Drainage design — right approach?

[Mr Punter]

Our EA flood level - on the river bank - is 1:100 years plus 300mm.

[epsilonGreedy]

8 minutes ago, Dreadnaught said:

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.

 

 

£5.5 million buys you an exciting solution to that problem. Can we all come around and test it?

 

http://news.bbc.co.uk/1/hi/wales/south_west/4305229.stm

[epsilonGreedy]

1 minute ago, Mr Punter said:

Our EA flood level - on the river bank - is 1:100 years plus 300mm.

 

 

Mine is +300mm also though in my case the concern is a flash flood rather than a flood plain type flood.

[Dreadnaught]

The level difference between the ground level of my site and the drains looks to me quite large. In the case of the foul drain, it looks to be 1½ m. In the case of the surface water drain, 2½ m. 

 

Is that a concern? A 1:40 slope would not do it, of course, the run is only about 20 m. I assume a drop needs to be achieved. How are these usually done, some form of chamber? I am also slightly concerned about working with such deep excavations.

[Temp]

Quote

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

 

It is but see "Drop Shaft or Back Drop Connection" on this page...

 

http://www.pavingexpert.com/drain05.htm#dropshaft

 

Allows you to keep below the max recommended falls on the pipes.

 

Edit: Note the use of a large radius bend at the bottom of the drop in the photo. I think this is safer than the tight bend shown in the diagram.

 

 

 

Edited September 4, 2018 by Temp

[Mr Punter]

3 minutes ago, Dreadnaught said:

The level difference between the ground level of my site and the drains looks to me quite large. In the case of the foul drain, it looks to be 1½ m. In the case of the surface water drain, 2½ m. 

 

Is that a concern? A 1:40 slope would not do it, of course, the run is only about 20 m. I assume a drop needs to be achieved. How are these usually done, some form of chamber? I am also slightly concerned about working with such deep excavations.

 

Dig it all as shallow as possible with a fall of 1:40 to 1:60 connected to a chamber near the boundary of your property, then you can connect to the sewer at any gradient you like as long as it is greater than 1:60.   So you may be at, say 600 invert at your last chamber and drop down from there to the 2.5m on the foul drain, where you may have a radiant like 1:1.

 

I think with surface water you may even be allowed to connect in with an internal backdrop but you would need to check.

[Dreadnaught]

Thanks @Temp. Fascinating link.

 

I note it says "anything deeper than 1000mm requires a brick-built or concrete section manhole". I assume that applies to both my connections, foul and surface. I see on my drain survey that for each drain there is a manhole only a few metres away. I guess that it is easier to run to the existing manhole(s) rather than intercept the drains at a point closer and make new ones.

[Mr Punter]

Depending on the main sewer size you may be able to connect via a saddle connection.

[epsilonGreedy]

6 minutes ago, Mr Punter said:

Dig it all as shallow as possible with a fall of 1:40 to 1:60 connected to a chamber near the boundary of your property, then you can connect to the sewer at any gradient you like as long as it is greater than 1:60.   So you may be at, say 600 invert at your last chamber and drop down from there to the 2.5m on the foul drain, where you may have a gradiant like 1:1.

 

 

Is that general industry practice? I thought foul drains could only be formed from shallow gradients plus vertical drops.

[PeterW]

2 minutes ago, Dreadnaught said:

Thanks @Temp. Fascinating link.

 

I note it says "anything deeper than 1000mm requires a brick-built or concrete section manhole". I assume that applies to both my connections, foul and surface. I see on my drain survey that for each drain there is a manhole only a few metres away. I guess that it is easier to run to the existing manhole(s) rather than intercept the drains at a point closer and make new ones.

 

That site is out of date. 

 

Anything below 1600mm invert needs a restrictor ring to 300mm, unless it has inbuilt ladder access which requires a concrete ring of min 1050mm width. 

 

450mm plastic can be used for depths up to 3200mm with the appropriate reducer ring. 

 

If you have an existing manhole, do you mean on the main sewer run or on your land ..? Whilst connections to existing manholes may appear easy on paper, if they are old brick built manholes then you may find there is no branches avaialble and you would need to replace the whole manhole. If it is less than 380mm (ST rules here) then you can saddle onto it down to 100mm where you need to use a branch. 

[PeterW]

2 minutes ago, epsilonGreedy said:

 

Is that general industry practice? I thought foul drains could only be formed from shallow gradients plus vertical drops.

 

Yep perfectly acceptable. You cannot use a backdrop though on a public sewer. 

[Dreadnaught]

11 minutes ago, Mr Punter said:

Depending on the main sewer size you may be able to connect via a saddle connection.

 

Ah, interesting. Makes sense.

 

7 minutes ago, PeterW said:

If you have an existing manhole, do you mean on the main sewer run or on your land ..?

