How ‘Suds’ are being retrofitted to a whole town

Sustainable drainage schemes or ‘Suds’ have become the go-to solution for managing rainfall run-off in new developments.

An example of the project’s ‘Suds’, or sustainable drainage scheme
An example of the project’s ‘Suds’, or sustainable drainage scheme

They slow surface water flow by natural means, such as ponds and swales, helping it to evaporate or soak safely into the ground.

But retrofitting Suds across a large built-up area is much harder. A pioneering project in Llanelli in south Wales shows it can be done – and the world's civil engineering community is paying close attention. The full story of progress to date is reported in the latest issue of the ICE Civil Engineering journal.

Preventing flooding

The old coastal town's inadequate combined sewage system had led to frequent flooding of property and pollution of local shellfish waters. The traditional solution of building extra storage would have cost £377m, but Welsh Water found they could achieve the same result for a quarter of the price using Suds.

According to lead author Christopher Ellis of Arup, 'A comprehensive catchment wide modelling assessment was undertaken to develop a pioneering, community-focused strategy based on retrofitting sustainable drainage systems.' While the systems are relatively small scale, mostly road-side swales and planters, they have performed better than expected as well as significantly 'greened' the town.

'New way of thinking'

Citing one of the swales Ellis says, 'During a one-in-one annual exceedance probability event, design phase model predictions indicated a 62% flow reduction would be achieved given the volume of attenuation provided. Post-construction monitoring indicated an additional 25% reduction in peak flow has been achieved during some storms.' And it gets better as the plants mature, he adds.

A total of 10 schemes have been completed so far at a cost of £15m, and these have already cut sewer overflows by nearly 40%. 'The successful implementation of early schemes has drawn the attention from many stakeholders throughout the UK and abroad,' says Ellis. 'The steps made in Llanelli will encourage a new way of thinking to make towns and cities better, more resilient places.'

For further information, please contact ICE Proceedings editor Simon Fullalove at [email protected].

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