Supply Chain - River Work

The town of Ammanford, Carmarthenshire has a history of flooding. Several significant flood events have been recorded since 1979 and most recently in 2009, with homes, businesses, roads, the Heart of Wales railway line and a local college inundated. Natural Resources Wales (NRW) commissioned Arup and Walters to design and construct a solution to reduce the community’s flood risk and improve fish passage. This project innovatively aligns solutions to reduce risk to the community from flooding with two other previously separate schemes; safeguarding a critical sewer crossing found to be in poor condition and addressing a significant barrier to migratory fish. Project scope

With limited public funding and the increasing need for multifunctional infrastructure enhancements, the scheme offers lessons in the delivery of multiple benefits. The scheme comprises:

Seven concrete flood walls. Two earth flood bunds. Four blockstone pre-barrages step water levels to allow fish passage, with adjacent ground lowering to increase flood capacity. Landscape planting and public realm enhancements, including nearby Bonllwyn Green and the Llandybie Community Woodland Project. Ecological enhancements for otters, bats, birds and invertebrates, focussed on improving connectivity, diversity and ecosystem resilience. Reducing flood risk to homes and businesses

Ammanford is located at the confluence of the rivers Loughor, Lash and Marlas. The rivers respond rapidly to rainfall and their channel capacity is limited. Much of the town is built on low-lying land adjacent to the river and overtopping is predicted at multiple locations. If nothing was done to reduce flood risk, 198 homes and 25 business would remain at risk of flooding from the 1 in 100 (1%) annual chance event.

NRW studies demonstrated a strong case for change. The study area was one of the top communities at risk of flooding from main rivers within Western Wales, and facing one of the largest increases in flood risk as a result of climate change. Flooding in the area was predicted to cost the UK some £7.9m in today’s prices over the next century.

With options to slow flows upstream infeasible, optioneering focused on better containment of flood waters in the river corridor. No single flood defence would be able to meet the project objectives, so a combination of raised defences throughout was adopted as the preferred solution.

The scheme was designed to reduce flood risk up to the 1% AEP event, allowing for the impact of climate change over the scheme’s 100-year lifetime; benefiting 349 homes and 37 businesses.

[caption id="attachment_14722" align="alignnone" width="1200"] (left) Project overview - Courtesy of Arup and (right) fish passage with adjacent ground lowering works - Courtesy of Walters[/caption] Improving fish passage

Separately, NRW had identified the case for improvements to Tir-y-Dail weir at the downstream extent of the scheme. The weir was a partial barrier for upstream migration of adult Atlantic salmon and trout, and a complete barrier to adult lamprey and juvenile European eel. Fish passage improvements would unlock 20km of river to migratory fish. The weir was in poor condition but encased a critical main sewer and enabled upstream river level gauging.

Scheme design and innovation

The design phase of the scheme involved comprehensive investigations and modelling to understand flood dynamics and identify effective solutions. Key design considerations and innovations are presented below.

Positive solutions to mitigate detriment

At the outset, the team identified a risk of conflict between the initially separate fish passage and flood scheme objectives. A further challenge was mitigating potential flood detriment to other areas. Flood modelling indicated that raising flood defences and making fish passage improvements to the Tir-y-Dail weir would cause unacceptable detriment elsewhere. Typical measures to mitigate the detriment were investigated but were found to be either ineffective or excessively costly (such as additional raised defences upstream) jeopardising the business case justification for the scheme.

The team identified and developed from first principles an innovative solution to locally address issues in proximity to the weir, including an area of ground lowering within the field adjacent to the existing weir and fish passage pre-barrages. This provided additional conveyance around the existing weir and proposed pre-barrages in more extreme flood events, mitigating potential detrimental increases in flood risk associated with both the flood defence and fish passage improvements.

To maximise the benefits of the ground lowering, soils generated from this area were used within the construction of the Tir-y-Dail flood bund, improving the scheme’s earthworks balance and reducing import volumes. The area was also landscaped to provide additional area of riparian habitat.

Balancing earthworks

Two factors made creating an earthworks balance on the project challenging. Firstly, the nature of the scheme and site constraints resulted in limited areas of fill. Secondly, the engineering and chemical parameters of some of the soils excavated were unfavourable.

[caption id="attachment_14724" align="alignnone" width="1200"] Tir-y-Dail flood bund following construction - Courtesy of Walters[/caption]

To minimise the volume of excavated soils that would need to be disposed of off site, the team first addressed the chemical properties of the made ground. Site-specific risk assessments were undertaken to derive appropriate re-use criteria for human health and controlled waters. These were developed into a material re-use strategy, which considered appropriate legislative frameworks and was able to demonstrate that the majority of made ground was chemically suitable for re-use in appropriate scenarios.

To address the imbalance in volumes, the design of the Tir-y-Dail flood bund was reviewed as the primary fill area within the scheme. Flood bunds are most commonly constructed from a homogenous clayey material. This would need to be imported due to a lack of suitable on-site sources. The flood bund was therefore designed to have a ‘cap’ of imported clay to ensure performance against flooding, while the main core was formed from variable site-won material. Slope stability assessment undertaken under various scenarios demonstrated that, as long as the clay cap material was appropriately controlled, the geotechnical parameters for the core material could be significantly reduced.

To allow the contractor flexibility of the site-won materials placed in the core of the bund depending on the sequencing of their works, a project-specific soil material class was created. This was based on a combination of observational and basic material classification controls. This approach resulted in a reduction of 270 wagon loads of soil movements, reducing carbon and impact on local residents.

Sensitive detailing of flood defences

Measures were adopted to ensure the flood defences were sensitively integrated into the local landscape.

Flood walls in publicly viewable areas were finished with brick cladding, form liner finish and imitation stone copings to match local architecture. Coping stones were designed to be structurally integral to the flood wall to minimise their overall height, as well as reduce maintenance and public safety risk. Pillar features were incorporated into the facing of brick cladding at regular spacings to provide visual interest along the expanse of the wall.

[caption id="attachment_14725" align="alignnone" width="1200"] (left) Lightweight shuttering utilised in constrained working corridors and (right) brick cladding works to flood walls - Courtesy of Walters[/caption]

Design of the Tir-y-Dail flood bund was tailored to mitigate its impact on the open views from a nearby public right of way. In plan, a curved alignment was specified to conserve an irregular field pattern. In section, slopes were slackened and rounded at the top and bottom to create a naturalistic landform.

A fundamental element of the scheme design was determination of the alignment and form of the flood defences to minimise tree loss along the riparian corridor. An arboriculturist, who was integral to all major design decisions, was embedded within the design team. Every tree was considered individually so that as many trees as possible could be maintained. The resulting designs/design adaptations included micro-siting of flood walls and localised modifications to flood wall foundations in the vicinity of root protection zones.

Ammanford Flood Risk Management: Supply chain - key participants Designer & ECC project management: ARUP Design sub-consultant: FiskTek Consulting Main contractor: Walters UK Ltd Environmental consultants: Yellow Sub Geo Ecology: Sylvan Ecology Arboriculture: Mackley Davies Associates Coping stone design: Quad Consult Floodgates: MM Engineering FRC works: Trueform Civil Engineering Ltd Waterproofing: Tywi Damp Proofing Treatments Metalwork: Cambrian Foundry Reinforcement: Express Reinforcement Asbestos disposal: Gavin Griffiths Brick & stonework: JW Masonry Surfacing: Laymak Ltd Rail contractors: Protech Infrastructure Rock armour: Tarmac Ltd Landscapes: Afan Group [caption id="attachment_14728" align="alignnone" width="1200"] Visualisation of Bonllwyn Green pocket park - Courtesy of Arup[/caption] Nature and people thriving together

The project is in the heart of Ammanford, with interventions required in a combination of residential, commercial and public spaces. Multiple rounds of public engagement supported the scheme development, including:

Public drop in events with questionnaires on specific issues and feedback forms. Online consultation during Coronavirus period, as well as letterbox information drops to ensure all parts of the community were engaged. Regular newsletters, press releases, social media updates and a project website were provided throughout the design and construction phase of the scheme. For public realm enhancements to Bonllwyn Green, visualisations and schematics were prepared to effectively communicate the scheme proposals and help local residents have their say on the design.

In line with NRWs strategic objectives, enhancements to public space and ecological habitats were fundamental to the scheme design, in addition to reducing flood risk. To provide biodiversity net benefit through increasing connectivity, diversity and ecosystem resilience of the high value riparian corridor, enhancements delivered as part of the scheme included:

Improved fish passage for multiple species through the existing weir on site. Otter ledges beneath two existing road bridges. Log piles for invertebrates, hedgehog nesting boxes and bat and bird boxes. Where tree loss was unavoidable, they were replaced at a ratio of 3:1. Appropriate native species of local provenance and were designed to maintain and enhance habitat connectivity, while maintaining an appropriate amenity setting for the local community. [caption id="attachment_14721" align="alignnone" width="1200"] Heol Hayden public realm improvements - Courtesy of Walters[/caption]

Improvements to public spaces in the vicinity of the flood defence works, but also within the wider community, were also delivered as part of the scheme as summarised below:

Where flood defences were constructed in public spaces and within the local college, the surrounding areas were enhanced with additional trees, swathes of bulb planting and new seating areas. Timber from trees felled as part of the scheme was reused to construct new benches. At the Heol Hayden Flood Wall, the need for improved delineation of space between an existing riverside footpath and the adjacent gardens which fronted directly on to it was identified through community engagement and the Landscape and Visual Appraisal. A new hedgerow with small clusters of trees better defined the space and provided additional privacy to the local residents. The existing Bonllwyn Green Park in the heart of the town was enhanced offering an amenity for local residents. The parks existing features were kept, but the space enhanced through avenue planting of specimen trees; drifts of bulb planting; interpretation boards; seating areas and informal play features formed from trees felled as part of the scheme. As part of the scheme, NRW worked in partnership with Carmarthenshire County Council to deliver the nearby Llandybie Community Woodland Project, which comprised1.58ha of community broadleaved woodland with public access. Protecting the environment

Environmental impacts were assessed through the Environmental Constraints and Opportunities Record (ECOR), with mitigation and enhancements integrated at an early stage into scheme development, heavily influencing the design process. Potential impacts upon trees, protected species, local community, archaeology, and local landscape, are some of the many that were assessed and mitigated.

