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build a swale on steep hillside?  RSS feed

 
Tracy Lee
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Location: NW Arkansas
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We just recently got 10 acres of mostly woods in the ozark/boston mountains of north west Arkansas. Along the whole edge of property is a steep hill. The highest point is in the south east corner of the property and it tapers down as it goes to the noth east corner but is still approx 100 ft high. We plan to put a cordwood house bermed on 2 sides in the northeast corner of the property. out from the house to the south and west I will be doing a large permaculture garden approx 150'x150'. Any ideas on how to slow down the rainwater that will come rouring down the hillside in a heavy fast rain. Because of the drought this summer we have not had the oppurtuninty to see what the water will actually do but see evidence from old runoff ditches. Along the bottom of the hill i would like to plant blueberries. Will it work to dig a ditch 18" to 24" deep and about 3' wide along the base of the mountain, fill that with logs, old hay etc, cover with dirt from the ditch and put all that behind a rock wall? While the hill above has some trees and foliage it was recently spot logged and there is some bare areas and i think the water could come barreling down in some of these rainstorms that drop 4-5 inches or more in an hour. my concern is that it would wash away my hugelkulture bed. Any thoughts on this? The ground does start to level out at the base of the hill but still tapers down slightly from that point the entire width of the area where i plan on doing my dream garden. Combination veggies, trees, edible perenials and flowers. I had thought about channeling that bed into another bed that could parralel it one terrace lower and continue that the entire garden. Also is there a permaculture designer out there that lives near our area? Would like some help in selecting the right plants for the area and how to work around excisting trees. The house will also be higher than the garden so we plan on foregoing the conventional septic system and do a compost toilet and a grey water system based on toby system in gaias garden. Fortunely we do have a backhoe. My other question is that i would like to lay logs across or create swales mid way up our hill in various spots and its to dangerous/steep to get our equipment up there, so how do you people do it that casually mention creating swales on hillsides? It would an enourmous amount of work to haul anything up that hill one armload at a time. Or to dig one with a shovel. There is no way to acces the top of our hill with equipment either so we cant drop it down. Any thoughts or comments appreciated.
 
Jordan Lowery
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How steep is the slope? Meaning how many ft of vertical for how many ft horizontal. This will dictate how big, wide, deep the swale is.
 
Tracy Lee
Posts: 52
Location: NW Arkansas
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Just walked out there and evaluated. Couldn't say exactly but i estimated that straight up the height of the hill is 30'-40'. If one was to lay a tape measure along the actually slope it looks to be around 55'-70'. Don't know mathematically how much drop that is per ft.
 
Jordan Lowery
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Usually people think smaller numbers here, for example I'm working on a slope right now that was 1:3 before all the terracing I did. so for every ft of rise there was three feet of horizontal run to meet back with the hill.
 
Paul Cereghino
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Perhaps getting a couple bales of straw down on bare spots on the hill slope will help prevent gully formation. Running goats on the straw can help poke it into the soil improving erosion control function. Seed into the straw with deep rooted grass and clover.

I suspect you'd need to also know rainfall intensity, and soil percolation, and the landscape beyond your property to size a swale on a steep slope. Capturing/concentrating water on a steep slope can be dangerous. Diffusing water with veg and roughness is traditional approach.

Once you capture water you end up having to manage it throughout the site. Consider ponds with an area of "live storage" to buffer the big storms (volume that fills up but that then drains into the swale system over a couple days. If no PC designer with serious experience on slopes (and in my opinion, a PDC doesn't qualify you to do erosion control or steep slope management), and you have capital than consider a design/build engineer with 'biotechnical erosion control' or 'ecological restoration' credentials, and make him or her deal with your design goals (retention, percolation, irregular surface, rather than drainage)
 
John Polk
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If the slope isn't too steep to run a tractor on, keyline plowing (sub-soiling) on contour would allow a lot of the runoff to soak in before it had a chance to flow down the slope.

