making my first swale

bart_2010(8/9 Italy)July 2, 2014

Hi,people! I think the best solution to my erosion/run-off problem in my sharply-sloping rose garden might be a swale; from what i'm seeing and reading on Internet, these seem like a GREAT idea. Too bad I didn't know about them 15 years ago when I started my garden,because now I'll have to work around what i already have.
Now for my questions. First of all, I understand the idea of digging the swale along the contour of the land. Is it really necessary to use an A-frame level for this? because frankly,in the area I am thinking to put my swale, the contour seems pretty clearly visible . It's very rough, uneven terrain, and I'm clearly going to have to bring in soil to complete the berm and make it all relatively even, but I guess the idea of using some kind of precision instrument just seems redundant, considering the nature of my site,and the small scale of my project: it will be dug by me with a pick and shovel,for example,and be no more than say, 10 meters long.
Secondly,I must decide at what level to make my swale. Do the plants on the up-hill side of the swale end out benefitting from the harvested water ,or are only the ones on the down-hill,berm side benefitted? Thanks in advance, bart

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nil13(z21 L.A., CA (Mt. Washington))

I responded to the level question on the other thread, but since you have this thread here we should probably not hijack the other one and talk about it here. You don't have to use an a frame level. I have used bubble levels, clear vinyl tubing, a frames, and transits. A masonry bubble level on a piece of straight wood about 1.5m long works pretty well. Just put a block on one side to create a specific slope. I did that and then checked everything later with a transit and the slope was spot on.

Downhill plants benefit more.

    Bookmark   July 4, 2014 at 2:42PM
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bart_2010(8/9 Italy)

Thank you very much, Nil13! I think I'll make an A-frame thingie, just because there's videos on YouTube showing how.
Now, on the other thread you mentiones the question of how slopy IS my slope,and as i said there, I'd guess around a 33 degree angle. Since i'm growing roses, i've always had to bring in lots of soil, since what I have is not very deep,so i've used the terracing concept already.
A few more questions: how deep must the trench of the swale be? How wide? How long? I've read from 3-5 feet deep and wide,but it seems to me that there's a lot of flexibility to the concept. One area of my garden is already somewhat planted up; here I was hoping to make ,say, 4 smallish swales (two to three meters long each). Could this work? regards, bart

    Bookmark   July 5, 2014 at 4:04AM
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nil13(z21 L.A., CA (Mt. Washington))

A 33deg slope is about 1.3m horizontal for every 1m vertical. That's pretty steep and hard to work with. I deal with slopes like that frequently. There are a couple of problems that have to be discussed. The first is landslides. Soil on steep slopes can slide when saturated. Soil and rock with high percolation rates are very helpful for avoiding slides. Not exceeding a 45deg angle (1:1) also helps. Just remember water and steep slopes can be a dangerous combination. Another problem is that since 1:1 is the steepest stable slope, you will not be able to make a very deep or wide swale. See the diagram. This means you can't hold much water before your swale overflows. Most of the swale suggestions you read about are for much shallower slopes. In order to create a stable 1m swale on a 1.3:1 slope you would have to build up something like 10 vertical meters of new 1:1 slope to create that space behind it. Not very feasible.

But there are other options. As you can see from the diagram, a small furrow can be created along the slope. So instead of making a swale that is level on contour, you make a furrow that descends across the countour at about 50:1. Then you make another furrow going the other way across the slope. Keep going back and forth from side to side until you make it all the way to the bottom of the slope. Then divert surface runoff to a non-erosive surface. That will greatly increase water infiltration on steep slopes while decreasing erosion.

To get the most out of steep slopes, you have to create terraces with some sort of retaining wall with proper drainage to deal with hydrostatic pressure.

    Bookmark   July 5, 2014 at 6:46PM
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bart_2010(8/9 Italy)

