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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
5.8 mm
3:00pm 22/08/2026
3:15pm 22/08/2026
1 in 1.3 years
79.45 %
30m
6.6 mm
3:00pm 22/08/2026
3:30pm 22/08/2026
1 in 1.1 years
89.64 %
45m
7.0 mm
2:45pm 22/08/2026
3:30pm 22/08/2026
1 in 1.1 years
93.82 %
60m
7.6 mm
3:30am 22/08/2026
4:30am 22/08/2026
1 in 1.1 years
94.96 %
90m
9.6 mm
3:15am 22/08/2026
4:45am 22/08/2026
1 in 1.1 years
90.98 %
120m
10.6 mm
3:00am 22/08/2026
5:00am 22/08/2026
1 in 1.1 years
91.34 %
3h
11.8 mm
2:45am 22/08/2026
5:45am 22/08/2026
1 in 1.1 years
93.35 %
4h
11.8 mm
1:15am 22/08/2026
5:45am 22/08/2026
1 in 1.0 years
97.77 %
6h
11.8 mm
11:45pm 21/08/2026
5:45am 22/08/2026
1 in 1.0 years
98.82 %
9h
12.0 mm
12:00am 22/08/2026
9:00am 22/08/2026
1 in 1.0 years
99.75 %
12h
17.4 mm
3:15am 22/08/2026
3:15pm 22/08/2026
1 in 1.0 years
96.66 %
18h
21.4 mm
12:15am 22/08/2026
6:15pm 22/08/2026
1 in 1.1 years
94.31 %
24h
21.4 mm
6:15pm 21/08/2026
6:15pm 22/08/2026
1 in 1.0 years
96.47 %
30h
21.4 mm
12:15pm 21/08/2026
6:15pm 22/08/2026
1 in 1.0 years
97.64 %
36h
21.4 mm
6:15am 21/08/2026
6:15pm 22/08/2026
1 in 1.0 years
98.3 %
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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