8 June 2008

An update on Tangjiashan

Posted by Dave Petley

I thought that this would be a good time to review the situation with Tangjiashan now that water has been flowing for over a day.

1. Water flow is now apparently about 21.5 cumecs – up from 10 cumecs at about the same time yesterday. This is still not enough to balance inflow, so the lake level continues to rise. Of course as the lake level increases the rate of flow should also increase, so inevitably this will go up.
2. It appears that for at least part of its course the flow is not following the man-made channel but has created its own route.
3. The rate of down-cutting is slow, perhaps because of the presence of boulders in the channel bed. These boulders, if they are persistent through the landslide mass, provide a source of hope.
4. There is some evidence that some water is preferentially flowing into the dam mass rather than through the whole length of the channel. This will be wetting up the landslide mass.

Some thoughts:
a. So far so good in terms of no disaster.
b. The Xinhua press releases have fairly consistently talked about draining the lake – the assumption being I think that the channel would slowly down-cut to allow this (?). This does not appear to be happening. I think that this may be bad news in the medium term as this will mean that somehow a channel will need to be constructed to cope with the peak flows in the rainy season. This is a very tough challenge.
c. The wetting up of the landslide mass may also be a threat. A big concern would be the possibility of a slope failure in the dam mass as this wetting occurs. This is pretty much the worse case scenario
d. Xinhua is reporting that “A relatively strong aftershock rocked the dam of the Tangjiashan “quake lake” for about 20 seconds at 6:51 p.m. on Sunday and caused massive landslides on surrounding mountains. The magnitude of the aftershock was not immediately available and its impact on the dam was under surveillance.” The USGS suggests that this was a magnitude 5.0 earthquake at a depth of 10 km (i.e. a large but not exceptional event). The “massive landslides” comment is hard to interpret, but deeply worrying. A large slide into the lake could cause an instant over-top situation. The dam is unlikely to survive this intact.

I have noticed that the international media reports appear to imply that now that the lake has flow through the sluice the problem is over. See for example the Washington Post, which reported that:

‘Quake Lake’ Drainage Eases Danger in China

This is wrong as the danger has not yet eased. If the lake level was falling then maybe, but it is still rising. As the Chinese media have rightly suggested, the situation remains critical. If the lake level does not start to at least stabilise, and preferably fall, soon then the danger level will rise substantially. This remains a deeply worrying situation.

As ever comments are welcome from all. The discussion on previous posts has been terrific – I hope that we can develop this further.