Llyn y Fan Fach reservoir refurbishment

Engineering meets heritage – Grade II-listed reservoir structure is refurbished with innovative engineering, preserving its historic legacy while securing its future for generations to come.  

Project overview

Llyn y Fan Fach reservoir, at the head of the Afon Sawdde, in Bannau Brycheiniog National Park, underwent a significant refurbishment following a statutory inspection under the Reservoirs Act 1975. The inspection identified structural concerns, particularly in the draw-off tower, a vertical structure that controls water release, and the spillway, which safely channels excess water away from the reservoir. These findings prompted a planned upgrade to ensure long-term safety and regulatory compliance. 

As a Grade II-listed structure, the reservoir required a refurbishment that balanced modern engineering standards with heritage preservation. Due to the site’s remote location, narrow access routes, and environmental sensitivities, the project team maximised off-site fabrication and used innovative construction methods.  

Key works included:

• The installation of modular glass-reinforced plastic (GRP) sections inside the tower to manage thermal movement and prevent water ingress
• Sealing existing cracks with a durable sealant
• Constructing a new reinforced concrete wall to replace the compromised valve tower wall  

The spillway was rebuilt using precast concrete units engineered to withstand extreme flood events, while its original 1910s stone appearance was carefully retained.  

Temporary over-pumping systems and stormwater management measures were implemented to ensure safe construction without compromising the integrity of the reservoir. 

The project was delivered in two phases between July 2022 and September 2023, with no reported accidents and excellent feedback from Welsh Water. It was signed off achieving ‘Perfect Delivery’.