Keywords

1 Introduction

The entrance area of the approach channel is usually located in the boundary sudden change, mainstream diffusion, dynamic and static water junction area, and it has been thought of as the throat of ships entering and exiting the approach channel (Li et al. 2016).In the entrance area, there exist oblique flow, backflow, bubble vortex, high-intensity eddy current and other complex flow patterns (Yang et al. 2016; Huang et al. 2017;Liu et al. 2021), which directly affect the navigation safety of ships. From the review of the current literature, it can be seen that the flow conditions at the entrance area of the approach channel are jointly restricted by factors such as flow rate, river trend, terrain, scheduling modes and anti-regulation (Han et al. 2014; Yu et al. 2014; Chen et al. 2008; Wu et al. 2016). There are important achievements on how to improve the complex navigation flow conditions and ensure the safety of ship navigation in past studies (Li et al. 2016; Yang et al. 2016; Wang et al. 2019). While the actual situation of each project varies greatly, and the problems of the navigation flow are different, it is necessary to improve and optimize the navigation flow conditions according to the actual situation of projects. The exit of the downstream approach channel of the navigation facilities of the Baise Water Conservancy Project is located about 700 m downstream of the Dongsun Hydropower Station, so the operation of Dongsun Hydropower Station has a direct impact on the navigation flow conditions of the approach channel. And because the entrance area of the approach channel is located in a narrow river area, the navigation conditions are also affected by the river terrain. According to Code for Master Design of Shiplocks, for IV ship lock, in the entrance area, the longitudinal flow velocity, transverse velocity and back flow velocity should be less than 2.0 m/s, 0.3 m/s, and 0.4 m/s, respectively. Therefore, based on the overall hydraulic model test of the large-scale and the simulation test of the self-propelled ship, the flow conditions at the entrance area of downstream approach channel are studied to propose the reasonable layout schemes of downstream approach channel from the perspective of hydraulics.

2 Methodology

2.1 Overall Physical Model Test

Navigation facilities of Baise Water Conservancy Project adopt the combination scheme of ship lock and ship lift, which consist of ship lock, intermediate channel, vertical ship lift and downstream approach channel (Fig. 1). The navigation route is arranged on the left bank of Baise Water Conservancy Project with total length of 4245 m. The designed navigation scale of the project is 2 × 500 t class and reconciling the 1000 t class single ship. The Dongsun Hydropower Station is located 6.5 km downstream of the Baise Water Conservancy Project, and the exit of the downstream approach channel of the navigation facilities of the Baise Water Conservancy Project is located about 700 m downstream of the Dongsun Hydropower Station, and it has an intersection angle of 20.203° with the downstream river. The downstream approach channel is arranged at the downstream of the auxiliary lock. The size of downstream approach channel in the preliminary plan is about 188 m long in a straight section, 34 m–60 m wide at the bottom, 110.0 m in the bottom elevation, 555 m in the centerline turning radius, 20.203° in the center angle, and 60 m wide at the entrance area. The right side of the downstream approach channel is provided with a permeable navigation embankment, the top elevation, length and insert plate height of which are121.04 m, 117 m and 6.94 m, respectively.

Fig. 1.
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The layout of Baise Water Conservancy Project

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The geometric scale of the model is 1:80. The upstream boundary of the model is taken to the dam site of Dongsun Hydropower Station, the downstream boundary is taken to 2000 m downstream of Dongsun Hydropower Station, and the upstream boundary of the approach channel is taken to the lower lock head of auxiliary lock. In this paper, an experimental study on the navigation flow conditions of the downstream approach channel was carried out according to the operation and scheduling modes of Dongsun Hydropower Station and the designed maximum navigation discharge (1500 m3/s).

The bottom flow velocities were measured by a DPJ propeller current meter with an incipient velocity of 1 cm/s and an accuracy of 0.01 cm/s.

2.2 The Simulation Test of the Self-propelled Ship

The ship simulation meets the requirements of gravity similarity and the scale is the same as that of the overall model to ensure the ship draft and the coordination of speed. The trajectories of the ship model are measured in real time by the multilateral wireless positioning method. Four wireless base stations are arranged on the shore, and one wireless tag module is respectively arranged at the bow and the stern of the ship model. The distance between the bow, stern and each base station is collected in real time and calculated by the computer to obtain the navigation trajectories and drift angles. The roll and trim of the ship model are transmitted to the computer by the wireless transmitter module. As shown in Fig. 2.

Fig. 2.
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Principle of multilateral measurement and positioning method

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A single ship is simulated in self-propelled ship test, and representative size of the 1000 t class single ship design is 67.5 m × 10.8 m × 2.6 m (length × width × full load draft), corresponding model size is 84.4 cm × 13.5 cm × 3.3 cm.

3 Results and Analysis

3.1 Velocity Distribution at the Approach Channel Entrance Area and Connecting Section

Velocity distribution in the approach channel entrance area and connecting section can be seen from Table 1 and Fig. 3. For condition 1–3, the flow velocities at the entrance area of downstream approach channel meet the specification requirements. For condition 4, the longitudinal and transverse flow velocity of individual measuring points on the right edge of the curved section of the downstream approach channel entrance area slightly exceeds the standard, but the flow velocities of the channel centerline and other areas can meet the specification requirements. For condition 5, the over-standard range of the longitudinal and transverse flow velocity in the local area on the right side of the curved section of the downstream approach channel entrance area is larger than that in condition 4, the backflow range on the left side of the entrance area also increased significantly from about 150 m in condition 4 to 350 m, and the flow velocity at the entrance area of downstream approach channel exceeded the standard obviously.

