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Residential swale construction, concerns and comments welcome

 
Posts: 31
Location: Lehigh Valley, PA zone 6b
6
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Hi all. I find myself in a bit of quandary with the space that's likely the best spot to grow food on our property. I'm looking at a model somewhat like Amy Stroess's Tenth Acre Farm, one that offers food production with significant aesthetic appeal.

We're on an urban lot of about 1/3 acre, and the only area with significant sun is the front yard, which faces due west. We're in eastern Pennsylvania, and our soil, where it isn't fill, is clay. Zone 6b. To avoid having to use city water for irrigation, I'd like to capture as much rainwater as possible.

The bed in question is a foundation planting along the front porch. There was nothing it in when we moved in late 2016. It had a really heavy weed barrier underneath some decomposing mulch. Last summer, we stuck in a very random assortment of edible annuals and flowering perennials, and they did better than expected, especially eggplant.  I expanded the footprint outwards about three feet, removed the weed barrier, and loaded it up with wood chips.

My plan is to trench out a swale/keyline running up the center of the bed planting on berms on either side. The trench would be lined with reclaimed weed barrier and filled with either gravel or wood chips, and it would serve as the path. I'm thinking gravel so that I don't have to dig it out every few years. That said, I'm pretty good at convincing tree services to dump their chips here. I'd love to hear people's thoughts on chips vs gravel. I have a line on a source for gravel at $15/ton, which doesn't seem terribly onerous to me.

Currently, the porch downspouts drain into some underground conveyance. These would transfer to draining into the swale. The property slopes towards the back, gently in most areas, with some very steep spots. The backyard is terraced above the adjoining property.

So, here are my concerns:

1. The changing climate in this part of the world means an increase in major rain events, so the swale would also need to allow for overflow drainage. That being said, I'd really like to give it the best possible change for as much water to infiltrate as possible. Because of the clay, I worry about digging the trench and inadvertently creating an impermeable barrier. Would broadforking the bottom of the trench do it? Would I be best served by planting a hundred diakons after digging it out and laying in the gravel next year?

2. If infiltration goes really well, would this swale risk creating a spring in my basement? The actual trench path of the trench is about 15 feet away from the foundation. It's an old house (by American standards) with a mortared stone foundation; we think we're pretty lucky that the basement stays dry with just a dehumidifier in the summer, and we REALLY don't want to that to change.

Here are some pictures that I hope clarify this book-length post. Any thoughts are more than welcome.

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gardener
Posts: 2917
Location: Upstate NY, zone 5
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Clay is clay; disturb it all you want, it will pack back down with freeze-thaw cycles and be impermeable. As long as the bottom of your gravel- or chip-filled swale has an outlet to drain by gravity, I think it will help maintain ground moisture around it without getting waterlogged. I'm not suggesting adding drain pipe, just continue the gravel around the house so that water has an outlet if it starts to over-fill. It's hard to tell from the photos - does the front yard slope toward the house at all? Or just around it? Keeping the swale at least 15 feet from the house foundation sounds pretty good.

I think gravel would give faster drainage than you may want. Wood chips  (with maybe some gravel at the bottom) would hold moisture while allowing any oversupply to filter downhill. When the wood chips decompose, they will be a mass of organic soil which will probably still allow gravity drainage at a slow rate.
 
Posts: 30
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It looks like you live nearby so you might have similar soil. I am near Broadheadsville PA. Generally what I see is that the water only infiltrates a short distance and then follows the clay layer. Where I have dug into the clay layer I generally end up with a place that is permanently muddy. An instant pre-sealed pond. The other thing I notice is that when it rains heavily, the water surfaces where the clay is close to the surface.
With this in mind, in your position I would put in a perforated drain pipe along the clay layer maybe two feet from the house and sloped to drain into the Swale. I would then make sure the overflow is below the drain pipe. I suspect that your Swale will be a pond for extended periods, so it might not work well as a path. Have you considered designing the Swale as an intentional pond?

Another general comment: it looks like your yard has a lot of shade on the south side as compared to the north, so you can get a nice variety of plants out of a small space.
 
