Att 1 - Engineering - RFI Reponse · MCU21/0117.01

                                                                                  23098 - SBSMP/4
                                                                                          Page 14




The DRAINS IL-CL hydraulic modelling results have demonstrated that the proposed mitigation
implemented will maintain pre-development flows (or less) for all design storms at the LPD located
north of the subject site.

To address the Water Quality and Drainage Code Performance Outcome PO2 design allowance for
climate change impacts, a 20% increase has been incorporated through the Climate change Rainfall
Multiplier function in the DRAINS program to address climate change considerations. This
adjustment is deemed to be an acceptable outcome to address AO2.4. The pre-development and
post-development mitigated with the 20% rainfall intensity increase peak flow rates were compared
for the catchment and are shown in Table 5-8 below.

             Table 5-8 - Comparison of Peak Flows – Inclusion of 20% rainfall intensity increase


                       Pre-Development Peak Flows           Post-Development Peak Flows (Mitigated) +
  Property
                              (Un-Mitigated)                      20% rainfall intensity increase
   State
                                  (m3/s)                                      (m3/s)

  50% AEP
   (m3/s)
                                   0.035                                     0.035
  20% AEP
   (m3/s)
                                   0.058                                     0.050

  10% AEP
   (m3/s)
                                   0.065                                     0.061

  5% AEP
   (m3/s)
                                   0.084                                     0.078
  2% AEP
   (m3/s)
                                   0.102                                     0.098

  1% AEP
   (m3/s)
                                   0.115                                     0.109

The DRAINS IL-CL hydraulic modelling results with the additional 20% rainfall intensity increase have
demonstrated that the proposed mitigation will maintain pre-development flows (or less) for all
design storms at the LPD located north of the subject site.

Refer to Appendix H for the pre-development and post-development DRAINS results and Appendix
I for the post-development mitigated DRAINS results.




Site Based Stormwater Management Plan
Proposed Townhouse Development at 5 Pearl Street, Cooroy