentration suspension with larger floc sizes. This sediment trapping occurred only on the neap tide when the estuary was highly stratified. An estimation of the gradient Richardson number indicates that stratification suppression is dominant below the halocline, whereas shear-induced instability occurs above the halocline. The turbulent kinetic energy balance demonstrates that the buoyancy flux dominates over viscous dissipation in turbulence destruction. Therefore, the trapping of suspended sediment with large floc sizes on the halocline is induced by both salinity stratification and buoyancy-induced instability. This finding can explain the role of salinity stratification in the mechanism for estuarine turbidity maxima and long-distance transportation of suspended sediment. & 2014 Elsevier Ltd. All rights reserved.

Keywords: River plume Sediment transport Flocculation The Pearl River Estuary

1. Introduction Stratified fluids in estuaries are induced by riverine fresh water discharge and saline-water intrusion. A significant salinity difference between the river plume and the underlying saline wedge can form a prominent vertical gradient, called a halocline. It has been accepted that the halocline can modify the mean flows and cause stratification suppression of turbulent mixing (e.g. Geyer, 1993; Fugate and Chant, 2005). However, the role of haloclines in sediment transport is still an open question in estuarine dynamics, although the saline front was found to play an important role in sediment trapping within the estuarine turbidity maxima (Uncles and Stephens, 1993; Burchard and Baumert, 1998; Geyer et al., 2001). The shear instability of the halocline and the resulting turbulent mixing in estuaries has been investigated in past years (Kay and Jay, 2003). Geyer and Farmer (1989) found that shear instability in the Fraser River Estuary is most apparent during the ebb tide when strong shear occurs over the length of the salinity intrusion. When the lower layer reverses and the shear between river plume and saline wedge increases, both shear instability and turbulent mixing are concentrated on the pycnocline, rather than in the bottom boundary layer. During flood tides, however, mixing is concentrated at the front of the salinity intrusion. Moreover, Tedford et al. (2009) investigated the occurrence n

Corresponding author. Tel./fax: þ 86 20 84113678. E-mail address: [email protected] (J. Wu).

http://dx.doi.org/10.1016/j.csr.2014.03.016 0278-4343/& 2014 Elsevier Ltd. All rights reserved.

of shear instability in the Fraser River Estuary. They found one sidedness of shear instability that is concentrated either above or below the density interface. Fugate and Chant (2005) showed that upper layer pycnoclines likely inhibit turbulence and bias log profile estimates even at a half-meter above the bed, where the water column is well mixed. The patchy high suspended sediment concentration (SSC) in the surface layer of water column is commonly associated with low-salinity river plumes above the halocline (Syviski et al., 1985; Wright et al., 1990). The SSC fields are patchy, exhibiting hot spots near the surface layer in a small mountainous estuary, and patches of high SSC near the surface were once thought to be prevented from settling by upwelling (Liu et al., 1999). Recently, Ralston et al. (2012) suggested that the along-estuary variation in salinity gradient and stratification occur at bottom salinity fronts at multiple locations within the salinity distribution. However, the specific structure of patchy high SSC and the underlying dynamics associated with salinity fronts are not yet clear. The objective of this paper is to investigate sediment trapping induced by the halocline of the river plume in the Pearl River Estuary. Field observations, including instrumented tripod mooring, will be presented in Section 3. Main results are given in Section 4, including mean flows, boundary layer flows, and the distributions of volume concentration and floc size. The discussions about mean flows and sediment trapping mechanisms appear in Section 5, and the conclusion is given at the end.

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J. Ren, J. Wu / Continental Shelf Research 82 (2014) 1–8

Fig. 1. Study area (a) and the mooring site (b) in the Modaomen River Estuary.

2. Regional setting

3.2. Data processing and calculation methods

The Modaomen Estuary is a main outlet of the Pearl River to the northern continental sea of the South China Sea (Fig. 1a). The estuary is dominated by fluvial forcing, with an annually mean river discharge of 88.39 109 m3. The estuary is separated into two sub-channels, one outlet flowing to the neighboring estuarine bay of Lingdingyang, called the Hongwan channel, and another outlet being a main channel of the Modaomen Estuary. The main channel is approximately 2 km wide, and 5–7 m deep. A river mouth bar is formed near the mouth. The Modaomen Estuary is modulated by a weak tide, with an annual mean tidal range of 1.08 m at the tidal gauging station of Sanzhao, offshore of the river mouth, and 0.83 m at the station of Denglongshan within the mouth (Fig. 1b). A reversing tidal current predominates in the estuary, with an ebbing current being stronger than the flood current. Waves become apparent offshore of the river mouth bar, with an annual mean wave height and period of 1.2 m and 5.49 s, respectively. The swell-dominated waves are southeast directed, and they are usually stronger in the winter than in the summer.

