Sunday, June 14, 2009

Permaculture, it’s hardly rocket science; or is it? Kristen, a good friend of our ours is, by trade, an engineer. A very special type of engineer, in fact. He spends his days in the exosphere (spatially, perhaps even space-ially and certainly intellectually) modelling the mathematics that keeps things like satellites in the right place and pointing in the right direction. Clever chap. He also has a young family and is passionate about permaculture.

Part 1 and then Part 2 of our experience digging a perma-culture swale got his grey cells tingling. Specifically, it was my question: “In all the information I read through in books and on the Internet, I couldn’t find any calculation or other guidance to size our single swale.” This is his clear, helpful and precise response:

For a full explanation, have a look at his own blog on the subject of swales. It’s in French but, if maths isn’t your strong subject, it might as well be in Double Dutch!

So now you know how to dimension a swale and, in answer to the question, “is permaculture rocket science”, the answer is “sometimes, yes”.


kristen said...

For those who do not speak French but do get math, I can explain the sizing formula here: h ~ sqrt(2rDp). Literally, it means : "the height of your swale above the initial terrain should be at least twice the maximum downpour rainfall (r) times the horizontal distance D between swales, multiplied by the initial slope (p)" (with a 1-p/q correction if your terrain has steep slopes) [note: all values in metric units, évidemment]

What it essentially means is that for instant summer downpours, your terrain can be considered impervious: therefore your swale has to be able to store all the water that falls between itself and the next uphill swale.

In fact, this is a worst-case assumption, when your soil is really impervious over a two-hour timescale. This might be the case if you inherit barren land that has been extensively plowed, but should not happen with healthy soils full of worm holes, tap roots and mole tunnels, even when they have a very high clay content and an impervious rock bed less than two feet below the surface like mine: last week we had a 50mm (2 inch) downpour over three hours and I believed I would find my swale half full (according to the above formula). In fact, it was dry: there had been absolutely no runoff.

kristen said...

erratum: "the height of your swale [...] should be at least THE SQUARE ROOT of twice [...]", otherwise the units are inconsistent.

Albert A Rasch said...

Wait a second...

If those calculations are correct, than a bumblebee can't fly...

Almost Neutered