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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
7.6 mm
12:00am 26/08/2026
12:15am 26/08/2026
1 in 1.0 years
95.45 %
30m
9.8 mm
12:00am 26/08/2026
12:30am 26/08/2026
1 in 1.0 years
96.18 %
45m
13.2 mm
11:30pm 25/08/2026
12:15am 26/08/2026
1 in 1.1 years
87.84 %
60m
15.4 mm
11:30pm 25/08/2026
12:30am 26/08/2026
1 in 1.2 years
82.11 %
90m
17.4 mm
10:45pm 25/08/2026
12:15am 26/08/2026
1 in 1.2 years
84.87 %
120m
26.2 mm
10:15pm 25/08/2026
12:15am 26/08/2026
1 in 2.2 years
46.17 %
3h
29.2 mm
9:30pm 25/08/2026
12:30am 26/08/2026
1 in 2.0 years
50.0 %
4h
29.2 mm
8:30pm 25/08/2026
1:00am 26/08/2026
1 in 1.5 years
68.28 %
6h
29.4 mm
8:30pm 25/08/2026
2:30am 26/08/2026
1 in 1.3 years
79.38 %
9h
30.2 mm
9:45pm 25/08/2026
6:45am 26/08/2026
1 in 1.1 years
90.4 %
12h
44.2 mm
10:15pm 25/08/2026
10:15am 26/08/2026
1 in 1.7 years
59.16 %
18h
27.0 mm
6:15am 25/08/2026
12:15am 26/08/2026
1 in 1.0 years
99.77 %
24h
29.4 mm
4:30am 25/08/2026
4:30am 26/08/2026
1 in 1.0 years
99.79 %
30h
54.2 mm
6:15am 25/08/2026
12:15pm 26/08/2026
1 in 1.4 years
71.31 %
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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