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Technical Excellence: Llanelli Rainscape project

The Llanelli Green Infrastructure Project, part of Dwr Cymru Welsh Water’s Rainscape initiative, is a genuine model for sustainable infrastructure and the first of its kind in the UK. In the first of a series of detailed studies of British Construction Industry Awards exemplars of excellence, we look at why this project took a unique retrofit approach to flood risk management and pollution reduction.

Damage caused by flooding cost the UK more than £1bn last year and existing sewerage and water systems are increasingly unable to cope with the effects of climate change and demands made by an ever growing population.

Regulations require Sustainable Drainage Systems (SuDS) to be incorporated into new developments. However, the Llanelli Rainscape Green Infrastructure Project is the first where so-called green infrastructure design principles have been employed to alleviate pressure on an already overstretched drainage network.

Llanelli has long suffered the effects of an ageing and outdated water network, resulting in regular flooding, and an increasing risk of damage to homes and businesses and pollution in the local water courses.
Without intervention, these problems would have continued to escalate, stifling the town’s development and its ability to grow and attract investment.

Swale at Queen Mary's Walk

Queen Mary’s Walk: Swale during rainfall

Traditional solutions to flooding and pollution problems might have called for building bigger pipes supplemented with buried storage systems to catch the water. However, in Llanelli, the volume of storage required was so large, that these traditional hard engineering options were ruled out as unbuildable, hard to maintain and not cost effective.

“By implementing those strategies, you’re only fighting a long term battle, it’s a short term sticking plaster type solution,” says Arup senior engineer Stephen Ollier, who was lead engineer on the project.

Project team

Client: Dwr Cymru Welsh Water
Consultant: Arup
Contractor: Morgan Sindall

 

In 2010, Dŵr Cymru Welsh Water commissioned contractor Morgan Sindall and consultant Arup to assess the extent of Llanelli’s issues and to develop a strategy to alleviate them. Some 18 months and £2M worth of investigation and hydraulic modelling investment enabled the design team to develop a level of technical catchment knowledge about the area. To give an idea of scale, the model size comprised 2,500ha of contributing area for the catchments of Llanelli and nearby Gowerton. For the Llanelli region, the model was informed physically by 308 flow monitors and 60 rain gauges.

“This allowed us to understand how the catchment responded to all different types of rainfall throughout the seasons,” says Ollier.

“There are the intense summer storms and the long wet winter periods with high groundwater levels and they have very different, but equally devastating effects.”

In addition to the flooding problem, another key driver was to reduce the impact of overflow spills from the town’s combined sewer - a single system that carries sewage and storm water.

“There are a lot of points in the sewer network where the foul sewer network is really struggling to cope with the flow it has in it. It allows itself to discharge partially treated, albeit diluted, flow to the local rivers and water causes,” says Ollier.

Llanelli flood risk map

Regulations state that to meet the water quality standards, “the equivalent of 500,000m3 of storm storage would be required”, says Dŵr Cymru Welsh Water project manager Lee O’Brien. This equates to 200 Olympic sized swimming pools and is clearly impractical to build. If the rainwater could be prevented from entering the system in the first place, the number of overflow discharges made would be less, and the quality of the water in rivers and streams would improve.

The Llanelli Rainscape project comprised well over 100 separate green infrastructure schemes in and around the urban landscape. Queen Mary’s Walk, Stebonheath School, and Glevering Street were the first three pioneering schemes built in 2013/14 at a capital cost of £3.31M and an estimated whole life cost of £3.69M. This represented a 57% whole life cost saving compared with the ruled out traditional solutions.
Stebonheath School

The green infrastructure project at Stebonheath School was needed as part of the improvement works to the surrounding area. The primary school is located on the top of a hill in Llanelli and as such, only suffered minor flooding. However, when a storm occurred, water would be collected in the sewers and flow down the hill.

“It’s that sudden impermeable response to rainfall, which gets in to the old pipe network and then by the time you get all that flow hitting the bottom of the hill at the same point, it comes out of the ground and out into peoples’ houses, causing flooding,” says Ollier.

