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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
1.2 mm
7:10am 20/07/2026
7:20am 20/07/2026
1 in over 2000 years
< 0.05 %
20m
1.4 mm
7:10am 20/07/2026
7:30am 20/07/2026
1 in over 2000 years
< 0.05 %
30m
1.4 mm
7:00am 20/07/2026
7:30am 20/07/2026
1 in over 2000 years
< 0.05 %
60m
1.6 mm
7:10am 20/07/2026
8:10am 20/07/2026
1 in over 2000 years
< 0.05 %
90m
1.6 mm
6:00am 20/07/2026
7:30am 20/07/2026
1 in over 2000 years
< 0.05 %
120m
1.8 mm
6:10am 20/07/2026
8:10am 20/07/2026
1 in over 2000 years
< 0.05 %
3h
2.4 mm
9:40am 20/07/2026
12:40pm 20/07/2026
1 in over 2000 years
< 0.05 %
4h
2.6 mm
9:40am 20/07/2026
2:10pm 20/07/2026
1 in over 2000 years
< 0.05 %
6h
4.0 mm
6:40am 20/07/2026
12:40pm 20/07/2026
1 in over 2000 years
< 0.05 %
9h
5.2 mm
5:00am 20/07/2026
2:00pm 20/07/2026
1 in over 2000 years
< 0.05 %
12h
5.8 mm
5:00am 20/07/2026
5:00pm 20/07/2026
1 in over 2000 years
< 0.05 %
18h
5.8 mm
5:00am 20/07/2026
11:00pm 20/07/2026
1 in over 2000 years
< 0.05 %
24h
5.8 mm
5:00am 20/07/2026
5:00am 21/07/2026
1 in over 2000 years
< 0.05 %
30h
5.8 mm
5:00am 20/07/2026
11:00am 21/07/2026
1 in over 2000 years
< 0.05 %
36h
5.8 mm
5:00am 20/07/2026
5:00pm 21/07/2026
1 in over 2000 years
< 0.05 %
48h
5.8 mm
11:10pm 19/07/2026
11:10pm 21/07/2026
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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