The Victorian Floodplain Management Strategy requires all councils to identify overland stormwater flood risks associated with a 1 in 100 year Average Recurrence Interval (ARI) rainfall event.
We identified the need for the study in 2014 in our Integrated Water Management Strategy (IWMS), which sets the strategic direction for managing water over the next decade.
The study used radar imagery and Council asset information to produce a combined topographic and hydraulic model of surface levels.
Once this information was gathered, we used Design Rainfall, a document derived from Geoscience Australia’s endorsed document Australian Rainfall and Runoff, to create simulated models of stormwater flows resulting from various ‘Design Rainfall’ events.
‘Design Rainfall’ is described as: ‘…a probabilistic or statistically-based estimate of the likelihood of a specific rainfall depth being recorded at a particular location within a defined duration.’
While we are required to designate areas at risk of flooding due to a 1 in 100 year ARI rainfall event, we also applied the effects of rainfall with lesser and greater frequency to the computer model. The study therefore includes assessments of flood levels associated with extremely rare rainfall and has also been tested against the forecast effects of climate change.
Thus the adopted flood extent represents a worst case scenario for the effects of a 1 in 100 year ARI storm.
We have correlated our study results against the results from Melbourne Water’s flood studies to provide us with confidence in their reliability.
The Flood Mapping Study shows that a 1 in 100 year ARI storm will overload gutters, downpipes and pipe drains to the extent that excess stormwater will flow overland.
Flood depths across individual properties will vary. The areas identified by the study represent sustained flow depths in excess of 50mm associated with a 1 in 100 year ARI rainfall event.
It is important to note that the City of Boroondara Flood Mapping Project is essentially a risk assessment of properties that may be affected by flooding during a 1 in 100 year ARI storm event. The lack of historical record of flood events at a particular location does not necessarily mitigate the risks that have been identified by the study.