18 May 2010
The GLOF into the lake behind the Attabad landslide yesterday appears to have raised the water level by something over two metres, in addition to the increase of about a metre from the normal inflow (please see the data on my monitoring site). The upshot is that the freeboard is likely be 4 metres or less. Inflow has increased substantially as well to about 93 cubic metres per second. It has not been possible to measure seepage in the last 24 hours as the site is now too dangerous.
All work on the spillway has ceased and the machinery has been withdrawn. The final form of the spillway is shown in this Focus image:
It pains me to say this, but I simply cannot see how this will resist the peak flow in the Hunza River. However, it has at least served the purpose of reducing the total volume of water involved in any flood that should occur.
The boat service has also now ceased as it would clearly be too dangerous should a breach start.
Water flow past the blockage may start through one of three key processes:
1. Overtopping and water flow through the spillway. In Tangjiashan the initiation of erosion started some hours after initial flow; this may or may not be the case here. Erosion may start on the downstream face or at the crest of the spillway, or there may be slope failures on the downstream face. It is also possible that the sides of the channel may fail, creating a new blockage, stopping water flow for a while.
2. A seepage induced failure. This is less likely than 1., but remains a distinct possibility;
3. Overtopping initiated by a landslide into the lake.
None of these can be ruled out at this stage, but 1. is the most likely. If so, the size of the resultant flood will depend upon the rate of erosion, if any.
I hope above all hopes that erosion happens slowly. My thoughts are with the people of Hunza.