Stormwater Management Report · MCU22/0087.01

    STORMWATER QUANTITY MANAGEMENT

    3.3. Pre-post comparison
As development of a site may results in increase in peak stormwater discharges, an analysis of the existing conditions
has been carried out as well as a comparison against the proposed development conditions. The proposed
development will result in net increase to the impervious area of only 33m², with most of this increase being through
a larger roof area, therefore the analysis will quantify the impacts of this increase to the downstream receiving
environment.

The site has been modelled using the DRAINS software to assess the peak flow hydrology off the site. In accordance
with ARR2019, an Initial Loss-Continuing Loss (IL-CL) model has been used to assess the proposed development
against the ensemble of storm temporal patterns for the East Coast North region. The models have been run using
the ensemble of storm events for durations from 5 minutes to 6 hours to determine the critical storm duration and
the mean peak flowrate from the storms.

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 for the location. A climate change factor of 1.2 has
been applied to the rainfall data to account for the expected changes in rainfall under 2100 climate conditions. Due
to the small nature of the development, a time of concentration of a minimum 5 minutes has been considered for
the site. Details of these parameters are provided in Appendix A.

The model used for DRAINS for the site is shown below in Figure 0.1 – DRAINS Model Layout. A single catchment is
used to represent the existing site and assumed future site conditions, with comparison of these results used to
establish the impact of the development on the existing flow rates.




                                          Figure 0.1 – DRAINS Model Layout

The configuration of the catchment areas for pre-development and post development areas are shown in Figure 0.2
– DRAINS Pre-development catchments and Figure 0.3 – DRAINS Post-development catchments respectively.




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