Keywords

1 Introduction

Since 2003, with the completion and operation of the Three Gorges Project (TGP) and other eight reservoirs, including Liyuan, Ahai, Jin'anqiao, Longkaikou, Ludila, Guanyinyan, Xiangjiaba and Xiluodu in the upper reaches of the Yangtze River, the average runoff from Yichang to Datong of the lower reaches of the Yangtze River has slightly reduced, while the sediment transport has reduced sharply. The data of hydrological stations in the Yangtze river below Yichang showed that sediment transport decreased by 4/5 – 2/3 on average from upper to lower reaches. Besides, the sediment environment changed significantly, resulting in continuous long-distance erosion in the lower reaches of the Yangtze River (LRYR).

Many scholars have studied the erosion and deposition characteristics, as well as the evolution laws of the downstream river bed caused by clean water discharge after the completion of upstream reservoirs (Dai and Liu, 2013). The analysis of the erosion and deposition laws in the main stream from Yichang to Datong conducted by Xu et al. (2011) showed that the erosion and deposition characteristics of the middle and lower reaches of the Yangtze River had changed from “upper erosion and lower deposition” and “deposition in benchland and erosion in channel” to “ erosion in both benchland and channel “ after the application of TGP. And the erosion was almost along the entire lower reaches. Through the analysis of prototype observation data, Zhu et al. (2018) concluded that the downstream of TGP had entered a state of strong erosion. Li et al. (2018; 2015) analyzed the sedimentation characteristics of the Three Gorges Reservoir area by means of sediment transport rate method and section analysis method respectively. Shi et al. (2020) analyzed the variation of flow and sediment and the characteristics of erosion and deposition in the reach from TGP to Gezhouba Project, and found that the relationship between flow and sediment in the reach changed significantly. The main composition of sediment became coarser after the operation of the projects, and the activity in each sub-reach was spatially distinct.

At present, relevant studies are mainly focused on the Three Gorges Reservoir area and Yichang to Datong reach. There are few studies on the erosion and deposition characteristics of the entire Hukou-Datong -Jiangyin reach. Furthermore, the relevant studies have not gotten the data of the regional flood in 2020 involved. On the basis of flow and sediment characteristics variation data of the hydrological stations in LRYR since TGP construction, we used the past and the latest river terrain data of the Hukou to Jiangyin reach to analyze the erosion and deposition in LRYR. Riverbed erosion and deposition was calculated below 4 different water levels (i.e. low, medium, bankfull and flood water level). Temporal and spatial variations of channel erosion and deposition in LRYR since the completion of TGP were analyzed. It could provide the latest reference data for river regulation, waterway regulation, shoreline protection and exploitation in the new period.

2 Overview of the Reaches

The lower reaches of the Yangtze River, from Hukou to Jiangyin, flows through Hubei, Jiangxi, Anhui and Jiangsu provinces. The length of the main stream is about 639.2 km. The river is alternated in wide and narrow, with many sandbanks along the river, which are generally in the shape of lotus nodes. According to the nodes or narrowed sections or specific tributaries of the estuary, LRYR is divided into fifteen sub-reaches (Table 1).

3 Flow and Sediment Variation in LRYR Before and After the Impoundment of TGP

There are two perennial hydrological stations, i.e., Jiujiang hydrological station and Datong hydrological station, in LRYR. The spatial differences of flow and sediment characteristics were statistically analyzed before and after 2003, when TGP impounded.

After the impoundment of TGP, the average annual flow runoff of Jiujiang hydrological station slightly decreased by 4.6%, from 7.522 × 1012 m3 to 7.177 × 1012 m3, while the sediment runoff significantly decreased by 67.8%, from 2.774 × 108 t to 8.929 × 107 t (Fig. 1a). The average annual flow runoff of Datong hydrological station slightly decreased by 2.5%, from 9.046 × 1012 m3 to 8.824 × 1012 m3, while the sediment runoff significantly decreased by 69.5%, from 4.2686 × 108 t to 1.3225 × 108 t (Fig. 1b). Overall, the decrease of sediment runoff was significantly greater than that of flow runoff at both hydrological stations.

Fig. 1.
figure 1

Annual flow and sediment runoff of Jiujiang and Datong hydrological stations (a. Jiujiang hydrological station; b. Datong hydrological station)

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4 Calculation and Analysis of Erosion and Deposition Changes in LRYR

4.1 Materials and Methods

Long range 1:10000 channel topographic maps of Hukou to Jiangyin in 1998 and every 5 years from 2001 to 2020 were collected. The the erosion and deposition of the river channel were calculated, using cross section method. The average interval of fixed cross sections was about 1.0 km, while that of curved reaches or reaches with large changes was reduced to about 500 m. Data extraction for cross sections was generally carried out according to 1:5000, i.e., the average horizontal point distance was 35 –40 m. The horizontal point distance was encrypted to about 20m in steep slopes and areas with severe local terrain changes.

