1/23/2007

Dam Blogging 1: A Dam Owner's Manual

This is the first in my series of posts about Wolf Creek Dam. I’m going to attempt to explain the basics of dam operations and failure methods.

I like to use a bathtub analogy to explain why a dam works. Your daily shower is a lot like a river. Imagine you’re standing at a specific place on the river. The shower head is standing in for all the water flowing toward you from up river. The tub drain is where the water ends up when it passes you. If your shower is working properly then all the water flowing out of the spigot goes down the drain. That’s how a normal river system works. If you really turn up the faucet, the water may start coming out of the faucet faster than it can get out of the drain, so it starts to puddle up in your tub. If you turn down the faucet again, the water drains out faster than it comes in, and it the puddle goes away. That’s how a river floods.

Now it gets more interesting. Imagine we’re building a dam on the river where you’re standing. We just plugged up the drain in your bathtub. The water is still coming in, but it can’t get out. So you get a puddle like a natural flood. Then the puddle starts to rise. If you don’t unblock the drain the water will just keep rising until it fills up the tub and runs off into the bathroom floor. The tub full of water represents the new reservoir we created behind the dam.

That’s dam operations in a nutshell. Barring a rain dance or a direct line to God, we can’t control the amount of water that flows into our reservoir/tub, but we can control how much drains out through our dam. When the dam is designed you have to carefully build in spillways to control how much flow gets out. If the water gets too high and is threatening to get out of the tub and into the bathroom floor you open up more drains.

Try Wikipedia for a discussion of dam types.

There are two basic failure mechanisms for a dam. They are also applicable to levees, and both were seen in New Orleans during Hurricane Katrina.

The first is overtopping. Just like it sounds, the water gets to high and goes over the top of the dam. Overtopping doesn’t have to mean failure, but it can fail the dam depending on what materials were used in construction. In the case of Wolf Creek Dam, overtopping would mean failure because then you’d have a lot of fast moving water flowing over the front of the dam which is soil and grass. The fast moving water would erode the dam until it just crumbles away in the water.

The second type of dam failure is due to infiltration and seepage. Overtopping is a situation where water goes over the dam, seepage is when water finds a way through the dam. Some slight seepage is normal in an earthen dam, but if there is a lot it can become a major problem. Seepage starts very small as the water finds a path to trickle through the dam. Once the water is through it can begin to erode the material of the dam and the pathway gets larger and larger. Eventually it can get large enough that the structural integrity of the dam is compromised and large chunks start to wash away or the embankment itself starts to slump and fall over like a sand castle when the tide comes in.

Seepage is the problem at Wolf Creek, and I'll tell you about it tomorrow.

3 Comments:

Anonymous The Librarian said...

Hunh . . . well so far, I understand and you aren't boring/pompous. Keep it up, I like to learn something new. A wise librarian once told me that any day she didn't learn something new was a wasted day. I liked that, sort of explains why I too am a librarian. Since I don't work a reference desk anymore, it is harder to find learning opportunities. That must be why I read blogs . . .

5:20 AM, January 24, 2007  
Anonymous Anonymous said...

I live about 30 miles below the Wolf Creek Dam and I have been researching to find information on a dam failure with a similar capacity of water. I haven't had much luck. Any suggestions?

5:31 PM, January 25, 2007  
Blogger W said...

I'm planning on writing about it later, but the closest I can think of off hand is the Teton Dam failure. Check wikipedia for a start.

12:44 PM, January 26, 2007  

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