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.
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.
Dr. Dave:Thank you very much for providing such good infirmation and professional analysis.Here are links of two videos showing the flow through the channel:http://news.qq.com/a/20080608/001343.htmhttp://news.qq.com/a/20080608/001503.htmThe interviewed man in the video is the top commander of the projetct.P.S.It appers that discussion about long term management of the dam is on the table also. The authrity has dispathed team and equipments digging small holes in the dam investigating the body, according some chinese medium.
A catastrophe is developing here. Risk of dam failure is the highest it has ever been.As the lake level rises the risk of collapse is obviously increasing exponentially.What is worrying is this:The water flow at the top of the dam will not sufficiently erode the top of the spill threshold to lower the lake level quickly enough, because the water velocity is slow. The slope of the spill down the free face of the dam is steeper, meaning faster water velocity and faster erosion which will cut away and weaken the dam. If the rate of this weakening is faster than the rate that the lake level is dropping to reduce the load, then dam failure is inevitable.A (the?) solution is to lower the dam spill height by digging it deeper. The Chinese engineers need to float earthmoving diggers over the top of the spillway (float on pontoons, and teether upstream with steel cables) and dig out the soil/rock, etc under the outflowing water at the top of the dam to lower the spill height. The water flow should flush this material away. The more diggers doing this the faster the job will be done. There is the problem of faster erosion down the free face of the dam (due to the extra water volume) increasing the rate of weakening, but unless the flow is enough to lower the lake level, dam failure is inevitable anyway.Slowing the water velocity down the dam face (hence the erosion rate) by putting boulders or other objects in the water flow should be done. (How about steel spikes dropped from a helicopter from enough height to spear into the dam and stay there standing upright?) If done optimally, then sediment released at the top of the dam by the diggers will deposit downhill and replace eroded material.Is it too late to prevent a dam collapse? Probably. Then the best scenario is to have the lake level as low as possible when collapse happens. That means spilling water as fast as possible.Theres no time to lose.Mike M
Dear All:I think same as Mike M, the most necessary thing to do is enlarge and dig the spill to increasing outflow. The relative stable spill can not lower the water level and reduce risk. During breaching process, the control section is spill section for this quit steep dam face. Once the flow over spill crest, the downstream channel developed by highly erosion on dam face.