History

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.

Exterior

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.

Reasons for Listing

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.

External Links

• Cadwr Listing
• Wikidata Q29504390

External links are from the relevant listing authority and, where applicable, Wikidata. Wikidata IDs may be related buildings as well as this specific building. If you want to add or update a link, you will need to do so by editing the Wikidata entry.

Recommended Books

• 
  Historic Environment Law: Planning, Listed Buildings, Monuments, Conservation Areas and Objects, Supplement 2014
• 
  Nottinghamshire (Pevsner Architectural Guides: Buildings of England)
• 
  London 4: North: North v. 4 (Pevsner Architectural Guides: Buildings of England)
• 
  The green guide for historic buildings: how to improve the environmental performance of listed and historic buildings
• 
  Maintaining and Repairing Old Houses: A Guide to Conservation, Sustainability and Economy
• 
  Berkshire (Pevsner Architectural Guides: Buildings of England)