“Start at the top of the hill and any water you take out doesn’t cause flooding or problems further down in the catchment,” adds O’Brien.

Stebonheath School

Stebonheath School: Newly improved green infrastructure playground

The school was to play a key role in preventing this.

Even though on the face of it, there was no incentive for the school to take part, the headmaster, Julian Littler, boldly embraced the concept of green infrastructure as opposed to a more traditional drainage solution.

Design competitions engaged the pupils too, and their ideas were included in the design process. This led to the inclusion of an outdoor nature learning area, attenuation pond and, an extremely popular, mini beasts hotel.

Several different elements of green infrastructure were incorporated. Planters were designed for their specific location some with soil and drainage stratas, to attenuate the water flowing into pipes and some, fully tanked solutions which store the water and remove it from the system.

Water butts collect water from the school’s roof, which in turn can be used to water surrounding allotments. A swale captures the water from the school car park.

Constructing the works in a tight timeframe presented a major challenge - the team had the six week summer holiday so as not to interrupt the day to day running of the school. The go ahead for the project was given three months before the start of the holidays, which left little time for design.

“The start of detailed design to being on site was about three months,” says Ollier. “Within that time, we had to complete all our survey, ground investigation and geotechnical analysis work. It was a rapid turnaround.”

To meet the start of term construction deadline, the programme had to be as slick as possible. The bespoke timber planters were prefabricated off site and lifted into place in a day and construction teams worked round the clock to get the work done.

Hydrograph showing swale impact

One of the areas the design team was very careful to manage was the future maintenance of all the elements.

“We didn’t want to leave something long-term that was difficult or costly [to maintain], so we worked with the school to make sure they were aware of everything we were doing and what the long-term maintenance implications would be,” says Ollier.

“The children get involved in certain aspects like fruit picking [from the trees which were planted] and cleaning out pond weed. The education benefits are huge.”

The scheme has been extremely successful. “The main benefit is probably the volume removal, which has exceeded our expectations, and we’re still working how and why that’s happening,” says Ollier. “We had a peak run off target of 49l/s and we’ve achieved 53 l/s.”

Littler could not be more enthused about the Rainscape scheme.

“It wasn’t so much the actual school site that was flooding. We were at the top so it was the other areas which we could help, but at the same time, it’s transformed our playground and made it look fantastic, so it’s worked both ways.”

Glevering Street and Queen Mary’s Walk

The Glevering and Queen Mary’s Walk schemes were a priority for reducing combined sewer overflow spills to the Loughor Estuary south of Llanelli.

Both schemes use above ground flow routing techniques to divert storm water runoff into new green infrastructure elements. The principle is to capture, treat and control runoff from an extensively urbanised, concreted over, catchment previously serviced by combined sewers. The green infrastructure elements are a mixture of landscaped swales, bio-retention planters and urban trees retro-fitted into existing highways and pockets of urban grassland (see box).

Llanelli drainage

Drainage: Outfall into swale

One of the major challenges faced by the team was working in an urban environment.

“Utilities were a major challenge - lots of sewers running through that area which we had to work around and divert,” explains Ollier.

“We did a lot of above ground tracing and looked at old drawings, we did everything we could to find out what was there, but it was very difficult.”

These challenges were not confined to the built up area. Under the swale at Queen Mary’s Walk, the team had to take into account a network of old mine workings below the common.

The swale at Queen Mary’s Walk is a long channel that holds the storm water, and allows it to very gradually filter back into the sewer network. It has a pinned, geosynthetic bentonite liner with self-healing properties due to the swelling and sealing ability of sodium bentonite. A perforated pipe runs along its length and forms part of the surface water network and is maintained by the council.

This impermeable liner has two functions. Firstly, it traps the water in the basin where the surrounding plants and trees have been planted to encourage the evapotranspiration of flows (see box).

