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Help me understand swales  RSS feed

 
Noel Baker
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So I live in zone 7 and the past couple years the lack of rainfall has been horrible for my garden. Then I learn a little tiny bit about permaculture and realize much of what I've been doing is wrong. I hear all these people talk about digging swales to help retain water. I see pictures of people digging swales but since I haven't taken a PDC, I'm not fully understanding the reasoning behind why they build them where they do, and what the effect is. To make matters worse, I've always been a "show me" type of learner, so here are some horrid ms paint pictures to try to help describe my question. Okay, so lets say the first picture, the white line across the middle is the highest point of elevation, and the south is higher than the north. So all rainfall is going to run up your monitor and to the left and right, as indicated by blue arrows.



So if I understand a swale, it just slows down the runoff, and keeps the ground downhill of the swale with more moisture? So if I'm trying to slow runoff would I dig my swales as indicated by the brown arc line? The rectangles represent where my garden would be. To keep things simple, lets not get into wether or not I'm watering a food forest, a hugelculture bed or just a plain old patch of ground....unless it really matters relating to the swale construction.



So when the spring rains come and wash all my dirt down the hill or form little ponds, wouldn't I just end up with a bunch of really big mud puddles (indicated by the blue area) that only compact the soil and kill whatever plant life is under them?



So I'm sure there's something I'm not understanding, what is it I'm missing, and if you could direct me to a video that would be peachy!
 
Ben Stallings
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Location: Emporia, KS
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Hi, Noel. I think what you're missing is that a swale is not a dam, it's a place for water to enter the ground. So if the swales are built correctly, the water will only pool up for a few hours at most; then it will be underground.
 
S Bengi
Posts: 1357
Location: Massachusetts, Zone:6/7, AHS:4, Rainfall:48in even distribution
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OK so your land is like a roof and the swale is a gutter collecting all that water.
I would then plant my garden all along that swale/gutter/ditch that is running parallel to the ridge line (N/S).
You want your swale to be at a 90 degree to the flow of water.

Now if for some reason you cant plant all along your yard you could use the swale/ditch to funnel the water to your garden area (two V shape ditch) and then use a spongy hugelculture-like soil thingy to hold the water in your garden.
 
Craig Dobbson
master steward
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Location: Maine (zone 5)
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Take it away Geoff

http://youtu.be/UFeylOa_S4c
 
Matt Grantham
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Location: Napa CA
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I like the Lawton video a lot, so thanks It does however really only show you the action from a cross section that is two dimensional and does not really give you a good sense in the dimension that run across the slope as you are moving up an down it. In other words how wide the swale mound should be or might be and whether the mounds has a linear perpendicular arrangement in relation to the slope of the hill or whether it has a more rounded perimeter. Also I remain unclear whether the soil in the swale mound should be any heavier or perhaps even rocky. I assume part of the idea is to force the overall water flow deep and that a less previous soil at the swale might help that process? I also understand that dense grass is recommended at the swale mound to prevent erosion and I asume to slow the water movement through it? The depth of the swale ditch and whether you can make general correlations between the slope of the hill and the depth of the swale ditch, etc. Also what kinds of plants will do well in the ditch iiself. In my case on the other thread we are tryoing to plant mostly fruit trees and so are interested in how to arrange soils to benefit them best Also if you keyline does this make swale use less impactful or necessary?

Sorry about all the questions, but hope to someday be able to answer more than i ask
 
Noel Baker
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Thank you! So my first question is "how deep do you make the swale". And the official permaculture answer is "it depends". I'm sure it depends on how steep the hill is, what kind of soil do you have, how much rainfall do you get and a host of other issues I'm sure.

I do get heavy spring rain (usually) and my first concern is erosion. That the swale is going to fill up, wash over the dirt mound and flood everthing down the hill. So is it possible you can dig the swale too deep? After watching Lawton's video, I assume so, especially since you're wanting to grow stuff in the soft earth above the ditch. If its too deep the water isn't going to soak into the root area right?

Lets say I cut my swale, plant all my goodies and sit and wait for rain. As the rain continues I nervously watch my swale and notice its holding too much water and I'm worried about it running over. Presumably I can always cut my swale out further on the ends, and for that matter dig a small holding pond for the water to run into just to keep the water from running over the soft dirt?
 
S Bengi
Posts: 1357
Location: Massachusetts, Zone:6/7, AHS:4, Rainfall:48in even distribution
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A Swale is not going to fix all your problems.
If you already have soil erosion problem the swale is only going to help not make it worse and if you dont already have on you should start now.
The swale not going to let anymore water run down your hill than what normally runs down your hill. Its not a dam that going to catastrophically fail.
As to how to fix your soil erosion problem, think root mat, mulch, terraces, vegetation over, anything that will brake the fail of the raindrop, slow the flow of the water on land, or roots closely holding the soil.

So you ahead and make your swale you can always come back and fix it.
The pond idea is pretty awesome you can also fill it with mulch and grow water-tolerant plants in it.
 
Alex Ames
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Location: Georgia
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There is another video from geoff lawton that explains that the swale is designed with an overflow system. He is doing it
in miniature in the dirt. I can't find it but it is out there. There is some precision required to get it right so the heavy rains
don't just over flow it and take it out as you fear.
 
