Keywords

1 Introduction

It commonly happens when building a new hydro-junction construction on a curved river of mountain area, while there are too many bends but few straight channels. The main characteristics of this type of river included shallow water, numerous beaches, more sharp bands and large surface slope (Jiang et al. 2022). This makes the flow conditions of approach channel and the entrance area more complicated, which is unfavorable for navigations (Yan 2021; Zhang 2020; Li 2019). Therefore, before constructing a new hydro-junction with a ship lock on curved rivers of mountain area, the navigation flow conditions should be studied deeply (He 2021; Sun 2016; Zhou 2008). Wang et al. (2021) compared how several engineering measures, including adjusting the height of the partition dam, adding partition walls and channel dredging, improve the navigable flow of a double-lane ship lock through the flood discharge variation test. Results showed that the comprehensive measure of adding partition walls and dredging the channel was effective in improving the flow conditions of the approach channel. There have been a large number of researches proposing several improvement measures for the navigable flow conditions. Shen et al. (2022) established a moving-bed partial model to predict the effects of sedimentation on the navigation and optimize the navigation condition of channels by adopting non-engineering measures such as dispatching the scouring sluice. Han et al. (2021) analyzed the flow conditions of the upstream and downstream approach channels and the connecting sections in the newly-built ship lock, and offered partial dredging as the improvement engineering measure. Besides, Wang et al. (2019) improved the flow conditions in the entrance area by adding spur dike complex and submerged dike complex. This paper aims to simulate the upstream flow of the montanic hydro-junction by building a physical hydraulic model, and improve the navigable conditions by proposing specific engineering measures.

Baihutan hydro-junction is the first hydro-junction built in the upper reaches of the “Changshanjiang River Development Key Projects” approved by the State Council. The navigation capacity of Baihutan is quite significant to the traffic volume of Changshanjiang River and Qujiang River. However, the bend in the upstream may limit and affect the navigation (Zhang 2020). This present study took the upstream of Baihutan hydro-junction as the research object, and built a normal physical hydraulic model to evaluate the flow conditions of upstream approach channel, entrance area and the bend connection area. Moreover, the remote-control ship model test is also applied for supplementary analysis. To sum up, engineering measures, including expanding the approach channel to concave bank to increase the cross-sectional area of the riverway, and adjusting the channel to avoid the top of the bend and accordingly reduce the angle between the mainstream and the axis of the channel, were proposed in this paper, which provide references for improving the navigation conditions of other hydro-junctions.

2 Hydro-Junction Information

The Baihutan hydro-junction is located in the Changshanjiang drainage basin in Zhejiang Province, China. The entire hydro-junction is lied in the straight section of the S-shaped riverway. Figure 1 shows the location and overall layout of the Baihutan hydro-junction. The layout of buildings from left to right successively includes the power station, sluice gates and ship lock. There are 17 sluice gates and the total net width of the flood discharge is 204 m with an 83-m bottom. The gates are divided into two parts by the longitudinal navigation walls. There are 8 gates on the ship lock side and 9 gates on the power station side, which are numbered 1#–17# from the power station to the ship lock by turns. The upper lock bay of the Baihutan Ship Lock is aligned with the axis of the sluice gates. Besides the lock chamber and the lower lock bay are located in the downstream of the dam axis. The available size of the lock chamber is 180 m × 23 m × 4 m (effective length × effective width × submergence), which is designed for navigating 500-ton ships. The levels of the ship lock and the navigation channel are Class IV. The approach channel of upstream is located in the upstream reservoir and connected by a curved section between the anchorage section and the navigation section. The anchorage section of upstream is 220 m long and 60 m width. The navigation section of upstream is 100 m long. In the initial design of upstream, the central angle of the bending section is 30°, the radius of centerline is 360 m, and the length of the centerline in the bending section is 172.8 m.

