The possibility of flooding from the Tangjiashan "quake lake", caused by China's May 12 earthquake, increased on Thursday even as water levels rose steadily to the point where engineers believe they may be able to open a drainage sluice.

The water level at the Tangjiashan lake in Beichuan County, Mianyang City, southwestern Sichuan Province had risen to 738.71 meters by 2:00 PM on Thursday, still 1.29 meters below the drainage sluice, according to the lake control headquarters.

Another 10,441 residents in a low-lying area about 50 km from the lake were evacuated on Thursday afternoon, and a traffic ban was enforced in downstream areas. More than 250,000 people have been relocated.

Emergency workers were also searching for residents who were missed in the evacuation.

Premier Wen Jiabao arrived in Mianyang on Thursday afternoon to oversee drainage of the Tangjiashan lake by helicopter.

"Now is a critical moment for the Tangjiashan quake lake, and the most important thing is to ensure there are no casualties," Wen said.

Emergency teams had been hoping to drain the lake on Thursday, but unforeseen factors, such as fresh landslides and higher than expected rainfall, had added to the risk of the lake breaching its banks and flooding downstream areas, said a spokesman with lake control headquarters.

The mud and rock dam -- caused by a landslide on the Jianjiang River -- was also in danger of collapsing under the pressure of the mounting volume of water behind it, and seepage was already occurring, said the spokesman.

More than 600 armed police and soldiers dug a 475-meter channel to divert water from the lake.

It was expected that the lake, which holds more than 200 million cubic meters of water, would start draining as soon as the water level reached the lowest point of the dam.

But a headquarters spokesman warned late on Wednesday that the chances of bursting were increasing due to uncertainties, including aftershocks, rain on the upper reaches and the instability of the mud and rock dam.

"Water has been seeping out of the dam body," he said.

A massive landslide could occur anytime on the upper reaches, which could bring about 20 million cubic meters of mud and rock into the lake, triggering huge waves.

"We must prepare for dealing with the worst scenario, but strive for the best results," said Chen Lei, Minister of Water Resources, at a meeting on Wednesday in Sichuan.

He said managed drainage was necessary to "ensure the safety of people downstream".

It was not clear when the draining would exactly start.

More than 250,000 people in Mianyang have been relocated under a plan based on the event that a third of the lake water breaches 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 fully opens.

The timing of the draining must be decided by water inflow, said ministry chief engineer Liu Ning.

The catchment area of the upper reaches covers 3,350 sq km. It is estimated that 2 millimeters of rain in that area would raise the lake level by 1 meter.

Sichuan Provincial Meteorology Bureau has forecast thundershowers in Beichuan on Thursday night and rain on Friday, with rainfall to peak on Friday night at 20 mm.

According to weather records, average rainfall in the Tangjiashan area in June stands at 140 mm, in July at 360 mm and in August at 330 mm. Rainfall of such levels would pose a grave danger.

The May 12 quake triggered massive landslides in Sichuan, blocking the flow of rivers and creating more than 30 unstable "quake-formed lakes" that threaten millions of people downstream.

The 8.0-magnitude quake centered on Wenchuan County, about 100 km southwest of Beichuan, has left more than 69,000 people dead, about 18,000 missing and millions homeless. More than 10,000 aftershocks have been reported since May 12.

(Xinhua News Agency June 6, 2008)