2160 IWMP Report_Jan2022

                                                                                Callaghan & Toth

The minimum carpark level is RL106.30, which is not affected by the Q100 flood level.

The applicant’s supplied flood certificate gives a minimum Q100 flood level of RL105.23 AHD
and a minimum floor level of RL106.7 AHD.

The MCU in 2013 detailed that the Bioretention basin has been modelled to act as a detention
basin. However, the increase in runoff from the additional roof area of 544m2 will be piped to
two (2) x 4,000 litre above-ground stormwater tanks which will attenuate the flow back to
predevelopment levels (see Figure 4.2). Runoff will then be directed to the biobasin which will
then treat the water to remove pollutants before releasing the stormwater into the existing gully
riparian zone.

HydroCAD was used to model a Q10 storm and Q100 storm back to predevelopment levels as
summarised in Table 6.1 and Appendix C.

This Material Change of Use application increases the roof area by 544m2 and as such, the
HydroCAD model has been updated to reflect this additional roof area. The hardstand areas
have not been reduced as a result of the building changes. The additional roof areas have been
modelled as footpath/landscape areas to ensure the same total square meterage is used in the
model.

TABLE 6.1:          DETENTION STORAGE – for 2022 MCU application
                                   PRE-             PRE-           POST            POST
     CATCHMENT                 DEVELOPMENT      DEVELOPMENT    DEVELOPMENT     DEVELOPMENT
                                   Q10              Q100            Q10            Q100
    Old Roof + New
     Roof (2022) +
                                0.13614m3/s      0.20514m3/s    0.13354m3/s      0.20421m3/s
    Footpaths/Land
         scape


7         CONCLUSIONS & RECOMMENDATIONS
The proposed increase in roof area for the new Kauri building can be treated by a treatment
train consisting of rainwater reuse tanks, bio-retention swale and bio-retention basin, which are
capable of removing the nutrients and heavy metals.

The existing car parking and new building can be built to flood free levels in accordance with
Council’s requirements.

The rainwater detention tanks and bioretention areas are able to attenuate the runoff
concentrations back to Pre Development levels and so the development will not have an impact
on the hydrology of the receiving waters.




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2160, Kabara Upgrade, Cooroy                  IWMP.doc                        January 2022