Suspended Sediment Concentration and Fluxes in the High-Turbidity Zone in the Macro-Tidal Hangzhou Bay

<jats:p>The turbidity maximum zone (TMZ) plays an important role in the morphology and ecosystems of estuaries. The distributions of TMZ in Hangzhou Bay (HZB) and mechanisms of sediment fluxes in TMZ have been studied by applying a fully calibrated three-dimensional baroclinic model that considers salinity, high turbidity, sediment flocculation, and density coupling between water and sediment. Based on the method of SSC thresholds, the TMZ is mainly distributed from YG (Yanguan, toponymy in HZB) to ZP (Zhapu, toponymy in HZB). S1 and S2 are the cross-sections within the TMZ. Along the estuary, there is an ascending-to-descending trend observed in the monthly averaged bed current shear stress, while the monthly averaged density displays a descending-to-ascending pattern. The orientation of the monthly averaged surface fluxes of suspended sediment within the TMZ shows a clockwise circular pattern. Moreover, the net flux of suspended sediment along the estuary at section S1 is seaward, while at the section S2, it is landward. The high bottom stress and the sediment fluxes facilitate the retention, deposition, and enrichment of sediment within the TMZ. The suspended sediment fluxes at the two cross-sections was primarily influenced by Eulerian transport, Stokes transport, and tidal pumping transport. At cross-section S1, the sediment transport is dominated by tidal pumping. At cross-section S2, advective transport controls the sediment transportation near the deep northern bank; while near the southern bank of cross-section S2, tidal pumping is still the dominant factor. The outcome of this study provides a foundation for TMZ research in similar macro-tidal turbid estuaries around the world.</jats:p>

section_content: The turbidity maximum zone (TMZ) is a turbid water body in which the sediment content is stably higher than that upstream and downstream and shows regular migration within a certain range in the estuary [1] [2] [3]. It was firstly discovered and defined by the French scholar Glangeaud in the Gironde Estuary, as cited in Burchard [4]. As an endemic phenomenon during the water and sediment transportation process in estuaries [5], the TMZ is crucial in shaping the estuarine morphology, shoals, and river mouth bars [6] [7] [8] [9]. Hence, it significantly impacts the planning and layout of estuarine areas, such as the construction of harbors and navigational channels, the fishery farming, and tourism [10] [11] [12]. 

Researchers have carried out numerous studies on the TMZ in estuaries worldwide, e. g., the Chesapeake Bay in USA [13], the Tamar Estuary in UK [14], the Gironde Estuary in France [15], the Chikugo River estuary in Japan [16], the Brisbane River estuary in Australia [17], the Sabaki Estuary in Kenya [18], and the Changjiang estuary in China [10]. Much interest has been paid to the mechanisms of TMZ in different types of estuaries. These mechanisms mainly include estuarine circulation [19, 20], tidal action [21, 22], sediment settling and starting hysteresis effects [23, 24], salt and freshwater mixing [12, 25], flocculation of suspended sediment [26, 27], and sediment resuspension [28, 29]. Estuarine circulation and tidal action are important macroscopic hydrodynamic processes that play a significant part in the formation and growth of TMZ in different estuaries. Other factors contributing to the development of TMZ are more localized and microscopic in nature. These factors primarily involve enhancing sediment retention, promoting aggregation trends, and maintaining suspension, reflecting the unique environmental characteristics of each estuary [3, 25]. 

In-depth analyses of the formation mechanism and changing patterns of TMZ have revealed that the development of TMZ in various types of estuaries is influenced by different dominant dynamic processes and major controlling factors [3, 25]. In general, weakly mixed estuaries are primarily driven by the estuarine circulation in the development of TMZ. In well-mixed estuaries, tides play the most important role. Partially mixed estuaries lie in between, where both the estuarine circulation and tides are indispensable in shaping the TMZ. The interaction among these drivers varies among estuary types, contributing to the diversity of TMZ development in different regions [3]. 

