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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
19.2 mm
11:15am 26/08/2026
11:30am 26/08/2026
1 in 11.1 years
9.03 %
30m
19.2 mm
11:00am 26/08/2026
11:30am 26/08/2026
1 in 3.7 years
27.1 %
45m
19.2 mm
10:45am 26/08/2026
11:30am 26/08/2026
1 in 2.4 years
41.3 %
60m
19.2 mm
10:30am 26/08/2026
11:30am 26/08/2026
1 in 2.0 years
50.97 %
90m
10.4 mm
8:00pm 25/08/2026
9:30pm 25/08/2026
1 in 1.0 years
98.97 %
120m
12.2 mm
7:30pm 25/08/2026
9:30pm 25/08/2026
1 in 1.0 years
98.55 %
3h
15.4 mm
7:45pm 25/08/2026
10:45pm 25/08/2026
1 in 1.0 years
97.46 %
4h
19.6 mm
7:00pm 25/08/2026
11:30pm 25/08/2026
1 in 1.1 years
95.02 %
6h
20.6 mm
5:30pm 25/08/2026
11:30pm 25/08/2026
1 in 1.0 years
96.47 %
9h
21.6 mm
3:30pm 25/08/2026
12:30am 26/08/2026
1 in 1.0 years
98.45 %
12h
23.4 mm
11:30am 25/08/2026
11:30pm 25/08/2026
1 in 1.0 years
98.59 %
18h
40.6 mm
5:30pm 25/08/2026
11:30am 26/08/2026
1 in 1.3 years
76.74 %
24h
43.4 mm
11:30am 25/08/2026
11:30am 26/08/2026
1 in 1.3 years
79.6 %
30h
49.2 mm
5:30am 25/08/2026
11:30am 26/08/2026
1 in 1.4 years
73.27 %
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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