Drainage design — right approach?

[Dreadnaught]

I have received a quote for the design of the drains for my site.

 

I am keen to do this early, well before my project starts, as I can ask for the works in the road done by my boathouse neighbour who is opening-up the road for their own purposes. They may well be willing to do this for me as in return I can give them access to my plot for site storage for the duration of their build. The timing would suit me as I need to go back to planning anyhow so wont be starting until after they finish anyway. 

 

I am told that they will be planning their work quite soon so I want to have my drain plan ready so I can hand it to them. At least that is my thinking. Am I going about this the right way?

 

Note that I have already got a detailed survey of the local drains, including cover and invert levels, etc., which was prepared for the boathouse neighbour by this same SE company, and which the boathouse kindly shared with me. Having said that I have all the invert levels, the foul manhole nearest my plot was UTL ("unable to lift") so I do not have that invert level but I have it for the next manhole along upstream.

 

I have been quoted by a local SE firm £1350 + VAT for preparing: (i) a foul drainage drawing showing external drain runs, manhole locations, and levels, etc. And (ii) a drawing showing the below ground surface water drainage for surface water to be discharged in to the adjacent stormwater drain via an attenuation tank. 

 

Given that this is the South East, I wonder if there is an SE-premium there?

I wonder, does drainage have to be designed by a local firm or can I go to anyone nationally? 

 

By the way, must I have an attenuation tank for surface water? My PP does not mention anything about drainage.

Edited September 1, 2018 by Dreadnaught

[Mr Punter]

There is nothing to stop you designing it yourself as long as it complies with building regs.  The attenuation tank is more likely to be Building Control or the sewerage undertaker who decides.  £1350 sounds steep so def shop around.

[Jeremy Harris]

My experience was that planning was concerned that I ticked a box saying that I'd install a SuDS compliant surface (and roof) water drainage system, but BC really weren't that interested, and didn't even accept my request that they come over and look at the 20 Aquacell crates I'd buried under the drive as an attenuation tank. 

[Mr Punter]

Also, you may find it better to just do the sewer connection and a new inspection chamber on your site and do the rest when or after you do the foundations.

[Russell griffiths]

Have you had a topo survey?

if you have then it can be designed by anybody anywhere in the country as the information should be on the topo

as long as you give finished floor height an invert level for connecting main sewer. 

 

My drainage design consists of

4 toilets, 1 kitchen outlet, 1 boot room sink outlet, 9 rainwater outlets, treatment plant location and discharge, 

Full design ready for building regs submission was £750 plus vat. 

[Dreadnaught]

11 hours ago, Russell griffiths said:

Have you had a topo survey?

 

I have an old one but need a new updated topo survey. That's next on my to do list. Have a quote for one. I have asked the surveyor to include some other things, such as the access road (which is narrow). The thinking was that I could provide the dimensions to suppliers to help them plan their access but I am now thinking that pushes up the price of the survey for no good reason and besides when I mentioned the restricted access to one of the timber frame companies they simply popped by when they were in the area (and pronounced the problem to be manageable).

 

The survey quote I have is for £1,350 plus VAT, and includes:
 
1. survey of any buildings - their external footprint, plus any external steps and ramps
2. survey of external building services such as down-pipes, gullies, etc.
3. survey of all walls, fences and hedges to show type and height
4. survey of all changes of surface material - tarmac, concrete, paving, grass, etc.
5. survey of all manholes and service covers, plus pipe sizes and invert levels where possible
6. survey of all other features and furniture, plus any overhead poles and cables
7. survey of major trees to show position, girth, spread and height only
8. survey of levels across the area at a maximum spacing of 10 metres, plus all changes of height and slope
9. the survey will include the access road and details of the public highway kerbs, etc.
10. we will include height details of neighbouring buildings if possible.

 

Is this OK?

 

By the way, does finished floor height include the floor make up & floor coverings? I am still working on the foundations system. And have not decided on floor coverings yet.

