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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
3.0 mm
11:00pm 23/08/2026
11:15pm 23/08/2026
1 in 1.0 years
99.22 %
30m
4.5 mm
10:45pm 23/08/2026
11:15pm 23/08/2026
1 in 1.0 years
98.71 %
45m
6.0 mm
10:30pm 23/08/2026
11:15pm 23/08/2026
1 in 1.0 years
97.9 %
60m
7.5 mm
10:15pm 23/08/2026
11:15pm 23/08/2026
1 in 1.0 years
96.03 %
90m
10.0 mm
10:15pm 23/08/2026
11:45pm 23/08/2026
1 in 1.1 years
92.06 %
120m
13.0 mm
9:30pm 23/08/2026
11:30pm 23/08/2026
1 in 1.2 years
86.69 %
3h
17.0 mm
9:00pm 23/08/2026
12:00am 24/08/2026
1 in 1.3 years
76.92 %
4h
19.5 mm
7:30pm 23/08/2026
12:00am 24/08/2026
1 in 1.3 years
75.96 %
6h
19.5 mm
6:00pm 23/08/2026
12:00am 24/08/2026
1 in 1.2 years
81.47 %
9h
23.5 mm
6:45pm 23/08/2026
3:45am 24/08/2026
1 in 1.3 years
77.5 %
12h
23.5 mm
3:45pm 23/08/2026
3:45am 24/08/2026
1 in 1.2 years
83.2 %
18h
24.0 mm
9:45am 23/08/2026
3:45am 24/08/2026
1 in 1.1 years
88.01 %
24h
24.0 mm
3:45am 23/08/2026
3:45am 24/08/2026
1 in 1.1 years
90.41 %
30h
24.0 mm
9:45pm 22/08/2026
3:45am 24/08/2026
1 in 1.1 years
92.18 %
36h
24.0 mm
3:45pm 22/08/2026
3:45am 24/08/2026
1 in 1.1 years
93.55 %
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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