Attachment C - Stormwater report

Figure 9 – A Comparison of the Runoff Volumes for the Pre- (left) and Post-Development (right) Scenarios.


5.6.         Regional Flooding
A hydrological model was developed by Hydrology & Water Management Consulting for the 1% AEP
Duration Independent Storm (DIS) event. The hydrological model indicates a peak flow rate of
approximately 470 m3/s is the critical design flow rate for the catchment at the site boundary. The design
hydrograph was utilised by Water Technology in a flood model to determine the peak water levels and
inundation extent for the site. The hydrographs of the 1% AEP design event for both the site and the
catchment as a whole are shown in Figure 10. The peak flow rate from the farm site represents a little
more than 1% of the peak flow in the whole catchment and therefore does not make a significant
contribution.

The 1% AEP DIS event hydrograph rises for approximately 8 hours before reaching the peak flow rate (and
therefore peak water level at the site). Analysis of the site hydrology reveals the critical storm duration is
the 180 minute storm. As illustrated by Figure 10, the critical hydrograph for the site has entered the
receiving waters and left the site well before the peak flow rate associated with the larger catchment
reaches the site. As a consequence, the increase in runoff volume associated with the greenhouse roof has
no impact on the peak water level for flooding associated with the larger catchment. It is therefore
considered that no adverse impacts associated with flooding will occur for neighbouring properties due to
the construction of the greenhouse.




Stormwater Management Plan – Lot 1 on SP108094, 305 Lake Macdonald Drive                                    Page 24

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