A6880 - Eng - June 2024

1. STORMWATER QUANTITY - Include allowance for climate change within the
stormwater design by incorporating a rainfall intensity increase of 20%.


We note that the Peritas report included an on-site stormwater attenuation tank based
on the below premise as agreed with Council officers and set out in the below Response
to Information Request dated 22 February 2024 by Place Design Group.


Based on subsequent discussions with Council’s development assessment team, the existing
drainage information from Council’s GIS map shows the existing stormwater pipes along the
site’s eastern boundary originate from no. 5 Commerce Court and continue southward to Rene
Street and appear to serve as lawful discharge points for these lots.
The Updated Stormwater Management Plan prepared by Peritas Group provided in Attachment
3 has assessed the impacts the proposed development will have on peak flows discharging
downstream of the site. The design objective is to mitigate increased flow rates above the 10%
AEP storm event and/or the existing downstream pipe capacity to ensure a non-worsening of
peak flows discharging from the site. It is proposed to restrict 1% AEP flow from the site to 16%
of the existing flow capacity of the 900mm diameter pipe at the point of discharge at 5
Production Street. This represents the proportion of the project site against total contributing
catchment. An onsite detention tank is proposed with total storage volume of 150m3.



100year ARI flows from all bus depot catchments will be piped to a proposed
underground attenuation tank located at the site’s eastern boundary and discharging
attenuated flows to the existing stormwater network along the eastern boundary of the
site in similar fashion to the proposals of the Peritas report but sized with allowance for
climate change rainfall intensity increases of 20% as required by Council.

Outflow from the tank will be throttled through a stainless-steel orifice plate such that
peak post development discharges are limited to an agreed nominal maximum of 0.297
m3/s (16% of existing pipe flow capacity) as set out in the Peritas report.

Assessment of pre and post-development flows is carried out with the assistance of a
simple unit hydrograph program in excel spreadsheet format with algorithms based on
the Rational Method. This spreadsheet has been developed by Alta Projects and readily
accepted by Council for previous projects. The spreadsheet calculations are with
reference to;

- Queensland Urban Drainage Manual (QUDM)
- Australian Rainfall and Runoff 2019 (ARR2019)




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A6880/24/ENG/Noosaville                                                2
June   SH/pl
                                    ALTA PROJECTS PTY LTD