H - Revised Integrated Water Management Plan by Cardno (inc MUSIC file) (13.09.19)

                                                                                   Integrated Water Management Plan
                                                                           Noosa Marketplace : 2-20 HOFMANN DRIVE



5.2.2       RAFTS Parameters
Catchment information was input to the RAFTS model and analysed using the same critical duration events
that were found in the existing conditions hydrological model.
As the runoff from the external north and north west catchments was assumed to be conveyed by the drainage
easement on the eastern side of the site, only the runoff from the site itself and small partial catchments to the
west were considered for the assessment of non-worsening compared to existing conditions. As expected, the
proposed development results in an increase in peak runoff from the site, with a peak flow of 2.7 m3/s
discharging from the site for the 1% AEP event without any mitigation measures. This is an increase of
approximately 1.95 m3/s when compared to the existing scenario.

5.2.3       Stormwater Detention
Increases in impervious area within the proposed development have resulted in an increase in local catchment
stormwater discharges. A rainwater detention tank is therefore proposed to maintain peak stormwater
discharges to values equal to or less than existing conditions. The design characteristics for the proposed tank
are shown in Table 5-2. These values can be altered subject to detailed design to ensure satisfactory
performance.
Table 5-2     Proposed Stormwater Detention Tank Parameters
 Attribute                    Value
 Discharge Pipe Diameter      1 by 375 mm dia
 Minimum Volume               1400 m3

Three additional rainwater tanks are proposed for the development as part of the stormwater quality treatment
train. These tanks are 50kL, 50kL and 10kL respectively in volume and are discussed in Section 6 of this
report. These tanks have been excluded from this modelling as they are assumed to be full at the
commencement of the rainfall event. In addition, they would have little impact upon runoff during larger AEP
events.




V190428 | 10 September 2019 | Commercial in Confidence                                                          14