KARACHI: Four months on with the summer melt approaching its peak, the lake formed behind the landslide dam has swelled to mammoth proportions. Extending upstream from the landslide dam the lake stretches for over 18.5 kilometres and has depths of over 100 metres in places. Due to the high inflow of water the level of the lake is rising at the rate of nearly a metre a day. Expecting the lake to overtop the dam, all work on the spillway has been suspended and the machinery withdrawn. Unfortunately the spillway, which was planned to be 40 metres wide by 30 metres deep and lined with boulders and rocks to minimise erosion, is not even close to this goal. The Pamir Times website reports that the spillway is closer to 14 metres in depth and five metres in width at its base and there is no rock reinforcement.
It is expected that the lake level will overtop the spillway, causing the spillway to erode, increasing its depth and the flow. The latter will increase the rate of erosion of the barrier, causing the whole thing to collapse in a short time. If this happens catastrophically, there is a strong likelihood of a massive flash flood barrelling down the valley, all the way to Tarbela.
I have been following this crisis on Dr Dave Petley’s blog (http://daveslandslideblog.blogspot.com) who is a professor at Durham University in England. He visited the landslide site and made a presentation to the NDMA in February. His recommendation, based on a study of similar events and flood modelling, is to closely monitor the flow through the spillway and to evacuate all inhabitants between Attabad and Tarbela, who are located less than 60 metres from the current river level.
All the evacuation efforts appear to be focused up to Gilgit at the moment. There is an urgent need to mobilise the people of Diamer and Indus Kohistan beyond Chilas, and make them aware of the imminent risk. The strength of a flash flood dissipates when its waters are able to spread out over a wide area. There is no opportunity for such dispersal before Tarbela. Should the dam collapse, the flash flood would be rapid and devastating and there will be no time to take any evasive action. Dr Petley’s blog records the account of the failure of landslide dam near Mount Cayley in Canada which created a debris flow that travelled at about 35 metres per second [126km/h]. The flow was sufficiently violent to propel wood splinters that became impaled in trees that they hit. It may be impossible to mitigate the flow from the lake but the government still bears the responsibility to save lives and property by taking timely action. The rest of the country would be welladvised to say a prayer for the well-being and safety of their countrymen in the coming days.
Published in The Express Tribune, May 23, 2010.