Terminology

Evapotranspiration: The total amount of water that has evaporated from the ground and transpired from plants to the atmosphere.
Bioretention areas: Shallow landscaped depressions that are typically under-drained and rely on engineered soils, enhanced vegetation and filtration to remove pollution and reduce runoff downstream.

It also stops the water seeping down and through into old mine workings below. The addition of the swale has enhanced biodiversity and helped regeneration in that area.

Throughout the process, stakeholder engagement was key to the overall project success.

“Our biggest challenge was coming up with these [green infrastructure] ideas and then trying to work with a number of different teams and key stakeholders to make sure that everyone was happy and engaged,” says Ollier.

 

Glevering street drainage

“We required planning for the swale and that brought about certain issues with historic plan of use, the potential impact on people’s views from their houses and walking routes, nearby football pitches and disabled access.

“Quite a lot of people were interested in what was happening outside their front doors and their back gardens.”

Car parking and the potential loss thereof was a hot topic. To try and ensure that needs were addressed, public consultation meetings were held monthly two years before work started on site.

This influenced the final design, with traffic management and staged construction works incorporated to minimise disruption. The layout of the green infrastructure elements was fitted around the needs of the people living and working in the affected areas.

Areas of new planting formed part of the green infrastructure and one of the new innovations used on the project was the use of deep root Silva cells, a technology for protecting tree roots.

Roadside planter, Glevering Street

Glevering Street: New roadside planter

Ollier says a lot of urban trees are put in by the side of the road and then the ground surrounding them is covered with tarmac. The soil beneath gets quickly compacted and the trees struggle to get water and air. The trees then dies within a few years.

“There are a lot of well established, mature trees and they’re fine, but when trying to put in new trees, there’s a low success rate when you don’t try to protect the root zone,” he says.

The cells are a structural buried crate system, which keep the soil uncompacted, allowing space for air and moisture. It is also able to store storm water but maintains the porosity of the soil so that when it rains, the soil around the tree roots fills up with water and then empty as the tree roots take the water in.

As well as the ground based planting, above ground planters were used where it was impractical to excavate.

“The planters were designed with heavy duty liners inside them,” says Ollier. “Within the planters there are geolite storage cells, which are a honeycomb storage system, so they store water and are a very strong, tough and effective way of providing storage. There’s then a drainage media and topsoil on top of that and planting on top of that.”

There is no doubt it has been a very successful scheme. The impact of an observed 70% peak flow reduction and 60% volume reduction from the sites is that 30,000m3 of rainwater has now been returned to the water cycle.

As a result Dŵr Cymru Welsh Water’s wastewater treatment costs have fallen significantly. The scheme itself has attracted lots of attention from the UK and abroad and is now being used as a benchmark for similar schemes to be rolled out across Wales. “It’s been great for morale, to be able to see your work, and other people can see it as well and everyone is really proud,” says Ollier
Llanelli, however, is only the tip of the iceberg. By 2050 it is predicted that 3.2M properties in the UK will be at risk of flooding from outdated urban drainage systems.

This is not to be taken lightly, and the solutions presented in Llanelli have provided a more resilient flood management resource. It is for this reason that Dŵr Cymru Welsh Water is now rolling the practices out across its business and promoting the Rainscape philosophy to the rest of the industry.

  • The Llanelli Green Infrastructure Project won the British Construction Industry Award 2014 for Civil Engineering Project of the Year (up to £10M).

BCIA Judges’ verdict

The Llanelli Green Infrastructure Project surprised the judges with its successful integration of sustainable drainage solutions into an urban environment.
The project is a first in using retrofitted green infrastructure schemes to reduce the load on old sewer networks - with a side benefit of improving the local environment through sensitive landscaping.

The judges recognised this project as an important example of how to deal with increasing urban drainage problems.

They said good collaboration and innovation throughout the project team was achieved through imagination and tenacity.

Quite simply the judges were delighted to see a soft sustainable drainage system working, and working well with proper consideration of future maintenance issues.

Verdict: Success beyond expectations

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