Lluest Wen: the near failure and subsequent repair of a Pennine type dam

The embankment dam at Lluest Wen reservoir in South Wales was a classic example of a Pennine type dam, with a slender puddle clay core. The dam's troubled history of settlement and leakage, culminating in a near dam breach 2 days before Christmas 1969, highlights the vulnerability of earthfill dams to internal erosion adjacent to appurtenant structures. The incident was a factor in the passing of the Reservoirs Act 1975 through Parliament. Nearly 50 years after the emergency and the completion of significant remedial and improvement works to the dam, it is appropriate to reflect on the causes of the incident and the performance of the works completed in the 1970s.