Three observations about this video:
- It shows that the major restriction on flow is still the large boulder at the saddle of the spillway;
- A second boulder has formed substantive constriction downstream as well, but this is not controlling the overall flow
- There is still considerable scour occurring, especially on the downstream side of the boulder, upstream of the boulder towards the neck of the channel, and elsewhere on the banks of the channel.
In case anyone was wondering, that expert is not me! Whilst I wouldn't completely rule out the possibility of a controlled blast, this would need to be undertaken with extreme care, and only after a full analysis of the consequences had been undertaken. I would in particular want to see an evaluation of the likely flood that the blast might trigger, and the potential for triggering further slides on the sides of the valley. However, this is not an excuse for inaction - NDMA really need to move quickly now to evaluate the range of options, and to communicate the pros and cons more widely.
As per my post yesterday, I will use the Hunza Monitoring Site to host updates on the situation at Attabad, and this site to provide a commentary. I will put a new set of data on the monitoring blog later this morning.
Blowing up the boulders will have little effect. Watch the first major rapids at the head of the spillway. Frame 1:47 of 3:29 in the YouTube video shows three v-shaped patterns. The elevation drop at this location is substantial. When this drop 'headcuts' through the spillway's crest structure, flows will increase dramatically.
ReplyDeleteYou can acutally see how erosion is happening at the top section. Whereas the water in the middle is blue, it's very brown because of erosion at the river banks.
ReplyDeletePakistan TV reported that as of this morning, the lake level had dropped by 6".
ReplyDeleteI Think the NDMA is right in rejecting the suggestion of the expert. If you see closely, the boulder is carrying the load of the complete hillside that is actually unstable landslide debris. As the boulder restricts downstream flow, the water curves towards the track, broadening the channel.
ReplyDeleteThe boulder would eventually get loose but hopefully after sufficient ground towards the track has widened to prevent another block by a land slide.
I believe that I see flow piping through behind the upper large boulder. Watch for this increasing and open channel flow on both sides of this boulder soon. I believe that the largest boulders are our friends to armor the channel to maintain gradual erosion and prevent a catastrophic flood.
ReplyDeleteI had earlier remarked on a large underwater boulder opposite a large boulder above an eddy pool. That boulder has now slid downstream. You can see it a bit downstream of the red flag. The video makes it plain that that pinchpoint does little to control the flow. The gradient is still a bit less below the red flag; so, the retrogressive erosion seems to have declined for now.
ReplyDeleteAt the top you see a laminar flow over what looks to be a slab, but in this case it's one extremely large boulder that could very well stay put and maintain the current lake level indefinitely -- at least until the retrogressive erosion undermines it or heavy glacier melt erodes a bank to a point where the flow bypasses this boulder and finds erodable material -- at which point things could happen very quickly.
It scares me to think that the authorities would contemplate allowing the downstream population to move back before the lake is substantially reduced as, left to nature, the level could reduce itself at any time although that might be years from now.
Below the "slab" you see the flow curling into the right bank and coming back over itself, plus it pushes up against the boulder on the right and curls over to the main stream. There's likely some tilt in the "slab" that directs some flow to the right.
It is too bad that drilling was not done to assess the material so that controlled blasting could be done as warranted.
Without the valley bottom, rerouting the KKH along steep slopes will require an enormous amount of blasting that will likely trigger further landslides, any of which may cause the dam to burst.
George
The discussion seems restricted to the boulders visible on the surface and their expected movement, owing the flow of water and erosion thereof, in the newly formed shallow channel.
ReplyDeleteBut the entire spillway is filled with hundreds of such big boulders, packed in between by the smaller boulders and the debris to have formed a strong long barrier.
All the efforts applied may reduce the lake level to an extent but looks like this lake is now there to stay for good.
Now that the lake is full, the material underlying the spillway is saturated and granular (granted that some of the grains are the size of buildings). A seismic event could trigger a rapid failure via liquefaction. There is a possibility that the "slab" at the top could suddenly become a natural bridge depending on the state of the material beneath it -- among other possibilities.
ReplyDeleteGeorge
George- No doubt about it, I would not want to live downstream. It's still the first couple of weeks, and the river is still removing quite a bit of material, so it's still quite early. I just hope we are beyond "that one rock," and to be quite honest, I don't know, maybe one rock is holding it together, but the length of the dam is far greater than any one rock.
ReplyDeleteIf liquefaction started, I suspect that the force from the reservoir would move much of the materials down, and once the static is broken, it's tough to get it back.
I like to know what's the best outcome to this situation. What is the best action if the dam holds?
@tropical: "What is the best action if the dam holds?" I'd say, pump the water out, siphon it off. The Japanese offered to do it but due to political corruption, officials hoping to get kickbacks from Pakistani companies declined the Chinese and Japanese technical expertise and resolved to do it themselves. 20 days were lost in meetings meanwhile village after village and fields, orchards and houses were submerged. Only 5 or so earth-moving machines were at work digging the spillway and no more than a few hours per working day. Preventing the flooding of villages should have been paramount in the order of business.
ReplyDeleteOrchards were vital to the local economy. I wonder if a tree that has been submerged several months can survive. If all these trees die it will be catastrophic. I would also hope that a progressive emptying of the lake will not wash away the topsoil.
Kamo
Looking at the photos at Pamir times that went up yesteday, my untrained opinion is that the area limiting flow has moved downstream almost to the large boulder that projects into the channel from the right side (as viewed when looking down the spillway in the direction of flow). There has been considerable erosion of the right bank just above this boulder (for about 30-40 meters) since the video was taken (cutting back several meters into the bank) which either allowed the channel to go around what was limiting it before, or merely decreased resistance to flow at this point due to the widening of the channel.
ReplyDeleteIt will be interesting to see what develops now at this projecting boulder. There does seem to be evidence of erosion on the opposite bank, so maybe the water will carve a channel around it.
But there is a lot of material on the steep banks of the channel, so this may take some time. Reports are the lake level is rising, and the increase in discharge volume seem modest to me (as if it reflects just the increased water level, and little if any channel cutting).
The temptation to dynamite the spillway ought to be mitigated by the general observation that the Hunza's flow will increase roughly 4-fold within a month -- to a nominal 20,000 cusecs. Material resistant to today's ~5,000 cusec flow may not be as effective then. The risk is an accelerating breakdown of the spillway. Ask yourself, how much of the 374 feet of impounded water would be a major flood downstream?
ReplyDeleteAnother aspect of Pamir Times recent photographs has received scant attention. That is the one that shows a flow under the right side of the spillway (as you look upstream). The photo bears the tag "Mouth of the spillway is wider than the main body, making it look like a funnel, with a bottle neck". Notice the slips of paper used to gauge the water's velocity. Notice also the extension cracks in the surrounding material. It seems to me that even without blasting, the spillway could become much wider quickly.