The Type and Layout of the Lock Gate and the Scheduling for the Babao Lock on the Estuary with Strong Tidal Bore

Abstract

The unique tidal bore of Qiantang River is called “the first tidal bore in the world”. The outlet of Hangzhou Babao lock of Beijing-Hangzhou Canal is located in the strong tidal bore area of Qiantang River estuary. This paper focuses on the characteristics of Qiantang River tidal bore, the selection of lock gates under the action of strong tidal bore, and the navigation safety control mechanism of lock. The results show that during the spring tide period, the maximum height of the head of tidal bore at Babao lock of Qiantang River can reach 2.5 m, and the maximum tidal bore pressure on the front of the lock can reach 90–100 kPa. In order to ensure the safe operation of ship lock, the design put forward for the Qiantang river side lock head adopts the miter gates and tidal gate combination plan, namely to put a flat gate on the outside of miter gate, with gate mouth 23 m wide and height of 13 m, to withstand the enormous impact of tidal bore in a large tide by closing the tide gate before the arrival of the tide and opening the tide gate afterward, while only using the miter gate as the lock operation gate the rest of the time during which there are no tidal surges. According to the patterns of tidal bore propagation, the intelligent prediction and early warning mechanism of tidal bore, the operation rules, and the automatic control system of miter gate and tide gate are studied and established to ensure the safe and efficient operation of the lock.

1 Preface

With a total length of 1,794 km, the Beijing-Hangzhou Grand Canal is China’s second “golden waterway”, only after the Yangtze River. In order to address the conflict between the rapid growth of the Beijing-Hangzhou Canal transport volume in Zhejiang section and the incompatibility of channel capacity, a second channel connecting the Beijing-Hangzhou Canal with the Qiantang River was started in October 2017 and will be completed by the end of June 2022, resolving the grand canal navigation bottlenecks in consideration of the grand canal cultural heritage protection and utilization. Hangzhou Babao ship lock is a key project connecting the southern end of The Beijing-Hangzhou Canal with the Qiantang River, which is capable of having 1000-ton ships navigating in the inland river. At the same time, a double-line lock is constructed, with the effective dimensions of the lock chamber being 300 m long, 23 m wide and 4.2 m threshold depth. The location of Babao Lock is shown in Fig. 1. The Qiantang River is famous for its unique tidal bore. The maximum height of tidal bore in tidal flood is 3 m, and the height of tidal bore on buildings can exceed 15 m. After tidal bore, the flow speed is generally 6–9 m/s, and the maximum is 10 m/s (Pan et al. 2008). The tidal bore at Sanbao lock is shown in Fig. 2. The outlet of Babao lock is located in the strong tide area at the mouth of Qiantang River. According to the research on the key technical issues of Babao lock gate and the in-depth research on the treatment scheme of Babao lock gate, the maximum tidal head height at the outlet of the lock can reach 2.5 m, and the maximum tidal bore pressure on the gate front can reach 90–100 kPa (Yang et al. 2016), which has a tremendous impact on buildings. Under such unusual circumstances, selection of lock gate is important as commonly used miter gate, triangle gate, and horizontal sliding gate cannot bear huge tidal impact. Therefore, in the project feasibility study and preliminary design stage, the key research is done on gate form and layout that can meet the safe operation conditions, and gate operation rules are formulated and intelligent safe operation control system of the lock is established according to the patterns of tidal propagation.

Fig. 1.

Location of Babao lock of Beijing-Hangzhou Canal

Fig. 2.

