STUDY ON HYDRAULIC PROBLEMS IN NAVIGATIONAL CHANNELS FOR LOW HEAD KEY HYDRAULIC PROJECTS

Wu Xiaoming ¹ and Deng Jiaquan ²

¹ The Scientific Research Institute of the pearl River Water Resources Commission

Ministry of Water Resources, Guangzhou, China. E-mail:xmwu1961@163.net

² The Pearl River Basin Water Resources Protection Bureau, Ministry of

Water Resources, Guangzhou, China. E-mail:dengjqx@hotmail.com

Abstract: The navigational hydraulic problems for a large key hydraulic project – the Liujiang key project in Guangxi province which is currently under pre-design are studied. Through the comparison of different schemes for the entrance and exit layouts of the ship lock, a layout scheme suitable for the project is optimized on a normal scale model which has geometry scale 1/100. Based on the analysis on generic navigational problems in the low head key hydraulic project, some common hydraulic characters are summarized. The study shows that the navigational problem at the entrance is mainly that the lateral flow velocity near the access wall excesses the limit. The methods for solving this problem are adjusting the integral upstream flow field and reducing the dynamic force of the upstream incoming water entering the ship lock. The navigational problem at the exit of the ship lock is mainly that when the discharge is small the lateral velocity created by the water flowing towards the downstream navigation channel excesses the limit. The methods to solve this problem are reasonably controlling the operation of the flood gates and coordinating suitable engineering measures. In addition, this study investigates the re-use of temporary construction diversion buildings and proposes the suggestion for retaining some parts of the temporary cofferdam as the permanent building to reduce the cost of the project.

Keywords: honghua key hydraulic project, navigational hydraulics, scale model experiment, access wall schemes

1    INTRODUCTION

Most of the key hydraulic projects built in the Pearl River Basin are low head projects. Due to different integral layout and various river morphological and flow conditions, it is difficult to design the navigational building according to an uniform standard. Generally, a scale hydraulic model is needed to carry out experiments, through which the navigational building scheme meeting the specific conditions and requirements is optimized. Examples of this kinds of experimental studies include recently built Feilaixia, Danzhu, Baishiyao, Honghua, Penglatan and Binchun key hydraulic projects. Geometry scales of these normal models are from 1/70 to 1/100 and they are used to investigate the navigational hydraulic problems in the ship lock according to the navigational requirement proposed by design departments. Through measuring the velocity and flow pattern the navigational conditions near the entrance and the exit of the ship lock are analyzed and different layout schemes for the entrance and the exit are compared and hence, the best layout scheme is optimized. At its pre-design stage, the large key hydraulic project – Liujiang hydraulic project in Guangxi province is one of the typical example of low head hydraulic project that we studied in recent years. Through this example the navigational problems commonly occurring in low head hydraulic  projects are analyzed and some common hydraulic characters are summarized.

2    GENERAL SITUATION OF THE PROJECT

The Honghua key hydraulic project is the last one in the planed nine cascading projects in the main channel of the Liujiang River. It is located at the Honghua village of the Liujiang county in Guangxi province and the Liuzhou City is 25 km upstream away from it. It is a multi-purpose project but the electricity generation and the navigation are emphasized. Currently, this project is under its pre-design stage. The integral layout of the building is shown in Fig. 1. The power station is located on the right-hand side of the river; the eighteen flood discharge gates (16 m wide for each) is located in the middle and the ship lock is located on the left-hand side of the river. The design flood discharge is 31400 m³/s.

The project is located in the middle of the bend of the river. The position of the project is shown in Fig.2. The river bed is of U-type and its deeper part is close to the left-hand side. The adjacent navigation channel at the downstream links with the deep channel of the river so that it can meet the requirement of the navigable depth when the discharge is small.

In the integral scale model experiment, one of the important tasks is to carry out the navigational hydraulic experiment. Under various navigational flow rates, the reasonability of the designed integral layout is investigated and the navigational buildings of the project are optimized through the experimental study in order to meet the requirements of the navigation.

This model is designed based on the similarity law of the gravitational force. The geometry length scale is 1/100; the velocity scale is 1/10; the discharge scale is 1/100,000; and the roughness scale is 1/2.154. The model scope covers 3 km natural river upstream of the dam and 2.5 km downstream.

