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Latitude: 52.2874 / 52°17'14"N
Longitude: -3.4325 / 3°25'57"W
OS Eastings: 302384
OS Northings: 266445
OS Grid: SO023664
Mapcode National: GBR 9M.Y3JJ
Mapcode Global: VH696.GR7T
Entry Name: Dolau Siphon Outlet House
Listing Date: 28 February 2005
Last Amended: 28 February 2005
Source ID: 84119
Location: In a field immediately NE of Dolau Chapel, approximately 1km W of Nantmel.
Traditional County: Radnorshire
Part of the Birmingham Corporation scheme to supply water to the city from reservoirs in the Elan Valley. The project began in 1892 with the construction of the reservoirs and opened in 1904. Chief engineer was James Mansergh, joined and later succeeded as project engineers by his sons Ernest Lawson Mansergh and Walter Leahy Mansergh. The water was conveyed principally by means of a subterranean aqueduct, but where the river valleys caused the ground level fell below the hydraulic gradient one of the solutions was to direct the water into siphons that carried the water under the valley floors. Siphons consisted of cast-iron pipes of 42-inch (10.7cm) diameter, the relatively small dimensions of the pipe being offset by the high velocity of flow. Each siphon was designed for 6 pipes but only 2 were built in 1904, the remainder being reserved for an increase in future demand. A third pipe of 60-inch (15.2cm) diameter was completed in 1939 and a 4th pipe of the same diameter was constructed in the 1950s. Each siphon has an inlet and an outlet house where water is channelled to and from the main aqueduct. The water flows into a bell chamber beneath the railed forecourt, then into the pipes situated below the inlet/outlet houses, which house valve controls.
A single-storey outlet house of brick with freestone dressings and rusticated quoins, on a battered rock-faced plinth. The roof is concealed behind a moulded cornice of reconstituted stone. The E side has central steel doors (beneath inscription bands now chiselled out) flanked by cross windows. The rear has 3 similar windows. The E front has forecourt railings with diagonal sides to a central gate.
Listed for its special architectural and historic interest as an integral component of one of the foremost civil engineering projects of the early C20 in Wales.
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