‘Overland stormwater flow’ and ‘1% Annual exceedance probability (AEP) rainfall’ are two key terms in the Flood Mapping Study.

Overland stormwater flow

‘Overland stormwater flow’ is commonly known as flash flooding, which occurs when extreme rainfall overloads gutters, downpipes and pipe drains and causes excess stormwater to flow over land - streets, laneways, and properties.  

This is very different from flooding that rises up from rivers and creeks. Areas that are at risk from rivers, creeks and main drains have previously been identified by Melbourne Water and are designated in the Boroondara Planning Scheme as a Special Building Overlay (SBO) or a Land Subject to Inundation Overlay (LSIO).

To find out if your property is already in one of these types of overlay:

  1. Visit VicPlan and search for the address of your property. 
  2. Follow the prompts to create and open a Planning Property Report.
  3. Read the report to see whether the property falls in any overlays.

1% Annual exceedance probability (AEP) rainfall 

1% Annual exceedance probability (AEP) rainfall is the preferred terminology used by the Bureau of Metrology and Melbourne Water for what was known as a 1 in 100 year Average recurrence interval (ARI) rainfall event.   

It refers to an infrequent high-intensity storm that has a 1% statistical probability of being exceeded in any one year.

This change in terminology is being adopted to emphasise the fact that such events are not cyclical. Therefore it is possible to have more than one 1% AEP occur in a given year or have several years go by without such severe rainfall.

It is also important to note that rainfall events are described by the intensity of the rainfall over a given duration. So a short duration high intensity rainfall event can have the same exceedance probability as a long duration low intensity storm.

It is generally the high intensity short duration storms that result in flash flooding within the City of Boroondara.


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