The ADV-based raw data needs to be pre-processed before turbulent parameters are estimated (for details see Wu et al. (2011)). A high-pass filtering was performed to cancel the low-frequency disturbance, and the phase-space thresholding method developed by Goring and Nikora (2002) was used to remove spikes from the velocity time series. Statistical distributions of turbulent velocity components and their second-order moments showed typical behavior of tidal BBL turbulence as described by Gross and Nowell (1985) and Kim et al. (2000). The shear production is defined as P ρ0 u0 w0 ∂U=∂z, the viscous dissipation D ρ0 ε, the buoyancy flux B g ρ0 w0 , where (U, ρ0 ) are mean velocity and fluid density, respectively; ðu0 ; w0 ; ρ0 Þ are the fluctuations of streamwise and vertical velocity, and density, respectively; g is the acceleration of gravity. The inertial dissipation technique was applied to estimate the rate of dissipation of turbulent kinetic energy (TKE) per unit mass (ε) (see Liu et al., 2009). Stratification and mixing arecritical issues in estuarine dynamics. The bulk stability of stratified flows can be described by the Brunt–Väisälä frequency or buoyancy frequency

3. Methodology 3.1. Instrumentation Field observations in the Modaomen Estuary were conducted from 15 to 24 December, 2010. A mooring survey marked as M2 (Fig. 1b) was continuously conducted over a spring-neap tidal cycle. An instrumented tripod was deployed at mooring site M2. The instrumental set was fastened to the bottom-mounted frame, including one Sontek Pulsed Coherence Acoustical Doppler Profiler (PC-ADP) with a central frequency of 1.5 MHz, two Nortek 100–200 Hz threedimensional (3D) point Acoustical Doppler Velocimeters (ADV), one 1 Hz XR-420 CTD, and two XR-620 OBS. In addition, shipboard profiling surveys were conducted on a mooring vessel close to the tripod. A Laser In-Situ Scattering and Transmissiometry (LISST-100B) was lowered vertically from surface to bottom. The LISST with an optical range of 5 cm, was used to observe volume concentration and fractional composition of flocs. The shipboard Aquadop profiler Nortek ADP with a frequency of 2 M was sampled every 5 min. for an average interval of 90 s with a bin size of 10 cm.

N2 ¼ ðg=ρÞð∂ρ=∂zÞ;

ð1Þ

where ∂ρ=∂z is the vertical density gradient, g is the gravitation acceleration. The dynamic stability of stratified shear flows is expressed by the gradient Richardson number: Rig ¼ N 2 =S2 ð2Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 where S ¼ ð∂u=∂zÞ þ ð∂v=∂zÞ is the mean shear, u and v are the streamwise and spanwise velocity, respectively. Classical theories (Miles, 1961, 1963) and experimental data (e.g. Tedford et al., 2009) showed that Rig o 0:25 indicates an unstable regime with intense mixing, and Rig 40:25 represents a stable regime where stratification may suppress turbulence.

4. Results 4.1. Properties of estuarine hydrology and suspended sediment The field survey at site M2 covers a cycle from neap to spring tide (Fig. 2a), with average tidal ranges being 0.69 m, 1.03 m and 1.27 m during the neap, moderate, and spring tides, respectively. The tidally oscillating variation of salinity in the near-bed layer, 0.25 m above the


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During the ebb tide, the mean flows are wholly seawards over the water column. During the flood tide, however, a two-layer exchange flow is dominant with an occasional occurrence of a three-layer exchange flow. The surface flows in the upper layer are seaward, roughly agreeing with the direction of winds and ebb currents, whereas the flows in the middle and bottom layers are landward. During early flood tide, flood currents appear landwards in the bottom layer, and then propagate upwards with the flood tidal phase, till they are significantly suppressed by stronger northern winds. The two-layer exchange flow during the flood tide is primarily due to the combined forcing of wind stresses and baroclinic pressure, which will be discussed in great detail in the following text. 4.3. Halocline and mid-depth concentration peak During the neap tide in the survey period, salinity-induced stratification occurs in the flood tide, while the well-mixed waters appear in the ebb tide (Fig. 5). The haloclines with a vertical thickness of 1–2 m are roughly distributed at 4–6 m below the surface (Fig. 5a–d). During the initial 8 h over the tidal cycle (Fig. 2b), the haloclines persist steadily with a salinity difference of 15 psu. An apparent peak of volume concentration appears on the halocline (Fig. 5a–d), forming a locally highconcentration patch in the mid-depth column. No mid-column concentration peak occurs in the weakly-mixed profiles (Fig. 5e–h). A good agreement in height of the halocline and the mid-depth concentration peak occurs only in the flood neap tide over the spring-neap tidal cycle, demonstrating sediment trapping induced by the halocline. 4.4. Floc size distributions Fig. 2. Time series of (a) tidal level above the Zhujiang datum corresponding to the zero point in the plot, (b) salinity, and (c) wind vectors at mooring site M2 from neap to spring tide from 15 to 24 December 2010.

