The North Wales branch of the Institution of Civil Engineers recently hosted a presentation by Binnies and William Hughes Civil Engineering, looking at reservoir improvements across one of Natural Resources Wales’ old mining reservoirs within the Gwydir Forest. Located in Conway Valley on Snowdonia National Park boundary, the Llyn Fuches Las reservoir is retained by a 4m high earthfill dam. It is an L-shape in plan with a length of 44m for the main dam and 200m for the subsidiary dam, and when full to its top water level of 152.40m, the reservoir has a capacity of 11,108m3.
Originally built for mine waste, the dam is believed to have been constructed in the 19th century. After failing in the 1960s it was rebuilt to provide some storage and flood prevention. A major storm in 2020 overtopped and damaged it, prompting a statutory inspection and safety measures under section 10(3) of the Reservoirs Act. As Richard Terrell, a Chief Reservoir Engineer at Binnies UK, explained, early geotechnical work confirmed the dam’s poor foundation materials – mainly silty sandy gravel with large cobbles – were unsuitable for a watertight structure..
Binnies then looked at two potential options to deal with the dam. They could either discontinue and remove it, or upgrade to meet modern engineering standards.
“The decision was made to upgrade it as we knew it provided some flood resistance and storage for the Lower Conway Valley,” Terrell explained, adding there were now two problems to solve: the effective water tightness of the dam to prevent internal erosion leading to dam failure; and improving the dam to safeguard it against overtopping.
The decision was taken to install a sheet pile core throughout the dam to provide a watertight cut off and prevent any further seepage underneath the structure. A new downstream shoulder was built that allowed compaction due to the existing embankment being weakened by tree roots. Reinforced grass was also used to protect against erosion on the downstream face.The next task was to solve the problem of the adequacy of the dam to pass floods. A flood study was undertaken, finding that the existing dam spillway lacked capacity to safely discharge the design flood.
After hydraulic modelling, it was concluded that a 5m wide spillway could pass the 1000-year event and 10,000-year event without overtopping. To future proof the dam it was also modelled against the probable maximum flood.
Getting underway
Andrew Hughes, Contracts and Framework Manager at William Hughes Civil Engineering, also gave a civils perspective of the project.
He explained that in May 2020 William Hughes had carried out repairs to the main embankment following storm damage, in the form of matting to provide erosion protection measures. Then from August 2021 to September 2023, the company provided early contractor involvement support, working collaboratively with the client, designer and key stakeholders to develop a coordinated buildable and deliverable solution.
Hughes said that a positive result from such early contractor involvement meant they were able to identify the unsuitability of the existing site access which would limit the size of the plant that could be delivered, particularly for piling operations. Consequently, an alternative access route was developed through the forest, connecting to the existing road network.
Substantial site clearance, including tree felling, road widening and access improvement in preparation for the main works, was carried out in April and May 2024. And then the main construction works were carried out over a 25-week period from 24 February 2025. This included: construction of a new reinforced concrete main and secondary spillway; construction of main embankment and outlet pipework; and installation of sheet pile core to entire dam with a total length of 210m.
Constraints
The project encountered various environmental and ecological constraints. These included relocation of a colony of small wood ants which were quite abundant on site and located within the site footprint. Nests were translocated to a safe area way from the works. In addition, the presence of bats nesting within adjacent historic mine workings which were very close to the site, resulted in mitigation measures being carried out in accordance with a bat licence. Unfortunately these impacted the start date for piling activity, protective fencing and noise restrictions. Then in January 2026, William Hughes Civil Engineering retuned to install an additional pedestrian access bridge to provide access to the embankment for inspection and maintenance during storm events.
Success of the project was attributed to early contractor involvement, regular design and construction team meetings, with everyone working together towards a common outcome.
Reference
https://www.ice.org.uk/events/recorded-lectures/fuches-las-reservoir-remediation-work
