Wenping Gong
email: wpgong@vims.edu

Jian Shen

Jianjun Jia

Xiaohai

Hainan

The impact of human activities on the flushing properties of a semienclosed lagoon

Keywords: Residence times, Flushing efficiency, Lagrangian particle tracking, Random walk model, ELCIRC, Hainan, China

In this study, a Lagrangian particle tracking model driven by hydrodynamic fields was used to investigate the changes of flushing properties in Xiaohai Lagoon resulting from natural evolution and influences of human activities. Comparisons of residence times between 1936 and 2001 indicate that the flushing efficiency of the lagoon has deteriorated greatly during the past 7 decades. Over this period, the average residence time of the lagoon has increased from 45 to 71 days, an increase of 59%. The Lagrangian residual velocity has decreased from 1936 to 2001 in the large portions of the lagoon, and the spatial distribution pattern of the residual current has changed significantly. The well developed horizontal circulations in 1936 disappeared. Instead, the horizontal Lagrangian residual velocity showed uniformly seaward motion with reduced velocity in 2001. Human activities have incurred great impacts on the deterioration of flushing efficiency. The reduction of river inflow imposed the most significant impact, causing an increase of average residence time by 33%. The land reclamation in the lagoon came as the second most significant factor, causing an increase of 15%. Closure of the North Opening had posed minor impact. The model results suggest that restoring the river inflow, as well as the dredging of the tidal channel and the inner lagoon, should be the top proprieties for future water quality management.

Introduction

Xiaohai Lagoon is located along the east coast of Wanning County, Hainan, China (Fig. 1). It has a surface area of approximately 30 km 2 with a mean water depth of 0.8 m (relative to mean sea level). Historically, it has been a main fishery port and an important habitat for some precious fish, shrimp, and crab along the eastern coast of Hainan Island.

In recent decades, aquaculture activities have been developed tremendously in the lagoon.

A tidal flat area of approximately 8 km 2 has been reclaimed for constructing shrimp pools, and a total of 5000 fishing cages have been installed along the tidal channel for fishfarming. With the rapid increase of aquaculture, a lot of environmental problems have arisen. More and more input of the land-based pollutants has been caused by deforestration and widespread use of pesticides and fertilizers. Additionally, there have been pollutants from shrimp pools, fish wastes and unconsumed fish feed and a decrease of the flushing efficiency of the tidal dynamics. Consequently, the eutrophication in the lagoon has become a concern. Occasional fish-kill events and subsequent huge economical losses have been reported.

Many studies have been conducted relevant to Xiaohai Lagoon's hydrodynamics, sediment transport, morphological evolution, and the inlet's stability and water quality [START_REF] Wang | The morphodynamics of Xiaohai Inlet and its stability analysis[END_REF][START_REF] Hu | Basic hydrologic characteristics of Xiaohai Lagoon in Hainan Island[END_REF][START_REF] Chen | The integrated environmental improvement of Xiaohai Lagoon, Wanning City[END_REF][START_REF] Wang | Self-adjustment and deterioration of Xiaohai Barrier-lagoon-tidal inlet system in Hainan Island[END_REF][START_REF] Gong | Feedback between tidal hydrodynamics and morphological changes induced by natural processes and human interventions in a wavedominated tidal inlet: Xiaohai, Hainan, China[END_REF]. [START_REF] Chen | The integrated environmental improvement of Xiaohai Lagoon, Wanning City[END_REF] estimated that the water renewal period is approximately 120 days during which time 99% of the original water mass is replaced. This is based on the water exchange rate defined as the ratio between the tidal prism and the total water volume in the lagoon during the spring tide period and assuming that the original water mass in the lagoon decreases exponentially. This simple estimation did not consider the factor of return flow [START_REF] Luketina | Simple tidal prism models revisited[END_REF]. Also it did not resolve the temporal and spatial variability of the flushing properties in the lagoon, which perhaps are most important for environmental management. A detailed study of variation of the flushing properties in the lagoon, both spatially and temporally, is urgently needed.

From a water quality management perspective, some major questions still remain regarding the status of the flushing efficiency of the lagoon at present compared with that in its pristine condition 7 decades ago. Also, what degree of changes of the flushing efficiency has the lagoon experienced, and what roles have human interventions played in causing these changes? We believe that proper management of the water quality in the lagoon for the future becomes possible only after these problems have been adequately addressed.

