Surface water flow rate

[Moonshine]

Looking at a new sewage connection, and may want to put my surface water down the main sewer to save doing a soak away.

The application form is a asking for surface water discharge rates based on 50mm/hr rain fall. 

How do I do about calculating it? 

[Ferdinand]

..

Edited October 9, 2020 by Ferdinand

[epsilonGreedy]

7 minutes ago, Moonshine said:

Looking at a new sewage connection, and may want to put my surface water down the main sewer to save doing a soak away.

The application form is a asking for surface water discharge rates based on 50mm/hr rain fall. 

How do I do about calculating it? 

 

 

Lucky you. In my part of the world there is a blanket ban on new builds discharging surface water in to the sewer.

[markc]

4 minutes ago, Ferdinand said:

There may be a formula in the Building Regs.

 

Or you can calculate it based on the surface area of your plot times 0.002m depth of water per hour to give a figure in cubic m per hour.

 

Are there any other factors you need to apply?

why .002? surely that would be 2mm depth per hour.

[Ferdinand]

Just now, markc said:

why .002? surely that would be 2mm depth per hour.

 

Correct.

 

My bad. Should be times 0.05 not 0.002.

[PeterW]

Same as the rainwater harvesting calculator 

 

Plan roof area x 0.05 gives you cubic metres / hour

 

so mine would be 3.85 cuM/H at 77 square metres

 

 

[Moonshine]

Thanks all, simple calc then.

 

Though for surface water it will be for all impermeable surfaces.

[Oz07]

Just do your roof if you want a chance of passing ?

[Mr Punter]

Is there any chance to include a soakaway or permeable paving?

[Moonshine]

41 minutes ago, Mr Punter said:

Is there any chance to include a soakaway or permeable paving?

 

There is, but trying to get my head round sizing of a soak away, I have been using bre365 but not sure of what rainfall rate to use 10 year, 30 year or 100 year return.

 

[jamieled]

For soakaway sizing it is the 10 year design event.

[Moonshine]

2 minutes ago, jamieled said:

For soakaway sizing it is the 10 year design event.

 

Thanks, have you got a source for that?

 

Based on 10 year, the soak away size could be accommodated, and maybe better to work with two, one to the front of the house, one to the rear, rather one big one to the front. 

[jamieled]

It's in my head ( sorry not helpful and it doesn't mean its correct). I just checked my work copy of the BRE digest which states the 10 year:

 

Inflow to the soakaway I = A x R where: A = the impermeable area drained to the soakaway; R = the total rainfall in a design storm (a 10-year return period should be used); calculation of R is shown in the box below.

[Moonshine]

17 minutes ago, jamieled said:

It's in my head ( sorry not helpful and it doesn't mean its correct). I just checked my work copy of the BRE digest which states the 10 year:

 

Inflow to the soakaway I = A x R where: A = the impermeable area drained to the soakaway; R = the total rainfall in a design storm (a 10-year return period should be used); calculation of R is shown in the box below.

 

Thanks, but I think that is older version, where as the 2016 version isn't as clear cut as that.

 

 

 

[jamieled]

It's possible (depending on where you are) that it is your local authority who will dictate what the design standards now rather than bre. Some of them publish their drainage standards.

 

[Moonshine]

8 hours ago, jamieled said:

It's possible (depending on where you are) that it is your local authority who will dictate what the design standards now rather than bre. Some of them publish their drainage standards.

 

 

I have just had a closer look at approved document H (I obviously didn't read it properly) which states ten years.

 

 

I wonder if build control may ever ask for more than that.

 

 

[PeterW]

So what is interesting is if you get the 1 in 10 official numbers you will find that they are less than the current average..! The last 3 years have caused so many problems for the data sets as more events are occurring of increased flooding and more intense rainfall. It needs a different sort of hydraulic design to cope which is what the water companies are now looking at as storm water attenuation becomes key rather than soakaway capacity.