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Journal of Geographical Sciences >

2022 , Vol. 32 >Issue 11: 2291 - 2310

DOI: https://doi.org/10.1007/s11442-022-2048-8

Research Articles

Morphological evolution of the channel-shoal system in the South Channel of the Changjiang Estuary during 1958-2018: Causes and future trends

  • LUAN Hualong ,
  • YAO Shiming ,
  • QU Geng ,
  • LEI Wentao
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  • River Research Department, Changjiang River Scientific Research Institute, Wuhan 430010, China

Luan Hualong, PhD and Senior Engineer, specialized in river evolution and management. E-mail:

Received date: 2021-11-11

  Accepted date: 2022-06-02

  Online published: 2022-11-25

Supported by

Natural Science Foundation of China-Ministry of Water Resources-China Three Gorges Corporation Joint Fund for Changjiang Water Science Research(U2040202)

National Natural Science Foundation of China(42006156)

National Natural Science Foundation of China(52009008)

Fundamental Research Funds for Central Public Welfare Research Institutes(CKSF2021530/HL)

Research Project on Major Scientific and Technological Issues in Watershed Water Management(CKSC2020791/HL)

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Abstract

The stability of estuarine channel-shoal systems is important for port utilization, navigation maintenance, habitat protection and ecosystem service functions. This paper uses the South Channel of the Changjiang (Yangtze River) Estuary as a typical example to investigate the channel-shoal adjustment mechanism and its future trend. The combined approaches of bathymetric data analysis and process-based modeling (Delft3D) are applied. Quantitative analysis of morphological changes indicates that the South Channel experienced remarkable channel-shoal adjustment during 1958-2018. Periodic evolution was identified, including shoal migration, incision and emergence under natural conditions before the mid-1980s. Since then, fluvial sediment decline and local human intervention have interrupted the periodic processes. After 1986, as river sediment discharge started to decline, the South Channel converted to net erosion, and both the mid-channel shoal at the bifurcation node and the tail of the Ruifeng Shoal showed significant scour. Process-based hydrodynamic simulations revealed that the northern rotation of the mainstream downstream of Wusong triggered the erosion of the Ruifeng Shoal, while unordered sand mining at the shoal tail in approximately 2002 enhanced shoal shrinkage. In addition, the self-adjustment of the transverse section shape resulted in abnormal accretion in 2002-2007. Afterward, the South Channel underwent overall erosion as sediment discharge decreased to a low level (<150 Mt/a). Five stages of channel-shoal pattern adjustment and accretion/erosion status during the past 60 years were defined, i.e., the accretion stage (1958-1965), remarkable channel-shoal adjustment stage (1978-1986), slow erosion stage (1986-1997), shoal scour and shrinkage stage (1997-2007) and overall channel-shoal erosion stage (2007-2018). Model prediction of the evolutionary trend indicates that overall erosion within the South Channel is most likely to continue in 2015-2050. Further adjustment of the South Channel under extremely low sediment discharge may threaten the riverbed stability and the sustainable development of this large-scale estuary. Future work on adaptive strategies for varying conditions is recommended.

Key words： channel-shoal pattern; morphological evolution; process-based modeling; South Channel; Changjiang Estuary

Cite this article

LUAN Hualong , YAO Shiming , QU Geng , LEI Wentao . Morphological evolution of the channel-shoal system in the South Channel of the Changjiang Estuary during 1958-2018: Causes and future trends[J]. Journal of Geographical Sciences, 2022 , 32(11) : 2291 -2310 . DOI: 10.1007/s11442-022-2048-8