 

On the foundation system, I had feared I would need a suspended floor with a sacrificial void for clay heave but I have just spoken to a local SE (the same SE as above) and, with a little urging from me, they have suggested it will be possible to design a insulated reinforced concrete raft/slab, MBC-style. The raft would be connected to piles. I have since gone back to MBC to ask them to ask their friendly SE to confirm this and am expecting a reply shortly.

 

Back to drains. Having skip read the building regs for draining, drainage does not look terribly complicated.

 

 

[Triassic]

38 minutes ago, Dreadnaught said:

 

55 minutes ago, Russell griffiths said:

Have you had a topo survey?

 

I have an old one but need a new updated topo survey.

 

Have you had an earth quake recently. Can’t imagine anything’s changed.

[Russell griffiths]

Have you had a flood risk Assessment, due to the river

ours stated minimum ffl we just worked off that

in the grand scheme of things this is house building not rocket science, 20-50mm up or down will not matter much it is a rough guide to determine if something is feasible 

on the day of the drain laying, two hungover loats will move it up or down and make it work. 

 

Also i found out when the survey guys came back to mine that their total station was only actually accurate to about 50mm, dependant on if they had a good signal to the satellite thingy, so height above see level was a bit hit and miss. 

[Ian]

@Dreadnaught

its easy to design private sewers that comply with the guidance laid out in the approved documents of Building Regs but it doesn’t always follow that you will then be granted permission to connect these to a public sewer.

 

Separate from Planning Permission and Building Regs approval there is a also a formal application that you must submit before you are allowed to make a connection to a public sewer. 

 

I’m working on a commercial project at the moment where UU are refusing our client permission to connect the site’s SW sewer to the public combined sewer in the road because the sewer in the road is already at its design capacity limit. Instead, they want our client to connect his SW sewer to a small stream that is located 400m away across a field that is not in our clients ownership.

[epsilonGreedy]

I did not view a drainage plan as a challenge that required a specific billable task to be offloaded to an external pro. Any decent main builder would be able to pace around your site and work up a plan in his head within 30 minutes.

 

As mentioned above you need to establish what flood risk conditions might influence your FFL, next is the invert level of your sewer interconnect, then you can work out the details.

 

You also need a big picture plan for all external trenchwork, where will your electric conduit enter the plot, can it live in the foul drain trench to save some money. How will other utilities such as gas be routed or will you need to accommodate a hole for an lpg tank. Next you should think about the more shallow stuff such as roof drainage and grey water. This entails designing your kitchen and thinking about any fancy sink drainage in an island counter. Do you want a sink in the garage, what about conduits for garden lighting and a powered driveway gate.

 

Swmbo moved the planned position for the garage sink so now I need to build an exit point elsewhere in the footings blockwork, this is an easy change with a suspended floor design, though you need to resolve the decision over your passiv slab or block & beam floor.

[Jeremy Harris]

12 minutes ago, epsilonGreedy said:

I did not view a drainage plan as a challenge that required a specific billable task to be offloaded to an external pro. Any decent main builder would be able to pace around your site and work up a plan in his head within 30 minutes.

 

As mentioned above you need to establish what flood risk conditions might influence your FFL, next is the invert level of your sewer interconnect, then you can work out the details.

 

You also need a big picture plan for all external trenchwork, where will your electric conduit enter the plot, can it live in the foul drain trench to save some money. How will other utilities such as gas be routed or will you need to accommodate a hole for an lpg tank. Next you should think about the more shallow stuff such as roof drainage and grey water. This entails designing your kitchen and thinking about any fancy sink drainage in an island counter. Do you want a sink in the garage, what about conduits for garden lighting and a powered driveway gate.

 

Swmbo moved the planned position for the garage sink so now I need to build an exit point elsewhere in the footings blockwork, this is an easy change with a suspended floor design, though you need to resolve the decision over your passiv slab or block & beam floor.

 

Not sure about "Any decent main builder would be able to pace around your site and work up a plan in his head within 30 minutes". 

