NEARING COMPLETION: China ’s Three Gorges Project
By Dr. AB Thapa
It is reported that China has completed the construction of the world's largest dam on May 20, 2006 , in Three Gorges area, central China 's Hubei Province , signifying accomplishment of the major structure of the mammoth Three Gorges Project ( TGP). The 185-meter-high and 2,309-meter-long Dam is the world's largest dam of reinforced concrete, with a total volume of 28 million cubic meters of concrete. The concrete placement of the Dam's main section was completed 10 months ahead of the schedule, which will enable the Dam to start its role in power generation, flood control and shipping improvement in 2008.
Sediment control was one of the major technical problems of the Three Gorges Project to be tackled. Comprehensive research methods, such as prototype observation, mathematical model computation, physical model tests, and analogue analysis on existing projects have been adopted for TGP's sediment study. These studies could be very useful for planning our Sun-Kosi Dam project characterized by a very big annual runoff by comparison with storage volume.
A Dream For Chinese People
Mr. Lu Youmei, President of China’s Three Gorges Project Development Corporation writes that the Three Gorges Project has been a dream for the Chinese people for more than 70 years as it is a key part of the scheme for harnessing and development of the Yangtze River. Chinese people’s relentless efforts over a very long period to harness the Yangtze River could serve us as a reminder that we too should take very seriously the development of our water resources since the projects to build dams on the Karnali or the Kosi rivers are almost comparable to the Three Gorges Project in magnitude and also in complexity.
The Yangtze River
The Chang Jiang or Yangtze is the largest river in China . It carries nearly 40% of the country’s annual runoff. It is also the third largest river in the world. At the dam site, the Yangtze brings an annual runoff of 438,000 million cubic meters, and an annual sediment discharge of 526 million cubic meters. The Three Gorges Project is located at Sandoupin in Hubei Province 1300 km from Shanghai and 660 km downstream from Chongqing.
The 660 km long Jingjiang Section in the middle reach of the Yangtze has been under serious threat of flooding. The dykes along this stretch of the river protect more than 15 million people, 1.56 million hectares of farmland and many industrial sites. At present these dykes breach whenever there is a flood greater than the one occurring once in ten years. Such floods result in a major disaster. After the completion of the Three Gorges Project, the flood control capacity of the Jingjiang Section could be raised from 10-year to once in 100-year flood.
The Three Gorges Project consists of a 185 m high concrete dam, two power plants, and a system of navigation facilities. The spillway dam section is located on the middle of the original main river channel, flanked by the power plant and non-spillway dam section, one on either side. The two power stations are at the toes of their respective dam sections. At the right bank there is room for an additional underground power plant to house additional generation units when the development is expanded later. Located on the left bank are the permanent navigation structures.
The spillway section of the dam has a total length of 483m. To facilitate sediment flushing and discharging high floods, there are 23 deep outlets and 22 surface bays. Each of the deep outlets has a dimension of 7m by 9m. The surface bays have a net width of 8m. At the downstream end of this section, a trajectory bucket is provided as an energy dissipater. The maximum flood-releasing capacity of the project is 113000 cubic meters/s.
Upon completion, the Three Gorges Project would become the largest hydropower station in the world, with a total installed capacity of 18,200MW and annual energy output of 84710 GWh from 26 sets of 700 MW generating units. The power generated would be fed to central and eastern China . Inland navigation is a very important component of the Three Gorges Project.
Gezhouba Reregulating Dam
The Gezhouba project is a component part of the Three Gorges Project. Its reservoir is to function as a re-regulating pool and a downstream navigation step for the Three Gorges Project. China decided to construct the Gezhouba dam before the Three Gorges Project for a number of reasons: to improve the navigation conditions of the gorge stretch; to produce hydroelectric power to meet the urgent demand in the area.
Inland Waterway
Inland waterways were very much developed in China from the time immemorial. Chinese had even built impressive canals between 3 rd century BC and the Ist century AD. Outstanding were the 90 miles long canal from the Han capital; Ch’ang-an to the Huang Ho; and the Pien Canal in Honan. Of later canals the most spectacular was the Grand Canal, the first 600-mile section of which was opened to navigation in 610. This waterway enabled grain to be transported from the lower Yangtze and Huai to K’ai-feng and Lo-yang. China’s past and recent experiences of developing inland navigation by using natural as well as artificial channels could be of great interest to Nepal as well India since both the countries, Nepal and India, have decided to carry out jointly a detailed study of the Kosi navigation canal linking Chatra in Nepal with the Ganges waterway.
