Reference Document - RAL23/0013.02 - Part 2 (As Published)

Table 2-8: Comparison of Peak Flows (Existing Vs. Mitigated)

      Event AEP             63.2%          50%          20%           10%             5%        2%         1%              1% CC
   Existing (m3/s)           0.33          0.43         0.72          0.92            1.21      1.55       1.86            2.57
  Mitigated (m3/s)           0.34          0.43         0.66          0.82            1.04      1.49       1.82            2.46
   Change (m3/s)             0.01          0.00        -0.06         -0.10            -0.17     -0.06      -0.04           -0.10



Table 2-9: Comparison of 1% AEP event across all durations (Existing Vs. Mitigated)

 Duration
                  10        15        20          25      30        45        60           90   120     180        270       360
 (min)
 Existing
                 0.98      1.68      2.13      2.45      2.57      2.49      2.45        2.29   2.16    2.04       1.77      2.02
 (m3/s)
 Mitigated
                 0.91      1.53      1.96      2.28      2.38      2.46      2.35        2.20   2.06    1.89       1.69      1.92
 (m3/s)
 Change
                 -0.07     -0.15     -0.17     -0.17     -0.18     -0.03     -0.09      -0.09   -0.11   -0.15      -0.08     -0.10
 (m3/s)


Table 2-7 provides a summary of the basin configuration and it is noted there is 0.19m freeboard from the
1% (2100) AEP flood level to the basin crest. A sensitivity analysis was undertaken to assess the impact of
100% blockage of both basin outlet configurations. Under this scenario the flood level rose to RL0.89m and
demonstrates the spillway is able to convey the 1% (2100) AEP flows under fully blocked conditions without
overtopping the embankment crest. Given this and the fact that the land downstream of the detention
basin will remain undeveloped it is considered that the 0.08m from the 1% (2100) AEP flood level (and
0.37m from spillway level) to basin crest level is acceptable.

It is noted that with the inclusion of 0.3m of Extended Detention Depth for the co-located bioretention
basin, the total Q20 depth for the basin is expected to be less than the 1.2m recommended by QUDM for
safety concerns.

Refer to Appendix B for summary of WBNM output.




23-0167SMP01-V3 | Stormwater Management Plan | 33 Pavilion Street, Pomona                                                  Page 24