 
Jennifer Wadsworth
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Location: Phoenix, AZ (9b)
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I know I'm late to this conversation, but if the slope is too steep (geoff lawton says 17% slope or more) you should NOT swale it due to the potential for collapse. Instead, plant it in trees to slow and spread water and start your swales where the slope is less than 17%.
 
R Hasting
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Location: Mineola, Texas
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Just wondering, the point of the swale to to hold water until it can soak in. It is just a hole in the ground, basically, nothing magical about it.

Now there is the zai method where you just dig holes in the ground every few feet, maybe a couple large shovel fulls of dirt for each hole. And that will act as a mini swale, but it will not disturb the earth so much as to risk collapse.

The downside is that it would be labor intensive unless you used a tractor with a 8" post hole auger attachment. Travel uphill, dig a hole 15" deep, drive up 5 feet, repeat, until you get to the top. Work your way down, start at the bottom, space over five feet, and repeat.
You could think of it is a massive lawn aerator. Within a few years, the hole will have completely filled in with organic material, and will have done their jobs. You may want to repeat this yearly until you have 20 inches of deep soil that holds all the water on the slope.

Downside, you now have a holy hillside, watch your step. not good for cows, probably fine for the sheep or goats.

Upside, this will absolutely work in much the same way as a swales and terraces, without the possibility of catastrophe. It would meet Paul Wheaton's "everything should be Wonky" test.


 
Jennifer Wadsworth
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Zai pits might work but depending on size of pit and angle of slope they could still destabilize an area. Zai pits (or infiltration pits/infiltration basins) are more suitable to flatter lands.

*IF* the slope is not more than 17 degrees or 2:1 in slope making swales a possibility, the fact that swales are long and continuous leads them to slow and seep otherwise erosive water flows.

Having said that, there is also "net and pan" which is kind of an "imprinting" method for steeper slopes, similar to what R was talking about before but more intensive. You can use this on slopes up to 26 degrees or 3:1 slope according to Brad Lancaster of Water Harvesting for Drylands and Beyond Vol 1-2 (specifically Vol 2 p. 42)
 
Dale Hodgins
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I like the bale approach. On a steep slope, I wouldn't dig.

A single sided gabion fence could be run on the same contour where a swale would go. Posts could be driven into the ground to 18 inches or so above the soil and wire placed against the uphill side. Few nails required, since wire will be pushed against the posts. Straw and twigs placed against the mesh, would trap soil and other debris that washes down slope. This would be a good spot to dump rubble rock, concrete scraps and tree waste. --- Over time, you'd have a rocky hugelkultur terrace that traps runoff and eroded soil. This could become a sort of hedgerow of cane berries and fruit and nut trees. Twenty years down the road, the posts and wire will rot away, but the terrace will remain anchored in tree roots. Not only would this be far less work, it would begin doing it's job immediately, without the process causing erosion that it should prevent.
 
Miles Flansburg
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What about dropping a row of trees along the hillside. Sort of a wooden berm across the slope.
 
Jennifer Wadsworth
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Miles - do you mean felling the trees and putting them along contour?
 
R Hasting
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Location: Mineola, Texas
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You could do what Sepp does and Terrace it. But if you do that, you will probably destroy a lot of vegetation. I think that the key here isn't whether to infiltrate water, you should. The key is to plant trees with deep tap roots and good soil holding ability. No conifers in this system.

That is the ultimate solution.

Of course it depends on the soil type and whether it is prone to slide.
In my area, soil and hillsides don't move at all because they are 95%+ rock. Never heard of a mudslide in Central Texas. We build houses here of the tops, sides and bottoms of cliffs with confidence. So this is my default perspective.

Jennifer, why do you think that a zai pit would not work on slope? The guys in Africa who are doing this are doing it on flatter land because it is what they have, and it can be done with a stick or a shovel.
I would never do zai on flat land. That is why God invented backhoes for swales.

We can apply lots of techniques to different problems. All of the techniques mentioned so far would certainly all help. I don't know that any one of them will give you an optimal solution. I am a big believer in diversity in that a cascading collection of techniques is probably better than one single silver bullet.

