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Latitude: 51.5262 / 51°31'34"N
Longitude: -3.1743 / 3°10'27"W
OS Eastings: 318632
OS Northings: 181464
OS Grid: ST186814
Mapcode National: GBR KK2.6V
Mapcode Global: VH6F0.XWRW
Plus Code: 9C3RGRGG+F7
Entry Name: Llanishen Reservoir Dam
Listing Date: 24 July 2009
Last Amended: 24 July 2009
Source ID: 87591
Building Class: Water Supply and Drainage
Location: To the east of Llanishen, accessed via a track running SE from the B4562, some 0.5km east of Llanishen station.
Community: Llanishen (Llanisien)
Built-Up Area: Cardiff
Traditional County: Glamorgan
Llanishen Reservoir forms part of the nineteenth century water-supply system for Cardiff. Supply was first in the hands of a private waterworks company who constructed a reservoir at Lisvane in the 1860s, but the Cardiff Borough Council assumed responsibility for the growing town's water supply in 1878, and commissioned a survey by its own engineer, John Avery Williams, to identify a suitable and sufficient water supply for the town. His report was presented in 1881, and the Taff Fawr scheme was the option favoured by the council. Early plans for Llanishen reservoir (also by J.A.Williams) pre-dated adoption of the scheme, but construction went ahead on the basis that it would be an integral part of it, as a storage reservoir receiving water impounded in the upper Taff valley. Llanishen was the first of the four reservoirs which comprised the scheme to be built, and construction began in 1884. It served as the primary source of water for Cardiff during construction of the Taff Fawr reservoirs, then began receiving water from the first of these in 1891.
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.
Listed as an integral part of the nineteenth century water-supply system for Cardiff. Together with structures associated with the impounding reservoirs in the upper Taff, the Llanishen Reservoir represents a major Welsh civic engineering scheme which has survived virtually intact.
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