Attabad: flow and erosion are continuing to develop

Report compiled by A Khokar  June 01, 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.

 Ref: Dr. David Petley Blog—20:16PM May 31, 2010

But the  Other Enginners are of the opinion that:

There are three scenarios of how the situation may develop over the next week or so:

1. The river cuts down through the spillway in a gradual manner leading to a controlled discharge of the lake.

2. The river cuts down rapidly through the spillway leading to an uncontrolled discharge of the lake.

3. The spillway remains stable and the lake continues to rise until the outflow through the spillway balances the inflow into the lake.

Scenario 1 is the ideal situation leading to a safe discharge of the lake. Taking Anon’s volume of 460M cu m for the lake, for a discharge through the spillway of 2000 cu m/sec (cumecs), it would take 2.6 days for the lake to be discharged. A discharge of 2000 cumecs though high is probably not exceptional for this area (GLOFs?)

Scenario 2 involves a rapid discharge of the lake in an hour or so, leading to an exceptional river flow and probably significant damage downstream. If people have been evacuated, there should hopefully be no loss of life, however there may be significant damage to property, land and livelihoods as well as damage to infrastructure such as bridges and roads.

Scenario 3 results in permanent flooding to the valley upstream of the landslide to approximately the current extent. There should be no loss of life, but there will be significant humanitarian impacts such as loss of property, land and livelihoods for the whole of the flooded area, as well as the need to replace over 17km of the KKH, including the potentially challenging stretch through the unstable landslide area. In the interim there will continue to be disruption to life in the upper valley until proper communication can be set up.

One other unfortunate circumstance of Scenario 3 is the fear that future flooding events could initiate a Scenario 2. The mean monthly river discharge at Dainyor 100km downstream of the landslide is over 5 times greater in July and August that in May. Peak flood discharges will exceed this mean monthly figure. Any flood in excess of that experienced previously could trigger an uncontrolled discharge of the lake. Therefore, in the interests of safety, evacuation of people downstream of the landslide may be required on a periodic basis for years to come.

Let us all hope that Scenario 1 comes to pass, but as Dave has stated now is not the time to lower the guard. The risk will remain for many days yet.

Dave Blog–8:28 PM, May 31, 2010