Sealed Judgement

 ARI Event                                            Peak Discharge (m3/s)

                                      Pre-development                   Post-development
                                                                   Incorporating Detention Tank
                                           (60min)                              (25min)

 1%                                          0.280                                0.179
                                           (60min)                              (25min)

 1% plus Climate Change                      0.360                                0.230
                                           (60min)                              (25min)


3.5.3         Concept Design of System

Although subject to detailed design, this section provides an overview of the management of
stormwater runoff from the site.

Runoff from roof areas will drain to rainwater tanks. Part of the volume of the tanks will be allocated
for detention. The detention component will be drained via a pipe to the bioretention system.

Figure 3-4 (an excerpt from ADG Drawing 22366-SK03 Revision 5) shows the potential location of
the pipe, noting that the final route would be determined as part of detailed design.

The bioretention system proposed to treat runoff (refer Section 4) will be located at existing ground
level. Based on detailed survey, the existing ground level is about 2.2 m AHD. It is proposed to set
the level of the basin at about 2.5 mAHD.

A bund will be constructed around the basin to provide for the extended detention of water above
the basin. For a 300mm depth, the bund would be at a level of 2.8 mAHD, resulting in an overall
bund height 600 mm above the existing ground surface.

The peak flow to the bioretention basin is 0.24 m 3/s for the 100-year ARI event. Given the low
quantum of the flow, it is not considered necessary to divert high flows away from the basin.

It is proposed to discharge this flow via a level spreader set equal to the level of the extended
detention depth.

Although considered to be a subject for detailed design, a 8-metre-wide level spreader would be
associated with a velocity of 0.5 m/s. It is considered that such a velocity will not give rise to scour
and that the downstream side of the bund can be adequately protected by vegetation.

The bioretention basin filter media is proposed to be 400 mm thick. This allows the design to either
allow for infiltration to the surrounding ground based on the permeability of the soil or the use of a
transition layer and drainage layer to drain the basin (a 100 mm transition layer and 200 mm
drainage layer would provide drainage pipes with an invert level of 1.8 mAHD which can drain to
natural surface).

Figure 3-4 and Figure 3-5 (excerpts from ADG Drawing 22366-SK03 Revision 5) illustrate indicative
layout and cross-section of the proposed bioretention basin and level spreader.




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