STORM WATER PIPE SIZING CALCULATOR FOR BUILDING SITES
This is a very simplified method of calculating
a pipe size, and takes no account of losses such as entry and exit losses,
and losses due to bends and fittings (it only calculates pipeline losses). These losses are assumed to be
compensated for, by allowing water to rise up a bit into the down pipe or
Also if the theoretical pipe diameter is greater than 4mm of an available pipe diameter, the next higher available pipe size is selected.
The velocity is calculated on this next higher pipe size.
The Formula used is Colebrook-White (k=0.015) as per the Australian Plumbing Code.
If using the drop down town list for rainfall intensity, the storm recurrence interval is 20 years.
Duration is 5mins for Australia. (NZ towns ARI 10 years, duration 10mins)
The coefficient of runoff is taken a 1. That is, 100% runoff, as in a roof.
If using the program for a different coefficient of runoff, multiply the catchment area by that coefficient before entering it
in the calculator. For example with a catchment area of 100sqm and a Coefficient of runoff of 0.6 (60%), the area to enter
into the program is 100 x 0.6 = 60 sqm.
Storm water design for Municipal systems is based on the Hydraulic Grade line, which takes into account all losses in the system.
e.g. through access chambers, junctions, catch pits etc. And also allows for pipes discharging below water level.
To learn about this, and storm water design principles in general
However this level of detail is not required for single straight pipes, and is not required under the Australian Plumbing Code.
The code makes allowances for losses by other means and a slightly different approach.
This method is explained more fully in the 'Notes & Instructions' on using
the multiple pipeline calculator.
HOW DOES THIS AFFECT DOWNPIPES
What if we use this method to size a pipe from a down pipe, and find that the resulting pipe is smaller than the downpipe itself.
(Assuming the Downpipe was sized in the Downpipe calculator)
A Down pipe size is more dependent on the perimeter of the pipe (circular
weir, or orifice) at the entrance, than the cross sectional area. And as such does not flow full.
Whereas the underground pipe maybe flowing full, thereby resulting in a smaller pipe.
Now it is very bad practise to reduce a pipe size downstream. Besides which it is against all codes and requirements and laws. It is also a potential blockage problem.
So, we can put in more DP's to reduce their size, or we can flatten the grade of the underground pipe to increase it's size,
or we can enlarge the Downpipe at the entrance to the diameter required by
the downpipe calculator, thereby providing a "Funnel" into the vertical pipe.
Or we could just enlarge the underground pipe and say nothing to anybody.
Note: Don't use this to design syphonic systems though, as a syphonic
systems need special fittings to get it started, and also to remove
How to Enter a Known Flow
The flow formula is :- flow = Area * Intensity / 3600 (Where coefficient of runoff is 1)
Make Area equal 3600, when divided by 3600 will cancel out, leaving the intensity equal to the required flow.
For example, say your known flow is 20 L/s, enter 3600 as the area, set the location to "I prefer to enter a know Intensity", and enter 20
as the intensity. the flow will be calculated to be 20 L/s.
What if ?
What if the storm intensity is greater than the pipe was designed for? or what if the pipe gets blocked?
Then, if there is a risk of serious damage, you may wish to design an overland flow path.