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When the stormwater inflow rate exceeds the user-defined High Flow Bypass amount (in units of m3/s), 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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The Low and High Flow Bypasses are assumed to occur simultaneously. So for a Low Flow Bypass of 2m3/s, a High Flow Bypass of 8m3/s, and inflow of 10m3/s:

 


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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The filter and media properties define the physical, hydraulic, chemical and biological characteristics of the filtration medium component of the bioretention system.  These are all critical for defining the predicted water quality performance of the system; the parameter values chosen should match what will realistically be used for construction of the system.  For further guidance, please refer to the guidelines available from FAWB (www.moanshhttps://www3.monash.edu.au/fawb/).

Filter Area

Defines the plan area of the filtration medium component of the bioretention system in m2. This can often be less than the surface area of the storage zone (where the extended detention depth has sloped sides).

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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 TypeMedian particle size (mm)Saturated Hydraulic Conductivity
(mm/hr)(m/s)
Gravel2360001x10-2
Coarse sand136001x10-3
Sand0.73601x10-4
Sandy loam0.451805x10-5
Sandy clay0.01361x10-5

Filter Depth

Defines the depth of the filter medium in metres. 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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Exfiltration from the bioretention system into the underlying soil is modelled by defining the exfiltration rate of these underlying soils (mm/hr). 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 TypeMedian particle size (mm)Saturated Hydraulic Conductivity
(mm/hr)(m/s)
Gravel2360001x10-2
Coarse sand136001x10-3
Sand0.73601x10-4
Sandy loam0.451805x10-5
Sandy clay0.01361x10-5

It is possible to record flux data for the bioretention system using Fluxes:

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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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