Spotlight | A catalyst for change


The Dolgarrog Dam disaster of 1925 was a cascade failure that changed the course of UK reservoir safety. It was the last dam disaster in the UK to result in loss of life and the final catalyst which heralded a new era in dam safety, construction and inspection. Here, a team of geomorphologists from the UK explain why geomorphological considerations must be an essential part of flood emergency planning and hazard assessment


the region. After the flood, the inundated areas were unrecognisable, and the main river (Afon Porth-llŵyd) had rerouted its path. Ten adults and six children lost their lives. This devastating flood was caused by the failure of two dams impounding the Eigiau and Coedty reservoirs on the Carneddau plateau, high above Dolgarrog. It was the last dam disaster in the UK to result in loss of life and the final catalyst that led directly to the Reservoirs (Safety Provisions) Act of 1930, which heralded a new era in dam safety, construction and inspection. This new legislation was applied to all reservoirs of a volume greater than ~ 22,730m3


and required all


reservoir construction to be designed and supervised by a competent engineer, with regular inspection and certification by a competent independent engineer every ten years. It was the first attempt in the UK to regulate the design, construction, and maintenance of reservoirs through statutory safety measures, and required water levels and leakages to be monitored.


Current research Our current research aims to reconstruct the


Above: Location Map of Dolgarrog in North Wales (elevation 10m above sea level) and the Afon Porth-llyd


JUST OVER ONE HUNDRED years ago, on 2 November 1925, a catastrophic dam-break flood carrying enormous boulders swept through part of Dolgarrog village and the hamlet of Porth-llŵyd, in north Wales, destroying homes, a bridge, and the local chapel. The flood entered the aluminium smelting works and power station, resulting in a power blackout across


Author details


● Professor Jeff Warburton, Department of Geography, Durham University, Lower Mountjoy, South Road, Durham, DH1 3LE, UK


● Professor Jamie Woodward, Department of Geography, The University of Manchester, Arthur Lewis Building, Oxford Road, Manchester, M13 9P, UK


● Professor Stephen Tooth, Department of Geography and Earth Sciences, Llandinam Building, Penglais Campus, Aberystwyth University, Ceredigion SY23 3DB, Wales


● Dr Hywel Griffiths Department of Geography and Earth Sciences, Llandinam Building, Penglais Campus, Aberystwyth University, Ceredigion SY23 3DB, Wales


hydrodynamics of the Dolgarrog flood and quantify the mechanisms of erosion and sedimentation related to the event. The disaster was notable, not only as a milestone in UK reservoir safety, but also due to the nature of the flood hazard which resulted from a cascade failure of two dams. The upstream Llyn Eigiau cyclopean concrete gravity dam, completed in 1911, impounded 4500 x103


m3 of water, was 1200m long, and had a


maximum height of 10.5m. The downstream Coedty embankment dam, constructed in 1924, impounded a small reservoir of 320 x103


m3 . Both dams were


constructed as part of a hydropower scheme providing electricity for the local aluminium smelting works in Dolgarrog, with excess production exported to the energy grid. At around 8:45pm on Monday 2 November 1925, Llyn Eigiau dam’s foundations failed. This was caused by a blow-out in the underlying sediments that breached the base of the dam (described in the aftermath as a ‘cauldron of emergence’) and resulted in a flood channel approximately 20m wide and 3m deep being eroded into the open moorland. The dam-break flood surged 4km down the Afon Porth-llŵyd to the Coedty reservoir, which filled in minutes. The Coedty dam was then overtopped by the inflow and failed catastrophically, releasing a flood estimated at ~1.7 million m3


. Within minutes, sediment-laden flood waters 10 | February/March 2026 | www.waterpowermagazine.com


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