RFI - stormwater report

2.4 Treatment Measures
Runoff from the development will be treated via a single end-of-line bioretention basins.
The locality of the treatment device is shown in Figure 2-1.
2.4.1 Bioretention System




Bioretention systems use ponding above a treatment surface to maximise the volume of
runoff flowing through the filtration media. They typically convey flows above the design
event through overflow pits or a weir. The treatment system operates by firstly filtering
surface flows through surface vegetation and then percolating runoff through prescribed
filtration media. This media provides treatment through fine filtration, extended
detention and biological uptake of nutrients.
As these systems take approximately 12-24hrs to drain, the maximum depth of pondage
allowed to occur over the filter area has been limited to 300mm, with a minimum filter
media depth of 500mm. Table 2-5 summarises the proposed design parameters for each
bioretention system for this development. It is to be noted that the Orthophosphate
content of the filter media has been reduced to 30mg/kg in line with recent advice from
Healthy Waterways.
(http://healthywaterways.org/initiatives/waterbydesign/stormwater).

Table 2-5: Bioretention Design Parameters

                                Parameter                         Bio Basin

                      Extended Detention Depth (m)                   0.30
                                                    2
                         Storage Surface Area (m )                   125
                                                2
                          Filter Media Area (m )                     125
                    Unlined Filter Media Perimeter (m)               0.01
                 Saturated Hydraulic Conductivity (mm/hr)            200
                             Filter Depth (m)                        0.5
                    TN Content of Filter Media (mg/kg)               400
             Orthophosphate Content of Filter Media (mg/kg)          30
                         Exfiltration Rate (mm/hr)                   0.0
                            Overflow Weir (m)                        0.5




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