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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
5.8 mm
12:30am 25/08/2026
12:45am 25/08/2026
1 in 1.0 years
96.25 %
30m
8.6 mm
12:15am 25/08/2026
12:45am 25/08/2026
1 in 1.1 years
93.31 %
45m
11.8 mm
12:00am 25/08/2026
12:45am 25/08/2026
1 in 1.2 years
83.81 %
60m
13.0 mm
11:45pm 24/08/2026
12:45am 25/08/2026
1 in 1.2 years
84.03 %
90m
17.2 mm
11:45pm 24/08/2026
1:15am 25/08/2026
1 in 1.4 years
69.61 %
120m
19.6 mm
11:15pm 24/08/2026
1:15am 25/08/2026
1 in 1.5 years
65.83 %
3h
21.6 mm
10:15pm 24/08/2026
1:15am 25/08/2026
1 in 1.5 years
68.15 %
4h
23.2 mm
9:30pm 24/08/2026
2:00am 25/08/2026
1 in 1.4 years
73.52 %
6h
23.4 mm
8:15pm 24/08/2026
2:15am 25/08/2026
1 in 1.2 years
82.7 %
9h
23.4 mm
5:15pm 24/08/2026
2:15am 25/08/2026
1 in 1.1 years
91.85 %
12h
23.4 mm
2:15pm 24/08/2026
2:15am 25/08/2026
1 in 1.0 years
95.9 %
18h
23.4 mm
8:15am 24/08/2026
2:15am 25/08/2026
1 in 1.0 years
98.4 %
24h
23.4 mm
2:15am 24/08/2026
2:15am 25/08/2026
1 in 1.0 years
99.0 %
30h
23.4 mm
8:15pm 23/08/2026
2:15am 25/08/2026
1 in 1.0 years
99.44 %
36h
23.4 mm
2:15pm 23/08/2026
2:15am 25/08/2026
1 in 1.0 years
99.76 %
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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