Intercepting Weir, At grid J3435 2438, Near Annalong, Newry, Co Down

This is an intercepting weir, constructed between 1948 and 1959 as part of the Slieve Bignian Tunnel scheme, a key element in Belfast's water supply and specifically relating to the Mourne Scheme. The weir is located near Annalong, Co Down, and is closely linked to the Bignian Tunnel complex (HB16/01/041). It has group value due to this association.

The structure is a vertical, reinforced-concrete weir built across a tributary of the upper Annalong River. Its purpose is to divert water through a south-westerly underground conduit to a small reservoir at the intake of the Slieve Bignian tunnel. When water levels are high, excess water spills over the top of the fixed-height weir. At the upper end, a set of vertical sluices are set between concrete abutments. Historically, when these sluices were closed, water could be diverted away from the weir, through a culvert on the left bank to a discharge orifice beyond the weir, allowing access to the conduit for maintenance. These sluice gates have since been removed, and this bypass is now redundant. Vertical concrete walls form the banks between the sluices and the weir. Concrete footbridges are situated at each end, each featuring two horizontal spans and metal two-bar handrails. A small metal pipe discharges from the bottom of the weir, with flow regulated by a valve accessible from the footbridge above.

The project was initiated by the Belfast City & District Water Commissioners and completed in 1952, with construction work carried out by A.M. Carmichael Ltd of Edinburgh, and engineering design by Binnie, Deacon & Gourley of Westminster. The weir remains in use by the Water Service for its original purpose. Original drawings are held by the Water Service at Westland House, Belfast (cat no. 2F1).