Stormwater Management Plan

6.3.3.1 Hydrologic Analysis

Rational method calculations have been completed for the development for the pre-development and
post-development conditions. The pre-development catchments runoff parameters are based on
detailed survey, aerial survey and satellite imagery.

The rational method calculations have been completed in accordance with the recommendations of
the Queensland Urban Drainage Manual (QUDM). These calculations are presented in Appendix D, with
the results summarised in Table 1 below.
                              Property                                                                             Value
        Average Recurrence Interval (Years)                                     2              5              10            20            50      100

                   Site Pre-Development                                      0.013          0. 018         0.022          0.026          0.033   0.038
                   Site-Post Development                                     0.016          0.023          0.028          0.033          0.042   0.045
                                                        Table 1 – Rational Method Flows

The DRAINS ILSAX computer model was used to calculate the pre-development and post-development
condition flows to compare against the calculated rational method peak flows. Runoff flows for Average
Recurrence Intervals of 2, 5, 10, 20, 50 and 100 years and durations of 5 minutes to 120 minutes.
                              Property                                                                             Value
        Average Recurrence Interval (Years)                                     2              5              10            20            50      100

                   Site Pre-Development                                      0.014          0.018          0.023          0.026          0.031   0.034
                   Site-Post Development                                     0.018          0.024          0.028          0.033          0.038   0.043
                                                          Table 2 – DRAINS ILSAX Flows

The peak flows calculated using the rational method and the DRAINS ILSAX model are considered to be
generally comparable. The variances in the result are as expected and are within tolerance, given the
differences between the DRAINS ILSAX model and the rational method. The flows calculated using the
DRAINS ILSAX model are considered more conservative and therefore it is considered appropriate these
flows are adopted for the purposes of detention tank analysis.




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Inspired engineering

22 May 2025
https://mpncnetau.sharepoint.com/sites/Administration/Shared Documents/10000-10999/10200-10299/10256/Report/22-05-25 SWMP_Issue A.docx