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River-sea transitions of sediment dynamics: A case study of the tide-impacted Yangtze River estuary
Estuarine, Coastal and Shelf Science 196 (2017) 207e216
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River-sea transitions of sediment dynamics: A case study of the tide-impacted Yangtze River estuary H.F. Yang a, b, S.L. Yang a, *, K.H. Xu b, c a
State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, China Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA c Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2016 Received in revised form 2 July 2017 Accepted 6 July 2017 Available online 8 July 2017
Hydrodynamics and sediment dynamics vary greatly in tide-dominated estuaries worldwide, but there is a paucity of data of large tide-dominated estuary systems due to difficulties of observation in a large spatial scale. In this study, we investigate sediment dynamic transitions in a 660-km long section between the tidal limit and mouth of the Yangtze River. We found that tidal effects are almost undetectable in the uppermost 100-km section, but the mean tidal range gradually increases downstream to nearly 3 m at the river mouth. Flow is generally unidirectional in the uppermost 400-km section, although its velocity changes in response to flood/ebb tidal dynamics; in the lowest 250-km section, flow is bidirectional, and ebb flow durations decrease towards the sea. In the lowermost 100 km, the ebb flow durations decreases to below 60%, and the flow is dominated by tidal currents. Salinity is only detectable in the lowest 100-km section due to the dominance of Yangtze River water discharge. Bed sediments mainly include sand in the uppermost 500-km section, whereas mud dominates in the remaining areas. In contrast, the median grain size of the suspended sediments was found to be greater in the lowest 100km section (8e13 mm) than in the upper sections (5e6 mm) due to strong exchanges between suspended and near bed sediments. The suspended sediment concentration (SSC) was found to be low (<0.1 g/L) and homogenous in the uppermost 100-km section, downstream of which the SSC increased rapidly to >1 g/L and both surface-bottom and intratidal variabilities occurred. The rates of sediment parameter changes were rapid in the river-sea transitional zone, and this zone may shift upstream and downstream in response to the relative contributions of the river, tides and waves. A conceptual model of the river-sea transition of sediment dynamics for the Yangtze estuary was established, and this model shed light on quantitative studies of sediment dynamics in other large tide-impacted estuaries worldwide. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Estuary Suspended sediment concentration Grain size River-sea interaction Yangtze estuary
1. Introduction Estuaries are popular sedimentary environments where rivers meet oceans. Physical, geological, chemical and biological processes in estuaries are generally more complex and dynamic than other areas on the Earth's surface (Holligan and Boois, 1993). Fluvial discharges, tides and waves are three main forces that affect estuaries. It is well known that the head of a estuary is controlled by the river, the main estuary in the middle experiences mixed energy from both the river and ocean, and the mouth in the lower section is mainly controlled by the ocean that is involved (Williams et al.,
* Corresponding author. E-mail address: [email protected] (S.L. Yang). http://dx.doi.org/10.1016/j.ecss.2017.07.005 0272-7714/© 2017 Elsevier Ltd. All rights reserved.
2013). The suspended sediment concentration (SSC) in the estuary is highly affected by catchment activities, such as soil leaching, floods, deforestation and forestation and dam constructions (Pont et al., 2002; Rothe et al., 2002; Yang et al., 2002, 2011). Oceanic forcings like tides, waves and wind can cause changes in the SSC through the process of mixing and resuspension in the estuarine area (De Jorge and Van Beusekom, 1995; Gensac et al., 2016). These processes play a key role in sedimentary processes, morphodynamics, and ecological systems and should also be taken into consideration in engineering construction (Newcombe and MacDonald, 1991; El-Asmar and White, 2002; Yang et al., 2011). Over the decades, many studies have been conducted on geomorphic processes in estuarine and coastal regions (e.g., Widdows et al., 2000; Andersen et al., 2005; Yang et al., 2008; Grabowski et al., 2011; Liu et al., 2014). Wright and Nittrouer
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(1995) proposed four stages of sediment dispersal in estuaries and adjacent coastal areas: supply via river plumes, initial deposition, resuspension and transport, and long-term net accumulation. Gibbs (1976) reported that Amazon River sediments are transported from its estuary to the continental shelf, but mud components can be separated and carried landward by onshore bottom currents. Fluvial water and sediment discharges, suspended particulate matter concentration and oceanic forcings were closely related to the formation and evolution of mud banks on the Amazon-dominated coast (Anthony et al., 2013; Gensac et al., 2016). For example, increases in riverine sediment resulted in a 150 km-long area of accretion, and water discharge from major rivers greatly affected the migration rates of these mud belts (Gensac et al., 2016). Estuary sizes vary considerably around the world, generally ranging from <1 km to >100 km (Dyer, 1997). Surveys of longitudinal transects along estuary head, main and mouth areas can provide valuable information on hydrodynamics and sediment dynamics. In tide-dominated estuaries, longitudinal transects can be surveyed hourly to capture temporal and spatial variations. These types of surveys can be applied for small estuaries such as the York River in the Chesapeake Bay and the Tagus, the Seine and the Mondego rivers in Europe (Avoine et al., 1981; Vale, 1990; Brotas and Plante-Cuny, 1998; Lillebø et al., 1999; Le Hir et al., 2001; Friedrichs, 2009). These types of surveys, however, are not practical for large tide-dominated estuaries (such as, the Amazon and Yangtze Rivers), as they are >500 km in length. Logistically it is difficult to use one research vessel to finish even one longitudinal survey during a single tidal cycle (~12 or 24 h). Although many studies have been performed on large river systems, there is still a paucity of data on hydrodynamics and sediment dynamics of large tide-dominated estuarine systems (e.g., Amazon, Yangtze, Ganges, and Mekong). Several studies have been conducted on the Yangtze River and its dispersal system over the past 20 years, but most of them have focused on either the 200 km-long funnel-shaped Yangtze River mouth only (Milliman et al., 1985; Li et al., 2012b; Liu et al., 2014) or the river drainage basin itself (Chen et al., 2007; Xu and Milliman, 2009; Luo et al., 2012). Although it is well known that river-ocean interactions occur along the low-gradient 660 km-long section of the Yangtze River, systematic and comprehensive measurements of this area are still limited, partially due to the aforementioned logistics challenges. In this study, we focus on the entire 660 km-long Yangtze estuary from Datong Station to the Yangtze subaqueous delta and study the interactions among the river, tides, waves, and SSC levels. We strive to filling in this knowledge gap by studying: (1) river flow speeds and directions, (2) suspended sediment concentrations, and (3) sediment grain sizes for the entire Yangtze estuary. We perform systematic and labor-intensive hourly surveys and samplings over more than one semi-diurnal tidal cycle (about 13e14 h) at nine stations. Our study sheds light on the quantitative studies of tide-dominated large river estuarine systems and our datasets can be compared with those of other tide-dominated rivers worldwide. With a length of ~6.3 103 km, the Yangtze River is the longest in Asia and is the third longest in the world. It originates from the Qinghai-Tibet Plateau, flows eastward, and debouches to the East China Sea. The Yangtze ranks fifth globally in terms of water discharge (~900 km3/yr) and fourth in terms of historical sediment load (~470 Mt/yr, Million tons per year) (Milliman and Farnsworth, 2011). In estuarine systems, the tidal limit is defined as the site at the most landward extent of spring tides during the dry season where the tidal range is null; the flood tidal current limit is defined as the farthest limit that a flood tide current (opposite to the dominant river flow direction) can reach. Datong Station (Fig. 1) is a
tidal limit station that is used to carry out water discharge and sediment load measurements for this study. 2. Study area The estuary of the Yangtze River is defined as a ~660 km-long section from Datong Station to the subaqueous delta at 30e50 m isobaths (Shen et al., 2003). Xuliujing Station is a key site that separates the estuary into two distinct parts (Fig. 1). From Datong to Xuliujing, the river channel is elongated (~510 km) and narrow and the river flows are fast (~0.56 m/s). Downstream from Xuliujing, the Yangtze River transforms to a broad and bifurcated estuary with four major outlets. Seaward of the river mouth (~90 km wide), subaqueous sand ridges form as a result of energetic tidal currents (Shen and Pan, 2001). At the seaward-most station in Jiuduansha (Fig. 1), mean and maximum tidal ranges are 2.7 and 4.6 m, respectively (GSICI, 1996). Wind-driven waves and swells are two major types of waves found in this estuary, and wave energy levels increase rapidly seaward. Long-term mean wave heights increase from 0.2 m at Gaoqiao Station (in the middle of the bifurcated estuary) to about 1.0 m at Yinshuichuan Station (~60 km seaward of Gaoqiao; Fig. 1) (Yang, 1999). During its geological development, the Yangtze Delta has shifted its depocenter southeastward since the 18th century. By the 1950s, >98% of Yangtze River discharge had entered the sea through the South Branch (Chen et al., 1985). Yangtze sediment loads into the East China Sea are mainly composed of suspended sediment (>99%) (Yang et al., 2002). When suspended sediment is transported downstream to the estuary, an Estuarine Turbidity Maximum (ETM) zone forms in the mouth bar area due to flocculation and canceling energy between the river and the sea (Jiang et al., 2013). Semidiurnal ebb currents are generally faster than flood currents. The depth-averaged velocity is ~1.4 m/s and the peak tidal current velocity can reach 2 m/s at the river mouth (Chen et al., 2014; Hu et al., 1988), which is generally comparable to that of the Amazon River (1e2 m/s) (Gensac et al., 2016). Over recent years, as a result of anthropogenic changes (particularly with the construction of the Three Gorges Dam -TGD) (Yang et al., 2015), the SSC at Datong Station decreased from 0.62 g/ L in 1958 to 0.35 g/L in 2000 (Yang et al., 2003) and then to only 0.17 g/L during the post-TGD period (after 2003). Numerous human activities have also happened in the Yangtze basin and estuary (e.g., water reservoir construction, sand mining, navigational dredging, shoreline protection and coastal reclamation) (Yang et al., 2006; Li et al., 2011; Jiang et al., 2012). It is likely that sediment dynamic processes have changed due to a variety of human activities. In this study, most observations were conducted in April of 2013, April of 2014 and October of 2015, when water discharge levels were close to the mean water discharge level of the past 65 years (averaged at 28,300 m3/s). 3. Materials and methods In this study, fieldwork was conducted at sites G and I in April of 2013, at sites A, B, C, D, E and H in April of 2014 and at site F in October of 2015 (Fig. 1; Table 1). It is well known that the river's effects are more prominent in JulyeAugust and that wind-driven waves play a more central role in JanuaryeFebruary (Chen et al., 2004; Xu and Milliman, 2009). To minimize the impact from seasonal variation, observations of all sites were made in April and October, during which river discharge and wind energy levels are both at moderate levels. To be specific, fluvial discharges in these two months are close to the mean water discharge, usually between 20,000 and 30,000 m3/s (See Table 1). These values differ from that in dry (~10,000 m3/s) and flood season (~50,000 m3/s). Thus our
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Fig. 1. Study area and observation stations in the Yangtze estuary from Datong Station to the East China Sea. SS is Sheshan and JDS is Jiuduansha.
