Keywords

1 Introduction

Tidal flats in estuarine and coastal areas are the important natural resources and distributed with ecosystems such as salt marshes and mangroves, which have strong carbon sequestration capacity and play an important role in the global carbon cycle (Falkowski et al. 2000). However, most tidal flats in the world are facing serious challenges from both natural change and anthropogenic activities (Wei et al. 2015). Over the past century, there has been a net global reduction of sediment flux into coastal oceans resulting from human impacts (Syvitski et al. 2005). The fluvial sediment decline, which triggers the regression of the subaqueous delta (Syvitski et al. 2009; Yang et al. 2011), is primarily caused by sediment retention in inland reservoirs (Walling and Fang 2003). Moreover, estuarine engineering projects, including waterway regulation and reclamation, also play an important role in altering estuarine hydrodynamics and controlling estuarine tidal flat morphodynamics (Spearman et al. 1998; Antoine et al. 2009; Dai et al. 2013; Song and Wang 2014).

The Yangtze Estuary (YE), located in eastern China (Fig. 1), is one of the world’s largest estuaries (Xie et al. 2009). It has been intensively influenced by human activities including altered river and sediment discharges in its catchment and local engineering projects in the estuary over the past half century (Luan et al. 2016). Due to the vulnerable natural conditions, intensive human activities and complex hydrodynamic and sediment systems, the tidal flats of the YE have attracted extensive attention as an ideal study area for studying on the temporal and spatial changes. Both sediment discharge and river flood events played important roles in the decadal morphological evolution of the YE (Luan et al. 2016), and the time-lag effect needs particular consideration in projecting future estuarine morphological changes under a low sediment supply regime and sea-level rise (Zhao et al. 2018). The Deep Waterway Project and reclamation projects were responsible for the polarization of seaward erosion and landward accretion, respectively (Wei et al. 2017). In addition, under favorable conditions (e.g., macrotidal range, strong tidal flow, flood dominance, sedimentary settling-lag/scour-lag effects, and increasing high-tide level), delta-front erosion of the YE can actually supply sediment to tidal flats, thereby maintaining the accretion rates in balance with relative sea-level rise (Yang et al. 2020). In general, previous studies mainly focused on the overall scouring and silting process of the YE riverbed and the evolution law and mechanism of some tidal flats, while the overall morphological evolution characteristics and driving factors of the YE tidal flats have not been systematically analyzed and discussed.

Fig. 1.
figure 1

Map of the Yangtze river basin (a) and Yangtze Estuary (b).

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The physical conditions in terms of the prevailing hydrodynamic forcing, bed sediment composition and channel–shoal patterns in the YE varied from the inner estuary to the outer estuary (He et al. 2015). From this point of view, we divided the study area into two sections that was the inner estuary and the mouth bar area (the outer estuary) by taking Hengsha Island as the boundary. Tidal flats of the YE mainly include East Chongming Shoal (ECMS), Hengsha Shoal (HSS), Jiuduan Shoal (JDS, including Jiangya Shoal) and East Nanhui Shoal (ENHS) located in the mouth bar area, as well as Baimao Shoal (BMS), Biandan Shoal (BDS), Xinliuhe Shoal (XLHS), Ruifeng Shoal (RFS) and Baozhen Shoal (BZS) in the inner estuary (Fig. 1). Since 1998, many large-scale waterway regulation projects, hydraulic engineering and reclamation projects (Fig. 1) have been conducted in the YE, which have an increasingly significant influence on the riverbed evolution of the estuary, they mainly include the Baimao Shoal Protection Project (BSP, 2012–2013), XinLiuHe Shoal Protection Project (XSP, 2007–2009), Qingcao Shoal Reservoir Project (QSR, 2007–2009), Deepwater Channel Regulation Project (DCR, 1998–2010), and reclamation projects such as East Hengsha Shoal (EHSR, 2003–2019), Outside Pudong Airport (OPAR, 2008–2010) and East Nanhui Shoal (ENSR, 2013–2017).

Based on the long-term bathymetric data, this study systematically analyzes the morphological changes of tidal flats in the YE. Combined with the fluvial discharge and sediment flux as well as the construction of estuarine engineering projects, the driving factors of the morphological evolution of the tidal flats are discussed. The insights obtained from this study are helpful for the understanding and prediction of tidal flats evolution trend and the protection of wetlands in the YE, and can be useful reference to other estuaries and coastal areas.

