Vegetated swales (bio-swales) or rain gardens are contours dug into the landscape that direct water through a site or property. Examples would include a swale that directs the downspout flow from your roof, or use of the sheet flows from hard surfaces like driveways, parking lots, or sidewalks. Swales incorporate plants at varying depths with varying preferences for water saturation of their roots. This technique allows water to slow down, cool off, and percolate into the ground.
To the left, is a vegetated swale or rain garden, in the city of Phoenix that was built to help reduce flooding. The round-about is interrupting and slowing down the flow to the storm drains. There is an inlet and an outlet to ensure that water will continue downstream and yet, the swale can provide other benefits, such as, aesthetics, shade, and improved water and air quality.