To put things into perspective, our dam is relatively shallow, peaking out at maybe two meters. It’s fed by a year-round stream that begins not far up the mountain from us, winding through the forest until it reaches the dam. The pond is oblong, hugging the forest on one side with the dam wall on the opposite side. The wall separates the pond from a cleared acre—done back in the ‘80s as well—which we are planning to develop into our cultivated site: house, greenhouse, gardens, food forest, fedge, and so on. The stream enters from the northwestern corner of the pond and currently drains into the floor of the southeastern corner.
Replacing the horizontal spillway pipe through which the stream currently flows would require digging up the thickest part of the dam and further compromising the wall’s integrity, which, despite the unfortunate situation, actually seems in good nick at the moment. We didn’t like the sound of this, and the cost would be astronomical. Some people suggested shoving a smaller pipe through the existing one, but with the inside collapsed, that seemed not possible, and sealing around the new pipe would be a nightmare. What’s more is that the existing pipe seems a bit small as it is.
All of that lead us to the option of putting in a new, higher horizontal pipe that would control the depth of the pond. However, we came to realize that this pipe would 1) be an eyesore, 2) likely would require equipment to pack in, and 3) called for materials to be bought and brought on site. Plus, it would be more limiting in its draining capacity than an open channel to control the water level. Ultimately, that lead us to doing away with a pipe altogether and putting in an earthen spillway.
After questioning a few people around town and revisiting our Geoff Lawton Online Course material, we settled a little further into the idea. It seemed a valid approach, but we ran into another issue we’d foreseen: concrete. We really didn’t want to pour concrete if it wasn’t absolutely necessary, and though most people seemed to suggest it was, we felt it was more a standard than what was possible.
The other issue that worried us was the backside of the spillway eroding away as water flowed across it. Our neighbor had mentioned having seen that happen before, only it was an instance in which someone had simply dug a spillway trench in the earth and left it as is. Eventually, in that instance, the stream just ate through the wall. We didn’t want that to happen, either.
What we had in mind was a level-sill spillway that was lined with rock that emptied onto a constructed cascade to ease the water back down to its original streambed. We’d been taking down an old house to repurpose the lumber for our own cabin, and it had a large stone chimney standing in the middle of it. We decided that we could use these rocks to line our channel, using a bit of clay to seal them into place.
While the thought seemed realistic, the issue was that we’d not heard of or found anyone who’d attempted such a thing. We were a little apprehensive about going rogue, so we spun our wheels for a while.
But, the rains in North Carolina this winter were relentless, and the dam overflowed several times in the span of a couple of weeks. The water came over the wall at basically ground level, after the dam wall had given way upslope, so we weren’t too worried about the wall itself eroding. However, the overflow was moving across the open field and causing a little problem when it neared the road. We needed to at least get our overflow situation sorted.
We began by constructing the cascade on the backside of the dam wall. The beauty of the situation was that a natural route existed, so we just had to get the water to the empty streambed without it causing damage on the way. This consisted of setting series of large stones running roughly on contour every couple of feet. Being visually particular, we did this with enough haphazardness to suggest it could have naturally occurred but with enough design that it should function well. Our goal was to continually slow and spread the water until it reached the streambed.
Then, we dug the spillway. The trench hole was about 30 inches (75 cm) wide by 20 inches (50 cm) deep. The original vertical pipe in the dam was a six-inch pipe, and a neighbor said he’d never seen the pond overflow when the pipe was intact. With that in mind, by the increase in volume our trench provided, we’d figured that we were giving the stream plenty of space to swell and move through safely. With the trench dug, we used large stones to set a level sill intake on the pond side and a level sill release on the exit side.
Finally, after finding the right reassuring article, we felt confident that our concrete-free version would work well. We lined the inside walls of the trench with flat stones stood vertically and the cracks mortared in with moist clay from the bit of the wall we’d removed. Atop the stones, we transplanted moss that had been growing on the ground where the trench now was. Then, we sifted mixed gravel from a pile of soil we had on site and lined the bottom of the trench with a thick, two-inch layer of that. We also added a similar layer of gravel between each level of the cascade to keep that soil in place.
Luckily, the article had addressed the exact issue we were dealing with: a rusted-out pipe beneath a dam. The article suggested avoiding concrete as it was a risk for frost damage. It also suggested avoiding landscaping fabric, which had also been suggested, because it was likely to be undermined. We’d not wanted to use plastic, either, so that had been a problematic choice from the start. The article suggested doing exactly what we’d been thinking, so we went for it. Of course, we’ll keep an eye on the situation and adjust if necessary.
While that all seems easy enough, this took us months of deliberation. We so wanted to get the situation right, and we walked away thinking we did. At the moment, the spillway has endured one good rain, during which the water didn’t breach the dam wall and the result of it moving through the spillway did nothing more than dust the gravel. We are now thinking through our add-on rice paddy, preliminary sediment pond (via gabion), and pipe removal. We hope to get started on that this spring.
Anyway, I thought it might be useful to some folks who may be facing similar quandaries to share what we’d learned in the earthen spillway project.