25 May 2010

Attabad – a bizarre race between two processes

Posted by Dave Petley

Yesterdays “coalition” resurvey of the spillway and water level now appears to provide a definitive freeboard value.  As of 8 am this morning the freeboard was 3.027 metres, representing an increase of water level of 1.09 metres in the last 24 hours.  This elevated rate of water level rise probably reflects the warmer weather in the area in the last few days, which will be driving snowmelt. 

I have recalculated the freeboard values assuming that the official numbers released today are correct, meaning the the early figures under-estimated the true freeboard figure.  The resulting graph is as follows:

At the current rate of increase the freeboard will be lost in two to three days, but as before we are reliant upon these figures being correct.  This episode of incorrect freeboard measurement does not reflect well on NDMA, given the importance of these numbers.  Of course, the fact that the saddle is located at a higher elevation means that the lake volume will be larger – i.e. it increases the size of the potential flood.

Of great concern to me though is the continued problem of closure of the spillway.  It is clear that the banks of the spillway are failing in multiple locations.  These failures are progressively closing the spillway, as this Focus image, taken of the spillway banks yesterday morning, shows:

The concern would be that the spillway closes completely before the water level reaches this level, which would effectively increase the freeboard once again, albeit probably briefly.  In effect we are in a high stakes  race between the rate of water level rise and the rate of closure of the spillway.  The strength of the clay material forming the spillway banks is clearly low, as we believed from the start.  Either way, the spillway is clearly completely incapable of handling the flow, so erosion is inevitable.  The one bright spot is that the failed banks may erode preferentially, allowing the channel to widen more rapidly than it down-cuts, as happened at Tangjiashan. 

Finally, lets not forget the hardship that this crisis is imposing on the local people.  The Pamir Times today puiblished this image of a camp for Internally Displaced People (IDPs):

I need say no more.