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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
5.6 mm
11:30am 24/08/2026
11:40am 24/08/2026
1 in over 2000 years
< 0.05 %
20m
11.0 mm
11:20am 24/08/2026
11:40am 24/08/2026
1 in over 2000 years
< 0.05 %
30m
13.2 mm
11:10am 24/08/2026
11:40am 24/08/2026
1 in over 2000 years
< 0.05 %
60m
13.8 mm
10:50am 24/08/2026
11:50am 24/08/2026
1 in over 2000 years
< 0.05 %
90m
15.0 mm
10:10am 24/08/2026
11:40am 24/08/2026
1 in over 2000 years
< 0.05 %
120m
19.2 mm
11:10am 24/08/2026
1:10pm 24/08/2026
1 in over 2000 years
< 0.05 %
3h
21.0 mm
10:10am 24/08/2026
1:10pm 24/08/2026
1 in over 2000 years
< 0.05 %
4h
21.8 mm
9:30am 24/08/2026
2:00pm 24/08/2026
1 in over 2000 years
< 0.05 %
6h
27.4 mm
7:30am 24/08/2026
1:30pm 24/08/2026
1 in over 2000 years
< 0.05 %
9h
28.4 mm
5:00am 24/08/2026
2:00pm 24/08/2026
1 in over 2000 years
< 0.05 %
12h
28.4 mm
2:00am 24/08/2026
2:00pm 24/08/2026
1 in over 2000 years
< 0.05 %
18h
29.8 mm
8:30pm 23/08/2026
2:30pm 24/08/2026
1 in over 2000 years
< 0.05 %
24h
29.8 mm
8:30pm 23/08/2026
8:30pm 24/08/2026
1 in over 2000 years
< 0.05 %
30h
29.8 mm
8:30pm 23/08/2026
2:30am 25/08/2026
1 in over 2000 years
< 0.05 %
36h
29.8 mm
8:30pm 23/08/2026
8:30am 25/08/2026
1 in over 2000 years
< 0.05 %
48h
29.8 mm
8:30pm 23/08/2026
8:30pm 25/08/2026
1 in over 2000 years
< 0.05 %
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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