How are lakes drained?

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I hear all the time that lakes are drained to build dams/create farmland, divert resources, etc. I imagine Mother Nature did not give each body of water a drain plug to pull, so how does it happen? Where does the water go? How do engineers make the water go? Is the process for refilling them similar?

In: Engineering
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When you have to drain an area of water, you have to create a reservoir for the water to move into. It has to be lower in elevation than the original body, and then you need to make a channel that connects the two bodies. The channel is cut at a downward slope from the original, creating a “slide” effect which causes all the water from the original body to drain into the lower elevation.

Refilling is a bit harder. You have to close off the channel and pump the water back up.

Assuming that you’re building a dam on a river, some sort of diversion will be built while the dam is being built. On the Hoover dam these tunnels became the spillways and hydroelectric tunnels on the finished dam.

Another possibility is something called a cofferdam that is used to hold back the water in a smaller area to make repairs or construct something in an area that would normally be under water.

On smaller projects the water is pumped out or siphoned (no pump needed once you get it started). I’ve seen a few farm ponds in my area get reworked. They’ll usually cut a channel in the dam to allow water to drain and fill it back in when they’re done. Ideally you’d leave a pipe with valve in the dam as an emergency release.