On September 23 2019 we started installing swales on our North facing slope at Rosheen Crescent 17 in Plettenberg Bay.
Swales as used in permaculture are designed to slow and capture runoff by spreading it horizontally across the landscape (along an elevation contour line), facilitating runoff infiltration into the soil. ... In arid climates, vegetation (existing or planted) along the swale can benefit from the concentration of runoff
The general idea behind digging the swale is that it should be about three-times as wide as it is deep, and the berm—the pile of excavated dirt—should be mounded to create the upper part of the bottom side of the swale. Make sure that the base of the swale is level so that the water disperses evenly.
On reasonably steep slopes where there were once deep rooted trees, the soil built up over centuries with the trees anchoring the top layers of soil to the bedrock or more solid substrate below. In heavy or prolonged rain events, even though the topsoil becomes saturated, lubricated and heavy, it stayed in place because of the deep rooted trees. When people arrived and remove trees for either subdivisions or to make the land more ‘agriculturally productive’ the deep roots slowly rot away and now there is nothing to hold saturated soils to the steep hillside. If one is designing swale systems on steeper slopes it would be wise to make them shallower to start with while also planting deep rooted trees immediately below them. Nut trees tend to have deeper roots–fruit trees do not. As the trees grow over the years the swales can be deepened if desired.