Swales for water retention
A swale can best be described as a ‘ditch’ on contour designed to hold rainwater and then disperse it into the surrounding soil. Traditionally rains would come and literally millions of litres of water would land up in rivers and eventually the oceans and invariably taking valuable topsoil with it. A swale system allows us to keep the water where we need it the most, in the ground.
The Swale on Contour
On Contour, simply means that the swale is dug exactly level which allows water absorption evenly on the land. The end of the swale is closed up in order to prevent water from flowing away.
Soil that is removed from the swale is placed on the downhill side (mini dam wall, almost) to form the Berm and is the perfect place to grow trees and deep rooting plants like strawberries, rhubarb, asparagus, comfrey as well as a variety of trees. The deep roots will keep the berm stable, as well as suck up the moisture from below so that the newly hydrated soil doesn’t become overly saturated.
The “A” Frame to get things level
If you don’t have access to fancy surveyor type equipment to ensure that your swale is completely level (on contour) a simple device called an A-frame can be made from a few pieces of wood. The two legs need to be of equal length, and about midway down, they’ll be connected by a third piece of wood, equidistant from the bottoms of the legs. The open end of the A to be between 1.2 to 1.5 meters apart.
- Mark the exact centre of the crosspiece and hang a plumb line from the centre point at the top of the A. When the plumb line hits that centreline on the crosspiece, then the two feet of the A frame are on level ground.
- Or, simply strap a water level (waterpas / Afrikaans) to the cross bar so that when it’s level the two pieces of ground upon which the A is resting are the same elevation.
Where to place your swale.
It is important to carefully determine the rough location of where to place a swale prior to digging. As a rule of thumb a swale should only be dug on ground with nothing more than 15% gradient, or a slope that climbs roughly 1 meter for every 7 meters it moves horizontally. Following this rule prevents mudslide problems that steeper gradients would cause, and that could potential be devastating to a property.
Other things to consider are that the longer the swale, the wider reaching the water absorption will be, and the higher its placement the more space in which the water will have to expand underground. So, ideally, a swale will be installed at the highest point possible but still low enough, down slope, to catch water run-off. From here, spread the water out on level plain by extending the swale on contour for as long as possible which will allow for water absorption evenly downhill.
Use the A-level to stake out the contour line and the exact route the swale will be taking. This will likely not be a straight line, but possibly curve through the landscape.
Using the marked contour line, dig vertically into the hill, piling the dirt on the downhill side of the swale to form the berm. The depth of the swale should remain the same and can be measured from the established contour line then levelled with a water level or the A-frame later. 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.
Other than catching and storing water on your land, the biggest benefit to having a swale is the growing potential of the berm. It will be mostly composed of rich topsoil that’ll be well hydrated. It’s important to plant on the berm immediately to prevent it from eroding. Trees will help to make sure the soil doesn’t get too saturated.
Once the base of the swale is satisfactorily level, a nice thick layer of mulch is a good idea. It will add nutrients to the water that is going into the soil below, as well as prevent evaporation. In fact, many people choose to fill up their swales in order to make convenient and logical access paths around the property. It’s just another viable purpose for having a swale.
The berm and the area adjacent to the berm is ideal for creating a food garden. With time, constant water retention in your swale system has the added benefit of re-hydrating the ground and natural ground water tables rising.