Is a 2100mm manhole unusual?

[SBMS]

We’ve had our SUDS compliant drainage plan designed by our engineer. Compacted clay so no chance of soakaways etc. I had asked to minimise attenuation tanks etc - to be compliant the engineer has designed for water to be attenuated in the surface water pipe work and oversized manholes. The big one is a 2100mm diameter by 1m deep manhole with a hydro brake limiting to 2.6l. This will run off into a distant stream (via an existing drain). 
 

I’ve looked online at the concrete ring which is about £900 so not obscenely expensive.
 

our last build was eye wateringly over engineered to the point of having the entire driveway effectively be the attenuation system with layers of stone, membrane and then crates on top. Very expensive. The manhole above looks like it’s about £900 so seemingly not too bad. 

 

Has anyone put something like this in before? It’s been sited in what will be the garden - how much of it has to be exposed? The full 2.1m, or can it be turfed over (or partially turfed over)?

 

[Dave Jones]

be cheaper by far to use 2m2 of crates.

[SBMS]

31 minutes ago, Dave Jones said:

be cheaper by far to use 2m2 of crates.

Editing my previous message… I was thinking of the radius not diameter so got you re the 2m2 of crates. Do sokeaway crates do the same job as a manhole?

Edited August 20 by SBMS

[Conor]

1 hour ago, SBMS said:

We’ve had our SUDS compliant drainage plan designed by our engineer. Compacted clay so no chance of soakaways etc. I had asked to minimise attenuation tanks etc - to be compliant the engineer has designed for water to be attenuated in the surface water pipe work and oversized manholes. The big one is a 2100mm diameter by 1m deep manhole with a hydro brake limiting to 2.6l. This will run off into a distant stream (via an existing drain). 
 

I’ve looked online at the concrete ring which is about £900 so not obscenely expensive.
 

our last build was eye wateringly over engineered to the point of having the entire driveway effectively be the attenuation system with layers of stone, membrane and then crates on top. Very expensive. The manhole above looks like it’s about £900 so seemingly not too bad. 

 

Has anyone put something like this in before? It’s been sited in what will be the garden - how much of it has to be exposed? The full 2.1m, or can it be turfed over (or partially turfed over)?

 

That's about twice the price it would be from a local concrete supplier. You'll also need the biscuit and an entry lid.

 

It's an easy way to build in attenuation, as you'd be digging and installing a chamber anyway. another way to do it is massively oversized pipework, or "S" pattern. Both would be more expensive than an off the shelf chamber.

 

The way we did it was unusual, we put in a normal chamber, but was extra deep (outlet way above the base) and on top of stone. Acts as a combined soakaway and attenuation system. It'll eventually silt up unless cleaned every 5 years or so. We could only do this due to the whole area around the house being on imported stone (basement excavation).

[SBMS]

3 minutes ago, Conor said:

That's about twice the price it would be from a local concrete supplier. You'll also need the biscuit and an entry lid.

 

It's an easy way to build in attenuation, as you'd be digging and installing a chamber anyway. another way to do it is massively oversized pipework, or "S" pattern. Both would be more expensive than an off the shelf chamber.

 

The way we did it was unusual, we put in a normal chamber, but was extra deep (outlet way above the base) and on top of stone. Acts as a combined soakaway and attenuation system. It'll eventually silt up unless cleaned every 5 years or so. We could only do this due to the whole area around the house being on imported stone (basement excavation).

Ah okay - that gets a bit closer then - a set of attenuation tanks to do 3.5m3 (the 2100mm chamber option) is about £500...so it might be a wash price wise or a few £100 more? Do you think the 'ease' of install balances the slight increase in materials (saving on labour costs for digging two holes for example?)

[Conor]

6 minutes ago, SBMS said:

Ah okay - that gets a bit closer then - a set of attenuation tanks to do 3.5m3 (the 2100mm chamber option) is about £500...so it might be a wash price wise or a few £100 more? Do you think the 'ease' of install balances the slight increase in materials (saving on labour costs for digging two holes for example?)