 

My experience has been that very few builders I've met (in fact none so far) have been either aware of SuDS or know of ways to design SW drainage in order to comply with it.  Foul drains they do know about, but any build today not only has to have some means of dealing with roof run-off (where old fashioned soakaways may or may not work, depending on the soil conditions) but any hard paved or surfaced area (drive, patio etc) needs to have a SuDS compliant drainage scheme.  Combined sewers are becoming a thing of the past, I think, as water companies cut back on investment.  It's far easier for them to just say that there is no capacity available and so force the use of a SuDS compliant SW drainage scheme.  Around here, there are no combined sewers anyway, so everyone has to have a SuDS compliant SW scheme for any new build or alterations to add impermeable surfaces to an existing dwelling.

 

I speak from experience when I say that working out our attenuation requirements and designing both the attenuation tank and controlled rate drainage from that certainly took a lot longer than 30 minutes.  The very experienced and competent chap I had to lay the drive and install the underlying SW drainage works hadn't come across the need to do this sort of work before either, and I've seen two new drives built in the village that are non-compliant, so could have enforcement action taken against them.

[Mr Punter]

14 hours ago, Ian said:

I’m working on a commercial project at the moment where UU are refusing our client permission to connect the site’s SW sewer to the public combined sewer in the road because the sewer in the road is already at its design capacity limit. Instead, they want our client to connect his SW sewer to a small stream that is located 400m away across a field that is not in our clients ownership.

 

I am not surprised.  How is the land currently drained?  Having surface water discharging into combined or, worse, foul sewers, is not considered the way forward re SUDs, flooding, etc.

[Temp]

Going back to the OP...  If short of time I would just get the boat house to do a basic connection and pipe run to your boundary or manhole just inside you boundary, worry about the onsite detail later. 

[Russell griffiths]

@Dreadnaught are you actually close enough to the river to discharge your roof rainwater directly?

 

our hydrology guy measured our rainwater runoff and calculated that at such a small amount could be discharged straight to a water course, so no suds needed. 

[Mr Punter]

26 minutes ago, Russell griffiths said:

@Dreadnaught are you actually close enough to the river to discharge your roof rainwater directly?

 

our hydrology guy measured our rainwater runoff and calculated that at such a small amount could be discharged straight to a water course, so no suds needed. 

 

And the guidance accords with this:

 

“Generally, the aim should be to discharge surface run off as high up the following hierarchy of drainage options as reasonably practicable:
1. into the ground (infiltration);
2. to a surface water body;
3. to a surface water sewer, highway drain, or another drainage system;
4. to a combined sewer.”

[Russell griffiths]

We have over 260m of roof catchment area, but when calculated after a heavy downpour, it is not a significant amount of water to discharge directly. 

 

[Dreadnaught]

42 minutes ago, Russell griffiths said:

are you actually close enough to the river to discharge your roof rainwater directly?

 

Not quite, Russell. Not directly, but almost so.

 

There is both a small access road and the boat houses between my plot and the river. However, in the access road directly in front of my plot there is a storm drain that discharges straight into the river just 40m away. It is to this that I plan to connect. So effectively I am discharging straight into the river.

[Dreadnaught]

On 01/09/2018 at 22:07, Russell griffiths said:

Have you had a flood risk Assessment, due to the river

 

Yes. It was done for the original planning permission in 2015. You are right, I must dig out the FFLs from that.

[Jeremy Harris]

One potential gotcha with discharging to a watercourse or body of water is the vagaries of the Environment Agency.  The people there dealing with surface water run off into the stream alongside our plot were a total PITA, and went so far as to insist on a planning condition that we didn't allow any water from the plot to run across the lane into the stream (a complete joke because in heavy rain the lane turns into a stream full of mud, horse poo, gravel etc all washed down from higher up the valley and straight into the stream).

 

If you can avoid dealing with the EA then I would, but the chances are they will be consultees during your planning application (they were for ours) and so may well poke their nose in with regard to drainage.

 

The bizarre thing is that the other bit of the EA I dealt with, that looked after licenses to discharge treated sewage into a watercourse were really helpful, and granted me a license to discharge to the stream within an hour or so of me contacting them.

[epsilonGreedy]

4 minutes ago, JSHarris said:

If you can avoid dealing with the EA then I would, but the chances are they will be consultees during your planning application (they were for ours) and so may well poke their nose in with regard to drainage.

