Att 1 - RFI Amended SWMP

                                                                                  23098 - SBSMP/3
                                                                                          Page 11




                                     Table 5-5 - Comparison of Peak Flows
                             XPSTORM          XPSTORM                 XPSTORM            XPSTORM
         XPSTORM Total
                          Post-development Post-development        Post-development       Total Post-
  ARI    Pre-development
                          Flows into Tanks Bypass Flows (Un -      Flows from Tanks   development Flows
(years) Flows (Catchment)
                3          (Un -Mitigated)     Mitigated)             (Mitigated)         (Mitigated)
              (m /s)
                                (m3/s)           (m3/s)                  (m3/s)             (m3/s)
   1            0.040               0.038                  0.021        0.019               0.040

   2            0.047               0.043                  0.024        0.021               0.046

   5            0.069               0.057                  0.036        0.026               0.062

  10            0.083               0.064                  0.043        0.029               0.072

  20            0.095               0.073                  0.050        0.039               0.089

  50            0.117               0.084                  0.070        0.039               0.109

 100            0.132               0.092                  0.086        0.039               0.125


As can be seen, the XPSTORM pre-development results are comparable to Rational Method for all
storm events, as they are generally within 10% to the Rational Method results. Therefore, the results
show that a reasonable validation in peak flows has been achieved from the XPSTORM model when
compared to Rational Method and is considered acceptable in assessing for mitigation of post
development flows.

Subsequently, the XPSTORM 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 northeast of the subject site.

To address the Water Quality and Drainage Code Performance Outcome PO2 design allowance for
climate change impacts, it is recommended that the OSD storage volume be increased by 20%, from
an initial capacity of 27.5kL to a revised capacity of 37.5kL, to address climate change
considerations. This adjustment to the proposed maximum tank volume for the proposed
development is deemed to be an acceptable outcome to address AO2.4.

The modified OSD tanks will now have the following properties:

    •   Available detention depth above reuse component = 1.820 m;

    •   Low flow orifice diameter = 0.070 m;

    •   High flow pipe diameter = 0.150 m;

    •   High flow pipe elevation above reuse component = 1.640 m;

    •   Total required detention volume = 28.20 kL (23.5 kL + 20%); and

    •   Total provided detention volume = 37.5 kL (5 x 7.5kL).

Refer to Appendix H for the pre-development XPSTORM results, Appendix I for the post-
development XPSTORM results and Appendix J for the XPSTORM post-development mitigated
results.




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