5179Flood Report_REV_3

                                                                                                  Callaghan & Toth



3.3        Model Determination Of The Required Detention Volume

The hydrologic modelling process at this RAL stage has determined only the required detention volumes
at various ARI’s from 1 to 100 years and has not calculated the exact dimensions of the outlet control
works that would accompany those volumes.

The required detention volumes were 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. model indicated that the adjusted storage on each data line produced the adjusted
basin outflow on that line and matched the pre-development discharges from the total catchment after
adding the unmitigated discharges from subarea #14. The resultant basin storages and total catchment
outflows for each ARI are summarised in Table2, below. The required 100-year storage requirement is
predicted to be 334 cubic metres, or almost exactly equal to the 335 cubic metre lower limit of the broad-
brush assessment in Section 3.1 of this report. The 334 cubic metres includes 128 cubic metres for no-
worsening of the 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                 334
      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 334 cubic metres is proposed to be provided partly in an open
offline detention basin and partly by backwatering the minor waterway upstream of the proposed footpath
crossing, as shown in Figure 3.3 on the following page of this report.

The detention volumes in Table 2 require the following detention water levels in Table 3 below, as
calculated in Item 8 of the calculations appended hereto. Those calculations allow for the detention
requirement being achieved over-and-above the normal (un-backwatered) flow volumes in the swale.


Table 3           Required Detention Water Levels

 ARI (years)          Req’d Volume (m3)           Req’d Water Level (m)
      100                    334                         101.26
      20                     243                         101.09
       5                     190                         100.98
       1                     128                         100.84

The required water levels can be controlled by a low-flow culvert of approximately 375 mm diameter at
the footpath crossing of the swale together with a weir outlet box in the basin for mid- to high-flows. A sag
in the footpath crossing can provide further high-flow control while also providing for emergency release
in higher flows or during debris blockage of the other outlets. These facilities have not been designed in
detail at this RAL stage of the application. A freeboard of at least 0.2 metre to the footpath will provide for
a greenhouse-induced 20 percent increase in storm rainfall intensities.




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2035 2 Pearsons Road – Flood Study – Revision 3                                           April 2021