9 July 2010
Increased landslide occurrence at Attabad
Posted by Dave Petley
Various media reports suggest that the rate of landslide activity has dramatically increased today on the slopes above the Attabad lake. For example, SAMAA is reporting that “Series of landsliding still continues in intervals in Ataabad, Hunza, causing difficutlties for the boat service, SAMAA reported Friday. Process of landsliding has started in intervals since last night in Hunza, Ataabad. Boat service however being continuing.”
A key concern remains the potential for a large failure to create a wave that triggers a rapid breach event. The ability for mass movements to create waves is best illustrated by this video of an ice collapse event:
This is a very small collapse event compared with a large rockfall. Large collapses are often preceded by increasing event rates (i.e. in this case increasing rates of rockfall activity). The best (and absolutely fantastic) example of this is a large ice collapse event captured in Argentina. Do watch this if you can!
This is not to say that such an event is coming in Hunza, or indeed that a wave would be large enough to trigger a breach, but the need for caution is clear.
I contacted my friends in Hunza. The landsliding is hapening very close to the barrier and on up-stream side ie where there is lake. This can be quite dangerous and can create a big splash wave and this could damage the fragile ( or not so fragile) barrier dam.I am sure FOCUS and NDMA must have gone into heightened state of alert and requested local population not to go near the barrier and also downstream population to maintain the 60 meter height distance from lake.we all wish them all the best
Three things:1. I did not see a single dam failure in those videos. The dam does seem to support an increase in flow.2. In regards to the commenter above, "The landsliding is hapening very close to the barrier and on up-stream side ie where there is lake. This can be quite dangerous and can create a big splash wave and this could damage the fragile ( or not so fragile) barrier dam." It should also be noted that the landslide could also reinforce the dam.3. It's easy to claim something might happen, even if the chances are very small. This is the same with "the big earthquake" claims and California. I know a guy who has been suggesting that the "big one is coming" for over 30 years. Since some possibility does exist, he will always be on the safe side, even if he bets wrong.The other side is to suggest that a big earthquake will not happen. This would have been a safe bet in Haiti until recently.Dr. Dave does point out, however, that there is a cost in making claims regarding hazards that do not occur. In the Hunza case, there are all the IDPs. If we knew, somehow, that the dam would not fail, then the situation would be handled differently.Erring on the safe side has its costs too.I once kept track of a weatherman's forecasts. Basically there was always, "A chance of rain." I suppose that's probably true, but if someone consistently claims a 10% chance of rain, then two things:1. It should rain about 10% of the time, within some level of randomness.2. Verifying the accuracy of low probability events is difficult.The weatherman was basically always leaving a chance that it could rain–he hedged his forecast a bit.
Tropical I am not sure as to your point. 1. The videos were intended to show the generation of waves from mass movements, not the mechanisms of failure of a dam.2. The site of the new slides will not add mass to the dam – trust me on this. 3. No-one is suggesting that a slide-induced wave is inevitably going to trigger failure. However, in this area this is a known mechanism for breach initiation – it was the cause of the breach in 1858 for example. Therefore, when an increased rate of mass movements starts to develop, given that we know that this is often the precursor to a large collapse, this is worth noting. In effect this is increasing the alert state, in the same manner as volcano observatories or tropical cyclone forecasts. It is prudent during these increased alert states to change behaviour – i.e. to ensure that no-one is immediately downstream, or on a boat, or on the dam. This is not being alarmist, just prudent.When the level of landsliding starts to decline then the alert state can be lowered.Surely you are not suggesting that one should just ignore a process that can indicate that a serious event could occur? That would be reckless in the extreme. I am reminded of the Vaiont landslide disaster of 1963 (also a landslide into a lake), and in particular the negligent behaviour of the authorities in ignoring the warning signs. This is not crying wolf, just prudent science- and experience-led disaster management.As you are aware I have advocated that people should be allowed back into the "red zone", protected by a proper warning system. However, when the alert status is elevated, as at present, these areas should be closed.
I would also NOT expect new masses to improve the stability of the dam, but more to block the spillway – which could cause again rising water levels. This would enlarge the problems in the higher valley as well as enlarging the danger for the lower valley, as the mass of water pressing against the dam would rise."Hoping for the best, but expecting the worst" (from A-HA, "Forever young")
Dear Tropical, the answer of Dave makes sense. New collapses of the slides are obviously a threat. It is not alarmist to mention it. Whatever the poor and unbearable situation of IDPs is, nobody can advise them to settle back home as long as the risk of landslides is high. I have the impression that, if there is no natural evolution of the situation from now onwards, no important decision can be seriously taken before mid August when the in-flow starts decreasing.