Fig. 1.
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Location of the Baihutan hydro-junction

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Fig. 2.
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Hydraulic model

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The entrance area of the approach channel in upstream of the Baihutan ship lock is located in the concave bank with a typical large-angle sharp bend. When the water flow enters the bend, the water level in the concave bank is higher than that in the convex bank and forms a banked-up water level on the concave bank under the effect of centrifugal force. This will make the surface of water body flows to the concave bank, because its centrifugal force is greater than the pressure in water. While the bottom of water body flows to the convex bank since its pressure is greater than the centrifugal force. As a result, the main circulation is formed on the profile side. The transverse flow velocity of the water surface in the entrance area near the top of the bend may be large and the navigable flow conditions may be more complicated under the influence of the circulation flow. Excessive flow velocity may cause drifting or twisting to the ship. In order to avoid adverse flow conditions such as oblique flow, backflow and whirlpools affecting the navigation safety, it is necessary to further demonstrate whether the ship can navigate safely and propose specific engineering measures to improve the navigable flow conditions.

3 Experiment Design

3.1 Hydraulic Model

The normal fixed-bed hydraulic model is designed on the basis of Froude similarity criterion, and its geometric scale is 1:80. The model simulates the whole hydro-junction building, the upstream channel, the upstream approach channel, the bend connection area and the downstream river channel. The layout of the hydro-junction, the distance of the model flow adjusting from turbulent flow to subcritical flow and the length of the start-up distance of the remote-control ship model are fully considered when delineating the simulation boundaries. The upper boundary of the model is set at the straight section from the upstream of the bend, 1400 m away from the hydro-junction building. The boundary of downstream is set 1700 m away from the building. The model is built on the basis of the measured topographic data using the terrain profile analysis method. The model elevation is controlled less than ±1 mm, the closure difference of triangle or polygon is within ±1′, and the channel roughness is 0.030–0.035. The surface of the model is made of cement mortar. The discharge is measured and controlled by using the YC101E type electromagnetic flowmeter produced by SYCIF company, and its deviation is less than 1%. The water level is measured by water level point gauges with 0.01 mm precision. The flow velocity is measured by the Vector type acoustic doppler velocimetry produced by Nortek company, whose range is 0.1 cm/s–400 cm/s. The overall hydraulic model is shown in Fig. 2.

3.2 Remote-Control Ship Model

The design of ship model is consistent with the hydraulic model, which can be controlled wirelessly. According to the actual operation status of the Changshanjiang River waterway, ship with a size of 67.5 m × 10.8 m × 2.2 m (length × width × designed draft) is chosen for simulation. The outline of the ship, the blades and the rudder blades are scaled down to build the ship model. The draft of ship model and the headway speed in still water are calibrated before the experiment to ensure the consistency of maneuverability between the model and the real ship.

3.3 Experiment Cases

The upstream approach channel of the Baihutan ship lock is designed in an open type, which has no navigation walls in the upstream. This will make it easier and faster for ships going in and out of the lock, and more economical for the construction cost. However, if the sluice gates near the ship lock side are opened, the navigation of ships will be affected. Under the condition of meeting the normal discharge demand for the power station, the rest flow rate should be discharged through the left sluice gates, which is equidistant opened, so as to guarantee positive navigable conditions. Based the layout of the sluice gates, the opening distance regulation of the sluice gates and the opening sequence of the sluice gates, balancing the navigable conditions of upstream and downstream as well as the scouring of the downstream river channel, a variety of opening operation schemes of sluice gate were compared before the experiment (Zhang 2020; Zhang et al. 2019). Finally, five different flood discharge cases and sluice gate schedule were determined (see Table 1).

Table 1. Experiment cases of hydraulic model.
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4 Navigable Flow Conditions in Initial Design

The riverbed topography of the upstream approach channel and the entrance area in Baihutan hydro-junction is complicated. The river narrows 110 m inward in the concave bank, which is 560 m away from the hydro-junction. When the ship goes down and enters the upstream approach channel, the maneuverability of the ship may be affected by the bed topography and the water flow in the bend. According to national standards of navigation conditions, the longitudinal flow velocity in the entrance area of the approach channel in Class IV should be less than 2.0 m/s, the transversal velocity should be less than 0.3 m/s, and the backflow velocity should be no more than 0.4 m/s. Moreover, the longitudinal flow velocity of the braking section and the anchorage section should be not more than 0.5 m/s, the transversal flow velocity should be not more than 0.15 m/s, the navigation section and the adjustment section of the approach channel should be still water.

