Treatment MeasuresBio-Retention Systems

The retention of stormwater with vegetation types is a method of reducing downstream flow velocities and subsequent drain sizes whilst facilitating treatment. Bio-retention systems combine various Water Sensitive Urban Design (WSUD) treatment types in one 'treatment train'. The system is designed to carry out primary and/or secondary treatment processes of stormwaters and retard flows. This retention or retardation can enable sediments to precipitate out of the water taking along with it some pollutants. The use of biological processes to 'treat' stormwater whilst facilitating conveyance and retention gives rise to the title of Bio-retention.

Comprehensive guidelines for the design of bio-retention systems can be found in Chapters 5 & 6 "Bioretention swales & Bioretention basins" in the WSUD Engineering Procedures available for purchase from CSIRO publishing.

This drawing indicates how bio-retention has been incorporated in a trench that will retard the water passing under a driveway that transverses the drain

This drawing indicates how bio-retention has been incorporated in a trench that will retard the water passing under a driveway that transverses the drain.
(Drawing Courtesy of: Contour Design Australia Pty Ltd.)

It is widely recognised that increased runoff and the discharge of polluted stormwater resulting from catchment urbanisation have a negative impact on the aquatic ecosystem of receiving waters. The management of urban stormwater for flow control and water quality improvement are now becoming standard design considerations in catchment development. Stormwater drainage solutions involving a combination of source and in-transit best management practices (BMPs) that are integrated into urban design, offer a means to protect receiving waters from stormwater discharged from urban development. Controlling stormwater pollutants at their source has the advantages of reduced hydraulic loading, greater ability to attenuate flows, reduced pollutant loads to downstream regional treatment facilities (such as wetlands and waterway protection/restoration works) and, in may cases, lower capital cost. A bio-retention system is one example of a source control method that can be integrated into the streetscape design to treat road runoff and roof runoff prior to discharging to receiving waters. Bio-retention systems are a combined detention, infiltration and collection system, and generally integrate a vegetated swale and infiltration trench as part of their design.

There is currently limited data on the performance and life cycle cost of bio-filtration systems. The Cooperative Research Centre for Catchment Hydrology is currently undertaking studies into this information.