31 May 2010
Flow through the spillway at Attabad has continued to develop during the day. However, as I suggested in my post this morning, some of the higher discharge values being quoted are without foundation. At 6 pm the discharge was about 360 cubic feet per second (10.2 cubic metres per second). Thus, the discharge graph looks like this:
At the moment there appears to be little evidence from these figures that large-scale scour and erosion has been initiated – the increase in flow appears to be the consequence of the increasing lake level, plus perhaps some lateral erosion.
However, erosion at the toe of the slope remains the major concern, with reports suggesting that the gully is continuing to erode back through the channel. This remains the most likely failure mode of the dam, but this may take a few more days to initiate. In addition, the increasing channel discharge may cause the flow velocity to increase to the point at which basal erosion starts. This has not yet happened, but could begin at any stage.
There have been various comments on the blog that the dam is not behaving as expected – i.e. erosion is not initiating – and that as a result measures should be taken to encourage erosion. I want to stress that this is not correct – from the start we knew that it could be that it could take some time for erosion to initiate. So far I see nothing surprising in the behaviour of this system, and nothing to suggest that erosion will not allow a lowering of the lake level in the next few days. At the same time, it is important to emphasise that the probability of a rapid breach has not diminished, and may in fact have slightly increased due to the headward erosion of the lower gully.
There is no need to panic, but at the same time there is also no justification for lowering the alert status. We all need to be patient and alert – this has a long way to go yet. If you are in the area please pay attention to the local alert system.