In weakly mixed and well-mixed estuaries, the tidal energy undergoes various processes during the landward propagation of the tidal wave from the outer seas. These processes include passing through the convergence of the estuaries, riverbed friction, and river-ocean interactions (e. g., backwater effect) [3, 30]. As a consequence, the flow velocity and shear stress induced by the current both experience an initial increase and then a subsequent decrease along the estuaries [31]. As the ebb tides accelerate from upstream to the estuarine mouth and then disperse into the sea, the current-induced shear stress and the flow velocity rapidly decrease [25]. The sedimentation and suspension of bed sediment are mainly influenced by this shear stress. Consequently, the sediment is distributed regularly within the estuarine, with the highest sediment concentration typically occurring near the mouth, leading to the formation of the TMZ. Additionally, the tidal asymmetry causes a net sediment transport toward the land, which is concentrated in the dynamic equilibrium zone [3]. The dynamic equilibrium zone refers to the region where the integrals of shear stresses induced by the ebb and flood tidal currents are balanced [32]. This zone experiences dominance from the flood and ebb tides and oscillates accordingly, contributing significantly to the development of the TMZ [33, 34]. It should also be noted that the study of the TMZ is still in its development. There are different understandings of their formation and evolution, influencing factors and mechanisms, which have not yet been unified [3]. 

Hangzhou Bay (HZB, Figure 1 ) broadly refers to the region where the Qiantang River discharges into the East China Sea. It is a representative example of macrotidal funnel-type estuary and is known for its highly turbid waters. The bay mouth spans about 100 km in width and gradually narrows to approximately 8 km at the bay head. The typical water depth at HZB is in the range of 8 to 10 m. HZB encompasses extensive muddy tidal flats along its southern coast. Notably, it is recognized as its powerful tidal bore, reaching heights of 9 m, which contributes to the high turbidity of its water. According to the field observation data, the suspended sediment concentration (SSC) in HZB could surpass 5 kg/m 3 at the surface layer [35]. Under the influence of runoff and topography, a significant amount of sediment is transported by powerful tides from the Changjiang River into the sea. This sediment accumulates from WJY (Wenjiayan) to ZP (Zhapu), creating a huge sand bar (Table A1 ). This sand bar stretches up to 130 km in length [36]. HZB has two prominent areas with high SSC: the GP (Ganpu) region and the regio near the southern coast; along with a low SSC area between the bay's mouth and ZP o the northern coast [38]. The SSC is higher during winter and lower during summer. Th island effect is associated with high values of SSC in HZB [39]. The sediment in HZB i predominantly composed of muddy chalk and silt, with coarser materials more prom nent near the northern and southern shores, while finer materials are concentrated in th central region. The D50 (median particle size) of fine and coarse material are approximatel 8 to 9 mm and 3 to 4 mm, respectively. The particle size transitions gradually from fine t coarse from the bay's mouth to its inner part [40]. 

According to the research conducted by Sun, the formation of the TMZ in strongl mixed estuaries in China could be attributed to several key factors [32]. One of the mai HZB has two prominent areas with high SSC: the GP (Ganpu) region and the region near the southern coast; along with a low SSC area between the bay's mouth and ZP on the northern coast [38]. The SSC is higher during winter and lower during summer. The island effect is associated with high values of SSC in HZB [39]. The sediment in HZB is predominantly composed of muddy chalk and silt, with coarser materials more prominent near the northern and southern shores, while finer materials are concentrated in the central region. The D 50 (median particle size) of fine and coarse material are approximately 8 to 9 mm and 3 to 4 mm, respectively. The particle size transitions gradually from fine to coarse from the bay's mouth to its inner part [40]. 

According to the research conducted by Sun, the formation of the TMZ in strongly mixed estuaries in China could be attributed to several key factors [32]. One of the main reasons was the current-induced shear stress pattern, which exhibited an increase and then a decrease. This pattern, combined with the oppositely net sediment transport arising from the asymmetry of the tidal wave in the upstream and downstream estuary, led to the aggregation of sediment in the TMZ. Flocculation and sedimentation processes further enhanced the tendency of sediment retention and enrichment within the TMZ. These mechanisms worked together to create and maintain the TMZ in strongly mixed estuaries in China. 

