Recently, I was considering whether digging a swale or two would be the right solution to rehabilitate a slope on my property where the vegetation dries out every summer. I mentioned this on social media, and discovered that most people aren’t aware that there are several kinds of swale—which leads to a great deal of confusion. This post attempts to provide some disambiguation.
There are several kinds of swales with different purposes, which include creating drainage in a conventional landscape design, diverting water to be stored in a pond, and raising the level of the water table. The word can also refer to a natural landscape feature. In any of these cases, a swale is a long, shallow depression in the earth that holds water.
Natural Swales
According to Merriam-Webster’s Collegiate Dictionary, a swale is “a low-lying or depressed and often wet stretch of land.” When I made a video on TikTok discussing human-made swales, a viewer was startled because “swale” was the word their family used to describe a natural swampy spot on the family farm. And they were perfectly correct. This use of the word far predates the more recent application in human-made design.
Drainage Swales
In conventional landscape design, especially in urban and suburban spaces, drainage swales are used to divert excess storm water without causing erosion. Imagine rain falling on a city full of paved surfaces. It runs along them, unable to penetrate the ground, until it comes to a steep slope overlooking a river. Unchecked, that water would cause severe erosion as it poured down the hill. It would also carry any pollutants it had picked up straight into the watershed.
Drainage swales manage that runoff in a much more controlled way. Unlike steep-sided ditches, drainage swales are much wider than they are deep. This encourages the water to slow and even pond temporarily, allowing some of it to infiltrate (that is, to sink into the soil). The swales are often planted with vegetation so the slow-moving water can be filtered of some pollutants.
Within urban design, the need for standardized swale terminology has been recognized. In 2020, Sujit Ekka and Bill Hunt at the North Carolina State Extension published “Swale Terminology for Urban Waterways,” identifying several subclasses of swales that manage stormwater: grass swales, which are planted with turf grass that provides a moderate amount of particle settling, pollutant filtration, and water infiltration; infiltration swales, which are grass swales with added check dams to hold water in place for a longer period of time; bioswales, which are more highly engineered to remove pollutants with layers of vegetation, soil mix, geotextile fabric, gravel, and pipes; and wet swales, which maintain wetland-like conditions most of the time.
Diversion Swales
What if you don’t want to dump the runoff from your land into a storm drain or watershed? What if you’re a farmer in a region where rain comes in a few big bursts followed by months of drought, and you’d really like to keep a handy reserve? That is when diversion swales come into play. They intercept water that is running down a hillside and move it intentionally to a destination, usually a pond. When these swales are dug, the earth being removed is mounded up on the downhill side of the trench to form a berm, sometimes also called a bund. Deep-rooted plants should be grown in the bund, to stabilize it and to use some of the water that now saturates the soil.
Imagine a sloped field where water runs quickly onto the neighbour’s property, taking valuable soil with it. A long, zig-zagging swale consisting of a trench and berm is dug across the face of the hillside, sloping downward at a very gentle rate of 1%. It carries water to a pond that has been sealed and dug deep enough to avoid losing too much water to evaporation. In a complex system, overflow from this pond might spill into another swale, which leads to another pond, and so on. When the dry season comes, the farmer will have plenty of water on hand.
The same principles can also be applied to distribute a large volume of water that has already been collected: a gently sloping diversion swale will move equal amounts of it to all parts of a growing area.
Contour Swales
Contour swales are used to improve the amount of water infiltrating into the earth, where it becomes groundwater. The effects of this can include preventing floods, replenishing aquifers, creating springs, making seasonal waterways flow all year, and improving the landscape’s drought resistance.
As with diversion swales, contour swales consist of a trench with a berm on its downhill side. Unlike diversion swales, they are not interconnected. Instead they form a series of irregular curving lines that stack horizontally across the face of a hill. Each swale is built on the contour of the land, meaning that every point along the base of the trench is the exact same height above sea level.
The result is a line that undulates with the topography of the hill. When rain falls, the level bottom of the swale allows the water to spread evenly across the whole expanse rather than running to one end or the other. Any sediment that has been washed downhill is caught and settles in the bottom of the swale rather than being carried away. When the first swale is full, the water sheets gently over the edge of the berm to fill the next swale down the slope. At the bottom of the trench, well-drained soil covered with gravel, compost, mulch, and/or vegetation allows the water to sink steadily into the earth during and after the rainfall event.
For an efficient system, moisture-loving plants can be grown in the trenches, and deep-rooted perennials, especially trees, should be grown in the berms. The effect is somewhat like terracing, but without the flat horizontal and vertical planes that turn terraced hillsides into a series of steps.
Which swale will serve your purpose? Here’s a handy chart to help you decide.
| I want to… | Drainage Swale | Diversion Swale | Contour Swale |
| …move runoff into the storm drain | X | ||
| …remove particles from stormwater | X | ||
| …reduce floods | X | X | X |
| …prevent erosion | X | X | X |
| …filter water before it reaches a destination | X | X | |
| …reduce pollution in urban waterways | X | ||
| …create a low-tech irrigation system | X | ||
| …reduce my dependence on groundwater or municipal water | X | X | |
| …recharge an aquifer | X | ||
| …create new springs in my pasture | X | ||
| …refill wells | X | ||
| …reforest a semi-arid landscape | X | X | |
| …become drought resistant | X | X |
Swales have an impressive list of applications in a broad range of circumstances—so broad that a landscape designer, a conventional farmer, and a permaculturist might all challenge some of my word choices in this post. There is not only a lack of standardization for the word’s use between these groups; there is often no inkling that anyone else is using the word “swale,” and using it differently.
So which kind was right for me? Well, actually, sometimes no kind of swale is the right fit for a situation—and that will be the subject of another post.
Cover image: “swales after a heavy fall of rain – working well” by Milkwood.net is licensed under CC BY-NC-SA 2.0.
Bibliography
Ekka, Sujit & Hunt, Bill (2020). “Swale Terminology for Urban Stormwater Treatment.” NC State Extension. https://content.ces.ncsu.edu/swale-terminology-for-urban-stormwater-treatment
Engels, Jonathon (2017). “Why We Use Swales and How to Do It Appropriately.” Permaculture Research Institute. https://www.permaculturenews.org/2017/03/31/use-swales-appropriately/
Jenkins, Emily. “Swales: Your Comprehensive Guide to Siting and Building Them.” New Life on a Homestead. https://www.newlifeonahomestead.com/swales/
Morrow, Rosemary (2006). Earth User’s Guide to Permaculture, 2nd ed. Kangaroo Press.
Tallarico, Giuseppe. “Swales? Or Not to Swale?” World Permaculture Association. https://worldpermacultureassociation.com/swales-or-not-to-swale/