RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

Figure 3-5: DRAINS Catchment Inputs




The pre-development flow rates for various AEP were calculated using DRAINS. The results are summarized in
Table 3-3.
Table 3-3: DRAINS Results for Existing Catchment

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

 Pre-Development Flow (m³/s)                  0.048          0.059   0.093         0.112   0.133    0.159        0.179



A comparative assessment was conducted to evaluate the variance between DRAINS results and the Rational
Method calculations. The differences between the two methodologies are presented in Table 3-4.



Table 3-4: Comparison Between DRAINS and the Rational Method

  Storm Event (AEP)             Rational Method (m³/s)               DRAINS (m³/s)                 Difference %

  63.2% (1-Year ARI)                       0.050                           0.048                       3%

   50% (2-Year ARI)                        0.059                           0.059                       1%

   20% (5-Year ARI)                        0.090                           0.093                       4%

  10% (10-Year ARI)                         0.110                          0.112                       2%

   5% (20-Year ARI)                        0.130                           0.133                       2%

   2% (50-Year ARI)                        0.165                           0.159                       3%

  1% (100-Year ARI)                        0.188                           0.179                       5%



The results obtained from the Rational Method and the DRAINS model show differences that are within acceptable
engineering tolerances and align with expectations given the inherent methodological differences between the
two approaches.



 45 MAPLE STREET, COOROY
                                                   24 June 2026                                           P a g e |9
 Stormwater Management Plan
 Client Reference: P0478-26-001
 Prepared for Myall Rise PTY LTD