2 Data and Methods

In order to analyze the topographic changes of tidal flats, we collected eight bathymetric maps (1998, 2002, 2010, 2013, 2016, 2017, 2018, 2019) of the study area (Fig. 1) from the Yangtze Estuary Waterway Administration Bureau, which were based on the chart datum. Using the GIS software package ArcGIS, all charts were digitized into Digital Elevation Model (DEM), and then spatial analysis and data statistics were performed. To assess the effects of river input, we collected annual runoff and suspended sediment discharge data at Datong station during 1950–2019 and the suspended sediment concentration (SSC) field survey data of fixed vertical line in the YE from 2002 to 2019. Meanwhile, the construction of estuarine engineering projects since 1998 had also been investigated.

Using a three-dimensional sigma coordinate coastal ocean model (SWEM3D), which was built based on unstructured C-grid and semi-implicit finite volume method by Shanghai Estuarine and Coastal Science Research Center, we simulated the changes of erosion and deposition trend of the tidal flats in the YE. By applying the semi-implicit method, SWEM3D can stay stable and efficient without the stability limitations of the surface gravity wave as well as the vertical diffusions. In addition, the finite volume method is used to calculate the surface gradient at each side which has no restriction on the grid orthogonality and makes the model more flexible for the complex coastal condition (Shen et al. 2014). The upstream boundary of the model is Datong Station of the Yangtze River, and the downstream boundary is −80 m deep area of the East China Sea. The total number of model triangular grids is 158,828, the minimum grid scale is about 30 m, the number of vertical layers is 10, and the time step length is 120 s. And the riverbed topography used in the model is a wide-range bathymetric map of the YE with a scale of 1:25000 in 1998. In order to discuss the influence of estuarine engineering projects and fluvial sediment decline on the erosion and deposition trend of the tidal flats, the following methods and indicators are used to calculate.

In this paper, the influence of the implementation of estuarine engineering projects on the morphological evolution of tidal flats is analyzed and calculated by using the indicators of erosion rate (\({f}_{e}\)) and deposition rate (\({f}_{d}\)), which are affected by hydrodynamic changes under fixed bed condition. \({f}_{e}\) and \({f}_{d}\) can indicate the probability of erosion and deposition of riverbed topography under certain hydrodynamic conditions, respectively (Cheng et al. 2020). The increase of \({f}_{e}\) and decrease of \({f}_{d}\) indicate that tidal flat tends to scour, and conversely, tidal flat tends to siltation. The formula of \({f}_{e}\) and \({f}_{d}\) are shown as:

$$ f_{e} = \mathop \smallint \limits_{0}^{{T_{1} }} m\left( {\frac{{\tau_{b} }}{{\tau_{e} }} - 1} \right)dt/15\quad \quad \quad \quad \tau_{b} > \tau_{e} $$
(1)
$$ f_{d} = \mathop \smallint \limits_{0}^{{T_{2} }} \alpha \omega C_{b} \left( {1 - \frac{{\tau_{b} }}{{\tau_{d} }}} \right)dt/15\quad \quad \quad \quad \tau_{b} < \tau_{d} $$
(2)

where, \({\tau }_{e}\) and \({\tau }_{d}\) are the critical deposition and starting stress at the bottom, which are 0.3 N/m2 and 0.4 N/m2 respectively; \(m\) is the erosion coefficient, set as 0.0001; \(\alpha \) is the settling probability, which is 1; \(\omega \) is the settling velocity of sediment at the bottom, take 0.2 mm/s; the above parameters are based on the field observation and laboratory test results of the YE (Qi et al. 2015). \({\tau }_{b}\) is actual bottom shear stress; \({C}_{b}\) is the unit SSC, set at 1.0 kg/m3, which means the influence of the changes of SSC is not taken into account; \({T}_{1}\) and \({T}_{2}\) are the duration of erosion and deposition, and the total calculation period is 15 d, which includes the full cycle process of spring-neap tide.

The upstream runoff process calculated by the model is the flow of Datong station from July 5 to 20, 2017, and the downstream boundary is the tide level of the offshore boundary in the corresponding period, which is calculated and given by the harmonic function.

