Att 2 SWMP

 STORMWATER QUANTITY

     3.1. Pre-Post Comparison
The site conditions were modelled using the Watershed Bounded Network Model (WBNM) software
in combination with Storm Injector. This combination allows the site to be modelled to determine the
critical storm events for the existing site and the proposed development to assess whether there is
an increase to the peak run-off and if so, to determine the level of stormwater detention required to
ensure peak outflows from the site are not increased.

The models have been run using the ensemble of storm events for 63.2%, 50%, 20%, 10%, 5%, 2%
and 1% Annual Exceedance Probability (AEP) for durations from 1 minutes to 12 hours to
determine the critical storm duration and the mean peak flowrate from the storms. The parameters
obtained from the Australian Rainfall and Runoff (ARR) Data Hub for the location and the rainfall
data obtained from the Bureau of Meteorology have been used to generate the storms.

As identified in the Noosa Shire Council Works Code, climate change impacts are to be accounted
for in the stormwater modelling. Discussions with Council have indicated that a 20% increase to the
Bureau of Meteorology rainfall data will provide an acceptable allowance, therefore the rainfall data
has been increased by 20% for all storm events to account for this.

Both the existing and developed site have been modelled as a single catchment representing the full
site with the actual impervious fractions determined from the site survey and the proposed design
plans. A catchment lag parameter of 1.6 has been applied along with a non-linearity parameter of
0.77 as recommended by the ARR Guide to Flood Estimation Book 7 Application of Catchment
Modelling Systems. The modelling parameters and rainfall intensities used are provided in Appendix
B WBNM Modelling Parameters.

The hydrographs and box plots of the different storm events for the critical storms for the existing
site and the proposed development site based on this modelling are provided in Figure 3.1 – Pre-
development site critical storm rainfall hydrographs showing peak flow rates and Figure 3.2 – Post-
development site critical storm rainfall hydrographs showing peak flow rates.




Project number 20210121
File RECOR Preliminary Stormwater Management Report - 20210121.docx, 2021-10-07                                 Page 9 of 26
Revision 0

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