Keywords

1 Introduction

Since the operation of 8 reservoirs (including Three Gorges reservoir, Liyuan reservoir, Ahai reservoir, Jinanqiao reservoir, Longkaikou reservoir, Ludila reservoir, Guanyinyan reservoir, Xiangjiaba reservoir, and Xiluodu reservoir) in 2003, the sediment runoff in the middle and lower reaches of Yangtze River has considerably declined. As a consequence, riverbed scour and deposition of the reaches has long been in the process of adaptive adjustment (Xu et al. 2019). Due to that scour and deposition of river channels downstream the TGR directly affects flood prevention, shipping and socio-economic development on both riverbanks (Yuan et al. 2014), researchers have paid great attention to investigating the characteristics of riverbed scour and deposition in the lower reach of the TGR has broken the original relative scour-deposition balance of river channels in the middle and lower reaches of Yangtze River, shifting the status from pre-impoundment “scour upstream, deposition downstream” and “scour in main channel, deposition in floodplain” to post-impoundment “scour in both main channel and floodplain”, and such all-the-way scour has developed to the Datong reach. The research of Hu et al. (2017) showed that, after the operation of the TGR, river channel scour kept developing downstream and parts of the reach swung frequently, leading to more complex evolution of scour and deposition. The Nanjing reach is in the lower reach of the Yangtze River, within which the bottomlands are quite well-developed and the evolutions of river channel scour and deposition are complex (Ling et al. 2021; Zang et al. 2021). Around 1985, the Nanjing reach shift from depositional to erosional. Then, after channel regulation for many years and under the effects of natural nodes and bank-protection projects, the overall river regime of the Nanjing reach was generally stable. However, parts of the channel still experienced intense scour and deposition variations (Qu et al. 2008; Wang et al. 2007). Associated with the construction and operation of the TGR and other reservoirs along the main stream, the adjustive characteristics of scour and deposition of the Nanjing reach would experience certain new changes, due to the impact of altered sediment content in upstream water. Therefore, there is an urgent need to re-evaluate the recent scour-deposition evolution pattern of Nanjing reach.

At present, most relevant studies mainly focus on the overall scour-deposition characteristics of Yichang-Datong reaches in the middle and lower reaches of the Yangtze River, while little attention has been paid to the spatiotemporal scour-deposition variation characteristics of the Nanjing reach across the 20 years. In the meantime, relative research has seldom included the data after the basin-scale big flood of the Yangtze River in 2020. This study will, based on analysis of the water and sediment characteristics of the Nanjing reach, use successive and the newest channel topographic data to calculate scour and deposition via four water levels (flooding, bankfull, medium, low), thereby analyzing the spatiotemporal scour-deposition variation characteristics of the Nanjing reach after the operation of the TGR. Research findings from this study would provide the latest scientific evidence for the protection and exploitation of the Yangtze River bank line in the new era.

2 Overview of the Nanjing Reach of Yangtze River

The Nanjing reach of the Yangtze River extends from the upstream Cihu stream outlet to the downstream Dadao stream outlet (Fig. 1), with a total length of 95.5 km. Within the Nanjing reach, the bottomlands are quite well developed, with the plane form being alternatively wide and narrow, lotus rhizome knot shaped, and bifurcated. From upstream to downstream, there are Xinjizhou branching channel, Meizizhou branching channel, Baguazhou branching channel, and Longtan bend channel segment (Wang et al. 2020). The mainstream direction exhibits a continuous wave shape, flowing through Xinshengzhou and Xinjizhou right branches, reaching the Qiba node on the left bank via Jiqian watercourse, turning right towards the head of Meizizhou at Dashengguan, entering Xiaguan and Pukou narrow sections via Meizizhou and Qianzhou left branch, turning to Yanziji on the right bank via the right border of the head of Baguazhou, transiting to Tianhekou on the left bank, bending towards the Xinshengyu port area on the right bank, turning to the Xiabatou node at the confluence segment of Baguazhou, finally turning to the Longtan bend on the right bank and entering the Yizheng watercourse via Sanjiangkou node.

