App C - Engineering Report · MCU25/0118

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Stormwater Management
The existing site has been developed in accordance with council’s approved development
OPW/25/0057. This development provides a stormwater outfall and water quality treatment for the site.
The minor storm event is defined as the 10% AEP storm plus climate change, with the major event
being the 1% AEP storm plus climate change.

4.1     Drainage Network & Legal Point of Discharge
Runoff from within the subject site is to be captured by a proposed underground pipe drainage network,
and is to be conveyed to an future stormwater pit (provided by the surrounding subdivision), which shall
form the Legal Point of Discharge for the development. The proposed and future surrounding network
(as per OPW/25/0057) is shown in in the civil engineering concept plans, refer Appendi A.

4.2     External Catchments
The site is not impacted by external drainage catchments, as per the approved Operational Works
drawings for the surrounding development (OPW/25/0057). A future cutoff drain shall protect the site
from runoff from the west, and the neighbouring lot to the south predominately grades towards the new
subdivision road. A new cut-off drain along the southern boundary will be provided to redirect any minor
flows from the south, as per the civil engineering concept plans (refer Appendix A).

4.3     Flooding
The site is not impacted by flooding, and is not covered by Council’s Flood Hazard Overlay.

4.4     Stormwater Quality Management
Stormwater quality is to be provided by a future bio-retention basin directly north of the subject site. The
bio-retention basin is to be built as a part of the surrounding subdivision works (refer OPW/25/0057)
and has been sized to cater for the fully developed catchment. As such, no additional stormwater quality
is proposed for the development.
4.4.1 Construction Phase Stormwater Quality Management
Stormwater quality management during the construction phase is critical to ensuring water quality
standards are achieved during the full life of the development. Implementing best practice Erosion and
Sediment Control techniques is imperative to managing the runoff quality affected by construction
works. Appendix B outlines best practice measures for stormwater quality during construction.

4.5     Stormwater Quantity Management
As per the approved Engineering Services Report for the surrounding subdivision (OPW/25/0057),
stormwater quantity management is to be provided on a lot by lot basis. As such, on-site stormwater
detention in the form of an OSD Tank shall be required for the proposed development.
A DRAINS model will be developed prior to OPW approval that will nominate a specific OSD tank size
and orifice arrangement capable of mitigating developed peak flows across a range of AEP storm
events back to existing levels.
For the purposes of MCU approval, an indicative OSD tank size of 30kL is nominated for the
development based on a rate of 90kL per hectare (typically accepted value for urban catchments in
South East Queensland). All roof and hard stand areas will be directed to the tank for on site flow
attenuation.
In addition to OSD storage, an indicative minimum rainwater resuse volume of 3,000L is required for
each dwelling, in accordance with the Sustainable Buliding Desing Code. A water balance equation wil
be completed prior to an OPW application to confirm the final volume to stormwater re-use for each
dwelling.


31 July 2025