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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
18.0 mm
11:30am 22/07/2026
11:45am 22/07/2026
1 in 1.5 years
64.78 %
30m
18.0 mm
11:15am 22/07/2026
11:45am 22/07/2026
1 in 1.1 years
89.65 %
45m
18.0 mm
11:00am 22/07/2026
11:45am 22/07/2026
1 in 1.1 years
95.2 %
60m
18.0 mm
10:45am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
96.59 %
90m
18.0 mm
10:15am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
98.19 %
120m
18.0 mm
9:45am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
98.47 %
3h
18.0 mm
8:45am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
98.84 %
4h
18.0 mm
7:15am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
99.21 %
6h
18.0 mm
5:45am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
99.48 %
9h
18.0 mm
2:45am 22/07/2026
11:45am 22/07/2026
1 in 1.0 years
99.78 %
12h
18.0 mm
11:45pm 21/07/2026
11:45am 22/07/2026
1 in 1.0 years
99.85 %
18h
18.0 mm
5:45pm 21/07/2026
11:45am 22/07/2026
1 in 1.0 years
99.96 %
24h
18.0 mm
11:45am 21/07/2026
11:45am 22/07/2026
1 in 1 years
100.0 %
30h
18.0 mm
7:00am 21/07/2026
1:00pm 22/07/2026
1 in 1 years
100.0 %
36h
18.0 mm
7:00am 21/07/2026
7:00pm 22/07/2026
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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