Table 1 Datasets for the field observations. Observation site Downriver distance from Datong (km)a Dates of observation Tidal phase Mean water depth (m) Tidal range (m) Water discharge in Datong (m3/s) A B C D E F G H I a
60 120 260 410 510 560 590 620 660
9-10 April 2014 8-9 April 2014 10-11 April 2014 6-7 April 2014 5-6 April 2014 20-21 October 2015 11-12 April 2013 13-14 April 2014 10-11 April 2013
neap neap neap mean mean neap spring mean spring
8.3 10.5 9.4 11.9 15.6 12.3 19.3 13.0 7.6
0.1 0.2 0.6 1.6 2.0 1.2 2.5 2.7 2.5
19,200 18,900 19,000 18,000 17,150 26,800 22,850 17,750 20,800
Datong is the tidal limit of the Yangtze River.
study approximately represents a long-term averaged condition of Yangtze estuary. During each fieldwork period, observation was made during at least one full semidiurnal tidal cycle (13e14 h). Observation sites were selected carefully, and key factors taken into account were flood tidal current limits, tidal limits, water depths, tidal ranges (Table 1), geomorphology features and spatial distributions. Daily water discharge was downloaded at the web site at http://yu-zhu.vicp.net, which is managed by the Changjiang (Yangtze) Water Resources Commission. To ensure that local water discharge levels corresponding to each observation are accurate, the time span of water discharge at the observation sites relative to
that at Datong Station was applied in calculations based on the measured velocity and distance. During each observation period, a 600-kHz Acoustic Doppler Current Profiler (ADCP) (Teledyne RD Instruments, Inc., Poway, California, USA) and an Optical Back Scattering (OBS) sensor (OBS3A, D & A Instrument Company, Washington, USA) were used to measure the hourly velocity and turbidity profiles, respectively. The ADCP was equipped with four flow sensors with a 0.5 m bin size and a blanking distance of 0.25 m. The Valeport-106 flow sensor (Valeport Ltd, Devon, UK) was used to measure the velocity and direction of the water surface layer to augment ADCP
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measurements. Current data collected using the ADCP (subsurface) and Valeport-106 (surface) were combined and then normalized according to the ratios of mean water discharge levels to actual water discharge levels and mean tidal ranges to measured tidal ranges at each site (Table 2). Two replicate samples were collected using 600-ml water bottles at six depths (the surface, 0.2H, 0.4H, 0.6H, 0.8H, and the bottom, where H is the depth of the water column) of each site and were used to measure SSC and particle size values in a laboratory at East China Normal University. In addition, river channel bed and seabed surficial sediments were collected from all nine sites. Water samples were filtered through 0.45-mm filters, dried and weighed following the method of Liu et al. (2014) to calculate SSC values. OBS calibrations were also performed in the laboratory to determine SSC values, and they were then compared with SSC values using water samples at six depths. Sediment grain sizes were analyzed using a Laser Diffraction Particle Size Analyzer, BeckmanCoulter Ls100Q (Beckman Coulter Inc., California, USA).
channel. The mean velocities of ebb tidal currents were also faster than flood tides at each site, and the differences varied from 0.15 to 0.25 m/s (Table 2). Mean water depths decreased from site G (19.3 m) to site I (7.6 m) as the river spread over the broad Yangtze mouth bar area (Figs. 1 and 2). At site I, the current rotation was clearly clockwise while shifting from flood to ebb tides (Fig. 2). 4.2. Sediment grain size 4.2.1. Bed sediment Bed sediments showed an overall downstream-fining trend from site A (~60 km downstream from Datong) to site I (~660 km downstream from Datong) (Fig. 3b). Sand content levels were found to peak at site A (~90%), and the lowest levels were found at site G (~4%) (Fig. 3a). The average D50 (median grain size) of riverbed sediment in five sites (A-E) was 164.1 mm (fine sand) but decreased to only 24.4 mm (medium silt) in four sites (F-I). This was especially the case in the section between sites E and F, where D50 decreased from 128.0 to 9.5 mm (Table 3; Fig. 3b).
4. Results 4.1. Hydrodynamics Approximately 60 km downstream from Datong, site A showed steady unidirectional currents without notable changes with water depths. The difference between surface and bottom water velocities here is small, generally <0.15 m/s (Fig. 2). The mean flow velocity after normalization under mean water discharge was recorded as 0.68 m/s (Table 2). Small fluctuations in water surface elevation (0.4 m between the peak and trough) occurred at site B, but this was still dominated by unidirectional flows moving to the east (Fig. 2). Compared with patterns found in site A, water velocity in site B decreased by 30% to 0.47 m/s (Table 2). River water flowed eastward in the same direction northeast in site C, but a low flow velocity period from hour 11 to 13 was found during the observation period. The velocity averaged at 0.28 m/s over hours 11e13 but was recorded as ~0.54 m/s for the rest of the period (Fig. 2). Slow currents (0.18 m/s) formed from hour 3 through hour 6 in site D, and a reversed current formed during this period. Except for this reversal period, water consistently flowed eastward at an average speed of 0.76 m/s (Fig. 2). Downstream from site E, current reversals were obvious (Fig. 2). Ebb tides in sites E, F, G, H and I occurred for approximately 8.2, 7.3, 7.1, 7.0 and 6.8 h, respectively, and thus were maintained for much longer periods than flood tides (4.3, 5.2, 5.4, 5.5 and 5.7 h, respectively) (Table 2), denoting the presence of strong tidal asymmetries. The ebb tide direction ranged from 100 to 135 flowing southeast, and the flood tide direction ranged from 281 to 313 flowing northwest, generally following the orientation of river Table 2 Normalized flow velocities and durations of tidal phases. Site
A B C D E F G H I
Ebb tide
Flood tide
Velocity (m/s)
Duration (hr)
Velocity (m/s)
Duration (hr)
0.68 0.47 0.49 0.60 0.74 0.57 0.98 1.10 1.15
12.5 12.5 12.5 12.5 8.2 7.3 7.1 7.0 6.8
0 0 0 0 0.56 0.36 0.77 0.87 0.85
0 0 0 0 4.3 5.2 5.4 5.5 5.7
Flow velocities are depth-averaged. The normalized flow is the flow occurring over the long-term averaged water discharge and tidal range.
