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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
5.6 mm
10:45am 21/08/2026
11:00am 21/08/2026
1 in 1.1 years
89.63 %
30m
8.8 mm
10:45am 21/08/2026
11:15am 21/08/2026
1 in 1.2 years
83.32 %
45m
11.6 mm
10:45am 21/08/2026
11:30am 21/08/2026
1 in 1.3 years
74.57 %
60m
14.0 mm
10:30am 21/08/2026
11:30am 21/08/2026
1 in 1.5 years
68.77 %
90m
16.4 mm
10:00am 21/08/2026
11:30am 21/08/2026
1 in 1.5 years
67.82 %
120m
18.0 mm
9:30am 21/08/2026
11:30am 21/08/2026
1 in 1.5 years
67.82 %
3h
18.8 mm
8:45am 21/08/2026
11:45am 21/08/2026
1 in 1.4 years
73.47 %
4h
25.2 mm
12:30pm 21/08/2026
5:00pm 21/08/2026
1 in 1.6 years
63.65 %
6h
34.8 mm
10:30am 21/08/2026
4:30pm 21/08/2026
1 in 2.3 years
43.79 %
9h
45.0 mm
8:00am 21/08/2026
5:00pm 21/08/2026
1 in 3.0 years
33.11 %
12h
46.4 mm
5:00am 21/08/2026
5:00pm 21/08/2026
1 in 2.7 years
36.96 %
18h
46.8 mm
11:15pm 20/08/2026
5:15pm 21/08/2026
1 in 2.2 years
46.18 %
24h
47.8 mm
8:45pm 20/08/2026
8:45pm 21/08/2026
1 in 2.0 years
50.73 %
30h
48.2 mm
5:30pm 20/08/2026
11:30pm 21/08/2026
1 in 1.9 years
53.42 %
36h
48.4 mm
1:15pm 20/08/2026
1:15am 22/08/2026
1 in 1.8 years
55.46 %
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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