Bioretention Systems

How Bioretention Systems Work

Bio retention systems treat stormwater by filtering runoff through planted
vegetation and percolating (drip feeding) the runoff through a filter
media, such as loamy sand. As the water is percolated through the soil,
pollutants are captured by fine filtration, absorption and biological uptake.

Bioretention System Example

Bioretention systems have a number of elements including:-

Extended Detention:- When stormwater enters the bioretention system, it
temporarily ponds to a depth of 200-400mm over the surface of the filter media.
This ponding depth, or the extended detention is created by raised field inlet pits
(overflow pits). Extended detention helps to manage flow velocities over the
surface of the filter media as well as increasing the overall volume of stormwater
runoff that can be treated by the bioretention system.Bio Retention System in place

Filter Media:- The layer of filter media provides the most treatment of pollutants through fine filtration and supporting the vegetation. The vegetation improves filtration, keeps the filter media porous, provides substrate for bio-film to form and takes up some nutrients and pollutants. The filter media should be deep enough to support vegetation. The typical depth for filter media is between 600-1000mm with a minimum depth of 400mm. The surface of the filter media is generally flat.

Transition and Drainage Layers: - Under the filter media, a ‘transition layer’ of coarse sand is used to prevent the filter media moving into the drainage layer and the perforated under-drains. The transition layer is typically 100mm deep.
The drainage layer is made up of fine aggregate (2-4mm) and is typically 200mm deep. The drainage layer collects treated water from the base of the bioretention system and delivers it into the perforated under-drains. The under-drains are perforated and typically slotted with PVC or ag pipe.

Note:- Our bio-retention sand and gravel blends fully comply with FAWB (Facility for Advancing Water Biofiltration) and Water By Design specifications and guidelines.

Hydraulic Structures (Overflow Pit):- During flood events that are above design of the bioretention system, stormwater flows are conveyed through overflow pits or bypass paths rather than over the filter media. Hydraulic structures protect the surface of the filter media from high velocities that can dislodge collected pollutants or scour vegetation.

Above excerpt taken from Water By Design ‘Construction and Establishment Guidelines: Swales, Bioretention Systems and Wetlands’, Version 1.1, April 2010.

‘Water by Design’ is a program of the South East Queensland
Healthy Waterways Partnership.



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