The ECOR informed the Environmental Action Plan which formed part of the works information for the scheme and enabled key environmental mitigation measures to be communicated and tracked during the construction phase. The mitigation and protective measures implemented were comprehensive, covering the local community, protected species and environmental quality.

[caption id="attachment_14723" align="alignnone" width="1200"] Llandybie community woodland project - Courtesy of NRW[/caption] Considerate construction

The works were spread across nine locations in Ammanford comprising residential, commercial and public areas. Each location presented unique challenges and constraints. To ensure that the project was executed safely and efficiently, significant effort was made to tailor methods of construction in each location.

The team’s focus on careful planning and attention to detail ultimately resulted in the successful completion of the project.

Early contractor investigation works captured potential clashes with existing utilities and structures. A collaborative working approach from all parties ensured potential issues were rectified before the main construction works commenced.

Working methodology, plant and equipment choices were tailored to the environment at each location to minimise disturbance. Due to the nature of restricted access in some locations, alternative construction methods were implemented. This included the use of a lightweight shuttering system that could be safely manually handled into position.

Collaborative working between the client, contractor and designers allowed early access for in-river works. Note only did this result in the fish passage works being completed with minimal impact on the ecosystem, but also reduced the overall construction programme by many months.

Summary

The Ammanford Flood Risk Management and Fish Passage Scheme is a testament to the importance of comprehensive planning, innovative design, and community involvement in flood risk management. By addressing the unique challenges of Ammanford, the scheme significantly reduces the flood risk for hundreds of properties and unlocks 20km of river to passage, as well as providing numerous other environmental enhancements. This case study highlights the critical role of such projects in building resilient communities and enhancing the environment.

The scheme was successfully tested whilst still under construction in September 2023.

[caption id="attachment_14726" align="alignnone" width="1200"] Flood event in September 2023 - Courtesy of Walters[/caption]

Starting on site in 2022, the conceived project was to construct a new fish passage around the Holme Sluice Gates on the River Trent, allowing bi-directional travel of all breeds of fish from salmon to coarse river fish. The existing gates were constructed in the 1950s and are currently operated by the Environment Agency to maintain navigable river levels within the Nottingham area. This paper follows up on one published by Water Projects in 2022, and looks at the construction and commissioning elements of the project. Background

When the Holme Sluice Gates structure was constructed, the River Trent was diverted from its original path around the Country Park through a newly excavated 80m wide cut channel. At the side of the sluice gates is the Holme Canal Lock with a further adjacent water structure of the Holme Pierrepont canoe slalom course; all of which create barriers for migratory fish back up the River Trent.

Project summary

As the sluice gates will remain in operation, the fish pass channel requires two bridges rated to 40t to be built to maintain access to that area. The channel is to be protected at the intake via a single floating boom and the water isolated from the channel by a combination of removable stop logs and an automated radial gate system.

[caption id="attachment_12348" align="alignnone" width="1200"] Aerial view of the completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]

The design of the fish pass has been provided by the Environment Agency’s designer, TEAM Van Oord (TVO), with Jackson Civil Engineering Ltd (JCE) undertaking design elements for the temporary works and the Mechanical, Electrical, Instrumentation, Control and Automation (MEICA) elements.

The fish pass design has a total flow rate of 4.4m3/s - 4.9m3/s with a varying water depth of 1.5m- 1.7m. Although the velocities within the vertical slots have a flow rate of 2m3/s, the pools created at each stanchion bay slows this to >1m3/s; allowing fish to rest as they travel through the pass.

Where we left off……

This paper follows on from an initial case study published by Water Projects in August 2022. In the time since, the piling works (which started in April 2022) were completed by Ivor King The Piling People, following a period of ground pre-auguring to ensure that the circa 10m AZ700-36 sheet piles could be driven with relative ease.

A total of 350 sheet pile pairs with a number of singular piles were driven into the ground to produce the working cofferdam.

Due to the inlet and outlet elements of the fish pass having to be installed within the river flow, it was imperative that the works hit the allowable in-river works period to avoid damage to spawning fish and suitable breeding grounds. This period ran from 15 June 2022 to 2 October 2022; these dates forming an X5 Contract Key Date. Work on the inlet/outlet sections started nine workdays after the period started, and all piling works were completed by the end of August 2022.

There were some significant challenges throughout this period, the depth and complexity of excavating safety behind the sheet wall to install tie piles at a depth of 4-5m requiring a significant portion of the river bank (50m3) to be removed stored and reinstalled and for a number of the tie bar being at a level below the surface of the river, meant a design change was needed to a welded beam solution on the backside of the piles.

[caption id="attachment_12352" align="alignnone" width="1200"] Modular wall shuttering in place for wall pour - Courtesy of Jackson Civil Engineering Ltd[/caption] Channel design concept

In summary, the channel concept included a full height channel wall at the inlet and outlet, joined on both sides by a 2m high dwarf FRC sheet pile wall. Around the top of the sheet piles, a non-structural concrete pile capping beam (chainage 24m - 150m) was required, at which point the design changed to a structural capping beam.

This design was driven by the ground pressures seen on the sheet pile walls in that the channel floor would act as the only prop needed in the non-structural sections, however, due to the increasing depth of the channel towards the discharge end the top of the channel would also need to be permanently propped as well as using the base slab.

The start of earthworks & FRC

Originally the staging for construction was to temporarily prop the sheets at the top throughout the channel run, excavate down to design invert level, cast the base slab and the install the capping beams. This would require the need for significant soffit support systems and working at height.

Through detailed project discussions with the principal designers and supply chain, the decision was made to excavate only the area needed for the capping beam and use the ground formation to act as the soffit support there for removing a significant health and safety risk. To allow access throughout the construction period a section of capping beam was left out (5m of the 400m required) which would require a conventional support system for install.

Earth removal started in earnest in mid-August 2022 reducing the ground to install 26 (No.) SBS 400-185Hyd struts and 300m linear lengths of SSB wailing beams provided by Mabey Hire Ltd.

[caption id="attachment_12345" align="alignnone" width="1200"] Excavated fish pass awaiting FRC works - Courtesy of Jackson Civil Engineering Ltd[/caption] Groundwater challenges

From the provided geotechnical report it was noted that groundwater was expected circa 2m below the ground surface and from engaging with a dewater drainage specialist it was possible that installing a dewatering system would require a significant number of well points discharging 25 l/s. The larger issue was that these wells would need to be installed through the capping beam (not aesthetically pleasing) or would be placed in the way of the bulk earthworks removal.

A number of trail pits were excavated to 4m to assess the realistic nature of the groundwater. Little groundwater was evidenced during these tests, therefore the project proceeded without the use of dewatering, with a mitigation measure that a local sump would be created to allow groundwater to be pumped our using a 6” end suction over pump, to aid in the drainage and working surface a 100mm thick layer of gravel (350 tonnes) was installed in the bottom of the channel to allow the water to flow beneath and work plant. The risk of significant groundwater did not materialise.

The installation of the temporary struts and excavation of bulk material were done in sections therefore allowing manoeuvrability of the excavator to reach between the piles. M2 Civils Ltd, the earthworks contractor, removed on average 24 road wagons of earth per day, totalling 1458 loads or 27,935 tonnes (13,760m3).

Holme Sluice Fish Pass: Supply chain - key participants Client designers: Team Van Oord Principal contractor: Jackson Civil Engineering Ltd Temporary works design: Waterman Group Mechanical design: KGAL Consulting Engineers Ltd FRC: STAM Construction Ltd Groundworks: M2 Civils Ltd Piling works: Ivor King The Piling People River works: Red7 Inshore Diving Ground support: Mabey Hire Ltd Electrical design & control equipment: IMAC Ltd Radial gate system & stop logs: Centregreat Engineering Ltd Eel tiles: Berry & Escott Engineering Services Fish monitors: FishTrack Precast concrete beams: Banagher Precast Concrete Ltd Tarmac & surfacing: CR McDonald Surfacing [caption id="attachment_12347" align="alignnone" width="1200"] Non running fish pass mid-distance - Courtesy of Jackson Civil Engineering Ltd[/caption] FRC

The FRC works was undertaken by STAM Construction Ltd and started in October 2023 with the installation of base slab using cut and bent reinforcement steel throughout the construction. This would then allow the high-level shoring props and wailing beams to be removed. Due to the nature of the design being effectively three straight runs of dwarf walls, the wall shuttering system (9m long by 2.5m high) was constructed on an adjustable wheeled cart, thus allowing the shutters to be installed, retracted, moved, and redeployed reducing the use of timber and removing significant lifting and manual handling risks.

