Llanelli Green Infrastructure Project

RainScape is Dwr Cymru Welsh Water’s innovative programme of works to deliver surface water management solutions to reduce sewer flooding and pollution. Here we take a look at their Llanelli Green Infrastructure Project, a scheme that has won many awards including ICE’s Chris Binnie Award for Sustainable Water Management 2014.

Stebonheath School, one of three scheme sites, features basins, planters and innovative storage products.
Stebonheath School, one of three scheme sites, features basins, planters and innovative storage products.

Project details

  • Location: Llanelli, Wales
  • Cost: £3.31m
  • Year of completion: 2014
  • Client: Dwr Cymru Welsh Water
  • Consultant: Arup
  • Contractor: Morgan Sindall

Challenge summary: Reduce CSO spill frequency, address flooding of properties, deliver resilient critical sewer infrastructure.

Challenge solution: Successful integration of sustainable drainage solutions into an urban environment, reduced load on old sewer network and wider benefits to community and environment.

Introduction

Opened in 2014, the Llanelli Green Infrastructure Project – part of Dwr Cymru Welsh Water's (DCWW) RainScape programme – is a model for sustainable drainage infrastructure and the first of its scale in the UK, providing solutions for flood risk management, pollution reduction and growth. The project received ICE's Chris Binnie Award for Sustainable Water Management in 2014.

The project features sustainable drainage solutions at three sites across the town – Stebonheath School, Queen Mary's Walk and Glevering Street – as the first three pioneering schemes of the RainScape programme. Built at a cost of £3.31m, the project as a whole represents a cost saving of 57% compared with "traditional" flood defence solutions.

Llanelli is a large coastal town of 58,000 people in the south of Wales. Due to its urbanised nature and ageing, overloaded sewer network, the community has long suffered flooding problems, with an increasing number of homes and businesses at risk. Pollution of local watercourses is also a problem, as number of Combined Sewer Overflows (CSOs) discharge into the adjacent Loughor Estuary (a protected tidal water body) – up to 2.36m3 of combined sewage annually from a single asset.

In 2010, DCWW commissioned Arup, in partnership with Morgan Sindall, to develop a catchment solution that would reduce the frequency of CSOs to an average of 10 per annum, address the flooding of 115 homes and deliver resilient critical sewer infrastructure able to withstand pressures of climate change impacts and population growth.

Over 100 green infrastructure (GI) schemes were identified for implementation as part of a long term catchment management strategy to address flooding and river pollution in Llanelli. In 2011, Welsh Water announced a £15m investment to target the top ten highest priority schemes collectively capable of reducing peak storm water runoff entering the combined sewer system by 25%. The project features three completed schemes which are now showing the benefits of this new, integrated, sustainable and resilient approach to water management.

Implementation of first schemes

The three initial schemes of the Llanelli Green Infrastructure Project were completed in 2014:

Glevering Street

Programme: Sep 2013 – Jun 2014
Cost (Design & Construction): £2,196k
Cost of "traditional" equivalent: £3.1m
Model prediction for peak surface water reduction (1 in 5 AEP): 300l/s

Stebonheath School

Programme: Jul 2013 – Sep 2013
Cost (Design & Construction): £448k
Cost of "traditional" equivalent: £2.0m
Model prediction for peak surface water reduction (1 in 5 AEP): 53l/s

Queen Mary's Walk

Programme: May 2013 – Sep 2013
Cost (Design & Construction): £813k
Cost of "traditional" equivalent: £2.4m
Model prediction for peak surface water reduction (1 in 5 AEP): 96l/s

These schemes were selected as the highest priority based on a multi-criteria analysis comprising key indicators such as surface water reduction, resolution of flooding, whole life costing, ease of construction and carbon footprint. They are all based in the centre of Llanelli and serviced by a combined sewer network. Each site is typical of how an urban environment has evolved within the catchment to develop a highly impermeable response to storms where rainfall quickly arrives within the sewer network.

