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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
3.4 mm
12:15am 21/07/2026
12:30am 21/07/2026
1 in 1 years
100.0 %
30m
4.8 mm
8:00am 21/07/2026
8:30am 21/07/2026
1 in 1 years
100.0 %
45m
6.4 mm
7:45am 21/07/2026
8:30am 21/07/2026
1 in 1 years
100.0 %
60m
8.8 mm
7:30am 21/07/2026
8:30am 21/07/2026
1 in 1.0 years
99.3 %
90m
10.2 mm
7:00am 21/07/2026
8:30am 21/07/2026
1 in 1.0 years
99.83 %
120m
13.0 mm
7:30am 21/07/2026
9:30am 21/07/2026
1 in 1.0 years
98.42 %
3h
18.0 mm
7:00am 21/07/2026
10:00am 21/07/2026
1 in 1.1 years
91.08 %
4h
19.4 mm
5:30am 21/07/2026
10:00am 21/07/2026
1 in 1.0 years
98.32 %
6h
22.2 mm
4:30am 21/07/2026
10:30am 21/07/2026
1 in 1.0 years
98.36 %
9h
28.6 mm
1:00am 21/07/2026
10:00am 21/07/2026
1 in 1.0 years
96.18 %
12h
35.2 mm
10:30pm 20/07/2026
10:30am 21/07/2026
1 in 1.1 years
91.04 %
18h
36.0 mm
6:15pm 20/07/2026
12:15pm 21/07/2026
1 in 1.0 years
98.93 %
24h
45.6 mm
12:15am 21/07/2026
12:15am 22/07/2026
1 in 1.0 years
96.03 %
30h
54.2 mm
10:45pm 20/07/2026
4:45am 22/07/2026
1 in 1.1 years
90.72 %
36h
55.6 mm
7:45pm 20/07/2026
7:45am 22/07/2026
1 in 1.0 years
95.58 %
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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