To facilitate this the project teams worked on providing a solution to allow the shuttering system to fly past the starter bars required for the wall and floor stanchions. This was achieved by utilising a ‘Startabar’ product whereby the RC bars are encased under a steel cover which is then removed, and the bars bent out to the correct position after the wall concrete has been poured.

To construct the wall and floor stanchions, conventional shuttering systems were employed. In total 329 tons of steel reinforcement was used and 2,250m3 of concrete were pumped.

Bridges

The construction of the fish pass has created an island of the operational sluice gates requiring 24 hour. To service this asset, two permanent 40T rated road bridges were designed and installed. A major project challenge was to sequence the bridge construction start for after the bulk excavation had past the bridge locations and have them finished before the temporary access created across the sheet piles needed to be excavated out. This includes the bridge abutments, deck and on/off ramps. The bridge construction utilised MY3/MY3E precast concrete beams spanning 12.6m utilising seven beams per bridge and used scaffold soffit support system to all for cast in situ sidings and deck. The MY3 beams were planned and delivered in a ‘Just in Time’ strategy allowing a direct lift and install from the wagon to their installation location.

Construction started September 2022 and finished October 2022 including the installation of the vehicle restraint system, notably the bulk earth movement completed by removing the temporary access to the island in December 2022.

MEICA

The water passage through the pass is controlled by an automated radial gate system, using local river levels and area flow gauges. The gate opens when the flow is greater than 49m3 and the level is above 20.34m AOD; should either of these readings become false the gate will close.

The gate spans the full width of the channel with a radius of 3.5m and stands closed at a vertical height of 1.9m. Using a single actuator the gate (10 ton self-weight) is lifted and lowered via a steel cable winch system with a full extent of travel being circa 4m vertically.

The radial gate solution was chosen by the designers as the best suitable control mechanism due to the following benefits:

Requiring less infrastructure than a vertical gate (no need for structures/lifting frame/gantry above the channel) all reducing the civil reinforcement design in those areas. Less energy to operate as loads are transferred through the pivots and the geometry of the gate means upwards water pressure will assist with lifting. Longevity of the seals as loads transferred through the pivots and not into guides and seals. No need for channels/slots in the walls/base of the channel which could become blocked with debris and would also affect hydraulics.

Even though the size of the fish pass requires significant volume of water to achieve a steady state, this is realised in approximately 90 seconds from the start of the gate opening even though the gate takes 8 minutes to fully clear the pass/river surface.

The isolation of the fish pass for maintenance purposes is undertaken by the installation of five 6.8m long stop logs. These are deployed using an 18m long lifting gantry with dual lifting hoist with a SWL of 2.5T and a hand cranked moveable storage cart.

While the pass is in operation there will be free bidirectional passage for aquatic life, however during non-operational periods eels are still likely to try and pass and with the water during non-operational period only filling the last third of the pass, there is the potential for the eels to become stranded in the pass. Therefore, the pass has been designed with eel chute compliant with the Eels Regulations 2009. The chute and subsequent rebates along the edges of the pass have been fitted with fixed plastic eel tiles therefore providing purchase to aid the eels in their journey. To assist the chute, which is connected to the radial gate and is lifted out of the water during gate operation, while the gate is closed there is a constant flow of water covering the eel tiles guided by the fall of the channel base and surface mounted fixed plastic lumber sections.

[caption id="attachment_12351" align="alignnone" width="1200"] Completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption] Pass/river interfaces

Extensive riverbank and river bend modifications were needed to ensure that the pass joined and exited the river and was engineered to prevent erosion. During the in-river works period of 2022 and detailed as a key date within the contract, specialist dive services were employed to dredge out the riverbank/bed and place 80-100T of scour protection amour using 5-300kg sections of rock.

This was all achieved using bank-side long reach excavators; therefore being more environmentally friendly, with greater efficiency than deployment of float pontoon platforms.

Due to the in-river works, stringent protection measures were placed on the project with regards to fish protection and silt migration. The project team deployed 100m of silt bubble curtains around the river work areas to reduce the any sit mitigation as well as undertaking on-site water quality monitoring for turbidity and dissolved oxygen both upstream and downstream, and within the works area. Data showed that the work area was a factor of 10 higher than the up/downstream areas, which remained within agreed permissible limits.

Further dive works included underwater cutting and removal of the cofferdam end sections to open the fish pass to the river. This solution was chosen as it reduced the risk of delays and damage to permanent structures during a vibration extraction method.

Monitoring fish usage

The fish pass is fitted with state of the art near-field communication hardware (passive induction transmitters) to facilitate the monitoring of fish passage. When the electronics are installed, the system will allow the client to understand how effective the pass is, the direction of fish travel and time spent within the pass.

[caption id="attachment_12350" align="alignnone" width="1200"] (left) Completed fish pass looking towards the intake structure and (right) fish pass in operation showing control radial gate - Courtesy of Jackson Civil Engineering Ltd[/caption] Conclusion

The project completed on 14 May 2024 having run for 27 months. The following is a statement from Simon Ward, Fisheries Specialist with the Environment Agency.

“After two years in construction, and several years of planning and design, the Colwick (Holme Sluices) Fish Pass on the River Trent is now complete and ready for fish to use enabling them to reach their spawning and feeding grounds. This flagship project opens up the River Trent and its tributaries for migratory fish, including salmon, trout and eels, making more habitat accessible for fish. Whilst a complex task the construction ran smoothly throughout and the partnership of designer, constructor and EA Fisheries technical staff working together is an exemplar of what can be achieved for the environment.

“The fish pass is part of the Environment Agency’s work to improve fish passage across the country and provides a significant step in restoring the River Trent catchment to its former glory for salmon and other coarse and migratory fish. The project also includes an eel pass to help support the critically endangered European eel; and a public viewing platform above the water, with highly visual interpretation boards to inform and advise visitors about the local wildlife in and around the river, including the fish that are expected to use the pass.”

Dunside Reservoir consisted of two reservoirs named Dunside Upper and Dunside Lower. They were opened in 1884 to supply drinking water to the surrounding area. They ceased operations in 2009 when Scottish Water deemed that it was not economically viable to continue to maintain the two impounding dams. The dams required weekly leakage monitoring and had to be inspected regularly by the Scottish Water Reservoir Engineer and Reservoir Panel Engineer (ARPE) to assess the dam conditions as part of the Reservoir Act 1975. The act states that any impoundment of 25,000m3 or more should be part of the Act. Scottish Water assessed the viability and future cost of the dams remaining within the Act and with agreement with the ARPE and SEPA deemed that they should be breached. Full abandonment/enabling works

Scottish Environmental Protection Agency (SEPA), the ARPE and Scottish Water decided that the breach would be a full abandonment. As a result, the reservoirs were drawn down to empty and the fish rescued and relocated to a place of safety under licence by the Clyde River Trust team by Spring 2023.

Design requirements

Mott McDonald was appointed as designers with the outline design scope being:

“Dunside Upper and Lower Reservoirs are to be removed so that they are no longer capable of storing water, and therefore no longer subject to the Reservoirs (Scotland) Act 2011. This will be achieved by the removal of both embankments, distributing the excavated material across the former reservoirs and profiling to match the existing ground, and forming a channel through the former embankments to reinstate the natural watercourse.”

[caption id="attachment_18268" align="alignnone" width="1200"] Dunside Reservoirs prior to breach - Courtesy of George Leslie Ltd[/caption] Scope of works

George Leslie Ltd was contracted by Scottish Water to carry out the reservoir abandonment works. The works were undertaken between May 2023 and January 2024:

Management of inflow to the reservoir, including temporary pumping to prevent refilling of the reservoirs during removal of each dam, and control of sediment to prevent discharges of water from the work site causing pollution of the receiving watercourses. Breach cut through the Dunside upper embankment (down to the original watercourse level). Work to be undertaken in a specified sequence. Breach cut through the Dunside lower embankment (down to the original watercourse level). Work to be undertaken in a specified sequence. Creation of a meandering channel through the base of each embankment, following the alignment of the original watercourse from historic drawings. Grouting of the outlet pipes through both embankments. Removal and disposal of valve towers/chambers, bridges, chambers, pipework. Infilling of the existing spillway channel. Hydroseeding of the breach cuts and material removed from the embankments. [caption id="attachment_18266" align="alignnone" width="1200"] Water management plan layout - Courtesy of George Leslie Ltd[/caption] Construction challenges

Flood risk: The earthworks required were not particularly unusual in that only some 60,000m3 of cohesive materials had to be cut and carted to designated on-site depositories for placing and compaction. To control the levels of the earthworks, Trimble GPS controlled bulldozers and excavators were used.

The excavations were carried in pre-agreed stages to set levels above AOD as a requirement of a Mott MacDonald study ‘Dunside Hydrology to Inform Construction Risk Review’ to negate theoretical risks arising of a flood event whilst works were in progress. In addition, a maximum legally permissible impounded water level at each stage was set by the ARPE. This required a minimum of eight 200mm pumps to be located on site throughout the construction period in order that they could be deployed at short notice in flood events.

Silt mitigation: In the 140 years the dams were in place, the natural downstream movement of sediments on the submerged watercourses had stopped, resulting in a substantial volume of silts in the deeper areas of the reservoirs.

George Leslie Ltd had to ensure that every practicable measure was utilised to minimise silt discharges to the watercourses and avoid harm to the ecosystems. These measures were required during the reservoir drawdown and throughout the construction period as the excavated and fill surfaces remained exposed to the elements.

