3 June 2008

Tangjiashan – is the dam leaking?

Posted by Dave Petley

Quite alarming reports started to emerge yesterday that the dam at Tangjiashan might now be “leaking”. For example, a correspondent in Japan, Hideo Ototake, sent me a link to this website, which suggests that a leak started at the bottom of the dam in the afternoon of 1st June. Reuters today is carrying this report, which states that:

“The largest “quake lake” formed by China’s most devastating earthquake in decades is not expected to start draining until Thursday due to a lack of rain, state media said. The water level at Tangjiashan was 735.78 meters on Monday, only 1.24 meters higher than a day earlier, because of a lack of rain and multiple leaks on the natural dam made of mud and rock, the newspaper said.”

Multiple leaks?

In addition, the Hideo Ototake also pointed me to a new and rather splendid aerial image of the lake that is available here (click on the image for a better view).

This image shows the lake and the dam, with the dry river valley below. As far as I can tell this image predates the construction of the channel. What is slightly alarming is the area at the toe of the dam that I have enlarged here:

This image might suggest that at the toe of the dam there is what appears to be a damp area? This is very unclear, but this is one interpretation. Is this the start of seepage?

There are strong reasons for concern here. If the dam is showing signs of seepage then there is a risk that the dam will fail through this mechanism. This is a worse case scenario as collapse under this mechanism can be very rapid indeed. Failure is not necessarily catastrophic, but there is a reasonable chance. So if the flow rate starts to increase through these seeps then the authorities will need to move all of the population in the affected areas without delay. Note also that if seepage has started then this suggests that the core of the dam is more permeable than I had anticipated. This is not good news at all when the overspill channel starts to erode.

Assuming that seepage failure doesn’t occur the spillway is expected to start handling flow on Thursday at the latest. It will be an interesting couple of days.