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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
4.6 mm
11:45am 16/07/2026
12:00pm 16/07/2026
1 in 1 years
100.0 %
30m
8.6 mm
11:30am 16/07/2026
12:00pm 16/07/2026
1 in 1.0 years
98.67 %
45m
11.2 mm
11:30am 16/07/2026
12:15pm 16/07/2026
1 in 1.0 years
96.4 %
60m
12.2 mm
11:15am 16/07/2026
12:15pm 16/07/2026
1 in 1.0 years
97.07 %
90m
15.4 mm
10:45am 16/07/2026
12:15pm 16/07/2026
1 in 1.1 years
93.61 %
120m
18.0 mm
10:15am 16/07/2026
12:15pm 16/07/2026
1 in 1.1 years
90.13 %
3h
21.2 mm
10:30am 16/07/2026
1:30pm 16/07/2026
1 in 1.1 years
89.34 %
4h
28.8 mm
10:30am 16/07/2026
3:00pm 16/07/2026
1 in 1.4 years
71.0 %
6h
33.6 mm
10:00am 16/07/2026
4:00pm 16/07/2026
1 in 1.5 years
65.5 %
9h
36.2 mm
10:00am 16/07/2026
7:00pm 16/07/2026
1 in 1.3 years
74.82 %
12h
37.4 mm
9:15am 16/07/2026
9:15pm 16/07/2026
1 in 1.2 years
85.26 %
18h
37.6 mm
9:15am 16/07/2026
3:15am 17/07/2026
1 in 1.0 years
95.36 %
24h
37.6 mm
4:00am 16/07/2026
4:00am 17/07/2026
1 in 1.0 years
98.36 %
30h
37.6 mm
10:00pm 15/07/2026
4:00am 17/07/2026
1 in 1.0 years
99.17 %
36h
37.6 mm
4:00pm 15/07/2026
4:00am 17/07/2026
1 in 1.0 years
99.76 %
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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