Reference Document MCU22/0102 Cubo Consulting Letter Final

23122_LO_01 – 34 Elm St, Cooroy – King Group


The total area within the property boundaries is 2014m2.

Proposed Development

A carwash and takeaway food outlet is proposed for the site.

A 2m wide road widening easement on the northern boundary removes 80.4m2 from the
finished site, and further council conditioned publicly accessible footway on the eastern
boundary next to the bowling club removes 100.6m2 from the total site area, leaving a final
effective total site area of 1,837m2.

845m2 of roof area is to be directed to 60kL of on-site rainwater tank capacity, with the
overflow and surrounding paved areas to be directed to council’s piped stormwater system
in Opal Street.

Rainfall

It is likely that the existing street storm water system was designed pre-2016, and so was
designed using the ARR87 IFD rainfall rates to have sufficient capacity for the then existing
motel and other upstream sites.

The new development design uses the ARR2016 IFD rates and ARR2019 methodology to
determine peak flowrates from the site. Fortunately, for the sake of comparison between pre
and post development, for larger events the ARR87 rates for this location are very similar to
the ARR2016 rates.

The table below shows the comparable rainfall rates for event of 5 minutes duration.


 ARI                      1          2             5            10           20           50          100
 ARR87 mm/h 5            123        156           194          215          245          285          315
 mins
 AEP                    0.632       50%          20%           10%          5%           2%           1%
 ARR2016 mm/h 5          120        135          183           215          245          285          315
 min

Noosa Council have also requested that climate change be taken into account for the design,
in this case described as a likely 20% increase on ARR2016 rainfall intensity rates by the
year 2100.

Onsite Detention

An on-site detention arrangement is proposed to ensure peak flowrates from the site do not
exceed pre-development peak flowrates.

A revised stormwater drainage design proposes to provide 3x 20kL linked rainwater tanks
collecting roof water, with the top portions of the tanks functioning as OSD.

Keeping the invert of the OSD system 150mm above the invert of the adjacent gutter
(108.28m AHD at the driveway in Opal St) and allowing for 500mm cover above the tanks
(min FFL 109.35m AHD), a height of 420mm is available for detention.

Based on the drawings provided by TMC, each tank has an area of 8.5m2, resulting in an
available OSD volume of 3 x 0.42 x 8.5 = 10.75m3.

This leaves 49m3 of storage volume for reuse on site in the car washing process.




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