Residence time is a convenient time-scale to represent the flushing properties of lagoons. There is a diversity of definitions and computation methods of this time-scale (see, for example, [START_REF] Bolin | A note on the concepts of age distribution and transit time in natural reservoirs[END_REF][START_REF] Zimmerman | Mixing and flushing of tidal embayments in the Western Dutch Wadden Sea, Part I: Distribution of salinity and calculation of mixing time scales[END_REF]Takeoka, 1984;[START_REF] Oliveira | Diagnostic modeling of residence time in estuaries[END_REF][START_REF] Shen | Calculation age and residence time in the tidal York River using three-dimensional model experiments[END_REF][START_REF] Wang | Residence time of the Danshuei River estuary, Taiwan[END_REF][START_REF] Delhez | Residence time in a semienclosed domain from the solution of an adjoint problem[END_REF][START_REF] Jouon | Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamic model[END_REF].

In this study the time scale is defined based on Takeoka (1984) as 'the residence time at a given location in the lagoon is the time it takes for a water parcel, initially located at the location considered, to leave the lagoon.' This definition stands for the spatial variability of flushing properties in the lagoon. The residence time for the whole lagoon can be expressed by a spatially averaged residence time [START_REF] Jouon | Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamic model[END_REF] or by the time at which the amount of released particles that leave the lagoon is 63.21% [( 11 - e )], 50%, or 99% of the total original amount of particles [START_REF] Oliveira | Diagnostic modeling of residence time in estuaries[END_REF]. In this paper, the spatial average residence time has been utilized.

In this study, a calibrated model was used to provide the hydrodynamic field to drive

Lagrangian particle tracking to study the residence times of the Xiaohai lagoon at present and 7 decades ago. Based on the analysis of model results, the cause of changes of the average residence time and the spatial distribution of residence times were examined. The impacts of human activities on the changes of lagoon's flushing efficiency were investigated, and some suggestions were proposed for the future water quality management.

Study site

Xiaohai Lagoon is located on Hainan Island, which is the largest tropical island in China. It lies along the eastern shore of Wanning County, Hainan Island, with a latitude ranging from 18°47' to 18°53'N and a longitude ranging from 110°26' to 110°34'E. It is a typically well-developed tidal inlet-lagoon system with a north-southward length of 10 km, and an east-westward width of 3.5 to 7.0 km. In 2001, it consists of a north sand barrier, a south sand barrier, a flood-tidal delta, a sand spit, a tidal inlet, a tidal channel, and an inner lagoon (Fig. 1). In this study, inner lagoon is referred to as the area where the tidal channel disappears and the tidal current is weak, and separated from the outer lagoon by the line which connects two coastline promontories from the tidal channel area (Fig. 1). Near the inlet throat, there are two bedrock islands, Neizhi and Waizhi, which are composed of granite rock.

Xiaohai is located within the tropical monsoonal climate zone. The average river discharge from the surrounding watershed to Xiaohai Lagoon was approximately 46.72 m 3 s -1 per year before the river diversion works completed in Taiyang River in 1972. After that, the river discharge decreased by about half to 26.46 m 3 s -1 . The discharges of the surrounding rivers are shown in Table 1.

The average tidal range is 0.71 m at Neizhi Island with an irregular diurnal type. The salinity at the outside of the lagoon varies from 26 to 34 ppt and varies from 9.5 to 14.0 ppt in the inner lagoon. The salinity is lower than 5 ppt near the river entrances. Due to the shallow water depth and low freshwater discharge, the water column is generally well-mixed except temporal occurrence of stratification during flood events (Hu et al., 1994). Therefore, a 2-D barotropic model is suitable in this study.

During the past 7 decades, Xiaohai Lagoon has endured dramatic morphological and hydrodynamic changes caused by both natural processes and human activities (Gong et al., submitted). Prior to 1972, natural processes controlled the evolution of the system.

There

Methodology

In this study, a Lagrangian particle tracking method was used to study the residence times in Xiaohai Lagoon. Lagrangian models appear as a very handy and naturally suited set of tools to investigate and model the transport pathways, mixing and estuary-ocean exchange of dissolved pollutants [START_REF] Dimou | A Random-walk, particle tracking model for wellmixed estuaries and coastal waters[END_REF]Bilgil et al., 2005;[START_REF] Burwell | Modeling residence times: Eulerian vs Lagrangian[END_REF].

