“Dear Friends at Pamir Times
I thought I should write in relation to the comments that you report today from the Deputy Commissioner of Gilgit district, Asad Zamin. First, I am pleased that the Gilgit Government is taking the issues associated with this landslide seriously, and I would like to emphasise that I am happy to continue to help in a voluntary capacity in any way that I can. I would welcome a discussion with the Gilgit Government about the hazard that this landslide poses.
In my report of early March 2010 I emphasised that the Government has acted appropriately in constructing a spillway. I maintain that this is the case, and I have huge respect for those who have been involved in constructing the structure, in difficult and often dangerous circumstances. My personal preference would be for the authorities to take action now to protect the spillway from erosion, but this may well be already planned for the next few weeks.
Unfortunately, although we understand landslide dams well from many previous examples, it is not possible to predict prfecisely what will happen when the water level reaches the spillway. It is entirely possible that the spillway will remain intact and that water will flow across the barrier, leaving the lake in place. However, there is also a possibility that the landslide will erode from the water flow, leading to a breach that allows the lake to drain. This could happen in a number of ways, ranging from a very slow release, which would be associated with a minor flood, to in the (very unlikely) worse case scenario a rapid release that could see a substantive wave travel downstream. I must emphasise that the latter scenario is possible but certainly not probable. Unfortunately we have no way to quantify the probabilities of the various outcomes, but such very rapid collapses are rare.
It is because of this uncertainty that I advocate being cautious about the range of possible events as the water nears the spillway. Putting suitable measures in place to protect the downstream population until we know how the dam is performing would be a prudent move. In all probability we will prove to have been overly cautious. However, this is a better outcome than being proven to have been insufficiently careful.
It is important that people downstream do not develop a sense of panic. A large flood is not inevitable – indeed it is unlikely – and with suitable plans it will be possible to move everyone out of the way should a flood develop. However, I do believe that it would be prudent to have appropriate plans in place to ensure the safety of the population. I should add that many of these measures are in place for the section of river to Gilgit thanks to the efforts of FOCUS – such as a 24 hour monitoring station, a warning system in downstream villages, and evacuation plans for the most endangered communities.
Finally, should the spillway survive the initial overtopping event then we will need to continue to be cautious to ensure that a flood does not develop at a later stage. This will require some thought and effort.
I would like to reiterate my willingness to help in any way that I can, and invite the DC to contact me at any stage to discuss the situation and my report. In my country it is normal during hazardous events to convene an expert group of scientists and engineers to advise the government as the situation develops. Such an approach might be worth considering in this case; I am certain that there are a number of suitably qualified individuals both within Pakistan and in the wider international community who would be delighted to help in this way.”
Professor David Petley
Wilson Professor of Hazard and Risk