The Suspended Sediment Concentration Distribution in the Bohai Sea, Yellow Sea and East China Sea

BIANChangwei1), 2), *,JIANG Wensheng1), Richard J. Greatbatch2), and DING Hui2)



The Suspended Sediment Concentration Distribution in the Bohai Sea, Yellow Sea and East China Sea

BIANChangwei,JIANG Wensheng, Richard J. Greatbatch, and DING Hui

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The distribution of the suspended sediment concentration (SSC) in the Bohai Sea, Yellow Sea and East China Sea (BYECS) is studied based on the observed turbidity data and model simulation results. The observed turbidity results show that (i) the highest SSC is found in the coastal areas while in the outer shelf sea areas turbid water is much more difficult to observe, (ii) the surface layer SSC is much lower than the bottom layer SSC and (iii) the winter SSC is higher than the summer SSC. The Regional Ocean Modeling System (ROMS) is used to simulate the SSC distribution in the BYECS. A comparison between the modeled SSC and the observed SSC in the BYECS shows that the modeled SSC can reproduce the principal features of the SSC distribution in the BYECS. The dynamic mechanisms of the sediment erosion and transport processes are studied based on the modeled results. The horizontal distribution of the SSC in the BYECS is mainly determined by the current-wave induced bottom stress and the fine-grain sediment distribution. The current-induced bottom stress is much higher than the wave-induced bottom stress, which means the tidal currents play a more significant role in the sediment resuspension than the wind waves. The vertical mixing strength is studied based on the mixed layer depth and the turbulent kinetic energy distribution in the BYECS. The strong winter time vertical mixing, which is mainly caused by the strong wind stress and surface cooling, leads to high surface layer SSC in winter. High surface layer SSC in summer is restricted in the coastal areas.

ROMS model; turbidity observation; seasonal variation; bottom stress; vertical mixing

1 Introduction

Sediment transport processes have significant influence on the economy and environment in the Bohai Sea, Yellow Sea and East China Sea (BYECS).The suspended sediment concentration (SSC) is one of the most important water quality parameters. SSC is also an important variable, which can reflect the distribution of the mud patches and the paths of the sediment transport in the BYECS.

The studies on sediment transport processes in the BYECS began in the early 1960s (Niino and Emery, 1961). Since the 1980s, a lot of investigations focusing on the SSC distribution and the sediment transport paths in the BYECS have been conducted (DeMaster., 1985; Guo., 1999; Lee and Chough, 1989; Martin., 1993; Milliman., 1985; Milliman., 1986; Milliman., 1987; Milliman., 1989; Pang and Wang, 2004; Yang., 1992). The traditional method for measuring SSC is to take samples by ship, which takes a long time to get samples over the whole BYECS and can only achieve low time-space resolution results. Recently,the Seapoint Turbidity Meter, which is usually attached to CTD (Conductivity, Temperature, Depth Sensors) and ADCP (Acoustic Doppler Current Profiler) measuring systems, and laser sediment particle sensing instruments (.., the LISST series products from Sequoia Scientific, Inc.) have been widely used to measure SSC (Bao., 2010; Bian., 2010; Peng and Gao, 2001; Yuan., 2008a; Yuan., 2008b). These instruments, however, are also unable to achieve high spatial resolution SSC data. Nowadays a lot of studies focus on the use of satellite remote sensing data (Wang and Jiang, 2008; Yuan., 2008c). High time-space resolution sea surface SSC results can be derived from such data; however, measuring SSC in the bottom layer is beyond the ability of satellite remote sensing due to the limited radiance penetration depth in sea water. Numerical models have also been used to simulate the SSC distribution in the BYECS since the 1990s (Jiang., 2004; Jiang., 2000; Li., 2010; Pang., 2004; Yanagi., 1996; Wang and Jiang, 2007). The numerical models can reproduce the principal features of the SSC distribution and demonstrate the dynamic mechanisms of the sediment transport processes. But the sediment transport processes are very complicated and the sediment transport models have to contain many simplifications and parameterizations. Therefore, numerical models cannot provide precise estimates of the SSC distribution or reliable quantitative estimates of the sediment transport flux.

