Swmp

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
The site has been modelled using the DRAINS software to assess the impact on peak flows, comparing the existing
site conditions to the proposed development.

In accordance with ARR2019, an Initial Loss-Continuing Loss (IL-CL) model has been used to assess the site against
the ensemble of storm temporal patterns for the East Coast North region. The loss parameters and temporal patterns
have been imported from the ARR Datahub, and rainfall intensities have been imported from the Bureau of
Meteorology IFD rainfall data allowing for a 20% increase to rainfall rates to account for future climate change
patterns.

The parameters obtained from the Australian Rainfall and Runoff (ARR) Data Hub for the location and the rainfall
IFD data obtained from the Bureau of Meteorology have been imported into DRAINS to generate the storm events.
The models have been run using the ensemble of storm events for 1EY, 50%, 0.2EY, 10%, 5%, 2% and 1% Annual
Exceedance Probability (AEP) for durations from 1 minutes to 12 hours to determine the critical storm duration and
the mean peak flowrate from the storms. Refer to Appendix B DRAINS Modelling Parameters and Results for details
of the parameters used and summary of the results from the modelling.

The configuration of the catchment areas for pre-development and post-development scenarios are shown in Figure
3.1 – DRAINS Pre-development catchment and Figure 3.2 – DRAINS Post-development catchments respectively.




                                           Figure 3.1 – DRAINS Pre-development catchment




                                          Figure 3.2 – DRAINS Post-development catchments


The results of the modelling against the range of design storms for the development under the existing and
proposed development conditions are shown in Table 3.1 Existing and proposed unmitigated site peak discharge
rates.

Project number 20230023                                                                                 Page 7 of 14
File 20230023 206 David Low Way SWMP.docx, 2023-10-17 Revision 1

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