Rainwater Drainage Basics

[MortarThePoint]

Due to a lack of solids, rainwater gradient is less critical than for foul drainage. The minimum gradient is 1:100. The lower the gradient the harder it is to control in the laying process, so I think my ideal would be 1:80. That said, I need to connect to a pipe that is 425mm below surface with a branch of pipework with a length of about 32m, so that's likely to need the 1:100 fall to be possible, since even that will leave the top of a 110mm pipe at the surface. I may have to rethink the far end of that branch since the other points are at a maximum distance of 20m so a 1:80 gradient leaves the top of a 110mm pipe at a depth of 425 - (20,000/80) - 110 = 65mm which may just work and a gradient of 1:100 would make it 115mm which would be fine.

 

Based on Diagram 3 on page 45 and Diagram 2 on page 43 of Part H, the following gradients of 100mm pipe can cater for up to the following areas of roof and/or paving:

 

Even 375m2 is a large area and way beyond my needs. You need to divide by 1.5 if it's roof area, so that become 250m2 which is still lots of area.

 

NB: Peak rainfall rate of 0.016 L/s on 1m is equivalent to 1mm or rain fall depth per minute (60mm per hour)

 

 

Pipe run distances to gully/hopper entry point to 110mm underground rainwater drainage pipework:

 

[MortarThePoint]

The downpipe at 27m is servicing 50m2 of roof whilst the one at 32m is servicing 26m2 which hasn't been constructed yet. I'm wondering about combining that second roof area with the one serviced by the furthest right downpipe which is only handling 12m2 of roof area.

 

The one at 27m can't be scrapped though so that's the main challenge. 27m @ 1:100 leaves a depth of 425 - (27,000/100) - 110 = 45mm which could just about work as it's a patio there.

[Nickfromwales]

Just use trapped gulleys on the furthest one, to manage leaf litter / other debris?

[MortarThePoint]

4 minutes ago, Nickfromwales said:

Just use trapped gulleys on the furthest one, to manage leaf litter / other debris?

 

Yes, that's a good though as that sort of gully would have its output near the surface. I'll have to work carefully to hold the 1:100 gradient though.

[Nickfromwales]

2 minutes ago, MortarThePoint said:

 

Yes, that's a good though as that sort of gully would have its output near the surface. I'll have to work carefully to hold the 1:100 gradient though.

Weak concrete pads at the start and end of the run, and then multiples more every 1200mm or less will make life easier here. Be mindful that you cannot compact / whack anything retrospectively, so do as much of that as you need to, prior to installing the pipe.

[MortarThePoint]

4 minutes ago, Nickfromwales said:

Weak concrete pads at the start and end of the run, and then multiples more every 1200mm or less will make life easier here. Be mindful that you cannot compact / whack anything retrospectively, so do as much of that as you need to, prior to installing the pipe.

 

That's a good thought as then it can't drift down in some spots and cause issues. I had wondered about sharing a trench for part of this section, but that feels like it's asking for problems and would stop me from adding any intermediate concrete supports since this pipe will be the higher of the two.

[saveasteading]

A general point re rainwater capacity.

The rainfall diagram above is becoming out of date, as some areas of the country are getting exceptional downpours.

The annual rainfall is irrelevant, but what happens over 15 minutes can cause problems to your system, or over days to the whole area of infrastructure and waterways.

So it is best to future-proof by overdesigning OR having somewhere else for the water to go.

So what if:

What if a downpipe is blocked?  does it escape safely over the gutter or go backwards and cause issues? Using 2 x dp set well apart or having a weir overflow prevents this.

What if your drain doesn't quite cope with the flow, or another drain downline is over capacity? An open gully as above allows the water to run onto the ground, but make sure that can run away from the building.

[MPx]

All good points  above.  We had our last set of soakaways sized "for the 100 year storm".  Which happened at least 5 or 6 times over the next decade.  In fact I did put in (more than) enough capacity - but still got caught out by the failures.  I had 8 downpipes on the house, 2 on the barn and 5 Acos across the drive.  My early warning was the one on the front of the barn which would always block first (leaves, helicopter seeds, muck).  That was the signal to check the Acos - cos if they blocked, a flash flood was a serious possibillity.  Clearing the downpipe head baskets was on my monthly jobs list - and in max drop season could be needed weekly.

[MortarThePoint]

Bottle Gullies have an output reasonably near the surface, though a P-trap is probably the best for that. Of the bottle gullies, UG425 from polypipe has the shortest distance between top of grate and output invert at 175mm (Osma 4D900 207mm, Floplast D515 262mm). The spigot output of Polypipe UG425 looks to be at least 60mm long which is greater then the insertion depth of most ring seal couplers

 

175mm + (27000/100) = 445mm so should be workable with 1:100 over 27m and an end invert of 425mm (would stick up 20mm).

[MortarThePoint]

If I needed to, I suppose I could use a section of 82mm (OD) drainage pipe where the invert is really shallow, but I'd rather not. 75mm (ID) pipe at 1:100 has a capacity of about 2.7L/s which would be enough for one downpipe (standard 63mm ID / 68mm OD downpipe has a capacity of 1.11 L/s according to Part H Table 2 page 40). It would buy me an extra 28mm of headroom, so shallowest top of pipe would 78mm rather than the 50mm with 110mm pipe. I guess it depends a bit on the make up of the patio whether that additional depth is needed. If it is, replacing the first 3m would be enough since it would drop 30mm (@ 1:100) in that distance.

[MortarThePoint]

Looking to use the Polypipe bottle gully with concrete antisplash surround (UG425AST). It's not cheap (~£50) but finishes things quite neatly. I had intended to use the concrete hopper (UG414AST) which has a square grate, but that has a 110mm vertical outlet that I would have sent into a rest bend but for levels.

 

I just need to decide how far to position it from the wall. Pulled away from the wall slightly would make access easier but leave a gap between the wall which would need dressing. If water blasted out of the downpipe it could in theory overshoot the concrete surround if it is too far back, but that feels pretty unlikely.

 

Positioned with concrete surround touching the wall, shoe dipping below the top of the concrete:

 

 

Positioned with concrete surround 25mm from the wall, bottom of shoe level with top of concrete:

 

 

Positioned with concrete surround 63mm from the wall so that shoe is just over the grate, bottom of shoe level with top of concrete: