5179Flood Report_REV_3

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     Callaghan & Toth




3.0              STORMWATER DETENTION REQUIREMENT


3.1              Broad-Brush Estimate Of Required Detention Volume

The proposed reconfiguration of the site, seen in Figure 3.2, will comprise 16 lots and an internal road
and a park. Lot 16 will retain its existing structures and other paved areas, thereby having no impact on
the site’s stormwater runoff. The remaining lots 1 to 15 and the internal road, will have a combined area
of 1.34 hectares. The required 100-year detention volume is expected to be in the order of 250 to 300
cubic metres for each hectare of a development platform of this lot density. For the abovementioned 1.34
hectare platform, this equates to a 100 year detention volume in the order of 335 to 402 cubic metres.

3.2              Hydrologic Model Set Up
For the sake of simplicity at this RAL stage of the development application, the required 100-year
detention volume has been determined by URBS modelling of the pre-development and post-
development catchment conditions, using the ARR87 design storm rainfall data with a simple
initial/continuing loss model. A more complex modelling procedure at the Operational Works stage, using
the Bureau of Meteorology’s 2016 design rainfall data, could result in a slightly smaller detention volume.

The pre-development catchment condition and the relevant catchment area and model subareas are
shown in Figure 3.1. The modelled catchment includes the existing development to the south, which
features a 525 mm diameter bypass pipe with a full-flow bypass capacity of 0.46 m3/sec (or slightly more
than Q5) to conduct lower flows away from the proposed development site. The total catchment area in
the model is 3.27 hectares, including 1.47 hectares in the adjoining development to the south. The
modelled catchment includes the site of the proposed open detention basin but excludes the area of a
proposed park in the north-east corner of the site, which will not be noticeably impacted by the proposed
development.
                                                                                                                                                                                                                                                                          2

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                                                       6




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                                                    11
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                                                                          RL: 110.614
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                                                                                                                                                                                                                                                                                                                                                                               10
                                                                                                                                                                                                                                                                                                                                                                      106




                                                                                                                                                       #4
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                                     #1                                                                                                                                                                                                                                                                                                                                                                                                                                       #13
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     102




                                                                                                                                                                                                                                                                                                                                                                #6
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    104
        STREET




                                                                                                                                                                                                                                                                                                                                                                                                                       525 DIA
                                                                                                                                                                                                                                                                                                                                                                                                                               BYPAS
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              CT




                                                                                                                                                                                                                                                                                                                                                                                                                                    S                           , 0.46 M
                                                                                                                                                                                                                                                                                                                                                                                                                                                                        3/S CAP
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                ACITY
                                                                                                                                                                                                                                        #3                                                                                                                       #5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             REN
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       BLUE W




                                                                                      #2

                                                                                                                                                                                                                                                                                                                                                                     CATCHMENT BDY                                                                                                0                                SCALE                            50 m

Figure 3.1                                                                                                       URBS Model of the Pre-Development Catchment Condition

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             7
2035 2 Pearsons Road – Flood Study – Revision 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                            April 2021