4.2.2. Suspended sediment The depth-averaged D50 of suspended sediments in sites A, C, F, G, H and I averaged at 5.5, 5.3, 6.2, 8.0, 8.7 and 12.8 mm, respectively. Clay and silt made up >90% of the suspended sediments (Fig. 3c). Median sizes changed little with increasing depth in sites A, C and F (Fig. 3d), and the differences between the maximum and the minimum roughly ranged from 0.3 to 0.6 mm. Median sizes in sites G, H and I showed a clear downward coarsening pattern (Fig. 3d) with ratios increasing from 1.0 to 2.2 (Table 3). Differences in the D50 between the water surface and near-bed suspended sediment were 3.0, 6.9 and 8.8 mm (Table 3). 4.3. Suspended sediment concentration Based on data derived from the water samples, we found no dramatic vertical change in SSC values for sites A, B, C, D, E and F, indicating a well-mixed environment. Standard deviations of the entire tidal cycle for each depth in these sites were also found to be negligible (Fig. 4). However, the SSC value and standard deviation in sites H and I increased dramatically with an increase in water depth (Fig. 4), and the difference between the water surface and near-bed layers reached 0.55 and 1.10 g/L (Table 3), respectively. These layers formed a smooth concave-upward profile that fits a logarithmic formula (R > 0.96) (Fig. 4). Based on the data derived from the OBS sensors, mean depthaveraged SSCs for the entire tidal cycle from sites A to I were recorded as 0.072, 0.069, 0.068, 0.083, 0.090, 0.074, 0.190, 0.273 and 1.107 g/L. The SSC value of the Yangtze mouth bar area (site I) was higher than that of the inner estuary (sites G and H) by more than one order of magnitude, and this was also the case for those of the river (site A, B, C, D, E and F). SSC values for sites A and B were mainly homogenous, intratidal temporal variations were found in site C, and intratidal changes became pronounced in each site downstream from site D (Fig. 4). Maximum depth-averaged SSC values for all of the observations were found during the first hour in site I at a value of 2.353 g/L. SSC values of the bottom layer were always greater than those of the surface layer, and the maximum difference between these two layers was recorded as 2.587 g/L at hour 4 in site I. 5. Discussion 5.1. Longitudinal current variations Water levels remained relatively stable in site A but varied in
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Fig. 2. Time series variations of current vectors. Arrows represent both current magnitudes (length) and directions (upward being north and right being east). Details are shown for Station D.
Fig. 3. Percentages of sand, silt and clay in (a) bed surficial and (c) suspended sediments; (b) median grain sizes along the Yangtze estuary from Datong Station to the East China Sea; and (d) vertical profiles of tide-averaged median size for suspended sediments.
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Table 3 Suspended sediment concentrations and median sizes of bed and suspended sediment. Site Median bed sediment size (um)
Suspended sediment concentration (g/L) Depthaveraged
Surface layer
Near-bed layer
Ratio (near-bed/ surface)
Depthaveraged
Surface layer
Near-bed layer
Ratio (near-bed/ surface)
A B C D E F G H I
0.072 0.069 0.068 0.083 0.090 0.074 0.190 0.273 1.107
0.068 0.051 0.064 0.066 0.057 0.068 0.122 0.100 0.616
0.081 0.077 0.089 0.087 0.119 0.083 0.256 0.647 1.720
1.2 1.5 1.4 1.3 2.1 1.2 2.1 6.5 2.8
5.5
5.6
5.6
1.0
5.3
5.1
5.3
1.0
6.2 8.0 8.7 12.8
5.9 6.2 5.6 7.6
6.5 9.2 12.5 16.4
1.1 1.5 2.2 2.2
220 128 219 125 128 10 8 53 27
Median suspended sediment size (um)
Fig. 4. Vertical profiles of tide-averaged SSC values with standard deviation and regression lines, and time series of SSC values based on OBS data collected from nine sites.
site B (Fig. 2), which likely suggests that the uppermost tidal effect reached somewhere between A and B. Flow directions in the section upperstream of site C were always seaward, whereas flood currents were found in the section downstream of site D (Fig. 2), suggesting that the flood tidal current limit was located between sites C and D. Counteraction and prop forces of the flood tidal current presumedly raised the water level rise and reduced low velocity during flood phase at site C. Considering bidirectional flows at site D, the flood tidal current limit must exist between sites C and D, which is basically in agreement with previous studies (Xu et al., 2012; Hou and Zhu, 2013). The abrupt decrease in current speed found at site F (Table 2) is attributable to the widened river channel in the South Branch and the spreading of energy (Fig. 1). As to the vertical velocity profiles against the height above the seabed, they all follow a quasi-logarithmic form. Velocities at water surface were usually smaller than those at sub-surface, mainly due to the friction between the air and the river water (Liu et al., 2014). 5.2. Longitudinal trends of sediment grain size The minimum suspended sediment grain size for all six layers was found in site C, and the maximum value was found in site I
(Fig. 3d). The difference between minimal D50 and maximum D50 was only 2.5 mm for the surface layer, but was 11 mm for the bottom layer (Table 3), suggesting that the influence of bed sediment resuspension weakens upward. The bed sediment grain size in site B decreased by ~92 mm relative to that of site A (Fig. 3b, Table 3). This resulted from weakened hydrodynamic conditions in site B, as the water speed was only 0.47 m/s, the slowest of all 9 sites (Table 2). Mud deposits were found in the lower 100 km section with evident bidirectional currents (Fig. 2), mainly resulting from the settling process of suspended sediments during the slack water period. Median bed and depth-averaged suspended sediment grain sizes in sites F and G were in the same order of magnitude (Fig. 3b) and were specifically recorded as 9.5 versus 6.2 mm and 8.4 versus 8.0 mm (Table 3), respectively. Considering very small differences in grain size and the dramatic sand-mud transitions between sites E and F (Fig. 3a and b), strong exchanges between bed and suspended sediments (especially near-bed sediment) are presumed to have occurred here, and suspended sediment could have settled in this area. Seaward of site G, stronger ocean dynamics should facilitate the resuspension of bed sediments and its exchanges with near-bed suspended sediment.
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Compared to the results in January 2010 at site I (Liu et al., 2014), the median depth-averaged suspended sediment grain size in April 2013 of this study coarsened from 6.6 to 12.8 mm, while the bed sediment grain size decreased from 98 to 27 mm. Presumably, flow velocity in natural rivers is higher in April than in January and bed materials are resuspended, which increased the suspended sediment grain size. As to the bed sediment grain size, sediments were mainly deposited in the estuary during flood season and were then flushed seaward during dry season (Yang et al., 1992). The surficial sediments turned coarser due to this winnowing effect.
5.3. Longitudinal changes in SSC values Upstream from site E (~510 km long), SSCs remained quite low (<0.1 g/L). The hydrodynamics in the section upstream of site E were considerably affected by fluvial discharge, where the tidal effect was relatively small (Chen et al., 2004). However, the section downstream from site E presented ~0.2, ~0.3 and ~1.1 g/L values over an approximately 150 km-long section along with steadily increasing ocean energy. This downstream pattern of SSC could be attributed to the resuspension and transport processes by tidal currents (Chen et al., 2004). The tidal range increased seaward from 0 m to >2.5 m (Table 1; Fig. 2), as the downstream stronger effects of tidal effects play a central role in sediment resuspension (De Jorge and Van Beusekom, 1995). The SSC showed evident tidal variations downstream of site F (Fig. 4), not only because of enhanced tidal energy but also weakened fluvial hydrodynamics. Net tide-averaged fluvial water velocity decreased from ~0.5 m/s in the river-dominated section to less than 0.2 m/s in the section downstream of site E (Fig. 5) due to a much broader river channel (Fig. 4). The highest SSC values largely corresponded to the fastest flood/ebb tide currents, but shifts in time were occasionally found (Fig. 4). For example, flood tide currents would speed up at the river mouth due to its funnel-shaped topography (Chen et al., 2014). Incipient velocity for the finer portion of bed sediments could occur long before the fastest flood tide currents. When the fastest flood tides were present, a coarser portion of bed sediment was resuspended and transported landward. Thus, two high SSC periods can be seen in site I during the flood tide (Fig. 4). In addition to general tidal effects and more specifically, nearbed water velocities, sediment grain sizes and wave impacts may also affect sediment resuspension considerably (Van Rijn, 1993; Taki, 2000; Soulsby, 2005; Wang et al., 2013). To facilitate a detailed analysis of bed sediment resuspension patterns, water level, near-bed water velocity and corresponding SSC variations were combined (Fig. 6). From sites A to F, the near-bed SSC values
Fig. 5. Net tide-averaged vertical velocity profile in the longitudinal transect. Arrows represent both current magnitudes (length) and directions (upward being north and right being east).
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remained very low (Fig. 6aef), but in stations downstream from site F, SSC values began to respond to water velocity variations (Fig. 6gei). Using site H as an example, SSC values were highly correlated with water velocities (blue) (P < 0.05) (Fig. 6h). Upstream from site F where bed sediments were mainly sandy (Fig. 3a and b), and little bed sediment was resuspended. However, with bed sediments turning into mud downstream from site F (Fig. 3a and b), the resuspension of bed sediments increased seaward. Although wave effect was relatively small throughout this region under normal weather conditions (Hu et al., 2009), a preliminary analysis of wave-induced sediment resuspension was conducted. Based on the wavelength equation for linear waves (Shi et al., 2015; Zhu et al., 2016):
L ¼ gT2 =2p tanhð2ph=LÞ (T: wave period, h: water depth) and the wave data collected on 11 September 2016 in the Yangtze estuary under normal weather condition (mean wave period: 4.68 s, mean water depth: 19.75 m), the mean wavelength was calculated as ~34 m using the iteration method. There is negligible effect beyond the wave base that is equal to half the wavelength (Woodroffe, 2002), and within this area, the maximum depth affected by fair-weather waves is ~17 m on average. Compared to water depths at sites G (19.3 m) and H (13.0 m), even under the same current velocity conditions, the shallowest water depth at site I (7.6 m) could induce more bed shear stress from waves, which may help increase resuspension. Along with presenting the fastest current velocity, relatively fine bed sediment, the near-bed SSC value (1.720 g/L) was much higher than that recorded at sites G (0.256 g/L) and H (0.647 g/L).
