Dreadnaught Posted February 18, 2019 Share Posted February 18, 2019 I have a quote for a SUDS-drainage design for my 20m x 20m plot of £1,350 + VAT. Is that high? Any recommendations for alternatives? (The plot is located in Cambridgeshire if that is relevant. I need to attenuate drainage from 120m2 of roofing to a max of 2 l/s.) Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 18, 2019 Share Posted February 18, 2019 Any reason you can’t do this yourself Is there no storm drain that goes to the river. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 18, 2019 Share Posted February 18, 2019 I did ours myself. Not hard to do, as it'll pretty much be dictated by the layout and topography of the plot and the soil conditions. My choices were pretty non-existent, as there was really only one possible solution, dig a big hole under where the drive was going to go and stick 20 crates in it, together with pipes out to all the downpipes. Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 18, 2019 Author Share Posted February 18, 2019 (edited) Thanks @Russell griffiths, @JSHarris . Yes, there is a lovely storm drain immediately outside may plot that drains into the river less than 40 metres away. But I just spoke to council drainage person and it seems that the new local plan requires SUDS down to 2 l/s anyway. I might be able to do it myself but am concerned about getting it right. I have a big no-dig zone so my amateur guess is I will need a biggish (1500l?) above-ground attenuation tank sited on firm footings outside the root-protection zone (as it will weigh a lot when full), like this… Edited February 18, 2019 by Dreadnaught Link to comment Share on other sites More sharing options...
Pocster Posted February 18, 2019 Share Posted February 18, 2019 Even I did my SUDS - real easy though a bit of back ache ? Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 18, 2019 Author Share Posted February 18, 2019 How would I calculate the drainage rate from my 120 sq.m roof area? I assume I need to find some standard figure for rainfall per sq.m in my area that will withstand scrutiny from the planners. Is that right? How then to convert that into a flow rate and a calculation for how big an attenuation tank I will would need to achieve a max flow rate into the storm-water sewer of 2 litres per second? Anyone know? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 18, 2019 Share Posted February 18, 2019 2 minutes ago, Dreadnaught said: How would I calculate the drainage rate from my 120 sq.m roof area? I assume I need to find some standard figure for rainfall per sq.m in my area that will withstand scrutiny from the planners. Is that right? How then to convert that into a flow rate and a calculation for how big an attenuation tank I will would need to achieve a max flow rate into the storm-water sewer of 2 litres per second? Anyone know? Use the chart and method in the building regs, Part H3, Section 1 1 1 Link to comment Share on other sites More sharing options...
Mr Punter Posted February 18, 2019 Share Posted February 18, 2019 (edited) Yes as @JSHarris says. Assuming the most negative you would have 0.016l/s/m2 so 0.016 * 120 = 1.92l/s. Can I have some of the £1,350? Edited February 18, 2019 by Mr Punter 2 Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 18, 2019 Author Share Posted February 18, 2019 3 minutes ago, Mr Punter said: 0.016l/s/m2 so 0.016 * 120 = 1.92l/s. Thanks! When I look at the map in the Building Regs for Rainfall intensities below for "design of gutter and rainfall pipes (litres per second per square metre)", why is the intensity higher in East Anglia (0.020 or 0.022) than say Pembrokeshire (0.016) when East Anglia has some of the lowest rainfall in the country? Seems counterintuitive. Link to comment Share on other sites More sharing options...
Andrew Posted February 18, 2019 Share Posted February 18, 2019 Just as a data point our SE is doing the foundation design, drainage design, highway access design and SUDS / surface water design for a total of £1680 + VAT. That's not broken down, so I'm not sure what part of that is the SUDS / surface water work. I don't think it's particularly challenging on our plot but I haven't had the design yet. Plot is in the North West and approx. 1000 sq. meters in size. 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 18, 2019 Share Posted February 18, 2019 9 minutes ago, Dreadnaught said: When I look at the map in the Building Regs for Rainfall intensities below for "design of gutter and rainfall pipes (litres per second per square metre)", why is the intensity higher in East Anglia (0.020 or 0.022) than say Pembrokeshire (0.016) when East Anglia has some of the lowest rainfall in the country? Seems counterintuitive. I agree, every time I've looked at this there seem to be apparent anomalies, but I suspect that's to do with it being an intensity map, rather than a rainfall map. Places on the West coast of Scotland, for example, are well known for being very wet, but they don't tend to get lots of very heavy rainfall, it just rains gently every other day... Link to comment Share on other sites More sharing options...
Mr Punter Posted February 19, 2019 Share Posted February 19, 2019 Oh dear I was using the paved are data. Should be 2.64! Link to comment Share on other sites More sharing options...
