Swales follow the contours around the base of a natural or created slope, redirecting storm water and filtering runoff as it sinks into the soil, instead of keeping it in one place, like a rain garden. Plants suck up the water along a swale’s gently sloping banks and sometimes down the center of the channel. Fast-draining soil maximizes drainage efficiency, and perforated pipe underneath the gravel can help with heavy water flow.
Small swales can direct gutter water to a dry well, while more substantial ones could run along the base of a hill to divert water around a low-lying house. Jan Johnsen, a landscape designer in Mount Kisco, New York, landscapes swales with a river rock lining and plants that thrive in moist conditions along the sides: evergreen ferns, sedges, winterberry, grasses, and Siberian and Louisiana irises. She also uses rugged prairie plants or other natives to fast-draining soil.
Creating a Swale
You can build a small swale yourself, but should hire a pro with earth-moving equipment for a long, wide one. Consult a landscape contractor or a civil engineer if you live near a bluff, have a septic tank nearby, or are on a slope that drops more than 1 foot over a horizontal distance of 20 feet.
A swale should carry water to a place where it can be released safely, such as a garden bed with good drainage or a buried dry well; allowing it to be absorbed on-site, rather than flow into a storm drain, is important for protecting natural waterways. The sides of the swale should flare so they extend out three to four times more than they are tall, and the first 8 inches of soil should drain well. If your soil drains quickly (at least ½ inch per hour), it can just be loosened. If water sinks in half that fast, amend it with 40 percent compost. Where drainage is slower still, replace soil with 60 percent screened sand and 40 percent compost. If the swale itself can’t be made big enough to handle all the water, consider excavating another 6 to 8 inches, lining the trench with filter fabric, laying perforated pipe, then covering it with round ¾-inch gravel. Top the gravel with at least 8 inches of a compost-rich soil mix.