Sunday, May 30, 2010

Retrogressive erosion at Attabad

Flow continues to increase at Attabad.  Inflow is still exceeding outflow and seepage, so the lake level continues to rise, to the detriment of the upstream communities. There has been some confusion about the statistics about outflow - it is now clear that NDMA discussions often use the combined spillway plus seepage figure, but that this is interpreted by the media as being the spillway flow.  According to the NDMA press conference tonight, combined seepage plus spillway is about 400 cubic feet per second (about 11 cubic metres per second), still less than 20% of inflow. The remaining 80% will mostly need to be accommodated by the spillway, so this has a long way to go yet.

Downcutting on the saddle of the spillway, where the gradient (and thus the water velocity) is low, appears to be limited at the moment.  Looking at the following two images, all of the action is on the downstream face.  This picture was taken between 8 am and 9 am local time this morning:

Note the clear erosion (deep downcutting) in the lower part of the channel, which appears to be driving some local instability too.  The following picture was taken just before dark (hence the lower quality), between 6 and 7 pm (local time):

Look carefully at the erosion in the lower part of the channel - the two waterfalls are clearly eroding back through the channel.  This is a common mode of failure for landslide dams - indeed I highlighted it a couple of days ago.  As the discharge through the channel increases we might seethe rate of retrogression of these faces increase.  On the other hand, initiation of downcutting in the channel might eliminate them altogether.

Certainly these features need to be watched very closely over the next few days.  IF (and it is a big if - by no means a certainty) these features started to erode through the saddle then a fairly rapid failure could develop.  However at the moment there is no need to worry unduly - this is just something to watch carefully.

Finally, failures continue to occur on the main scarp above the dam.  This one happened early this evening:

It did not result in a wave.

1 comment:

  1. Dr Dave, great blog. Isn't the lower spillway erosion expected since that's where the less-compacted spoils of the spillway dredging were deposited?

    Sam Crawford
    Bellingham, WA