Retrofit GI is the foundation for all three schemes; principally intercepting storm discharge and directing it away from the combined sewer network via pipes and overland channels. Innovative attenuation basins and planters then utilise specially selected soils, plants and trees to slow the flow whilst improving water quality and encouraging evapotranspiration.

Placement of the bespoke attenuation units, all of which adopt the same hydraulic principles, has been carefully considered to integrate into and enhance the existing local built environment. The desire was to provide wider community benefits, which go beyond flood alleviation by providing open green space in otherwise dull environments, improving local air quality, encouraging local biodiversity enhancement, and promoting health and wellbeing.

Queen Mary's Walk

Queen Mary's Walk is a suburban street located between a residential estate and public playing fields. Previously the roofs, roads and other hard standing areas within this subcatchment drained to a 225mm combined sewer running along QMW, contributing 129 l/s to the network during a 1 in 5 Annual Exceedance Probability (AEP) rainfall event. Consequently, the scheme was identified as a key contributor to pollution and flooding.

At Queen Mary's Walk
At Queen Mary's Walk, stormwater is conveyed into a 100m bioswale located next to local playing fields

The basis of the design at QMW involves conveying storm flows into a 100 metre long bioswale located alongside local playing fields. This required diversions of storm and road drainage into a new storm sewer feeding the new swale. The swale controls pass forward flow via a perforated underdrain which sits beneath the swale topsoil surface, allowing a maximum of 4 l/s to enter the combined sewer downstream. The swale also provides water quality benefits through natural processes such as filtration.

Historical mining activities, contamination and the presence of clay rich Glacial Till within the vicinity of QMW limited the potential for infiltration into underlying soil layers and bedrock. To mitigate the risk of local subsidence & contaminant mobilisation, an impermeable Geosynthetic Clay Liner (GCL) was used. The process of evaporation and evapotranspiration allows the system to completely remove some flow from the combined sewer network in the absence of infiltration.

Stebonheath School

Stebonheath Primary School previously contributed 59 l/s into the combined sewer network (1 in 5 AEP). The school grounds offered little in the way of green space, and localised flooding was common. A GI scheme at the school brought with it the potential to turn an otherwise bland playground into something green and vibrant, creating an exciting place and educational resource to encourage children to come and learn in the centre of Llanelli.

Design proposals for Stebonheath were refined through interactive sessions with the staff and pupils at the school. The final design consisted of a selection of basins and planters, whereby attenuation volume was maximised through the use of innovative storage products, such as Silva Cell, designed to increase soil void ratios. Collectively, 100m³ of sustainable attenuation volume has been constructed at the site, limiting peak flows into the sewer system to just 6 l/s (1 in 5 AEP).

Stebonheath School
Stebonheath School, one of the three sites of the Llanelli scheme, previously suffered regular localised flooding and had little green space.

A key challenge faced at Stebonheath School was programme. Construction was limited to just six weeks to fit within the summer holiday period, which in turn presented design challenges. This leant itself to off-site fabrication, for example bespoke downpipe timber planters which were delivered to site in two halves, generating 14m3 of attenuation volume within a matter of hours on site. The innovative design avoids the need for excavation whilst providing aesthetic benefits. In addition the planters are maintainable by the school pupils themselves instilling a sense of ownership whilst benefiting both the school and Welsh Water alike.

Glevering Street

The Glevering Street sewer flooding scheme was part funded by the European Regional Development Fund via Welsh Government and is the largest scheme in the strategy to date, with construction work spanning nine highly urbanised streets. The dense nature of housing and streetscape within this area, which feeds solely into a combined sewer system, meant that previously peak flows leaving the catchment were in excess of 800l/s (1 in 5 AEP). In addition seven properties in the immediate vicinity of this scheme were on Welsh Water's Definitive Flood List.

The scheme ethos at Glevering Street is based on the conveyance of surface water using overland routing, combined kerb drainage and dish channels; all of which direct runoff towards a network of roadside basins and planters. Collectively this will reduce peak storm flows by over 300l/s (1 in 5 AEP).

