Abstract Description: The Eigiau and Coedty Dam failures of 1925, known as the Dolgarrog Incident, represent a landmark event in dam safety history. This disaster, which resulted in 16 fatalities and extensive destruction in North Wales, directly led to the passage of the Reservoirs (Safety Provisions) Act of 1930, establishing the foundation of modern dam safety regulation in the United Kingdom (UK). This paper and presentation revisit these cascading failures to explore their technical causes, lessons learned, and their enduring influence on dam safety practices and regulations. The impact of this event is seen today in the modern risk assessment process.
Constructed by Aluminium Corporation Limited (ACL) for hydroelectric power generation at Dolgarrog, Eigiau Dam was a concrete gravity dam with a maximum height of 10 meters (32.8 feet) located upstream of Coedty Dam, an embankment dam with a maximum height of 11 meters (36.1 feet). On November 2, 1925, the failure of Eigiau Dam, due to a shallow and inadequate clay foundation, triggered the overtopping and subsequent failure of Coedty Dam. Coedty Dam’s limited spillway capacity and storage volume offered no attenuation of the floodwave, which released over 1,216 acre-feet of water in the first hour, devastating the village of Dolgarrog.
The UK Environment Agency has studied these failures extensively, and published literature identifies critical technical issues. Eigiau Dam, described as “intrinsically unsafe,” was constructed at a shallower foundation depth than originally designed. Interestingly, a 1924 incident at the nearby Cowlyd Dam, also built and owned by ACL, did not prompt a reevaluation of Eigiau Dam’s conditions. These oversights highlight the need for rigorous geological assessments, competent engineering oversight during construction, and the evaluation of risks posed by construction delays or work stoppages.
The regulatory response to this disaster was transformative. The Reservoirs (Safety Provisions) Act of 1930 mandated periodic inspections by licensed engineers, established standardized design and construction practices, and required risk assessments for large reservoirs. This framework became a model for modern dam safety programs and demonstrated the enduring importance of proactive regulations to prevent failures. This presentation will provide a detailed technical analysis of the failures and examine how the lessons learned influenced regulatory advancements. By revisiting this historical case, the dam safety community can gain a deeper understanding of the interplay between technical challenges and the regulatory framework, ensuring that these lessons continue to inform safer dam design, construction, and management in the future.
Learning Objectives:
Describe the Eigiau & Coedty dam failure event and the downstream consequences of the failures in Dolgarrog.
Identify overlooked causes/risk-drivers of the failure and provide the connection to resulting regulations.
Demonstrate the current impact of the failure incident on current dam safety regulation & risk assessment processes.