5.4. Conceptual estuarine model Although tidal influence reaches 660-km upstream from the river mouth in the dry season, or the tidal limit, tidal effects (mainly reflected in the tidal range) are generally detected in the lower 560km. In other words, tidal influence is generally insignificant in the 100-km close to the tidal limit. Wave effects reached a maximum value ~660 km downstream from Datong due to the existence of shallow water depths (Fig. 7). Salinity was only detected in the lower 100 km-long section of this estuary with a dramatically increasing trend in general, though the limit of saltwater could reach was greatly influenced by wind speeds and directions (Li et al., 2012a). Though these energy levels (river, tide and wave) vary seasonally and annually, a preliminary partition (~660 km long) under mean discharge condition based on the results of this study is helpful for understanding transitions from fluvial to marine processes in the Yangtze estuary. In light of conceptual models introduced in previous studies (Woodroffe, 2002; Williams et al., 2013), a refined and more quantitative model specific to the Yangtze estuary was created (Fig. 7). The first section (~510 km long) is dominated by the river, the last section (70 km long) is controlled by marine forcings like tides and waves, and the transition zone (~80 km long) is affected by mixed river and ocean energy (Fig. 7). The mean depth-averaged SSCs for the tidal cycle remained at <0.1 g/L for the river-dominated section, increased gradually in the transitional section to ~0.2 g/L, and increased considerably to ~1.1 g/L in the marine-dominated section (Fig. 7). Bed deposits changed from sand in the riverdominated section to mud in the marine-dominated section, with the finest sediments located in between (Fig. 7). Salinity levels remained negligible in the upper 560-km section but increased drastically in the marine-dominated section, reaching 17 ppt in the open-water area (Fig. 7). These variations in the transition zone
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Fig. 6. Water levels and current speeds (based on ADCP data) and SSC values (based on OBS calibrations) recorded 1.5e2 m above the bed (no current speed data below this height are available due to ADCP data limitations) at nine sites.
Fig. 7. Conceptual diagram of variations in relative energy, mean depth-averaged SSCs of the tidal cycle and surficial sediment distributions from Datong to the outer river mouth.
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were nonlinear and started from different sections. 5.5. Deviations in flow velocities due to the time lag Flow velocity in the estuary is mainly affected by two factors, including tidal range and fluvial discharge. The flow velocities shown in Table 2 were normalized with long-term averaged tidal range and fluvial discharge, according to statistical relationships between flow velocity and tidal range/fluvial discharge at each site from historical data. The flow velocities shown in Fig. 2 were based on in situ measurements. Because the measurements at sites F, G and I were conducted in different years from measurements at the other sites, there must be deviations in flow velocities. To evaluate the deviations, we compared tidal ranges and fluvial discharges among the different measurement periods. The effect of tidal range on the flow velocities was not taken into consideration when compared to the scenario at sites A, B and C, since the tidal ranges in the upper stream were very small. The fluvial discharge when the measurement at site F was conducted was ca. 40% higher than those at sites A, B, and C (Table 1). Comparison of flow velocity and duration between ebb and flood tides suggested that fluvial discharge contributed 55% of the seaward flow. That is, the ebb flow velocity measured at site F would be ca. 20% higher than the level under a scenario in consistent with sites A, B and C. Considering that measurements at D, E and H were conducted during mean tides, the flood (and ebb) velocity at site F would be ca. 40% (and 18%) lower relative to the scenario in consistent with sites D, E and H. Similarly, the higher fluvial discharge at site F would have increased the ebb velocity by 28%, in comparison with the scenario at sites D, E and H. Thus, the combined effect of different tidal range and fluvial discharge would have increased the ebb flow velocity at site F, in comparison with the scenario at sites D, E and H. In the same way, we found that the mean deviation in flow velocity at sites G and I was 6% higher during ebb tide compared to the scenario at sites A, B and C. The mean deviations in flow velocity at sites G and I were 40% higher during flood tide and 34% higher during ebb tide compared to the scenario at sites D, E and H. 5.6. Challenges and future work We used one research vessel to collect hourly data in the 660km long Yangtze estuary and our data provided multiple short ‘snapshots’ at 9 sites over different time periods. Due to logistical constraints, we could not collect data at all 9 sites concurrently. Doing measurement from Spring tide to Neap tide is a great idea and can be a part of future work. In the future, optical and acoustic sensors may be deployed on buoys or tripods at these sites to measure parameters on an hourly basis. This will help scientists collect high resolution data in time and space. Our work is focused on the long-term averaged conditions under the impact of moderate levels of river flow, winds and waves. River flooding, as well as energetic winds and waves, can shift the transition zones dramatically as shown in Fig. 7. In the future, wave data should be collected in the field to better quantify wave-induced sediment resuspension. In such a large and dynamic tide-impacted Yangtze estuary, more studies should be focused on estuarine circulation as well as the propagation of waves and tides. Coupled hydrodynamics and sediment transport models can also better quantify sediment transport directions and fluxes in this estuary and refine the temporal and spatial resolutions. 6. Conclusions Positioned between the tidal limit (site of the most-upstream effects of spring tides in the dry season) and the outer mouth, the
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transitional zone from the Yangtze River to the East China Sea is a 660-km long section which accounts for >10% of the river's total length. The river-sea transition in sediment dynamics under mean river discharge and tidal range conditions occurs mainly in the downstream sections. Tidal effects on the water levels were not detected in the uppermost 100 km section from the tidal limit. The tidal range increased downstream from 0 m to more than 2.5 m in the remaining 560 km. Bidirectional currents occurred in the lowest 250-km section due to the effects of flood tides. In the upper section with unidirectional currents, the water velocity remained relatively stable at approximately 0.5e0.7 m/s. Meanwhile, the remaining 150 km-long section with evident bidirectional currents, exhibited a water velocity varying from 0 to 2 m/s. Compared to the sandy deposits in the upper 510 km-long section, muddy deposits were found in the lower bidirectional-current section because of the settling process of suspended sediments during the slack water period. The SSC was maintained at 0.1 g/L in the upper 510 km section and increased to 1.1 g/L in the lower 150 km section, reflecting strong exchanges between suspended and bed sediments due to intratidal flow velocity variability. Salinity was not detected in the upper 560 km but increased to 17 ppt in the remaining section. Variations in the parameters were generally not linear, including a stable trend in the upstream and a rapid changing trend in the downstream; however, rapid changing zones vary with parameters and shift upstream and downstream in time. Time-series observations and modelling work are needed in the future to help better quantify sediment transport direction and fluxes in the Yangtze estuary. These findings revealed the interactions of fluvial discharge and oceanic forcings and have direct implications to relevant studies in biochemistry, ecology and geomorphology. Conflict of interests The authors declare that they have no conflict of interests. Author contributions H.F.Y. wrote the draft of the manuscript and prepared the figures, S.L.Y. conceived the study and contributed to the improvement of the manuscript, and K.H.X. helped revise the figures and improved the writing. All authors reviewed the manuscript. Acknowledgments This study was funded by the Ministry of Science and Technology of China (2016YFA0600901), the Natural Science Foundation of China -Shandong Joint Fund for Marine Science Research Centers (U1606401) and the Natural Science Foundation of China (41130856). The authors are very grateful to the Editor and multiple anonymous reviewers for their constructive suggestions and comments in multiple rounds of reviews. References Andersen, T.J., Lund-Hansen, L.C., Pejrup, M., Jensen, K.T., Mouritsen, K.N., 2005. Biologically induced differences in erodibility and aggregation of subtidal and intertidal sediments: a possible cause for seasonal changes in sediment deposition. J. Mar. Syst. 55 (3), 123e138. Anthony, E.J., Gardel, A., Proisy, C., Fromard, F., Gensac, E., Peron, C., Walcker, R., Lesourd, S., 2013. The role of fluvial sediment supply and river-mouth hydrology in the dynamics of the muddy, Amazon-dominated Amap aeGuianas coast, South America: a three-point research agenda. J. S. Am. Earth Sci. 44, 18e24. Avoine, J., Allen, G.P., Nichols, M., Salomon, J.C., Larsonneur, C., 1981. Suspendedsediment transport in the Seine estuary, France: effect of man-made modifications on estuarydshelf sedimentology. Mar. Geol. 40 (1e2), 119e137. Brotas, V., Plante-Cuny, M.R., 1998. Spatial and temporal patterns of microphytobenthic taxa of estuarine tidal flats in the Tagus Estuary (Portugal) using pigment analysis by HPLC. Mar. Ecol. Prog. Ser. 171, 43e57.
