Why Is There the Need of a Second Damming of the Three Gorges?

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Last updated at: (Beijing Time) Wednesday, October 30, 2002

Why Is There the Need of a Second Damming of the Three Gorges?

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Three Gorges to Maintain Original Beauty, Splendor

On October 17, the pivotal project acceptance group of the Yangtze Three Gorges Second-phase Project Acceptance Committee under the State Council announced that conditions for damming the diversion channel were available for the Three Gorges Project, approving that the damming of the diversion channel be implemented in November.

At 3:00 p.m. on November 8, 1997, the great river damming of the Yangtze River Three Gorges Project that attracted world attention was successfully closed. Why is a second damming of the diversion channel is necessary this time?

To get an answer to this question, this reporter recently paid a special trip to call on Xu Keda, an expert on the Three Gorges and director of the Technology and Equipment Department under the State Council.

The damming this time aims to make preparations for the third-phase project

The proposal on the construction of the Three Gorges Project was voted through at the Fifth Plenary Session of the Seventh National People's Congress in 1992. Construction of the project will take a total work period of 17 years to complete in three phases.

Three Gorges Downstream Cofferdam Destroyed

The first phase began in 1993 and to be completed in five years, this includes preparatory work for construction, the building of the first-phase earth-stone cofferdam, a horizontal concrete cofferdam and the digging and building of an open diversion channel on the right bank, with the completion of the damming of the great river, which is familiar to the vast number of readers, as the hallmark.

The second-phase project began in 1998, a flood-discharge dam section and a power plant dam section on the left bank built under the protection of the second-phase earth-stone cofferdam after the damming of the great river, with the completion of the preliminary water retention, a permanent ship lock and navigation of the reservoir, the first batch of generating units put into production and power generation as the hallmark, which will take six years.

The third-phase project will start in 2004, a crushed stone-concrete cofferdam will be built under the protection of the third-phase cofferdam, check-dam power generation will go side by side with the construction of a power plant dam on the right bank, it will also take six years, all of this work will be completed in 2009.

Xu Keda said the damming to be carried out in mid-November this year is the damming of the diversion channel built in the first phase, it aims to make preparations for the third phase, hence it is also called the third-phase damming.

Trial Operation of The Permanent Lock in Three Gorges

He called the reporter's attention to the use of such a term, the damming made last time is not called "the damming of the Yangtze", but rather it is called "the damming of the great river", because it was not cut off the entire water surface. The river surface of Sandouping, a site selected for the Three Gorges Dam, is about 1,200 meters wide, there is a Zhongbao Island near the right side, it is about 300 meter wide from Zhongbao Island to the right bank, the first-phase project includes the digging of an open diversion channel on this 300-meter wide river section after the construction of a cofferdam. While the 900-meter-wide main channel of the Yangtze River-the "great river" was dammed, the first-phase cofferdam had already been demolished, so as to allow the split-flow of the Yangtze river water through the open diversion channel. The third-phase damming of the river water will not be cut off, because the second-phase upstream and downstream cofferdams had begun to be torn down respectively in May and July this year, now river water can flow through the bottom holes of 22 diversion channels in the water-discharge dam section. Ships can sail through the temporary ship locks built on the left bank.

The damming this time is more difficult than that of the great river last time

The diversion channel to be dammed this time is the "artificial Yangtze River" dug in the first-phase project. Work involved in the first-phase project includes constructing the upstream and downstream cofferdams, dig this silted river section deep and wide, build a 3,215 meter-long, and 350-meter-wide open channel which is laid up with concrete on both sides and at the bottom. This makes it easier for the split-flow of the river water and for the safe navigation of ships, because with the cofferdams built on the great river, this 300-meter-wide open channel will replace the original 900-meter-wide main channel.

Then, what difficulties are involved in the damming of the open diversion channel? Xu Keda told the reporter Comparison of the difficulties involved in the damming depends mainly on two indexes -"runoff" and "drop". A large water flow and a huge head drop are bound to increase the velocity of flow.

Take the damming runoff first. After all the 22 river-bed diversion holes of the water-discharge dam are open, the river water flowing through them can only reach a little bit over one-fifth. So the designed flow through the open channel will be greatly enhanced, reaching 10300 cubic meters/second, nowadays the largest damming runoff successfully practiced abroad is 8100 cubic meters/second of a Brazilian hydropower plant. The flow was 8480 cubic meters/second when the great river Three Gorges was dammed last time.

Again take the fall of the dammed water head. The drop of the upstream and downstream water level at the designed "long kou or dragon mouth" is 4.11 meters, while that of the Brazilian project is 3.86 meters. The largest drop at the "dragon mouth" was 0.66 meter when the great river was dammed last time.

A two-way damming is adopted this time, this means materials are thrown simultaneously into both sides of the river, the materials down the river should be able to stand firm there. This adds one more "difficulty" to the damming of the diversion channel-the open channel is "artificial", the bottom side is almost on the same level of the concrete baseboard. That is to say, the damming is to be carried out on smooth riverbed this time.

