You might be wondering, “What is a swale drain?” Adding a swale can improve your land quality and increase biodiversity–a win-win for everyone! Read on to learn how to build a swale for drainage with some planning and a little elbow grease.
Water management is a critical component to conservation, and when used effectively, it can significantly increase the production value and habitat quality of your land. Strategically placed swales are one of the easiest ways to enhance your drought resilience and vegetate the more arid zones in your landscape.
Swales are shallow trenches or waterways created to channel and trap or slow the movement of surface water. From the short linear depression that channels runoff from your downspouts to a water garden or pond, to a much larger earthwork that’s installed transverse to the slope in your south 40, swales can help you keep more of the water that falls on your place, on your place!
Swales can be vegetated or not. Vegetated swales are arguably more beneficial, because they have silt- and water-filtering capabilities, but if you’re in arid zones, your swales may begin as bare earthworks. In time, they may become vegetated, produce food, and increase habitat diversity.
In addition to cleansing water and delivering it to storage facilities, swales also contribute to recharging the groundwater in their immediate vicinity and downhill. Many so-called “bioswales” are created level to capture and retain surface runoff so more water will percolate to charge the groundwater. Installing a swale can help convert a dry slope to a highly vegetated, moist habitat.
How to Build a Swale for Drainage
When installing swales, having the right tools makes all the difference. You can install small-scale swales, such as in a sloped backyard, with hand tools, but you’ll want to bring in some machinery if your project is of a moderate scale. Large-scale projects often require hydraulic excavators and dozers.
Large or small, a swale project should be marked out so you know where to dig. You can use surveying tools, laser levels, and the like to lay out the swale along larger contours. Those tools will allow you to build level or sloped earthworks. For smaller projects, or even for the larger ones, you can build an A-frame level to manually mark your swale line. In either case, take some care with this step, because the final result will depend on how level or mildly sloped your swale is. Once marked to your satisfaction, it’s time to collect the tools, labor, and fortitude to begin the build.
How Big to Dig Swales
Before you start digging in earnest, be aware that most successful swales are at least three times as wide as they are deep. They can be wider to no ill effect, but if they’re narrower, the surface area for water percolation and retention volume will be diminished. You’ll want to keep the swale edges sloped very gradually to avoid erosion, although in practice, this isn’t always easy to accomplish with hand-dug swales. With a few heavy rains, it usually sorts itself out.
Whether you’re using a pick, shovel and mattock, backhoe, plow, or dozer, the general approach is to excavate a trench along your contour line while pulling the soil to the downslope side of the trench to create a berm. One edge of your swale will be the slope of the undisturbed land, while the other will be the berm.
Swales can range in width from a few feet to 20 or more feet, depending on specific circumstances. On gently sloped swales, you’ll want the outlet to be in an area that’s relatively flat and well-vegetated to prevent erosion, or to have it feed into a pond or series of them. Bioswales will need one or more flat spots in the berm where water can overflow during times of heavy runoff. These spillways will protect the integrity of the berm and should be located where the overflow is least likely to cause erosion.
For larger projects, a loader-backhoe tractor will likely be the fastest, most versatile method. If the slopes are sufficiently gentle, you might be able to find an old terracing plow to run along the contour using your tractor. If you’re good with a dozer, or have a friend who is, it’s possible to install 1,000 feet of swale in a day or two in prime conditions. Likewise, it might make sense to hire a pro with a large hydraulic excavator for bigger projects on steeper slopes.
Adding Vegetation to Swales
Plant the swales’ berm and downhill from it with soil-holding grasses, forbs, and trees appropriate to your bioregion. Many folks choose fruit trees, while others use shrubby species that can be grazed, such as willows, or berries, which provide food. It all depends on your circumstances and needs.
Plant the swale itself when sloped; choose a grass mixture proven in agricultural waterway projects. Shrubs can also effectively reduce erosion issues within the swale. In any case, the vegetative focus should add diversity and habitat to the landscape that’ll serve wildlife and humans alike. On small projects, hand-planting works well. On larger projects, you can use your tractor with a seeder or drill to plant the berm. Planting trees can be as easy as sticking willow or cottonwood twigs into moist soil in early spring, or you can use hand tools and power diggers to plant larger-rooted specimens in virtually any season.
Creating swales for water conservation, habitat creation, and climate modification can be rewarding in both the long and short run. And with the right tools and equipment, it’s a project you can tackle confidently. Remember, small steps can make a big difference, and the best time to get started was yesterday.
Legal Considerations
Before embarking on any earthworks project or water-diversion initiative, research ordinances in your municipality. In some places, surface water is considered a public commodity, so any modification to the “normal” flow needs approval. County agencies, the Army Corps of Engineers, and other organizations might have a say in the matter. For example, in some places, collecting rainwater isn’t legal, although it’s likely fine to swale the water to divert it to your garden.
In some instances, your swale might divert runoff that sufficiently reduces the value of your neighbor’s pond or the flow of a creek, and thus it could capture the attention of the Army Corps. The Corps has universal permits that encompass smaller projects and don’t require engineering drawings to get. In some areas, bioswales might become classified as wetlands, which come with their own set of rules and regulations. In any case, a bit of research coupled with a risk-and-benefit analysis is in order. You’d hate to invest in a project only to have it torn out because of some technicality, or to lose control of a portion of your land to outside conservation interests.
Oscar H. Will III and his wife, Joanna, run a diversified sheep ranch, where they’ve relied on miles of swales to prevent erosion, to recharge the groundwater, to increase amphibian populations, and to modify the prairie plant and soil matrices to enhance diversity.
Originally published as “Diggin’ Swales” in the July/August 2023 issue of Grit magazine and regularly vetted for accuracy.