The discharge across the spillway at Attabad is still increasing. Whilst some reports suggest that it is higher, the Pamir Timeshttp://pamirtimes.net/ suggests that the flow at 2 pm local time was 250 cubic feet per second (7.08 cubic metres per second). If this is correct, the discharge time graph looks like this:
However, the situation is now somewhat confused as the NDMA report from 6 pm on 30th May (i.e. yesterday) was that total outflow was 900 cusecs (25 cubic metres per second). Assuming that 250 cusecs is seepage, this would give a spillway flow of 650 cusecs (18.4 cubic metres per second). This seems to be surprisingly high. Maybe the NDMA information has the wrong time stamp?
Therefore, at the moment I am a little confused as to what is happening. I will try to clarify this in the next few hours.
The latest news coming from there reports total outflow through spillway and seepage to be 800 cusecs, 400 cusecs each.
ReplyDeleteAlthough not an expert on channel flow, my expectation about what flow would be now is the opposite of yours: I would have expected it to be even higher, increasing more than it increased over the prior interval.
ReplyDeleteFlow is proportional to the pressure head, which increases as the lake level rises, and also the conductance of the channel, which is also increasing due to the elevated water level (the x-sectional profile of the spillway at the choke point grows as the water level rises). In a circular pipe, the conductance is proportional to the radius to the 4th power, so even modest increases in the height of the water channel can have substantial effects.
Thanks a lot Raheel... great find!
ReplyDeletemr khokar.. which madrassa did you attend? I hope not the Binori masjid one !! large hoses ?
ReplyDeletewhy is it unfortunate that major erosion is not happening soon?
though agree with you that blasting in mountains is not advisable ever.
The spillway has a sufficient size to eventually discharge the quantity of water entering the lake, it will just take some more days and probably a couple more metres of lake rise to achieve this steady state.
ReplyDeleteIdeally, what needed to be done was to excavate a channel from the crest down the downstream face of the landslide and line it with large boulders. The size of the boulders would need to be such that they would not be eroded/dislodged under the maximum spillway discharge. Unfortunately this would logically be the peak river flow which is I assume several hundred cubic metres/second. This would also mean that upstream of the landslide would remain flooded for the foreseeable future.
Creating such a lined channel would involve huge resources and take time, probably more time than was available between the landslide and now.
Engineering a controlled erosion of a river channel through the landslide does not seem feasible. Downcutting is dependent upon both the size of particles in the channel bed and the velocity of the water. As soon as downcutting starts, the discharge from the lake is increased, raising the flow rate and logically increasing the rate of downcutting. This would seem to be a self-perpetuating and accelerating phenomenon unless the channel encounters significantly coarser material (such as large rock blocks) or the channel widens faster than it can downcut. There are too many variables to engineer this process.
One method to obtain a reasonably accurate flow rate would be a monitoring station a safe distance downstream of the dam with the subtraction of any small tributaries between.
ReplyDeletemy dear mr anonymous, there is no safe distance downstream....
ReplyDelete