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RainScape - an Integrated SuDS Solution

Dwr Cymru Welsh Water's RainScape initiative has seen an integrated, large scale sustainable drainage solution retrofitted in Llanelli's densely populated urban area

Sustainable urban drainage (SuDS) is rising up the agenda for water companies in this AMP cycle, as a cost-effective, environmentally friendly way of reducing pressure on combined sewer networks. But SuDS solutions on a significant scale – especially when retrofitted in existing urban centres - require effective co-operation between a host of stakeholders, including local authorities, developers and regulators, as well as the water utilities. Can this be achieved?

One example that suggests that it can is Dwr Cymru Welsh Water’s RainScape initiative, which has made £15M of improvements in Llanelli and Gowerton during AMP5, and will be expanded to the tune of another £25M in AMP6. The Llanelli schemes are the centrepiece of a sustainable drainage programme which will see around £60M spent on SuDS across Wales in the 2015-20 period.

As a sloped, coastal town with traditional terraced houses and few gardens and green spaces, Llanelli is a particular hotspot for surface run-off: its combined sewer network, which serves 53,000 people, has nearly as much storm water in its network as Swansea, which has three times the population. With nearly 40 low-lying houses vulnerable to sewage flooding, and shellfish waters in the Loughor Estuary at risk from CSO discharges and diffuse pollution, Welsh Water decided to focus on an integrated SuDS solution for the area back in 2010/11.

“The standard approach to the problem would have been to build big storage tanks, combined with increased pump capacity,” explains Fergus O’Brien, RainScape Strategy Manager at Dwr Cymru Welsh Water. “But when we looked at that we realised the amount of storage we would have needed in this area was enormous - over 200,000 cubic metres of storm tanks in Llanelli WWTW alone - and even then, the tanks would never drain before the next storm came along and filled them up again. So technically it was never feasible to take that approach.

“We were already adopting a sustainable drainage approach for some small, localised schemes at the time to protect customers from flooding. So we decide to extend our analysis and modelling across the whole of Llanelli and Gowerton, and ask - what would you have to do in order to deliver a fully sustainable approach?”

After studying international examples – such as Malmo in Sweden and Portland, USA – O’Brien knew that retrofitting SuDS was possible despite the constraints of the landscape. While it only had limited SuDS spending approved by Ofwat for AMP5 - £3M for three specific schemes - Welsh Water’s conviction about the concept, combined with its ethos as a not-for-profit organisation, meant its board was willing to back an enlarged programme and provided £15M for the purpose.

A thorough analysis of the sewer network and catchment using advanced modelling techniques allowed O’Brien and his colleagues to identify where to concentrate their efforts. Designers Arup and contractors Morgan Sindall are key partners in RainScape, and were involved throughout in developing and implementing solutions.

An early success was at Stebonheath Primary School in the town, where permeable paving, strategically placed plant boxes and a redesigned playground featuring green elements were combined to achieve a dramatic reduction in surface run-off. Where previously run-off from the site would have been 53 litres per second in a one-in-five-year storm, it is now just 2 litres per second, and a total of 3 million litres a year have been removed from the sewer network. Completed in summer 2013 and costing £500,000, the scheme achieved a high level of community engagement, with children asked to play a part in the design process.

“The headmaster, Julian Littler, was fantastic and is a very enthusiastic champion; the school governors got engaged in this very quickly as well, and the children too,” says O’Brien. “We were able to go in there and hold a session explaining what it was all about. We gave the children sheets of paper and asked them to draw a picture of what they’d like to see. We worried they’d come back with slides and swings and things, but they understood what we were doing immediately. The woodland walk, the pond, all these were things that they actually identified and were beneficial to what we were proposing to do. They have now become advocates for the idea of the programme and that engagement that we got there, that ownership, means that those assets should continue to operate well and look great for years to come.”

The programme’s biggest piece of infrastructure so far is the £850,000 swale in Queen Mary’s Walk. Completed in Autumn 2013 and with a capacity of 220 cubic metres, the gravel and grass swale takes water from five nearby road drains and slows its entry to the network with the help of plants and shrubs. It was designed to remove 4400 cubic metres of surface water a year via the natural process of evapotranspiration, but has been performing even better than expected, O’Brien explains.

Source: WWT

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