Swale

When rain falls faster than the ground can take it in, the excess slides across the surface in a thin sheet and is lost downhill. A swale catches that water and holds it still long enough to soak in, so it refills the soil where it landed instead of running away. The level bottom is what defines it: a channel built with a slope along its length carries water off somewhere else, which is a diversion rather than a swale. The soil dug out is heaped downhill into a bank, and that bank is half the point, since it gathers organic matter as it ages and is usually planted as the growing bed.

Siting decides whether it works. Banks like these, built along the level, suit ground from flat to about 5 percent, a fall of 1 m in 20. They stand 20 to 40 cm (8 to 16 in) high, sit about 10 m (33 ft) apart on gentle ground and 5 m (16 ft) on steeper, and are tied across every few metres so water cannot run along the channel and gather at one point. The exclusions matter more than the dimensions: not on slopes much over 10 percent, not in landslide-prone ground, not directly above a steep face, and not where the wet-season water table comes within 60 cm (24 in) of the channel bottom. A swale puts water into the ground, so ground already wet or already moving is where it must not go.

The classic failure is overtopping: the channel fills, spills over the bank and cuts through it. A failed swale does more damage than none, because the released water runs unimpeded downhill and scours. The slower failure is silt, which settles on the bottom and seals it, and a channel that no longer soaks water away is just a shallow pond. Clearing silt is most of the work: an inspection every three months, deposits cleared at least yearly and sooner once they reach 10 cm (4 in), a breach repaired at once, and grass left to root into the bank, because those roots are what stop the second breach.