1 Introduction

Channel-shoal systems are important morphological elements in tidal basins and estuaries. The morphodynamics of these systems are primarily controlled by river inputs, astronomical tides, wind waves and their interactions (Galloway, 1975; Caldwell et al., 2019). Under steady forcing, estuarine channel-shoal systems can present dynamic equilibrium or quasiperiodic behavior as their intrinsic characteristics, which can be affected by littoral currents and storm events (Kleinhans et al., 2015). Their stability is of high significance for flood protection, freshwater utilization, navigation maintenance, land resources, habitats for living creatures and the ecosystem service function within estuaries and deltas, which have enormous relevance to regional socioeconomic development and environmental protection (Pont et al., 2002; Ericson et al., 2006). However, increasing human interventions in recent decades, including sediment trapping in upstream reservoirs and estuarine engineering projects, have caused dramatic adjustments in channel-shoal patterns worldwide (Syvitski et al., 2009; Giosan et al., 2014). Understanding the physical mechanisms and future trends of estuarine channel-shoal systems under changing external forcing has become one of the main concerns for both scientific research and sustainable estuarine management.
The basic confirmation of channel-shoal systems is characterized by interlaced ebb and flood channels with shallow shoals between them, as observed in the Western Scheldt Estuary and many other estuaries (Van Veen, 1950). The formation of this multiple-channel pattern can be explained by a positive feedback mechanism between the tidal flow, sediment transport and erodible bed (Schuttelaars and De Swart, 1999). Hibma et al. (2004) used a 2D process-based model (Delft3D) and simulated the formation processes of channel-shoal patterns in a schematized estuary, which showed qualitative agreement with field observations. Van der Wegen and Roelvink (2012) reproduced the channel-shoal pattern in the Western Scheldt Estuary from a flat bed with a fixed basin geometry. A series of idealized numerical studies revealed the relationship between estuarine channel-shoal patterns and controlling factors, including prevailing forces, sediment properties and estuarine geometry (Nardin and Fagherazzi, 2012; Yu et al., 2012; Leonardi et al., 2013; Guo et al., 2015; Braat et al., 2017). Extensive on-site observations and case studies have identified various morphological features under different types of physical forces. Under the decreasing fluvial sediment supply in most large rivers globally (Walling and Fang, 2003), dramatic changes in estuarine channel-shoal patterns have occurred, and human interventions have outstripped natural evolution (Syvitski and Saito, 2007). For instance, training wall construction and dredging activities for navigation purposes interrupted the natural evolution in both the Mersey Estuary and Ribble Estuary (England) and induced sedimentation within their channel-shoal systems (Thomas et al., 2002; Van der Wal et al., 2002; Blott et al., 2006). The closure of the secondary basins in the Western Scheldt Estuary (Netherlands) triggered lateral channel-shoal migration as weaker tidal currents in the main channel and more rotary near secondary basins (Nnafie et al., 2018). Intensive dredging-dumping activities in the same estuary also altered the stability of the channel-shoal system from multiple ebb-flood channels to a single channel (Jeuken and Wang, 2010). In the long term, channel-shoal systems can adapt to human interventions and maintain overall stability, which is demonstrated by the evolution of channel-shoal systems in Qinzhou Bay (SW China) under the combined effects of channel dredging, land reclamation and long-range jetties (Wang et al., 2014). The above previous studies have enriched our knowledge of channel-shoal morphodynamics in response to natural forcing and human interventions. However, the combined effects of altered river inputs and local human interventions on large-scale estuarine channel-shoal systems are less understood, especially on their quasiperiodic morphological processes, which is the main motivation underlying the present study.
The three-level bifurcated Changjiang (Yangtze River) Estuary (Figure 1), as the mouth of the largest and longest river in southeastern Asia, includes several large-scale channel-shoal systems with various bathymetric patterns. Due to the closure of the Three Gorges Dam in 2003, the fluvial sediment-transport process has changed significantly. In addition to the significant decline in suspended sediment discharge since 2003, the riverbed of the middle-lower Changjiang has become the major source of sediment being transmitted downstream, and estuarine shoals and offshore sediment supply have become major sediment sources for the river estuary (Dai et al., 2018). Considering remarkable fluvial sediment decline and intensive estuarine engineering projects in recent decades, the Changjiang Estuary provides a typical example to address the above issue. Xie and Yang (1999) revealed the formation process of the Jiuduansha Shoal between the North and South Passages, and Li et al. (2016) demonstrated that the evolution of the Jiuduansha Shoal since 1997 was closely linked to the construction of the training walls along the North Passage. Dai et al. (2013) analyzed bathymetric changes in the North Passage and identified primary features, including deposition in the groyne-sheltered areas and deepening along the navigation channel. Wei et al. (2017) explored the morphological response of the Nanhui Shoal to river damming, estuarine hydraulic engineering and reclamation projects. Mei et al. (2018) analyzed the recent evolution of the North Channel and found erosion in the upper part and deposition in the lower part due to the closure of the Three Gorges Dam and land reclamation. Lou et al. (2019) explored the evolution of the Biandan Shoal and Xinqiao Channel and relevant factors over the past two decades. Dai et al. (2016) found that local land reclamation along the banks of the North Branch was directly responsible for the infilling of this tidal-dominated distributary of the Changjiang Estuary. It is suggested that river input changes and estuarine engineering projects played an important role in morphological changes in channel-shoal systems within the Changjiang Estuary, although variable features were identified in different areas.
Figure 1 Map of the study area (a. Locations of the South Channel (red rectangle) and Changjiang River Basin (white area); b. South Channel with bathymetry observed in 2018; white dashed lines denote the domain for the morphological calculation of the South Channel.)
As the second bifurcation of the Changjiang Estuary, the South Channel consists of the main channel (ebb-dominated), the Changxing Channel (flood-dominated) and the Xinliuhe Shoal and Ruifeng Shoal (Figure 1b). The stability of the channel-shoal system is of high importance for navigation maintenance and Shanghai harbor operation at the southern bank. Zhu et al. (2014) analyzed the initial formation processes of the Ruifeng Shoal since the 1950s and the channel volume variations during 1998-2012. Zhu and Luo (2015) found that the shrinkage of the Ruifeng Shoal in the early 2000s was largely related to unordered sand mining at the shoal tail, which resulted in immediate deposition in the main channel (Yan et al., 2010). To date, the morphological evolution processes (periodic or not) of the channel-shoal system since its formation and the controlling factors have not been fully revealed. After the Qingcaosha Reservoir and the regulation project of the Xinliuhe Shoal were completed in 2009, the bifurcation of the North and South Channels was artificially fixed (Figure 1b). How the channel-shoal system responds to continuous fluvial sediment decline after the closure of the Three Gorges Dam and local engineering projects is another question that remains unclear. Therefore, this study combined bathymetric data analysis and process-based modeling (Delft3D) to quantify the evolution processes of the channel-shoal system within the South Channel during 1958-2018 and identify the controlling factors and driving mechanisms. The evolutionary trends until 2050 are predicted using a previously well-calibrated and validated numerical model (Luan et al., 2017). The evolution mode in the South Channel on a decadal timescale and implications for integrated management are discussed.

