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UPDATED: August 2, 2010 NO. 31 AUGUST 5, 2010
Project to Protect
The Three Gorges Dam holds up two flood flow peaks in less than two weeks

FLOODING OUT: Water sluices at the Three Gorges Dam, Hubei Province, on July 28, when flood peak with a water flux of 56,000 cubic meters per second reached there (CHENG MIN)

China's Three Gorges Dam on the swollen Yangtze River stood its second test this year on July 28, holding back flood flows peaking at 56,000 cubic meters per second, the second greatest this year.

The peak arrived at the dam at 8 a.m., said engineers with the world's largest hydroelectric project in Yichang of central China's Hubei Province. The dam buffered the flood by discharging water at a rate of 40,000 cubic meters per second, holding up 16,000 cubic meters in a second.

The water level of the reservoir behind the dam rose to 158 meters at the same time, about 17 meters under its maximum capacity of 175 meters, they said.

On July 20, the dam held up against its first major flood-control test, Chen Fei, General Manager of the China Three Gorges Corp., told Xinhua News Agency.

The flow on the Yangtze's upper reaches topped 70,000 cubic meters a second on that day, the highest level since the dam was completed and 20,000 cubic meters more than the flow during the 1998 floods that killed 4,150 people.

"The peak flow is high, but it has not exceeded the designed capacity of 100,000 cubic meters of water per second," said Cao Guangjing, Chairman of the China Three Gorges Corp.

"The dam can withstand the challenge easily," he said. It is designed with 23 discharge holes and only nine opened to embrace the flood peak.

Withstanding tests

Heavy rains had increased water flow at both the mainstream and tributaries of the upper reaches of the Yangtze River, with inflow from the Minjiang, Tuojiang and Jialing rivers rising fast, said Wang Jun, Director of the Hydrology Bureau of the Yangtze River Water Resources Commission.

The Yangtze River Flood Control and Drought Relief Headquarters had been closely monitoring water in the river and adjusting the storage of the Three Gorges reservoir to help relieve mounting flood pressure along the middle and lower reaches of the Yangtze River, headquarters director Wei Shanzhong said.

On July 19, all ferry services were halted at the dam, which had been used exclusively by reservoir construction and maintenance vehicles, was temporarily opened to other vehicles so passengers and cargo could be transported.

Ferries near the Gezhouba Dam, on the lower reaches of the Three Gorges, were still operating as the flow was 40,000 cubic meters a second, below its designed capacity of 60,000 cubic meters per second.

On July 22, vessels resumed passing through the Three Gorges Dam, after it was closed for three days, the Three Gorges Navigation Administration said.

Ship locks on the dam were reopened as water flows fell to about 45,000 cubic meters per second.

On July 23, the continuous deluge pushed the water level of the reservoir behind the dam to its peak this year—158.86 meters, about 13.86 meters above the reservoir's water-releasing level, and the water volume stored reached 7 billion cubic meters, said engineers of the dam.

The Three Gorges Dam can store a maximum of 22.1 billion cubic meters of floodwater.

As of 8 a.m. on July 25, the water level fell to 157.84 meters, according to data from the State Flood Control and Drought Relief Headquarters.

The water level was still 12.84 meters above the alarm level and was expected to increase again due to rainfall and water inflows from tributaries in the upper stream.

On the evening of that day, the Yangtze River Flood Control and Drought Relief Headquarters increased the water discharge volume from 34,000 to 40,000 cubic meters per second to lower the water level in the dam in anticipation of the coming flood.

On the morning of July 26, the water flow rate at the Three Gorges Dam hit 40,500 cubic meters per second after a three-day lull. Vessels' transit through the dam was suspended for the second time in July as engineers braced for the second floodwaters of the year to hit.

At 8 a.m. on July 28, the flow peaked at 56,000 cubic meters per second.

Flood capacity

The flood control capacity of the Three Gorges Dam is designed to be limited, said Zhao Yunfei, Deputy Director of the China Three Gorges Corp.'s Cascade Dispatch Center, on July 22 amid mounting concerns over the project's ability to manage floods.

The multi-billion-dollar dam was designed to withstand floods with water flow of 83,700 cubic meters per second and mainly to protect the Jingjiang River area and the Chenglingji region in Hubei and Hunan provinces, Zhao said.

"The dam's flood-control capacity is not unlimited. It has a capacity of 22.1 billion cubic meters and protects, by design, a limited area," he said.

Zhao's explanation came amid growing concern over the controversial dam, which officials hope will play a pillar role in flood control and clean power generation.

In the past few days, the public have been hotly debating online, posting four reports on the dam's flood-control capacity.

The first, released in June 2003, claimed that the dam "could fend off the worst flood in 10,000 years." The second, four years newer, changed that number to "the worst flood in 1,000 years." In October 2008, the number was again modified to "the worst flood in 100 years."

On July 20, a report on state broadcaster China Central Television's website, titled The Three Gorges Dam's Capacity to Store Floodwater Is Limited, urged the public not to "lay all hopes on the dam."

Cao, who has been engineer with the corporation since 1985, said the dam's flood-control facilities are complete and need no adjustment.

"The peak flow this time was historical. But the frequency, peak period and volume of water were all comparatively limited—the worst in about 20 years. The dam is far from displaying its full potential," he said.

China had spent 181.5 billion yuan ($26.5 billion) on the dam project by the end of 2008. But critics claim that the project has caused, or will cause, consistent landslides and severe erosion downstream.

Facts and Numbers

70,000 cubic meters

The Three Gorges Dam held back flood flow peaking at 70,000 cubic meters a second on July 20, the highest level since the dam was completed

158 meters

At present, the water level of the reservoir behind the dam rises to around 158 meters, compared to its maximum capacity of 175 meters

7 billion cubic meters

The water volume stored in the Three Gorges Reservoir reached 7 billion cubic meters on July 23 while the reservoir has a storage capacity of 22.1 billion cubic meters

9 discharge holes

The Three Gorges Dam is designed with 23 discharge holes with only nine needed to open to embrace the flood flow

(Source: Xinhua News Agency)

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