To understand the distribution of erosion and deposition in the river channel, water levels at the discharge of 10000, 30000, 45000 and 60000 m3/s (i.e., low, medium, bankfull and flood water level) were used to calculate the erosion and deposition. Since LRYR is as long as 639.2 km with a large drop of water head, the calculated water level of each sub-reach was determined by previous mathematical model results (Table 1).

4.2 Analysis of Calculation Results of Erosion and Deposition Characteristics in Hukou-Datong Reach

In 2003, TGP was completed and put into operation. From 2011 to 2016, 8 reservoirs in the upper reaches of the Yangtze River were successively completed and put into operation. The main stream reservoir group was basically completed and initial water storage has been achieved since 2016. According to the operation period of the reservoir group, the 23 years since 1998 were divided into 5 periods: i) before the operation of TGP (1998–2001); ii) shortly before and after operation of TGP (2001–2006); iii) initial period of TGP operation (2006–2011); iv) operation period of main stream reservoir group (2011–2016); v) after the operation of the main stream reservoir group (2016–2020). The variation characteristics of erosion and deposition in each reach were analyzed during each period.

Table 1. Water level values used for erosion and deposition in Hukou to Jiangyin reach
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Table 2. Statistics of erosion/deposition amounts in Hukou-Datong reach
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Table 3. Statistics of erosion/deposition intensities in Hukou-Datong reach
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Table 4. Statistics of proportion of erosion/deposition in flood channel in Hukou-Datong reach
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As shown in Table 2, 3, 4 and 5 and Fig. 2, the accumulative performance of river bed in Hukou-Datong reach was erosion. However, before the impoundment of TGP, the river bed of Hukou-Datong reach was generally deposited except the low water level channel. After the impoundment of TGP in 2003, the river bed of the Hukou-Datong reach turned into erosion and then presented a unidirectional erosion trend. The specific characteristics were as followed.

  1. i.

    Shortly before and after operation of TGP (2001–2006), the deposition mainly occurred in channel while the erosion in benchland. Both the amount and intensity of erosion increased with the elevation of water level, indicating that the erosion was mainly in the benchland during that period.

  2. ii.

    During the initial period of TGP operation (2006–2011), the erosion, which occurred in both channel and benchland, was relatively slight.

  3. iii.

    During 2011 to 2016, erosion gradually increased with the successively operation of the reservoirs upstream. The amounts and intensities of erosion were very close among 4 different water levels, which indicated that the erosion mainly occurred below the low water level.

  4. iv.

    After the operation of the main stream reservoir group (2016–2020), erosion rate reduced, but the average erosion intensity was still higher than the pre-2011 level.

  5. v.

    As shown in Table 4, erosion increased with the increase of calculated water level since 2006. The erosion amount below low, medium and bankfull water level channel accounted for 86.67%, 88.25% and 91.14% of flood water level channel, respectively. The data indicated that erosion occurred both in channel and benchland, and mainly below bankfull water level.

  6. vi.

    In general, during the 14 years from 2006 to 2020, the average annual erosion intensity below bankfull water level increased by 79.94% compared with the period before the impoundment of TGP (1998–2001).

Fig. 2.
figure 2

Erosion/deposition amount and intensity in Hukou-Datong reach (deposition: +; erosion: -) (a. accumulative erosion/deposition amount; b. erosion/deposition intensity)

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4.3 Analysis of Calculation Results of Erosion and Deposition Characteristics in Datong-Jiangyin Reach

As shown in Table 5  and  6 and Fig. 3, the performance of river bed in Datong-Jiangyin reach was unidirectional erosion. Before the impoundment of TGP, the river bed of Datong-Jiangyin reach experienced erosion in general. However, the erosion amount decreased with the elevation of water level, indicating that the benchland experienced slight deposition (Table 3, 1998–2001) (Table 6).

Table 5. Statistics of erosion/deposition amounts in Datong-Jiangyin reach
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Table 6. Statistics of erosion/deposition intensities in Datong-Jiangyin reach
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Table 7. Statistics of proportion of erosion/deposition in flood channel in Datong-Jiangyin reach
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The river bed of the Datong-Jiangyin reach experienced unidirectional erosion, but the characteristics were different from the former period.

  1. i.

    Shortly before and after operation of TGP (2001–2006), the amount and intensity of erosion increased with the elevation of water level. The erosion, which occurred in both channel and benchland, was relatively slight.

  2. ii.

    During 2006 to 2011, erosion gradually increased to about 2.5 times of the former period.

  3. iii.

    With the successively operation of the reservoirs upstream during 2011 to 2016, the erosion rate reduced, but still twice of that during 2001 to 2006.

  4. iv.

    After the operation of the main stream reservoir group (2016–2020), the erosion sped up once again below all the 4 water levels.

