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SUDS work, but only in the right circumstances

Eddie Mewies highlights the challenges for the civils sector as companies strive to implement viable and consistent Sustainable Urban Drainage Systems while regulation remains fluid.

As the Flood and Water Management Act remains partially implemented, our sector is currently in limbo and under increasing pressure as more construction projects come on stream.

Critical elements such as the ‘National Standards’ for Sustainable Urban Drainage Systems and the introduction of the SUDS approving body (SAB) have yet to be enacted, meaning over the past 12 months there has been a huge national variation in requirements and attitudes towards sustainable drainage design.

As a result, it is extremely difficult to provide consistent advice to clients and we have had to learn to adapt accordingly.

SUDS only work well in the right conditions

While there are varying levels of complexity and techniques, in basic terms SUDS work extremely well when the ground has infiltration potential. Where there is relatively impermeable subgrade such as clay soils, infiltration invariably doesn’t work.

“We’ve found that statutory bodies often tend to view SUDS piece by piece or as one ‘tick in a box’, which is wrong”

What we are experiencing, however, is a constant pressure from planning authorities, the strategic flood authority (SFA), the Environment Agency, sewerage undertaker, client or any combination of the above to implement SUDS, regardless of the ground conditions.

The most suitable solution for each development

It goes without saying that surface water run-off must be attenuated to prevent a downstream risk of flooding. Where infiltration is not viable, is it still worth using SUDS techniques?

This depends on your interpretation – for example, large-scale developments need a significant volume of attenuation.

The best solutions are those that fit well into the topographical constraints of a site and where these have been the first consideration before determining site layouts. Unfortunately, this often doesn’t happen.

The SUDS treatment train, as outlined in CIRIA document C697, is often regarded as the model to follow but it does not give consideration to individual site constraints. This typically leads to two or three processes being required for developments.

“The best solutions are those that fit well into the topographical constraints of a site and where these have been the first consideration before determining site layouts”

The first is often the main attenuation feature, typically the pond or basin. Secondly, as the use of permeable paving is becoming a popular train process, it is imperative that the design is carefully thought out, as its effectiveness can be hampered by site constraints and topography.

For example, if a permeable surface slopes towards a building then the risk from an extreme storm needs to be considered.

Other viable options

Our view on the SUDS treatment train is that it is quality not quantity that matters.

One thoughtfully designed pond, adopted by the SAB, with settlement, adsorption, filtration and ecological benefits is better than a basic grassed dry basin or mish-mash of permeable paved drives, the performance of which depends on the ongoing maintenance and attitudes of homeowners.

We’ve found that statutory bodies often tend to view SUDS piece by piece or as one ‘tick in a box’, which is wrong.

Sustainability should mean considering the practicality of future maintenance, which is why it is essential both the national standards and SAB are introduced sooner rather than later and our industry can move forward with consistently designed, encompassing SUDS systems that present long-term benefits for all.

Eddie Mewies is managing director of MEC consulting development engineers

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