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River-sea transitions of sediment dynamics: A case study of the tide-impacted Yangtze River estuary H.F. Yang a, b, S.L. Yang a, *, K.H. Xu b, c a
State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, China Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA c Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2016 Received in revised form 2 July 2017 Accepted 6 July 2017 Available online 8 July 2017
Hydrodynamics and sediment dynamics vary greatly in tide-dominated estuaries worldwide, but there is a paucity of data of large tide-dominated estuary systems due to difficulties of observation in a large spatial scale. In this study, we investigate sediment dynamic transitions in a 660-km long section between the tidal limit and mouth of the Yangtze River. We found that tidal effects are almost undetectable in the uppermost 100-km section, but the mean tidal range gradually increases downstream to nearly 3 m at the river mouth. Flow is generally unidirectional in the uppermost 400-km section, although its velocity changes in response to flood/ebb tidal dynamics; in the lowest 250-km section, flow is bidirectional, and ebb flow durations decrease towards the sea. In the lowermost 100 km, the ebb flow durations decreases to below 60%, and the flow is dominated by tidal currents. Salinity is only detectable in the lowest 100-km section due to the dominance of Yangtze River water discharge. Bed sediments mainly include sand in the uppermost 500-km section, whereas mud dominates in the remaining areas. In contrast, the median grain size of the suspended sediments was found to be greater in the lowest 100km section (8e13 mm) than in the upper sections (5e6 mm) due to strong exchanges between suspended and near bed sediments. The suspended sediment concentration (SSC) was found to be low (<0.1 g/L) and homogenous in the uppermost 100-km section, downstream of which the SSC increased rapidly to >1 g/L and both surface-bottom and intratidal variabilities occurred. The rates of sediment parameter changes were rapid in the river-sea transitional zone, and this zone may shift upstream and downstream in response to the relative contributions of the river, tides and waves. A conceptual model of the river-sea transition of sediment dynamics for the Yangtze estuary was established, and this model shed light on quantitative studies of sediment dynamics in other large tide-impacted estuaries worldwide. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Estuary Suspended sediment concentration Grain size River-sea interaction Yangtze estuary
1. Introduction Estuaries are popular sedimentary environments where rivers meet oceans. Physical, geological, chemical and biological processes in estuaries are generally more complex and dynamic than other areas on the Earth's surface (Holligan and Boois, 1993). Fluvial discharges, tides and waves are three main forces that affect estuaries. It is well known that the head of a estuary is controlled by the river, the main estuary in the middle experiences mixed energy from both the river and ocean, and the mouth in the lower section is mainly controlled by the ocean that is involved (Williams et al.,
* Corresponding author. E-mail address: [email protected] (S.L. Yang). http://dx.doi.org/10.1016/j.ecss.2017.07.005 0272-7714/© 2017 Elsevier Ltd. All rights reserved.
2013). The suspended sediment concentration (SSC) in the estuary is highly affected by catchment activities, such as soil leaching, floods, deforestation and forestation and dam constructions (Pont et al., 2002; Rothe et al., 2002; Yang et al., 2002, 2011). Oceanic forcings like tides, waves and wind can cause changes in the SSC through the process of mixing and resuspension in the estuarine area (De Jorge and Van Beusekom, 1995; Gensac et al., 2016). These processes play a key role in sedimentary processes, morphodynamics, and ecological systems and should also be taken into consideration in engineering construction (Newcombe and MacDonald, 1991; El-Asmar and White, 2002; Yang et al., 2011). Over the decades, many studies have been conducted on geomorphic processes in estuarine and coastal regions (e.g., Widdows et al., 2000; Andersen et al., 2005; Yang et al., 2008; Grabowski et al., 2011; Liu et al., 2014). Wright and Nittrouer
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(1995) proposed four stages of sediment dispersal in estuaries and adjacent coastal areas: supply via river plumes, initial deposition, resuspension and transport, and long-term net accumulation. Gibbs (1976) reported that Amazon River sediments are transported from its estuary to the continental shelf, but mud components can be separated and carried landward by onshore bottom currents. Fluvial water and sediment discharges, suspended particulate matter concentration and oceanic forcings were closely related to the formation and evolution of mud banks on the Amazon-dominated coast (Anthony et al., 2013; Gensac et al., 2016). For example, increases in riverine sediment resulted in a 150 km-long area of accretion, and water discharge from major rivers greatly affected the migration rates of these mud belts (Gensac et al., 2016). Estuary sizes vary considerably around the world, generally ranging from <1 km to >100 km (Dyer, 1997). Surveys of longitudinal transects along estuary head, main and mouth areas can provide valuable information on hydrodynamics and sediment dynamics. In tide-dominated estuaries, longitudinal transects can be surveyed hourly to capture temporal and spatial variations. These types of surveys can be applied for small estuaries such as the York River in the Chesapeake Bay and the Tagus, the Seine and the Mondego rivers in Europe (Avoine et al., 1981; Vale, 1990; Brotas and Plante-Cuny, 1998; Lillebø et al., 1999; Le Hir et al., 2001; Friedrichs, 2009). These types of surveys, however, are not practical for large tide-dominated estuaries (such as, the Amazon and Yangtze Rivers), as they are >500 km in length. Logistically it is difficult to use one research vessel to finish even one longitudinal survey during a single tidal cycle (~12 or 24 h). Although many studies have been performed on large river systems, there is still a paucity of data on hydrodynamics and sediment dynamics of large tide-dominated estuarine systems (e.g., Amazon, Yangtze, Ganges, and Mekong). Several studies have been conducted on the Yangtze River and its dispersal system over the past 20 years, but most of them have focused on either the 200 km-long funnel-shaped Yangtze River mouth only (Milliman et al., 1985; Li et al., 2012b; Liu et al., 2014) or the river drainage basin itself (Chen et al., 2007; Xu and Milliman, 2009; Luo et al., 2012). Although it is well known that river-ocean interactions occur along the low-gradient 660 km-long section of the Yangtze River, systematic and comprehensive measurements of this area are still limited, partially due to the aforementioned logistics challenges. In this study, we focus on the entire 660 km-long Yangtze estuary from Datong Station to the Yangtze subaqueous delta and study the interactions among the river, tides, waves, and SSC levels. We strive to filling in this knowledge gap by studying: (1) river flow speeds and directions, (2) suspended sediment concentrations, and (3) sediment grain sizes for the entire Yangtze estuary. We perform systematic and labor-intensive hourly surveys and samplings over more than one semi-diurnal tidal cycle (about 13e14 h) at nine stations. Our study sheds light on the quantitative studies of tide-dominated large river estuarine systems and our datasets can be compared with those of other tide-dominated rivers worldwide. With a length of ~6.3 103 km, the Yangtze River is the longest in Asia and is the third longest in the world. It originates from the Qinghai-Tibet Plateau, flows eastward, and debouches to the East China Sea. The Yangtze ranks fifth globally in terms of water discharge (~900 km3/yr) and fourth in terms of historical sediment load (~470 Mt/yr, Million tons per year) (Milliman and Farnsworth, 2011). In estuarine systems, the tidal limit is defined as the site at the most landward extent of spring tides during the dry season where the tidal range is null; the flood tidal current limit is defined as the farthest limit that a flood tide current (opposite to the dominant river flow direction) can reach. Datong Station (Fig. 1) is a
tidal limit station that is used to carry out water discharge and sediment load measurements for this study. 2. Study area The estuary of the Yangtze River is defined as a ~660 km-long section from Datong Station to the subaqueous delta at 30e50 m isobaths (Shen et al., 2003). Xuliujing Station is a key site that separates the estuary into two distinct parts (Fig. 1). From Datong to Xuliujing, the river channel is elongated (~510 km) and narrow and the river flows are fast (~0.56 m/s). Downstream from Xuliujing, the Yangtze River transforms to a broad and bifurcated estuary with four major outlets. Seaward of the river mouth (~90 km wide), subaqueous sand ridges form as a result of energetic tidal currents (Shen and Pan, 2001). At the seaward-most station in Jiuduansha (Fig. 1), mean and maximum tidal ranges are 2.7 and 4.6 m, respectively (GSICI, 1996). Wind-driven waves and swells are two major types of waves found in this estuary, and wave energy levels increase rapidly seaward. Long-term mean wave heights increase from 0.2 m at Gaoqiao Station (in the middle of the bifurcated estuary) to about 1.0 m at Yinshuichuan Station (~60 km seaward of Gaoqiao; Fig. 1) (Yang, 1999). During its geological development, the Yangtze Delta has shifted its depocenter southeastward since the 18th century. By the 1950s, >98% of Yangtze River discharge had entered the sea through the South Branch (Chen et al., 1985). Yangtze sediment loads into the East China Sea are mainly composed of suspended sediment (>99%) (Yang et al., 2002). When suspended sediment is transported downstream to the estuary, an Estuarine Turbidity Maximum (ETM) zone forms in the mouth bar area due to flocculation and canceling energy between the river and the sea (Jiang et al., 2013). Semidiurnal ebb currents are generally faster than flood currents. The depth-averaged velocity is ~1.4 m/s and the peak tidal current velocity can reach 2 m/s at the river mouth (Chen et al., 2014; Hu et al., 1988), which is generally comparable to that of the Amazon River (1e2 m/s) (Gensac et al., 2016). Over recent years, as a result of anthropogenic changes (particularly with the construction of the Three Gorges Dam -TGD) (Yang et al., 2015), the SSC at Datong Station decreased from 0.62 g/ L in 1958 to 0.35 g/L in 2000 (Yang et al., 2003) and then to only 0.17 g/L during the post-TGD period (after 2003). Numerous human activities have also happened in the Yangtze basin and estuary (e.g., water reservoir construction, sand mining, navigational dredging, shoreline protection and coastal reclamation) (Yang et al., 2006; Li et al., 2011; Jiang et al., 2012). It is likely that sediment dynamic processes have changed due to a variety of human activities. In this study, most observations were conducted in April of 2013, April of 2014 and October of 2015, when water discharge levels were close to the mean water discharge level of the past 65 years (averaged at 28,300 m3/s). 3. Materials and methods In this study, fieldwork was conducted at sites G and I in April of 2013, at sites A, B, C, D, E and H in April of 2014 and at site F in October of 2015 (Fig. 1; Table 1). It is well known that the river's effects are more prominent in JulyeAugust and that wind-driven waves play a more central role in JanuaryeFebruary (Chen et al., 2004; Xu and Milliman, 2009). To minimize the impact from seasonal variation, observations of all sites were made in April and October, during which river discharge and wind energy levels are both at moderate levels. To be specific, fluvial discharges in these two months are close to the mean water discharge, usually between 20,000 and 30,000 m3/s (See Table 1). These values differ from that in dry (~10,000 m3/s) and flood season (~50,000 m3/s). Thus our
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Fig. 1. Study area and observation stations in the Yangtze estuary from Datong Station to the East China Sea. SS is Sheshan and JDS is Jiuduansha.
