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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
2.8 mm
10:15am 19/08/2026
10:30am 19/08/2026
1 in 1 years
100.0 %
30m
4.6 mm
10:00am 19/08/2026
10:30am 19/08/2026
1 in 1 years
100.0 %
45m
6.4 mm
9:45am 19/08/2026
10:30am 19/08/2026
1 in 1.0 years
99.78 %
60m
8.2 mm
9:30am 19/08/2026
10:30am 19/08/2026
1 in 1.0 years
98.81 %
90m
12.6 mm
9:00am 19/08/2026
10:30am 19/08/2026
1 in 1.1 years
90.49 %
120m
15.0 mm
8:45am 19/08/2026
10:45am 19/08/2026
1 in 1.1 years
87.24 %
3h
21.2 mm
8:45am 19/08/2026
11:45am 19/08/2026
1 in 1.5 years
68.07 %
4h
28.4 mm
7:45am 19/08/2026
12:15pm 19/08/2026
1 in 1.8 years
54.82 %
6h
32.8 mm
6:45am 19/08/2026
12:45pm 19/08/2026
1 in 1.9 years
52.98 %
9h
37.0 mm
4:15am 19/08/2026
1:15pm 19/08/2026
1 in 1.6 years
61.03 %
12h
44.6 mm
7:00am 19/08/2026
7:00pm 19/08/2026
1 in 1.8 years
54.43 %
18h
53.0 mm
8:30pm 18/08/2026
2:30pm 19/08/2026
1 in 1.8 years
55.7 %
24h
64.8 mm
7:30pm 18/08/2026
7:30pm 19/08/2026
1 in 2.1 years
47.36 %
30h
66.6 mm
2:15pm 18/08/2026
8:15pm 19/08/2026
1 in 1.8 years
54.24 %
36h
66.6 mm
8:15am 18/08/2026
8:15pm 19/08/2026
1 in 1.6 years
62.45 %
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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