Swale
A ditch dug dead level, along a contour line, to stop runoff and let it soak into the soil.
Explanation
On a slope, rainwater rushes downhill and carries soil away. A swale (baissière in French) stops it: it is a ditch dug exactly level, along a contour line, with its soil forming a mound on the downhill side. Water spreads along its whole length and soaks in instead of running off; the mound, planted with trees or shrubs, benefits from this reserve. A technique described by Bill Mollison (Permaculture: A Designers' Manual, 1988) and by Brad Lancaster (Rainwater Harvesting for Drylands and Beyond, vol. 2, 2nd ed., 2019).
Refined definition
Unlike a drainage ditch, a swale does not carry water away: its bottom is level, so that water stays and soaks in. It applies one of Brad Lancaster's eight principles: “slow, spread, and infiltrate the flow of water”. It is meant for sloping ground, where water runs off.
Key points
Mark it out level, never by eye
Everything depends on the level: a bottom that tilts sends water to a low point, where it overflows and gouges. The contour is marked with a water level (clear hose), an A-frame or a laser level, then dug keeping the bottom level. The Managing water in the garden guide details the method.
Always plan the overflow
A heavy rain will eventually fill the swale. Lancaster makes it a principle: always plan an overflow route, and manage that overflow as a resource, towards another swale, a pond or a planted bed, rather than letting it spill anywhere.
Start at the top
Another of Lancaster's principles: start at the top (highpoint) of your watershed and work your way down. Swales stepped down the slope stop water as early as possible, before it gathers speed; it is the idea behind keyline, at garden scale.
Watch out for
- • A swale dug without a level concentrates water at one point and can worsen the erosion it was meant to stop.
- • Infiltrated water keeps moving through the soil: on a steep slope, near a house or on unstable ground, have the project checked by a professional before digging.
- • On clay soil, water soaks in slowly: dig shallower and wider, as the same guide advises.
Examples
- • A swale across a grassy slope, its mound planted with soft fruit
- • A swale's overflow feeding a pond further down
- • Swales stepped from the top to the bottom of the slope
Sources
- Mollison, B. (1988). Permaculture: A Designers' Manual. Tyalgum : Tagari Publications.
- Lancaster, B. (2019). Rainwater Harvesting for Drylands and Beyond, vol. 1 (3e éd.) et vol. 2 (2e éd.). Tucson : Rainsource Press.