# Let’s Get Hydroelectric

China’s Three Gorges Dam is the world’s largest hydroelectric station.  It is a structure that stands as a testament to the tenacity of human ingenuity.  First envisioned in 1919 and finally completed in 2012, the dam spans the Yangtze River and measures 2.3 kilometers (1.4 miles) long and 185 meters (607 feet) tall.  Almost 30 million cubic meters (just over 35 million cubic yards) of concrete and 463,000 tons of steel were used to erect the massive structure.

Three Gorges Dam
With a ridiculous reservoir that runs for 660 kilometers (410 miles), the dam raises water to 175 meters (574 feet) above sea level.  The amount of water displaced when the dam is full weighs about 39.3 TRILLION kilograms (or around 86 TRILLION pounds).  Yeah, it’s huge.  But what if I told you that the most unbelievable factoid isn’t about the dam’s substantial dimensions?

### Believe it or not, the filling of the Three Gorges Dam has slowed down the rotation of the Earth!

What does a dam have to do with the Earth’s rotation?  What it comes down to is something called moment of inertia.  Essentially, this is a property of rotating bodies that can tell us how easy (or difficult) it is to spin something.

Yangtze River
The moment of inertia of an object depends on a few things.  One, it depends on an object’s shape and the point about which it’s rotating.   It also depends on an object’s mass and how it’s distributed.

For our purposes, let’s focus on the mass distribution property of moment of inertia. If two balls are the same size but different masses, the lighter ball rotates more easily than the heavier one.  What happens, though, if we displace some of the mass?

My favorite way to visualize mass displacement on a rotating body is to imagine a figure skater.  They displace their mass by bringing in or extending their arms upon rotation to speed up or slow down.  If you’ve got a good visual, you can see “arms in” is fast and “arms out” is slow.  If the weight moves away from the axis of rotation, the rotation is slowed.  This is pretty much what the Three Gorges Dam has accomplished by displacing all that water!

Mass is displaced on the Earth every day but only when it is done on such a colossally large scale does it have the power to effect a measurable change.

Brought to you by Melissa Radey, B.Sc., Staff Scientist at Science North

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6 Comments to “The Science of the Three Gorges Dam”
1. Vijay Mehra says:

Wonder what would happen if there was a calamity and the Dam busted ???

2. I think it is totally insane to slow our Earth. Damn. I beg to differ.

• If this is slowing the rotation of the planet, can the US split the difference and stop doing daylight savings time?

Also what implications does slowing the earths rotation have on the biosphere? I mean some animals are very in-tune with the planet, what effect could this have on them, or humans for that matter, we are more attuned to the planet than we realize.

3. It have many power that!

4. As an engineer, yes it would throw off the moment of inertia. However, the simile of an ice skater isn’t correct. Since there’s nothing on the exact other side of the planet to balance it, mathematically its off balance. The simile should be more akin to a new year’s eve noisemaker with a weight on one side and nothing on the other. However also given the size and mass of the planet in proportion to the size and mass behind the dam, yes there will be a difference in the spin/travel of the planet but it will be minuscule and nearly undetectable.

5. Over 9000! Oh wait that’s out of place…