A hydrodynamic model of Xiaohai Lagoon (Gong et al., submitted) was utilized to generate the hydrodynamic fields for driving the Lagrangian particle motion. The depthaveraged barotropic model of ELCIRC [START_REF] Zhang | A cross-scale model for 3D baroclinic circulation in estuary-plume-shelf systems: I. Formation and skill assessment[END_REF] Lagrangian particle tracking was performed in a 2-D mode, using a 4 th order Runge-Kutta (RK) scheme for particle advection. This was combined with a random walk model of horizontal eddy diffusion that enables a particle-based statistical treatment of turbulent mixing. The position of a particle is given as:

t d z t u t x t t x h n ∆ + ∆ + = ∆ + * * 2 * ) ( ) ( ( 1
)
Where x is the particle's position, t ∆ is the time step, u is the velocity vector, h d is the horizontal diffusion, n z is a random vector normally distributed with an average value of zero and unit standard deviation, which was obtained based on Press et al. Because small portion of particles located near both eastern and western corners of the inner lagoon were almost trapped completely, 180 days were used as truncation without losing significance. The diffusion coefficient was set as 0.01 m 2 s -1 uniformly based on [START_REF] Oliveira | Diagnostic modeling of residence time in estuaries[END_REF]. The definition of 'once-through' instead of 're-entrant' residence time [START_REF] Oliveira | Diagnostic modeling of residence time in estuaries[END_REF] was used because the current is strongly ebb-biased in the inlet and a long-shore current prevails outside the inlet (see later sections). The time at which the particle left the inlet was recorded as the residence time of that particle. For the particles that still stayed in the lagoon at the end of the simulation, the residence times were specified as 180 days. The results obtained for eight particle release simulations were averaged to represent the residence times in the lagoon in each scenario.

The change of residence times from 1936 to 2001

The Although the particle traveled through a very large cross-sectional area, it moved out of the lagoon rapidly. The well-developed horizontal circulation was clearly seen in this transport pattern and played a role in decreasing the average residence time of the lagoon.

The impacts of human activities on the change of residence times

The natural evolution of the system had been interrupted greatly by human interferences since 1972. To concentrate the flow in the South Opening, an effort was made to close the North Opening artificially. Meanwhile, Taiyang River was diverted directly into the ocean for irrigation and river flood mitigation purposes that resulted in the river inflow to the lagoon being reduced by 50%. The old flood-tidal delta and nearby tidal flats were reclaimed and merged into land area. As a result, the southern tidal channel disappeared. A large bend was formed from the inlet throat to the inner lagoon along the tidal channel. Impacted by these human activities, the dynamics of the lagoon had been altered. It was expected that the flushing ability of the lagoon would also be changed.

To In the side corners of the inner lagoon, the residence times became longer.

Influence of river flow reduction

The

Influence of closure of the North Opening

The flushing efficiency of a lagoon depends on both the tide and freshwater water discharges. The characteristics of tidal propagation from the outside into the lagoon can be changed once the morphology of the inlet is changed. However, the influence of

Influence of land reclamation

Increase of land reclamation in the lagoon will decrease the total volume of the lagoon.

Additionally, the change of morphology of the lagoon due to land reclamation can have a profound impact on the dynamics of the lagoon. With the increase of human activities of land reclamation, the southern tidal channel and the nearby tidal flats became land, which resulted in narrowing of the entrance to the inner lagoon and decreasing the intertidal zone. Consequently, it restricted the tides from propagating into the lagoon and reduced the tidal prism.

To understand the influence of land reclamation on residence time of the lagoon, a scenario was conducted by merging the southern tidal channel and the nearby tidal flats With the decrease of tidal prism, the relative effect of river discharge became more important. The impact of river discharge from the Beipo River on the residence time can be seen along the central axis of the inner lagoon, which separated the eastern and western regions of the lagoon.

