Info:
This page tells you how to find the required design rainfall intensities for all the free roof gutter, eaves gutter, box gutter and downpipe sizing calculators found here.

These programs have a drop down list of all the major towns in Australia and New Zealand.
Selecting a town from the list will insert the rainfall intensity required to design everything relating to storm water drainage for that particular aspect of a building site.

Other Plumbing Codes
Although the calculators are based on the Australian and New Zealand Plumbing Code, they can be used for anywhere in the world where water flows downhill. The physics of water flow does not change. (Well not enough to affect us, the viscosity and gravity may very slightly which affects things, but not enough to make any significant difference to our sizes. So these programs may not be suitable for use on other planets.

The significant thing that changes around the world is the required design rainfall Intensity. For this requirement you must refer to the Plumbing Code applicable to your area. Other differences maybe the allowable freeboard for box gutters etc. These programs use approximately 60mm for freeboard and wave motion. Other codes may, or may not, rely more heavily on flashing.

Anyway Australia has desert areas, tropical rainforest areas, and cyclone areas. So basically all possible storm conditions. The Australian Code is based on experiments and backed up standard Hydraulic formulas, so we know it will work for all these conditions.

Now most codes these days are based on performance criteria. That is, if you can prove your design will work, and not cause any damage, or you have used recognised design methods, (or other recognised Codes) your design should be approved.
For instance the Uniform Plumbing Code (USA) section '301.2.2 Compliance', states "The Administrative Authority may approve any such alternative, provided the Administrative Authority finds that the proposed design is satisfactory and complies with the intent of this Code".

So, moving on, you are looking for the required storm frequency sometimes called the average recurrence Interval (ARI), or the return period, and is expressed in terms of "once in so many years" eg 1 in 20 years, or 1:20 or Q20.
Then you must find the required duration stipulated in your Plumbing Code. This is usually in the form of minutes, or hours.

With these two pieces of information you can now find the required design rainfall intensity based on historical records and calculated by the local Bureau of Meteorology.

For towns not on any of the drop down lists, and for everywhere else in the world read on...

Other Places not listed :
There is the ability to use any intensity you want. To do this set the location choice to "I prefer to enter a known intensity".
You now need to find the required storm frequency, and duration, from your applicable plumbing Code.
Storm frequency and duration for Australia and New Zealand is:-

• Box gutters : ARI 100 (NZ 50) = 1% AEP* (2% NZ)
• Eaves gutters : ARI 20 (NZ 10) = 5% AEP* (10% NZ)
• * Surface (dwellings) ARI 10 = 10% AEP*
• Surface no danger of damage: ARI 2 = 50% AEP*
• Duration Aust- 5 mins ; Duration NZ- 10 mins.

From the International Plumbing Code and the Uniform Plumbing Code (USA):-

• Storm frequency - 100 years (all catchments) = 1% AEP
• Storm Duration - 1 hour (all catchments)

Some notes on AEP :
You may see this terminology starting to creep in on BOM sites and other texts.
AEP means Average Exceedance Probability.
ARI (average recurrence Interval) was too confusing for Lawyers, the media, and the general public.
It seems that everyone thought a 1 in 100 year storm would occur exactly once every 100 years. On the dot!

But in reality it is just a design terminology to take a guess at the storm intensity that on average may occur at that frequency.

So 1% = 1/100 = 1:100
Therefore a 1% AEP = 1:100 year ARI.
2% AEP = 2:100 = 1:50 ARI
5% AEP = 5:100 = 1:20 ARI and so on.

So for the benefit of the Lawyers out there, a 5% AEP (Average Exceedance probability) means there is a 5% chance of a 1:20 year storm occurring every year. A 1% AEP means there is a 1% chance of a 1:100 year storm occurring every year.

Anyway, you are now looking to find Intensity Frequency Duration curves. (IFD or words to that effect) These figures can be obtained from the graphs in the relevant Plumbing Codes, or from here:-

Intensity, Frequency, Duration curves. (AUST). For Australian towns.
Intensity, Frequency, Duration curves. (NZ) For New Zealand towns. (frequency 10 years, duration 10 mins)
Intensity, Frequency, Duration tables. (USA) For USA. (frequency 100 years, duration 1 hour)

Some of these sites allow you to enter the town name, and others require the latitude and longitude of the desired location. This is easy, into Google type "coordinates (then your town name)" the latitude and longitude will come up on top of the page. Copy and paste into the Bureau of Meteorology web site and that's it.

For other places in the world, your local Authority, Bureau of Meteorology, Consulting Engineer, or Hydraulic Consultant may be able to advise.


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