Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Drain brains

SUSTAINABILITY: Sustainable drainage - For a notoriously wet country, the UK is exceptionally bad at managing rainwater run-off. Andrew Mylius looks at some alternatives to conventional drainage.

The annual visitation of swirling murky waters on villages and towns across the UK is now such a dependable fixture that flooding this year has barely registered on the news agenda.

Many parts of the country are still battling to make good the £700M of damage done during the inundation of winter 2000-01, while residents of Bewdley, Kidderminster, Malton, Norton and a host of other market towns have been forced again to evacuate their houses.

However, many others who suffered last year will be heaving sighs of relief that so far 2002 has been relatively dry.

Flooding's fall from the headlines will be temporary, though.

Climate change predictions make for worrying reading.

While summers will become more parched, winters are set to get wetter. Over the course of the next century, the frequency and severity of winter flooding is expected to increase.

To exacerbate the situation, across the UK 3.8M new houses are needed by 2021, according to the Town & Country Planning Association - 2M of those will be in the South East.

UK drainage systems are already failing to cope with the volume of water that sluices off rural hillsides, thousands of kilometres of road and off the streets and roofs of every developed area. They are hopelessly underdesigned to deal with greater volumes still. Put bluntly, the UK has an acute drainage problem, and it is going to get worse.

Plenty of coverage has been given to the Government's new planning policy guidance, PPG25, warning developers and local authorities not to build on flood plains. By contrast, guidance on tackling the causes of flooding through alternative approaches to drainage design is almost unknown.

Sustainable drainage solutions, abbreviated to SuDS, have been knocking around for centuries. The idea is basic: instead of ducting surface water through grates and pipes direct into the nearest stream or river, you encourage it to soak into the ground, to evaporate and be taken up by plants, or simply delay its release, preventing the kinds of sudden rush that contribute to flooding (see graph, page 22).

SuDS of various kinds are widely used in the US and Canada and are fairly popular with highways engineers in northern Europe. But SuDS have all but disappeared from the UK landscape over the last century.

Bringing sustainable drainage back to the UK involves 'a rethink of first principles', says Environment Agency diffuse pollution project manager Phil Chatfield.

Key to the idea of SuDS is use of a range of solutions in combination. 'What you have here is a toolbox of techniques. There will be three to four alternatives to conventional drainage that you can call on. To an engineer it is quite challenging - the engineer has to determine what's most appropriate to the site, ' Chatfield says. It should be possible to design development that produces no more run-off than a greenfield site. If SuDS were deployed on all new build projects and retrofitting were carried out, major strides could be made toward reducing flood risk, he adds.

Few of the available SuDS designs can be described as truly 'hard' engineering, but perhaps the softest approach to managing run-off is that advocated by environmental consultant Chris Baines - aforestation. Hillsides, forming the UK's largest rainwater catchments, have become denuded of large plants through grazing by generations of sheep, says Baines.

Trees and shrubs play a crucial role in slowing the rate at which rainfall hits the ground, and help slow the velocity of runoff, so limiting erosion. He argues that water companies and estate owners should be taking advantage of the depressed state of agriculture to fence sheep off upland areas of the UK, allowing native trees and shrubs to become re-established.

Baines also champions the idea of urban woodland. Urban trees retain water as it falls and, if water is allowed to infiltrate the ground, will suck it up as well.

Leading promoters of SuDS the Environment Agency, its northern counterpart the Scottish Environment Protection Agency (SEPA), and construction industry research bodies CIRIA and the Building Research Establishment back more 'engineered' designs such as porous paving, gravel filled infiltration trenches, grassy depressions known as swales, infiltration basins and retention ponds (see diagrams).

Every SuDS is designed to hold water and allow it to soak into the surrounding ground.

Even if ground underlying an area of porous paving is highly compacted clayey material, for example, use of a high void space aggregate sub-base will act as a sponge. The pavement structure can be made safe against saturation by providing French drains connecting to a swale or pond.

Infiltration trenches, all but invisible on the surface, are used to drain run-off from non-porous pavement - they are suited to car parks or road kerbs. Filled with high void aggregate, trenches can provide significant storage capacity and work by allowing water gradually to seep into the surrounding ground. Like porous paving, they can be proofed against overflow by linking to a retention pond.

Swales and infiltration basins are perhaps the most elementary SuDS designs around - grassy indentations that are normally dry but where water will naturally pond. As with all other solutions, the aim is to let water soak into the ground. And as maximum capacity is neared, they should release excess water incrementally rather than all at once. It is a similar story for retention and balancing ponds - they work the same way but remain wet year round.

Where the run-off period for conventional drainage is short, producing a sharp peak, run-off from SuDS is lower and takes place over a longer period. They have potential to help moderate the flow of streams and rivers, reducing the dramatic fluctuations in height and flow rate seen at present. SuDS also help recharge aquifers - despite being fabled as a sodden isle the UK's water table has been falling in recent years, notes Environment Agency Thames region technical officer Prosper Paul.

