1 June 2008

Tangjiashan Lake, Beichuan – so now we wait…

Posted by Dave Petley

Xinhua is reporting that the spillway at Tangjiashan Lake is now finished and that the soldiers constructing it have withdrawn, leaving just a few hardy and brave souls to raise the alarm.

First, we must recognise the awesome task that has been accomplished here – 135,500 cubic metres of debris have been shifted to create a channel that 475 m long and “up to” 10 m wide. To do this in the circumstances is utterly extraordinary, putting the responses of most other countries to disasters to shame. But where does it leave the situation at Beichuan and downstream?

First, I have tried to collate some images of the channel. All originate from Xinhua. In the first, excavators are shown digging the channel. Now, this is unfinished so great care is needed, but note:

  1. The material is very fragmented and soft. There are almost no boulders in the channel bed and in the walls of the channel;
  2. The channel is very narrow indeed. It was unfinished when the image was collected, so might not be the final product, but the walls have been well-graded, so it probably is not far from it (?);
  3. The gradient is quite steep, meaning that water flow is likely to be rapid.

In the second image, soldiers are seen working on the channel banks. Rocks are being moved in chains – it appears that this might be to armour the channel bed? Again, the channel is very narrow and steep, and interestingly it also has a very clear bend in it.

The final image shows one end of the channel, which here is broad and quite shallow. Again the material is fine grained, with almost no obvious boulders. The channel gradient is quite low.

There is little more information available to me at present. The channel is expected to start to carry water in the next three to four days. It seems to me (and to one or two other landslide people that I have spoken to ) that the channel will almost certainly erode rapidly once it is carrying water. The steepness of the gradient and the fine grained material appear to almost guarantee this. It seems likely therefore that the authorities have decided that the release of much or all of the water in the lake is inevitable. The key question will be how quickly that erosion occurs, and thus how big the flood wave actually is. The fine-grained nature of the material suggests to me that it could well be rapid, although this could be tempered by the size of the blockage. Note that the blockages downstream will also play a role, allowing the water to pond and then break through, which is likely to increase the maximum size of the flood wave. It seems likely that the flood wave will be very large, but frustratingly it is difficult to say just how big this will be. Evacuating as many people as possible downstream seems to be to be prudent.

Xinhua remains somewhat intriguing in its reporting, saying: “Two other plans require the relocation of 1.2 million people if the half of the lake volume was released, or 1.3 million if the whole landslide blockage is washed away.

I cannot quite understand what this means. It seems to me that the river will in due course cut a channel right through the dam – i.e. all of the water will be released. The key question is how quickly this occurs. Past experience suggests that this could be anything from a small number of hours to months or even years. If I was a betting man I would tend towards sooner rather than later. I hope that the authorities understand that if a catastrophic collapse were to start it would happen in a very short period of time. Thus, the evacuation would need to be very rapid indeed. Hopefully (and probably I guess?) this won’t be the case, but…

We must also be very careful to ensure that we understand that this is just one problem of many. We mustn’t lose sight of the fact that:
1. There are c.27 other remaining dangerous landslide dams. All need to be dealt with. It would be an extraordinary achievement to mitigate them all, so an unexpected collapse of at least one is surely likely. It worries me that there is so little information about other areas. Given the numbers of landslides that we know have occurred, I wonder if we are even sure that all of the valley blockages have been identified. I guess we have to trust the Chinese on this, and they have some incredibly competent people, but concerns linger.
2. Landslides continue to occur in the aftershocks. These represent a substantial hazard.
3. We are getting closer to the wet season. In heavy rainfall landslides will be a substantial problem, as will the movement of sediment in floods and debris flows. Dealing with the valley blockages is clearly the immediate crisis, but there are many more just around the corner.

Finally, I will be in and out of my office for the next few days but can be contacted by email. I would really like to hear from anyone who can provide more info, and am willing to discuss the above in more detail as and when appropriate. I intend to continue to post on here as much as I can.