epsilonGreedy Posted February 19, 2019 Share Posted February 19, 2019 16 hours ago, Dreadnaught said: I have a quote for a SUDS-drainage design for my 20m x 20m plot of £1,350 + VAT. Is that high? Any recommendations for alternatives? Borrow mine which came with my plot purchase and was one of the satisfied conditions. Or fast forward to the answer and no need to wait millions of years. The answer is 42 or with some penny pinching rounding down = 4 cubic meters. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted February 19, 2019 Share Posted February 19, 2019 14 hours ago, Dreadnaught said: why is the intensity higher in East Anglia (0.020 or 0.022) than say Pembrokeshire (0.016) when East Anglia has some of the lowest rainfall in the country? Seems counterintuitive. Days of non stop Welsh drizzle does not = intensity. We get more summer tropical downpours in the east I guess. Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 19, 2019 Author Share Posted February 19, 2019 (edited) 1 hour ago, Mr Punter said: Oh dear I was using the paved are data. Should be 2.64! Ah, I think you were right the first time @Mr Punter. Reading through it all, the rain-intensity figures for drainage come from Diagram 2, which also mentions "underground rainwater drainage", not Diagram 1 which is what I showed and is for roof drainage. As follows under "Section 3: Surface water drainage" … Quote 3.8 Design rainfall intensities of 0.014 litres/ second/m2 may be assumed for normal situations. Alternatively the rainfall intensity may be obtained from Diagram 2. From diagram 2… So I should use 0.014 or 0.016 l/s. Now, I just need an exact measure of the roof area including the overhangs from my architects drawings. I suspect it is a bit greater than 120 m². But even if the drainage rate exceeds 2 l/s I suspect it won't do so by much so a little attenuation is all that I will need to hopefully fall into compliance and satisfy the drainage person in the local planning department. The drainage person was adamant that any attenuation device would need to be "permanently installed". I presume they have instances of a modified water butt doing the job which was whipped away as soon as their backs were turned. I am not sure what permanently installed means in this context. And she also said that a water butt would not do anyawy as it could be full on the day of a deluge and so do no attenuating. This has been very helpful. BuildHub continues to amaze! Thanks to everyone. Edited February 19, 2019 by Dreadnaught 1 Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 19, 2019 Author Share Posted February 19, 2019 38 minutes ago, epsilonGreedy said: Borrow mine which came with my plot purchase and was one of the satisfied conditions. Thanks @epsilonGreedy, yes please. That could help me put my home-brew numbers in a form that they might like & expect. Presentation matters! Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 19, 2019 Author Share Posted February 19, 2019 34 minutes ago, epsilonGreedy said: Days of non stop Welsh drizzle does not = intensity. We get more summer tropical downpours in the east I guess. Good point. As @JSHarris also mentioned. That sounds like the answer. It is interesting to consider why the figures in Diagrams 1 and 2 in the Building Regs are so different. Both measure rain intensity but one is for gutter sizes and the other for underground drainage pipes. It is unclear to me why it pays to oversize gutters but not underground pipes. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted February 19, 2019 Share Posted February 19, 2019 1 hour ago, Dreadnaught said: Thanks @epsilonGreedy, yes please. That could help me put my home-brew numbers in a form that they might like & expect. Presentation matters! Sent as a private message. 1 Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 22, 2019 Author Share Posted February 22, 2019 With your collective help, I have progressed a little further my drainage but now would welcome a bit more help. My roof area is 168 m2. The rainfall intensity is 0.014 litres/second/m2 from Building Regs. This gives a surface water drainage rate of 2.35 litres/second. I need to get this down to 2.0 litres/second for planning so am thinking to use an attenuation tank like the one below. This one happens to be 1,500 litres. How do I calculate the size of tank I need in a way that would convince my local council's drains person? Or is this the point where I need to involve an drains engineer? Any help appreciated. Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 22, 2019 Author Share Posted February 22, 2019 After I posted this above, one of the online suppliers of tanks offered to do the necessary calculations for me for £200 + VAT, which would be reimbursable against the cost of a tank. Sounds tempting. Is that a good deal? Link to comment Share on other sites More sharing options...
PeterW Posted February 22, 2019 Share Posted February 22, 2019 OK so the M-10 M-60 rainfall values are what you are interested in. If you assume that M-10 is 600 seconds, then you will have to store 0.35l/sec or 210 litres. If you took the exceptional M-60 rainfall (ie 3600 seconds) you need to store 1,260 litres. I would expect that you could use the M-10 to calculate the minimum and then suggest that using the M-10 +200% would be a reasonable factor, so you would need 630 litres. I can write this up properly, and based on your quote above will charge you an attractive £150.... This may help... 1 Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 22, 2019 Author Share Posted February 22, 2019 Thanks @PeterW. That's very helpful information. Am reading it all now! Link to comment Share on other sites More sharing options...
epsilonGreedy Posted February 22, 2019 Share Posted February 22, 2019 3 minutes ago, Dreadnaught said: Thanks @PeterW. That's very helpful information. Am reading it all now Is the type of tank shown in your photo suitable? It is more than a matter of capacity as the tank needs to soak away or drain somewhere at an attenuated rate. Underground crates are often used wrapped in a water permeable fabric. Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 22, 2019 Author Share Posted February 22, 2019 5 minutes ago, epsilonGreedy said: Is the type of tank shown in your photo suitable? It is more than a matter of capacity as the tank needs to soak away or drain somewhere at an attenuated rate. Underground crates are often used wrapped in a water permeable fabric. Good question. I think it is suitable. But I will double check with the supplier. Unfortunately, I can't have a crate-based soak away because none of my site is 5-metres from a building (a building regs restriction I think). Therefore the rain water will go into said tank and then drain slowly (at 2 l/s) into storm water drain in the road immediately in front of my plot and thence into the river just 40m away. Anyhow that's what I hope planning will agree to. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted February 22, 2019 Share Posted February 22, 2019 8 minutes ago, Dreadnaught said: ... Therefore the rain water will go into said tank and then drain slowly (at 2 l/s) into storm water drain in the road immediately in front of my plot and thence into the river just 40m away. Anyhow that's what I hope planning will agree to. Sounds like a plan, I guess you have the gradient to drain from the bottom of the tank into the storm water drain? Link to comment Share on other sites More sharing options...
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