The exercise to drain the likely to burst Tangjiashan lake – a landslide-dammed lake formed after the 8-magnitude earthquake in Sichuan – has entered its final countdown as some 200,000 living down streams were evacuated on May 30.   

As of the night of May 30, the hydro-power paramilitary team in Tangjiashan said the digging of draining channels to mitigate avalanche of flood had basically been completed, and personnel working at the site would also evacuate in stages.

The team person in-charged Yuan Xi was quoted in a report, released on the website of Sichuan Water Resources Administration (SWRA) – the agency entrusted to overseeing the mitigation work – as saying that preparatory work to drain and divert the flow of the lake to prevent its bank from bursting was "a success".

Between 6am on May 31 and 12pm on June 1, all personnel except a few ground observers working on risk mitigation at the Tangjiashan site would leave the scene, mainly on foot and some air lifted. There were over 600 workers and some 1,000 soldiers on duty.

The workers had already started retreating and digging concurrently. Explosives were also planted at a strategic location to create a small opening when detonated, and let the water pressure to burst the earth and rocks around the mouth of the draining channels.

Tangjiashan is one of the 34 barrier lakes formed after the May 12 quake, it is also classified as the most dangerous. Its water level had been rising over one meter per day on average, and as of Saturday (May 31) morning the water level stood at 733.42 meters, contained 184 million cubic of water.

Though having the capacity like a large-size dam, the Tangjiashan barrier lake was unlike any man-made dam with evenly high and strong banks.

Instead, the measurement between the highest and lowest point of the banks was a drop of 41 meters, posing huge risk of water overflowing and bursting its bank, especially when coupled with days of heavy rains, according to expert analysis posted on SWRA relief watch website. 

The nearest settlement to the Tangjiashan barrier lake was reportedly Beichuan's Tongkou town, all its 1,858 residents had been evacuated to safer ground, and 6,800 residents in the nearby Jiangyou's Qinglian town had been moved to higher ground.

Meanwhile, some 200,000 people in further downstream in Mianyang city had also been moved to higher ground on Friday, according to various state media reports.

The Friday's evacuation exercise was according to the "one-third collapsed" contingency formula. The authorities had come up with three contingency plans based on the degree of bursting and the possible route of flows as a result, and thus the affected areas and number of people. The formulas included one-third, half and total collapsed, and the number of evacuation would be 158,600, 1.2 million and 1.3 million people respectively.

If the water in Tangjiashan barrier lake was to burst its bank, an alert would be issued 24-hour in advance. If its banks totally collapsed, the avalanhce of water would take about four hours to travel down to Mianyang city, and if only half of it collapsed, the water would take between five and six hours to hit the city.

Apart from concern over the life of people, local media had also reported that some 5,000 tons of dangerous chemical were located downstream of four barrier lakes in Mianyang city. The authorities claimed the chemical had been either transferred or secured.

Todate, the Sichuan earthquake had already claimed over 68,000 lives, and tens of thousands more were still missing, and millions made homeless.
Researched and written by Lin Li, Liu Peng