The effect of fluvial sediment decline is calculated by using the indicator of deposition rate (\({{f}_{d}}^{\mathrm{^{\prime}}}\)), which is affected by SSC change under fixed bed condition, without considering the changes of riverbed topography and hydrodynamics. The decrease of \({{f}_{d}}^{\mathrm{^{\prime}}}\) indicates that tidal flat tends to scour, otherwise tends to siltation. \({{f}_{d}}^{\mathrm{^{\prime}}}\) is calculated as follows:

$$ f_{d}^{^{\prime}} = \mathop \smallint \limits_{0}^{{T_{2} }} \alpha \omega C_{s} \left( {1 - \frac{{\tau_{b} }}{{\tau_{d} }}} \right)dt/365\quad \quad \quad \quad \tau_{b} < \tau_{d}$$
(3)

where, \({C}_{s}\) is the actual SSC at the bottom calculated by the model, and the total calculation period is 365 d.

Fluvial sediment decline is represented by the following conditions: the SSC of Datong station decreased from 0.4 kg/m3 in 1986–1997 to 0.2 kg/m3 in 1998–2019. According to the calculation, the SSC decrease process in the YE caused by fluvial sediment decline tends to be stable in about one year under the condition of fixed bed. Therefore, the calculation results of the second year are selected to observe the changes of the erosion and deposition trend of tidal flats. The upstream runoff is the average discharge process of Datong station from 1998 to 2019, and the downstream tide is represented by the offshore boundary tide level calculated by harmonic function from 2018 to 2019.

3 Results

3.1 Area Changes of Tidal Flats

The calculation results of tidal flats area in the YE from 1998 to 2019 are shown in Fig. 2. During the last two decades, the tidal flats demonstrated different evolution processes. As mentioned above, the evolution characteristics of tidal flats were analyzed according to the upper and lower reaches of the YE, which were inner estuary and mouth bar area.

BMS, a typical central shoal (Fig. 1b), located in the main channel of the upper reaches of South Branch (SB), and the BSP was completed in 2013. Over the past 20 years, BMS shows a change of “scouring first, silting later, and then stabilizing” (Fig. 2a). The –5 m tidal flat of BMS was scoured from 1998 to 2013 with an area decrease rate of 0.7 km2/yr, and turned into siltation from 2013 to 2016 with an area increase rate of 1.0 km2/yr, which changed little from 2016 to 2019. The erosion of tidal flat above −2 m began in 2002, which was later than the lower tidal flat above −5 m. After that, the process of erosion and deposition was basically the same.

BDS is a relatively large tidal flat on the north side of the main channel of SB (Fig. 1b), which shows a general trend of siltation and expansion recently, especially the higher flat (Fig. 2b). During 1998–2019, the area of higher flat above 0 m and +2 m increased by 2 and 3 times respectively. However, the area of lower tidal flat above −5 m and −2 m only increased by 7% and 8%, and began to decrease in 2010 and 2016, respectively.

XLHS is a central shoal in the lower reaches of the SB (Fig. 1b), and the XSP was completed in 2009. Its +2 m higher flat developed and silted rapidly from 2010 to 2016, with an area growth rate of 0.5 km2/yr, and tended to be stable from 2016 to 2019. The 0 m and −2 m tidal flats of XLHS were slowly silted from 1998 to 2010, and quickly silted from 2010 to 2016 with an area growth rate of 0.8 km2/yr and 0.3 km2/yr respectively, and remained stable from 2016 to 2019. However, the area of −5 m lower tidal flat decreased at a rate of 0.2 km2/yr from 1998 to 2010 and remained almost unchanged thereafter (Fig. 2c).

RFS and BZS are located in South Channel (SC) and North Channel (NC) of the inner estuary, respectively, and both of them are close to the thalweg of ebb channels (Fig. 1b). The area of the two tidal flats had declined significantly since 1998, and the lower tidal flats are the main ones (Fig. 2d and e). Take the −5 m isobath as an example, the area of RFS and BZS decreased at a rate of 0.9 km2/yr and 0.4 km2/yr, respectively.

It can be seen that each tidal flat in the inner estuary presents different erosion and accretion characteristics due to the different engineering layouts and local hydrodynamics and sediment conditions. Similarly, the variation of the tidal flats in the mouth bar area also shows the spatial difference.

ECMS is located on the north side of the mouth bar area, between the North Branch (NB) and North Channel (NC), and its upstream is covered by Chongming Island (Fig. 1b). Tidal flats above + 2 m and 0 m of ECMS expanded from 1998 to 2010, and then remained almost unchanged, while the tidal flats above −2 m and −5 m experienced a slowdown deposition process during 1998–2016, and slight erosion occurred during 2016–2019 (Fig. 2f).