3    NAVIGATIONAL REQUIREMENTS AND TEMPORARY UILDINGS

The planed navigational targets are as follows: the navigation class is III; the largest ship passing by is 300t; the length of ship group is 87m. The entrance and exit region is 75m×271.5m(see fig.1). Requirements of navigation are as follows: at the entrance and exit region, the maximum lateral velocities V_T at the mouths of the access channel should be less than 0.25m/s and the maximum longitudinal velocities V_L at the mouths of the access channel should be less than 1.5m/s; the circular velocities V_C should be less than 0.4m/s^[1].

In order to discharge the incoming water of the river and maintain navigation while construction, the construction of the project employs the two-stage diversion method. The diversion layout of the first stage is shown in Fig.3. The middle of the river is the diversion channel; the two sides of it are strong solid longitudinal enclosing weirs. To ensure the construction safety, the longitudinal weirs are constructed in reinforce concrete as temporary buildings which will usually be removed after the project is completed. To removing the weirs under the water is a time-consuming and trouble work. A new subject is to study if the temporary weirs become the permanent buildings so as to reduce the amount of the concrete and the cost of the project.

4    COMPARISON AND ANALYSIS OF SCHEMES FOR THE UPSTREAM BUILDING

The purpose of the experiment for comparing the schemes of the navigational buildings in the upstream of the ship lock is to select reasonable and economic scheme for the navigational buildings which meets the requirements of the navigation at the entrance of the ship lock. The proposed scheme of the buildings by the designing department is indicated in Fig.1. The experiment shows that this designed scheme can not meet the navigation condition at the entrance region; under the maximum navigational discharge Q=13600m³/s, the measured lateral velocity near the head of the access wall reaches 0.75m/s which excesses largely the limit of 0.25m/s.

4.1    Flow pattern experiment and analysis

From the river geometry and the integral layout of the project we can see that there are two disadvantageous factors: the entrance of the ship lock is located on the concave bank of the river bend; and the entrance is close to the flood discharge gate on the right-hand side. To know how these factors influence the navigation, the experiment is carried out under different navigational discharges for measuring the flow fields and flow patterns and hence the navigational problems at the entrance of the ship lock are analyzed. The experiment shows that the flow in the project area is of character as: the upper layers of the water flow towards the concave bank or the entrance region forms an oblique flow near the head of the access wall. Because of the oblique flow the lateral velocity near the head of the access wall can not meet the requirement. Therefore, the key issue to solve the navigational problem at the entrance becomes to reduce the oblique flow velocity at the head of the access wall.

Through the comparison of the flow patterns between three cases i.e, under three characteristic discharges: the maximum navigational discharge Q=13,600m³/s, the minimum navigational discharge Q=8,800m³/s and the maximum controlled navigational discharge Q=15,000m³/s, it is shown that when Q=15,000m³/s although the discharge is large, the influence on the navigation is not so serious as the gate is opened partly and the water level rises, which decreases the upstream velocity. It is also shown that when Q=13,600m³/s, the flow condition is the worst and can not meet the navigational requirement. This indicates that the difficulty for navigation in the area of the ship lock entrance is that the lateral flow velocity near the head of the access wall can not meet the requirement when Q=13,600m³/s.

4.2    Experiment for comparison of the schemes and analysis

In order to meet the navigational requirement and optimize the schemes for the upstream access wall and supplementary measures, the experiment for comparison of the multi-schemes is carried out. Based on the above experimental results of the flow pattern, this experiment is carried out under the maximum navigational discharge Q=13,600m³/s and the emphasis is put on measuring the circular velocity near the heat of the access wall. The positions of the measured points are shown in Fig.4.

l         When we construct the schemes the main considered factors are as follows:

l         Re-using original longitudinal diversion weirs to reduce the cost of the project;

l         Changing the length and direction of the upstream access wall;

l         Partly closing the gate to reduce the velocity in the near field;

l         Adding a separating pier at the head of the access wall to destroy oblique flow;

l         Building double access walls to separate the flow;

l         Building a groyne at upstream to guide the flow.

Table 1 shows the results of the lateral and longitudinal velocities near the head of the access wall for various schemes. It can be seen that there are five schemes, i.e., schemes 5, 8, 12, 13 and 16 (see Table 1 and Fig.5), meeting the condition V_T≤0.25m/s. The mechanism in the reduction of lateral velocities in these schemes is analyzed as below.