2 Changjiang Estuary and the South Channel

The Changjiang Estuary is a large-scale dynamic system with three-level bifurcation and four outlets. The domain of the estuary spans from the head of Chongming Island to the outer subaqueous delta with a length of more than 180 km, and the channel width increases from ~5.7 m at the inner estuary to over 90 km at the mouth bar area. Multiple channel-shoal systems can be found within the estuary at scales from several kilometers to dozens of kilometers. A large amount of freshwater and suspended sediment reaches the Changjiang Estuary. The annual mean runoff and sediment load at the Datong station (tidal limit) were 893 km3 and 368 Mt, respectively, during 1951-2015 (CWRC, 2018). Over the past half century, river water discharge remained relatively stable, except for several wet or dry years, whereas sediment load abruptly declined after the closure of the Three Gorges Dam in 2003 (Figure 2). The mean and maximum tidal ranges in the Changjiang Estuary are 2.66 and 4.62 m, respectively (Yun, 2004). Under the combined effects of river flow and tidal current, the Changjiang Estuary can be classified as a joint river- and tide-controlled estuary.
Figure 2 Variations in annual river runoff and sediment load at the Datong station (tidal limit) since 1950
The South Channel is the second-level bifurcation spanning from the head of Xinliuhe Shoal to the head of the Jiuduansha Shoal (Figure 1b). The length and widths of the South Channel are approximately 39 km and 5.5-7.5 km, respectively. The Huangpu River enters the South Channel at Wusong. The main channel is ebb-dominated due to the large amount of river discharge, while the Changxing Channel is flood-dominated. The Ruifeng Shoal and these two channels jointly form the channel-shoal system in the South Channel. The bifurcation of the North/South Channel is regarded as the most complicated and unstable area within the entire estuary. Regulation projects, including the Xinliuhe Shoal protection works and the submerged breakwater at the Nanshatou Channel (Figure 1b), were carried out in 2007 and completed in 2009. Since then, the bifurcation site of the South Channel has been fixed. Gaoqiao, as one of the largest harbors in Shanghai, is located on the southern bank, suggesting the importance of the South Channel on regional economic development.

3 Methods

Bathymetry data covering the South Channel observed over multiple years from 1958 to 2018 were collected (Table 1). Most of the measurements were carried out after river flooding. The water depth and positions were measured by an echo sounder and a GPS system, respectively. Depth samples in each year refer to the theoretical low-tide datum. The scales of bathymetry maps vary from 1:25,000 to 1:120,000, and the sample density is acceptable for calculating erosion and accretion volumes (Blott et al., 2006). Using the GIS technique, a digital elevation model (DEM) with a 50×50 m grid resolution is produced using the kriging interpolation method. Three typical transverse sections at different areas are presented based on the DEMs. Erosional/depositional patterns are obtained by subtracting a later DEM grid from an earlier grid. The domain for calculating channel volume, mean depth and hypsometry curves is defined as shown in Figure 1b.
Table 1 Information on the bathymetric dataset used in this study
No. Year Scale Source Survey month
1 1958 1:100,000 NGDCNH 8-0
2 1965 1:100,000 SWB 4-1
3 1973 1:50,000 SWB 3-1
4 1978 1:120,000 NGDCNH -
5 1986 1:50,000 SWB 5-9
6 1994 1:120,000 NGDCNH -
7 1997 1:50,000 YEWAB 12
8 2002 1:25,000 YEWAB 12
9 2007 1:50,000 YEWAB 8
10 2010 1:10,000 YEWAB 8
11 2013 1:10,000 YEWAB 8
12 2018 1:10,000 YEWAB 2

Note: NGDCNH: Navigation Guarantee Department of the Chinese Navy Headquarters; SWB: Shanghai Waterway Bureau, Ministry of Transport of China; YEWAB: Yangtze Estuary Waterway Administration Bureau, Ministry of Transport of China

