21 May 2010
Seepage at the Attabad landslide
Posted by Dave Petley
Focus have today kindly provided a new set of measurements of the state of the Attabad landslide. I will update the monitoring site shortly, but in the meantime here is the graph of the development of seepage. The plot of seepage against time suggests that the trend in the rate of development of seepage changed on the last few days. Note how the most recent point does not fit on the curve that one would extrapolate from the previous days data::
However, when the same data are plotted against lake depth this discontinuity in the trend disappears:
This might suggest that the seepage is being drive primarily by the water depth, and that the little disconnect in the current trend suggests that seepage is likely to increase substantially in the final stages before over-topping. This is probably supported by the suggestion that sinkholes are opening up on the dam by the Pamir Times yesterday, although the image that they provided to illustrate this does not really seem to show sinkholes:
Those large boulders are nice to see since they will not wash out like sand, and if enough are locked together for a long enough stretch, this will minimize any failure.
So, now as water is merely 6 feet below spillway, within two days things may start shaping up.How would you comment, Dave?
@tropical Those "large boulders" are not unmovable. I live on the coast and we see huge boulders moved by wave action. A similar force would be at work at Attbad, and they could become projectiles… I too hope for a slow release, but am holding my breath over the next few days.Anyone have any news of the latest water levels and seepage?
Pete- I understand that no boulder is unmovable, but I don't expect much tidal action, nor would I expect much in the way of waves. Also if there are enough boulders, the dam should hold. It's only a question is are they large enough, in both size and number, to hold?
Yesterday after the prime minister visited Altit, local men marched in protest six hours north to sit on the dam and spillway. "If we die, we die." They feel that adequate aid has not reached them. Then their families apparently went to join them. This is increasingly dangerous! The dam may erode right out from under them, even before spillover. We are watching the freeboard with great concern, but the erosion of the sediment around the large boulders and in the south side of the mudflow is dramatic… without armoring, it seems the spillway and entire dam will similarly erode.
@tropical Neither tidal action nor waves will be needed to move them – simply the force of the flow from that huge lake once it starts through that tiny, unarmoured spillway. Once it gets going, it will remove first the sediment, then the smaller rocks, and then the boulders. No way will they hold.
With regard to those large boulders, they only really occur on the upstream end of the spillway, where there is a coarse rockslide deposit. Most of the channel does not have these large boulders, and where they do occur they are surrounded by fine materials (see the images I posted today). Unfortunately, they are not going to offer much protection, and the flow rates once erosion starts will be sufficient to move them.