Keywords

1 Introduction

The Changjiang River Estuary is the largest estuary in China which locates at north of Shanghai, its evolution has attracted much more attentions in past decades. Meanwhile, water load of the Changjiang River Basin into estuary is also the first one in China, and relevant sediment load is the second one that just only being less than Yellow River, in result, its water and sediment load always play an important role in evolution of the estuary. For many years, due to complexity and importance, evolution of the Changjiang Estuary is still the hot topic of researches till now.

In general, the Changjiang Estuary is divide into three branches. The first branch is the South Branch and North Branch, the second branch is the South Channel and North Channel, and the third branch is the South Passage and North Passage (Fig. 1). So we can see that the South Channel locates at the middle position of the estuary. It directly connects the upper South Branch and the lower South and North Passage. Hence, its evolution will directly impact the upper and lower reaches, especially the lower South and North Passage.

For many years, because of importance of the Changjiang Estuary, lots of researches have focused on its evolution characteristics on hydrodynamics (Wang et al. 2010; Shi and Lu 2011), suspended sediment transport (Shi et al. 1997), bed load movement (Wu et al. 2009), sand bodies variation (Yang et al. 2003), geomorphology (Zhao et al. 2018), engineering influence (Jiang et al. 2012; Chen et al. 2001) and so on (Qiu and Zhu 2013).

However, owing to afforestation after 1980s, the fluvial water and sediment load into the estuary had appeared variation after then. And till 2000s, owing to continuous construction of Three gorges Dam, South to North Water Diversion and many other conservancy projects in the Changjiang River Basin, the fluvial water and sediment load into the estuary have appeared more significant variation again in the latest 20 years. In result, the Changjiang Estuary also has inevitably emergent new evolution tendency (Chu et al. 2013), correspondingly, as an integral part of the estuary, the South Channel’ geomorphology also appears new evolution characteristics.

In this background, based on historical underwater topography data in each 10 years during past half century, this paper tries to find out former decades and recent geomorphology evolution tendency and characteristics of the South Channel.

Fig. 1.
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Location of the South Channel in the Changjiang Estuary

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2 Data and Method

In order to discover evolution characteristics of the South Channel in the Changjiang Estuary during the latest half century, underwater topography data in different period during the past 50 years were collected and edited. Their detail periods were respectively in 1973, 1981, 1994, 2007, 2016, and their datum were unified to the Theoretical Lowest Low Water Datum (Table 1).

Then, by the GIS software, their erosion and sedimentation between adjacent years were calculated through subtracting underwater topography of former year from that of later year.

Meanwhile, in order to further reveal regime evolution of the South Channel, thalwegs and cross sections in each period were also extracted and analyzed. Wherein, each thalweg represented the deepest location along the main channel in the South Channel during each period. And cross sections were respectively set at inlet, middle and outlet of the South Channel, and their locations were shown in Fig. 2a as pink line across the channel.

Table 1. Topography data information
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Fig. 2.
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Location of cross sections in the South Channel

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3 Results and Discussion

3.1 Erosion and Sedimentation Characteristics

In order to reveal the evolution tendency of the South Channel, Fig. 3 gave out erosion and sedimentation characteristics of the South Channel during the past 50 years.

As the Fig. 3 shown, there were three stages during 1973~2016 for evolution of the South Channel. In the first stage among 1973~1981, the channel presented obvious sedimentation with value about 1.0~9.7 m, especially in the central region on longitudinal direction along the reach, but manifested erosion with value about −1.0~−13.6 m along both side channels. Wherein, the erosion pattern ran through the whole main deep channel closed to south bank, but it was broken at inlet in the other channel closed to north bank.

In the second stage during 1981~1994, the erosion and sedimentation region turned around, wherein obvious erosion with value about −1.0~−9.8 m mainly appeared at the central region along the reach, and the maximum erosion mainly located at inlet and lower reach. Meanwhile, along both side channels, it presented notable sedimentation with value about 1.0~10.6 m, especially in the upper reach closed to south bank. But it should be concerned that in this stage the south and north bank all present local scouring along banks with maximum value nearly −15 m, which would threat the embankments on both banks. This phenomenon might be caused by building of ports along both banks, and decreasing of sediment load from upper basin due to afforestation in the Changjiang Basin after 1980s (Wang et al. 2008b).

In the third stage, till initial 2000s, the erosion and sedimentation regions varied again. The central region along the reach presented partial sedimentation with value about 1.0~8.1 m, and both side channels presented partial erosion with value about −1~−11.5 m. In this stage, the sedimentation region was nearly equal to that of erosion, but in the upper and middle reach, the sedimentation played a dominant role. This may be caused by several big floods in 1990s, which transported large amount of sediment into lower reach and estuary and then deposited in the estuary gradually (Yu et al. 2009). After 2010s, following continuous impacts of constructions of the Three Gorges Dam and others water conservancy projects, fluvial sediment load into estuary continuously decreased. In result, the erosion and sedimentation pattern gradually varied in a certain extent but without qualitative change. The erosion in the central region along the reach increased to −1.0~−6.1 m, and the sedimentation decreased to 1.0~4.1 m. Meanwhile, the erosion region along both side channels increased distinctly, wherein the erosion region ran through the whole reach with value about −1~−13.2 m, especially in the lower reach where the erosion took the dominant function.

