Try a free trial of the Extra Features
This free version is limited
to only 4 catchment Areas.

Indicates a calculated field. User cannot change.
Indicates a calculated (or assumed) field. User can modify.
Indicates a User changed value.
Indicates a downstream pipe is smaller than an upstream pipe. This is not good.

To Calculate: Press the 'Calculate Pipe Size' button. This can be done on any amount of input data.

Choose a location, or enter the intensities for the various storm recurrence intervals.
(duration 5mins Aust, 10mins NZ)


STEP 2:  Enter Catchment Areas, and select catchment type. Remainder will calculate.

Catchment Areas

Catchment Areas Continued

STEP 3 :
Give each Pipeline 'End', and 'Junction', a Letter. (not a number, numbers are for Catchments). Example.
Join all pipes, and Catchments to pipes, by entering the relevant Upstream and Downstream letters or numbers. All other input is optional (or calculated) at this stage.
Order of entry does not particularly matter. As long as everything joins to everything else. (once only). However it is always best to enter the pipe sections in the order of the flow.
But don't worry too much as the program will usually tell you if you've missed something, or doubled up.


*A smaller pipe would work but the velocity would exceed the max allowable of 2m/s.

STEP 4 :
Now you can check the answers. Check the design flow against pipe capacity. If the pipe seems well oversized, try increasing the grade. Sometimes this may give a smaller pipe, if the allowable velocity of 2m/s is not exceeded. However a steeper grade may also lower all the downstream pipework. Which is more expensive and may not drain.
If a pipe is too big to enter a kerb, or get under or over an obstacle, try increasing the number of pipes, or try a rectangular pipe. A steel rectangular hollow section (RHS) is always good for entering a kerb.
To calculate the invert levels refer below.

The program generally conforms to the method outlined in the Australian Plumbing Code AS/NZS 3500.3:2015.
However it differs in the following manner.:-
The code calculates the total equivalent Impervious area draining to each pipe section, then uses a suitable rainfall ARI from Table 5.4.3 to calculate the design flow for all pipes.
(The equivalent impervious area is the sum of areas of each catchment multiplied by its coefficient of runoff).

This could mean that box gutters designed for a storm return periond of 100 years are draining into pipes of a storm return period of 10 years.

Although this may sound strange, it would be very unusual to have two different storm intensities occuring at once on a small building site. So a suitable storm intensity for all pipes must be chosen, normally around 1 in 10 years if no damage is likely to occur.

However it could also lead to the case of having large pipes from the roof draining into smaller underground pipes, which is not allowed.
So to make life easier and avoid such problems, this program calculates the runoff from each catchment for the required storm, then sums the runoff (not the equivalent impervious area) into each pipe section. Hopefully this minimises the possibility of larger pipes into smaller pipes, and reduces overflowing during larger storms. But it does mean that the design will have a slightly longer ARI than the Code requirement.
Which is a good thing.

Allowable intensities as per Australian Plumbing Code:-

  • Box gutters : ARI 100, for 5 mins (NZ ARI 50, 10 mins)
  • Eaves gutters : ARI 20, for 5 mins (NZ ARI 10, 10 mins)
  • Surface (dwellings) ARI 10, for 5 mins (NZ ARI 10, 10 mins)
  • Surface no danger of damage: ARI 2 for 5 mins (NZ ARI 10, 10 mins)
  • International, and Uniform Plumbing Code ARI 100,1hr all catchments)

  • ARI: Average recurrence Interval
  • C: Coefficient of runoff
C - Impervious surfaces : 0.9
C - Pervious Surfaces : 0.3 - 0.8
Default C: 0.8 (User can modify)

If you wish to take advantage of these smaller allowable Intensities for places not listed in the program,
Click Here for how to do this.

Using the Program:

The pipeline sections can be entered in any order, for instance the main line can be entered first, then all the branches, or you can stop at each junction and enter the branch. As long as all the catchments enter the pipe system, and all the pipe sections connect to each other, with ultimately only one outlet.

It is important to give the catchment areas numbers and the junctions (nodes) letters. This enables the program to differentiate between the two.

A catchment draining directly to a field inlet (gully pit, junction pit, RWO etc) does not have an associated pipe. Whereas a downpipe draining to a field inlet does have a pipe. To avoid confusion as to what is a catchment area and what is a downpipe, the program will determine a flow and a pipe size for both instances. The non existent pipe size can obviously be ignored.
Alternatively, use the catchment 'number' (not the catch pit 'letter') as the U/S point of the pipe.
When determining invert levels however, everything will still work out, because the nonexistent pipe will not have a length, or an upstream IL, so nothing will appear in the IL box.

For a large project its surprisingly easy to stuff it up at this stage; but don't worry, the program is keeping an eye out for such things, and will hopefully tell you what went wrong and how to fix it.

Catchment Types
Selecting the 'catchment type' will automatically allot the correct rainfall intensity and coefficient of Runoff.
Each 'catchment type' has a different allowable rainfall intensity, which relates to the severity of damage should a problem occur.
So a box gutter has the highest intensity, and 'no damage' has the lowest.

A 'no damage' catchment is one that probably doesn't need a drain at all, but a drain is put in so that water doesn't flow across a walkway or driveway etc. If all drains to this area are completely blocked, water will overflow to a point of lawful discharge without damage.

