DEFINITION
A rain garden is a topographic depression designed to receive surface run-off. Compost-amended soil acts like a sponge holding stormwater while microorganisms and bacteria in the soil remove pollutants. Surface vegetation enhances evapotranspiration and pollution removal. Geotechnical conditions will determine whether stormwater can infiltrate or will overflow during the peak of a rain event. Even in limited spaces, small rain gardens are very efficient in improving water quality because it is the first part of the run-off that carries the majority of pollutants during a storm. After a few hours there shouldn't be any water on the soil's surface. Run-off from non-polluting surfaces typically requires only a medium of crushed rock or gravel to allow infiltration.
Rain gardens can be a technique used in Low Impact Design.
Generally, rain gardens serve as collection point for stormwater and are usually limited to runoff from a single lot.
Rain gardens, by facilitating infiltration, serve two purposes.
First, they allow the potential downsizing of the stormwater infrastructure (e.g. inlets, piping, detention & retention facilities) that must be constructed to serve development.
Second, the quality of water that enters the facility may be eventually enhanced. This is primarily due to the 'filtering' that it receives as it percolates through the ground.


AKA
infiltration garden, biofiltration garden

PERFORMANCE CRITERIA
improvement in water quality and slowing of stormwater
One important to consider the type of plants that will be used. Plants should be native species and tolerant of water.
For assistance in plant selection, contact the nearest agricultural extension office.

ILLUSTRATION



Typical Rain garden section

CASE STUDIES
Oxbow Park, Seattle
Glencoe Elementary School, Portland
Cedar River Watershed Education Center, Cedar Falls
Helen Gordon Child Development Center, Portland
High Point, Seattle
New Harbor, Malmo
FUPAM, Sao Paulo
Sidwell Friends Middle School, Washington, D.C.
Mount Tabor Middle School, Portland
St. Margaret Mary School Native Garden, Chicago
Kitsap Homebuilders LID demo project, Bremerton
Northgate Green Parking Lot, Seattle
EPA Headquarters, Washington, D.C.
8th Avenue Industrial Strength Natural Drainage, Seattle, WA
Grass Lawn Park, Redmond, Washington

ADDITIONAL RESOURCES
How to build a Rain Garden, by Stewardship Partners
Raingarden Network: http://www.raingardennetwork.com/
http://www.wikihow.com/Create-a-Rain-Garden (explains how to build a residential rain garden)
Washington State University Extension Rain Garden Brochure (2 page version): http://www.pierce.wsu.edu/Water_Quality/LID/Raingarden_brochure.pdf
Washington State University Extension Rain Garden Handbook (43 page version): http://www.pierce.wsu.edu/Water_Quality/LID/Raingarden_handbook.pdf
LID Center's rain garden templates for Chesapeake Bay Watershed: http://www.lowimpactdevelopment.org/raingarden_design/
Puget Sound Rain Gardens (WSU): http://raingarden.wsu.edu/
American Rivers' Rain Garden Video

Raingarden at theConnectiicut NEMO project in the winter

Toronto's Green Parking Lot Guidelines

Where to place raingardens?

How to design raingardens.