20210041 27 Bottlebrush Ave Stormwater

            3. PROPOSED DEVELOPMENT
                    3. 1. P R O P O S E D D E V E L O P M E N T D R A I N A G E P A T T E R N

            The proposed development increases the impervious area of the site by approximately
            167m² (from 223 m2 to approximately 390m²).

            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
            allowing for a climate change factor of an additional 20% increase to rainfall depths to
            account for future climate change patterns. Due to the small nature of the development, a
            minimum time of concentration of 5 minutes has been used for each contributing area.

            The model used for DRAINS for the site is shown below in Figure 3.1 – DRAINS model
            Layout. A single pre-development catchment represents the existing site, whereas the
            developed site is represented by the sum of the ground catchment and the roof catchment,
            allowing the detention tank to be introduced to analyse the mitigated developed site
            conditions.




                                                     Figure 3.1 – DRAINS model Layout

            The configuration of the catchment areas for pre-development and post development areas
            are shown in Figure 3.2 – DRAINS Pre-development catchments and Figure 3.3 – DRAINS
            Post-development catchments respectively. The developed flow rates were determined
            from the sum of flow values at the source catchments prior to any impacts from the
            attenuated model.




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