Understanding China’s Flood Situation

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As you probably are aware, China is currently facing one of its worst floods in recent history. This post is to give a geographical perspective on the situation.

Background

If you were mistaken as we were that the Yangtze river flows straight from its catchment area to the sea, then this post should disabuse you of that view. The Yangtze river meanders through Southern China and divides near Hunan, with one tributary going towards Shanghai and the other towards Guangdong.

Towns and Cities.

Along the entire “flood plain” as it was prior to the construction of the 3 Gorges Dam, are cities like Anhui, Shanghai, Hubei, Hunan, Jiangxi. Behind the marker for the 3 Gorges Dam,  of which we can share some facts.

At maximum capacity, where it is at now, the displacement, or difference in height between the water behind the dam and the river downstream is about 110m. Thats about a 40 storey building, or about half the height of Maybank Tower at Dataran Maybank. The water behind the dam is at 170m . The dam capacity is about 40km3, or about 40 billion tonnes of water.

3 Gorges Dam is discharging 10x the peak load of the Pahang River over the past 30 years.

The key issue is now is that the 3 Gorges Dam is at peak capacity and is forced to perform maximum discharge. This amounts to 60,000 m3 per second.

Yangtze River Basin. Credits Wikipedia.

To give you an idea of what this means, consider our mighty Pahang River.  Over the past 30 years between 1973 – 2003, based on a study by the Kyoto University , the maximum discharge recorded at the Pahang River was 6,300 m3 per second. I vaguely remember the floods of 1983 being bad and that was a peak discharge of 2,700 m3. So currently, the discharge from the 3 Gorges Dam downstream is 10x the peak discharge of the Pahang River over the last 30 years.

Here is the situation in the the areas affected

Anhui

Guzhen Town of Yu’an District, Jul 20. Credits: Xinhua

Jiangxi

Wuhan

Wuhan

Analysis

The 3 Gorges Dam is currently no longer playing a role in flood prevention. It is actually making things worse because it has to discharge water at 66,000 m3 to prevent the waters from submerging the dam.

While there has been numerous comments about the structural stability of the 3 Gorges Dam, there currently is no conclusive evidence that the structure is about to fail. If you follow some of the news headlines, you may heard that authorities in China reported last Saturday that the 3 Gorges Dam has “moved, leaked and distorted.” This has been picked up by some media to claim that the 3 Gorges Dam is on the verge of collapse.

To be honest, dams to move, leak and get distorted – albeit infinitesimally (within the range of 25 mm) and there are dam models that actually account for that – both radial distortion and tangential distortion.  So without providing the figures one cannot form an opinion about whether the 3 Gorges Dam is about to break or not. Please bear this in mind when getting news about the 3 Gorges Dam. The thing to note however is that with a wall of water 180m in depth stretching 2 km impinging on the dam and the underlying eddy currents at the base of the dam, you really need a sophisticated computer model to figure out the forces that are being exerted on the dam. This will be within the realm of finite element analysis. Also the density of the water may play a factor due to the hydro-static pressure (density x gravity x height for those who did STPM Physics or Matrikulasi).

This is an over simplification

What I really view is that different parts of the dam are subjected to different forces with different vectors along the face of the dam. There is definitely a major component which is pushing against the dam, but there could be shear forces due to the flow of the water.

 

There however has been criticisms that the structure lacks quality control as the team which built was also the team that implemented quality control. If the 3 Gorges Dam does collapse, the entire area marked in red could be devastated by the floods. Literally, the fate of China as a nation, and the world’s 2nd largest superpower rests on the structural integrity of the 3 Gorges Dam.

 

*Note: No comments mocking the fate of China will be entertained. Remember, those who gloat over disaster will not go unpunished.


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