bed, is significant, and its amplitude decreases from neap to spring tides (Fig. 2b). A burst of strong northward wind occurred in the neap tide, with mean wind velocity being approximately 10 m/s (Fig. 2c). These strong winds may significantly modify the vertical structures of velocity and saltwater intrusion. Time series of vertical distributions of axial velocity, salinity, volume concentration, floc size and gradient Richardson number demonstrate some distinct properties of flows, salinity and sediment over the neap to spring tidal cycle (Fig. 3). A highly-stratified regime appears during the neap tide, whereas a well-mixed regime occurs during the spring tide (Fig. 3b). During the strong current period of the flooding neap tide (Fig. 3a), higher concentration flocs with larger sizes appear in the mid-depth halocline, rather than in the upper or lower layer (Fig. 3b–d). During the moderate and spring tides, however, the halocline is apparently weakened and persists for a shorter period, and the volume concentration appears higher during the low-water stage, and the floc sizes are vertically uniformly distributed. The gradient Richardson number indicates the distinct stability of stratified shear flows during the neap and spring tides (Fig. 3e). During the neap tide, a stable regime appears below the halocline, and an unstable regime occurs above the halocline. During the spring tide, a stable regime occurs in the ebb tide, and an unstable regime occurs in the flood tide. These results clearly indicate that suspended sediment trapping by the halocline does not occur in the moderate and spring tides, but only in the neap tide when intense stratification is induced by salinity intrusion.

Typical profiles of volume concentration and salinity (Fig. 5) indicate the evident appearance of the mid-depth concentration peak and the halocline during the flood tide, but no occurrence during the ebb tide. Fig. 6 shows typical compositional distributions of floc fractional sizes under intense stratification during the flood tide (Fig. 6a–f) and well-mixed conditions during the ebb tide (Fig. 6g–l). Three depths represent three layers of the water column, i.e. the surface layer, mid-depth or on the halocline, and the bottom layer, respectively. The compositional curves of floc sizes can be used to estimate the statistics of grain-size mixture, and also to infer the relative importance of flocculation versus deflocculation in the estuary (Wu et al., 2012). Under intense stratification during the flood tide (Fig. 6b and e), the compositional curves are negatively skewed and the flocculation is dominant over deflocculation, forming large flocs. In the mixing layers (Fig. 6a, c, d, and f), however, the compositional curves are flat, and there is a dynamic equilibrium between flocculation and deflocculation. Under the well-mixed regime during the ebb tide (Fig. 6g–l), the compositional curves are all negatively skewed and flocculation is predominant over the whole water column. Therefore, the halocline induced by salinity stratification can change the fractional composition of the floc mixture by modifying the relative role of two contrasting processes of fine-grained sediment particles: flocculation and deflocculation. This forcing can enhance the deflocculation in the upper and lower mixing layers, and promote the flocculation on the halocline.

5. Discussions

4.2. Exchange flows

5.1. The influences of winds and stratification on mean flows

Mean flows during the neap tide were examined to understand flow structures for sediment trapping by the halocline (Fig. 4).

The analytic solutions of Hansen and Rattray (1965) will be applied to examine the influences of winds on mean flows and

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Fig. 3. Times series of (a) axial (along-channel) velocity (m s 1), (b) salinity (PSU), (c) volume concentration ðμl=lÞ, (d) floc size ðμmÞ, and (e) gradient Richardson number at mooring site M2. A positive velocity in (a) indicates a flood (up-estuary) current, and a negative one represents an ebb (down-estuary) current. The profiles marked H1 to H4 and M1 to M4 in (b) will be denoted as those typical ones in Figs. 5 and 6.

