Reference Document MCU22-0102 Cubo Consulting Letter Final

23122_LO_01 – 34 Elm St, Cooroy – King Group


Modelling

A model of the site was prepared using DRAINS Version 2023.11.8718.30178, with site
specific rainfall data published by the BoM.

Catchments used the Horton method with post-development catchments based on the
design drawings by TMC, which can be summarised as:

    •     845m2 of roof area, 100% impervious
    •     718m2 of pavement, 100% impervious
    •     274m2 of landscaping, paved seating area, and small driveway areas, 38%
          impervious

All roof areas are directed to the rainwater/OSD tanks, which discharge into the site drainage
network that also collects the pavement areas, directs them through the treatment train, then
discharges into the piped stormwater network in Opal St.

The various landscaping, paved seating areas, and small driveway areas outside the site
network are assumed to drain uncontrolled across the boundary.

The rainwater tanks were assumed to be full to the OSD outlet invert, with outflow controlled
by a 105mm orifice on the outlet of Pit 1.

Two further catchments were added to model the natural state – 1,857m2, 0% impervious –
and the previous development – 1,857m2, 77% impervious.

The model was run for several events from 1EY to 1% AEP, for 5 minutes duration to 2
hours.

Results

The table below shows the peak flow rates for each event from the site for the natural state,
previous development, and proposed arrangement.

 Event       Natural State       Previous           Post               Comment
             (L/s)               Development        Development
                                 (L/s)              (L/s)
 1EY                       37             47                 33                Developed flows less than
                                                                                     predeveloped flows
       50%                 40                 54                  37           Developed flows less than
                                                                                     predeveloped flows
       20%                 63                 78                  53           Developed flows less than
                                                                                     predeveloped flows
       10%                 79                 94                  61           Developed flows less than
                                                                                     predeveloped flows
        5%                 93                109                  68           Developed flows less than
                                                                                     predeveloped flows
        2%               108                 130                  78           Developed flows less than
                                                                                     predeveloped flows
        1%               121                 146                  83           Developed flows less than
                                                                                     predeveloped flows
 1% x 1.2                148                 178                  97           Developed flows less than
                                                                                     predeveloped flows
DRAINS output for the model is attached.
The drainage design for the site can be further refined at the detailed design stage.




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