Process-based numerical modeling (Delft3D) is applied to explore the hydrodynamics and sediment transport in the South Channel. The model domain covers the entire Changjiang Estuary from the tidal limit (Datong) to the East China Sea. The grid size varies from ~300 m within the estuary to ~3000 m near the offshore boundary, which is sufficient for simulating the morphological behaviors of the Changjiang Estuary with acceptable accuracy. The time step of the hydrodynamic model is 2 minutes according to the CFL criteria. Major physical driving forces are considered in the model, including astronomical tides, river flow and wind waves. The offshore boundaries are driven by 8 main astronomic components (M2, S2, K1, O1, N2, K2, P1, and Q1) derived from a well-validated large model covering the East China Sea (Ge et al., 2013). Currents across the boundaries are calculated by the model. The tangential component of coastal currents is absent due to the unnecessary complexity in morphological modeling when prescribing both astronomical tides and coastal currents along the offshore boundaries (Van der Wegen et al., 2011). Wave parameters are calculated by SWAN (http://www.swan.tudelft.nl) using monthly climatological wind data (1995-2005). The wave information that is obtained each hour is coupled offline with the flow and morphological model. River inputs are schematized in the model based on monthly averaged water and sediment discharges. The morphological acceleration factor technique is used to simulate and predict decadal morphological evolution. The values of morphological factors depending on the amplitude of water discharge are determined after sensitivity analysis. The model considers multiple sediment fractions (cohesive and noncohesive), which are based on grain size and compositional analyses of bed sediment samples in the estuarine area. The model has been fully calibrated and validated against observed water levels, tidal currents and morphological changes in the Changjiang Estuary (Luan et al., 2017). In this study, hydrodynamics in the wet season (65,000 m3/s) and dry season (15,000 m3/s) were simulated under different bathymetric conditions. Residual currents are computed by averaging the model results of hydrodynamics within one month. To predict the morphological evolutionary trends of the South Channel, four future scenarios are designed to predict the evolutionary trend under continuous river sediment decline. Scenarios 1-2 and 3-4 predict the evolutionary trend in 2015-2035 and 2035-2050, respectively. The sediment discharge in Scenarios 1 and 3 is 125 Mt/yr, while the sediment discharge in Scenarios 2 and 4 is 100 Mt/yr, representing an extremely low level (Yang et al., 2014). The initial bathymetry of Scenarios 3-4 uses the prediction result of Scenario 1 since the sediment discharge is close to the present level.

4 Results

4.1 Adjustment of the channel-shoal system

The observed bathymetry during 1958-2018 indicated that the channel-shoal pattern within the South Channel experienced remarkable adjustment (Figure 3). In 1958, the South Channel consisted of a wide main channel and a short Ruifeng Shoal (Figure 3a). The upstream bifurcation area was largely obstructed by several middle-channel shoals (named the Liuhe Shoal). Afterward, the Biandan Shoal and Liuhe Shoal migrated downstream with anticlockwise rotation of the Xinqiao Channel until the late 1970s (Figures 3b-3d). The entrance of the South Channel became wider and aligned with the flow direction. Meanwhile, the head of the Ruifeng Shoal emerged with the Liuhe Shoal, and the tail accreted downstream. The typical channel-shoal pattern, with the main channel, the flood-dominant Changxing Channel and the Ruifeng Shoal has formed since the 1970s. During 1978-1986, the Biandan Shoal was incised, and its lower part emerged into the shallow shoal near the head of Changxing Island (Figure 3e). After this remarkable adjustment of the channel-shoal pattern in the South Branch, the bifurcation area of the South Channel was obstructed again by the middle-channel shoals (named Xinliuhe Shoal) (Figure 3f). The Ruifeng Shoal continuously accreted downstream, and the length of the shoal body was over 20 km in 1997, which was the longest in its lifespan to date (Figure 3g). Since then, the Ruifeng Shoal has experienced continuous scour and shrinkage. The scour initially occurred at its middle part, and the -5 m isolines were cut through in 2002 (Figure 3h). The downstream end of the -5 m isolines moved upstream for more than 11.5 km from 1997 to 2018. Another notable feature was the erosion in the bifurcation zone of the South Channel, and the small middle-channel shoal disappeared after 2010 (Figures 3i and 3j). The Xinliuhe Shoal gradually migrated downstream before 2009, when its head was controlled by submerged jetties (Figure 3j). The south side of the Liuhe Shoal has incised since 2013, resulting in the separation of a small shoal from the main body of the Xinliuhe Shoal and a small ebb channel (Figures 3k and 3l).
Figure 3 Observed bathymetry of the South Channel from 1958 to 2018
Variations in the three typical sections also suggested the adjustment of the channel-shoal pattern. Section 1 is located at the entrance of the South Channel and has shown complicated variations (Figure 4a). The mainstream moved northward from 1958 to 1978 and then back to southward from 1978 to 2013. After 2013, a secondary ebb channel formed with the lowest bed level of approximately -15 m in 2018 due to the Ruifeng Shoal incision on its southern side. The Nanshatou Channel was also deepened to approximately -10 m, resulting in shrinkage of the entire sand body. The section shape experienced a periodic variation, namely, “U-W-U-W”, in the study period. Section 2 is located upstream of the South Channel and has shown relatively less significant variation than Section 1 (Figure 4b). The south side of the upper Ruifeng Shoal experienced periodic erosion and accretion along with upstream channel-shoal adjustment. The head of the Changxing Channel was largely deepened from 0 m in 1958 to -15 m in 2018. Section 3 is located in the middle of the South Channel and reflected the formation of the Ruifeng Shoal from 1958 to 1978 (Figure 4c). The shoal body showed limited change from 1978 to 1997. Afterward, significant scour in the middle and lower parts of the Ruifeng Shoal occurred, and the section shape converted from “W” to nearly “U”. Along with the adjustment of the section shape, the marginal shoal at the southern bank experienced immediate accretion, which threatened the safety of Gaoqiao Harbor. After 2007, the main channel showed continuous erosion, and the bed level in 2018 was almost the same as that in 1997.
Figure 4 Variations in three typical sections in the South Channel from 1958 to 2018 (see Figure 3a for the locations)