A challenge faced in the design of this scheme, recognised through engagement with the local community, was the potential loss of parking caused by roadside GI.

Engagement with the local community was important at the Glevering St site
Engagement with the local community was important at the Glevering St site

The solution was to strategically place the planters and basins in locations less frequently used for parking, such as side streets, reengineered junctions and in areas with existing parking restrictions in place; the number of planters was also reduced from 100 to 30 following engagement with the local community.

Construction of retro-fit GI in highways is likely to involve interaction with existing below ground services. The design approach taken in Glevering Street was to supplement significant up front trial trench information with flexibility in design and materials/products; this has allowed for significant on-site modifications to suit local conditions.

 

Adoption

Adoption can be a critical barrier for many retro-fit GI projects; cost, responsibility and maintenance are often the three key issues. This process was facilitated by working alongside Carmarthenshire County Council (CCC) from inception stage. Discussions and agreements started with the GI overall strategy; this then progressed to agreeing individual detailed site layouts and the use of bespoke technologies.

Performance of Retrofit GI

The peak flow reduction impact of each of the GI schemes is being monitored and assessed using rain gauges, pipe flow monitors and water depth monitors. To date it has been shown that the GI systems are not only meeting, but surpassing targets set within the solution development stage. For instance during a typical annual storm event at Queen Mary's Walk (1 in 1 AEP) an additional 20% peak flow reduction has been achieved beyond that predicted. This additional hydraulic benefit can be attributed to the green elements; the process of soil retention properties and evapotranspiration brought about by plants and trees.

Performace graph
Hydrograph showing impact of bioswale at Queen Mary’s Walk during 1 in 1 return period – courtesy of Arup. Click to enlarge.

Through infiltration, evaporation and evapotranspiration, it is estimated that 32,000m3 of water will be removed from the combined sewer network annually from these three schemes; this is water which now simply returned to the water cycle rather than being pumped and treated at great energy and cost.

The immediate observed performance is promising; as the plants and trees mature the system will improve further; trees will intercept, capture and store even more rainfall, and plant growth will increasingly slow and treat stormwater runoff.

Long term monitoring will provide Welsh Water with a more comprehensive understanding of how the climate and seasons affect the performance of the green infrastructure assets.

Costs

The early green infrastructure schemes implemented in Llanelli provide an effective, sustainable method of reducing peak flows from urbanised subcatchments.

GI has offered a significant cost benefit over traditional hard engineering solutions. The combined projected whole life cycle cost of Stebonheath School, Queen Mary's Walk and Glevering Street is projected to be approximately £3.5m; delivering a peak flow removal of 477 l/s.

The cost of delivering an equivalent traditional "storage and upsize" solution, approximately 1092m3 plus local pipe upsize, was estimated at £7.5m.

Conclusion and Benefits to Society

The GI systems are proving to be hydraulically effective, physically robust, cost effective, sustainable, carbon efficient and providing community benefit.

Rosemary Cripps, Water & Environment Engineer at Arup, accepts the ICE Chris Binnie Award for Sustainable Water Management Award 2014
Rosemary Cripps, Water & Environment Engineer at Arup, accepts the ICE Chris Binnie Award for Sustainable Water Management Award 2014

Post monitoring indicates that additional hydraulic benefits have been achieved beyond those originally targeted. Planters which have been installed for over six months are now flourishing and seeing an improvement in performance as smaller rainfall events never actually reach the receiving sewer downstream of the GI elements. The cost effective delivery of flow removal and subsequent flood alleviation and pollution reduction has led Welsh Water to adopt the Llanelli strategy as best practice approach for implementation across Wales.

Community benefits have been achieved including the creation of unique learning environments, improved recreational spaces and enhanced biodiversity. Collaboration with the community ensured that customers had the opportunity to understand the purpose of the work and any associated short term disruption, and were given the opportunity to influence the design of the RainScape scheme.

The award winning approach adopted in Llanelli has been recognised as a blueprint for sustainable and affordable water management by Welsh Government and the water industry as a whole.

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