2 June 2010

Attabad – so why hasn’t it breached yet?

Posted by Dave Petley

Several people have emailed to ask why the lake has yet to breach.  Well, I would like to emphasise two things – first, that every landslide is different, so predicting the behaviour of any particular slide is effectively impossible.  Second, there is a peculiarity about the spillway that appears to be delaying the breach event.

Regular readers (of which there are now several thousand each day I think – thanks to you all) may remember that a few days ago I noted the presence of a large boulder in the channel, right at the crown of the spillway:

 Well it is this boulder that is preventing the scour from propagating upstream and that is, in effect, protecting the upper part of the channel against further erosion, as this image shows.  The boulder is circled:

The lower part of the channel is continuing to erode with some power, and the channel is widening.  The red lines show substantial cracks suggesting further potential widening.  The steepest part of the channel is continuing to erode backwards, such that sooner or later the boulder will start to be undercut.

Meanwhile, there appears to be an increasing level of frustration about the very confused spillway discharge statistics emerging from NDMA.   This is understandable.  If we take the last four daily reports we get:
29th May 18:00: 150 cusecs (cubic feet per second) from the spillway, 350 total discharge
30th May 18:00: 900 cusecs total discharge
31st May 18:00: 900 cusecs total discharge
1st June: 18:00: 1050 cusecs discharge at Ganesh Bridge.

This is a quite bizarre mixture of measurements, but we should all understand that the NDMA team dealing with this are under huge pressure too. 

Meanwhile, the more reliable measurements of inflow suggest that the rate is continuing to increase linearly with time, probably because of the choke provided by that boulder.  However, I am very surprised by just how linear this increase is proving to be, so please treat this with some caution:

The lake level upstream continues to rise, increasing both the damage and the amount of water in the lake.  In the last 24 hours the lake level rose by 75 cm, perhaps suggesting that the rate of inflow is further increasing. 

Finally, several people have emailed to ask why I have not covered the multiple landslide accidents around the world in the last few days, including the multiple slides in Central America (and that amazing sinkhole), the landslides in China, the non-fatal slide in New Zealand, etc.  I will try to catch up on all of those things as and when this event ends – please accept my apologies.  You will understand that I have to maintain my day job whilst doing this, plus yesterday and today I am moving house!