Bales of hay, along contour would also help, as would brush piles or logs along contour. But nothing beats water holding capacity like a little pool of water.
Even digging a six inch deep swale every few feet would give you so much more water seepage. You don't have to do it with a track hoe.

I still like the idea of pocking the land with 8 inch diameter, foot deep holes to hold water and erosion and organic matter. And plant the heck out of it.
 
Jennifer Wadsworth
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R Hasting wrote:Jennifer, why do you think that a zai pit would not work on slope? The guys in Africa who are doing this are doing it on flatter land because it is what they have, and it can be done with a stick or a shovel.
I would never do zai on flat land. That is why God invented backhoes for swales.


Hey R Hasting - thanks for the question.

Just to reiterate - what I said was "Zai pits might work but depending on size of pit and angle of slope they could still destabilize an area. Zai pits (or infiltration pits/infiltration basins) are more suitable to flatter lands. "

"Imprinting" and "net and pan" techniques are basically zai pits with stabilizing properties built into them that are designed to work on slopes.

Brad Lancaster has a really great chart in his "Rainwater Harvesting for Drylands and Beyond - Vol. 2" pp. 42-43 (which he's given me permission to post on my blog but I haven't gotten to yet...) that categorize earthworks and the type of situation it is best suited for. It goes something like this:
--Berm and basin/swales - good for sloped land UP TO 3:1, 18 degrees or 32.5% grade
--Terrace - good for sloped land UP TO 2:1, 26 degrees or 48.8% grade
--Infiltration basin (essentially what zai pits are) - good for flat or gently sloped land (note that the original poster stated they had a "steep hillside")
--Imprinting (a variation on a zai pit) - good for flat OR sloped land up to 2:1, 26 degrees or 48.8% grade - these are basically micro pits with mini berms on the downslope side that are placed close together.

There are always things that are going to work better in certain situations than others. That's why I expressed doubts about zai pits (as I know them from "The Man Who Stopped the Desert"), not as impossible, but rather, possibly not the best choice. I don't think we ever did find out the exact slope of this property so it would be hard to make a determination lacking this knowledge. I know in the Loess Plateau they used net and pan and terracing very effectively on slopes. Geoff Lawton mentioned several times in his online PDC that he likes to put any slope that's more than 18 degrees into trees (no water harvesting other than the trees' natural water moving/storing capacity). This holds moisture in that high, steep part of the property.

Other factors that need to be considered in making a decision (and which we also don't know) are things like type of soil, rockiness, existing vegetation, etc.

And finally - I just want to say - "thank goodness for back hoes!"
 
Miles Flansburg
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Jennifer Wadsworth wrote:Miles - do you mean felling the trees and putting them along contour?


Yes.
 
Dale Hodgins
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I have a very steep, south facing slope that is barely walkable. It was logged before I bought the place and grows broom and some butterfly bush and other bushes. Some cottonwood regeneration was well under way until high winds toppled them. They were 8 inches in diameter and 50+ ft tall. I bucked them into manageable chunks and laid them on the slope.

The hope is that the debris will catch leaves and other stuff from washing to the valley below. It's quite a tangle now and seems to be working. By shading the soil and adding organic matter, I hope to create an environment suitable for useful plants. Grapes do well here. For now it is a good spot to dump materials including rocks that I'm piling on any spot level enough to hold them.

I didn't take photos of the stuff. This is a shot looking to the valley floor.
IMAG1228.jpg
[Thumbnail for IMAG1228.jpg]
 
Jennifer Wadsworth
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Nice photo, Dale. I always kick myself for not taking "before" shots. You just can't go back and recreate "before"....sigh.
 
Patrick Mann
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--Berm and basin/swales - good for sloped land UP TO 3:1, 18 degrees or 32.5% grade
--Terrace - good for sloped land UP TO 2:1, 26 degrees or 48.8% grade


Why is terracing applicable to steeper slopes than berming? I would have expected the opposite. Unless terracing implies retention walls.
 
