Regular readers (of which there are now several thousand each day I think - thanks to you all) may remember that a few days ago I noted the presence of a large boulder in the channel, right at the crown of the spillway:
Well it is this boulder that is preventing the scour from propagating upstream and that is, in effect, protecting the upper part of the channel against further erosion, as this image shows. The boulder is circled:
The lower part of the channel is continuing to erode with some power, and the channel is widening. The red lines show substantial cracks suggesting further potential widening. The steepest part of the channel is continuing to erode backwards, such that sooner or later the boulder will start to be undercut.
Meanwhile, there appears to be an increasing level of frustration about the very confused spillway discharge statistics emerging from NDMA. This is understandable. If we take the last four daily reports we get:
29th May 18:00: 150 cusecs (cubic feet per second) from the spillway, 350 total discharge
30th May 18:00: 900 cusecs total discharge
31st May 18:00: 900 cusecs total discharge
1st June: 18:00: 1050 cusecs discharge at Ganesh Bridge.
This is a quite bizarre mixture of measurements, but we should all understand that the NDMA team dealing with this are under huge pressure too.
Meanwhile, the more reliable measurements of inflow suggest that the rate is continuing to increase linearly with time, probably because of the choke provided by that boulder. However, I am very surprised by just how linear this increase is proving to be, so please treat this with some caution:
The lake level upstream continues to rise, increasing both the damage and the amount of water in the lake. In the last 24 hours the lake level rose by 75 cm, perhaps suggesting that the rate of inflow is further increasing.
Finally, several people have emailed to ask why I have not covered the multiple landslide accidents around the world in the last few days, including the multiple slides in Central America (and that amazing sinkhole), the landslides in China, the non-fatal slide in New Zealand, etc. I will try to catch up on all of those things as and when this event ends - please accept my apologies. You will understand that I have to maintain my day job whilst doing this, plus yesterday and today I am moving house!
Good luck moving and thanks for all your hard work!
ReplyDeleteExcellent information and commentary as usual.
ReplyDeleteI note from yesterday's photo that there is an area of white water downstream of the area circled as the bottleneck, whereas at the location shown the channel is quite wide and the water flow looks smooth. I also wonder if the boulders shown in the two photos are the same. Difficult to be sure despite the excellent quality of the photos. The material beneath the second boulder looks quite cohesive as shown by the imprint of the teeth of the excavator. This would presumably be harder to erode.
If the lower location is the bottleneck, then this brings it closer to the active area of erosion increasing the risk of undermining of the boulder.
As stated previously, measuring the discharge from the spillway must be very difficult, but if the data is reasonably correct, the outflow needs to increase substantially before the lake inflow is balanced.
I would sure like to see today's picture, bigger and without the red lines.
ReplyDeleteThe large rock that leads the right side of the spillway looks like it will be an island soon. The lake is rising enough to make that boulder moot. I would not be surprised if the channel shifts to right before that boulder is undercut.
Dave,
ReplyDeleteThanks so much for all the updates about this event. Here in New Zealand we hear absolutely no other news about this and I've followed your updates with great interest.
And to all those affected, my thoughts and prayers are with you.
Kind regards
Phil
Those large boulders have always been of particular interest, at least to myself...
ReplyDelete1 What is nature of former river slopes?
ReplyDelete2 What is nature of slide material?
Why so many apparent terraces in photos of the downstream areas?
Suggest that slide debris is poorly sorted varying from rock flour silt with some clay from weathered to disintegrated rock to intact rock seams fractured during the slide.
Former channel deposits likely consist of larger cobbles, boulder and stone sizes with sand and gravel infill or open voided honeycomb structure. As levels within the impoundment area rise either progressive piping and collapse of the former river banks and/or differential head are increasing the seepage portion of the flow. It may be that the flow paths are long enough that the slide is initially stable against piping and stability failure. Down cutting of the softer erodible materials in the excavated spillway (was the spoil placed on the adjacent slope) will eventually result in a breach assuming sufficient inflow is available. Look at Freed's Dambreak Model and adjust for material mix. Looking at the photos you may end up with a rock fill with the porosity of swiss cheese instead of a Grand Teton failure mode.
GRider
Your work is appreciated so much. Thanks for everything, Dave, and good luck with the move. Don't forget to leave the kettle and teabags out!
ReplyDeleteif the NDMA is measuring water at Ganish bridge ,they might be doing it wrong as the water from Ultar glacier adds up there
ReplyDeleteThe photo on 4/6/2010 it looks the water will not able to cut enough to the downstream because
ReplyDeleteof boulders and heavy rocks
please if you visit Gojal and make measurement the in flow of water to the lake.I understand
the water in flow will not remain in same quantity due to weather changing.
My on assessment by naked eye. the Gojal will remain under the flooding up to the end of August.
Hikayat shah
I must commend your committment to our worries and the confidence you inspire in all of us.On behalf of my country I thank you from the depth of my heart.
ReplyDeleteWish you and your family good luck in your new house. Pakistan is also now your house, it would be an honour to have you amongst us,you have given a lot of insight and motivation to our people to analyse the technical aspects of Geological phemenon in our region.Please do not forget us. My apologies for the amatuer remarks of our senior government officials( Bureaucrats) who Know everything but Geology!
Regards
Kaiser Hameed Khan Islamabad Pakistan