Table 2. Measured flow velocity in initial design.
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Results showed that when the flood discharge was 102 m3/s–800 m3/s, the maximum velocity in the navigation section and the adjustment section of approach channel is 0.04 m/s, and the water surface is the still water without fluctuations (see Table 2). The maximum velocity in the anchorage section is 0.60 m/s, and most of the flow velocity is within the range of 0.45 to 0.53 m/s. The maximum longitudinal velocity is 0.59 m/s, and the maximum transversal velocity is 0.11 m/s. Compared with the above-mentioned areas, the water flow velocity in the entrance and the connection area of the bend, is larger. The maximum longitudinal velocity in the bend is 1.27 m/s, the maximum transversal velocity is 0.22 m/s, and the transversal velocity in all measuring points is less than 0.30 m/s. Therefore, when the flood discharge is less than 800 m3/s, the flow velocity in the entrance area of the upstream approach channel and the bend connection area can both meet the requirements for navigation.

When the flood discharge is 1000 m3/s, the navigation section and the course adjustment section of the approach channel are still water areas. The maximum velocity of the anchorage section is 0.83 m/s, the maximum longitudinal velocity is 0.77 m/s, and the maximum transversal velocity is 0.27 m/s. In the bend connection area, the maximum longitudinal flow velocity is 2.00 m/s and the maximum transverse flow velocity is 0.37 m/s. When the discharge reaches 1200 m3/s, the flow velocity in the navigation section, the course adjustment section and the anchorage section of the approach channel can still guarantee the navigation safety. But the transverse flow velocity in the bend connection area is larger, and the transverse flow velocity in some individual measuring points can reach 0.48 m/s. From the perspective of the characteristics of riverway layout, the cross-sectional area of water body decreases, and the flow velocity increases under the effect of the shrinkage in the bend connection area. In addition, the location of the shrinkage is too closed to the top of the bend. It can be observed that the angle between the mainstream and the axis of the designed channel is about 10° when the flood discharge is 1000 m3/s, and the angle is about 14° when the flow discharge reaches 1200 m3/s. Larger angle makes the projection of the mainstream flow vector velocity in the normal direction of the channel centerline bigger, which causes larger transverse flow velocity in this area.

Fig. 3.
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The upbound and downbound sailing tracks in initial design.

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In order to explore the influence of the large transverse flow velocity on the actual maneuvering of the ship in this area, a remote-control ship model was built to conduct the simulated tests under the 1000 m3/s flood discharge. Figure 3 shows the sailing tracks of the upbound and downbound of the upstream ships in the Baihutan ship locks when the flood discharge meets 1000 m3/s. Figure 3 indicated that when the ship goes up, the ship can be well controlled in all the navigation section, the course adjustment section and the anchorage section. However, the ship was found to drift toward the concave bank in the entrance area and the connecting section of the bend. Thus, the ship needs to be steered the full rudder in advance to avoid collision with the concave bank and deviation from the predetermined route. Ships need to be frequently steered in 0+100 m to 0+250 m away from the entrance. The transverse flow velocity in this area is about 0.25–0.42 m/s and exceeds the standard value. The ship enters the predetermined route in 350 m away from the entrance. The sailing track has large radian in the in the bend connection. The speed of the ship relative to the bank is about 2.6 m/s, and the maximum drift angle in the entrance area is 23°, which is larger than the limited value of 15°. Besides, the transversal swing angle of the ship is less than ±1.5° and has no obvious longitudinal tilt. When the ship goes down, the left rudder should be turned ahead to enter the approach channel. The speed of the ship relative to the bank is about 3.2 m/s, the maximum drift angle in the entrance area is 18°, and the drift angle is about 10°. The sailing track deviates to the concave bank, which is dangerous for sailing. Therefore, the transverse flow velocity exceeded the standard value in the entrance area and the bend connection area in the initial design, which make it infeasible for ships to navigate safely.