Studies of the sediment transport processes demon- strate that the suspended sediment in the BYECS is mainly from the discharge of the Changjiang River (Yangtze River), the Huanghe River (Yellow River) and the mud patches formed by these rivers. The studies also reveal the distribution and seasonal variation of the SSC in the BYECS. However, there are still different opinions on the dynamic mechanisms of the SSC distribution. Based on the limited observational SSC data, it is hard to reveal the dynamic mechanism of the sediment transport processes in the BYECS completely. Previous model studies did not consider all the important factors which can significantly influence the SSC distribution, such as the subtidal current, tides and waves, or were limited to the Bohai Sea area. In this paper, a numerical model which combines the effects of the subtidal current, tides, waves and river input is applied to study the detailed dynamic mechanisms of the SSC distribution in the whole BYECS. A large quantity of observational SSC data, which reproduces the distribution and seasonal variation of the SSC in the BYECS, is used to verify the model results.

2 Study Area

The Huanghe River and the Changjiang River are the main sources of the suspended sediment in the BYECS. Every year, about 1.08×10t Huanghe River sediment and 4.78×10t Changjiang sediment are discharged to the BYECS (Milliman and Meade, 1983; Milliman., 1985). 70%-90% of the river-discharged sediment is deposited near the river mouth areas and forms the Huanghe River and the Changjiang River mouth area mud patches, and the rest is carried to the open seas and forms mud patches in the BYECS (Alexander., 1991; Bornhold., 1986; Yang and Liu, 2007).

The subtidal currents in the BYECS are very complicated due to the effects of winds, ocean water intrusion (.. from the Kuroshio) and the non-linear effect of the tides and waves. Fig.1 shows a schematic diagram of the most important subtidal currents in the BYECS. The currents play a significant role in the sediment transport processes. The Bohai Sea Coastal Current transports Huanghe River sediment to the Yellow Sea (Bao., 2010; Bian., 2010; Milliman., 1986). In winter, the wind-induced East China Sea Coastal Current carries the Changjiang River mouth sediment to the coasts of the Zhejiang and Fujian provinces forming the offshore mud patches (Milliman., 1989). Some researchers argue that the offshore sediment transport from the Changjiang River and the coasts of the Zhejiang and Fujian provinces is impeded by the Taiwan Warm Current (Guo., 2002; Liu., 2007; Milliman., 1989; Yuan., 2008c). The Huanghe River used to flow into the southern Yellow Sea from 1128 to 1855 and produced the Old Huanghe Delta mud patch. Some authors (Bian., 2010; Milliman., 1985; Milliman., 1989; Yuan., 2008c) argue that in winter, the Yellow Sea Coastal Current brings the Old Huanghe Delta sediment to the East China Sea shelf forming the mud patch to the southwest of Cheju Island, while part of the sediment continues to be carried to the north by the Yellow Sea Warm Current to form the mud patch in the Yellow Sea Trough. The sources of the mud patch in the Okinawa Trough are still unclear, but there is a consensus that the strong Kuroshio Current blocks the shelf sediment from being transported to the Okinawa Trough.

The wind-generated waves are important for the sediment resuspension in the BYECS, especially in the near shore areas. In winter, there are strong winds with wind speeds of 5-10ms, while in summer, the winds become weak with wind speeds of 4-7ms(Sun, 2006). The high energy winter time wind waves can induce strong bottom stress which tends to stir the deposited sediment back into the water and leads to a very high SSC in the coastal areas (Bian., 2010; Li., 2010; Wang and Jiang, 2008). The tidal current is strong in the near shore areas, with the largest tidal current speed more than 150cms, which can readily resuspend the deposited sediment back into the water (Larsen., 1985; Milliman., 1985; Pang., 2004; Sternberg., 1985).

Fig.1 Schematic illustration of topography, currents and fine-grain particulates distribution in the BYECS. The lines with an arrow represent: the Bohai Sea Coastal Current (BSCC), Yellow Sea Warm Current (YSWC), Yellow Sea Coastal Current (YSCC), Taiwan Warm Current (TWC), East China Sea Coastal Current (ECSCC) and Korea Coastal Current (KCC) (Guan, 1994; Yuan et al., 2008c). Shaded areas show the fine-grain sediment distribution, with darker color indicating mud areas (Saito and Yang, 1995). This is the current field in winter; in summer the ECSCC and KCC flow northward and the YSCC disappears.

3 Data and Method

3.1 Observation Data

In this paper, the field data in 4 cruises (June–August, 2006; January–February, 2007) are used to study the SSC distribution in the BYECS. Fig.2 shows the locations of the survey stations. The Seapoint Turbidity Meter, which was attached to the CTD system, can measure the turbidity in water by detecting back-scattered infra-red light (880nm) from the suspended sediment. The turbidity has a linear relation with the SSC, so it can represent the SSC in the BYECS (Bao., 2010; Qin and Li, 1982; Su., 2001); a linearly fitted relationship formula from Bao. (2010) is used to convert the observed turbidity data to the SSC in this study.