HSS is located between NC and North Passage (NP), east of EHSR (Fig. 1b). The area of HSS at different elevations changed from expanding to shrinking, and the tidal flats above 0 m, −2 m and −5 m changed from deposition to erosion in 2016, 2016 and 2010, respectively (Fig. 2g). Among them, the area of –5 m lower tidal flat decreased by nearly 13% from 2010 to 2019.

JDS, the youngest tidal flat and the largest natural preservation zone in the YE (Fig. 1b), located between the NP and the South Passage (SP). The area of tidal flat above +2 m, 0 m and −2 m increased by 178%, 70% and 13% from 1998 to 2019, respectively. While the tidal flat above −5m changed from deposition to erosion in 2010, and its area decreased by about 4% from 2010 to 2019 (Fig. 2h).

ENHS is located on the northeast side of OPAR and ENSR, adjacent to the ebb channel of NP (Fig. 1b). It's obvious that the area of −5 m lower tidal flat continued to expand before 2010 and decreased significantly after 2010, when the reclamation project was constructed. Moreover, the erosion of −2 m lower tidal flats began in 2016 (Fig. 2i).

From the above, most tidal flats in the YE have presented an erosion trend successively in the past 20 years, which mainly occurred in lower tidal flats. Furthermore, the erosion of tidal flats in the inner estuary occurred earlier and more obvious than that in the mouth bar area.

Fig. 2.
figure 2

Area changes of the tidal flats with respected to the benchmark water level.

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3.2 Erosion and Deposition Distribution of Tidal Flats

The erosion and deposition distribution were obtained from the DEM by differencing the two years (Fig. 3). The results indicate that the YE experienced major changes over the 20 years from 1998 to 2019, and the spatial distribution of erosion and deposition varied considerably from the inner estuary to the mouth bar area. In general, the range of erosion and deposition in the inner estuary was significantly greater than that of the mouth bar area. From the perspective of spatial distribution, erosion mainly occurred in the thalweg of ebb channels and the areas of the bifurcation (the head of central shoal), while deposition mainly occurred in the areas of weaker hydrodynamics such as the sheltered waters of the estuarine engineering projects and the convex bank side of the channel.

It can be seen in Fig. 3 that BMS, XLHS and JDS, as typical central bar, had experienced different levels of erosion at the head of tidal flats and deposition at the higher mudflat in the past 20 years, what's more, the erosion of tidal flats in the inner estuary was more obvious. The southern edge of BDS, which is adjacent to the ebb channel of SB, showed severe erosion, and the scoured sediment transported downstream and deposited at the tail of BDS. Both RFS in the ebb channel of SC and BZS in the concave bank side of the NC were obviously eroded. ECMS on the north side of the mouth bar area had been mildly silted up during 1998–2019, with only the local area along the southern edge scoured slightly. The mid-lower part of HSS was slightly deposited, while the upper part and the north and south edges of the tidal flats were scoured. In addition to the head erosion mentioned above, JDS also showed obvious characteristics of northern edge deposition and southern edge erosion. ENHS located outside the OPAR and ENSR reclamation projects had been eroded slightly from 1998 to 2019.

Fig. 3.
figure 3

Bathymetric changes of the Yangtze Estuary during 1998–2019 (positive values are for deposition and negative values are for erosion).

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3.3 Shape Changes of Tidal Flats

The shape changes of tidal flat can not only directly reflect the vertical differences in the erosion and deposition process, but also indicate the characteristics of steepening or flattening. According to the shape changes of the tidal flats in the YE (Fig. 4), although the vertical erosion and deposition patterns of the tidal flats were obviously different in the past 20 years, they all showed a trend of gradual steepening.

BMS deposited in the higher mudflat and eroded in the lower mudflat during 1998–2002, then erosion and siltation occurred throughout the tidal flat, respectively, in 2002–2013 and 2013–2019 (Fig. 4a), the statistical results show that BMS steepened obviously before 2013, and leveled out slightly afterwards (Table 1). BDS has been steepening in the past 20 years, and the shape variation characteristics of BDS are as follows: the higher tidal flat above 0 m deposited continuously, the tidal flat between −1 m and −4 m changed from siltation to micro-erosion in 2016, and the −5 m lower tidal flat changed from deposition to erosion in 2010, which occurred earlier (Fig. 4b). XLHS steepened significantly from 1998 to 2019. Before 2010, the higher mudflat above 0 m of XLHS remained stable with rapid erosion of the lower mudflat below −4 m; after 2010, the higher mudflat above 0 m rapidly expanded with little change of the lower mudflat below −3 m (Fig. 4c). The significant erosion of RFS and BZS began in 1998 and 2002 respectively, among which the erosion of lower mudflat was more intense than that of the higher mudflat (Fig. 4d and e), therefore, the steepening characteristics of RFS and BZS were obvious.

