2018-05-14 GC Agenda Attachment 1 to Item 6 Lake MacDonald Dr Community Reference Group Investigations Report

above the 1 in 100 year ARI design storm level. Another example may be a small road crossing
may be required to be designed to be drivable for a 1 in 10 year ARI design storm event.

More recently, the flooding industry has moved away from adopting this type of terminology to
describe flood events, as it was found to be potentially misinterpreted that once an event of a
particular size (i.e. 1 in 10 year event) had occurred it would not occur again for that period of time.
This is not correct. To address this, the industry now prefers to describe these events by the
percentage (%) chance of the event occurring in any given year. This is referred to as the Annual
Exceedance Probability (AEP). Therefore; in general terms a 1 in 100 year event may be described
as having a 1% AEP which is a 1% chance of the event occurring in any given year. Similarly a 1 in
10 year event has a 10% AEP i.e. a 10% chance of occurring in any given year and hence a 1 in 1
year event has a 100% AEP as it is expected to happen at least once a year.

The upgraded Six Mile Creek Flood Study will provide a new suite of design flood levels for the
main floodplains and waterways of Six Mile Creek, including Cooroy Creek and Cooroora Creek
(Pomona).




Lake McDonald Flooding Community Reference Group Report                                        Page 10