Stormwater Treatment & Water Quality Management Plan_April2014

                                                                                  Callaghan & Toth


5.       FLOODING ASSESSMENT SUMMARY
The flood study for the site was undertaken by Brisbane Stormwater Management Pty Ltd
(BSM). The results of the flood study and assessment are summarised below.

5.1      Existing Conditions

The site is traversed by two tributaries of Six Mile Creek, described as the eastern and western
tributaries.

The site is subject to ponding of floodwaters from the adjacent watercourses and the Q100 year
ARI flood levels (see Drawing Number 1225-33). The proposed lots are above the Q100 year
inundation line. The new road from Blue Wren Court is partly below the Q100 inundation line.

The flood assessment from BSM (See Appendix 4) reports on the hydrologic and hydraulic
performance of the site including upstream and downstream catchments.

5.2      Proposed Drainage System

Some of the increased stormwater runoff from the proposed residential areas is proposed to be
mitigated by conventional detention volumes in two bio-retention basins, but most of the
increased runoff is proposed to be attenuated by an increase in the floodway storage on the
site. The increased floodway storage will be achieved by deliberately “roughening” the
floodways (by revegetating the riparian zones, as required by Council) and by constructing a
restrictive footbridge across the eastern tributary to back-up the flood profiles in that tributary
floodway.

There will also be some regrading of the invert of the western tributary and the removal of an
artificial hump in the eastern tributary channel to achieve some more storage of flood flows
during the 100yr events.

The road surface level is proposed to tbe drained to an underground drainage system capable
of taking Q5 ARI flows. The overflow will discharge to the Bioretention Basins / Swales where it
will attenuate flows to pre-development conditions.

The Bioretention Basins have been modelled to act as a detention/infiltration basin. The basin
size and weir size is sufficient to reduce the increase in runoff for Q1 to Q100 storms, back to
predevelopment levels as shown in Table 5.1.

         TABLE 5.1:         Pre-Development and Post Development Discharges at node 15-16

                               Q1         Q2            Q5                Q10             Q100
      Location
                              (m3/s)     (m3/s)        (m3/s)            (m3/s)           (m3/s)

Pre-Development                 6.8       8.4           13.8             17.3              26.0
Post Development                6.7       8.3           13.5             17.2              25.1




55 Lake MacDonald Drive – Stage 1 to 5                                                      pg. 21