3 Water and Sediment Characteristic Variations of the Nanjing Reach

Variations in the conditions of incoming water and sediment are directly linked to the adjustment of scour and deposition. The Nanjing reach is a tidal reach, and the multi-year measured data of Nanjing tidal level station show that the runoff effect of the upper reach of Yangtze River is the main factor affecting the riverbed scour-deposition evolution of Nanjing reach. The Datong hydrometric station is the control station of Yangtze River runoff. According to statistics, the amount of water inflow into the Yangtze River from main stream downstream the Datong hydrologic station accounts for approximately 3% of the total water discharge of the station. Therefore, the amount of incoming water and sediment in the lower reach of Yangtze River is determined using the data of Datong hydrologic station (Table 1).

Before the impoundment of the TGR, the multi-year average runoff of Datong station was 9051 × 108 m3, and the sediment runoff was 4.27 × 108 t. After impoundment of the TGR, the incoming water and sediment from upper reach of Yangtze River declined in certain years. Compared to the pre-impoundment average values, the multi-year average average runoff between 2003–2020 for the Datong station varied slightly, being approximately 3.1% lower. However, since the reduced incoming sediment from upstream of the reservoir and sediment detention by the TGR, the sediment load of downstream of the reservoir declined greatly. Between 2003–2020, the multi-year average sediment load of Datong station was 1.34 × 108 t, falling by 68.6% in comparison to the pre-impoundment value. Meanwhile, the multi-year average sediment concentration decreased to 0.151 kg/m3.

In 2020, another basin-scale big flood took place in the Yangtze River, following previous two in 1954 and 1998. The big flood in 2020 led to historical high water levels in the Nanjing reach of the Yangtze River. In 2020, the maximum water discharge of Datong station was 83800 m3/s, and the multi-year average runoff was 11180 × 108m3, increasing by 23.5% and 27.5% compared to the pre-impoundment and post-impoundment multi-year average runoffs, respectively. Of the of entire year 2020, there were 48 days when the water level of Nanjing Xiaguan station exceeded the warning water level. While of the 48 days, there were 6 days when the water level was higher than the highest water level ever recorded, reaching a maxima of 10.39 m (on 21 July), which was 0.17 m higher than the previous historic high. The multi-year average sediment load of Datong station in 2020 was 1.64 × 108 t, remarkably decreasing by 61.6% compared to the pre-impoundment multi-year average value, but increasing by 22.4% in comparison to the post-impoundment multi-year average sediment load (Ling et al. 2021).

Table 1. Statistical table of runoff and sediment characteristics of Datong hydrologic station.
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4 Computational Analysis of Scour-Deposition Variations of the Nanjing Reach

4.1 Study Area, Data Source and Computing Method

This paper chose the section of the Nanjing reach as the study area, being the segment from Xinjizhou (Xinshengzhou) branch to Longtan-Yizheng watercourse (Fig. 1), with a total length of 95.5 km. To facilitate computational analysis, this study divided the Nanjing reach into 9 sub-segments, namely left branch of the Xinjizhou (Xinshengzhou) reach (L1), right branch of the Xinjizhou (Xinshengzhou) reach (R1), main reach upstream the Meizizhou bifurcated reach (M1), left branch of the Meizizhou reach (L2), right branch of the Meizizhou reach (R2), main reach upstream the Baguazhou bifurcated reach (M2), left branch of the Baguazhou reach (L3), right branch of the Baguazhou reach (L3), and Longtan and Yizheng reach (M3). Based on the segmentation, this study analyzed the scour-deposition conditions of the entire Nanjing reach and the 9 sub-segments.

Fig. 1.
figure 1

Schematic of the study area.

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The data used include 1:10000 fluvial geomorphology and observational data of fixed sections of the Nanjing reach in 1998, 2001−2020 at a 5-years interval, and 2021. This study adopted the commonly used cross-section method to calculate the scour and deposition of the Nanjing reach. The interval between two fixed sections is about 1.0 km, and reduced to 500 m for bending sections and sections with great changes. In total, there are 128 sections. Generally, the river cross-sections are extracted using the ratio of 1:5000, namely that the mean transverse point-to-point distance is 30−40 m, and reduced to 20 m for scarp and local areas with dramatic topographical changes.

