Raingardens and bioretention facilities have much in common: both use soil and vegetation to collect, slow, and filter stormwater runoff. That is why the terms are frequently used interchangeably — even by stormwater agencies. Generally, however, rain garden describes a smaller and less engineered landscape feature, while bioretention refers to a stormwater practice designed to meet defined performance requirements.
A raingarden is typically a shallow, planted depression that receives runoff from a roof, driveway, sidewalk, or other relatively small drainage area. It may be intended to reduce runoff, encourage infiltration, filter pollutants, and create an attractive planted space. Some raingardens use amended or engineered soil, many with underdrains, but the name by itself does not indicate that the feature was engineered for regulatory stormwater compliance.
| Typical Characteristic | Raingarden | Bioretention Facility |
|---|---|---|
| Common Setting | Residential lots and landscaped areas | Streets, parking lots, developments, and public infrastructure |
| Design Basis | May follow general sizing and planting guidance | Designed for a defined drainage area, storm event, or regulatory objective |
| Soil | Existing, amended, or engineered soil | Soil media evaluated or specified for defined performance goals |
| Drainage | May infiltrate naturally or include an underdrain | May use infiltration, an underdrain, internal storage, or an impermeable liner |
| Performance | Manages and filters runoff, but not necessarily for regulatory credit | Designed for specified water-quality, volume, or flow-control performance |
Bioretention is generally a more formally designed stormwater control measure. Depending on the site and local requirements, its components may include defined inlets, pretreatment, specified soil media, surface ponding, subsurface storage, an underdrain, a liner, controlled outflow, and an overflow route. Not every bioretention facility includes every component; the design should respond to local soils, climate, drainage conditions, pollutants, and project objectives.
Neither size nor the presence of an underdrain definitively separates the two terms. A small facility can be engineered bioretention, while a comparatively large, planted depression may simply be called a raingarden. The most important questions are how the facility was designed, what runoff it receives, what performance it is expected to provide, and whether it meets the applicable local stormwater criteria.
A Silva Cell system can be configured as engineered bioretention by directing runoff into soil held beneath pavement. Inlets, soil media, storage, drainage, and overflow components can then be designed around the project’s stormwater goals. Although this type of subsurface system may not look like a conventional surface raingarden, it uses many of the same soil- and plant-based processes to manage and treat runoff.
Sources: U.S. EPA, Types of Green Infrastructure; U.S. EPA, Bioretention Design Handbook; and Minnesota Pollution Control Agency, Bioretention Types and Terminology.
Great article. It is good to see that folks are making a distinction between the two. In Vermont we’re focusing more on the distinction between LID, green infrastructure, and green stormwater infrastructure. It’s a fun process that will hopefully benefit us as we push these concepts forward. Keep up the good work.