08/1128 - Response to Information Request [As Published] · 132008.1128

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29. Sealing bio-retention basin is not considered necessary. Stormwater
is treated through infiltration in the bio-retention basin to meet water quality
objectives. Thus deep seepage may occur with minimal environmental
disruption and act to minimise the ecological impact of the proposed
development.

30. Indicative construction checklists and maintenance schedules were
extracted from the Healthy Waterways Guidelines. Please refer to attached
copy of these Guidelines. Also attached are the Bioretention Construction
and Bioretention Maintenance Guidelines also extracted from the Healthy
Waterways Guidelines dated 1% June 2006.

31, The natural surface contours were used to determine the pre-
developed peak stormwater discharges. The estimated time of concentration
was 18 minutes for Lot 6 and 22 minutes for Lot 1. The time of concentration
is reduced to 8 minutes, post-development. The resulting increases in rainfall
intensities were used for design purposes.

Rainwater tanks were assumed to be full at the onset of critical storms, for
worst-case scenario assessment.

32. Stormwater modelling was reviewed. The areas used for calculation
were as reported in Section 2.6.2. The areas reported on Dwg 2383-01 SK01
are slightly larger due to variation in alignment of catchment boundary,
including areas which are not impacted by the development.

33. The freeboard requirements were determined in accordance with the
Queensland Urban Drainage Manual (QUDM) Table 5.06.1.

34. The anticipated pH range for treated stormwater is 4 — 6.5. It is
proposed that water quality monitoring be undertaken as part of regular
maintenance schedule, ongoing maintenance be undertaken as required to
ensure the desired treatment functionality is maintained.

35. MUSIC modelling has been undertaken in accordance with the Gold
Coast City Council MUSIC modelling guidelines. This provides the best
available assessment of water quality objectives. Further review of analyses
and RPEQ certification will be undertaken as part of the Operational Works
application.

36. Stormwater discharge is proposed to be treated through the use of
rainwater tanks, multiple gross pollutant traps and the bio-retention basins.

37. Road network forms major overland drainage channels. Suitable
overland channels are proposed where major flow paths not confined to
roadway. Overland flow paths were included in the XPSWMM model, and set
to convey major flow when the minor piped drainage system became
overloaded.

38. At initiation of overflow to National Park, depth of flow in channel is
4.0m. Cross sectional area is 3.0m?.

Gil