To understand the scour and deposition conditions of different parts of the river channel, the current study uses water levels corresponding to water discharge of 10000 m3/s, 30000 m3/s, 45000 m3/s and 60000 m3/s to calculate the sediment scour and deposition amount in low-water channel, medium-water channel, bankfull water channel and flood water channel, respectively. Since the analyzed reach is quite long and the upstream-downstream water level gradient is large, this study refers to previous numerical simulation results to determine the calculated water levels. Table 2 lists the calculated water levels for different segments.

Table 2. Calculated water levels of control sections for computing scour and deposition of Nanjing reach.
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4.2 Variations of Channel Scour and Deposition

To facilitate description, this study, according to the operations of the TGR and the reservoir group at main stream of upstream Yangtze River, divides the period into 6 stages: pre-impoundment (1998.10–2001.10), impoundment of TGR (2001.10–2006.10), early operation of TGR (2006.10–2011.10), successive operation of main stream reservoirs (2011.10–2016.10), utilization of the main stream reservoir group (2016.10–2020.11), and the period after the basin-scale big flood (2020.11–2021.03). The spatiotemporal variation characteristics of channel scour and deposition for each stage are described below.

After impoundment, the TGR had intercepted over 70% of the inbound sediment, thereby greatly reducing the amount of outbound sediment and leading to prominent scour-deposition adjustment of main stream in middle and lower reaches of Yangtze River. In particular, since 2006, the Nanjing reach has experienced rather intense scour. During 2006–2021, the multi-year average scour amount of bankfull channel for the entire reach reached 874.3 × 104 m3/a, being approximately 16 times of the multi-year average value for 2001–2006. Scour has become dominant for the whole Nanjing reach, especially for the Longtan and Yizheng reach. Figure 2 and Table 3 compare the amount of scour or deposition of Nanjing reach in different periods.

Prior to the impoundment of the TGR (1998.10–2001.10), owing to the channel-forming effect of the Yangtze River flood in 1998, the Nanjing reach experienced intense scour. For the whole reach, the total sediment scour amount corresponding to low-water channel, bankfull channel and flood channel was 6405 × 104 m3, 6346 × 104 m3, and 7384 × 104 m3, with the multi-year average scour intensity being 2.36 × 104 m3/(km\(\cdot\)a), 122.15 × 104 m3/(km\(\cdot\)a) and 25.77 × 104 m3/(km\(\cdot\)a), respectively. The overall scour-deposition status was “scour in both main channel and floodplain” and dominated by low-flow channel scour.

Since impoundment of the TGR (2001.10–2021.03), the total bankfull channel scour of the entire Nanjing reach was 13368 × 104 m3, corresponding to a multi-year average scour amount of 668.4 × 104 m3/a and a multi-year average scour intensity of 7.00 × 104 m3/(km\(\cdot\)a). Along the Nanjing reach, scour amount was higher at the main reach upstream the Meizizhou bifurcated reach, the left branch of the Meizizhou reach, and Longtan-Yizheng reach, with the total scour amount of the three reaches accounting for 80.3% of the total scour amount of the whole Nanjing reach. Of this period:

During the cofferdam impoundment period (2001.10–2006.10), the scour of Nanjing reach slowed down, with the total bankfull channel scour amount being merely 254 ×104 m3, corresponding to the scour intensity of 0.53 × 104 m3/(km\(\cdot\)a). According to the distribution along the Nanjing reach, the scour is the most intense at the right branch of the Xinjizhou reach.

