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

Diffusing demands

Water SUDS

Watch out, the European Water Framework Directive is clamping down on diffuse water pollution. That means the run-off from the street or the farm. Alan Sparks reports.

The construction industry is still struggling to get the message of flooding - that paving over greenfield land increases run-off, and that more run-off makes for more floods.

The other side of the same coin - that more run-off equals more waterborne pollution - hasn't even registered on the radar yet.

But it will.

One of the key concerns of the European Water Framework Directive is water quality, and from 2012 it will be stamping down on the sources of pollution, not just from industry and water companies, but from agricultural land and storm drains discharging run-off from city streets and highways.

'The issue of diffuse pollution is of equal importance to that of discharge levels. This form of pollution was never considered a relevant design issue in the past, but from 2012 it will be, ' says Aidan Millerick, managing director of UK drainage software company MicroDrainage.

Drainage designs have historically been all about gratings and pipelines, geared to get volumes of surface water into watercourses or sewers as quickly as possible.

Fortunately in recent years the most enlightened drainage designers and environmentalists have been pushing sustainable drainage systems (SuDS) to reduce the run-off peaks that contribute to flooding. These are typically ditches, swales, ponds, or porous paving, which attenuate or retain water, allowing infiltration into the ground.

'And when we analyse the effects of SuDS, we find that these can also be a highly effective way of dealing with diffuse pollution, ' Millerick says.

Diffuse pollution is generally at its most acute after a dry spell, during which roads become coated with hydrocarbons such as fuel and oil drip from cars, rubber and carbon from car tyres and exhausts, and other sediments. Agricultural land may build up a surface layer of manure or fertiliser. A moderate downpour will sweep up these pollutants.

Figures from Scotland reveal that urban runoff accounts for 11% of total pollution in rivers, and 31% of seriously polluted watercourses.

'Diffuse pollution has to be prevented from reaching watercourses, ' emphasises Millerick.

'The Directive insists this has to be done, and SuDS are the only game in town to achieve it.'

Swales, soakaways and ponds slow the rate of flow, allowing sediments to settle out, while thick grasses provide filtration.

If water can be held for some time, naturally occuring bacteria will also attack and break down organic pollutants and hydrocarbons.

'Basically, you need to give nature long enough to deal with any pollutants. Simulating the processes of a greenfield site is good, ' Millerick sums up.

He says that porous paving systems, in which water is retained in an engineered subbase, are particularly effective: bacteria break down hydrocarbons, phosphates and nitrates.

In the past porous paving was frequently underdesigned, leading to scepticism in the drainage industry. 'However, with modern design guidance they are often far more substantial and extremely effective.'

Porous paving is also seen as a good way of overcoming an apparent contradiction between planning guidance and building regulations. UK building regulations demand SuDS for roof and paved area drainage, but these generally require space. Meanwhile, planning guidance calls for higher density housing.

Porous paving at least ensures that hard surfaced areas take care of themselves.

SuDS are often most effective when used in combination: 'Each solution will have its strengths and by using a combination you are designing a robust drainage system offering the best protection from pollutants, ' says Millerick.

The cleansing potential of SuDS has only recently been appreciated and designing for removal of diffuse pollution is at an early stage. 'Much of the research and knowledge are being brought over from the US where SuDS design is much further advanced than in Europe, ' Millerick says.

The Water Framework Directive is a comprehensive piece of pollution and abstraction legislation requiring higher standards throughout Europe.

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.