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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
5.8 mm
7:50pm 24/08/2026
8:00pm 24/08/2026
1 in 1.2 years
80.9 %
20m
5.8 mm
7:40pm 24/08/2026
8:00pm 24/08/2026
1 in 1.0 years
98.64 %
30m
5.8 mm
7:30pm 24/08/2026
8:00pm 24/08/2026
1 in 1.0 years
99.97 %
60m
11.6 mm
7:30pm 24/08/2026
8:30pm 24/08/2026
1 in 1.1 years
90.2 %
90m
17.2 mm
7:00pm 24/08/2026
8:30pm 24/08/2026
1 in 1.6 years
63.89 %
120m
21.6 mm
6:30pm 24/08/2026
8:30pm 24/08/2026
1 in 2.0 years
48.92 %
3h
27.2 mm
5:30pm 24/08/2026
8:30pm 24/08/2026
1 in 2.3 years
42.74 %
4h
33.2 mm
5:30pm 24/08/2026
10:00pm 24/08/2026
1 in 2.5 years
40.69 %
6h
37.2 mm
4:00pm 24/08/2026
10:00pm 24/08/2026
1 in 2.4 years
42.07 %
9h
45.0 mm
1:30pm 24/08/2026
10:30pm 24/08/2026
1 in 2.4 years
41.2 %
12h
50.4 mm
1:30pm 24/08/2026
1:30am 25/08/2026
1 in 2.3 years
42.7 %
18h
56.8 mm
2:00pm 24/08/2026
8:00am 25/08/2026
1 in 2.0 years
49.0 %
24h
61.2 mm
8:00am 24/08/2026
8:00am 25/08/2026
1 in 1.8 years
55.48 %
30h
63.2 mm
2:30am 24/08/2026
8:30am 25/08/2026
1 in 1.6 years
64.24 %
36h
63.2 mm
8:30pm 23/08/2026
8:30am 25/08/2026
1 in 1.4 years
72.81 %
48h
63.2 mm
8:30am 23/08/2026
8:30am 25/08/2026
1 in 1.1 years
87.05 %
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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