Previous studies have investigated the hydro-sediment characteristics of HZB using in situ measurements [41, 42], modeling methods [43] [44] [45] [46], and remote sensing [47, 48]. However, less focus has been placed on understanding spatial distributions of the TMZ and the features and mechanisms of sediment transport within it. This study builds and fully validates a three-dimensional numerical model based on the FVCOM (Finite Volume Coastal Ocean Model). Given the high turbidity of HZB, the model incorporates the impact of SSC on water density, and the flocculation of fine sediment is also considered. The spatial distribution and possible formation mechanisms of TMZ are illustrated using model results. The numerical model was described in Section 2. Section 3 provides spatial and temporal variations in high SSC water bodies. The possible formation causes of TMZ are analyzed in Section 4. Section 5 presents the conclusion of the article.
section_name: Introduction

section_name: Materials and Methods

section_content: Model descriptions are available in Supplementary Materials [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]. Please refer to Chen et al. for more detailed information of the model [49].
section_name: Hydro-Sediment Model

section_content: The model domain consists of unstructured triangular grids, comprising 114,211 elements and 60,441 nodes. The size of the element cells is variable, from about 100 m in the bay to 30 km at the open boundary of model (Figure 1e ). The vertical direction is divided into seventeen sigma layers evenly. The numerical model used a cold start and was simulated from 1 December 2012 to 31 March 2013, as the observation data used for validation were collected in that year. The first three months were dedicated to model stabilization, while the last month was utilized for analysis. The model considered the effects of three runoff streams, namely the CR, the QTR, and the CER (Figure 1b ). Within the study domain, the CR and QTR contribute to over 80% of the river discharge (datasets are available in http://www. mwr. gov. cn/sj/tjgb/zghlnsgb/, accessed on 1 January 2023) [61]. According to the field data, the annual-averaged discharge and sediment load of QR is 19. 8 × 10 9 m 3 and 2. 5 × 10 6 t, respectively. In contrast, it is 896. 4 × 109 m 3 in discharge and 389. 8 × 10 6 t in sediment load in CR [62]. It was forced with various inputs, including river discharges, tides, winds, and salinity. Tides are the dominant forcing in the bay [63]. The model was compelled with different tides and salinity using the TPXO7. 2 and HYCOM (HYbrid Coordinate Ocean Model). Various tides consisted of two long-period components (MF, MM), three shallow-water components (M4, MS4, MN4), four diurnal components (K1, O1, P1, Q1), and four semidiurnal components (M2, S2, N2, K2) [64]. To expedite model stabilization, the salinity initial field was based on HYCOM. Wind data were sourced from the NCEP (National Centers for Environmental Prediction). The time step of external mode was 5 s. And the time step of internal mode was 0. 5 s. Table 1 presents the key parameters of the model. Only cohesive suspended sediments were taken into consideration, because they comprised the predominant portion of suspended sediment and were generally distributed across the seafloor in HZB [65]. A thorough description of the sediment parameters configuration and their rationale can be found in the article by Li et al. [44].
section_name: Model Configurations

section_content: Three stations (T1 to T3) routinely collected elevation data in March 2013. An additional two stations, N1 and N2, were utilized to measure velocity and SSC data during both spring and neap tides (Figure 1b ). Another sixteen stations collected depth-and-monthly averaged salinity in December 2006 (Figure 1d ). Measured salinity data are from Wang et al. [37]. The validation of the model was performed by comparing the simulated and observed tidal elevation (Figure S1I ), tidal currents (Figure S1II ), and SSC (Figure S1III ) at the field stations. To assess the performance of model, two parameters were computed to measure the discrepancies between the observation data and the model results. The correlation coefficients (CCs) and the Willmott skill score (WSS) were provided as follows [66] :

where η mod represents the simulation results, and η obs refers to the observation results. σ mod and σ obs are the standard deviations of the calculated and observed values, respectively. The closer the absolute values of the CCs and WSSs are to 1, the better the results of the simulation. The performance levels of WSS were categorized as: >0. 2, some credibility; >0. 5, high credibility [67]. The model results for water levels showed excellent agreement with the field data (Figure S1I ). The CCs of the C2 and C3 stations were significantly higher than 0. 97, and the WSSs were no less than 0. 97 (Table S1 ). However, the C1 station, being at the top of HZB and affected by the tidal bore, had lower CC and WSS values, specifically 0. 761 and 0. 841, respectively. Regarding the current velocities at V1 and V2 stations, the model verification results were satisfactory (Figure S1II ). During both spring and neap tides, the CCs and WSSs for current speed, as well as the CCs and WSSs for current direction, exceeded 0. 87, 0. 91, 0. 88, and 0. 95, respectively (Table S1 ). Furthermore, the verification results of SSC at each layer of the V1 and V2 stations accurately reflected the temporal trends and effectively reproduced the vertical SSC characteristics (Figure S1III ). The average CCs for spring and neap tides of SSC were 0. 687 and 0. 499, respectively, with corresponding WSSs of 0. 695 and 0. 505 (Table S1 ). The simulated salinity of the model was in high agreement with the observed salinity, with errors not exceeding 2 psu (Table 2 ).
section_name: Model Validation

