Keywords

1 Introduction

The Three Gorges Project (TGP) has brought comprehensive benefits since its impoundment in 2003. Nevertheless, the water impoundment also changed the flow-sediment regime of downstream rivers, leading to general scour, and variations in transects, longitudinal slope, riverbed composition as well as river regime.

The Tai**kou waterway, located at the **gjiang reaches of the Yangtze River, is the first sandy-shoal waterway in the downstream of TGP project. Variations in erosion and deposition there are uncertain, and thus, the navigation conditions are changing all the time, involving a variety of research. Liu Ka et al. (2018) established a two-dimensional simulation, and illustrated that northern distributaries of Yanglinji would be silted and thus, the navigation condition would become severe; Jia Dongdong et al. (2017) built a three-dimensional dynamic model to verify that slide collapse of riverbanks could lead to scouring; Cao **kou waterway after the impoundment of 175m in the Three Gorges and analysis of waterway maintenance and safeguard measures. China Water Transp. Channel Technol (02)" href="#ref-CR1" id="ref-link-section-d34837061e821">2017) proposed that TGP was difficult to maintain because the bridge anti-collision setting cannot meet the year-round navigation requirements; Kong **kou Waterway">

2 Overview of the Tai**kou Waterway

The Tai**kou waterway is located in the **gjiang reach, starting from Chenjiawan and down to Yuhe**, about 475 km to 492 km from Hankou waterway. The north bank of the Tai**kou waterway lies in **gzhou city, which belongs to straight, meandering, and anabranching channels controlled by ecological revetment. The Tai**kou waterway consists of two bifurcation reaches, with Yanglinji point bar as the boundary. Its upstream part is divided into north and south channels by the Tai**kou mid-channel bar, and the downstream part is divided into north and south branches by the Sanba mid-channel bar, where the **gzhou Yangtze River Bridge is located.

Both north branch and south branch of the downstream part are used as navigable channel, the north one being the main branch. In recent years, however, the main flow during low water shifted from the north branch to the south branch. The south branch has developed into the main channel, which moved left to the middle and formed a new navigable channel. The original channel gradually lost their functions due to siltation. The dimension of the Tai**kou channnel is 3.5 m × 150 m × 1000 m (Figs. 1 and 2).

Fig. 1.
figure 1

River regime of the Tai**kou waterway.

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Fig. 2.
figure 2

Schematic diagram of pier setting of the **gzhou Yangtze River Bridge.

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After the impoundment of the TGP project, the Sanba mid-channel bar shrinks rapidly, facing the threat of disintegration again. In this regard, several river training projects were undertaken to ensure the good navigation, including the urgent protection measures of the Sanba mid-channel bar and the reinforcement of the urgent protection projects (or the first-stage training project of the Shashi waterway), and the protection project for the middle-north and upper-north parts of the Lalingzhou point bar.

3 Varation Flow-Sediment Regime

After TGP impoundment, the flow-sediment regime downstream has changed significantly, and the Tai**kou waterway has been greatly affected. In this section, we took Yichang station and Shashi station as study cases to illustrate the changes of flow-sediment regime in the Tai**kou waterway.

3.1 Flow Variations

The interannual flow is changeable due to the randomness of the hydrological process, but there is no change in trend, and the commissioning of the upstream terrace reservoir does not have a continuous effect on the downstream annual discharge. As to yearly distribution, as the TGP project advances, the discharge decreases gradually, especially during dry period (i.e., January to April), with discharge increases slowly, and recession period (i.e., September and October), with discharge decreases significantly (Fig. 3).

Fig. 3.
figure 3

Annual average discharge changes in major hydrological stations.

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3.2 Variations in Sediment Load

Under TGP project, sediment load has been reduced significantly, and it reduced more after the impoundment. Influenced by upstream cascade reservoirs, sediment load from downstream further decreases. Even though annual discharge is high, sediment load from the downstream remains unchanged. (Fig. 4).

Fig. 4.
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Variations in sediment load of major hydrological stations.

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4 Riverbed Evolution and Analysis

4.1 Before TGP Impoundment

Before the 1980s, the Tai**kou waterway was close to the left bank due to the large areas of Lalinzhou point bar and Sanba mid-channel bar. Its upstream had only one channel, while the downstream was divided into two branches. However, after the 1980s, the operation of the Gezhouba Dam changed the form of the Tai**kou waterway. Tai**kou mid-channel bar gradually silted up, while Lalinzhou point bar and Sanba mid-channel bar gradually shrank, with two branches in both its upstream and downstream. The downstream was often in a scattered multi-channel form due to 1998 China floods.

4.2 After TGP Impoundment

The Tai**kou waterway shows a planar form of compound channels with artificial nodes. At present, the waterway is suffering cumulative, intense scouring, but the river regime is stable.