Jennifer Wadsworth
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R Hastings and Patrick Mann - you got me thinking (and poking around the web). I've found a bunch of techniques referred to by different terms, which might be part of the problem... Also when does a pit become a mini-terrace, or an imprint become a pit??

Apparently not all terraces have retaining walls - I don't think much of the Loess Plateau restoration used retaining walls but rather a set of imprints (like miniature zai pits!) for stabilizing backcuts with vegetation.

Here are some interesting photos of water harvesting earthworks on steep slopes:

From: The Imprinting Foundation



From: Before and after of earth restoration from the ground up The images below are of the Loess Plateau




I also checked out some more images of zai pits. I always understood them to be larger, more like the infiltration basins we use here all the time in Phoenix and thus suited for flat to gently sloped land - but indeed, the term "zai pit" is used for many different types of "holes" including imprinting-style mini earthworks too!

From: Conservation Tillage Systems

Matengo pits
Structures called "matengo" pits are used in the Southern Highlands of Tanzania. The pits can be up to 1m x 1m and 30 cm deep, but the actual size is determined by depth of the soil and ease of digging. The pits are laid out on sloping land forming a grid to cover the entire surface. Soil taken from the pits is used to form ridges around the pits.
Crops are grown on the ridges and the weeds and crop residues are thrown into the pits. A rotational system is usually practiced using crops such as maize, beans and sweet potato, and the pits are regularly moved and new ridges built where the organic matter has accumulated. The pits also serve as structures to conserve water.

Zai basins
"Zai" basins are used in semi-arid areas to concentrate manure and runoff into basins or pits where the crops, e.g. sorghum, are planted. Zai pits have traditionally been used in the dry regions of Burkina Faso and Mali in West Africa. The pits are about 15-20 cm deep and 30 cm in diameter, with 1 m between the rows.
Topsoil from the excavation is mixed with manure and put back in the pit where a few cereal seeds are then planted. The pits also concentrate rainfall runoff around the plants, thus improving moisture supply to the roots.




 
Dale Hodgins
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Jennifer Wadsworth wrote:Nice photo, Dale. I always kick myself for not taking "before" shots. You just can't go back and recreate "before"....sigh.


Many plastic surgeons would beg to differ.
 
Dan Grubbs
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Just thinking out loud here for the veterans to pitch in on in this swale or no swale discussion. But, here is my thinking ... in the possibility of considering all earthworks shouldn't we be doing it in concert with soil improvements on the area. Since one particle of uncompacted humus can retain 4 particles of water, it seems that we would want to put just as much energy and effort into optimizing our soils as we do earthworks. Healthy soil is by far the cheapest way to store water because of this factor, besides all the other very beneficial outcomes of hydrated soil. Whatever appraoch to earthworks one takes: swales, berms, ripping, terracing, it seems that they can dramatically be enhanced by taking proactive soil-improvement steps with water retention in mind.

If I'm off base here, be merciful with this rookie.

Dan
 
Jennifer Wadsworth
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Hey Dan - thanks for weighing in!

No matter what kind of water harvesting you do - be it simply planting stabilizing trees on really steep slopes or adding some kind of earthworks - the net result is increased water absorption in the landscape from running the water energy through the system more slowly.

Increased water absorption naturally provides a more hospitable environment for all the critters that live in the soils. More critters will be automatically making more and better soil.

Add to the fact that both trees and earthworks attract nutrient drop from various sources such as nutrients from wind (in the trees, airborne nutrients drop out about 12-15 feet into the forest, in earthworks, air swirls in the depression and drops nutrient), from animal manures (animals sitting in trees, dropping manure, animals in the earthworks foraging in the collected leaf drop, etc), from plants, etc.

Nutrient drop + slowing and sinking water = kick ass soils with little work on our part once the system is set up. It's a sweet deal.

As for being a rookie - aren't we all? I think there's no end to learning this great stuff - your question was awesome!

 
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