Sunday, June 22, 2008

Iowa City Flood 2008

Photo to the left shows the flooded Iowa River at the renowned University of Iowa hydraulics lab on June 14.

I visited the lab twice in the past. Folks there actually used a real fish to test how it would move up the fish ladder. The fish ladder is installed at many dams, and is a life passage for migratory fish such as salmon.

(Photo Credit: James Hemsley)

Tuesday, June 10, 2008

Update 6 (Final Update): A Great Success in "Quake Lake" Draining! Congratulations to Chinese Engineers! (唐家山堰塞湖排水获巨大成功! 热烈祝贺中国工程师们!)

A deep channel has cut through the landslide dam by the "natural" current. See photo to the left for the initially cut through channel around 9:00 a.m., June 10, Beijing time, with lake water surface elevation of about 742 m and discharge rate of about 1,200 cubic meters per second. Channel inlet bottom elevation was cut down from 740 m to about 720 m (about 20 meters lower) and channel was cut wider from about 10 m to about 150 m. The lake water level was brought down from its highest at 742.96 m (8:00 p.m., June 9, Beijing time) to the present reading of 719.48 m (8:00 p.m., June 10, Beijing time). As a result, over 100 million cubic meters of the lake water was drained out of the lake. The population at risk of future downstream flooding has been reduced from 1.3 million (upon catastrophic collapse of the landslide dam) to 50,000.

Although the peak discharge rate was very high at 6,420 cubic meters per second during the course of channel cutting and lake water draining (and was rapidly increased to this peak value from 500 cub. m/s within three and half hours, from 7.56 a.m. to 11:30 a.m. on June 10), the downstream area flooded by this release of water was far less than the planned 1/3 dam collapse inundation zone. Note over 200,000 people were already evacuated about 10 days ago, in preparation for the possible 1/3 dam collapse scenario. Therefore, I would define this initial draining of the "quake lake" water as a great success! The engineers' original drainage scheme including the sluice channel design worked.

Congratulations to the Chinese engineers for their great success!

P.S.: I am also glad that my projection of the unnecessary second, deeper drainage channel to "force" an additional outflow was correct (see my Update 5 posted on June 8). Construction of the second channel was stopped at 6:00 p.m., June 9 as the lake outflow through the first already-constructed channel increased to 81 cubic meters per second (cms) and was approaching the lake inflow rate of 115 cms.

Photo credit and info source: Xinhuanet

Sunday, June 8, 2008

Update 5: Lake Started to Drain through Sluice Channel "Naturally" and Second being Dug to "Force" Additional Draining ("自然"道流开始了,正在开挖第二条导流槽强行增大排水量)

At midnight of June 6 (Beijing time), the lake water level rose to 740 m, the highest bottom elevation of the constructed drainage/sluice channel. The outflow was anticipated to start at that moment. However, a 0.4 m high dirt block was later constructed in the channel in order for soldiers to make the channel wider, deeper, and steeper (no specific channel dimensions were given). The soldiers were dispatched back to the dam to modify the channel in the afternoon of June 6.

At 7:08 a.m., June 7 (Beijing time), the "natural" outflow started. The reported flow rates were 2 cubic meters per second at 2:15 p.m. (with current velocity of 1.5 m/s) and 10 m3/s at 6:00 p.m. (at 741.02 m lake level). On June 8, the reported flow rates were 20 m3/s at noon (at 741.82 m) and 25 m3/s at 7:15 p.m. These inflow rates were much smaller than the reported rate of inflow to the lake, more than 100 m3/s.

The landslide dam was stable, but was not getting removed. The drainage channel was not scouring significantly and not getting much wider and deeper due to the relatively small outflow rate. The outflow rate is relatively small thus far since it would take some time to build up the lake water level/head above the channel bottom to push the water through. The slow water level rise is a result of the relatively small upstream inflow rate (due to the relatively small rainfall in the upstream watershed). The outflow rate was also reduced by partial blockage of the channel by debris floating down from the lake.

