David Petley’s analysis of potential failure scenarios for landslide lake in Hunza

1. This is a landscape littered with large landslide scars

In the Attabad area alone Shroder (1998) identified six previous very large landslides:

“Multiple overlapping rockslides have repeatedly thundered into the Hunza River near Atabad, several with serious and destructive consequences. The six slope failures of interest here are: (1) older Serat rockslide; (2) younger Seratrockslide; (3) Ghammessar slope failure and lake; (4) Ghammessar breakout flood and retrogressive slump failure; (5) 1962 Ghammessar rockslide and lake; and (6) 1991 Sulmanabad rockfall.”

Both upstream and downstream this pattern is repeated – there are literally hundreds of large rockslide scars in this landscape, many of which will have blocked the valley in a similar way to Attabad.

So how many landslide dams are left? There are fragments and remains of them in many locations, but there are very few intact landslide dams. This suggests that most such valley blockages eventually fail, although not necessarily rapidly. There are no real grounds at this stage to assume that the dam at Attabad is exceptional.

2. We are still some way from peak flow

The data from David Archer that I presented in an earlier post suggests that we are probably three weeks or so from the peak flow, which may well be 30-50% greater than at present. Whilst there are grounds for optimism that the structure may survive such flows, it is far from certain that this will be the case.

3. Landslides into the lake are a real threat

Landslides continue to occur on the walls of the valley. A large slide still has the potential to create a wave that could trigger a rapid collapse. This threat has not diminished. We believe that this was the failure mode for the 1858 landslide dam just downstream at Salmanabad. There is a need for proper assessment of this threat before one could sound the all-clear. I hope that NDMA are on the case.

4. The dam is still losing volume

Images of the downstream area of the river show that the water is still carrying a substantial amount of sediment, as this Pamir Times image from a week or so ago shows:

The loss of volume implies that the dam is weakening with time, but it is not clear how fast or where. Nonetheless, until this ceases the potential for failure remains.

5. This is a river with GLOFs

GLOFs are glacial lake outburst floods, which are flash floods created by the collapse of lakes dammed by glaciers or moraines high in the mountains. GLOFs create short duration, very large magnitude floods. The Hunza suffers GLOFs on a regular basis. Such an event would lead to a greatly increased flow rate over the spillway, threatening its stability.

6. Earthquakes

This is an area of high seismic hazard. A substantial earthquake would threaten the dam in a number of ways. First, the dam itself could undergo slope failure and collapse. Second, the earthquake could create a seiche (standing waves) in the lake that could overtop the dam, inducing failure. Third, an earthquake could trigger further slope failures into the lake, causing waves. The likelihood of an earthquake is low, but the consequences could be very serious.

Please do not believe that the boulders rule out the possibility of the release of the lake. This is not the case, despite their size. If the flow velocity and volume is sufficiently high then this dam can still fail. Unfortunately it is impossible to say when and how this might occur, or how rapidly such an event might develop. The chances of a very rapid failure are comparatively low, but are not negligible by any means.

Complete at Professor David Petley’s Blog


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5 Responses to David Petley’s analysis of potential failure scenarios for landslide lake in Hunza

  1. Nisar Malik says:

    No matter how and when the barrier gives way, no matter how big or small the remaining lake, one thing is clear; the people of the valley have to change their lives to adapt to the immediate and the long term effects of this disaster.
    To wait and see is neither prudent nor sensible. Before you know it the summer will change to winter and a new set of problems will surface.
    It is time that all the communities address the fact that life will not become normal in a hurry and the longer the people look to others the more harsher their lives will become.
    Address IDP’s and their living conditions for the coming winter NOW!
    Create food, fuel and medicine supply chains and storage for the summer but also for the coming winter. For communities as well and individual house holds.
    Address winter traffic on the lake – edges of the lake (if still there) will freeze!
    Even if the lake clears – it will not clear completely – the roads will not be made over night and heavy traffic will remain suspended till next year (scary? but a real possibility!)
    PLEASE, plan for the future and not just for the few months ahead!

  2. Asrar Ahmed says:

    Thank you once again Dr Petley for your interest in Attabad landslide and people of teh area. We request you to keep on up-dating your blog with technical data and input. You are a trusted Geologist expert and people have trust in your coments. Thank you FOCUS for inviting Dr Petley to Attabad.

    I wish we in Pakistan build up our local capacity and expertise in better understanding landslides // GLOF etc.

