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
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 & 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)
Eaves gutters : ARI 20 (NZ 10)
Surface (dwellings) ARI 10
Surface no danger of damage: ARI 2
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)
Storm Duration - 1 hour (all catchments)
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:-
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.