Tidal bore at Sanbao lock

2 Tidal Bore Characteristics

2.1 The Tides

The tides at the Qiantang River estuary are irregular semidiurnal tides. There are two tides every day, and there are two changes between spring tides and neap tides every month. The tides can reach the Fuchun River Hydropower Station about 120 km upstream of Babao. According to the long-term tidal level observation station near the gate of Babao lock, the measured perennial average high tide is 4.43 m (1985 national elevation benchmarks. Same below), the perennial average low tide is 3.65 m, the maximum tidal range is 4.28 m, and the perennial average tidal range is 0.8 m. The tidal current in the section near the entrance is a reciprocating current, lasting from 1 h 11 min to 4 h 10 min for a flood current and 7 h 42 min to 12 h 15 min for an ebb current. Except for the flood in Meiyu season, the flow velocity of floods is greater than that of ebbs, which is generally less than 1.5 m/s. When large tidal range occurs every five years, the average flow velocity of floods at perpendicular line can reach 3–4 m/s. In September 2021, the measured maximum surface flow velocity of floods was 2.71 m/s, and the maximum ebb tide flow velocity was 1.28 m/s. Figure 3 and Fig. 4 respectively show the validation diagram of secondary tide level measurement process and tidal current process near the lock entrance in September 2021.

Fig. 3.

Tide level process verification diagram

Fig. 4.

Validation diagram of tidal flow proc

2.2 The Tidal Bore

The tidal bore of Qiantang River is formed in the area of Jianshan, and gradually increased in the upstream process, reaching the maximum in the area of Yanguan-Daquekou, and then gradually decreased in intensity. When the tide is strong, the head of the surge can reach Wenjiayan, and the whole journey is about 90 km. The strength of tidal bore is consistent with the size of tidal range: when the tidal range is large, tidal bore is strong; and when the tidal range is small, the tidal bore is weak. If the tidal range is less than 1.0 m, there will be almost no tidal bore.

2.2.1 The Height of the Tidal Bore

Traditionally, the height of the tidal head is used to represent the strength of the tidal bore. According to the observation data of tidal bore over the years and the relationship between tidal height and tidal range, the maximum height of tidal bore near Babao lock is about 2.5 m. According to the actual measurement, the velocity of the fast water measuring point following the head of the tidal bore is generally 6–9 m/s, with a maximum of 10 m/s and a duration of about 15 min. According to the Babao data measured in September 2001, when the tidal range was 2.7 m and the height of the tidal bore was 1.8 m, the tidal bore velocity was about 7.34 m/s. In September 2021, when the measured tidal range was 2.5 m, the height of tidal bore was 1.3 m, and the propagation velocity of tidal bore was about 6.64 m/s.

2.2.2 The Pressure of Tidal Bore

The tidal bore propagates rapidly, and the impact force on the gate greatly exceeds the hydrostatic pressure. The pressure of tidal bore is closely related to tidal bore height. According to the physical model test of tidal bore pressure at Babao lock, the distribution of tidal bore pressure corresponding to different frequencies and tidal bore height is shown in Fig. 5.

It can be seen from Fig. 5 that the maximum pressure of tidal bore acting on the front of the gate is 92 kPa, and the action elevation is the low tidal level before the tidal bore, and the height decreases linearly upward and downward. The tidal bore pressure has also been observed in other projects on the Qiantang River and the results are similar.

Fig. 5.

Tidal action of gate at 2% tidal height

3 Lock General Layout and Gate Selection

In the stage of project feasibility study and preliminary design, detailed research and demonstration was carried out on the selection and layout of the gate of the Babao lock, as detailed below.

3.1 General Layout of Lock

The new double-line lock is arranged in parallel, the center distance of the lock chamber is 55 m, and the effective scale of the double-line lock chamber is the same at 300 m long, 23 m wide, and the threshold water depth is 4.2 m. The plane scale of the side gate head of Qiantang River is 43 m wide and 46.5 m long. The main ship lock is set up in the north of Xiasha Road, the main road of the city, and the boundary of the head is about 450 m away from the Qiantang River dike. The length of the straight section of the downstream approach channel is 438 m, among which the length of the navigation adjustment section is 138 m, the length of the ship section is 300 m, and the width is 110 m.The downstream approach channel entrance is set up with two guide dikes, with lengths of 200 m and 163.7 m respectively. The lock management area is set up on the west side of the lock, and auxiliary buildings such as the lock operation and management building, maritime management building and substation will be built in the lock area. The general layout of Babao Lock is shown in Fig. 6.

Fig. 6.

General layout of the Babao lock

3.2 Lock Operating Conditions

• 1) Characteristic water level of the Babao Lock

  The designed maximum navigable water level of Qiantang River channel is 7.0 m, and the designed minimum navigable water level is 2.7 m. The water level is set at 9.3 m according to the high water level for a 300-year flood standard.

