I did that!
A074754 – 229 – 300 – Drainage Layout
…Unless it doesn’t work in which case someone else did it!
Today I received a pdf of the drainage design I did back from the drafty in Nottingham who drew it for me (see link above).
The site is an old National Grid gas storage site. A large part of it was sold off to Barrett homes who then built houses on it. In doing so they destroyed the drainage outlet and so now the site floods. A lot. So much so that the water is spilling over a small retaining wall at the north of the site and into a timber yard. The timber yard then floods and they claim against National Grid for the damage to their stock. National Grid have been saying to them for ages (the houses were build nearly 10 years ago) that they would do something about it and are only getting around to it now because they want to demolish the northern gas storage tank and the ground it too wet and soft to get the plant across.
To the west of the site and atop a retaining wall is another development. The drainage behind the retaining wall discharges onto the National Grid site and is causing the majority of the flooding. A manhole was once constructed there but never connected to anything. We’ve devised a ground drain to deal with the surface water while also lowering the ground water. This feeds into a new underground drainage system which then discharges into the river the far side of the road in the east.
The drainage design was pretty straight forward. I designed a similar temporary drainage solution during Phase 2 when the basement flooded (there is a blog about that one too). For this one all the leg work had been done for me, a drainage specialist in another office had run simulations and done other clever computer things and advised on a 150mm dia pipe running at a minimum fall of 150mm.
So all I had to do was draw some straight lines on a site diagram and specify the manhole types, invert levels and a few other bits and pieces that i’s learnt when doing it on site. Doing it on site previously actually gave me a huge advantage, I knew to specify a rocker pipe connection, a geotextile membrane and rocker pipe connections. All that stuff which is easily overlooked but important to making it work and not looking like a knob.
National Grid want to sell this bit of land as soon as the gas storage site is gone (probably to Barrett again) and so don’t want any on-going maintenance issues. Therefore the design must comply with the Anglian water adoptable sewers regs. So with a bit of reading I made sure the manholes, falls, diameters and all the other bits met the spec.
Now comes the hard part… In order to discharge into the river I need two things:
- Consent from the Environment agency to discharge into the river – this is actually easier than you’d think. We tested the water so we know it’s clean enough, so all we do is show them how we’ll be working safely next to the river and how we won’t constrict the flow. They then give us consent.
- Section 50 consent to dig up the road from the highways agency. This is more difficult. I’ve submitted our design Colchester tomorrow to meet with some bloke to take about crossing the road. Apparently they can only talk to me on site, they wouldn’t even tell me what depth they wanted it under the road in order to miss other services until I’ve met the bloke. Not sure what all the cloak and dagger stuff is about but it’s a day out of the office I suppose! The really kicker is that before they’ll give you consent you have to give them a cheque for however much they think it’ll cost to reinstate the road in case you dig a massive hole then leave it (or go bust).They also charge you for maintenance of whatever service you’re instating despite the fact that they also ask you to prove it will be adopted by the relevant undertaker.
I’m also told the bloke in question is mega boring, I’ll let you know how it goes…
Holdfast Training Services Trial Blog 
Your straight lines look good to me… I’m interested about the whole Highway’s Agency process. I assume when you say ‘you have to give them a cheque’ that this is a bond of some form that comes from NG as the client and dealt with by a project manager/architect. Is this your firm or some other outfit?
Sounds like a good ‘yes saheb’ day tomorrow
It is. They will also accept a bond when the numbers get big. although on my boss’s previous job the client just wrote a cheque for £600k! We’re project managing the process too, so we ask the client for the cheque payable to the highways agency and pass it on.
Guz, Looks like you have been busy. Manhole looks familiar! A few general questions – what is the catchment area/ where is the timber yard (am I being blind)? It looks like mostly to west of the green perforated pipe, but is there some from the housing area too – if so, will there be issues with vehicles contaminating the water? Does the fly tipping have to be cleared before consent is given or are test results sufficient for discharge consent? It looks like the perforated pipe almost runs along a ditch which presumably is the idea to have the water drain in.
On a more detailed point, presumably the perforated pipe is plastic – how does it join into the manhole? Some sort of band seal to a solid pipe, or just cast straight in?
The catchment area in principle id pretty small, just that shown on the drawing, but because it also has inflow from the drain behind the retaining wall the flow rates get much bigger. The timer yard is off the drawing to the north. The housing area in the east has it’s own surface water drainage so we don’t need to worry about that bit. I was worried about contaminants in the water so we had it tested by the environmental team and they said it came back clean, so no dramas.
The seal into the manhole is literally cast in. It’s a plastic corrugated pipe so has enough flexibility to be cast in but also enough surface texture to bond to the concrete. Winner all round.
I guess this will be given to a contractor who will make the sensible changes and save himself some cash…
I’d expect to see the geotextile wrap to the slot drain lapping over itself on top of the tench with 4″ of single size cover over that rather than turned back to the sides. That way you can dig out and replace the top every time it gets completely clogged up and won’t have fines throughout your drain.
Sensibly you might have put a single chamber on the corner and another one on the site boundary with a straight line between – chambers cost money!
Have you looked at the opening size for your chamber and thought about the depth (1800-600-150=not much room to work in so I’d look to 1200×600 and bin the step irons!
Do you know what invert level you need to make at the river outlet? Have you asked EA for standard headwall details? Are they restricting your discahge in terms of flow rate or discharge velocity?
Good point on the top wrap on the textile, I might change that. Thank you.
Regarding the number of chambers – I did that originally but had to add in others to ensure the system was adoptable by Anglian water.
And again, the manhole design is prescriptive from Anglian water. The key here was to make it adoptable so that National Grid could be guaranteed to be able to sell the land once the gas storage tank is gone.
No restrictions on discharge provided the water is clean as the natural path for all the water would have been into the river anyway. The river is tidal in that area with a 1.2 meter maximum difference between the low and high tide. We’ll sit within these so the outlet requires a covering flap to ensure that the sewer doesn’t flood at high tide.