A reservoir safety project in Gwynedd has won the Institution of Civil Engineers (ICE) George Gibby Award.

The Llyn Celyn Reservoir Auxiliary Spillway Project was delivered by Welsh Water in partnership with Mott MacDonald Bentley and Natural Resources Wales.

It involved a major upgrade to Wales’ largest reservoir by volume, combining technical innovation with careful environmental management, stakeholder collaboration and community engagement. 

A new auxiliary spillway features 4.5-metre-high reinforced concrete tipping gates, the largest known system of its kind in the UK.

Designed to operate during exceptionally rare flood events, the system strengthens the reservoir’s ability to manage extreme conditions. 

The project team overcame engineering and environmental challenges, including a 70-degree soil-nailed excavation, complex concrete delivery requirements and the need to carry out the work within the sensitive landscape of Eryri National Park. 

Advanced digital technology was used, including detailed geophysical surveys, two-dimensional hydraulic modelling and intelligent machine-guided construction. Two-thirds of the spillway was constructed as a culverted structure, helping to minimise visual impact and preserve the surrounding natural landscape. 

Sustainability was a key consideration throughout the project. Through innovative design and advanced modelling, the team reduced the amount of construction required, lowered carbon emissions and reused excavated materials on site, helping minimise the environmental impact of the scheme while delivering long-term resilience. 

Karen Morris, Programme Delivery Manager at Welsh Water, said: "Winning this award is a fantastic achievement and a testament to the dedication, expertise and collaborative working of everyone involved in the project. 

“Safety has been at the heart of the scheme, but we have also worked hard to deliver it in a way that respects the surrounding environment and provides long-term resilience for communities. 

“We are extremely proud of what has been achieved alongside our partners, contractors and stakeholders, and we are grateful for the support shown by the local community throughout the project.” 

Michael McAree, Design Manager at Mott MacDonald Bentley, said: "We are delighted the Llyn Celyn project has received this recognition. Its success reflects the strength of our partnership with Welsh Water, the ingenuity of our engineering teams and our shared commitment to delivering safe and sustainable infrastructure. 

“The innovative spillway design and digital approach used during delivery have set high standards for the sector.” 

Stephen Shakespeare, Welsh Water’s Dam Safety Manager, said: "Winning this award is a proud moment for everyone involved in the Llyn Celyn project and recognises the skill, commitment and collaboration behind this important work. 

“The scheme has strengthened the long-term safety and resilience of one of Wales’ most important reservoirs. The new auxiliary spillway introduces the largest reinforced concrete tipping gate system of its kind in the UK, providing additional capacity to safely manage exceptionally rare flood events and helping to protect communities and the environment for generations to come. 

“What makes this project particularly special is that it combines engineering excellence with environmental stewardship. Through innovation, sustainable design and close partnership working, we have delivered a major safety enhancement while respecting the unique landscape of Eryri National Park. We are incredibly proud that this achievement has been recognised by ICE Wales Cymru.”