The jury is out on how much SuDS cost to build and maintain compared to conventional drainage. Depending on what design is opted for, the International Association on Water Quality estimates that SuDS are anywhere between 10% and 50% cheaper. SuDS construction is often a lot more straightforward than piped drainage, though land take may be greater if swales, basins or ponds are used, says Chatfield. For this reason those developers who have shown interest in SuDS favour designing for 1:30 year rainfall, while the Environment Agency is arguing that they should be built to cope with a 1:100 year event, requiring larger capacity. Based on the ratio between cost and environmental benefit, however, much of the £1.7bn being spent by UK water companies on combined sewer overflows under the present round of capital spending could arguably be more usefully spent on SuDS.

SuDS have not been used in the UK for long enough to gauge how often different designs need maintenance or how much it costs - a working group to study whole life cost and performance has been set up by the Environment Agency with principal water companies, CIRIA, Department of Transport, Local Government & the Regions, Department of the Environment, Food & Rural Affairs, the National Assembly for Wales, Local Government Association and highways authorities.

But compare the task of cutting grass in a swale once a year with that of sucking out the gully pots that line UK roads and car parks every six months and draw your own conclusions on cost, recommends Chatfield.

SuDS in action The Environment Agency Thames region has drawn up a list of 50 sites designated for sustainable drainage schemes as part of a bid to improve knowledge of the design, construction and performance of SuDS. It is compiling a national database that will swell data from Scotland and a number of well known English projects.

SuDS have been used on a contaminated site at the M40 Wheatley service station. The developer was not allowed to let water infiltrate the ground for fear of mobilising contaminants in the soil matrix. An impermeable geotextile barrier was installed, and rainwater run-off controlled instead through retention in a gravel layer beneath porous paving, and balancing in ponds.

Royal Bank of Scotland recently completed construction of a car park at its headquarters paved with permeable asphalt. In addition to retention of water in the gravel base layer, significant infiltration into the ground has been found to take place, even though the sub-soil is clay. Retail giant Tesco is also adopting SuDS for construction of car parks.

Highways Agency design codes now address SuDS and sustainable drainage solutions are being deployed on the M6 Toll by concessionaire CAMBBA.

Reed beds have been constructed at Heathrow for treatment of glycol - used to de-ice aircraft wings - which is caught up in run-off from the airport's apron. Reed beds have also been retrofitted at the Environment Agency's Preston office.

Wessex Water's headquarters at Claverton Down, Bath, has combined SuDS with water recycling to produce a zero run-off development.

And South Gloucestershire Council and Leeds City Council stand out among English local authorities for promoting the application of SuDS.

Fighting pollution The Environment Agency favours SuDS not just for their potential to reduce flood risk, but for their efficiency in reducing waterborne pollution and improving river water quality.

According to SEPA, 20% of river pollution by length stems from urban drainage. Despite tightening regulation of the water industry, 'we're not seeing the kinds of improvement in the quality of rivers that we should', Chatfield notes.

Rainwater run-off, in the main flows, without treatment straight into watercourses, carrying with it oil, heavy metals, detergents and toxic chemicals, rubber dust, eroded soils, animal excrement and organic matter such as leaf litter or paper. That is to say nothing of pollutants from drains, toilets or private sewers that are misconnected to surface water drains. Pollution levels equivalent to 50/50 dilution of raw sewage in clean water have been measured at surface water outfalls.

Vegetation in SuDS filters out particles and absorbs toxins.

Settlement takes place when water is collected in swales or ponds, while pollutants of all types adhere to fines in the soil matrix as water seeps away.

Vegetation can be harvested periodically and disposed of to ensure that pollutants are not re-released into the environment when it dies and decomposes.

Silts can be removed periodically.

And bacteria occurring naturally in the soil will munch away on organic compounds, including oil and hydrocarbons, rendering them harmless.

Main barrier to the widespread adoption of SuDS, it seems, is lack of familiarity - sustainable drainage is not taught on most engineering degree courses, while clients and engineering design departments tend to be conservative, opting for flawed but well known conventional drainage solutions.

'However, even if you can get over that professional lack of familiarity, getting people to take SuDS up, whether they are planners, local authorities or water companies, hits problems, ' Chatfield observes: SuDS are not legally classified as 'sewer', making it difficult for utilities companies to justify them as legitimate capital expenditure to water regulator Ofwat. Utilities firms are reluctant to adopt SuDS constructed by private developers, Paul reports. Water companies and local authorities also want better information on SuDS' long term maintenance costs.

Attitudes are shifting, though.

Some private developers are starting to ask: why pay to connect surface drains to the main sewer network? SuDS are being built on large housing and business estates, with the developer or managing agent taking on responsibility for maintenance. Meanwhile, PPG25, dealing with development in flood plains, says that SuDS should be adopted by local authorities as their default surface drainage option. Part H of the Building Regulations, covering drainage and coming into effect next month, states that rainwater should be directed into an infiltration system rather than a watercourse or sewer. And the Department of Trade & Industry is weighing in with calls for data on whole life costing.

INFOPLUS Environment Agency:

www. environmentagency. gov. uk. Contact regional technical officer Prosper Paul, tel 0118 953 5699, e-mail prosper. paul@environmentagency. gov. uk, Ciria guidance on SuDS can be found at www. ciria. org. uk/suds Scottish Environment Protection Agency SuDS guidance can be found at www. sepa. org. uk/guidance/urb an-drainage/index. htm

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.

Related Jobs