Stormwater management plan 20220156 9 Kauri St SWMP

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
The proposed development will result in an increase in peak flows from the site due to the significant change in
impervious area in the developed state. Hydrologic and hydraulic 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 1EY, 50%, 0.2EY, 10%, 5%, 2% and 1% Annual
Exceedance Probability (AEP) for durations from 1 minutes to 12 hours to determine the peak flow rates under
varying storm conditions. Refer to Appendix B DRAINS Modelling Parameters and 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 south east, therefore the model has been prepared using a single node to represent the overall
conditions of the site. The model used to represent the development is shown below in Figure 3.1 – DRAINS model
layout.




                                                     Figure 3.1 – DRAINS model layout

The existing site’s time of concentration is calculated by the software using the kinematic wave equation based on
flow path characteristics measured from the site survey, with a minimum time of concentration of 5 minutes for the
site. Due to the reduced travel time and typically piped drainage conditions of the developed site, a time of
concentration of 5 minutes has been conservatively assumed to represent the site. 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 20220156
File RECOR Stormwater Management Report - 20220156.docx, 2022-08-08 Revision 0                                Page 8 of 26

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