For all impervious catchments other than roofs, (concrete, tiles, bitumen areas etc) the coefficient of runoff is 0.9 (clause 5.4.6(a)(ii)).
'Box G' stands for box gutter and 'Eaves G' stands for Eaves gutter.

Pervious surfaces include grass, rocks, gravel, sand, etc. The coefficient of runoff can vary markedly for these areas. The program default is 0.8. However the user can change this to anything (except greater than 1). The figure of 0.8 is high in the scheme of things, but will allow for all those concrete paths, and roadways to be included in the catchment. Also the figure is still a best guess. However the user can lower this, for say large sandy areas, to see if it makes much difference to the pipe sizes.

Pipe Sizes:
The program will calculate the flow in each pipe section, and the required theoretical pipe size, using the minimum allowable grade for that pipe diameter. Learn more   If there is no available pipe of that exact diameter, the program selects the nearest available larger pipe and displays the nominal diameter (DN). It also shows the capacity of that pipe at the entered grade, and the velocity when flowing full with the 'design' flow.

The program uses the Colebrook-White formula with roughness coefficient "k" = 0.015 as per the Code.
This is only suitable for smooth bore pipes as in plastic, copper or stainless steel.

If the calculated flow requires a pipe larger that 600 mm dia. then the theoretical pipe size is displayed, It is then up to the user to find the nearest available suitable size.
Pipe sizes will not go below 90mm, as this is considered the minimum for storm water drainage for building sites.

If the pipe capacity is much greater than the design flow, meaning the theoretical pipe size is much smaller, then steepening the grade a bit may be just enough to reduce the pipe size.
However no downstream pipe can be smaller than an upstream pipe (for blockage reasons).
The program will tell you if this situation exists by highlighting the offending pipes in red like so.
Pipe highlighted in red
If you have to use a number of smaller pipes to enter a kerb, do this in a grated pit, situated so that any blockages can flow out of the top of the pit without causing damage. Do not use manifolds, junctions, etc.

Box culverts and rectangular pipes can also be calculated. This is done using the Hazen Williams formula with a coefficient of friction suitable for concrete pipes (140).

There is also the capability of using more than one pipe. This is usually necessary when draining large areas to the kerb and channel, where the outlet pipe is 150mm or 225mm and the pipe diameter into the kerb is limited to 100mm or less (usually).
In this case we could split the flow (in an access chamber) into a number of pipes. Galvanised steel RHS are usually good.

Some local Authorities limit the number of pipes to 3 in any one location. (Locations separated by 6m). Other Authorities limit the flow, or the velocity, at any one point into the kerb.
This is required so that water doesn't overshoot the gutter and flow across the road, being a hazard to traffic.

Calculating Invert Levels (IL):
Should you wish to do this, the length of each pipe section must be entered, and also the starting Invert Level for each branch line.
The lowest invert level draining to each point is calculated. If there is no change in pipe size upstream to downstream, this invert level becomes the next downstream pipeline's upstream invert level. If there is a change in pipe size, ie the D/S pipeline is larger, then the obvert levels are lined up, and the invert level drops by the difference in pipe diameters. The obvert level is the level of the inside top of the pipe.
The reason for this, is that the obvert is where the water level is when the pipe is full. so to avoid raising the water level during a pipe size change, it is best to lower the IL.

Finally, check that the invert level of the outlet pipe is equal to, or above the proposed point of discharge, and all pipes have the required amount of cover.

The program calculates pipe sizes using the minimum grade for that pipe diameter. This is suitable for flat sites. However if you have a sloping site, it is best to calculate the grade of the surface between each pipe section, and use that in the table. This will give more economical pipe sizes, and will also give invert levels that will help to keep the pipe underground.

Also if a grade is entered that is flatter than the allowable, the program will change it back to the allowable minimum.

Any grades that have been made steeper than the minimum allowable, are highlighted in pink like so
Pipe highlighted in pink
This can occur through user input, or various trials where the grade is kept from the last calculation. To get the minimum grade back, delete the pink one and recalculate.

To limit energy losses, the Code suggests a desirable maximum velocity of 1.5m/s with an absolute maximum of 2m/s.
The flow velocity is calculated using the full pipe diameter at the design flow.
If the velocity exceeds 2m/s, the pipe size is increased until the velocity falls below 2m/s.
If this situation occurs the new velocity is shown with an asterisk ' * '.

More than 40 Pipes
The program has calculation fields for up to 40 pipes. If your project has more than this, you can drain the first 40 pipes into the next 40 and so on.
First, note the final (maximum) flow of the first 40 pipes, and save the screen shot for your records. Delete all data and start a new calculation. (Press the 'delete all entered data' button).

The object is to create a catchment that has this flow, and enter it into the new catchment table. To do this, any of the catchment variables can be changed.
The easiest way is to give the catchment an area of 3,600 sqm, a coefficient of runoff of "1" and an intensity equal to the flow required. To see how this works refer to the runoff equation.

Now the best thing, just cut and paste the table/s and insert into your drawing, and that's it. All the information is there for the Plumber to construct.

Also, should you which to design an overland flow path, or put all this water into an open channel, this can be done here.

Further Notes and Instructions.
Copyright Ken Sutherland 2015
If you find that the programs save you heaps of agro, please feel free to buy me a few beers.