20210153 - Maple Street SWMP

                    Table 3.1 Existing and proposed unmitigated site peak discharge rates

                                                                  Maple Street
                                                                                              Qchange
                                 A.E.P                Qexisting      Qproposed    Qchange      (%)
                                                      (m³/s)         (m³/s)       (m³/s)
                                             1EY        0.045          0.073       0.028         62%
                                            50%         0.054          0.085       0.031         57%
                                         0.2 EY         0.103          0.128       0.025         24%
                                            10%         0.129          0.149         0.02        16%
                                              5%        0.159          0.174       0.015           9%
                                              2%        0.196          0.199       0.003           2%
                                              1%        0.215          0.223       0.008           4%



As expected, the site has increased the total peak flows from the site for all storm events. Attenuation
storage will be required to mitigate the outflows to ensure that the development does not exceed
the predevelopment levels at the downstream receiving environment.

The mitigated post-development model splits the catchment into a roof area, which drains to an on-
site detention (OSD) area, and the remainder of the site being treated through the bioretention area.
An iterative process was used to determine a suitable configuration of storage volume, low-flow, and
high flow pipe outlets for the OSD to deliver the required reduction of the flow rates from the site.
Conceptually, the OSD has been represented as rainwater tanks, however the final configuration of
the OSD is to be developed as part of an operational works approval.

The result of the iterative sizing process determined that the OSD storage will need to provide an
attenuation capacity of 50m³ for the proposed roof areas, and the maximum possible roof area is to
be plumbed to the tank. A low flow outlet consisting of a 55mm orifice plate is to be provided the
base of the tank, and a 150mm high flow outlet located at 1.8m above the base of the tank of the
tank provides discharge for larger storm events.

The bioretention area has been modelled based on the sizing determined through MUSIC modelling
to deliver the required stormwater quality improvements. A 20m² area is modelled with two low flow
100mm diameter slotted ag pipe at the base of the garden, and the inlet pit operating as a weir inlet
overflow route. A 600mm media filter depth and 200mm drainage layer are modelled with a typical
porosity of 0.3, along with the extended detention depth of 200mm which provides the storage
capacity for the bio-garden.

Using this mitigated development model, the proposed developed flows were again compared to
the existing state as shown below in Table 3.2 Existing and proposed mitigated site peak discharge
rates.




Project number 20210153
File RECOR Preliminary Stormwater Management Report - 20210153.docx, 2022-03-31                                     Page 11 of 26
Revision 0

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