Approved Document MCU22-0144 & OPW22-0207 · MCU22/0144

The development of the site will therefore increase the total peak flows from the site under all storm events.
Attenuation is required to mitigate the outflows to ensure that the development does not exceed the
predevelopment levels to minimise impacts to the downstream environment.

To determine the attenuation requirements, the developed site was duplicated an on-site detention (OSD) tank
receiving the developed catchment outflows. An iterative process was used to determine a suitable configuration of
storage volume, low-flow orifice and high flow overflow weir for the OSD 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 54m³ to ensure adequate capacity to achieve attenuation. A low flow outlet consisting of a 132mm orifice
plate is to be provided the base of the OSD, and an overflow 0.3m wide overflow weir discharging to the outlet 300m
from the top of the tank for larger storm events.

Using this OSD combination for the 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.

                               Table 3.2 Existing and proposed mitigated site peak discharge rates

                                                                       Wyandra Street
                                                                                                         Qchange
                                           A.E.P               Qexisting      Qmitigated    Qchange       (%)
                                                               (m³/s)         (m³/s)        (m³/s)
                                            1EY                 0.021          0.02         -0.001         -5%
                                            50%                 0.026          0.023        -0.003        -12%
                                           0.2 EY               0.044            0.04       -0.004         -9%
                                            10%                 0.060          0.059        -0.001         -2%
                                             5%                 0.075          0.073        -0.002         -3%
                                             2%                 0.094          0.092        -0.002         -2%
                                             1%                 0.106          0.106           0           0%



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




     3.1. Stormwater Harvesting
In accordance with the Sustainable Building Code PO2 and AO2.1, a minimum of 3,000L of rain water storage has
been provided per dwelling. The 10 units will be supplied by a 30,000L in ground rainwater tank, nominally a
Humes Rainvault RV3000-01-00 or approved equivalent located within the common space of the development.

The maximum possible roof water from the development is to be drained into the tank, with downpipes to be fitted
with first flush divertors of minimum 20L/100m² of roof area captured. The roofwater will drain to a manhole fitted
with a VF3 volume filter (or equivalent) capable of filtering up to 1,100m² of roof area before it passes to the reuse
tank.

The reuse tank will be plumbed for internal and external non-potable water demands including external irrigation
fixtures and wash down areas, internal toilets and laundry areas. An automatic switching device is to supplement
the tank storage to ensure that water (town water supply) is available should the tank storage be exhausted.

Overflows from the tank will be connected to the internal drainage network upstream of the OSD tank to ensure site
run-off remains attenuated.




Project number 20220156                                                                                              Page 11 of 28
File RECOR Stormwater Management Report - 20220156.docx, 2023-01-13 Revision 2

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