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The diffuser tunnel at Darlington was bitchy. Normally, the warm water discharge is just handled by a channel out into the lake. They had done a lot of hydraulic modeling on this outflow, and for the most part it was quite benign to the lake. That's because the warm water would stay on top and most of the heat energy would go to the atmosphere. Lots of fishies like the warm water, and it's usually the best fishing in the lake! The water is usually cleaner coming out than going in, because of all the zebra mussels in the pipes!
But once in a while, physics comes into play. Water has the maximum density at about 4 deg. C, and then fluffs up for freezing. That's why the whole lake seasonly 'turns over', when the temperature changes. If the water is near freezing, and the warm water comes in, it can be denser, and plunges along the bottom. This could kill fishies in their nests.
For this reason, we had to diffuse the warm water. The tunnel was about twice as long as the intake. It had a reducing diameter, and had diffuser pipes shooting out the top. As such, it was more difficult to excavate.
First, the pipes had to be drilled in, grouted, and closed with a cap. When the tunnel was excavated, these pipes were opened from underneath, so that only the steel cap was between you and certain death! The tunnel was lined with concrete and filled with water. Then the caps were taken off the top of the pipes.
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