Llanishen Reservoir Dam

The Llanishen Reservoir Dam is a significant structure built to enclose a flat lowland site, covering an area of 238,000 square meters and stretching almost a mile in length. Like other reservoir dams in the area, including Cantref, Beacons, and Llwyn Onn, it features a clay core and mass earth bund construction with sloping embankments. The inner embankment is faced with pitched stone, while the outer embankment is turfed.

Notable features include the inlet gauge chamber from the Taff Fawr and the outflow control valve platform, both of which are faced in rusticated ashlar with heavily dressed plinths, string courses, and copings. Similar dressed stone is also used for the Nant Fawr wash channel and spillway, the culvert exits, the stepped weir, and the scour tunnel exit.