5 Optimized Navigable Flow Conditions

Adjusting the layout of channels, excavating local riverbed and adjusting shorelines are effective measures to promote the navigable flow conditions. There are two main reasons for the excessive flow velocity in upstream of the initial design. Firstly, the entrance area of the upstream approach channel is located in the sharp bend section of river, and the connection between the entrance area and the bend is too close to the top of the bend. Thus, the smooth navigation of ships was restricted by the bend. Secondly, the narrowed channel at the top of the bend leads to faster flow velocity, and the elevation of the bed in the concave bank is higher, which causes deflexion of water flow when passing through the top of the bend, and increase the angle between the mainstream and the axis of the channel. To reduce this angle and slow down the flow velocity, the following measures were taken: 1) Change the angle of the bending section between the anchorage section and the navigation section into 15°, and extend the approach channel toward the concave bank to expand the cross-sectional area of the riverway. 2) Extend the channel 150 m to the south, and connect the channel and the approach channel with a 72° curve to avoid the top of the bend. 3) Widen the inner side of the channel from the original 35 m on both sides to 35 m on the outer side and 70 m on the inner side. The differences between these two channel designs were demonstrated in Fig. 4.

Fig. 4.
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Comparison of channel layout between improved design and initial design.

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Fig. 5.
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Rudder angle, drift angle and ship speed process curves of upbound and downbound.

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Table 3. Measured flow velocity in improved design.
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The flow velocity in improved design is shown in Table 3. Results indicated that when the flood discharge is 1200 m3/s, the navigation section and the course adjustment section is still water. The maximum flow velocity in the anchorage section is 0.42 m/s, the maximum longitudinal flow velocity is 0.39 m/s, and the maximum transverse flow velocity is 0.11 m/s. Besides, in the connecting area between the entrance area and the bend, the maximum longitudinal flow velocity is 1.75 m/s, and the maximum transverse flow velocity is 0.22 m/s. The angle between the mainstream and the axis of the channel reduced significantly, and the transverse velocity dropped about 56% in the same position and the same flow rate. The flow velocity in the entrance area, the approach channel and the bend connection area meet the requirements for navigation. The upbound and downbound sailing status parameters of the ship model are shown in Fig. 5. Results of the ship model test revealed that when the flood discharge is 1200 m3/s, the sailing track of upbound was smooth. Moreover, the ship was able to enter the upstream channel successfully under simple manipulations, among which the maximum drift angle throughout is 13°, the maximum drift angle of the entrance area is 10°, the average frequency of steering is 2 times/min, and the drift angle between the entrance area and the bend connection area is within 10°. What’s more, the sailing track of downbound was also smooth, among which the maximum drift angle throughout is 12°, the maximum drift angle throughout is 6°, and the average frequency of steering is 3 times/min. Furthermore, no obvious tilt was observed and the ship could navigate easily.

To sum up, by adopting engineering measures including adjusting the layout of channels appropriately, changing the angle of bending section, expanding the approach channel and et al., the angle between the mainstream and the axis of the channel was reduced, and the navigable flow conditions in upstream were improved, which made it feasible to meet the navigation requirements under the flood discharge of 102–1200 m3/s.

6 Conclusions

This study evaluated the navigable flow conditions in the upstream of Baihutan hydro-junction by building a normal physical hydraulic model and a remote-control ship model. The shrinkage of the riverway in the entrance area leads to a large angle between the mainstream and the axis of the channel, which is the main reason for the high flow velocity in the entrance area and the bend. In the initial design with 1000 m3/s flood discharge, the maximum longitudinal flow velocity in the entrance area and the bend is 2.00 m/s, the maximum transverse flow velocity is 0.37 m/s, and the angle is about 10°. This affects the normal navigation, and engineering measures need to be taken to improve the flow conditions.

Reducing the angle between the mainstream and the axis of the channel is the main method to decrease the transverse flow velocity. What’s more, other measures including adjusting the channel to avoid the top of the bend, reducing the central angle of the bending section from 30° to 15° and extending the concave bank 150 m away from the approach channel to increase the cross-sectional area, were all found effective in improving the flow conditions.

The maximum navigation discharge of the upstream in the initial design is 800 m3/s. After conducting the engineering measures mentioned above, the maximum navigation discharge increases to 1200 m3/s. In summary, the status parameters of the ship model indicated the effect of water flow on the ship. The sailing tracks and process curves of rudder angle, drift angle and ship speed were also proved to play an auxiliary role in the analysis of navigation conditions.