Calculating Flow Rates

[JCB123]

Morning all....one for the clever-clogs.

Just had an acceptance from Severn Trent Water to discharge 7.5 l/sec into their surface water sewer.

At a pipe size of 150mm dia. @ 1:150 fall, how can I calculate the equivalent area of the roof of my building this equates to.

I really have no idea but probably should....

Thanks!

[tonyvid]

Not a clever clog....but doesn't the actual rainfall amount have a serious impact there? I would just "average" the 7.5L/sec over a week

[LJTS]

Link below to roof drainage design

http://www.roofconsult.co.uk/articles/drainage.htm

Calculation below

http://www.roofconsult.co.uk/calculations/drainage...

[Tom_C76]

You're looking at the problem backwards. What you need to do is work out the discharge from your roof, then put in a holding tank sized to suit and a very small outlet pipe to control the discharge rate. 7.5l/s is much less than you would get from a 150 dia pipe.

Try printing off the nomograph on page 5 of this pdf http://www.peterborough.gov.uk/pdf/env-pl-dev-esta... which I think will give a flow in your pipe of about 10l/s.

(ed for speeling)

Edited by Tom_C76 on Friday 7th January 09:43

[TomE]

Where in the country are you (approximately will do!), and how big is your roof?

You may be looking at needing to put some attenuation in to limit the outfall to the agreed discharge rate, but with the above info a quick guesstimate can be made with available rainfall data.

[cv01jw]

As TomE said your location is important, as is the area of your roof.

Have Severn Trent asked to see calcs etc to prove you are discharging 7.5 l/s or less? If so, you need to clarify what design horizon you need (ie. 1in30, 1in100 years etc).

if you need these calcs let me know and I will do my best to help - can use our modelling software over lunch if I need to.

James

ETA: By the way, looking in our hydraulic design tables a 150 pipe at 1/150 will give you approx 14.5 l/s running full bore.

Edited by cv01jw on Friday 7th January 09:59

[JCB123]

Thanks for the replies...

Roof plan area totals 1640m2, at 22.5 deg pitch.
Location is east of England.

Basically we are wanting to harvest the water, but site conditions mean the tank will be very deep, so we want to drain off site as much as we can, then harvest the rest.

So if 7.5 l/sec equates to 500m2, we will then design the harvesting tank to take 1140m2....

[cv01jw]

JCB123 said:

Thanks for the replies...

Roof plan area totals 1640m2, at 22.5 deg pitch.
Location is east of England.

Basically we are wanting to harvest the water, but site conditions mean the tank will be very deep, so we want to drain off site as much as we can, then harvest the rest.

So if 7.5 l/sec equates to 500m2, we will then design the harvesting tank to take 1140m2....

Right. Building regs state 0.014 litres/second/m² as the 'normal' runoff rate for surface water design. this is based on a 50mm rainfall event.

So, backworking that, with a permissible discharge of 7.5 l/s you could drain a roof area of 535m², leaving you with 1140m² to harvest.

1140m² equates to 16 litres/second to be harvested/stored.

[JCB123]

cv01jw said:

JCB123 said:

Thanks for the replies...

Roof plan area totals 1640m2, at 22.5 deg pitch.
Location is east of England.

Basically we are wanting to harvest the water, but site conditions mean the tank will be very deep, so we want to drain off site as much as we can, then harvest the rest.

So if 7.5 l/sec equates to 500m2, we will then design the harvesting tank to take 1140m2....

Right. Building regs state 0.014 litres/second/m² as the 'normal' runoff rate for surface water design. this is based on a 50mm rainfall event.

So, backworking that, with a permissible discharge of 7.5 l/s you could drain a roof area of 535m², leaving you with 1140m² to harvest.

1140m² equates to 16 litres/second to be harvested/stored.

Super - so my guestimates weren't far off....

cv01jw - have an e-beer on me!

[TomE]

As a general rule of thumb for the east of England you could say that rainfall is about 50mm/hr so for a 7.5l/s discharge rate, you can drain approx 540m2.

As I say though, that is a guesstimate, you would need to run that through some modelling software if it needed to be accurately calculated.

ETA- Gah beaten to it! Real work got in the way halfway through my calcs!

Edited by TomE on Friday 7th January 10:39

[cv01jw]

JCB123 said:

cv01jw said:

JCB123 said:

Thanks for the replies...

Roof plan area totals 1640m2, at 22.5 deg pitch.
Location is east of England.

Basically we are wanting to harvest the water, but site conditions mean the tank will be very deep, so we want to drain off site as much as we can, then harvest the rest.

So if 7.5 l/sec equates to 500m2, we will then design the harvesting tank to take 1140m2....

Right. Building regs state 0.014 litres/second/m² as the 'normal' runoff rate for surface water design. this is based on a 50mm rainfall event.

So, backworking that, with a permissible discharge of 7.5 l/s you could drain a roof area of 535m², leaving you with 1140m² to harvest.

1140m² equates to 16 litres/second to be harvested/stored.

Super - so my guestimates weren't far off....

cv01jw - have an e-beer on me!

For info, Building regs relies on BSEN 752 for much of the drainage technical criteria. BSEN 752 states that in normal circumstances a design rainfall duration of 5 minutes is acceptable, so you would need to store 5 minutes worth of runoff at 16 l/s (as a ballpark estimate).

This would equate to 4.8m³ of storage, which you could either harvest or discharge to the SW system at 7.5 l/s using a hydrobrake or similar.

[Tom_C76]

JCB123 said:

Thanks for the replies...

Roof plan area totals 1640m2, at 22.5 deg pitch.
Location is east of England.

Basically we are wanting to harvest the water, but site conditions mean the tank will be very deep, so we want to drain off site as much as we can, then harvest the rest.

So if 7.5 l/sec equates to 500m2, we will then design the harvesting tank to take 1140m2....

If you want/need calcs for Severn Trent you could send me a PM, we do flood and storm water modelling.

TC