The images seem to show that the key constraint remains the point of the large boulder as before, although the water has now eroded around this, leaving it stranded in the middle of the channel:
The channel is clearly widening on both sides through slope failures, increasing the capacity of the channel, but there is still remarkably little sign of down-cutting. Meanwhile downstream, the dam is continuing to lose mass through erosion, a process that we must watch carefully:
This loss of volume is very slowly weakening the barrier. There are no signs at the moment that this is a cause for real concern, but close monitoring is needed as the flow rate increases.
The boulder sticking straight up does not appear to me to be the principal obstruction of the total flow. Note the smooth nonturbulent flow up to the boulder. This shows that there is little dissipation of kinetic energy. At the upstream side of the boulder the water curls up to about the height of the boulder. The difference between water heights at the up- and down-stream sides of that boulder is a rough measure of the drop from lake level to that point. At the left (near) side we see a drop over a ledge/slab about 1/4 to 1/3 the height of the boulder yielding a drop above the boulder some 3/4 to 2/3 the height of the boulder.
ReplyDeleteRemember that this boulder was originally part of a massive slab leaning on the right (far) band that fractured as it was undermined. I originally thought that this upright slab was the somewhat finger of the massive slab, but these photos show something that rotated up or downstream as it departed its parent slab -- a large piece of which I suspect lies immediately upstream.
George
Below the second midstream boulder down the spillway, there seems to be a transition in gradient. The recent photos show three ledges/slabs at this point that I suspect have halted the
ReplyDeleteretrogressive erosion. The right bank shows a number of slabs, mostly pale yellow in this section; however the two prominent midstream boulders are dark grey with light veins.
At this point I would be extremely cautions about landing boats on the right bank ramp close to the spillway as one photo shows a distinct crosscurrent that could suck a boat down into the spillway. I have seen exactly that happen similar situation.
George
George- Considering the right-hand side is the active slide side, and that the road is on the left-hand side, I would guess that there won't be too many right-hand side landings. If workers or someone wanted to take photos on the right-hand side, then the lake would be a way to cross from one side to the other.
ReplyDeleteDivalent-
ReplyDeleteIn 3386 there is a person on the left-hand bank. I use that for a scale approximation.
From that, I am going to guess the boulder is about 8 meters.
Also in 3386, it appears that the bottleneck is near the standing man. See 3382 also. The man is standing near where the blasted rock was. If someone suggested the bottleneck was about 30 meters downstream from the man, I would consider it reasonable.
Let's look at 3400 & 3405.
In 3400 it looks like there is about a five meter drop in elevation, plus or minus. I make this approximation based on the rock being 8 meters high, and the river height changes significantly there, including the shift to total whitewater.
That said, looking at 3405 reminds me of when I visited the Tacoma Narrows in 2005. I happened to be there during a near maximum tidal flow event. I forget what the speed of the water was. Oh, I guess I should also say that the Tacoma Narrows is definitely, without question, a bottleneck. It would be difficult for me to begin to imagine the peak maximum flow, which is shut off after about 6 hours.
Needless to say, the water was riding high up pylons on Narrows Bridge (the replacement to Galloping Gurdy). Admittedly the pylons are in the bottleneck area, but the water is very deep (I'd guess 500-800 feet in the middle area, about 150 feet deep at the pylons).
I hope it is safe to suggest that the pylons don't significantly restrict the total flow.
Yes, while everyone else was drinking wine and eating cheese, I was looking at the flow rates and water impact on the pylons.
Some things that need to be noted, however:
1. There is some whitewater on the right-hand side near the start of the spillway.
2. There is some whitewater near where the man is standing.
The saddle point might be back near the man, and the narrow channel is relatively flat, but sloping down, so the restriction is mainly the width and depth of the channel.
It might also be that the saddle point is 30 meters, plus or minus, downstream from the man, but the total flow from the depth and rock placement are such that the whitewater shows up near the shores.
I guess we should also define saddle point. If the saddle point occurs in a wide area, it might be less restrictive than a narrow area with bed elevation, which brings us back to this rock.
It's tough when all we have is very limited information...
Oh, one more thing:
ReplyDeleteLook at the right-hand elevation drop in 3383. It looks like the right-hand side drops well before the center rock; plus the upstream V is quite obvious.
I can agree that the saddle about 30 m downstream of the man in 3382 governs the flow and level. Note the two V's by the man and 30 m down. Both V's are straddled by eddies. The top of each eddy is just below the downstream edge of an obstruction. Note also that eddies on the side of a V are at a maximum height differential at the top and that this differential decreases towards the bottom of the V. The flat cross-sectional profile at the top of these two Vs indicate a deep bottom. Earlier pictures at low flow were strongly suggestive of underwater slabs at the main V and below, but the recent increased volume obscures these bottom features.
ReplyDeleteIt surprises me that the first midstream boulder is still sticking straight up beam on to the current after it dropped into its present position. It may be anchored by a fortuitous slot between horizontal remnants of the parent slab that was earlier leaning on the right bank.
My concern about boat landings on the right at present flow levels stems from when the locals from upstream landed there to widen the banks.
George
I wish you had seen the site of slide before it happened on 04th of Jan 2010. You would have reached the conclusion that i have reached long ago, that the lake is there to stay without any danger of bursting for a long time to come. Please join me in recommending to covert this disaster to an opportunity - by using it for generating 40 MW in winters and 400Mw for the three months in summers.
ReplyDeleteMr Beg are u from pak fauj? nice idea..
ReplyDeleteWhy can't you access images? Have the authorities knobbled the FOCUS people so they can't give you any information? It seems madness if they have - as the comments section of your various posts show, your blog is by far and away the best source of pure data, information, analysis and reasoned debate of the situation at Attabad.
ReplyDeleteYes, the right bank has eroded and the cracks indicate that more erosion will happen. If/when bank erosion works its way around the underlying slabs at the top, downcutting could quickly effect major changes in the situation. It seems that peak flow is a month away at twice the current level; so, there is a distinct probability that this will happen.
ReplyDeleteHindsight tells me that geologic investigation could have disclosed the slabs in time to have drilled down and broken them up with charges so that the lake could have been discharged at a low flow period. That and sufficient excavation equipment could have spared a large portion of the upstream population and enabled the downstream people to live normally after the lake level was reduced.
Of course an earthquake could change the situation in a blink of the eye and reports of fault strains indicate that this may happen sooner than later.
George
come on anonymous.... pak fauj is the best..knows everything abt everything... how abt constructing a kargil and siachen victory towers on top of the dam?
ReplyDelete