The UK's Largest Spillway at Llyn Celyn Reservoir - CSF's Progress on the Project

The start of 2024 saw some of CSF's talented team arrive in beatiful Bala in North Wales to commence work on a new spillway at Llyn Celyn reservoir. A Welsh Water project in conjunction with JN Bentley Ltd.

Llyn Celyn reservoir was built, to supply water to Liverpool, between 1960 and 1965. This highly controversial project involved the flooding of the Tryweryn valley, including the village of Capel Celyn, and adjacent farmland, displacing its former residents. In recent years the remains of the village have been exposed due to extreme dry weather when water levels are low.

In contrast the spillway project is designed to mitigate against a different sort of extreme weather - flooding. In 2020 a statutory inspection was carried out which confirmed that the reservoir was in good condition with no immediate safety concerns. However, the report also advised that the consequence of the dam being breached could mean catastrophic damage and potential loss of life downstream. Whilst the likelihood of this ever happening are still extremely rare (1 in 10,000 year flood event), it's undeniable that climate change is bringing ever more extreme weather events across the globe.

Following the report, Natural Resources Wales in conjunction with Welsh Water, and following various ecological and ground investigations, developed the proposal to build an auxiliary spillway at the crest of the dam. This will run next to the main car park off the A4212, and will divert floodwater down a hillside, protecting local people. This has since been approved by planners at Eyrie National Park.

Llyn Celyn Spillway Works - plans
Outline Plans for the New Spillway and Associated Works.

CSF, arrived on site in January 2024, once the phase 1 area ground works had been completed. By the end of January we are all on the way to concrete pour 1 & 2 having completed the pours to the blinding strips and the installation of formwork and beam supports. Please see our progress photos below, and follow our updates on our website as the project progresses over the next 12 months.

Anti Flotation Blinding
Formwork for Anti Flotation Blinding
Phase 1 Concrete Pour
Phase 1 pour preparation.
A cold day in North Wales