Saturday, May 31, 2008

Update 2: Final Constructed Drainage/Sluice Channel Dimensions (最后建成导流明渠尺寸) - Landslide Dam Removal at Tangjiashan "Quake Lake"

By 22:00 May 31 (Beijing time), the entire channel was finalized and completely constructed (see photo to the left for a view of the channel inlet segment). The final channel dimensions are: Total length = 475 m; bottom elevation of inlet segment = 740 m, bottom width > 7 m; bottom elvation of the middel segment = 740 m, bottom width > 7 m; bottom elevation of outlet segment = 739 m, bottom width > 10 m.

The channel was further changed to make the bottom wider in the downstream segment than the upstream segment and the bottom slope milder in the upstream segment than the downstream segment.

[Other reports: 1)"Leaving a 475-meter-long channel up to 10 meters wide on the giant blockage". 2)The total channel length as 475 m, inlet elevation down to 740 m, and outlet elevation down to 738 m. 3)Inlet channel bottom elevation of 739.15 m - at the inlet edge I assume. 4) Near 4 meters wide at the narrowest location from a CCTV reporter (Bai Songyan)'s body measurements]




[其它报导: 1)475 米长 宽至 10 米。2)“泄洪槽总长475米,泄洪槽进口高程降至740米,出口高程降至738米。”3) 泄流槽(进口)高程739.15米 - 我想是在进口边缘。4)中央电视台记者白岩松用身体测量最窄处近四米宽。]

Info source: Xinhua; Photo credit: Reuters

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。


Info source:

Thursday, May 29, 2008

The Most Daring Landslide Dam Removal in Human History: Proper Design and Construction of Drainage Channel is Key to Success (唐家山堰塞湖-泄洪槽设计和建造是成功关键)

It just came to my full realization that the Chinese authorities are actually trying to release the water and remove the landslide debris/dam simultaneously at the Tangjiashan "quake lake." Apparently, keeping the dam in place was not an option. The dam removal and water release operation will start within several days!

A drainage/sluice channel is being dug through top of the dam, and is almost completed (see photo to the left). Once the water starts to flow in the channel, water current will scour the channel bottom and the channel sides. The channel will become wider and deeper while water level in the "quake lake" will be lower and lower. The channel bottom and sides will scour because of steep slope of the channel being dug.

If it works perfectly, a good mixture of debris/soil and water will be flowing gradually downstream, water in the lake will be draining down slowly, and the dam itself will disappear steadily. In this scenario, the flooding downstream will be minimal, although some debris/soil will deposit along the way.

However, if scour of the channel is not gradual and initial release of the water triggers a catastrophic collapse of the dam, entire volume of the lake water will rush downstream rapidly, leading to devastating downstream flooding and destruction.

Therefore, this daring engineering has to be designed and executed extremely carefully. A detailed hydrodynamic and sediment transport analysis (including computer modeling and physical modeling which Chinese engineers are fully capable of conducting) would help properly design and construct the channel. A reinforcement at downstream end of the channel and/or diversion of the released water far away from toe of the dam may also be necessary.

The best wishes to the Chinese endeavors!!!

The channel dimensions are: Channel length = 300 m, bottom width = 13 m, top width = 50 m, inlet bottom elevation = 742 m, outlet elevation = 740 m (i.e., an elevation drop of 2 m over the length of 300 m, a steep slope that will definitely result in soil scour once the water flow starts), upstream depth = 8 m, downstream = 12 m. Total volume of the debris/dam = 20,370,000 cubic meters. At 8:00 a.m. of May 29, 2008 (Beijing time), lake water level = 730.13 m (rising), water depth = 62.58 m, lake water volume = 161,000,000 cubic meters.

Three downstream evacuation plans were planned: 1/3 of the dam opened up instantly, 1/2 of the dam opened up instantly, and the entire dam opened up instantly (the catastrophic collapse). Nearly 200,000 downstream people are being evacuated based on the scenario of 1/3 dam collapse. More than one million people are on alert, and they would have to be evacuated if the total collapse, in addition to inconceivable property and natural environment damages. A lot is at stake, and it is obviously highly desirable to have a well engineered slow dam removal and water release.

The Chinese Short Version:

从媒体报道得知,唐家山堰塞湖的泄洪槽(导流槽)已基本上修完。 湖水面在近日就会上升至槽(渠)底开始排水。

因为新修的渠比较陡 (300 米距离内底部高程降2米),放水一开始,渠道就会冲刷。我想这是中国工程师预期的效果,既边排放湖水,边冲走堰塞土堆/坝。

我相信中国的水利专家已经为决策高层设计了最佳的渠道高程,断面,和坡度。修造最佳的渠道可以达到缓慢放水,平稳冲涮(除坝)的目的。最佳设计可以先通过水动力学与泥沙传输原理简单计算得出,然后最好再用复杂的数学模型及/或大型物理模型验证。万一这一步还没做,需马上请国内专家算, 以作渠道调整。我在美国也可以帮



Photo credit and info source:

Monday, May 19, 2008

The Highest Annual Rainfall in the World: Wailua River on Kauai Island, Hawaii

A photo that I took of the Wailua River on the island of Kaua'i, looking downstream toward the Pacific Ocean. The headwater area of this river receives the highest annual rainfall in the world, at 460 inches (11,700 mm).

I was in Hawaii last week attending the ASCE/EWRI Environmental and Water Resources Congress. I presented a paper and moderated a technical session on certification guidelines for stormwater manufactured treatment devices, and later presided over a committee meeting on the same topic.

While enjoying the nice weather and beautiful scenery in Hawai'i, I was saddened by the devastating news of earthquake in China.

My ablum contains additional photos of the Hauai Island.