The likelihood of a flood from the Attabad landslide – by Dr. David Petley
Source Dave’s landslide blog:
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?
here is a media report in which a government official (wrongly) says the target depth of 30 metres has been achieved
http://tribune.com.pk/story/13714/spillway-in-attabad-lake-complete-to-ease-pressure/
GILGIT: Work to build a spillway in the Attabad ‘landslide’ lake was stopped on Sunday after authorities said that the target of a 30-metre deep cut has been achieved.
“As against our initial target of cutting the debris down to 18 meters, we decided to cut it down to 30 meters as the inflow of water was slow,” said a senior government official.
He added that the water is expected to overflow the banks of the dam by the end of the month. The lake now covers an area of 17 kilometres, leaving 25,000 people stranded.
The water level has reached alarming levels of 353 feet owing to the rapid meltdown of glacier in the last two days.
Assistant Commissioner Hunza, Zamir Abbad, told The Express Tribune via telephone that the evacuation of vulnerable people from downstream villages will be completed by the 20th of this month. He said that people are being mentally prepared to move to safer places in time to avoid any catastrophe.
“As per the marking of the National Engineering Services Pakistan, more than 500 people from vulnerable villages – Faqirbad, Mayoon, Hussainabad, and Khaan – are being moved,” he said.
He said that boat service has not been stopped yet but a decision to this effect will be taken in a few days.
Chinese and Frontier Works Organisation [FWO] engineers began cutting the 196,000 cubic metre-high debris from the Attabad Dam in early January after a massive landslide blocked the once fast flowing Hunza River at Attabad, creating a lake that submerged at least three villages as it expended.
At least 19 people were killed in the landslide that also swept away a major portion of Karakoram Highway that links Pakistan with China.
Officials said that more than 20 villages have been vacated and people moved to safer places.
According to experts, more than 36 villages downstream can come underwater if the dam breaks its banks.
In addition, more than 13,000 people can be displaced. Officials said that the army has been assisting the evacuation of communities living on both sides of the Hunza River in Gilgit and Hunza and Nagar districts.
According to the latest information, three upstream villages – Ayeenabad, Shishkat and many commercial areas in Gulmit town – have been flooded while a major portion of the Karakoram Highway has also submerged.
Some experts fear that debris from mountains on each side of the spillway could again close the lake’s passage.
A total of 18 relief camps have been set up in Gilgit and Hunza. Of these, nine are in Hunza Nagar and nine in Gilgit.
There are reports of traders near Pak-China borders having been evacuated from Sost in the wake of warning from the government, sources said.