Supply Chain - River Work - Trash Screens

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:

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.

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.

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.

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.

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.

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:

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:

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:

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.

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.

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.

The Don Regulators comprise three regulator gates on the River Rother that are owned, operated, and maintained by the Environment Agency (EA). The purpose of the regulators is to reduce the risk of flooding to settlements within the River Don catchment by attenuating flows in the River Rother. JBA-Bentley has completed the design and installation of the first regulator gate. The detailed design of the second gate is currently being undertaken, while the third regulator is being modelled as part of the optioneering phase. In addition to the installation of the first regulator gate, a bespoke full-channel fish pass has been installed directly downstream to improve migratory fish passage.

The Meadowgate Regulator is located within the Rother Valley Country Park. The historic regulator consisted of a single fish belly bottom-hinged tilting gate, operated by a single electrically powered hydraulic ram.

In 2023, JBA-Bentley replaced the gate with a top-down vertical control gate, improving the resilience of the structure. Alongside the regulator gate, a new control building and EA Office have been constructed. Both buildings have green roofs and EV charging. The gate was commissioned in February 2024 and is ready to operate during a flood event.

The gate is operated to ensure a maximum of 60m3/s forward flow is always maintained. When flows exceed 60m3/s the new gate is lowered and water levels in the Rother rise and begin to spill into the washlands located within the Rother Valley Country Park, which can retain approximately 1.5 million m3.

The removal of the bottom tilting gate provided an opportunity to improve fish passage on the River Rother through the installation of a unique full-channel width fish pass. This fish pass is designed with 4-tonne reinforced concrete ‘fish tiles’ featuring varied-height upstands supplied by Craven Concrete Ltd.

The tiles were designed with the aid of computational fluid dynamics modelling to ensure they influenced the nature of the flow to allow for varied fish species to migrate upstream while also withstanding hydrodynamic forces during extreme events. The fish species focused on are coarse fish, European eels, brown trout, large migratory salmonids, and lamprey.

The 40m long stilling basin has been removed by building up the existing concrete channel bed and grading out the levels to create a uniform slope. The ‘fish tiles’ have been laid onto the regraded channel. This is the first fish pass of its kind in the world.

The fish pass installation required the full channel as a dry working area. To achieve this, an innovative syphon system, designed by Dutch water specialist Vanderkamp UK, was installed. The solution involved complex temporary works and contingency planning.

To form the upstream dam, the new regulator gate was dropped to bed level, allowing upstream water levels to build up. The gate was designed to operate this way; hence no further calculations were required. Four syphon intake pipes were installed upstream of the gate to divert up to 12m3/s of flow out of the channel for 115m, around the working area.

The syphons were primed using electric vacuum pumps which expelled all the air from the system to draw water through the pipes. By utilizing the existing gate and power supply, the syphons had a far lesser environmental impact than a typical cofferdam and over-pumping solution. The total embodied carbon associated with the syphon solution was 464 tons.

Due to the scale of the flows being diverted, the loads imposed on the riverbanks by the pipes when full were considerable. JBA-Bentley and Vanderkamp UK worked together to design a temporary support system involving ground-bearing improvements, steel and timber supports, and retaining structures. Checks were also carried out on the existing channel wall where the forces from the syphons acted upon it.

In addition to work to ensure the stability of the syphons, an energy dissipation system was designed and installed at the syphon outlets to combat scour, and a bubble curtain was installed upstream to deter fish from entering the syphons. The installation of the system was done via a temporary crane pad. Due to the nature of the site and its space constraints, this was a challenging design. JBAB worked closely with the landowner, Rother Valley Country Park, to balance the desire to allow continual access into the park with ensuring the crane was safe to carry out the required lifts. To achieve this, the crane had to be placed in close proximity to the riverbank. Slope stability modelling and bearing capacity calculations were undertaken to ensure the crane could be operated safely.

Recent storm events have recorded flows higher than the syphon capacity. To manage the risk of this occurring during the programmed works, a contingency plan was developed.

The basis of the plan was developed from a bespoke JBA flow forecasting system, which forecasted the flow over a 2-day period. Using the data from the forecasting system, flow monitoring data provided by Vanderkamp UK, upstream gauge data, and the storage capacity of the washlands upstream of the gate, JBAB could make informed decisions on whether the gate needed to be raised to allow flow to pass forward.

This decision was made with the aid of hydraulic calculations and decision flow charts within the contingency plan, allowing for quick and effective decision-making, ensuring flood risk was not impacted upstream and the safety and well-being of the site team working in the channel was never put at risk.

The Canklow Regulator is the furthest downstream of the three Regulators, approximately 3.5km upstream from the confluence of the River Rother and the River Don at Rotherham.

The existing structure consists of a single electrically powered vertical lifting sluice gate operated by an electric wire rope hoist, comprising a drive motor gearbox assembly, cross shaft with integral rope drums, and support bearings.

To improve resilience and the operation and maintenance of the asset, JBA-Bentley is completing the detailed design and build of a new vertical regulator gate and associated MEICA and civils infrastructure.

The MEICA structure consists of a 20.3-ton vertical gate, mounted on a new steel bridge and working platform, which shall also house 13.5 tons of drive equipment for operating the gate.

Two 4kW motors, working in a duty/standby configuration, will provide drive to the gate via bevel and gearboxes, which turn a single driveshaft spanning the width of the gate. On this driveshaft, one at either end of the gate, are rack and pinion gear assemblies, which convert the rotation of the driveshaft to the vertical motion of the gate. The total gear ratio from motor to driveshaft is an approximate reduction of 4410:1, allowing relatively small motors to be specified.

To support the new gate structure over the next 50 years, a new anchor restraint system has been designed. A buried sheet-piled ‘dead-man’ wall will be installed with ten 36mm anchor rods, evenly spaced, connecting to a waling beam on the wet side of the existing channel wall. This will be mirrored on both riverbanks.

The new infrastructure replicates the existing system, which has been determined to be past its design life and insufficient in supporting the new gate.

The existing superstructure of the control houses will be demolished, the foundations will be retained and extended where required to support the new gate structure using cast-in bolt sets.

A new permanent crane pad will be constructed on the south bank for the construction, maintenance, and decommissioning of the gate. This will double as parking spaces for EA operatives when not being used by a crane.

The JBA-Bentley team worked collaborative to optimise the delivery. This meant that site constraints were identified, and the design was completed with buildability and safety as the focus. A key constraint on the scheme is the proximity of the Northern Power sub-station and overhead power lines. The design of the dead-man walls, crane pad, and steelwork platform was completed to ensure that the plant and temporary works required for construction and installation were outside of the powerline exclusion zones.

The Woodhouse Mill Regulator is situated between Canklow and Meadowgate. It uses a single electrically powered vertical lifting gate to retain water. This gate is operated by an electric wire rope hoist, which includes a central drive motor and gearbox assembly, half shafts, spur and pinion driven rope drums, and support bearings, all counterweight assisted.

JBA-Bentley is currently carrying out hydraulic modelling on behalf of the EA as part of the optioneering phase. The options include passive solutions, such as a baffle or orifice plate spanning the width of the channel to continue to utilize the upstream storage. Due to the size of the Don catchment, the modelling required three individual models to be run together:

Multiple return periods are being considered, resulting in hundreds of models runs. The modelling also uses a continuous simulation approach, which adds to the complexity of the model runs.

The upgrading of infrastructure of the Don Regulators will ensure continued flood risk benefits into the future for the Don Catchment and surrounding area with improved resilience and reliability of the new assets. Alongside the reduction in flood risk, the environment and biodiversity will improve in the River Rother as a result of the Meadowgate Fish Pass.