  3. Amin Beg says:

    thank you Professor for scaring us again!!

    It seems, we the affectees, are, as they say caught between the devil and the hard rock!

    The government claims they are following what the ‘experts’ are advising them. We are not sure what level of expertise they are banking on (how many landslide experts, hydrologists, structural engineers, seismologists FWO, NDMA and NESPAK have pooled as panel of experts?).

    while for academic reasons, it is well appreciated to monitor the lake, study and assess the multiple risks etc., for humanitarian and strategic reasons, it is very important that NDMA and the GB government, take calculated and bold steps, let me say, in a Chinese way, to take immediate measures to release the water in controlled manner.

    Waiting for the 50% peak flow to do this for us, or a landslide into the lake or a quake jerk, means more destruction, more risks and in the aftermath, more uncertainities, more unpredictability.

    The experts need to go beyond, academics, and propose credible, actionable strategies to minimize and ultimately eliminate the risk.

  4. Brigitte in Paris says:

    I could not agree with you more, Amin Beg. From Paris I am following the developments and biting my fingernails thinking about the time pressure and the helplessness the affected villagers must be feeling. I have a feeling of urgency that scientists who are expert at observation detached from humanitarian concerns do not have.
    I read today on Professor Petley’s blog that he is advising more “wait and see” and waiting until the winter to pump. However if the water freezes it will not be possible to pump. Moreover the freeze that will surround all the submerged property will cause even more damage. In my opinion the lake must absolutely be emptied before the winter! And the bigger it gets, the more dangerous, so the sooner it is emptied, the better.
    I keep thinking that pumping and siphoning the water is the solution because it will be gradual and controllable, unlike a sudden outburst if the dam should break.
    For pumping, instead of some huge pipe, it could be better to have several smaller diameter pipes which would be easier to prime and manage. They could even be primed with a manual pump, not necessarily electric, and once the water starts flowing there is no need for human intervention, only careful watching.
    I would recommend the villagers to insist with the authorities that they want to work on the problem instead of being forbidden to do anything . Tell the NDMA to consider them as a resource and they could work in shifts almost round the clock. The villagers, because they are motivated to return to their villages, would do a better job than those who are only working for the money.

  5. Baig Ali says:

    Dear All!

    ;I read today on Professor Petley’s blog that he is advising more “wait and see” and waiting until the winter to pump. However if the water freezes it will not be possible to pump. Moreover the freeze that will surround all the submerged property will cause even more damage. In my opinion the lake must absolutely be emptied before the winter! And the bigger it gets, the more dangerous, so the sooner it is emptied, the better.
    I keep thinking that pumping and siphoning the water is the solution because it will be gradual and controllable, unlike a sudden outburst if the dam should break.
    For pumping, instead of some huge pipe, it could be better to have several smaller diameter pipes which would be easier to prime and manage. They could even be primed with a manual pump, not necessarily electric, and once the water starts flowing there is no need for human intervention, only careful watching.
    I would recommend the villagers to insist with the authorities that they want to work on the problem instead of being forbidden to do anything . Tell the NDMA to consider them as a resource and they could work in shifts almost round the clock. The villagers, because they are motivated to return to their villages, would do a better job than those who are only working for the money;

    I do agree 100% with Briggiti based on physical visit of the site. I here endorse only two technical suggestions for the concerns Officials and individuals.

    Dragger would be the most feasible machine of its type-that would be use for removing the silted cannals of width more than 375 ft. Very easible available with the irregation Departments of the Country.

    The immediate action is required to increase the flow up to 30,000 cusec-as the highest flow of the river of Hunza remainsfrom 15th July to 15th August. The maximum flow record is the same as noted above.

    Through-small controll blasting this flow could be increased up to the maximum level with the special available techniques as commonly used and practised. If the flow has not increased up to the maximum capacity that would be a desaustars for the upper stream people and area. In lower area the water level safely increased up to the highest level as its very clear from commonly observation of eye.

    Based on my experience and inspite of all the geological arguments I don’t agree with the idea David Pettly that the dame will burest. Major portion of the debrise is rocks, boulders with silty clay and combination of molten megma. So, this composion of of rocks, boulders and silty clay and the eroupted magma will and never errousion would happen.

    If the channel Dragger and controlled blasting used-this will be assurance for the safety of the lower and upper area of this deam.

    Engr. Baig Ali
    03005125309

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