1/30/2007

Dam Blogging Part 2: Just The Facts

So now you’re educated about dams. The real question is… So what?

If you’ve paid any attention to the news in the last two weeks you’ve heard about the Wolf Creek Dam up in south-central Kentucky. It’s on the Cumberland River upstream of Nashville about 120 miles as the river meanders. It’s huge. Biggest reservoir east of the Mississippi River and ninth biggest in the country. It’s so big that if it failed it could seriously flood Nashville. And it’s leaking.

Wolf Creek Dam is a combination of dam types. It’s part concrete with spillways and generators for hydro power, but it’s also over a mile long and 200 feet tall. That’s a lot of concrete, even at prices in the 1940s. So rather than build it all in concrete, an earth embankment was used. Lots and lots of clay was brought in to built the embankment portion. Clay was used because it’s semi-impermeable (water can’t flow through it very well).

It was designed in the late 1930’s, but construction was delayed by World War II, and wasn’t completed until 1952. By the late 1960’s they started seeing problems with water getting under and through the dam. This was caused by a less than ideal foundation. The dam is built on limestone and the whole area is typical karst geology. Limestone can dissolve in water and karst geology is a description of the tendency of limestone to have seams and ‘tunnels’ in it filled with dirt. You see a lot of sinkholes in karst areas and have a lot of water flowing below ground.

What was happening at Wolf Creek is pretty simple. Water was seeping into the limestone and being carried in the ‘tunnels’ and seams under and around the dam. At some points the limestone ‘tunnels’ were near the ground surface, and the flowing water eroded enough of the dirt on top of the limestone to form sinkholes. A sinkhole is a lot like a rat. When you see one there are a lot more you can’t see. And they don’t do anything but grow. Just like a crack in a window makes a window a lot weaker, sinkholes are usually just the beginning of a major problem for a dam.

So the Corps of Engineers jumped into action. They built a concrete wall through the center of the earthen portion of the dam. They drilled it into the rock under the dam to try and cut off the ‘tunnels’ in the limestone. But it was the early 1970’s and money was tight. So they built the wall shallower and shorter than it really should have been. They managed to save the dam for a time, but the problem came back.

And that brings us to today. The Corps of Engineers has been keeping an eye on the dam ever since the work in the 70’s. In the last few years they’ve noticed a lot more water flowing under the dam and the sinkholes and wet spots are starting up again. So now we have to fix it again, and try and be a little more thorough this time.

Next up we have the payoff post which describes what this dam could do to Nashville.

1 Comments:

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4:53 PM, September 15, 2007  

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