AP image captioned: Water from the Tangjiashan "quake lake" flows down a landslide site in the quake-devastated Mianyang City, southwest China's Sichuan Province.Today, the following Reuters image has been released, apparently showing the same location:
Reuters image entitled: Water flows through a sluice channel of the Tangjiashan quake lake in Tangjiashan, Sichuan Province June 8, 2008 in this picture distributed by China's official Xinhua News Agency. The water level in the quake lake stood at 741.82 metres above sea level at midday on Sunday, still 1.45 metres higher than the sluice, with the lake's volume exceeding 240 million cubic metres, Xinhua News Agency reported. Picture taken June 8, 2008.The caption says that the picture was taken yesterday. Clearly there was massive erosion on the downslope face. The flow yesterday was reported as c. 10 cumecs, whereas today it is estimated to be 50 cumecs. If these dates are correct then the erosion rate must now be very rapid.
A further image shows the site from above:
Reuters image entitled: Water flows through a sluice channel (R) of the Tangjiashan quake lake as another channel is being constructed in Tangjiashan, Sichuan Province June 8, 2008 in this picture distributed by China's official Xinhua News Agency. The water level in the quake lake stood at 741.82 metres above sea level at midday on Sunday, still 1.45 metres higher than the sluice, with the lake's volume exceeding 240 million cubic metres, Xinhua News Agency reported.Note two things here. First, the magnitude of the erosion. Second, as suspected yesterday, the team on the dam appears to have filled in the channel that they cut before (see the bulldozers on the left - compare this with the first picture (above) in this post).
The critical issue is how quickly this erosion is moving upslope. I suspect rather rapidly now as the flow in the channel increases and the water level in the lake rises.
Dear Dave:
ReplyDeleteThe water level is 742.62 at 10:00 9 June. They were reported 10 seepage point with 3 quit large discharge. the overflow discharge was 57 cms on that time.
The new direction for Chinese government is reduced dam height to 720 which can reduce 100 million cm volume. Since the channel erosion from the bottom part, I guess that some channel development was connected to the seepage channel too. I agree with you that with this highly steep dam face 80m per 803 m, the erosion rate should be very quickly. I hope that everything goes to right direction, otherwise ????
Dear Jennyf,
ReplyDeleteThanks for your comments.
Think of the lake like a basin of water with a leaky plug. If the leak in the plug lets out more water than the taps are letting in then the basin will empty, and vice-versa. In the case of the lake, the outflow is less than the inflow because the channel is not deep and wide enough for the height of the water to drive enough flow through. As the lake deepens the height of water in the channel will increase, so the flow will also increase, but the question is when this will occur. Ideally the flow should gently erode the bed at the crest of the dam, progressively dropping the level of the lake. There is no evidence that this is happening. The danger is that the steep part of the channel erodes backwards up the channel, causing it to suddenly deepen when it reaches the top of the channel. At this poinbt there will be a sudden and very dangeroud release. This is one of the major concerns right now.
Hope that helps
Dave
Dear Dave-
ReplyDeleteI appreciate your blog, it appears to be the most curent and authoratative on the web. I am wondering when we can start predicting the breakdown of the dam itself as the erosion works it way back up the slope. Is there any hydrology mathematics to determine this, or at least a rule of thumb?
Thanks - Brian
It looks to me like the "channel" they filled in was actually a buttress for the side of the channel. Maybe they knew the dump-truck area was likely to collapse, so they abandoned the trucks and put in a buttress of compacted earth to keep the bank on that side stabilized and try to keep the channel from enlarging too fast. Just a guess, though.
ReplyDeleteHow far below the dam top is the spillway? Looking at photos like the one in Friday's post the spillway was not particularly deep and with the lake surface now 2.5m above the channel and still rising is there still a danger of overtopping? After the channel began to flood the diggers were tasked with creating a second channel and I had assumed that the picture in your last two posts were them grading in preparation for that rather than shoring up the original channel. If a second channel can be opened then the same discharge can run at half the velocity which will considerably reduce the erosion and spread the points of erosion.
ReplyDeleteJJ
National Geographic is hosting a 6/9 video clip sourced from the AP, probably via Xinhua. There's closeup footage of the drainage channel, as well as a few shots of the dump trucks and some anti-tank missiles and explosives fired.
ReplyDeletePoint well taken. My perspective is shaped not by an actual view of the situation, but by the very limited information available on the internet. While, I think I raised a valid concern based on the information available, I overstated the case above.
ReplyDeleteHere's a good photo showing where the water is beginning to go down the sluice:
ReplyDeletehttp://www.hindu.com/2008/06/10/stories/2008061054631300.htm
attributed to Xinhua.
Dear andy,
ReplyDeleteThe peak discharge increase to 1210m3/sec at 9:00 (10th June, Beijing time) and water level drops to 742.18m. We can guess the erosion rate is very high at this moment from rapid increase in discharge from 377 to 1210 within three hours. I think such a rapid increase in discharge is not good in terms of downstream hazard.