24 December 2008

Ash flowslide at Knoxville, Tennessee

Posted by Dave Petley

A few months ago Shaanxi Province in China suffered a dreadful flowslide when the dam holding back mine wastes collapsed, releasing an avalanche of material onto the town below. About 260 people were killed. On 22nd December, what appears to be a similar failure occurred at Knoxville in Tennessee, USA, when the retaining wall holding back coal ash from the Tennessee Valley Authority Kingston Steam Plant in Harriman power station collapsed, releasing an estimate 2 million cubic metres of waste, which then flowed down slope (image from Knoxnews).

Fortunately, the area below was only sparsely inhabited, so in this case no-one has been killed, but about a dozen homes have been rendered uninhabitable and, as the picture above shows, a train was also hit (image from Knoxnews).

Microsoft Virtual Earth has this rather nice black and white image of the site, which I have annotated to show the salient features (click on the image for a better view in a new window):

The image below shows the crown of the failed area (see annotation) – comaprison with the above image confirms which part of the storage pond has failed (image from Knoxnews):

However, as the image below shows, the failure affected a very large part of this lower set of ponds (image from Knoxnews):

My current interpretation of what has happened is shown below, which is an annotated zoom-in of the MicrosoftVirtual Earth image above:

Interestingly, the mass of mobile ash also appears in places to have caused the ground to fail without flowing. This image shows the access road to the site – note how the material has moved forward over the road without fluidising (image from Knoxnews):

That such a failure should occur is extraordinary, given that the danger of flowslides has been known for over 40 years. The only fortunate aspect of this is that no-one was killed, which seems to be a matter of luck given the volume and mobility of this landslide. However, coal ash is an unpleasant material (which is why it is stored in ponds like this), sometimes containing lad, arsenic and mercury amongst other heavy metals, although in very low concentrations. The major issue will probably be dealing with the sludge before it enters the watercourses.

Accidents like this should not be allowed to happen – they are utterly avoidable. I hope that a review is underway to ensure that there is no repeat.