8 July 2010
Can Attabad fail?
Posted by Dave Petley
A number of commenters here have suggested that the structure of the dam is such that it cannot now fail due to the presence of the large boulders. So I thought I’d write a short post exploring whether this is actually the case. I believe that it can still fail, and indeed that eventually it probably will, but I have no idea as to the likely time-frame. The reasons that I believe that a failure is still highly possible are as follows:
1. This is a landscape littered with large landslide scars
In the Attabad area alone Shroder (1998) identified six previous very large landslides:
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
Meanwhile, NDMA reported that the lake level rose 2 inches (5 cm) yesterday, although it does appear to have fallen the day before (the link on the NDMA website to the report yesterday is dead). Local people are reportedly protesting about government action and the limited amount of compensation by camping close to the dam.
Unfortunately though the flow of information from the site remains very limited.
Reference
Shroder J.F. 1998. Slope failure and denudation in the western Himalaya, Geomorphology, 26 (1-3), 81-105. DOI: 10.1016/S0169-555X(98)00052-X.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Very good article regarding situation created at Atabad.What if some more rain happens on upper part of the river, which can cause flood situation in the lake, during coming moon soon.
The 04 January landslide was so large that the debris filled the river and then went on to rise on the Southern side of the river. Hence the debris on Southern side was higher than the Northern side. Your photograph very clearly explains this. The high volume of debris has remained at the landslide point for more than six months and logically should have settled in permanent strata. The huge earth moving vehicles have moved all over the area which would have settled the debris further. The dam was constructed naturally over a period of time since 2002 onwards and only a small gap was open for the river to flow. The height of dam was already more than 20m or so before the landslide. The spillway has been constructed towards the North of the landslide area where the height of the debris was the lowest. If the dam fails by GLOFs or any earthquake (very rare but still a possibility) the debris on the South will come down and block the spillway one more time. If the dam does not fail and there is GLOF then the situation is worse. So the resolution of the Gojal Lake by natural means seems to be very remote. The GB government should get more experts like Dr Dave and others to work for resolution of the lake at the earliest.
Hi Dave – what has happened to the FOCUS team up in their eyrie? Are they still there? The sub-aerial photos they were so kind to provide gave fantastic views of the spillway and were very useful for comparing day-by-day shots. Any chance of some more? There was one particular massive rock I was watching quite high in the side of the spillway below the access track (I called it Falcon Rock because it looked like a falcon's head) which provided a very useful monitoring point. I wonder if it's still there or if it has eroded out already.
Is the FOCUS team still monitoring the lake data or due to safety reasons they have moved out?. Irrespective of the fact that they have moved out from near the barrier due to safety reasons,they have done excellent work being an NGO. Very few NGOs working in DRR sector have geotechnical capacity that FOCUS has and the commitment with which they are working helping the IDPs etc. Bravo FOCUS!
According to local blogs, the IDPs are desperate to return to their villages and cannot wait for Nature to take its course, all the more since the govt is not offering them any financial help that would allow them to start a new life somewhere else. Therefore they can only hope to return.Which brings back the issue of pumping the water out of the lake. I really don't see why it shouldn't be tried. The technology exists for pumping large volumes and with this the problem could be solved in a relatively short time. Pumps would empty the lake in a progressive manner which would preserve the structures which are presently submerged.Should a sudden, violent discharge occur, the bridges, roads, buildings, even the topsoil would be washed away, making the area unfit to live and grow anything for a long, long time.Whereas pumping would ensure the safety of all concerned, including the downstream IDPs who would see the danger of a disastrous flash flood recede as well.From a human point of view, this situation is extremely stressful. Women especially are tormented by the uncertainty and many are despondent.Brigitte
Responding to suggestion given by Brigitte of "pumping".The estimated volume of water is about 600 million cubic meters.The electric supply in th e area is erratic at best and will not be able to supply huge drain.And the pumps: how do you transport them up there in Attabad. highway 101 does not exist there.I understand the president of Pakistan has sought technical help from Govt. of China and this may offer some viable options.OR in next 3-4 weeks, the increased flow due to glacial melt may trigger progressive failure.Let us hope and pray the local population is safe.
In response to Anonymous above, electric supply would be needed only to get the pumps started. Once the water is inside the pipes, the siphoning action will do the job with no electic input whatsoever.The pumps could be transported by helicopter.Brigitte
Brigitte,The largest pumping station in the UK which is currently under construction will be capable of pumping 100 cumecs against a few metres water head. To achieve this discharge there are six pumps housed in a purpose built pumping station.The current discharge through the spillway is around 500 cumecs therefore such a pumping station might lower the lake level a metre, but it would not be able to lower the lake level below the "lip" of the spillway. Even in the dry season the inflow into the lake probably exceeds 100 cumecs.I agree that in theory, a siphon could operate without pumping, but you would need large pumps to get the siphon primed.
Anonymous, Winter inflows were in the order of 30 cumecs. Seepage outflow is currently about 200 cumecs, such that the water level may naturally fall below the spillway level in the winter anyway. Add to that some pumping effort and it may be possible to bring the lake level down during the dry season, allowing the spillway to be either reinforced or deepened. This will require a substantial effort though.This all assumes that the lake survives the next few weeks, which is hard to predict.
With siphons the water could be safely removed from the lake 24/7 and I am certain this would have a noticeable effect on the water level even in the short term. Moreover it would provide water for the downstream communities which must be parched in the absence of the usual water flow. At the very least it would decrease the threat to the downstream villages because the bigger the lake gets the more threatening it is. Siphoning would also have a positive psychological effect because ppl would know it is on-going night and day without human intervention.Once the water level has come down it would be possible to deepen and widen the spillway which is not otherwise possible.Villagers are willing to contribute effort for the restoration of their communities. Why should they be left out when they are dying to participate? It is a big political error to forbid them from doing anything, saying only the pros are allowed to work, and then the pros have short working hours, long week ends and only 1/4th the machinery that is required.Brigitte
Dave,Certainly lake inflows of the order of 30cumecs are more managable, but will still need large pumps and pipes to pump or siphon the water over the dam.I had not realised that the seepage had increased so much. At the time of the overtopping at the end of May the seepage was reported as 100 cusecs or so, i.e. a few cumecs. If the seepage is now 200 cumecs then this indicates roughly a fifty-fold increase for very little additional head. If the seepage is this great, then as you say, during the winter season the lake water level would be expected to fall.
Dave,Just realised you might have meant that the seepage through the dam is currently 200 cusecs i.e. about 6 cumecs, which would represent about 20% of the lake inflow in the winter months.