 

 

They influenced the drainage spec for my plot before I purchased it. I have to install 4.5m3 of storm crates for about 160m2 of roof area. In addition my drive must be gravel.

 

At the end of the day it looks like £2500, £700 for the crates, £1200 for the regulator and 1.5 days for a crew of two to dig and bury the tanks.

 

 

[PeterW]

2 minutes ago, epsilonGreedy said:

£1200 for the regulator 

 

What regulator..? Should just be an attenuation tank that holds the water and then allows to soak away slowly. 

 

With gravel drives, just be careful if you use MOT1 under the gravel as a sub base as if you compact it hard, it becomes like concrete and doesn’t allow infiltration. 

[epsilonGreedy]

9 minutes ago, PeterW said:

What regulator..? Should just be an attenuation tank that holds the water and then allows to soak away slowly. 

 

 

There is a more complex exit route to an open pond.

 

9 minutes ago, PeterW said:

With gravel drives, just be careful if you use MOT1 under the gravel as a sub base as if you compact it hard, it becomes like concrete and doesn’t allow infiltration.

 

 

I am starting off with 150mm of 2 inch clean as they say in these parts which is more porous than MOT1 I understand. Presumably it is the "fines" that tend to gum up the surface drainage?

 

I am going to give the drive a year to settle so no compacting planned at this stage. The digging contractor offered to drive over it multiple times with his monster jcb. 

Edited September 4, 2018 by epsilonGreedy

[Dreadnaught]

I found my proscribed FFL from the Environment Agency for 1:100 flood risk.

• The original planning permission for the old house design (not a passive house) had an internal FFL of 6.42 m ODN
• The plot is on average 6.29 m ODN
• The EA FFL is a minimum of 6.13 m ODN
• The EA 1:100 flood risk is 5.83 m ODN
• The storm drain in the road adjacent to my plot:
  • cover level 5.9 m ODN
  • invert level 3.8 m ODN
• The storm drain discharges into the river at invert level 3.41 m ODN

It occurs to me that fall from the plot to the drain is some 2½ metres. That strikes me as quite a lot.

 

 

Edited September 4, 2018 by Dreadnaught

[Jeremy Harris]

The EA proscribed my FFL as well, and the level for the garage FFL and any car parking area.  They seem to work to the house FFL being a minimum of 1.5m above the 1:100 year flood risk level, as far as I can tell from the planning conditions they imposed on us.  It's completely OTT in our case, as our stream is spring-fed from a source about a mile or so away and is less than 1m wide and about 200mm deep at the most.  It has been known to back up because of weed, debris etc getting stuck under the bridges after very heavy rain and for there to be an inch or two of flood water over the lane, but older residents reckon that has only happened a couple of times in the last 50 years or so, and even then the flood level was a good 2m below the level of our house.  Quite what it would take for this small chalk stream to rise a couple of metres I don't know, but I suspect it's just the EA applying a standard "one size fits all" approach, rather than looking at specific circumstances.

 

It's a nuisance for us, as it's meant having the house higher than it needed to be and that meant having a very steep drive, that continues to be a nuisance even now.

[Dreadnaught]

I just added above the FFL recommended by the EA. They recommend a FFL of 6.13 m ODN for a 1:100 risk level of 5.83 m ODN, a difference in our case of just 300mm, rather less than your 1.5m.

 

In our case I spoke to people who had lived locally for 20 years or more. They said that flood water has never even reached the access road (which is at about 5.9m ODN), let alone my building which will be at 6.42 or above with my no-dig and passive-raft foundation. However, with global warming, perhaps the future will not be like the past.

 

The annoyance for me is that I may need to put an evacuation door in the rear of my property (I have no garden). The EA considers the access road to be flood prone and so assumes I may need to high-tail-it across my neighbours' back gardens in the event of a flood. Rather ridiculous I think. Perhaps they are envisioning a Krakatoa-style flood with mere moments to respond. I would have thought a sedate egress in wellies would be more than sufficient!

Edited September 4, 2018 by Dreadnaught