  The designed maximum navigable water level of the canal channel is 2.4 m, and the designed minimum navigable water level is 0.6 m. The water level is set at 3.5 m according to the high water level for a 100-year flood standard.

• 2) Lock working head

  The lock operates as a unidirectional head, and the maximum head is 6.4 m. At this time, the Qiantang River side is 7.7 m, and the canal side is 0.6 m. The maximum check head of the side gate of Qiantang River is 8.3 m, the side gate of Qiantang River is 9.3 m and the side gate of gate chamber is 1.0 m (the normal water level of the canal), so the flood control and tidal gate should be closed.

• 3) Annual navigating days

  According to the meteorological, hydrological and sediment conditions, and taking the maintenance needs of the lock into consideration, the designed annual navigation time is set at 310 days.

• 4) Tidal bore effect

  The cumulative rate of tidal range greater than 1.0 m was 40% on average for many years, with an average of 12 days of tidal bore per month and 2 semidiurnal tidal bores per day.

3.3 Selection and Layout of Lock Gate

3.3.1 Characteristics of Various Gate Types

Under the hydrologic conditions of the Babao lock, the main types of gate available are triangular gate, miter gate, flat lifting gate and sliding gate. Under normal circumstances, miter gate is most commonly used for unidirectional head lock, and triangular gate is most commonly used for bidirectional head lock. Both of them have the characteristics of economical and efficient operation, with a width of 12 m to 34 m in various specifications; Flat lifting gates and sliding gates can be adapted to bidirectional head and are used in some locks with widths ranging from 12 m to 23 m. Triangular gate, flat lifting gate and sliding gate can adapt to the action of bidirectional head and certain dynamic water load.

Facing the special hydrological and sediment conditions, the side gate of Qiantang River of the Babao lock is not ideal to be used as tide blocking gate because the miter gate and triangular gate can not bear the huge impact of tidal bore during the spring tide. For the sliding gate, when the gate is closed, it can bear a certain dynamic water effect. However, due to the high sediment content in Qiantang River during the spring tide or flood, the track groove is subject to silting, which needs to be cleaned regularly, and the underwater maintenance of the flat car at the bottom of the hoist is difficult, which affects the passing capacity of the lock, so the sliding gate scheme is not appropriate.

Flat lifting gate has the structural bearing capacity of tidal bore impact and can avoid the influence of gate groove silting, so it is a reasonable choice of tide blocking working gate. Located at about 7 km upstream of Qiantang River at the Babao lock, the Sanbao lock connects the Beijing-Hangzhou Canal to the Qiantang River with the width of the lock being 12 m, and the vertical lifting flat gate is adopted, which can withstand the impact of huge tidal bore. Thirty years of operation shows that this type of gate is safe. However, the net width of the gate of the Babao lock is 23 m, the height is nearly 13 m, the mass of the gate reaches 220,00 kg, and the lifting height is about 16 m. According to the designed capacity, the lifting times reaches 26 times a day, which is not ideal in terms of operational costs and work efficiency. Moreover, the raised gate is hoisted high in the air, resulting in poor visual and landscape effects.

3.3.2 Gate Selection and Layout

According to the characteristics of various feasible gate types and the use requirements of tide gate, the design adopts the combination scheme of “miter gate + tide gate”. The gate head structure is lengthened, and a flat tide gate is set up on the outside of the miter gate, as shown in Fig. 7. Usually, when there is no tidal bore, only the miter gate is used. During the tidal bore, the tide gate is used. The tide gate is also used as a flood control and maintenance gate.

• 1) Miter gate

• Gate head is 23 m wide, the miter gate scale is 13.58 × 1.4 × 9.0 m (width × thickness × height), the gate top elevation is 7.0 m, the threshold elevation is −2.5 m, the designed working head is 6.4 m, bearing unidirectional head, and the panel is arranged towards the Qiantang River side. Miter gate open and close in still water, without considering the impact of tidal bore.