section_content: This section is divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.
section_name: Results

section_content: During spring tides, water bodies with a bottom SSC greater than 6 kg/m 3 were defined as high SSC water bodies (Figure 2 ). During neap tides, water bodies with a bottom SSC greater than 3 kg/m 3 were defined as high SSC water bodies. At the onset of the spring flood, high SSC water bodies were primarily distributed from LCG to GP (Figure 2a ). As the tide continued to advance (Figure 2b-e ), these high SSC water bodies gradually moved landward, causing their area to shrink (Figure 2d, e ). The area of high SSC water bodies fluctuated during the spring ebb, and the high SSC water bodies mainly distributed from YG to JS (Figure 2f-l ). Due to the higher flow velocity during the spring flood (Figure A1a,e) in the bay compared with that during spring ebb (Figure A1c,g), the area of high SSC water bodies during the spring ebb was much smaller than that during the spring flood. 

Compared with spring tides (Figure A1 ), the difference in flow velocity during neap tides was smaller (Figure A2 ). During neap tides, high SSC water bodies tended to stay between YG and ZP (Figure 3 ). There was a sand bar underwater. The area of the high SSC water bodies was relatively small during neap tides. In comparison to spring tides (Figure 2 ), the changes in the location and area of high SSC water bodies were also small during neap tides (Figure 3 ).
section_name: Variations in High SSC Water Bodies

section_content: Various methods were employed to study the TMZ, including field investiga numerical modeling, and remote sensing. Among these methods, the most comm used approach involved setting thresholds for water quality parameters, such as sed
section_name: Distribution of the TMZ

section_content: Various methods were employed to study the TMZ, including field investigations, numerical modeling, and remote sensing. Among these methods, the most commonly used approach involved setting thresholds for water quality parameters, such as sediment content, to delineate the extent of the maximum-turbidity zone within the estuary [3]. The threshold value of the TMZ could vary in different regions. 

Currently, the threshold value of the TMZ in HZB has not been definitively determined. In a previous study conducted by Ye et al. [68], the threshold value of 0. 7 kg/m 3 was utilized as the criterion for identifying the surface high-turbidity zone in HZB. Figure 4 displays the distribution of the TMZ in HZB for different threshold values of SSC. As the threshold value of SSC of the TMZ increased, e. g., from 0. 7 kg/m 3 to 1. 2 kg/m 3, the area covered by the TMZ in HZB gradually decreased. 

Under different threshold values, the TMZ in HZB was mainly distributed from YG to ZP. Combined with the distribution of high SSC water bodies during spring (Figure 2 ) and neap tides (Figure 3 ), it could be seen that the TMZ in HZB was mainly distributed from YG to ZP. With the occurrence of ebb and flood tides, the location of the TMZ may move toward the head or the mouth of the bay, and the area of the TMZ may also be changed accordingly. 

was utilized as the criterion for identifying the surface high-turbidity zone in HZB. 4 displays the distribution of the TMZ in HZB for different threshold values of SS the threshold value of SSC of the TMZ increased, e. g., from 0. 7 kg/m 3 to 1. 2 kg/m 3, th covered by the TMZ in HZB gradually decreased. 

Under different threshold values, the TMZ in HZB was mainly distributed fro to ZP. Combined with the distribution of high SSC water bodies during spring (Fig and neap tides (Figure 3 ), it could be seen that the TMZ in HZB was mainly distr from YG to ZP. With the occurrence of ebb and flood tides, the location of the TMZ move toward the head or the mouth of the bay, and the area of the TMZ may a changed accordingly.
section_name: Distribution of the TMZ

section_content: Two cross-sections, S1 and S2, located in the TMZ, were selected to analyze th iment fluxes in the high-turbidity zone (Figure 1c ). The cross-section S1 was locate the head of HZB, and the water depth gradually increased from the northern to the ern bank. The sediment fluxes at the cross-section S1 during neap and spring tid shown in Figure 5. Additionally, the associated residual currents are presented in 6. 

