How An Infamous Hydroelectric Dam Changed Earth’s Rotation
It provides clean energy, but at what cost?
A Major Impact
“Powerful” doesn’t really do the amazing (and very controversial) Three Gorges Dam justice. Since the $30 billion project was announced, Chinese officials have faced heavy scrutiny from both scientists and environmental activists alike. Many believe that the dam will ultimately result in catastrophe. Some concerns include the dam trapping pollution, spawning earthquakes and landslides, uprooting citizens (more than 1.3 million people have already been forced to relocate), and destroying historical locations, along with the habitats of endangered animals.
The government finally conceded that the project was ill conceived — after years of dubbing the dam one of the most spectacular pieces of engineering in Chinese history — but the damage is already done.
However, the dam isn’t all bad. Its last 32 generators (each capable of generating 50 MW of power) went into action at the end of July last year, and the gushing water produced has enough power to generate about 22.5 million kilowatts (22,500 megawatts) of energy (the estimates vary), which is equivalent to about 15 nuclear reactors. It doesn’t cause concerns about radioactive materials being unleashed either, which is a very good thing, especially after events like the Chernobyl and Fukushima nuclear disasters, so the negative effects of its construction can be ignored by most, mainly due to the fact that it’s a clean, effective way of rendering energy for a booming population.
The implications of the dam extend far beyond both its impact on the environment and its ability to provide much needed energy, though. It’s also changing how the Earth itself rotates.
The Rotation Situation
Wondering how a dam could possibly have an impact on the Earth’s rotation? Here’s a wonderful source that breaks it down further:
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).
Not to worry, though. Earth’s rotation changes frequently, with many different variables added into the equation. First, we have the Moon gradually receding from the Earth, changing Earth’s rotation ever so slightly. Earthquakes (like the mega quake in Japan back in 2011) also help along the process (the same quake changed Earth’s rotation by 2.68 microseconds). Furthermore, every five years or so, the length of the day increases and decreases by about a millisecond, or about 550 times larger than the change caused by the Japanese earthquake.
Still, this knowledge begs many interesting questions. Namely, how far is too far? Individually, these things don’t make much of a difference one way or another, but together, who knows? So Futurism readers, what are your thoughts?
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