20220237 73 Southern Cross Pde SWMP

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
As the development proposes to increase the impervious area of the site, there is likely to be an increase in peak
flows discharged from the site. The site has therefore 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 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.

Due to the small site area, the pre and post development conditions have been modelled each as a single catchment
made up of the impervious areas as measured from survey and the proposed architectural design. A minimum time
of concentration of 5 minutes has been nominated for all surfaces within the catchment. The configuration of the
catchment areas for pre-development, post-development and split post development for roofwater to an attenuation
tank 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




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Project number 20220237
File 20220237 73 Southern Cross Pde SWMP.docx, 2022-10-31 Revision 1
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