4.2 Erosional and depositional patterns

Erosional and depositional patterns during 1958-2018 suggested intensive bed-level changes in the South Channel at decadal and interannual time-scales (Figure 5). The depocenter at the tail of the Biandan Shoal and the erosion zone at the head of the Liuhe Shoal were clearly demarcated from 1958-1978 (Figures 5a-5c), reflecting downstream shoal migration under ebb river flow. However, the locations of the depocenter and erosion zone were reversed in 1978-1986 (Figure 5d), which resulted from shoal incision and emergence. This period was thereby characterized by the most dramatic changes in terms of channel-shoal patterns. Afterward, the depocenter and erosion zone returned to their previous locations, although the amplitudes of bed-level changes decreased. Alternate deposition and erosion occurred along the main channel and at the Ruifeng Shoal before 1997. Since then, continuous erosion has occurred in the middle and lower parts (Figures 5g-5k). Simultaneously, the main channel and the marginal shoal at the southern bank turned to notable accretion during 1997-2007. After 2007, the entire channel-shoal system converted to overall erosion, especially at the southern side of the Ruifeng Shoal in 2013-2018 (Figure 5k).
Figure 5 Erosional and depositional patterns of the South Channel in different periods from 1958 to 2018

4.3 Variations in channel volume and mean depth

The channel volume and mean depth showed synchronous variations during the study period, indicating sediment accretion or loss (Figure 6). The channel volume and mean depth of the defined domain (Figure 1b) decreased in 1958-1965 and increased in 1965-1978. Net accretion (0.71×109 m3) in the former period mainly occurred at the Liuhe Shoal and downstream part of the South Channel, while net erosion (-0.46×109 m3) in the latter period occurred mainly at the head of the Liuhe Shoal due to its downstream migration (Figures 5a-5c). The channel volume and mean depth decreased from 1978-1986 when the incised lower Biandan Shoal emerged into the Liuhe Shoal. From 1986 to 1997, the channel volume and mean depth slightly increased. It is noteworthy that the channel-shoal system in the South Channel seemed to present periodic variation, and the shoal incision and emergence in approximately 1986 was regarded as the end of the last cycle and the beginning of a new cycle. The channel volume and mean depth increased in 1997-2002 and then decreased abruptly in 2002-2007. However, erosion (-0.27×109 m3) in the former period and accretion (0.25×109 m3) in the latter period were no longer at the Xinliuhe Shoal but at the Ruifeng Shoal and the main channel, respectively (Figures 5g-5h). From 2007 to 2018, the channel volume significantly increased from 6.69×109 m3 to 8.06×109 m3, indicating 1.36×109 m3 of sediment loss. The water depth also increased from 7.9 m to 9.8 m.
Figure 6 Variations in mean water depth and channel volumes along the South Channel (see the domain in Figure 1b)

4.4 Changes in hypsometry curves

Hypsometry curves of the South Channel in various years provide more detailed morphological changes over a continuum of water depths (Figure 7). All the areas were shallower than -20 m (mostly over -15 m). Accretion occurred over the full depth range in 1958-1965 (Figure 7a). Areas shallower than -2 m retained accretion in 1965-1978, while areas deeper than -2 m converted to erosion. From 1978-1986, the shallow shoal and deep main channel divided by -10 m isolines showed accretion and erosion, respectively, whereas the evolutionary pattern reversed from 1986-1997. The entire study domain underwent net accretion in the former period and net erosion in the latter (Figure 6). Shallow shoals (mainly the Ruifeng Shoal) were eroded in 1997-2002, and the main channel (<-8 m) accreted rapidly in 2002-2007 (Figure 7b). In 2007-2010, erosion occurred over the full depth range, with stronger erosion in shallow areas (>-10 m). In 2010-2018, the main channel (<-8 m) showed significant erosion. Overall, the water surface area over the full depth range increased from 1997-2018, indicating overall erosion in the past two decades.
Figure 7 Hypsometry curves of the South Channel from 1958 to 2018 (see the domain in Figure 1b)

