LATE RFI - Revised SWMP

 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 therefore been modelled using the DRAINS
software to determine peak flows under both scenarios to identify any attenuation requirements for the site.

An assessment of the potential future development of both lots has been undertaken to determine the impact of an
increased impervious catchment on the peak flow conditions. Each future lot will be a low-density residential site,
with lot area of over 1,000m². In accordance with Table 4.5.1 of QUDM and based on assessment of the surrounding
land use, an impervious fraction of 40% of the future lot will be assumed to be developed. Of this, it is assumed 70%
will be roof area made up of the dwelling, sheds and garage, 20% will be the driveway accesses to the properties
and 10% will be other impervious ground area such as pools, patios and footpaths.

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.

A single catchment has been used to represent the existing and developed site conditions, with the areas for
impervious surfaces determined using survey data and architectural designs. The time of concentration for pre and
post development conditions has been calculated by DRAINS based on the estimated flow paths as follows.

     •    Pre-development

                o    EIA – 2 minute lag time for older roof drainage system + 26m overland flow at 3.5% on lawn

                o    RIA – 75m overland flow at 6.7% on gravel

                o    PA – 57m overland flow at 8.8% on lawn

     •    Post-development

                o    EIA – 1 minute lag time for new roof drainage system + 26m overland flow at 3.5% on lawn

                o    RIA – 50m overland flow at 8% on lawn

                o    PA – 50m overland flow at 8% on lawn

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




Project number 20220217
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File 20220217 Pavilion St SWMP Rev1.docx, 2023-04-19 Revision 1

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