20220206 48 Elm St SWMP

To determine the attenuation requirements, the developed scenario was remodelled with the impervious surfaces
discharging to an OSD tank prior to leaving the site, and a small portion of the site consisting of the drainage
easement and the entry into the site are modelled to bypass the OSD tank, discharging directly to Pearl Street.
Details of these catchments are shown below in Figure 3.4 – DRAINS Post-development catchments.




                                 Figure 3.4 – DRAINS Post-development catchments for attenuation

An iterative process was used to determine a suitable configuration of storage volume and outlet arrangement for
the proposed On Site Detention (OSD) tank to deliver the required reduction of the flow rates from the site. The
result of the iterative sizing process determined that the OSD storage will need to provide an attenuation capacity
of 13m³ to ensure adequate capacity to achieve attenuation. A 115mm diameter low flow orifice outlet at the base
of the OSD tank was determined to adequately attenuate flow from the 0.5EY to 1% AEP events. A small amount of
stormwater will surcharge from the access grate of the OSD tank during the 2% and 1% AEP events. This flow will
travel as overland flow across the carpark and ultimately to Pearl Street.

It is proposed to install Humes Stormtrap in-ground OSD tank consisting of two standard VI Modules for the
development. This will provide 18.8m² of storage area, which at 0.7m high will provide 13.2m³ of attenuation storage
for the development.

The proposed OSD arrangement was modelled in DRAINS to compare flows to the existing condition. Results from
DRAINS is summarised in Table 3.2 Existing and proposed mitigated site peak discharge rates.

                                Table 3.2 Existing and proposed mitigated site peak discharge rates

                                                             Qexisting   Qmitigated    Qchange      Qchange
                                        AEP (2100)
                                                             (m³/s)      (m³/s)        (m³/s)        (%)
                                           0.5 EY             0.026       0.026        0.000         0%
                                             10%              0.044       0.035        -0.009        -20%
                                             5%               0.050       0.039        -0.011        -22%
                                             2%               0.057       0.049        -0.008        -14%
                                             1%               0.063       0.061        -0.002          -3



As the total peak flows are equal to or below the existing site conditions for all storm events, the proposed OSD tank
will provide the mitigation required for the peak flow storm events from the development.


     3.2. Existing Drainage Network
As discussed in Section 2.3.1, there is an existing 300mm diameter roofwater pipe along the eastern boundary of
the site that collects roofwater from the southern adjoining property (Lot 44 on SP196689). There is an existing
grated inlet pit in the verge of Pearl Street adjacent to the north eastern corner of the site. The 300mm diameter

Project number 20220206                                                                                       Page 9 of 19
File 20220206 48 Elm St SWMP, 2023-02-03 Revision 1

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