After TGP impoundment, the north channel in the upper part rarely changes, while the south changed deeply, with increasing divided water ratio, and thus, has become the main channel. For the upper part, changes are obvious and intense: the scour and deposition ranges in the south and north branches are relatively large, but the plane position of the north branch is relatively stable. In recent years, the south branch has moved northward and widened year by year, becoming the main branch. In general, the Tai**kou main channel maintains‘a south-south (upstream-downstream) direction. The variations in divided water ratio of the north-south branches as well as the north-south channels of the Tai**kou waterway are shown in Fig. 5.

Fig. 5.
figure 5

Variations in divided water ratio.

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4.3 Analysis of the Adjustment

The reduction of sediment load intensifies the scouring of the convex bank during flood seasons. After the impoundment of TGP project, the mid-year discharge has long retention and short recession period, which greatly limits the siltation of the convex bank. The Yanglinji point bar does not match the sediment transport capacity of the north channel. Thus, the hydrodynamic conditions for generating the Yanglinji point bar still exist in the inlet of the north branch.

Under the action of unsaturated water flow, the development of the branching channel in straight channels depends on the good headwater conditions, and the branches with good headwater conditions tends to develop. The development of the two branches is repeated. Under the action of unsaturated water flow, when slope of the south branch is greater, the south branch would develop with strong discharge, which can be transformed into hydrodynamic conditions.

5 Navigation Obstruction and Dredging Projects

5.1 Characteristics of Navigation-Obstruction

The current characteristics of Navigation-Obstruction in the Tai**kou waterway are closely related to the construction of the **gzhou Yangtze River Bridge. The 1998 China Floods and the operation of the TGP project has greatly influenced Sanba channel bar. It was rapidly washed away and shrank, making it difficult to restore. Accordingly, the main flow in the north branch is very important for good navigational conditions because bridge piers are very closely distributed in the south branch, thus limiting navigation. However, after 2012, the main flow during low water shifted from the north branch to the south branch, bringing great challenges to navigation. It is obvious that the north branch cannot be used as main waterway anymore.

The cause of the current obstruction in the Tai**kou waterway is that the bridge abutment setting of **gzhou Yangtze River Bridge cannot adapt to the natural evolution of rivers. The designed navigational channel in the south branch have been severely deposited, while the non-designed navigation bridge channel limit the utilization of better natural water depth conditions.

5.2 Recent Maintenance

During the initial period of the TGP project (2003–2006), the Tai**kou waterway was frequently changing, with unstable channels and adverse navigation conditions. Dredging and maintenance work were urgently needed during the dry season every year.

Since 2016, the dredging and maintenance of north branch of downstream has been continued. The dredging area is mainly concentrated in the transition area from the branch section of Tai**kou mid-channel bar to the branch section of Sanbatan mid-channel bar, and there are two transitional forms: Tai**kou South Branch-Sanbatan North Branch (2# channel) and Tai**kou North Branch-Sanbatan North Branch (1# channel), which can be divided into three stages: (1) North-South transition, centralized maintenance of 2# channel; (2) 1# channel and 2# channel are constructed alternately, navigable alternately, and the two channels are equally important; (3) North-north transition, drainage and stabilization of channels, and efforts to dredge 1# channels.

5.3 Variations in Navigation Conditions

In 2018, due to the more unfavorable water conditions, the Yanglinji point bar shifted down significantly and the Sanba bar mid-channel bar were connected into one, making the navigational conditions in the north branch very bad after the floods, and channel 2# was fully blocked. In 2018–2019, the maintenance method of alternate construction and alternate navigation of channel 1# and channel 2# was adopted to achieve year-round ship passage in the northern branch, and the survey map in March 2019 showed that the 4 m line of channel 1# was close to penetration and the width of the upper section 3 m line reached more than 180 m; the 5 m line of channel 2# was penetrated and the narrowest width was about 180 m. In July 2019, the 3.5 m line under the most depleted water level of channel 1# was penetrated, and the width could reach about 200 m. Channel 2# was slowly silted up after dredging, and the width of the 3.5 m line under the most depleted water level in the shallow area was less than 150 m. Before the flood in 2019, the channel maintenance department will change the main branch to channel 1#, mainly to maintain channel 1#, channel 2# gradually silt waste. By March 2020, after many years of maintenance and dredging, the diversion ratio of channel 1# increased to more than 20%, and the wandering beach body - Yanglinji point bar was gradually moved out. Even if the impact of the unfavorable water period in 2020 is encountered, the impact on the channel conditions of the Tai**kou waterway is not significant due to the low upstream sand source, and the amount of dredging and maintenance in 2020 is comparable to that of the previous year.

In July 2021, the width of the 3.5 m line under the driest water level of the north branch reaches more than 200 m, but from the comparison of August 12–28, 2020 and September 16–17, 2020 flushing and siltation, channel 1# to the north branch is dominated by siltation, with about 1.08 million square meters of siltation and about 300,000 square meters of flushing, with a net siltation of 780,000 square meters. Considering that the flooding period from July to September is the period of the largest siltation in the northern branch of Tai**kou, according to the large scale measurement map of the shallow area in late August 2021, the shallow area of the northern branch has increased significantly compared with July, and the change trend of the development of the southern branch and the shrinking of the northern branch has not been reversed, and the northern branch is still in the process of slow siltation.

