Bio-retention basins and constructed wetlands are areas that temporarily hold and filter stormwater runoff before it enters creeks and waterways. Bio-retention basins and constructed wetlands are used in urban areas, where rain washes pollutants off hard surfaces such as houses and roads and into stormwater drainage systems. These systems seek to replicate some of the natural functions of the water cycle that are lost through urbanisation.
How do they work
First, the water enters a vegetated area that has been densely planted with species such as native trees and grasses such as sedges.
This slows the water and traps pollutants, using physical and biological processes. Some pollutants, such as nitrogen, are harmful to waterways but can be beneficial for plants.
- Some ponding of water is normal. These systems will hold water on the surface for a short time while it filters through. The system will have a high flow bypass channel that diverts excess water.
- Lots of plant growth is also normal. As natural, densely planted areas they may appear 'overgrown' compared to mown parks. Thick vegetation is important for the function of these systems.
Then the water filters down through a 'filter media' layer such as sandy loam. This fine filtration also traps pollutants as the water passes through.
Finally, the water collects in perforated drainage and flows to downstream creeks and waterways.
Benefits to bio-retention basins
Benefits to bio-retention basins include:
- filtering pollutants out of stormwater before it enters creeks and waterways
- slowing the velocity of water which helps reduce erosion of downstream creek banks
- habitat for native wildlife such as frogs and other species
- low maintenance plantings with the system essentially 'self-watering'.
Refurbishment is important
Bio-retention basins need maintenance such as the removal of debris and accumulated sediment.
After a number of years it may be necessary to do a refurbishment to ensure the system functions properly. This can involve removing the surface filter media and the accumulated sediment, replacing the surface filter media and replanting the basin.
Other checks can be done for potential issues such as erosion, blockage of pipes, overflow pits and weirs, and structural integrity of embankments and retaining walls.
Bio-retention basins are often within public spaces such as open space or within streets so it is important to provide landscape design that complements the surrounding area.
What does it look like?
There are different design configurations that could be used, depending on the site features and landscape.