The World’s Largest Hydroelectric Power Station Is Slowing The Earth’s Rotation

The word ‘powerful‘ doesn’t even begin to describe this amazing (yet quite controversial) mega structure in China – the Three Gorges Dam.

The controversy doesn’t revolve entirely around the cost of the project (which totals to about $30 billion) but has also received criticism from the public, as many believe that the dam will ultimately result in a mass catastrophe.

Some of these concerns include the dam trapping in pollution, creating earthquakes/landslides and destroying historical locations along with the habitats of endangered species.

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Recently, the government agreed that these problems were quite serious (after years of calling the dam a ‘modern wonder‘ and the ‘best in Chinese engineering‘) but unfortunately, the damage is already done.

Looking on the bright side, the stream of water produced by the dam has enough power to generate about 22.5 million kilowatts (22,500 megawatts) of energy, which is equivalent to about FIFTEEN nuclear reactors and, of course, it doesn’t cause concerns about radioactive materials being unleashed.

Due to all these benefits, the disastrous effects are often ignored by most – mainly due to the fact that it’s a clean, effective way of rendering energy for a booming population.

So how does this massive structure affect the rotation of the Earth? The answer is best explained by Cutler Cleveland from The Energy Watch:

Three Gorges Dam crosses the Yangtze River in Hubei province, China. It is the world’s largest hydroelectric power station by total capacity, which will be 22,500 MW when completed.

When the water level is at maximum, it will flood a total area of 632 km2 of land. The reservoir will contain about 39.3 cubic km (9.43 cubic miles) of water. That water will weigh more than 39 trillion kilograms (42 billion tons).

A shift in a mass of that size will impact the rotation of the Earth due to a phenomena known as “the moment of inertia”, which is the inertia of a rigid rotating body with respect to its rotation. 

The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. The longer the distance of a mass to its axis of rotation, the slower it will spin. You may not know it, but you see examples of this in everyday life. 

For example, a figure skater attempting to spin faster will draw her arms tight to her body, and thereby reduce her moment of inertia. Similarly, a diver attempting to somersault faster will bring his body into a tucked position.

Raising 39 trillion kilograms of water 175 meters above sea level will increase the Earth’s moment of inertia, and thus slow its rotation. However, the impact will be extremely small.

NASA scientists calculated the shift of such a mass will increase the length of day by only 0.06 microseconds, and make the Earth only very slightly more round in the middle and more flat on the top. It will also shift the pole position by about two centimeters (0.8 inch). 

Note that a shift in any object’s mass on the Earth relative to its axis of rotation will change its moment of inertia, although most shifts are too small to be measured (but they can be calculated).

So all in all, the change is minimal. But it’s still quite an impressive feat – not many mass structures are able to move the Earth!

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