A Water Management Plan, detailing an extensive regime of settlement lagoons and swales, filtration through matting and bags, and finally lagoon flocculation using QP33 was developed and implemented prior to any release of water to existing watercourses. QP33 is an organic coagulant, which is supplied and installed by Taytech Environmental Ltd. This coagulant is a primary chemical used in suspended solids settlement. At the time of writing (August 2024) QP33 was the only flocculant chemical approved by SEPA.

[caption id="attachment_18270" align="alignnone" width="1200"] (left) Dunside Reservoirs during construction July 2023, and (right) after breach - Courtesy of George Leslie Ltd[/caption]

Sediment filter bags from Murlac Land & Marine Environmental Solutions are made of a 90 micron non-woven high strength (400 gsm) geotextile, which catch silt, sediment and oil, whilst releasing clean water. The Murlac sacs were connected to the de-silters to catch any finer silt before discharge.

During the construction phase, turbidity levels throughout the site and at designated points were monitored using turbidity meters and Secchi tubes. An Environmental Clerk of Works visited site at least once a week and compiled a report with recommendations and actions if required, which was made available to SEPA.

The weather throughout July and August was generally cool and wet and as a consequence, all the earthmoving plant was required to stand down for long periods of time rather than generate turbid runoff. The water management, however, was required throughout the construction period and utilised a significant amount of resource and consequent cost.

Ecological constraints: Ecological constraints were present on site, such as nesting birds and otters. Exclusion zones were set up to protect ecological features, as part of George Leslie Ltd’s commitment to minimise their environmental impact.

Hydroseeding: George Leslie Ltd was required to carry out hydroseeding as soon as was practicable, and as early in the growing season due to hilly nature of the site, on any finished surfaces in order to encourage early growth to minimise the area of exposed bare earth.

Dunside Reservoirs Abandonment: Supply chain - key participants Designers: Mott MacDonald Main contractors: George Leslie Ltd Ecological surveys: JK Ecology Drone surveys: Whitehouse Studios Fish rescue: Clyde River Foundation Removal of tower: PMP Utilities Temporary bridges: Groundforce Pumping supply: Selwood Silt mitigation: TayTech Ltd Silt filter bags: Murlac Land & Marine Environmental Solutions Silt mitigation fences: Hy-Tex UK Ltd Hydroseeding: Aitchison Civils materials: Keyline Civils Tankering services: Enviroclean [caption id="attachment_18269" align="alignnone" width="1200"] Reinstatement of the watercourse - Courtesy of George Leslie Ltd[/caption] Reinstatements and formation of watercourses

Following the earthworks being completed, the formation of the watercourse was carried out. Hydro-morphologists from Mott McDonald and SEPA had consulted historical maps, from prior to the original dam construction.

George Leslie Ltd was tasked with reforming the watercourse as closely as possible to the original course to replicate the sinuosity to appear more natural and less engineered. Prior to flows being gradually re-introduced over a prolonged period, coir fabric was installed temporarily on the burn banks to minimise scouring whilst vegetation growth took place.

The Clyde River Trust carried out invertebrate surveys of the stream beds prior to works and then subsequently one year after completion.

Carbon savings

One of the main targets within the contract was to utilise initiatives consistent with The Scottish Water Net-Zero Strategy. Initiatives George Leslie Ltd implemented included using batteries within the site accommodation and welfare buildings, and recycling and using arisings from the works by processing, crushing and grading to meet specifications. Scottish Water are planning to undertake tree planting throughout the area.

Conclusion

This contract is one in a series of similar future planned works and as such the lessons learned during the works are to be used in the planning and design of future dam breaching works.

Llyn Bran is located in Denbighshire, north-east Wales and was a natural lake that was raised in 1899 by a concrete dam. The reservoir was used to supply water to the North Wales Hospital in Denbigh. The hospital closed in 1995 and Llyn Bran has not been used for drinking water since. As part of a separate discontinuance scheme at Llyn Anafon, Llyn Bran was identified as a ‘compensatory site’ for the loss of habitat at Anafon. One element of turning Bran into a compensation site was the removal of the dam, as well as other ecological enhancements. Through careful restoration work, Llyn Bran will eventually become a Special Area of Conservation. The reason for Llyn Bran discontinuance Llyn Bran Reservoir is required as a ‘compensatory site’ for the loss of habitat area that will be caused by discontinuing the dam at Llyn Anafon Reservoir, as required by the Habitats Directive. Llyn Anafon Reservoir was originally a natural lake which was raised by 1.4m in 1931 through the construction of an embankment dam. The use of this site for local water supply ended in 2002. A 2016 S10 inspection of Llyn Anafon resulted in a MITIOS (Measures in the Interests of Safety) to “develop a long-term solution to the poor and deteriorating condition of the dam”. In January 2019, it was agreed with relevant regulators that to provide a long-term solution, the reservoir would need to be decommissioned, with the dam structure removed and the lake lowered by 1.4m. [caption id="" align="alignnone" width="1200"] Llyn Bran before discontinuance - Courtesy of Welsh Water[/caption] Llyn Anafon is nationally and internationally protected for specific conservation features. This means prior to any project taking place, a full Habitats Regulation Assessment (HRA) had to be undertaken. The outcome of this assessment was that it was not possible to avoid adverse impacts to protected features and as such, the only route to proceed with the project was under the “Imperative Reasons of Overriding Public Interest (IROPI)” (Article 6.4 of the Habitats Directive) on the grounds of public safety. The Anafon Dam Discontinuance Scheme is the first IROPI case of its kind in the UK and has recently been approved by Welsh Government. To satisfy the IROPI case, Welsh Water was required to secure ‘compensatory measures’ for the loss of habitat at Anafon. Using a set of criteria agreed with National Resources Wales (NRW), Llyn Bran was identified as the most suitable site to provide the necessary compensation. A key element of the work was to ‘restore and maintain hydrological functioning’ i.e., removal of the dam to create a natural lake and restore connectivity to the downstream watercourse. Although the primary reason for discontinuing Llyn Bran was for compensation, it also had the added benefit of removing a dam which was no longer needed for operational purposes. [caption id="" align="alignnone" width="1200"] Llyn Bran dam before discontinuance - Courtesy of Welsh Water[/caption] Scope of works Create temporary construction access using bog mats. Draw down the reservoir by means of siphon pipes and pumps. Remove silt from the area immediately upstream of the dam. Demolish the entire dam and remove arisings for disposal off-site. Restore the watercourse channel downstream of the retained natural lake, incorporating silt management and retention measures. Create new water vole habitat along the shore areas of the retained natural lake using coir matting. Demolish outbuildings and associated structures with all building waste removed for disposal off-site. Remove exposed sections of pipework. Buried pipework to be sealed and filled by grouting. Install new roadside fencing along the north end of the reservoir. Install rip-rap protection around the upstream end of the B4501 culvert. [caption id="" align="alignnone" width="1200"] Ground conditions - Courtesy of Welsh Water[/caption] As with all discontinuance schemes the various surveys, permits, consents and negotiations in advance of the physical works were particularly complex, taking more than two years to complete. An overview of the consents and constraints required is below: Impoundment Licence and SSSI consent from Natural Resources Wales (NRW). Permitted Development Certificate from Denbighshire CC. Ordinary Watercourse Consent from Denbighshire CC. Protected species licence required for trapping and temporarily removing water voles (this was needed before permitted development could be granted). Annex 1 Habitats present around lake margins. Reservoir drawdown rate limited to 120 l/s. HRA level 1 & 2 required. Peat bog deposits along route of temporary access track. Bat surveys required prior to demolition of valve house & boat house. Challenges & constraints Site access With no formal access track to the dam, and the surrounding land being designated as Annex 1 Habitat due to the presence of peat, access was one of the main challenges. With 140 peat depths probes taken along the route of the proposed access track, and working closely with NRW, a methodology was developed for installing over 280m of bogmats to the dam without the need to excavate and remove the top layer of peat. This minimised the potential damage to the peat and made site reinstatement far easier. [caption id="" align="alignnone" width="1200"] (left) Bog mat access track and (right) amphibious excavator used to load barge with coir matting - Courtesy of Welsh Water[/caption] Peat The extent of peat encountered on site was far greater than expected once construction began. This resulted in an entire change in the contractors methodology for the coir matting. It was initially planned to use Bog Master excavator and tracked dumper to access the western shore of the reservoir, however it quickly became clear this was not practical. Thanks to the contractor being proactive, the methodology was quickly changed to using an amphibious excavator and barge, to transport the coir matting across the reservoir. Water voles Water voles were identified on site during the early site survey stages, meaning a licence was required to temporarily remove the voles during construction. Six water voles were captured by specialists and were transferred to a facility in Kent, where they were looked after while the work took place. A condition of the licence was that there must be 5-days with no water voles captured before it could be concluded that all voles were removed from site. Rather unluckily, there were two occasions where a vole was captured at the end of the fifth day, meaning the process took far longer than expected and the licence had to be extended. The voles are due to return home to Llyn Bran this summer. [caption id="" align="alignnone" width="1200"] Water voles were captured and transferred to a facility in Kent for the duration of the works - Courtesy of Welsh Water[/caption] Compensation flow A condition of the NRW impoundment licence was for a continuous compensation flow of at least 20 l/s for the duration of the works. This meant pumping to the watercourse, which had to be continually monitored. It was hoped to use a cloud-based telematics tool that makes it easy to monitor the pumps off-site but unfortunately, due to lack of signal, this did not work meaning William Hughes Civil Engineering needed to attend site over the weekends to check on the compensation flows. The summer of 2022 proved to be exceptionally dry, which crated a challenge in meeting compensation flows whist also not allowing the remaining lake to become too low. Flood risk The existing reservoir provided significant flood attenuation meaning the discontinuance increased the flood risk to a downstream highway culvert. Initially, there were concerns from the highways department that the culvert would need to be upsized, which would have added significant costs to the project. Following a detailed flood risk and stability assessment, Welsh Water were able to prove that the increased flood risk would not adversely affect the culvert and only minimal slope protection work around the culvert was required. [caption id="" align="alignnone" width="1200"] Amphibious excavator forming the new channel connecting to the downstream watercourse - Courtesy of Welsh Water[/caption] Llyn Bran Reservoir Discontinuance Project Construction timeline: 16 May 2022 - 4 November 2022 Project cost: £1.7m Drivers: Dam safety | Habitats Directive | Reducing costs | Ecological compensation measures Supply chain - key participants Principal Contractor: William Hughes Civil Engineering Design Partner: Stillwater Associates Environmental Consultants: Ricardo Energy & Environment Ecology/water vole licensing: Wildwood Ecology Ltd Silt management: Salix River and Wetland Services Amphibious excavator hire: Land and Water Plant Opportunities & Savings Forestry culvert At the design stages there were concerns about the condition of a nearby downstream forestry culvert and its ability to accommodate the reservoir outflows during emptying. Expertise within Welsh Water was utilised by requesting a CCTV survey of the culvert via the wastewater team. The team provided a prompt and efficient service and enabled us to confirm the capacity and condition of the culvert. [caption id="" align="alignnone" width="1200"] Cores being drilled through concrete dam to allow any extreme storm flows to pass through - Courtesy of Welsh Water[/caption] Dam safety mitigation During detailed design, the Dam Safety Team suggested coring holes though the concrete dam once the reservoir was drawn down to the required level. This would mitigate the risk of the reservoir refilling during severe weather, before the dam was fully demolished. Fortunately, on this occasion the weather was favourable and the cores were not needed. This approach will be considered for any future concrete dam discontinuances. Waste removal It was agreed with NRW that the stone from the haul road and compound could be transferred to a nearby quarry, just down the road from site. This was done under a waste transfer notice, saving costs. The arisings from the dam were removed off site. The dam was demolished far easier and quicker than expected, making the removal of concrete straightforward. [caption id="" align="alignnone" width="1200"] Dam demolition in progress - Courtesy of Welsh Water[/caption] Coir matting Due to the silt being drier than anticipated, there was spare rolls of coir matting left over. Rather than this being wasted, it was moved to nearby Alwen WTW where it is being stored before being used for silt management on the upcoming Cilcain Discontinuance Scheme which is due to start this spring. The future for Llyn Bran The discontinuance will give significant biodiversity opportunities in relation to the marginal areas around the retained natural lake and with the restored connectivity to the watercourse. The retained lake will also be used as a receptor site for aquatic plants translocated from Llyn Anafon. Long term ecological monitoring of the lake is required to ensure the compensatory criteria are met. This includes water quality and macrophyte monitoring as well as botanical surveys to check the natural regeneration of exposed shore habitats. A condition of the water vole licence is that activity is monitored until 2034. Through careful management and monitoring of the site, it is hoped that Llyn Bran will become recognised as a Special Area of Conservation.

