Altnaheglish Reservoir, Banagher Glen, Dungiven, Co Londonderry

Altnaheglish Reservoir is a large water storage facility created at the upper end of Banagher Glen on the Altnaheglish Burn in County Londonderry. It was formed by a mass concrete gravity dam constructed between 1930 and 1934, which was filled in November 1934 and officially opened in November 1935.

The dam is 110 metres long on crest and concave curved in plan to a radius of approximately 153 metres, developing a degree of arching against the abutments. It stands 42 metres high above foundation level, making it the highest dam in Northern Ireland and the eighth highest in the British Isles. The dam was founded on soft mica schist bedrock. Aggregate from an adjacent gneiss quarry was used for concrete production on site. The downstream side of the wall was originally stepped, though this stepping was later concealed by remedial works. Edward Sandeman of London acted as design consultant on the dam.

In March 1915, M A Robinson, Engineer for Londonderry Corporation and practicing architect, recommended the development of Banagher Glen as a major new water supply for the City of Derry. An Act of Parliament in 1918 enabled work to commence with construction of the access road, which extended over six miles and included a number of bridges. When M A Robinson died in 1929, Walter Criswell, who was resident engineer, was appointed City Engineer and, with W S Osman, was responsible for the design and construction of the remaining work.

Spalling of the downstream face was reported in 1952 and increased by 1959. Investigations were undertaken in 1960 and 1964, leading to the water level being lowered in 1967. Ferguson & McIlveen were appointed as consultant engineers and prepared reports recommending substantial remedial works. These remedial works, completed in 1987 with B Mullan & Sons as contractors, consisted of a new grout curtain and pressure relief system constructed from within the original inspection galleries that ran along the length of the dam wall. A series of 200mm diameter pressure relief wells in both the original upstream and new downstream transverse galleries were installed, fitted with protective liners to facilitate long-term maintenance. Well spacings and depths were designed to eliminate uplift pressure on the base of the high central section. The works included a rockfill berm support placed against the dam wall on the downstream side, a new side overflow spill weir on the north abutment to discharge probable maximum flow, the extension of existing eduction and scour galleries under the new berm, a bridge and access road onto the new downstream face, and a new dam crest constructed over the central spill weir section to give a continuous crest profile. The extensive use of galvanised steel safety railings and stairways around the later remedial works creates a utilitarian effect. The reservoir contributes greatly to the environment of the upper Banagher Glen and provides an essential utility.