A milestone for the Three Gorges8 October 1998
The condition of the Yangtze river has been of life-threatening importance this year, as it has been the source of massive flooding. The Three Gorges dam project is intended, among other things, to reduce the danger from flooding: Rushu Wang* offers an update on the project
The river diversion is the key point in the construction of any water resources project. At the Three Gorges Project (TGP) on China’s Yangtze river, the river diversion is being accomplished in phases.
The total duration of the project construction is planned at around 17 years, taking account of the time required for preparation. The river diversion is divided into three stages, which are scheduled as follows:
•Preparation period and stage I construction — five years (1993-1997).
•Stage II construction — six years (1998-2003).
•Stage III construction — six years (2004-2009).
In the first stage, now completed, the sub-river on the right side of the islet was closed, the open diversion channel was completed and the longitudinal cofferdam (roller compacted concrete — RCC) was constructed in the pit. The temporary shiplock was finished on the left bank of the river.
During this period, river flow and navigation continued to pass along the original main river course.
In the second stage, construction will be started. The stage II transverse cofferdam will be built up on both the upstream and downstream sides. Together with the completed RCC longitudinal cofferdam, they will form the stage II construction pit. In addition the spillway, the intake dam and the power plant on the left bank will be undergoing construction. Simultaneously, the permanent shiplock and shiplift will be constructed on the left bank.
During this stage river flow will be diverted through the open channel; barge fleets and passenger vessels will be able to pass through either the open channel or the temporary shiplock.
In the third stage, the upstream stage III RCC cofferdam will be built to cut off the open channel and raise the reservoir to 135m in elevation. The left bank power station and permanent shiplock will be put into operation in this phase, as will the stage III construction pit which will make it possible to build the right bank dam and power plant.
During this period, the river flow will be discharged through the permanent bottom outlets and the temporary diversion outlets in the spillway, while boats will pass through the permanent shiplock.
Closure of the Yangtze
The closure of the Yangtze was completed on 8 November 1997.
This milestone marks the successful completion of the stage I works and the beginning of the stage II works. Achieving this milestone was no small task: the design closure discharge is 14000-19400m3/s and the maximum water depth reaches 60m.
The closure dike is located on the downstream side of the stage II upstream cofferdam. Work on the dike began on October 1996, after that year’s flood season, and the gap in the upstream dike had reached 460m by the time of the flood season in 1997. Closure of the gap began on 26 October. The final closure began at 9:00am on 8 November 1998 and was completed at 3:30pm in the afternoon.
The phased closure time was based on a reliable short-term hydrological forecast, the navigation of the Yangtze river and the sediment scouring in the open diversion channel on the right bank.
The stage II cofferdam
The stage II earth-rock cofferdam is located on the left bank of the main river bed. This cofferdam is the most important temporary water-retaining structure of the TGP and the key item in ensuring the safe construction of the main stage II works in the dry pit, and hence the power generation from the first units.
The design standard of the upstream cofferdam is a 100-year flood, with a discharge of 83,700m3/s. The maximum allowed is a once in 200-year flood with discharge of 88,400m3/s. The total length of the upstream and downstream transverse cofferdam is 2438m in axis. It has an earth and rock fill of 10.32M m3 and impervious area of 92,200m2, of which the concrete cut-off wall is 83,450m2. Its maximum height is 74m. As regards the stage II cofferdam, there is a large amount of difficult work to be undertaken during a short timescale. The main technical characteristics of the cofferdam are given in the Table below.
According to the construction schedule for the stage II cofferdam, the single cut-off wall was formed in April and finished in July 1998. The cofferdam was filled to flood control elevation before the flood season in June, and it was also in that month that limited pumping from the pit began. At the time of writing (July) three peak floods in the early part of the month have been met successfully. Preparations are being made for higher floods.