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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
10m
0.4 mm
4:50am 23/07/2026
5:00am 23/07/2026
1 in 1 years
100.0 %
20m
0.8 mm
7:10am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
30m
1.0 mm
7:00am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
60m
1.0 mm
6:30am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
90m
1.0 mm
6:00am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
120m
1.0 mm
5:30am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
3h
1.6 mm
4:30am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
4h
1.6 mm
3:00am 23/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
6h
1.6 mm
1:40am 23/07/2026
7:40am 23/07/2026
1 in 1 years
100.0 %
9h
1.6 mm
10:30pm 22/07/2026
7:30am 23/07/2026
1 in 1 years
100.0 %
12h
2.2 mm
4:00am 23/07/2026
4:00pm 23/07/2026
1 in 1 years
100.0 %
18h
2.4 mm
4:40am 23/07/2026
10:40pm 23/07/2026
1 in 1 years
100.0 %
24h
2.6 mm
5:40pm 22/07/2026
5:40pm 23/07/2026
1 in 1 years
100.0 %
30h
2.6 mm
11:40am 22/07/2026
5:40pm 23/07/2026
1 in 1 years
100.0 %
36h
2.6 mm
6:50am 22/07/2026
6:50pm 23/07/2026
1 in 1 years
100.0 %
48h
2.6 mm
6:50am 22/07/2026
6:50am 24/07/2026
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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