RFI - Attachment 3 - Stormwater Management Plan

 STORMWATER QUANTITY MANAGEMENT

     5.1. Site Discharge Impacts
The proposed development will result in an increase to the total impervious area of the site that will result in
increased run-off generation compared to existing conditions. An assessment of the existing and proposed peak
flow hydrology has been carried out to determine the impacts of the development upon the downstream waterways.

The site has been modelled using the DRAINS software to assess the peak flow hydrology off the site from the
existing and proposed site. A simplified catchment has been modelled for both existing and developed sites
containing the watershed for the development area as a single catchment, with impervious areas based on aerial
imagery for the existing site, and the proposed design layout for the developed scenario. No attenuation has been
modelled from either the rainwater tanks nor the bioretention area proposed for the developed site to conservatively
assess peak run-off under saturated conditions.

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. A climate change factor multiplier of 1.2 has been applied to
the design storm intensities to simulate conditions under the predicted 2100 climate conditions.

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. Details of the parameters used are
provided in Appendix B - DRAINS Modelling Results.

A time of concentration for the pre-developed catchment has been determined based on overland sheet flows from
the top of catchment, with an additional 2 minutes travel time to allow for roof drainage to arrive at ground level.
The time of concentration for the fully developed site has been determined based on a standard inlet time of 13
minutes (Urban residential areas with land slopes of 3 to 6% at the top of catchment), and subsequent pipe travel
time to the outlet into the bioretention area. Due to the increased impervious area and predominately piped
drainage, this has resulted in a reduction to the overall time of concentration for the catchment. The time of
concentrations have been assessed as

     -    Tc (Existing site) – 22 minutes

     -    Tc (Fully developed site) – 15 minutes

The configuration of the catchment areas for pre-development and post development areas are shown in Figure 5.1
– DRAINS Pre-development catchments and Figure 5.2 – DRAINS Post-development catchments respectively.




                                           Figure 5.1 – DRAINS Pre-development catchments




Project number 21020                                                                                       Page 27 of 36
File RECOR Concept Stormwater Management Report - 21020.docx, 2022-09-05
Revision 0


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