The lock approach entrance area is the link between the ship lock and the river, and it also acts as the transition zone from quasi-static flow in the approach channel to dynamic flow in the river. The flow conditions in this area directly affect the safety of ships entering and leaving [1
]. For the upstream entrance, the river cross-section shrinks, and for the downstream entrance, the river cross-section widens. Therefore, the water flow in the river is contracted or expanded. Owing to the influences of special boundary conditions, there often exists a strong oblique flow or backflow at the lock approach entrance area. These unfavorable flow patterns pose great risks to navigation safety; therefore, it is necessary to take structural or non-structural measures to improve the navigation conditions. One of the main structural measures is the construction of an auxiliary diversion structure [5
]. Currently, the most commonly used auxiliary diversion structures include navigation wall, diversion piers, and partition dykes.
The navigation wall (i.e., guide wall) directs the water of the river side to flow parallel to it; thus, the water fluctuation in the approach channel can be effectively weakened. This measure has been widely used in engineering practices [9
] such as the Lehmen Lock on the Moselle River and Rehlingen Lock on the Saar River in Germany; Belleville Lock on the Ohio River, Ozark Lock on the Arkansas River, and Columbia Lock on the Ouachita River in the United States [11
]; and Xintan Lock on the Qijiang River [12
], Fengdongzi Lock on the Qujiang River [13
], Xinzheng Lock on the Jialing River [14
], Guigang Lock on Xijiang River [15
], Naji Lock on the You River [16
], and Tugutang Lock on Xiangjiang River in China [17
Diversion piers, which block the flow by the piers and divert the flow through the gaps of the piers, can effectively alleviate the oblique flow, reduce the backflow range and the deflection angle of flow at the entrance area, as well as weaken the intensity of cross-flow and backflow. This measure has been adopted more widely in China [18
], such as for the Yuliang Lock on the You River, Dadingzishan Lock on the Songhua River [19
], Dayuandu Lock [20
], and Zhuzhou Locks and Changsha Locks [21
] on the Xiangjiang River.
Partition dykes are commonly used in ship locks in mountainous areas [22
]. The straight section of mountain river is relatively short; thus, the lock approach is generally arranged in a curved or slightly curved section where the angle between the mainstem of the river and the navigation route tends to be large, and the oblique flow at the lock approach entrance area is relatively strong. The upper portions of the partition dykes are used to block the flow, and the lower portions are used to divert the flow to effectively control the amount of diverging flow into the approach channel entrance area and limit the surface flow, thereby weakening the intensity of the surface oblique flow. This measure has been adopted successfully in various cases, including in the Yinpan Lock on the Wujiang River [23
] and Yakou Lock on the Han River [24
For the ship lock projects in the narrow and rapid river reaches in mountainous areas, there is a great difference in water level between upstream and downstream of hydro-junctions, and the river cross-sections are narrow, resulting in odious flow conditions. Owing to the constraints induced by the complex river regime and boundary conditions, the layout of the lock approach channel is constrained. When combined with the influences of the operating conditions of the constructed sluice gates, power stations, and other structures, the navigation conditions of the ship locks deteriorate, resulting in a low guaranteed rate of navigation that is extremely difficult to improve. However, most of the above-mentioned measures for improving navigation conditions can hardly take effects in the more severe navigable flow conditions under such multi-factorial constraints [25
], and new diversion structures need to be developed.
The Wuqiangxi Lock is a typical mountain lock located in a narrow and fast-flowing river section, which experiences extremely harsh flow conditions. The obstruction in the downstream lock approach channel entrance has always been a bottleneck that restricts the operating efficiency of the Yuan River high-grade inland waterway. In this study, based on the ship lock renovation project of the Wuqiangxi hydro-junction, we propose a new type of diversion structure, the pile-foundation permeable dislocation-type diversion vanes (also referred to as the diversion vanes). Its effects on improving navigation condition were investigated by model tests, and its practical application research on the Wuqiangxi Lock was carried out. The new auxiliary diversion structure has the advantages of effectively suppressing oblique flow, stable structure, and easy maintenance, providing a novel and effective approach to alleviating the problem of oblique flow at the lock-approach entrance area
The effect of the diversion structure on improving the navigation condition of lock approach entrance area was investigated based on the Wuqiangxi Lock, which is a typical ship lock severely affected by strong oblique flow. The following describes in detail the profile of the Wuqiangxi hydro-junction, model design, test conditions, and evaluation methods for performance on improving navigation conditions.
