To AS/NZS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions".
(This program will calculate the size of inground storm water pipes suitable for the connection of roof drainage downpipes)

This program is a simple stormwater pipe size calculation designed to be useful in the majority of small building sites.
To do this, certain parameters have been allocated. eg:-

Storm frequency for eaves gutters as per Code   1 in 20 years (if you choose to use the location list)(10yrs NZ)
Time of concentration  5 minutes, as per code requirements for Aust. (10mins NZ)
Pipe class   PVC, for small diameters this is recommended.
Coefficient of runoff   1 , ie 100% runoff from a roof, as per Code requirement.

There are only three parameters required to successfully size any pipe:-
  • Flow
  • Grade
  • Internal surface roughness
The effect is as follows:-
 More flow requires a larger pipe size.
 Flatter grade requires a larger pipe size.
 A rougher pipe internal surface requires a larger pipe size.

PVC and copper are smooth. Concrete, fibre cement, earthenware, cast iron etc are rougher. The program uses PVC

The program calculates the theoretical storm water pipe size required to carry the flow when the pipe is flowing full. For the formulas used click here

However, pipes are not made in increments of 1mm, pipes are generally made in increments of 75mm (3 inches). Therefore the available pipe size is also shown. If the theoretical pipe size is within 4mm of a smaller available pipe, the suggested pipe size is rounded down, otherwise it is rounded up to the next larger available pipe size.

For pipes larger than 600mm diameter the 'theoretical' pipe size is shown.

Pipe grade can be expressed in a number of different ways. It is expressed in the industry generally as follows :-
  • Gravity pipes as in Storm water or sewerage - Vertical to horizontal, as in 1 in 100, or 1:100.
  • Pressure pipes as in Water supply - Is the hydraulic grade, expressed as a decimal. eg the above 1:100 would be 1/100 = 0.01.
  • Municipal Stormwater sometimes but rarely - As a percentage, eg the above 1:100 would be 1%.
  • Never in degrees.
The program requires pipe grade to be entered in the form of 1 unit vertical to x units horizontal, written as 1 in x, or 1:x. For instance if the grade was 1 in 100, or 1:100, enter 100 in the box. Entering 1 in the box will give a grade of 1 in 1, that is 45 degrees.

A slope of 1:200 is flatter than a slope of 1:100.

Pipe grade is dependent on the ground surface grade, juggling the pipe it miss obstacles and other pipes, the level of the outfall and depth and soil cover requirements. For a first trial use the grade of the surface
If the surface is flat, the code requires the use of the minimum allowable grades as shown below :-

90 1:100 1:90 225 1:200 1:350
100 1:100 1:120 300 1:250 1:350
150 1:100 1:200 375 1:300 1:350

Note, for Australia a 150 dia pipe requires a grade of 1:100, the same grade as required for a 100 and a 90 dia pipe.
This is interesting as larger pipes generally require a flatter grade.
By the way, the purpose of a minimum grade is hopefully to ensure a self cleansing velocity during periods of low flow.
Also, the Code only refers to the "Nominal Diameter". The actual Internal diameter may be slightly more or less, depending on the material chosen.

There are also requirements for the depth of the pipe, generally known as soil cover requirements. This is normally dependent on the location of, the material of the pipe, and the likely vehicle loads. The heavier the vehicle the deeper the pipe must be. Cover requirements range from 100mm for single dwellings with no vehicle loads, to 500mm under roads. Please consult the plumbing code for greater clarification.
Pipes under public roads come under different codes and Council requirements. Please consult a Civil Engineer to design pipes in this location.

Storm Return Period, Time of Concentration and Coefficient of Runoff:
The time of concentration is the time it takes water to travel from the furthermost point in the catchment to the point under investigation. For building sites in Australia this is taken as 5 mins. (NZ 10 mins).

The coefficient of runoff is the percentage of storm water that actually runs off. eg a coefficient of runoff of 1, means 100% runoff, This program uses "1" ie 100% runoff as on a roof, or other impervious surface.

The program uses an average recurrence interval (ARI) of 1 in 20 year (5 min duration) storm for Australia, and (10mins 1:10 year NZ) for the towns on the drop down list.

However, you may use the program to determine the storm water pipe size for any condition, if you know the rainfall intensity, and the coefficient of runoff. Multiply the catchment area by the coefficient of runoff before entering it in the program. eg 50% runoff, is the same as 50% catchment area with 100% runoff.

If using your own intensity, you must select the location choice to:- "I prefer to enter a known intensity"

Intensity for towns not listed
These are two great sites. Look for the Intensity, Duration, frequency curves (IDF) or words to that effect.
For Australia Time of concentration (TOC) 5mins ARI 20 years.
New Zealand Time of concentration (TOC) 10mins ARI 10 years.

Intensity, Frequency, Duration curves.. For Australian towns.
and for New Zealand from here Also, your local Authority, Consulting Engineer, or Hydraulic Consultant may be able to advise.

Learn much much more

Ken Sutherland

DISCLAIMER: It is the users responsibility to ensure input data is calculated and entered correctly, and that the program is suitable for the roof configuration required.

Copyright 2015 Ken Sutherland