Keywords

1 Introduction

The river bend section is a common plane form of mountainous rivers. The navigable safety problems of the junctions located on the narrow and the curved river sections are relatively serious. The main difficulty in junction layout lies in the complex navigable flow conditions of entrance area, especially river regime with large areas of flood plain. River regulation is an important technical means to ensure safe navigation (Guo et al. 2015).

The navigable junction was generally arranged on the straight transition section or curve in the middle of the two curves for curved river. The entrance area of upstream and downstream approach channel was located on the convex or concave bank of the river thanks to the limitation of terrain, which caused field concern to the layout of the navigable buildings. The comparison of flow conditions between concave bank and convex bank demonstrated that it was advisable to adopt a dispersed arrangement on the side of the convex bank when the junction was located in the downstream bend segment for the narrow continuous bend river segment with sharp bends in the upstream and reverse bends in the downstream (Pu et al. 2012). For mountainous rivers, some studies noted that navigable buildings should be arranged on concave banks (Cao and Zhou 2012). On basis of the comparison of the ship locks arrangement on the left or right banks of Naji Junction, it was more appropriate to be located on the mainstream side of the concave bank (Zheng and Chen 2005). Furthermore, improvement measures of navigable flow conditions should be taken according to the type of river bending (Yu et al. 2014). Han et al. (2014) proposed the junction position should be adjusted if the entrance area of approach channel was set on the convex bank on basis of Yongning Project. Li et al. (2016) carried out the optimization experiment of flow conditions in the upstream and downstream approach channels for ship lock arranged on the concave bank, and proposed the optimization measures of the boundary excavation and the setting of the permeable guidance wall.

In fact, the method of sluice operation had a significant impact on the navigable flow conditions (Wu et al. 2016). For the junctions which the ship lock was built on the convex bank, opening of the remote sluice hole could effectively reduce the backflow and oblique flow in the entrance area of the approach channel (Zhang et al. 2021). The simulation of navigable flow conditions adopted physical model test and numerical simulation (Ahmed et al. 2010; Zhang et al. 2018). On basis of the overall hydraulic model experiment of Zhaoxian Project, this paper analyzed flow regime of entrance area and connecting section on the concave bank of curved river section downstream, and proposes the engineering improvement measures.

2 Project Summary

Zhaoxian Junction, the fourth cascade project of the Changshan River cascade development in the upper reaches of the Qiantang River, was located in Changshan Country, Quzhou. The engineering layout from left to right was: ship lock, sluice and power station, as shown in Fig. 1. The left section of sluice set 13 holes with the baseboard elevation of 66.00 m, and left section of sluice set 8 holes with the baseboard elevation of 68.00 m. The net width of sluice was 252 m.

The lock chamber of Zhaoxian Junction was located on the left bank. The length of ship lock was 256 m, of which the head of the upper lock was 36 m, the lock chamber was 190 m, and the head of the lower lock was 30 m. The upstream approach channel was open without separation levee, and the layout was “straight in and curved out”. The length of straight section of upstream approach channel was 330 m, of which the navigation adjustment section was 140 m, the breasting section was 190 m, and the bottom width of the upstream approach channel was 60 m. The downstream approach channel adopted the layout of “straight in and curved out”. The total length of the approach channel was 330 m containing 140 m navigation adjustment section and 190 m breasting section, and the bottom width of the downstream approach channel was 60 m.

The characteristics of Zhaoxian Junction embodied: 1) the sluice and power station protruded to the right bank, and flood channel was relatively narrow. 2) There was a large area of flood plain on the right bank with the 66.30–73.80 m elevation, which was not conducive to discharge flood and should be partially dredged (see Fig. 2). 3) There are bends in vicinity of downstream entrance area and the Wuli High-speed Bridge, therefore, flow conditions were more complicated.

Fig. 1.
figure 1

Layout of Zhaoxian Junction

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Fig. 2.
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Flood plain of Zhaoxian Junction

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3 Experimental Setup and Methodology

3.1 Experimental Setup

The test model was designed at the geometric scale of 1/80 on basis of Froude similarity criterion according to the similarity law of model, the terrain conditions of junction upstream and downstream and the laboratory site. The Zhaoxian Junction was located the gentle S-shaped river section in the middle of the slightly curved river channel, therefore, the upper boundary of physical model was arranged at 1600 m upstream of sluice taking into account the smooth inflow and navigable flow conditions.

Wuli High-speed Bridge, situated in Zhaoxian Junction downstream with 40 m navigable net width, adjoined neighbor the bend of downstream approach channel, which contributed to complex flow conditions. Consequently, the lower boundary of physical model should be taken to 2160 m downstream of sluice considering the length of sufficient flow adjustment.

The section method is used for terrain production, and the triangulation wire system is employed for plane stakeout where the triangle closure error hardly exceeded ±1’. The model elevation was measured by a level and checked during the production process, and the installation elevation error of the section was controlled within ±1 mm. For complex terrain, the intensive section was added and processed separately in order to improve the production accuracy. The junction, including sluice, ship lock and power station, was made of PMMA. The surface of the river was made of cement mortar. The overall model was photographed in Fig. 3.

The flow velocity measurement adopted the acoustic Doppler flow meter produced by Nortek Company with a range of 0.1–400 cm/s. The flow discharge was measured by a standard rectangular thin-walled weir with an error of less than 1%; the velocity was measured by a water level stylus with an accuracy of 0.02 mm.

