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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
15m
7.0 mm
3:30am 02/08/2026
3:45am 02/08/2026
1 in 1 years
100.0 %
30m
9.0 mm
12:00am 02/08/2026
12:30am 02/08/2026
1 in 1 years
100.0 %
45m
11.0 mm
12:00am 02/08/2026
12:45am 02/08/2026
1 in 1 years
100.0 %
60m
12.0 mm
12:00am 02/08/2026
1:00am 02/08/2026
1 in 1 years
100.0 %
90m
17.0 mm
2:15am 02/08/2026
3:45am 02/08/2026
1 in 1 years
100.0 %
120m
21.0 mm
3:30am 02/08/2026
5:30am 02/08/2026
1 in 1 years
100.0 %
3h
30.0 mm
2:15am 02/08/2026
5:15am 02/08/2026
1 in 1.0 years
99.97 %
4h
34.0 mm
1:15am 02/08/2026
5:45am 02/08/2026
1 in 1.0 years
100.0 %
6h
46.0 mm
11:45pm 01/08/2026
5:45am 02/08/2026
1 in 1.0 years
99.78 %
9h
60.0 mm
8:15pm 01/08/2026
5:15am 02/08/2026
1 in 1.0 years
99.39 %
12h
69.0 mm
5:45pm 01/08/2026
5:45am 02/08/2026
1 in 1.0 years
99.37 %
18h
69.0 mm
11:45am 01/08/2026
5:45am 02/08/2026
1 in 1.0 years
99.92 %
24h
69.0 mm
5:45am 01/08/2026
5:45am 02/08/2026
1 in 1 years
100.0 %
30h
69.0 mm
11:45pm 31/07/2026
5:45am 02/08/2026
1 in 1 years
100.0 %
36h
69.0 mm
5:45pm 31/07/2026
5:45am 02/08/2026
1 in 1 years
100.0 %
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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