Posts: 4
Location: Cincinnati, OH
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I'm glad you found my swale posts at TenthAcreFarm.com useful. :) The swale referenced in my articles was dug into solid clay, and as mentioned above, it works because the slope allows the water to continue draining downhill and away from the house when the system fills up and the water can't infiltrate the clay. This is essential for the system to work. Gravity is your friend.

Sowing with daikons or broadforking is a good idea. You could also puncture the clay throughout the trench with a digging fork. Then sprinkle some compost throughout to fill those holes, which provides food and oxygen to attract soil organisms to work on that clay layer, soften it, and allow deeper infiltration of the water.

I'm glad you're making sure to plan an overflow. If your swale trench fills up, where will the water go? Where do you want it to go? Can you direct it into a rain garden (like I did in my system), into a diversion trench directed to another garden bed, or simply away from the house?

Concerns: The uphill planting berm closest to the house might not be a great idea. You want to encourage water to run away from the house, and a thick berm like this close to the house might collect more moisture and insects than you want. 15 feet away from the house should be fine as long as gravity is your friend and the trench is correctly on contour.

As far as filling the trench, I found pluses and minuses to each type of material. For function, gravel and large rocks topped with wood chips (that you can see in my TenthAcreFarm.com article photos) worked fantastic. My swale functioned like a dream for about five years until I moved. But I had figured the trench would need to be dug out occasionally (every 8-10 years?) to aerate the clay bed and to be sure the material in the trench was not too compacted to do its job. Digging out gravel and rocks is not that fun.

Alternatively, I worry about decomposing wood chips holding too much water. By the time I moved, I wondered if that was causing a mosquito issue. If the trench was only wood chips, then you could dig it out every year, use that compost in the garden, and refill with new chips. You could also think about planting that trench with deep rooted, native rain garden wildflowers (bee balm, coneflower, blazing star, etc.). They will encourage a good soil biology that would continue to decompact the clay for you and soak up any water that might otherwise sit there if the clay barrier issue isn't worked out. Also, flowers = pretty landscaping!
 
Daniel Ackerman
Posts: 31
Location: Lehigh Valley, PA zone 6b
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Thanks all!. Yep, Paul, I'm nearby, in Easton. The porch starts at least 12 feet from the house, so with the back berm and good contour, I'm not too worried about surface flooding. And yeah, I definitely have a lot of microclimates! I just with a few more of them included sun, but hey, we work with what we have!

Glenn, thanks for the thoughts on the clay. I like that you think that it'd only be mostly impermeable, but the berms on either side would still get the water infiltration. Yes, the front yard does slope gently towards the house, and the rest of the yard slopes past it. Ground level is a full story lower in the back. I can't quite figure out the drainage, but they've done a decent job with gutters and outflow. I just want to tap into it for irrigation without flooding my basement.  And I actually hadn't considered the flow rate of wood chips vs gravel.

Amy, I've enjoyed your blog and book very much, and have found them both inspirational and useful. Thank you for that! And thanks for your followup on your swale construction. We do have mosquitoes here, and I don't want to do anything to exacerbate the problem. Still, mosquitoes need pooled water (although not much), so I doubt that the water being held in the wood chips themselves would be all that problematic. I can't plant on top of the trench, since I also plan on using it as a path. I'll lay large flat stones above the wood chips for easier walking/child guidance.

The overflow will be guided into the next section of landscaping, which is on a pretty steep slope. I'm going to terrace it with some stones I've been scavenging, trench in some swales and paths, and maybe (biiig maybe)  add some stairs that'll could potentially double as a waterfall in extreme rain events, with some kind of sediment trap upstream. I think once my kids are a bit older, I'll add a rain garden in the front, but right now, having more grass is more useful.

Taking it all in, I think I'll probably use wood chips for the trench and observe closely. The porch has several downspouts, so I can incorporate them into the system one at a time and see how they do in various rain events. If I find digging out the chips after a year or three too onerous, I'll look for another solution.

Once again, thank you all for your thoughts and feedback!

Daniel
 
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