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# calculating flow capacity in a stormwater downpipe

## calculating flow capacity in a stormwater downpipe

(OP)
Hello all,

I have been tasked with designing a stormwater drainage plan for a single residential home. I am hoping you guys can help me figure out the flow capacity of different downpipe diameters. It has been a long time since I have done Fluid Mechanics in school.

Now I realize this problem will vary with change in pressure, storm intensity etc... I am hoping I can find something reasonably simple (rule of thumb) to find a flowrate at maximum capacity.

There is a local supplier who has rectangular downpipes at 100 x 75mm and claims these have a flow capacity of 4.4L/s. I could base my number of downpipes on this value, but I would like to see if I can use a few less (bigger) downpipes to please the architect as the lot is quite big (1200m2).

### RE: calculating flow capacity in a stormwater downpipe

Do you mean downspouts? Going vertically from the roof to ground?

You'll want to model like a horizontal orifice, to ensure your desired head doesn't exceed the gutter or parapet depending on roof type.

### RE: calculating flow capacity in a stormwater downpipe

(OP)
That is exactly what I mean.

Do you have a model I could follow to calculate flow capacity based on the pipe diameter?

### RE: calculating flow capacity in a stormwater downpipe

The downspout itself isn't going to control. The size of the flat orifice through which the water falls to get into the downspout is going to control. For that, use the orifice equation.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

### RE: calculating flow capacity in a stormwater downpipe

due to two-phase flow (air and water), unless the downspout pipe is ventilated well or very large such that viscosity and surface tension are negligible, you may get "slug flow" which could be less than the orifice or weir capacity. For vertical pipes, I believe maximum flow is achieved when the water flows down the walls of the pipe and air rises back up through the center of the annulus.

Unless the gutter is fairly deep or the downspout is quite small, I doubt you would ever get orifice flow conditions. I would use the weir equation with a sizable factor of safety to account for clogging and for air slug flow to determine the capacity of a downspout.

A couple of references for slug flow:
https://dspace.mit.edu/bitstream/handle/1721.1/614...
http://www.thermopedia.com/content/38/?tid=104&...

### RE: calculating flow capacity in a stormwater downpipe

You can buy open on part of one side, metal, rectangular downspouts. They work well in cold climates and give a nice ice sculpture to the side of a building. Thinnk of rectangle with 1/3 of one wide side gone.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

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