Compared with the tidal flats in the inner estuary, the steepening trend of the tidal flats in the mouth bar area is relatively gentle. The entire tidal flat of ECMS continued to expand from 1998 to 2016, with slight erosion from 2016 to 2019 (Fig. 4f). Meanwhile, the steepness of ECMS remained stable (Table 1). HSS silted up and expanded before 2010, then the lower mudflat began to erode in 2010–2016, and the entire tidal flat scoured and shrank in 2016–2019 (Fig. 4g), with little change in the steepness. During the last 20 years, the higher mudflat of JDS was subjected to significant accretion, while erosion appeared in the lower mudflat after 2010, so the steepening trend of JDS was relatively obvious (Fig. 4h). ENHS silted up before 2010, and then the lower mudflat below −2 m began to erode (Fig. 4i) and the whole tidal flat steepened.

Fig. 4.
figure 4

Shape changes of the tidal flats.

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Table 1. Steepnessa of the tidal flats in the Yangtze Estuary.
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4 Discussions

4.1 Impact of Estuarine Engineering Projects

The large-scale estuarine engineering projects in the YE include waterway regulation projects such as BSP, XSP and DCR, reclamation projects such as EHSR, OPAR and ENSR and hydraulic engineering such as QSR (Fig. 1b). The changes of erosion rate and deposition rate calculated by the numerical model (Fig. 5) show that the morphologic evolution of the tidal flats in the YE was significantly affected by the construction of large-scale wading projects.

Waterway regulation projects can not only control the river regime and form deep channel, but also play the role of protecting the shoal. BSP and XSP are the waterway regulation projects located at the SB-NB and SC-NC bifurcations respectively. Under the protection of BSP and XSP, the deposition rates of BMS and XLHS within the submerged breakwater of the projects increased and the erosion rates decreased, basically consistent with the actual erosion and deposition status of the two tidal flats (Fig. 3). DCR is located at the SP-NP bifurcation and its downstream. Due to the larger scale, DCR had obvious effects on HSS and JDS. Figure 5 shows that the deposition rates of HSS and JDS increased significantly, especially in the southern edge of HSS and the northern edge of JDS near DCR. Meanwhile, the erosion rate of the southern edge of HSS decreased slightly. However, the southern edge of the upper part of JDS tended to erosion under the influence of the increased diversion ratio of the SP after the implementation of DCR.

OPAR and ENSR narrow the width and enhance the hydrodynamic forces of SP (Cheng et al. 2020). Affected by the above changes, the erosion rate of ENHS outside the reclamation projects increased, while the erosion rate and the deposition rate of central southern edge of JDS increased and decreased, respectively. The above two areas showed an erosion trend, which is consistent with the actual erosion characteristics (Fig. 3). In addition, EHSR caused a slight increase in the erosion rate of the upper part of HSS near the dike. QSR is a large water source project located at the SC-NC bifurcation, which resulted in the increase of erosion rate on the north side (concave bank) of the main channel in the upper section of NC and intensified the erosion of BZS. The above changes show that the reclamation and reservoir projects occupy the higher tidal flat, and at the same time cause the erosion of the nearby lower tidal flat.

Due to the shielding effect of BSP extending downstream and the restriction of XSP and QSR on the flow from SB into NC, the deposition rate of BDS increased slightly and the erosion rate of the lower southern edge of BDS decreased (Fig. 5), which contributed to the stable deposition of BDS in the past 20 years (Fig. 3). It can also be seen from Fig. 5 that the deposition rate of ECMS generally increased, which should be related to the weakening effect of DCR and EHSR on the tidal dynamics in the northern waters of the projects. However, the impact of estuarine engineering projects on the erosion and deposition trend of RFS was relatively slight.

