15 May 2010

The likelihood of a flood from the Attabad landslide

Posted by Dave Petley

The water level at Attabad is now within ten metres of the base of the spillway at the saddle, and continues to rise at about 90 to 100 cm per day.  Thus, water flow through the spillway is now about ten days away, or possibly less. 

So what will happen when water flow starts?  Regular readers will know that I have been continually pressing the authorities to assume that a large flood will occur, even if this was an outside possibility.  My recommendations were, and still are, to evacuate everyone downstream of the dam located within 60 metres of the river level. Past experience suggests that the flood could be large all the way to Tarbela – i.e. that standard attenuation models don’t apply here.

Throughout this crisis I have maintained that a catastrophic flood was possible but not probable.  This was very much based upon the plans for the spillway, which we were assured would be 30 metres deep, up to 40 m wide, and with a low longitudinal gradient.  We were even assured that the base would be armoured to prevent erosion.

The reality is rather different.  A couple of days ago the Pamir Times published this image of the spillway (the annotation is from them – I would like to formally thank them for highlighting the monitoring site I run):

Those building the spillway have admitted that it is just 5 metres wide and 14 metres deep.  None of us should be under any illusions – this is unlikely to be large enough to cope with the flows that the river experiences in the summer floods.  Furthermore, even though the flow from seepage that has developed below the spillway over the last few days is a fraction of the current inflow (seepage = 510 litres per second; inflow = 77,750 litres per second), the water flowing from the seepage point has been able to erode the dam materials.  This does not bode well in terms of the landslide dam resisting erosion.

The increase in seepage rate remains a source of  concern too, both in terms of the potential for an earlier water release and the likelihood of internal weakening of the dam structure.

All of this means that a flood is now likely, hopefully of just a moderate size, but possibly larger.  Once again I urge, as I have throughout this crisis, that the authorities assume that the flood will be large and thus ensure that everyone within 50 metres of the river between Attabad and Tarbela is protected. 

Finally, I just cannot understand why the authorities are not getting expert advice to help them to deal with this.  Help is being offered – why are they not accepting it?