Saturday, May 31, 2008

Update 1: Channel Dimensions Changed - Two Meter Deeper and Bottom Slope Milder (明渠改深及底坡缓了)- Landslide Dam Removal at Tangjiashan "Quake Lake"

The channel inlet would be made two meters deeper, and the channel outlet one meter deeper. Over almost the same channel length, the bottom slope would be much milder at about half of what originally planned. The changed channel dimensions are: Length = 320 m, top width = 50 m, bottom width = 8 m. Depth at upstream end = 10 m, that is, the dam was excavated from the original top surface elevation of 750 m down to 740 m. Depth at downstream end = 13 m, that is, the dam was excavated from the original top surface elevation of 752 m down to 739 m. The channel is expected to be completed by noon (New York time) or midnight (Beijing time) today.

At 14:10 Beijing time, the lake water level was 734 m, still 6 m below the channel bottom. The water level rising speed was 1.5 m per day. The lake level is expected to rise up to the channel bottom in three or four days and the water would start to discharge downstream through the channel.

Change to the deeper channel was attributed to a desire for an earlier water release (because of the decreased channel bottom elevation). They were able to make this change because the construction was several days ahead of schedule. The original plan to blow open an entrance to the drainage channel using explosives was also scraped since it was no longer necessary. The change to a milder bottom slope was not explained.

Due to complex and likely unknown soil properties of the massive landslide dam, there would still be a possibility of catotrophic collapse even with a proper design and construction of the drainage channel.

All the 190,000 people expected to be affected by the 1/3 dam collapse scenario have been evacuated.

I would define success as the downstream water level rises to or below that projected for the 1/3 dam collapse scenario. I am of course hoping for a smaller water level rise downstream (thus less damages) if the water release and dam removal would indeed occur slowly.

Yesterday, I sent my best wishes and my thoughts (or reminder) on proper design and construction of the drainage channel to the China Ministry of Water Resources. This is possibly the best thing that I could do for my motherland within the distance and time constraints.




我认为如果排水除坝过程中下游水位不超过三分之一溃坝方案线, 就是成功。当然, 我希望湖水会真的慢慢下降,下游水位会上升更底而损失更小。


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