You can build a small swale yourself, but for a long, wide one you’ll want to hire a pro with earth-moving equipment. 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.