4.5 Numerical modeling of hydrodynamics and future evolutionary trends

Modeled residual currents show different patterns in 1997 and 2007 (Figure 8). In 1997, the mainstream was divided into two channels by the Xinliuhe Shoal, and the residual flow in the main channel was along the southern shoreline, while the flow direction in the secondary channel presented an acute angle with the shoreline. After the two ebb streams converged near Wusong, the currents were reflected by the marginal bank and rotated northward toward the tail of the Ruifeng Shoal (Figures 8a and 8b). This has probably been the main reason for the erosion of the Ruifeng Shoal since 1997. After the Xinliuhe Shoal merged with the Ruifeng Shoal, the secondary channel shrunk, although the ebb flow at the channel head was still notable in 2007. The residual currents expanded in the South Channel, and the main streamline moved northward due to scour at the Ruifeng Shoal (Figures 8c and 8d). The Xinliuhe Shoal was scoured into several small shoals in 2007, and residual flow among the subchannels was disordered. Residual flow along the Changxing Channel showed flood-dominant features, which may cause sediment accumulation in dry seasons. The amplitude of residual flow under high river discharge was much larger than that under low discharge.
Figure 8 Residual currents under different river discharge and bathymetric conditions
Numerical prediction of the evolutionary trend during 2015-2050 indicates that the South Channel will be continuously dominated by overall erosion. In 2015-2035, under present sediment discharge, accretion will occur at the head of the Xinliuhe Shoal and the Qingcaosha Reservoir, while the Nanshatou Channel and Changxing Channel will be eroded and connect with each other (Figure 9a). Erosion at the south side of the Xinliuhe Shoal will also continue, and further shoal incision will result in separation of a small shoal. The main channel will convert to accretion, whereas the flood-dominant Changxing Channel will be largely deepened. In 2035-2050, the erosion and accretion pattern is similar to the previous period with decreased overall erosion intensity (Figure 9b). Accretion at the shoal head will also decrease, and the separated shoal from the Xinliuhe Shoal will move downstream. Lower sediment discharge will obviously enhance the erosion (Figures 9c and 9d). Notably, the erosion enhancement in 2035-2050 is less than that in 2015-2035, suggesting approaching the dynamic equilibrium of the South Channel.
Figure 9 Model predictions of erosional/depositional patterns (a-b) and the differences between modeling scenarios (c-d)