Since 2006, the riverbed scour of Nanjing reach has prominently increased (Figs. 2c–2f, Table 3). During 2006–2021, the total bankfull channel scour amount of Nanjing reach reached 13114 × 104 m3, and the multi- year average scour intensity was 9.15 × 104 m3/(km\(\cdot\)a), being approximately 16 times of the multi-year average scour intensity between 2001–2006. From the early operation of the TGR to the successive operation of reservoirs at the main stream of Yangtze River (2006.10–2016.10), the total bankfull channel scour amount was 12150 × 104 m3, while the multi-year average scour intensity was 12.72 × 104 m3/(km\(\cdot\)a), being much greater than that of the impoundment period. Meanwhile, the scour amount was the highest at Longtan and Yizheng reach, while the scour intensity was the strongest at the Main reach upstream the Meizizhou bifurcated reach. After the utilization of the main stream reservoir group (2016.10–2020.11), almost the riverbed along the entire Nanjing reach has experienced deposition, with the total bankfull channel deposition about being 1713 × 104 m3 and the multi-year average deposition intensity being 4.48 × 104 m3/(km\(\cdot\)a). Also, the deposition amount was the highest at the left branch of the Xinjizhou reach, accounting for 57.9% of the deposition of the whole Nanjing reach. After the basin-scale big flood in 2020 (2020.11–2021.03), the Nanjing channel section again experienced intense scour, the total bankfull channel scour amount reached 2677 × 104 m3, the multi-year average scour intensity was 28.03 × 104 m3/(km\(\cdot\)a), and the scour was the strongest at the right branch of the Xinjizhou reach. It is clear from these figures that the operation of the TGR has gradually intensified the riverbed scour of the Nanjing reach. Such continuous scour would threaten the stability of wading projects on both sides of the riverbank and the security of flood prevention. Therefore, it is necessary to enhance the monitoring.

Fig. 2.
figure 2

Scour and deposition conditions of Nanjing reach during 1998–2021.

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According to the spatial distribution of scour of Nanjing reach (Fig. 3, Table 3), it is clear that during 2006–2021, when the riverbed scour was quite prominent, the river cross-sections demonstrated an overall trend of “scour in both main channel and floodplain”, with bankfull channel scour being dominant, accounting for 85% of flood channel scour amount. Meanwhile, scour was more intense on the left river bank, and the corresponding mean value of the multi-year average scour intensity of each river reach was 8 × 104 m3/(km\(\cdot\)a), being about 1.14 times of that of the right river bank. Along the Nanjing reach, under the effect of perennial deposition of upstream Maanshan reach, the right branch (main branch) of Xinjizhou reach showed an overall trend of deposition. However, the left branch of Xinjizhou reach and the downstream reaches demonstrated an scour pattern, with multi-year average scour being larger in the upper reach and smaller in the lower reach. Also, the multi-year average scour was the greatest at the Longtan-Yizheng reach (313 × 104 m3/a), and the multi-year average scour intensity was the strongest at Main reach upstream the Meizizhou bifurcated reach (16.13 × 104 m3/(km\(\cdot\)a)).

Table 3. Comparison of bankfull channel scour amount of different parts of the Nanjing reach during 1998–2021.
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Fig. 3.
figure 3

Comparison of bankfull channel scour along the Nanjing reach after the operation of the TGR (2006.10–2021.03).

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5 Conclusions

  1. (1)

    Since the impoundment of the TGR, the discharge of clean water has gradually intensified the scour of the Nanjing reach. During the cofferdam impoundment period of the TGR (2001–2006), the scour was relatively weak, with the total bankfull channel scour amount of the entire Nanjing reach being 254 × 104 m3, and the multi-year average scour intensity being 50.8 × 104 m3/a. Since 2006, the scour apparently became more intense. In particular, between 2006.10–2020.11, the total bankfull channel scour amount reached 10437 × 104 m3, and the multi-year average scour amount was 745.5 × 104 m3/a, which was approximately 14 times of the multi-year average scour amount for 2001–2006. After the basin-scale flood in 2020, during the period of 2020.11–2021.03, the scour amount of the Nanjing reach further increased to 2677 × 104 m3. The continuous scour may potentially threat the stability of wading projects on both river banks of Nanjing reach and the security of flood prevention, so monitoring should be strengthened in the future.

  2. (2)

    During 2006–2021, the Nanjing reach demonstrated an overall trend of “scour in both main channel and floodplain”, and bankfull channel scour was dominant, with the scour amount being 85% of that of the flood channel scour amount. Along the Nanjing reach, the scour on the left bank was relatively intense, with the mean value of the multi-year average scour intensity of each river reach being about 1.14 times of that on the right bank. Moreover, the left branch of Xinjizhou (Xinshengzhou) reach and its downstream reaches demonstrated an scour pattern, with multi-year average scour being larger in the upper reach and smaller in the lower reach. Also, the multi-year average scour was the greatest at the Longtan and Yizheng reach, while the multi-year average erosion intensity was the strongest at the main reach upstream the Meizizhou bifurcated reach.