Bioretention systems (also known as biofiltration systems or raingardens) promote the filtration of stormwater through a vegetated filter media (NOTE: whilst the user has the option to specify a non-vegetated system, it is strongly recommended that bioretention systems always be vegetated). Non-vegetated systems will generally give poor water quality outcomes and are more likely to clog prematurely. The type and composition of filter medium determines the effectiveness of the pollutant removal, as does the choice of vegetation.
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Exfiltration from the bioretention system into the underlying soil is modelled by defining the exfiltration rate of these underlying soils. Representative exfiltration rates for different soil types are provided in the table below. The water that seeps from the bioretention system is lost from the catchment, and cannot re-enter the system downstream. Contaminants in the water that is lost to exfiltration are removed from the bioretention system, along with the exfiltrated water and are also lost from the catchment. NOTE that in MUSIC , exfiltration from the ponding zone of the bioretention system is also taken into account.
| Soil Type | Median particle size (mm) | Saturated Hydraulic Conductivity | |
|---|---|---|---|
| (mm/hr) | (m/s) | ||
| Gravel | 2 | 36000 | 1x10-2 |
| Coarse sand | 1 | 3600 | 1x10-3 |
| Sand | 0.7 | 360 | 1x10-4 |
| Sandy loam | 0.45 | 180 | 5x10-5 |
| Sandy clay | 0.01 | 36 | 1x10-5 |
Vegetation Properties
The presence of vegetation in a bioretention system is very important for the nutrient removal performance of a bioretention system, as is the type of plants used. From research undertaken by FAWB, certain species of plants can be far more effective at removing nutrients than others. As such, it is recommended that bioretention systems be planted with plants that have been shown to be effective in nutrient removal, however in certain circumstances, it may be necessary to use plants which may not be optimal. This section allows the user to choose whether effective nutrient removal plants are to be used. Guidance on which plants may be most suitable for bioretention systems should be obtained from organisations with experience in bioretention systems e.g. in Australia, such guidance is provided by FAWB (see www.monash.edu.au/fawb) and in the UK, guidance can be obtained from CIRIA (www.ciria.org).
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Defines the porosity (voids ratio) of the filter media. MUSIC provides a ‘tooltip’ with guidance on the appropriate values to use, depending on the filter media specified. The filter media is normally made up of sand or loamy sand. The following general values are recommended:
Media Type | Typical Porosity |
|---|---|
Loamy sand | 0.35-0.4 |
Sandy loam | 0.35-0.4 |
Sand | 0.3-0.4 |
Gravel | 0.3-0.4 |
Scoria | 0.5-0.6 |
Porosity of Submerged Zone
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Evapotranspiration losses
The method of calculating evapotranspiration from bioretention systems has been enhanced. In the previous version of MUSIC, evapotranspiration was a function of soil moisture and a term, Emax, which is used to describe the maximum daily evapotranspiration rate. The Emax value was derived from experiments using biofilter columns planted with Carex appressa, and was applied as a constant rate. More recently, field experiments on biofilters (see for example Hamel et al. 2011 and 2012) have enabled the MUSIC team to develop a more sophisticated and precise prediction of evapotranspiration, taking into account seasonal variation. We have done this by developing a ratio between potential evapotranspiration (PET) and the measured ET. This scaling factor has then been used to develop a seasonally-adjusted Emax figure on a monthly basis, such that: Emaxj = scaling factor * PETj, where Emaxj is the Emax value for month j, and PETj is the potential evapotranspiration for month j.
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