In summary, BSP and XSP were the key factors for BMS and XLHS to maintain the deposition of higher mudflats under the environment of significant erosion of tidal flats in the inner estuary, and DCR was the main cause of the siltation at the north side of JDS and southeast side of HSS. Meanwhile, the shielding effect of the estuarine engineering projects could promote the deposition of BDS and ECMS located on the north side of the estuary. However, QSR and EHSR, ENSR and other reclamation projects tended to cause erosion of adjacent lower tidal flats outside the projects.

Fig. 5.
figure 5

Changes of erosion rate (a) and deposition rate (b) caused by estuarine engineering projects (results of numerical simulation, red represents the deposition trend and blue represents the erosion trend).

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4.2 Impact of Fluvial Sediment Decline

The YE is rich in water and sediment. Over the years, the runoff of the Yangtze River basin has not changed much, but due to the impact of water and soil conservation and dam construction in the basin, the amount of sediment carried by runoff into the YE has decreased significantly (Liu et al. 2017). As shown in Fig. 6, the annual sediment discharge of Datong station began to decline after the mid-1980s, from an average of 471 million tons per year in 1955–1985 to 349 million tons per year in 1986–1997, with a reduction of 25.5%. Since 1998, the decline rate of sediment discharge has accelerated. The annual sediment discharge of Datong from 1998 to 2019 was 175 million tons per year on average, 48.6% less than that from 1986 to 1997. During this period, after the operation of the Three Gorges Project (2004–2019), the annual sediment discharge remained at a low value, with an average of only about 128 million tons per year.

Fig. 6.
figure 6

Changes of the annual runoff and sediment discharge at the Datong station during 1950–2019.

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Due to the fluvial sediment decline, the SSC in the lower reaches of the Yangtze River and the YE has been decreasing (Yang et al. 2013; Liu et al. 2017). The field data showed that the SSC of NG station in SC and BG1 station in NC decreased in fluctuation from 0.87 kg/m3 and 0.78 kg/m3 in August 2002 to 0.15 kg/m3 and 0.22 kg/m3 in August 2019, respectively (Fig. 7). From 2002 to 2009, 2010 to 2014, and 2015 to 2019, the average SSC of NC1 station in SP was 1.32 kg/m3, 1.06 kg/m3 and 0.30 kg/m3 respectively, while the average SSC of BC1 station in NP was 0.61 kg/m3, 0.48 kg/m3 and 0.26 kg/m3 respectively. The average SSC of NC2, BC2 and BG2 stations in the turbidity maximum zone decreased from 1.39 kg/m3, 1.44kg/m3 and 0.85 kg/m3 in 2012–2014 to 0.49kg/m3, 0.60kg/m3 and 0.39kg/m3 in 2016–2019, respectively. It can be seen that due to the time-lag effect (Zhao et al. 2018), the decrease of SSC in the mouth bar area is later than that in the inner estuary. In general, under the influence of the significant decrease of sediment discharge in the basin, the SSC in the YE had been declining successively from the inner estuary to the mouth bar area in the past 20 years.

Fig. 7.
figure 7

Changes of SSC at typical survey stations in the Yangtze Estuary (Location is shown in Fig. 1)

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For more than half a century, the tidal flats of the YE have been dominated by sedimentation. However, the decrease of SSC in the estuary would enhance the scouring capacity of the current, which would slow down the deposition rate of estuary and even make the transition to erosion (Liu et al. 2017). Studies have shown that when the annual sediment discharge of Datong is 260–280 million tons, the deposition and erosion of the YE subaqueous delta are in equilibrium (Yang et al. 2003; Yang et al. 2014). During the formation and development of the Yangtze River Delta, the average annual sediment discharge of Datong was 184–228 million tons (Li et al. 2004). The critical annual sediment discharge of Datong is about 254 million tons when the lower reaches of SB in the YE changes from silting to scouring (Cheng et al. 2020). From 1998 to 2019, the average annual sediment discharge of Datong is only 175 million tons, which is obviously lower than the critical annual sediment discharge proposed by previous scholars, and the estuary tends to be scoured on the whole (Liu et al. 2021).

