Posted: Fri Nov 18, 2005 2:04 pm
I have a 150 dia combined drain, 9.0m long, which needs to be altered. Is there a maximum fall for this? I need to install it at 1:8 to avoid a foundation!!
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Drainage falls - Maximum/minimum?
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Posted: Sun Nov 20, 2005 4:46 pm
There is no maximum - just a minimum.
Posted: Tue Nov 22, 2005 8:59 pm
There's no official maximum but there used to be an unwritten rule not to go steeper than 1/10. This was because there were fears of scour and hammer occuring (damage to pipes and joints due to high water velocities) although personally I've never heard or seen any reliable research which has proved this actually occurs in practice.
I know quite a few drainage engineers that won't go steeper than 1/10 though......but this is probably them just sticking to tradition. I'd say you should be fine with 1/8.
I know quite a few drainage engineers that won't go steeper than 1/10 though......but this is probably them just sticking to tradition. I'd say you should be fine with 1/8.
Posted: Wed Nov 23, 2005 9:21 pm
I don’t know if its just around here but building control certainly believe you can have to much fall on a system
We installed a new build system that ran through a steep banking before connecting to a sewer, we had to lay the 150mm line at a set fall until the line was 600mm from the surface at which pint we installed a new chamber with backdrop to take the line deeper and off we set again, in all we put three 2mtrs chambers in which added greatly to the labour and materials costs, as it happens it was an architects house so as they say , every cloud and all that.
We installed a new build system that ran through a steep banking before connecting to a sewer, we had to lay the 150mm line at a set fall until the line was 600mm from the surface at which pint we installed a new chamber with backdrop to take the line deeper and off we set again, in all we put three 2mtrs chambers in which added greatly to the labour and materials costs, as it happens it was an architects house so as they say , every cloud and all that.
Posted: Wed Nov 23, 2005 11:39 pm
It's not unusual for BCOs to insist on a shallower fall and a backdrop chamber in preference to a steep fall, but there's still no max fall. According to SfA and/or Part H, you could, in theory, have a pipe laid vertically, which is, after all, what a backdrop chamber has.
An old Drainage Inspector I knew in Bury used to waffle on about attenuation, and could make a fairly good case for 'slowing down' the flow before it hit the ETW or the Irwell (is there a difference?) but it was just his own theory. Given that he was 'God' when it came to getting drains passed and adopted in Bury MBC, we generally did what he asked!
An old Drainage Inspector I knew in Bury used to waffle on about attenuation, and could make a fairly good case for 'slowing down' the flow before it hit the ETW or the Irwell (is there a difference?) but it was just his own theory. Given that he was 'God' when it came to getting drains passed and adopted in Bury MBC, we generally did what he asked!
Posted: Thu Nov 24, 2005 12:17 pm
Tony McC wrote:An old Drainage Inspector I knew in Bury used to waffle on about attenuation, and could make a fairly good case for 'slowing down' the flow before it hit the ETW or the Irwell (is there a difference?) but it was just his own theory. Given that he was 'God' when it came to getting drains passed and adopted in Bury MBC, we generally did what he asked!
I don't think that would be really all the relevant now anyway. Any major sewer system is designed using a computer these days (usually MicroDrainage is used in England) and this takes into account the rate of flow through the pipes and can optimise them for the best slope.
On a small system (say a domestic one) any attenuation you'd get by changing the level of the pipes would probably be negligable when looking at the whole system...