The Severn Estuary is renowned for its tidal range of 12-14 meters, the second largest in the world, and as such poses significant challenges for civil and marine works. In late 2024, a critical infrastructure upgrade project was completed near Nash, Newport, replacing a long sea outfall pipeline originally constructed in 1960 to extend its operational life. The project involved installing a 1.4km 560mmØ high-density polyethylene (HDPE) outfall pipe to replace the deteriorated concrete-coated steel pipe. Situated within one of the UK’s most ecologically sensitive regions and a Site of Special Scientific Interest (SSSI) in the Severn Estuary, the project not only secured infrastructure integrity but also adhered to stringent environmental protection measures. Project challenges and environmental safeguards

Kaymac Marine & Civil Engineering Ltd was appointed as the principal designer and contractor for the £8.4m outfall replacement project.

The Severn Estuary is designated as a Site of Special Scientific Interest (SSSI), which protects its tidal flats, salt marshes, and diverse wildlife, which required the project team to introduce and adhere to comprehensive environmental safeguards.

[caption id="attachment_19920" align="alignnone" width="1200"] The in-land RC chamber connecting the new and existing pipe (during construction) - Courtesy of Kaymac Marine & Civil Engineering Ltd[/caption]

Sediment control measures such as careful selection of plant and equipment, implemented in collaboration with National Resources Wales (NRW), minimised water quality impacts during the excavation and pipe-laying activities.

Kaymac Marine & Civil Engineering Ltd drew upon its deep industry expertise and practical experience to drive the design direction and successfully secure all necessary permits and permissions for the project. Working closely with the local council and NRW, Kaymac led the process of obtaining the required Marine License, Environmental Permits, and Planning Permissions, ensuring full compliance with all regulatory requirements.

Through comprehensive site investigations and advanced geophysical and bathymetric surveys, Kaymac Marine & Civil Engineering Ltd identified critical site-specific characteristics that directly influenced the project’s design. This informed approach allowed the team to craft tailored solutions that addressed regulatory concerns while optimising project outcomes.

Kaymac’s practical experience played a crucial role in addressing the expectations of the environmental regulators. By proactively collaborating with NRW and other stakeholders, the team was able to satisfy stringent environmental requirements, demonstrating how the proposed designs minimised ecological impacts while maintaining structural and operational integrity.

[caption id="attachment_19919" align="alignnone" width="1200"] New valves within the inland chamber - Courtesy of Kaymac Marine & Civil Engineering Ltd[/caption]

This expertise and experience, coupled with a solutions-focused mindset, not only streamlined the approval process but also ensured that the final design adhered to both regulatory standards and the project’s overarching goals.

To maintain quality standards and to ensure the works could be undertaken within the licencing timeframes, Kaymac Marine & Civil Engineering Ltd engaged suppliers early on, securing essential equipment and materials; whilst conducting regular necessary quality visits and inspections to ensure technical and Quality compliance.

Severn Estuary LSO Replacement: Supply chain - key participants Principal Designer & Contractor: Kaymac Marine & Civil Engineering Ltd Designer: Pebble Engineering Ltd Specialist design: Morek Engineering Ltd Specialist design: H2NA Ltd HDPE pipe: Pipelife Norge AS Precast concrete collars: Poundfield Precast Precast concrete mattresses: Subsea Protection Systems Duckbill check valves: Measurit Technologies Valves: Scott Parnell Landside temporary works & shoring MGF Ltd Huennebeck Construction materials Scott Parnell Neal Soils Wright Readymix Stone Supplies IPP Floating plant Jenkins Marine Kraken Marine Services P&D Environmental Construction plant hire Land & Water Plant Hire Flannery Plant Weldex RW Christopher Crane Hire Ltd United Rentals Atlas Winch & Hoist Services Ltd A1 Surveys MTK Breakers LGS Gap Hire Services HV Fusion Severn Environmental Services ABP Newport & Barry [caption id="attachment_19913" align="alignnone" width="1200"] Trench excavation prior to the first section of pipe installation - Courtesy of Kaymac Marine & Civil Engineering Ltd[/caption] Detailed design & planning

A comprehensive understanding of the physical forces that dominate the ocean environment, their origins, and the likelihood and intensity of these forces over the lifespan of an outfall is critical to its effective design, regardless of the construction materials used.

These forces can compromise the outfall’s integrity either directly or indirectly, such as through seabed erosion or by destabilising its foundation. Additionally, sediment transport can obstruct diffuser orifices, potentially leading to system failure. These factors were considered and mitigated during the planning and design phases to ensure the long-term functionality for this particular environment.

Kaymac Marine & Civil Engineering Ltd appointed Pebble Engineering Ltd as designers and worked collaboratively to develop the construction methods and subsequently the detailed design of the outfall.

Both the construction method and the detailed design needed to align and complement each other, with regard to compliance of the aforementioned environmental and planning regulations, the seabed conditions present on site and the anticipated physical conditions (such as sea, wind, wave, tidal and current) of the works location.

Delivery & preparation of HDPE pipe

The new high-density polyethylene (HDPE) long sea outfall pipe, measuring 1.4km in length and 560mm in diameter, was manufactured by Pipelife Norge AS in Norway and was towed 1,134 nautical miles to Newport in 11 sections under a single trip.

Upon arrival at ABP Newport Docks, each pipe section was stringently inspected, tested and then later prepared for installation.

The preparation works involved the installation of over 260 bespoke precast concrete ballast collars that were designed to ensure stability of the pipeline against the hydrodynamic forces of the watercourse and the natural buoyancy of the HDPE material, once it had been sunk to its installed position within the trench.

[caption id="attachment_19915" align="alignnone" width="1200"] Pipe being prepared for connection - Courtesy of Kaymac Marine & Civil Engineering Ltd[/caption] Innovative installation techniques in a tidal environment

The use of specialised low pressure bearing amphibious excavators handled intertidal trenching at low tide, while a GPS-equipped dredging vessel undertook the offshore trenching requirements. The installation spanned an approximate 28-day period, working shift patterns under challenging tidal conditions, with trenches reaching depths of approximately 1.5–2 meters.