 

Main sewer run in the access road. There is a storm-drain manhole and an "unable-to-lift" foul sewer manhole, both within 10 m of my boundary in the road.

Edited September 4, 2018 by Dreadnaught

[Dreadnaught]

5 minutes ago, PeterW said:

If it is less than 380mm (ST rules here) then you can saddle onto it down to 100mm where you need to use a branch. 

 

The foul sewer is ⌀ 100 mm. The surface water drain is ⌀ 300 mm.

[Jeremy Harris]

Interesting that the EA have set FFLs at +300mm above the 1:100 flood risk level elsewhere, yet seemed to go OTT with us.  I've just checked and they actually made us build 2m above the 1:100 year level, according to their own data, not 1.5m as I thought.  This is what they stipulated be a condition of our planning consent in their consultation response:

 

Quote

11. Finished floor levels within the dwelling shall be set no lower than 83.6m AOD and the for the garage/parking area set no lower than 83.1m AOD.

Reason: to minimise flood risk

 

The stream normally flows at about 80.9m AOD, and the EA provided me with a flood risk map showing that the 1:100 level was 81.6m AOD.  The lane next to the stream at it's lowest point at the end of our drive is 81.45m AOD, so about 150mm below the 1:100 level.

 

I argued strongly with them that we should be allowed to drop the house and garage down by at least 1m, just so we wouldn't have such a steep drive, which would still have had the house FFL at 82.6m AOD, 1m above their 1:100 level, but they were absolutely adamant that they would not accept that.

 

It's another case of one set of rules being applied to some and another set of rules being applied to others, by the same government department.

[epsilonGreedy]

21 minutes ago, PeterW said:

450mm plastic can be used for depths up to 3200mm with the appropriate reducer ring. 

 

 

Pleased you posted that, I was beginning to think that my foul drain was not to regs, as it is 1.6m deep built with sections of 450mm rings.

Edited September 4, 2018 by epsilonGreedy

[Dreadnaught]

39 minutes ago, PeterW said:

Anything below 1600mm invert needs a restrictor ring to 300mm

 

For those like me who did not know, the ring is for safety to stop a child falling down the hole. The stipulation is in Building Regulations Part H.

Edited September 4, 2018 by Dreadnaught

[Temp]

Depending on the lie of the land it might be better to have a manhole half way along the 20m run. If the fall is 2.5m over 20m then 1:40 uses up 0.5m leaving 2m. Perhaps use two 1m high drops, one near the house and one at the mid point or part way along. Lots of options if that doesn't work for some reason. Increasing the number of manholes can reduce the depth of the trench for the pipe as well as for the manhole.

[Jeremy Harris]

2 hours ago, epsilonGreedy said:

 

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.

 

 

 

 

Keep an eye on Ebay for crates.  I managed to buy 20 off heavy duty Aquacell crates for a ludicrous price - I think I paid a fiver each for them.  A local contractor had a pile of them left over from a highways job.  IIRC, he charged me as much to deliver them as I paid him for the crates.

 

You quite often seem to get soakaway crates come up on Ebay, probably worth also looking at places like Gumtree, too.

[epsilonGreedy]

1 minute ago, JSHarris said:

Keep an eye on Ebay for crates.  I managed to buy 20 off heavy duty Aquacell crates for a ludicrous price - I think I paid a fiver each for them.

 

 

It looks like mine will need to domestic drive grade able to take the weight of light vehicles. I will need to delay laying these until bricks and tiles are onsite as a minimum though at that sort of price I would be prepared to store them.

[Jeremy Harris]

The ones I bought were the blue Aquacell ones, so fine under a drive.  They are rated at 56 tonnes/m² so a bit of an overkill for a domestic drive.

[Mr Punter]

1 hour ago, epsilonGreedy said:

 

Is that general industry practice? I thought foul drains could only be formed from shallow gradients plus vertical drops.

 

No, as long as it is over 1:60 there is no maximum. You should, however, provide access / inspection points at the change in gradient.

[Dreadnaught]

5 minutes ago, Mr Punter said:

No, as long as it is over 1:60 there is no maximum. You should, however, provide access / inspection points at the change in gradient.

 

So could I just lay a drain at 1:8 over 20 m to achieve my 2.5 m drop, no chambers needed? Presumably not.

 

(Or 1:13 for the foul drain?)

Edited September 4, 2018 by Dreadnaught

[Russell griffiths]

Surely a fire escape spec window would be good enough for flood escape 

no door needed if you don’t want it

if your single storey most of your bedroom windows will be fire escape types. 

Im sure I will be exiting the building far quicker in a fire than a flood. 

[Dreadnaught]

Good point @Russell griffiths. The only problem is that I don't have any windows at all from either of the bedrooms and also none from other rooms on those sides of the house. Instead I have lots of large roof lights.

[Russell griffiths]

Rope ladder ?