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Swale vs french drain, in clay soil

 
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I have a back yard about 120 ft square with a slope of about 2.3%. I'm considering manipulating the topography to help accomplish two goals: first, I'd like a large flat area for kids to run around in (ideally with an edible ground cover that takes foot traffic, but that's a question for another forum). Second, I'd like to retain topsoil and control erosion. There seem to be two main options:

1. Import a large 3-D triangle of topsoil and grade so that the yard becomes mostly flat. Then, near the edges, build some combination of diversion ditches and French drains to divert water away from the house and also channeled nicely in such a way as to preserve our topsoil and not create erosion problems for our downhill neighbors.

2. Build a couple of swales on contour and plant them appropriately (with a combination of water-loving plants). Probably import some topsoil anyway, to reduce the slope, but not all the way to level.

I'm not averse to importing topsoil anyway because 95% of the topsoil was removed 12 months ago (before we purchased the property) to clear out a massive bamboo infestation (fence to fence, 25 feet high, 120 feet square, no rhizome barriers). What's left is sandy clay with very poor nutrient content and very low organic matter. We are putting on cover crops to build the soil and reduce erosion, but we may well import some compost and topsoil into this system to increase organic matter content a bit faster. This question is more about what we should consider as we choose to build swales on contour, versus a diversion ditch + French drain, given our goals.

We are in Durham, NC (Zone 7b) with sandy clay soil. In our not very scientific percolation tests, parts of the yard drain pretty fast (less than a minute) and other parts much more slowly (1-2 hours).

In addition to general advice about considerations, I would love to be able to look at some swales and earthworks constructed by other permaculture growers, in our area (Durham / Raleigh / Chapel Hill, if there are growers on this forum who would be willing to give us a brief tour. (Or I also travel quite a bit between NC and Connecticut, so anywhere close to the I-95 corridor could work well also.) I've been studying permaculture off and on for >10 years, but this is both the first time I have land of my own and also the first time I've tried to garden in clay soil, so I'm feeling unexpectedly out of my element.

Many thanks!
Rob

 
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Welcome to Permies, Rob!

To be sure I understand you, you have 2.3% elevation change over 120 feet? You mean it drops 33" from one side to the other? That doesn't sound like something to require earthworks. I have a drop of about 8' across a distance of 100' and other than a French drain around the house and some retaining blocks and old RR ties, I don't need much in the way of earthworks. I haven't noticed much erosion in the 5 years I have been here, nothing I can't correct with a little shoveling and mulching once in a while.

I'd say that building the topsoil so that it can hold during heavy downpours is your first order of business. Get a hold of tree trimming services and have them dump their chipped brush on your property. It's going to take about 15 loads or so to get enough chips to put 4" down on your back yard. And that 4" is a start. It will all decay away in 3 years (4 years tops), maybe by that time you can build up enough soil so that it doesn't require as many inputs.

Are you considering seeding it with crimson clover in the fall?
 
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Maybe also check out some information on rain gardens. Might give some perspective on just water management, and add permaculture/swale logic on that.

http://www.bae.ncsu.edu/topic/raingarden/stormwater.htm
 
Rob Fetter
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Thanks, John and Dave. Yes - over 120 feet I have about a 3 foot drop. A couple of permie friends had suggested swales, so I was going down that path. But if you've done fine with RR ties, retaining walls and a French drain on an 8% slope then that makes me more optimistic about using those methods. Thanks for the tip on getting chipped brush - there are a lot of trees in our area so with luck we will find a tree trimming service that will provide chips at low cost. (Any tips on finding such a place - or just call around?)

I am pretty sure I'll use oats + hairy vetch in the fall. From what I understand I can plant hairy vetch a bit later in the fall than crimson clover, and the extra 2 weeks will make a difference in how much benefit we can get from the summer cover crop (cowpeas).



 
John Elliott
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Rob Fetter wrote: (Any tips on finding such a place - or just call around?)



Call your electric company and find out who they subcontract their line maintenance to. Around here, they do it in the winter months so they don't have a whole lot of leaves running through the chipper. Last January the line maintenance company called me (I have a standing order for chipped brush with our electric co-op) and I got 5 loads (about 10 cu yd. each). Between that and the clean up from the ice storm in February, I've got plenty of mulch.

But you should be able to call any tree trimming service and they will be glad to help you. EPA frowns on chipped brush going into landfills, and in some states you can't take it to a landfill, it has to be recycled or composted.
 
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