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Very basic swale design question

 
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So, from what I’m reading, if I oversimplify to an extreme, swales are dug into the contour lines of a property with the berm on the downslope side of the swale. So, for instance, if I wanted to plant food on my property, using a modified Ruth Stout method, I would do best to swale and berm on the contour lines instead of in straight rows?
Here’s a picture of one section of my property from the GIS mapping website.
595B260A-44A4-4CB7-A085-D585FA4D35D6.jpeg
[Thumbnail for 595B260A-44A4-4CB7-A085-D585FA4D35D6.jpeg]
 
pollinator
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That's my understanding as well.
 
pioneer
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One thing to consider is that cold air also falls downhill. If you have cold weather, I would consider filling the swale with some kind of mulch and limit the size of the downhill berm. I would shape it in a way that air may flow over the top (smooth transitions and low angle), otherwise your growing season may be significantly shortened. Water should still fill the space in between the mulch then soak into the ground.
 
pollinator
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Well, sort of.  If you build perfectly on contour then you are really building a series of dams on the hillside.  In a rain event, that water has no place to go except through the swale ... and if does that you've got a problem!  Also, its possible that the highest swale captures all of the water coming down the slope and holds it - and although "stacking" water up high isn't a bad idea the swales further down the slope are deprived of that water.

So you can instead build the swale slightly off contour, and have them zig-zag down the slope.  Water coming down is then allowed to slowly wander along each swale before spilling into the next and changing direction.
 
Cookie White
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Thanks, ya’ll! Its so exciting to finally be mildly understanding a permaculture principle 😂
 
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Just coming in to say that I've read or listened to info about swale building in a few places now. This is by far the clearest explanation yet, at least for me. Thank you all!
 
pollinator
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Cookie,

You have it right.  Ditch on contour with berm (excavation) on the downhill side of the trench.  As pointed out, have a spill way for water to release before finding its own way over through or under the berm.  

Could you elaborate more on how you did your maps with self designated contours, please?  I am familar with GIS maps and such but have never figured out how to specify the contour interval of my choosing and impose it on an image.  I would very much appreciate knowing how you did this.
 
pollinator
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Eliot Mason wrote:Well, sort of.  If you build perfectly on contour then you are really building a series of dams on the hillside.  In a rain event, that water has no place to go except through the swale ... and if does that you've got a problem!  Also, its possible that the highest swale captures all of the water coming down the slope and holds it - and although "stacking" water up high isn't a bad idea the swales further down the slope are deprived of that water.

So you can instead build the swale slightly off contour, and have them zig-zag down the slope.  Water coming down is then allowed to slowly wander along each swale before spilling into the next and changing direction.



I would be really cautious here. A swale is, by definition, on contour, otherwise it's a meandering water channel. But, if built as described here, a large rain event could turn it into a torrent of water and produce some pretty dramatic erosion.

An up-slope swale does not deprive the lower parts of the slope of water. One must presume it rains down slope as well. Additionally, the whole point of a swale is, in fact, to stop the water from creating surface flow, to soak it into the ground, so that it seeps down slope THROUGH the ground, thus rehydrating the landscape.

If one is concerned that the water might overflow or erode the swale berm, remember that a swale has a compacted sill that allows for a gentle release of water in a way that is not harmful to the landscape. If there are several swales down the slope, care should be taken to direct the overflow sills from one swale to the next, and ultimately to a retention area. But this is a contingency for catastrophic precipitation. The whole point of a swale is to keep the water soaking into the hillside, not flowing off of it.
 
Eliot Mason
pollinator
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I generally accept Nathanael's clarifications, and I could have written my post better to reflect his points.  Sadly, my dictionary defines a swale merely as "a low or hollow place, especially a marshy depression between ridges"  and the Appropedia (https://www.appropedia.org/Swales) doesn't even mention contour so I don't find the appeal to a definition helpful.  

So regardless of name, lets talk about function.

My caution is that simply creating any earthwork on contour is problematic as it may turn into an inadvertent dam and that can lead to dam failure.  Allowing the water to escape via a well designed spillway or a "meandering water channel" prevents a dam from forming yet slows the progress of the water.  The relative benefits of slightly flowing vs captured water depend, I think, on local conditions and goals.

My reference to the top most swale capturing all the water needs clarification... yes, rain fall hits the whole of the hillside and thus you can calculate the volume of a swale to match 100 year rain events.  I made the error of presuming there is some surface water flow above the top most swale, and if it is functioning as a dam would prevent that surface water from moving down the hill.  If the sole purpose is infiltration then mission accomplished, but if the swales are planted then distributing the water is probably desireable.  In an instance on my property I'm considering berms (not really swales as we are discussing them), but a consideration is that near the tree line the effective rate of precipitation is greater as the canopy pushes the water outwards.
 
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Location: 10 Miles ESE Walla Walla WA 46°N 118.11°W (Elev. 2195 ft)
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Thanks for the discussion. I am considering both swales and keyline development on my upper slope.

I would repeat Jack's request for details on creating a custom topo line map. Google maps on Terrain loses the detail when I zoom in.
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