Table 1 Datasets for the field observations. Observation site Downriver distance from Datong (km)a Dates of observation Tidal phase Mean water depth (m) Tidal range (m) Water discharge in Datong (m3/s) A B C D E F G H I a
60 120 260 410 510 560 590 620 660
9-10 April 2014 8-9 April 2014 10-11 April 2014 6-7 April 2014 5-6 April 2014 20-21 October 2015 11-12 April 2013 13-14 April 2014 10-11 April 2013
neap neap neap mean mean neap spring mean spring
8.3 10.5 9.4 11.9 15.6 12.3 19.3 13.0 7.6
0.1 0.2 0.6 1.6 2.0 1.2 2.5 2.7 2.5
19,200 18,900 19,000 18,000 17,150 26,800 22,850 17,750 20,800
Datong is the tidal limit of the Yangtze River.
study approximately represents a long-term averaged condition of Yangtze estuary. During each fieldwork period, observation was made during at least one full semidiurnal tidal cycle (13e14 h). Observation sites were selected carefully, and key factors taken into account were flood tidal current limits, tidal limits, water depths, tidal ranges (Table 1), geomorphology features and spatial distributions. Daily water discharge was downloaded at the web site at http://yu-zhu.vicp.net, which is managed by the Changjiang (Yangtze) Water Resources Commission. To ensure that local water discharge levels corresponding to each observation are accurate, the time span of water discharge at the observation sites relative to
that at Datong Station was applied in calculations based on the measured velocity and distance. During each observation period, a 600-kHz Acoustic Doppler Current Profiler (ADCP) (Teledyne RD Instruments, Inc., Poway, California, USA) and an Optical Back Scattering (OBS) sensor (OBS3A, D & A Instrument Company, Washington, USA) were used to measure the hourly velocity and turbidity profiles, respectively. The ADCP was equipped with four flow sensors with a 0.5 m bin size and a blanking distance of 0.25 m. The Valeport-106 flow sensor (Valeport Ltd, Devon, UK) was used to measure the velocity and direction of the water surface layer to augment ADCP
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measurements. Current data collected using the ADCP (subsurface) and Valeport-106 (surface) were combined and then normalized according to the ratios of mean water discharge levels to actual water discharge levels and mean tidal ranges to measured tidal ranges at each site (Table 2). Two replicate samples were collected using 600-ml water bottles at six depths (the surface, 0.2H, 0.4H, 0.6H, 0.8H, and the bottom, where H is the depth of the water column) of each site and were used to measure SSC and particle size values in a laboratory at East China Normal University. In addition, river channel bed and seabed surficial sediments were collected from all nine sites. Water samples were filtered through 0.45-mm filters, dried and weighed following the method of Liu et al. (2014) to calculate SSC values. OBS calibrations were also performed in the laboratory to determine SSC values, and they were then compared with SSC values using water samples at six depths. Sediment grain sizes were analyzed using a Laser Diffraction Particle Size Analyzer, BeckmanCoulter Ls100Q (Beckman Coulter Inc., California, USA).
channel. The mean velocities of ebb tidal currents were also faster than flood tides at each site, and the differences varied from 0.15 to 0.25 m/s (Table 2). Mean water depths decreased from site G (19.3 m) to site I (7.6 m) as the river spread over the broad Yangtze mouth bar area (Figs. 1 and 2). At site I, the current rotation was clearly clockwise while shifting from flood to ebb tides (Fig. 2). 4.2. Sediment grain size 4.2.1. Bed sediment Bed sediments showed an overall downstream-fining trend from site A (~60 km downstream from Datong) to site I (~660 km downstream from Datong) (Fig. 3b). Sand content levels were found to peak at site A (~90%), and the lowest levels were found at site G (~4%) (Fig. 3a). The average D50 (median grain size) of riverbed sediment in five sites (A-E) was 164.1 mm (fine sand) but decreased to only 24.4 mm (medium silt) in four sites (F-I). This was especially the case in the section between sites E and F, where D50 decreased from 128.0 to 9.5 mm (Table 3; Fig. 3b).
4. Results 4.1. Hydrodynamics Approximately 60 km downstream from Datong, site A showed steady unidirectional currents without notable changes with water depths. The difference between surface and bottom water velocities here is small, generally <0.15 m/s (Fig. 2). The mean flow velocity after normalization under mean water discharge was recorded as 0.68 m/s (Table 2). Small fluctuations in water surface elevation (0.4 m between the peak and trough) occurred at site B, but this was still dominated by unidirectional flows moving to the east (Fig. 2). Compared with patterns found in site A, water velocity in site B decreased by 30% to 0.47 m/s (Table 2). River water flowed eastward in the same direction northeast in site C, but a low flow velocity period from hour 11 to 13 was found during the observation period. The velocity averaged at 0.28 m/s over hours 11e13 but was recorded as ~0.54 m/s for the rest of the period (Fig. 2). Slow currents (0.18 m/s) formed from hour 3 through hour 6 in site D, and a reversed current formed during this period. Except for this reversal period, water consistently flowed eastward at an average speed of 0.76 m/s (Fig. 2). Downstream from site E, current reversals were obvious (Fig. 2). Ebb tides in sites E, F, G, H and I occurred for approximately 8.2, 7.3, 7.1, 7.0 and 6.8 h, respectively, and thus were maintained for much longer periods than flood tides (4.3, 5.2, 5.4, 5.5 and 5.7 h, respectively) (Table 2), denoting the presence of strong tidal asymmetries. The ebb tide direction ranged from 100 to 135 flowing southeast, and the flood tide direction ranged from 281 to 313 flowing northwest, generally following the orientation of river Table 2 Normalized flow velocities and durations of tidal phases. Site
A B C D E F G H I
Ebb tide
Flood tide
Velocity (m/s)
Duration (hr)
Velocity (m/s)
Duration (hr)
0.68 0.47 0.49 0.60 0.74 0.57 0.98 1.10 1.15
12.5 12.5 12.5 12.5 8.2 7.3 7.1 7.0 6.8
0 0 0 0 0.56 0.36 0.77 0.87 0.85
0 0 0 0 4.3 5.2 5.4 5.5 5.7
Flow velocities are depth-averaged. The normalized flow is the flow occurring over the long-term averaged water discharge and tidal range.
