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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
4.0 mm
2:45am 24/08/2026
3:00am 24/08/2026
1 in 1.0 years
97.64 %
30m
6.0 mm
2:45am 24/08/2026
3:15am 24/08/2026
1 in 1.0 years
95.88 %
45m
6.0 mm
10:15pm 23/08/2026
11:00pm 23/08/2026
1 in 1.0 years
97.85 %
60m
8.0 mm
10:15pm 23/08/2026
11:15pm 23/08/2026
1 in 1.1 years
95.07 %
90m
10.0 mm
10:00pm 23/08/2026
11:30pm 23/08/2026
1 in 1.1 years
91.85 %
120m
12.0 mm
9:15pm 23/08/2026
11:15pm 23/08/2026
1 in 1.1 years
88.88 %
3h
16.0 mm
9:15pm 23/08/2026
12:15am 24/08/2026
1 in 1.2 years
80.7 %
4h
18.0 mm
7:45pm 23/08/2026
12:15am 24/08/2026
1 in 1.2 years
80.96 %
6h
22.0 mm
9:15pm 23/08/2026
3:15am 24/08/2026
1 in 1.4 years
73.89 %
9h
24.0 mm
6:15pm 23/08/2026
3:15am 24/08/2026
1 in 1.3 years
76.38 %
12h
24.0 mm
3:15pm 23/08/2026
3:15am 24/08/2026
1 in 1.2 years
82.05 %
18h
24.0 mm
9:15am 23/08/2026
3:15am 24/08/2026
1 in 1.1 years
88.13 %
24h
24.0 mm
3:15am 23/08/2026
3:15am 24/08/2026
1 in 1.1 years
90.54 %
30h
24.0 mm
9:15pm 22/08/2026
3:15am 24/08/2026
1 in 1.1 years
92.27 %
36h
24.0 mm
3:15pm 22/08/2026
3:15am 24/08/2026
1 in 1.1 years
93.56 %
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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