Lake Vyrnwy Dam

The Lake Vyrnwy Dam is a large gravity dam built in the late 19th century. It is 358 metres long and 44 metres high, and features a striking carriageway formed by 31 masonry arches, each spanning 7.3 metres. The dam’s design incorporates a deliberate sense of massiveness, influenced by the neo-Baroque style and the work of Sir John Vanburgh. To ensure structural stability, the base of the dam is exceptionally wide, measuring 39 metres. The dam is constructed from substantial, irregularly shaped blocks of locally quarried stone, primarily rubble with facing of fine-grained freestone and vermiculated dressings, particularly on the towers. The design is symmetrical, with groups of four towers located at either end of the nineteen-arch central section, which acts as the main overflow area. A full-length pierced stone parapet and piers run along each arch. The towers are capped by corbelled pyramidal roofs, with a pierced upper stage above a pronounced bracket cornice. The main piers of the towers exhibit rustication to create a ribbed appearance, while the doorways are tapered in an Egyptian style. The towers on the south-east side have heavily key-blocked round-arched openings with pierced and bracketed balustrades. Two inscriptions are carved into the frieze on this side: OPUS INCHOATUM MDCCCLXXX and OPUS ABSOLUTUM MDCCCXC.

Three commemorative plaques are set into the rock face to the north of the dam, one of which, positioned on the axis of the dam road, marks the laying of the foundation stone in 1881 by Lord Powis.

At the base of the dam, below the towers, are two transverse discharge tunnels fitted with hydraulically operated valves controlled from a chamber near the water level. The cast-iron valve controls were manufactured by Guest and Chrimes of Rotherham. The dam includes tapering circular shafts with cast-iron staircases providing access from the towers. A drainage tunnel network was integrated into the design to prevent water pressure build-up beneath the base, a feature believed to be a pioneering element for dams of the time.