20220218 22 Grant SWMP · MCU22/0172

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
As the development proposes to increase the impervious area of the site by approximately 177m² (from 257m² to
434m²), there is likely to be an increase in peak flows from the site. 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 depths 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 24 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 impervious areas and composition of each site has been determined using survey data and architectural
designs. The configuration of the catchment areas for pre-development and post-development conditions 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



Project number 20220218                                                                                       Page 7 of 14
File 20220218 22 Grant SWMP.docx, 2022-09-15 Revision 1

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