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

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village

                                 Table 5.1: Stormwater Detention Basin Concept Details
      Basin Details                                        Northern Basin                     Southern Basin
      Detention Storage1                                       1940m3                             1640m3
      Min. Detention Level                                   RL 99.60m                          RL 101.50m
      Embankment Level                                       RL 101.00m                          103.00m
      Detention Area1                                          2200m2                             1980m2
      Low Level Outlet & Invert Level2                5 x 375mm – RL 99.60m              5 x 375mm – RL 101.50m
      Outlet Pit Size & Invert Level2                2.4m x 0.9m – RL 100.25m           2.4m x 0.9m – RL 102.40m
                                                       1 x 50mm – RL 99.20m               1 x 50mm – RL 101.00m
      Multiple Pipe Outlet Dia
                                                      1 x 750mm – RL 99.15m              1 x 450mm – RL 101.20m
      & Invert Levels2
                                                      1 x 825mm – RL 99.15m              2 x 525mm – RL 101.20m
      Overflow Weir Width                                        4m                                3.75m
      Overflow Weir Level                                    RL 100.70m                         RL 102.5m
                         1 - Details based on Overflow Weir Level.
                         2 – Refer to Outlet Structure details on Drawing No. B07056-SK08 in Appendix A.
                         Note: Concept details subject to final design.

5.2                Mitigated Scenario Results
The mitigated scenario model was created by entering the storage characteristics and stage discharge
relationships of each detention basin into the developed scenario model. All ARI storms and durations
previously listed were modelled. Results for the mitigated scenario and comparison to the existing site peak
discharge rates are detailed in Table 5.2.
                  Table 5.2: Comparison of Existing and Mitigated Scenario Peak Discharge Results
                                       Northern Basin                                Southern Basin
 ARI Storm Event                                         Change in                                         Change in
                          Existing       Mitigated                       Existing       Mitigated
                                                         Peak Flow                                         Peak Flow
        2 Year              1.12            0.94           -0.18           0.95            0.95                0
        5 Year              1.56            1.47           -0.09           1.32            1.16              -0.16
       10 Year              1.82            1.76           -0.06           1.54            1.28              -0.26
       20 Year              2.16            2.13           -0.03           1.83            1.68              -0.15
       50 Year              2.62            2.43           -0.19           2.22            2.13              -0.09
       100 Year             2.91            2.69           -0.22           2.46            2.46                0


Results indicate that both basins are effective in reducing the increase in peak discharges from the development
back to existing flows. Maximum Water Surface Levels (WSL) in the Northern and Southern detention basins
during the peak 100 year ARI storm are 100.72m and 102.71m respectfully. This indicates that a minimum
freeboard of 300mm to the top of each basin’s embankment can be achieved.

For the Southern Basin, discharge across the overflow weir is limited to a depth of 210mm. The velocity depth
product (V.D) of the flow over the weir is 0.16m2/s. Both depth and V.D are in accordance with QUDM. While no
discharge across the Northern Basin overflow weir occurs for the 100 year ARI storm, a weir is provided to
control overflow should such an event occur. Please note the sag in the accessway is flatter and wider than the
weir. Therefore, the V.D product will be lower than QUDM requirements.




B07056.W-01A.doc                                                                                               Page 10