During spring tides (Figure 5a, d ), the magnitude of suspended sediment flu five times higher than that during neap tides (Figure 5b, e ). The along-estuarine susp
section_name: Suspended Sediment Fluxes in the TMZ

section_content: Two cross-sections, S1 and S2, located in the TMZ, were selected to analyze the sediment fluxes in the high-turbidity zone (Figure 1c ). The cross-section S1 was located near the head of HZB, and the water depth gradually increased from the northern to the southern bank. The sediment fluxes at the cross-section S1 during neap and spring tides are shown in Figure 5. Additionally, the associated residual currents are presented in Figure 6. 

During spring tides (Figure 5a, d ), the magnitude of suspended sediment flux was five times higher than that during neap tides (Figure 5b, e ). The along-estuarine suspended sediment fluxes (Figure 5f ) were approximately four times greater than the lateral suspended sediment fluxes (Figure 5c ). In the section S1, all lateral suspended sediment fluxes during spring tides were directed southward. The middle of the section (2 to 5 km from the north bank) had higher lateral suspended sediment fluxes compared with those near the both banks, reaching up to 0. 2 kg/m 2 /s (Figure 5a ). Additionally, lateral suspended sediment fluxes were greater in the surface layer than in the bottom layer at the middle of the section (Figure 5a ). The along-estuarine suspended sediment fluxes were seaward on the northern bank and landward on the southern bank, with higher values observed on the northern bank than on the southern bank (Figure 5d ). 

During neap tides (Figure 5b,e), the lateral suspended sediment fluxes at the middle of the section were southward in the surface layer and northward in the bottom layer (Figure 5b ). The spatial variations in the along-estuarine suspended sediment fluxes during neap tides (Figure 5e ) were similar to those during spring tides but with smaller magnitudes (Figure 5d ). 

ing neap tides (Figure 5e ) were similar to those during spring tides but with smaller nitudes (Figure 5d ). 

The suspended sediment fluxes were influenced by both lateral and along-estu residual currents (Figure 6 ), in combination with the distribution of SSC. At the midd the section, there was a lateral residual current pattern characterized by northward tom flow and southward surface flow (Figure 6a-c ). The lateral residual currents i surface layer were stronger than those in the bottom layer. Additionally, the along-est residual currents had higher magnitudes compared to the lateral residual currents (F 6). 

The SSC was considerably higher near the bottom layer. The interaction betw these residual currents and the spatial variation of SSC played a crucial role in gover the patterns of suspended sediment fluxes in the section. The cross-section S2 was characterized by a sand bar in the middle of the section a deep channel (15 m) near the northern bank. The sediment fluxes at the cross-sectio during neap and spring tides are displayed in Figure 7, and the associated residua rents at the cross-section S2 are presented in Figure 8. 

The along-estuarine and the lateral suspended sediment fluxes had the same o of magnitudes (Figure 7 ). The suspended sediment fluxes in the deep channel on The suspended sediment fluxes were influenced by both lateral and along-estuarine residual currents (Figure 6 ), in combination with the distribution of SSC. At the middle of the section, there was a lateral residual current pattern characterized by northward bottom flow and southward surface flow (Figure 6a-c ). The lateral residual currents in the surface layer were stronger than those in the bottom layer. Additionally, the along-estuary residual currents had higher magnitudes compared to the lateral residual currents (Figure 6 ). 

The SSC was considerably higher near the bottom layer. The interaction between these residual currents and the spatial variation of SSC played a crucial role in governing the patterns of suspended sediment fluxes in the section. 

The cross-section S2 was characterized by a sand bar in the middle of the section and a deep channel (15 m) near the northern bank. The sediment fluxes at the cross-section S2 during neap and spring tides are displayed in Figure 7, and the associated residual currents at the cross-section S2 are presented in Figure 8. 

The along-estuarine and the lateral suspended sediment fluxes had the same order of magnitudes (Figure 7 ). The suspended sediment fluxes in the deep channel on the northern bank were directed northward and seaward, respectively. On the southern and northern banks, the along-estuary and the lateral suspended sediment fluxes were in opposite directions. During spring tides (Figure 7a, d ), the spatial variations in suspended sediment fluxes were similar to those during neap tides (Figure 7b, e ), but the magnitude of suspended sediment flux was seven times higher during spring tides than during neap tides. 

