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Llanishen Reservoir Dam

A Grade II Listed Building in Llanishen, Cardiff

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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

Grade: II

Source: Cadw

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.

County: Cardiff

Community: Llanishen (Llanisien)

Community: Llanishen

Built-Up Area: Cardiff

Traditional County: Glamorgan

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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.

Reasons for Listing

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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  • II Century Methodist Church
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  • II Cyncoed Methodist Church (Church only)
    In its own grounds at the corner of Westminster Crescent
  • II The Court School including attached chapel
    At the E end of Llanishen, near the railway station. Set back from the road in its own grounds, the front facing N.
  • II The Hollies including former coach-house range
    At the E end of Llanishen, near the railway station. Set back from the road in its own grounds, the front facing N.
  • II Hill House
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  • II* Church of St. Isan
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  • II Church Inn
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