4.2.2. Suspended sediment The depth-averaged D50 of suspended sediments in sites A, C, F, G, H and I averaged at 5.5, 5.3, 6.2, 8.0, 8.7 and 12.8 mm, respectively. Clay and silt made up >90% of the suspended sediments (Fig. 3c). Median sizes changed little with increasing depth in sites A, C and F (Fig. 3d), and the differences between the maximum and the minimum roughly ranged from 0.3 to 0.6 mm. Median sizes in sites G, H and I showed a clear downward coarsening pattern (Fig. 3d) with ratios increasing from 1.0 to 2.2 (Table 3). Differences in the D50 between the water surface and near-bed suspended sediment were 3.0, 6.9 and 8.8 mm (Table 3). 4.3. Suspended sediment concentration Based on data derived from the water samples, we found no dramatic vertical change in SSC values for sites A, B, C, D, E and F, indicating a well-mixed environment. Standard deviations of the entire tidal cycle for each depth in these sites were also found to be negligible (Fig. 4). However, the SSC value and standard deviation in sites H and I increased dramatically with an increase in water depth (Fig. 4), and the difference between the water surface and near-bed layers reached 0.55 and 1.10 g/L (Table 3), respectively. These layers formed a smooth concave-upward profile that fits a logarithmic formula (R > 0.96) (Fig. 4). Based on the data derived from the OBS sensors, mean depthaveraged SSCs for the entire tidal cycle from sites A to I were recorded as 0.072, 0.069, 0.068, 0.083, 0.090, 0.074, 0.190, 0.273 and 1.107 g/L. The SSC value of the Yangtze mouth bar area (site I) was higher than that of the inner estuary (sites G and H) by more than one order of magnitude, and this was also the case for those of the river (site A, B, C, D, E and F). SSC values for sites A and B were mainly homogenous, intratidal temporal variations were found in site C, and intratidal changes became pronounced in each site downstream from site D (Fig. 4). Maximum depth-averaged SSC values for all of the observations were found during the first hour in site I at a value of 2.353 g/L. SSC values of the bottom layer were always greater than those of the surface layer, and the maximum difference between these two layers was recorded as 2.587 g/L at hour 4 in site I. 5. Discussion 5.1. Longitudinal current variations Water levels remained relatively stable in site A but varied in
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Fig. 2. Time series variations of current vectors. Arrows represent both current magnitudes (length) and directions (upward being north and right being east). Details are shown for Station D.
Fig. 3. Percentages of sand, silt and clay in (a) bed surficial and (c) suspended sediments; (b) median grain sizes along the Yangtze estuary from Datong Station to the East China Sea; and (d) vertical profiles of tide-averaged median size for suspended sediments.
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Table 3 Suspended sediment concentrations and median sizes of bed and suspended sediment. Site Median bed sediment size (um)
Suspended sediment concentration (g/L) Depthaveraged
Surface layer
Near-bed layer
Ratio (near-bed/ surface)
Depthaveraged
Surface layer
Near-bed layer
Ratio (near-bed/ surface)
A B C D E F G H I
0.072 0.069 0.068 0.083 0.090 0.074 0.190 0.273 1.107
0.068 0.051 0.064 0.066 0.057 0.068 0.122 0.100 0.616
0.081 0.077 0.089 0.087 0.119 0.083 0.256 0.647 1.720
1.2 1.5 1.4 1.3 2.1 1.2 2.1 6.5 2.8
5.5
5.6
5.6
1.0
5.3
5.1
5.3
1.0
6.2 8.0 8.7 12.8
5.9 6.2 5.6 7.6
6.5 9.2 12.5 16.4
1.1 1.5 2.2 2.2
220 128 219 125 128 10 8 53 27
Median suspended sediment size (um)
Fig. 4. Vertical profiles of tide-averaged SSC values with standard deviation and regression lines, and time series of SSC values based on OBS data collected from nine sites.
site B (Fig. 2), which likely suggests that the uppermost tidal effect reached somewhere between A and B. Flow directions in the section upperstream of site C were always seaward, whereas flood currents were found in the section downstream of site D (Fig. 2), suggesting that the flood tidal current limit was located between sites C and D. Counteraction and prop forces of the flood tidal current presumedly raised the water level rise and reduced low velocity during flood phase at site C. Considering bidirectional flows at site D, the flood tidal current limit must exist between sites C and D, which is basically in agreement with previous studies (Xu et al., 2012; Hou and Zhu, 2013). The abrupt decrease in current speed found at site F (Table 2) is attributable to the widened river channel in the South Branch and the spreading of energy (Fig. 1). As to the vertical velocity profiles against the height above the seabed, they all follow a quasi-logarithmic form. Velocities at water surface were usually smaller than those at sub-surface, mainly due to the friction between the air and the river water (Liu et al., 2014). 5.2. Longitudinal trends of sediment grain size The minimum suspended sediment grain size for all six layers was found in site C, and the maximum value was found in site I
(Fig. 3d). The difference between minimal D50 and maximum D50 was only 2.5 mm for the surface layer, but was 11 mm for the bottom layer (Table 3), suggesting that the influence of bed sediment resuspension weakens upward. The bed sediment grain size in site B decreased by ~92 mm relative to that of site A (Fig. 3b, Table 3). This resulted from weakened hydrodynamic conditions in site B, as the water speed was only 0.47 m/s, the slowest of all 9 sites (Table 2). Mud deposits were found in the lower 100 km section with evident bidirectional currents (Fig. 2), mainly resulting from the settling process of suspended sediments during the slack water period. Median bed and depth-averaged suspended sediment grain sizes in sites F and G were in the same order of magnitude (Fig. 3b) and were specifically recorded as 9.5 versus 6.2 mm and 8.4 versus 8.0 mm (Table 3), respectively. Considering very small differences in grain size and the dramatic sand-mud transitions between sites E and F (Fig. 3a and b), strong exchanges between bed and suspended sediments (especially near-bed sediment) are presumed to have occurred here, and suspended sediment could have settled in this area. Seaward of site G, stronger ocean dynamics should facilitate the resuspension of bed sediments and its exchanges with near-bed suspended sediment.
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Compared to the results in January 2010 at site I (Liu et al., 2014), the median depth-averaged suspended sediment grain size in April 2013 of this study coarsened from 6.6 to 12.8 mm, while the bed sediment grain size decreased from 98 to 27 mm. Presumably, flow velocity in natural rivers is higher in April than in January and bed materials are resuspended, which increased the suspended sediment grain size. As to the bed sediment grain size, sediments were mainly deposited in the estuary during flood season and were then flushed seaward during dry season (Yang et al., 1992). The surficial sediments turned coarser due to this winnowing effect.
5.3. Longitudinal changes in SSC values Upstream from site E (~510 km long), SSCs remained quite low (<0.1 g/L). The hydrodynamics in the section upstream of site E were considerably affected by fluvial discharge, where the tidal effect was relatively small (Chen et al., 2004). However, the section downstream from site E presented ~0.2, ~0.3 and ~1.1 g/L values over an approximately 150 km-long section along with steadily increasing ocean energy. This downstream pattern of SSC could be attributed to the resuspension and transport processes by tidal currents (Chen et al., 2004). The tidal range increased seaward from 0 m to >2.5 m (Table 1; Fig. 2), as the downstream stronger effects of tidal effects play a central role in sediment resuspension (De Jorge and Van Beusekom, 1995). The SSC showed evident tidal variations downstream of site F (Fig. 4), not only because of enhanced tidal energy but also weakened fluvial hydrodynamics. Net tide-averaged fluvial water velocity decreased from ~0.5 m/s in the river-dominated section to less than 0.2 m/s in the section downstream of site E (Fig. 5) due to a much broader river channel (Fig. 4). The highest SSC values largely corresponded to the fastest flood/ebb tide currents, but shifts in time were occasionally found (Fig. 4). For example, flood tide currents would speed up at the river mouth due to its funnel-shaped topography (Chen et al., 2014). Incipient velocity for the finer portion of bed sediments could occur long before the fastest flood tide currents. When the fastest flood tides were present, a coarser portion of bed sediment was resuspended and transported landward. Thus, two high SSC periods can be seen in site I during the flood tide (Fig. 4). In addition to general tidal effects and more specifically, nearbed water velocities, sediment grain sizes and wave impacts may also affect sediment resuspension considerably (Van Rijn, 1993; Taki, 2000; Soulsby, 2005; Wang et al., 2013). To facilitate a detailed analysis of bed sediment resuspension patterns, water level, near-bed water velocity and corresponding SSC variations were combined (Fig. 6). From sites A to F, the near-bed SSC values
Fig. 5. Net tide-averaged vertical velocity profile in the longitudinal transect. Arrows represent both current magnitudes (length) and directions (upward being north and right being east).
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remained very low (Fig. 6aef), but in stations downstream from site F, SSC values began to respond to water velocity variations (Fig. 6gei). Using site H as an example, SSC values were highly correlated with water velocities (blue) (P < 0.05) (Fig. 6h). Upstream from site F where bed sediments were mainly sandy (Fig. 3a and b), and little bed sediment was resuspended. However, with bed sediments turning into mud downstream from site F (Fig. 3a and b), the resuspension of bed sediments increased seaward. Although wave effect was relatively small throughout this region under normal weather conditions (Hu et al., 2009), a preliminary analysis of wave-induced sediment resuspension was conducted. Based on the wavelength equation for linear waves (Shi et al., 2015; Zhu et al., 2016):
L ¼ gT2 =2p tanhð2ph=LÞ (T: wave period, h: water depth) and the wave data collected on 11 September 2016 in the Yangtze estuary under normal weather condition (mean wave period: 4.68 s, mean water depth: 19.75 m), the mean wavelength was calculated as ~34 m using the iteration method. There is negligible effect beyond the wave base that is equal to half the wavelength (Woodroffe, 2002), and within this area, the maximum depth affected by fair-weather waves is ~17 m on average. Compared to water depths at sites G (19.3 m) and H (13.0 m), even under the same current velocity conditions, the shallowest water depth at site I (7.6 m) could induce more bed shear stress from waves, which may help increase resuspension. Along with presenting the fastest current velocity, relatively fine bed sediment, the near-bed SSC value (1.720 g/L) was much higher than that recorded at sites G (0.256 g/L) and H (0.647 g/L).