A key milestone involved towing the first 250 meter section of the pipe from ABP Newport, navigating the currents and flows of the Severn Estuary, to the installation site, approximately three nautical miles away. This segment, equipped with 50 concrete collars, was positioned over the trench as the tide receded, sunk under the guidance of divers from Kaymac Marine & Civil Engineering Ltd to ensure accuracy, before being backfilled with the previously excavated spoil.

Finally, the precast articulated concrete mattresses from Subsea Protection Systems Ltd were installed by utilising a crane barge to provide additional scour protection and stability to the outfall. Such mattresses have been used successfully for decades to protect oil and gas pipelines and since the early 1990s, they have been increasingly used to protect ocean outfalls.

Throughout the project, Kaymac employed a flotation and submersion technique whereby each 250 meter long section was towed into position and the seaward end of the previously installed pipe length was recovered to the working platform, before connecting to the new length and then sinking within the new pre-excavated trench.

[caption id="attachment_19916" align="alignnone" width="1200"] First section of the pipe within the trench during low tide ahead of backfilling - Courtesy of Kaymac Marine & Civil Engineering Ltd[/caption]

Ensuring an accurate and safe sinking procedure in the Severn Estuary required considerable planning efforts from the delivery team, taking into consideration tidal movements and slack water periods. Each sinking activity was planned meticulously to ensure that a constant submersion rate could be implemented, assisted by a number of key specialist vessels being present to position and secure the pipe section in place whilst the slack water period approached.

Slack water periods in this location are relatively short and as such, the rate of pipe submersion needed to be planned and controlled by adjusting both the water inflow and air release rates. To avoid a bending radius that could become smaller than its allowable minimum and further, to prevent the pipe wall buckling, the rates needed to be carefully monitored and controlled, all whilst considering the load imposed by the ballast weights, the varying tidal water depth and the pulling forces applied to the outfall by the marine plant.

The pipe installation continued in four separate sections, totalling 1.47km, with a diffuser head separately installed through diving operations at the seaward end. A backhoe dredger was used to place the previously excavated material on to the pipe.

Following the backfilling, approximately 210 of the precast articulated concrete mattresses in total, each weighing 8.3t in air, were laid over the installed lengths of pipe from the crane barge. The backfilling and mattress installation were monitored and supervised by Kaymac Marine & Civil Engineering Ltd’s dive teams, ensuring the accuracy and efficiency of both activities throughout the scheme.

Civil operations, plant isolations and final connection

To prevent disruption to the client during their day-to-day operations, the marine activities to install the replacement outfall were carried out alongside the existing operational outfall. Simultaneously, inland operations were undertaken by the skilled civils teams from Kaymac Marine & Civil Engineering Ltd, who installed a temporary sheet piled cofferdam to excavate and expose the existing valve and land-based outfall arrangement.

Two short operational isolations were required; the first to replace the existing land based valve and connecting pipes, and the second to connect the new replacement outfall to the existing infrastructure. Again, these isolations were planned precisely around suitable tidal windows, to maximise working efficiency and to minimise interruption to the plant operations.

During the first isolation, the existing land-based single non-return valve was replaced with new valves and a T-piece arrangement system that would permit future inspection and maintenance requirements.

Once the land based replacement and connection works were complete, the replacement valve arrangement was housed within a newly constructed reinforced concrete chamber, built alongside the normal operations of the plant. The chamber was designed to permit safe future access around the valve arrangement and to retain a section of the existing flood defence embankment.

The second isolation took place while the land-based civil works were underway to construct the new chamber. Working at low tidal periods, a bespoke HDPE pipe section was fabricated that considered both the vertical and horizontal changes in direction to align with the newly installed outfall. This section was installed to connect the newly installed outfall to the existing pipeline ahead of removing the isolation.

[caption id="attachment_19918" align="alignnone" width="1200"] Old pipe being removed during low tide - Courtesy of Kaymac Marine & Civil Engineering Ltd[/caption] Conclusion

The long sea outfall replacement project stands as a model of responsible marine engineering within sensitive ecosystems.

By balancing complex engineering requirements with rigorous environmental protection, Kaymac Marine & Civil Engineering Ltd are proud to have played a key role in improving Newport’s existing infrastructure; blending innovative marine engineering techniques with stringent environmental safeguards to protect and enhance the unique ecology of the Severn Estuary.

Duston Mill Pumping Station in Northampton is a critical Anglian Water asset supplying raw water to Pitsford Reservoir, a primary water source for nearby communities and industries. The pumping station plays a pivotal role in water abstraction and distribution, ensuring the consistent and reliable supply of water resources to meet the growing demands of households, businesses, and agricultural operations. Nestled in an area rich in biodiversity, the station’s surrounding environment is home to numerous aquatic and terrestrial species. Among these is the European eel (Anguilla anguilla), a species classified as critically endangered due to a significant decline in population over the past few decades. Background

The river system around Duston Mill is part of a vital migration route for eels, making the preservation of this habitat a key priority for ecological conservation efforts.

In addition to its ecological significance, Duston Mill’s location within a dynamic river system presents unique operational challenges. Seasonal variations in river levels, potential flooding events, and the need for uninterrupted water supply create a complex balancing act between operational efficiency and environmental responsibility. These factors make Duston Mill a prime candidate for innovation in water management infrastructure.

[caption id="attachment_19464" align="alignnone" width="1200"] Hydrolox eel screen installation - Courtesy of Anglian Water’s @one Alliance[/caption] The issue

The European eel population has experienced an alarming decline of up to 95% since the 1970s, attributed to a combination of factors including overfishing, habitat loss, climate change, and barriers to migration. These barriers, which include water intake systems and hydropower facilities, are a significant threat as they hinder the eels’ natural life cycle by preventing migration and causing injury or death during intake processes.

To combat this crisis, the Eels (England and Wales) Regulations 2009 were introduced, mandating measures to protect eel populations, particularly at water abstraction points. Therefore compliance with these regulations are critical, as failure to address the requirements can exacerbate the decline of eel stocks and result in penalties for the water companies.

Existing intake structure

At Duston Mill, the existing intake structure was assessed and found to be non-compliant with these regulations. The intake consists of two channels, each equipped with angled baffles, flat bar coarse screens, and fine mesh cup screens.

While these components provide basic screening functionality, they do not meet the stringent standards required to prevent eel entrainment and impingement.

Time-limited exemptions granted by the Environment Agency allowed continued operation while plans for compliance were developed. However, these exemptions are set to expire, necessitating immediate action.

Adding to the complexity, the station is the sole source of water supply to Pitsford Reservoir, requiring that abstraction operations continue uninterrupted during the upgrade. This dual challenge of maintaining operational continuity while implementing a compliant solution underlines the importance of careful planning and execution.

[caption id="attachment_19461" align="alignnone" width="1200"] Hydrolox eel screen installation - Courtesy of Anglian Water’s @one Alliance[/caption] Duston Mill Pumping Station: Supply chain - key participants Project delivery: @one Alliance Mechanical/electrical PSSC works: Waveneys Eels screens: Hydrolox Steel access metalwork: Global Energy Group MCC: Paktronic Engineering Co Ltd Roof survey: Claret Civil Engineering Ltd The solution

To address these challenges and bring the station into compliance, a robust solution was developed by the Anglian Water’s @one Alliance. The centrepiece of the project was the installation of a modern positive exclusion screening system, that meets the Eels (England and Wales) Regulations 2009 and supports broader environmental conservation efforts.

The solution involved replacing the outdated coarse and fine screens, with two high-performance Hydrolox traveling band screens. These were designed to operate with a 2mm slot size and an approach velocity of ≤0.4 m/s, ensuring optimal protection for yellow and silver eels.

The screens will operate in a duty/duty configuration, allowing each to handle 50% of the design flow and maintaining redundancy and reliability. To support the new screens, a steel structure was constructed on the intake frontage, providing a secure foundation and incorporating access platforms and handrails to facilitate maintenance and inspections. Additionally, an automated wash water system, powered by duty/standby submersible pumps delivering 5.94 l/s of water at 4 bar pressure, will ensure uninterrupted performance by efficiently clearing debris from the screens.

[caption id="attachment_19460" align="alignnone" width="1200"] Dive operations - Courtesy of Anglian Water’s @one Alliance[/caption]

The solution also included integrating the new system with existing automation and telemetry controls through a new motor control center (MCC) and human-machine interface (HMI). The installation was carried out in phases to maintain the continuous abstraction, with one intake channel remaining operational while the other is upgraded, ensuring a steady water supply to Pitsford Reservoir throughout the project.

To minimise disruption to the aquatic environment, environmental safeguards were included like air-lifting localized silt deposits and removing existing coarse screens and baffle plates to prepare the site for installation. These measures ensure that the project aligned with both regulatory compliance and ecological preservation goals.

Environmental & operational benefits

From an environmental perspective, the new screening system will play a critical role in supporting eel population recovery. By preventing the impingement of eels, the from Hydrolox system allows these species to continue their natural migration and life cycles.

Compliance with the Eels (England and Wales) Regulations 2009 will contribute significantly to reversing the severe decline in eel stocks, promoting ecological balance and enhanced biodiversity.

The protection of eels and other aquatic species fosters a healthier river ecosystem, benefiting a wide range of wildlife. Additionally, the project aligns with the goals of the Water Industry National Environment Programme (WINEP), meeting regulatory requirements while demonstrating a strong commitment to environmental stewardship.