Discussion

Sensitivity of diffusion coefficient

The accuracy of the residence time calculation using Lagrangian particle tracking depends on a number of factors. The release time of the particles with respect to tidal phase is one of them [START_REF] Awaji | Water mixing in a tidal current and the effect of turbulence on tidal exchange through a strait[END_REF][START_REF] Oliveira | Diagnostic modeling of residence time in estuaries[END_REF]Bilgil et al., 2005). Usually, the residence time calculated from an ebb time release is shorter than that calculated from a flood time release. The tidal range can also play a role in residence time. In a common sense, the larger tidal range during the spring tide increases tidal prism and, thus, increases tidal velocity and leads to a shorter residence time than occurs during the neap tide. However, the study of [START_REF] Oliveira | Diagnostic modeling of residence time in estuaries[END_REF] demonstrated that this is not always the case. In their study, larger tidal ranges do not always lead to faster removal of particles and the removal rate depends on the release time. Another important factor is the horizontal diffusion coefficient. As stated by Oliveira et al. (2006a), the choice of the horizontal diffusion coefficient in the particle tracking simulations should be based on the physics represented in the model and on the horizontal grid resolution. A coefficient of 0.01 was used in their study. Another study in the Guadiana Estuary by Oliveira et al. (2006b) showed that the differences in residence times between the two simulations with different diffusion coefficients are small compared to the variability related to the release times, and are much smaller than those associated with a reduction of river flow. In contrast, the simulation experiments conducted by Bilgil et al. (2005) showed that the presence of diffusivity is of prime importance to the exchange. Their results showed that allowing even weak diffusion is sufficient to cause a significant decrease in the fraction of particles retained, but increasing the diffusivity by a factor of 10 leads to little further exchange, except in those areas dominated by diffusion.

In this study, the residence time has been calculated by averaging eight simulations with particles released at slack before ebb, peak ebb, slack before flood, and peak flood during both spring and neap tides. Therefore, the effects of the release time relative to tidal phase and the tidal range have been taken into consideration. On the other hand, the sensitivity of the model results with regard to the diffusion coefficient needs to be examined further. It suggests that the use of a small diffusion coefficient in the model simulations is reasonable.

Influence of hydrodynamics on the residence time

Measures to improve the flushing efficiency of the lagoon

From the above studies, the flushing efficiency of Xiaohai Lagoon has deteriorated greatly from 1936 to 2001, with the average residence time increasing 59 %.

For the improvement of flushing efficiency of the lagoon, firstly, completely or partially restoring the river inflow to its level before 1972 can be the most effective way; secondly, combined with other harnessing measures for improving the stability of the tidal inlet and mitigating river inundation disaster, dredging of the tidal channel and the inner lagoon can also make great difference. Reopening the North Opening can have little effect on improving the flushing efficiency of the lagoon and should not be considered as an effective management option.

Conclusion

In this study, the hydrodynamic model with Lagrangian For the factors that caused these changes, human activities have incurred great impacts on the deterioration of flushing efficiency. The reduction of river inflow imposed the most significant impact, causing an increase of average residence time of 33%. The land reclamation in the lagoon came as the second most significant factor, causing a increase of 15%, while closing the north inlet posed a relatively minor impact. For the future water quality management, restoring the river inflow and dredging of the tidal channel and the inner lagoon should be placed at the top proprieties. List of tables:

  were two openings of the inlet, the North Opening and South Opening, separated by Neizhi Island. The inlet was characterized by a southward extension of the north sand barrier due to the predominant southward longshore sediment transport, resulting in narrowing and shoaling of the North Opening. The flood-tidal deltas underwent a series of mergings and expansions. In contrast, the south sand barrier extended northwardly mainly during the summer season when SE waves prevailed. The relatively large runoff during summer had a great effect on flushing out the sediments and pollutants. After 1972, human interventions changed the morphology of the system. The North Opening was closed, and the morphological evolution was characterized by a continuous extension of the south sand barrier and narrowing of the inlet. A large portion of tidal flats was reclaimed for constructing shrimp pools. With the morphological evolutions, the tidal hydrodynamics has experienced great changes as well. The tidal propagation from the nearshore into the lagoon has been impeded and the tidal amplitude in the lagoon has decreased, which resulted in a decrease of the spring tidal prism from 3.68×10 7 m 3 in 1936 to 0.5×10 7 m 3 in 2001. The velocity asymmetry has changed from ebb-dominant to flood-dominant, and the relative importance of the river inflow to the tidal effect has increased. These morphological and hydrodynamic changes must have led to significant modifications of the flushing efficiency of the lagoon.

  scheme for the advection term, a large time step of 60 s was used in the simulation. In

