Attabad – a bizarre race between two processes

 

 Tuesday, 25 May 2010

Water filling the lake is continuously rising in  the newly formed lake. It was expected that soon the water will start over flowing the spillway. But as soon as the water freeboard level[1] is over come and water may move for the spillway and start draining the lake excess water; spillway starts filling up agian by the falling mountains debris.

 Mean while seepage at various place down the spillway which is very porous in nature from small openings and holes  is said to be most dangerous that it may cause a sudden breach of dam so formed and there may be sudden outburst.

 National Disaster management authority(NDMA) along with the local administration with the help of Pak army have managed to evacuate the population from the downstream areas.

Yesterdays “coalition” resurveys 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 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, let’s not forget the hardship that this crisis is imposing on the local people.  The Pamir Times today published this image of a camp for Internally Displaced People (IDPs):

 Notes

 [1]Freeboard: Vertical distance from the normal water surface to the top of a confining wall….i.e. spillway height of Attabad Lake left to be filled that water will start over flowing after its filling.