Request to Amend Application and Response to DSDIP's Information Request · MCU14/0009

2.4 Treatment Measures
Runoff from the development will be treated via one (1) bioretention system. The
locality of the treatment devices are detailed in Appendix 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
ponding allowed to occur over the filter area has been limited to 100mm, with a
minimum filter media depth of 400mm. Table 4 summarises the proposed design
parameters for each bioretention system for this development. Indicative
bioretention locations are highlighted in Appendix 1. The extended detention depth
has been conservatively estimated as 10% more than the filter area.
Table 4: Bioretention Basin Design Parameters

                                 Parameter                      Values
                       Extended Detention Depth (m)              0.10
                                                      2
                          Storage Surface Area (m )              27
                                                  2
                            Filter Media Area (m )               24
                     Unlined Filter Media Perimeter (m)          0.01
                  Saturated Hydraulic Conductivity (mm/hr)       200
                               Filter Depth (m)                  0.4
                     TN Content of Filter Media (mg/kg)          400
               Orthophosphate Content of Filter Media (mg/kg)    50
                          Exfiltration Rate (mm/hr)              0.0
                             Overflow Weir (m)                   2.0

2.4.2 Sediment Forebays
Sediment forebays are not proposed as part of the sites treatment due to the small
size of the development that is contributing to the bioretention system (<2ha). In
line with Bioretention Technical Design Guidelines – Version 1 (WaterbyDesign,
2012), it is recommended that pretreatment of runoff using sediment forebay’s are
not required in situations where the contributing developed catchment is less than
2ha (refer Table 14 of the guideline).


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