Purpose-built raingardens have been used successfully overseas and in New Zealand for over 15 years.
Stormwater runoff enters a raingarden through vegetation layers at the surface, before it soaks into soil media. More information on soil media can be found in the Auckland Council Technical Report TR2013/011 Media Specification for Stormwater Bioretention Devices. Depending o0.n underlying subsoils, stormwater may then infiltrate to groundwater or be collected and piped to a discharge point. Ponding on the surface of the raingarden is called 'live storage' and is designed to dissipate over a period of 24 hours as a function of media permeability and evapotranspiration rates.
In many locations where conventional landscape areas would occur, raingardens can be used instead. This includes roadside verges, traffic islands, parking areas and around existing catchpits. Landscape design is an important consideration for the construction of raingardens, since attractive features encourage landowners to take pride and stewardship over the maintenance of these facilities. Maintenance plans should be provided to the landowner to promote the use of correct maintenance procedures. Specific responses to landscape are discussed in Auckland Regional Council Technical Report TR2009/083 Landscape and Ecology Values within Stormwater Management (Lewis et al., 2010) including:
- Incorporating raingardens into the existing landform
- Reinforcing existing or visible landscape patterns
- Enhancing urban ecology and natural character values
- Contributing to streetscape amenity and providing a visual cue for traffic calming
- Integrating, framing or complementing architectural forms and landscape spaces.