Operationally, the project brings improved efficiency, reliability, and adaptability. The advanced design of the new system, combined with automation, will streamline operations by reducing energy consumption and minimising Anglian water’s maintenance costs. The automated wash water system ensures debris buildup is efficiently managed by maintaining optimal flow rates. The duty/duty operational configuration offers redundancy, allowing abstraction to continue uninterrupted during maintenance or equipment failures.

The robust design, with a 40-year horizon for civil structures and a 15-year horizon for mechanical components, guarantees long-term resilience and reliability. Furthermore, the system’s ability to accommodate peak abstraction flows of up to 1,053 liters per second, and its scalability for higher river levels, ensures adaptability to future demands.

[caption id="attachment_19463" align="alignnone" width="1200"] Hydrolox eel screen installation - Courtesy of Anglian Water’s @one Alliance[/caption] Community & economic benefits

From a community and economic standpoint, the benefits are equally significant. Continuous abstraction during construction ensures an uninterrupted water supply to communities and industries dependent on Pitsford Reservoir. The phased installation approach minimised disruptions, maintaining trust and reliability among customers and stakeholders.

Proactively addressing compliance during AMP7 also provides a cost-effective solution, avoiding the potential for higher expenses and operational constraints in AMP8 and beyond.

The project enhances public perception by showcasing a clear commitment to environmental sustainability, strengthening the organization’s reputation among regulators, local communities, and environmental advocates. These collective benefits highlight the comprehensive value of the project in achieving environmental, operational, and societal goals.

Conclusion

The £2m upgrade at Duston Mill Pumping Station represents a landmark initiative in integrating ecological responsibility with operational excellence. By addressing the non-compliance issues and implementing cutting-edge eel protection measures, the project ensures adherence to legal requirements while contributing to the conservation of a critically endangered species. Beyond regulatory compliance, the initiative highlights the broader benefits of sustainable water management, from enhanced biodiversity to improved operational resilience and community trust.

As Duston Mill transitions to this new system, it sets a benchmark for similar projects, demonstrating that sustainable practices and technological innovation can coexist to meet both human and environmental needs. This forward-thinking approach not only safeguards the future of the European eel but also exemplifies the role of modern water management in creating a harmonious balance between development and conservation.

Springfield Surface Water Abstraction (SWA) is a Southern Water pumping station located on the banks of the River Medway in Maidstone, Kent and transfers water to Eccles Lake. The intake is positioned flush with the riverbank and consists of two openings into an underground concrete intake structure, with a busy riverside towpath on top. The picture above is before any works started. There are two coarse bar screens, one in front of each opening and a fixed timber boom in the river in front of the intake to deflect large debris. The flow travels approximately 25m by gravity into the pump house building, then is pumped to Eccles Lake and from there under gravity to Burham Water Supply Works. To comply with The Eel Regulations, Southern Water had to upgrade the intake so that the maximum bar-spacing on the intake was 2 mm and the velocity of the water approaching the screen did not exceed 0.25 m/s. This is to prevent juvenile ‘glass eels’ from being sucked into the intake which would prevent them from moving upstream into the Medway catchment and beginning the long and complex life cycle of the European eel. The 0.25 m/s approach velocity could be achieved by using only half of the existing intake, with the other half blocked up using a concrete stop-log. GoFlo Screens were awarded the contract to supply and assist with the installation of the eel screens at this location, by Trant/BTU Utilities Ltd. The site is particularly complicated because it is in use 24/7, and cannot be turned off for more than a day or two before disruption to Burham Water Supply Works would occur. Coupled with this, the site cannot be de-watered, so would have to be installed by divers in water that is 4.5 metres deep, with very low underwater visibility. The new GoFlo screens are too large to fit inside the existing concrete structure, so had to be accommodated outside of the existing concrete structure, in a prefabricated stainless steel ‘intake box’ that sits on the existing underwater concrete apron. GoFlo designed and manufactured the GoFlo screens, intake box, walkways and debris trough. Everything had to be designed to be simple and modular so that the underwater elements could be installed by divers. The elements supplied by GoFlo are shown below along with the basic specification: Screen drive: 2.2 kW 3-phase WIMES-spec motor/gearbox Maximum bar spacing: 2mm (Single) screen mesh area: 4.652m2 Angle of inclination (from horizontal): 72.5° Construction material: 304 stainless steel [caption id="" align="alignnone" width="1200"] GoFlo 3D CAD model of the proposed screens, intake box and walkways[/caption] Proposed design The overall design of the system was developed collaboratively between the main contractor (Trant/BTU), the client (Southern Water), Atkins Global who were the M&E designers working for Trant/BTU and GoFlo Screens. GoFlo also worked closely with Commercial and Specialised Diving Ltd to incorporate their ‘diver design friendly’ feedback into the design to make it easier for the divers to install, which was especially important considering it would be installed in the middle of winter. Before the detailed design could begin, the intake and apron area in front had to be cleared of silt and debris. Once in the water the divers found a tangled mess of silt, fishing line, a large quantity of steel railing, two motorbikes, a pushbike and various other debris! Once the debris was cleared, a detailed site survey was undertaken to determine how flat the apron was, the exact profile of the vertical concrete piers which the GoFlo intake box would fix onto (a special 5-metre-long profile gauge was manufactured to achieve this), and check how far the apron extended out into the river. A number of differences were found between the original construction drawings from the 1960s and what was actually built. Following the site survey, a detailed 3D model of the intake was constructed in Inventor software which formed the basis of the GoFlo design. The principle of the GoFlo design was a modular construction, comprising of: A fixing frame that fixed to the concrete structure and was used as a template for drilling the fixing holes. A sacrificial ‘measuring frame’ which bolted to the fixing frame to allow all of the key levels to be measured (hence adjusted) from above-water. 2 x intake box sides which bolted to the fixing frame (assembled above-water, after the fixing frame fixing holes had been drilled and the fixing frame removed). The standard GoFlo baseplate and pivoting frame (guide rails), 2 off. The standard GoFlo screen(s). Coarse debris screen/walkway that integrated with the Intake box/debris trough and return channel. The modules were pre-assembled in our workshop to ensure that everything fitted together, prior to arrival on-site. The individual items, and the complete assembly are shown below. [caption id="" align="alignnone" width="1200"] (i) The fixing frame (ii) the temporary measuring frame, fixed to the fixing frame (iii) an intake box side - two are required, and (iv) the baseplate and pivoting frame[/caption] [caption id="" align="alignnone" width="1200"] (i) Course debris screen (ii) the walkway assemblies (iii) debris trough and return channel with covers (iv) the completed design[/caption] The whole design was based on simplifying the works on site, particularly in regard to making the works as straightforward as possible for the divers. Key design features include: Two reference brackets installed on the above-water concrete upstand, which provided two precise reference points for the fixing frame to locate onto at top end. [caption id="" align="alignnone" width="1200"] (left) Reference brackets fixed to existing concrete structure and (right) fixing frame in position against existing structure[/caption] The fixing frame, which once positioned acted as a drilling template for all of the fixings into the concrete structure. At each fixing position there is an 18.5 mm diameter hole, to allow an 18 mm masonry drill bit to pass through which was used as a guide for drilling. These 18 mm diameter holes, once the fixing frame was removed, had Hilti HKD-SR stainless-steel M16 fixings installed. For precise positional adjustment, the fixing frame had 2 x M16 height adjusting bolts close to the toe of the intake apron, 2 x M16 stand-off adjusting bolts at the bottom of the vertical and two temporarily-fixed adjustment blocks at the bottom of the vertical frame members to allow the frame to be pushed/pulled both laterally and longitudinally, and once in position locked in that position for the fixings to be drilled. These were removed once the intake box was finally installed. [caption id="" align="alignnone" width="1200"] (left) Close up of the baseplate of the fixing frame, showing the levelling bolts and (right) temporary adjustment blocks used for positioning the fixing frame[/caption] The fixing frame was designed to stand-off the concrete by a nominal distance of 20 mm to allow for unevenness in the concrete structure. This gap was taken-up at each fixing point using a stainless-steel shim stack. The height of the required shim stack was measured after the fixings holes had been drilled, but before the fixing frame was removed so that the shim stacks for each fixing point could be pre-prepared. Fine adjustment was still possible by adding/removing shims later as required. Once the fixing holes had been drilled, the fixing frame was removed, then the intake box sides bolted on, the GoFlo baseplates and pivoting frames attached. This allowed the whole intake box to be lifted in as an assembled unit and fixed in place using the pre-prepared fixings and shim stacks. Apart for the minor adjustments to levels, this saved a lot of complex underwater assembly work. Once the intake box was fixed in position, the coarse debris screen and walkway were fixed onto the intake box, then the GoFlo screens were installed. So, that was the principle – now let’s looks at the actual installation! Construction The photos below show the actual parts being installed on site. Bear in mind that the water is 4.5 metres deep, and the GoFlo screen 6.1 metres long. The first photo shows the fixing frame lying on its back, having the measuring frame bolted to it prior to being lifted in. The second photo shows the assembled fixing frame and measuring frame being lifted into position. You can see that the river was very high after several days of heavy rain. [caption id="" align="alignnone" width="1200"] (left) Fixing frame and measuring frame being assembled and (right) fixing frame and measuring frame being lifted into the intake – note the very high water levels![/caption] The fixing frame and measuring frame in position, ready for the fixing holes to be drilled. The mild steel measuring frame was sacrificial, but provided above-water horizontal references for a spirt level, which was essential during levelling. Once the drilling was completed, the fixing frame and measuring frame were removed from the water, then the intake box sides, GoFlo screen baseplates and pivoting frames assembled together. [caption id="" align="alignnone" width="1200"] (left) Fixing frame and measuring frame in position and (right) the intake box being assembled prior to final installation[/caption] [caption id="" align="alignnone" width="1200"] (left) The fully assembled intake box, (middle) lifting the assembled intake box into the intake with a 60-tonne crane, and (right) the assembled intake box was attached to the concrete structure by divers which took several days[/caption] Next, the coarse bar screen was lifted into position, closely followed by the GoFlo Screens. GoFlo screens are lifted in vertically and use the pivoting frames as guide rails to guide the screen precisely onto the baseplate. [caption id="" align="alignnone" width="1200"] (top left) Lifting the course bar screen into the intake box, (bottom left) lifting the first GoFlo screen into the intake box, and (right) fitting the front part of the walkway structure onto the intake box[/caption] The screens are designed to discharge the collected debris into a dedicated trough behind the screens. The trough is made from stainless steel (as are the screens, intake box and coarse bar screen). The trough has a high back to catch the spray (2.5 bar water pressure) that is emitted from the screen spray-bars behind the mesh belt. Each screen has a spray bar with multiple nozzles fed by a remote water booster pump located inside the main building. Debris on the surface of the screen mesh as it rotates falls off as it runs over the top sprockets, while any remaining ‘sticky’ debris is blasted off by water the jets from the inside of the screen and into the trough. [caption id="" align="alignnone" width="1200"] (top left) Fitting the debris trough and return channel, (bottom left) the spray boom assembly which fits inside the screen mesh, and (right) completed walkway structure attached to the intake box[/caption] The debris trough includes a dedicated water feed (10 litres/second) to flush collected material back into the river downstream of the screens. The green fence posts are to support the security fence to protect the public from the machinery. [caption id="" align="alignnone" width="1200"] (left) The debris flushing flow pipework underneath the debris trough and (right) the security fencing being fitted[/caption] 10 cm bar-spaced coarse debris screen protects the GoFlo screens from large object damage, and keeps swimmers out. Due to the proximity of the screens to the footpath and the possibility of ice formation in winter months, a fully enclosed debris trough was required to confine the spray mist. The covers are hinged to enable full unrestricted access to the trough. [caption id="" align="alignnone" width="1200"] (top left) View from the-screens, looking out towards the river, (top right) the lids covering the debris trough, (bottom left) further lids covering the debris return channel and (bottom right) the completed installation, inside the security fencing[/caption] [caption id="" align="alignnone" width="1200"] The completed installation, looking upstream[/caption]   For more information: GoFlo Screens Ltd | +44 (0)1453 884400 | www.gofloscreens.co.uk  