Fig.2 Distribution of survey stations and the path of typhoon ‘Ewiniar’. (a) Summer cruise; (b) winter cruise. Dots indicate stations and the solid line with circles shows the path of typhoon ‘Ewiniar’.

3.2 Model Description

The ROMS model is a three-dimensional, free-surface, terrain-following numerical model, which is widely used in the regional sea simulation community. The model domain in this paper covers the whole BYECS (Fig.3) with 5min resolution in latitude and longitude and with 20 layers in the vertical direction. ROMS supports a variety of vertical mixing schemes (Warner., 2005); the popular Mellor-Yamada Level 2.5 Turbulence Closure is used here to represent the vertical mixing(Mellor and Yamada, 1982).

The model bathymetry is interpolated from the 1 min resolution topography data of Choi (1999). However, this dataset only covers the area north of 24˚N in the model domain, so the bathymetry south of 24˚N is interpolated from the ETOPO1 dataset (Amante and Eakins, 2009). At the surface, the model is forced by a monthly climatology of wind stress, heat flux and freshwater flux (DaSilva., 1994), while the bottom heat and freshwater fluxes are set to zero. The model includes 11 rivers in the BYECS (Fig.3), and the river runoff and sediment discharge data are from Dai. (2007) and Chinese River Sediment. The eight principle tidal constituents M2, S2, N2, K2, K1, O1, P1 and Q1 in the BYECS are used as tidal forcing in the model, and the tidal data are derived from the Global Inverse Tide Model dataset (Egbert and Erofeeva, 2002). At the lateral open boundaries, horizontal sea water velocity and sea surface elevation are specified from climatology obtained from the SODA dataset (Carton and Giese, 2008) together with climatological potential temperature and salinity assembled from the World Ocean Atlas 2005 (WOA05) dataset. To reach ocean state equilibrium, the hydrodynamic model is integrated for 10 years before introducing the sediment module, after which the model is run for another 5 years. Only the final year is used for analysis.

The wave forcing data used in the ROMS model are derived by running the Simulating Waves Nearshore (SWAN) wave model offline. The SWAN model uses 6-hourly wind which is derived from the Common Ocean-ice Reference Experiments (CORE) Normal Year Forcing Data (Large and Yeager, 2009) to calculate the wind-generated wave height, wave direction and bottom wave period in the BYECS and outputs these instantaneous wave data every 6h. The ROMS wave module uses these wave data to calculate the near-bed orbital velocity and the wave-current shear stresses which have influence on sediment resuspension.

In the sediment module, 3 sediment classes representing clay, silt and sand sediment are analyzed. Parameters used for the 3 sediment classes are listed in Table 1. The initial suspended-sediment mass in the hydrodynamic model cells is set to be zero. The deposited-sediment is represented by the sediment bed, which is a three-dimensional array with user-defined constant layers (1 bed layer is used here, with bed thickness 10m in this model case) beneath each horizontal hydrodynamic model cell. Initially, deposited-sediments are given only at the Huanghe River mouth, the Old Huanghe Delta and the Changjiang River mouth areas shown in Fig.3. The mass of each sediment class in each bed cell is determined by the sediment density, bed thickness, bed porosity and sediment fraction. When the calculated bottom stress is bigger than the critical shear stress for erosion (see Table 1), the top bed layer deposited-sediment is suspended back to the water cell at a rate given by the surface erosion rate (5.0×10kgms). On the other hand, when the bottom shear stress is smaller than the critical shear stress, the suspended-sediment in the water cell is deposited in the top bed layer at a rate depending on the particle settling velocity. Details of the calculation method of both the hydrodynamic model and sediment-transport module are described by Warner. (2008).

The sediment settling velocity is a key variable in the sediment suspension processes. In reality, the settling velocity depends on the sediment properties, surrounding SSC, turbulent mixing and interaction with other particles. The standard ROMS sediment module simplifies the sediment settling velocity to be constant in time. The simplification makes it difficult to quantitatively simulate the sediment transport processes in the BYECS. Therefore, a fine-grain sediment settling velocity calculation formula (Mehta and McAnally, 2008), which considers the influence of free settling, flocculation settling and hindered settling, is added to the model to estimate the settling velocity of the clay sediment. The estimation of the silt and sand sediment settling velocities is based on the Stokes’ Settling Law.