The direction of the lateral residual currents at the middle of the section was southward, while the opposite directions were near the northern and southern banks (Figure 8a-c ). The predominant direction of the along-estuarine residual currents was seaward (Figure 8d, e ). The SSC at the section S2 exhibited a large difference in surface and bottom levels, and in spring tides and neap tides (Figure 9a-c ). The interaction between residual currents and the spatial variability of the SSC also played a crucial role in the pattern of suspended sediment fluxes at this section.
section_name: Suspended Sediment Fluxes in the TMZ

section_content: The along-estuarine variation of monthly averaged current-induced shear stress HZB is shown in Figure 10a. In general, the bed current shear stress in HZB exhibite trend of increasing and then decreasing along the estuary. Moreover, the current-indu bed shear stress in the TMZ was larger than that in the upstream and downstream are The along-estuarine variation of monthly averaged surface density in HZB is shown Figure 10b. The monthly averaged surface density showed a decreasing and then incre ing trend along the estuary. Additionally, the monthly averaged surface density in TMZ was smaller than that in both the upstream and downstream areas. The along-estuarine variation of monthly averaged current-induced shear stress in HZB is shown in Figure 10a. In general, the bed current shear stress in HZB exhibited a trend of increasing and then decreasing along the estuary. Moreover, the current-induced bed shear stress in the TMZ was larger than that in the upstream and downstream areas. The along-estuarine variation of monthly averaged surface density in HZB is shown in Figure 10b. The monthly averaged surface density showed a decreasing and then increasing trend along the estuary. Additionally, the monthly averaged surface density in the TMZ was smaller than that in both the upstream and downstream areas. 

The monthly averaged surface residual currents in HZB are shown in Figure 6a, b. The monthly averaged surface residual currents in part of the region where the TMZ was located exhibit a planar vorticity in a clockwise direction (Figure 11b ). The monthly averaged surface-suspended-sediment fluxes in HZB are shown in Figure 11c, d. Generally, the region where the TMZ was located has upstream net sediment transport within a month. The direction of the monthly averaged surface-suspended-sediment fluxes in the TMZ region exhibited a clockwise vorticity and negative divergence (Figure 11d ), which results in a tendency for suspended sediment to be retained and enriched within the TMZ. The divergence of monthly averaged surface-suspended-sediment fluxes in HZB is shown in Figure 10c. Positive and negative divergence alternated within the TMZ. The magnitude of monthly averaged surface-suspended-sediment fluxes (Figure 11c ) and the divergence (Figure 10c ) in the TMZ was larger than that in both the and downstream areas.
section_name: Possible Causes of the Surface TMZ

section_name: Discussions

section_content: The along-estuarine variation of monthly averaged current-induced shear stress in HZB is shown in Figure 10a. In general, the bed current shear stress in HZB exhibited a trend of increasing and then decreasing along the estuary. Moreover, the current-induced bed shear stress in the TMZ was larger than that in the upstream and downstream areas. The along-estuarine variation of monthly averaged surface density in HZB is shown in Figure 10b. The monthly averaged surface density showed a decreasing and then increasing trend along the estuary. Additionally, the monthly averaged surface density in the TMZ was smaller than that in both the upstream and downstream areas.
section_name: Possible Causes of the Surface TMZ

section_content: Suspended sediment flux is the amount of suspended sediment flowing over a given area per unit time, also known as the suspended sediment transport rates in hydrological surveys [69]. The mechanism of suspended sediment flux decomposition involves breaking down the suspended sediment flux into multiple dynamic terms, allowing for the calculation and analysis of each term. This approach helps to explore the degree of influence that different dynamic factors have on suspended sediment transport [70]. We utilized the suspended sediment flux mechanism decomposition method proposed by Dyer [71], which has been successfully applied in several estuaries worldwide [20, 71]. Descriptions of the suspended sediment flux mechanism decomposition method are available in supplementary materials. 