3.1. Project Overview
The Wuqiangxi hydro-junction is located on the mainstem of the Yuan River, which runs through south-central China and offers a combination of benefits such as power generation, flood control, and navigation. Its location and layout are illustrated in Figure 4
. With a reservoir capacity of 4.2 billion m3
and an installed capacity of 1200 MW, it is one of the largest hydropower stations in China, with a capacity of more than one million kilowatts. The hydro-junction is composed of a concrete gravity dam, power station on the right bank, dam spillway, and three-step continuous ship lock on the left bank. The total water level difference that the ship lock must overcome is 60.9 m. The downstream approach channel of the ship lock is connected to the river through the entrance area, and approximately 1 km downstream of it is the Lanziwan Bridge.
The Wuqiangxi Lock was officially opened for operation in February 1995. However, as the hydro-junction is located on a typical river reach in a mountainous area, the river channel is narrow and deep, and the water-level variation is large. The navigable flow conditions of the downstream approach channel entrance area of the ship lock are poor owing to the effects of the river boundary conditions, operating conditions, and the existing project layout. The actual maximum navigable flow rate is considerably lower than the designed maximum navigable flow rate. To change the unfavorable navigation situation, it is necessary to upgrade the Wuqiangxi Lock, and the design of the diversion structure becomes a key problem.
3.2. Model Design and Validation
The physical model (1:100 scale) was designed in accordance with the gravitational similarity criterion. The length of the simulated prototype river channel is approximately 5.2 km, of which approximately 1.2 km is upstream of the dam site, and approximately 4.0 km is downstream of the dam site. The prototype width of the model is 400–900 m. The prototype and model image of the Wuqiangxi hydro-junction are shown in Figure 5
In the experiments, the river flow field was observed using a flow-field real-time measurement system (VDMS, see Figure 6
), which can realize the large-scale synchronous measurement of non-contact surface flow field using a digital camera and particle tracking measurement technology, and the measurement error was less than 5%. Velocities in several key locations were measured by the Nortek Vectrino II profiler, a widely used acoustic doppler current meter with a measurement accuracy of ±0.5% or ±1 mm/s. The water level was measured by an ultrasonic level transmitter with a measurement accuracy of ±0.1 mm and a sampling frequency of 40 Hz.
The physical model was verified using the on-site measured water levels and cross-section flow velocities downstream of the dam under typical flow rates. The water levels in the model were basically consistent with those in the prototype, and the water level error was within ±0.1 m (as shown in Figure 7
), which is relatively small compared with the water depth. The flow velocity distribution in the model section was also consistent with that in the prototype; thus, the physical model could meet the requirements of similar resistance and flow motion.
3.3. Experimental Conditions
The model test experimental conditions are listed in Table 1
, and the water surface curves downstream of the dam at different flow rates are shown in Figure 8
. The flow rate varies from the minimum navigable discharge rate to the maximum discharge rate. As the flow rate increases, the water level gradually rises, and the water surface gradient also gradually increases. The water surface gradients at flow rates of 2500 and 7800 m3
/s are 0.2‰ and 0.47‰, respectively.
Three types of tests were conducted: (a) under original condition (there is a large and deep scour pit near the head of the navigation wall, as shown in Figure 4
); (b) under improved conditions of different diversion structures; and (c) under riverbed-dredging condition and diversion piers. For the last two types of tests, the scour pits were backfilled.
3.4. Evaluation Methods
Usually, the navigable flow conditions in the entrance area of the lock approach channel are characterized by the extreme values of the flow velocity in each direction on the water surface. For the project case used in this study, the designed maximum ship tonnage of the Wuqiangxi Lock is 500 t, and the corresponding lock grade is Class IV. According to the “Navigation Standard of Inland Waterway (GB50139-2014)” [27
] that was promulgated in China, to ensure the navigation safety of ships in and out of the lock, when the ship lock is classified as Class IV, the longitudinal flow velocity parallel to the navigation line, transverse flow velocity perpendicular to the navigation line, and backflow velocity of the water surface should not be greater than 2.0 m/s, 0.3 m/s, and 0.4 m/s, respectively. In the model tests, these values were used as reference values to determine whether the improved navigation conditions meet the requirements and gain significant effects.