According to the numerical model calculation results (Fig. 8), it can be seen that the decrease of SSC at Datong reduces the deposition rate of tidal flats in the YE, that is, the tidal flats tend to be scoured. Specifically, the erosion trend of the tidal flats in the inner estuary is more obvious than that of the mouth bar area. The erosion trend of BMS, BDS, XLHS and RFS in the inner estuary is generally more significant. At the same time, the erosion trend of the coastal side of the tidal flats in the mouth bar area is also obvious along the line of “ECMS-HSS-JDS”. However, the erosion trend of tidal flats in the mouth bar area is relatively light, which is due to the time-lag effect of sediment accumulation in the turbidity maximum zone (Zhao et al. 2018). In addition, the erosion trend of lower tidal flats is generally more significant than that of higher tidal flats, especially that of tidal flats in the mouth bar area (Fig. 8), which accords with the law that the scouring of estuarine riverbed is first deep and then shallow under the condition of fluvial sediment decline (Liu et al. 2017; Cheng et al. 2020). It should be noted that the above characteristics of tidal flats erosion trend are only preliminary simulation results of the influence of fluvial sediment decline at Datong under fixed bed conditions, and the influence of complex coupling changes of “SSC, hydrodynamics and riverbed topography” on the tidal flat evolution trend needs to be further explored.

We speculate that, the erosion of tidal flats in the YE occurred from the inner estuary to the mouth bar area and from the lower mudflat to the higher mudflat successively, which was closely related to the successively decrease of SSC in the estuary caused by the fluvial sediment decline in the Yangtze River Basin.

Fig. 8.
figure 8

Changes of deposition rate caused by fluvial sediment decline (results of numerical simulation, the darker the color and the more obvious the erosion)

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4.3 Evolution Pattern of Tidal Flats

The above analysis results show that, under the fluvial sediment decline and estuarine engineering projects, the morphological evolution of tidal flats in the YE showed significant spatio-temporal differences in the past two decades, but it also followed a certain rule. The evolution pattern of tidal flats is summarized as follows.

In the first place, sediment decline in the basin had dominated the erosion of tidal flats in the YE from 1998 to 2019, the sequence of erosion was that the inner estuary preceded the mouth bar area, and the erosional location was mainly in the lower mudflat and gradually extended upward.

Secondly, different engineering boundaries and hydrodynamic conditions controlled the evolution trend of tidal flats and the distribution of erosion and deposition in the Yangtze Estuary under the macro environment of fluvial sediment decline. The tidal flats (BMS, XLHS) in the inner estuary protected by the projects were characterized by continuous erosion and obvious deposition, before and after the implementation of the projects, respectively; the tidal flats (RFS, BZS) without engineering protection and close to the mainstream of ebb current in the inner estuary were obviously eroded; the tidal flats (JDS, HSS) in the mouth bar area protected by the projects were characterized by first deposition and then erosion in the low mudflat and stable deposition in the high mudflat; the tidal flat (ENHS) in the mouth bar area without engineering protection silted up at first and then scoured as a whole; the tidal flats (BDS, ECMS) on the north side of the main channel changed from silting to scouring later and to a lesser degree.

5 Conclusions

The study reveals the morphological evolution characteristics of tidal flats in the YE under the influence of fluvial sediment continuous decline and the estuarine engineering projects in recent two decades.

Affected by the fluvial sediment decline, the SSC in the YE experienced a significant decline since 1998. However, due to the time-lag effect, which was caused by sediment accumulation in the turbidity maximum zone, the SSC in the mouth bar area decreased later than that in the upstream area. As a result, the tidal flats of the Yangtze Estuary presented a situation of erosion on the whole, Moreover, the tidal flats in the inner estuary were earlier and more obvious than those in the mouth bar area, and the tidal flats adjacent to the mainstream of ebb current were earlier and more obvious than those on the north side of the main channel (weaker hydrodynamics area). Meanwhile, the erosion place of tidal flats extended from the lower mudflat to the higher mudflat, resulting in the steepening of the tidal flats.

The estuary wading projects controlled the change of evolution trend and the distribution of scouring and silting position in the adjacent tidal flats. Among them, the estuarine waterway regulation projects were beneficial to the deposition of tidal flats while enhancing the erosion of deep channels. However, the reclamation and reservoir projects rented the higher tidal flats and caused the erosion of the lower tidal flats nearby.

We speculate that, under the influence of human activities such as dam construction and water-soil conservation, the sediment supply in the Yangtze River Basin may keep decreasing and maintain a lower level in the future, and without the implementation of targeted protection projects, the tidal flats in the YE will face a risk of further erosion. Therefore, we suggest that scientific and effective measures such as protection projects and ecological siltation-promotion should be taken as soon as possible to protect the tidal flats and improve the ecological service function of the wetlands in the Yangtze Estuary.