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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
2.4 mm
11:15am 30/07/2026
11:30am 30/07/2026
1 in 1 years
100.0 %
30m
4.4 mm
11:15am 30/07/2026
11:45am 30/07/2026
1 in 1 years
100.0 %
45m
6.2 mm
11:00am 30/07/2026
11:45am 30/07/2026
1 in 1 years
100.0 %
60m
7.8 mm
11:00am 30/07/2026
12:00pm 30/07/2026
1 in 1 years
100.0 %
90m
10.8 mm
10:30am 30/07/2026
12:00pm 30/07/2026
1 in 1.0 years
99.97 %
120m
13.4 mm
10:15am 30/07/2026
12:15pm 30/07/2026
1 in 1.0 years
99.66 %
3h
17.6 mm
10:15am 30/07/2026
1:15pm 30/07/2026
1 in 1.0 years
98.58 %
4h
20.4 mm
8:45am 30/07/2026
1:15pm 30/07/2026
1 in 1.0 years
99.1 %
6h
21.4 mm
8:30am 30/07/2026
2:30pm 30/07/2026
1 in 1.0 years
99.85 %
9h
22.2 mm
5:15am 30/07/2026
2:15pm 30/07/2026
1 in 1 years
100.0 %
12h
23.6 mm
2:30am 30/07/2026
2:30pm 30/07/2026
1 in 1 years
100.0 %
18h
25.2 mm
2:00am 30/07/2026
8:00pm 30/07/2026
1 in 1 years
100.0 %
24h
26.2 mm
8:00pm 29/07/2026
8:00pm 30/07/2026
1 in 1 years
100.0 %
30h
27.0 mm
1:45pm 29/07/2026
7:45pm 30/07/2026
1 in 1 years
100.0 %
36h
27.6 mm
8:00am 29/07/2026
8:00pm 30/07/2026
1 in 1 years
100.0 %
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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