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Latitude: 51.7925 / 51°47'32"N
Longitude: -3.4349 / 3°26'5"W
OS Eastings: 301133
OS Northings: 211398
OS Grid: SO011113
Mapcode National: GBR YM.Y8M1
Mapcode Global: VH6CQ.D6YQ
Entry Name: Llwyn-on Reservoir dam, including valve house and spillway (partly in Hirwaun community)
Listing Date: 6 August 2002
Last Amended: 29 July 2009
Source ID: 81190
Building Class: Water Supply and Drainage
Location: Prominently sited on the W side of the A470 Merthyr Tydfil to Brecon road approximately 6km NW of Merthyr Tydfil.
County: Merthyr Tydfil
Town: Merthyr Tydfil
Community: Vaynor (Y Faenor)
Traditional County: Brecknockshire
Llwyn-on 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. Llwyn-on Reservoir was the last of the four reservoirs comprising the scheme to be built, and detailed design work was carried out by Charles H Priestley, on a bigger scale than envisaged in the original scheme. It was begun in 1912, but work was delayed after the outbreak of war in 1914, and it was not completed until 1926.
Dam is clay core and earth bund construction, with pitched stone to inner face of embankment, terminating in a battered rock-faced masonry wave-wall forming parapet to roadway; turfed outer face. Circular two-stage valve house offset to east side of the dam, where cast-iron gates with flanking railings form a break in the parapet. The valve house has an embattled parapet on a corbel table and higher corbelled stair-turret to south-east. It has a pointed arched entrance with replacement doors, and metal plaque in tympanum recording the opening of the reservoir in 1926. The sides have windows replaced in original openings. Above the lower stage are narrow loops lighting the stair and the upper stage.
The spillway is on the east side of the dam, where the dam is terminated by a short pier with a rounded end. The road is carried by a 2-arch bridge over the spillway, with elliptical arches and pointed cutwaters to the central pier. Beyond this the parapet wall curves round with the road where it forms a revetment to a steep bank further east. The spillway is stone-paved and has a revetment wall to the bank on the east side. Similar revetments enclose the stilling basin, overflow channel and weirs below the dam. Tunnel portal alongside spillway at base of dam, with rusticated stone voussoirs and parapet. A later C20 syphon spillway with submerged intake is in the centre of the dam.
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. The Llwyn-on Reservoir dam is a prominent landmark of definite quality and character.
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