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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
7.0 mm
9:00am 18/08/2026
9:15am 18/08/2026
1 in 1 years
100.0 %
30m
12.0 mm
9:00am 18/08/2026
9:30am 18/08/2026
1 in 1 years
100.0 %
45m
14.0 mm
8:45am 18/08/2026
9:30am 18/08/2026
1 in 1 years
100.0 %
60m
15.0 mm
8:45am 18/08/2026
9:45am 18/08/2026
1 in 1 years
100.0 %
90m
21.0 mm
8:00am 18/08/2026
9:30am 18/08/2026
1 in 1.0 years
99.72 %
120m
25.0 mm
7:30am 18/08/2026
9:30am 18/08/2026
1 in 1.0 years
99.13 %
3h
27.0 mm
6:45am 18/08/2026
9:45am 18/08/2026
1 in 1.0 years
99.45 %
4h
31.0 mm
7:15am 18/08/2026
11:45am 18/08/2026
1 in 1.0 years
99.41 %
6h
32.0 mm
5:45am 18/08/2026
11:45am 18/08/2026
1 in 1.0 years
99.78 %
9h
36.0 mm
2:45am 18/08/2026
11:45am 18/08/2026
1 in 1.0 years
99.82 %
12h
42.0 mm
7:15am 18/08/2026
7:15pm 18/08/2026
1 in 1.0 years
99.74 %
18h
54.0 mm
3:15am 18/08/2026
9:15pm 18/08/2026
1 in 1.0 years
99.14 %
24h
57.0 mm
9:15pm 17/08/2026
9:15pm 18/08/2026
1 in 1.0 years
99.49 %
30h
58.0 mm
3:15pm 17/08/2026
9:15pm 18/08/2026
1 in 1.0 years
99.79 %
36h
61.0 mm
9:15am 17/08/2026
9:15pm 18/08/2026
1 in 1.0 years
99.81 %
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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