• 2) Tide gate

• The width of the gate is 23 m, the height of the gate top is 11.5 m, and the height of the bottom sill is –1.5 m.The flat gate is 13 m high, 24.3 m wide and 2.81 m thick. When closed, both sides are stuck in the gate head slot. The structure of the tide gate meets the requirements of strength, stiffness and stability under the most adverse conditions, and the safety of the upper gate stuck in the gate slot is desirable. The tide gate is usually stored in the side gate storehouse of the head, and is hoisted into the working gate slot by the trolley hoist.

Fig. 7.

Arrangement of gate along Qiantang River

3.3.3 Operation and use of Miter Gate and Tide Gate

During the normal operation of the lock without tidal bore, the miter gate is used as the working gate to block water. When the tidal range of Qiantang River is greater than 1 m and there is a tidal bore, the tide gate should be closed about 0.5 h before the arrival of the tidal bore, and the gate should be used to resist the impact of the tidal bore. The tide gate should be opened 1.5 h after the tide to make the Qiantang River navigable. Because the tide gate is only used in the middle and high tide, and is opened and closed only once per tide, and the use frequency is relatively not high, the height of the gate frame can be greatly reduced by lifting the gate to a certain height and moving horizontally to the gate chamber afterward. At the same time, the flat tide gate can also be used as the maintenance gate, and also as the accident gate when the miter gate is running, which further improves the safety of the project.

Considering the operation characteristics of the side gate of Qiantang River, that there are 12 days of tidal bore every month on average, and twice a day, the tide gate is used twice a day, and the gate is closed for 2 h each time. Only the miter gate is used during other times not affected by tidal bore. Under the circumstances that the lock is in operation 24 h a day, the number of ships passing locks is 26 times a day, and the miter gate is only required for the remaining 24 times except for the use of the tide gate twice. In addition, there is no tidal bore during 18 days of each month, so there is no need to use the tide gate.

4 Lock Safe Operation Control Measures

4.1 Navigation Control of Ship Entry and Exit

When the tidal range of Babao area is greater than 1.0 m, the ship entry and exit of the lock will be affected by tidal bore. According to the analysis of measured data, the accumulation rate of tidal range greater than 1.0 m is 39.7%. 280 tides out of 705 tides in a year have tidal bore phenomenon, during which ships need to take tide shelter measures for navigation. The tidal bore can travel 30 to 40 km upstream of Babao during strong tides, during which the ships passing the lock should shelter in the anchorage of the deep water area of the upstream, which is about 35 km from the Babao ship lock to the Zhijiang anchorage, and the tidal bore propagation time is about 70–80 min. In order to prevent ships from encountering tidal bore in the passage from Zhijiang to Babao lock, according to the operation experience of Sanbao lock in anti-tidal bore for more than 30 years or the operation rules of anti-tidal bore, ships entering and leaving the gate of Babao lock and navigation on Qiantang River are restricted for several hours before and after high tide. The navigation control time of ships passing the lock at Babao Lock is shown in Fig. 8.

• 1) Lock exit time: stop exit for a total of 5 h: 3.5 h before high tide (to ensure that the ship can safely reach the anchorage before the tide head given the single ship speed of 8–9 km/h) and within 1.5 h after high tide (flow rate > 2.0 m/s). Lock exit time within a tide (about 12.4 h) is during 1.5 h to 8.9 h after high tide, 7.4 h in total.

• 2) Lock entrance time: considering that entering the lock should be stopped 0.5 h before the tide, ships should be able to descend within 0.5–1 h after the tidal bore reaches the anchorage of Zhijiang (2–2.5 h after the tidal head of Babao ship lock). Based on the calculation of single ship speed of 10 km/h, the actual time for entering the lock of Babao ship is generally 5 h after the high tide, and the time for entering the lock within a tide is during 5 h to 11.9 h after the high tide, a total of 6.9 h.

  According to the above control of gate entry and exit time, in theory, the ship lock can operate for about 20 h every day when there is tidal bore (tidal range > 1.0 m).

  During the small tide season when the tidal range is less than 1.0 m, as long as the water depth of the lock entrance and channel is allowed, ships entering and leaving the lock and sailing in the Qiantang River are not affected by the tidal bore, and they can theoretically pass for 24 h a day.

