The Yangtze River Estuary is one of the three major estuaries in China. From below Xu LiuJing, it diverges to the north and south sides of ChongMing Island, which are the South Branch and the North Branch respectively. The hydrodynamic conditions are complex and variable, the geomorphology is evolving rapidly, and the deposition process is complex. Hangzhou Bay is located in the Yangtze River Delta region, adjacent to the Yangtze River Estuary, and has a trumpet shape and flat bottom topography, with an average water depth of 8-10 m. It is one of the estuaries with the highest concentration of suspended sand in the world, and the measured concentration of suspended sand at the top of the bay was as high as 10.6 kg/m³ (Xie et al., 2009). The sea area of YangShan Port is located in the sea area of the rugged island group, which is a trumpet-shaped water area composed of two island chains, one in the north and one in the south, mainly composed of the Large and Small YangShan, with the Small YangShan, Huo Gai Tang, large rock reef, small rock reef, Pearl Hill and large turtle islands in the north island chain, and the Large YangShan, large Shan Tang and Shuang Lian Shan islands in the South Island chain. The sea is influenced by the sediment from the Yangtze River Estuary, which has a high sand content all year round, and the strong currents in the island group make the sediment movement more active (Dong, 2011).

In this study, data collection is divided into fixed vertical lines data collection and large surface observation data collection. Five fixed vertical lines were deployed in the Yangtze River Estuary, including C1 to C4 near the mouth gates of the North Branch, the North Port, the North Channel and the South Channel of the Yangtze River Estuary, and C5 near the mouth of Nanhui in Shanghai. Three fixed vertical lines, H1 to H3, were deployed in the northern part of Hangzhou Bay. One vertical line was deployed in the harbor area of YangShan Port, and two vertical lines were deployed on the east and west sides of the island, Y1 to Y3 respectively. The large surface observation in the Yangtze River Estuary covers the Shanghai waters outside the Yangtze River Estuary, with 6 large surface observation sections, totaling 36 sampling points, numbered CD1~CD36, in the northern part of Hangzhou Bay, 4 large surface observation sections are deployed to monitor the changes of suspended sand concentration in the north coast waters, totaling 16 sampling points, numbered HD1~HD16, as shown in Figure 1 .

Continuous monitoring was carried out at 11 fixed vertical lines in the Yangtze River Estuary, the north coast of Hangzhou Bay and Yangshan Port, respectively, to collect information on the stratified flow velocity and suspended sand concentration at spring, middle and neap tides. Sampling dates for spring tides are 15-16 June 2018; 29 May-30 May 2019; and 7-8 June 2020. Sampling dates for middle tides are 11-12 June 2018; 1-2 June 2019; and 3-4 June 2020. Sampling dates for neap tides are 7-8 June 2018; 4-5 June 2019; 30-31 May 2020. The velocity and direction data were obtained based on the ADCP, and were observed every 15 minutes for 3 minutes during the observation process; the SSC data were obtained based on the OBS, and were observed once every hour during the whole observation period. Water depths of up to 10 meters are sampled by the 3-point method, and water depths of 10 meters or more are sampled by the 6-point method. At the same time, 1000ml horizontal samplers were used to collect water samples to rate the OBS sampling results. The sampling frequency was as follows: the water samples were collected at the four periods of flood peak, flood slack, ebb peak and ebb slack in the first half of each fixed vertical line (4 times in total), and the water depth was within 10m according to the 3-point method, and above 10m according to the 6-point method. The water samples were taken at a relative water depth of 0.8H every hour during the rest of the period. The water samples were taken back to the laboratory to obtain suspended sand concentrations based on the weighing method. The acquisition of data from large surface observations is simpler than that of fixed plumb line data, using ADCP to obtain stratified flow velocity and direction, and only one observation at each point; using OBS to SSC data, and only one observation at each point during the whole observation period, while collecting water samples to rate the OBS sampling results. The sampling frequency is slightly different from that of fixed plumb line, specifically: CD1-CD4, CD6- CD8, CD11-CD13, CD19-CD21, CD29-CD31 and other 16 points, the water depth within 10 m was sampled by the 3-point method and above 10 m by the 6-point method; for the remaining 20 points, only water samples at 0.8H were collected. The water samples collected were also analyzed in the laboratory by the weighing method.

Since the OBS collects turbidity value, it is necessary to collect calibration water samples to analyze SSC in order to deduce the SSC of the measurement point, and then calculate SSC of the measurement point through the OBS turbidity value according to the OBS rate calibration formula (OBS turbidity value ~ SSC relationship curve). The field rate calibration is carried out according to the requirements of the “OBS-3A Calibration and Calibration Technical Guide”. Table 1 shows the R2 of OBS turbidity values and SSC fitting curves at some points, indicating a high correlation between the two, with R2 greater than 0.8.

