storm water pipe size calculator

7/29/2019 Storm Water Pipe Size Calculator

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Storm Water Pipe Size CalculatorTo AS 3500.3.2:1998 " Stormwater Dr ainage 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

NOTES:

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 coderequirements for Ausralia

Pipe class= PVC, for small diameters this is

recommended.

Coefficient of runoff= 1 , ie 100% runoff from a roof, as per

Code requirement.

IMPORTANT NOTEIf 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 mustuse 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 tomatch 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.

PIPE SIZES:
http://www.roof-gutter-design.com.au/swpipe/swpipe_a.phphttp://www.roof-gutter-design.com.au/swpipe/swpipe_a.phphttp://www.roof-gutter-design.com.au/swpipe/swpipe_a.php


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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 arerougher. 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:

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 :-

PIPE

SIZE

MIN GRADE

AUST

MIN

GRADE NZ

PIPE

SIZE

MIN GRADE

AUST

MIN

GRADE NZ

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.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.


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PIPE COVER:

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 townson 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 aknown intensity"

I would also be interested in any modifications, or suggestions that you would like

incorporated.


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If you need more info, or you would like other areas of Australia, or New Zealand,

added to the list, pleaseClick here to send email

Regards

Ken Sutherland

B.Tech MIEAust CPEng RPEQ
http://void%280%29/http://void%280%29/http://void%280%29/http://void%280%29/

storm water pipe size calculator

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