The suspended sediment fluxes averaged over two spring-neap tidal cycles at two cross-sections were decomposed by the mechanism. The results are shown in Figure 12. The Eulerian residual current represents the average of flow velocity, which illustrates the uneven distribution of flow magnitude throughout the tidal cycle. On the other hand, the Stokes residual current indicates the non-uniformity of water transport during the tidal cycle. When combined (T 1 + T 2 ), the Eulerian transport term and the Stokes transport term form the advective transport term, accounting for transport caused by the Lagrange residual flow [72]. The fluctuations in SSC over the tidal cycle are influenced, on one hand, by advective sediment transport, and on the other hand, they are primarily driven by the tidal pumping effect (T 3 + T 4 + T 5 ). 

aged surface-suspended-sediment fluxes in HZB are shown in Figure 11c, d. Generally, the region where the TMZ was located has upstream net sediment transport within a month. The direction of the monthly averaged surface-suspended-sediment fluxes in the TMZ region exhibited a clockwise vorticity and divergence (Figure 11d ), which results in a tendency for suspended sediment to be retained and enriched within the TMZ. The divergence of monthly averaged surface-suspended-sediment fluxes in HZB is shown in Figure 10c. Positive and negative divergence alternated within the TMZ. The magnitude of monthly averaged surface-suspended-sediment fluxes (Figure 11c ) and the divergence (Figure 10c ) in the TMZ was larger than that in both the upstream and downstream areas.
section_name: Mechanisms of Sediment Fluxes in the TMZ

section_content: Suspended sediment flux is the amount of suspended sediment flowing over a given area per unit time, also known as the suspended sediment transport rates in hydrological surveys [69]. The mechanism of suspended sediment flux decomposition involves breaking down the suspended sediment flux into multiple dynamic terms, allowing for the calculation and analysis of each term. This approach helps to explore the degree of influence that different dynamic factors have on suspended sediment transport [70]. We utilized the suspended sediment flux mechanism decomposition method proposed by Dyer [71], which has been successfully applied in several estuaries worldwide [20, 71]. Descriptions of the suspended sediment flux mechanism decomposition method are available in supplementary materials. The suspended sediment fluxes averaged over two spring-neap tidal cycles at two cross-sections were decomposed by the mechanism. The results are shown in Figure 12. The Eulerian residual current represents the average of flow velocity, which illustrates the uneven distribution of flow magnitude throughout the tidal cycle. On the other hand, the Stokes residual current indicates the non-uniformity of water transport during the tidal cycle. When combined (T1 + T2), the Eulerian transport term and the Stokes transport term form the advective transport term, accounting for transport caused by the Lagrange residual flow [72]. The fluctuations in SSC over the tidal cycle are influenced, on one hand, by advective sediment transport, and on the other hand, they are primarily driven by the tidal pumping effect (T3 + T4 + T5). 

At the section S1, the magnitudes of Eulerian transport and Stokes transport were substantial when compared with tidal pumping transport (Figure 12a ). Specifically, on the southern bank, Eulerian transport could reach up to 6 kg/m/s, while Stokes transport could reach up to 4 kg/m/s. However, Eulerian transport and Stokes transport exhibited opposite directions within the main part of the section S1. As a result, the overall value of advective transport at section S1 remained relatively modest. The principal mechanism driving sediment transport at section S1 was tidal pumping. 

The southern and northern banks of section S2 had different sediment transport characteristics (Figure 12b ). On the northern bank, both water depths and flow velocities were large. In the deep channel, the Eulerian transport attained its maximum, reaching a magnitude of up to 7 kg/m/s, while the tidal pumping transport was negligible. Despite the Stokes transport opposed the direction of Eulerian transport, its magnitude was smaller than that of Eulerian transport. Consequently, the advective transport was still large. Within the deep channel of the northern bank, advective transport was the principal mechanism governing sediment transportation. In contrast to the northern bank, the southern bank had relatively lower water depths and smaller flow velocities. This led to reduced magnitudes of both Eulerian transport and Stokes transport, whereas tidal pumping transport had a more pronounced magnitude. Therefore, tidal pumping was the dominant mechanism governing sediment transport on the southern bank. At the section S1, the magnitudes of Eulerian transport and Stokes transport were substantial when compared with tidal pumping transport (Figure 12a ). Specifically, on the southern bank, Eulerian transport could reach up to 6 kg/m/s, while Stokes transport could reach up to 4 kg/m/s. However, Eulerian transport and Stokes transport exhibited opposite directions within the main part of the section S1. As a result, the overall value of advective transport at section S1 remained relatively modest. The principal mechanism driving sediment transport at section S1 was tidal pumping. 