4. Results and Discussions
4.1. Flow Conditions under Current Situation
The flow-field distribution under typical flow rate at the entrance area of the approach channel and the velocity extremums of surface flow at different flow rates are shown in Figure 9
Greater flow rates result in greater velocity extremums of surface flow in entrance area and more severe navigable flow conditions. The oblique flow intensity within the approach channel entrance area of the ship lock is extremely high. With the exception of the minimum navigable flow rate of 390 m3/s, the extreme values of the transverse flow velocity in the entrance area under the other flow rates all exceeded the standard limits, and the extreme value of the transverse flow velocity under the maximum navigable flow rate of 7800 m3/s reached 0.75 m/s. When the flow rate Q ≥ 5000 m3/s, the extreme values of the longitudinal and back flow velocities within the approach channel entrance area also exceeded the reference limits.
The poor flow conditions of the current situation are related to the river boundary conditions and complex riverbed morphology (as shown in Figure 4
and Figure 9
). The layout of the riverbed below the dam is “low on the left and high on the right” with a “deep pool on the left and reef on the right”. As a result of the influence from the suction flow of the deep pool on the left side and the convex bank on the right side, there is a strong oblique current at the entrance area of the downstream approach channel that causes the transverse flow velocity and return flow velocity on the surface of the water within the entrance area to substantially exceed the permitted specification values.
4.2. Flow Conditions with Different Diversion Structures
For the three types of auxiliary diversion structures—diversion piers, partition dykes, and diversion vanes—multiple layout schemes were studied through model tests separately by changing the structural parameters except the total length, and the optimal arrangement of each diversion structure was obtained based on the optimal effect in terms of improvement in the navigation flow conditions. The optimal arrangement and surface flow field of different diversion structures are shown in Figure 10
, and the extreme values of surface velocity at the entrance area of the approach channel are shown in Figure 11
The optimal arrangement of diversion piers is as follows: Nine piers with the same length of 20 m were installed, with the space between the first six piers being 10 m and the last three piers being 20 m. In contrast to the current situations, the results of diversion piers scheme show that the oblique flow and backflow intensity within the entrance area of the approach channel were significantly reduced. The extreme values of the longitudinal flow velocity and backflow velocity can meet the specification requirements in the test flow rate range; however, when the flow rate Q ≥ 3500 m3/s, the transverse flow intensity was still relatively high, and the maximum value of the transverse flow velocity was 0.49 m/s.
The optimal arrangement of partition dykes is as follows: Fifteen sections comprising 16 bearing platforms and 15 inserted vanes were installed, with a distance of 20 m between the bearing platforms and a clearance height of 3.5 m below the inserted vanes. In comparison with the diversion piers scheme, the results of the partition dykes show that the extreme values of longitudinal flow velocity and backflow velocity at the entrance area were reduced further. However, the improvement in the extreme values of the transverse flow velocity was unsatisfactory. When the flow rate exceeded 5000 m3/s, the transverse flow velocity increased sharply, with the extreme values of transverse flow velocity under the maximum navigable flow rate reaching 0.81 m/s. Under a high flood flow rate, the oblique flow intensity entering the entrance area of the approach channel was also large, and the diffused flow through the clearance below the inserted vanes into the entrance area led to the formation of an upwelling flow, resulting in poor flow patterns such as vortex and bubble swirls, thereby causing an increase in the transverse flow intensity at the entrance area.
