RFI - Stormwater management (flood report)

             Treatment Measures
      Runoff from the development will be treated through two bioretention basins or flow split to a
      number of devices. The potential locality of the treatment devices servicing the
      development is shown in Figure 3-1.

      3.3.1. 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 3-5 summarises the proposed design parameters for the main
      bioretention system for this development. As noted above this may be split into several
      devices on detailed design. 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 3-5: Bioretention Design Parameters

                                          Parameter                         Bioretention Basin
                                                                                    A

                               Extended Detention Depth (m)                       0.30
                                 Storage Surface Area (m2)                         80
                                   Filter Media Area (m2)                          80
                            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




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                                 | 17