(

  1994). Due to the relatively large timestep used in the hydrodynamic model, the timestep was divided into multiple sub-time steps to track particles for meeting the Courant-Friedrichs-Levy (CFL) condition. During every sub-time step, the movement of the particle due to advection was calculated by the 4 th order RK scheme following Blanton (1993). Combined with a quick search algorithm embedded in ELCIRC for determining the destination cell into which the particle moved, the velocity at the particle's position was obtained by linear interpolation if the particle was in a triangle cell or by bilinear interpolation if in a quadrilateral cell. When the particle moved across a land boundary or from a wet cell to a dry cell, only the tangential velocity at the edge was used to prevent the particle being transported outside of the land boundary or moving into land areas. In this study, the model was configured to represent both 2001 and 1936 bathymetries and river discharges, respectively, for different modeling scenarios. The bathymetries of the lagoon in 2001 and 1936 are shown in Figs.1 and 2, respectively. In each case, the model was run with the particles released at slack before ebb, peak ebb, slack before flood, and peak flood during both spring and neap tides. For each simulation, the particles were placed in every nodal point in the lagoon and the simulation lasted for 180 days.

  Fig.5ashows the trajectories of selected particles released at different parts of the

  Fig. 6a. The average residence time was 73 days. It can be seen that the particles could

  change of freshwater discharge can lead to a change of advection and residual circulation. Freshwater inflow is one of the dominant factors of residence time change in large estuaries(Shen and Lin, 2004). It was expected that residence time of the lagoon would change correspondingly resulting from the 50% reduction of river inflow from 1972 to 1975. A scenario with restoration of river discharge to the level before 1972 was simulated. The result is shown in Fig.6b. The difference of residence between increase of river and baseline condition is shown in Fig.7a. It can be clearly seen that the residence time of the whole lagoon decreased significantly with the increase of river inflow compared to the baseline result (Fig.6a). The average residence time of the lagoon became 54 days, showing that the average residence time increased 33% after reducing the river inflow. Compared to the baseline situation, the particles in the inner lagoon and along the tidal channel could transport out of the lagoon more rapidly. A sharp transition of residence time was evident along the central axis linking the end of the tidal channel and the entrance of Beipo River. The residence time was shorter along the east side of the lagoon than along the west side. Except for the particles in the side corners of the inner lagoon, most of the particles could move out the lagoon during the simulation period. The influence of the shallow area between the end of the tidal channel and the inner lagoon on residence time in the central portion of the inner lagoon was reduced, probably resulting from strong advection enhanced by the increase of freshwater discharge.

  closure of the North Opening on the lagoon residence time is not clear. A scenario with the North Opening open was conducted. In order to amplify the effect, we set the width of the North Opening to be 140 m, which is about 2 times of the width in 1972. The result is shown in Fig. 6c. The average residence time became 72 days, which was slightly reduced compared to the result of the baseline condition. The distribution pattern of the residence times was similar to the baseline result. The main change took place at the northern tidal channel. With the North Opening open, particles at the northern tidal channel could transport through the North Opening freely to open sea. However, for most of the lagoon, the changes were minor, showing that closing the North Opening had a minor effect on the flushing efficiency of the lagoon.

  to the southern bank. The new geometry resembled the condition of 2001 except that the water depth in the tidal channel and the inner lagoon was larger than in 2001. With the land reclamation, the southern tidal channel no longer existed and the only channel that connected the inlet and the inner lagoon was through the northern tidal channel. The model result after land reclamation is shown in Fig. 6d. The difference between increase of land reclamation and baseline condition is shown in Fig. 7b. The average residence time was 84 days, an increase of 15% compared to the baseline result. The contours of residence time generally shifted seaward. The 25-day contour lines before land reclamation were virtually replaced by the 50-day contour lines. The areas, where residence time exceeded 150 days, increased significantly in the east corner of the lagoon.

  The above model simulations used a uniform diffusion coefficient of 0.01 m 2 /s. To test the sensitivity of the selection of the diffusion coefficient on the residence time, we conducted a series of model experiments with respect to the use of different diffusion coefficients. A baseline simulation was conducted using 2001 bathymetry without freshwater discharge and without diffusivity. All particles were released at slack before ebb during spring tide. The simulation lasted for a total of 90 days. The average residence time of all particles is 79 days. Model results with the use of different diffusion coefficients are shown in Fig. 8. It can be seen that including a weak diffusion can sharply decrease the average residence time, while further increasing the diffusivity results in a slight decrease of the average residence time. Model experiments involving the annual mean river inflow in 2001 were also conducted. The results with different diffusion coefficients are also shown in Fig. 8. It can be seen that the average residence time of the lagoon is greatly reduced. The results show that the impact of horizontal diffusivity on residence time is insignificant compared to the influence of river discharge.