Nil, you are so kind and generous to take out time to help a complete stranger; thank you so much! I wish I wasn't so dumb,not understanding the "degrees" stuff (what is 50:1,asks Bart the Dummy?) but your drawing is mega-helpful!!! since you DO understand this stuff written down, whereas I DO need an image. Now, from your drawing, i realize that my slope may not be quite as sharp as that,though it's pretty close: it levels out more or less at the height of the dotted line in the drawing (there's a bull-dozed dirt road cut into the side of the hill at that level, done by the previous owner,which is part of the reason for the bad erosion problems, methinks...) ,so the angle is more or less as you've seen it in your mind's eye.
Actually, it's good news to me to know that I can't do deep or wide swales; digging meter-deep-and-wide ditches is a lot of work for a middle-aged, albeit strong,lady to do on her own,though as I assured my DH, I've done harder work for my roses...not to mention the fact that I am by no means sure that the soil is sufficiently deep all throughout the slope to permit such stuff! I gather that what you're saying is that I should think in terms of constructing small, narrow trenches: basically sort of just terraces the entire slope. I was thinking to use a lot of branches to make mounds at the foot of each trench,covering them with gooey clay,holding them in place with short, thick branches stuck vertically in the soil,hopefully directing said trenches in such a way as to diffuse the run-off from the road throughout the slope. Fact is, I've seen the power of the run-off water. I tried to construct "ecologically friendly" steps from the road down into the garden,basically just using the branches-and-earth technique, but it was entirely eroded away within a couple of months once the rainy season began; I was forced to make cement steps ( couldn't do it with rocks; my soil is very, very rocky, but they are these very soft, friable rocks that break up very quickly ;there just are not many true, hard stones ...being an artist, I have tried to make the cement steps aesthetically appealing,but the problem remains). So the next step is to diffuse the run-off into the garden,making sort of mini-swale-like trenches. Did I understand correctly? Thanks once more... regards, bart

    Bookmark   July 6, 2014 at 4:47AM
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nil13(z21 L.A., CA (Mt. Washington))

That diagram I made is very rough and I was just doing the math in my head, so it might not be spot on. Here is the wikipedia page for slope that has some nice charts and explanation. 50:1 is just a ratio that describes the slope. In this case 50 horizotal units (meters, feet, whatever) for every 1 vertical unit.

You are probably right about the road causing erosion problems. There are two basic actions when dealing with surface drainage, concentration and diffusion. Roads concentrate flow. Water comes off the hills, hits the road and starts going sideways meeting up with more water that came down to the road. If the road is well designed, it will take all that concentrated runoff to a drain where it can't do any damage. If it isn't well built, there will be a high spot somewhere in the road that will at like a dam and create a pond. The water level in that pond will raise until it finds a new way down which may be in a bad location like your rose garden. The other action is diffusion. That is what builders do with new houses. They move soil around so it is nice and even with no little valleys or ridges so that the water runs off the property in a nice thin sheet. This works pretty well with shallow slopes but it is difficult to keep steep slopes as even as is necessary. Steep slopes inevitably form little valleys which concentrate more and more flow as they get deeper. So if steep slopes are going to concentrate flow eventually no matter what, you miht as well decide where that is going to happen with the tiny swales.

Here is a link that might be useful: Slope

    Bookmark   July 6, 2014 at 12:23PM
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nil13(z21 L.A., CA (Mt. Washington))

Since you are a visual person, I diagramed some pics of my hillside.

Here is a link that might be useful: Drainage control on slopes

    Bookmark   July 6, 2014 at 6:40PM
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bart_2010(8/9 Italy)

This is great; thank you so much, Nil13!!! I'm getting a lot of ideas. The slope will come first, of course, but I'm looking at the flat part at the bottom of my garden,too, as an excellent place to make a swale;much neede, too,since it floods a bit there when it rains a lot.This is so exciting; promises to help really improve my soil as well as conserve the precious water...regards, bart

    Bookmark   July 8, 2014 at 3:07AM
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Bromus

Do the work while it's raining so you can see precisely where the rain naturally wants to channel and pool. This also makes the leveling tools relatively unnecessary. Think like the wilderness; cascade and pool, cascade and pool.

Taking water off a roadway is a pretty serious undertaking (impervious road surfaces collect ALOT of water). You are right in using concrete/rocks to get that kind of water-volume down the slope sans erosion. Nature's cascades are rocky too!

Use coarse woody debris (logs) to help construct your catchments; they are super water/carbon buffers for your soil microbes (and thus your plants).

Again, I think going out in the rain and actually looking where the water flows FROM and TO is really key though. It seems stupidly simple, but so many people I work with want to sit in their office when it rains, looking at topos, drawing 2d pictures, overconfident of their ability to engineer nature.

*Going with the flow*, you can change the water course pretty profoundly, and with *minimal shoveling*. On the flat, think broad, shallow catchments; this means you only have to build a dam wall a few inches high. Stringing together multiple smaller catchments is much more effective than building a single, large catchment.

-B

    Bookmark   September 16, 2014 at 11:12AM
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bart_2010(8/9 Italy)

Good suggestion,Bromus The ditches I made to catch the water flowing off the road seem to be working pretty well,though they still need finishing off,and I won't really be able to say until we (hopefully) get some serious autumn rain.
I have just begun digging in the uncultivated part of my garden. Using the A-frame is very easy,so even though this, too, is a pretty sharp slope, I still want to find out the contour,as a learning experience, if nothing else. bart

    Bookmark   September 19, 2014 at 6:05AM
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