7 June 2008

Updated: Flow begins through the channel at Tangjiashan

Posted by Dave Petley

Various media sources are reporting that water began to flow through the channel at Tangjiashan today. For example, Reuters is reporting that:
“Chinese troops began easing pressure on a dangerously swollen “quake lake” on Saturday, with water gushing into a man-made sluice in an operation monitored by satellite.”

The dam and channel now has to survive two critical phases. The first lies in the next day or two, with increasing volumes of flow through the sluice. The key issue will be the rate of erosion of the channel bed. If the rate of erosion is slow, then we can breathe a sigh of relief. Ideally the channel bed will slowly erode, releasing the water at a manageable rate. It could be that no bed erosion will occur, in which case the worry would be the second critical phase, which will be the first really intense rainfall event in the rainy season. This of course would trigger a flood down the river that would seriously challenge the channel. Personally, I suspect that this is comparably unlikely as erosion of the channel bed looks probable.

Thanks to Edward Vielmetti for his many updates over the last few hours. He has just pointed out that Xinhua has recently stated that:

“The long-awaited drainage of China’s Tangjiashan “quake lake” started at 7:08 a.m. Saturday, when its water flowed into a manmade sluice channel. A Xinhua reporter at the commanding center saw water passing the sluice channel via satellite monitor. The flow was rapid, steady and gradually increasing in volume. The channel bifurcated, and some of the water diverged into the separate small channel at 9:30 a.m. Soldiers of the armed police force are still working at the exit of the sluice channel to expand the channel and make it steepy so that the flow could be speeded up.”

The statement that they wanted to make the channel “steepy” (presumably steeper?) is concerning if true – the aspiration should be to slow the flow down to reduce the erosion rate, not to speed it up.

Updated: Xinhua is now reporting that:
The dam of Tangjiashan quake lake faces no danger of collapsing in the foreseeable period, a top military commander at the site said Saturday. “The dam has no collapsing danger in the foreseeable period,” said Fan Xiaoguang, deputy commander of the Chengdu Military Area Command.

I interpret this as being an indication that, so far at least, the rate of erosion is low. If true then this is would be very good news indeed.