Att8 - SWMP

 STORMWATER QUANTITY MANAGEMENT

     5.1. Site run-off assessment
The proposed development site and its catchments are large rural allotments with predominately pervious
catchments. Relative to the size of the catchments, the change in impervious surfaces is expected to have minimal
impact.

The site has been modelled using the DRAINS software to assess the peak flow hydrology off the site from the
proposed development areas. The contribution of upstream catchments being conveyed through the development
has also been modelled to determine the impact of the development relative to the total catchment.

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 24 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 20% increase to rainfall intensities
has been applied to account for future climate change impacts upon the variability of rainfalls.

A simplified catchment has been modelled containing the watershed for each catchment, with impervious areas
based on survey and the proposed designs. Time of concentrations for each catchment have been determined in
accordance with QUDM typically made up of a sheet flow component for the upper catchment and modified stream
velocity method through the lower catchment. Details of the parameters used are provided in Appendix B - DRAINS
Modelling Results.

Results of the modelling for Catchments B and C1 are presented below in Table 5.1 and Table 5.2 below, as these
are the catchments changed as a result of the development.

Table 5.1 Catchment B Modelling Results

                                                           Catchment B
          A.E.P                                                                                                   Qchange (%)
                               Qexisting (m³/s)           Qproposed (m³/s)           Qchange (m³/s)
                      1EY                     5.23                            5.21                -0.02                   -0.4%
                     50%                      6.16                            6.17                 0.01                    0.2%
                    0.2 EY                    9.98                       10.10                     0.12                    1.2%
                     10%                     11.98                       12.05                     0.07                    0.6%
                      5%                     14.20                       14.20                         0                   0.0%
                      2%                     17.50                       17.50                         0                   0.0%
                      1%                     20.38                       20.33                    -0.05                   -0.3%




Project number 20220146                                                                                                    Page 13 of 20
File RECOR Stormwater Management Plan- 20220146.docx, 2022-09-06 Revision 0

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