• 3) According to ship model sea trial and real ship sea trial, the safe navigable flow condition of ship lock entrance and exit area is recommended. The restricted flow rate of ship entrance and exit door is: single ship is no more than 2 m/s; The ebb tide velocity of the towing fleet shall not exceed 1.2 m/s, and the flood tide velocity shall not exceed 1.5 m/s.

Fig. 8.

Operation diagram of Babao lock

4.2 Intelligent Control of Lock Safe Operation

In order to ensure the operational safety of Babao lock and the navigation safety of ships, the project establishes a set of intelligent control system for the safe operation of the lock, which as the whole realizes the sharing of hydrological information in the upper and lower reaches of Qiantang River, accurate prediction of tidal bore arrival time, intelligent dispatching of ship crossing and safe control of gate operation.

• 1) Accurate prediction of tidal bore

  According to the historical hydrological data for many years from the anchorage of the Qiantang river estuary to upstream hydrological tide stations, compared with analysis of the measured data at Babao, correlation of the observed data at the Babao lock and. the hydrological stations of the Qiantang River has been established. Through short-term tidal prediction model algorithm, time and height of tides arrived at the Babao lock can be accurately predicted, and GIS (geographic information system) technology is applied to dynamically display the real-time position, high tide and height of tidal bore in real time on the map.

• 2) Intelligent tidal bore warning

  The system realizes the tidal bore warning function. When the tidal range is greater than 1 m and there is tidal bore, the system automatically sends out the tidal bore warning two hours and one hour before the tidal head reaches the gate of the Babao lock, and warns the lock management and control personnel to close the tide gate 0.5 h before. After the prediction and early warning system is associated with the lock control system, it can automatically control the closure of the tidal flat door.

• 3) Intelligent dispatching of ships passing the lock

  Ships that plan to pass through the Babao lock shall apply in advance through the ship lock passing dispatching system platform of the channel network. AIS base stations are set up along the lock and channel, which can monitor and position each ship in real time and release tidal bore dynamic information for ships.The navigation dynamics of ships sailing on the upstream anchorage of Qiantang River and the time of their arrival at the Babao lock are monitored, and the risk of ships crossing is predicted by combining with the real-time dynamic information of tidal bore, so as to carry out intelligent dispatching for safe ship crossing.

• 4) Automatic control of gate operation

  When the tidal range of Qiantang River is greater than 1 m and there is tidal bore, the tidal gate of the Babao lock should be closed 0.5 h in advance, and the opening and closing of the tidal gate should be intelligently linked with tidal bore prediction and warning. When the tidal range is less than 1 m and there is no tidal bore, only miter gate is used. The operation control model and control system of miter gate and tide gate are systematically studied to ensure the efficient and safe operation of lock.

5 Conclusion

• 1) The Babao lock exit is located in the area of strong tide of Qiantang river estuary area, and usually there is tidal bore in Qiantang river when tidal range is greater than 1 m. The average cumulative rate of total range greater than 1 m is 40% for many years, the largest tidal bore tide height can reach 2.5 m, and the maximum tidal bore pressure on the head on the building can reach 90–100 kPa, having a strong impact on the lock gate.

• 2) The head of the lock facing the Qiantang River adopts the combination scheme of “miter gate + tidal gate”, that is, a flat tidal gate is set up on the outside of the miter gate to resist the huge impact of tidal surge. Usually when there is no bore, the tidal gate is placed in the head side door bank, and only the miter gate is used as the working gate. During the period of medium and large tides from June to November, there are 12 days affected by tidal bore on average every month, with 2 tidal bores every day. The tidal gate will be closed about 0.5 h before the arrival of tidal bores, and the tidal gate will be opened 1.5 h after the arrival of tidal bores, and the tidal gate will be used for navigation only twice a day.

• 3) In order to ensure the efficient and safe operation of the lock, accurate prediction of tidal bore arrival time, intelligent warning of tidal bore, and intelligent dispatching of ships, operation control system of double gates of miter gate and tide gate is studied and established, and integrated intelligent dispatching and safe operation control system of navigation lock in river section of strong tidal bore is constructed.