Spotlight |


Right: Eigiau dam plan, dam cross-section and peat core of the dam substrate


slope instability or overtopping. UK Climate Change Projections (UKCP18), which illustrate a range of future climate scenarios until 2100, predict a greater chance of hotter, drier summers and warmer, wetter winters, both with more extreme weather events. Models predict that intense rainstorms that cause flash flooding will be five times more likely by 2080. Steep UK upland catchments underlain by hard impermeable rocks, where much of the UK’s water storage infrastructure is located, are especially vulnerable to flash flooding. In late July 2019, the evacuation of more than


Below: Long profile of the Afon Porth-llyd


1500 residents from Whaley Bridge downstream of Toddbrook reservoir in Derbyshire, England, was ordered. Toddbrook had received a month’s rain (160mm) between 27-31 July, and swollen inflows overtopped the dam’s emergency spillway, causing erosion of the concrete lining. This erosion exposed the dam’s core, raising fears of a breach2


.


Following the Toddbrook reservoir incident, the UK government commissioned an Independent Reservoir Safety Review Report, which identified systemic weaknesses in the reservoir safety regime and called for a new hazard classification and safety management system.


Geomorphological considerations As a team of geomorphologists who work on river


processes and landforms, we are interested in the landscape-changing effects of such dam-breach floods, and how a combination of substrate properties, catchment topography and flood hydraulics can amplify the dam-breach flood hazard to communities. The 1925 Dolgarrog disaster serves to highlight the dangers of ignoring such factors, treating them cursorily, or viewing them in isolation. We advocate strongly that such geomorphological considerations be an essential part of flood emergency planning and hazard assessment.


References


1. Woodward, J, Warburton, J. and Tooth, S. (2025) Dam disasters of the 1920s made reservoirs safer – now the climate crisis is increasing risk again The Conversation. https://doi.org/10.64628/AB.q5vupyhtx


2. Balmforth, D. (2020) Toddbrook Reservoir Independent Review Report. PB 14612 https://assets.publishing.service.gov.uk/ media/5e6f3f56d3bf7f2690785d52/toddbrook- reservoir-independent-review-reporta.pdf


12 | February/March 2026 | www.waterpowermagazine.com


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