Fig. 3.
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Model photo of Zhaoxian Junction

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3.2 Experimental Cases

Table 1 listed the experimental cases for navigable flow conditions of the Zhaoxian Junction. The water level of downstream of sluice was controlled on basis of the numerical simulation of stage-discharge relation curve. Preliminary experiment ensure the method of sluice operation.

Table 1. Experimental cases of Zhaoxian Junction
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4 Results and Discussions

4.1 Downstream Flow Conditions of Initial Layout

For Q = 1000 m3/s with the opening of #1–#6 holes in the left section, there would be backflow within the range of 200 m downstream from the entrance area of the downstream approach channel. The longitudinal velocity of the approach channel generally exceeded 2.0 m/s within the range of 240 m to 320 m downstream from the entrance area of approach channel, and the lateral velocity in the left section was relatively large, which seldom met the navigable requirements.

The reason for the above phenomenon was that the downstream level of river channel was lower than 70.3 m, and the flood plain (higher than 71 m elevation) on the right bank was not completely submerged, which resulted in concentrated flow discharged between ship lock and flood plain. The concentrated flow rushed straight towards entrance area and connecting section of downstream approach channel, where developed a large-scale backflow in vicinity of the entrance area of the downstream approach channel, as shown in Fig. 4. Additionally, the mainstream in concentrated on the channel in vicinity of the Wuli High-speed Bridge area due to the high terrain on the right bank of the river channel, which led to large longitudinal velocity and large flow resistance of upward ships. Therefore, the rudder efficiency was poor, and the control difficulty increased.

Fig. 4.
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Downstream flow regimes of initial layout (Q = 1000 m3/s)

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4.2 Downstream Navigable Flow Conditions for Sluice Moving to Left

The sluice was moved 32 m to left due to the flood plain on downstream right bank. Table 2 listed downstream navigable flow conditions. The data demonstrated lateral velocity exceeded navigation requirements in 200 m–360 m downstream from entrance for Q = 600 m3/s–1000 m3/s. In general, navigable flow conditions had improved with sluice moving to left, whereas navigation requirements remained unsatisfied. It is mainly manifested in the following aspects: 1) flow out of sluice was not smooth, and the overflow on the floodplain was small. The blocking effect of the floodplain on the right bank precipitated flow to the left, which easily caused scouring of the riverbed in vicinity of lock wall and separation levee. 2) Concentrated flow rushed straight towards entrance area and connecting section of downstream approach channel as similar to Fig. 4. Therefore, the dredging on the right bank of the river channel was mainly considered in the optimization of downstream navigable flow conditions.

Table 2. Navigable flow conditions with sluice moving 32 m to left
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4.3 Dredging Downstream Flood Plain to 67 m Elevation

Fig. 5.
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Flow regimes of downstream entrance area with dredging flood plain to 67 m elevation (Q = 800 m3/s)

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Figure 5 photographed flow regimes of downstream entrance area with dredging downstream flood plain to 67 m elevation, and Table 3 listed the data of navigable flow conditions. It could be seen that the dredging of flood plain partly decreased flow velocity and improved flow regimes, however, the main trough was still close to ship lock. The flow diffusion into entrance area brought about relatively large flow velocity. Moreover, blocking effect on the right bank upstream from Wuli High-speed Bridge still existed due to the insufficient dredging range of flood plain, which narrowed the flow cross-section in entrance area downstream and mainstream of the river in vicinity of entrance area was biased towards the left bank. From the data point of view, the lateral velocity still exceeded navigable requirements in a certain range, and we could continue to further deepen the dredging range.

Table 3. Navigable flow conditions with dredging flood plain to 67 m elevation
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4.4 Dredging Downstream Flood Plain to 67 m Elevation

Figure 6 photographed flow regimes of downstream entrance area with dredging downstream flood plain to 67 m elevation, and Table 4 listed the data of navigable flow conditions. It could be seen that for Q = 800 m3/s–1000 m3/s with dredging downstream flood plain to 65 m elevation, the longitudinal velocity and backflow velocity in entrance area and connecting section of the approach channel met navigable requirements, and lateral velocity slightly exceeded navigable requirements within the range of 80 m–200 m downstream from the entrance. Nevertheless, the maximum lateral velocity was less than 0.39 m/s, and the scope of influence was slight. Therefore, the navigable flow conditions of the entrance area and connecting section basically met the requirements under this condition.

Table 4. Navigable flow conditions with dredging flood plain to 65 m elevation
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Fig. 6.
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Flow regimes of downstream entrance area with dredging flood plain to 65 m elevation (Q = 1000 m3/s)

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5 Conclusions

  1. 1)

    Zhaoxian Junction was located on the gentle S-shaped river section in the middle of the slightly curved river channel. The downstream approach channel was located on the concave bank, and there was flood plain on downstream right bank. Relatively large intersection existed between the centerline of the approach channel and the mainstream direction, which resulted in large longitudinal velocity and lateral velocity and affected the safety of ship navigation. It was necessary to take engineering measures to improve the navigable flow conditions in the entrance area to ensure the safety of ships entering and leaving the lock.

  2. 2)

    By moving the sluice to the left and dredging the downstream flood plain, longitudinal velocity, lateral velocity and backflow velocity in the entrance area could be significantly reduced, and the navigable flow conditions could be effectively improved. The experimental results showed that the maximum navigable flow discharge of the initial and left-moving layout could only reached 400 m3/s. By means of effective measures, For Q ≤ 1000 m3/s, hydraulic indexes of the entrance area and connecting section of the upstream and downstream approach channels met the navigation requirements, and navigable flow conditions had been significantly improved.