The water level of the Tangjiashan "quake lake" in southwest China was continuing to rise dangerously on Sunday despite the operation of a manmade drainage channel since Saturday morning.
The drainage plus natural leakage of the lake was about 31 cubic meters per second, while the inflow was 3.7 times more than that at 3:15 p.m., but its impact on the lake dam is not obvious, according to an official with the Tangjiashan quake lake relief headquarters.
The water level in the lake stood at 742.13 meters above sea level as of 9 p.m. on Sunday, still 1.76 meters higher than the sluice, and the lake's volume was 242.4 million cubic meters.
The water level in the lake was expected to reach 742.55 meters and the lake's volume would reach 242.5 million cubic meters at 8 a.m. on Monday, according to an expert with the Tangjiashan quake lake relief headquarters.
"Controlling the lake outflow is critical for the dam's safety, " Zhang. "If the water flows out too slowly, the accumulating inflow will increase pressure on the barrier, but too strong an outflow will also erode the diversion channel and lead to the dam collapse."
A moderate rainfall hit the lake at 6:50 p.m., and a minute later a strong aftershock rocked the dam for about 20 seconds and caused massive landslides on surrounding mountains, a Xinhua reporter at the dam said.
The 4.8-magnitude aftershock jolted Beichuan County at 6:51 p.m., according to the China Earthquake Administration.
The aftershock's impact on the dam is being monitored.
The sluice appeared to be still operating smoothly after the aftershock and rain, the reporter said.
In addition, military engineers have fired missiles to blast boulders in the channel to accelerate drainage.
The water flow through the sluice channel has widened from less than five meters to about 10 meters.
"Generally speaking, construction of the lake's drainage projects is going well, but the lake remains dangerous for hundreds of thousands of people downstream," said General Ge Zhenfeng, Deputy Chief of the General Staff of the People's Liberation Army (PLA), who is supervising the project.
"It will take us a few days to eliminate the potential danger of the lake," said the general.
Some 600 armed police and soldiers worked for six days and nights to dig a 475-meter channel to divert water from the lake.
Soldiers are still widening and deepening the sluice channel with the help of 30 bulldozers and excavators. They are also digging a second sluice channel on another side of the lake barrier.
The soldiers had finished building a third of the new channel, which required the removal of about 60,000 cubic meters of earth and stone, said Liu Yongjian, a PLA officer in charge of the channel.
"We have also prepared underwater blasts to deepen the channels for accelerated drainage," said Liu.
Rao Xiping, head of the Beichuan hydro-meteorological station, said the lake dam remained stable as the drainage continued.
"We have found no obvious expansion of the sluice holes nor fissures in the dam. There is no sign of dam collapse either," said Rao. The staff of his station and soldiers are keeping 24-hour patrol along the dam.
Water Resources Minister Chen Lei warned that increasing rainfall, aftershocks, landslides and leakage were still threatening the barrier.
Rainfall of 400 mm or 500 mm, well above normal levels, was forecast upstream in June and July, posing a risk for the swollen lake, he said.
Landslides could pour another 17 million cubic meters of rock and earth into the lake, threatening a dam burst and workers on the barrier could be swept away, he said.
According to the lake relief headquarters, the lake area had seen nine aftershocks measuring two to 3.2 magnitude in the 24 hours since 1 p.m. on Saturday. However, the possibility of a strong aftershock measuring six or above on the Richter scale directly endangering the dam was slim.
Geologists with the Chinese Academy of Sciences arrived at the dam on Sunday morning by helicopter to conduct a geological study.
"We plan to take dam samples with drills from eight spots on the dam for a long-term study," said Pu Zhiyong, an expert.
He said experts had set up monitoring systems, like cameras, on the two sides of the dam.
Military forces finished building a second helicopter landing site on Sunday on flat ground halfway up a 200-meter hill next to the Tangjiashan dam. The first airfield was located on the dam.
About the size of two basketball courts, the new airfield can accommodate both Mi-171 and Mi-26 helicopters to take off and land, according to the officer in charge of the construction team.
If the dam was in danger of bursting, the airfield would enable helicopters to evacuate personnel, he said.
The Tangjiashan "quake lake" was formed after a massive quake-triggered landslide from Tangjiashan Mountain blocked the Tongkou River, which ran through the Beichuan County, one of the worst-hit areas in the quake.
The swollen lake is the largest of more than 30 quake lakes in Sichuan following the May 12 quake, posing a threat to 1.3 million people downstream.
More than 250,000 people in low-lying areas in Mianyang have been relocated under a plan based on the assumption that a third of the lake volume breached its banks.
Two other plans require the relocation of 1.2 million people if half the lake volume is released or 1.3 million if the barrier is fully opened.
The drainage has not affected drinking water sources for the 1.3 million residents downstream, but the Mianyang municipal government has dispatched fire engines to take drinking water to the quake-affected towns and villages. Samples taken from four monitoring stations showed the water was potable,the municipal water environment monitoring center said on Sunday.
"It may take another day for the the drainage water to flow into the Fujiang River, the major water source of Mianyang," said an official with the center.
"It's too early to determine how serious the water sources of Mianyang will be affected by the drained lake water, but we will keep a close eye on the situation," he said.
China's longest oil pipeline, winding from Lanzhou through Chengdu to Chongqing, is 60 kilometers downstream from the lake.
Liu Xiaozhong, director of pipeline protection, said, "The pipeline is still safe. We have not found a significant changes of water flow here."
As of 2 p.m. on Sunday, more than 200 Petro China staff and 30 items of large equipment were assembled to protecting the pipeline. Soldiers had begun to prepare building a floating bridge on the river near the pipeline to transfer heavy machinery.
With a capacity of 6 million tonnes of oil each year, the pipeline provides Sichuan and Chongqing Municipality with 70 percent of their oil.
If the line was cut, refined oil in storage could only supply Sichuan for three days, whereas repair work would take 30 days.
The May 12 quake had killed 69,136 people nationwide as of Sunday noon, the State Council Information Office said.
A total of 374,061 people were injured and 17,686 others remained missing after the 8.0-magnitude quake that jolted southwestern Sichuan Province and neighboring regions.
(Xinhua News Agency June 9, 2008)