stratification. Assuming a simple momentum balance between pressure gradient and frictional force, Hansen and Rattray (1965) derived an analytic solution of mean velocity in a steady partiallymixed estuary under surface winds: 1 gh ∂ρ=∂x 3 1 τw h ð1 9η2 þ 8η3 Þ þ u0 ð1 η2 Þ þ ð1 4η þ 3η2 Þ; 48 2 4 ρK ρK ð3Þ 3

uðzÞ ¼

where ρ is water density, ∂ρ=∂x is horizontal pressure gradient induced by fluid density, g is gravitational acceleration, h is water depth, η ¼ z=h represents relative water depth, K is vertical eddy viscosity, uðzÞ and u0 are mean velocity and depth-averaged velocity due to the freshwater discharge, and τw is the wind-

induced surface stress. The analytic solution of velocity (Eq. (3)) is composed of three parts: horizontal gradient of density forcing, river discharge forcing, and wind-induced frictional forcing. This solution depicts the possible influences of winds on estuarine circulation and salinity distributions. Fig. 7a and b shows the vertical profiles of velocities derived from the analytic solutions under different wind stresses and baroclinic pressures. The two-layer exchange flows may be formed in the flood tide under the combined forcing of horizontal gradient of density and seaward winds. Wind stresses can cause significant adjustment on the exchange flow. Seaward winds can stop the surface flood currents or even change their direction to the seaward (Fig. 7a), thus intensifying the gravity circulation. This mechanism will enhance the stratification of the river plume.


The wind stresses can also form three-layer exchange flows, as demonstrated in Fig. 7b. An evident difference between analytic solution and data exists (Fig. 7c). This indicates that another forcing besides baroclinic pressure and winds can modify the velocity profile. Both bottom friction and unsteadiness of tidal forcing may have a significant role in the momentum balance. However, highly-stratified fluid may decrease the flow velocity, and even change the flow direction in the sub-surface or near-bed layer (Fig. 7c).

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5.2. Turbulence mixing on the halocline The first typical halocline in the neap tide appeared 2–4 m above the bed, high over the top of the tripod. Several subsequent haloclines occurred approximately 1 m above the bed, within the observational range of the tripod instruments, so that turbulence mixing and stratification on the halocline can be observed. Fig. 8 shows time series of dissipation, axial velocity and the salinity difference between surface and bottom layers. A semi-diurnal tide oscillation of dissipation is evident, which indicates the driving force of turbulence by tides. Turbulent mixing under intense stratification is small compared with that under well-mixed conditions. This is the well-known effect of turbulence suppression by stratification. However, high-concentration suspensions with large flocs concentrated on the halocline of the river plume occur, instead of in the lower layer. The depression effect of salinity stratification on near-bed turbulence generation has been examined (Geyer, 1993; Geyer et al., 2001), and the role of salinity stratification (the freshwater–saltwater interface) in the ETM sediment trapping was identified (Uncles and Stephens, 1993). These widely recognized arguments, however, cannot be applied to completely understand this phenomenon of locally high concentration suspension in the mid-depth column. The examination of the turbulent kinetic energy (TKE) balance may provide another important mechanism for the trapping effect of sediment suspension in the halocline.

5.3. Turbulent kinetic energy balance

Fig. 4. Velocity profiles under strong northern winds at site M2 in the neap tide. A positive velocity indicates a flood (up-estuary) current, and a negative one represents an ebb (down-estuary) current.

For tidally energetic stratified flows in estuaries, the time derivative and the diffusion terms are usually small in relation to the production (P), buoyancy (B) and dissipation (D), except for short transients at the onset of ebb and flood (Rippeth et al., 2003;

Fig. 5. Typical profiles of suspended sediment volume concentration ðμl=lÞ and salinity (PSU) demonstrating the mid-depth concentration peak and the halocline. The profiles (H1–H4) denote the highly-stratified conditions in the presence of haloclines, and the profiles M1–M4 represent the well-mixed conditions in the absence of haloclines.

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Fig. 6. Typical histogram of floc particle sizes (percentage versus phi) and their Gaussian fit (solid curves). Profiles H1 and H2 represent the highly-stratified cases, while Profiles M1 and M2 indicate the well-mixed cases. The left, middle and right histograms at H1 and H2 denote the distribution roughly in the surface layer, halocline, and bottom layers, respectively.

MacDonald and Geyer, 2004; Peters et al., 2005; Liu et al., 2009; Wu et al., 2011). Under the assumption of local TKE equilibrium, as a first-order approximation, we expect P ¼ B þ D. Based on field observations by ADV (Fig. 9), we obtain D ¼0.21P during the neap tide, in which D ¼0.17P when sediment trapping is the most significant under intense stratification. The flux Richardson number Rf ¼ B=P represents the fraction of TKE that is converted into potential energy through mixing against the density gradient. Therefore the values of Rf are 0.79 in the neap tide and 0.83 under intense stratification, respectively. These values are much larger than that (Rf ¼ 0.47) under well-mixed conditions in the Huangmaohai Estuary of the Pearl River Estuary (Liu et al., 2009). These estimates indicate that buoyancy flux is a major contribution to the TKE balance, compared with viscous dissipation. The support mechanism for suspended load is turbulence. In the classical Rouse's theory of diffusion, sediment suspension was assumed to be induced by gradient diffusion. The present

observations indicate that the buoyancy flux should be another important source for sediment suspension. This may be the mechanism of sediment trapping on the halocline, whose energy comes from the buoyancy flux. Time series of dissipation (Fig. 8) indicates that turbulent mixing becomes weaker under intense stratification, and it becomes stronger for vertically uniform flows. In a word, turbulent mixing induced by the strong buoyancy flux, rather than viscous dissipation, is the TKE mechanism for sediment trapping on the halocline.