Alex Ames
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Location: Georgia
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Alex Ames wrote:There is another video from Geoff Lawton that explains that the swale is designed with an overflow system. He is doing it
in miniature in the dirt. I can't find it but it is out there. There is some precision required to get it right so the heavy rains
don't just over flow it and take it out as you fear.


According to Paul's latest podcast it in "Introduction to Permaculture DVD" with Geoff Lawton.
 
Jonathan Teller-Elsberg
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Location: Norwich, Vermont
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Noel, it looks as though you have some basic confusions about swales that don't seem to have been addressed by the other responses.

1. A swale is a ditch, plus berm on downhill side, that is on contour across the slope. (If you aren't clear on what "on contour" means, definitely ask!)

2. The purpose of the swale is to slow water flow down the slope to (A) enable more water to soak into the soil rather than run off, and (B) reduce the soil erosion that is associated with water runoff.

Swales are particularly useful in arid environments, where every drop of water counts -- especially on a slope, since slopes tend to dry out faster than level ground. But even in humid, temperate locales they can make a noticeably positive impact on plant growth, as I've seen during a visit to the Whole Systems Research Farm in the Mad River Valley in Vermont, http://www.wholesystemsdesign.com/wsrf/. There I saw side by side plantings of black locust, some planted directly into the slope and some on the berms of swales. Though the trees on the swales were 1 year younger (if I remember correctly, 3 vs. 4 years old), they were clearly larger and more vigorous.

Because swales encourage water to enter the soil rather than quickly run off, your plants can get some benefit even if they are a ways downhill from the swales, as you draw in your second picture. However, greatest benefit occurs for plants that are planted onto the berm or only slightly downhill from it. These have ready access to the lens of water that forms in the soil at the swale and below it.

Over time, sediments and falling leaf litter will fill a swale, converting it to a naturalistic terrace. This is A-OK and you do not need to trench it back out. By the time this happens, your soil should have improved and root systems will have established well enough that water will more easily infiltrate into the soil even without an explicit swale, and the plants will be less dependent on the water gains provided by the explicit swale. (A terrace of this sort will also slow water and help it infiltrate, even if not quite as thoroughly as a proper swale.)

Regarding you worry about pooling of water and such, swales should be sized so that they capture some, but not too much water at any one contour line. If your spring rains are significant, the solution is to construct more swales, each smaller in size, and closer to one another up and down the slope. That way no one swale is handling too much water. Under normal (large'ish rain) circumstances, the water that collects in a swale should fully soak into the soil within a few hours, or day or two at most. Swales should have spillways built into them to allow overflow an appropriate exit, thus avoiding "catastrophic" breakdown of the berms.

By far, the best discussion of swales that I have found is in Brad Lancaster's book Rainwater Harvesting for Drylands and Beyond, Volume 2, pages 66 and following. Lancaster calls them "berms 'n basins," in part to avoid confusion over the term swale, which in permaculture is limited to a ditch on contour but in other realms means a diversion ditch -- a ditch with slope to it that diverts water to some other location. (For similar reasons, in Edible Forest Gardens, Volume 2 Jacke and Toensmeier call them "infiltration swales" to make their meaning clear.) The following PDF regurgitates some of basics from Lancaster's book, but I recommend reading his full take on swales before you proceed with digging anything: http://www.riverlink.org/documents/CH-3-1BermsandSwales.pdf.

Unfortunately, even Lancaster's explanations leave me with some unanswered questions, in particular w/r/t determining the size and spacing of swales, but I'm a lot better off having read his stuff on the topic than I was before.

Note: per Lancaster, swales are the wrong choice if your slope has rise/run ratio steeper than 1:4, aka 25% slope, aka slope of 14 degrees. Anything steeper than that requires a more substantive earthworks to avoid risk of mudslides.

Best of luck!
Jonathan
 
Jonathan Teller-Elsberg
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Location: Norwich, Vermont
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After I complained yesterday about not finding an easy source of info on determining swale sizes and spaces, I decided to construct such a thing. Using Lancaster's formulas, I built a spreadsheet that allows quick calculation of related swale sizes and spacings. You can start either with a certain size in mind and learn the associated spacing, or start with a spacing in mind and learn the associated size(s). I have posted it towards the bottom of http://www.terrapermadesign.com/home/garden-and-landscape-tips/.
 
Elissa Teal
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Location: Detroit, Michigan
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Jonathan Teller-Elsberg wrote:After I complained yesterday about not finding an easy source of info on determining swale sizes and spaces, I decided to construct such a thing. Using Lancaster's formulas, I built a spreadsheet that allows quick calculation of related swale sizes and spacings. You can start either with a certain size in mind and learn the associated spacing, or start with a spacing in mind and learn the associated size(s). I have posted it towards the bottom of http://www.terrapermadesign.com/home/garden-and-landscape-tips/.


Jonathan, would you please re-post the link for your spreadsheet? The one that you gave above is not working. TIA.
 
Meghan Orbek
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Location: Yonkers, NY/ Berkshires, MA USA
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Yeah, Jonathan! That sounds great but the link isn't working! Can you please repost it?
 
John Pollard
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Location: Ozarks
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Meghan Orbek wrote:Yeah, Jonathan! That sounds great but the link isn't working! Can you please repost it?


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