9 Feb 2011


I've always had this nagging feeling that there was something amiss in the world of SUDS. I thinks the whole acronym thing is a clue here. SUDS stands for Sustainable Urban Drainage Systems. I have even seen it written as Sustainable (Urban) Drainage Systems. It's that Urban bit that stands out. Why in particular should a drainage system be Urban, rather than rural or suburban?

And if it is urban, should SUDS be restricted to urban areas? Are SUDS necessarily a green solution to the problem of surface water run-off? Or just a good solution for densely packed areas where flooding might be a problem?

Maybe it's that word Sustainable at work again, but SUDS have somehow been incorporated into the Code for Sustainable Homes as a must-have feature. You might think that a good old fashioned soakaway would be just the ticket but the Code calls for something a little more sophisticated than this. It requires you to provide robust hydraulic design calculations referred to in key guidance documents such as The SuDS manual (CIRIA C697, 2007) and Preliminary rainfall runoff management for developments (EA/DEFRA, 2007).

Blimey. Sounds like you need an expert! Indeed you do. Suitable professionals may be found in a variety of disciplines, such as engineering, landscape design or hydrology.

Now experts have a habit of recommending complicated solutions and the Code experts have cooked up a real beauty here. It now seems that if your site doesn't pass the porosity test for normal soakaways, then the Code requires that you fit a rainwater harvesting system to act as a holding tank for surface water run-off. Now rainwater harvesting systems are all fine and dandy (or at least many people think they are) but not even their biggest fans think that they are to be confused with SUDS, as their purpose is quite different.

A SUD system is basically just an engineered-but-still-dumb soakaway, whereas as rainwater harvesting system needs a good deal of management and lots of bits added onto it in order to function correctly. As well as being expensive to install (maybe £3,000 per property), they also have to be maintained.

But the way it stands, if you are on a site where conventional soakaways will not work then, in order to get to Code Level 3, you will have to fit Rainwater Harvesting. Not to deliver low levels of water use within the household (for which it is designed) but to act as a SUDS-compliant soakaway (for which it isn't). And that would seem to include sites well away from urban areas.

Have a look at this video of some of the recent Queensland flooding and ask yourself how your rainwater harvesting system would have coped. Or, for that matter, just about any SUDS system you could dream up.


  1. Another interesting post Mark.

    We did something rather like this on our very heavy non-draining site. I built a rather ramshackle brick tank (essentially a hollow patio) to collect our rainwater because it seemed getting it to run down the drain was going to be tricky and ugly due to awkward levels. It's crude but it works.

    I don't suppose it would count as kosher because it's a very DIY system and there were no 'calculations' at all never mind robust ones. If only we'd consulted an expert, eh?

    Is there no end to these self-serving regulations and jobs for the boys?

  2. There will be no end to self-serving regulations and jobs for the boys because that is the system that has been stealthily created. PFI is the worst example, but you see such banditry everywhere now.

  3. Thanks for flagging this up Mark but it is even more complicated than you describe!

    It is not just for Code level 3 as Sur 1 is a mandatory requirement to get any Code level at all but as Housing Associations have to achieve Code 3 (rather than nil point) then it is a particular issue for them.

    Site I am looking at can't have soakaways (for reasons we have not gone into) but does have Severn Trent storm sewer that is happy to take whatever the few houses can throw at it. The engineers have designed a large attenuation tank to control flow rates down to green field runoff rates in 1:100 year storm events plus climate change prediction and that is all fine and dandy.

    Problem is Sur 1 compulsory credit (for greenfield sites) requires the total VOLUME during said storm to be reduced. ideally by soakaways but if not then by RWH unless it can be shown that tanks can't be fitted etc.

    Having wasted days many years ago building a spreadsheet model for the EA to prove, among other things, that RWH helps reduce storm flows and then finding it doesn't, I assumed this was all just a Code assessor getting the wrong end of the stick.

    I suggest that as people watch the flooding video (or look at pictures of Tewksbury under water etc etc) imagine just how much different it would have been if all the buildings had rainwater harvesting tanks and people had flushed the loo a few times before the rain set in. OK common sense can be wrong but a hydrogeological model gives the same answer - it makes no measurable difference.

    As with Carbon we are blowing silly amounts of limited cash on stuff that doesn't work and taking our eye off the mind numbingly huge environmental issues that are about to hit us.

    What to do? Many of us have flagged up these issues to the powers that be but we are just a few voices among the many stakeholders and open rational has debate has to happen on blogs like this.

  4. David OlivierFebruary 11, 2011

    This is what a country gets by privatising everything and doing no public srector buildings research. Little is done in the national interest; rather, most actions turn out to serve the interests of people who are determined to earn a living from designing or selling complicated and expensive solutions and trying to discourage or exclude cheap and simple ones.

    Personally I've noted that my site whose soil is rich in organic matter had little surface water accumulation even before the soakaway was built. Being a slow self-build, this means a lot more than one season.

    The soil is a silt loam - heavy, but not intractable. The water-holding capacity of clay soils, if percolation is aided by high organic matter content, is at least as high as that of loams or sandy soils.

  5. RWH systems are several orders of magnitude smaller than they would need to be in order to provide flood attenuation. Even the innumerate cretins at BRE should be able to work this out. Acceptable runoff rates are measured in litres/second/hectare. How exactly is a 6 litre toilet flush going to help with this?

    But the best bit about SUDS? They don't actually prevent flooding either! Take a look at the data from the 'flagship demonstration project' at Lamb Drove. Retains flows at very low rainfall intensities, but has almost identical performance to a neighbouring non-SUDS site after a fairly insignificant rainfall event.

    Not to mention the fact that they won't protect against fluvial, coastal, tidal, or groundwater flooding...

  6. Yes, I agree, many contractors have the habit of over engineering things and mostly for their benefit. Best to get a couple quotes.

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