Intercepting Weir and Tunnel, At Grid J3415 2425, Near Annalong, Newry, Co Down

This water supply infrastructure comprises an underground tunnel, intercepting weir, and associated structures completed between 1948 and 1952 to divert water from the headwaters of the Annalong River into the Silent Valley reservoir, serving Belfast's water supply.

History and Development

The Belfast City & District Water Commissioners originally planned to construct a reservoir in the Annalong Valley to supplement the Silent Valley reservoir, which opened in 1933. However, geological difficulties encountered during the Silent Valley's construction led to abandonment of this plan in 1945. A tunnel solution was adopted instead, to carry water under Slieve Bignian from the Annalong River headwaters to Silent Valley. Work began in 1948 with simultaneous driving from both ends; the two headings met in 1950. A waterlogged fissure delayed progress by approximately seven months until pressure grouting with cement could proceed. The contractors were A.M. Carmichael Ltd of Edinburgh, and the consulting engineers were Binnie, Deacon & Gourley of Westminster. The tunnel was completed and opened by Viscount Brookborough, Prime Minister of Northern Ireland, on 28 August 1952. The structures remain in use by the Water Service for their original purpose.

The Tunnel

The tunnel runs beneath Slieve Bignian and is approximately 3.4 kilometres long, 2.3 metres high and 2.4 metres wide. It is roughly equal in length to the Donard Tunnel under Millstone Mountain on the Silent Valley–Newcastle section of the Mourne Conduit. The tunnel has a fall of 5.5 metres along its length. The bottom is lined with concrete along its bed and to a height of 1.2 metres on either side. A granite datestone above the voussoir crown reads 1952, repeated at the Silent Valley outlet. The tunnel has a maximum capacity of 410 million litres per day, though average daily flow is approximately 23 million gallons. It is the longest tunnel in the province and remains unique in Northern Ireland as an imaginative engineering alternative to constructing a dam lower down the river.

The Weir and Associated Structures

An inclined fixed weir of reinforced concrete adjoins the tunnel entrance, over which stands a concrete footbridge. The weir intercepts the river and diverts water into the tunnel, featuring an almost-horizontal apron on its downstream side and a concrete parapeted footbridge with five horizontal spans. The riverbanks immediately above the weir are pitched with dressed granite blocks set vertically in irregular courses and coped with large saddle-backed blocks. A concrete footbridge at the top of the pond created by the weir has piers that form emplacements for vertical sluice gates, which may never have been installed; the bypass intake on the left bank appears not to have progressed beyond a masonry roundel in the abutment wall.

The tunnel intake is augmented upstream by discharge from an underground culvert fed by a secondary impounding weir across a nearby tributary. Impounded water from the south-west side of the pond enters the tunnel with flow controlled by two vertical valves. Excess water spills over the weir back into the river.

At the south-west corner of the main weir pond stands a small flow meter house built of strap-pointed granite blocks with a flat concrete roof. Its south face bears two plaques: one commemorating the opening by Viscount Brookborough on 28 August 1952, and the other recording the tunnel's technical specifications and purpose.