Altnaheglish Reservoir

The Department's description

Exterior description and setting

A large reservoir created at the upper end of Banagher Glen on the Altnaheglish Burn formed by a mass concrete gravity dam 110 metres long on crest and concave curved in plan to a radius of approximately 153 metres to develop a degree of arching against the abutments. The dam was founded on soft mica schist bedrock and aggregate from an adjacent gneiss quarry was used for concrete production on site. It is built to a height of 42 metres above foundation level making it the highest dam in N Ireland and 8th in these islands.

On the downstream side the wall was stepped but this stepping was concealed by more recent remedial works. The remedial works consisted of a new grout curtain and pressure relief system constructed from within the original inspection galleries which ran within the dam wall along its length. A rockfill berm support was placed against the dam wall on the downstream side. A series of 200mm diam. pressure relief wells in both the original (upstream) and new (downstream) transverse galleries were installed . These wells are fitted with protective liners to facilitate long term maintenance and the well spacings and depths are designed to eliminate uplift pressure on the base of the high central section of the dam.

The works included a new side overflow spill weir on the north abutment to discharge the probable maximum flow around the enlarged structure, the extension of the existing eduction and scour galleries under the new berm and a bridge and access road onto the new downstream face. A new dam crest was constructed over the central spill weir section, which gives a continuous crest profile.

See section of dam wall attached.

Historical information

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 passed in 1918 enabled the work to commence with the construction of the access road. 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 design and construction of the remaining work. The dam was constructed between 1930 and 1934 and the reservoir filled in November 1934 and opened officially the following November. The construction of the access involved a road of over 6 miles and a number of bridges.

Edward Sandeman of London acted as design consultant on the dam and he was also involved in the Silent Valley Dam between 1929 and 1932.

Spalling of the downstream face of the dam wall was reported in 1952 and increasing in 1959. Investigations were put in hand in 1960 and 64. The water level was lowered in 1967. Ferguson & McIlveen were appointed as consultant engineers and reports prepared. The work described above was put in hand. B Mullan & Sons were contractors and work was completed in 1987.

I H R No. 1925

References

Primary Sources
1. Short paper prepared by Water Service Western Division.

Evaluation

An impressive reservoir dam construction and engineering works which not only provides an essential utility but contributes greatly to the environment of the upper Banagher Glen. The initial works of the mass concrete curved dam wall created the massive visual impact of the reservoir and iemphasises the engineering achievement. The later rock reinforcement of the dam wall did not detract from this,The extensive use of galvanised steel safety railings and stairways around the later remedial works creates an unfortunate utilitarian effect.
Listing is not considered appropriate for civil engineering works of this type.