Fig.3 Model domain. The circles represent the river mouth locations in the model; the shadow areas show the initial fine-grain sediment locations.

Table 1 Parameters used for the 3 sediment classes

4 Results

4.1 Observed SSC Distribution

Fig.4 shows the observed SSC distribution in the BYECS. The summer and winter SSC distribution of the surface and bottom layers are used to study the sediment transport processes in the BYECS. During the CTD system deployment in the survey, a minimumdistance was main- tained between the CTD system and the seafloor to prevent damage to the instrument. Usually, this distance is several meters in the continental shelf areas and dozens of meters, even more than 100 m in the outer shelf sea areas. So the bottom layer adopted here is at the deepest depth of the CTD deployment, it is not the actual bottom layer.

The horizontal SSC distribution shows that high SSCappears in the coastal areas of the BYECS while the outer shelf seas have lower observed SSC. The coastal areas have a large amount of fine-grain sediment shallow water depth,which makes the sediment readily suspendable. However, the SSC in the outer shelf seas is extremely low due to the deep water depth and coarse-grain sediment coverage. The vertical SSC distribution shows that the bottom layer SSC is higher than the surface layer SSC. This is because the settling velocity of the sediment and the stratification of the ocean inhibit the suspended sediment moving to the surface layer. The seasonal variation of the SSC distribution shows that the winter SSC is higher than the summer SSC. In winter, the strong wind-waves can enhance the bottom stress which entrains the seabed sedimentinto the water column and the strong vertical mixing carries the suspended sediment to the surface layer. In summer, the weaker winds cannot keep the sediment suspended, which leads to a lower SSC in the BYECS. However, there are extreme high SSC levels in the bottom transects S03, S04 and SFJ in summer. Bian. (2010) demonstrated that these extraordinarily high SSCs in summer are caused by the typhoon-induced wavesand swells. Previous observations in summer showed much lower SSC in these areas(Milliman., 1989; Pang and Wang, 2004; Yang., 1992).

To sum up, there are 3 principal characteristics of the SSC distribution in the BYECS.First, the SSC in the coastal area is higher than the SSC in the outer shelf sea area; second,the SSC in the surface layer is lower than that in the bottom layer; third, the winter time SSC is higher than the summer time SSC. The SSC distribution characteristics based on the turbidity data presented here are consistent with previous observed SSC results (Guo., 2002; Milliman., 1989; Milliman., 1986; Pang and Wang, 2004; Wang and Jiang, 2008; Yang., 1992; Yuan., 2008c).

Fig.4 The observed SSC distribution in the BYECS.

4.2 Modeled SSC Distribution

Fig.5 shows the January and July (representing the winter and summer) SSC distribution in the surface and bottom layers in the 5th model year. A comparison between the modeled SSC and observed SSC (Fig.4) shows that the modeled SSC can represent the SSC distribution in the BYECS.

In summer, the modeled SSC distribution (Figs.5a, 5b) shows that turbid water (SSC>100mgL) covers the coastal areas of the East China Sea and the bottom Bohai Sea. A bottom turbid water plume extends from the East China Sea coastal area to the middle shelf area, and some sediments also spread to the Korea Peninsula coastal area. The surface layer high SSC is only found in the coastal areas. The modeled SSC distribution is similar to that observed (Fig.4). There was no observation station in the Bohai Sea during the cruises (Fig.2). A comparison between the modeled SSC and other observed SSC in the Bohai Sea (Jiang., 2002) shows that in summer the modeled bottom SSC is extremely high, up to 10 times larger than the observed SSC. The parameters for the sediment model (Table 1) have not been widely tested for their suitability in the BYECS, which may be the reason for the higher modeled SSC.

In winter, the modeled SSC distribution is consistent with the observed SSC distribution: the surface layer SSC (Fig.5c) is lower than that in the bottom layer (Fig.5d); high SSC is found in the coastal areas in the BYECS; there is turbid water (SSC>20mgL) to the north and east of the Shandong Peninsula near shore areas; in the East China Sea, the turbid water plume reaches 126˚E; in the southwest East China Sea, the high SSC spreads to Taiwan Island and shrinks back to the coastal areas around 26˚N. Like the extremely high SSC in the bottom Bohai Sea water in summer, the model does not simulate the SSC very well in the mentioned East China Sea middle shelf area. And the modeled SSC is about 2-5 times larger than the observed SSC in the East China Sea middle shelf area.