  5. v.

    As shown in Table 7, the erosion amount increased with the increase of calculated water level since 2006. The data indicated that erosion occurred both in channel and benchland, and mainly below bankfull water level.

  6. vi.

    In general, during the 14 years from 2006 to 2020, the average annual erosion intensity below bankfull water level increased by 67.35%, compared with the period before TGP impoundment (1998–2001).

Fig. 3.
figure 3

Erosion/deposition amount and intensity in Datong-Jiangyin reach (deposition: +; erosion: -) (a. accumulative erosion/deposition amount; b. erosion/deposition intensity)

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4.4 Temporal and Spatial Variations of Erosion/Deposition Intensity Along LRYR Below Bankfull Water Level

Based on the analysis in the previous section, erosion and deposition mainly occurred below bankfull water level. So, we set the calculation water level to bankfull level and analyzed the temporal and spatial variations of erosion/deposition intensity along LRYR.

The temporal characteristics of erosion/deposition were summarized as below. (Fig. 4).

  1. i.

    Before the operation of TGP, the riverbed of the Hukou-Datong reach experienced deposition, while that of Datong-Jiangyin reach experienced erosion. The extent of erosion/deposition was relatively small.

  2. ii.

    The riverbed in LRYR performed as slight erosion in general during 2001 to 2006.

  3. iii.

    The erosion intensity gradually increased since 2006, and increased further with the successively operation of the reservoirs upstream since 2011.

  4. iv.

    During 2006 to 2020, the erosion in Hukou-Datong reach was about 1.37 times of that in Datong-Jiangyin reach.

Fig. 4.
figure 4

Amount of erosion/deposition per km and average erosion/deposition intensity in LRYR (deposition: +; erosion: -) (a. amount of erosion/deposition per km; b. average erosion/deposition intensity).

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Figure 5  and  6 showed the erosion and deposition of each sub-reach in LRYR below bankfull water level. The characteristics were summarized as blow (Fig. 6).

  1. i.

    Upstream of Nanjing in LRYR, the annual average erosion/deposition intensity increased and decreased alternately. Accumulatively, the sub-reaches performed as erosion, with erosion intensity ranging from 3.74–26.8 × 104 m3/km, except the Ma’anshan reach, Anqing reach and Taiziji reach.

  2. ii.

    Three reaches, i.e., the Ma’anshan reach, Anqing reach and Taiziji reach, experienced deposition accumulatively, with relatively small deposition intensities of 8.40 × 104 m3/km, 4.35 × 104 m3/km and 7.39 × 104 m3/km, respectively.

  3. iii.

    The erosion intensities of Nanjing and its downstream reaches increased along the river, ranging from 7.81 × 104 m3/km to 34.55 × 104 m3/km.

  4. iv.

    The erosion and deposition intensities of many reaches were close to that of the reaches about 100 km upstream, such as Zhenyang reach and Wuyu reach (126.1km upstream), Wuyu reach and Datong reach (93.3 km upstream), Nanjing reach and Heishazhou reach (80.4 km upstream), Guichi reach and Madang reach (115.9 km upstream), etc.

Fig. 5.
figure 5

Amount of erosion/deposition per km of each sub-reach in LRYR (bankfull channel; deposition: +; erosion: −)

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Fig. 6.
figure 6

Average annual erosion/deposition intensity along LRYR during 2006 to 2020 (bankfull channel; deposition: +; erosion: -)

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5 Conclusions

  1. i.

    The relationship between flow and sediment in LRYR has changed significantly since TGP and the reservoir group were put into operation. Annual flow runoff decreased slightly, and the process of runoff showed a reaction of flood peak cutting and valley filling. The sediment runoff decreased sharply, and was significantly planarized in flood season. The sediment runoff of Datong hydrological station has slightly increased during dry season, which was caused by discharge increase and erosion in the upstream reaches during dry season water. However, there was little difference in variation characteristics between Jiujiang hydrological station and Datong hydrological station. Overall, sediment variation far exceeded that of flow.

  2. ii.

    Hukou-Jiangyin reach experienced both erosion and deposition the during 1998 to 2006, and unidirectional erosion in both benchland and channel after 2006. The average annual erosion intensity was significantly increased over time. The erosion in Hukou-Datong reach was about 1.37 times of that in Datong-Jiangyin reach. Erosion and deposition mainly occurred below bankfull channel, and the proportion of erosion between low water level channel and flood channel was only about 10–20%.

  3. iii.

    During 2006 to 2020, the sub-reaches performed as erosion accumulatively except for the Ma’anshan reach, Anqing reach and Taiziji reach. Erosion mainly occurred below bankfull channel. The annual average erosion intensity fluctuated along the river upstream of Nanjing in LRYR, while that of Nanjing and its downstream reaches increased along the river. The erosion and deposition intensities of many reaches were close to that of the reaches about 100 km upstream.