Storm Water Pipe Size Calculator
To AS 3500.3.2:1998 "Stormwater Drainage Acceptable Solutions".
(This program will calculate the size of inground storm water pipes suitable for the connection of roof drainage downpipes)
LOAD AND RUN PIPE SIZE CALCULATOR
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 requirements
||= 1 in 20 years (if you choose to use the location list)
|Time of concentration
||= 5 minutes, as per minimum code requirements for Ausralia
||= PVC, for small diameters this is recommended.
|Coefficient of runoff
||= 1 , ie 100% runoff from a roof, as per Code requirement.
If using this in conjunction with any downpipe sizing program, please be aware that the possibility exists for the surface water pipe size
to be of a smaller diameter than the downpipe size. This can occur from a number of reasons,
a) DP numbers are rounded up. eg if you theoretically require 1.2 downpipes you must use 2.
This means that each DP will have more capacity than required and will be overdesigned, this may result in a larger pipe size
than the equivalent surface water pipe.
b) The surface drainage pipe size is dependant on the slope, ie a steep slope will result in a smaller pipe.
You must ensure that you never decrease the pipe size downstream. The reason for this is to do with potential blockage problems.
Your choices are to either increase the surface water pipe size to match the largest downpipe size, or increase the number of downpipes
until a smaller size is reached to match the surface water required pipe size.
It is the designers job to juggle the parameters to get the most economical compromise.
eg more DP's verses larger pipe size. each case is diferent and depends on the relevant pipe lengths of each.
Stormwater Pipe sizes are determined by three parameters, FLOW, GRADE, and PIPE 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.
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.
Pipe grade is dependant on the level of the pipe outlet to achieve drainage, the grade of the surface, avoiding obstacles and other pipes,
and 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 :-
||MIN GRADE AUST
||MIN GRADE NZ
||MIN GRADE AUST
||MIN GRADE NZ
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.
I believe this to be a misprint and should be 1:150, 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.
Be aware that the minimum depth of soil over a pipe is also a requirement (pipe cover). This is normally affected by the location of,
and material of, the pipe, and the likely vehicle loads. The heavier the vehicle loads, 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.
RAINFALL INTENSITY, TIME OF CONCENTRATION and COEFFICIENT OF RUNOFF:
The time of concentration is the time it takes water to travel from the furtherest point in the catchment to the point under investigation.
To generate peak flow from a catchment, a storm must last at least this long. Now the longer a storm lasts, the less is the average intensity.
eg a storm may bucket down for 5 mins, but is not likely to keep up such an intensity for hours.
The coefficient of runoff is the percentage of storm water that actually runs off. eg a coefficient of runoff of 1, means 100% runoff,
a coefficient of runoff of 0.5 means 50% runoff, the rest soaks in, or is held in puddles or ponding etc.
The program uses the minimum time of concentration of 5 minutes and a 1 in 20 year storm, to determine the rainfall at the towns
on the drop down list. And a coefficient of runoff of 1.
However, you may use the program to determine the stormwater pipe size for any condition, if you know the rainfall intensity for your
time of concentration, and the required storm return period eg 1 in ? years for the area you wish to drain, and the coefficient of runoff
for the type of surface involved.
Multiply the catchment area by the coefficient of runoff before entering it in the program.
If using your own intensity, you must select the location choice to:- "I prefer to enter a known intensity"
I would also be interested in any modifications, or suggestions that you would like incorporated.
If you need more info, or you would like other areas of Australia, or New Zealand, added to the list, please
B.Tech MIEAust CPEng RPEQ
Other Hydraulic Sizing Calculators
Copyright 2006 Ken Sutherland