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You can find out more about Design Rainfall and IFD here.
Observations for this analysis :
Maximum observed rainfall by duration, estimated return period and AEP
Duration
Rainfall
Start
End
Years
AEP
60m
7.0 mm
1:00pm 10/08/2026
2:00pm 10/08/2026
1 in 1 years
100.0 %
120m
12.4 mm
1:00pm 10/08/2026
3:00pm 10/08/2026
1 in 1.0 years
99.54 %
3h
17.0 mm
12:00pm 10/08/2026
3:00pm 10/08/2026
1 in 1.0 years
97.82 %
6h
27.2 mm
10:00am 10/08/2026
4:00pm 10/08/2026
1 in 1.1 years
87.76 %
9h
31.4 mm
9:00am 10/08/2026
6:00pm 10/08/2026
1 in 1.1 years
88.52 %
12h
36.0 mm
2:00am 10/08/2026
2:00pm 10/08/2026
1 in 1.2 years
86.8 %
18h
52.0 mm
12:00am 10/08/2026
6:00pm 10/08/2026
1 in 1.6 years
61.54 %
24h
57.8 mm
4:00pm 09/08/2026
4:00pm 10/08/2026
1 in 1.6 years
60.73 %
30h
66.6 mm
10:00am 09/08/2026
4:00pm 10/08/2026
1 in 1.9 years
52.09 %
36h
70.4 mm
6:00am 09/08/2026
6:00pm 10/08/2026
1 in 1.9 years
52.3 %
Note: A '1 in 1.0 years' estimate means that it is almost certain to occur at least once in a year, and could occur much more than once per year on average. For smaller values like this the Annual Recurrance Interval (ARI) may be of more relevance, but it is complex to calculate for common events.
IFD describes how rainfall intensity relates to how long it lasts and how often it happens. It is the basis for design rainfall used in engineering and flood studies.
- Duration: the length of the rainfall burst, from a few minutes to several days.
- AEP (Annual Exceedance Probability): the chance, in any one year, of a rainfall of that depth or greater. A 1% AEP event has a 1 in 100 chance each year - what is often loosely called a '1 in 100 year' event.
- The curves show design rainfall depths for a range of AEPs. The observed maximums for this location and period are plotted against them so you can see how rare the event was.
Design rainfall data is sourced from the Bureau of Meteorology.
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