6. Conclusions Sediment trapping by the halocline of a river plume was examined from the neap to spring tidal period in the Pearl River Estuary. Flows and sediment transport during the neap tide were concentrated on in this study, when both a mid-column concentration peak and halocline


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Fig. 7. Theoretical curves of velocity derived from the analytical solutions of Hansen and Rattray (1965). (a) Velocity profiles affected by various density gradients (0, 0.05, and 0.05) and wind stresses (0, 0, and 0.25) for the corresponding curves (1 to 3); (b) velocity profiles affected by winds with different magnitudes and directions ( 1, 0.6, 0.3, 0, and 0.2) for the corresponding curves (1 to 5); (c) comparison between theoretical (dashed) and observational (solid) curves of velocity.

Fig. 8. Time series of the dissipation rate, axial velocity and salinity difference between surface and bottom layers at mooring site M2. The dissipation was estimated by ADV.

Fig. 9. Comparison between production and dissipation (a) during the neap tide and (b) under intense stratification.

appear. Field observations indicate that high concentration occurs on the halocline in the neap tide when intense stratification occurs. Density stratification during the neap tide can decrease the flow velocity in the upper and lower layers. The halocline induced by salinity stratification can change the fractional composition of the floc mixture by modifying the relative role of flocculation and deflocculation. This forcing can enhance the deflocculation in the upper and lower mixing layers, and promote the flocculation on the halocline.

The local TKE balance indicates that the buoyancy flux is a major contribution to turbulence destruction, compared with viscous dissipation. Therefore, the trapping of suspended sediment with large floc sizes on the halocline is induced by both intense stratification and buoyancy-induced instability. This finding of sediment trapping by the halocline will have an important implication in the dispersal of suspended sediment in the estuaries and even on the continental shelf sea. It is an important mechanism for sediment trapping in the

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estuarine turbidity maxima, and is probably a critical process for a long-distance transportation of riverine sediments off the river mouth.

Acknowledgments Great thanks to Yun Bao, Xiaoling Yin, Rikui Yang for their help in field observations. This work was jointly supported by National Basic Research Program of China (2013CB956502), National Natural Science Foundation of China (41276079, 41176067, and 40976053), Water Conservancy innovation Research Project of Guangdong province in 2009 (2011370004209292), and the Younger Teacher Cultivation Project of Sun Yat-Sen University (10lgpy06). References Burchard, H., Baumert, H., 1998. The formation of estuarine turbidity maxima due to density effects in the salt wedge. A hydrodynamic process study. J. Phys. Oceanogr. 28 (2), 309–321. Fugate, D.C., Chant, R.J., 2005. Near-bottom shear stresses in a small, highly stratified estuary. J. Geophys. Res. 110, C03022, http://dx.doi.org/10.1029/ 2004JC002563. Geyer, W.R., Farmer, D., 1989. Tide-induced variation of the dynamics of a salt wedge estuary. J. Phys. Oceanogr. 19, 1060–1672. Geyer, W.R., 1993. The importance of suppression of turbulence by stratification on the estuarine turbidity maximum. Estuaries Coasts 16 (1), 113–125. Geyer, W.R., Woodruff, J.D., Traykovski, P., 2001. Sediment transport and trapping in the Hudson River estuary. Estuaries 24 (5), 670–679. Goring, D.G., Nikora, V.I., 2002. Despiking acoustic Doppler velocimeter data. J. Hydraul. Eng. 128 (1), 117–126. Gross, T.F., Nowell, A.R. M., 1985. Spectral scaling in a tidal boundary layer. J. Phys. Oceanogr. 15, 496–508. Hansen, D.V., Rattray, J.M., 1965. Gravitational circulation in straits and estuaries. J. Mar. Res. 23, 104–122. Kay, D.J., Jay, D.A., 2003. Interfacial mixing in a highly stratified estuary 1. Characteristics of mixing. J. Geophys. Res. 108, 3072, http://dx.doi.org/ 10.1029/2000JC000252.

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