20220206 48 Elm St SWMP

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
The proposed development will result in a small increase in peak flows from the site due to an increase in impervious
area. Hydrologic modelling of the site has been carried out using the DRAINS software to assess the impact on peak
flows by comparing the existing site conditions to the proposed development.

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 0.5EY, 10%, 5%, 2% and 1% Annual Exceedance
Probability (AEP) for durations from 1 minutes to 3 hours to determine the peak flow rates under varying storm
conditions. Refer to Appendix B DRAINS Modelling Results for details of the parameters used and summary of the
results from the modelling.

The existing and developed states can effectively be represented as a single aggregate catchment discharging
towards the northeast, therefore the model has been prepared using a single node to represent the overall
conditions of the site. A developed model for an attenuated scenario has also been prepared which separates the
area of the site draining to the internal drainage network from the ground areas bypassing the system. The DRAINS
model used to represent the development is shown below in Figure 3.1 – DRAINS model layout.




                                                      Figure 3.1 – DRAINS model layout

The site’s time of concentration has been nominated conservatively as 5 minutes due to the small site area and high
existing impervious areas. The configuration of the catchment areas for pre-development and post-development
conditions are shown in Figure 3.2 – DRAINS Pre-development catchment and Figure 3.3 – DRAINS Post-
development catchment respectively.




Project number 20220206                                                                                 Page 7 of 19
File 20220206 48 Elm St SWMP, 2023-02-03 Revision 1

P +61 7 5211 1003             A 75 Noosa Drive, Noosa Heads Qld 4567       E [email protected]    recorconsulting.com