5.4 Analysis of the Effect of Dredging Project

  1. i.

    In the context of relatively limited engineering management measures, maintenance dredging is an effective measure to ensure the smooth flow of waterways. The external conditions of the Tai**kou waterway are complex, with various constraints as well as difficult implementation. In this context, dredging has become a necessary tool to ensure the smooth flow of waterways.

  2. ii.

    Adjusting the river flushing and siltation area by “digging and throwing” is one of the effective ways to cope with the adverse effects of TGP storage on the middle and lower reaches of the waterway. By dredging the navigable channel and throwing the dredged soil to the beach body, non-navigable branches and other areas that need to be controlled, it enhances the undercutting of the target channel on the one hand, and helps to alleviate the unfavorable phenomena such as beach scouring and branch development on the other hand, which is one of the effective ways to cope with the adverse effects of TGP clear water discharge on the navigation channel.

  3. iii.

    The trenching design idea of the diversion plastic tank has a certain contribution to the formation of a stable navigation path. From the last five years, under the initial control of the remediation project, after continuous dredging, the northern branch of the Tai**kou waterway diversion overall showed a small increase in the trend, the channel conditions have all shown some improvement, dredging maintenance volume also generally decreased slightly, the difficulty of channel maintenance has also been reduced.

6 Forecast of the Evolutionary Trend

Under the combined effect of the guarding project, shore protection project and dredging project, the river potential of the Tai**kou waterway is basically stable, but with the change of water conditions, bars and branches are still subject to change. Combined with the historical flow-sediment regime and the evolution of the riverbed, the future evolution trend of the Tai**kou waterway is predicted as follows.

  1. i.

    The Tai**kou mid-channel bar: Since 2018, with the increase in the fight against illegal sand mining, the current river form has the trend of repairing the impact of sand mining, mainly in the form of local sand mining deep pits gradually silt flat; in the past two years only the remaining beach body still maintain an obvious scouring trend.

  2. ii.

    Transition section and Sanba mid-channel bar: In recent years, the evolution of the transition section and Sanba mid-channel bar has actually been strongly influenced by dredging activities. The dredging project for the north branch is essentially a process of gradually removing the Yanglinji point bar, which not only stabilizes the diversion of the northern branch to a certain extent, but also retards the further development of the south branch. Combined with the dredging in recent years, continuous dredging can avoid the complete decline of the northern branch; considering the weakening of the upstream sand recharge conditions, the possibility of generating large-scale Yanglinji point bar is small, and the north branch has the conditions for maintenance.

  3. iii.

    From the changes in the river during the maintenance and dredging in the past two years, the maintenance and dredging has ensured the overflow capacity of the north branch and maintained the current channel scale, but the trend of siltation in the north branch and development in the southern branch is still difficult to reverse, and the dredging only eases the degree of siltation in the north branch.

  4. iv.

    From the variations in the yearly divided water ratio of the Tai**kou waterway, it can be seen that before 2014, the ratio of the north branch was mostly above 30%, and the channel conditions were better; since 2014, the ratio of the north branch decreased sharply and has been maintained at less than 30%. Therefore, it can be inferred that when the divided water ratio of the north branch is greater than 30%, the channel conditions will improve significantly.

7 Conclusions

This paper presents a comprehensive analysis of the Tai**kou waterway based on the new water and sand conditions after impoundment of TGP, comparison of riverbed evolution before and after impoundment, and changes in channel conditions, with the following main conclusions.

  1. i.

    After TGP is impounded, the average annual flow of the Tai**kou waterway remains basically unchanged, but the distribution of flow changes within the year, limiting the re-siltation of the beach during the receding period. At the same time, the sand content of the Tai**kou waterway has been significantly reduced, resulting in the general scouring of the channel, which currently maintains the mainstream orientation of the Tai**kou south branch-Sanba bar south branch.

  2. ii.

    The obstruction factor of the Tai**kou waterway is that the navigable bridge piers of **gzhou Yangtze River Bridge cannot adapt to the evolution trend of rivers. Since the impoundment of the TGP project, a series of dredging projects have been carried out and the channel conditions have stabilized. In the current situation, dredging is still a feasible measure to keep the waterway open. Although the north branch, where the main navigable bridge piers is located, continues to shrink, it still has the conditions for maintenance.

  3. iii.

    The river can basically remain stable in the future period. Under the strong influence of the dredging project, the divided water ratio of north branch was stabilized and the channel conditions were improved, but the trend of siltation in the north branch remained irreversible. In addition, maintaining the divided water ratio of the north branch at 30% or more is conducive to the improvement of waterway conditions.