5 Discussion

5.1 Causes for the morphological evolution of the channel-shoal system in the South Channel

The South Channel is located in the transition zone from a confined channel to a wide open subaqueous delta, and the channel-shoal system is influenced by both fluvial and tidal forces (Guo et al., 2015). Under natural conditions, the channel-shoal system in the lower South Branch showed periodic evolution processes with simultaneous downstream shoal migration, shoal incision/mergence and upstream channel shifts in sequence (Chen et al., 1988). In 1958, the Liuhe Shoal was a long and narrow sand body, and its head was elongated upstream into the middle section of the South Branch (Figure 3a). From 1958 to 1965, the head of the Liuhe Shoal moved downstream with transverse shoal accretion and expansion (Figure 3b). In this period, the South Channel involved rapid accretion due to a high river sediment supply. Afterward, the head of the Liuhe Shoal entered the South Channel, and erosion occurred at the entrance of the South Channel due to downstream shoal migration. This resulted in net erosion of the South Channel during 1965-1978 (Figure 5b), although sediment discharge was relatively high. It is suggested that the morphological evolution of the South Channel was affected by upstream bifurcation migration. Sudden adjustment of the channel-shoal pattern occurred after 1978 (Figures 3d, 3e and 5d). From 1978-1986, the South Channel rebounded to net accretion, which implied that the adjustment of the channel-shoal system turned to a new cycle after remarkable channel-shoal adjustment.
Human interventions from both the upstream reach and local areas also played an important role in channel-shoal adjustment. Before the mid-1980s, river sediment discharge was at a high level, with an annual mean value of 466 Mt/a in 1951-1980 (Figure 2). Moreover, the ebb flow diversion ratio of the South Channel increased from 40% in 1958 to 55% in 1988 (Figure 10). Thus, a large amount of sediment entered the South Channel in this period, contributing to the rapid accretion of the South Channel, especially in 1958-1965. After 1986, the channel volume and mean depth showed an overall increase (especially after 2007), indicating net sediment erosion. This has been directly caused by the decline in fluvial sediment discharge since the mid-1980s, and the sediment load dropped to a low level after the closure of the Three Gorges Dam, with an annual mean value of 133 Mt/a in 2003-2018 (Figure 2). The observed suspended sediment concentration within the South Channel started to decrease after 2007 (Liu et al., 2017). The ebb flow was unsaturated, which triggered riverbed erosion. The driving mechanism of fluvial sediment decline on the morphological evolution of the Changjiang Estuary has been revealed in many previous studies (Yang et al., 2011; Luan et al., 2016, 2021; Luo et al., 2017). However, it is noteworthy that the channel volume in 2002-2007 showed an abrupt decrease under low sediment supply. Accretion mainly occurred in the main channel and the southern marginal shoal (Figure 5h) from water depths of 8-15 m (Figure 7b). This abnormal feature was primarily controlled by the self-adjustment of the transverse section shape. Due to the erosion of the Ruifeng Shoal, the mainstream moved northward, and ebb flow velocity along the main channel and the southern shoal decreased. Subsequently, suspended sediment tended to be deposited in the main channel, which induced serious depositional problems in Gaoqiao Harbor (Yan et al., 2010). As the annual mean sediment discharge decreased to less than 150 Mt after 2007, the erosion capacity due to low sediment supply overlapped the effect of channel-shoal self-adjustment. Therefore, the South Channel rebounded rapidly to overall erosion.
Figure 10 Variations in the ebb flow diversion ratio of the South Channel since 1958
The South Channel is the deep navigation channel, and Shanghai Harbor (Gaoqiao) is located on its south bank. Therefore, local human activities showed increasing impacts on channel-shoal adjustment. The amount of sand mining in 2003-2005 at the middle and end of the Ruifeng Shoal was greater than 60 Mm3 (Li, 2014), and Zhu et al. (2016) estimated that the accumulative sand mining within the South Channel until 2012 was probably more than 100 Mm3. It is reported that sand mining at the tail of the Ruifeng Shoal contributed to shoal erosion and shrinkage during 1997-2002, while decreasing the river sediment supply further enhanced the erosion and led to overall erosion in the South Channel afterward. The impact of dredging or sand mining on channel-shoal stability has also been identified in other tidal estuaries. For instance, Swinkels et al. (2009) found that the connecting channel between the ebb and flood channels in the Western Scheldt Estuary tended to disappear after intensive dredging activities, which may further induce the instability of the entire channel-shoal system. Jeuken and Wang (2010) estimated the critical dredging and dumping amount to be 5%-10% of the sediment transport capacity of the entire channel-shoal system based on stability analysis. This raises the question of whether the Ruifeng Shoal can recover to the original scale after sand mining without river sediment decline. This study indicates that the northward rotation of the mainstream near Wusong due to erosion in the bifurcation area induced the ebb flow directly heading toward the middle of the Ruifeng Shoal tail. Thus, transverse flow across the shoal increased, which resulted in shoal erosion. Therefore, the shrinkage of the Ruifeng Shoal seems to be inevitable due to the combined effects of sand mining, river sediment decline and increased transverse flow. In addition, the Xinliuhe Shoal protection works stabilized the bifurcation and broke the periodic evolution processes of shoal migration and incision. Significant erosion of the South Channel after 2011 was also attributed to navigation dredging along the main channel.

5.2 Morphological evolution stages of the South Channel over the past 60 years

Based on the above quantitative analysis of the morphological evolution of the South Channel in the past 60 years, five stages of the accretion/erosion status and the channel-shoal pattern adjustment are identified (Figure 11). The first stage (1958-1978) was the natural evolution period, which featured downstream migration and lateral expansion of the Liuhe Shoal. The South Channel experienced net accretion in 1958-1965 and net erosion in 1965-1978. In the latter period, erosion mainly occurred at the entrance area after the shoal head entered the South Channel due to bifurcation migration, and the lower part of the South Channel was dominated by accretion. The second stage (1978-1986) showed remarkable adjustment of the channel-shoal pattern with shoal incision and emergence, which was the cumulative result of downstream shoal migration in the first stage. Because of the emergence of the lower Biandan Shoal into the Liuhe Shoal, the South Channel rebounded to net accretion. In the third stage (1986-1997), the channel-shoal system entered a new evolution cycle with downstream migration of the Xinliuhe Shoal, and the Ruifeng Shoal accreted to its largest scale since its formation. The periodic evolution process was also observed in the North Channel (Mei et al., 2018). However, because the fluvial sediment started to decline after the 1980s, the South Channel slowly involved net erosion in this stage. The fourth stage (1997-2007) was the erosion period of the Ruifeng Shoal and subsequent accretion at the main channel and southern marginal shoal. In this stage, the area and volume of the Ruifeng Shoal above the 5 m isobaths decreased by 33.0% and 38.8%, respectively. Both fluvial sediment decline and sand mining contributed to rapid shoal scour. Self-adjustment of the channel-shoal system resulted in abnormal accretion in 2002-2007. The third and fourth stages were during the transition period from natural-driven to human-driven effects. The fifth stage (2007-2018) was the overall erosion period when the sediment discharge decreased to a low level. Both the shoals and the main channel experienced erosion. More than half of the Ruifeng Shoal above the 5 m isobaths was eroded, and the main channel and the Changxing Channel below the 5 m isobaths also show continuous deepening. After the completion of the Xinliuhe Shoal protection works in 2009, the bifurcation site and the head of the Xinliuhe Shoal were fixed. Therefore, the morphological evolution of the channel-shoal system in the last stage was dominated by human interventions from both the river basin and the estuarine area. It is concluded that multiple causes are responsible for the adjustment of the channel-shoal system in the South Channel, and each factor showed different contributions in different stages. In the early two stages, natural processes, including sediment deposition and bifurcation migration, dominated. In the third stage, fluvial sediment discharge began to decline, and the South Channel converted to slight net erosion. In the fourth stage, the continuous decline in sediment discharge led to rapid erosion, although the self-adjustment of the channel-shoal system converted the channel from net erosion to net accretion in a short period (2002-2007). Under an extremely low sediment supply in the last stage, channel dredging and engineering work enhanced the scour and shrinkage of the channel-shoal system.
Figure 11 Schematic framework of the channel-shoal adjustment in the South Channel during 1958-2018

