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Latitude: 51.8279 / 51°49'40"N
Longitude: -3.4576 / 3°27'27"W
OS Eastings: 299647
OS Northings: 215371
OS Grid: SN996153
Mapcode National: GBR YL.W2YR
Mapcode Global: VH6CJ.0BY2
Entry Name: Cantref Reservoir dam, including valve house, draw-off tower and spillway (partly in Llanfrynach)
Listing Date: 6 August 2002
Last Amended: 29 July 2009
Source ID: 26829
Building Class: Water Supply and Drainage
Location: On the W side of the A470 Merthyr Tydfil to Brecon road approximately 10km NNW of Merthyr Tydfil.
County: Rhondda Cynon Taff
Locality: Nant Ddu
Traditional County: Brecknockshire
Cantref 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. Cantref Reservoir was the second reservoir to be built (after Llanishen), and the first of the three impounding reservoirs intended for the upper Taff. Construction started in 1886, and the reservoir was opened in 1892.
Dam is clay core and earth bund construction, with pitched stone to inner face of embankment terminating in battered rock-faced masonry wave-wall with simple cast-iron railings; turfed outer face. Offset to the east side of the dam is the single storey draw-off tower of coursed rock-faced stone. Flat roof concealed behind parapet. Round headed doorway with replacement doors, and metal plaque in tympanum recording the opening of the reservoir in 1892. Moulded cornice above doorway, and single window, renewed in original round-headed opening, to rear.
At the west end of the dam is a spillway, where the dam is terminated by a short pier with rounded end, of rock-faced stone facing the reservoir, and dressed stone to spillway. A segmentally-arched bridge crosses the spillway at the end of the dam. Spillway comprises stone-paved channel with dressed stone revetment to west bank below a rock-faced dwarf wall with railings and gateway. The channel continues down-stream in a series of weirs with stepped and then ramped dressed stone revetments with free-stone copings. Auxiliary syphon spillway added in later C20.
Listed as an integral part of the nineteenth century water-supply system for Cardiff. Together with structures associated with the other impounding reservoirs in the upper Taff and the Llanishen Reservoir, the Llwyn-on Reservoir represents a major Welsh civic engineering scheme which has survived virtually intact. Cantref is the earliest of the impounding reservoir dams and is contemporary with a water treatment works immediately downstream (in Llanfrynach Community).
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