Scheme 16, i.e., extending longitudinal weir to position (0-250m) and building a groyne on left-hand bank: The experiment shows that the groyne plays an obvious rule in guiding the flow and reduces the water entering the entrance, and hence reducing the oblique flow at the head of the access wall. However, the disadvantage of this scheme is that the groyne reduces the width of the navigational channel.

Scheme 8, i.e., combining design access wall with part of the longitudinal weir (0-093m~0-170m) to form double access walls, and closing 5 flood gates near ship lock: It employs double access walls. A part of water in entrance region flows out of the lock from the gap between the two walls to reduce the oblique flow strength near the head of the wall. Meanwhile, five flood discharge gates near the ship lock are closed to force the main flow keeping far away from the entrance so as to reduce the flow velocity near the entrance. However, this scheme increases a heavy gate operating work.

The other three schemes have similar form, i.e., adding a suitable length of the upstream access wall. Under the combined effects of the access wall and the upstream morphology on the left-hand bank, the lateral velocity can be reduced. By considering both the experimental results and the cost of the project, the recommended scheme is Scheme 12 which retains the original longitudinal weir and adding its length to position (0-310m).

5    COMPARISON AND ANALYSIS OF SCHEMES FOR OWNSTREAM BUILDINGS

The downstream navigation in the ship lock is closely related with the ways of water release. The experiment considers the following two different ways: (1) the flood gates are totally open and (2) the flood gates are partly open.

Under the condition (1), the experiment shows that for different discharge the velocity and the flow pattern in the exit region of the ship lock meet all requirements based on the designed downstream guiding wall. When the diversion weir is used as the downstream guiding wall the velocity and flow pattern in the exit region still meet all requirements. Therefore, there are no problems for the navigation when the flood gates are totally open.

When the flood gates are partly open, the flow conditions in the navigation channel downstream and the exit region can be adjusted if the they do not meet the requirement. The experiment suggests that the irregular or non-uniform opening of flood gates be avoided and the experiment shows that the worst case for the navigation happens when the right-hand-side gates are partly opened (while the left side 10 gates are closed). In that case, due to the small discharge and low water level, the water flows towards the deep channel on the left-hand side and hence the lateral diffusion of the water from the left-hand side to the right-hand side occurs (see Fig.6). This situation can be improved if retaining part of the longitudinal weir as the extension of the separating wall. The upstream access wall and the downstream navigational guiding wall as well as the separating wall recommended by the experiment are shown in Fig.7.

6    CONCLUSIONS

Through the study of the navigational problems for the typical project general characters for low head key hydraulic projects can be summarized as follows:

The navigational problems for low head hydraulic projects can be considered from the two different operating ways i.e., the way that the flood gates are totally opened and the way that the flood gates are partly opened.

 The problem for the navigation in the upstream entrance region of the ship lock is that the lateral velocity near the head of the access wall does not meet the requirement under the maximum navigational discharge. The problem for the navigation in the downstream exit region of the ship lock is that the lateral velocity near the exit does not meet the requirement while the flood gates are partly opened.

To solve the navigational problem at the upstream of the ship lock, the integral upstream flow field should be adjusted to reduce upstream water entering the entrance region of the ship lock. To solve the navigational problem at the downstream exit region of the ship lock, flood gates should be operated reasonably and suitable engineering measures should be cooperated.

The temporary longitudinal weirs could be retained as access walls of the ship lock to reduce the cost of the project.

References

[1]    Dong Shiyong and Song Weibang. Navigational Buildings. Chinese Hydraulic and Electric Express. 1998. Beijing.

Table 1  Circular flow velocities near the head of access wall under various schemes

         Unit: m/s

Note: (1) “(0-087)” means the cross-section number, i.e., this point is 87m away from dam upstream; 

       (2) “+30m” means the extension of the building to 30m;

       (3) “10°” means the anti-clock angle between the adding building and the original building.

Fig. 1    Layout of the hydraulic project

Fig. 2    Experimental river reach

Fig. 3    Layout of the first stage diversion

 Fig. 4    Positions of measured points near the head of the access wall

Fig. 5    Some layout schemes for the upstream access wall

Fig.6    Flow patter in separating flow

Fig.7    Recommended layout of the project