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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
5.6 mm
3:15pm 17/07/2026
3:30pm 17/07/2026
1 in 1.2 years
82.22 %
30m
9.8 mm
3:15pm 17/07/2026
3:45pm 17/07/2026
1 in 1.8 years
57.11 %
45m
12.0 mm
3:15pm 17/07/2026
4:00pm 17/07/2026
1 in 1.9 years
51.5 %
60m
14.2 mm
3:15pm 17/07/2026
4:15pm 17/07/2026
1 in 2.2 years
44.57 %
90m
16.2 mm
3:00pm 17/07/2026
4:30pm 17/07/2026
1 in 2.2 years
45.17 %
120m
17.2 mm
2:45pm 17/07/2026
4:45pm 17/07/2026
1 in 2.0 years
49.31 %
3h
21.6 mm
1:30pm 17/07/2026
4:30pm 17/07/2026
1 in 2.5 years
40.17 %
4h
23.8 mm
12:45pm 17/07/2026
5:15pm 17/07/2026
1 in 2.3 years
44.43 %
6h
23.8 mm
11:15am 17/07/2026
5:15pm 17/07/2026
1 in 1.8 years
55.1 %
9h
26.6 mm
8:15am 17/07/2026
5:15pm 17/07/2026
1 in 1.7 years
57.7 %
12h
28.2 mm
4:15am 17/07/2026
4:15pm 17/07/2026
1 in 1.6 years
61.5 %
18h
31.0 mm
11:15pm 16/07/2026
5:15pm 17/07/2026
1 in 1.6 years
64.28 %
24h
31.6 mm
5:15pm 16/07/2026
5:15pm 17/07/2026
1 in 1.4 years
69.5 %
30h
31.6 mm
11:15am 16/07/2026
5:15pm 17/07/2026
1 in 1.3 years
74.59 %
36h
31.6 mm
5:30am 16/07/2026
5:30pm 17/07/2026
1 in 1.3 years
79.12 %
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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