The most common bioretention practices are raingardens, tree pits and planter boxes.
These systems are discrete landscape areas that use specific soils and plant materials to manage stormwater effects. Tree pits are essentially raingardens with a single tree rather than smaller foliage plants. Planter boxes are usually lined bioretention areas which receive point source runoff from rooftops or adjacent hard surfaces. The stormwater management functions provided by bioretention practices include:
- Filtration of stormwater through surface vegetation
- Settlement of sediments by surface ponding
- Detention of stormwater volumes in soil media and surface ponding
- Filtration of contaminants through soil media
- Microbial treatment processes in contact with soil and root horizons
- Infiltration to ground and evapotranspiration.
Bioretention practices are usually a secondary treatment practice, meaning the inclusion of a filter strip or gross pollutant trap for pre-treatment will make them less vulnerable to sediment loading. This requires specific maintenance regimes during operational phases, and protection measures from sediment loads during construction phases. For more information on maintenance, refer to Auckland Regional Council Technical Report TR2010/053 Operation and Maintenance of Stormwater Treatment Devices in the Auckland Region.
Bioretention practices can be integrated seamlessly into existing landscapes or proposed planting schemes. The ecological value of bioretention practices is limited by climatic effects and isolation within the catchment. However, these practices may still provide a refuge for urban wildlife and a means to promote native species dispersion into the urban environment. This is particularly true of bird species. There are a number of opportunities to maximise ecology values for bioretention practices, including:
Using diverse native plant species to provide food for birds and insects all year round
Applying multiple tiers of planting, from ground cover species to canopy and emergent trees, providing a range of habitat niches of cover and food. Multi-tiered planting can also form microclimates to accommodate species that are sensitive to climatic extremes.
Including threatened plant species if conditions are appropriate and if there is a means to support their ongoing population and dispersal
Incorporating rocks or driftwood to provide habitat for lizards and skinks. Rotting logs, twigs and leaves provide habitat for both insects and lizards.
Utilising the potential to form a transitional edge to existing vegetation. In such cases it is also important to consider the existing hydrology of adjacent vegetation and ensure this is not impacted by rainwater collection in the practice.
Note: Technical design guide documents provide detailed advice for design and construction of devices