From the aforementioned comparison between the modeled SSC distribution and the observed one, it can be concluded that the modeled SSC is higher than the observed in some areas. However, the principal features of the SSC distribution in the BYECS are well reproduced in the modeling. The model results can be used to study the dynamic mechanisms of the SSC distribution in the BYECS.

Fig.5 The modeled SSC distribution in the BYECS. (a) Summer surface layer; (b) Summer bottom layer; (c) Winter surface layer; (d) Winter bottom layer.

5 Discussion

5.1 Dynamic Mechanisms for the Sediment Erosion Processes

In the BYECS, the sediment erosion processes are mainly forced by the bottom stress, which ismainly induced by the high waves and strong currents. The tidal currents dominate the current-induced bottom stress pattern in the BYECS, for the averaged maximum speed of the tidal currents is 40-120cmssFig.6 shows the current-wave induced bottom stress distribution in the BYECS.

The summer time total bottom stress distribution is sim- ilar to that in winter (Figs.6a, 6b). The total bottom stress in most areas of the BYECS is larger than 0.1Nm, which can resuspend the fine-grain sediment with grain size smaller than 0.032mm (Table 1). Therefore, high SSC is found in the areas covered by fine-grain sediment in the model. There are two regions of strong bottom stress (>0.4Nm) in the BYECS: one is the near shore area of the West Korea Peninsula, another extends from the west coast of the East China Sea to the middle shelf of the East China Sea along the 50misobath.In these areas, even the fine-sand sediment (grain-size: 0.128mm) can be readily entrained into the water column by the strong bottom stress. Therefore, the SSC in these areas should be very high. However, the simulated SSC is lower than expected in the nearshore areas of the West Korea Peninsula. This is because no fine-grain sediment was assumed to be presented here in the model initial condition. Due to the lack of fine-grain sediment, the model does not show high SSC in the near shore areas west of the Korea Peninsula. The observed low SSC in these areas also supports this view(Wang and Jiang, 2008; Yuan., 2008c). Weak bottom stress is found to the northeast of the Shandong Peninsula, the middle Yellow Sea and the Okinawa Trough areas. These areas provide a suitable environment for sediment deposition. When sediment is deposited in these areas, the weak bottom stress is not able to resuspend the sediment again.

The comparison between the wave-induced bottom stress (Figs.6c, 6d) and the current-induced bottom stress (Figs.6e, 6f) shows that the latter is much stronger than the former. This means the tidal current plays a more significant role in the sediment erosion processes than the wind waves do. Field observations in the Yellow Sea also showthat even though the wind forcing persisted through- out the whole observation period, there was no appreciable wind-wave induced bottom stress and the tidal-current induced bottom stress was the main mechanism for sediment resuspension(Yuan., 2008a). The strong wave-induced bottom stress is restricted to the very near shore areas. The influence of the waves on the sediment resuspension can reach 100m isobath in the East China Sea in winter and the influence shrinks back to 50m isobath in summer. The wave-induced bottom stress is not simulated very well in the modeling. The wave-induced bottom stress is supposed to be much stronger in winter than in summer due to the strong winds in winter. However, there is no obvious difference between the summer and winter wave-induced bottom stresses.

Fig.6 Bottom stress in the BYECS. (a) Total summer bottom stress; (b) Total winter bottom stress; (c) Summer wave-induced bottom stress; (d) Winter wave-induced bottom stress; (e) Summer current-induced bottom stress; (f) Winter current-induced bottom stress.

5.2 Dynamic Mechanisms for the Sediment Vertical Movement Processes

The vertical transport of sediment in the water column is the same as for tracers like potential temperature and salinity except that there is an additional settling velocity for the sediment. The vertical mixing dominates the vertical movement of the suspended sediment in the water column, so only the effect of the vertical mixing on the SSC distribution is discussed in this section.

The surface mixed layer is the upper boundary layer of the ocean, which is well mixed by the wind stress, surface cooling and ocean interior physical processes. It is a leading indicator for the vertical mixing in the upper ocean. There are various of ways to define the mixed layer depth. In this paper, the depth at which the density changes from the ocean surface by 0.125 (sigma units) is defined as the mixed layer depth. Fig.7 shows the summer and winter mixed layer depth in the BYECS. In summer, only the very near shore areas and the Old Huanghe Delta area are well-mixed. On the other hand, the mixed layer depth in the continental shelf area is less than 6m and in the outer shelf seas the corresponding depth is around 10m. The decreased wind stress and the net heat and fresh water input increase the stratification of the upper ocean in summer, which inhibits the bottom layer sediment from being entrained to the surface layer. In winter,the well-mixed areas extend to the 20m isobath and the whole Bohai Sea is vertically uniformly mixed.The mixed layer depth in most areas of the BYECS exceeds 30m.The strong vertical mixing in winter is mainly caused by the strong wind stress and the surface cooling, while the vertical mixing isweakened by thestratification in summer. The strengthened vertical mixing transports the bottom layer sediment to the sea surface and leads to higher surface layer SSC in winter than in summer.

