7 June 2008

What is going on at Tangjiashan?

Posted by Dave Petley

First thanks to both Ripen and Hideo Ototake for their emails and images on this issue.

I remain somewhat confused about what is going on at Tangjiashan. We know that the sluice now has flow within it – that is not in doubt. But what is happening with that flow is very hard to understand.

First, let’s work out the flow rate needed to maintain a constant lake level. The earthquake happened on 12th May and now it is 7th June – this is 26 days. In this time the lake has, according to Xinhua, accumulated 229.5 million cubic metres of water. By my calculation this is approximately 102 cubic metres per second.

So, to maintain the level of the lake at a constant point the channel plus seepage should be transporting this volume of water. More than this and the lake will slowly empty, less and it will fill (this is certainly not rocket science!).

Now let’s look at the images that are available at the moment. In this post I highlighted one that appears to show a low level of flow. The image below has also appeared here. This clearly shows a level of flow that is well below 100 cubic metres per second (from now on let’s call this a cumec for convenience). Furthermore, Xinhua has now said that:
“Experts calculated that the water flows at seven to eight cubic meters per second, far more than the previous two cubic meters per second.”

Now the figure of less than 10 cumecs is absolutely consistent with the image above, which is clearly of this order. There is no way that this is the 100 cumecs required to balance average inflow.

So where is the excess water? I cannot work this out! Possibilities are (as I see them – there may be more):
1. In-flow is only <10 cumecs now because it has not rained for a day or so. I do not believe that this is conceivable;
2. The lake is filling so slowly that only <10 cumecs is escaping. This is possible – but suggests that there is a great deal more flow to come (which means that the current situation is a "phoney war");
3. The excess is being dealt with by seepage through the dam (in which case then the flow is now essentially out of control – this is not necessarily bad news, but it could be);
4. The volume of the lake has been seriously over-estimated (this does not seem likely).

There may be other options. Suggestions please?

Whatever, something does not add up here, and that is very worrying to my mind…

Finally, Ripen asks what the channel is on the right side of the above image? Is this a former attempt to cut a channel by the army? Is it erosion by seepage (there appears to be water within it)? Is it a natural feature associated with the landslide (I don’t believe this). Or is it something else (again, suggestions welcome please)?

In conclusion, something feels wrong here. I hope it is just that I am not getting a good picture of what is going on, which means that I am missing something vital. This could well be the case, or…