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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A conceptual diagram of the bioretention system properties in MUSIC is presented below:
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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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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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
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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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