The vertical average SSC is calculated by the following equation:

Six-point method:

Three-point method:

When it is located near the diverted flow, the flow velocity is low and the flow direction is turbulent. It may be unreasonable to calculate the average sand content of the vertical line by weighting the flow velocity, so it is calculated by weighting the sand content of the measurement point, i.e.

In the above equation: C S P is mean sand content of the vertical line (kg/m³); C i is weighting coefficient (1, 2, 2, 2, 2, 2, 1 for the six-point method and 1, 1, 1 for the three-point method); C s i is sand content of the measurement point (kg/m³); V x i is average flow velocity at the measurement point (m/s) and V P is average flow velocity of vertical line (m/s).

The SSCs in the three regions of the Yangtze River Estuary, Hangzhou Bay and Yangshan Port from 2018 to 2020 were counted, and the characteristics of SSCs under different tidal types of neap, middle and spring tides were analyzed using three indicators: surface SSC (C(0H)), near-bottom SSC (C(1H)) and vertical average SSC (C_SP), as shown in Tables 2 – 4 , where C(0H), C(1H) and C_SP are the average values of multiple single SSC vertical profiles, and N is the number of single vertical profiles. It can be seen that for the same region, the size ranking of C(0H), C(1H) and C_SP for different tide types is all: neap tide< middle tide< spring tide. And from the three large regions, the highest SSC is in Hangzhou Bay, followed by Yangshan Port, and the lowest in Yangtze River Estuary, and with C_SP in 2019, for example, Hangzhou Bay is 1.28 times higher than Yangshan Port and 2.10 times higher than Yangtze River Estuary. To investigate the effect of different tidal states such as high and low tides on the SSC, the magnitude characteristics of the SSC at high and low tides in the three regions were also analyzed using three indicators, C(0H), C(1H) and C_SP as shown in Tables 2 , 3 , 5 . The results show that the regularity of differences between the SSC at flood and ebb tides is not obvious in the three regions. Taking 2018 data as an example, in the Yangtze River Estuary area, the SSC during flood tide is slightly higher than that during ebb tide. In the waters of Yangshan Port, the SSC at flood tide is slightly lower than that at ebb tide. Similarly, analyzing the data from 2019 and 2020, it was found that there is no significant regularity in the differences in SSC between flood tides and ebb tides. That is to say, when analyzing the impact of flood tides and ebb tides on SSC, we also need to comprehensively analyze hydrodynamic parameters such as flow velocity.

Taking the 2019 data as an example, the correlation coefficients between the SSC in each layer of the three regions and the vertical average SSC were calculated separately to form the correlation coefficient matrix shown in Table 4 . It can be seen that the SSC in two adjacent layers show a high correlation. Taking the Yangtze estuary as an example, the correlation coefficient between C(0H) and C(0.2H) is 0.8598, the correlation coefficient between C(0.2H) and C(The correlation coefficients between C(0H) and C(0.2H) were 0.8598, C(0.2H) and C(0.4H) were 0.8861, C(0.4H) and C(0.6H) were 0.8799, and C(0.6H) and C(0.8H) were 0.8819. There is also a high correlation between the SSC in each layer and the vertical average SSC. In Hangzhou Bay, for example, the correlation coefficients between the SSC in each layer and the vertical average SSC are above 0.7. The correlation coefficient between the SSC and the SSC in each layer from the surface to near-bottom decreases gradually, but the correlation with the vertical average SSC is larger, which is consistent with the findings of existing studies (Wang, 2008). Whether sampled by the six-point method or the three-point method, the correlation between the SSC of each layer in the vertical direction shows a similar regularity, i.e. the SSC of two adjacent layers has a high correlation, and there is a high correlation between the SSC of each layer and the vertical average SSC, but the correlation coefficient between the surface layer and the SSC of each layer from the surface layer to the near-bottom layer gradually decreases.

We analyzed the spatial and temporal distribution of SSC in the Yangtze River Estuary and its adjacent waters based on fixed vertical observation data and large-scale observation data from 2018 to 2020, as shown in Figure 2 . In terms of spatial dimension, as shown in Figures 2A–C , the distribution of SSCs in the Yangtze River Estuary and the north of Hangzhou Bay is slightly different. For the Yangtze River Estuary, SSC shows a decreasing trend as the offshore distance increases. However, in the north of Hangzhou Bay, the SSC of each sampling point has little difference. The reason for the analysis may be that outside the Yangtze River estuary, the water depth also shows an increasing trend with the increase of offshore distance, and the water area is vast and less affected by land. In the north of Hangzhou Bay, the overall offshore distance is small, and it is still distributed in the interior of Hangzhou Bay.

