Llanishen Reservoir Dam

The flat lowland site of the Llanishen reservoir necessitated a continuous encircling dam, enclosing an area of 238,000 square metres, almost a mile in length. Like the other reservoir dams of the scheme (Cantref, Beacons and Llwyn Onn), it uses a clay core and mass earth bund construction with sloping embankments. The inner embankment is faced in pitched stone, and the other embankment is turfed. The inlet gauge chamber from the Taff Fawr and the outflow control valve platform are faced in rusticated ashlar with heavily dressed plinths, string courses and copings. Similar dressed stone is used on the Nant Fawr by wash channel and spillway, the culvert exits, the stepped weir and scour tunnel exit.