Appendix C - Flood Report_Sept2020

                                                                                                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. The result is summarised in Table 2, 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.


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
351 m cubic metres and an EcoAid underground facility with 47 m cubic metres, as shown in Figure 3.3,
hereafter.

The 100 year water depth in the basin, above the top of the sand, is expected to be 1.05 metres, which will
provide for a weir overflow depth of 0.15 metres and 0.2 metre freeboard to the crest of the embankment.
The freeboard is approximately 20 percent of the water depth and the basin should therefore be able to
contain the inflows from a greenhouse-induced 20 percent increase in storm rainfall intensities.




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2035 2 Pearsons Road – Flood Study                                                  September 2020