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SUDS drainage design quote? High?


Dreadnaught

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4 minutes ago, epsilonGreedy said:

I guess you have the gradient to drain from the bottom of the tank into the storm water drain?

 

Fortunately that I do. 

I am a bit concerned about what such a tank should stand on. It will be 1.5 tonnes when full of water on a base that is 120 by 78 cm.

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5 hours ago, Dreadnaught said:

 I am a bit concerned about what such a tank should stand on. It will be 1.5 tonnes when full of water on a base that is 120 by 78 cm.

 

 

I was going to say look at the spec for an underground LPG tank, adjust for the density of LPG but add back the extra weight of the tank but then I realized the cylindrical shape of the tank would be supported by the back fill.

 

Anyhow I make your pressure per cm2 to be 0.16 of a kg so not a major civil engineering challenge. What soil type do you have a meter down?

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6 minutes ago, epsilonGreedy said:

Anyhow I make your pressure per cm2 to be 0.16 of a kg so not a major civil engineering challenge. What soil type do you have a meter down?

 

Thanks @epsilonGreedy

 

image.thumb.png.aec8db2e8d7148d6b0047b22cc91992e.png

 

And then Gault Clay underneath all the way to the centre of the Earth (or at least that's what the borehole that went 10m down suggests) :) 

Edited by Dreadnaught
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I doubt the Gault goes down that far, as I believe the deepest layer in the UK is only around 100m deep, and where you are it's probably around the same depth as here, about 40m or so.  We have Gault under our house (we built on the top of the layer) and found that it's about 40m deep, with about 12m of Lower Greensand underneath it (where our water comes from) and the Wealden Group underneath that (we could tell when our driller hit that, as it's as hard as hell, and the drill pretty much stopped going anywhere).

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18 hours ago, Dreadnaught said:

 

Fortunately that I do. 

I am a bit concerned about what such a tank should stand on. It will be 1.5 tonnes when full of water on a base that is 120 by 78 cm.

 

2 no 90cm x 60cm 50mm pressed council slabs sat on whatever is underneath.

 

About £6 each.

 

Permanent installation? Set in minimal mortar, or glue.

 

PS I think the roof numbers are higher due to water velocity running into gutters, and that roofs do not divide roof area uniformly across drainage routes, so it is a increased to reflect possible disproportionate maximum intensity for a single drainage route.

 

F

Edited by Ferdinand
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A serious question.

 

How is the flow attenuated down in this setting? 

 

I know how it is done on a balancing pond, ‘cos I had to have one designed ... they use an electronically controlled outlet with a flow regulator.

 

But if it is just a big water tank with an in and and out, how is that done? is there some kind of pressure controlled outlet that gives the max allowed flow when the tank is full, or is there some other clever mechanical or electronic gubbins?

 

@JSHarris does not have that challenge, as his SUDS is a soakaway-with-storage iirc.

 

Ferdinand

Edited by Ferdinand
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32 minutes ago, JSHarris said:

Just a small pipe leading from the attenuation tank that drains it away and that can't flow at more than the stipulated rate, I expect.

 

That would be my pressure controlled outlet ,,, ?.

 

And pipe diameter was how they regulated the continuous water supply to individual properties in Ancient Rome.

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1 hour ago, Ferdinand said:

2 no 90cm x 60cm 50mm pressed council slabs sat on whatever is underneath.

 

About £6 each.

 

Thank you. That's nice and simple.

 

1 hour ago, Ferdinand said:

Permanent installation? Set in minimal mortar, or glue.

 

Yes,  I suspect the council will want evidence of permanence. I was mentioned, unprompted, on the phone when I called.

 

1 hour ago, Ferdinand said:

But if it is just a big water tank with an in and and out, how is that done? is there some kind of pressure controlled outlet that gives the max allowed flow when the tank is full, or is there some other clever mechanical or electronic gubbins?

 

 

Interesting question. Here's a picture of the device. It connects to the outlet. The hose is suspended vertically in the water to terminate just below the surface by the floating ball. This ensures a constant head of water so constant pressure so constant flow rate. The top of the hose connected to the ball, with the brass connector, has a mesh filter. Clever!

 

To adjust outlet flow rate, if desired, I assume you can adjust the length of the link to the ball. A longer linkage would increase the flow rate, shorter would reduce, as this defines the pressure from the head of water above.

 

image.thumb.png.67e0f9aadc000eb6a3344aedee2fafc5.png

Edited by Dreadnaught
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  • 2 weeks later...

Original poster here. My negotiation continues with the drainage lady in the council. The good news is that, I have met the need for 2-litres-per-second discharge rate with an attenuation tank. Thanks for all your help with that!

 

My next requirements from her are:

  1. I must pre-treat the rainwater. Not sure what that means. I'm guessing it is to remove fine silt from the roof. Anybody know?
  2. And I must ensure that the first 5 mm of any rainfall event does not enter into a sewer.

For (2) by my calculation 5 mm represents 875 litres of rainwater. The obvious solution would be the oh-so-often-specified 1 m³ soakaway crate buried somewhere that is 5 metres from the house* such as under the drive. But in my case that's not allowed as I have a root-protection zone so there is no location on my land that is 5 metres from a building.

 

Any other thoughts or ideas?

  • Above ground soakaways don't exist do they (i.e. a flower bed)?
  • I could try and convince one of the neighbours to allow me to bury a crate under their lawn. I can imagine their faces when I ask.
  • As a last resort, I could convert my pitched roof into a green roof. The drainage lady has said that that would work as a solution but its a drastic re-design.

* a building regs requirement I believe.

 

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