13. Appendix I - Integrated Water Mangement Plan

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



4.2.2       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. The same lag time of 1 minute was used in the
developed case scenario between the upstream and downstream boundaries of the site catchment.
As the runoff from the external catchments was assumed to be conveyed by the drainage easement on the
eastern side of the site, only the peak runoff from the site itself was 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 3.3 m3/s discharging from the site for the 1% AEP event without
any mitigation measures. This is an increase of approximately 1.5 m3/s when compared to the existing
scenario.

4.2.3       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 0. These values can be altered subject to detailed design to ensure satisfactory performance.
Proposed Detention Storage Tank Characteristics
Table 4-2
 Attribute                    Value
 Discharge Pipe Diameter      1 by 750 mm dia
 Minimum Volume               1500 m3

Three additional rainwater tanks are proposed for the development as part of the stormwater quality treatment
train. These tanks are 50kL, 25kL and 2kL respectively in volume and are discussed in the following section
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.

4.3         Model Results
The proposed storage system was incorporated in the XP-RAFTS model which was run for the same events
as discussed previously. The peak discharges from the site for pre-development conditions and for post-
development conditions, with the rainwater storage included are shown in Table 4-3.
Table 4-3     Peak XP-RAFTS Discharge
                                                   Peak Flow Rate
 Event          Pre-Development Peak Discharge (m3/s)        Post-Developed (Mitigated) Peak Discharge (m3/s)
 (AEP)          (Site Only)                                  (Site Only)
 1%             1.84                                         1.25
 2%             1.48                                         1.14
 5%             1.31                                         0.93
 10%            0.97                                         0.88
 20%            0.80                                         0.76
 39%            0.68                                         0.66

As shown above, the proposed tank results in a decrease in peak discharges from the site for all AEP events
modelled in the mitigated developed scenario. Table 4-4 shows the storage volume for each modelled event.




V190428 | 15 May 2019 | Commercial in Confidence                                                               12