Installing a 2100mm ring is a fair bit trickier than a 600mm plastic ring! I used a 5t digger for our 900mm rings and there wasn't much breathing room. Tbh best speaking to a ground worker.

[Alan Ambrose]

That's an interesting approach, each ring would be ~2T.

 

>>> how much of it has to be exposed?

 

I assumed all of it, but the designer should have figured this out along with the levels, falls etc. In essence, you need minimum 1:100 fall from the furthest away source along the length of the pipe and into the tank inlet, which is usually towards the top of the tank.

 

As others have said, that's around 3.5m^3 of tank. Is that enough? - my SE is coming up with 27m^3 for mine.

Edited August 21 by Alan Ambrose

[saveasteading]

It also needs a big hole, a concrete base, a top (biscuit) , and an access cover. And a hydyobrake.

And it is heavy. 

£3k at a guess.

 

The inlet will be near the top. The outlet at the bottom. Have you got the fall for it?.

15 hours ago, SBMS said:

eye wateringly over engineered 

Meaning you think it was? It is unlikely to have been  oversized.

Crates de-skill the construction and are usually better value if there is a driveway anyway.

We can't comment without the details and circumstances If both projects.

[Alan Ambrose]

Sorry, I meant I assumed none of it would be exposed.

[Dave Jones]

16 hours ago, SBMS said:

Editing my previous message… I was thinking of the radius not diameter so got you re the 2m2 of crates. Do sokeaway crates do the same job as a manhole?

 

forget about the manhole your 'engineer' just like spending your cash.

 

In a nutshell you need some m2 of storage or attenuation as they call it to hold the water back before it discharges. 

 

crates do this for a fraction of the cost. if you want to add a chamber for some reason crack on but not needed with crates.

[SBMS]

9 hours ago, saveasteading said:

It also needs a big hole, a concrete base, a top (biscuit) , and an access cover. And a hydyobrake.

And it is heavy. 

£3k at a guess.

 

The inlet will be near the top. The outlet at the bottom. Have you got the fall for it?.

Meaning you think it was? It is unlikely to have been  oversized.

Crates de-skill the construction and are usually better value if there is a driveway anyway.

We can't comment without the details and circumstances If both projects.

It was over engineered. Our house was one of 3 self builds at the site and we inherited the scheme that had been approved and discharged with the council. The owners of the site had responsibility for implementing the scheme. Our builders, the ground workers and another drainage engineer I asked to have a look at it said it was entirely unnecessary and some crates would have sufficed. Fundamentally 500 sqm of driveway was used as attenuation. This was dug down, lined with membrane and then filled with stone with a hydrobrake at the end to discharge. Ironically it was an absolute silty mud pit by the end of the build when the crates went down and likely totally impermeable below the crates. In any event…
 

Agree with your comments on the chamber, seems really expensive way to do it

[SBMS]

7 hours ago, Dave Jones said:

 

forget about the manhole your 'engineer' just like spending your cash.

 

In a nutshell you need some m2 of storage or attenuation as they call it to hold the water back before it discharges. 

 

crates do this for a fraction of the cost. if you want to add a chamber for some reason crack on but not needed with crates.

Thanks @Dave Jones - I spoke with the engineer today after these comments and he basically said we could go with either. He pointed me at some manufacturers of plastic manholes that were cheaper but fundamentally said we need 4m3 of attenuation with a hydrobrake and said we could use crates or a manhole. He suggested speaking to our groundworkers to price up the most cost effective route. Seems like a pragmatic response at least. 

[SBMS]

9 hours ago, Alan Ambrose said:

That's an interesting approach, each ring would be ~2T.

 

>>> how much of it has to be exposed?

 

I assumed all of it, but the designer should have figured this out along with the levels, falls etc. In essence, you need minimum 1:100 fall from the furthest away source along the length of the pipe and into the tank inlet, which is usually towards the top of the tank.

 

As others have said, that's around 3.5m^3 of tank. Is that enough? - my SE is coming up with 27m^3 for mine.

