I remain somewhat confused about what is going on at Tangjiashan. We know that the sluice now has flow within it - that is not in doubt. But what is happening with that flow is very hard to understand.
First, let's work out the flow rate needed to maintain a constant lake level. The earthquake happened on 12th May and now it is 7th June - this is 26 days. In this time the lake has, according to Xinhua, accumulated 229.5 million cubic metres of water. By my calculation this is approximately 102 cubic metres per second.
So, to maintain the level of the lake at a constant point the channel plus seepage should be transporting this volume of water. More than this and the lake will slowly empty, less and it will fill (this is certainly not rocket science!).
Now let's look at the images that are available at the moment. In this post I highlighted one that appears to show a low level of flow. The image below has also appeared here. This clearly shows a level of flow that is well below 100 cubic metres per second (from now on let's call this a cumec for convenience). Furthermore, Xinhua has now said that:
"Experts calculated that the water flows at seven to eight cubic meters per second, far more than the previous two cubic meters per second."
Now the figure of less than 10 cumecs is absolutely consistent with the image above, which is clearly of this order. There is no way that this is the 100 cumecs required to balance average inflow.
So where is the excess water? I cannot work this out! Possibilities are (as I see them - there may be more):
1. In-flow is only <10 cumecs now because it has not rained for a day or so. I do not believe that this is conceivable;
2. The lake is filling so slowly that only <10 cumecs is escaping. This is possible - but suggests that there is a great deal more flow to come (which means that the current situation is a "phoney war");
3. The excess is being dealt with by seepage through the dam (in which case then the flow is now essentially out of control - this is not necessarily bad news, but it could be);
4. The volume of the lake has been seriously over-estimated (this does not seem likely).
There may be other options. Suggestions please?
Whatever, something does not add up here, and that is very worrying to my mind...
Finally, Ripen asks what the channel is on the right side of the above image? Is this a former attempt to cut a channel by the army? Is it erosion by seepage (there appears to be water within it)? Is it a natural feature associated with the landslide (I don't believe this). Or is it something else (again, suggestions welcome please)?
In conclusion, something feels wrong here. I hope it is just that I am not getting a good picture of what is going on, which means that I am missing something vital. This could well be the case, or...
Indeed - in fact the flow in the Thames at Henley is slightly less than 100 cumecs actually (about 75 cumecs I understand). I think that your comment that this is just the initial low flow is right. If not, then as you say there is something else going on.
ReplyDeleteI have to admit to being confused as to how this is meant to work. The channel lip is at an alt of 740m and water entering the lake after this point will be able to exit (allowing for a small additional head of water (C) to drive the flow through the sluice say 740 + 1.5). All well and good but then what? If this was a man made concrete dam this balance would be maintained indefinitely but the level would never drop. Obviously that is not what is going to happen here, the rock and soil are going to erode and release the bulk of the lake the question is how, when and how quickly. If you could control the erosion so the channel maintained its shape but gradually cut down through the land slide until it reached the old river bed you would have a perfectly controlled release. If it erodes primarily from the down stream end then the dam mass reduces without lowering the 740m+C or mass of water so the potential energy remains constant until catastrophic failure. Reality is presumably somewhere between these extremes. Given that the dam is made of friable soil with some rocks what is the normal method of erosion and failure for this type of structure? Are there any good videos/time lapse photography of previous similar attempts (obviously this channel over the top is not the first time this has been done) or film of simulations (computer or experimental)?
ReplyDeleteThanks JJ
"Finally, Ripen asks what the channel is on the right side of the above image? Is this a former attempt to cut a channel by the army? Is it erosion by seepage (there appears to be water within it)? Is it a natural feature associated with the landslide (I don't believe this). Or is it something else (again, suggestions welcome please)?"
ReplyDeleteMy guess is it is the edge of the slide. The river was coming around the hairpin and the slide flowed from off screen left in the photo ‘slopping’ across the river bed. Under this interpretation the darker material (left and centre) was deposited by the slide while the slightly redder material is the original bank (and seems to be of a piece with the bank as it extends below the landslide). I would be very surprised if this feature was man made as news coverage has been going on about the ‘600 men working 24/7 to dig the channel’ but by the look of this structure it would have needed a far more herculean earth moving effort than the channel. Water in the channel I take to be seepage finding its path of least resistance. If this natural groove was already in place I wonder why it was not used as the basis for the spillway channel.
JJ
JJ, I am sure that this is the edge of the slide in terms of the fact that the red material is the bank and the grey material is the landslide mass. However, the edge of the grey material looks to me to have suffered erosion or some other form of removal - surely this sort of sharp edge would not be formed by movement of the landslide? Note how similart this looks to the much smaller area of erosion to the left where there is channel flow. The question is what caused this apparent mass erosion?
ReplyDeleteThe ongoing excavation for the channel can be seen in the left centre of the image - the excavators are visible if you look carefully. I note also that there are quite few ecavators etc at the top of the channel on the right.
I remain confused!
Kung and Dave thanks for your explanations.
ReplyDeleteHaving made some guesses earlier I will now change up a gear and speculate wildly on a subject I know nothing about.
Dave just to be sure we are talking about the same thing. The area of the dam between the new man made channel and the red/grey boundary looks like a discarded fletcher’s knapped flint with the sharp end pointed down stream. There seems to be ridge down the middle with the sides (particularly on the right of the photo) getting very steep at the down stream end. Where it butts onto the slope of the old red bank it forms a very pronounced V. What caused this?
I do not see how it could be caused by erosion there has not been time unless you allow for an very unlikely scenario involving two slide events with a dam failure between.
When granular materials flow they behave much like liquids and in a quake the shaking can cause liquefaction in sandy soil.
The landslide flows down into the valley and its momentum causes it to slosh up against the step red wall and washes back settling with the ridgeline as a frozen interference wave crest. Final smoothing and polishing done by aftershocks and recent rains.
I did warn you that I would be speculating wildly. JJ