27 May 2010
Attabad: the freeboard is now reportedly about 1,3 metres
Posted by Dave Petley
The latest NDMA figures suggest that the freeboard is down to about 4.3 feet (1.31 metres) as of 3 pm today. That is a reduction of about 70 cm on the day, meaning that we are probably less than two days from the point of overtopping:
I am reasonably confident of these values as the Focus geologists took this picture yesterday of the spillway:
The circled boulder is the crest of the spillway, so water flow will start when this point is reached. Of course the one potential problem is a slope failure on the spillway walls, which could block the channel. This is a real possibility, but we won’t know until tomorrow.
Close up the channel at this point looks like this:
Unfortunately the large boulder will resist lateral cutting, initially at least, which is likely to increase the rate of down-cutting. This is unfortunate. Once the water reaches the top it will initially flow down this channel, crossing the summit of the dam:
And then will flow down here:
The disturbance here is from collapse of the banks induced by the seepage. Finally, it will flow down the main river channel:
The terrace in the centre left of the image is at 60 metres above the river level. Previous (comparatively recent) floods have not reached this level, which is half the height of the dam. This is the level that we believe is comparatively safe from the flood.
Any estimates of seepage?I tried plotting the seepage/depth on a loglog plot. It was not very linear, as might be possibly expected from Darcy's law, which would (very roughly) predict seepage proportional to the square of the depth [(area)*(dP) or (L*W)*(gH)*K]. In fact, the seepage appeared to increase faster than expected, perhaps due to fines washing out and increasing permeability. I'm not sure what the water depth is now (114 m?) but extrapolating from the previous data, 114 m depth would predict a substantially greater flow (anywhere from 10 to 30 m3, depending on whether all points or the last ones are used). Then again, Darcy's law might be completely inappropriate.
Thanks David for up to hour information. its really interesting to see the pic it might take another week if the banks of the spill way collapse, hope so it will take a smooth passage but things are really concerning once the summit is down.
Attabad Water Bomb: Siphon the water out and Drain the lakeReportedly the lake level continues to rise. The estimates from satellite imagery suggest that the volume may have now exceeded 500 million cubic metres into Attabad Lake as weather is getting warmer and glaciers are melting fast. But the expected decrease of freeboard is not being achieved to facilitate the overtopping (over flow) of water from over the spill way as was expected to top over by May 24/25th.A caveat certainly applies that estimated volume of some 500 million cubic meters of water is contained in such a narrow Attabad lake of loose eroding soil which is highly dangerous. Daily increase of water into lake is not attaining the rise in the water level and is not automatically translated into reduction of freeboard. Keeping the nature of soil the deformation and swelling of lake bed may give the way to water to travel within the lake to multi burst points.Reportedly the spill way made of debris from the slides is weak in nature but being at the turning point of the River is sufficiently strong at least to block the water as the mass of water is seemingly contained by the curved sides near the mouth of the lake. The subject blockage expected to serve as spill way is made of lose clay debris and already many seepage holes are discovered in it. There is likely hood that massive volume of water is already pushing out and up lifting the lake floor on to the sides and loosening the weak clay deposit under the spill way; it may collapse and lake may bursts out.The expert may tell us better but to keep the spill way intact in order to prevent the downstream wide spread disaster and devastation, it is but necessary that an adequate water draining arrangements by laying the mega poly pipes or other flexi artificial Siphons System over the spill way may be the only solution to drain the lake as well as save the situation and its sudden outburst which may damage even the downstream Tarbela dam is prevented.