Appendix C - Flood Report_Sept2020

                                                                                                  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 19 lots (18 new lots) and an
internal road and a park. Lot 19 will retain its existing structures and other paved areas, thereby having no
impact on the site’s stormwater runoff. The remaining lots, 1 to 18 and the internal road, will have a
combined area of 1.41 hectares.

The required 100 year detention volume is expected to be in the range 250 to 300 cubic metres for each
hectare of a development platform of this lot density. For the abovementioned 1.41 hectare platform, this
equates to a 100 year detention volume in the range 352 to 423 cubic metres.


3.2       Hydrologic Model Set Up

For the sake of simplicity at this Lot Reconfiguration 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, will most likely 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 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.

The total catchment area in the model is 3.3 hectares, consisting of 1.47 hectares in the adjoining
development to the south, 0.11 hectares of external property at the Pearsons Road frontage and 1.72
hectares within the site itself.




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2035 2 Pearsons Road – Flood Study                                                    September 2020