Therefore, under the condition that the total discharge of Dongsun Hydropower Station not exceed 1500 m3/s and the power station is fully powered, the flow conditions at the entrance area of the downstream approach channel can meet navigation requirements.

Table 1. The velocity at the entrance area of the downstream approach channel in the preliminary scheme
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Fig. 3.
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Velocity distribution at the entrance area of approach channel

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3.2 Wave Characteristics of River Channel and the Entrance Area

In the model test, the fluctuation of the water surface of downstream was measured. A total of 6 measurement points were arranged, including 4 points in the main channel which are located in sequence: 0 + 64 m, 0 + 384 m, 0 + 640 m, 0 + 960 m; and 2 in the entrance area of approach channel which are 120 m and 250 m away from the auxiliary ship lock respectively (Fig. 4). The measurement results are shown in Table 2.

It can be seen that the wave height in the entrance area has exceeded the requirement under the conditions of 4 and 5.

Fig. 4.
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Arrangement of wave height measuring points

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Table 2. The wave height of the downstream channel and the entrance area in the preliminary scheme
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3.3 The Navigation Parameters of the Ship Model

A simulation test of a self-propelled ship was carried out, and navigation parameters can be seen from Table 3. Under all test conditions, the ship model can smoothly pass through the entrance area to enter and exit the approach channel. While in condition 5, the maximum drift angle of the whole journey has exceeded the requirements, which occurs about 360 m downstream of the gate. Besides, when the ship passes through the river channel shaped “S”, the track of the ship is affected by the contracted flow, so it is necessary for steer in time to overcome the transverse flow and straighten the course.

Table 3. Navigation parameters of the ship model
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What can be learned from navigation flow condition test and self-propelled ship model test is that the preliminary layout scheme of approach channel fully meets the navigation requirements under the designed maximum navigation discharge of 1500 m3/s, and the only shortcoming is that it is necessary for steer in time to overcome the transverse flow due to the influence of the protruding rock point on the right bank downstream of the entrance area.

4 Optimized Layout of Approach Channel and Its Navigation Conditions

In order to solve the above problems in the preliminary scheme and improve the navigation capacity of the downstream approach channel, the optimized layout schemes were proposed by adjusting the course and dredging. The flow velocity of each optimized scheme is shown in Table 4.

In Optimized Scheme 1: Adjust the downstream route under the condition that the axis of the ship lift and the auxiliary ship lock remain unchanged. Compared with the original scheme, the straight section of the approach channel is offset to the left bank by 3.7° starting from 470 m from the lower gate of the auxiliary ship lock, and the route of the optimized scheme is laterally shifted to the left by 23 m at about 780 m away from the lower gate, shown in Fig. 5. After the adjustment of the route of downstream approach channel, the characteristic flow velocity of the entrance area of downstream approach channel in condition 3 meets the requirements of the navigation specification, but the local characteristic flow velocity on the right side in conditions 5 and 6 exceeds specification requirements.

In Optimized Scheme 2: On the basis of optimized scheme 1, excavation and dredging were carried out at the rocky point on the right bank about 500 m downstream of the entrance area, as shown in Fig. 6. After the dredging, for condition 4, the longitudinal and transverse velocity of individual measuring points on the right edge of the bend section of the downstream entrance area slightly exceeds the standard, but the characteristic velocity of the channel centerline and other areas can meet the specification requirements, so it is considered that the navigation conditions are met according to the use experience of the built navigation facilities. For conditions 5 and 6, the flow velocity in a large range on the right side of the center line exceeds the specification requirements.

In Optimized Scheme 3: In order to explore the possibility of increasing the maximum navigable discharge to 1700 m3/s which is once in 5 years, on the basis of optimized scheme 2, excavation of the protruding rocky point in upstream of the right bank was carried out, as shown in Fig. 7. After the excavation, only the velocity index of local area of the outer boundary of the bend section of the downstream entrance area exceeds the standard, and the area is small. In addition, the maximum wave height in the entrance area is not greater than 0.1 m. Although the maximum drift angle of the whole journey slightly exceeds the requirements (Table 5), the ship can enter and exit the entrance area smoothly, and the number of steering times per minute is less than 2 times. Therefore, it is reasonable to take 1700 m3/s as maximum navigable discharge.

Table 4. The velocity at the entrance area of the downstream approach channel in the optimized scheme
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Table 5. Navigation parameters of the ship model with the discharge of 1700 m3/s
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Fig. 5.
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Comparison of the route of preliminary plan and optimized scheme 1 (shaded)

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Fig. 6.
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Excavation and dredging at the rocky point on downstream of the right bank

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Fig. 7.
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Excavation and dredging at the rocky point on upstream

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5 Conclusions

From the review of the test result, the following can be concluded:

  1. (1)

    In the preliminary scheme, under the condition that the discharge of the power station is 1500 m3/s at full power, the flow pattern, velocity, wave height, and navigation parameters all can meet the requirements of the specification, and the ship can smoothly enter and exit the downstream approach channel. The only shortcoming is that it is necessary for steer in time to overcome the transverse flow due to the influence of the protruding rock point on the right bank downstream of the entrance area.

  2. (2)

    After excavation and dredging of two rock points on the right bank, the maximum navigable discharge can be increased from 1500 m3/s to 1700 m3/s, so as to meet the requirements of navigable flow once in five years.