Sandown Surface Water Abstraction (SWA) is a pumping station abstraction from the River Yar on the Isle of White, which feeds raw water into the Sandown WTW. The pumping station is several hundred metres from the water treatment works, and sits alongside the popular and busy Red Squirrel Trail walking route and cycle path. Along with many other water intakes throughout the UK, Southern Water had to upgrade the water intake screens to be compliant with the Eel Regulations, which meant a bar-spacing that did not exceed 2mm and an approach velocity that did not exceed 0.25 m/s. The existing intake structure was built in the early 1980s and consisted of interlocking steel sheet pile walls, capped with a concrete slab. Manholes on top of the slab provided access to the sump pumps, valves and pipe/cable ducts. The river bed in front of the structure is a flat concrete slab, which could provide a good base for the eel screens to sit on if needed. [caption id="" align="alignnone" width="1200"] (left) Concrete slab on top of the sheet-pile pump and valve enclosure and (right) sheet-pile walls that make up the pump enclosure[/caption] [caption id="" align="alignnone" width="1200"] (left) View inside the pump chamber, looking at the sheet piling adjacent to the river. Below the waterline is the 0.2m high x 3m wide slot where the river water is drawn through. (right) close-up of intake drawing showing ‘letterbox’ slot[/caption] The existing intake did not have any screening; hence fish, eels and other objects could potentially enter the pump chamber. CMDP was the main contractor for the project, and GoFlo worked closely with them from the early design stages to the final commissioning to ensure an optimal solution was developed. With a maximum abstraction flow rate of 0.278 m3/ second, the approach velocity through the existing intake slot in the sheet pile wall would exceed the maximum specified in the Eel Regulations. This meant that would it would not be possible to install the GoFlo screens inside the existing pump building and would have to be installed externally to the existing building. An added complication was that the intake could not be shut down for more than 8 hours without severely disrupting the water supply to the Isle of White, which would not be acceptable, so the solution had to be designed to be installed into a working intake using divers. GoFlo had a similar issue at another Southern Water site (Springfield), and for that site designed a fabricated ‘intake box’ system that could be installed by divers. This system worked well, so the principle was used for Sandown, even though the water depth at Springfield was 4.5m, whereas at Sandown it is typically only 600mm. The width of the screens at 3.55m has been determined by the minimum depth of water in the river (0.438m) while still having to meet the 0.25 m/s approach velocity under the Eel Regulations while abstracting the maximum flow rate. Site survey The design process started with a detailed survey of the intake area. As is the case with many sheet-pile structures, nothing was exactly plumb or square, so because the fabricated intake box we would have to manufacture had to fit perfectly, first time, we had to enter the river and make plywood templates that fitted the profile of the existing structure perfectly. At the same time a detailed level-survey was conducted on the river bed concrete slab. We took the wooden templates away and turned them into precise profile data points that could be used to build an accurate 3D model of the existing structure in our CAD system. Based on that, we could design the intake box to fit perfectly. Design Due to how the in-pans and out-pans of the sheet piling were positioned, the bolted connections onto the Intake Box at the upstream end was on an out-pan and at the downstream end on an in-pan, so the left and right-hand intake box sides being different widths (1560mm and 1450mm). [caption id="" align="alignnone" width="1200"] (left) Intake box side drawing and (right) plan drawing showing new intake box attached to existing sheet-pile structure[/caption] The intake box is a stainless steel fabricated box made up of the two sides and a baseplate that were assembled into a finished box on-site. The structure is designed to be a rigid structure so it could support the screens in the inclined ‘operating’ and vertical ‘service’ positions. The Intake Box structure also had the walkways fixed to it, which in turn have the security fencing fixed to them, so had to be designed to be very strong. A course bar screen with 50 mm bar-spacing was incorporated into the intake box, just upstream of the screens. This bar screen will protect the fine GoFlo screen meshes from large debris which could deform or puncture the screen face during flood flow conditions, but also keep any swimmers away from the moving parts! An advantage of having the screens perpendicular to the river flow will reduce the possibility of the bar screen collecting debris; the river passing flow tending to pull material away from the bars. [caption id="" align="alignnone" width="1200"] (left) Drawing of intake box with course bar screen attached to upstream face and (right) drawing showing GoFlo screens, debris trough and intake box attached to existing structure[/caption] The debris captured by the screens is returned to the river via the debris trough and return channel, but about 5m downstream, so the likelihood of material being pulled back into the screens is minimised. The walkway structure incorporates davit sockets and removable winches. The winches are used for pulling the screens from their inclined ‘operating’ position to the vertical ‘service’ position. From here, the screen’s motor/gearbox and drive system can be accessed safely. Should a motor need to be taken off the screen, a portable davit can be mounted in a dedicated floor socket and the motor lifted up and over the handrail onto a trolley for transport away from the area. [caption id="" align="alignnone" width="1200"] Complete 3D model of proposed GoFlo system attached to the historic structure[/caption] Installation This started with the river bank and floor being cleared of any silt and stones etc. Next, the intake box was assembled, including adding some temporary diagonal braces to keep the sides an baseplate square and to prevent the structure flexing during lifting. Once the intake box was in the river and fixed in position the diagonal braces were removed. The intake box was fixed down to the concrete apron using stainless steel M16 resin-fixed anchors, and to the vertical sheet pile wall by drilling a tapping M12 bolted fixings. Installing such a structure in a flowing river was a hazardous operation, especially with divers in the water to guide it into position, drill and install the fixings into the wall and river bed. Underwater visibility was very poor due to unseasonal heavy rainfall and increased river flow. The divers had to be in constant radio contact with the dive support team on the bank. The divers also had a live video feed to the team in the control room so they could be guided to the fixing locations. [caption id="" align="alignnone" width="1200"] (left) Lifting the intake box into the river and (right) lifting the first of the GoFlo screens into the intake box[/caption] Once the intake box was fixed in place the two GoFlo screens were lifted into position. This is a relatively quick operation due to the integrated ‘pivoting frame’, which are guide rails allowing the screen to be lifted in vertically and accurately guide the screen into position, even when the intake is flooded with zero visibility. At this point the screens were connected to a temporary control panel and a wooden debris trough was constructed to allow the screens to be put into operation. The screens had to be put into service immediately because of the looming deadline to make the Intake Eel Regulations compliant. [caption id="" align="alignnone" width="1200"] (left) The first screen in position on the intake box and (right) both GoFlo screens in position in the intake box[/caption] A few months later the walkways, debris trough and security fencing were fitted, which all bolted directly to the intake box. The assembly was all designed in detail using 3D CAD software by GoFlo, then the constituent parts laser-profiled and CNC folded, finally being welded together. The walkways were installed in two days, and the debris trough in a similar time, which is testament to the accuracy of the design and fabrication undertaken by GoFlo. [caption id="" align="alignnone" width="1200"] Completed GoFlo screen installation[/caption]   For more information: GoFlo Screens Ltd | +44 (0)1453 884400 | www.gofloscreens.co.uk