A6880 - Eng - June 2024

Calculation of peak flows for the site have been developed based on several variables;

- Catchment Areas and Fractions Impervious Areas - Pre Developed v Post Developed
- QUDM Tables – Frequency factors and C10 values for pervious v impervious areas
- Analysis of 10 separate Temporal Patterns (East Coast North Region) for each rainfall
event, with selection of the median temporal pattern used as the design temporal
pattern for each rainfall event as per ARR2019 guidelines
- IFD Design Rainfalls (with 20% increases applied for Climate Change)
- Storm Durations ranging from 5 to 120 minutes

Using BOMs Design Rainfall Data System, IFDs and Temporal Patterns were
downloaded and imported into the model. Refer to Appendix C for the IFD chart used
within the model.

The Bus Depot catchment has an area of 6,095m2 and an estimated conservative pre-
development fraction impervious of 0.8, (assumed C10 = 0.86).

Post-development 6,095m2 of the Bus Depot site will be directed to an underground
attenuation tank located beneath the bus access driveway.

Post development fraction impervious of the overall Bus Depot site will increase to 0.9
with a standard inlet time of 5 minutes adopted.

Using these parameters as inputs the excel spreadsheet is used to calculate and compare
pre and post-development flows for a range of storm from 5 to 120 minutes and a range
of ARI’s from 1 year to 100 years.

RFI tables and model results are included at Appendix C.

Table 5.3.1 below summarises results and demonstrates that non-worsening (maximum
outflow of 297m3/s) may be achieved with an attenuation storage volume of 203m3 and
a staged orifice plate. Refer engineering drawings at Appendic B for proposed tank and
Orifice Plate outlet configuration.

Emergency overflow discharge will be provided via weir overflow within the
attenuation tank catering for full orifice blockage under 1% AEP conditions.




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A6880/24/ENG/Noosaville                                                3
June   SH/pl
                                ALTA PROJECTS PTY LTD