Is it enough? He’s done the storm calculations and says it is - that’s what we are paying him for, so got to assume that his climate adjusted 1 In 100 year storm figures are correct? One thing I have noticed is some engineers count the driveways (even if permeable) into modelling/attenuation figures and some don’t - did yours (assuming your driveway areas are permeable?)

[Dave Jones]

10 hours ago, SBMS said:

Thanks @Dave Jones - I spoke with the engineer today after these comments and he basically said we could go with either. He pointed me at some manufacturers of plastic manholes that were cheaper but fundamentally said we need 4m3 of attenuation with a hydrobrake and said we could use crates or a manhole. He suggested speaking to our groundworkers to price up the most cost effective route. Seems like a pragmatic response at least. 

 

10 of these https://constructionmegastore.co.uk/products/hydrocell-soakaway-crate-1000mm-l-x-500mm-w-x-400mm-h?variant=48103923679543&currency=GBP&srsltid=AfmBOoq6Rb7ITFnUEkMFUNQvOpwsem7jo3VVDE_iu7HFCm7o2IVHE2nPb0s

1 of these https://drainstore.com/stormwater-solutions/jfc-hydro-valve-vortex-flow-control/

 

couple hours with a mini digger, some pea shingle and a roll of membrane. 

 

Change from £1000

 

 

[Dave Jones]

20 hours ago, Alan Ambrose said:

 

I assumed all of it, but the designer should have figured this out along with the levels, falls etc. In essence, you need minimum 1:100 fall from the furthest away source along the length of the pipe and into the tank inlet, which is usually towards the top of the tank.

 

As others have said, that's around 3.5m^3 of tank. Is that enough? - my SE is coming up with 27m^3 for mine.

 

27 cube ! Is it for a block of 25 flats ? Thats mad, so your 'SE' is saying you need 27 thousand litres of storage to catch the water that used to fall on the footprint of your building ? 

 

I'd be calling BS on that.

 

Ref fall, attenuation unlike a soakaway can be online or offline so flow can be top or bottom to suit.

[Alan Ambrose]

Well this is topical as I'm hoping to put most of my surface water drainage to bed today. I'll add to the thread if the OP is willing as it's on topic.

 

>>> 27 cube ! Is it for a block of 25 flats ? Thats mad, so your 'SE' is saying you need 27 thousand litres of storage to catch the water that used to fall on the footprint of your building ?

 

The 'SE', as you put it, is a fairly well known firm of SEs. I suspect that many SEs use an external web service anyway as it's a specialised calc. Of course, to be able to compare houses & attenuation tanks you need to know the catchment areas (roof & hard surfaces e.g. driveway), the ground conditions & the allowed nozzle rate. The location, and therefore storm rates, will make some difference too. I'm on clay with an apparent high GW level so similar to @SBMS.

 

I thought my SE's number sounded high and I was interested in how it was calculated, so I had another calc done by Freeflush (dumb name I think) for £100. I should say that I am very inquisitive and interested in how everything works and am treating the house build as a big learning exercise. I was also looking for clues in order to modify the SE's proposed design a little. I supplied the roof and drive areas and the location, Freeflush did the rest. As it happens I also had the FEH13 & 22 numbers from WHS which I puchased for £7. Freeflush came up with 26 m^3 and used the FEH13 numbers - so very similar to the SE. The FEH22 numbers will be a little different. Freeflush's calc sheet below and I've also added the FEH13 numbers from WHS for anyone who is curious. The big result I got back was that both the SE and Freeflush had treated the permeable driveway and patio as impermeable as @SBMS suggested above. @SBMS - do you have your areas for comparison? It would be good to bottom out why we have such different attenuation volumes.

 

 

FEH_Point_Rainfall_FEH13_AM (anonymised).xlsx

Edited August 22 by Alan Ambrose

[saveasteading]

13 minutes ago, Alan Ambrose said:

treated the permeable driveway and patio as impermeable

That may be reasonable in that the  stored volume of water will not permeate thriugh your clay at any useful rate. Ie critical storms these days are very heavy downpours, but tne soakaway amount will remain slow. 

[Alan Ambrose]

@saveasteading - yep, that sounds a reasonable assumption.