Fig. 3.
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Erosion and sedimentation between adjacent years

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3.2 Cross Sections Evolution

Evolution of cross sections in different period were shown in the Fig. 4.

As Fig. 4 shown, it could be found that every cross section had its own variation characteristics and they also undergone three stages for river regime. In the first stage during 1973~1981, the central shoal mainly presented sedimentation but erosion in both side channels. Then in the second stage during 1981~1994, the central shoal turned to distinct erosion especially in the upper reach but obvious sedimentation in both side channels. And in the third stage after 1990s, the central shoal still remained erosion trend but both side channels also changed into overall erosion.

In addition, it should be concerned that owing to implementation of afforestation and large scale conservancy projects, fluvial water and sediment load had appeared obvious variation during 1973~2016 especially the sediment load decreasing obviously after 1980s, hence the evolution of central shoal and both side channels appeared their respective characteristics in past 50 years.

For the cross section at inlet of the South Channel, the whole profile pattern was basically stable in past half century, but there were still many detailed changes. Firstly, in past 5 decades, its location of main deep channel moved about 275 m southward, but its elevation just only changed −1.1 m. Secondly, its central shoal had obvious erosion with average value about −4.6 m in past 50 years, which may be caused by continuous decreasing of sediment load from river basin due to afforestation and construction of large scale water conservancy projects.

And for the middle cross section, during 1973~2016, the initial pattern with obvious compound channel changed into inconspicuous compound channel pattern, where the central shoal became not very significant than that before. Meanwhile, the main southern channel moved about 450 m northward with elevation decreasing 0.4 m. Accordingly, the central shoal presented firstly sedimentation and then erosion tendency and lastly scoured about −1.3~−5.7 m in the past decades.

Lastly, for the outlet cross section, being similar with middle cross section, its initial typical compound channel pattern in 1973 also changed into inconspicuous compound channel pattern in 2016. And the deepest location of the main southern channel moved about 1775 m northward with elevation deepening about 6.1 m. Meanwhile, the central shoal also presented firstly sedimentation and then erosion trend, in past 50 years, its elevation decreased about −5.8 m on average. Consequently, the central shoal also presented not significant at present.

Fig. 4.
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Variation of cross sections in different years

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3.3 Thalweg Evolution of the Main Channel

In order to reveal main southern channel evolution in the South Channel, Fig. 5 presented variation of thalwegs in different period.

As Fig. 5 shown, from 1973 to 2016, average elevation of thalwegs along the whole channel presented an overall fluctuating but slight decreasing trend. During 1970s to 1980s, average elevation of thalwegs decreased with value from −14.0 m to −16.6 m. Then till 1990s, owing to erosion of longitudinal shoals in the center region of the South Channel and returning deposition into channel, average elevation of thalgweg consequently increased to −13.4 m. And then following the construction of many large scale water conservancy projects such as the Three Gorges Dam, South to North Water Diversion and so on after 2000s, sediment load into estuary decreased obviously year by year but annual water load still remained stable. In result, average elevation of thalwegs started turning to decreasing trend in the whole till nowadays. Its value had changed to −15.9 m till 2016. But, integrating location of thalwegs in different period shown in Fig. 4, it could be found that elevation change of main channel in different period were always due to channel swing but not directly erosion.

In addition, comparing variation of central shoal and main channel, it could be found that variation of average elevation of thalwegs were all below 0.3 m/a during 1973~2016, erosion and sedimentation range of main channel was distinctly smaller than that of central shoal.

Fig. 5.
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Evolution of thalwegs in different period

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4 Conclusion and Discussion

In sum, the evolution of the South Channel has experienced three stages in past 50 years, which mainly presented at exchange of erosion and sedimentation regions. Wherein, the variation of central shoal along the South Channel was the most distinct, and the main deep channel also presented variation in location but average elevation of its thalweg changed in smaller range than that of central shoal.

Meanwhile, according to fluvial water and sediment load variation characteristics in past decades, it could be preliminary found that there may be close relationship between the geomorphology evolution of the South Channel and the incoming water and sediment load from upper basin. From 1973~2016, the fluvial water load basically remained stable, but the fluvial sediment load presented obviously decreasing tendency after 1980s and then after 2000s its decreasing trend became more obvious. Correspondingly, the central shoal along the reach appeared erosion in general after 1980s, its elevation decreased obviously, especially in the middle and lower reach. Moreover, the main deep channel maintained a relatively stable state, though its location existed swing. Hence, to some extent, it could be preliminarily inferred that evolution of the central shoal along the South Channel might be mainly influenced by fluvial sediment load which had shown a decreasing tendency from 1980s till now, but evolution of the main deep channel might be mainly influenced by fluvial water load which always presented basically stable from 1970s till nowadays.