Wednesday, April 08, 2009

We’ve been so busy lately, with the long length of the “list of things to do” ever unchanging, it seems that we’ve just been chasing our own tails. Click to play the video above, where one of our cats helpfully enacts this metaphor for your clearer understanding. All this proper work has taken a toll on my blog writing and so I’ve plenty of things to tell you about: first up, our newly dug swale.

Permaculture swales: Part 1
If you want to grow anything, you need water but what’s available is often either too much or too little, so needs some careful management. Have a look at this photo of our vegetable plot at the end of May last year to see it doing a passable imitation of Venice. In contrast, we were expecting an average rainfall of 60 mm of rain last month yet only received about half of that. And rather than being evenly distributed over the month, two thirds fell on one day. The longest gap between rain days was just over two weeks, leaving our total water storage of 4 tonnes bone dry and forcing us to turn to potable tap water to keep seedlings and other new plantings alive.

Just uphill of our vegetable plot, we’ve surveyed, dug and planted a swale—a horizontal ditch that follows the contour—which we hope will deal with both of the problems I’ve mentioned, i.e., avoid the inundation of our veggies but also storing that valuable water underground. How so? Our main field slopes gently downhill towards the vegetable plot; the earth has a large clay content, so when we get heavy rain, the water will run off the land, rather than infiltrate it. The swale now provides a barrier to this run off but, unlike a land drain, the idea isn’t then to just get rid of the water, draining it by gravity along buried pipes to the drainage ditches at the edge of our property that run along the road. We want to utilise this water by storing it … underground.

I’ll tell you more about how swales work in Part 2 but will now explain how we dug our own. The swale is dug on contour, that’s to say it’s a horizontal ditch, and so we need to join all the points at the same level by surveying the land. Even without professional tools, it’s possible to survey accurately. Read how we made a Bunyip water level and used it to mark a level line on our barn (we measured off that line to install a French drain). On my permaculture course, we learnt to use both of these devices. The first photo demonstrates the bunyip water level and, in the second, Patrick Whitefield shows us how the A-frame works. For our swale, we constructed an A-frame, with an adaptation.

The idea of an A-frame is that the two legs of the triangle are equal and a line with a weight (plumb line) hangs off their intersection. The connecting bar, attached at the same point on each leg, is marked at its centre. When the plumb line cuts the centre mark, the two legs are at exactly the same height. We use this property to walk the A-frame across the land, shifting the moved leg up or down the slope until the plumb line shows they are level. We then mark that point and swivel the other leg around, repeating the levelling, and so on, leaving a line of pegs that mark out the contour. One problem with the plumb line is that the bob swings like a pendulum and takes a while to settle, or needs a partner to steady it at the bottom. We got round that by attaching a spirit level at eye height on the crosspiece, whose bubble settles quickly, so speeding up the process. Photo shows Gabrielle using our A-frame.

Another issue is that if a device relies on levelling with regard to the last point, rather than the original datum, a slight inaccuracy each swing of the legs, gets multiplied over a long distance. A long bunyip, with one end always held on the start point, can be more accurate. We used the A-frame to mark out the contour and then, when I’d dug out the swale, I hammered in pegs of wood and used a 2.5 metre straight edge with spirit level to check the level of the bottom of the ditch (see photo). Ultimately, the most accurate way of checking your swale is level is to look at it when it first fills with water. The surface of the water will always be at the same height, so if you have a reasonably even puddle the length of your swale, you’ve surveyed and dug your swale well. You can adjust the swale after it’s rained, so it really is quite easy to get it spot on.

More on how swales work, how we dug ours and what we planted it up with in Part 2.


kristen said...

I have dug my first swale in November last year. I did the surveying alone with a water level, one end of which was tied to a post.

But on second thought I judged I would have been better off with an A-frame, as I found it hard to handle the water level alone.

Then on third though, I have issues with the A-frame. In addition to requiring a damping device for the plumb, the A-frame is stopped by obstacles (trees, shrubs), whereas a water level can go anywhere. Moreover, the standard deviation of the measurement error builds up like sqrt(N) (N being the number of measurements from the reference), then the sqrt(longer) your swale, the larger your level error.

My best choice would be a water level with a big reservoir at the reference end. Indeed, if the section of the big reservoir is 1000 times that of my hose, then a 1-m level change in my hose will only result in a 1-mm level change in the reservoir (hence negligible). This allows the water level to measure heights instead of just levels, making it a more versatile tool.

Anyhow, I can testify it feels really good the first time the rain starts to fill the swale, when you see that the water is horizontal with respect to your work.

Mark Angelini said...

I'm glad I've found this article, because I was wondering how one would keep the entire swale level if you are only going off the last marked point. Good stuff, I am also getting some good ideas with regards to using the bunyip, thank you!

Stuart and Gabrielle said...

Your welcome, Mark.