In terms of time dimension, Figure 2D shows the spatial distribution of SSCs at 11 fixed vertical line positions from 2018 to 2020. In the northern part of Hangzhou Bay, the H1 vertical line near the Jinshanzui hydrographic gauging station has lower suspended sand concentrations than the H2 and H3 vertical lines to the north of the station, and the H1 suspended sand concentrations do not change much between 2018 and 2020, while the vertical lines H2 and H3 have higher concentrations in 2019 compared to 2018 and 2020; the suspended sand concentrations at the three locations in Yangshan Port do not differ much. The vertical lines Y1 and Y2 have lower concentrations in 2019 compared to 2018 and 2020, and the vertical line Y3 has lower concentrations in 2020 compared to 2018 and 2020. The data for Y3 in 2020 are vacant due to equipment and other reasons.

The vertical profiles of SSC at the 11 sampling sites in the three regions are diverse. Based on morphological characteristics and existing studies, the instantaneous SSC profiles in the study area can be classified into six basic types ( Figure 3 ), namely L-shaped profile, Stepwise profile, Vertical profile, Slanting concave-up profile, Slanting concave-down profile and Quasi-slanting line profile (Dong et al., 2011). Taking the three vertical lines C5, H3 and Y1 as examples, Figures 1 – 3 in the Appendix show the vertical distribution curves of the three vertical lines during the four tidal states of flood peak, flood slack, ebb peak and ebb slack during neap, middle and spring tides. Among all the profiles, only about 40% of the basic types can be identified, and nearly 60% of the profiles cannot be classified into the basic types, making it difficult to describe the trend. It is probably a transitional pattern from one basic type to another.

The average SSC profile was obtained by averaging multiple single instantaneous SSC profiles and fitting a curve regression. The R² for most of the sites was above 0.99, and the fit functions were mostly linear and logarithmic in form. When the mean profiles of all single profiles at all points in the three regions were analyzed, the fit functions were in logarithmic form for the Yangtze River Estuary and linear form for Hangzhou Bay and Yangshan Port ( Figure 4 ), and the R² was greater than 0.99 for all three regions.

In order to investigate the relationship between flow velocity and SSC, a comparison of SSC and flow velocity is plotted as shown in Figure 5 , which shows that the distribution of SSC in each trough of the Yangtze estuary and the northern part of Hangzhou Bay is highly consistent with the distribution of flow velocity. During the neap, middle and spring tides, the SSC and velocity show a similar planar distribution. The SSC increases linearly from C1 at the mouth of the Northern Branch to C3 in the Northern Trough, with a lower concentration in the Southern Trough at C4, and then increases again at C5 near the mouth of the Nanhui, with the corresponding flow velocities showing similar distribution characteristics. In the northern part of Hangzhou Bay near Jinshanzui hydrographic station, the H1 vertical line has a lower SSC, while the H2 and H3 vertical lines to the north of the H1 vertical line have a higher SSC. Similarly, the flow velocities are lower on the H1 vertical line and higher on the H2 and H3 vertical lines. Although the locations of the verticals are different, the magnitude of SSC and flow velocity during the small, medium and large tides basically follow the pattern of spring tide > middle tide > neap tide.

There is usually a close relationship between the variation of SSC and current velocity during a tidal cycle in estuaries and estuarine bays. Figure 6 shows the variation curves of SSC and current velocity during a complete tidal cycle in three periods: C5 neap tide, H2 neap tide and H2 spring tide, where the red line indicates the variation of current velocity and the yellow line indicates the variation of SSC. It can be found that the changes in flow velocity and SSC are basically synchronous and show a positive correlation during a complete tidal cycle, i.e. when the flow velocity increases, the SSC increases and when the flow velocity decreases, the SSC also decreases.

In order to further investigate the relationship between SSC and current velocity, the SSC and current velocity at each layer in the vertical direction during a complete tidal cycle are shown in Figure 7 , using the C5 tide as an example. The type of tide in the Yangtze estuary is an informal semi-diurnal tide, and this characteristic can be clearly seen in the curves of flow velocity, with two low tides and two high tides occurring in a full tidal cycle, and varying at high tide and low tide. Correspondingly, SSCs also show two peaks and two troughs within a completed tidal cycle, and they occur at roughly the same time as the peak and trough of the flow, but the peaks and troughs are less pronounced as the SSC is influenced by other hydrodynamic factors at the same time. In the vertical direction, the SSC increases from the surface layer to the bottom layer, while the flow velocity decreases in the opposite direction from the surface layer to the bottom layer. The reason for this is that when the bottom flow velocity increases, the sediment at the bottom is re-suspended due to scouring, which increases the SSC in the surface and middle layers.