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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| In MUSIC, the bioretention node can be used for modelling both a ‘traditional’ bioretention system (with an underdain) and a vegetated infiltration system (without an underdrain). For modelling an infiltration system with vegetation, the bioretention node should thus be used. |
Refer to Modelling Bioretention System Treatment Performance for more details on how bioretention systems are modelled in MUSIC.
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More details on the algorithms used for treatment performance in the bioretention system are provided in Modelling Bioretention System Treatment Performance.
A conceptual diagram of the bioretention system properties in MUSIC is presented below:
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Conceptual diagram of bioretention system properties.
Inlet Properties
The inlet properties define the flow rate at which the stormwater begins to bypass the treatment device.
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When the stormwater inflow rate exceeds the user-defined High Flow Bypass amount, only a flow rate equal to the High Flow Bypass (less that specified in any Low Flow Bypass) will enter and be treated by the bioretention system. All of the stormwater flow in excess of the High Flow Bypass amount will bypass the bioretetention system and will not be treated. The high flow bypass may be used where inflows to the system are restricted. For example, a diversion pipe into the system may constrain flows.
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Storage Properties
The Storage Properties define the physical characteristics of the surface storage above the infiltration medium of the bioretention system.
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| NOTE: Because the hydraulic conductivity of filter media will reduce over time (due to the input of sediments, etc), it is recommended that a value of 50% of the design value be considered as a conservative estimate of the realistic long-term hydraulic conductivity of the system. |
| Soil Type | Median particle size (mm) | Saturated Hydraulic Conductivity | |
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| (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 |
Filter Depth
Defines the depth of the filter medium. This depth should exclude the drainage layer and transition zone, unless they form part of a Submerged Zone (see Infiltration and Outlet Properties).
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Infiltration and Lining Properties
The infiltration and lining properties of the base only are specified in this section (details of lining of the walls are specified by specifying the Unlined Filter Media Perimeter in the Filter and Media Properties section.
Is Base Lined
Tick the "Yes” box if the filter is lined with an impermeable liner and the "No” box if it is not. Note that unless an exfiltration rate is entered, no change in performance will result if the base is unlined.
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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 | |
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| (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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The advanced tab under bioretention system displays the hydraulic characteristics for the overflow weir structure, and the parameters that describe the treatment processes in the bioretention system, including the media soil type, its porosity (along with that of the submerged zone, if present) and the coefficient of horizontal flow (exfiltration through the sides).
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k and C* Values
The first order kinetic model adopted in the USTM for the surface storage component of the bioretention system is described by definition of k, the exponential decay rate constant and C*, the background concentration. The rate at which each contaminant is treated, and the background concentration for each contaminant will be different within a bioretention system and different values should be adopted for each contaminant.
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The overflow weir carries a discharge when the water level in the bioretention system pond exceeds the Extended Detention Depth. The overflow weir is modelled as a sharp broad crested weir whose discharge equation is given by:
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The default number of CSTR cells in a bioretention system is three. For more details on CSTR cells, refer to Number of CSTR Cells.
Porosity of Filter Media
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 |
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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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The change means that MUSIC will provide more accurate predictions of evapotranspiration and can thus provide more precise information on the impact of biofiltration systems on the water balance and flow regime.
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NOTE: Advanced Properties are uneditable by default. You can make it editable for each project by clicking on Edit and Project Options. This will present you with a Project Options Screen as displayed below:
Now, click on MUSICX Treatment Node Settings and tick on Editable as shown above. |
Pollutant Removal through the Filtration Medium
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For more information, please refer to the see the section on Modelling Bioretention System Treatment Performance, where a full description is provided.
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