Intercepting Weir and Tunnel

The Department's description

Exterior description and setting

This tunnel runs under Slieve Bignian and feeds the Silent Valley reservoir with water from the headwaters of the Annalong River. A granite datestone above the voussoir crown reads '1952' (the year of its opening); the same date appears at its Silent Valley outlet.
Adjoining the tunnel entrance, over which there is a concrete footbridge, is an inclined fixed weir of reinforced concrete to intercept the river and divert some of it into the tunnel. It has an almost-horizontal apron on its downstream side and a concrete parapeted footbridge of five horizontal spans over. The riverbanks immediately above the weir have been pitched with dressed granite blocks set vertically in irregular courses and coped with large saddle-backed blocks. At the top end of the pond created by the weir is another concrete footbridge, the piers of which form emplacements for vertical sluice gates. These gates may never have been installed as the bypass intake on the left bank seems not to have been progressed beyond a masonry roundel in the abutment wall. The intake to the tunnel is augmented a short distance upstream from this weir by the discharge from an underground culvert from a secondary impounding weir across a nearby tributary (HB 16/01/040).

The impounded water discharged from the south-west side of the pond into the tunnel, the flow being controlled by two vertical valves. Any excess spilled over the weir back into the river.

At this south-west corner there is also a weir flow meter housed in a small building of strap-pointed granite blocks with a flat concrete roof. Its south face bears two plaques: one records the opening of the tunnel by Viscount Brookborough, Prime Minister of N. Ireland, on 28 August 1952, and the other gives technical details of the tunnel's statistics and purpose.

Historical information

The Belfast City & District Water Commissioners originally planned to construct a reservoir in the Annalong Valley to augment the one opened at the Silent Valley in 1933. However, geological problems encountered with the construction of the latter forced the abandonment of this plan in 1945. Instead, it was decided to excavate a tunnel under Slieve Bignian to divert the headwaters of the Annalong River into the Silent Valley. This scheme was commenced in 1948, with simultaneous driving from both ends; the two headings met in 1950. A waterlogged fissure delayed work by some seven months until it could be pressure grouted with cement. The contractors were Messrs A.M. Carmichael Ltd of Edinburgh and the consulting engineers were Messrs Binnie, Deacon & Gourley of Westminster. The tunnel was completed in 1952 and is still used for its original purpose by the Water Service.

The tunnel is approximately 3.4km long, 2.3m high by 2.4m wide. It is about the same length as the Donard Tunnel under Millstone Mountain (on the Silent Valley – Newcastle section of the Mourne Conduit). The bottom of the tunnel, along the length of which there is a fall of 5.5m, is lined with concrete along its bed and for a height of 1.2m on either side. It has maximum capacity of 410 million litres per day; however, the average daily flow is only 23 million gallons.

Primary Sources:
1. Plaque at flow gauge station and also on valve tower house at Silent Valley Reservoir.
2. Original drawing of the tunnel and weir in DOE Water Service, Westland House, Old Westland Rd, Belfast (cat. no. 2F1).

Secondary Sources:
1 Belfast & District Water Commissioners, ‘The Mourne Scheme: development of the Silent Valley and Annalong Valley catchments’ (Belfast, 1959) pp. 25-7.

Criteria for listing: architectural interest

A. Style
B. Proportion
C. Ornamentation
E. Spatial Organisation
F. Structural System
G. Innovatory Qualities
J. Setting
K. Group value

Criteria for listing: historic interest

V. Authorship
Y. Social, Cultural or Economic Importance
X. Local Interest

Evaluation

The structures uncluding the tunnel are of some technical note as the tunnel is the longest in the province (the longest railway tunnel, near Goraghwood, Co Armagh, is only a mile long) and innovatory in being an imaginative alternative to constructing a dam lower down the river. In both respects, it is unique in N. Ireland. It now forms a key element of Belfast's water supply and besides its group value, it is also of significance as a public utility.