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 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
As the development proposes to increase the impervious area of the site by approximately 119m² (from 247m² to
366m²), there is likely to be a minor increase in peak flows 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 6 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 model used to represent the development is shown below in Figure 3.1 – DRAINS model layout.




                                                      Figure 3.1 – DRAINS model layout

The configuration of the catchment areas for pre-development, post-development and a split post development (to
allow for roof water to an attenuation tank) are shown in Figure 3.2 – DRAINS Pre-development catchment, Figure
3.3 – DRAINS Post-development catchments and Figure 3.4 – DRAINS Split Post-development catchment
respectively.




Project number 20220130
File RECOR Preliminary Stormwater Management Report - 20220130.docx, 2022-05-19                                Page 8 of 16
Revision 1


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