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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
28.0 mm
8:15pm 24/07/2026
8:30pm 24/07/2026
1 in 19.4 years
5.14 %
30m
28.0 mm
8:00pm 24/07/2026
8:30pm 24/07/2026
1 in 6.5 years
15.37 %
45m
28.0 mm
7:45pm 24/07/2026
8:30pm 24/07/2026
1 in 4.0 years
25.02 %
60m
28.0 mm
7:30pm 24/07/2026
8:30pm 24/07/2026
1 in 3.0 years
32.94 %
90m
28.0 mm
7:00pm 24/07/2026
8:30pm 24/07/2026
1 in 2.0 years
49.17 %
120m
28.0 mm
6:30pm 24/07/2026
8:30pm 24/07/2026
1 in 1.7 years
60.01 %
3h
28.0 mm
5:30pm 24/07/2026
8:30pm 24/07/2026
1 in 1.3 years
76.08 %
4h
28.0 mm
4:00pm 24/07/2026
8:30pm 24/07/2026
1 in 1.1 years
90.61 %
6h
28.0 mm
2:30pm 24/07/2026
8:30pm 24/07/2026
1 in 1.0 years
96.11 %
9h
28.0 mm
11:30am 24/07/2026
8:30pm 24/07/2026
1 in 1.0 years
98.81 %
12h
28.2 mm
8:30am 24/07/2026
8:30pm 24/07/2026
1 in 1.0 years
99.6 %
18h
56.0 mm
7:45pm 24/07/2026
1:45pm 25/07/2026
1 in 1.1 years
91.21 %
24h
56.0 mm
1:45pm 24/07/2026
1:45pm 25/07/2026
1 in 1.0 years
95.68 %
30h
56.2 mm
7:45am 24/07/2026
1:45pm 25/07/2026
1 in 1.0 years
97.21 %
36h
56.2 mm
7:30am 24/07/2026
7:30pm 25/07/2026
1 in 1.0 years
98.22 %
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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