5.4. Conceptual estuarine model Although tidal influence reaches 660-km upstream from the river mouth in the dry season, or the tidal limit, tidal effects (mainly reflected in the tidal range) are generally detected in the lower 560km. In other words, tidal influence is generally insignificant in the 100-km close to the tidal limit. Wave effects reached a maximum value ~660 km downstream from Datong due to the existence of shallow water depths (Fig. 7). Salinity was only detected in the lower 100 km-long section of this estuary with a dramatically increasing trend in general, though the limit of saltwater could reach was greatly influenced by wind speeds and directions (Li et al., 2012a). Though these energy levels (river, tide and wave) vary seasonally and annually, a preliminary partition (~660 km long) under mean discharge condition based on the results of this study is helpful for understanding transitions from fluvial to marine processes in the Yangtze estuary. In light of conceptual models introduced in previous studies (Woodroffe, 2002; Williams et al., 2013), a refined and more quantitative model specific to the Yangtze estuary was created (Fig. 7). The first section (~510 km long) is dominated by the river, the last section (70 km long) is controlled by marine forcings like tides and waves, and the transition zone (~80 km long) is affected by mixed river and ocean energy (Fig. 7). The mean depth-averaged SSCs for the tidal cycle remained at <0.1 g/L for the river-dominated section, increased gradually in the transitional section to ~0.2 g/L, and increased considerably to ~1.1 g/L in the marine-dominated section (Fig. 7). Bed deposits changed from sand in the riverdominated section to mud in the marine-dominated section, with the finest sediments located in between (Fig. 7). Salinity levels remained negligible in the upper 560-km section but increased drastically in the marine-dominated section, reaching 17 ppt in the open-water area (Fig. 7). These variations in the transition zone
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Fig. 6. Water levels and current speeds (based on ADCP data) and SSC values (based on OBS calibrations) recorded 1.5e2 m above the bed (no current speed data below this height are available due to ADCP data limitations) at nine sites.
Fig. 7. Conceptual diagram of variations in relative energy, mean depth-averaged SSCs of the tidal cycle and surficial sediment distributions from Datong to the outer river mouth.
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were nonlinear and started from different sections. 5.5. Deviations in flow velocities due to the time lag Flow velocity in the estuary is mainly affected by two factors, including tidal range and fluvial discharge. The flow velocities shown in Table 2 were normalized with long-term averaged tidal range and fluvial discharge, according to statistical relationships between flow velocity and tidal range/fluvial discharge at each site from historical data. The flow velocities shown in Fig. 2 were based on in situ measurements. Because the measurements at sites F, G and I were conducted in different years from measurements at the other sites, there must be deviations in flow velocities. To evaluate the deviations, we compared tidal ranges and fluvial discharges among the different measurement periods. The effect of tidal range on the flow velocities was not taken into consideration when compared to the scenario at sites A, B and C, since the tidal ranges in the upper stream were very small. The fluvial discharge when the measurement at site F was conducted was ca. 40% higher than those at sites A, B, and C (Table 1). Comparison of flow velocity and duration between ebb and flood tides suggested that fluvial discharge contributed 55% of the seaward flow. That is, the ebb flow velocity measured at site F would be ca. 20% higher than the level under a scenario in consistent with sites A, B and C. Considering that measurements at D, E and H were conducted during mean tides, the flood (and ebb) velocity at site F would be ca. 40% (and 18%) lower relative to the scenario in consistent with sites D, E and H. Similarly, the higher fluvial discharge at site F would have increased the ebb velocity by 28%, in comparison with the scenario at sites D, E and H. Thus, the combined effect of different tidal range and fluvial discharge would have increased the ebb flow velocity at site F, in comparison with the scenario at sites D, E and H. In the same way, we found that the mean deviation in flow velocity at sites G and I was 6% higher during ebb tide compared to the scenario at sites A, B and C. The mean deviations in flow velocity at sites G and I were 40% higher during flood tide and 34% higher during ebb tide compared to the scenario at sites D, E and H. 5.6. Challenges and future work We used one research vessel to collect hourly data in the 660km long Yangtze estuary and our data provided multiple short ‘snapshots’ at 9 sites over different time periods. Due to logistical constraints, we could not collect data at all 9 sites concurrently. Doing measurement from Spring tide to Neap tide is a great idea and can be a part of future work. In the future, optical and acoustic sensors may be deployed on buoys or tripods at these sites to measure parameters on an hourly basis. This will help scientists collect high resolution data in time and space. Our work is focused on the long-term averaged conditions under the impact of moderate levels of river flow, winds and waves. River flooding, as well as energetic winds and waves, can shift the transition zones dramatically as shown in Fig. 7. In the future, wave data should be collected in the field to better quantify wave-induced sediment resuspension. In such a large and dynamic tide-impacted Yangtze estuary, more studies should be focused on estuarine circulation as well as the propagation of waves and tides. Coupled hydrodynamics and sediment transport models can also better quantify sediment transport directions and fluxes in this estuary and refine the temporal and spatial resolutions. 6. Conclusions Positioned between the tidal limit (site of the most-upstream effects of spring tides in the dry season) and the outer mouth, the
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transitional zone from the Yangtze River to the East China Sea is a 660-km long section which accounts for >10% of the river's total length. The river-sea transition in sediment dynamics under mean river discharge and tidal range conditions occurs mainly in the downstream sections. Tidal effects on the water levels were not detected in the uppermost 100 km section from the tidal limit. The tidal range increased downstream from 0 m to more than 2.5 m in the remaining 560 km. Bidirectional currents occurred in the lowest 250-km section due to the effects of flood tides. In the upper section with unidirectional currents, the water velocity remained relatively stable at approximately 0.5e0.7 m/s. Meanwhile, the remaining 150 km-long section with evident bidirectional currents, exhibited a water velocity varying from 0 to 2 m/s. Compared to the sandy deposits in the upper 510 km-long section, muddy deposits were found in the lower bidirectional-current section because of the settling process of suspended sediments during the slack water period. The SSC was maintained at 0.1 g/L in the upper 510 km section and increased to 1.1 g/L in the lower 150 km section, reflecting strong exchanges between suspended and bed sediments due to intratidal flow velocity variability. Salinity was not detected in the upper 560 km but increased to 17 ppt in the remaining section. Variations in the parameters were generally not linear, including a stable trend in the upstream and a rapid changing trend in the downstream; however, rapid changing zones vary with parameters and shift upstream and downstream in time. Time-series observations and modelling work are needed in the future to help better quantify sediment transport direction and fluxes in the Yangtze estuary. These findings revealed the interactions of fluvial discharge and oceanic forcings and have direct implications to relevant studies in biochemistry, ecology and geomorphology. Conflict of interests The authors declare that they have no conflict of interests. Author contributions H.F.Y. wrote the draft of the manuscript and prepared the figures, S.L.Y. conceived the study and contributed to the improvement of the manuscript, and K.H.X. helped revise the figures and improved the writing. All authors reviewed the manuscript. Acknowledgments This study was funded by the Ministry of Science and Technology of China (2016YFA0600901), the Natural Science Foundation of China -Shandong Joint Fund for Marine Science Research Centers (U1606401) and the Natural Science Foundation of China (41130856). The authors are very grateful to the Editor and multiple anonymous reviewers for their constructive suggestions and comments in multiple rounds of reviews. References Andersen, T.J., Lund-Hansen, L.C., Pejrup, M., Jensen, K.T., Mouritsen, K.N., 2005. Biologically induced differences in erodibility and aggregation of subtidal and intertidal sediments: a possible cause for seasonal changes in sediment deposition. J. Mar. Syst. 55 (3), 123e138. Anthony, E.J., Gardel, A., Proisy, C., Fromard, F., Gensac, E., Peron, C., Walcker, R., Lesourd, S., 2013. The role of fluvial sediment supply and river-mouth hydrology in the dynamics of the muddy, Amazon-dominated Amap aeGuianas coast, South America: a three-point research agenda. J. S. Am. Earth Sci. 44, 18e24. Avoine, J., Allen, G.P., Nichols, M., Salomon, J.C., Larsonneur, C., 1981. Suspendedsediment transport in the Seine estuary, France: effect of man-made modifications on estuarydshelf sedimentology. Mar. Geol. 40 (1e2), 119e137. Brotas, V., Plante-Cuny, M.R., 1998. Spatial and temporal patterns of microphytobenthic taxa of estuarine tidal flats in the Tagus Estuary (Portugal) using pigment analysis by HPLC. Mar. Ecol. Prog. Ser. 171, 43e57.
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