B07056.W-01A Complete - Stormwater [As Published] · 132007.2398

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village

          Table 7.3: Average Annual Runoff Volume Comparison for Existing and Developed Scenarios
                                  Scenario              AVRC               AARV (ML/year)
                                    Existing             0.30                  45.0
                                  Developed              0.49                  73.5
                                   Increase                -                   28.5
                   Note: AVRC adopted based on Table 4.11.1 of PRSC Subdivision Development Guidelines.

The above results indicate that runoff from the site will be increased by 63% as a result of the development.

7.3.2              Mitigated Scenario

To determine the reduction in the volume of runoff by the proposed measures, four separate water balance
models for each of the underground rainwater tanks were undertaken to determine the amount of water that
could be captured and reused from roof areas.

The results of the analysis are presented below in Table 7.4, with detailed results presented in Appendix E.
                                        Table 7.4: Water Balance Model Results
                 Tank                               Size (kL)         Reduction in Runoff (ML/year)
                 Northern                              250                         6.88
                 Central                                39                         0.90
                 Residential Aged Care                 50                         1.68
                 Southern                              350                         9.22
                 Total Reduction                         -                        18.68


The above results indicate that a reduction in runoff volume of 18.68 ML can be achieved by adopting the tank
sizes and management scheme proposed. Therefore, the proposed annual runoff is 54.8 ML. While the
proposed management scheme does not reduce the increase in flows to below existing volumes, it should be
noted that:
    •    The increase in runoff volume has been significantly mitigated;
    •    Equivalent Person (EP) occupancy rates for ILU’s and Apartments as required by The Noosa Plan for
         Retirement or Special Needs Housing are low at 1.5 (as modelled). This has an impact on water
         balance models through lower demand and hence lower rainwater use for toilets and laundries.
         Therefore, normal residential EP rates of around 2.5 would reduce the developed annual runoff further;
    •    Overflow from the underground rainwater tanks will flow into the two WQCP’s. This overflow is required
         to maintain water levels in the ponds for submerged plant species and continuous turnover of the water
         bodies for stormwater quality treatment; and
    •    Reduction of runoff volumes via evaporation within the WQCP’s has not been accounted for in the
         analysis.

On this basis the proposed measures to reduce runoff volume from the development is considered to be
appropriate.




B07056.W-01A.doc                                                                                          Page 21