博文
Zipingpu Dam Upstream of Chengdu Secured
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05-19 12:18 Caijing Magazine
All odds stacked against, the Chengdu dam has been relieved of rising pressure.
By staff reporter Li Hujun
Experts announced May 18 that the Zipingpu dam upstream of Chengdu is functionally intact, removing concerns that a breach could deluge nearby cities and endanger more than ten million urban and rural residents.
The magnitude 8 earthquake on May 12 caused a death toll of at least 33,000 in China. Damage was also severe on innumerable structures and created cracks on the Zipingpu dam, critically standing merely 10 kilometers upstream from the capital city of Sichuan province.
The damaged dam posed two main challenges – inoperable gates blocking the supply of drinking water for the nearby Chengdu and Dujiangyan cities; and the grave concern that a collapse or breach would flood Chengdu.
The intensity of the May 12th earthquake was greater than the designed safety parameters of the Zipingpu dam. An expert from the Nanjing Hydraulic Research Institute told Caijing that Zipingpu was designed to withstand earthquakes below level 8 on the Mercalli intensity scale, however the recent quake reached level 11, on a scale ranging between 0 and 12.
“Had we known earthquakes like this could happen in this area, we wouldn't have built a dam of this height (156 meters),” said the expert. “It's really a wonder that the dam survived the jolt.”
“If no special conditions occur, water should pass the dam smoothly, and the safety of people downstream is ensured,” said Li Hong, manager of Zipingpu Development Ltd.
A deputy minister and an expert team from the Water Resources Ministry was dispatched to Zipingpu on the night of May 12, to live and work there for the next six days.
The looming risk was ominous. Zipingpu had been undergoing a routine safety check when the earthquake stuck and damaged part of the dam's power generation facilities. As an added effect, it also ended up blocking water from the Min River.
The immediate risk was a shortage in the water supply. Zipingpu's reservoir provides drinking water for the cities Dujiangyan and Chengdu. Engineers decided to bypass power generation and send water through the spillways at 90 cubic meters per second to reduce pressure on the dam.
A new problem quickly arose. Rain had fallen in the region for several days and water was adding into the reservoir at 600 cubic meters per second, much more than the designed outflow capacity of 370 cubic meters per second. The 90 cubic meters per second of extra efflux would not be sufficient to balance the equation.
If the water continued to accumulate in the reservoir, rising pressure would break the weakened dam and swamp Chengdu.
After an initial check, experts decided as early as May 12 that, despite some cracks and distortion at the bottom, the dam was basically safe.
Engineers and dam workers then manually opened the flood gates, increasing the outflow to 800 cubic meters per second. After several days of work, engineers completed repairs on damaged facilities May 17 and left the site May 18.
Restoring the Zipingpu Dam back to normal operation was critical. The Zipingpu reservoir is fed by the Min river and numerous tributaries in the mountainous watershed. The earthquake, its aftershocks and resultant mud and rock slides created natural dams in the mountains - perhaps hundreds of them. When these dams fill and eventually burst, Zipingpu must be able to withstand the cascading floodwaters.
Now, Zipingpu is only 30 percent full and has the capacity to absorb flooding, should there be any.
http://www.caijing.com.cn/20080519/63348.shtml
https://blog.sciencenet.cn/blog-3598-26997.html
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