  Model simulation shows that the average residence time has increased greatly from 1936 to 2001. This change should be largely attributed to the corresponding hydrodynamic changes in the lagoon. A comparison of tidal propagation from the nearshore coast to the inner lagoon is shown in Fig. 9. The model results show that tidal dissipation increases continuously. The tidal energy dissipation was less than 1/10 in 1936 due to its relatively large depth and the open connection between the lagoon and the sea. In 1975, the amplitude of K 1 decreased to approximately 1/3 of its value outside the inlet, as did the amplitude of O 1 . The amplitude of 2 M in the inner lagoon was about 1/6 of that outside the inlet. In 2001, the amplitudes of 1 K and 1 O have decreased to be about 1/7-1/8 of their values outside the inlet, and the amplitude of 2 M in the inner lagoon was about 1/14 of its value outside the inlet. With the narrowing and shoaling of the tidal inlet, the dissipation of tidal energy increasingly resulted in poor circulation of the water in the inner lagoon. Along with the tidal energy dissipation, the tidal prism of the lagoon experienced the same trend of decrease as the tidal amplitude. In 1936, the spring tidal prism of the lagoon was 3.68 ×10 7 m 3 . In 1975, it decreased to 1.114×10 7 m 3 , and in 2001 it decreased to 0.5×10 7 m 3 . With the above notable changes of the tidal dynamics and morphological evolutions, the Lagrangian residual current also changed significantly. In 1936, strong horizontal circulation was well developed in the lagoon. Although the particles took complex pathways to be transported out of the lagoon, the residence time was much less due to strong residual current. In 2001, the residual current became uniformly seaward, but much weaker. Particles moved back-and-forth very slowly, and this resulted in the increase of the residence time in the lagoon. It is noteworthy that the spatial variability of the flushing properties of the lagoon is important. For example, the spring tidal prism decreased from 1.114×10 7 m 3 in 1975 to 0.5×10 7 m 3 in 2001 and the volume of the lagoon below the mean low water decreased from 3.836×10 7 m 3 in 1975 to 2.887×10 7 m 3 in 2001. The estimated flushing times ((V+P)/(P+R), where V is the water volume below mean low water, P is the tidal prism, R is the river discharge during a tidal cycle) were 3 and 5 days in 1975 and 2001, respectively. However, based on our model results, the average residence time in 1975 was longer than that in 2001. The difference can be attributed to the spatial variation of the flushing capability. Since the lagoon is not spatially homogeneous, using a simple method to estimate the flushing time is not accurate. The decrease of the average residence time from 1975 to 2001 can be partially attributed to the change of bathymetries, such as the disappearance of the shoal between the tidal channel and the inner lagoon, which could have decreased the residence times at the tidal channel area and the inner lagoon, thus decreasing the average residence time in 2001.

  particle tracking was used to investigate the changes of flushing properties in Xiaohai Lagoon and the impact of human activities on these changes. The simulations of the 1936 and 2001 circulations demonstrated the flushing efficiency of the lagoon has deteriorated greatly from 1936 to 2001. The average residence time increased from 45 to 71 days, with an increase of 59%. The general Lagrangian residual velocity has decreased from 1936 to 2001, and the residual current pattern has changed from the existence of strong horizontal circulations in 1936 to a simple weak seaward current in 2001. Consequently, the spatial distributions of the residence times also endured great changes.

Fig. 1 .Fig. 8 .Fig. 9 .

 189 Fig.1. A map of Xiaohai Lagoon and its location

  Fig.3

Table 1 .

 1 The river discharge into Xiaohai Lagoon (based on the Wanning Hydraulics

	Department, 1991)

Acknowledgement

This study is funded by Grant 40266001 from Chinese National Science Foundation.

We would like to thank Mr. Mac Sisson of Virginia Institute of Marine Science for his help on improving this manuscript. This is contribution number xxxx of Virginia Institute of Marine Science, The College of William and Mary.