Appendix C - Stormwater Management Plan

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
An assessment of the existing and proposed site hydrological conditions has been carried out to determine the
impacts of the development upon peak flows from the site. The site has been modelled using the DRAINS software
to determine peak flows under both scenarios in order to identify any attenuation requirements for the site.

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.

To assess the peak flows arising from the development, the site has been broken into the following catchment area

     •    Pre-development

               o     Existing Roof – Piped drainage from existing building at west of site to pit

               o     Existing Car Park – Overland surface flows from car parking and adjacent shed outlet to pit

               o     Existing Undeveloped – The remaining pervious areas bypassing the stormwater pit

     •    Post Development

               o     Existing roof – unchanged

               o     Existing car park – unchanged

               o     Smokehouse roof – Piped drainage from proposed roof to pit

               o     Remaining Area – typically pervious area bypassing pit

The configuration of the catchment areas for pre-development and post-development areas are shown in Figure 3.1
– DRAINS Pre-development catchments and Figure 3.2 – DRAINS Post-development catchments respectively, and
the existing and proposed DRAINS layouts are shown in Figure 3.3 – DRAINS Pre-development Layout and Figure
3.4 – DRAINS Post-development Layout.




                                            Figure 3.1 – DRAINS Pre-development catchments




Project number 20220213
File 20220213 Reserve St SWMP.docx, 2022-09-19 Revision 1                                               Page 7 of 16

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