(Xinhua)
Updated: 2008-06-06 19:14

MIANYANG - Engineers hoped new rains at China's Tangjiashan "quake lake" Friday afternoon would help raise the water level to reach the height of a manmade drainage sluice soon, to prevent a dam burst and flooding downstream.

The surface of the lake had risen to 739.7 m as of 5 p.m., only 0.3 m below the sluice, according to the quake lake control headquarters in Mianyang.

A moderate rain started falling over the lake at about 4:20 p.m., said Rao Xiping, head of the Beichuan county station of the Mianyang Municipal Hydrology Bureau, at Tangjiashan.

Rao had expected that drainage would start after 4 p.m. but the weather was fine in the morning and early afternoon at Tangjiashan.

It was unclear how long it would take for the water level to rise to the sluice.

The Sichuan Provincial Meteorology Bureau forecast thundershowers on Friday afternoon and moderate rain at night in Beichuan. The rainfall on Tangjiashan lake was likely to reach 15 mm in the night.

It was expected the lake, which holds more than 220 million cubic meters of water, would start draining as soon as the water reached the lowest point of the drainage sluice.

Rao and 13 other experts on hydrology and communications have stayed at Tangjiashan for more than a week to monitor the situation.

More than 600 armed police and soldiers had left last week after digging a 475-m channel to divert water from the lake, which was inaccessible by road and can only be reached by foot or air.

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 fully opened.

Premier Wen Jiabao flew to Mianyang Thursday afternoon and then boarded a helicopter to Tangjiashan to oversee the drainage operation.

"The lake's condition is even more complicated than expected, but we must remove the hidden threat as soon as possible, because many evacuees are still living in tents," he said.

Engineers have warned that the dam could burst and cause a flood, citing unforeseen factors such as fresh landslides, heavier than expected rain and the instability of the mud and rock dam.

The timing of the drainage operation must be decided by water inflows, Liu Ning, chief engineer of the Ministry of Water Resources, has said.

The catchment area of the upper reaches covered 3,350 sq km. It was estimated that 2 mm of rain in that area would raise the lake level by 1 m.

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 grave danger.

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

Tangjiashan was caused by a blockage of the Jianjiang River, just 3.2 km from the county seat of low-lying Beichuan.

The 8.0-magnitude quake centered in Wenchuan County, about 100 km southwest of Beichuan, 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.