SWMP Technical Memo Climate Change as submitted with Further Advice response

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landscaped areas. The revised classification was applied consistently to the like-for-like pre-development
and post-development climate scenarios.

4.3 External stormwater pipe
The downstream external DN375 RCP link was refined in the DRAINS model using the lesser proposed grade
shown on the current operational works longitudinal section. Drawing C052, Revision C, identifies the final
external connection grade as approximately 4.77%. This amendment more accurately represents the
proposed constructed system and its hydraulic routing and conveyance characteristics.

4.4 Online 15 kL rainwater harvesting tank
The current Revision C drawings include a 15 kL online rainwater harvesting tank upstream of the 75 kL
main OSD tank. Drawing C052 notes that the tank is to detain 15 kL before overflow continues to the
downstream detention tank, and Drawing C054 shows both tanks in the coordinated layout. The online
tank provides rainwater harvesting and reuse, regular operational drawdown and additional upstream
attenuation before overflow reaches the main tank.
The two tanks provide 90 kL of nominal gross tank volume across the site. Importantly, the climate-change
compliance result reported below does not rely on the full 15 kL harvesting volume being empty at the start
of the design storm: the modelled 75 kL main OSD tank remains within capacity on its own. The harvesting
tank therefore provides additional resilience while also supporting Council's rainwater harvesting and reuse
objectives.


5. Climate-adjusted modelling results
DRAINS was run using the Full Unsteady hydraulic model. The ensemble analysis identified the 25-minute
burst, Storm 4, as the critical storage case for the climate-adjusted 1% AEP event. The updated results are
summarised below.
Table 3 - Updated 1% AEP + 20% climate-change results

   Result                                           Model output               Assessment

   Pre-development discharge                                 0.151 m3/s        Like-for-like climate scenario

   Post-development unmitigated discharge                    0.197 m3/s        Before detention

   Post-development mitigated discharge                      0.148 m3/s        After the 75 kL OSD controls

   Change from pre-development                           -0.003 m3/s (-2.0%)   Non-worsening achieved

   Critical storage case                                   25 min, Storm 4     1% AEP ensemble

   Peak main-tank storage                                 Approx. 74.0 m3      Approx. 98.7% of 75 kL

   Peak main-tank level                                     RL 117.98 m        Storage curve upper level RL 118.00 m

   Controlled outlet split                               0.088 + 0.060 m3/s    Total 0.148 m3/s

   Modelled overflow                                         0.000 m3/s        No overflow reported



   Assessment. The controlled climate-adjusted discharge of 0.148 m3/s is below the corresponding
   pre-development discharge of 0.151 m3/s. Peak storage remains below the 75 kL modelled capacity
   and no overflow occurs. The current main tank sizing and outlet controls therefore remain suitable
   for the required climate-change design case.


P0478-26-001 | Cooroy Social Housing | RFI Item 4       Page 3 of 6