Rain Gardens and Bioswales
Rain gardens and bioswales are simple landscaping features used to slow, collect, infiltrate, and filter stormwater. Differences between these systems are subtle and the terms often are used interchangeably to describe systems that achieve the end goal of reducing stormwater runoff and improving stormwater quality. There is, however, a difference.
Rain gardens are smaller or residential systems. These gardens have a slight depression to help collect water and are vegetated with plants that can withstand moisture regimes ranging from flooded to dry. The existing soil is often used in rain gardens if it provides adequate water infiltration rates, although native soils can also be amended with some sand or compost if needed. Several resources are available for homeowners and landscapers interested in installing their own gardens including: installation and maintenance guides, incentive programs, and rain garden classes.
Bioswales achieve the same goals as rain gardens by slowing and filtering stormwater, but are designed to manage a specified amount of runoff from a large impervious area, such as a parking lot or roadway. Because they need to accommodate greater quantities of stormwater, they often require use of engineered soils and are deeper than rain gardens. They are also linear systems that are greater in length than width. Like rain gardens, they are vegetated with plants that can withstand both heavy watering and drought.
The effectiveness of both rain gardens and bioswales increases with increased contact time between soil and stormwater, and increased vegetative cover. This is all best achieved by using soils that can adequately slow down, infiltrate, and retain water, as well as support plant life. In areas where nutrients are a concern to water quality, soils capable of retaining high amounts of phosphorus or nitrogen should be selected, along with plants that use nutrients very efficiently.
Phosphorus retention can be improved by adding water treatment residuals that increase the level of the phosphorus-binding elements, iron and aluminum, in soil. Increasing the carbon content of soil with wood mulch can help to retain nitrogen.