Slieve Bignian tunnel, Silent Valley Reservoir, Head Road, Kilkeel, Newry, Co Down, BT34 4PU

Slieve Bignian Tunnel

The Slieve Bignian tunnel is a water conveyance structure forming part of the Mourne Scheme, which supplies Belfast with water. It is the longest tunnel in Northern Ireland and represents a novel engineering solution to the challenges encountered in harnessing the waters of the Annalong River. The tunnel is of national interest as a major component of this significant water infrastructure project, and adds considerable interest to the landscape at its point of emergence.

The tunnel was conceived after the Belfast Water Commissioners' original plan to construct a reservoir in the upper reaches of the Annalong Valley proved unviable due to major geological problems encountered during construction of the Silent Valley dam. The alternative scheme diverted the Annalong River into the Silent Valley reservoir through a tunnel driven beneath Slieve Bignian. Excavation commenced in April 1948, with contractors Messrs A.M. Carmichael of Edinburgh working under the direction of consulting engineers Messrs Binnie, Deacon and Gourley of Westminster. The work was carried out simultaneously from both ends, and the two headings met in December 1950. The tunnel measures 3.4 kilometres (2 1/8 miles) in length with an overall fall of 5.5 metres (18 feet). Its internal dimensions are 2.3 metres (7 feet 6 inches) wide by 2.45 metres (8 feet) high, with concrete lining to a height of 1.25 metres (4 feet). Viscount Brookeborough, Prime Minister of Northern Ireland, formally opened the tunnel on 28 August 1952.

The tunnel emerges from the west side of Slieve Bignian through a segmental opening edged with stepped vee-jointed concrete blocks. The rock above the opening is faced with vee-jointed blocks in regular courses, above which sits a section of shuttered concrete bearing the date 1952. The wall is topped with ashlar granite blocks. The intake to the tunnel lies in the Annalong Valley. Water flows along a concrete-lined open channel into a circular concrete pipe positioned beneath the road and bank, which feeds into the reservoir. The parapets on either side of the road are constructed of roughly dressed granite blocks in regular courses with quarry-faced copings. The east parapet bears a bronze plaque commemorating the tunnel's opening in 1952. The pipe exit is embellished with a horseshoe-shaped trimming of vee-jointed cement blocks, with the bank similarly faced. The water discharges onto a shallow apron pitched with large granite blocks.