RFI - Stormwater management (flood report)

      The existing drainage capacity of Pavilion Street is approximately equivalent to the 10% AEP
      storm event, with flow greater than this overtopping the verge and entering the site. To
      account for this, the peak flow for SUB1-Ex was determined for the 10%, 5%, 2% and 1% AEP.
      The portion of flow greater than the 10% AEP was converted into a ratio, which was then
      applied to the catchment area as detailed below in Table 2-1. This ensured that flows from
      SUB1-Ex greater than the 10% AEP were routed through the site.
      Table 2-1: Summary of Existing Catchment Parameters

       Sub-catchment                            Area (ha)                          Imperviousness (%)
       SUB1                                       3.407                                     11
       SUB1-Ex                                    0.287                                     50
       SUB1-Ex (<10% AEP)                          0.0*                                     -
       SUB1-Ex (5% AEP)                           0.047*                                    50
       SUB1-Ex (2% AEP)                           0.095*                                    50
       SUB1-Ex (1% AEP)                           0.138*                                    50
      * area factored based on flow rate for events greater than 10% AEP

      The WBNM model has been run for events from 1 to 100-year ARI and for durations of 20
      minutes to 3 hours. A WBNM catchment lag factor of 1.6 was applied which is within the
      recommended range. An initial and continuing loss of 10mm and 2.4mm/hr has been
      applied refer to Rational Method Check). The resulting peak flows for the critical duration are
      provided in Table 2-2 for each event at the outlet reporting locations.
      Table 2-2: Existing Catchment Peak Flows

       Event AEP             63.2%       50%        20%        10%          5%       2%           1%    1%CC

       SUB1 (m3/s)            0.34       0.42       0.64       0.80         0.95     1.12        1.27   1.59

      2.1.2. Rational Method Check

      Pre-development peak flows have been checked against peak flows determined using the
      Rational Method (RM) for SUB1, as shown in Table 2-3. A time of concentration of 20.6
      minutes was determined for SUB1 rational method. It can be seen there is less than ten
      percent difference across all assessed events, indicating that the WBNM model flows are
      suitable for use. Refer to Appendix B for a summary of rational method parameters used.
      Table 2-3: Comparison of WBNM and Rational Method Peak Flows

       Event AEP               63.2%       50%        20%       10%         5%       2%          1%     1%CC

       Rational (m3/s)          0.34       0.40       0.61      0.75        0.89    1.12         1.29   1.52
       WBNM (m3/s)              0.34       0.42       0.64      0.80        0.95    1.12         1.27   1.59
       Difference (%)            2.3        3.0       4.4       6.3         6.3      0.1         -0.9   -0.7




21-0166SMP01-V1 | Stormwater Management Plan | 25 Pavilion Street, Pomona                                      |7