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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
30m
14.2 mm
8:30am 21/07/2026
9:00am 21/07/2026
1 in 2.2 years
46.03 %
60m
14.2 mm
8:00am 21/07/2026
9:00am 21/07/2026
1 in 1.4 years
72.24 %
90m
14.2 mm
7:30am 21/07/2026
9:00am 21/07/2026
1 in 1.2 years
86.47 %
120m
14.2 mm
7:00am 21/07/2026
9:00am 21/07/2026
1 in 1.1 years
91.8 %
3h
14.4 mm
6:00am 21/07/2026
9:00am 21/07/2026
1 in 1.0 years
96.89 %
4h
17.2 mm
6:00pm 21/07/2026
10:30pm 21/07/2026
1 in 1.0 years
95.87 %
6h
21.0 mm
5:30pm 21/07/2026
11:30pm 21/07/2026
1 in 1.1 years
91.13 %
9h
27.2 mm
5:00pm 21/07/2026
2:00am 22/07/2026
1 in 1.2 years
82.34 %
12h
30.8 mm
3:00pm 21/07/2026
3:00am 22/07/2026
1 in 1.3 years
78.87 %
18h
30.8 mm
9:30am 21/07/2026
3:30am 22/07/2026
1 in 1.1 years
89.19 %
24h
31.8 mm
3:00am 21/07/2026
3:00am 22/07/2026
1 in 1.1 years
91.75 %
30h
34.0 mm
9:00pm 20/07/2026
3:00am 22/07/2026
1 in 1.1 years
91.3 %
36h
44.0 mm
2:00pm 20/07/2026
2:00am 22/07/2026
1 in 1.3 years
75.12 %
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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