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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
8.0 mm
5:45pm 16/07/2026
6:00pm 16/07/2026
1 in 1.0 years
99.78 %
30m
13.0 mm
5:30pm 16/07/2026
6:00pm 16/07/2026
1 in 1.0 years
98.75 %
45m
13.0 mm
5:15pm 16/07/2026
6:00pm 16/07/2026
1 in 1.0 years
99.77 %
60m
14.0 mm
5:00pm 16/07/2026
6:00pm 16/07/2026
1 in 1.0 years
99.82 %
90m
15.0 mm
4:30pm 16/07/2026
6:00pm 16/07/2026
1 in 1.0 years
99.92 %
120m
15.0 mm
4:00pm 16/07/2026
6:00pm 16/07/2026
1 in 1 years
100.0 %
3h
16.0 mm
5:15pm 16/07/2026
8:15pm 16/07/2026
1 in 1 years
100.0 %
4h
18.0 mm
3:45pm 16/07/2026
8:15pm 16/07/2026
1 in 1 years
100.0 %
6h
19.0 mm
2:15pm 16/07/2026
8:15pm 16/07/2026
1 in 1 years
100.0 %
9h
24.0 mm
4:45pm 16/07/2026
1:45am 17/07/2026
1 in 1 years
100.0 %
12h
30.0 mm
5:15pm 16/07/2026
5:15am 17/07/2026
1 in 1.0 years
99.91 %
18h
34.0 mm
11:15am 16/07/2026
5:15am 17/07/2026
1 in 1.0 years
99.95 %
24h
44.0 mm
5:15am 16/07/2026
5:15am 17/07/2026
1 in 1.0 years
99.68 %
30h
45.0 mm
12:45am 16/07/2026
6:45am 17/07/2026
1 in 1.0 years
99.83 %
36h
45.0 mm
5:15pm 15/07/2026
5:15am 17/07/2026
1 in 1.0 years
99.93 %
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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