REPS - Updated SWMP

     4.1.3       Mitigation Scenario
For the Mitigation Scenario, the only update made to the Developed Scenario model was to represent the approved / constructed detention
basin. It is noted that the small detention basin within the townhouse site has not been represented within this assessment, which is
considered a more conservative approach (and is consistent with the original assessment. A summary of the elevation-storage data for the
detention basin is provided in Table 4.5 with a summary of the outlet configuration and key details provided below:

•     Outlet Summary:

      –    Pipe 1 = 1 x 225mm at RL1.56m.

      –    Pipe 2 = 900 x 900mm culvert at RL1.56m.

      –    Weir = 5m at RL2.72m.

TABLE 4.5: ELEVATION-STORAGE DATA FOR APPROVED / CONSTRUCTED DETENTION BASIN

    Elevation (m RL)                                                    Volume (m3)

    1.56                                                                0

    1.7                                                                 81

    1.9                                                                 206

    2.1                                                                 344

    2.3                                                                 494

    2.5                                                                 659

    2.7                                                                 841

    2.8                                                                 1019


A comparison of the existing and mitigated peak flows at the Site outlet is presented in Table 4.6. The results show that a minor increase in
peak flows is expected in the smaller events (<10% AEP) and the 1% AEP, with reductions in the 5% and 2% AEP events. As the catchment
details have not changed significantly between the approved SMP and this update, it is considered the minor increases are a result of the
change in the hydrologic method from ARR1987 to ARR2019 and the change to the losses. As such, it is considered that these minor increases
would not result in actionable nuisance, and the outcome remains consistent with the approved SMP. Further to this, the Site’s bioretention
system will provide some additional detention in the smaller flood events to help in managing this further. Further to this, the gap flow from
the townhouse site (refer 4.3) is approx. 0.11m3/s in the 1% AEP, which would report south downs Starboard Street (away from basin) and
improve results further in the 1% AEP events.

TABLE 4.6: PEAK DISCHARGE COMPARISON – EXISTING VS MITIGATED SCENARIOS


    Scenario              63.2% AEP         50% AEP       20% AEP            10% AEP        5% AEP           2% AEP           1% AEP

    Existing (m3/s)       0.61              0.72          1.11               1.41           1.68             1.99             2.23

    Mitigated (m3/s)      0.65              0.77          1.16               1.42           1.64             1.92             2.27

    Difference (m3/s)     0.05              0.05          0.05               0.01           -0.04            -0.07            0.03



A full summary of peak flow results is presented in Appendix C.




73 EUMUNDI - NOOSA ROAD, NOOSAVILLE I BBNE00529_0002_0001-REP-001-2                                                                        15