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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
5.2 mm
5:15pm 22/08/2026
5:30pm 22/08/2026
1 in 1.1 years
88.58 %
30m
7.6 mm
5:00pm 22/08/2026
5:30pm 22/08/2026
1 in 1.2 years
83.18 %
45m
8.4 mm
4:45pm 22/08/2026
5:30pm 22/08/2026
1 in 1.1 years
87.02 %
60m
10.6 mm
5:15pm 22/08/2026
6:15pm 22/08/2026
1 in 1.3 years
76.4 %
90m
15.6 mm
5:00pm 22/08/2026
6:30pm 22/08/2026
1 in 1.8 years
55.99 %
120m
17.0 mm
4:45pm 22/08/2026
6:45pm 22/08/2026
1 in 1.8 years
57.1 %
3h
17.2 mm
4:45pm 22/08/2026
7:45pm 22/08/2026
1 in 1.5 years
67.28 %
4h
18.4 mm
2:15pm 22/08/2026
6:45pm 22/08/2026
1 in 1.4 years
72.46 %
6h
18.8 mm
2:00pm 22/08/2026
8:00pm 22/08/2026
1 in 1.3 years
78.8 %
9h
19.0 mm
1:30pm 22/08/2026
10:30pm 22/08/2026
1 in 1.1 years
88.09 %
12h
19.0 mm
10:30am 22/08/2026
10:30pm 22/08/2026
1 in 1.1 years
92.15 %
18h
30.0 mm
1:15am 22/08/2026
7:15pm 22/08/2026
1 in 1.5 years
67.32 %
24h
31.4 mm
12:15am 22/08/2026
12:15am 23/08/2026
1 in 1.4 years
69.44 %
30h
31.4 mm
12:15am 22/08/2026
6:15am 23/08/2026
1 in 1.4 years
73.56 %
36h
31.4 mm
6:30pm 21/08/2026
6:30am 23/08/2026
1 in 1.3 years
76.76 %
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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