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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
17.6 mm
9:15am 29/07/2026
9:30am 29/07/2026
1 in 6.7 years
14.96 %
30m
21.4 mm
9:15am 29/07/2026
9:45am 29/07/2026
1 in 4.7 years
21.06 %
45m
21.4 mm
9:15am 29/07/2026
10:00am 29/07/2026
1 in 3.2 years
31.64 %
60m
21.4 mm
9:15am 29/07/2026
10:15am 29/07/2026
1 in 2.4 years
41.11 %
90m
21.4 mm
9:15am 29/07/2026
10:45am 29/07/2026
1 in 1.8 years
54.81 %
120m
21.4 mm
9:15am 29/07/2026
11:15am 29/07/2026
1 in 1.5 years
65.13 %
3h
21.4 mm
9:15am 29/07/2026
12:15pm 29/07/2026
1 in 1.3 years
79.28 %
4h
21.4 mm
9:15am 29/07/2026
1:45pm 29/07/2026
1 in 1.1 years
91.24 %
6h
21.6 mm
3:45am 29/07/2026
9:45am 29/07/2026
1 in 1.0 years
95.86 %
9h
21.6 mm
12:45am 29/07/2026
9:45am 29/07/2026
1 in 1.0 years
98.62 %
12h
21.8 mm
9:45pm 28/07/2026
9:45am 29/07/2026
1 in 1.0 years
99.31 %
18h
21.8 mm
3:45pm 28/07/2026
9:45am 29/07/2026
1 in 1.0 years
99.86 %
24h
21.8 mm
10:15am 28/07/2026
10:15am 29/07/2026
1 in 1.0 years
99.99 %
30h
21.8 mm
7:00am 28/07/2026
1:00pm 29/07/2026
1 in 1 years
100.0 %
36h
21.8 mm
7:00am 28/07/2026
7:00pm 29/07/2026
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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