Thread by @jamewils

Yangtze deluge tests limits of Three Gorges Dam

Top engineer says the dam failed to regulate flood despite deluge being less severe than worst-case parameters

asiatimes.com/2020/07/yangtz…

Three Gorges Dam drain water at 37,000 cumecs since Tuesday, after the reservoir’s water level increased by more than eight meters in 48 hours to 163.5 meters. The height of the dam is 185 meters.  

To get a perspective, Idukki Dam discharged 1500 cumecs in 2018 floods

Now listen...

Stories fantasizing the dam being close to collapse pop up annually.

But according to experts, even in face of severe floods and torrential rain this year, the dam will stay firm and strong as ever, if not stronger, rather than deformed or collapsed.

The one whose comment is quoted by Asia Times is not a hydrologic, but he is Fan Xiao, chief engineer with Sichuan’s Provincial Bureau of Geology and Mineral Resources.

So listen to what an expert hydrologist is saying here.

Listen to Wang Hao, an academician of the Chinese Academy of Engineering, and honorary director of water resources department with China Institute of Water Resources and Hydropower Research (IWHR)

He says China puts in place five lines of defense when it comes to flood prevention and control, i.e. interception in the main streams, control in tributaries, protection by embankments, flood detention basins in low-lying areas and management of urban development along rivers.

As of Tuesday, the dam had intercepted 10.7 billion cubic meters of floodwater with a maximum peak clipping ratio (the ratio by which flood is intercepted) of 46.7 percent in the latest round of floods, the Three Gorges Corp (CTG), operator of the mega project.

The powerful floodwater will not bring down the Three Gorges Dam, which has 23 deep spillway holes and 22 surface spillway holes to efficiently release flood water and hence stay firm, according to Wang.

"China's largest recorded flood in the past 2,500 years was in 1870 with a peak flow of 105,000 cubic meters per second. The Three Gorges Dam is designed to withstand more, even a peak flow of 124,300 cubic meters per second," said Wang.

Wang also introduced that there is still 21.1 BCM of flood control capacity on stand-by in the Three Gorges Reservoir, and risks in the Yangtze River basin are under control.

To get a perspective, understand that Almatti dam capacity is 6 BCM & Idukki 1.46 BCM

Salient features of TGD

The total water catchment area is about 10 lakh sq km.

The surface area of the reservoir
will reach 1,084 sqkm and the land area inundated is 632 sqkm—almost twice the original water surface area.

The average annual
runoff to the TGD is 451 BCM.

This ribbon- or river-like, rather than a lake-like, reservoir have a total length of over 600 km and an average width of 1.10 km.

With the Normal Pool Level (NPL) at 570 ft (175 m) above MSL, the total storage capacity of the reservoir
is 39.3 BCM.

Now understand its flood hydrology

Crest level of the dam 185 m Maximum Water Level 181 m
NPL or FRL 175 m with 39.3 BCM

During the flood season, the flood control limit water level is 145 meters, thus have flood control negotiating storage capacity of 22.15 BCM.

Its spillways are designed to negotiate a PMF or 1 in 10,000 year flood of 124,300 cumecs, and the maximum discharge possible through 1,16,000 cumecs, it is world's biggest!

Our Idukki spillway max capacity is 5040 cumecs

The total installed capacity of the power station is 22,500 MW with world's largest single generator of 700 MW, and the annual average power generation capacity is 88,2000 MU.

While Idukki 780MW with 6x130 MW and annual generation of 2000 MU