In recent years the Yangtze river is extensively used for water transportation. The capacity of the Yangtze river waterway constitutes 78% of the China’s total inland rivers transportation capacity. Despite such extensive uses, the navigation condition of its natural course could not fully meet the growing requirements of China’s fast growing economy. There were bottlenecks from Chongqin in Sichuan province to Yichang in Hubei province. Here, the river flows through an area of high mountains and deep valleys, with considerable turbulence and dangerous shoals, which had limited the development of the Yangtze waterway. Upon completion of the Three Gorges Project, the reservoir backwater would reach Chonqin and the annual transportation capacity for freight would be increased to 50 million tons, with costs reduced by 35-37 percent. For half of each year when the reservoir water level is high, 10000 ton ships would be able to sail right up to the Jiulongpo port of Chongqing. During dry seasons, the navigation conditions of the river downstream of the dam will also be improved, due to an increase in the low water flow.
Permanent double-line locks would be provided as the main navigation facilities after the project is commissioned. The locks would meet the requirements of a single direction downstream freight volume of 50 million ton, the projected volume for the year 2030, and they would allow passage of 10000 ton ship fleets. A vertical ship lift is also provided as an express passage for passenger traffic and also for certain specific types of ship.
Permanent Navigation Locks
The permanent locks comprising large-scale, multi-step structures would be provided, with the highest total head of any lock now in operation or under construction anywhere in the world. The effective lock chamber dimensions of 280m x 34m x 5m have been adopted. The minimum dimensions of the approach channels are controlled as follows: length of straight approach line entrance 930m, radius of bends 1000m, bottom width 180m, minimum upstream water depth 6,0m and minimum downstream water depth 5.5m. The maximum allowable river flow for navigation is 56700 cu.m/s, with allowable maximum water velocity at the mouth of 2.0m/s (longitudinal) and 0.3m/s (transverse).
The double-line five-step flight locks are situated on the left bank. The length of the main navigation structures is 1607m, with an upstream approach channel 2113m long, downstream approach channel 2722m, and a total lockage route length of 6442m. Mitre gates are used to operate the lock. The height of the mitre gates at lock heads 1 and 2 is 37m and at lock heads 3 – 6 it is 39.75m. During opening/closing, the maximum submerged depth of the gate at the first lock head is 35m. At the first lock head, the filling valve is of the normal tainter type and at lock heads 2 -6, all are reversed tainter gates.
The method of operating the double-line lock under normal conditions is that one line carries up bound traffic while the other down bound traffic. During the periods when one lock is shut down for repairs, the other line of locks passes traffic in one direction, changing over to pass the traffic in the opposite direction at specific intervals. The filling or emptying time for the Three Gorges Project lock chamber would be 12 – 13 min. The time interval( or cycle time) for successive groups of ships to pass through would approximately be one hour. Annual one-way capacity would amount to 51.52 million tons. Transit time for a group of ships to pass through the five flight ship locks would be 2.35 hours and the transit time for a group of ships, from entering the mouth of an approach channel to leaving the mouth of another approach channel is expected to be 3.14 hours. Annual water consumption, excluding flood season, would amount to 1.7 billion cubic meters.
Vertical Ship Lift Layout
The ship lift of the Three Gorges Project is a counterbalanced vertical ship lift with steel cable hoists, with effective dimensions of 120m x 18m x 3.5m, the same as those of the Gezhouba No.3 lock. Temporary ship lock has been provided to continue navigation even after the Yangtze river course is blocked by coffer dam. The effective dimensions of the lock chamber are 240m x 24m x 4m.
Operating Water Levels
During October, at the end of the wet season, the reservoir will be filled to Normal Pool Level. This is the highest normal operating level of the reservoir. Water levels will be maintained at Normal Pool Level until stored water is needed during critical dry periods to increase outflows for navigation and power.
During the winter season, the power plant will operate at full head when at the Normal Pool Level but with reduced output depending on the dry season inflows. Daily peaking operations will be possible with reregulation in the Gezhouba reservoir to ensure steady flows downstream.
A Lesson To Learn
The volume of the Three Gorges Project storage reservoir is too small to handle the enormous volume of the Yangtze river sediments. It would not take long time to fill up the reservoir. China has made considerable headway with the studies to find ways to preserve the live storage volume of the reservoirs. Their findings are reported in the Journal of the Hydraulic Research published by the American Society of Civil Engineers of the USA. It is being said that the unique mode of the Three Gorges reservoir operation, in particular the low pool level release of silt-laden normal flood under possibly the largest water surface gradient allows most (85%) of the live storage between minimum operating level and the full supply level be preserved for permanent use. In Nepal we could expect to encounter a similar problem in planning the Sunkosi-1 high dam despite the fact that it is a far smaller river by comparison with the Yangtze river. Needless to say that in future the information on the Three Gorges Project reservoir operation would be very valuable to us in planning the Sunkosi-1 or any other similar projects.
(Dr. Thapa writes on water resources)
