| Spotlight
plunged 230m down the final 2500m of the Afon Porth-llŵyd into the village. Enormous boulders up to 5m in length were excavated from the bedrock gorge and swept onto the lower valley slopes and Afon Conwy floodplain, depositing an extensive fan of debris, destroying buildings and burying infrastructure. Following the disaster, the Coedty dam was rebuilt.
The Eigiau dam was never reinstated but has been left as a reminder of the disaster to the present day. Several key factors contributed to the failure of the upper Eigiau dam, most notably: ● The shallow depth of the foundations, which were grounded in a weak glacial clay below a thick layer of peat.
● The summer prior to failure had been very dry, resulting in the exposure of the base of the dam wall and shrinkage of the peat and desiccation of the clay substrate, which were then rapidly rewetted during a wet October.
● The very poor quality of the cyclopean concrete used in the construction of the dam, which was full of voids and structural flaws.
The long profile of the Afon Porth-llŵyd, from its
source at Llyn Eigiau to its eventual confluence with the Afon Conwy, contributed significantly to the impact of the disaster. The overall long profile form is convex upward, in as much as the gradient of the channel increases dramatically downstream of the Coedty reservoir. Therefore, the force of the dam-break flood released
from the headwaters, and the ability to entrain debris and sediment, increased downstream rather than decreasing along a long profile of reducing gradient. In the UK uplands, this long profile form is not uncommon, and many upland reservoirs are also located close to large areas of population where
downstream settlements and critical infrastructure could be affected by a dam failure.
Important legacy
An important legacy of the Dolgarrog disaster, and the legislation that followed, is that it helped to ensure that no further fatalities by dam failures have occurred. However, there is increasing concern that given ageing dam infrastructure in the UK and a changing climate, there is the potential for an increased risk of future failures. Most reservoirs in upland Britain were constructed in the 19th century under hydrological conditions that no longer hold1
. Embankment dams and older masonry dams can be especially vulnerable to erosion, seepage,
Above: Historical image from 4 November 1925 - The breach in the Coedty dam upstream of Dolgarrog. Photo: Smith Archive/Alamy
Below: Drone image (2024) of the breached Eigiau dam and downstream erosion channel
www.waterpowermagazine.com | February/March 2026 | 11
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