RFI - 20210153 Maple St SWMP

                                 Figure 3.4 – DRAINS Post-development catchments


The results of the modelling against the range of design storms for the development under the
existing and proposed development conditions are shown in Table 3.1 Existing and proposed
unmitigated site peak discharge rates. The external catchment is included in these figures to assess
the overall impact to the receiving system in Myall Street.



                     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       0.058         0.087          0.029         50%
                                           50%         0.068         0.100         0.032         47%
                                         0.2 EY        0.125         0.151         0.026         21%
                                           10%         0.156         0.177         0.021         13%
                                             5%        0.192         0.208         0.016           8%
                                             2%        0.235         0.241         0.006           3%
                                             1%        0.262         0.270         0.008           3%



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.


Project number 20210153
File RECOR Preliminary Stormwater Management Report - 20210153.docx, 2022-03-31                                     Page 13 of 27
Revision 1

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