Att 2 SWMP

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.

                     Table 3.1 Existing and proposed unmitigated site peak discharge rates

                                                         16 Rene Street
                                                                                      Qchange
                                A.E.P        Qexisting     Qproposed      Qchange      (%)
                                             (m³/s)        (m³/s)         (m³/s)

                               63.20%        0.031         0.034       0.003         10%
                               50%           0.038         0.040       0.002         5%
                               20%           0.062         0.065       0.003         5%
                               10%           0.077         0.082       0.005         6%
                               5%            0.093         0.094       0.001         1%
                               2%            0.106         0.108       0.002         2%
                               1%            0.116         0.121       0.005         4%


As is expected, the increased impervious area will generate a higher peak storm discharge that will
require attenuation to ensure no worsening of the current run-off from the site.

The post-development model was updated to provide storage through a rainwater tank with
attenuation storage connected to the new roof area, as well as the storage provided by the
bioretention area for surface run-off. An iterative process was used to determine a suitable
configuration of storage volume, low-flow, and high flow pipe outlets for the rainwater tank to deliver
the required reduction of the flow rates from the site.

The tank has been modelled as a 7,500L rainwater detention tank. A low flow outlet with 25mm orifice
plate is provided at half the tank height with the volume below the low flow assumed to be full at the
time of the stormwater. A high flow 90mm diameter outlet at the top of the tank acts as the overflow
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 5m² area is modelled with a low flow
100mm diameter slotted ag pipe at the base of the garden, and the inlet pit operating as a weir inlet.
A 600mm media filter depth with an assumed porosity of 0.3 and the extended detention depth of
200mm provides the storage capacity for the bio-garden.

Using this combination of parameters, has been found to achieve the desired results as shown below
in Table 3.2 Existing and proposed mitigated site peak discharge rates. The resulting hydrograph is
shown in Figure 3.3 – Mitigated post development site critical storm rainfall hydrographs showing
peak flow rates.




Project number 20210121
File RECOR Preliminary Stormwater Management Report - 20210121.docx, 2021-10-07                                   Page 11 of 26
Revision 0

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