Despite draining more than 100 acres of watershed, we have never had water go over the top of the dam at Lake Marguerite. There are two reasons for this. One is the emergency spillway, which directs high water around the side of the dam (sort of). The other is the primary overflow system. On our recent visit I finally managed to take some pictures of the inlet so you can get an idea of how it works.
This is a concrete drum that is about four feet in diameter. (There must be a standard dimension for these things, but I didn’t measure it.) It is set vertically in the ground near the top of the dam. (The top of the dam is only about a foot and a half higher than the top of the drum. The lake is behind me from the perspective of the photo. Had the water been higher, I would have been standing in it.) The top of the drum is set lower than the lip of the emergency spillway. Thus excess water will drain here first rather than over the earthen spillway. I’m sure you can see how, when the water rises to its lip, it pours into the drum. The bottom of the drum is sealed with concrete so it holds the water, and our builder left an impression of the sole of his boot in it. I like that personal touch.
Before taking the photo, L and I got busy with the grass whip and cleared away the scrub that was growing around it, as you can probably tell. We also cleared away the flotsam — mostly sticks — that had accumulated against the screen. The presence of the flotsam shows that this primary overflow system gets put to use, though L and I have never been present when the lake was so high that this happened. It would be cool to see though.
The water then accumulates in the drum until it reaches the black hole at the back. This is a twelve-inch, corrugated plastic pipe that exits the drum and slopes down just below the surface of the earth of the dam. At the base of the dam, near the pecan plantation, the black pipe reappears, as shown here:
Clearly the water comes out of here with a great deal of force, and poor Pablo has some work to do to correct the erosion that has resulted. L and I have filled that ditch with rocks, and as you can see, they’ve been pushed aside with no difficulty. This gouged out area is not in the dam itself but in the floor of the pecan plantation. It probably is not compromising the integrity of the dam, but the USDA man suggested we make a modification to move the erosion farther from the base of the dam. He recommended that we get another length of the black corrugated pipe and attach it to the outlet here. (There are supposed to be special gaskets for this joinery, and I hope they hold so the extension of the pipe doesn’t get blasted off.) Since all we have is free time and money to burn, we’ll get right on that. This is a job for the fall, after the first frost but before the ground freezes.
You will also notice (in the top photo) a white pipe emerging in the center of the drum. This is part of the drainage system. We have a drain valve that we can open to let water out of the lake on purpose. So far, we’ve had no reason to do this since retaining water has been our priority, but it will likely be a handy thing to have in the years ahead.
Imagine a gigantic letter “T” made out of white PVC pipe. If you invert the “T” the part you see in the drum is its base, which is now on top. (Does that make sense?) The horizontal part of the “T” goes from a drain set in the floor of the lake at its deepest spot to the valve and its outlet on the pecan side of the dam. Thus it passes under the dam. The drain is a 55-gallon plastic drum with about a zillion one inch diameter holes drilled in it. The PVC pipe enters the plastic drum at its base.
The builder told me when he first built a drain valve system in his own dam that he had his son open the valve to see how well it worked. The boy did this and then closed it. Yet the flow was so strong — the pressure was so great — that the valve and the end of the drain pipe were blown off and their entire lake drained away.
From that lesson he began adding the vertical part of the “T” to act as an expansion area to absorb the pressure from a sudden closing of the valve. Now, he tells me, when that same action is repeated, the pressure sends a geyser of water shooting out of the top of the pipe emerging from the drum. I’d like to see that sometime, but I’m afraid that it would blow the screen off of the top of the drum as well. It would then sink to the bottom of the lake, and I’d never see it again.