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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
3.6 mm
3:45pm 16/07/2026
4:00pm 16/07/2026
1 in 1 years
100.0 %
30m
7.0 mm
3:30pm 16/07/2026
4:00pm 16/07/2026
1 in 1.0 years
99.99 %
45m
8.8 mm
3:30pm 16/07/2026
4:15pm 16/07/2026
1 in 1.0 years
99.83 %
60m
9.8 mm
3:30pm 16/07/2026
4:30pm 16/07/2026
1 in 1.0 years
99.86 %
90m
13.8 mm
2:45pm 16/07/2026
4:15pm 16/07/2026
1 in 1.0 years
98.25 %
120m
17.4 mm
2:15pm 16/07/2026
4:15pm 16/07/2026
1 in 1.1 years
93.74 %
3h
20.4 mm
1:30pm 16/07/2026
4:30pm 16/07/2026
1 in 1.1 years
94.24 %
4h
25.8 mm
2:15pm 16/07/2026
6:45pm 16/07/2026
1 in 1.1 years
88.65 %
6h
32.2 mm
10:30am 16/07/2026
4:30pm 16/07/2026
1 in 1.3 years
75.27 %
9h
39.6 mm
10:30am 16/07/2026
7:30pm 16/07/2026
1 in 1.5 years
68.8 %
12h
40.4 mm
10:30am 16/07/2026
10:30pm 16/07/2026
1 in 1.2 years
80.49 %
18h
40.6 mm
9:30am 16/07/2026
3:30am 17/07/2026
1 in 1.1 years
93.89 %
24h
40.6 mm
4:15am 16/07/2026
4:15am 17/07/2026
1 in 1.0 years
97.83 %
30h
40.6 mm
10:15pm 15/07/2026
4:15am 17/07/2026
1 in 1.0 years
98.86 %
36h
40.6 mm
4:15pm 15/07/2026
4:15am 17/07/2026
1 in 1.0 years
99.47 %
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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