1 June 2010
Attabad – continued retrogression. Close to the next stage?
Posted by Dave Petley
The latest images from Attabad suggest that the retrogressive erosion in the channel continues to accelerate, and that this is now by far the most likely failure mechanism. A breach now looks to me to be inevitable. This image, taken this afternoon, shows how the spillway has developed:
Note the people on the road for scale.
Compare it with yesterday’s image:
The waterfall has now smoothed out, probably because the water is now eroding the landslide mass rather than the excavated material from the spillway, to create a set of rapids that are clearly eroding back up the channel. The head of the rapids are close to the saddle. The key point is probably the location where the channel become notably steeper – this is where scour accelerates. One this point is reaches and passes the saddle, the rate of flow will start to increase and we might well see the breach developing. Unless the top of the rapids is being impeded by a large boulder or similar, this will probably develop quite quickly.
It remains hard to know how quickly this will develop once the saddle is reached, but downstream communities need to be prepared for a rapid breach.
It's coming.. soon!
Dave,Comparing the two photos, in particular the large boulder on the true left bank of the lake, I get the impression that the level of the water in the lake may already be starting to draw down. This would be a positive sign suggesting that outflow is now balancing or exceeding inflow. Obviously retrogressive erosion of the channel is still a major concern.
Thanks David your continues study on the lake situation is very informative and a preparatory guidelines for the downstream population. now we are entering a week where things are really unpredictable at any point a big erosion has to take place. Downstream community should really prepare for it.
i feel probably that the erosion is not widening the spillway as much as it is deepening it ?? not sure though….
The Boston Globe's “The Big Picture” has picked up the story: http://www.boston.com/bigpicture/2010/06/landslide_lake_in_pakistan.html.