Attachment 5 - Stormwater Managment Plan · 132007.12.8

Figure 3: MUSIC Model Layout

2.4 Treatment Measures
Runoff from the development will be treated via a bioretention system. The locality
of the treatment device is shown 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
pondage allowed to occur over the filter area has been limited to 300mm, with a
minimum filter media depth of 400mm. Table 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 5: Bioretention Design Parameters

                                  Parameter                       Basin
                        Extended Detention Depth (m)               0.30
                          Storage Surface Area (m2)                 27
                                                   2
                            Filter Media Area (m )                  27
                      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
                                                                    30
                                   (mg/kg)
                         Exfiltration Rate (mm/hr)                  0.0
                               Overflow Weir (m)                    2.0

2.4.2 Sediment Forebays
A sediment forebay is only required to pre-treat runoff if their contributing catchment
is greater than 2 ha. As the total site being treated is only 0.202ha, a sediment forebay
is not being proposed.
                                                                                       9