The southern and northern banks of section S2 had different sediment transport characteristics (Figure 12b ). On the northern bank, both water depths and flow velocities were large. In the deep channel, the Eulerian transport attained its maximum, reaching a magnitude of up to 7 kg/m/s, while the tidal pumping transport was negligible. Despite the Stokes transport opposed the direction of Eulerian transport, its magnitude was smaller than that of Eulerian transport. Consequently, the advective transport was still large. Within the deep channel of the northern bank, advective transport was the principal mechanism governing sediment transportation. In contrast to the northern bank, the southern bank had relatively lower water depths and smaller flow velocities. This led to reduced magnitudes of both Eulerian transport and Stokes transport, whereas tidal pumping transport had a more pronounced magnitude. Therefore, tidal pumping was the dominant mechanism governing sediment transport on the southern bank. 

The along-estuary net suspended sediment fluxes and suspended sediment transport due to different mechanisms at two cross-sections in two spring-neap tidal cycles are listed in Table 3. The along-estuary net suspended sediment fluxes at the two cross-sections were in opposite directions. Specifically, the net suspended sediment flux at the section S1 was seaward, while at the section S2, it was landward. This led to easier sediment retention and deposition between the section S1 and section S2, which promoted the development of TMZ. The suspended sediment transport at two cross-sections was primarily governed by Eulerian transport, Stokes transport, and tidal pumping transport. There were also minor contributions from gravitational circulation and the varying shapes of vertical velocity and SSC profiles within the tidal cycle. Eulerian transport and Stokes transport exhibited comparable magnitudes but opposite directions at both cross-sections. Moreover, sediment transport resulting from tidal pumping largely outweighed advective sediment transport at both sections, with their directions opposing each other. The advection ratios for Sections 1 and 2 were 0. 08 and 0. 06, respectively, signifying that tidal pumping assumes a more substantial role in suspended sediment transport along the estuary in HZB (Table 3 ). 

Suspended sediment transport caused by Eulerian transport at section S1 was landward, whereas at section S2, it moved seaward. In the case of Stokes transport, suspended sediment transport at both sections S1 and S2 was seaward, while at section S2, it was landward. Additionally, suspended sediment transport induced by tidal pumping at section S1 was seaward, but at section S2, it moved landward.
section_name: Mechanisms of Sediment Fluxes in the TMZ

section_content: A three-dimensional baroclinic numerical model has been established utilizing the FVCOM and fully validated using the field data. The model has adeptly replicated the hydro-sediment dynamics in macro-tidal turbid HZB that considers salinity, high turbidity, sediment flocculation, and water-sediment density coupling. The distribution of the TMZ in HZB is analyzed and discussed. The main conclusions are as follows. 

(1) The locations and areas of high SSC water bodies change with the flood and ebb tides during spring and neap tides. At different thresholds, the area of TMZ varies. However, they are predominantly situated from YG to ZP. 

(2) Along the estuary, there is an ascending-to-descending trend observed in the monthly averaged bed current shear stress, while the monthly averaged density displays a reverse pattern. The direction of the monthly averaged surface-suspended-sediment fluxes, where the TMZ is situated, displayed a clockwise circular pattern. The alongestuary net suspended sediment flux at section S1 is seaward, while at section S2, it is landward. All these dynamic characteristics collectively facilitates the retention, deposition, and enrichment of sediment within the TMZ. 

(3) The interaction between the residual currents and the spatial variation of SSC plays a crucial role in governing the patterns of suspended sediment fluxes in the two sections. The suspended sediment flux at two specific cross-sections is primarily influenced by Eulerian transport, Stokes transport, and tidal pumping transport. At cross-section S1, the primary mechanism driving sediment transport is tidal pumping. At cross-section S2, situated within the deep channel of the north bank, advective transport is the principal governing mechanism for sediment transportation. However, on the south bank of crosssection S2, tidal pumping remains the dominant mechanism governing sediment transport. 

The above conclusions provide data for the future study in estuarine geomorphology. More data are needed to strength the conclusions in future studies.
section_name: Conclusions

section_name: Acknowledgments:

section_content: Data Availability Statement: Not applicable.

section_content: We are grateful for H. Q. for providing salinity data.
section_name: Acknowledgments:

section_name: Supplementary Materials:

section_content: The following supporting information can be downloaded at: https: //www. mdpi. com/article/10. 3390/jmse11102004/s1.
section_name: Supplementary Materials:

section_name: Author

section_content: The authors declare no conflict of interest.
section_name: Conflicts of Interest:

section_name: Appendix A