When inspections found a group of 150-year-old reservoirs in Wales’s Snowdonia National Park to be failing, owners Natural Resources Wales (NRW) appointed Arup to design a solution to ensure their safety in the long term.

Reservoirs are some of our best-appreciated examples of civil engineering, supplying water for homes and industry, offering habitat for wildlife and providing amenity value for visitors. But if dams fail, they pose a risk of catastrophic flooding for those downstream and potential criminal, civil and reputational consequences for reservoir owners.

Restoring failing dams with minimal impact and maximum effectiveness  

Using a restorative green infrastructure approach, we focused on reusing and strengthening existing structures to minimise the project’s impact on the local environment. The solutions have been tailored to each site, protecting habitats and ensuring NRW can efficiently maintain the reservoirs in future. 

The scheme has made the reservoirs safe for visitors and local communities while respecting their unique historical, ecological and amenity value within the national park landscape. NRW is now using our approach as a blueprint for projects of this type.

A challenging site with a historic legacy to protect

All three reservoirs sit within the Gwydir Forest Special Area of Conservation (SAC), which includes many Sites of Special Scientific Interest (SSSI). While the reservoirs, originally constructed to serve the area’s metal mining industry, leave a legacy of protected historic structures, they can also leave potentially contaminated silts within the dams and reservoirs, which must be contained to protect downstream watercourses.

We brought together a team of civil and geotechnical engineers, working in collaboration with environmental and ecological specialists, to design the safe decommissioning of one dam and the repair of two others. This work involved strengthening embankments, improving overflow capacity and creating a way to lower water levels for maintenance or emergencies. By bringing this expertise together, we ensured landscape and habitat features were integral to our designs rather than a secondary consideration.

The work was being carried out as Measures in the Interests of Safety under the 1975 Reservoirs Act and had to be completed to a fixed timescale. To maximise efficiency without sacrificing quality, the design team developed standard solutions – dam buttressing, spillway material selection and elements of drainage design – that could be adapted to suit each site, reducing time spent on redesign work and cutting material costs. 

New spillway in operation at llyn cyfty reservoir 
Creating improved spillways, we selected materials that minimised environmental and aesthetic impacts.
Gwydir Reservoir
The project aims to sustain water levels for wildlife and private supply.

Reusing existing structures to reduce embodied carbon

Instead of removing and rebuilding the dams, our practical solution supported existing structures by building new buttresses. The buttresses provided long-term embankment stability and widened the narrow crests at the top of each dam that had previously prevented safe access for maintenance. This approach also minimised environmental impacts – requiring less new material, which reduced the project’s embodied carbon, and enabling existing water levels to be maintained, which protected habitats and reduced the chance of contaminated silts entering the water.

Where new materials were needed, such as when creating improved spillways, we selected materials that minimised environmental and aesthetic impacts. Locally-quarried stone was used instead of concrete, and we designed restorations to mimic historic outflows, minimising disruption to protected habitats. Material excavated during construction was reused in landscaping, saving the need to remove substantial quantities of inert soils off site. 

This multi-site engineering project has benefitted from the work of Arup, applying their technical expertise throughout all stages of appraisal, design and construction. Combining reservoir safety principles with the conservation and enhancement of the natural and historical environment, these reservoirs are sustained for current and future generations. 

Andrew Basford

Specialist: Projects & Programme Delivery, Natural Resources Wales

Safety solution systems for remote area access

The Gwydir Forest is a well-used public amenity, popular with walkers and cyclists. We needed robust processes to manage risk from the public inadvertently accessing dangerous live works or coming into contact with unexpected site traffic in remote areas with difficult access for emergency services. Our measures included haulier traffic management to prevent unauthorised access, slow-down points and no-go areas to avoid walkers and cyclists, and a 24hr emergency response plan. NRW is now using the public safety risk assessment system we developed across its entire reservoir portfolio.

As well as keeping the public safe during construction, our designs ensure that dams can be maintained and inspected safely in the future. We improved access routes, reduced embankment gradients, and installed solar-powered telemetry and remote cameras, ensuring that spillway operation can be checked 24/7 without unnecessary travel to site.