Binnies supported Natural Resources Wales (NRW) in delivering critical safety improvements to Llyn Fuches Las Reservoir, a historic mining reservoir located within the Gwydir Forest in North Wales. Constructed in the late nineteenth century, the reservoir forms part of Wales’ mining heritage and plays an important role in attenuating flood flows within the Conwy catchment.
Following a significant overtopping event in 2020, inspections identified that the reservoir could not safely pass extreme flood events and that sections of the embankment were deteriorating. NRW required a solution that would achieve compliance with modern reservoir safety legislation while preserving the site’s environmental, ecological and heritage value.
Delivered between March and August 2025, the project strengthened the reservoir’s resilience, protected sensitive habitats and enhanced flood protection for downstream communities, while avoiding the environmental impacts associated with traditional reconstruction.
Our Role
Binnies was appointed as lead designer and Principal Designer, providing reservoir engineering, hydraulic design, structural design and construction support throughout the project lifecycle. We worked closely with Natural Resources Wales, Arcadis and William Hughes Civil Engineering to develop a proportionate, sustainable solution tailored to the unique challenges of the site.
Our services included:
- Reservoir safety assessment and optioneering
- Detailed engineering design
- Hydraulic and flood risk modelling
- CDM Principal Designer duties
- Environmental risk management
- Construction support and technical assurance
- Sustainable design and value engineering leadership
Project highlights
Flood standard achieved: 1-in-1,000-year flood event
Extreme resilience standard: 1-in-10,000-year flood event
Programme performance: delivered ahead of schedule
Main embankment length: 45m
Reservoir surface area: 9,000m2
Subsidiary embankment length: 135m
Challenges and solutions
The reservoir is located within a sensitive ecological setting that supports rare and protected species, including Lesser Horseshoe bats and wood ant colonies. It also sits within a historic mining landscape containing elevated levels of heavy metals within the embankment. Large-scale reconstruction would have generated significant volumes of contaminated waste and increased environmental disturbance.
To address these challenges, Binnies developed an innovative hybrid engineering solution that combined several proven reservoir engineering techniques into a single integrated design. This included installing sheet piling through the existing embankment to create a new watertight core, constructing a reinforced capping beam and delivering a new spillway capable of safely managing extreme flood events.
The solution strengthened the reservoir without excavating contaminated material, significantly reducing waste, vehicle movements and environmental impact.
Construction was complicated by narrow forestry access roads, remote working conditions and challenging weather. Shortly before mobilisation, a major storm event damaged the downstream spillway, creating additional scope that had to be incorporated without affecting programme delivery.
Early contractor involvement enabled the project team to optimise piling depths, construction sequencing and access arrangements, ensuring works progressed safely and efficiently despite the constraints.
Extensive ecological mitigation measures were implemented throughout construction. Low-vibration techniques, seasonal working restrictions, exclusion zones and habitat enhancement measures protected important bat roosts and other ecological receptors. Monitoring confirmed continued bat presence during and after construction. Additional enhancements included woodland habitat improvements and the relocation of wood ant colonies.
The completed scheme demonstrates how thoughtful, site-specific engineering can successfully balance public safety, environmental stewardship and long-term resilience.
Benefits and outcomes
The upgraded reservoir now meets modern Category B reservoir safety requirements and provides improved protection for downstream communities within the Conwy catchment. The design safely passes a 1-in-1,000-year flood event, withstands a 1-in-10,000-year flood event and accommodates the Probable Maximum Flood scenario.
The design avoided excavation of contaminated mine material, removing the need for off-site disposal and significantly reducing environmental risk. All excavated material was reused on site for landscaping and reprofiling works, delivering additional carbon and cost savings.
The project retained and stabilised a historic reservoir asset while preserving wetland habitats and protecting important ecological receptors. Locally sourced timber and stone were incorporated throughout the design to minimise embodied carbon and ensure the completed works blended naturally into the surrounding landscape.
Through close collaboration between NRW, Binnies, Arcadis and William Hughes Civil Engineering, the project was delivered ahead of programme, within budget and with zero reportable health and safety incidents.
Enhancing lives, communities and the environment
By combining reservoir safety improvements with biodiversity protection, heritage preservation and sustainable design principles, the Llyn Fuches Las Reservoir Safety Improvements project provides a resilient, low-carbon solution that will continue to benefit communities and the environment for generations to come.