5.3 Implications for future work

Model prediction of the decadal evolutionary trend of the South Channel in this study shows significant implications for the stability of the channel-shoal pattern and the sustainable use of the Shanghai harbors. The Xinliuhe Shoal will show accretion at both the head and tail in 2016-2035, whereas it will revert to a recession under an extremely low sediment supply in 2035-2050. The separation of a small shoal from the south side of the Xinliuhe Shoal will induce deposition in the main channel, which will threaten the navigation channel and complicate the channel-shoal pattern. Moreover, the erosion along the Nanshatou Channel in 2035-2050 will decrease the flow diversion of the main channel, which is also unfavorable for the stability of the navigation channel. With the continuous erosion at the Ruifeng Shoal and the Changxing Channel, the shape of the transverse section will be further modified. The channel-shoal system may gradually develop from a two-channel type (“W” shape) to a single one (“U” shape). Deposition will occur in the main channel and influence the safety of Gaoqiao Harbor. Nevertheless, the uncertainty of process-based morphodynamic modeling should be considered due to the uncertainty of model parameters and future scenarios. Therefore, systematic research on the accumulative impacts of extremely low sediment discharge and local human interventions is recommended to provide scientific guidance for the sustainable management of the South Channel and the entire Changjiang Estuary under changing environments.

6 Conclusions

The causes of the channel-shoal adjustment in the past 60 years and the future evolutionary trend until 2050 within the South Channel of the Changjiang Estuary are investigated by combining bathymetric data analysis and process-based modeling (Delft3D). Quantitative analysis of the morphological processes indicated that the channel-shoal system showed remarkable changes from 1958-2018. The main conclusions are listed below.
(1) The lower Biandan Shoal, Xinqiao Channel and Liuhe Shoal (Xinliuhe Shoal) showed periodic evolution under natural conditions, with downstream shoal migration, incision and emergence under natural conditions before the mid-1980s. The South Channel experienced net accretion in 1958-1965 and net erosion in 1965-1978 and then rebounded to net accretion in 1978-1986 due to periodic evolution. Abundant sediment supply contributed to the overall accretion of the South Channel during 1958-1986.
(2) Since the mid-1980s, river sediment discharge has started to decrease, and the periodic process has been interrupted by human interventions. Although the Xinliuhe Shoal continued to migrate downstream, the channel-shoal system converted to net erosion, especially at the bifurcation area and the tail of the Ruifeng Shoal. A modeling study indicated that the increased transverse flow due to upstream erosion triggered the shrinkage of the Ruifeng Shoal. The self-adjustment of the transverse section shape contributed to accretion in the main channel and southern marginal shoal in 2002-2007. After 2007, the South Channel underwent rapid erosion as the sediment decreased to a low level.
(3) Unordered sand mining at the Ruifeng shoal tail in approximately 2002 aggregated shoal erosion. The shoal protection works at the head of the Xinliuhe Shoal that were completed in 2009 fixed the bifurcation site and restrained shoal migration, whereas the controlled ebb flow due to the regulation works tended to incise the southern side of the Ruifeng Shoal.
(4) Five stages of morphological evolution of the South Channel are identified, i.e., the accretion stage in 1958-1978, remarkable channel-shoal pattern adjustment stage in 1978-1986, slow accretion stage in 1986-1997, shoal scour and self-adjustment stage in 1997-2007 and overall channel-shoal erosion stage in 2007-2018.
(5) Evolutionary trend prediction implied that overall erosion within the South Channel will continue in 2015-2050, especially under extremely low sediment supply. Several changes may influence the stability of the navigation channel and the safety of Gaoqiao Harbor.

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