While clearing out or breaking down floating debris on the lake surface, we could simply wait for the lake water to rise several more meters so that all the inflow (more than 100 m3/s) would be pushed through the channel. We could also wait for the next rain storm that would lead to a higher outflow rate. The high outflow rate (more than 100 m3/s) would scour the existing channel deeper and wider and would eventually form a relatively stable channel through the landslide dam. If we are lucky enough, there might not be much of the landslide dam left afterward. This was exactly what Chinese engineers originally planned for, I assume.

However, understandably, Chinese leaders, engineers, and especially over 200,000 residents who have been relocated to the high ground since about ten days ago (plus other over one million residents on high alert for emergency evacuation) are running out of patience.

To increase the outflow rate, it was decided yesterday to dig another, deeper channel several meters away from the existing channel. These two channels are anticipated to merge together once the dirt barrier between the two channel is washed away during a high flow.

At the end of the second channel digging, the soil/debris pile at the inlet would have to be removed by a remotely-controlled explosion. Since the inlet bottom elevation would be below the lake water level at that time, once the inlet is blast open, water will gush out from the "quake lake", in addition to that being released out from the first channel.

My four concerns for digging the second channel, in comparison to waiting for a "natural" enlargement of the first channel:

1) Engineers and soldiers are working in a highly risky environment;

2) The explosion used to open the channel inlet, though of relatively small strength, might still trigger a catastrophic collapse of the landslide dam;

3) The sudden gush of water, immediately after the explosion, might create an impact or shear force too strong for the dam to stand, leading to a catastrophic collapse;

4) The wider channel (the existing plus the new) would lead to a lower lake level (a smaller head) in front of the channel and a smaller current velocity. The smaller current velocity may not be sufficiently large to continue scour the channel(s) and to eventually remove the dam.

My best wishes to the additional efforts to release water from the landslide dam at Tangjiashan, but maybe a better option is to leave where it is and let the first channel do the entire job, "naturally!"

Therefore, I would like to suggest a halt to construction of the second channel on the dam.


1。现有的导流渠正冲得越宽越深,泄流量正变得越来越大,也就是说,正在平稳走向全功能发挥作用。我估计再过一至两天,水位就会涨到一定高程(最多涨到 744 米),排水流量就可以达到每秒一百多立方米,泄水量就会大于进水量,湖水位就会开始降落而越来越底。 继续开挖第二条导流渠就没有必要了。




Info source: Xinhuanet

Tuesday, June 3, 2008

Update 4: Possible Upstream Landslide to Trigger Dam Collapse (如上游滑坡会瞬间引起溃决) - Landslide Dam Removal at Tangjiashan "Quake Lake"

Within the upstream "quake lake" area, a massvie mountain slope is at risk of sliding down. If the upstream landslide does occur, a huge surge will be created in the lake, leading to an instant collapse of the existing landslide dam. This was warned by Mr. Liu Ning (family name given name), the Chief Engineer of China Ministry of Water Resources.


Info source: Xinhuanet

Monday, June 2, 2008

Update 3: Significant Seepage Observed (大量水渗过堰塞体) - Landslide Dam Removal at Tangjiashan "Quake Lake"

Rate of water seepage suddenly increased from the bottom of the landslide dam at 14:50 on June 1 (Beijing time). Fortunately, up to 17:00 on June 2, the seepage flow rate was stabilized to approximately 10 cubic meters per second. I estimated this as about 10 percent of upstream inflow rate to the "quake lake." In my opinion, this seepage rate is still quite large. I hope this would not lead to a "piping" failure of the landslide dam before the start of the natural outflow from the completed drainage /sluice channel.

At 14:00 on June 2, the lake water level was at 735.78 m. Since not much rain is in the forecast for the coming days, the natural water discharge from the lake is now expected to occur on June 5.

“1日14时50分,唐家山堰塞湖坝底一处渗漏出现流量忽然加大的情况,但到2日17时为止,流量基本稳定在10个流量左右。” 我估算这相当于进湖水流量的十分之一。我认为这渗流量太大了。但愿不会(在自然导流之前)发生堰塞体内管涌而溃堤。


Info source: Xinhuanet