"So, the damming of the open diversion channel is harder than the damming of the great river last time. It is so far the hardest in water conservancy projects at home and abroad," said Xu to the reporter.

In-depth study to ensure absolute safety

To guarantee safety of the open diversion channel, the Three Gorges Project Construction Commission under the State Council has long before started organization and made arrangements, the Three-Gorges Corporation as well as the Yangtze River Water Conservancy Committee, in a rigorous attitude, conducted intensive and meticulous scientific studies on the choice, runoff, drop, the weight and size of thrown down materials, damming and construction technologies, etc.

The Yangtze Academy of Sciences specially made a large damming hydraulic model and conducted hundreds of simulated experiments on damming under various circumstances and drew a scientific conclusion. They had studied lots of technical documents and overcome various difficulties. Besides, they also prefabricated extra large stones and concrete pieces.

"Additionally, with the successful experiences we gained in the Gezhouba damming on the Yangtze River and the damming of the great river, all the Three Gorges project builders and Chinese experts on river damming are full of confidence in damming the open diversion channel," said Xu Keda with a smile.

It is reported that Gezhouba Group Company is responsible for the construction of the upstream cofferdam, and the armed police hydraulic Three Gorges Headquarters is responsible for construction of the downstream cofferdam, both of them had participated in the damming of the great river last time, they are both crack forces in this field.

The time limit for the damming is set for three-five days

At the long-term hydro-meteorology forecasting seminar concerning the damming of the open diversion channel held in mid-October, prediction was made on hydro-meteorology during the period of river damming: the possibility for the occurrence of extraordinary flood is slim.

Xu Keda told the reporter that currently at the construction site on both banks of the Three Gorges diversion channel, everything is ready, once an order is given by the State Council, the Three Gorges builders will certainly work another miracle of river damming.

Related work (1)

Second-phase relocation work set to be completed at the end of the year

The Yangtze Three Gorges Project has thus far relocated 646,000 people from the reservoir area, 140,000 of whom were moved out and settled down elsewhere. The second-phase migration and the clearance work of the reservoir bottom of the Three Gorges Project will be completed by the end of this year.

Thirteen fully- and partially-submerged cities and county towns in the reservoir area of the Three Gorges Project have been basically rebuilt, 13 completely new cities have sprung up in the reservoir region, their basic functions, including land occupancy, transportation, communication, power and water supplies, all surpass the original levels.

Statistics show that a total of 31.22 million square meters of houses, 819.9 km of highways, 11,170 meters of large and medium-sized bridges, 44 ports and harbors, 1.61 million meters of transmission and transformer electric lines have been rebuilt in the reservoir region, bringing epoch-making change in the outlook of the reservoir region.

With a view to improving the ecological environment and raising people's living standards in the reservoir area, the State Council had made two major adjustments of the migration policies for the Three Gorges reservoir region in 1999. These included increasing the number of rural people from the reservoir area to be moved out and settled elsewhere; intensifying the structural adjustment of enterprises to be moved out from the reservoir area. By the end of September 1, 140,000 rural people from the reservoir area had been moved out and settled elsewhere, 70,000 of them were organized directly by the State, they were migrated to 11 coastal provinces (municipality) such as Shanghai, and Jiangsu, Zhejiang, Shandong, Fujian, Guangdong, Hunan, Hubei, Jiangxi, Anhui and Sichuan provinces, another 45,000 people were organized by Hubei Province and Chongqing Municipality to move out to 24 provinces and cities nationwide. At the same time, structural adjustment was completed for 1,011 enterprises to be moved out.

Related work (2)

The work of clearing the bottom of the second-phase water-storage reservoir proceeds smoothly.

The scope of the work for clearing the bottom of the second-phase water-contention reservoir is the 135-meter water level in front of the dam linked to the reservoir bottom under the backwater line (also called the second-phased migration line) of flood that happens once every 20 years. The contents of clearing include hygiene, solid waste, building and wood clearing. The task of clearing the bottom of the second-phase reservoir bottom is quite arduous, it covers 12 prefectures, counties and seven county seats as well as many other towns and other towns, enterprises and institutions.

The clearing of the second-phase reservoir bottom involves many points and wide areas and large scopes, high technical standard and requirement are set for the work. What's more, the reservoir region is an area of high incidence of infectious diseases, endemic diseases, as well as enterological nematode , malaria and filariasis, so effective hygienic clearance must be carried out. The rat eradication work and the epidemic prevention in the burial ground of numerous men and livestock over a large area must not be neglected. The clearing of solid wastes involves not only a huge amount of work, but also great difficulties, thus increasing the arduousness in clearing reservoir bottom. According to construction arrangements, self-checking of the work of clearing prefectural and county reservoir bottoms should be completed at the end of January 2003, provincial and municipal preliminary checking in mid-March 2003, and final checking at the end of April. At present, the clearing work is progressing without a hitch.

By People's Daily Online

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