 

@Dave Jones >>> so flow can be top or bottom to suit.

 

So, you think this detail into crates will work OK? I'm concerned about the inlet pipe getting backed up as the tank fills and causing the inlet flow rate to diminish a lot.

 

[Dave Jones]

250m2 building is pretty massive ? 500m2 on 2 floors ?

 

Use permeable paving, no  one uses conc or tarmac these days.

 

The bare site must be one giant puddle of 25000Litres when it rains ?

[Alan Ambrose]

Single storey, so 250m^2 total. Plan is to use permeable, I just have to convince the SE that the tank doesn't need to live under the driveway.

 

It is fairly wet in the winter 😀 - I think that has a lot to do with the ditch at the back of the plot being converted to a piped culvert years before we got the plot, so it can't drain right now. On my to do list is to sort out land drainage to the culvert.

[saveasteading]

I am a great fan of swales. If you have the room and the topography.  This simple hollow can take loads of water and it disappears by percolation and evaporation.

Nature does the work, and little reaches the drainage infrastructure downstream...and it is easy and cheap.

[saveasteading]

3 hours ago, Alan Ambrose said:

the inlet pipe getting backed up as the tank fills

That would slow the flow significantly. The most efficient flow in a drain is at about 3/4 full and of course unrestricted downstream.  So you would need a bigger pipe, or the water may start to pop up manhole covers and squirt out of joints/ pour out of gutters.

[SBMS]

10 hours ago, Alan Ambrose said:

do you have your areas for comparison?

 

@Alan Ambrose, our ground floor sqm footprint is 206m2 (garage + ground floor).  My basic maths:

 

my 206m2 / your 470m2 = 43%. So of your 24.7m3 storage would be around 43% at 10.6m3.

Then there's the discharge rate - mine is 2.6l/s whereas yours is 1.9l/s.

my 2.6 / your 1.9m2 = about 136%. So that 10.6m3 is about 7.8m3.

 

I'm not sure why the figures I've tried to normalise above are still double when removing the 'permeable' area, and adjusting for the lower discharge rate.. However I suspect that the simulation isn't linear (i.e. half the sqm doesn't yield half the required attenuation) and is possibly more like an exponential curve... someone smarter than me can answer that?

 

What I would say is counting your permeable areas as impermeable seems up for debate and might drastically reduce your storage requirements.  Secondly - how did they calculate your discharge rate? Our site is currently brownfield and they adjusted it 'up' as if it was greenfield (so there is a betterment), but still theres a 30% difference.. maybe one to investigate?

 

And finally, the great irony in all this, is that currently, every single bit of our site - where the house is going, the driveway and our garden - totally 1500sqm - is currently a thick concrete slab (brownfield, former piggery and stores) with some rudimentary drainage discharging to the local beck about 160m away.  The site never floods, and neither does the beck, so anything we do - probably even without attenuation - might be entirely unnecessary??

Edited August 22 by SBMS

[saveasteading]

It depends on rainfall rates as well. This is not related to annual rainfall.

For example SE England gets occasional very intense storms, hence flooding in London, Tunbridge Wells and Hastings recently.

Allow 0.022 litres/ second/m2 for London but just 0.016 along the more consistently wetter west coast.

 

2 hours ago, SBMS said:

The site never floods,

 

The point of the retention is to protect downstream. eg The Glastonbury Levels get flooded because water reaches there so quickly form the catchment area. Development. and farmers ploughing downhill. and streams being cleared cause the same rain to flow faster to one place.  A particular location may have local rainfall effects but the map used is more general with rainfall isopleths, one of my favourite words.

 

I would cut and paste the chart, but the new 'improved' Windows 11 doesn't seem to have  a 'snip' function.

[Alan Ambrose]

>>> It depends on rainfall rates as well.

>>> isopleths

 

ooo, errr

 

Here's the overall rainfall map and a 'storm map' I found in here: https://www.sciencedirect.com/science/article/pii/S2212094721000372. If I've understood correctly, in the 2nd map the East of UK gets v. few 50mm storms compared with the West.

 

 

 

 

Edited August 24 by Alan Ambrose