Fig.7 Mixed layer depth distribution in the BYECS. The blank area means the water column is well-mixed from surface to bottom. Different colorbars are used in summer and winter.

The turbulent kinetic energy is also an important variable, which indicates the strength of the vertical mixing. The stratification in the BYECS impedes the vertical mixing of the seawater so it traps the sediment in the sea bottom layer. If the turbulent kinetic energy is strong enough to overcome the potential energy of the stratification, the bottom layer sediment can be entrained to the sea surface by the strong vertical mixing. Fig.8 shows the vertical turbulent kinetic energy distribution along 32˚N. In summer, the strong turbulent kinetic energy (larger than 3.0×10s) is found only in the near shore area. The turbulent kinetic energy in the surface layer and the outer shelf seas is very weak. In winter, the bottom layer turbulent kinetic energy is very strong in the continental shelf areas west of 125.5˚E. The turbulent kinetic energy in the surface layer and the outer shelf sea areas is also stronger in winter than the summer, which means there is stronger vertical mixing in winter than in summer.

Even though the strong bottom stress in summer can entrain the sediment into the water column, the stratification inhibits the upward transport of the sediment. As a result, the high SSC in summer is found only in the bottom layer and the high SSC water in the surface layer shrinks to the coastal areas (Fig.5). In winter, the strongwind stress and the surface cooling lead to a deeper mixed layer and stronger turbulent kinetic energy at the bottom. Therefore, the winter water column is well-mixed and a large amount of sediment is entrained to the sea surface, which causes the high SSC in winter in the surface layer (Fig.5c).

Fig.8 Vertical turbulent kinetic energy distribution along 32˚N.

6 Conclusions

In this paper, the SSC distribution and sediment transport processes are studied based on the observed SSC data and ROMS model results. The observed SSC distribution shows that the SSC in the outer shelf seas is much lower than in the coastal area, the SSC in the surface layer is much lower than in the bottom layer, and the SSC is higher in winter than in summer. Even though the modeled SSC is higher than the observed SSC in some regions in this study, the comparison between the observed and modeled SSC results shows that the model can reproduce the pattern of the SSC distribution in the BYECS.

The dynamic mechanisms of the sediment resuspension and vertical movement processes in the BYECS are analyzed based on the modeled bottom stress, the mixed layer depth and the turbulent kinetic energy. The study shows that the current-induced bottom stress is much stronger than the wave-induced bottom stress, which means that the tidal current is the most important factor for the sediment resuspension processes in the BYECS. The vertical mixing strength, which determines the vertical SSC distribution, is studied based on the mixed layer depth and the turbulent kinetic energy in the BYECS. In winter, the large mixed layer depth in the upper ocean and the high turbulent kinetic energy near the bottom lead to strong vertical mixing which can transport the bottom layer sediment to the sea surface. In summer, the high SSC is trapped in the bottom layer and the near coastal areas due to the strong stratification and the weak turbulent kinetic energy.

There are some limitations in our study. For example, the modeling does not show obvious seasonal variation of the wave-induced bottom stress. The vertical distribution of the SSC in the BYECS is not simulated well due to the parameterized sediment settling velocity in the sediment module. The climatological wind stress used in this study cannot capture episodic events such as winter storms and summer typhoons that can resuspend a large amount of sediment in a short time. Moreover, the constant critical erosion stress and surface erosion rate also have negative effects on the sediment model results. The sediment model still needs to be improved in future to get more accurate sediment transport processes results.

Acknowledgements

This paper is supported by the China Scholarship Council and the National Basic Research Program of China (973 Program 2010CB428904 and 2005CB- 422300). We thank GEOMAR for support and the provision of computing facilities during the time this work was carried out. We are grateful for the work of the developers of the ROMS model.

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(Edited by Xie Jun)

10.1007/s11802-013-1916-3

ISSN 1672-5182, 2013 12 (3): 345-354

. Tel: 0086-532-82031226 E-mail: bianchangwei@ouc.edu.cn

(December 6, 2011; revised February 21, 2012; accepted January 22, 2013)

© Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2013