The optimal arrangement of diversion vanes is as follows: Ten sections comprising 11 bearing platforms and 10 inserted vanes were installed, with a distance of 20.8 m between the bearing platforms. The transverse gap, bottom clearance height, and deflection angle of the inserted vanes were 2.5 m, 3.5 m, and 16°, respectively. Although the layout length of the diversion vanes was shorter than that of the diversion piers and partition dykes, they were more effective in improving water flow conditions. Under all the test operating conditions, in addition to the longitudinal flow velocity and backflow velocity extreme values that could meet the specification requirements, the cross-flow intensity at the entrance area was also significantly reduced, and the transverse flow velocity was only slightly in excess of the specification requirements at individual points under the maximum navigable flow rate, with a maximum value of 0.33 m/s. The significant difference in transverse velocity extremums between partition dykes and diversion vanes schemes indicates that the flow into the channel through gaps between the inserted vanes has favorable effects on reducing the transverse velocity. There are two reasons for this: (a) by deflection resulting from the inserted vanes, the incidence angle of the surface oblique flow into the channel is reduced, and then, the longitudinal component of the oblique flow velocity increases, while the transverse component decreases. (b) The oblique flow through the gaps can effectively suppress the upwelling of the flow through the clearance and tends to drive it to flow downstream.
In addition, according to the model test results, the channel transverse flow from the entrance area of the approach channel to the downstream bridge also decreased, and the extreme value of the transverse flow velocity under the maximum navigable flow decreased from 0.61 m/s to 0.52 m/s. Under the diversion vanes scheme, the navigable flow conditions at the entrance area of the ship lock approach channel and its downstream channel were significantly improved.
4.3. Flow Conditions with Riverbed Dredging and Diversion Piers
Dredging of the riverbed was also considered according to the morphological characteristics of the downstream riverbed, that is, the reef explosion scheme. It was found that the navigable flow conditions can be improved to the greatest extent by dredging the riverbed and installing the diversion pies simultaneously, and the riverbed-dredging scope and diversion piers layout are shown in Figure 12
. The dredging scope of the riverbed was 1.5 km long and 80 m wide, and four diversion piers with a length and a gap of 20 m were installed at the entrance area of the approach channel.
shows a comparison of the navigable flow conditions between the optimal riverbed-dredging scheme and optimal diversion vanes scheme. The extreme values of flow velocities in all directions are slightly larger for the riverbed-dredging scheme than for the diversion vanes scheme, but they are still able to meet the overall specification requirements. Riverbed dredging can also improve navigable flow conditions at the entrance area of the approach channel. However, the riverbed-dredging scheme requires extensive blasting, a large amount of underwater construction work, and high investment, whereas the diversion vanes scheme affords the advantages of easy construction and uninterrupted navigation during the construction period. If the two options are compared in terms of cost-effectiveness, the diversion vanes option can save approximately CNY 43 million in investment.
In summary, based on the improvement effect on the navigable flow conditions at the entrance area of the downstream approach channel and cost effectiveness, the developed diversion structure (diversion vanes) is a solution with great potential for improving the navigation conditions of mountain ship locks.
The intensity of oblique flow and backflow at the approach channel entrance area of ship locks on the mountain rapids reaches are usually strong in some cases, and the existing traditional diversion structures are not suitable or inefficient for improving navigation condition. Thus, a new type of diversion structure—the pile foundation permeable dislocation-type diversion vanes structure—was developed. It evolved from diversion piers and partition dykes and combined their advantages. The results of model tests show that the new diversion structure can significantly reduce the velocity extremums of transverse flow and obtain better effects on improving flow condition than other existing diversion structures. The working mechanisms of the new diversion structure were confirmed; that is, the structure integrates the functions of flow blocking and flow diverting, enabling the flow-blocking function of the upper inserted vane, flow-diverting function of the clearance below the inserted vanes, and flow-diverting function of the gaps between the inserted vanes to control the diverging flow rate and the angle of the water flow as well as to weaken the oblique flow in the entrance area downstream, thereby achieving the purpose of improving the flow conditions. In addition to being effective in improving navigation conditions, it is also cost-efficient. Its application in a typical mountain ship lock project—the ship lock renovation project of the Wuqiangxi hydro-junction—is quite successful, which has been confirmed by on-site hydrological observations and real-ship seaworthiness tests. In conclusion, the proposed diversion structure is a promising and effective solution to the navigation obstruction problem caused by fast-flowing waters at the approach channel entrance area of ship locks in narrow mountainous reaches. However, to promote the application of the new diversion structure in engineering practice, further studies are needed on the systematic influence law of diversion vanes structural parameters on the navigation conditions as well as detailed and sound principles for structure design.