Chinese roulette

Are the Chinese authorities are playing roulette with lives of their citizens? The LA Times reports:

The Chinese government warned today that as many 1.2 million residents may have to be evacuated because they could be inundated by a swelling "barrier lake" formed by the May 12 earthquake. [. . .]

Tonight, about 600 engineers and soldiers had gathered at the blockage of the lake and were planning to work through the night to remove debris from a sluice, the agency said. Soldiers also might set off small-scale blasts to help drain the water.

Tens of thousands of people already have been evacuated. The warning issued today said that in the worst-case scenario, in which the entire barrier collapsed, about 1.2 million people would be ordered to move to higher ground, including some in central Mianyang.

This last line doesn't make any sense to me. On what basis does the PRC government assume -- should the entire barrier collapse -- that 1.2 million people will have sufficient time to move to higher ground? What if they are sleeping?

The Chinese government has a lot of faith in the ability of its engineers to release the waters of the new lake by use of explosives. But with so many lives at stake, the authorities should not take anything for granted.