Broken dam, Cwm Eigiau
The giant wall which crosses the valley here was a dam built in the early 20th century to supply water to a hydro-powered aluminium factory in the Conwy Valley. The dam broke in 1925, with disastrous results. The photos of the dam, under construction and completed, are shown here courtesy of Conwy Archives Service – from an album made by a visitor to the factory c.1920.
Llyn Eigiau, a natural lake, was surveyed in 1896 for potential conversion to a reservoir to supply water to Llandudno. In the following decade, the Aluminium Corporation Ltd created reservoirs here and at Coedty to supply water to its power station in Dolgarrog, which still operates and originally powered the adjacent aluminium factory.
For years, water seeped under the dam, which had shallow foundations. The dry summer of 1925 was followed by a wet autumn, and the water seepage increased. On the stormy night of 2 November, a large rectangular hole opened up in the ground beneath the wall and water gushed out.
Instead of following the natural watercourse (which would have avoided a serious problem downstream), the water was diverted by a constructed channel which led to Coedty reservoir. It exposed the Coedty dam’s concrete core, which suddenly collapsed. The reservoir’s contents, in the form of a huge wave, hurtled down a gorge, carrying giant boulders and other debris towards the village of Dolgarrog.
The wave flattened and damaged homes and other buildings, causing the deaths of 10 adults and six children were killed. You can see their details on our page about the disaster memorial in Dolgarrog.
As a result of this and other reservoir tragedies, the Reservoirs Act 1930 was enacted to improve design, construction and inspection of all large reservoir dams.
The Coedty dam was later rebuilt but the Llyn Eigiau dam was reduced in length and made safe by cutting out the remainder of the undermined section of wall – leaving the wide gap you see today. The dam’s spillway was lowered, to reduce the volume of water impounded in the lake.
With thanks to David Greatrex, former Dŵr Cymru Reservoir Safety Manager