5179Flood Report_REVISION_2_7Feb2021

                                                                                              Callaghan & Toth



3.3        Model Determination Of The Required Detention Volume

The hydrologic modelling process at this Lot Reconfiguration stage has determined only the required
detention volume and has not calculated the exact dimensions of the outlet control works that would
accompany that volume.

The required detention volume was determined by adding a tabulation of storage-discharge pairs to the
model of the post-development condition and then successively adjusting the storage values in that
tabulation until the peak outflow rates exactly matched those from the model of the pre-development
condition, for ARI’s from 1 year to 100 year. The result is summarised in Table2, below. The required 100
year storage requirement is predicted to be 398 cubic metres, or just over the middle of the range of the
broad-brush assessment. This includes 128 cubic metres for no-worsening of 1 Year peak discharges to
the downstream waterway, as required for downstream waterway stability.


Table 2:       Required Detention Volumes

     ARI             Pre-Dev. Outflow             Post-Dev. Outflow (m3/sec)        Storage Volume
   (years)               (m3/sec)                                                    Required (m3)
                                                  No Basin         With Basin

     100                     0.93                  1.06               0.93                 398
      20                     0.62                  0.74               0.62                 285
       5                     0.43                  0.53               0.43                 216
       1                     0.16                  0.21               0.16                 128




3.4        Provision of Required Detention Volume
The required 100 year detention volume of 398 cubic metres cannot be achieved in a single storage
facility without displacing protected vegetation or depreciating the lot yield or pedestrian access to the
creek. The volume is therefore proposed to be provided in two facilities, comprising an open bio/detention
basin with 384 cubic metres of storage above the extended bio-retention depth and an oversized
stormwater pipe with 22 cubic metres of nett detention capacity, as shown in Figure 3.2 and in Figure
3.3 hereafter.

The 100 year water depth in the basin, above the top of the bio-sand, is expected to be 1.25 metres, or
1.05 metres above the extended bio-retention depth, which will provide 0.25 metre freeboard to the crest
of the embankment. The freeboard is approximately 25 percent of the water depth above the extended
bio-retention level and the basin should therefore be able to contain the inflows from a greenhouse-
induced 20 percent increase in storm rainfall intensities. The nett capacity of the oversized pipe (1050
mm diameter) reflects the part-full HGL along part of the length of that pipe and excludes the volume of
what would otherwise be a 600 mm diameter pipe.




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2035 2 Pearsons Road – Flood Study – Revision 2                                    Feburary 2021