by Kirby Fry
This article covers what a swale is, the functions of a swale, how to lay out a system of swales, and one way to build a swale using a shovel and a rake.
I. What is a Swale
The soil that comes out of the ditch or swale as you excavate it becomes the berm just below and parallel to the swale. When you hear someone refer to “berms and swales” they are usually referring to two parts of the same thing.
Berms and swales are one type of earthworks used in permaculture design to conserve soil and water. Other types of earthworks include terraces, tree boomerangs, ponds, gray water and rain water harvesting beds, and constructed wet lands.
As a fan of geometry, I think of a berm and swale as resembling a sine wave which many of us learned about in high school. The low, concave part of the sine wave is the swale, and the high convex part of the sine wave is the berm. To help me remember which is which I think of the swale as the sway back of a horse.
The point at which a swale goes from being concave to convex is the point of inflection.
II. The Functions of a Berm and Swale
The functions of a berm and swale in permaculture design are to:
Collect and harvest surface water runoff from the slope above it and make that water available to a perennial agriculture below it.
A swale should:
- collect surface water runoff from three times its own surface area, effectively tripling the year’s annual rain fall in the swale.
- Infiltrate surface water runoff into the soil, the water table and the aquifer.
- Reduce the net soil run off on any site to zero.
- Provide at least a 4’ wide raised bed (the berm) for growing perennial food trees and crops on top of.
III. How to Lay Out a System of Swales
To layout and design swales and berms, first find the steepest slope on your property. Some lots and properties have very subtle drainages but the water is almost always running from one side of the property to the other. Start layout from the highest side or point of the area being worked with just above the steepest slope.
Take twelve paces straight downhill. Place a landscaping flag at this point, right at the toe of your shoe. This flag marks the point of inflection of the first and highest swale on the property. Spacing the swales out along the steepest slope on the property ensures that they will not bunch together somewhere else.
Take twelve more paces straight downhill from that point and place another flag. Repeat until you run out of space. The landscaping flags should be about 30’ apart. Many yards can only accommodate two or three swales which is fine because there are lots of other really great earthworks to install in the smaller spaces and sometimes it makes sense to bunch swales closer together, maybe six to eight paces apart.
Step back and look over this layout and choose which berm and swale to build first. It is best to begin at the top of the slope but that does not always end up being the case due to logistical considerations like availability of water or proximity to a house.
Now use a hose level, “A” frame, transit, or laser level to lay out contour lines starting from each of the flags just laid out. A contour line joins points of equal elevation, so swales are essentially built along points of equal elevation.
To adequately mark each contour line you will need a flag laid out on contour about every 12’ to 16’ or so. It is useful to lay out several different contour lines even if they don’t get built into swales in order to best see the lay of the land and the flow of the surface water runoff.
Observe the contour lines that have been laid out, sometimes you may need to move the whole system uphill or downhill a bit to avoid trees or other obstructions, sometimes you may also need to straighten out the middle flags of a contour line, but don’t move the end flags as that would significantly affect how the structure holds water.
To lay out a swale and berm in preparation for digging it I use three different colors of landscaping flags – the most available colors at hardware stores are pink, green, and yellow. Lay out the point of inflection with green flags. Measure uphill 8’ right above each green flag and place a pink flag – that is now the top of the swale. Measure downhill 6’ right below each green flag and place a yellow flag – that is now the bottom of the berm.
The berm and swale can vary in width but the wider they are the gentler they are and the better they re-vegetate. Unless there isn’t enough room, the swale should be 8’ wide and the berm should be 6’ wide. The berm is narrower than the swale because it requires a greater surface area in order to excavate enough dirt to make an adequate planting bed or berm.
IV. How to Build a Swale Using a Shovel and Rake
Now dig down the very center of the swale with a long handled round headed shovel a full shovel depth, 12” if possible. Throw the dirt towards what will be the downhill side of the berm just below the swale. Do this along the entire length of the swale, it may take three or more passes. Then dig out the downhill side of the swale connecting the point of inflection marked by the green flags to the bottom of the trench that was just dug down the center of the swale. Then dig out the uphill side of the swale connecting the uphill side of the swale marked by the pink flags to the bottom of the trench.
All of the dirt removed from the swale is tossed immediately downhill to make up the berm which should be at least 12″ high giving the berm and swale a 2′ difference in height.
Rake the berm out with a bow rake, trying to burry rubble and clods of dirt under the highest part of the berm. Remove unwanted grasses and debris from the edges and surface of the berm. Shape the berm into what will be a raised garden bed.