MCU14/0106 Appendix E - Engineering Report

Noosa Civic
Stormwater Management Plan



Table 2-2         Developed Catchment Properties

        CATCHMENT ID             Catchment area (ha)        Percentage Impervious
                                                                     (%)
                  D                     4.47                        85.00
                   E                    1.53                        5.00
                  F1                    1.92                        33.66
                  F2                    0.95                        59.57
                  F3                    2.78                        67.60
                  G1                    2.11                        25.27
                  G2                    1.82                        76.55
                  G3                    0.80                        82.17
                  G4                    1.68                        75.21
                  H                     3.21                        28.45
                Total                   21.27                       55.14



To ensure that the peak flow calculated by the model was reasonable, the existing case model was validated
against a Rational Method calculation. The calculations completed in relation to the Rational Method are
presented in Appendix B. The Rational Method was found to predict peak flows of 3.9 m3/s in Catchment A
and 1.7 m3/s in Catchment B for the 100 year event. In comparison, the WBNM model provided peak flow
estimates of 4.2 m3/s in Catchment A and 1.8 m3/s in Catchment B for the existing case. Given this close
level of agreement, it was considered acceptable to use the WBNM model to calculate peak flows for the
existing and developed cases and to determine the size of detention basins required to offset the increase in
peak runoff.
It is proposed that on-site mitigation of local flows will be provided by a standard detention basin at the
south-western corner of the site in addition to utilising flood storage within the car parks. The location and
conceptual footprint of the proposed basins is shown in Figure 5. For the purposes of the development
application assessment, the levels were based on the preliminary engineering levels.
With reference to Section 3, it is intended to install bioretention systems within the basin and the car parks.
In accordance with best practice, it was assumed that the extended detention zone of the bioretention
systems was full at the commencement of each rainfall event.
Table 2-3 lists the stage-storage relationship ultimately adopted for the detention basin. For the analysis, the
size of the basin and the outlet grate size and weir were varied until an acceptable result was obtained for
the developed case. Ultimately, the outlet consisted of a 450mm diameter low flow pipe located immediately
above the extended detention zone of the basin and an overflow weir length of 35 metres located 1.6m
above the invert of the outlet.




QIC Noosa Civic Pty Ltd                             Version 1                                     November 2014
Appendix D - SMP.docx                                                                                   Page 3